Chem Soc Rev TUTORIAL REVIEW - University of Toronto

Chem Soc Rev

TUTORIAL REVIEW

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Received 00th January 20xx,

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

Shining light on CO2: From Materials Discovery to Photocatalyst, Photoreactor and Process Engineering

Yuchan Donga, Paul Duchesnea, Abhinav Mohana, Kulbir Kaur Ghumanb, Paul Kantc, Lourdes Hurtadoa, Ulrich Ulmera, Joel Y.Y Lohd, Athanasios A. Tountasa, Lu Wanga,e, Abdinoor Jellea, Meikun Xiaa, Roland Dittmeyer c, Geoffrey A. Ozina

A celebration of the 77th birthday of Professor Geoffrey Ozin

Heterogeneous catalysis, a process in which the reaction of gaseous or liquid chemical reagents is facilitated at the surface

of a solid material, is responsible for the majority of industrial-scale chemical and fuel production reactions. The energy

required to drive these reactions has historically been derived from the combustion of non-renewable fossil fuels and carries

an unavoidably large carbon footprint. More recently, the development of environmentally responsible and sustainable

chemical industries is increasingly motivated by greenhouse gas-induced climate change, thus creating demand for eco-

friendly heterogeneous catalytic processes. This includes innovative approaches enabled by renewable forms of energy,

such as the electrification of chemical and petrochemical processes, utilization of CO2 as a feedstock and the incorporation

of light into catalytic reactions.

Herein we review the conversion of solar energy to chemical energy using CO2, and describe how the photophysical and

photochemical properties of nanostructured metal oxide photocatalysts have been engineered to efficiently incorporate

light into heterogeneous gas-solid CO2 hydrogenation reactions. Realizing high photonic and energy efficiencies in these

systems has demanded innovation in not only photocatalyst engineering, but also photoreactor and process engineering.

Rather than exclusively providing an in-depth discussion of the chemistry and science within each individual study, this

Tutorial Review highlights the multidisciplinary character of photocatalysis studies by covering the four essential

components of a typical research work in this field (materials engineering, theoretical modelling, reactor engineering and

process development) via case studies of the archetypal indium oxide catalyst materials. Through advances in these four

components, progress has been made towards the ultimate goal of industrializing the production of CO2-derived chemicals

and fuels.

Introduction

The connection between CO2 and global warming via the

greenhouse gas effect can be traced to the early works of

Joseph Fourier (1827), John Tyndall (1859), and Svante

Arrhenius (1896).1 The idea of using TiO2 semiconductor

photocatalysts and photoelectrodes to imitate natural

photosynthesis emerged as a mainstream topic of research well

before climate scientists began to seriously discuss the growing

concentration of anthropogenic CO2 in the troposphere and its

deleterious effects on global climate.2 This work inspired the

rapid growth of international research into water splitting and

CO2 conversion via solar-powered thermochemical,

electrochemical and biochemical means.3-5

Various reviews have been published regarding the applications

and mechanisms of CO2 photocatalysis, especially in the

aqueous phase.6,7 However, gas-phase heterogeneous

photocatalytic CO2 reduction methods, with their potential for

ready integration into existing thermocatalytic processes, have

remained relatively less explored. By replacing combustion-

a. Solar Fuels Group, Department of Chemistry, University of Toronto, 80 St. George, ON, M5S 3H6, Canada, email: [email protected], www.solarfuels.utoronto.ca

b. Centre Énergie Matériaux Télécommunications, Institut National de la Recherche Scientifique, 1650 Boul. Lionel-Boulet, Varennes, Quebec, Canada J3X 1S2

c. Karlsruhe Institute of Technology (KIT), Institute of Micro Process Engineering (IMVT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen

d. Department of Electrical and Computing Engineering, University of Toronto, 10 King’s College Road, Toronto, ON M5S 3G4, Canada

e. School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong, China 518172

Key learning points 1. Material engineering strategies and new reactor designs for enhanced performance in the heterogeneous photocatalytic hydrogenation of CO2. 2. Computational modelling design strategies for studying the function of defects, light and heat in photocatalytic CO2 hydrogenation processes. 3. Comparison of thermochemical, photochemical and photothermal CO2 utilization and future opportunities. 4. Key decision-making criteria for assessing photocatalytic process viability and potential.

mailto:[email protected]

Tutorial Review Chem Soc Rev

2 | Chem. Soc. Rev.,2020, 00, 0-0 This journal is © The Royal Society of Chemistry 20xx



driven heating with solar irradiation, gas-phase CO2

hydrogenation reactions such as reverse water gas shift,

methane dry reforming, methanol synthesis, methanation, and

hydrocarbon production could thus be driven under relatively

mild conditions. Doing so would reduce the carbon footprint of

future refineries while also offering a renewable source of

chemicals and fuels from CO2. However, a significant challenge

to the technological relevance and cost-effectiveness of

photocatalytic CO2 hydrogenation is the low efficiency or

stability of existing photocatalysts, which necessitates both

advancements in materials design and a comprehensive

understanding of the photocatalytic process.

Metal oxides are omnipresent in the field of gas-phase

heterogeneous CO2 catalysis, due largely to their stability

relative to other chemical compositions. Among these oxides,

wide-bandgap (UV-absorbing) insulators notably absorb only a

small fraction of incident solar radiation, which has a marked

impact on their photochemistry.8 As recommended by the US

National Renewable Energy Laboratory (NREL), the AM1.5

spectrum is commonly used to represent the solar spectrum at

sea level on Earth, as it also takes into consideration the variable

angle of the sun due to Earth's rotation. The distribution of

wavelengths present in solar radiation resembles that arising

from a black body and is further modified by absorption as they

pass through the Sun's corona and the Earth's atmosphere. As

such, emission intensity is highest at wavelengths just below

500 nm, drops rapidly in the UV as it approaches 140 nm and

more gradually diminishes into the infrared toward 4,000 nm.

In the case of archetypal TiO2, for instance, its large bandgap

(3.2 eV) means that barely 5% of the incident solar energy can

be utilized to create electron-hole pairs. Moreover, once

electron-hole pairs are generated, the energetics and dynamics

of these charge carriers determine their ultimate fate, be it

relaxation via physical processes to generate heat/light or

transfer to adsorbed reactants to enable chemistry.9 Therefore,

in order to fully utilize solar energy and achieve high catalytic

activity, an effective CO2 photocatalyst should be able to: 1)

generate excited charge carriers capable of surviving long

enough to perform catalytic reactions (i.e., microseconds to

seconds), and 2) provide active sites for the adsorption and

transformation of CO2 and other reactant molecules.

