A review on advances in photocatalysts towards CO2 conversion

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A review on adva

SrmUHiCms(tmH

ment) in the Youngone Group oBangladesh. Beginning on 19 Septetime ‘Faculty Member’ in the depaand Polymer Science, SUST, Scontinues today. He is now con“Photocatalytic Conversion of CO2

University of Malaya.

aDepartment of Chemical Engineering, Facu

50603 Kuala Lumpur, Malaysia. E-mail: ashbDepartment of Chemical Engineering & P

Science & Technology, 3114 Sylhet, Banglad

Cite this: RSC Adv., 2014, 4, 20856

Received 28th February 2014Accepted 8th April 2014

DOI: 10.1039/c4ra01769b

www.rsc.org/advances

20856 | RSC Adv., 2014, 4, 20856–208

nces in photocatalysts towardsCO2 conversion

Sreejon Dasab and W. M. A. Wan Daud*a

The present situation reveals the dependence on fossil fuels, which is seriously accountable for two major

impediments: (i) global warming due to increasing atmospheric carbon dioxide (CO2) and (ii) the alarming

consumption of energy assets. The reduction of green CO2 in terms of producing solar fuels would be an

expedient accomplishment to resolve both problems, simultaneously. The review classifies different

categories of photocatalysts applied in foregoing photocatalytic CO2 conversion processes with detailed

information concerning operating conditions, preparation techniques and physical properties of

catalysts, radiation sources, and selectivity. The categories are concentrated on metal oxides, sulfides,

phosphides, and p-type and nonmetal oxide semiconductors. In addition, their modification by doping

co-metals, noble metals, transition metals and non-metals for visible light response is emphasized.

Besides, for harnessing solar fuel, the recent prospect and advancement of novel sensitized catalysts by

dye elements, phthalocyanines and quantum dots (QDs) are also highlighted in this review. This

technology needs more efficient solar active catalysts to increase production rates as well as selectivity.

The recent scenario indicates that massive prospects and opportunities still exist in this area for further

investigation on catalyst selection.

reejon Das is a postgraduateesearch student in the Depart-ent of Chemical Engineering atniversity of Malaya, Malaysia.e graduated with Distinctionsn 2009, from the Department ofhemical Engineering and Poly-er Science, Shahjalal Univer-ity of Science and TechnologySUST), Sylhet, Bangladesh. Hehen began his career as a teamember (Environment, Safety,ealth and Chemical Manage-f Industries Ltd., Chittagong,mber 2010, he joined as a full-rtment of Chemical Engineeringylhet, Bangladesh, where heducting his research work ininto Hydrocarbon Fuels” at the

lty of Engineering, University of Malaya,

[email protected]

olymer Science, Shahjalal University of

esh

93

1. Introduction

World energy consumption has deep implications for human-ity's social-economic-political sphere. For this reason, theglobal energy map is changing with potentially extensiveconsequences for energy markets and trade, and also by a

Professor Wan Mohd Ashri BinWan Daud earned his bachelor'sdegree in chemical engineeringin 1991 at Leeds University,Leeds, UK and his master'sdegree in chemical engineeringin 1992 from the University ofSheffield, Sheffield, UK. Heearned his PhD degree inChemical Engineering in 1996 atthe University of Sheffield. Since2005, he has worked asProfessor of Chemical Engi-

neering at the University of Malaya. His research elds includeenergy, biomass conversion and the synthesis of catalyst materials,catalysis, zeolites, polymerization process, separation process(adsorption, activated carbon, and carbon molecular sieve);ordered mesoporous materials, and hydrogen storage materials.Professor Wan Mohd Ashri Bin Wan Daud has written approxi-mately 90 research papers that have been published.

This journal is © The Royal Society of Chemistry 2014

http://dx.doi.org/10.1039/c4ra01769b

http://pubs.rsc.org/en/journals/journal/RA

http://pubs.rsc.org/en/journals/journal/RA?issueid=RA004040

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growing world population.1–3 On the other hand, the world isgradually becoming aware that essential alterations will berequired to meet the growing demand for energy. Maximumenergy sources are inadequate to satisfy the growing globalenergy demand, which will double by 2050 and triple at the endof this century.4 According to the U.S. Energy InformationAdministration,5 energy consumption rates remained vigorousin several developing countries, specically in Asia (>4%). InNorth America, Europe and the Commonwealth of IndependentStates, consumption fell by 4.5%, 5% and 8.5%, respectively,because of economic breakdown.

Energy produced from fossil fuels has great importancebecause the latter can be burned (oxidized to carbon dioxideand water), producing signicant amounts of energy per unitweight.6 Non-renewable fossil fuels yield takes millions of years,and these assets are being exhaustedmuch faster than new onesare being made. Fossil fuel consumption is also threatening tothe environment. During the period from 1961 to 2010, theBritish Petroleum Statistical Review of World Energy 20137

estimated that the rate of energy consumption has shown anupward trend. Worldwide movement towards renewable energyproduction is therefore in progress to meet increased energyrequirements. The main renewable sources such as sunlight,wind, tides, waves, geothermal heat, biomass and nuclearenergy offer electricity with zero carbon dioxide emissions.8

Energy conversion from waste is one of the supportable sourcesto convert the unwanted biomass into 7 to 10 TW of energy.9–11

Other renewable sources like wind, hydroelectric and tide/oceancurrents provide almost 2.1 TW, 1.5 TW and <2 TW of energyrespectively.12 Conversely, these renewable sources are quitelimited to meet the overall energy demand.

To full the energy demand, nuclear energy, which generallyis the part of the energy of an atomic nucleus released by fusionor ssion or radioactive decay could be a promising renewablesource. Currently, approximately 14% of the world's electricenergy is provided by the nuclear energy.13 On the other hand,the protection of nuclear reactors is a forthcoming limitation ofnuclear chain-reaction control, and safe storage of theextremely radioactive nuclear waste is still an unidentiedpathway towards a low fossil-fuel consumption future.14 Naturaldisasters like earthquakes may have harmful impacts on theearth from the nuclear plants. Moreover, nuclear power is notable to produce liquid fuel for transportation purposes. Unlikenuclear energy, another sustainable and abundant source ofenergy is required without the introduction of harmful envi-ronmental effects.

Renewable solar energy is used to harness the sun's energyand convert it into usable products through solar heating,15,16

solar photovoltaics,17,18 solar thermal electricity,19 solar archi-tecture,20,21 and articial photosynthesis.22 The satisfaction ofglobal energy needs for an entire year is achievable by har-nessing solar energy on the Earth's surface. Solar fuel is formedby utilizing light to produce chemical energy23,24 by articialphotosynthesis, or thermochemical reaction. The developmentof inexpensive, limitless and clean solar energy technologiesmust have massive and long-term benets. This progress willintensify global energy reserves through dependence on an


original, innite and mostly import-independent resource,increase sustainability, decrease pollution, lower the costs ofmitigating climate change, and keep fossil-fuel prices lowerthan through other means.

Besides energy needs, sufficient physical evidence suggeststhat climate change is mainly responsible for carbon dioxideemissions from fossil-fuel burning.25–27 As stated by the Inter-national Energy Agency (2012),28 the goal of decreasing climatewarming by 2 �C is becoming more difficult and costly eachpassing year. All tolerable carbon dioxide emissions would belocked-in by the remaining energy infrastructure if appropriatesteps are not taken before 2017. Already, global carbon dioxidegrowth in the atmosphere was amplied by around 1.5 ppm(8 billion tons) per year from 1975 to 2002. Basically, theatmospheric carbon dioxide level inated much more quicklyby 2.25 ppm (12 billion tons) per year. Every year, more than 10billion tons of carbon dioxide is added to our atmosphere.29 Asestimated by the U.S. Department of Energy,30 natural processescan only absorb about half that amount.

Abundant CO2 can be an asset by using appropriate appli-cations in solar-fuel transformation. The recycling process ofCO2 can regulate global warming as well as fuel demand.31

Besides photocatalytic conversion of CO2, other conversionprocesses can turn CO2 into fuels illustrated in Fig. 1, but thedisadvantages in those processes are the high temperature andhigh electrical voltage requirements to break down the CO2

molecules, limitations of raw materials, high-cost operation,and unsustainability. Photocatalytic transformation of CO2 intosolar fuel is more sustainable and favorable than other tech-nologies because of zero addition of additional energyconsumption and environmental deterioration. In the begin-ning, photocatalytic CO2 conversion was conducted by an arti-cial lamp. Recently, interest has grown in the harnessing ofsolar power by advanced photocatalytic processes. The stabilityof catalysts and mild operating conditions are among the mostsignicant reasons for choosing this technology.32,33 However,more intensive study and improvement are required to selectactive solar-driven novel catalysts and effective architecture ofsolar photoreactors for industrial applications. Moreover, thereare some unanswered questions in this technology with theselectivity of the photocatalytic reactions and limited efficiency.

This paper concentrated on advancements in the photo-chemical transformation of CO2 into hydrocarbon fuels. Thesecond chapter of this paper provides a snapshot of currentdevelopments in the photocatalytic reduction of CO2 on varioustypes of semiconductors. Pure metal oxide semiconductors arecheap and the most commonly used photocatalysts in thistechnology like TiO2. To a large extent, this can be ascribed to itsband structure, which is acceptable, but certainly not optimalfor the reduction of CO2 driven by solar energy. The wide bandgap (3.2 eV) limits the absorption of light into the UV range. Themodication of metal oxide semiconductors by supportingmaterials helps to control the catalysts' size from crystal growthand facilitate the response towards visible light. The two mostcommon methods for extending the absorption range to visiblelight are sensitization and doping. Doping metal on semi-conductors acts as an electron trap to provide the electrons to

RSC Adv., 2014, 4, 20856–20893 | 20857


Fig. 1 Well-known CO2 transformation paths with potential products.34

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the CO2. Besides, non-oxide semiconductors initially received alot of attention. For example, the valence band of sulphidesemiconductors is made of 3p orbitals of the sulfur atoms and isshied upwards, compared with those of the oxide analogues,while the conduction band electrons are also more reductive.35

Interest has grown in the involvement of exible substratesbecause they deliver a high surface area for catalysts on thelms of substrates. In the mainstream, the review categorizesthe different types of photocatalysts used in previous photo-catalytic conversions of CO2 processes (with detailed informa-tion regarding the operating conditions, preparationtechniques and physical properties of catalysts, radiationsources and selectivity) based on metal oxides, suldes, phos-phides, and p-type and non-metal-oxide semiconductors. Inaddition, these processes highlighted their modications bydoping co-metals, noble metals, transition metals and non-metals for visible light responses. Besides, the recent prospectand advancement of novel sensitized catalysts by dye elements,phthalocyanines and quantum dots (QDs) for harnessing solarfuels are prominent in this review.

2. Photocatalytic CO2 transformationbased on catalysts

The transformation of CO2 into fuel by using solar lightirradiation is an effective method because there is no addi-tion of extra energy and no negative inuence on the envi-ronment. The immediate requirement in this technology is todevelop visible light-sensitive photocatalysts, which areprominent in CO2 recycling. Different types of photocatalystshave been already introduced by many researchers in thistechnology. Some of the catalysts exhibited high conversionrates and selectivity under visible light irradiation, whileother catalysts were not feasible for visible light response andpresented low yield rates. Researchers are still trying hard toadvance the properties of catalysts in terms of solar fuelproduction. The reaction mechanism of selective photo-catalysts under light irradiation is illustrated in Scheme 1. Aconcise classication of photocatalysts is also shown inFig. 2, based on the recent development in photocatalytic CO2

transformation.

20858 | RSC Adv., 2014, 4, 20856–20893

2.1 Metal oxide photocatalysts

Metal oxides as light-sensitive catalysts have been applied inseveral processes such as the breaking down of organic andinorganic materials to valuable products,23,36,37 water treat-ment,38,39 and self-cleaning40,41 processes. The leading approachof using metal oxide catalysts in CO2 conversion to carbona-ceous fuel was introduced by Inoue et al.42 Variations in metaloxide photocatalysts have already been introduced by inte-grating foreign elements and other compounds with differentsupportive substrates43–46 for lessening its particle size,controlling its crystal growth, and increasing its surface areaand pore volume. Various metal oxide photocatalysts, whichwere frequently studied in many photocatalytic CO2 reductionprocesses, are characterized in this section with sufficientinformation.

2.1.1 Surface unmodied metal oxide photocatalysts. TiO2

is a frequently used metal oxide in the photocatalytic trans-formation of CO2 into hydrocarbon fuel because it is an inert,corrosion-resistant and inexpensive semiconductor. In nature,TiO2 is established as three familiar minerals such as rutile,anatase and brookite. Among them, rutile and anatase phasesindicate better efficiency under light radiation. Mutually,anatase and rutile phases reveal denite band gap energy ataround 3.2 eV and 3.0 eV, respectively. The rutile phase iscapable of absorbing visible light because of its comparativelylow band gap energy, while the anatase form only displays itsresponse to ultra-violet (UV) irradiation. Thus, the rutile phaseis not used due to its ineffective photoactivity; the best photo-activity can be achieved by combining anatase with a slightamount of rutile.47 A clear idea of metal oxide semiconductorsused in various studies for photocatalytic CO2 transformation isrepresented in Table 1. The light absorption by titanium-dioxide semiconductor is effective, while the semiconductorexhibits high surface area. The surface area of TiO2 is indirectlyproportional to the size of particle; conversely, the size ofthe particle is directly proportional to the band gap energy. Thesemiconductor shows high band gap energy as a result of theshiing of valence band energy to lower energies aer a certaindecrease in the particle size, whereas band energy is intenselyshied to higher energies.48 On the other hand, signicantlynarrow band gap energy leads to inadequate redox potential toachieve oxidizing and reducing reactions. Therefore, it is



Scheme 1 Photocatalytic reaction mechanism of (a) an unmodified metal oxide semiconductor; (b) supported metal oxide semiconductor; (c)doped semiconductor; and (d) dye sensitized semiconductor under light irradiation.

Fig. 2 Classification of photocatalysts on CO2 transformation intofuel.

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compulsory to x a structure that exhibits a settlement betweenthe particle size and band gap energy range.49 Tan et al. inves-tigated a combination of 80% anatase and 20% rutile forms as aphotoactive catalyst.50 In Scheme 1(a), the photocatalyticconversion mechanism of CO2 on unmodied metal oxidesemiconductor is sketched.

Commercial Degussa P25 is found to be slightly less activethan modied TiO2 (anatase and rutile).51 It is indicated thatunmodied TiO2 (P25) metal oxide semiconductors generallyhave high surface area as well as an affinity to organic impuri-ties on their surfaces to facilitate the adsorption of organiccompounds, which have the potential to perform as both an


electron donor and a carbon source for the products.52 Besides,this type of metal oxide catalyst expresses better methaneproduction, compared with the Degussa P25 treated by calci-nation and washing where CO is the major product.52,53 Invarious studies, product dissimilarity is also observed due to thetreatment procedure on anatase TiO2 in several methods.54–56

For instance, treatment by the acidic solutions (1 M nitric acid)is suitable for formic acid formation due to the protonation ofreaction intermediates into the reaction medium.54 Treatmentwith 2-propanol acts as a hole scavenger in CO2 photoreductionwith anatase TiO2, where the methane formation rate wasconsiderably higher. In addition, it was observed that inabsence of 2-propanol, no methane was found.56 Moreover, thesynthesis of TiO2 by the sol–gel method removes the presence ofsmall brookite nanoparticles that normally arise in low-temperature formation reactions and considerably hinder thephase alteration to rutile at elevated temperatures.48,57

Some prospective metal oxide photocatalysts other than TiO2

were reported in many investigations. Inoue et al. studied threemetal oxides, TiO2, ZnO and WO3, to understand their photo-catalytic characteristics under UV light radiation.42 It was foundthat the yield of methyl alcohol increased as the conductionband becomes more negative with respect to the redox potentialof H2CO3/CH3OH, whereas methyl alcohol was not produced inpresence of the WO3 catalyst, which has a conduction bandmore positive than the redox potential of H2CO3/CH3OH.However, WO3 possesses low band gap energy (2.8 eV) withrespect to visible light irradiation. G. Xi et al. synthesized theW18O49 catalyst in the form of nanowires with band gap energy

RSC Adv., 2014, 4, 20856–20893 | 20859


Tab

le1

Advance

sin

surfac

eunmodifiedmetalo

xideca

talystsforCO

2photoreductiona

Researcher

(year)

andref.

Catalyst/ba

nd

gapen

ergy

Catalysttreatm

ent

tech

nique

Reactan

ts(amou

nt)/rad

iation

source/light

intensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),ph

ysical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg

catalyst)/effi

cien

cy(%

)Com

men

ts

G.X

iet

al.

(201

2)58

W18O49/2.7

eVOne-po

tsolution

-ph

asemethod

CO2(saturated

),water

vapo

r(—

)/visibleligh

t(>42

0nm)/—

(343

/—/—

/6/—

),(—

/—/nan

owires

with1

nm/—

/sup

ported

)

Methan

e/66

6b/—

�Ultrathin

nan

owires

indicate

strongligh

tab

sorption

from

thevisibleligh

tto

theNIR

region

s�A

lloxygen

vacancies

intheultrathin

nan

owires

areus

edup

bycarbon

dioxidemolecules,a

ndno

methan

emolecules

areprod

uced

X.L

iet

al.

(201

2)67

HNb 3O8/3.66

eVHyd

rothermal

synthesis

CO2(saturated

),water

vapo

r(7

kPa)/

350W

Xelamp/—

(318

/—/—

/4/—

),(0.1/

39.4/1.12nm

particle/—

/sup

ported

)

Methan

e/3.58

/—�K

Nb 3O8an

dHNb 3O8nan

obelts

show

greater

activities

forCO2ph

otored

uction

tomethan

ethan

commercial

TiO

2,a

ndtheKNb 3O8an

dHNb 3O8

particlessynthesized

byconvention

alsolid-state

reaction

�Theproton

icacidityalso

giveshigher

photocatalytic

activity

totheHNb 3O8nan

obelts

�Axed-be

dqu

artz

tubu

larreactor(159

mL)

isplaced

horizon

tally

,andthecatalystsarepu

tinside

onaat

quartz

plate

J.W.L

ekse

etal.

(201

2)69

CuG

aO2/2.6

eVStoich

iometric

mixing

CO2(saturated

),water

vapo

r(—

)/30

0W

Xearclamp/—

(—/—

/7/—

/—),(0.2/—

/60

–80mesh

particles/—/sup

ported

)

Carbo

nmon

oxide/9b/—

�Allo

yingtheB-sitewithFe

toform

CuG

a 1�xFe xO2

(x¼

0.05

,0.10,

0.15

,0.20)

createsresp

onse

toward

visiblean

dnear-infrared

region

s�A

gas-tigh

tcellis

builtby

stainless

steelconat

an

gecompo

nen

tstted

withtw

oinlet/ou

tlet

valves,a

ndon

eUVqu

artz

view

port

Stockan

dDunn

(201

1)68

LiNbO

3/3.6

eVPu

lverized

witha

mortaran

dpe

stle

toprod

uce

particles

CO2(30%

),H

2O

(10

mL)/natural

sunligh

tor

Hg

lamp/64

.2mW

cm�2

(—/—

/—/6/2),(—

/—/

1mm

particle/—

/su

pported)

Form

icacid/7.7

c /—

�TheLiNbO

3is

kept

onaplaten

(1.26�

10�5m

2)

abovethewater

togive

aga

s–solidcatalyticreaction

�MgO

dope

dLiNbO

3givesan

energy

conversion

efficien

cyof

0.72

%is

lower

than

that

of2.2%

for

LiNbO

3

�Theferroe

lectricmaterialassistan

cescarrier

sepa

ration

andgivesan

improved

catalytic

performan

ceY.Z

hou

etal.

(201

1)65

Bi 2WO6/2.69eV

Solidstate

reaction

and

hyd

rothermal

method

CO2(saturated

),H

2O(1

mL)/300

WXearclamp(>42

0nm)/—

(298

/—/—

/5/—

),(0.1/

5.6/nan

oplatesof

9.5

nm

thickn

ess/—/

supp

orted)

Methan

e/1.1/—

�Bi 2WO6nan

oplatesdisp

layexcessivepo

tential

intheap

plicationof

visibleligh

ten

ergy

�Ultrathin

geom

etry

ofthenan

oplateshelps

charge

carriers

tomovesw

ily

from

theinterior

tothe

surfaceforph

otored

uction

�Catalysts

areeven

lydisp

ersedon

theglassreactor

withan

area

of4.2cm

2

N.M

.Dim

itrijevicetal.

