ww.sciencedirect.com

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 3

Available online at w

journal homepage: www.elsevier .com/locate/he

Production of syngas from steam reforming andCO removal with water gas shift reaction overnanosized Zr0.95Ru0.05O2Ld solid solution

Vijay M. Shinde, Giridhar Madras*

Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 012, India

a r t i c l e i n f o

Article history:

Received 29 May 2013

Received in revised form

8 August 2013

Accepted 15 August 2013

Available online 14 September 2013

Keywords:

Steam reforming

Hydrogen production

CO removal

Water gas shift

Tetragonal ZrO2

* Corresponding author. Tel.: þ91 80 2293232E-mail addresses: giridhar@chemeng.iisc

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.08.0

a b s t r a c t

This study presents the synthesis, characterization, and kinetics of steam reforming of

methane and water gas shift (WGS) reactions over highly active and coke resistant

Zr0.95Ru0.05O2�d. The catalyst showed high activity at low temperatures for both the re-

actions. For WGS reaction, 99% conversion of CO with 100% H2 selectivity was observed

below 290 �C. The detailed kinetic studies including influence of gas phase product species,

effect of temperature and catalyst loading on the reaction rates have been investigated. For

the reforming reaction, the rate of reaction is first order in CH4 concentration and inde-

pendent of CO and H2O concentration. This indicates that the adsorptive dissociation of

CH4 is the rate determining step. The catalyst also showed excellent coke resistance even

under a stoichiometric steam/carbon ratio. A lack of CO methanation activity is an

important finding of present study and this is attributed to the ionic nature of Ru species.

The associative mechanism involving the surface formate as an intermediate was used to

correlate experimental data.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction whiskers even at very low S/C ratios [4]. Among these noble

There is renewed interest in hydrogen production due to ad-

vances in fuel cell technology, which is an efficient way of

extracting energy fromhydrogen [1,2]. The steam reforming of

natural gas is the most economical and efficient catalytic

process for the production of hydrogen. Industrially, this re-

action is catalyzed over nickel supported Al2O3 catalyst

around 850 �C with steam to carbon (S/C) molar ratio in the

range of 2e5 [3,4]. The high temperature and low S/C ratio

(<1.4) result in sintering and deactivation of catalyst mainly

due to coke deposition via cracking of methane [5]. However,

noble metals (Ru, Rh, Pd, Ir and Pt) over various supports are

highly active and more resistant to the formation of carbon

1; fax: þ91 80 23601310..ernet.in, giridharmadras2013, Hydrogen Energy P70

metals, Ru and Rh have been reported to be the most active

catalysts and the propensity of deposited carbon decreases in

the order of Ni [ Rh > Ir w Ru w Pd at 500 �C [6e8]. Further,

Ru is cheaper that Rh and, therefore, it is desirable to explore

the activity of Ru based catalysts for steam reforming

reaction.

The reformer stream contains 8e15 vol% of CO depending

on the operating conditions and this stream cannot be directly

sent to the polymer-electrolyte fuel cell (PEFC). At high CO

concentration, the adsorption of CO on the noble electrode

takes place irreversibly and the performance of the fuel cell is

affected by this phenomenon. Therefore, CO needs to be

removed below 20 ppm in order tomake it suitable for fuel cell

@gmail.com (G. Madras).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 313962

application. Thus, the water gas shift (WGS) is an integral

component of fuel processing for fuel cell applications, which

not only reduces the CO content but also enriches the

hydrogen content. The WGS reaction is slightly exothermic.

Therefore, the reaction is equilibrium limited at high tem-

peratures and kinetically limited at low temperatures.

Therefore, the WGS reaction is usually carried out in two

stages: high temperature shift (350e500 �C) with Fe2O3/Cr2O3

catalyst and low temperature shift (200e250 �C) with Cu/ZnO/

Al2O3 catalyst [9]. However, these catalysts are not suitable for

fuel cell applications because of their pyrophoricity, sensi-

tivity to start-up/shut-down cycles, lengthy and cumbersome

activation procedure [10,11].

