Ni supported high surface area CeO2–ZrO2 catalysts for hydrogen productionfrom ethanol steam reforming

Mohamed A. Ebiad,* Dalia R. Abd El-Hafiz, Radwa A. Elsalamony and Lamia. S. Mohamed

Received 14th February 2012, Accepted 5th July 2012

DOI: 10.1039/c2ra20258a

The catalytic activity of nano-sized x%Ni/Ce0.74Zr0.26O2 (x = 0, 2, 10 and 20wt%) catalysts have been

investigated to develop highly active catalysts for ethanol steam reforming (ESR) into hydrogen. The

structure and surface properties of the catalysts were tested by XRD, TPR, HRTEM and BET

surface areas. The effect of reaction temperature from 200 uC to 600 uC was studied in a flow system

at atmospheric pressure with an ethanol/water molar ratio of 1 : 8. Selectivity was calculated for the

catalytic products H2, CO, CO2 and CH4, as well as the intermediates C2H6, C2H4, C3H8, CH3CHO

and CH3COCH3, at different reaction temperatures. It was found that complete conversion of

ethanol with considerable amounts of H2 was obtained at 400 uC over all catalysts. H2 was produced

at a very low temperature (200 uC) over 10% and 20% Ni loadings, while a maximum H2 selectivity

(75%) is reached at 600 uC over the 2%Ni/Ce0.74Zr0.26O2 catalyst; this is most likely due to the small

nickel particle size (2–4 nm) in 2%Ni, which results in enhancement of the metal–support interactions.

Thermal decomposition of ethanol in an ethanol/water mixture under the same reaction conditions,

but in the absence of catalyst, was also studied. HRTEM of the spent catalyst (8 h ESR) shows the

deposition of carbon in the form of carbon nanotubes (CNTs).

1. Introduction

Hydrogen is a potentially very attractive source of energy since

its combustion produces only water and energy. Molecular

hydrogen is a clean burning fuel, can be stored as a liquid or gas,

is distributed via pipelines, and has been described as a long term

replacement for natural gas, so increased attention is focused on

hydrogen production technologies.1 Several catalytic processes

for hydrogen production from ethanol have been investigated,

including steam reforming (SR), partial oxidation, and auto-

thermal reforming.2

Ethanol is a promising future bio-fuel because of its relatively

high hydrogen content (on a molar basis), non-toxicity, ease of

storage and handling safety. Ethanol can be readily produced

from renewable feedstocks such as sugar cane, municipal solid

waste and agro-waste. Another advantage of using ethanol is

that the CO2 produced during ethanol steam reforming (ESR) is

equal to the CO2 required for biomass growth and thus provides

a closed cycle for CO2 consumption. Therefore, no net pollutants

are released into the environment.

Nickel-based catalysts are widely used in commercial reform-

ing processes and are recognized as appropriate catalyst systems

in SR for their low cost, relatively high activity towards C–C

bond cleavage and hydrogenation/dehydrogenation reactions.3

The study by Kugai et al. 4 has shown that ethanol is adsorbed

on the surface of Rh and Ni metals as ethoxide species, which

form an oxametallacycle intermediate and favors C–C bond

rupture. Ni has a high activity for C–C bond and O–H bond

breaking, and has high activity for hydrogenation, facilitating H

atoms to form molecular H2.5

Supports also play important roles in the SR of ethanol, as

supports help in the dispersion of the metal catalyst and may

enhance the metal catalyst activity via metal–support interac-

tions. Supports may promote migration of OH groups toward

the metal catalyst in the presence of water at high temperature,

facilitating SRR.6

In Ni-based catalysts, CeO2 has been used as either an

effective promoter or support because of its characteristic oxygen

storage capacity (OSC), which allows it to store and release

oxygen, leading to the presence of highly active oxygen for the

oxidation of CO and hydrocarbons. This makes the catalyst

more active in many carbon formation-related reactions of

hydrocarbons, such as SR, dry reforming and oxidation. The

specific role of CeO2 in SR involves improving the dispersion of

the active phase and enhances interaction with the supported

metal.7

During the reforming process, in addition to the reactions

on Ni surface, the gas–solid reactions between the gaseous

components presented in the system (C2H6, C3H8, C2H4, CH4,

CO2, CO, H2O, and H2) and the lattice oxygen (Ox) on the ceria

surface also take place. The reactions of adsorbed surface

hydrocarbons with the lattice oxygen (Ox) on the ceria surface

(CnHm + Ox A nCO + m/2(H2) + COx2n) prevent the formation of

carbon species on Ni surface from hydrocarbons decompositionEgyptian Petroleum Research Institute, Cairo, 11727, Egypt.E-mail: m_a_ebaad@yahoo.com

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reaction (CnHm « nC + m/2H2). Moreover, the formation of

carbon via the Boudouard reaction (2CO « CO2 + C) is also

reduced by the gas–solid reaction of carbon monoxide (produced

from SR) with the lattice oxygen (CO + Ox « CO2 + Ox21).8

Ceria easily forms solid solutions with transition metals and

other rare-earth elements. It has been established that introduc-

tion of zirconium into the ceria lattice greatly enhances the

surface area, thermal stability and oxygen storage capacity,

resulting in superior catalytic properties. Hence, ceria–zirconia

solid solutions have been investigated with huge interest among

other ceria-based mixed oxides.9,10 Seog et al.11 have been

confirmed that Ni/Ce0.8Zr0.2O2 shows a higher BET surface area

than Ni/CeO2 and better thermal resistance than Ni/ZrO2

during the reduction process at 600 uC. Electron paramagnetic

resonance (EPR) spectroscopy has identified three types of

paramagnetic defects in CeO2–ZrO2 oxides,12 viz. electrons,

quasi-free or trapped in anion vacancies, O2 ions and O22

ions.13 These species arise by the following interactions:

Ce3+ + O2 A Ce4+(O22) (1)

Ni+ + O2 A Ni2+(O22) (2)

Ce4+ + O22 A Ce3+(O2) (3)