(201

1)53

TiO

2(D

egus

saP-25

Aeroxide)/

3.08

eV

TiO

2was

lein

D2O

inorde

rto

exch

ange

boun

dwater

CO2(saturated

),water

vapo

r(2.8

mmol)/10

0W

Hg

lamp(365

nm)/11

mW

cm�2

(298

/1/—

/2.6/—

),(0.01/

55/nan

oparticle/—/

supp

orted)

Methan

e/4d/—

�Itis

prop

osed

that

thetw

o-electron

andon

e-proton

reaction

sareresp

onsibleforinitialstep

intheredu

ctionof

CO2on

thesu

rfaceof

TiO

2

�Aclosed

reactoris

used

havingacircular

pot

sampleholde

rwithadiam

eter

of1.8cm

20860 | RSC Adv., 2014, 4, 20856–20893 This journal is © The Royal Society of Chemistry 2014

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Tab

le1

(Contd.)

Researcher

(year)

andref.

Catalyst/ba

nd

gapen

ergy

Catalysttreatm

ent

tech

nique

Reactan

ts(amou

nt)/rad

iation

source/light

intensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),ph

ysical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg

catalyst)/effi

cien

cy(%

)Com

men

ts

T.Y

uiet

al.

(201

1)52

TiO

2(D

egus

saP-25

)/3.08

eVTreated

bycalcinationan

dwashing

CO2(—

),H

2O(1.5

mL)/500

WHgarc

lamp(>31

0nm)/—

(278

/—/4.1/5/—

),(0.15/—/nan

oparticle/

0.1/su

spen

ded)

Carbo

nmon

oxide/

0.35

/—�C

H4was

themainprod

uctincase

ofPd

–TiO

2,and

CO

form

ationis

sign

icantlyredu

ced,

compa

red

withthat

onthepretreated

TiO

2

�Isotope

labe

lingsp

ecies

that

CO2an

dCO32�are

themaincarbon

sourcesof

theCH

4prod

uction

�Catalysts

areke

ptin

aroun

d-sh

aped

quartz

vessel

of6cm

diam

eter

Q.L

iuet

al.

(201

0)66

Zn2G

eO4/4.5

eVSo

lvothermal

method

CO2(—

),water

vapo

r(—

)/UV

ligh

t/—

(—/—

/—/16/—),

(—/28.27

/nan

oribbo

nof

7nm

thickn

ess/—/

supp

orted)

Methan

e/0.41

/—�U

ltra-lo

ngan

dultrathin

geom

etry

oftheZn

2GeO

4

nan

oribbo

nsh

owshighpe

rforman

ceon

photocatalytic

activity

�Therate

ofCH

4prod

uction

over

theZn

2GeO

4

nan

oribbo

ncouldbe

sign

icantlyen

han

cedby

load

ingof

PtK.K

ociet

al.

(200

9)48

TiO

2(anatase)/

3.0eV

Sol–gelprocess

CO2(—

),NaO

H(0.2

M,1

00mL

solution

)/8W

Hg

lamp(254

nm)/1.41

mW

cm�2

(—/1.08/—/24/—),(0.1/

106/14

nm

nan

oparticle/0.00

1/su

spen

ded)

Methan

e/0.30

8/—

�Astirredba

tchan

nular

reactor(380

cm3)with

quartz

inner

tube

andstainless

steelsh

elltube

isus

edin

this

expe

rimen

t�T

heyields

isob

served

insu

chan

orde

r:H

2>CH

4>

CH

3OH

$CO

Y.L

iuet

al.

(200

9)63

BiVO4

(mon

oclinic)/

2.24

eV

Microwave

assisted

hyd

rothermal

method

CO2(—

),H

2O(100

mL)/300

WXearc

lamp(>40

0nm)/—

(273

/1/—

/1/—

),(0.20/—/

nan

oparticles/—

/su

spen

ded)

Ethan

ol/110

/—�P

hotocatalystBiVO4exhibitsvisible-ligh

tresp

onse

�Intense

irradiationcreateshug

eam

ountof

C1

interm

ediate

speciesan

chored

ontheBiVO4,w

hich

dimerizes

toform

ethan

ol�C

atalysts

aredisp

ersedin

water

andmag

netically

stirredinside

thereactor

L.Yuliatiet

al.

(200

8)59

Ga 2O3(Kishida

chem

icals)/4.7

eV

Clean

ingat

13.3

kPaof

oxygen

atmosph

ereat

1073

Kfor1hou

r

CO2(200

mmol),CH

4

(200

mmol)/30

0W

Xe-lamp(220

–300

nm)/9mW

cm�2

(473

/1/—

/3/—

),(0.2/2/—

/—/sup

ported

)Hyd

rogen,c

arbo

nmon

oxide/4.13

,1.8/—

�Sup

plem

entary

thermal

energy

enhan

cesthe

reaction

betw

eenmethan

ean

dcarbon

dioxideby

stim

ulatingthethermal-activationstep

sin

the

reaction

mechan

ism

�Aclosed

quartz

reactor(30cm

3)wherethe

catalystsaresp

read

over

aat

bottom

ofthereactor

(14cm

2)

Xia

etal.(20

07)70

TiO

2(D

egus

saP-25

)/3.08

eVCalcined

at72

3K

for2h

CO2(1

mmol),water

vapo

r(5

mmol)/15

WUVlamp(365

nm)/—

(298

/—/7/5/—

),(—

/50/

nan

oparticle/—/

supp

orted)

Methan

e/15

/—�A

ppropriate

amou

ntof

MWCNTsas

supp

orts

for

TiO

2couldextrao

rdinarilyad

vance

theeffi

cien

cyof

theph

otocatalytic

reaction

�Catalysts

areke

pton

apieceof

tran

sparen

tglass

andthen

putinto

ahom

emad

estainless

steel

reactor

C.-C

.Loet

al.

(200

7)62

ZrO2(Proch

em)/

5.0eV

ZrO2is

immob

ilized

bycoated

glass

pelle

ts

CO2(99.8%

),H

2

(95%

)/15

WnearUV

lamps

(254

nm)/—

(316

/1.1/—

/2/—

),(0.25/—/100

mesh

powde

r/—/sup

ported

andim

mob

ilized

)

Carbo

nmon

oxide/

0.51

/—�A

circulated

,packe

d-be

dph

otocatalytic

reaction

system

ismad

efrom

aqu

artz

tube

with48

0mm

lengthan

d22

.5mm

inner

diam

eter

�One-site

Langm

uir–Hinsh

ewoo

dkinetic

mod

elis

successfully

tted

tosimulatetheph

otored

uction

rate

This journal is © The Royal Society of Chemistry 2014 RSC Adv., 2014, 4, 20856–20893 | 20861

Review RSC Advances

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ishe

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Tab

le1

(Contd.)

Researcher

(year)

andref.

Catalyst/ba

nd

gapen

ergy

Catalysttreatm

ent

tech

nique

Reactan

ts(amou

nt)/rad

iation

source/light

intensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),ph

ysical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg

catalyst)/effi

cien

cy(%

)Com

men

ts

S.S.

Tan

etal.

(200

6)50

TiO

2(80%

anatasean

d20

%rutile)/3.0

and3.2eV

Heated-up

to47

3Kin

oven

CO2(saturated

),water

vapo

r(saturated

)/1.6W

UV

ligh

t(253

.7nm)/—

(298

/1/—

/48/—),(100

/50

/4mm

porous

pelle

t/—/sup

ported

)

Methan

e/0.01

4/—

�Acylindrical

quartz

tube

xed-be

dph

otocatalytic

reactorhavingdimen

sion

s30

0mm

(len

gth)a

nd74

mm

inner

diam

eter

isop

erated

for48

hcontinuo

usly

�Ago

odredu

ctionyieldis

attained

ascompa

red

withim

mob

ilized

catalyststhroug

hthethin-lm

tech

nique

andan

choringmethod

G.R

.Dey

etal.

(200

4)56

TiO

2(anatase)/

3.0eV

Susp

ension

ofTiO

2in

presen

ceof

0.5M

2-prop

anol

CO2(—

),H

2O

(5mL)/light(350

nm)/—

(298

/1/—

/10/—),

(0.005

/—/325

mesh

powde

r/—/sus

pende

d)

Methan

e/6/—

�Itis

observed

that

surface-ad

sorbed

andin

situ-

generated

CO2areliab

leformethan

eform

ation

throug

hph

otored

uction

byTiO

2

�Inpresen

ceof

saturatedO2,themethan

eyieldis

lower,c

ompa

redwiththat

inaeratedsystem

while

CO2yieldis

develope

d�A

quartz

cell(16.5mL)

havingaga

s-pu

rginginlet

andaga

ssamplingpo

rtis

used

torunthe

photored

uction

ofCO2

K.T

eram

ura

etal.(200

4)61

MgO

(Merck)

andZrO2/7.0

eVan

d5.0eV

Hyd

rated,

ltered

andcalcinated

inairat

873Kfor3h

CO2(150

mmol),CH

4

(50mmol)/50

0W

ultrahighpressu

reHglamp/—

(293

/1/—

/5/—

),(0.3/

110/10

0mesh

powde

r/—/sup

ported

)

Carbo

nmon

oxide/2.4

and0.48

resp

ectively/—

�Itis

foun

dthat

thesu

bstrate-mod

ied

insu

lating

materialshow

sreason

ableactivityin

CO2redu

ction

�Catalysts

areke

pton

aon

theat

bottom

ofa

quartz

reactor(closedstatic

system

)withcapa

city

18.9

mL

A.H

.Yah

aya

etal.(200

4)71

NiO

/——

CO2(saturated

),de

ionized

water

(70

mL)/highph

oton

ux

mon

ochromatic

ligh

t(355

nm)/—

(—/—

/7/1.5/—

),(0.3/—

/po

wde

r/—/sus

pende

d)Methan

ol/393

/—�T

heform

ationof

methan

olfrom

CO2is

areversible

processan

dde

grad

estheph

otocatalytic

processto

generatehyd

rogenan

dCO2ag

ain

�APy

rexcellwithop

tical-g

rade

quartz

windo

wsis

used

forph

otored

uction

ofCO2having35

mm

diam

eter

and12

0mm

length

S.Kan

ecoet

al.

(199

9)54

TiO

2(anatase)/

3.0eV

Pretreated

bybo

ilingin

1M

nitricacid

and

thorou

ghly

rinsing

CO2(saturated

),H

2O

(5mL)/990

WXelamp(>34

0nm)/

96mW

cm�2

(308

/88.82

/3/5/—

),(0.05/8.7/23

0nm

nan

oparticle/0.01

/su

spen

ded)

Form

icacid/1.8/—

�Add

ingof

acidic

solution

srather

than

pure

water

issu

perior

forform

icacid

prod

uction

�Stainless

steelvessel

(557

.5mL)

withaglass

windo

wis

used

tocarryou

ttheph

otored

uction

ofCO2

S.Kan

ecoet

al.

(199

7)55

TiO

2(anatase)/

3.0eV

Purity

99.9%

and

treatedby

1M

nitricacid

with

distille

d-deion

ized

water

CO2(saturated

),H

2O

(57.5mL)/990

WXelamp(340

nm)/96

mW

cm�2

(293

/64.15

/—/30/—),

(0.05/8.7/23

0nm

particle/—

/sus

pende

d)

Form

icacid/0.28/—

�Thereis

noform

ationof

gaseou

sredu

ction

prod

ucts,a

ndform

icacid

isen

tirely

foun

din

the

aque

oussolution

�Theinternal

surfacewas

completelycoveredby

Te

onto

preven

tcontaminationfrom

thestainless

steelan

ditscatalyticeff

ecton

redu

ction


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4


Tab

le1

(Contd.)

Researcher

(year)

andref.

Catalyst/ba

nd

gapen

ergy

Catalysttreatm

ent

tech

nique

Reactan

ts(amou

nt)/rad

iation

source/light

intensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),ph

ysical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg

catalyst)/effi

cien

cy(%

)Com

men

ts

M.A

npo

etal.

(199

5)72

AnataseTiO

2

(JRC-TIO

)an

drutile

TiO

2(JRC-

TIO

)/3.47

and

3.32

eV

Degassedan

dcalcined

at45

2� C

CO2(0.15mmol),

water

vapo

r(0.25

mmol)/75

Whigh-

pressu

reHglamp

(>28

0nm)/—

(275

/—/—

/8/—

),(—

/16

and51

/1mm

nely

powde

red/—/

supp

orted)

Methan

e/0.03

and0.02

resp

ectively/—

�Theph

otocatalytic

reaction

sarecondu

cted

ina

quartz

cellwithaat

bottom

(60mL),w

herethe

catalystsarepu

ton

atran

sparen

tpo

rous

Vycor

glass

�TiO

2(anatase)

withlargeba

ndga

psan

dman

ysu

rface–O

Hgrou

psexhibithigheffi

cien

cyforthe

form

ationof

CH

4

F.So

lymosian

dI.Tom

bacz

(199

4)73

TiO

2/—

Pure

TiO

2from

British

Drog

Hou

se

CO2(saturated

),H

2O(120

mL)/500

Whighpressu

reXe

lamp/—

(333

/1/—

/—/—

),(0.3/

18/nan

oparticle/0.00

25/

susp

ende

d)

Form

icacid/2/—

�APy

rexglasscellwithaPy

rexwindo

wis

used

tocondu

cttheph

otocatalytic

reaction

having

surrou

ndingwater

jacket

tocontrol

the

tempe

rature

ofcirculatingwater

byan

ultra-

thermostat

�Anincrease

intheelectron

concentrationof

TiO

2

enhan

cestheph

otored

uction

T.Inou

eet

al.

(197

9)42

TiO

2,Z

nO,a

nd

WO3/3.0,3

.2,

and2.8eV

Purity

99.5

to99

.99%

CO2(saturated

),H

2O(100

mL)/500

WXeor

Hglamp/—

(298

/1/5/7/—

),(1/—

/20

0–40

0mesh

powde

r/—/sus

pende

d)

Form

alde

hyd

e/16

,17

.14,

and0/—

�Methan

olform

ationrate

isincreasedby

increasingilluminationpe

riod

incase

ofTiO

2

�Incase

ofsemicon

ductor

electrod

e,ithas

been

sugg

estedthat

thech

arge

tran

sfer

rate

betw

een

photog

enerated

carriers

andsolution

speciesis

relatedto

theen

ergy

levels

betw

eenthe

semicon

ductor

andredo

xag

ents

�Aglasscellwithqu

artzwindo

wisus

edto

carryou

tthereaction

aT/P/pH/t/Q

E:tem

perature

(K)/pressu

re(atm

)/pH

/illu

minationpe

riod

(hou

rs)/qu

antum

efficien

cy(%

).W/SA/PS/SC

/MP:

weigh

tof

photocatalyst(g)/su

rfacearea

(m2g�

1)/ph

otocatalyststructure

(diameter)/semicon

ductor

concentration(g

mL�

1)/mod

eof

photocatalyst.‘—

’:not

men

tion

edin

theoriginal

pape

r.bpp

mh�1g�

1catalyst.c

mmol

h�1.d

Turnover

num

ber.


Review RSC Advances

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RSC Advances Review

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of 2.7 eV for the photocatalytic conversion of CO2 to methaneunder visible light irradiation.58 The diameter of W18O49

nanowires is approximately 0.9 nm and hold a large number ofoxygen vacancies. These oxygen vacancies indicate anoutstanding competency on photochemical carbon dioxidereduction over the visible light region.

Metal oxides like Ga2O3, ZrO2, and MgO were reported invarious studies as having wide band gaps, compared with otherphotocatalytic metal oxides.59–61 In those studies, methane andhydrogen were used as reductants for photoreduction of CO2,and CO was the major product.61,62 In the case of ZrO2, highphotoreduction of CO2 is obtained when H2 is used rather thanCH4. Moreover, it is found that some intermediates are formedwhen H2 is used for CO2 photoreduction over ZrO2 and MgO asa surface formate species, whereas the acetate is generated inlieu of the formate when CH4 is used as reductant over ZrO2.60,61

It was also observed that the surface bidentate formate speciesshowed high photoactivity for photocatalytic reduction of CO2

over ZrO2 and MgO.Recently, visible light-responsive and other oxide catalysts

fabricated through various routes like hydrothermal, sol-vothermal, and solid state reactions are widely used in thephotochemical reduction of CO2 because of high yield rates.Microwave-assisted, hydrothermally derived monoclinic BiVO4

catalysts show higher ethanol yields (110 mmol h�1 per g ofcatalyst) than that of tetragonal BiVO4 due to anchored CO3

2� tothe Bi3+ sites on the external surface through a weak Bi–O bondfor receiving the photogenerated electrons effectively from the V3d-block bands of BiVO4. The asymmetric behavior of themonoclinic phase around the Bi3+ ion is more effective than inthe tetragonal phase. That is why, the Bi3+ ion shows a strongerlone pair behavior in the monoclinic phase for increasing thetendency of the Bi–O bond formation with CO3

2�.63,64 Y. Zhouet al. and Q. Liu et al. reported visible light-responsive, 9.5 nmthick Bi2WO6 nanoplates and UV-driven single-crystallineZn2GeO4 nanobelts with a thickness of 7 nm, respectively.65,66

The ultrathin geometry encourages the charge carriers totransfer swily from inside to the external surface for effectiveparticipation in the CO2 photoreduction reaction. It also sepa-rates the photogenerated electron and hole progressively, ulti-mately decreasing the recombination rate.

It has been reported that hydrothermally prepared KNb3O8

and HNb3O8 nanobelts show high yields of methane, comparedwith the same catalysts derived from conventional solid statereactions and commercial TiO2 (Degussa P25).67 In the case ofhydrothermal synthesis, this nanobelt-like morphology andprotonic acidity give higher photochemical activity for methaneproduction through hydrogen bonding, which is facilitated bythe separating and trapping of photogenerated carriers at theinterlayer surfaces of HNb3O8 and KNb3O8. Ferroelectric mate-rials can be used as remarkable substitutes for the standardsemiconductor photocatalysts. Ferroelectric characteristics leadto driving the electrons and holes apart because they possessinternal dipoles. Therefore, this phenomenon decreases theprobabilities of the recombination of carriers and also inhibitsthe reaction of redox products, driving the equilibrium towardsproduct formation. For this ferroelectric behavior, the polar

20864 | RSC Adv., 2014, 4, 20856–20893

compound LiNbO3 was used by Stock and Dunn for photo-catalytic CO2 conversion to formic acid under an Hg lamp ordirect natural sunlight irradiation.68

2.1.2 Supported metal oxide photocatalysts. Several typesof supportive materials were used in this technology, such aszeolite, mesoporous SiO2, kaolinite, Al2O3, montmorillonite andother pillar materials. Table 2 demonstrates the texturalbelongings of the silica supports and the TiO2-loaded sampleswith specic surface area, pore volume, andmaximum pore sizedistributions.46 The mesoporous, SiO2-prepared material MCM-41 shows higher specic surface area having an average poresize of 2.3 nm. The pore structure of supported materials is avital factor because it manages the transportation of reactantsand products to and from the surface, and controls the size ofthe active surface area.74 The detailed information of supportedmetal oxide semiconductors investigated in previous studies arehighlighted critically in Table 3. Titania charging on the silicamatrix reduces the surface area of the supports and alsodecreases the pore volume, as the titania content increases. Thereduction of pore volume indicates that the loading of semi-conductor particles saturates the maximum space of theordered channels within the support, which helps to control thecatalysts' size from crystal growth. High catalyst loading cancause pore blockage inside the channels, although the inter-connection among the parallel pore channels should have theability to reduce this effect.75 The common photocatalyticreaction path for supported metal oxide semiconductor isillustrated in Scheme 1(b).