On the other hand, noble metal based catalysts, mainly Pt

supported on ceria or ceria based mixed oxides have exten-

sively been studied as promising low temperature shift cata-

lysts [12,13]. Au supported on reducible oxides such as CeO2,

TiO2, Fe2O3 have shown remarkable activity for WGS reaction

[14]. However, there is uncertainty about the feasibility of

these catalysts for practical applications [15]. Further, Pt

supported ceria catalysts are vulnerable to deactivation with

time on stream mainly due to over reduction of support, sin-

tering, loss of surface area and the formation of stable surface

carbonate [16,17 and references therein].

ZrO2 offers few advantages over the ceria support due to its

high thermal stability and unique acidic and basic properties

[18]. Previously, we have studied WGS reaction over combus-

tion synthesized zirconia and Pt substituted ZrO2was found to

be highly active [19]. In another study, excellent activity and

stability was observed over Pt/ZrO2 catalyst due to the lack of

Pt sintering [20]. The polymorphic structure dependent ac-

tivity has also been studied for WGS reaction and the tetrag-

onal phase of ZrO2 was reported to be more active phase than

the monoclinic phase [21]. The difference between activities

was attributed to the high reactivity of the hydroxyl groups

over tetragonal phase [22]. Further, Ru supported catalysts are

also active forWGS reaction. However, very little attention has

been paid to examine the activity of the Ru based catalysts due

to the drawback of CO methanation under WGS conditions

[23]. Previously, we have shown that the Ru based catalysts, if

developed properly, can suppress the CO Methanation reac-

tion [24]. Therefore, it is very interesting to investigate the

WGS reaction over Ru supported tetragonal ZrO2.

Coke deposition, sintering and agglomeration of the active

metal are the main causes of catalyst deactivation in steam

reforming reaction [25]. Therefore, an intimate contact and

strong metal support interaction between metal and support

is necessary for the development of stable catalyst [5]. For

example, the high stability of Rh based catalyst depends on

the interaction between Rh and oxygen of the support, which

anchors the Rh particle on the support and inhibits noble

metal sintering [25]. The importance of strong metal support

interaction between the noble metal (Au or Pt) and the oxygen

of ceria has also been demonstrated for WGS reaction and it

has been claimed that the ionically dispersed Au or Pt are the

most active species for WGS reaction [26e29]. Further, the

activity of the catalyst depends on the dispersion of active

phase. The fine dispersion not only increases unsaturation of

the surface metal atoms but also provides more steps and

kinks sites on the surface. It has been reported that the energy

barrier for CH4 dissociation, which is considered to be a rate

determining step decreases with increasing coordinative

unsaturation of the surface metal atoms. It means that the

rate of reaction depends on the extent of metal dispersion and

interaction with the support [30]. Therefore, high dispersion

and ionic nature of active phase are essential for high activity

and stability.

The ionic substitution of metals in the lattice of metal ox-

ides offers advantages such as complete metal dispersion

within an oxide support, high oxygen storage and release, and

redox exchange between substituted metal and metal oxide

[31]. Further, ionic substitution of metal also creates oxide ion

vacancies and activation of molecules such as CH4, CO and

H2O over oxide vacancies have been demonstrated [30]. The

high rate of reaction over ionic catalysts compared to the

same metal impregnated oxide for various reactions like CO

oxidation, NOx reduction andWGS reaction has been reported

in literature [32,33]. Therefore, ionic substitution of metal in

support could be another alternative to maximize the metal

support interactions.

This study reports the synthesis and characterization of

nanosized Zr0.95Ru0.05O2�d solid solution by low temperature

sonication method using diethylenetriamine (DETA) as a

complexing agent. The performance of the solid solution was

investigated for both the steam reforming andWGS reactions.

The detailed kinetic studies including influence of gas phase

product species, effect of temperature and catalyst loading on

the reaction rates have been investigated. The catalyst sta-

bility and resistance towards coke deposition were also

investigated for steam reforming of CH4.

2. Experimental

2.1. Synthesis and characterization

Nanostructured Zr0.95Ru0.05O2�d solid solution was synthe-

sized using a sonochemical method at room temperature.