The oxygen storage capacity of CeO2–ZrO2 catalysts was

correlated to the concentration of the oxide ion species.13

Although it had been postulated that CeO2 promotes NiO

reduction, no experimental evidence had been reported in the

literature so far. The work of Srinivas et al.12 offers such a

support, part of the Ni+ ions subsequently react with aerial

oxygen and form Ni2+(O2?2) species, as described:

Ce4+ 2 O22 2 Zr4+ « Ce3+ 2 O2 2 Zr4+ (4)

Ce3+ + Ni2+ A Ce4+ + Ni+ (5)

Ni+surf + O2 A Ni(O2?2) (6)

Three types of Ni species in NiO/CeO2–ZrO2 samples were

observed by Srinivas et al.,12 (I) Ni substituted in the lattice

forming Ni(O2?2) ions, (II) nanosize crystallites of NiO forming

super paramagnetic metallic Ni and (III) larger particles of NiO

forming ferromagnetic Ni. The sample containing 1% Ni

contained species I and II, the sample containing 5% Ni con-

tained all three species; while samples containing 40% Ni

contained species I and III. The reducibility of these species

decreased in the order: I > III > II.

Jones et al.14 suggested the elementary steps of ethanol

decomposition in order to gain some insights into the reasons

behind the product distribution. In the decomposition pathway

of the ethoxy species Ni has a unique effect. It abstracts H from

the CH3-group making a stable oxametallacycle intermediate

(adsCH2–CH2–Oads). The stoichiometry of the reaction for

maximum hydrogen production is given by:

CH3CH2OH + 3H2O A 6H2 + 2CO2 (7)

Our study aimed at developing the amount of H2 produced

from ESR over x%Ni/Ce0.74Zr0.26O2 catalysts. This goal was

intended to firstly demonstrate the occurrence of ESR over the

used catalysts by performing the reaction in the absence of the

catalysts, i.e., thermal decomposition of ethanol. Secondly, to

study the effect of the support and different Ni loadings on

product distribution in the ESR reaction, and finally, to study

the distribution in the gas and liquid phases to clarify the

mechanism from intermediate distribution at different tempera-

tures.

2. Experimental

2.1. Catalyst preparation

The Ce12xZrxO2 mixed oxide supports were prepared by co-

precipitation with ammonia using an aqueous solution of cerium

nitrate (Ce(NO3)2?6H2O, 99% Fluka) and zirconium oxychloride

(ZrOCl2?8H2O, 98% Aldrish). The weight ratio of CeO2/ZrO2

was 4 which corresponds to an x of 0.26. Aqueous ammonia

solution (28–30%) was added dropwise to the aqueous solution

containing (0.1 M) Ce and (0.1 M) Zr salts with constant stirring

until the pH was 9–10. After precipitation, the obtained

hydroxide was filtered, washed thoroughly with deionized water

until chloride free (tested with silver nitrate Ag2NO3), and then

dried overnight at 120 uC. The obtained solid was calcined in air

at 600 uC for 5 h.

Ni loading was carried out by an incipient wetness impregna-

tion method using an aqueous solution of (Ni(NO3)2?6H2O 99%

Aldrich) dissolved in deionized water. Ni loadings were 0, 2, 10

and 20 wt%. After impregnation, the catalyst was dried

overnight at 120 uC and then calcined at 600 uC for 6 h.

2.2. Characterization methods

Thermal gravimetric analysis (TGA) was carried out using

SETARAM Labsys TG-DSC16 equipment in the temperature

range of room temperature up to 1000 uC under a nitrogen flow,

in order to follow the thermal stability of the prepared catalyst.

X-Ray diffraction analysis (XRD) was carried out by Shimadzu

XD-1 diffractometer using Cu-target and Ni-filtered radiation,

to trace the various changes in the crystalline structure and the

different phases accompanied preparation method. Sample

powders were packed in a glass holder, then measurements were

taken of the diffraction intensity by step scanning in a 2h range

between 5u and 70u. The phase identification was made by

comparison with the Joint Committee on Powder Diffraction

Standards (JCPDS). The average crystallite size of the samples

was determined from the XRD peaks using the Scherrer

equation.

High resolution transmission electron microscopy (HRTEM)

and energy dispersive X-ray (EDS) spectroscopy were conducted

using a JEOL 2100F TEM at an accelerating voltage of 200 kV.

To prepare the TEM samples, a dilute particle–ethanol colloidal

mixture was ultrasonicated for 30 min and a drop of solution was

placed on a carbon coated Cu TEM grid.

The textural properties were determined from the N2 adsorp-

tion–desorption isotherms measured at liquid nitrogen tempera-

ture (2196 uC) using a NOVA2000 gas sorption analyzer

(Quantachrome Corporation) system. All samples were degassed

at 200 uC for 17 h in a nitrogen atmosphere prior to adsorption

to ensure a dry clean surface. The adsorption isotherm was

8146 | RSC Adv., 2012, 2, 8145–8156 This journal is � The Royal Society of Chemistry 2012

constructed as the volume adsorbed (V cm3 g21) versus the

equilibrium relative pressure P/Po, where P is the equilibrium

pressure and Po is the saturated vapor pressure of nitrogen.

Total amount of acidity was estimated from weight loss

measurements of adsorbed pyridine using SETARAM Labsys

TG-DSC16 equipment. First, a platinum crucible containing

50 mg of the sample was placed in a shallow porcelain plate and

inserted into a glass reactor adapted to a tubular furnace. The

sample was dehydrated in dried N2 (100 ml min21) at 120 uC for

2 h, cooled to 70 uC, and then gaseous pyridine diluted in N2 was

allowed to pass through the sample for 1.5 h. The temperature

was held at 70 uC under N2 for 1 h to remove the physically

adsorbed pyridine. After that, the samples were analysed by

TGA.

Temperature programmed reduction (TPR) measurements were

carried out to investigate the redox properties (the ease of

reducibility of the metal oxide) over the resultant materials. The

experiments were performed in automatic equipment (ChemBET

3000, Quantachrome). Typically, 100 mg of the pre-calcined

sample was loaded into a quartz reactor and pretreated by

heating under an inert atmosphere (20 ml min21 nitrogen) at

200 uC for 3 h prior to running the TPR experiment, and was

then cooled to room temperature in N2. Then the sample was

submitted to a constant rate of heat treatment (10 uC min21 up

to 1000 uC) in a gas flow (80 ml min21) of the hydrogen/nitrogen

(5/95 vol%) mixture as a reducing gas. A thermal conductivity

detector (TCD) was employed to monitor the amount of

hydrogen consumption.