A catalyst on supported materials has been used in manyphotocatalytic aqueous systems due to their immobilization interms of easier separation of the catalyst aer the photocatalyticreactions. It is reported that supported photocatalysts exhibitlower activities than powdered TiO2 materials. However, treatedphotocatalysts show their high activity because of cyanidecontent and represent the dicyanoaurate photoreductionactivity in order to reect TiO2/GrSiO2 > Degussa P25 > TiO2/SBA-15. Depending on the treatment processes, TiO2/SBA-15materials achieved up to eight times of high activity, comparedwith the Degussa P25 TiO2.76 The addition of Cu species onTiO2–SiO2 catalysts increased the overall CO2 conversion effi-ciency by suppressing the electron–hole recombination andincreasing multi-electron reactions,77 compared with only TiO2–

SiO2 matrices.78 It is reported that the addition of 0.5% Cushows high specic surface area, but the addition of 1% and 3%Cu successively reduces the specic surface area. A multi-walledcarbon nanotube (MWCNT)-supported TiO2 composite catalystis introduced by Xia et al. due to its unique electronic proper-ties,70 and the results show a high conversion rate when aproper amount of MWCNTs is used as a support for TiO2.Moreover, the MWCNT-supported composite catalysts are ableto lessen the accumulation of the catalyst particles and reducethe recombination of electron–hole pairs by moving it along thetubes. Carbon is used as a catalyst support in gas–solid reactionphases with suitable temperature and pressure under which thecarbon support remains stable. In hydrogenation reactions,carbon-supported iron catalysts with high dispersions wereused at 1 atm and <700 K because of carbon stability under this



Table 2 Textural properties of different types of silica-supported samples46

Catalysts' compositionSpecic surfacearea (m2 g�1)

Pore volume(cm3 g�1)

Maximum of the poresize distribution (nm)

MCM-41 SiO2 1051 0.83 2.3SBA-15 SiO2 640 0.96 7.5SBA-15/TMB SiO2 601 1.62 20.0Commercial SiO2 317 1.59 27.5100% TiO2 reference 9 — —20% TiO2 MCM-41 294 0.57 16.020% TiO2 SBA-15 532 0.78 7.040% TiO2 SBA-15 442 0.69 6.560% TiO2 SBA-15 349 0.60 6.520% TiO2 SBA-15/TMB 517 1.40 18.540% TiO2 SBA-15/TMB 414 1.08 18.560% TiO2 SBA-15/TMB 304 0.78 18.520% TiO2 commercial 299 1.16 22.540% TiO2 commercial 246 0.94 25.060% TiO2 commercial 179 0.67 26.0

Review RSC Advances

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condition.74 The stability of activated carbon can be controlledby ZnO catalysts. The high reaction rate of the gasication of theactivated carbon is carried out with CO2 by ZnO loading in lowtemperatures.79 The presence of an acidic medium in mixedoxide systems is suitable to achieve additional knowledge onthe physical nature of photocatalysts.80 The oxide-supportedcatalysts are treated by an acidic medium due to the presence ofan oxygen species in the reaction system that leads to aformation of basic nature of the catalyst, which was reported byM. Subrahmanyam et al.81 Zeolites are microporous, alumi-nosilicateminerals having porous structures as traps for mole-cules to analyze. Investigation shows that the TiO2 particles arehighly distributed inside the pore channels of zeolite withtetrahedral coordination, which has high selectivity for theformation of CH3OH.82 At the same time, it was also found thatPt loading on TiO2 catalysts in the presence of zeolite supportencourages the charge separation and increases the formationof CH4 in lieu of CH3OH by stimulating the reaction between thecarbon radicals and H atoms formed on the Pt metals toproduce CH4.

Recently, montmorillonite has been loaded with TiO2

structures to increase the surface area of TiO2 particles.45 Ulti-mately, in this research, the particle size of the catalyst wasreduced from 18.73 to 13.87 nm and allowed efficient chargeseparation. The band gap of TiO2 was also reduced to 3.07 eVand could permit visible light irradiation during photoreduc-tion of CO2 in the presence of water vapor. Effective chargetransfer between the two semiconductors and the response tovisible light are found in the FeTiO3–TiO2 catalyst due to itsunique band structure (2.7 eV).83 When the loading of FeTiO3

increases on TiO2 from 10% to 50%, it shows a decrease insurface area accordingly from 55.8 to 35.7 m2 g�1. Due to thesmall surface area, a high content of FeTiO3 on TiO2 provides alow production rate. Kaolinite, which is a clay mineral with alayered silicate having one tetrahedral sheet connected throughoxygen atoms to one octahedral sheet of alumina octahedral,84

is another promising supporting material in this technology. Akaolinite/TiO2 composite decreases the crystallite size of


anatase TiO2. Kaolinite increases the effective surface area forTiO2 by avoiding TiO2 aggregates in suspension and preventsthe recombination of electron–hole pairs. It also hinders crys-tallite growth and decreases particle sizes from 26 nm to 18nm.85 23.2% AgBr is highly dispersed on the surface of TiO2

nanoparticles, which exhibit strong absorption ability in thevisible-light region and stability during repeated operationsbecause of transferring photoexcited electrons from conductionbands of well-dispersed AgBr to that of TiO2.86 RuO2 is a favor-able material which shows high electrical conductivity andelectrochemical stability, intermediate hydrogen over-potentialand the capacity to reversibly adsorb hydrogen for CO2 reduc-tion.87 RuO2 supported on TiO2 nanotube-composite-modiedPt electrodes showed higher activity on electrochemical reduc-tion of CO2 to methanol, compared with RuO2-modied Ptelectrodes, according to the investigation of J. Qu et al.88 It isalso found that RuO2–TiO2 nanotube-modied electrodes aremore promising electrocatalysts than that of RuO2–TiO2 nano-particle-modied electrodes due to the high dispersion of RuO2

on the TiO2 nanotubes surface with more surface area, creatingmore active sites. Photoreduction rates can be improved by thedeposition of Pd metal on the surface of RuO2–TiO2, wherephotoelectrons are trapped by Pd, and photoholes are connedat the RuO2/SO3

2� site.89 WO3 and CeO2 were also applied onTiO2 support to investigate photocatalytic activity,73,90 whereCeO2–TiO2 shows the visible light responsiveness. The loadingof Rh on TiO2 containing 2% WO3 leads to the formation ofmore formic acid, compared with WO3–TiO2.73

Some other investigations were published based on the CuO,which is highly dispersed on TiO2.91,92 By using XRD analysis, it isfound that the dispersion capability of CuO in the vacant sites ofTiO2 was approximately 4.16 Cu2+ nm�2. The high percentage ofCuO loading on TiO2 slightly reduces the surface area of catalystparticles, and this slight change in surface area is negligible tothe reactivity of the catalyst.91 On the other hand, 1 wt% CuO–TiO2 exhibits high band gap energy (3.02 eV) with lowerproduction yields than that of 3 wt% CuO–TiO2 (2.88 eV).91

Calcination temperature is also important in terms of production

RSC Adv., 2014, 4, 20856–20893 | 20865


Tab

le3

Advance

sin

supportedmetalo

xideca

talyst

forCO

2photoreductiona

Researcher

(year)

andref.

Catalyst

compo

sition

/ba

ndga

pen

ergy

Catalyst

prep

aration

tech

nique

Reactan

ts(amou

nt)/rad

iation

source/light

intensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),ph

ysical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg

catalyst)/selectivity(%

)Com

men

ts

M.T

ahiret

al.

(201

3)45

20wt%

mon

tmorillonite

mod

ied

TiO

2/

3.07

eV

Sol–gelmethod

CO2(saturated

),water

vapo

r(—

)/50

0W

Hglamp(365

nm)/—

(393

/0.20/—/4/—

),(0.05/82

.62/13

.87nm

particle/6.33/

supp

orted)

Methan

e/44

1.5/—

�Loa

dingmon

tmorilloniteon

toTiO

2structure

amplied

surfacearea,red

uced

particle

size

and

allowed

efficien

tch

arge

sepa

ration

�H2O/CO2feed

ratio¼

0.35

givesthemaxim

umyieldformethan

e(441

.5mmol

gcat�

1h�1)

�Stainless

steelreactorhavinglengthof

9.5cm

,width

of3.75

cman

dtotalvolum

eof

106cm

3witha

quartz

windo

wforthepa

ssingof

ligh

tirradiations

Q.D

.Truon

get

al.

(201

2)83

20%

FeTiO

3–

TiO

2/2.7

eVFa

cile

hyd

rothermal

method

CO2(—

),NaH

CO3

(0.08M,3

0mL

solution

)/50

0W

Xe

lamp(>30

0nm)/—

(—/—

/—/3/—

),(0.05/

51.3/5

nm

particle/—

/su

spen

ded)

Methan

ol/0.46/—

�Jun

ctioneff

ecto

ftwosemicon

ductorsan

dnarrow

bandga

pof

FeTiO

3sh

owextrao

rdinaryactivity

unde

rbo

thvisiblean

dUV-vis

ligh

tirradiationwith

asu

prem

eyieldof

CH

3OHthat

isthreetimes

higher

than

that

from

bare

TiO

2or

Degus

saP2

5�A

Pyrexglasstube

reactorwherethetempe

rature

ofthesolution

iske

ptconstan

tby

awater

bath

D.-S

.Lee

etal.

(201

2)95

1wt%

NiO

–InNbO

4/—

Solid-state

reaction

and

wetness

impregnation

method

s

CO2(saturated

),KHCO3(0.2

M,5

0mLsolution

)/50

0W

halog

enlamp(900

nm)/14

3mW

cm�2

(298

/—/7/20/—),

(0.14/—/1

mm

particle/—

/dispe

rsed

andsu

spen

ded)

Methan

ol/1.4/—

�Catalystshow

shighesta

ctivitydu

eto

thepresen

ceof

core–shelltype

Ni0an

dNiO

onthesu

rfacean

dthepresen

ceof

asm

allam

ountof

Nb 2O5as

aprom

oter

unde

rvisibleligh

tirradiation

�Photocatalytic

redu

ctionis

carriedou

tin

acontinuo

usow

reactorwithaPy

rexglassdo

wn-

windo

wtype

irradiationcell(75mL)

Q.L

iuetal.(20

12)99

1wt%

Ptan

d1

wt%

RuO

2

load

edon

Zn1.7GeN

1.8O/2.6

eV

Solvothermal

routean

dnitridation

CO2(saturated

),H

2O(0.4

mL)/300

WXearclamp(>42

0nm)/—

(298

/1/—

/14/0.02

4),

(0.1/32.33

/0.5–2

mm

bundlesh

ape/—/

supp

orted)

Methan

e/11

.5/—

�This

hyb

ridph

otocatalystallowsCO2redu

ction

into

CH

4in

thepresen

ceof

H2Oun

dervisibleligh

tirradiation

�Photocatalysts

areeven

lypu

tat

thebo

ttom

ofa

Pyrexglassreactorwithan

area

of4.2cm

2

K.K

ocıet

al.

(201

1)85

Kao

linite

mod

ied

TiO

2

compo

site/—

Thermal

hyd

rolysis

CO2(saturated

),NaO

H(0.2

M,1

00mLsolution

)/8W

Hglamps

(254

nm)/—

(—/1/7/24/—),(1/40/18

nm

particle/0.001

/su

pported)

Hyd

rogen/0.187

/—�P

rodu

ctionof

methan

ean

dmethan

olwere

advancedover

aka

olinite-mod

ied

TiO

2compo

site

than

over

commercial

TiO

2(D

egus

saP-25

)�A

ddingof

TiO

2nan

oparticles

into

theka

olinite

structurecaus

edade

crease

ofan

atasecrystallite

size �Inhibittherecombinationof

electron

–holepa

irs

andstop

theform

ationof

TiO

2ag

gregates

insu

spen

sion

�Theph

otocatalytic

redu

ctionis

condu

cted

ina

hom

emad

eap

paratusus

ingastirredba

tchan

nular

reactorwithasu

spen

dedcatalyst


RSC Advances Review

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16


14


Tab

le3

(Contd.)

Researcher

(year)

andref.

Catalyst

compo

sition

/ba

ndga

pen

ergy

Catalyst

prep

aration

tech

nique

Reactan

ts(amou

nt)/rad

iation

source/light

intensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),ph

ysical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg


)Com

men

ts

M.A

.Asi

etal.

(201

1)86

23.2

wt%

AgB

r–TiO

2/2.9eV

Dep

osition–

precipitation

method

CO2(saturated

),KHCO3(0.2

M,1

00mLsolution

)/15

0W

Xelamp(420

nm)/—

(298

/74/8.5/5/—),(0.5/

50/5

nm

particle

/—/sus

pende

d)

Methan

e,ethan

ol/

25.72,

15.57/—

�AgB

r–TiO

2sh

owsitsstrongab

sorption

and

stab

ilityin

therepe

ated

uses

unde

rvisible-ligh

tregion

dueto

thetran

smission

ofph

otoe

xcited

electron

sfrom

thecondu

ctionba

ndof

well-

disp

ersedAgB

rto

that

ofTiO

2

�Photocatalytic

redu

ctionis

condu

cted

ina

stainless

steelv

esselw

ithvalves

forevacua

tion

and

gasfeed

ingin

whichan

O-ringsealed

glasswindo

wis

kept

atthetopforfocu

singligh

tP.-Y.L

iouet

al.

(201

1)93

1wt%

NiO

–InTaO

4/2.6

eVIm

pregnation

method

CO2(saturated

),water

vapo

r/30

0W

Xelamp/10

0mW

cm�2

(303

/1/—

/6/0.057

),(0.12/3.58

/compo

site/—

/su

pported)

Acetaldeh

yde/0.3/—

�Toincrease

theam

ountof

catalyst

load

ing,

amon

olithph

otoreactor

isus

eddu

eto

itsmultiple

chan

nels

�Surface-carvedpo

lymethylmethacrylate

optical

be

rscantran

smitan

dscatterligh

teff

ectively

toilluminatethecatalyst

inside

thech

annels

�Qua

ntum

efficien

cywas

sign

icantlyen

han

cedin

themon

olithreactor,compa

redwiththeop

tical-

be

rreactor

S.Yan

etal.(20

11)98

3wt%

RuO

2–

Zn2GeO

4/4.65eV

Low-tem

perature

(100

� C)solution

phaseroute

CO2(saturated

),H

2O

(—)/ultra-violet

irradiation/—

(—/—

/—/5/—

),(—

/14.8/

rod-sh

aped

nan

ostruc

ture/0.58/

supp

orted)

Carbo

nmon

oxide/

17.9

b/—

�Facilesolution

phaseroute-synthesized

catalysts

havelowcrystalde

fects,

highsp

ecicsu

rfacearea

andbe

ne

cial

microstructureon

itssu

rface

S.Qin

etal.(20

11)92

1wt%

CuO

–TiO

2/3.2eV

Labo

ratory

synthesis

CO2(saturated

),methan

ol(30mL

solution

)/25

0W

highpressu

reHg

lamp(365

nm)/—

(300

/1–60/—/6/—

),(0.03/—/13.8nm

particle/—

/sus

pende

d)

Methyl-formate/16

02/—

�Methan

olactsas

asacricial

reag

enttoreactw

ith

theph

otog

enerated

holes

inthevalence

band

�Hetero-junctionph

otocatalystredu

cesthe

recombinationof

theelectron

san

dholes

�Theph

otocatalytic

reaction

iscondu

cted

ina

slurry

reactorhavingaat

topcoverof

thevessel

ismad

eof

Pyrexglass

YingLi

etal.

(201

0)77

SiO2su

pported

0.5wt%

Cu–

TiO

2/3.1eV

One-po

tsol–gel

method

CO2(saturated

),water

vapo

r(—

)/Xe

lamp(250

–400

nm)/

2.4mW

cm�2

(273

/1/7/4/1.41),(0.1/

386.2/<1

00mm

powde

r/—/sup

ported

)

Carbo

nmon

oxide/45

/—�E

xperim

entis

carriedou

tin

acontinuo

us-ow

reactorhavingstainless

steelwallwithaqu

artz

windo

wat

thetop,

andtheinner

cavity

is6.0cm

indiam

eter

and2.5cm

deep

�Highsu

rfacearea

ofmesop

orou

ssilica

subs

trate

sign

icantlyim

proves

CO2ph

otored

uction

becaus

eof

improved

TiO

2disp

ersion

andhighCO2an

dH

2O

adsorption

onthecatalyst

�Cusp

eciessu

ppress

theelectron

–hole

recombinationan

den

han

cingmulti-electron

reaction

s


Review RSC Advances

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16


7


Tab

le3

(Contd.)

Researcher

(year)

andref.

Catalyst

compo

sition

/ba

ndga

pen

ergy

Catalyst

prep

aration

tech

nique

Reactan

ts(amou

nt)/rad

iation

source/light

intensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),ph

ysical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg


)Com

men

ts

X.-H

.Xia

etal.

(200

7)70

TiO

2–

MWCNT/—

Sol–gelan

dhyd

rothermal

method

s

CO2(1

mol),water

vapo

r(5

mol)/15

WUVlamp

(365

nm)/—

(298

/—/7/5/—

),(0.1/50/

nan

oparticle/—/

supp

orted)

Ethan

ol,formicacid/30,

19/—

�Catalystisplaced

over

apieceof

tran

sparen

tglass

andthen

laid

into

ahom

emad

estainless

steel

reactor

�MWCNTsin

thecompo

site

catalystscanmitigate

theaccu

mulationof

TiO

2pa

rticlesan

dde

crease

the

recombinationof

electron

–holepa

irs

�MWCNTshaveen

han

cedpe

rforman

cein

the

photocatalytic

reaction

sas

supp

orts

forTiO

2,

compa

redwithactivatedcarbon

sP.-W

.Pan

andY.-W

.Chen

(200

7)94

1.0wt%

NiO

–InTaO

4/2.6eV

Solidstate

reaction

andwet

impregnation

method

CO2(saturated

),KHCO3(0.2

Maq

ueou

ssolution

)/50

0W

halog

enlamp

(365

nm)/—

(298

/—/—

/20/—),

(0.14/—/1–2

mm

particlesan

dpe

llets/—

/su

spen

ded)

Methan

ol/1.4/—

�Con

tinuo

usmod

edo

wn-windo

w-typ

eirradiation

cellreactormad

eof

Pyrexglass(75mL)

isus

edto

carryou

ttheph

otoreaction

�Themethan

olyieldincreaseswiththeam

ountof

NiO

co-catalyst

�Catalysts

haveab

ilityto

redu

ceCO2to

methan

olun

dervisibleligh

tillumination

J.Quet

al.(200

5)88

RuO

2–T

iO2

nan

otube

mod

ied

Ptelectrod

e/—

Sol–gelmethod

andlabo

ratory

synthesis

CO2(saturated

),NaH

CO3(0.5

M,5

0mLsolution

)/electrochem

ical

redu

ction/—

(298

/—/—

/—/current

efficien

cy60

.5c ),

(0.001

/—/25nm

nan

oparticlecompo

site

electrod

e/—/sup

ported

)

Methan

ol/—

/—�H

igheffi

cien

cyan

dselectivityforelectrochem

ical

redu

ctionof

CO2de

pends

onthesu

rfacestructure

ofthenan

otub

escompo

site

electrod

e�E

xperim

ents

aredo

nein

astan

dard

three-

electrod

eelectrochem

ical

cellwhereasaturated

calomel

electrod

eisus

edas

thereference

electrod

eNasution

etal.

(200

5)91

3%CuO

–TiO

2/

2.88

eVIm

proved

impregnation

method

CO2(saturated

),KHCO3(1

M,3

00mLsolution

)/10

WUVblackligh

tlamp

(415

–700

nm)/2.45

mW

cm�2

(333

/—/—

/6/19.23

),(0.3/45.8/23

nm

particle/—

/sus

pende

d)

Methan

ol/442

.5/19.23

�Slurrytype

photocatalytic

reactorconsistsof

ahorizon

talstainless

steelvessel

withan

inner

diam

eter

andheigh

tof

thevessel

140an

d50

mm,

resp

ectively

havingaplatecoverat

thetopof

the

vessel

ismad

eof

Pyrex

�Thepo

sitive

values

ofEaindicate

that

desorption

ofprod

ucts

istherate-limitingstep

inthe

photosyn

thetic

form

ationof

methan

olN.G

okon

etal.

(200

3)79

5wt%

ZnO

onactivatedcarbon

/3.2eV

Impregnation

method

CO2(saturated

),—/Xelamp(<40

0nm)/34

000mW

cm�2

(873

/—/—

/—/—

),(0.10/—/150

–300

mm

particle/—

/sup

ported

)

Carbo

nmon

oxide/2�

105 /—

�Sam

pleis

kept

inqu

artz

tube

reactor(8

mm

diam

eter)is

supp

ortedfrom

both

side

sby

alum

ina

hon

eycombcoveredwithqu

artz

woo

lan

dirradiated

byasolarfurnacesimulator

�This

isahigh-tem

perature

operationfor

photocatalytic

conversionof

CO2

T.-F

.Xie

etal.

(200

1)89

Pd–R

uO2

supp

ortedon

TiO

2/—

Labo

ratory

synthesis

CO2(saturated

),NaO

HwithNa 2SO

3

(0.05M)/45

0W

Xe

short-arclamp/—

(298

/1/—

/—/—

),(0.02/—/22nm

particle/—

/sus

pende

d)

Form

ate/72

.3c /—

�Surface

nob

lemetal

(Pdan

dRu)

depo

sition

isa

costly

andecon

omically

unfeasible

forindu

strial

purposes

�Reactionis

carriedou

tin

a30

mLPy

rexglass

cuvettean

dstirredcontinuo

usly

byamag

netic

stirringba

r


RSC Advances Review

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16



Tab

le3

(Contd.)