To this end, extending photocatalyst’s light absorption to the

visible range, suppressing the recombination of charge carriers

and optimizing surface active sites are important objectives in

gas-phase photocatalysis studies. However, merely controlling

the structure, composition, size, and shape of nanostructured

metal oxides is insufficient; it is also necessary to tailor surface

defects such as coordinately unsaturated metal and oxygen

sites, and hydroxide groups. Those defects which act as active

sites create mid-bandgap states that broaden the optical

absorption range and extend the lifetimes of photogenerated

charge-carriers, thereby encouraging photochemical reactions

rather than non-productive, photophysical recombination

processes.10,11 Beyond utilizing solar light to produce

photoexcited electron-hole pairs for surface reactions

(photochemical processes), photocatalysts can convert light

energy to heat through localized surface plasmon or carrier

relaxation (thermochemical processes), which can improve the

overall energy efficiency of the catalytic process by mitigating

the need for external heating.53 Photothermal catalysis,

combining both thermochemical and photochemical catalysis

within a single system, thus offers the opportunity to further

boost photocatalysis efficiency.10

One major goal of this Tutorial Review is comparing a library of

photocatalytic nanomaterials for heterogeneous CO2

hydrogenation and providing an overview on their synthesis-

structure-property relationships. Understanding the photo-

chemistry and photophysics underpinning their excited-state

surface reactivity with gaseous H2 and CO2 under light

irradiation was also a priority, necessitating the study of

catalytic activity, selectivity, reaction kinetics, stability trends,

percent conversion, and photonic and energy efficiencies in

order to provide parallel comparisons. While many excellent

reviews of different classes of materials and materials

engineering methods for heterogeneous CO2 photocatalysis

have been written in the past few years, the literature generally

lacks a coherent and uniform comparison of catalytic

performances due to differences between testing systems and

rate calculation methods.12 Herein we choose indium oxide as

an exemplary photocatalyst for illustrating the concepts and

principles pertinent to designing a high-performance

heterogeneous CO2 photocatalyst, and investigate the

effectiveness of each materials engineering method.

Considering the interdisciplinary nature of photocatalytic

studies, which involve chemists, physicists and engineers with

different educational backgrounds in materials design,

computational modelling and reactor engineering, principles

underlying theoretical studies and new reactor engineering

strategies are also introduced to provide a comprehensive

picture and valuable tutorial experience.

Engineering of Metal Oxide Materials

Distinct forms of a metal oxide material can be obtained by

tailoring its defects (e.g., stoichiometric MxOy versus non-

stoichiometric MxOy-n), polytype (e.g., rutile, anatase, or

Figure 1. Jablonski scheme for engineered forms of metal oxides. Illustration of the photo-physical processes of solar irradiation of metal oxide engineered through chemistry to contain mid-gap electron acceptor defect states below the CB edge (oxygen vacancy [O]v, coordinately unsaturated meta (M’) and isomorphic substituted metals (M*’) and mid-gap hole acceptor defect states above the VB (hydroxyl OH and coordinately unsaturated oxygen O’).10,13 Recreated with permission from Refs 10 and 13. Copyright 2019 Royal Society of Chemistry and 2016 National Academy of Sciences.

Chem Soc Rev Tutorial Review

This journal is © The Royal Society of Chemistry 20xx Chem. Soc. Rev.,2020, 00, 0-0 | 3



brookite TiO2), superstructure (e.g., assembled-nanocrystal

nanorods), isomorphic substitution (e.g., M*zMx-zOy), and/or

heterostructure formation (e.g., MxOy/M* or MxOy/M*Oy).

Various microscopic, diffractive, and spectroscopic analytical

techniques, including time-resolved and in situ measurements,

have been applied to elucidate the physical and chemical

dynamics of photogenerated charge carriers and compare the

effect of materials engineering. These analyses enable the

formulation of a Jablonski scheme depicting photophysical and

photochemical processes deemed to follow the absorption of

bandgap and sub-bandgap solar irradiation, as shown in Figure

1.10,13 Here, the progenitor metal oxide photocatalyst can be

engineered to contain mid-gap electron-acceptor defect states

below the conduction band edge (i.e., oxygen vacancies, [O]v;

coordinately unsaturated metal, M’; and isomorphic

substituted metals, M*’) as well as mid-gap hole-acceptor

defect states above the valence band (e.g., hydroxyl, OH, and

coordinately unsaturated oxygen, O).

As shown in the aforementioned Jablonski scheme, photo-

generated electrons and holes can follow both fast (bandgap)

and slow (sub-bandgap) decay pathways. The latter processes

involve carriers trapped in defect states with lifetimes

sufficiently long for them to participate in chemical reactions

catalysed by active sites such as surface frustrated Lewis pairs

(SFLPs); in the case of defect-laden indium oxide In2O3-x(OH)y,

SFLPs are composed of proximal Lewis-basic In-OH sites and

Lewis-acidic In’/M’ sites.14 The photocatalytic activity of In2O3-

x(OH)y can then be modified by various materials engineering

methods, including altering the Lewis acidity/basicity of the

SFLP components and the distance between them as shown in

Figure 2 and Table 1.15

Nanocrystals and Superstructures

Like many metal oxide semiconductors, the optoelectronic

structure of In2O3 can be hugely impacted by its crystal phase,

size, morphology, doping elements, and defect concentrations,

which makes it an incredibly versatile material. Generally, In2O3

is an n-type semiconductor with direct and indirect band gaps

of 3.7 and 2.6 eV, respectively. The n-type conductivity is

attributed to its oxygen vacancy ([O]v) defects, which increase

electron density at In sites.16

Nanostructures tend to favour the formation of defects due to

their high surface-to-volume ratio, and studies have shown that

the energy required for [O]v formation is lower at surfaces than

in bulk.17 Thus, many studies aimed at tuning the electronic

properties of metal oxide photocatalysts have focused on

synthesizing nanoscale materials with different sizes and

morphologies. For instance, highly crystalline spherical In2O3

nanocrystals were prepared by thermal decomposition of

In(acac)3/oleylamine mixture in an argon atmosphere. In2O3

particles of 4, 6 and 8 nm in diameter were obtained by tuning

the ratio of In(acac)3 to oleylamine, with size-dependent

photoluminescence being exhibited due to quantum

confinement effects.18 In2O3 nanocrystal nanorods with

different lengths (from 800 nm to 1,830 nm) have also been

synthesized via slow hydrolysis of InCl3 in urea solution, with

their photocatalytic properties being compared through CO2

hydrogenation. The longest nanorods performed best due to

longer charge carrier lifetimes (4.81 μs for 1,830 nm rods versus

1.78 μs for 800 nm rods) induced by the nanocrystal nanorod

superstructure, which allowed electrons to migrate between

nanocrystals and avoiding recombination with holes.19

The interaction between reactant molecules and defects on the

metal oxide surface also hugely impacts catalytic activity.