Reagent grade zirconyl nitrate [ZrO(NO3)2, Thomas Baker,

India], ruthenium chloride [RuCl3, Spectrochem, India],

diethylenetriamine (DETA) [(C4H13N3) S.D Fine, India] were

used without further purification. In a typical experiment, 5 g

of ZrO(NO3)2 and 0.5 g of RuCl3 were dissolved in 100 ml of

deionized water. 5 ml of DETA was then added to the solution

and the solution turned to gel immediately. The resulting gel

was irradiated with high intensity ultrasound radiation (Ti

horn, 20 kHz, 125 W/cm2 at 60% efficiency) for 4 h. The tem-

perature during the sonication increased to 60 �C. After soni-cation, the product was centrifuged, washed repeatedly with

water-ethanol mixture, and then dried in hot air oven at

120 �C for 3 h. The solid product was further calcined at 450 �Cfor 30 min to remove (if any) the residual of chloride/nitrates

and the dry powder product is termed as “as-prepared com-

pound”. 5 at% Ru impregnated on ZrO2 was also prepared for

comparison. RuCl3 corresponding to 5 at% was added to an

aqueous dispersion of ZrO2 (synthesized by sonication

method) and reduced by hydrazine hydrate (99%, S.D Fine,

India). The product was then washed with water-ethanol

mixture, filtered, and dried at 110 �C for 5 h. Finally, the

product was heated at 450 �C before the reaction.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 3 13963

The crystal structure of the synthesized compound was

studied by X-ray diffraction (XRD) on Philips X’Pert Diffrac-

tometer operated at 40 kV and 30 mA with Cu Ka radiation

(l ¼ 1.54056 �A) in the 2q range of 20e80� at 0.02� min�1. Theprofile fitting, lattice parameter refinement and microstruc-

ture analysiswere performed on pure tetragonal phase of ZrO2

using JANA 2000 program suite. The size and morphology of

the synthesized compound were observed by transmission

electron microscopy (TEM) on a FEI Technai 20 operating at

200 kV. The powdered sample was ultrasonically dispersed in

ethanol and dropped onto the carbon coated Cu grid (300

mesh). Energy dispersive X-ray (EDX) analysis was also carried

out on the same instrument with a fine probe. Raman spectra

were recorded on a NXR-FT Raman module (Thermo Scienti-

fic, USA) with the spectrum resolution of 4 cm�1 equipped

with Ge detector and Nd: YVO4 laser. The spectra of the

sample before and after the reaction were recorded at room

temperature. X-ray photoemission spectra (XPS) were recor-

ded in a Thermo Scientific Multilab 2000 equipped with Al Ka

radiations source (1486.8 eV). All spectra were charge cor-

rected with the C (1s) peak observed at 285 eV and they are

accurate within �0.1 eV. BET surface area of the sample was

measured with a Quantachrome NOVA 1000 gas adsorption

analyzer. Before the measurement, the sample was degassed

at 150 �C for 5 h under vacuum.

2.2. Catalytic activity measurements

Catalytic tests were performed in a fixed bed continuous flow

reactor in a packed bed quartz reactor tube (30 cm in length

and 0.4 cm of ID) under atmospheric pressure using 100 mg of

catalyst (80e100 mesh) diluted with an inert glass beads. The

reactor is fixed in a tubular electrical furnace and the tem-

perature of catalyst bed was measured by a K type thermo-

couple inserted in the middle of packed bed. Prior to the

activity test, the catalyst was reduced at 650 �C for 2 h under a

mixture of 5% H2 in N2 at a flow rate of 50 ml/min. The steam

reforming reaction was carried out with the gas mixture

consisting of 3 vol% of CH4 and the balance wasmade up of N2

keeping the total flow rate to 100 ml/min. All experiments

were performed at the gas hourly space velocity of 80,000 h�1

(on dry basis). Water was pumped at a constant flow rate of

0.08 ml/min to the evaporator using HPLC pump (Waters 410)

and the generated steam (3 ml/min) was mixed with the re-

action mixture. A stream trap was placed to condense the

surplus steam at the downstream of the reactor. The dry

product gas mixture was analyzed by a gas chromatograph

equipped with a FID (incorporating a methanator) and TCD

detectors. Hayesep-A columnwas used to separatemixture of

CH4, CO and CO2 and amolecular sieve 5A columnwas used to

separates mixture of H2, and N2. The reforming reaction was

carried out isothermally at several temperatures. Methane

conversion and CO selectivity in steam reforming were

calculated

CH4 conversion ð%Þ ¼�CCO þ CCO2

��CCO þ CCH4 þ CCO2

�� 100 (1)

CO selectivity ð%Þ ¼ CCO�CCO þ CCO2

�� 100 (2)

C is the concentration of each component in the effluent

stream. The rate of formation of (CO þ CO2) was almost equal

to the rate of disappearance of CH4 indicating the rate of

carbon deposition was negligible.