Researcher

(year)

andref.

Catalyst

compo

sition

/ba

ndga

pen

ergy

Catalyst

prep

aration

tech

nique

Reactan

ts(amou

nt)/rad

iation

source/light

intensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),ph

ysical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg


)Com

men

ts

M.S

ubrahman

yam

etal.(199

9)81

10wt%

TiO

2–P

dsu

pportedon

Al 2O3/—

Impregnation

method

CO2(saturated

),KHCO3(0.5

Msolution

)/0.25

WHg

arclamp/—

(—/—

/7.5/6/—

),(—

/—/Particle/—/

susp

ende

d)

Acetone/8.5d/—

�Theexpe

rimen

tis

condu

cted

inawell-m

ixed

heterog

eneo

usba

tchtype

reactor

�Catalysto

nsu

pporta

ccom

plishes

thecondition

ofago

odph

otocatalystto

prod

uceC1an

dC2

compo

unds

B.-J.L

iuet

al.

(199

8)78

TiO

2–SiO2

matrices/—

Sol–gelmethod

CO2(saturated

),lithium

nitrate

and

prop

an-2-ol(20an

d1mmol

dm�3)/50

0W

Hgarclamp(280

nm)/10

00mW

cm�2

(—/—

/—/8/0.16),

(—/290

/5.3

nm

spherical

particle/3.89/

supp

orted)

Form

ate/0.5d/—

�Itisde

cide

dthat

redu

ctionreaction

ofnitrate

ions

istherate-determiningstep

oftheform

ationof

urea

�Theph

otored

uction

iscarriedou

tus

ingaqu

artz

cell(9

cm3 ),w

hosetopis

sealed

witharubb

erseptum

andthetran

sparen

tTiO

2–SiO

2lm

coated

onaqu

artz

plate(2

cm2)was

vertically

subm

erged

insolution

sM.A

npo

etal.

(199

7)82

1.1wt%

TiO

2–Y

zeolite

cavities/—

Ion-exchan

gemethod

CO2(24mmol),water

vapo

r(120

mmol)/75

Whigh-pressureHg

lamp(>28

0nm)/—

(328

/1.31x

10�9/—

/6/—

),(0.15/—/pow

der/—/

supp

orted)

Methan

ol/5/—

�Based

onTi3+ ,H

atom

san

dCradicals,a

molecular–scale

reaction

mechan

ism

has

been

prop

osed

�Highly

disp

ersedisolated

tetrah

edraltitanium

oxidesp

eciesde

libe

rate

theactive

sitesforreaction

system

sSo

lymosiet

al.

(199

4)73

2%WO3–T

iO2/—

Wet

impregnation

method

CO2(saturated

),H

2O(120

mL)/500

Whighpressu

reXe

lamp/0.68

mW

cm�2

(333

/1/—

/5/—

),(0.3/18/

nan

oparticle/0.00

25/

susp

ende

d)

Form

icacid/1.4/—

�Reactionis

carriedou

tin

aPy

rexglasscellwhere

thereaction

tempe

rature

iscontrolledby

asu

rrou

ndingwater

jacket

inwhichthetempe

rature

ofcirculatingwater

isregu

latedby

anultra-

thermostat

K.O

gura

etal.

(199

2)90

0.5wt%

CeO

2–TiO

2/—

Heatingco-

precipitation

CO2(0.13atm),H

2O(0.032

atm)/50

0W

Xelamp(<37

0nm)/—

(298

/1/—

/18/—),(1/111

/nan

oparticle/—/

susp

ende

d)

Hyd

rogen/0.26/—

�Photocatalytic

reaction

ispe

rformed

inan

appa

ratusof

closed

circulatingsystem

(1.5

dm3 )

andvisibleligh

tirradiationis

appliedfrom

the

bottom

viaarectan

gularqu

artz

prism

aT/P/pH/t/Q

E:tem

perature

(K)/pressu

re(atm

)/pH

/illu

minationpe

riod

(hou

rs)/qu

antum

efficien

cy(%

).W/SA/PS/SC

/MP:

weigh

tof

photocatalyst(g)/su

rfacearea

(m2g�

1)/ph


(diameter)/semicon

duc

torconcentration(g

mL�

1 )/m

odeof


’:not

men

tion

edin

theoriginal

pape

r.bpp

mh�1 .

cpp

m.d

mmol

h�1.


Review RSC Advances

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16



RSC Advances Review

Publ

ishe

d on

13

May

201

4. D

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oade

d by

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f M

alay

a on

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08/2

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rates. 1 wt% CuO–TiO2 calcined at 450 �C represents greateractivity than that of calcined at 350 �C and 550 �C.92

In addition, supported materials other than TiO2 are foundat reasonable photocatalytic activity. The use of NiO on severalmetal oxides such as InTaO4 and InNbO4 are found in somepublications for CO2 reduction to fuel.93–95 1 wt% NiO–InTaO4

catalysts have low band gap energy of 2.6 eV, which is highlyresponsive to visible light and can be used in different types ofreactors like slurry, optical ber and monolithreactors.93,96,97 Incase of NiO–InNbO4 catalyst, it shows also better activity due tothe presence of core–shell-type Ni0 and NiO on the surface andthe presence of a small amount of Nb2O5 as a promoter undervisible light irradiation.95

Zn2GeO4 is an important metal oxide catalyst in photo-conversion technology, which is frequently used as a support forother oxide catalysts to enhance photocatalytic CO2 conversion.Zn2GeO4 catalyst contains some superior properties like lowcrystal defects, extraordinary specic surface area and favorablemicrostructure on the surface of the catalyst, but the band gapenergy is high enough (4.65 eV) to show the visible lightresponse.98 Sheaf-like, hyperbranched Zn2GeO4 nano-architecture was developed by Q. Liu et al., which was furthermodied by NH3 ow to a yellow Zn1.7GeN1.8O solid solution forCO2 conversion into carbonaceous fuel in the existence of waterat ambient conditions under visible light irradiation.99 Themodied catalyst shows low band gap energy (2.6 eV), which isused as a support for 1 wt% Pt and 1 wt% RuO2. Moreover, theloading of Pt and RuO2 on Zn1.7GeN1.8O gives a high conversionrate, which is able to arrest the electron–hole pairs by prevent-ing the recombination of electrons and holes.

2.2 Modied/unmodied non-oxide catalysts

Non-oxide semiconductor photocatalysts are selected for CO2

recycling because of their low band gap energy to facilitate thephotocatalytic response to the visible light region, and highconversion efficiency is achieved for their unique photo-behavior. Elaborately major ndings and characteristics of non-oxide semiconductors upon photocatalytic reactions aredescribed in Table 4.

2.2.1 Metal sulphide semiconductors. It is investigatedthat semiconductor catalysts are one of the supreme sources fordirect solar energy conversion. Many wide-ranging reviewsalready have been published on the advancement of oxide ornon-oxide semiconductor photoactive materials.100–103 It wasfound that apart from the metal oxide semiconductor catalysts,metal sulphide semiconductor materials have effective photo-activity because of their outstanding ability to absorb the solarspectrum with high energy yields.104 Metal sulphides havecomparatively high conduction band states more appropriatefor enhanced solar responses than for metal oxide semi-conductors, which are facilitated by the higher valence bandstates consisting of S 3p orbitals.104 It was reported that toabsorb the entire UV and visible light region of solar irradiation,a ZnS–AgInS2–CuInS2 solid solution was capable due to anabsorption edge of up to 800 nm.105 Fujiwara et al. investigatedZnS–DMF(OAc) for the CO2 photoreduction process.106 They

20870 | RSC Adv., 2014, 4, 20856–20893

found that the close interaction of acetate ions to Zn atomsinhibited the creation of sulfur vacancies as catalytic sites forCO production. Moreover, surface dimethylformamide (DMF)solvated Zn atoms give more efficiency to HCOO� formation.On the other hand, the addition of excess Zn2+ increases the sizeof ZnS–DMF(OAc) through the reaction between excess Zn2+ andthe surface of ZnS–DMF(OAc) nanocrystallites. ZnS depositedon montmorillonite (MMT) was also fabricated by Kocı et al.with particle sizes of 3 nm to 5 nm, which provided signicantlyhigher productivity, compared with the commercial TiO2

Degussa P25.107,108 They used cetyltrimethylammonium (CTA)with ZnS nanoparticles to inhibit the formation of biggeragglomerates. Different percentages of ZnS loading on porousSiO2 matrices were also applied to investigate the improvedphotocatalytic yields.109

In 1979, Inoue et al. introduced non-oxide semiconductorcatalysts (CdS, GaP and SiC) for the photocatalytic conversion ofCO2 to fuel.42 Among them, CdS shows comparatively betteryields in CO2 conversion. It is found from Table 4 that thenarrow band gap (2.63 eV) CdS supported on montmorilloniteexhibits a lower conversion yield than that of 3.89 eV ZnSdeposited on MMT, but the CdS–MMT system has high stabilityin presence of CTA.108,110,111 Not only that, here CTA acts as abinder among CdS and MMT particles. Photocatalytic conver-sion of CO2 on CdS semiconductors with modied surfaces byvarious types of thiol compounds was performed in presence of2-propanol solvent.112 In this study, the major products foundwere formate and carbon monoxide. The ratio of formate tocarbon monoxide is highly dependent on the solvent andincreases with increasing the dielectric constant of the solvent.Moreover, the surface modier is used in this system to x thesurface Cd2+ sites, thus decreases the adsorption of CO2c

� onCd2+ sites, which means that the formate yield will be increasewith increasing the surface exposure of the modier. Anotherstudy shows that extra Cd2+ addition to the CdS–DMF systemforms sulfur vacancies on the surface of nanocrystallitesbecause of the adsorption of excess Cd2+ on the surface.113 Overthe surface of sulfur vacancies, CO2 takes electrons sequentiallyaer developing a Cd2+OCOCO2 complex to react with anotherCO2, leading to the formation of CO as reported in the electricalCO2 reduction to CO.114

Yuan et al. investigated CdS–TNT loaded with Pt to carry outthe photochemical reaction under visible light irradiation, butthey found that the worsening effect of photochemical reactionoccurs due to the adsorption of intermediate products (CO, O2

etc.) on Pt and the oxidation of Pt.115 Another approach was ledby Li et al. for visible light-responsive photocatalytic CO2

reduction on modied CdS with 15% Bi2S3.116 The investigationshows that the Bi2S3 catalyst itself has higher photochemicalactivity and visible light response than that of CdS. The additionof Bi2S3 increased the photochemical performance of CdS, anda maximum yield was obtained 120 mmol h�1 g�1 catalyst. Cd–ZnS was investigated to compare the photocatalytic behaviorwith solid solutions of ZnS–CdS microcrystals.117 Cd–ZnS showsthe highest activity than that of ZnS–CdS because Cd is moreactive in improving the quantum efficiency of the photochem-ical reduction of formate due to its ability to activate CO2 more



Tab

le4

Advance

sin

modified/unmodifiednon-o

xideca

talystsforCO

2photoreductiona

Researcher

(year)an

dref.

Catalyst/ba

nd

gapen

ergy

Catalyst

treatm

ent/prep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingcondition

s(T/P/pH/t/Q

E),

operatingvariab

les

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rgcatalyst)/

efficien

cy(%

)Com

men

ts

K.K

ociet

al.

(201

1)108

ZnSde

positedon

MMT/3.89�

0.03

eVNa 2S,CTAan

dZn

(AcO

) 2mixingby

vigo

rous

stirring

CO2(saturated

),NaO

H(0.2M

solution

)/8W

Hg

lamp(254

nm)/—

(273

/1/—

/24/—),

(0.1/—

/nan

ocom

posite/

0.00

1/su

spen

ded)

Methan

ol,m

ethan

e/1.41

,1.33

/—�I

tis

analyzed

that

prod

uctyields

depe

nd

onthereactordiam

eter

andon

thevolume

oftheliqu

idph

ase

�Slurry-type

two-stirredba

tchan

nular

reactors

isus

edwiththreequ

artz

glasstube

sof

differen

tdiam

eters(3.5,4

.0an

d4.5cm

)placed

inside

thereactors

�Perfectmixingison

eof

themostimpo

rtan

tfactorsin

slurry

reactors,a

ndthis

mixingis

difficu

ltin

annular

reactors

J.Yua

net

al.

(201

1)115

Ptload

edon

CdS–

TNT/—

Alkalinehyd

rothermal

method

andim

mersing

TNTin

Cd(CH

3COO) 2

withheattreatm

ent

CO2(500

0pp

m),

water

vapo

r(—

)/UV-

visan

dvisibleligh

tirradiation/—

(298

/—/—

/6/—

),(—

/—/nan

otub

e/—/

supp

orted)

Methan

e/78

00c /—

�Ads

orptionof

interm

ediate

prod

ucts

(CO,

O2)on

Pt,a

ndtheoxidationof

Ptmightbe

thereason

sforthede

teriorationof

photocatalytic

reaction

P.Prau

set

al.

(201

1)110

6wt%

CdS

–mon

tmorillonite/

2.63

�0.09

eV

Dep

ositionof

originated

CdS–CTA

micelleson

mon

tmorillonite

CO2(saturated

),NaO

H(0.2

mM

aque

oussolution

/8W

Hglamp(254

nm)/—

(—/—

/6.7/24/—),

(0.1/—

/5nm

particle/

0.00

1/su

spen

ded)

Methan

e/0.93

/—�M

ontm

orillonite(M

MT)serves

asacarrier

ofCdS

nan

oparticles

that

wereattach

edto

itsexternal

surface

�Astirredba

tchan

nular

reactorisus

edwith

asu

spen

dedcatalyst

X.L

iet

al.

(201

1)116

15%

Bi 2S 3–C

dS/1.28

eVHyd

rothermal

direct

reaction

CO2(saturated

),NaO

H–N

a 2Saq

ueou

ssolution

(200

mL)/

500W

Xelamp(200

–70

0nm)/—

(—/—

/—/5/—

),(0.2/24/

12nm

particles/17

/su

spen

ded)

Methan

ol/120

/—�R

esults

show

that

theph

otocatalytic

activityan

dvisibleligh

trespo

nse

ofBi 2S 3

are

higher

than

thoseof

CdS

�Reactionis

carriedou

tin

aXPA

-II

photochem

ical

reactorwithamag

netic

stirrer,aqu

artz

cool

trap

,anda

conde

nsation

tube

Bartonet

al.

(200

8)121

p-GaP

/2.24eV

—CO2(—

),py

ridine(10

mM)/20

0W

Hg–Xe

arcligh

t(365

nm)/

0.92

mAcm

�2b

(—/—

/5.2/—

/44),

(—/—

/sem

icon

ductor/

electrod

es/—

/sup

ported

)

Methan

ol/—

/10.9

�Anaq

ueou

sph

otoe

lectroch

emical

cellis

used

whereligh

ten

ergy

istheon

lyen

ergy

used

tocarryou

tthereaction

Zhan

get

al.

(200

4)118

MnS/3.0eV

Labo

ratory

synthesis

CO2(saturated

),de

oxygen

ated

water

(500

mL)/450

Wmed

ium-pressureHg

arclamp(200

–400

nm)/0.6mW

cm�2

(308

/—/7.5/0.5/4.5),

(0.5/—

/particles/0.001

/su

spen

ded)

Form

ate/22

0d/—

�Results

represen

tsomeof

thepreb

iotic

syntheses

that

havearisen

via

photoe

lectroch

emical

reaction

son

semicon

ductingminerals

�Insu

fficien

tredu

ctiven

essof

condu

ction-

bandelectron

sis

alimitationin

this

system

�Thereaction

iscarriedou

tinacommercial

photochem

ical

reactorconsistingof

a0.5L

glassreaction

vessel

withawater

jacket

for

tempe

rature

control


Review RSC Advances

Publ

ishe

d on

13

May

201

4. D

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71


Tab

le4

(Contd.)

Researcher

(year)an

dref.

Catalyst/ba

nd

gapen

ergy

Catalyst

treatm

ent/prep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingcondition

s(T/P/pH/t/Q

E),

operatingvariab

les

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rgcatalyst)/

efficien

cy(%

)Com

men

ts

Y.S

hioya

etal.

(200

3)127

Ti-m

esop

orou

ssilica

thin

lm

(hexag

onal)/—

Solven

tevap

oration

method

CO2(36mmol),water

vapo

r(180

mmol)/10

0W

highpressu

reHg

lamp/0.26

5mW

cm�2

(323

/9.8

�10

�10/—

/6/

0.28

),(0.05/90

0/1.9nm

pore

size/—

/sup

ported

)

Methan

e,methan

ol/7.2,

1.98

/—�A

dvan

cedtran

sparen

tTi-c

ontainingthin

lm

isus

edas

photocatalysts

havingun

ique

andhighph

otocatalytic

activity

�Thelm

shavehexag

onal

pore

structure

withhigher

photocatalytic

activity

than

the

Ti-M

CM-41po

wde

redcatalyst

even

withthe

samepo

restructure

�Aqu

artz

cellwithaat

bottom

(88cm

3)is

used

tocarryou

tthereaction

K.Ike

ueet

al.

(200

2)128

Ti-c

ontaining

porous

silica

thin

lm

s/—

Solven

tevap

oration

method

CO2(36mmol),water

vapo

r(180

mmol)/10

0W

Hglamp/0.26

5mW

cm�2

(323

/9.8

�10

�10/—

/8/

0.28

),(0.05/—/thin

nan

olm

/—/sup

ported

)

Methan

e,methan

ol/8.75,

2.12

/—�S

elf-stan

dingpo

rous

silica

thin

lm

swith

hexag

onal

pore

structures

areus

edas

photocatalysts

andsh

owhigher

photocatalytic

activity

than

thesamepo

re-

structured

Ti-M

CM-41po

wde

rcatalyst

�Theconcentrationof

thesu

rfaceOH

grou

pshas

arole

inthehighselectivity

�Reactioniscondu

cted

onaqu

artz

cellwith

aat

bottom

(88cm

3)linke

dto

aconvention

alvacu

umsystem

K.Ike

ueet

al.

(200

1)129

Ti–b-zeo

lite

Hyd

rothermal

synthesis

method

CO2(36mmol),H

2O

(180

mmol)/10

0W

high-pressureHg

lamp(>25

0nm)/

0.26

5mW

cm�2

(323

/9.8

�10

�10/—

/6/—

),(0.05/63

0/0.25

–0.50mm

particles/—/sus

pende

d)

Methan

e/5.5/11

�Ti–b-zeo

lite

show

sthetitanium

oxide

speciesarehighly

disp

ersedin

their

fram

eworks

inatetrah

edralcoordination

state

�H2Oattraction

tothezeolitesu

rfacerunto

astronginue

nce

onthereactivity

and

selectivity

�Theprop

erties

ofthezeolitecavities

are

impo

rtan

tfactorsto

regu

late

thereactivity

andselectivity

Ulaga

ppan

and

Frei

(200

0)130

Tisilicalite

(TS-1)

molecular

sieve/—

Prom

oter-in

duced

enhan

cemen

tmethod

CO2(saturated

),methan

ol(—

)/26

6nm

pulsed

Nd:Y

AG

laserat

10Hz/30

mW

cm�2

(298

/0.006

6/—/4/—

),(0.022

/—/266

nm

sieve

nan

ocom

posite/—

/su

pported)

Form

icacid,c

arbo

nmon

oxide/—/—

�TS-1wafer

isheldin

asm

allinfrared

vacu

umcellwithCaF

2windo

ws,

whichis

xedinside

anOxfordcryostat

mod

elOptistat

�This

stud

yrstrepresen

tsthe

unde

rstandinginto

theinitialstep

sof

CO2

photored

uction

inafram

eworkTim

olecular

sieve

Hinog

amiet

al.

(199

8)125

Cu,

Ag,

Au–

(p-Si)/

1.12

eVMetal

particlesare

depo

sitedon

p-Si

electrod

e

CO2(saturated

),KHCO3(0.1

M)/

tungsten-halog

enlamp/10

0mW

cm�2

(298

/1/6.8/—

/—),

(—/—

/particles

onSi

electrod

e/—/sup

ported

)

Carbo

nmon

oxide,

hyd

rogen/—

/—�P

hotoe

lectroch

emical

redu

ctionis

carried

outon

p-type

siliconelectrod

esmod

ied

withsm

allmetal

(Cu,

Ag,

orAu)

particles

�Reactionis

condu

cted

onH-shap

edPy

rex

cellwithareference

calomel

electrod

ean

da

Ptplateas

thecoun

terelectrod

e


RSC Advances Review

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16



Tab

le4

(Contd.)