Defect-laden indium oxide (In2O3-x(OH)y) nanocrystals bearing

both [O]v and OH defects were prepared via thermal

dehydroxylation of indium hydroxide in air, and showed

impressive photocatalytic activity toward CO2 hydrogenation.20

This synthesis approach of converting hydroxide to oxide-

Figure 2. Synthetic pathways and relationships between different forms of defected indium oxide.





hydroxide allowed the proportions of hydroxides and oxygen

vacancies to be carefully tuned, via the temperature and

duration of the heat treatment, to achieve the best

photocatalytic activity.21 In sites adjacent to [O]v acted as the Lewis

acid and the proximal OH defect acted as Lewis base, together

forming an SFLP active site able to heterolytically dissociate H2,

generating a protonated hydroxide and indium hydride that

subsequently reacted with CO2 to form CO.14 These protonic

and hydridic hydrogen species formed from hydrogen cleavage

can be detected through 1H magic angle spinning (MAS) NMR

and diffuse reflectance infrared Fourier transform spectroscopy

(DRIFTS) measurements.22

Isomorphic Substitution

Tuning the chemical nature, concentration, and position of

dopants in the metal oxide lattice is another effective method

of modifying the optoelectronic properties of metal oxide. The

bixbyite type structure of cubic In2O3 (𝑰𝒂�̅�) can be viewed as

defected fluorite with one-quarter of the oxygen sites being

vacant. These vacant sites provide space for extrinsically doped

atoms. For instance, tin-doped In2O3 (Sn-In2O3, ITO) is a

transparent conductor widely used in electronic devices. Due to

the 4+ valence of Sn4+, each Sn4+ replacing In3+ in the lattice

contributes one free electron to the conduction band.

Meanwhile, the charge imbalance between Sn4+ and In3+ needs

to be compensated by the incorporation of oxygen anions into

some of the structural anion vacancy sites of In2O3.23

Additionally, the two distinct indium positions in cubic In2O3

(24d and 8b) differ in their bonding with neighbouring oxygen

atoms, thus providing further opportunity for site-selective

substitution.

Given that reaction rate and selectivity of In2O3-x(OH)y are both

apparently dictated by the SFLP, these properties could be

advantageously tailored through chemical means by adjusting

the SFLP geometry, as well as the Lewis acidity and Lewis

basicity of its In and In-OH sites, respectively, via isomorphic

substitution.15 One study of Bi3+-substituted In2O3-x(OH)y

nanocrystals, for instance, indicated that differences in atomic

size, electronegativity and degree of Bi3+ substitution provided

a quantitative means of tuning both the distance between Lewis

acid and base sites, as well as the Lewis acidity/basicity of the

SFLP.24 Elemental substitution also creates new mid-gap states

within the electronic bandgap, which generates visible-

Table 1. Performance metrics and experiment parameters for selected indium oxide photocatalyst materials. *1 sun intensity = 100 mW/cm2

Material Target

Product

Rate

(μmol h-1 g-1)

Selectivity

(%) Reactor

H2/CO2

Ratio

H2 /CO2 Flow Rate

or Pressure

Temperature

(°C) Light Source

c-In2O3-x(OH)y

nanocrystals20 CO 0.25 100 Batch 1:1 15 psi/15 psi 150

1000 W metal halide bulb

(2.2 suns*)

c-In2O3-x(OH)y

nanocrystals20 CO 15 100 Flow 1:1 3 sccm/3 sccm 150 300 W Xe lamp (22 suns)

c-In2O3-x(OH)y

nanorods19 CO 1.2 100 Batch 1:1 15 psi/15 psi 150

1000 W metal halide bulb

(0.8 sun)

Bi3+-substituted

In2O3-x(OH)y24

CO 1.32 100 Batch 1:1 15 psi/15 psi 150 1000 W metal halide bulb

(0.8 suns)

Bi3+-substituted

In2O3-x(OH)y24

CO 100 100 Flow 1:1 1 sccm/1 sccm 170 300 W Xe lamp

(~8 suns)

rh-In2O3-x(OH)y27 MeOH 180 13 Flow 3:1 6 sccm/2 sccm 270

130 W Xe lamp

(10 suns)

c-In2O3-x29

CO 24000 100 Batch 1:1 15 psi/15 psi

No external heat

(~260)

300 W Xe lamp

(20 suns)

c-In2O3-x29

CO 2000 100 Flow 1:1 1 sccm/1 sccm 300 300 W Xe lamp

(8 suns)

In2O3-x(OH)y

on SiNW31 CO 22.0 100 Batch 1:1 15 psi/15 psi

No external heat

(~150)

300 W Xe lamp

(20 suns)

In2O3-x(OH)y

on Nb2O534

CO 1400 100 Batch 1:1 13.5 psi/13.5 psi No external heat

(~60)

300 W Xe lamp

(25 suns)

In2O3-x(OH)y on

quartz rod44 CO 246.2 100 Annular (flow) 5:1 5 sccm/1 sccm 200

120 W Xe lamp

(~2 suns)

In2O3-x(OH)y

on Cu foam45

CO 1.5 88 Annular (flow) 1:1 3 sccm/3 sccm 185 Solar simulator AM 1.5G

(~2.5 suns)

In2O3-x(OH)y on Ni foam45

CO 32 81 Annular (flow) 1:1 3 sccm/3 sccm 185 Solar simulator AM 1.5G

(~2.5 suns)





wavelength photon absorption. According to external quantum

efficiency (EQE) measurements, shallow to deep trap states

around 550 nm were present in Bi3+-substituted In2O3-x(OH)y

samples. With increasing Bi3+ substitution levels, the main

photocurrent absorption edge was red-shifted from 430 nm to

470 nm, indicating that enhanced light absorption was induced

by Bi3+ substitution. Thus, tailoring the chemical, electronic and

optical properties of metal oxides via isomorphic substitution

provides a rational and systematic approach toward optimizing

their photocatalytic activity and selectivity for CO2

hydrogenation.

Polymorph Selection

Another strategy that can enable optimization of gas-phase

heterogeneous CO2 photocatalyst performance is materials

engineering of polymorphism. The polymorphs of TiO2 have

been heavily studied and compared regarding their

photocatalytic properties, as has been summarized in many

reviews.25 In2O3, the new rising star in photocatalysis, has two

primary crystal phases defined by space group symmetries 𝑰𝒂�̅�,

(cubic) and 𝑹�̅� (rhombohedral), with the former being widely

studied both experimentally and theoretically.26 Cubic In2O3,

the most stable form of In2O3, is a bixbyite-type structure with

two distinct In sites and one O site atom occupying 8b, 24d, and

48e Wyckoff positions, respectively. It can be obtained by

various synthetic methods such as direct oxidation of indium

metal, or calcination of indium hydroxide and indium salts.