The catalytic activity of Zr0.95Ru0.05O2�d for WGS reaction

wasmeasured at atmospheric pressure in terms of percentage

of CO conversion. The WGS reaction was carried in the same

reactor employing 2 vol% CO balanced of N2 with a total gas

flow of 100 ml/min over 250 mg catalyst in the range of

140e500 �C. The gas hourly space velocity of 60,000 h�1 (on dry

basis) was kept constant for all experiments. The influence of

H2O/CO ratio on CO conversion was also studied and it was

found that at a given temperature, CO conversion was

increased with H2O concentration. However, at higher H2O

concentration (>flow rate 5 ml/min), the conversions

remained independent of H2O concentration. Therefore, the

flow rate of water vapor was maintained at 5 ml/min. Further

details about the experimental setup can be found elsewhere

[34].

3. Results and discussion

3.1. Structural studies

XRD patterns of the as-synthesized compound are shown in

Fig. 1. All the peaks were indexed to tetragonal structure of

ZrO2 (space group: P42/nmc) and no diffraction peak corre-

sponding to Zr-DETA complex or Ru-DETA complex or Ru

metal or RuO2 were observed. The y scale of the XRD patterns

was also magnified 10 times in the range of 30e50� and no

peaks either due to Ru or RuO2 were found. This implies that

the substitution of Ru metal in the ZrO2 forming a single solid

solution can be represented by the formula Zr0.95Ru0.05O2�d.The changes in the lattice parameter on the substitution of Ru

metal in ZrO2 were obtained by profile refining using JANA

2000 suite program. The Pseudo-Voigt and Legendre poly-

nomial (no of terms ¼ 15) were used as peak width and

background function, respectively. The observed, calculated

and difference XRD patterns of the compound are shown in

Fig. 1. The satisfactory values of the reliability parameters Rp

and Rwp were obtained by fitting the XRD patterns to the

tetragonal phase while poor correlation was obtained by

fitting the XRD patterns to the monoclinic phase. The lattice

parameter, crystallite size and the reliability parameters are

summarized in Table 1. A slight decrease in the lattice

parameter of Zr0.95Ru0.05O2�d compared to unsubstituted ZrO2

also confirms ionic substitution. The broad X-ray line width is

indicative of nanometer sized crystallites and the average

crystallite size was determined to be 8 nm using the Scherrer

equation with taking into account the full width at half-

maximum (FWHM) of the most intense peak.

The XRD pattern of Zr0.95Ru0.05O2�d was also recorded after

3 cycles of WGS reaction and profile refined XRD patterns is

shown in Fig. 1(b). The XRD patterns are identical to the as-

synthesized compound and could be indexed to the parent

tetragonal structure of ZrO2. Diffraction peaks due either Ru or

RuO2 were not observed in spent catalyst. Therefore,

Zr0.95Ru0.05O2�d compound retained its structure even after 3

cycles of WGS reaction.

Fig. 1 e Profile refined XRD patterns for Zr0.95Ru0.05O2Ld (a) as-synthesized (b) after 3 cycles of WGS reaction, respectively.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 313964

Due to the presence of nanocrystals, XRD peaks are broad

and it is difficult to distinguish the tetragonal and cubic

structure of ZrO2 on the basis of XRD only. Therefore, the

crystal structure of Zr0.95Ru0.05O2�d was also confirmed using

FT-Raman spectroscopy. Fig. 2 shows the FT-Raman spectra of

the compound before and after the reaction. Peaks mainly

observed at 145, and 260 cm�1 are characteristic of tetragonal

phase and a single peak at 490 cm�1 is the characteristic of

cubic phase, respectively [35]. No strong band was observed at

w260 cm�1, and it can be due to changes in disorder of the

oxygen sublattice of Ru stabilized ZrO2 [35]. However, the

presence of the peak near 145 cm�1 and the absence of the

peak near 490 cm�1 confirm that Zr0.95Ru0.05O2�d crystallizes in

tetragonal structure with a space group of P42/nmc.