2.3. Catalytic activity

The experimental system used in this work is schematically

shown in Fig. 1. Experiments were performed at atmospheric

pressure in a continuous fixed-bed down flow vertical tubular

reactor consisting of a quartz tube, 300 mm in length and 10 mm

i.d. Another silica tube of 500 mm length and 14 mm i.d.

attached to the first one served as the outlet to facilitate fast

removal of the reactor effluent. The catalyst was placed on a

quartz wool bed inside the reactor. 2 g of catalyst diluted with

the same sized quartz particles was used for catalytic tests. Prior

to a run, the catalyst was reduced in situ at 600 uC for 2 h under a

hydrogen flow rate of 20 cm3 min21, where the catalyst showed

thermal stability up to 1000 uC (TGA, not shown). A water/

ethanol mixture (EtOH/H2O = 1 : 8 mol/mol) premixed in a

separate container was fed into the evaporator (at 150 uC) by a

pump (Model RP-G6; FMI, USA) with a flow rate of 0.2 ml

min21. Nitrogen gas was fed into the evaporator with a flow rate

40 cm3 min21 to uniformly carry the steam. This steam was then

fed into the reactor. The reaction temperature was varied from

200 uC to 600 uC. The thermal decomposition of ethanol was

studied under the same reaction conditions but in the absence of

catalyst.

The product stream was analyzed by the use of two gas

chromatographs (GC) (Agilent 6890 plus HP, Varian Natural

Gas Analyzer type C model CP-3800). The first one, equipped

with a molecular sieve 5 A capillary column (Molsive 5 A, 15 m

in length, 0.53 mm in internal diameter and 40 mm in film

thickness) and TCD with (He) as the carrier gas, was used for the

determination of H2. The first one was also used to analyze the

composition of condensed liquid products on the capillary

column (HP-30 50% phenyl/50% dimethyl polysiloxane, 30 meter

in length, 0.53 mm in internal diameter and 0.32 mm in film

thickness) using a flame ionization detector (FID) with N2 as the

carrier gas and isopropanol was used as the internal standard.

The second GC equipped with TCD and FID, three columns are

used to accomplish the single channel analysis of the gas. Firstly,

a molecular sieve column (4 ft in length, 1/8 inch in internal

diameter and Molsieve 13X with a 45/60 mesh size) for oxygen,

nitrogen, methane and carbon monoxide separation. Secondly, a

Hayesep P column (6 ft in length, 1/8 inch in internal diameter

and Hayesep P with an 80/100 mesh size) for separation of

methane, carbon dioxide, ethane. The two packed columns,

molecular sieve and Hayesep P, were attached with TCD. The

third capillary column (CPSil 5, 60 meter in length, 0.25 mm in

internal diameter and 1 mm in film thickness) used for separation

of methane, ethane, ethylene, acetaldehyde, ethanol, acetone,

was attached with FID. Helium gas was used as carrier gas with

constant pressure 20 psi, 23.5 psi and 13.4 psi for the CPSil 5,

Hayesep P and molecular sieve columns, respectively. The

quantitative analysis of the gas mixture is based on an external

standard, as the GC response is calibrated using a suite of

certified reference external standard gas samples with known

compositions and is used to calculate the response factor of each

component in the products. Selectivity values were calculated as

the molar percentage of products obtained, excluding water;

Selectivity of product = (mol of product/total mol of H2 and

carbon-containing products in the outlet) 6 100.

Fig. 1 Schematic diagram of experimental setup: (1) feeding tank

containing the water/ethanol mixture; (2) pump (Model RP-G6; FMI,

USA); (3) evaporator; (4) catalyst packed reactor; (5) cooler; (6) gas

collector; (7) liquid collector (8) hydrogen and nitrogen cylinder; and (9)

heating and temperature controller.

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3. Results and discussion

3.1. Catalyst characterization

3.1.1. X-Ray diffraction analysis. The X-ray diffraction pattern

of the CeO2–ZrO2 support, as well as the Ni/CeO2–ZrO2

catalysts calcined at 600 uC with various values of Ni loading

(2–20%), are represented in Fig. 2.

The XRD patterns of the CeO2–ZrO2 support (Fig. 2) showed

a number of peaks corresponding to 2h = 28.5, 33.3, 47.4 and

56.4, which are assigned to a typical cubic fluorite structure of

Ce0.74Zr0.26O2 [ref. JCPDS card (00-028-0271)]. No separate

phase due to CeO2 or ZrO2 was detected, which is ascribed to the

insertion of zirconium ions of a smaller radius (0.084 nm) into

the cubic lattice of CeO2 (0.097 nm) and the formation of a

homogenous Ce12xZxO2 solid solution (ss).15 This cubic phase

CeO2–ZrO2 has the maximum degree of structural defects and

oxygen storage capacity as reported by Kumar et al.16

The XRD patterns of the x%Ni/CeO2–ZrO2 catalysts impreg-

nated with different Ni loadings all showed peaks corresponding

to the cubic fluorite structure of the CeO2–ZrO2 solid solution.

No separate NiO phases were found by XRD for 2%Ni/CeO2–

ZrO2. The peak position of CeO2–ZrO2 was invariant with the

Ni content, and the peak width changed only marginally. At

higher Ni loadings (10 and 20% Ni), three new peaks at about

37u, 43u and 62u (2h) [ref. JCPDS card (00-001-1239)] indicating

a NiO phase were observed. The peak intensity was obviously

stronger with increasing Ni loading.