Rhombohedral In2O3 is a corundum-type structure with one In

site and one O site at the 12c and 18e Wyckoff positions,

respectively. It has been rarely studied, as it was thought to be

a metastable phase at ambient temperature and pressure, and

was typically prepared by treating cubic In2O3 at high

temperatures and pressures. Recently however, nanoscale

rhombohedral indium oxide (rh-In2O3-x(OH)y) with [O]v and OH

defects has been prepared through the calcination of InOOH,

which exhibits significantly higher activity and superior stability

while also favouring the production of CH3OH over CO. Though

the band gap of rh-In2O3-x(OH)y is slightly larger than that of c-

In2O3-x(OH)y (3.01 eV versus 2.89 eV, respectively), detailed

studies have observed gains in catalytic performance relative to

cubic bixbyite In2O3-x(OH)y. Here, the increased distance

between the acidic and basic components of SFLP sites was

observed on switching from the cubic to rhombohedral

polymorph, thereby resulting in enhanced reactivity.27

Non-stoichiometry

Photocatalyst performance can also be tuned by introducing

additional defects to enhance light utilization and reactant

activation. Hydrogenated black TiO2-x exhibited a substantially

narrowed bandgap compared with stoichiometric TiO2 due to

intra-band transitions;28 likewise, In2O3 was hydrogenated to

create non-stoichiometric, ultra-black c-In2O3-x, in which oxygen

vacancies gave rise to mid-gap states that could accommodate

charge-balancing electrons.29,52 As the non-stoichiometry (x)

increased, the pale-yellow colour gradually became dark grey,

eventually turning black, with the band gap narrowing from

2.66 eV to 2.36 eV.29 According to UV-VIS-NIR reflectance

measurements, this modification to In2O3 could enhance its

absorption of the solar spectrum and even achieve intense and

uniform light absorption of ~90% between 250 and 2,500 nm.

Compared with pristine In2O3, photoconductivity measurements

showed that ultra-black c-In2O3-x also exhibited faster photo-

saturation upon light irradiation and slower photocurrent

decay, which is attributed to increasing optical absorption of

visible and near infrared light by an increasing number of mid-

gap and conduction electronic states.

In a batch reactor with a light intensity of approximately 20

suns, black c-In2O3-x exhibited a CO production rate more than

2,400 times that of (pale-yellow) In2O3. The superior catalytic

activity of black c-In2O3-x arises from its surface oxygen

vacancies, which abstract O from CO2 to form CO and are

subsequently regenerated by reaction with H2 to form H2O,

thereby completing the catalytic cycle. This photochemistry is

also enhanced by the photothermal and plasmonic effects

associated with the aforementioned mid-gap and conduction

electron states, respectively.

Hybrids

The catalysts discussed thus far are single-component

materials. They still face some challenges regarding industrial

applications, particularly in terms of material cost and stability

on larger scales, where coke and water vapor formation can

cause deactivation. Thus, it is necessary to introduce greater

photothermal and plasmonic effects into the system to further

improve the performance of metal oxide photocatalysts while

maintaining their active sites. These needs can be met by

incorporating metal oxide photocatalysts into binary

architectures. In such catalysts, photo-chemical, photothermal,

and plasmonic components are integrated synergistically to

enhance the overall efficiency of solar energy conversion.

Binary heterostructures of In2O3-x(OH)y that incorporate

broadband light-absorbing materials can photothermally or

photochemically accelerate the rates of catalysed reactions.

Examples include using black Si nanowire arrays to form In2O3-

x(OH)y/SiNW and niobium pentoxide nanorods to form In2O3-

x(OH)y/Nb2O5. These Si nanowire supports, with a band gap of

1.1 eV, provided a photothermal activity enhancement to the

photocatalytic In2O3-x(OH)y nanocrystals whose band gap is 2.9

eV. More specifically, the vertical configuration of the black Si

nanowires, as shown in Figure 3a-c, enabled efficient

broadband light harvesting across the near-infrared, visible and

ultraviolet wavelength ranges, absorbing over 85% of incident

solar intensity by minimizing reflective losses.30 As a result of

this enhancement, the hybrid photocatalyst was able to reduce

CO2 to CO at a rate of 22.0 μmol g cat-1 h-1, a 6-fold rate increase

over that of an identical In2O3-x(OH)y catalyst on a roughened

glass substrate.31

Owing to the traditional application of indium oxide catalysts

under predominately thermal conditions, many oxide supports

that have been studied in the field of CO2 heterogeneous

thermocatalysis can also provide valuable lessons for designing

better photocatalysts. For instance, supporting In2O3 on ZrO2

has been shown to prevent catalyst deactivation by CO.32 Unlike





the copper/zinc-oxide/alumina (CZA) system in which CO

addition enhances performance via the water gas shift (WGS)

reaction, up to 80% of the CO2 feedstock could be substituted

with CO to promote the generation of vacancies in In2O3/ZrO2.

Thus, the catalyst remained stable for 1,000 h under

thermocatalytic conditions, avoiding the otherwise rapid

deactivation than would typically occur within 1 h.3,32,33 The

electronic effect of ZrO2 not only provided sintering resistance,

but was also believed to generate a new type of vacancy and a

different reaction pathway by abstracting oxygen from the

active indium phase. Similarly, Nb2O5 nanorod supports in

Figure 3d served to increase the concentration of oxygen

vacancies relative to pristine In2O3-x(OH)y, thereby prolonging

charge-carrier lifetimes by around 30%, according to time-

resolved fluorescence spectroscopy measurements, and

enhancing their chemical reactivity.34

Computational Modelling

Throughout the development of metal oxide-based catalysts,

computational modelling has played a key role in improving our

understanding of a material’s surface defects and its interaction

with reactant molecules, which guides further materials

engineering and unveils catalytic pathways. Point defects (such

as oxygen vacancies, hydroxide groups, etc.) in metal oxides can

change the potential energy and configuration of the lattice.

Knowledge of the distortion created by these defects can,

therefore, play a crucial part in optimizing the material’s

properties and catalytic reactivity. One of the most relevant and

extensible approaches to modelling point defects in metal

oxides is density functional theory (DFT). This quantum

mechanical approach allows the description of a many-body

electronic ground state in terms of single-particle equations,

providing accurate information about defect formation

energies and electrochemical properties, and yielding valuable

insights into the crucial ground state mechanisms of chemical

reactions.14

Experimental studies on defect-laden In2O3-x(OH)y nanoparticles

demonstrated photocatalytic CO2 reduction by H2 at

temperatures as low as 150 °C using both ultraviolet and visible

light.20 The exact role of [O]v and OH defects, and the

mechanism responsible for gas phase CO2 reduction in the

presence of light and temperature, can be unveiled using proper

computational modelling designs. Periodic slab models of In2O3

with no defects, In2O3-x with only [O]v defects, In2O3-x(OH)y with

only OH defects and In2O3-x(OH)y with both defect types were

used to represent the pristine and defected indium oxide

surfaces. An exhaustive set of DFT simulations then analysed

the interactions of each surface with CO2 and H2 molecules,

resulting in the identification of a catalytically active SFLP site

formed by proximal In (Lewis-acidic) and In-OH (Lewis-basic)

moieties on the metal oxide surface, Figure 4a-b.