Bright field and high resolution (HRTEM) images of

Zr0.95Ru0.05O2�d are given in Fig. 3. The bright field image

shows that the average particle size is w9 nm, which is in

close agreement with the size determined by XRD. Selected

area electron diffraction pattern (see inset of Fig. 3(a)) clearly

shows high crystalline nature of Zr0.95Ru0.05O2�d nanoparticlesand the ring patterns can be assigned to tetragonal structure

of ZrO2. The absence of the rings either due to Ru or RuO2

confirms the formation of the single phase solid solution.

Further, HRTEM image (see Fig. 3(b)) indicates the presence of

many nanocrystals showing (111) lattice fringes of tetragonal

ZrO2. The energy dispersive X-ray (EDX) analysis was per-

formed on the same instrument and the analysis shows that

Zr and Ru are present in the compound with molar ratio of

0.96:0.04 which is close to the molar ratio taken during the

synthesis. Therefore, the combined studies of XRD, Raman

and TEM confirm the substitution of Ru in ZrO2 lattice. The

BET surface area for Zr0.95Ru0.05O2�d was found to be 104 m2/g.

Table 1 e Cell parameters of Zr0.95Ru0.05O2Ld obtained by profi

Compound Cell parameters (�A)

a/b c

Zr0.95Ru0.05O2�d (before reaction) 3.599 5.175

Zr0.95Ru0.05O2�d (after reaction) 3.606 5.176

The oxidation state of Ru and Zr in Zr0.95Ru0.05O2�d beforeand after WGS reaction was determined using XPS. The re-

sults of XPS were also used to understand the reaction

mechanism for the WGS reaction. All the binding energies

were adjusted to the C (1s) peak observed at 285 eV and this

was also verified with the oxygen peak observed at 530 eV.

Core level Ru (3d) spectra in Zr0.95Ru0.05O2�d before and after

the reaction are shown in Fig. 4(a). The binding energies of Ru

(3d) and C (1s) fall in the same range. Therefore, Ru (3p)

spectrum was also analyzed to assign the oxidation state of

Ru. The XPS spectrum was broad and therefore, the Ru (3d)

peak was de-convoluted to obtain C (1s), Ru (3d5/2) and Ru (3d3/

2) states. The binding energy of Ru (3d5/2) in Zr0.95Ru0.05O2�d is

slightly higher (0.4 eV) than the binding energy of Ru in RuO2,

indicating that the RueO distance and the location of the ox-

ygen atoms in Zr0.95Ru0.05O2�d is different from that of the Ru

ions in RuO2 [36]. The Ru (3p3/2) spectrum (not shown) also

showed slightly higher binding energy (463.1 eV) than that of

Ru (3p3/2) spectrum in RuO2.Therefore, it can be concluded

that that the Ru is in the 4þ oxidation state before the reac-

tion. The XPS spectrum of Ru (3d) afterWGS reaction is shown

in Fig. 4(a) and mixed oxidation state of Ru (Ru0 and Ru4þ) was

observed, which indicates that the Ru was in partially reduced

state after the reaction.

Fig. 4(b) shows the Zr (3d) photoelectron peak of

Zr0.95Ru0.05O2�d before and after the reaction. The binding

energy of Zr (3d5/2) and Zr (3d3/2) are 182.9 and 185.3 eV,

respectively and this corresponds to the 4þ state of Zr [37]. No

appreciable variation in the Zr spectrum was observed indi-

cating no reduction of Zr took place during the reaction.

Therefore, XPS results show that the participation of lattice

oxygen is not possible during the reaction and thiswas used to

le refinement.

Rp Rwp c2 Crystallite size (nm)

2.62 3.42 1.02 8

3.2 4.26 0.92 10

Fig. 2 e FT-Raman spectra of Zr0.95Ru0.05O2Ld (a) before and

(b) after WGS reaction.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 3 13965

deduce the reaction mechanism for WGS reaction over

Zr0.95Ru0.05O2�d catalyst.