Table 1 summarizes the crystallite sizes and lattice parameters

of the prepared catalysts. The crystallite size of the support is in

the range 11.5–9.4 nm, which confirms that a nano-crystallite

size is achieved by the co-precipitation method. In addition, the

crystallite size of NiO in the (2%Ni) catalyst is too small to

measure, this indicates that the NiO species are finely dispersed

on the support beyond the XRD detection limit (,3 nm), this is

confirmed by the decrease in support lattice parameters, due to

the incorporation of Ni2+ with a small ionic radius of (0.072 nm)

into the support lattice.11,17 The dispersion and particle sizes

were not identified by XRD, therefore HRTEM was utilized

later (section 3.1.2) to discuss the presence and the size of NiO in

the 2%Ni/CeO2–ZrO2 catalyst. The size of the NiO crystallites

increased from 24.5 to 26.3 nm with increasing Ni content in the

samples (Table 1).

3.1.2. High resolution transmission electron microscopy. The

HRTEM and STEM/EDS photographs obtained from the

CeO2–ZrO2 and Ni/CeO2–ZrO2 catalysts are shown in Fig. 3.

Analysis of the photographs for CeO2–ZrO2 (Fig. 3a) revealed

particles with defined dimensions, having a grain size ranging

from 10–19 nm. As per the literature,18 and taking into account

the slight contraction of the crystal lattice, which is expected by

partial substitution of Ce ions by smaller Zr ions, the cubic

fluorite structure is quite obvious.19 Although no separate NiO

phases were discernible by the XRD of 2%Ni/CeO2–ZrO2,

STEM/EDS elemental mapping and HRTEM images (Fig. 3b)

identified nickel clusters that were well dispersed within the

CeO2–ZrO2 matrix. Therefore, at low Ni content, nickel oxide

was present in CeO2–ZrO2, but, in nanoparticle form (2–4 nm).20

The 20%Ni/CeO2–ZrO2 (Fig. 3c) system is not homogeneous. In

fact, two forms of crystals are observed, corresponding to the

presence of the CeO2–ZrO2 fluorite structure and nickel oxide, as

confirmed from XRD and STEM/EDS. The grains of NiO are

readily recognizable by their opaque hexagonal shape, in

contrast to the CeO2–ZrO2 particles, which are cubic and

transparent. The mean size of the NiO aggregates is estimated to

be in the range 57–95 nm and those of the CeO2–ZrO2 particles is

in the range 8–12 nm.

3.1.3. Surface areas. All samples displayed type IV isotherms,

as illustrated in Fig. 4, according to Brunauer’s classification and

are characterized by well-developed mesoporous structures

including micropores.21 The shape of the hysteresis loop is type

H2, according to the IUPAC classification, which usually has

non uniform sized or shaped pores.22

Table 1 summarizes the BET surface area, pore volume and

pore radius of the CeO2–ZrO2 and x%Ni/CeO2–ZrO2 catalysts,

the surface area was found to be 133.5 m2 g21 for the

CeO2–ZrO2 support, which decreases with increases in Ni

loading from 2–20 wt%. This might be a consequence of

crystallite rearrangement during metal impregnation, as already

reported by Alonso et al.,23 and/or due to the blockage of some

pores by NiO particles.24 These surface areas are higher than

those reported by other authors who have prepared ceria–

zirconia supported nickel catalysts.25,26

The pore volume of the CeO2–ZrO2 support increases after

impregnation with 2% Ni from 0.112 to 0.155 cm3 g21, this may

be attributed to low Ni loading, which is present in the form of

nanoparticles, as illustrated from HRTEM (Fig. 3), so it may be

incorporated into the support pore and increase the pore volume.

On the other hand, as the Ni loading increases up to 20% the

pore volume decreases, due to blocking of the pores by

agglomerated NiO at the surface of the support.

3.1.4. Temperature programmed reduction (TPR). TPR is a

simple and quite useful method to obtain information on the

reactivity of lattice oxygen towards hydrogen in CeO2–ZrO2

supports, as well as the Ni reducibility in x%Ni/CeO2–ZrO2

catalysts. As illustrated in Fig. 5, the CeO2–ZrO2 support shows

two reduction peaks, at 450 and 600 uC. Since pure ZrO2 did not

show any reduction peak in this wide temperature range, it is inFig. 2 XRD patterns of fresh CeO2–ZrO2 and x%Ni/CeO2–ZrO2

catalysts (ss: solid solution).

8148 | RSC Adv., 2012, 2, 8145–8156 This journal is � The Royal Society of Chemistry 2012

agreement with the reported result.25 All the consumed hydrogen

contributes to the reduction of Ce4+ to Ce3+. According to the

reduction mechanism of CeO2 proposed by Pantu and

Gavalas,27 the TPR profile of pure CeO2 shows a two-peak

pattern at 503 uC and 752 uC, due to surface and bulk reduction.

The outermost layers of the smaller crystalline CeO2 are first

reduced at lower temperatures due to a lower enthalpy of

reduction, and then bulk reduction occurrs at higher tempera-

tures. In context, the TPR profiles of the mixed CeO2–ZrO2

oxides show a main broad reduction feature, which is in

agreement with the promotion of the reduction of the mixed

oxide, this means that the presence of Zr weakens the Ce–O bond

in the solid solution of CeO2–ZrO2 and makes the CeO2

component more easily reduced.28 The low temperature peak

at 450 uC is due to the reduction of the surface layer of CeO2 and

the higher one at 600 uC is due to that of the material in the

bulk.15

The TPR profile of the 2%Ni/CeO2–ZrO2 catalyst possesses

two peaks: the first one is a peak at 370 uC, and the other peak is

at 430 uC. The first peak is attributed to the reduction of NiO to

Ni0 which is lower than the reduction at 490 uC in pure NiO.29 It

is obvious that the reduction peak of NiO is shifted to a lower

temperature when it is deposited on the high surface area

CeO2–ZrO2 support; due to appropriate interaction between the

metal and support. This interaction is an electronic interaction

between CeO2 and Ni because Ce is rich in d-electrons and Ni

has unfilled d-orbitals, where the unfilled d-orbitals of the Ni atoms

accept d-electrons from Ce, resulting in an increase in d-electron

density of the Ni atom.30 The second peak at 430 uC corresponds to

the reduction of surface CeO2 in the CeO2–ZrO2 support.