To clarify the behaviour of these active sites in the presence of

light and heat, thus mimicking the real photocatalytic condition,

sophisticated linear response time-dependent DFT (LR-TDDFT)

and meta-dynamics calculations can be used respectively.35,36

Unlike standard DFT, the TDDFT approach is currently incapable

of dealing with large numbers (i.e., hundreds) of atoms. Cluster

models were thus used as an alternative to the computationally

expensive periodic slab calculations typically used to study

surfaces and processes mentioned earlier. Cluster models are

based on the high degree of localization of the perturbation

caused by the existing point defects in the lattice. A

representative defect and its close surrounding were selected

to create the cluster model. Further, a sufficiently large size of

the vacuum layer in the cell was implemented to avoid the

cluster-cluster interaction that causes artefacts via the

interactions of overlapping wave functions. To represent the

surfaces, “saturators” should be used in order to deal with the

dangling bonds of the surface atoms, which have a negative

Figure 3. a) SEM images and b) EDX mapping of evenly coated In2O3-x(OH)y/SiNW hybrid films; c) Schematic illustration of the effect of nanostructuring on the film’s interaction with incident light. Blue arrows represent incident solar irradiation and the red shading for the evenly coated and bilayer In2O3-x(OH)y/SiNW films illustrates photothermal heat generation;31 d) STEM image and EFTEM elemental (In) mapping of In2O3-x(OH)y/Nb2O5; e) Illustration of the synthesis of In2O3-x(OH)y nanocrystals grown on the surface of Nb2O5 nanorods.34 Reprinted with permission from Refs 31 and 34. Copyright 2016 American Chemical Society and 2019 Wiley.





effect on the entire model. Hydrogens are commonly used as

saturators, as they act as both an electron supplier and logical

terminator capable of reducing such adverse effects.

For instance, cluster models representing pristine and defected

In2O3 surfaces were thoroughly tested by probing the

convergence of their optical properties with those calculated

from standard periodic models and experiments. These cluster

models were then utilized to identify the detailed excited-state

properties of pristine and defected In2O3 surfaces, which

extended understanding of the enhanced CO2 reduction rate for

defected In2O3-x(OH)y in the light relative to the dark. In

particular, it was found that the SFLP in In2O3-x(OH)y tends to

trap photoexcited electrons and holes at its Lewis-acidic In and

Lewis-basic OH sites, thereby enhancing the respective hydridic

and protonic character of these excited SFLP moieties and their

reactivity towards CO2, relative to the ground state, Figure 4d. 35

To elucidate the role of elevated temperatures (e.g., 180 °C) in

photocatalyst’s activity, an in-depth temperature-dependent

(20 °C versus 180 °C) computational study was conducted on the

defected In2O3-x(OH)y system via well-tempered meta-

dynamics-biased AIMD simulations.36 First, ab initio molecular

dynamics (AIMD) was performed on the Born-Oppenheimer

surface and then implemented a meta-dynamics approach that

applied a periodic bias to the system to encourage efficient,

self-avoiding exploration of the free-energy landscape. This

work suggested that the SFLP site was structurally altered under

high-temperature conditions, providing the means to break the

CO2 reduction reaction into two less-energetic steps with

slightly reduced reaction barriers.

Overall, the detailed electronic level information obtained at

ground state, excited state and at various temperatures can

provide much-needed guidance in the selection of elemental

composition, materials design and surface structural

engineering, thereby enabling the discovery of next-generation

gas-phase CO2-reduction photocatalysts. These computational

modelling results, together with validation from experimental

studies, can lead to valuable insights to further enhance the

activity of the metal oxide photocatalyst via different material

engineering methods mentioned in the previous section.

Photoreactor Design, Modelling, and Photocatalyst Testing

Photoreactor design plays an important role in all stages of

photocatalysis studies, from photocatalyst screening and

kinetic studies to high-performance demonstrators for

industrial scale-up. Thus, achieving high photonic and energy

efficiencies in catalytic CO2 hydrogenation is not only a

formidable materials science challenge, but also a demanding

engineering task that requires a detailed understanding of the

relevant transport processes.37

The CO2-reduction activity of promising new photocatalysts is

usually screened in simple reactor geometries under batch or

flow conditions, with the aim of limiting light and thermal

gradients to allow comparisons of different catalysts under

variable wavelength, illumination intensity, temperature,

pressure, and/or gas flow rates. Commonly used illumination

sources include solar simulators, Xe lamps with filters, LED light

sources, and lasers. Batch reactions are typically performed in a

Figure 4. a) Scheme of SFLP in In2O3-x(OH)y;35 b) Side view of bixbyite In2O3 supercell with (111) orientation and the upper surface structure of (111) terminated In2O3;14 c) Overall proposed mechanism for the CO2 + H2 → CO + H2O reaction on In2O3-x(OH)y;35 d) Schematic illustration of the origin of the difference in the experimental activation energy (ΔEa) for the RWGS reaction CO2 + H2 → CO + H2O involving the ground state SFLP in the dark and excited state SFLP in the light.35

Reprinted with permission from ref 14 and ref 35. Copyright 2015 Royal Society of Chemistry and 2016 American Chemical Society.





pressurized reactor vessel with an optically transparent window

and externally heat controller to control sample temperature.

Samples are dispersed as a thin powder layer onto a flat

substrate, such as quartz filter paper, and then placed into the

reactor. In contrast, flow reactor testing is commonly

performed using a packed bed/pellet of the catalyst powder in

tubular quartz capillaries, followed later by testing in reactors

with more well-defined conditions for catalyst characterization,

such as a thin-slit reactor or a Harrick cell.

While working with a well-defined reactor system is an

important starting point, comparing production rates on

different photocatalysts studied in different reactors and by

different research groups remains a challenge due to the lack of

widely-adopted standardized reactor operating conditions (e.g.,

illumination intensity and spectral distribution of light source),

incomplete reporting of reactor operating conditions, different

conventions used in normalizing production rates, and a lack of

reference calibrations.38

At a minimum, best practices in conducting photocatalyst

testing and reporting rates should be adopted. These include:

1) validation of new photoreactors with a known reference

catalyst; 2) pre-reaction cleaning of the catalyst to minimize

surface adsorbed organic contamination, coupled with 13C-

labelled CO2 reaction testing to verify the origins of the product

carbon species; 3) estimation of the photon penetration depth

for the catalyst sample and measuring potential temperature

gradient across the sample by making reactor modifications to

accommodate multiple temperature measurements, so that

photochemical and photothermal rate contributions may be

decoupled; and 4) normalization of reported rates over the

illuminated specific surface area, and reporting a

photochemical and photothermal turnover frequency where

appropriate, in preference to mass normalized rate.

Simulation studies on the transport processes of heat, mass,

momentum, and radiation within photoreactors at length scales

from nanometres (catalyst particles) to meters (the reactor) can

help guide optimization of photoreactor performance. One

approach is to use simple continuum Monte Carlo ray-tracing

(MCRT) methods coupled with finite element methods

(FEMs).39,40 Effects of continuum substructures (catalyst

structure) are also considered in such continuum models using

appropriately chosen physical parameters (scattering and

absorption coefficients, etc.).