3.2. Steam reforming activity and kinetic study

Fig. 5 shows the influence of CH4 conversion and CO selec-

tivity over the catalyst. At low temperature, CO selectivity is

low, indicating a large contribution fromWGS. Nearly 93%CH4

conversion with 92% CO selectivity was observed at 700 �C.The rate of reaction and activation energy was determined by

performing experiments with different weights of the cata-

lyst. The reaction was carried out with composition of 3% CH4

and balance N2 with the total gas flow of 100 ml/min. The rate

of reaction was determined under absence of any transport

artifacts and the temperature of the reactor was varied such

that the differential reactor approach was maintained. The

following equation was used to calculate rate of reaction

rate ðrÞ ¼ F� xW¼ x

W=F(3)

Fig. 3 e (a) Bright field image and (b) HRTEM image of Zr0.95Ru0

where F is the flow of the gas in mol/s, W is the weight of the

catalyst in g and x is the fractional CH4 conversion. The vari-

ation ofW=FCH4 with the fractional conversion of CH4 is shown

in Fig. 6(a). The plot is linear up to 50% conversion and the

rates of reactionwere calculated at various temperatures from

the slope of the curve. The variation of rate of reaction as a

function of temperature is shown in Fig. 6(b). The apparent

activation energy of steam reformingwas calculated using the

Arrhenius plot and the corresponding plot is given in the inset

of Fig. 6(b). The activation energy for steam reforming reaction

was found to be 92 kJ/mol.

The effect of concentration of CH4, steam and CO on the

rate of reaction was investigated in a differential reactor with

50 mg of catalyst in the range of 400e550 �C under atmo-

spheric pressure. The total flow and space velocity was kept

constant for all experiments and reactor temperature was

varied in suchway that the total conversion of CH4was always

below 30%. The concentration of CH4 was varied from 1 to 4%

keeping the steam concentration constant at 2%. The con-

centration of steam was varied between 1 and 4% keeping the

concentration of CH4 constant at 2%. The rate of reaction in

the presence of CO was also examined independently. The

concentration of CO was varied between 0.25 and 0.75%

keeping the concentration of CH4 and steam constant at 2%.

Fig. 7(a) and (b) shows the variation of rate of CH4 conversion

as function of CH4 and steam concentrations, respectively.

The rate of reaction increases with CH4 concentration while

the rate of reaction is almost independent of steam concen-

tration. The order of reaction with respect to concentration of

CH4 was obtained by plotting ln(rate) versus ln ðCCH4 Þ and was

found to bew1 at all temperatures. The first order dependence

of the rate of reaction on concentration of CH4 and zero order

dependence on the steam concentration are also consistent

with the literature [25,38]. Therefore, it can be concluded that

the overall reaction is first order with respect to concentration

of CH4 and zero order with respect to steam concentration.

rCH4¼ kCCH4

(4)

Fig. 7(c) shows the effect of concentration of externally

added CO to the reaction mixture of CH4 and steam on the

rate of reaction. It can be seen from Fig. 7(c) that the rate of

reaction remains almost constant. This indicates that the

.05O2Ld with indexed electron diffraction in the inset of (a).

Fig. 4 e Core level XPS of (a) Ru (3d) and (b) Zr (3d) in Zr0.95Ru0.05O2Ld before and after the reaction.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 313966

adsorption of CO over the catalyst surface is negligible and the

activation of CH4 was unaffected due to presence of fed CO.

This indicates that the activation of CH4 is the slowest step

compared to the activation of co-reactant molecules. These

results are consistent with the studies reported by Wei and

Iglesia for steam and dry reforming of methane over Ru sup-

ported on various supports [7,38]. The kinetic study over

Rh/ZrO2 at 400 �C also shows that the reaction is first order in

CH4 and the intrinsic activity did not depend on the steam to

carbon ratio [25].

The kinetics of CH4 steam reforming was studied exten-

sively over noble metals (Pt, Ru, and Rh), Ni-based catalysts.

However, the reaction mechanism has still remained contro-

versial and contradictory results can be found in the litera-

ture. This is mainly due to the nature, morphology of the

support and different experimental conditions. The kinetic

model including LangmuireHinshelwood, power laws, and

expressions based on microkinetic analysis were used to

describe the experimental data [6,39]. Generally, the dissoci-

ation ofmethane is considered to be the rate determining step

Fig. 5 e Variation of CO selectivity and CH4 conversion with

temperature for steam reforming reaction. The dotted line

shows the equilibrium conversion.

[4,40,41]. The mechanism involves the decomposition of CH4

to C* in a series of elementary steps followed by reaction be-

tween adsorbed oxygen derived from steam disassociation.