Researcher

(year)an

dref.

Catalyst/ba

nd

gapen

ergy

Catalyst

treatm

ent/prep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingcondition

s(T/P/pH/t/Q

E),

operatingvariab

les

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rgcatalyst)/

efficien

cy(%

)Com

men

ts

B.R

.Egg

insetal.

(199

8)111

CdS

collo

ids/2.4eV

Labo

ratory

synthesis

CO2(saturated

),TMACl(solution)/

med

ium

pressu

reHg

lamp/2.5�

10�3

einsteinspe

rh

(—/—

/4/42/0.48

),(0.3/—

/2–4

nm

nan

oparticle/—/

susp

ende

d)

Glyoxylate/53

e /—

�Photored

uction

usingZn

O,S

iC,B

aTiO

3

andSr

TiO

3in

absence

oftetram

ethylam

mon

ium

ionsprod

uced

form

atean

dform

alde

hyd

e�I

nthis

system

,theeff

ectivenessof

hole–

acceptor

compo

unds

issh

ownto

berelated

totheirredo

xpo

tentials

B.-J.L

iuet

al.

(199

8)112

CdS

surface

mod

ied

bythiol/—

Labo

ratory

synthesis

CO2(—

),2-prop

anol

(1M)/50

0W

high

pressu

reHgarclamp

(300

nm)/10

00mW

cm�2

(298

/—/—

/7/—

),(0.72/—/50nm

particle/—

/sus

pende

d)

Form

ate,

carbon

mon

oxide/1.5,

0.75

f /—�I

tis

foun

dthat

form

ateto

carbon

mon

oxideprod

uction

becamegreaterwith

increase

ofthesu

rfacemod

ication

ofCdS

�Aqu

artzcellof

a7.0cm

3capa

cityisus

edto

carryou

tthereaction

withsealed

topby

arubb

erseptum

H.F

ujiwaraet

al.

(199

8)106

ZnS–DMF(OAc)/—

Colloidal

ZnS–DMFis

prep

ared

from

deaeratedDMFsolution

ofZn

(OAc)

2

CO2(saturated

),de

aeratedDMF

solven

t(2

mL)/UV

ligh

t(>29

0nm)/—

(298

/—/—

/5/—

),(10g/—

/2nm

nan

oparticle/—/

susp

ende

d)

HCOO�/—

/—�E

xcesszincacetateincreasestheeffi

cien

cyof

thesystem

andpreven

tstheform

ationof

sulfur

vacancies

ascatalyticsites

�Tog

ether,H

COO�an

dCOareprod

uced

inthepresen

ceof

excess

zincpe

rchlorate

inthis

system

H.F

ujiwaraet

al.

(199

7)113

CdS

surface

mod

ied

byDMF/

2.4eV

Labo

ratory

synthesis

CO2(—

),TEA(1

M)/

300W

halog

entungstenlamp(>40

0nm)/—

(298

/—/—

/—/—

),(—

/—/nan

ocrystallites/—/

susp

ende

d)

Carbo

nmon

oxide/—/—

�Add

itionof

excess

Cd2

+to

thesystem

increasestheph

otocatalytic

activity

�Sulfurvacancies

supp

ortedby

insitu

Cd

chan

gesthecoordinationnum

bers

ofcadm

ium–sulfuran

dcadm

ium–o

xygen

�Aclosed

Pyrextube

(8mm

diam

eter)is

used

tocondu

cttheph

otored

uction

H.Inou

eet

al.

(199

5)117

0.02

5mol%

Cd–

ZnS/3.66

eVPh

otod

eposition

method

CO2(saturated

),NaH

CO3(50mL,

1.5

�10

�3M

solution

)/50

0W

high-pressure

Hgarclamp/—

(—/—

/5.5/1/32.5),

(—/200

/nan

oparticle/—/

susp

ende

d)

Form

ate/10

.5f /—

�Itis

foun

dthat

solidsolution

sof

ZnS–CdS

microcrystals

donot

show

highactivities

for

theph

otored

uction

ofCO2

�Theprod

uction

ofCO

was

observed

fora

CdS

molefraction

of0.5–0.67

Coo

k,R.L

.etal.

(198

8)126

Cu–

(p-SiC)/—

Susp

ension

sof

p-SiC

andCupa

rticles

CO2(—

),KHCO3(0.5

Maq

ueou

ssolution

)/Hglamp(>27

5nm)/8

mAcm

�2

(313

/1/—

/—/6),(0.1/—

/pa

rticles/—/sus

pende

d)Methan

e/0.63

/—�I

tis

foun

dthat

form

alde

hyd

ecanbe

redu

cedin

anaq

ueou

selectrolyteto

give

methan

e�P

hotoe

lectroch

emical

(PEC)redu

ctionof

CO2is

donein

this

system


Review RSC Advances

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16



Tab

le4

(Contd.)

Researcher

(year)an

dref.

Catalyst/ba

nd

gapen

ergy

Catalyst

treatm

ent/prep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingcondition

s(T/P/pH/t/Q

E),

operatingvariab

les

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rgcatalyst)/

efficien

cy(%

)Com

men

ts

Aurian-Blajeni

etal.(198

3)124

p-GaA

s/—

—CO2(saturated

),KCl

(0.5

Msolution

)/15

0W

Xelamp/—

(298

/8.5/—

/—/—

),(—

/—/sem

icon

ductor

electrod

e/—/sup

ported

)

Form

icacid/170

f /—�A

photoe

lectroch

emical

autoclave,

tted

withaqu

artz

windo

wisus

edto

carryou

tthe

reaction

�Ads

orptionof

theCO2on

the

semicon

ductor

surfaceclearlyplaysan

impo

rtan

trole

intheprocess

�Theelectrod

esarenot

stab

lean

dhave

worsened

noticeablywithin

severalhou

rsInou

eet

al.

(197

9)42

CdS

,GaP

andSiC/

2.4,

2.3an

d3.0eV

Purity

99.5

to99

.99%

CO2(saturated

),H

2O

(100

mL)/500

WXeor

highpressu

reHg

lamp/—

(298

/1/5/7/—

),(1/—

/20

0–40

0mesh

powde

r/—/sus

pende

d)

Form

alde

hyd

e,methan

ol/

(29,

17),(14,

16)an

d(14,

76)/—

�Mag

netic

stirredglasscellwithqu

artz

windo

wis

used

forph

otored

uction

ofCO2

�Photoe

xcited

electron

sin

themore

negativecondu

ctionba

ndpo

seshigher

abilityto

redu

ceCO2

M.H

alman

n(197

8)119

p-GaP

singlecrystal/

2.25

eV—

CO2(saturated

),K2HPO

4solution

(0.05M)/high

pressu

reHglamp/

2.63

mW

cm�2

(298

/—/6.8/90/—),

(—/2.4

�10

�5/

semicon

ductor

electrod

e/—/sup

ported

)

Form

icacid/50g/0.61

�Closedbu

bble-walledbo

rosilicate

glass

beak

er(30mL)

withthermostatis

used

asa

photoe

lectroch

emical

reactor

aT/P/pH/t/Q

E:tem

perature

(K)/pressu

re(atm

)/pH

/illu

minationpe

riod

(hou

rs)/qu

antum

efficien

cy(%

).W/SA/PS/SC

/MP:

weigh

tof

photocatalyst(g)/su

rfacearea

(m2g�

1)/ph


(diameter)/semicon

duc

torconcentration(g

mL�

1 )/m

odeof


’:not

men

tion

edin

theoriginal

pape

r.bCurrentde

nsity.c

mmol

h�1.d

mM.e

mmol

dm�3.f

mmol.g

mM.


RSC Advances Review

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16



Review RSC Advances

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16


effectively in the photoreaction. Zhang et al. introduced anothermetal sulphide semiconductor catalyst (MnS), which can beused under the solar spectrum more competently due to lowband gap energy (3.0 eV).118 Similar to CdS, MnS semiconductormaterials also have the distinct behavior of containing high-reducing conduction band electrons which are satisfactorilyactive for encouraging the reduction of CO2.118 That is why, MnSsemiconductors deliver higher quantum efficiency than othercommon catalysts such as TiO2.

2.2.2 Metal phosphide semiconductor. The use of catalystsfor photochemical purposes without incorporating co-catalystssuch as p-type GaP and p-type InP have also been studied due tonarrow band gaps to facilitate the visible light response.119,120

Basically, in the photoelectrochemical CO2 conversion process,p-type metal phosphides (GaP) were used mainly in methanolproduction but needed remarkably high over-potentials.121

However, high faradaic efficiencies for the photo-electrochemical reduction of CO2 to CO have also been inves-tigated, while electrolytes are used in nonaqueous forms.Nevertheless, high over-potential is necessary to obtain highfaradaic efficiency.122 Barton et al. found that p-type GaP wasable to obtain faradaic efficiency of 100% while converting CO2

to methanol at low potentials, which was more than 300 mVbelow the standard potential of �0.5 V versus the saturatedcalomel electrode (SCE).121 Habisreutinger et al. reported thatp-type GaP semiconductor exhibits low band gap energy(2.24 eV) and possesses high reducing conduction band elec-trons to ease the reduction of CO2.35 On the other hand, for CO2

photocatalytic conversions, it was detected that p-type InPprovided high selectivity for formic acid in a photo-electrochemical cell.123 Moreover, according to Kaneco et al.,p-InP is an active substantial for the photoelectrochemicalreduction of CO2, which revealed higher positive potential of0.2–0.4 V than that of p-Si and p-GaAs photocathodes in theelectrolysis of methanol electrolyte.120

2.2.3 Other non-oxide semiconductors. Aurian-Blajeniet al. were carried out in the aqueous CO2 photo-electrochemical reduction by using p-GaP and p-GaAs asphotocathodes.124 In case of p-GaP, the Faradaic yield decreaseswhen the reaction is carried out at a more negative potential,whereas the opposite behavior is shown in the case of p-GaAselectrode. The different behavior of the two electrodes isprobably connected to the greater stability of GaP towardsanodic decomposition, while in the case of the unstable GaAs,the higher number of negative potentials provide a degree ofcathodic protection. Modied p-type silicon (p-Si) electrodeswith Cu, Ag, and Au foreign elements have been investigated byHinogami et al., and were determined to have extraordinaryactivation energy for CO2 photoreduction.125 Metal elementdeposition on p-Si electrodes not only induce the photo-catalytic activity but also produce high photovoltages. More-over, CO2 photochemical reduction is activated by an upwardshi of surface band energies of p-Si in terms of achievingenergy levels, which is identical between the semiconductorand solution reactants.125 In another study, p-type SiC modiedby Cu particles was also investigated to determine the photo-electrochemical ability in CO2 reduction.126


Ti containing mesoporous silica thin-lm materials havebeen introduced in the photochemical reduction of CO2 due totheir unique and high photocatalytic performances.127 Spectro-scopic analysis proved that Ti ions were highly dispersed insilica networks, and the silica thin lms acted as effectivephotocatalysts during the photoreactions. Besides, the hexag-onal pore construction inside the lms revealed superiorphotosensitivity compared with the same pore-structured Ti-MCM-41 powdered catalyst.127,128 Moreover, FTIR investigationfound that Ti containing silica thin lm-possessed surface OHgroups in various concentrations had a signicant inuence onCH3OH formation.128 Ikeue et al. also studied Ti containingb-zeolite treated with OH� and F� ions as structure-directingagents (SDA) showed hydrophilic and hydrophobic properties,respectively.129 Hydrophilic properties show high photocatalyticefficiency due to the easy interaction of H2O molecules with thetetrahedrally coordinated titanium oxide species on Ti–b-zeolite(OH) than on Ti–b-zeolite (F).

2.3 Doped photocatalysts

To introduce impurities into an extremely pure semiconductor,doping of foreign elements is deliberately inserted for thepurpose of controlling its electrical properties. Due to largeband gaps, unmodied semiconductor photocatalysts cannoteffectively absorb solar radiation with photon energy 1.7–3.1 eV.131 To decrease its band gap energy, surface modicationof the photocatalysts has been attempted by applying stressalong the so direction of a layered semiconductor,131 intro-ducing oxygen vacancies in semiconductors,132 or incorporatingforeign elements.133–139 The schematic photocatalytic view of adoped semiconductor is illuminated in Scheme 1(c). Not onlythat, the recent advancement in different types of doped metaland non-metal elements incorporated with semiconductorsfound in various research for the photocatalytic conversion ofCO2 has been reported in Tables 5 and 6, respectively.

2.3.1 Noble metal doped semiconductors. Dopingmetal onsemiconductor acts as an electron trap to provide electrons toCO2. The photoexcited electrons are shied from the conduc-tion band of the semiconductor to doped metal to leave theholes on the surface of semiconductor.37 Noble metals,including Ru, Rh, Pd, Ag, Pt, Cu, Ni and Au, have beencommonly used with semiconductors as dopants due to Fermi-level or electron-accepting areas at an energy just beneath theconduction band of semiconductors for the photochemicalreduction of CO2. Among those metals, Pt was widely used inmany investigations. For example, the TiO2 semiconductor wasdoped with 0.5 wt% Pt by wet impregnation to avoid very lowphotocatalytic activity.140,141 More homogeneous dispersion of0.5 wt% Pt on the TiO2 surface drives the reaction rate swily inthe both liquid and gas phases.142 It is found that Pt loaded onTiO2 transforms active sites from Ti4+ to Ti3+ by shiing elec-trons from TiO2 to Pt, and this phenomena facilitates visiblelight response.143 Another investigation shows that Pt doping onanchored titanium oxide catalysts within the Y-zeolite cavitiesencourages charge separation and increases the formation ofCH4 in lieu of CH3OH by promoting the reaction between the

RSC Adv., 2014, 4, 20856–20893 | 20875


Tab

le5

Advance

sin

catalyststhat

aremodifiedbymetald

opingforCO

2photoreductiona

Researcher

(year)/ref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration/

treatm

enttech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/Yield

(mmol

h�1pe

rg


)Com

men

ts

Q.Z

hai

etal.

(201

3)167

1.7wt%

Cu–

0.9wt%

Pt–T

iO2/—

Photod

eposition

tech

nique

CO2(saturated

),H

2O

(4.0

mL)/200

WXelamp(320

–780

nm)/—

(323

/2/—

/4/—

),(0.02/—/7.3

nm

particles/—/

susp

ende

d)

Methan

e/33

/85

�Cu 2Ode

livers

sitesfortheactivation

ofCO2

inpresen

ceof

H2O,w

hilePt

extracts

the

photog

enerated

electron

sfrom

TiO

2

�Astainless-steel

reactorwithaqu

artz

windo

won

thetopis

used

tocarryou

tthe

reaction

B.D

.Man

kidy

etal.(201

3)169

Ag–Pt–T

iO2/—

Galvanic

replacem

ent

reaction

andsol–gel

method

CO2(—

),water

vapo

r(—

)/10

0W

Hg

lamp(330

nm)/—

(298

/1/—

/6/—

),(1/—

/nan

oparticles/

—/sup

ported

)

Methan

e/—/80

�Com

binationof

both

bimetallicco-

catalystsan

dAg–SiO2nan

oparticles

increase

prod

uctyields

anden

han

cedactivity

incompa

risonto

nativeTiO

2

�Aba

tchreactoris

used

withthreeop

tical

windo

ws;

oneglasswindo

wat

thetopfor

photoirrad

iation

andtw

oZn

Sewindo

wson

twoside

sP.

Lietal.(20

12)145

0.5wt%

Pt–N

aNbO

3/

3.29

eVTyp

ical

furfural

alcohol

derived

polymerization–

oxidationprocess

CO2(saturated

),H

2O

(3mL)/300

WXearclamp/—

(—/0.78/—/8/—

),(0.1/28.6/po

wde

r/—/

supp

orted)

Methan

e/4.9/—

�Itis

foun

dthat

H2de

velopm

entan

dCO2

redu

ctionover

cubicNaN

bO3arenearly

twiceof

thoseover

orthorhom

bicNaN

bO3

�Cub

icNaN

bO3canbe

qualied

toits

unique

electron

icstructure,

whichis

useful

forelectron

excitation

andtran

sfer

�Catalystsarepu

tonasm

allglass

cellinside

thePy

rexreaction

cellwithga

sclosed

circulationsystem

H.S

hian

dZ.

Zou

(201

2)144

Pt–K

NbO

3/3.1eV

Solidstatereaction

CO2(saturated

),H

2O

(3mL)/300

WXelamp/—

(—/—

/—/6/—

),(0.1/

3.4/1mm

particle/—

/su

spen

ded)

Methan

e/70

b/—

�Itis

detected

that

KNbO

3disp

lays

ahigher

photocatalytic

activity

than

NaN

bO3du

eto

thenarrower

bandga

pan

dhigher

mob

ile

charge

carriers

�Catalysts

areun

iformly

puton

thebo

ttom

ofasm

allg

lass

cellthat

isplaced

inaPy

rex

glasscell

K.Iizuk

aet

al.

(201

1)154

1wt%

Ag–

BaL

a 4Ti 4O15,an

dAg–

SrLa

4Ti 4O15/3.79an

d3.8eV

Polymerizab

lecomplex

method

and

impregnationmethod

CO2(saturated

),H

2O(360

mL)/400

Whighpressu

reHg

lamp/—

(—/—

/—/7/—

),(0.3/—

/10nm

particles/—/

susp

ende

d)

Carbo

nmon

oxide,

form

icacid/(17

.33,

—)

and(7.7,4

.3)/—

�Agco-catalystpe

rformsas

aCO2redu

ction

site

toform

CO

�Develop

men

tof

O2in

astoich

iometric

ratio(H

2+CO:O

2¼

2:1

inamolar

ratio)

show

sthat

water

isus

edup

asaredu

cing

reag

ent

�Aninner

irradiationcellmad

eof

quartz

isus

edto

carryou

ttheph

otocatalytic

redu

ctionof

CO2


RSC Advances Review

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16


c-

014


Tab

le5

(Contd.)

Researcher

(year)/ref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration/

treatm

enttech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/Yield

(mmol

h�1pe

rg


)Com

men

ts

C.-C

.Yan

get

al.

(201

1)162

0.05

wt%

Ti-S

BA-15/—

Synthesized

byltration

,washing,

drying,

and

calcinationat

550

� Cfor6hou

rs

CO2(saturated

),water

vapo

r(76

mmol)/12

0W

high

pressu

reHglamp

(280

nm)/15

0mW

cm�2

(313

/1/—

/7/—

),(0.05/81

/nan

o-po

rous

,3.2

nm

pore

size/—

/sup

ported

)

Ethan

e,methan

e/0.02

,0.01

6/—

�Acombinationof

CO

andH

2O

lead

sthe

highestam

ounts

ofCH

4,C

2H

4,a

ndC2H

6,

whereasamixture

ofCO2an

dH

2lead

sthe

lowestprod

uction

rate

oftheseprod

ucts

�Amultipleba

tchcylindrical

photoreactor

(50mL)

isus

edto

condu

ctthereaction

T.Y

uiet

al.

(201

1)52

2wt%

Pd–T

iO2/—

Photochem

ical

depo

sition

CO2(saturated

),H

2O

(1.5

mL)/500

Whigh-pressureHg

arclamp(>31

0nm)/—

(278

/—/4.1/5/—

),(0.15/—/

nan

oparticle/0.1/

susp

ende

d)

Methan

e/0.56

/—�L

engthyirradiationsh

owsde

activation

oftheph

otocatalysis

ofPd

–TiO

2du

eto

the

partialo

xida

tion

ofthede

positedPd

toPd

O�C

atalysts

arepu

tin

aroun

d-sh

aped

quartz

vessel

withan

inner

diam

eter

of6cm

N.Z

han

get

al.

(201

1)146

1wt%

Pt–Z

n2GeO

4/4.5

eVIonexch

ange

method

CO2(saturated

),H

2O

(—)/Arc

Xe

lamp(635

nm)/—

(298

/—/14/12

/0.2),

(—/90.5/20

0nm

particle/—

/su

pported)

Methan

e/28

.9b/—

�Ion

exch

ange

synthesized

catalyst

show

sactivity

inph

otored

uction

ofCO2,com

pared

withZn

2GeO

4synthesized

byasolid–state

reaction

C.W

anget

al.

(201

1)170

[ReI(CO) 3(dcb

py)Cl]-

Zr6O4(O

H) 4(bpd

c)/—

Mix-and-match

synthetic

strategy

CO2(—

),Acetonitrile

(2mL)

andTEA(0.1

mL)/

450W

Xelamp

(>30

0nm)/—

(—/—

/—/16/—),

(0.02/27

7/metal–

orga

nic

fram

eworks/—

/su

spen

ded)

Carbo

nmon

oxide/42

/—�T

hesestab

lean

dpo

rous

metal

complex-

derivatives-do

pedmetal

orga

nic

fram

eworks

arevery

active

catalystsforarange

ofreaction

srelatedto

solaren

ergy

application

C.A

mpe

lliet

al.