With an increase in the metal loading from 2% to 10%, the

intensity of the peaks at 370 uC and 430 uC increased and shifted

towards higher temperatures (460 uC and 660 uC, respectively),

which is significantly due to increasing NiO content. Also

another small peak at about 360 uC was observed, which might

be attributed to the reduction of free NiO particles 31 or the

hydrogen spillover effect.32 Further increase in metal loading to

20 wt%, resulted in the peak at 460 uC to be shifted towards

higher temperatures and merged with the second peak, which is

attributed to agglomerated NiO.25

3.2. Catalytic Activity

The effect of reaction temperature (200–600 uC) and metal

loading on ESR and product selectivity over the CeO2–ZrO2

support and x%Ni/CeO2–ZrO2 catalyst (x = 2, 10, 20) are

studied. The catalytic products were H2, CO, CO2 and CH4. The

presence of these products indicates that steam reforming (SR)

and the water gas shift (WGS) reaction took place simulta-

neously in the reactor.33 Minor amounts of by-products, such as

CH3CHO, C2H4, C2H6 and CH3COCH3, were also observed;

their concentration exhibiting a strong dependence on reaction

temperature. As expected, ethanol conversion increased with

increasing temperature and with an increase in metal loading.

Nearly complete conversion was obtained with nickel loaded

catalysts at 400 uC. The high activity and variety of products is

mainly attributed to the complex composition and multi-reactive

sites, such as metal Ni, acidic/basic centers, and oxygen vacancies

in the CeO2–ZrO2 solid solution.

3.2.1. Homogeneous contribution. Reforming of ethanol under

the same reaction conditions, but in the absence of catalyst, was

studied to completely understand the role of the catalyst in the

ESR reaction. The blank experiment shows that only 8.8–17.5%

of ethanol was converted at temperatures in the range 400–600

uC (Table 2), in the absence of catalyst, which is probably due to

ethanol thermal decomposition. The distribution of products

cannot be attributed to the ESR reaction, where water is being

produced and not consumed. The production of water can be

explained by ethanol dehydration yielding ethylene, which is a

coke precursor.

On the basis of the obtained products, after we systemically

investigated the variation of gaseous products, it was found that

the amount of H2 and CO increased while CH4 and CO2

decreased as the reaction temperature rises. The following

reactions may describe the changes in gaseous products with

temperatures in our reaction system.

CH3CH2OH A CO + H2 + CH4 (8)

CH4 A C + 2H2 (9)

2CO A C + CO2 (10)

The first reaction (eqn (8)) is the most endothermic one and it

is thermodynamically favorable,34 so the high temperature

favors the production of H2 and CO. Therefore, the higher the

temperature, the larger the amount of H2 and CO obtained. The

reaction in eqn (9) is also a thermodynamically favorable

reaction, thus the synthesized CH4 in eqn (8) will partially

decomposed to produce carbon and hydrogen. This is the reason

that the amount of CH4 decreased when the reaction tempera-

ture increased. The most exothermic reaction is the Boudouard

reaction (eqn (10)), which is favored at low temperatures, and

leads to a decrease in the amount of CO2 when increasing the

reaction temperature.

Table 1 Textural properties for the prepared Ni/CeO2–ZrO2 catalyst

CatalystAcidity(mmol g21)

Crystal size(CeO2–ZrO2)a

(nm)

Crystalsize (NiO)a

(nm)

Unit cellparameter(CeO2–ZrO2) (nm)

BETsurface areab

(m2 g21)

Porevolumec

(cm3 g21)Poreradiusc (A)

CeO2–ZrO2 740.8 10.4 – 0.542 133.5 0.112 18.22%NiO/CeO2–ZrO2 574.1 11.5 – 0.538 82.9 0.155 40.310%NiO/CeO2–ZrO2 160.1 10.4 24.1 0.541 67.8 0.128 45.920%NiO/CeO2–ZrO2 63.3 9.4 26.1 0.542 41.9 0.1 31.9a According to XRD analysis using Scherrer equation. b BET surface area calculated from the linear portion of the BET plot in the relativepressure range of P/Po = 0.05–0.35. c Pore radius and total pore volume estimated using BJH method from the isothermal desorption data.

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 8145–8156 | 8149

3.2.2. CeO2–ZrO2 support. During catalytic ESR, ethanol

decomposes mainly via two different routes:35 either by

dehydration forming ethylene, or by dehydrogenation forming

acetaldehyde. These two intermediate products further catalyti-

cally decomposed and are steam reformed to an equilibrated

mixture of methane, carbon dioxide, carbon monoxide, hydro-

gen and water (Fig. 6).

When CeO2–ZrO2 was used as a catalyst for ESR, the

products distinguished were mainly H2, CO, CH4, C2H6, C3H8,

C2H4, CH3CHO, and CH3COCH3, as shown in Table 3. At low

temperatures (200–400 uC), ethylene, acetaldehyde and acetone

were produced from side reactions, such as ethanol dehydro-

genation and dehydration, and acetaldehyde decomposition. A

higher selectivity toward ethylene reached 61%, resulting from

Fig. 3 HRTEM and STEM/EDS photographs obtained for the fresh CeO2–ZrO2 and x%Ni/CeO2–ZrO2.

8150 | RSC Adv., 2012, 2, 8145–8156 This journal is � The Royal Society of Chemistry 2012

ethanol dehydration according to the reaction in eqn (11).

Generally, ethanol dehydration to ethylene is mainly promoted

by acidic sites. Carbon deposition in the form of carbon

nanotubes (CNTs) (as shown in HRTEM image (Fig. 10)),

results when large amounts of ethylene are produced. This was

ascribed to the high acidity (741 mmole g21) of CeO2–ZrO2

catalysts as shown in Table 1, ethylene (C2H4) was ascribed as a

severe carbon precursor resulting in rapid coke formation during

the reforming process (eqn (12)).

C2H5OH A C2H4 + H2O (11)

C2H4 A 2H2 + 2C (12)

The formation of acetone (at 300 uC) proceeds through the

dehydrogenation of ethanol to acetaldehyde, the aldol

condensation to 3-hydroxybutanal, its oxidation to 3-hydro-

xybutyrate using the oxygen provided by the catalyst, and

finally, the thermal decarboxylation and dehydrogenation of

this product (generally by reaction shown in eqn (13)).28 C2H4

and CH3COCH3 are highly undesired products in the ESR

reaction since they are precursors of coke, leading to catalyst

deactivation.