The subsequent H abstraction is much faster than the initial

CeH bond dissociation and this leads to low coverage of CHx*

intermediates on the surface [42]. DFT was used to investigate

the reaction pathways and kinetics over Ni (111). It was shown

that CH* is themost important intermediate and reactions like

CH*þ O*/ CHO* and CH*þOH*/ CHOH* are critical steps in

the mechanism [43,44]. In another study, it has been argued

that CHO species may be formed from CH2O or CH and O, and

dissociation of these result in CO and Hwhich then desorbs to

CO and H2 [43]. It should be noted that CH4 dissociation is still

the rate determining step. Further, the CHO* species was also

proposed as an intermediate for H2 induced CO dissociation

on nickel surface and the mechanism with this intermediate

showed the square root dependence on hydrogen pressure

[45]. However, it can be shown that if the CHO forms active

intermediates, the rate of steam reforming reaction should

decrease with addition of the CO/H2 in CH4eH2Omixture. Wei

et al. argued that this kind of step introduces intricacy in the

mechanism and these intermediates cannot be proved using

spectroscopic analysis due to their low steady state concen-

tration [7].

In another study, O2 assisted CH4 dissociation

(CH4* þ O* / CH3* þ OH*) was suggested to be a possible re-

action mechanism [46]. However, the energy barrier for O2

assisted CH4 dissociation is higher than unassisted CH4

dissociation [47]. Further, this would result in a more complex

reactionmechanism and the rate of reactionwould depend on

the surface concentration of O*. It means that the rate of re-

action should depend on concentration of co-reactants and

products which is contradictory to the present findings.

Therefore, O2 assisted CH4 dissociation is unlikely to initiate

the steam reforming reaction.

We studied the cracking of CH4 at 550 �C in absence of H2O

and only H2 was observed in the outlet stream via adsorptive

dissociation of CH4. Then, H2O vapor with N2 was passed over

the catalyst at the same temperature and CO and CO2 was

indeed observed. This shows that methane dissociates to

carbon and gaseous H2 on Rumetal. The gasification of carbon

Fig. 6 e (a) Variation of fractional conversion of CH4 withW=FCH4 and (b) rate of reaction as function of temperature for steam

reforming reaction.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 3 13967

with O* species extracted from steam adsorption yields in the

formation of CO and CO2. Based on the qualitative trends

presented in this study, the following reaction stepswere used

to propose the reaction mechanism.

CH4 þ �/CðadsÞ þ 2H2 (5)

H2Oþ �4OðadsÞ þH2 (6)

CðadsÞ þOðadsÞ/COþ 2� (7)

This mechanism involves the following main steps (a)

dissociative adsorption of CH4 on Ru surface (Equation (5))

resulting in H2 and surface carbon, (b) dissociative adsorption

of steam on the Ru surface (Equation (6)) and (c) gasification of

the surface carbon by O* species extracted from steam

(Equation (7)). Based on the present results and available

literature, it was found that the rate of reaction was first order

with respect to CH4 concentration, which implies that the

overall reaction is controlled by dissociative adsorption of

CH4.

The stability and anti-coke property of the catalyst was

tested at 650 �C with 3% CH4 and balance of N2 keeping total

flow rate of 100 ml/min over 100 mg of catalyst (S/C ratio w1).

Fig. 8 shows the time on stream CH4 conversion and CO

selectivity for 24 h over the catalyst. No appreciable deacti-

vation of the catalyst was observed and conversion of CH4

(w74%) and selectivity of CO (w81%) remained nearly con-

stant over 24 h of testing. Therefore, the present catalyst is

highly active, stable and carbon resistant for the production of

syngas from CH4 under the low H2O/CH4 ratio. Thus, the

present catalyst is promising candidate in comparison with

the industrial reformer catalysts (mainly Ni based catalysts)

which deactivate rapidly due to deposition of coke on the

catalyst surface under low S/C ratio [48]. Further, the stability

of the catalyst under stoichiometric steam to carbon ratio (w1)

is important because it can avoid the extra cost associated

with steam recycling and results in syngas with H2/CO ratio

close to 2.5e2.8, which is desirable for methanol and

FischereTropsch synthesis.