According to 2D MCRT/FEM simulation results on three typical

reactor designs in Figure 5, fixed-bed capillary reactors

irradiated from one side showed large gradients in their

absorption intensity fields, Figure 5a-b. However, significant

improvement could be achieved using thin-slit reactors, as

evidenced by their highly uniform optical absorption intensity

fields, Figure 5c. Likewise, better insulation and improved

thermal contact of the catalyst with the heating block

effectively homogenized the temperature field. Nevertheless,

all reactors exhibited reduced optical absorption due to

reflective losses at phase boundaries, absorption by reactor

components, and light scattering from the catalyst. Thus,

mitigating these losses would require minimizing phase

boundaries and using reflective reactor components to redirect

light toward the catalyst, thereby emphasizing the importance

of the reactor/catalyst geometry and alignment. One strategy

to meet this demand is through integration of proper

noncatalytic photocatalyst supports into the system.

Photoreactor and Support Archetypes

Within a photoreactor, photocatalysts can be immobilized onto

non-catalytic supports whose individual geometry and material

properties can be controlled to affect photon absorption,

thermal conductivity, surface adsorption, charge transfer, and

other catalyst properties that affect production rate and selectivity.

For example, thermally conductive materials such as Al have

been chosen for efficient heat transfer where additional

Figure 5. Cross-sections showing simulated optical absorption and temperature fields for two tubular designs similar to those used for catalyst testing of In2O3-

x(OH)y systems (a and b) and one thin slit design for catalyst testing under well-defined conditions currently under development (c). Reactor designs include transparent “windows” (light blue), metallic bodies (light grey), photocatalysts (yellow), external heating element (dark grey).

Figure 6. Continuum of macro porous catalyst supports for effective mass and light transport from regular to random geometry. a) Channels of square geometry for a monolith structure, with catalyst powder deposited in channels or coated on channel walls, with scattering waveguide illuminating surface. Alternately, film coated directly onto a waveguide and placed in channels, b) Interconnected random network of pores (reticulated) coated with film of

catalyst, illuminated either centrally or externally through a solar

concentrator.





thermal energy is supplied to the reaction.41 Also, as a reflective

support, Al can increase the absorption of photons in the

photocatalyst layer.

When considering reactor scale-up, optimizing the light

distribution and gas-catalyst contact while keeping the reactor

footprint low requires photocatalyst immobilization onto

macroscopic supports that offer a high surface area to reactor

volume ratio. Packing structures of a regular geometry

(monoliths) and random geometry (reticulated foams) have

been shown to improve mass transfer and process energy

efficiencies by up to an order of magnitude in conventional

thermochemical processes.42 Planar photoreactors may

themselves be miniaturized and micro-structured

(“microreactors”) to reduce the length scale for heat transfer

and light transport to (and from) the catalytically active sites.

Beyond the improvements in external heat and mass transfer

efficiencies over randomly packed beds (Figure 6), such

structures allow improved light penetration, and can also be

coupled with optical fibres, which overcome efficiency losses

usually associated with under-absorption of the catalyst sub-

surface layer.43

For instance, quartz waveguide rod supports were used to

maximize optical absorption by In2O3-x(OH)y via internal

reflection, with red (732 nm) to green (535 nm) lasers being

readily propagated, Figure 7a-c.44 Diffuse transmittance

measurements, Figure 7e, revealed that the catalyst coating

increased visible light side scattering by ~21% versus an

uncoated quartz rod. This waveguide behaviour allowed more

efficient light absorption by defect states extending from the

conduction and valence band edges of In2O3-x(OH)y. The CO

production rates on a coated waveguide were significantly

higher (by factors of 8.7 and 8.1, respectively) than a similarly

coated planar substrate when UV or green cut-off filters were

used to reject wavelengths shorter than 500 nm or 620 nm. The

advantage of using optical fibre waveguides is the efficient

packing of optical support-catalyst within a reactor volume. This

maximizes catalytic film surface area, while utilizing the gaps

between fibres as narrow gas flow channels.

Additionally, commercial Ni and Cu metal foams were used as

porous supports to maximize the catalyst loading and surface

area-to-volume ratio of In2O3-x(OH)y while also increasing light

penetration and contact with reactant gases.45 CO production

rates were optimized by systematically tuning foam porosity,

surface roughness, reactant gas velocity, and reaction

temperature. In2O3-x(OH)y on nickel foams exhibited greater

redox stability and higher CO production rates than In2O3-x(OH)y

on copper foams. Elsewhere, it was found that when using

In2O3-x(OH)y deposited on an oxidized nickel foam, a superior CO

production rate was observed. Under solar illumination of 247

mW/cm2, the CO production rates of In2O3-x(OH)y on the

oxidized nickel foam were 1.3 times higher than under dark.

Potential of Photocatalytic Processes

The engineering challenges facing photocatalytic CO2

hydrogenation are not limited to photocatalyst and reactor

Figure 7. a-c) Waveguide behaviour of In2O3-x(OH)y -coated quartz rod supports, the wavelength of light used were 405 nm, 532 nm and 645 nm.44 d) Integrating sphere setup for quantifying optical leakage by coated waveguides. e) Transmittance spectra of coated and uncoated waveguides, with and without an aluminium foil end cap.44

Figure 8. Five important interdependent factors influencing the implementa-tion potential of a photocatalytic process.





development. When considering the potential implementation

of a photocatalytic reaction at industrial scale, the entire

process chain, including upstream and downstream unit

operations, must be compared with other process designs (e.g.,

conventional thermal, electrochemical, or photo-

electrochemical processes). A multitude of interdependent

factors influence the outcome of this comparison, as shown in

Figure 8, which will be briefly introduced in this section.

Economics and Overall Efficiency

Even the visionary among us must admit that investors expect a

return on their capital investment; in other words, the product

value must exceed the process costs. While the product value is

determined by the dynamics of the target market, the process

costs are inevitably related to the use of available resources. To

ensure economic feasibility, all resources must therefore be

used efficiently, and the assessment and optimization of

efficiencies in all process subsystems becomes indispensable.

Unsurprisingly, in most energy applications the efficient use of

the employed energy resources is of utmost importance.

An exact definition of an energy efficiency metric that is

applicable to a broad variety of processes – and which would

facilitate a fair comparison – is difficult for several reasons. One

reason is that the significance of different forms of energy (e.g.,

chemical, thermal, mechanical, electrical, radiative) varies

across process designs. Further complicating matters is the fact

that, fundamentally, different forms of energy have different

thermodynamic and economic value. A given amount of

thermal energy (at, for instance, 100 °C) is of lower value than

an exact same amount of electrical energy. This is because

thermal energy can execute less thermodynamic work than

electrical energy. One best-practice approach enabling the

calculation of efficiencies while considering the thermodynamic

value of different forms of energy, thereby linking the use of

resources to the economic costs of process steps, is the concept

of exergy.46 Exergy is defined as the maximum useful work that

can be extracted from a system when it comes in

thermodynamic equilibrium with its environment.