2C2H5OH + H2O A CH3COCH3 + CO2 + 4H2 (13)

On the other hand, ethanol could be converted to acetalde-

hyde (eqn (14)),36 which is sequentially decomposed and

converted into syngas (CO and H2) through dehydrogenation

as the temperature is raised to 600 uC. The formation of

hydrogen is based on the reactions of ESR and water-gas shift

(eqn (15)).

C2H5OH A H2 + CH3CHO (14)

CO + H2O A CO2 + H2 (15)

Another important product of ESR over the CeO2–ZrO2

catalyst is C3H8, the reactivity of the surface towards higher

hydrocarbons was related to the tendency to form multiple

bonds between one carbon atom and the surface.37 CH4 was

adsorbed on the surface, forming an intermediate surface

hydrocarbon species with the lattice oxygen (OOx) at the CeO2

surface, as illustrated schematically below. Using the Kroger–

Vink notation, Vo?? denotes as an oxygen vacancy with an

effective charge 2*, and e2 is an electron which either more or

less localized on a cerium ion or delocalized in a conduction

band.38

CH4 adsorption

CH4 + 2* A CH3* + H* (16)

CH3* + * A CH2* + H* (17)

CH2* + * A CH* + H* (18)

CH* + *A C* + H* (19)

Fig. 4 N2 adsorption–desorption isotherms for CeO2–ZrO2 and x%Ni/CeO2–ZrO2 catalysts.

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 8145–8156 | 8151

Co-reactant (H2O and CO2) adsorption

H2O + 2* « OH* + H* (20)

OH* + * « H* + O* (21)

CO2 + 2* « CO* + O* (22)

Redox reactions of lattice oxygen (Oox) with C* and O*

C* + OO* A CO* + Vo?? + 2e2 (23)

Vo?? + 2e2 + O* « Oo

x + * (24)

Inhibitory effects of CO and H2

H2 + 2* « H* + H* (25)

CO + * « CO* (26)

CO* + Oo* « CO2* + Vo?? + 2e2 (27)

As the temperature increased from 400 to 600 uC, the increase

in selectivity of methane, carbon monoxide and carbon dioxide

keeps increasing the selectivity of hydrogen, which indicates that

the predominant reactions are the reforming reaction (eqn (7)) of

ethanol and water.36 On the other hand, at high temperatures,

ethane and propane undergo decomposition,39 or SR 40 to

produce hydrogen, methane, ethylene, and carbon (eqn (28)–

(34)):

C2H6 A C2H4 + H2 (28)

C2H4 A 2H2 + 2C (29)

C2H6 + 2H2O A 5H2 + 2CO (30)

C2H6 + H2 A 2CH4 (31)

C3H8 A 3/2C2H4 + H2 (32)

C3H8 + 3H2O A 3CO + 7H2 (33)

C3H8 A 2C + CH4 + 2H2 (dehydrogenation) (34)

C + 2H2 A CH4

3.2.3. Metal loading. For all x%Ni/CeO2–ZrO2 catalysts, the

main reaction products were H2, CO, CO2 and CH4. With

increasing temperature, the selectivity to H2 and CO2 increased,

where the selectivity to CH4 passed through a maxima and the

selectivity to CO passed through a minima. On the other hand,

selectivity to ethylene, acetaldehyde and acetone formation was

negligible (Table 3).

At 200 uC; only the 10 and 20 wt% catalyst gave H2. These

unexpected results clearly show evidence the there are higher

amounts of reduced nickel available for reaction, as illustrated in

the TPR analysis, and is the main factor in ESR for hydrogen

production. On the other hand, Hernandez et al.,41 suggested

that nickel is the phase which is mainly responsible for hydrogen

production, although the ZrO2 support reduces CO formation.

This data is a totally different picture compared to the results

obtained in earlier studies on ESR, which stated that tempera-

tures higher than 230 uC are necessary to carry out the ESR

reaction. In a similar study, Duhamel, et al.42 investigated

Table 2 Product distribution from ESR of ethanol in the absence of catalyst

Reaction temperature (uC) Ethanol conversion (wt%)

Selectivity (%)

H2 CH4 CO CO2 Ethane Ethylene

400 8.8 0.00 42.59 2.51 12.64 35.56 6.70500 10.6 7.18 25.39 17.71 3.14 17.54 29.04600 17.5 10.78 29.21 32.13 0.00 24.84 3.04

Fig. 5 TPR patterns of Ni/CeO2–ZrO2 catalysts with different Ni

loading.

Fig. 6 Ethanol reaction pathways.

8152 | RSC Adv., 2012, 2, 8145–8156 This journal is � The Royal Society of Chemistry 2012

hydrogen production from ethanol in the presence of water

(H2O/C2H5OH = 3) over cerium–nickel CeNixOy (0, x ¡ 5)

mixed oxide catalysts. The influence of different reaction

temperatures showed that a stable activity, i.e., ethanol conver-

sion and H2 selectivity, was obtained at a very low temperature

(200 uC) when the solid is previously in situ treated in H2, in a

temperature range between 200 and 300 uC. After such a

treatment, the solids studied are hydrogen reservoirs, called

oxyhydrides, with the presence of a hydrogen species of hydride

nature in the anionic vacancies of the solid.42 David et al.43

indicated that reduction experiments under SR conditions, in the

presence of both H2 and hydrocarbons, have shown an increased

reducibility of nickel ions in the presence of hydrocarbons and

alcohol.

On the other hand, from the analysis of product distribution

obtained at 200 uC, it can be observed that methane selectivity is

equal to CO selectivity, suggesting that ethanol decomposition to

CO, CH4 and H2 (eqn (8)) is the main reaction at 200 uC, while

the lower selectivity to CO2 could be produced either by the

WGS reaction (eqn (15)) or by ESR (eqn (7)). As the temperature

increases from 200 to 300 uC, CH4 and CO2 selectivity increases

while CO selectivity decreases. This means that at 300 uC the

ESR (eqn (7)) instead ethanol decomposition (eqn (8)) becomes

significant and low quantities of CO are produced by the

reaction in eqn (35).