(201

0)142

0.5wt%

Pt–T

iO2/—

Sol–geldipc

oatingfor

TiO

2lm

andwet

impregnationto

add

Pt

CO2(—

),NaO

H(1

Maq

ueou

ssolution

)/60

Wsolarlamp/—

(313

/19.3/—/7/—

),(—

/—/nan

oparticle/

—/sup

ported

)

Hyd

rogen/5.3

c /—

�Req

uire

developm

entof

thecu

rren

tlimits,

relatedto

thede

sign

oftheph

otoa

nod

ean

delectrocathod

eto

increase

activity

�Photoe

lectroch

emical

solarcellisde

sign

edby

Plexiglasan

deq

uipp

edwithaqu

artz

windo

wan

d5.7cm

2irradiated

area

A.N

ishim

uraet

al.

(201

0)159

Cr–TiO

2/—

Sol–gelan

ddip-

coatingmethod

CO2(5.76mmol),

H2O

(5.56mmol)/Xe

arclamp(185

–200

0nm)/43

.67mW

cm�2

(343

/9.8/—

/72/—),

(—/—

/lm

/3.9/

supp

orted)

Carbo

nmon

oxide/

1285

/—�T

hetotallayernum

berof

Cr-do

pedTiO

2

lm

coated

isup

to7

�Cr–TiO

2lm

isinserted

into

thestainless

pipe

,equ

ippe

dwithqu

artzglassdisc

xedto

thetopof

thestainless

pipe

Ngu

yenan

dWu

(200

8)165

1wt%

Cu–

Fe–T

iO2

lm

s/2.93

eVDip-coa

tingmethod

ordo

ctor

blad

etech

nique

CO2(saturated

),water

vapo

r(—

)/15

0W

UVAlamp(320

–50

0nm)/22

5mW

cm�2

(348

/—/5/4/0.001

),(—

/47.74

/lm

of87

8nm

thickn

ess/—/

supp

orted)

Methan

e,ethylen

e/0.06

,0.35

/—�A

continuo

uscircular

Pyrexglassreactor

(216

cm3)withaqu

artz

windo

w,w

here

catalyst

coated

opticalbe

rsarega

thered

inside

it�F

eas

aco-dop

antisindicatedto

redu

cethe

photop

rodu

ctionof

methan

ein

this

expe

rimen

t


Review RSC Advances

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16


77


Tab

le5

(Contd.)

Researcher

(year)/ref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration/

treatm

enttech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/Yield

(mmol

h�1pe

rg


)Com

men

ts

Ngu

yenan

dWu

(200

8)166

0.5wt%

Cu–

Fe–(TiO

2–

SiO2)/2.95

eVSo

l–gelmethod

CO2(saturated

),H

2O

(—)/natural

sunligh

t/2.05

mW

cm�2

(348

/—/—

/6/0.018

2),

(0.5/44.7/53

nm

thickn

esslm

/—/

supp

orted)

Methan

e/0.27

9/0.01

52�I

ntrod

ucingFe

metal

into

TiO

2–SiO

2lattice

show

sthefullvisibleligh

tab

sorption

and

supe

rior

prod

uction

ofmethan

e,compa

red

withthat

oftheba

reTiO

2–SiO

2–acac

�Acontinuo

uscircular

Pyrexglassreactor

(216

cm3)withaqu

artz

windo

w,w

herethe

catalyst

coated

opticalbe

rsarega

thered

inside

itJ.-S.H

wan

get

al.

(200

5)163

0.29

wt%

Ti-S

BA-15

(Si/Ti¼

270)/—

Mod

ied

hyd

rothermal

method

CO2(36mmol),water

vapo

r(180

mmol)/

100W

highpressu

reHglamp(>25

0nm)/—

(323

/9.8

�10

�10/—

/12

/—),(0.05/10

40/

6.9nm

pore

size/—

/im

mob

ilized

)

Methan

e,methan

ol/

0.31

,0.081

/—�T

i-SBA-15sh

owsreactivity

muc

hhigher

than

bulk

TiO

2

�Cationic

rhen

ium(I)

complex

into

amesop

orou

sAlM

CM-41materialby

ion-

exch

ange

method

show

svisibleligh

tresp

onse

�Aqu

artz

cell(88cm

3)withaat

bottom

isus

edto

carryou

tthereaction

G.G

uanet

al.

(200

3)158

Pt–K

2Ti 6O13withFe

-ba

sedcatalyst

supp

ortedon

Y-typ

ezeolite/—

Steam

dealumination,

impregnationan

dph

otochem

ical

depo

sition

method

CO2(saturated

),H

2O

(4mL)/300

WXelamp,

orconcentrated

sunligh

t/—

(298

/0.76/—/6/—

),(0.3/—

/pow

der/—/

supp

orted)

Hyd

rogen/15.2/—

�Thereaction

tempe

rature

expressively

inue

ncedby

concentratingthesolar

irradiation,reach

ing60

0Kan

dincreasing

theyields

�Catalystsareke

pton

wet

quartzwoo

l(awet

bedforthecatalystlayer)in

anop

ticalq

uartz

tube

cell

I.-H

.Tsenget

al.

(200

2)150

2wt%

Cu–

TiO

2/—

Mod

ied

sol–gel

process

CO2(—

),NaO

H(300

mL,

0.2N

aque

ous

solution

)/Hglamp

(254

nm)/—

(323

/—/7/6/—

),(0.3/

26/20nm

particle/—

/su

spen

ded)

Methan

ol/20/—

�Resultsh

owsthat

ahigher

positive

zeta

potential

atpH

7canlead

higher

activity

�Acylindrical

quartz

reactorwithacapa

city

of30

0mLis

used

tocarryou

tthereaction

M.A

npo

etal.

(199

8)171

1.0wt%

Pt-Ti-MCM-

48(Si/Ti¼

80)/—

Hyd

rothermal

and

impregnation

method

s

CO2(24mmol),water

vapo

r(120

mmol)/

highpressu

reHg

lamp(>28

0nm)/—

(328

/1.3

�10

�9/—

/—/—

),(—

/—/>2nm

pore

size/—

/sup

ported

)

Methan

e/12

/—�T

hech

arge

tran

sfer

excitedstateof

highly

disp

ersedtitanium

oxidesp

eciesplaysan

impo

rtan

trole

intheredu

ctionof

CO2an

dsh

owsahighselectivityfortheform

ationof

CH

3OH

�Aqu

artz

cellwithaat

bottom

isus

edto

redu

ceCO2withH

2O

M.A

npo

etal.

(199

7)82

1wt%

Pt–(TiO

2–Y

-zeolite)/—

Impregnationmethod

CO2(24mmol),water

vapo

r(120

mmol)/75

Whigh-pressureHg

lamp(>28

0nm)/—

(328

/1.3

�10

�9/—

/6/—),(0.15/—/

powde

r/—/

supp

orted)

Methan

e/0.08

/—�T

hetitanium

oxidesp

eciesarehighly

disp

ersedwithin

thezeolitecavities

andexist

inatetrah

edralcoordination

�Add

ingPt

tothean

chored

titanium

oxide

catalystsen

courag

esthech

arge

sepa

ration

withincreasingCH

4yields

inplaceof

CH

3OH


RSC Advances Review

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Tab

le5

(Contd.)

Researcher

(year)/ref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration/

treatm

enttech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/Yield

(mmol

h�1pe

rg


)Com

men

ts

Solymosian

dTom

bacz

(199

4)73

1wt%

Rh–T

iO2/—

Wet

impregnation

method

CO2(saturated

),H

2O(120

mL)/500

Whighpressu

reXe

lamp/0.68

mW

cm�2

(333

/1/—

/5/—

),(0.3/

18/nan

oparticle/

0.00

25/sus

pende

d)

Form

icacid/1.6/—

�Anincrease

intheelectron

concentration

ofTiO

2en

han

cestheprod

uction

oforga

nic

compo

unds

�APy

rexglasscellis

used

tocarryou

tthe

photocatalytic

redu

ctionof

CO2withH

2O

K.A

dachiet

al.

(199

4)151

5wt%

Cu–

TiO

2/—

Impregnationmethod

CO2(27.09

atm),

H2O

(30mL)/450

WXelamp/—

(298

/1/5.45/48

/—),

(0.5/8.7/230

nm

particle/—

/su

spen

ded)

Methan

e/0.02

/—�L

owyieldan

dcompu

lsorysepa

ration

ofthega

seou

sprod

ucts

may

create

draw

backs

forthis

system

�Catalysts

areplaced

inastainless-steel

vessel

(42.5mL)

withaqu

artz

windo

wto

run

theph

otocatalytic

redu

ctionof

CO2

J.C.H

emminger

etal.(197

8)172

SrTiO

3–P

t/3.2eV

—CO2(0.019

atm),

water

vapo

r(0.022

atm)/50

0W

high

pressu

remercu

rylamp/—

(315

/1/—

/0.5/0.01),

(—/—

/particle/—/

supp

orted)

Methan

e/—/—

�Aba

tchtype

isolationch

ambe

ris

used

tocarryou

tthereaction

andthesamples

are

kept

onasm

alldisk

of1cm

2area

�Theph

otoan

dthermal

processesareeasily

distingu

ished

bytheltersus

edin

this

expe

rimen

t

aT/P/pH/t/Q

E:tem

perature

(K)/pressu

re(atm

)/pH

/illu

minationpe

riod

(hou

rs)/qu

antum

efficien

cy(%

).W/SA/PS/SC

/MP:

weigh

tof

photocatalyst(g)/su

rfacearea

(m2g�

1)/ph


(diameter)/semicon

duc

torconcentration(g

mL�

1 )/m

odeof


’:not

men

tion

edin

theoriginal

pape

r.bpp

mh�1g�

1catalyst.c

mmol

h�1g�

1catalyst.


Review RSC Advances

Publ

ishe

d on

13

May

201

4. D

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02:3

0:16



Tab

le6

Advance

sin

non-m

etal-dopedan

dmodifiedca

talystsforCO

2photoreductiona

Researcher

(year)

andref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg


)Com

men

ts

Q.Z

han

get

al.

(201

1)190

10wt%

Ido

pedTiO

2

at37

5� C

/3.0

eVHyd

rothermal

method

CO2(saturated

),water

vapo

r(0.022

atm)/45

0W

Xelamp

(>40

0nm)/23

3mW

cm�2

(—/—

/—/3.5/—

),(0.2/137

.6/<45

mm

nan

oparticle/—/

supp

orted)

Carbo

nmon

oxide/

2.4/—

�Sam

ples

arepu

ton

aglass-be

rlter

atthe

bottom

ofthecylindricalph

otoreactor

(58cm

3)w

ith

stainless

steelwalls

andaqu

artz

windo

w�H

ighCO2redu

ctionactivity

isde

tected

forI–TiO

2

unde

rvisibleligh

tthan

undo

pedTiO

2un

derUV-vis

irradiation

�Too

highan

iodine-do

pinglevelmay

create

recombinationcenters

andthus

lower

the

photocatalytic

activity

T.M

.Suz

uki

etal.(201

1)182

N–T

a 2O5an

chored

withRu-dp

bpy/2.4eV

Directassembly

method

CO2(saturated

),MeC

N:T

EOA(5

:1,

4mLsolution

)/50

0W

Xelamp(410

–750

nm)/—

(—/—

/—/60/—),

(0.01/—/20–40

nm

nan

oparticle/—/

susp

ende

d)

Form

icacid/60b/—

�Itis

foun

dthat

theph

otocatalytic

activity

and

stab

ilityof

thehyb

ridcatalyst

depe

nds

onthe

chem

ical

structureof

thean

chor

grou

p

C.-W

.Tsaiet

al.

(201

1)184

0.42

wt%

Ndop

edInTaO

4with3.2wt%

Ni–NiO

core

shell/2.28

eV

Impregnationmethod

CO2(saturated

),de

ionized

water

(50

mL)/Xelamp/—

(298

/—/—

/2/—

),(0.1/—

/40nm

particles/—/

susp

ende

d)

Methan

ol/165

/—�N

itrogendo

pingsh

owsvisibleligh

tresp

onsive

photocatalytic

activity

withad

dition

ally

high

absorban

cean

dgivesap

proxim

atelytw

icetheyield

ofun

dope

don

es,w

hiletheco-catalytic

method

givesab

outtriple

theyield

�Reactionis

condu

cted

onastirred-type

,continuo

us-ow

reactorhavingawindo

w-typ

eirradiationcellthat

ismad

eof

Pyrexglass(200

mL)

L.-M

.Xueet

al.

(201

1)185

Cdo

pedTiO

2/—

Immersion

and

calcinationmethod

sCO2(saturated

),H

2O

(10mL)/175

Whighpressu

reHg

lamp/—

(—/—

/2/6/—

),(1/—

/nan

opow

der/—/

susp

ende

d)

Form

icacid/439

/—�T

hedo

pedcarbon

show

slow-ban

dga

pan

dexpe

nds

theab

sorption

ofvisibleligh

tregionun

der

thesimulated

daylightlamp

�Athree-concentric

cylindrical

quartz

reactoris

used

havingou

term

ostlayerforreaction

pool,the

middlelayerforqu

artz

cold

trap

andtheinnermost

layeras

theligh

tsource

S.Sa

toet

al.

(201

0)183

N–T

a 2O5linke

dwith

Ru-dcb

py/2.4

eVThermal

treatm

ent

method

CO2(saturated

),MeC

N:T

EOA(5

:1,

4mLsolution

)/Xe

lamp(405

nm)/—

(298

/—/—

/20/1.9),

(0.005

/—/

nan

oparticles/—

/su

spen

ded)

Form

icacid/70/75

�Visible

ligh

tis

used

toirradiate8mLtest

tube

scontainingtheph

otocatalysts

�Thelinka

gebe

tweenthecomplex

andthe

semicon

ductor

greatlyincreasesthereaction

rate

O.K

.Vargh

ese

etal.(200

9)168

Nitrogen-dop

edtitania

nan

otub

ewith

copp

er/—

Anod

izationan

dan

nealedat

460or

600

� Cfor3h

CO2(saturated

),water

vapo

r(saturated

)/su

nligh

t/10

0mW

cm�2

(317

/<0.06

8/—/3.5/—

),(—

/—/nan

oparticle/

—/sup

ported

)

Methan

e/4.4/—

�CO2to

hyd

rocarbon

prod

uction

rate

unde

rou

tdoo

rsolarligh

tis

atleast20

times

higher

than

usingUVillumination

�Sam

ples

areload

edinto

astainless-steel

cham

ber

havingan

O-ringsealed

quartz

windo

wat

thetop

forad

mittingsolarradiation

�Ide

nticalconstructionof

twoch

ambe

rshaving

volumes

7.5cm

3an

d8.6cm

3areus

edto

allowfor

twosamples

simultaneo

usly

aT/P/pH/t/Q

E:tem

perature

(K)/pressu

re(atm

)/pH

/illu

minationpe

riod

(hou

rs)/qu

antum

efficien

cy(%

).W/SA/PS/SC

/MP:

weigh

tof

photocatalyst(g)/su

rfacearea

(m2g�

1)/ph


(diameter)/semicon

duc

torconcentration(g

mL�

1 )/m

odeof


’:not

men

tion

edin

theoriginal

pape

r.bTurnover

num

ber.


RSC Advances Review

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arbon radicals and H atoms formed on the Pt metals.82 Pt-loaded alkali niobates ANbO3 (A ¼ Na, K) photocatalysts werereported by Li et al.144 Results showed that KNbO3 exhibitedhigher photocatalytic activity than NaNbO3 because of havinglow band gap energy (Eg) and high mobile charge carriers. Here,Pt acts as a co-catalyst to provide the reactive site for hydrogengas production easily in terms of low H2 over-potential on the Ptsurface.145 It is also noticeable that increasing Pt proportionfrom 0.5 wt% to 1.0 wt% promotes the formation of hydrogengas. The photocatalytic performance of micro- or mesoporousZn2GeO4 loading with various percentages of Pt (from 0.5 to 2.5wt%) was investigated by N. Zhang et al., from which the resultsevaluated the highest activity for 1.0 wt% Pt.146

Ishitani et al. reported that Pd doped on TiO2 performedmore effective photocatalytic activity than that of other metalsloaded on TiO2, and it also increased the formation of CH4

instead of CO from CO2.147 On the other hand, Pd on the surfaceof TiO2 partially oxidized to form PdO during the progression ofCH4 formation, which led to the deactivation of the Pd–TiO2

catalysts.52 Solymosi and Tombacz assumed that adsorptionand photocatalytic behaviors of Rh were signicantly inuencedby the existence of an electronic interaction at the Rh–TiO2

interface.73 Moreover, the catalytic activity of Rh was affected bydoping with altervalent cations (W6+) due to the variation of theelectron concentration of the TiO2.148 Cu is another noble metal,which is eminent for its availability and low cost. Not only that,Cu loading on TiO2 was a prevalent approach in photocatalyticconversion of CO2 because of its high yield through the loweringof re-electron–hole recombination probability. According toTseng et al., an optimal Cu-loaded titania (2 wt% Cu) was foundto have the highest photochemical activity, which providedlower uorescence intensity and resulted in higher methanolformation.149,150 Depending on the amount of Cu loading onTiO2, the formation of the hydrocarbon products has beenchanged also.151 Ag-doped TiO2 was also investigated in thistechnology reduced the band gap energy of TiO2 and shied theband edge from the UV region to the visible region, according tothe amount of Ag added.152,153 Ag co-catalyst-loaded on ALa4-Ti4O15 (A ¼ Ca, Sr, and Ba) has been published, and Ag acts as aCO2 reduction site to generate CO as reported in electro-chemical reduction of CO2, where Ag was used as an electro-catalyst.154 Moreover, the plasmon effect has been developed byAu nanoparticles doped with TiO2 through increasing thevisible light response.155–157

2.3.2 Transition metal doped semiconductors. To improvethe photocatalytic activity and efficiency, transition metals arealso investigated in many studies due to the high visible lightconversion of CO2.158 The interesting thing about transitionmetals is that their valence electrons to exhibit severalcommon oxidation states. Among the transition metals, Co,Mn, Ni, Cr, Mo, Fe, V, and Ti are commonly used as dopingelements in photocatalytic technology to enhance the semi-conductor performances under visible light. For visible lightresponsibility, Nishimura et al. synthesized Cr-doped TiO2

lm by the sol–gel and dip-coating methods.159 They statedthat a suitable percentage of metal doping on semiconductorsimproved catalytic activity, whereas excess percentages of


dopant metal beyond the optimal level decreased the photo-catalytic performances because of charge recombination.However, the H2 generation rate was more advanced for Fe-doped TiO2 than Cr-doped TiO2 because of avoiding therecombination in trapping both electrons and holes by Feparticles, whereas Cr can only trap one type of chargecarrier.160 The experiment was done to characterize thedifferent types of transition metals (V, Fe, Ce, Cu, Cr) dopedTiO2, prepared by Pan et al.161 They reported that V and Fewere placed in the substitutional sites of TiO2, and Ce ionswere dispersed in the interstitial sites, although Cr and Cuaccumulated on the surface. The activity order was found insuch a manner: Fe–TiO2 > V–TiO2 > Cr–TiO2 > Ce–TiO2 > TiO2

> Cu–TiO2, which indicates that the local structure and type ofdopant have vital contributions on photocatalytic technology.