Another, more-technical difficulty in assessing energy

efficiencies of processes is that the overall and subsystem

efficiencies and other process performance metrics are

interdependent. A reactor with low conversion, for instance,

will entail a large energy demand in subsequent product

purification steps. Therefore, complex, and often non-linear,

process models are needed to assess the overall efficiency of a

process in advance. Aspen® or MATLAB Simulink® are

commercial tools providing a framework for such modelling.47

Despite the challenges in comparing different forms of energy,

researchers and engineers in the field of energy applications

often use simplified process energy efficiencies, 𝜼 , as shown

in Equation (1). Of course, such efficiency definitions do not take

account of the different thermodynamic or economic values of

different forms of energy.

The numerator represents the sum of useful energy outputs. It

comprises the sum of energy contained in the products (given

by the sum of the products of mass streams �̇�𝒊 and higher

heating values 𝑯𝑯𝑽𝒊 of products 𝒊), and the energy of useful

extracted energy streams �̇�̇𝒋. This can be, for instance, heat flux

at elevated temperature. The denominator is the sum of all

energy inputs. It comprises the energy added to the process via

chemical energy of the reactants (represented by sum of the

products of mass streams �̇�𝒌 and higher heating values 𝑯𝑯𝑽𝒌

of all reactants 𝒌 ) and other energy streams �̇�𝒍 , such as a

photon or heat flux, or electrical energy.

If solar energy was the only energy input and there was only one

useful extracted energy stream (e.g. chemical energy in the

form of hydrogen in an exemplary case of water splitting), the

energy efficiency in equation (1) would be called solar-to-

hydrogen efficiency, which is a commonly reported efficiency

metric in the field of solar fuels.

Exemplary Aspects in Assessments of Photocatalytic CO2

Hydrogenation Processes and Process Steps

By analogy to the overall energy efficiency, energy efficiencies

for process steps can also be defined. Unsurprisingly, the solar-

to-product or solar-to-fuel efficiency ( 𝜼𝑺𝑻𝑿 ) of the synthesis

step is the crucial parameter in assessing photocatalytic CO2

hydrogenation processes.47 𝜼𝑺𝑻𝑿 is defined by the photonic

efficiencies of solar light collection and transport in the

photoreactor, and the intrinsic photon efficiency of the

photocatalyst employed. The experimental and/or theoretical

assessment of the photon transport efficiency of a photoreactor

including the light collector is non-trivial. Possible approaches

include chemical actinometry following recommended

protocols from IUPAC supplemented with optical simulations.

Reactor design, in particular the geometry of light

concentration and/or guiding elements, drastically influences

photon transport efficiency in a photoreactor. Further

complications are the interdependencies between photon

absorption efficiency of a reactor/catalyst combination and

radiation transport properties of the employed photocatalyst

systems.

The determination of the intrinsic photon efficiency of the

employed photocatalyst is no less challenging than the

assessment of photon transport efficiencies. State-of-the-art

methods applied in heterogeneous catalysis to quantitatively

determine kinetic properties, such as rate constants of a given

catalyst, are not applicable in photocatalysis.48 This is due to the

dependence of the photochemical rate constant on quantum

yield, incident light intensity, and light transport properties,

which must be taken into account alongside the “traditional”

thermodynamic state variables of temperature and pressure.

One method that may reveal information on the intrinsic

photon efficiency of photo catalyst systems is photocurrent

measurements. Photocurrent measurements provide insight

into charge accumulation/recombination in an illuminated

photo catalyst and further indicate reactivity with chemical

precursor species.[29,35,54] Some of the best practices associated

with photocurrent measurements are: 1) a “sanity check” that 𝜼 =

∑ �̇�𝒊𝑯𝑯𝑽𝒊𝒊 𝝐 𝑷𝒓𝒐𝒅𝒖𝒄𝒕𝒔 + ∑ �̇�𝒋𝒋 𝝐 𝑶𝒖𝒕𝒑𝒖𝒕

∑ �̇�𝒌𝑯𝑯𝑽𝒌𝒌 𝝐 𝑹𝒆𝒂𝒄𝒕𝒂𝒏𝒕𝒔 + ∑ 𝑬𝒍𝒍 𝝐 𝑰𝒏𝒑𝒖𝒕 (1)





the measured photocurrents are not above the integral of the

absorptance and the solar spectrum; 2) considering the

conversion efficiency from photons to thermalized charge

carriers by including the optical losses of unabsorbed photons

and the reduction in energy of the absorbed photons, as well as

electrical contact losses; 3) evaluating the fill factor using a

circuit model and an accompanying impedance spectroscopy

measurement to separate resistive and capacitive behaviour;

and 4) localizing charge-carrier behaviour with a geometric

diffusion-recombination model that profiles the illumination

absorption depth, carrier concentrations, and trap state

distribution under steady-state conditions.

To simplify, the apparent quantum yield or photonic efficiency

(𝜻𝒑) has been introduced as a metric to determine the efficiency

of a given photocatalyst:

𝜻𝒑(𝝀) = 𝒓𝒂𝒕𝒆/𝑰𝟎(𝝀), (2)

where 𝝀 is the wavelength of the incident light and 𝑰𝟎

represents the light intensity arriving at the inside of a flat front

window of the photoreactor. The photonic efficiencies of

different experiments and laboratories are, however, only

comparable if an identical amount of light has been absorbed in

each experiment. Due to the wide range of photoreactors used

in different research laboratories, this is unlikely. Standardized

testing methods based on, for instance, ISO oxidations of NOx

(ISO 22197-1:2016) or acetaldehyde (ISO 22197-2:2011) have

therefore been developed and should be applied to determine

apparent quantum yields. Rigorous experimental protocols and

thorough characterization of the employed photocatalysts,

photoreactors, and light sources are instrumental to ensure

accurate and reproducible results.

The photoreactor system represents the core element of a

photocatalytic CO2 conversion process. However, CO2 capture is

another important step that must be considered during the

assessment of the process. Since CO2 capture from emission

streams or from atmosphere, as well as the transportation of

CO2, are crucial and energy-intensive steps in CO2

hydrogenation processes, their influence on the overall

efficiency metric deserves a detailed assessment. A multitude

of adsorption and absorption processes designed for different

CO2 sources, including flue gas streams and the atmosphere, are

available.49 Special attention should be paid to the possibility of

integrating waste heat streams from process subsystems to

cover the significant low-temperature heat demand of most

CO2 capture processes. At the same time, the cost of CO2

requires smart process engineering of the subsequent synthesis

steps to convert as much of the captured CO2 into products as

possible. Using multiple passes of the gas stream over the

catalyst might address incomplete conversion if one-pass

conversion is not satisfactory. However, such a recycle will again

entail an energetic cost that must be considered.3,47

Another important aspect in the assessment of photocatalytic

processes is the land area, which is required to harvest sunlight.