CH4 + H2O « CO + 3H2 (35)

The selectivity to hydrogen obtained for the 10 and 20 wt%

catalysts was comparable, as shown in Fig. 7, from this figure we

can also observe that at lower reaction temperatures (, 400 uC)

the 10%Ni/CeO2–ZrO2 is the most selective catalyst toward

hydrogen production (H2 selectivity is 37% at 200 uC). Whereas,

at higher reaction temperatures (> 400 uC), with an increase in

metal loading, the selectivity of hydrogen substantially

decreased. 2%Ni/CeO2–ZrO2 is the most selective catalyst

towards hydrogen production (H2 selectivity is 75% at 600 uC),

because it has a high specific surface area and large pore volumes

(Table 1). Also, the small particle size of nickel in 2% Ni loading

catalyst results in enhanced metal–support interactions, which

accounts for the high activity of the catalyst.44

The presence of strong metal–support interactions (SMSIs)

would prevent the nickel metal from sintering at moderate

temperatures, thus increasing the dispersion of nickel and the

length of the interfacial boundaries between Ni and the support.

The presence of nickel with a small particle size guarantees high

activity of the catalyst by its high J activity in C–H and C–C

bond cleavages, while the abundant interfacial boundary

between nickel and support facilitates oxidization of the

carbon-containing intermediates to COx or other oxygenates

by the active oxygen species on the surface of the support.44 This

has also been demonstrated in our study. Whereas, the XRD

pattern of the 2%Ni/CeO2–ZrO2 catalyst did not show a

significant nickel peak; which encourages the strong metal–

support interaction. HRTEM shows high nickel dispersion

(Fig. 3). This was also established by the TPR data, where the

reduction property of the 2%Ni/CeO2–ZrO2 catalyst seems to

cause the peak corresponding to CeO2–ZrO2 to shift downward,

and thus, overlaps the NiO reduction peak at about 460 uC. This

indicates that a strong interaction between Ni and CeO2–ZrO2 or

Ni incorporation into CeO2–ZrO2 makes ceria more reducible,

which helps to produce mobile oxygen during the reforming

reaction. The participation of the lattice oxygen would be

subsequently supplemented with the oxygen from the water

molecules and lead to high activity in SR reaction.45 As reported

by Arias et al.,46 for ceria’s ability to store and release oxygen is a

crucial feature in its superior performance. Because oxygen

desorption from ceria occurs at very high temperatures,47

oxygen-storage requires contact between ceria and a precious

metal, with the reduction of ceria occurring through reaction

with a reductant on the precious metal.48

For all catalysts, when the temperature is increased from 500 to

600 uC, methane selectivity decreases, carbon dioxide selectivity

remains constant while hydrogen and carbon monoxide selectivity

Table 3 Product distribution from ESR

— Reaction temperature (uC) Converted ethanol (%)

Selectivity (%)

H2 CH4 CO CO2 C2H6 C2H4 C3H8 CH3CHO CH3COCH3

CeO2–ZrO2 200 63 0.00 29.70 0.00 35.23 12.48 0.00 6.48 0.00 0.00300 70 6.35 0.94 0.00 31.32 2.69 38.68 0.94 2.63 12.29400 92 5.08 3.63 0.88 13.85 3.22 61.44 4.17 7.17 0.37500 98 19.90 4.29 1.30 11.81 2.57 47.41 3.21 6.30 3.05600 100 25.88 23.28 8.31 22.12 1.73 17.91 0.20 0.30 0.10

2%Ni/CeO2–ZrO2 200 82 0.00 36.94 0.00 4.01 28.19 0.66 16.22 10.47 0.00300 96 50.72 14.96 17.23 14.17 0.05 0.13 0.03 1.74 0.84400 100 54.80 16.87 1.54 26.76 0.00 0.00 0.00 0.02 0.02500 100 72.49 4.05 4.36 19.10 0.00 0.00 0.00 0.01 0.01600 100 75.34 0.39 8.00 16.27 0.00 0.00 0.00 0.00 0.00

10%Ni/CeO2–ZrO2 200 98 37.56 34.80 15.91 1.29 0.75 0.09 0.30 8.11 0.00300 98 40.61 31.90 0.26 25.60 0.73 0.01 0.36 0.30 0.00400 100 56.45 16.60 1.08 25.87 0.00 0.00 0.00 0.01 0.00500 100 70.21 3.16 3.27 23.35 0.00 0.00 0.00 0.00 0.00600 100 70.32 0.64 5.28 23.76 0.00 0.00 0.00 0.00 0.00

20%Ni/CeO2–ZrO2 200 96 36.88 25.33 29.01 1.11 0.05 0.05 0.22 7.13 0.00300 96 40.11 22.54 25.15 11.97 0.02 0.00 0.01 0.14 0.00400 100 54.13 22.35 2.06 21.36 0.09 0.00 0.00 0.01 0.00500 100 67.35 6.72 5.98 19.87 0.05 0.00 0.01 0.00 0.00600 100 71.74 3.13 8.80 16.29 0.02 0.00 0.00 0.00 0.00

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 8145–8156 | 8153

increase. CO produced from the reverse water gas shift reaction

(WGSR) was expected (eqn (36)) to increase with increases in

temperature.49 The decrease in CO2 and CH4 yields with increased

temperature was ascribed to the RWGS, SR of CH4 (eqn (35))

and/or dry reforming, resulting in increase in the selectivity to H2

and CO2.50

CO2 + H2 A CO + H2O (36)

CH4 + CO2 A 2CO + 2H2 (37)

On the basis of intermediate distribution at lower and higher

reaction temperatures (Table 3), we can conclude that ethanol

dehydrogenation to acetaldehyde (eqn (14)) and ethanol decom-

position to methane and carbon monoxide (eqn (8)) were the

primary reactions at low temperatures. At higher temperatures,

where all the ethanol and the intermediate compounds, like

acetaldehyde and acetone (eqn (38) and (39)), were completely

converted into hydrogen, carbon oxides and methane, SR of

methane and WGS became the major reactions, which is in

agreement with Zhang et al.51

CH3CHO A CH4 + CO (38)

CH3CHO + H2O A 2CO + 3H2 (39)

A bi-functional mechanism for ESR was proposed by

Aupretre et al.,52 as shown in Fig. 8. According to this

mechanism, ethanol may be activated on the metal particle or

on the support acidic and basic sites to produce the intermediate

products (step 1). Acetaldehyde is produced on both the support

(basic sites) and the metal. Ethylene is only produced on the

support, with an essential role of acidic sites isn olefin formation.