The loading of transitional metal species on the frameworknanostructure is a common practice for increasing the highdispersion of those species. Moreover, for surface immobiliza-tion, titanium species are conned onto mesoporous silica.This phenomenon is also effective for photocatalytic reactionsof titanium particles as their photocatalytic activities intenselyrely on the structures of active sites.162 It was found that highand exceptional photocatalytic reactivity was obtained for thereduction of CO2 with H2O to generate CH4 and CH3OH underUV irradiation, compared with the bulk TiO2.163

2.3.3 Co-metal doped semiconductors. The doping ofbimetallic elements together on semiconductor photo-catalysts has been validated as a potential method to advancethe visible light response of photocatalysts. Suitable co-dopant combinations are necessary to enhance the photo-catalytic activity of semiconductors because all of the co-dopant congurations are not appropriate for this tech-nique.164 Fe as a co-dopant in the Cu–TiO2 catalyst has beeninvestigated for photocatalytic reduction of CO2 with H2O toethylene at a quantum yield of 0.024%.165 An efficient chargetransfer mechanism occurred in this combination betweenTiO2 and Cu as well as Fe as co-dopants. However, Cu co-doped with Fe–TiO2 on SiO2 support has also been examinedunder concentrated natural sunlight with high quantum yield(0.05%) and full absorption of visible light and compared toonly Cu co-doped on the Fe–TiO2 counterpart.166 The photo-catalytic performance of the Cu doped on Pt–TiO2 catalysts isinuenced by the content of Cu loading. The high percentageof Cu loading expressively decreases the formation of H2 andimproves the formation of CH4 and CO.167 Varghese et al.reported that Pt may have an effect on H2O activation,whereas Cu plays an important role in activating CO2 to CO.168

Bimetallic Ag–Pt were used with the TiO2 semiconductorcatalyst as reported by Mankidy et al. who obtained a selec-tivity of CH4 of almost 80%, compared with TiO2 of 20%selectivity.169 Their investigation found that Ag species had astrong surface plasmon absorption band in the UV-vis region,while the Pt species did not show that behavior. However, thecombination of bimetallic Ag–Pt has the ability to tune theelectronic properties during photochemical reactions. Notonly that, combined Ag–Pt can accumulate the electrons by Agand transfer the interfacial electron charge from Pt.

RSC Adv., 2014, 4, 20856–20893 | 20881


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2.3.4 Non-metal doped semiconductors. It has beenbroadly conrmed that doping or co-doping of semiconductorphotocatalysts with non-metals such as C, N, S, B and F hasshown considerably low band gap energy, compared with metaldoping in different experiments, prominent to efficient pho-tocatalytic activity during visible light irradiation.173–180 InTable 6, the update of major ndings and detailed informationregarding non-metal-doped semiconductors is given elabo-rately to evaluate their progress in the photocatalytic conver-sion of CO2. Compared with other non-metals, nitrogen-dopedsemiconductors show the most active results due to the narrowband gap.181 A high rate of solar conversion of carbon dioxide inthe presence of water vapor to methane and other hydrocar-bons is attained from nitrogen-deposited titania nanotubearrangements, as reported by Varghese et al.168 Particularly, lowwall thickness of nanotube arrays promotes the transfer ofefficient carriers to the adsorbing species. From the experi-ment, it was also found that the incorporation of Pt and Cunanoparticles on the external surfaces of nanotube arrange-ments further increases the hydrocarbon yield under theoutdoor global AM 1.5 sunlight with 100 mW cm�2 intensity.Both Suzuki et al. and Sato et al. studied p-type, photoactive,nitrogen-doped Ta2O5 linkage with metal-complex electro-catalysts, which revealed outstanding photochemical conver-sion of CO2 to formic acid under visible light irradiation byincreasing the photoexcited electron transfer from theconduction band of the semiconductor to the metalcomplex.182,183 The co-catalytic (N-doped InTaO4 modied withNi–NiO core–shell nanostructure) approach by Tsai et al. notonly intensely increases solar light absorbance but alsocompetently reduces the electron–hole recombination.184 Theinvestigation with carbon-doped TiO2 was done by Xue et al.because doped carbon narrows the band gap, shis theabsorption to the visible light region and facilitates the chargeseparation efficiency.185 Hiroshi et al. reported that the hydro-philic behavior of C-doped TiO2 semiconductor exhibitedvisible light response, which was initiated from the localized C(2p) formed above the valence band.186 S-doped TiO2 has theability to reduce the band gap energy and inhibits the alter-ation of anatase phase into rutile phase at elevated tempera-tures.187 Correspondingly, the doping of F on a semiconductorcatalyst performs better by decreasing the band gap energy.188

Results showed that iodine doping on semiconductor cata-lysts exhibited higher photochemical activity due to the closerionic radii of I5+ and Ti4+ for substituting lattice titanium thanother non-metal-doped elements like nitrogen, carbon, boron,and sulfur.189 Moreover, the replacement of Ti4+ with I5+ beginscharge inequality to the formation of Ti3+ surface states that isable to arrest the photoinduced electrons and prevent chargerecombination in the photocatalytic conversion of CO2.190

Zhang et al. found that iodine-doped TiO2 semiconductorscalcined at 375 �C reduced particle sizes signicantly andincreased photochemical activity.190 It was also observed that10% iodine deposition on TiO2 harvested maximum yield undervisible light irradiation compared with the 5% iodine dopingon TiO2.

20882 | RSC Adv., 2014, 4, 20856–20893

2.4 Sensitized photocatalysts

For photocatalytic CO2 conversion, photosensitization of thecatalyst is a promising technique which makes the catalystmore sensitive during light irradiation. This practice is gener-ally conducted in photochemical applications under the lightsources that have particular wavelength deciencies for photo-excitation. The mechanism generates photoexcited electrons inthe conduction band of photosensitizers under the light sourceof particular wavelengths, and the electrons are directly shiedto the conduction band of catalysts to make them photoactiveunder visible light irradiation.191 According to Tahir and Amin,photosensitization exclusively depends on the suitable sensi-tizer band gap that could absorb visible light irradiations, andthe sensitizer conductance band needed to be higher (morenegative) than the catalyst's conductance band to transferelectrons towards the catalyst.181 Some informative character-istics are represented in Table 7 for different types of sensitizedphotocatalysts, which are applied in many studies forincreasing photocatalytic activity in order to obtain high photonefficiency and selectivity.

2.4.1 Dye sensitized photocatalysts. Dye sensitized photo-catalysts are widely used in photochemical technology becauseof their ability to create photoactive electrons easily by visiblelight hitting the dye compound. This phenomenon helps totransfer those photoactive electrons from the dye to the cata-lysts' conduction band for activation during the visible lightirradiation.37 An effective response to visible light and efficientelectron transfer can enhance fuel yields. The charge separationcan be effectively achieved by adding a co-catalyst to trap theelectrons, which is depicted in Scheme 1(d).

Li et al. reported that charging of Pt and RuO2 on dyesensitized hexagonal nanoplates to form micro-octahedronZn2SnO4 enabled the accumulation of photoexcited electronsand holes, respectively.192 Dye sensitized, Pt loaded, nanosized{010} dominant anatase TiO2 rods were used to obtain excep-tional surface atomic structure for optimum adsorption of dyemolecules and encourage electron transmission from exciteddye molecules to anatase TiO2.193 These occurrences alsoupgraded the current density and solar energy conversion effi-ciency of that system. Another investigation found thatRuII(2,20-bipyridyl-4,40-dicarboxylate)2-(NCS)2 (basically knownas N3-dye) sensitized Cu–Fe loading on the P25 photocatalystwas able to absorb the wavelength of 400–800 nm from solarirradiation.194 Moreover, the results show that N3-dye is notsuitable under articial light (wavelength range is 320–500 nm)irradiation because before the irradiation, it is dispersed on thematrices of the photocatalysts; as a result, the dye does not showits full response to the light source but using concentratednatural sunlight N3-dye remarkably increases methane yield upto over 100%. Similar results were found by Wu and Chiou inthat N3-dye gave effectual charge transfer with higher photo-reduction under concentrated sunlight.195 Moreover, it was alsofound that N3-dye showed its stability for up to 6 h under botharticial light and concentrated sunlight. Ozcan et al. usedperylene diimide dyes, which have revealed similar visible lightresponse to the RuII(2,20-bipyridyl)chloride hexahydrate.196 In



Tab

le7

Advance

sin

sensitize

dca

talyst

forCO

2photoreductiona

Researcher

(year)

andref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/Yield

(mmol

h�1pe

rg

catalyst)/effi

cien

cy(%

)Com

men

ts

Z.Li

etal.

(201

2)192

Dye-sen

sitized

Zn2Sn

O4with1wt%

Ptan

d1wt%

RuO

2/

3.87

eV

Nuc

leationprocesses

andsolid-state

reaction

CO2(saturated

),water

vapo

r(0.4

mL)/300

WXearc

lamp/10

0mW

cm�2

(—/—

/—/10/—),(0.1/

7.71

/nan

oplates/—/

supp

orted)

Methan

e/86

.7b/3.1

�Catalysts

areeven

lyke

ptat

thebo

ttom

ofa

Pyrexglasscell,

andthereaction

iscarried

outin

aga

s-tigh

tcondition

�Theun

itarygeom

etry

oftheph

otoa

nod

eim

proves

thesolaren

ergy

conversion

J.Pa

net

al.

(201

1)193

Dye-sen

sitizedTiO

2

(anatase)

with1wt%

Pt/3.27eV

Hyd

rothermal

treatm

ent

CO2(saturated

),water

vapo

r(—

)/solarligh

t/—

(—/—

/7/10/7.73

),(—

/25/nan

o-sized

rods

/—/sup

ported

)

Methan

e/2.5/—

�Due

todistinctivesu

rfaceatom

icor

electron

icstructure,

thenan

osized

rods

show

ahigher

activity

inconvertingCO2into

CH

4an

dacompa

rableen

ergy

conversion

efficien

cyW.-N

.Wan

getal.

(201

1)212

SensitizedTiO

2–SiO

2

compo

site

with0.01

mol%

Cu/1.2eV

FuARmethod

CO2(saturated

),de

ionized

water

(3.0

g)/Xearclamp(250

–40

0nm)/2.4mW

cm�2

(—/—

/—/8/—

),(0.02/

81.71/26

5nm

compo

site

particles/—/

supp

orted)

Carbo

nmon

oxide/20

/—�S

ize,

compo

sition

andpo

rosity

ofthe

compo

site

particlesaretailored

bycontrollingtheprecursorconcentration,

stoich

iometricratioan

dtempe

rature,

resp

ectively

�Thereactoris

cylindrically

builtwitha

stainless-steel

wallan

daqu

artz

windo

w60

mm

indiam

eter

and25

mm

deep

,vertically

facingthesolarligh

ter

W.H

ouet

al.

(201

1)211

SensitizedTiO

2lm

swithAu/2.5eV

Sol–gelprocess

CO2(saturated

),water

vapo

r(—

)/UV

lamp(254

nm)/20

mW

cm�2

(348

/—/—

/15/2.3�

10�2),(—

/—/400

nm

thickn

esslm

/—/

supp

orted)

Methan

e/22

.4c /—

�Surface

plasmon

sof

theAunan

oparticles

create

intense

electrom

agnetic

elds

toen

han

cesu

b-ba

ndga

pab

sorption

inthe

TiO

2,a

ndim

provetheph

otocatalyticactivity

inthevisiblerange

�Reactionis

condu

cted

onasealed

51.6

mL

stainless

steelreactor

withaqu

artz

windo

w,

wheretheph

otocatalytic

lm

sareke

pton

thecatalystholde

r,whichison

thebo

ttom

ofthereactor

D.L

uoet

al.

(201

1)213

SensitizedTiO

2

particleswith2wt%

Cu–

Ce/—

Wetness

impregnationmethod

CO2(saturated

),NaO

H(0.2

M,1

50mLsolution

)/12

5W

UVlamp(365

nm)/—

(343

/—/—

/16/—),(0.15/

2.96

3/po

wde

r/—/

susp

ende

d)

Methan

ol/11.3/—

�Itisfoun

dthat

Ceatom

saff

ectthereaction

moreprofou

ndlythan

Cuatom

sdo

,andCe

atom

sactivate

H2O

andCO2molecules

�Reactionis

carriedou

tin

ahermetic

high-

pressu

rereaction

stillequ

ippe

dwithheating

andmag

netic

stirringsystem

T.W

.Woo

lerton

etal.(201

1)209

EnzymeCODHI-

mod

ied

TiO

2with

ruthen

ium

bipy

ridyl/—

Stan

dardsystem

protocol

CO2(98%

),CH

4

(2%)/25

0W

(24V)

tungsten-halog

enbu

lb(420

nm)/45

mW

cm�2

(293

/—/6/4/0.07),

(0.005

/50/nan

oparticles

(<10

0nm)/—/

susp

ende

d)

Carbo

nmon

oxide/

255/—

�Reactionis

carriedou

tin

apressu

revessel

containingthesu

spen

sion

was

sealed

tigh

tly

witharubb

erseptum

�Mod

ied

enzymeis

efficien

tforCO2

conversionan

dsensitizedto

visibleligh

t


Review RSC Advances

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16


83


Tab

le7

(Contd.)

Researcher

(year)

andref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/Yield

(mmol

h�1pe

rg

catalyst)/effi

cien

cy(%

)Com

men

ts

Y.L

iet

al.

(201

0)77

SensitizedTiO

2with

0.5wt%

Cuon

SiO2/

3.1eV

One-po

tsol–gel

method

CO2(saturated

),water

vapo

r(—

)/Xe

lamp(250

–400

nm)/

2.4mW

cm�2

(273

/1/7/4/1.41),(0.1/

386.2/<1

00mm

powde

r/—/sup

ported

)

Carbo

nmon

oxide/45

/—�H

ighsu

rfacearea

mesop

orou

ssilica

subs

tratehighly

improves

photored

uction

dueto

better

TiO

2disp

ersion

andincreased

adsorption

ofCO2an

dH

2O

onthecatalyst,

andad

dition

ofCusp

eciesincreasesmulti-

electron

reaction

s�A

cylindrical

continuo

us-ow

reactoris

used

withastainless

steelwallan

daqu

artz

windo

wwitha6.0cm

diam

eter

anda2.5cm

depth

K.K

ocıet

al.

(201

0)152

SensitizedTiO

2

particleswith7wt%

Agdo

ping/2.74

eV

Sol–gelprocess

CO2(saturated

),H

2O

(100

mL)/8

WHglamp(254

nm)/—

(—/—

/—/24/—),

(—/79.7/1.65

nm

particles/0.00

1/su

spen

ded)

Methan

e/0.36

/—�A

gim

purity

inside

theTiO

2sh

isthe

absorption

edge

tovisibleligh

tan

dform

sSchottkyba

rrierat

themetal–sem

icon

ductor

interfaceto

redu

ceelectron

andholes

recombination

�Thereaction

iscondu

cted

onastirred

batchan

nular

reactorwithasu

spen

ded

catalyst

T.W

.Woo

lerton

etal.(201

0)208

EnzymeCODHIwith

RuPmod

ied

TiO

2/—

—CO2(98%

),CH

4

(2%)/tungsten-

halog

enlamp(420

nm)/45

mW

cm�2

(293

/—/6/4/—

),(0.005

/—/

nan

oparticles/—

/su

spen

ded)

Carbo

nmon

oxide/

250/—

�Photocatalytic

reaction

iscarriedou

tin

aPy

rexpressu

revessel

(9mL)

withgentle

stirring

�Reactionis

focu

sedon

atw

o-electron

pathway

toavoidthethermod

ynam

ically

difficu

ltstep

involved

inon

e-electron

CO2

activation

sC.W

anget

al.

(201

0)200

CdS

e–Pt–T

iO2/—

Wet

impregnation

method

CO2(0.000

4atm),

water

vapo

r(0.004

atm)/30

0W

Xearc

lamp(>42

0nm)/10

0mW

cm�2

(—/—

/—/6/—

),(0.3/50/

hetero-structured

/—/

supp

orted)

Methan

e,methan

ol/

0.00

19,0

.000

137/—

�Qua

ntum

dot-sensitizedTiO

2

heterostruc

turedmaterials

arepro

cien

tfor

photored

uction

ofCO2us

ingvisibleligh

t�R

esults

exhibitthat

germ

anecanbe

used

onph

otored

uction

reaction

sforCO2capture

andreus

eZ.

Zhao

etal.

(200

9)203

0.7wt%

Co(

II)Pc–

TiO

2/—

Insitu

synthesized

byad

vancedsol–gel

method

CO2(—

),NaO

H(100

mL,

0.1N

aque

ous

solution

)/50

0W

tungsten-halog

enlamp/—

(—/—

/—/10/—),(0.15/

75.23/11

.35nm

particle/—

/sus

pende

d)

Form

icacid/150

/—�T

heyieldof

insitu

CoP

c–TiO

2is

muc

hhigher

than

thoseof

TiO

2an

dph

ysical

absorbed

CoP

c–TiO

2

�Reactionis

condu

cted

inaPy

rexglasscell

(100

mL)

withan

opticalwindo

wZ.

Zhao

etal.

(200

9)204

0.5%

CoP

c–TiO

2/—

Sol–gelmethod

CO2(saturated

),NaO

H(0.1

N,100

mL

aque

oussolution

s)/

500W

tungsten-

halog

enlamp/—

(673

/—/—

/6/—

),(0.15/

90.7/11.02

nm

particle/

0.1/su

pported)

Form

icacid/57/—

�CoP

c–TiO

2prep

ared

bysol–gelsh

ows

relatively

highph

otocatalytic

activity,

compa

redwithTiO

2synthesized

bysol–gel

method

�APy

rexcellwithan

opticalwindo

w(100

mL)

isus

edto

condu

cttheph

otocatalytic

reaction


RSC Advances Review

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16



Tab

le7

(Contd.)

Researcher

(year)

andref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/Yield

(mmol

h�1pe

rg

catalyst)/effi

cien

cy(%

)Com

men

ts

H.-C

.Yan

get

al.

(200

9)214

Sensitized45

wt%

Cu–

TiO

2on

SBA-15/

3.2eV

Sol–gelsynthesis

and

hyd

rolysis–

conde

nsation

process

CO2(saturated

),NaO

H(0.1

N,5

50mLaq

ueou

ssolution

)/40

0W

med

ium-pressure

halidelamp(365

nm)/—

(315

/—/7/8/—

),(0.05/

436/5.5nm

particles/—/

susp

ende

d)

Methan

ol/475

/—�L

oadingam

ountof

TiO

2on

mesop

orou

sSB

A-15compo

site

photocatalysts

control

the

crystallize

size

ofthesu

pportedTiO

2

particlesan

dthemesop

orou

sstructureof

thecatalyst

JeffreyC.S

.Wu

(200

9)194

Dye-sen

sitizedCu–

Feload

edon

P25/—

Dip-coa

tingmethod

CO2(saturated

),water

vapo

r(—

)/su

nligh

t(400

–80

0nm)/20

mW

cm�2

(—/—

/—/—

/—),

(—/—

/lm

compo

site/—

/su

pported)

Methan

ol/—

/—�D

ye-sen

sitizedCu–

Feload

edon

P25

photocatalystcancompletelyus

etheligh

ten

ergy

of40

0–80

0nm

from

sunligh

tan

dsign

icantlyincrease

itsph

otoa

ctivity

Ngu

yenan

dWu

(200

8)166

SensitizedTiO

2with

0.5wt%

Cu–

Feload

edOnSiO2/2.95eV

Sol–gelprocess

CO2(saturated

),H

2O

(—)/natural

sunligh

t/2.05

mW

cm�2

348/—/—

/6/0.018

2),

(0.5/44.7/53

nm

thickn

esslm

/—/

supp

orted)

Methan

e/0.27

9/0.01

52�A

nop

tical-

berreactorconsistsof

acontinuo

uscircular

Pyrexglassvessel

(216

cm3)w

ithaqu

artz

windo

win

whichcatalyst-

coated

opticalbe

rsareinserted

�Cu–

Feload

edon

TiO

2–SiO

2–acac

catalyst

show

sthesu

perior

photoa

ctivity,

compa

red

withthat

ofba

reTiO

2–SiO

2–acac

coun

terpart

JeffreyC.S

.Wu

etal.(200

8)215

SensitizedTiO

2with1

wt%

Agcoated

onop

ticalbe

r/3.5eV

Thermal

hyd

rolysis

method

CO2(1.13atm),H

2O

(0.03ba

r)/UVligh

t(365

nm)/10

000

mW

cm�2

(348

/—/—

/11/0.00

013),

(—/71.66

/sph

erical

particlesof

12nm/—

/su

pported)

Methan

ol/4.1/—

�Anop

tical-

berph

otoreactor,b

uilt-in

216x

catalyst-coa

tedbe

rsaresu

pportedon

circular

plates

andirradiated

throug

hthe

quartzwindo

whavingalargeexternalarea

ofop

ticalbe

rto

disp

erse

catalysts

�Optical-be

rde

livers

amed

ium

totran

smit

ligh

tun

iformly

throug

hou

tthereactor

compa

redwithatrad

itional

packed

-bed

reactor

TheV.N

guyen

etal.(200

8)195

N3dy

e–0.5wt%

Cu–

0.5wt%

Fe–T

iO2/3.11

eV

Dip-coa

tingmethod

CO2(saturated

),water

vapo

r(—

)/su

nligh

t/20

mW

cm�2

(348

/—/—

/6/—

),(—

/48.7/lm

compo

site/—

/su

pported)

Methan

e/0.61

7/—

�Acontinuo

uscircular

Pyrexglass(216

cm3)

opticalbe

rph

otoreactor

isirradiated

byconcentrated

sunligh

tthroug

haqu

artz

windo

w�N

3-dy

ehighly

increasestheph

otoa

ctivity

unde

rconcentrated

natural

sunligh

tan

dde

livers

efficien

tch

arge

tran

sfer

Z.Zh

aoet

al.