Land area dedicated exclusively for solar processes significantly

impacts process economics and is directly linked to 𝜼𝑺𝑻𝑿 of the

synthesis step: when 𝜼𝑺𝑻𝑿 is low, then a large area of land is

required to supply the photons for the photocatalytic reaction,

and vice versa. If implemented in world-scale production

facilities, and even if 𝜼𝑺𝑻𝑿 is high, land use will remain one of

the major drawbacks of all solar energy technologies including

photovoltaic and biomass-based approaches, as they all require

either a direct or indirect sunlight harvesting process step.

However, there are futuristic concepts avoiding, or at least

reducing, the need for additional land utilization. For example,

a retrofitting of existing infrastructures has been proposed,

including buildings equipped with photovoltaic panels, fully

autonomous units for extracting CO2 from ventilation air

streams, and the on-site conversion to green fuel precursors.50

A tool that connects all the different aspects discussed above is

the techno-economic assessment (TEA). TEAs based on detailed

process modelling may reveal bottlenecks in process designs by

allocating economic costs to each process step. For example,

TEAs of different solar methanol process designs have revealed

that, if the overall photon efficiency is low, the major cost-

driving factor in a combined electro- and photochemical

methanol synthesis plant is the photoreactor-catalyst

combination.3 In encouraging contrast to the economic metrics

cited for solar methanol, TEAs of photocatalytic hydrogen

processes suggest that cost-competitive hydrogen production

can be achieved simply via photoelectrochemical water splitting

in low-cost plastic baggie reactors filled with photocatalyst

slurry.51

At this point, it is important to address how thresholds must be

met regarding crucial process parameters, such as the overall

energy efficiency, so that a photocatalytic production process

could compete with established fossil resource-based

production. Exemplary values can often be extracted from the

literature; for instance, at 10% solar-to-hydrogen efficiency for

water splitting (using the above-mentioned low-cost baggie

reactors) may yield cost-competitive hydrogen51. From a

broader perspective, this question is more difficult to address,

as the answer depends strongly on the boundary conditions

chosen for the TEA. Such boundary conditions are highly

dependent on the existing political framework, including the

implementation of CO2 taxes, the local availability of resources

such as average solar irradiance and CO2 sources, and the locally

and globally achievable revenues. A photocatalytic process with

limited solar-to-product efficiency may be feasible in a sunlight-

rich area with numerous local consumers, whereas another

area with lower solar irradiance and inexpensive CO2 sources

may see non-solar conversion processes as economically

preferable.

Further complication arises because TEAs for photocatalytic

processes must estimate prices for non-commercial

components that are currently under development. Prototype

photoreactors and concentrating optics are currently still

expensive, and their application entails a higher cost for solar

chemicals and fuels. Fortunately, though, mass production of

these process components could soon be established, resulting

in decreased component costs and improved process

economics. This demonstrates that the TEA is, therefore, also





dependent on assumptions made regarding the economy of

scale and the future component-development costs.

In addition to economic aspects, the greenhouse gas (GHG)

reduction potential can be an important process selection

criterion. To accurately assess the GHG reduction potential of a

photocatalytic CO2 conversion process, the GHG footprint of

reactants (CO2, H2O), installations (process unit operations) and

products in a global network of supply chains and consumers

must be considered. Lifecycle assessment (LCA) is a tool

commonly used to perform these analyses. Standardized LCA

procedures and guidelines have been developed, in particular

ISO 14040 and ISO 14044. During the design phase of an LCA

study, special attention should be devoted to the definition of

scope and a careful selection of system boundaries.

Inconsistencies should be avoided within a given study and

when comparing the environmental impacts of competing

processes to inform unbiased and rationally guided decisions.

Imperatively, in the perspective of a state of global climate

emergency the worlds resources should be invested into

technologies which, for a given effort, promise the highest GHG

emission reduction potential.

Although photocatalytic CO2 conversion processes are not yet

cost competitive with fossil-driven production, they certainly

are among the hot candidates to meet demands of a global

society of environmentally conscious consumers.

Conclusions

The development of different kinds of renewable-energy-

powered CO2 refineries stands to help curb greenhouse gas

emissions over the next decade and play an important role in

ameliorating climate change. To realize this utopian vision of a

sustainable future, the discovery and optimization of highly

active, selective and stable CO2 photocatalysts is imperative.

Catalytic metal oxides are capable of filling diverse roles in solid-

state chemistry, physics and engineering, as well as being

employed in the production of many industrial products,

processes and devices. This prevalence of metal oxides is

traceable to their myriad compositions, structures and forms,

variations of which bestow upon their versatile properties,

functionality and utility.

As has been discussed in this Tutorial Review, even greater

variation in the properties of metal oxides can be achieved by

modifying their stoichiometry via doping, isomorphic

substitution, aliovalent modifications and non-stoichiometry, as

well as by introducing nanostructures, heterostructures,

superstructures, support materials and polymorphism. This

strategy exemplified by indium oxide in its various guises has

furthered the understanding of gas-phase heterogeneous CO2

photocatalysis while facilitating advancements in innovative

photocatalyst discovery and engineering. The development of

novel photoreactor designs has enabled more accurate

performance testing of metal oxide photocatalysts while

providing the groundwork for the up-scaling and future

industrialization of CO2 photocatalysis. These advancements

could well lead to future implementation of solar refineries for

the manufacture of eco-friendly chemicals, pharmaceuticals,

polymers, materials, and fuels from recycled CO2, thereby

toward combating climate change.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

G.A.O. is a Government of Tier 1 Canada Research Chair in

Materials Chemistry, Nanochemistry and Solar Fuels. Financial

support for this work was provided by the Ontario Ministry of

Research Innovation (MRI); Ministry of Economic Development,

Employment and Infrastructure (MEDI); Ministry of the

Environment and Climate Change (MOECC); Ministry of

Research, Innovation and Science (MRIS); Connaught

Innovation Fund; Connaught Global Challenge Fund; Best in

Science (University of Toronto) and the Natural Sciences and

Engineering Research Council of Canada (NSERC). L.H. received

a Postdoctoral Fellowship SECITI (066/2017), P.D. was awarded

an NSERC Postdoctoral Fellowship and U.U. won an Alexander

von Humboldt Postdoctoral Fellowship.

Author contributions

Y.D., P.D., L.W., A.J. and A.A.T. wrote the Engineering of Metal

Oxide Materials section; K.K.G. wrote the Computational

Modelling section; A.M., P.D., U.U., J.Y.Y.L., P.K. and R.D. wrote

the Photoreactor Design, Modelling, and Photocatalyst Testing

section. P.K., U.U. and R.D. wrote the Potential of Photocatalytic

Processes section. Y.D. and G.A.O. wrote the Introduction and

Conclusion sections. Y.D. coordinated the cooperation of all co-

authors and synthesized the final draft. P.D. proofread the draft

and assisted in the coordination. A.M. and M.X. assisted in the

graphic design. G.A.O. supervised this project and approved the

final manuscript. All authors participated in discussions and

revisions of the draft together.

Notes and references

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TOC Figure materials engineering, theoretical modelling, reactor engineering and process development of gas-phase photocatalytic CO2 reduction exemplified by indium oxide systems.

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