The water molecules are adsorbed on the support in the form of

mobile hydroxyl groups (step 2). The support would promote the

migration of these OH groups towards the metal particle (step 3).

As a consequence, hydrophilic supports, with high oxygen

mobility, are expected to act as promoters in SR reactions.53

Furthermore, the mobile oxygen species supplied by the support

may participate in SR reactions and prevent the formation of

coke on the metal particles.54 Ethanol dehydration to ethylene is

mainly promoted by the acidic sites. Coke deposition results in

deactivation of the catalysts when large amounts of ethylene are

produced. On the other hand, ethanol could be converted to

acetaldehyde, which is sequentially decomposed and converted

into syngas through dehydrogenation. Two different mechan-

isms have been suggested for the dehydrogenation of alcohol: (i)

the ‘‘carbonyl’’ mechanism, in which the hydrogen from the

hydroxyl group and from the a-carbon atom are split, either

simultaneously or in two steps; (ii) the ‘‘enolic’’ mechanism,

which involves splitting of the b-hydrogen.55 While it is well

documented that splitting of the a-hydrogen with formation of

surface alkoxide is the rate determining step on oxide catalysts,55

little information is available on ethanol dehydrogenation on

metal catalysts.

3.3. Characterization of used catalyst

To gain some insight into the state of the catalyst after the ESR

reaction, the XRD and HRTEM analysis of CeO2–ZrO2 and

10%Ni/CeO2–ZrO2 catalysts after 8 h of ESR reaction were

carried out. It should be mentioned that the XRD and HRTEM

of the fresh catalyst were taken after calcination, whereas for the

used catalysts these analyses were done after in situ reduction

followed by the reforming reaction. The CeO2–ZrO2 patterns

(Fig. 9) reveal the presence of carbon nanotubes, as evidenced by

reflections [(100) and (004)] at 43u and 53u, respectively [ref.

JCPDS card (00-058-1638)]. Other reflections (002), at 26.2u is

associated with carbon nanotube structures that are not clearly

Fig. 7 Variation of H2 selectivity with temperature over x%Ni/CeO2–

ZrO2 catalysts.

Fig. 8 Mechanism of ethanol steam reforming.52

Fig. 9 XRD pattern of spent catalyst.

8154 | RSC Adv., 2012, 2, 8145–8156 This journal is � The Royal Society of Chemistry 2012

evidenced because they overlap with the reflection assigned to

the CeO2–ZrO2 solid solution. The absence of low angle

reflections indicates the formation of thick MWNTs.56 This

matter will be pursued by HRTEM examination (Fig. 10). The

HRTEM micrograph of this sample reveals that the deposited

carbon appears either as large filaments with catalyst particles,

or as a rather uniform carbon coating covering the catalyst

particles.

From the XRD of the 10%Ni/CeO2–ZrO2 catalyst (Fig. 9), we

observed that all peaks corresponding to NiO completely

disappeared and other reflections at 44.5u and 51.8u appeared,

which were assigned to planes (111) and (200) respectively, of

metallic Ni. Whereas the peaks corresponding to the CeO2–ZrO2

mixed oxide were almost unaffected. Although, no separate

carbon nanotube phases were discernible by XRD of the used

10%Ni/CeO2–ZrO2 catalyst, the HRTEM image (Fig. 10)

identified CNTs deposited within the catalyst matrix. The

CNT reflections overlap with other reflections assigned to Ni-

containing species also present in these samples. The center of

the filaments in Fig. 10 seem hollow, which suggests that they are

multiwalled nanotubes. Deactivation likely occurs because the

active Ni nanoparticles become increasingly encapsulated by

graphitic carbon.57 Suelves et al.56 suggested that the catalyst

becomes deactivated when deposited carbon prevents reactant

diffusion toward the Ni surface, and diffusion is prevented in

higher extents by carbon-coating than by filaments.

4. Conclusion

The catalytic performance of a nickel supported ceria-zirconia

catalyst on the ESR reaction was investigated at different

temperatures and different nickel loadings to understand the role

of the active metal and support oxides in the reaction pathways.

The thermal decomposition of ethanol in the absence of catalyst

produced low amounts of CO2, which reached zero at 600 uC,

indicating the absence of the SR reaction. For all catalysts, the

major products formed include H2, CH4, CO and CO2 at higher

temperatures (400–600 uC) and intermediate compounds

CH3CHO, C2H4, C2H6 and CH3COCH3 appear at lower

temperatures (200–300 uC). Compared to nickel support

catalysts, the ceria-zirconia support shows a significant amount

of activity, and reaches 61% of ethylene, this is due to the high

acidity of the support (740.8 mmole g21). The main reaction

route of dehydrogenation of ethanol, followed by decomposition

of acetaldehyde, and subsequently the competition of SRM and

WGSR, were proposed for ESR. Surprisingly, while the 10%Ni/

CeO2–ZrO2 and 20%Ni/CeO2–ZrO2 catalysts showed a con-

siderable amount of H2 at a very low temperature (200 uC), the

2%Ni/CeO2–ZrO2 catalyst showed high selectivity to H2 at

600 uC. XRD and HRTEM images indicated that the interaction

between the nickel metal and the support is greatly strengthened

by the small nickel particle size in the 2%Ni/CeO2–ZrO2 catalyst,

which accounts for its high selectivity. Characterization of the

spent catalyst shows that higher amounts of carbon deposition

on the CeO2–ZrO2 support was observed, as shown by HRTEM.

The deposited carbon appears as either large filaments with

catalyst particles, or as a rather uniform carbon-coating that

covers the catalyst particles.

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