(200

7)205

1wt%

Zn(II)Pc–

TiO

2/—

Sol–gelm

ethod

andin

situ

chem

ical

synthesis

tech

nique

CO2(saturated

),NaO

H(100

mL,

0.1

Naq

ueou

ssolution

)/50

0W

tungsten-halog

enlamp/—

(—/—

/—/10/—),

(0.15/—/28nm

particle/—

/sus

pende

d)

Form

icacid/98/0.37

�Results

show

that

form

icacid

prod

uction

issign

icantlyincreasedby

load

ingZn

Pc,

andhavingahigher

selectivitythan

metal

catalystsfortheredu

ctionof

CO2

�Amag

neticstirredPy

rexglasscell(100

mL)

withan

opticalwindo

wis

used

tocarryou

tthereaction


Review RSC Advances

Publ

ishe

d on

13

May

201

4. D

ownl

oade

d by

Uni

vers

ity o

f M

alay

a on

14/

08/2

014

02:3

0:16



Tab

le7

(Contd.)

Researcher

(year)

andref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/Yield

(mmol

h�1pe

rg

catalyst)/effi

cien

cy(%

)Com

men

ts

O.O

zcan

etal.

(200

7)196

BrG

ly-ptload

edon

TiO

2/—

Dip-coa

tsol–gel

tech

nique

andwet

impregnationmethod

CO2(0.88atm),

water

vapo

r(0.03

atm)/75

Wda

ylight

lamp/58

mW

cm�2

(—/0.9/—

/3/0.022

8),

(—/—

/>1mm

thicklm

compo

site/—

/su

pported)

Methan

e/0.2/—

�Perylen

edi-im

idede

rivativesareus

edto

show

ligh

tharvestingcapa

bility

similar

tothetris(2,2-bipyridyl)ruthen

ium(II)ch

loride

hexah

ydrate

�Thereaction

iscondu

cted

onaglass

cham

berconnectedto

avacu

umline

�Itis

foun

dthat

visibleligh

tpe

rforman

ces

ofthedy

esarein

thefollo

wingorde

rin

the

presen

ceof

Pt:R

ubpy

>BrAsp

>BrG

lyTsengan

dWu

(200

4)216

SensitizedTiO

2with2

wt%

Cu/—

Sol–gelproced

ures

andpo

st-treatmen

tsCO2(saturated

),NaO

H(0.2

Naq

ueou

ssolution

)/Hglamp(254

nm)/—

(—/—

/7/30/—),(0.3/—

/25

nm

particle/—

/su

spen

ded)

Methan

ol/17/—

�Itisfoun

dthat

photoa

ctivityde

clines

when

Cu(

I)ch

angesto

Cu(0)or

aggregates

aer

redu

ctionwithH

2

�Reactionis

carriedou

tin

acylindrical

quartz

reactorwithacapa

city

of30

0mL

�Itisiden

tied

that

theph

otoa

ctivityof

Ag–

TiO

2is

lower

than

thoseof

Cu–

TiO

2du

eto

thestrongaffi

nitybe

tweenAgclus

ters

and

photoe

lectrons

Hiroseet

al.

(200

3)198

Co(bp

y)32+sensitized

withRu(bp

y)32+

supp

ortedon

Na

onlm

/—

Labo

ratory

prep

aration

CO2(saturated

),DMF&TEOA(30mL

solution

)/Xe

lamp/—

(—/1/<1/46

/—),(0.1/—

/pa

rticleson

polymer

lm

/—/sup

ported

)

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nmon

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gcatalyticactivity

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than

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otolysis

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ctthereaction

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ing

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ated

cycles

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Ishitan

iet

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(199

3)147

SensitizedTiO

2

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)pa

rticleswith2%

wt

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ical

depo

sition

method

CO2(saturated

),H

2O

(1.5

mL)/500

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reHg

lamp(310

nm)/—

(278

/0.86/—/5/—

),(0.15/—/pow

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ende

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e/0.3/—

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estedthat

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uctdistribu

tion

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thesort

ofmetal

onthe

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2

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artz

vessel

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edto

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otored

uction

Tham

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7)217

25%

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5%RuO

x

sensitizedon

TiO

2/3

eV

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ratory

prep

aration

CO2(0.05atm,1

mL),H

2(0.6

atm,1

2mL)/150

Whigh

pressu

reXelamp

(310

–435

nm)/—

(298

/1/—

/5/0.59),(0.1/

55/sph

erical

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rticle/3.8/sus

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d)

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at

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andyields

Lehnan

dZiessel

(198

2)197

Ru-bipyridine-Co(

II)

chloride

/—Stan

dard

labo

ratory

synthesis

CO2(700

mL),H

2O

(6mL)/100

0W

Xeor

Hgozon

e-free

oriel

lamp(400

nm)/—

(303

/1/8.5/26/—),

(—/—

/—/—

/sus

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d)Carbo

nmon

oxide/

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observed

that

addition

offree

bipy

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decreasesCO

generationbu

tincreasesH

2prod

uction

�Theph

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reaction

iscarriedou

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a50

mLroun

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ask

aT/P/pH/t/Q

E:tem

perature

(K)/pressu

re(atm

)/pH

/illu

minationpe

riod

(hou

rs)/qu

antum

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cy(%

).W/SA/PS/SC

/MP:

weigh

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photocatalyst(g)/su

rfacearea

(m2g�

1)/ph


(diameter)/semicon

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mL�

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mmol.


RSC Advances Review

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this study, visible light performance of the dyes was in thefollowing order in presence of Pt: Rubpy > BrAsp > BrGly.

Much attention has been given to [Ru(bpy)3]2+ cation

because of its unique optical features. The combination of[Ru(bpy)3]

2+ and cobalt(II) chloride has been investigated overCO2 photoreduction, where Ru and Co complexes act as aphotosensitizer and a catalyst, respectively.197 Absorbing photonenergy, the [Ru(bpy)3]

2+ cation transforms to the triplet state,denoted by [Ru(bpy)3]

2+*. It was observed that the extra additionof bipyridine intensely reduced CO formation but raised the H2

yield. On the other hand, cobalt(II) chloride acts as an effectivemediator for both CO and H2 productions and highly endorsesCO yield. Co(bpy)3

2+ sensitized with Ru(bpy)32+ supported on a

cation exchange Naon lm has been synthesized to carry outthe photocatalytic reduction of CO2 for a period of more than400 h under atmospheric pressure.198 High stability of the metalcomplexes is obtained by the suppression of the decompositionof the Ru complex xed on the polymer surface. In this system,[Ru(bpy)3]

2+ was immobile on a cation exchange polymer and[Co(bpy)3]

2+ was mixed in a di-methylformamide (DMF) and tri-ethanol amine (TEOA) as a solvent and reducing agent,respectively. It is assumed that the photoinduced Ru complextransmits an electron to Co(II) complex, which acts as a reactionmedium of CO2, and at that time, a tertiary amine (sacricereagent) reduced the oxidized Ru(III) complex.198

2.4.2 Quantum dots (QDs)-sensitized photocatalysts. Aquantum dot-sensitized photocatalyst is one of the advancedapproaches in photochemical conversion of CO2 because of itsintrinsic optical and electrical behavior. These characteristics arestrongly inuenced by the size of quantum dots materials.Generally, chalcogenide metals (including CdSe or ZnS) are usedwith the semiconductors to facilitate the photon transformationof CO2 into hydrocarbon fuels. In Section 2.2.1, there is a briefdiscussion about the metal sulde (CdS and ZnS) semi-conductors, which also have such unique optical properties asquantum dots materials. To understand the size reliance on thephotochemical reduction of CO2, Wang et al. developed a PbSQDs-sensitized Cu–TiO2 catalyst of three different sizes of PbSQDs samples (3 nm, 4 nm and 5 nm).199 Among them, 4 nm PbSQD-sensitized Cu–TiO2 displayed the maximum CO2 trans-formation rate due to the absorption of visible spectrum (l > 610nm) and charge separation properties. It is found that small-sizeddots can facilitate the blue edge of the spectrum, while the large-sized dot is closer to the red edge spectrum. Particularly, small-sized QDs inject electrons swily to the conduction band edge ofTiO2 and large-sized QDs cover the visible region.200 Moreover,the electron transfer rate was very fast (less than 1 ns) from theexcited PbS QDs state to the TiO2 conduction band state.201

Another investigation was conducted on the CdSe QDs-sensitizedPt–TiO2 composite heterostructure, which revealed a response tothe visible light region (l > 420 nm).200

2.4.3 Phthalocyanine sensitized photocatalysts. For visiblelight irradiation, metal phthalocyanines are also applied in CO2

photoreduction, which have outstanding semi-conductivity andchemical stability.202 The CoPc–TiO2 nanocomposites are char-acterized by Zhao et al.203,204 It was observed that excited CoPcmolecules emitted the electrons to the conduction band of TiO2


during visible light irradiation; this phenomenon thenimproved the electron–hole separation process to increase thephotocatalytic efficiency. From Table 7, it is found that theincorporation of 0.7% CoPc into TiO2 exhibited a high yield rateof formic acid (about 150 mmol g�1 h�1) than that of 0.5% CoPcaddition to TiO2. In addition, their investigation was conductedon ZnPc–TiO2 and found that ZnPc has a higher selectivity offormic acid than metal catalysts by decreasing the recombina-tion possibility of the hole–electron pairs.205 It is important thatthe excess loading of ZnPc can reduce the photocatalytic activitybecause the active TiO2 surface sites can be blocked by addi-tional metal phthalocyanines, and the optimal amount isobserved, 1.0 wt% ZnPc. However, according to the observationby Wang et al., the band gap of TiO2 can be minimized from2.95 eV (at 0.6 wt% ZnPc) to 2.79 eV by loading 5 wt% ZnPc.206

2.4.4 Other photosensitizers. Enzymes have an effectivesensitization facility to the red shi of TiO2, compared withother sensitizers.207 Woolerton et al. handled carboxydothermushydrogenoformans (Ch)-derived carbon monoxide dehydroge-nases (CODHI) enzyme to functionalize the RuP-sensitized TiO2

nanoparticles for CO2 photoexcitation.208,209 Anaerobic organ-isms, Ch contains ve forms of CODH enzymes, each havingactive sites of [Ni4Fe-4S]. The subunit of these enzymes (Ni–CODHs) possesses a buried active site that is attached to theprotein exterior by a chain of [4Fe-4S] clusters. Electrons caneasily access or leave the enzyme by distal cluster (D-cluster)that is joined between the two subunits. In this experiment,visible light-excited RuP sensitizer nanoparticles transferphotoinduced electrons to TiO2 for easing its solar light acti-vation.209,210 These electrons can easily get into the CODHenzyme through the D-cluster and also move through a second[4Fe–4S] cluster to the active site where the CO2 reduction hastaken place.

Recently, co-loaded novel metals are using the semi-conductor catalysts as photosensitizers. For example, sensitizedTiO2 lm with Au nanoparticles is modied by Hou et al. forturning CO2 into fuels.211 Here, Au nanoparticles are photoex-cited by the plasmon effect (the generation of robust electricelds by surface plasmon resonance of the Au nanoparticle),which shis the photoinduced electrons from the Au to theTiO2. Another approach of sensitized TiO2 with Cu onsupportive SiO2 is applied in terms of solar fuel productionfrom CO2.77,212

2.5 Flexible substrate based photocatalysts

In photochemical reactions, the involvement of exiblesubstrates have generated interest because they deliver a highsurface area for catalysts on the lm of substrates such as pol-yethyleneterphthalate (PET), polyethylenenaphtalate (PEN),polyethylene (PE) and polypropylene (PP).218 However, thedegradation of polymer substrates by solar irradiation dependson the operating period of atmospheric exposure.219 This pho-todegradation is carried out by a combination of photolysis andphotocatalytic oxidation.218 The formation of volatile productsdue to photodegradation is another limitation in the case ofpolymeric substrates.220 Table 8 represents the full information

RSC Adv., 2014, 4, 20856–20893 | 20887


Tab

le8

Advance

sin

flexible

substrate

catalystsforCO

2photoreductiona

Researcher

(year)

andref.

Catalystcompo

sition

/ba

ndga

pen

ergy

Catalystprep

aration

tech

nique

Reactan

ts(amou

nt)/

radiationsource/

ligh

tintensity

Ope

ratingvariab

les

(T/P/pH/t/Q

E),

physical

prop

erties

(W/SA/PS/SC

/MP)

Major

prod

uct/yield

(mmol

h�1pe

rg


)Com

men

ts

W.K

imet

al.

(201

2)223

Na

onlayeron

1.0wt

%Pd

-dep

osited

TiO

2/—

Labo

ratory

synthesis

CO2(saturated

),H

2O

(—)/UVligh

t(>30

0nm)/—

(—/—

/3/5/—

),(—

/—/nan

oparticle/

1.5�

10�6/

susp

ende

d)

Methan

e/7.6b/—

�Na

onlayeren

han

cesproton

-cou

pled

,multiple-electron

tran

sfer

reaction

s,stab

ilizes

interm

ediatesan

dinhibitsthere-

oxidationof

theCO2redu

ctionprod

ucts

byincreasingthelocalproton

activity

within

thelayer

�Palladium

isselected

toretard

the

recombinationof

Hatom

sto

minim

izethe

prod

uction

ofH

2

J.Jensenet

al.

(201

1)218

TiO

2–P

ET/—

Spraycoating

CO2(25mol),H

2(75

mol)/Osram

ligh

tbu

lb/120

mW

cm�2

(308

/—/—

/230

/—),

(—/—

/lm

with13

0mm

thickn

ess/—/

supp

orted)

Carbo

nmon

oxide/

0.00

13/—

�Polym

ersu

bstrates

areap

pliedin

this

stud

ywithhigh-activearea

of10

0cm

2en

able

toachieve

catalyst-covered

lm

within

ash

orttimean

dat

areason

able

prod

uction

cost

�UVde

grad

ationof

polymer

subs

trates

(photocatalytic

oxidation)canbe

supp

ressed

,but

not

avoide

d,by

using

oxygen

-freecondition

sT.A

raiet

al.

(201

1)224

Cu 2Zn

SnS 4

mod

ied

byruthen

ium

complex

polymer/1.5

eV

Chem

ical

polymerization

method

CO2(saturated

),H

2O

(—)/visible

ligh

tirradiation

(200

–400

nm)/—

(—/—

/—/3/—

),(—

/—/photocathod

e/—/sup

ported

)

Form

icacid/0.49c/>

80�S

ulde

semicon

ductor

that

show

sanarrow

bandga

pwithhighselectivityin

photoe

lectroch

emical

CO2redu

ction

�Theinsertionof

Seinto

thecatalyst

enhan

cesholemob

ilityan

dincreasesthe

photocurrent

Hiroseet

al.

(200

3)198

Co(bp

y)32

+with

Ru(bp

y)32+su

pported

onNa

onlm

/—

Labo

ratory

prep

aration

CO2(saturated

),DMFwithTEOA(30

mLsolution

)/Xe

lamp/—

(—/1/<

1/46

/—),(1/—

/pa

rticleson

polymer

lm

/—/sup

ported

)

Carbo

nmon

oxide/

25b/—

�Catalytic

activity

show

smorethan

400h

unde

ratmosph

eric

pressu

re�P

hotoreactionis

carriedou

tin

aga

stight

photolysis

photoreactionvessel

(Pyrex)

�This

system

allowseasy

han

dlingan

drecovery

oftheRucomplex

withrepe

ated

cycles,u

singCO2refreshing

aT/P/pH/t/Q

E:tem

perature

(K)/pressu

re(atm

)/pH

/illu

minationpe

riod

(hou

rs)/qu

antum

efficien

cy(%

).W/SA/PS/SC

/MP:

weigh

tof

photocatalyst(g)/su

rfacearea

(m2g�

1)/ph


(diameter)/semicon

duc

torconcentration(g

mL�

1 )/m

odeof


’:not

men

tion

edin

theoriginal

pape

r.bmmol.c

mM.


RSC Advances Review

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of exible substrate based photocatalysts, which are gatheredfrom important investigations related to CO2 photoconversion.

Some aromatic polyesters such as PET, PEN and similarexible substrates act in different ways under light irradiation,compared with olens (PE and PP). Aromatic polyesters are ableto absorb light with a long wavelength (<330 nm) due to thearomatic rings of carbonyl groups.220 Another important factoris the oxygen diffusion constant of exible substrates. Higheroxygen diffusion constants may lead to oxidation in all parts ofthe surface.221 Comparatively, polymer degradation is lesser onthe surface of the PET because of its restriction to diffuseoxygen, whereas the diffusion is higher in the PE.218,221,222

Use of a Naon (peruorinated polymers with sulfonategroups) thin lm on Pd-loaded TiO2 nanoparticles was intro-duced by Kim et al. due to its stability against photocatalyticoxidation, and hence increases the photochemical conversionof CO2 under visible light irradiation.223 To provide multipleelectrons, the Naon layer can increase local proton activityinside the layer. It also helps to stabilize the intermediates andprevent the re-oxidation of the CO2 reduction yields because itinhibits direct interaction between products and surfaces of thecatalysts. Another investigation shows that Naon lm acts as agood supportive material for catalysts due to cation exchangeability.198 This Naon-supported Co(bpy)3

2+ and Ru(bpy)32+

system can increase the catalytic activity to a maximum of 400 hin atmospheric condition because of obstruction to the break-down of [Ru(bpy)3]

2+ complexes that are trapped on the cationexchange polymer surface, and hence the separation of ligandsis minimized. Low-band gap (1.5 eV) Cu2ZnSnS4 modied by theruthenium complex polymer is a new addition to determine thephotochemical performance of CO2 reduction under the visiblelight region, which shows high selectivity of the formate(>80%).224

3. Conclusions

The transformation of CO2 towards hydrocarbon fuels is asubstitute approach to regulate both fuel crisis and globalwarming. Various inquiries have been undertaken to evaluatethe photocatalytic performance over CO2 transformation intohydrocarbon fuels. This technology was found to be morepromising, compared with other approaches such as thermo-chemical, electrochemical, and biological transformationtechnologies. In this paper, a general overview is emphasized onthe behavior of different types of photocatalysts with respect toCO2 transformation into fuel. In various investigations, it wasobserved that most of the studies were carried out in the pres-ence of articial light sources. Recently, researchers havefocused on direct solar fuel production by CO2 conversion. Forsuch a phenomenon, the selective catalysts must have thedesired band gap energy for visible light response. Previously,unmodied oxide and non-oxide semiconductors are widelyused with or without supportive materials in this technology.However, some of them have high band gap energy and lowconversion yields. To facilitate, some techniques have beenfollowed to minimize band gap energy by doping the semi-conductors with foreign elements like novel metals, transition


metals, co-metals, and non-metals. It also inuences the chargetransfer rate and reduces the electron–hole recombination.Further, different substrate materials were used to achieve highsurface area and the catalysts' immobilization during the pho-tocatalytic irradiation. Recently, different types of sensitizationtechniques are incorporated to improve the photocatalyticperformance under visible light irradiation like dye sensitiza-tion, enzyme-based sensitization, QDs sensitization, andphthalocyanine sensitization. Both doped and sensitizedsemiconductors were employed for higher conversion. Theselectivity of products not only depends on the catalysts'compositions but also on the choice of reductant and thesolvent used in the catalytic system. Besides water, organicsacricial reducing reagents are the most common reductantsused in the photochemical CO2 conversion process. Althoughwater is available and affordable, there are some limitations likelow CO2 solubility, resulting in water splitting rather than CO2

reduction. Instead, methane and hydrogen were used assignicant reductants in many investigations reported in thisreview. Other factors are denitely considerable for evaluatingphotocatalytic performance on CO2 reduction such as catalysts'dosage, reactants' ratio, reaction temperature, time, systempressure, pH, light intensity and wavelength. A clear idea isessential for understanding the reaction mechanisms togetherwith charge carrier dynamics within the catalysts and at theirinterfaces with the metal co-catalysts. From this review, it isclear that the existing techniques are insufficient to make themapplicable for the industrial production of fuels. As a result, theselection of solar-driven photocatalysts is essential for indus-trial solar fuel harvesting from CO2.

Acknowledgements

The authors acknowledge nancial support from project RP015/2012D and D000011-16001, Center of Research Grant Manage-ment, University of Malaya, 50603 Kuala Lumpur, Malaysia.

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A review on advances in photocatalysts towards CO2 conversion

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