3.3. WGS activity and kinetic study

WGS reaction was also carried over Zr0.95Ru0.05O2�d in the

temperature range of 140e500 �C with reaction mixture con-

sisting of 2% of CO and balance of N2 with the total flow rate

100 ml/min. Fig. 9(a) shows the variation of concentrations of

CO, CO2, H2 and CH4 with temperature. All the concentrations

were normalized by the initial concentration of CO and nearly

complete conversion (w99%) of CO with 100% selectivity to-

wards the hydrogen production was obtained below 290 �C.CO conversion increased in the temperature range of

140e400 �C and CO conversion decreased due to the reversible

nature of WGS reaction above 420 �C. The results also indicate

that CO conversion at 500 �C reached the equilibrium value

(98%) under the present feed gas conditions [49]. Notably, CO

methanation activity was not observed and CO2 and H2 were

the only products detected in the product stream (detection

limit <20 ppm).

Further, the unique role of ionic Ru substitution was also

investigated by performing WGS reaction over 5 at% Ru metal

impregnated ZrO2 catalyst. The gas mixture consisting of 2%

CO and balance of N2 with a total flow of 100 ml/min was

passed over 250 mg of catalyst. Fig. 9(b) shows the variation of

normalized concentrations of % CO, CO2 and H2 with tem-

perature. A nearly complete conversion of CO to CO2 (w99%)

was observed around 320 �C. The possible side product such as

methane was also observed above 320 �C. Therefore,

Zr0.95Ru0.05O2�d exhibits higher catalytic activity and selec-

tivity towards H2 production than those of Ru impregnated

ZrO2 catalyst and this is attributed to strong metal support

interactions and ionic nature of Ru in ZrO2.

The rate of reaction and activation energy forWGS reaction

over Zr0.95Ru0.05O2�d was determined by plotting fractional CO

conversion with W/FCO at different temperatures (see

Fig. 10(a)). The rate of reaction at various temperatures was

calculated from the slope of the curve. The total flow rate of

gas was kept constant at 100 ml/min and the weight of the

catalyst (W) was varied. The temperature of the reactor was

varied such that the differential reactor approach was

Fig. 7 e (a) The effect of concentration of CH4 on the rate of

methane conversion at a constant CH2O [ 8.17 3 10L7 mol/

cm3 and (b) effect of H2O concentration on the rate of

methane conversion at a constant CCH4 [ 8.17 3 10L7 mol/

cm3 (c) effect of CO concentration on the rate of methane

conversion at a constant CCH4 [ 8.17 3 10L7 mol/cm3 and

CH2O [ 8.17 3 10L7 mol/cm3, respectively.

Fig. 8 e Time on steam methane conversion and CO

selectivity for steam reforming reaction.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 313968

maintained. The variation of rate of reaction with tempera-

ture is shown in Fig. 10(b) and it can be seen that the rate

of reaction is very high over Zr0.95Ru0.05O2�d catalyst. The

activation energy was calculated using Arrhenius equation

(see inset of Fig. 10(b)) and found to be 32 kJ/mol. The com-

parison of various catalysts in term of rate of reaction and

activation energy is given in Table 2. Therefore, the present

compound is promising in removing poisonous CO from the

reformate hydrogen for fuel cell applications.

A series of experiments were performed to investigate the

effect of concentration of CO and H2O on the rate of reaction.

All experiments were performed under isothermal conditions

with a differential reactor approach. Further, all experiments

ensured that the kinetic data is free from internal and external

mass transfer resistance. The dependence of the rates of re-

action on the reactant concentration was measured by

changing the concentration of one reactant by keeping the

concentration of other reactant constant. The composition of

COwas varied from1 to 3% at a constant steamcomposition of

2.75% and the composition of steamwas changed from 1 to 7%

at a constant CO composition of 2% (all on volume basis). The

total gas flow rate of the reaction mixture and loading of the

catalyst were kept constant at 100 ml/min and 50 mg,

respectively. The dependence of reaction rate on the con-

centration of CO and H2O at various temperatures is shown in

Fig. 11(a) and (b). At low concentration of CO, the rate of re-

action exhibits positive dependency on the concentration of

CO with the apparent order of reaction close to 1.

The rate of reaction shows a maximum at higher concen-

tration of CO,which indicates there is an optimumcoverage of

CO and steam on the catalyst surface. Therefore, it is expected

that CO and steam undergo competitive adsorption on the

catalyst for the same active sites under the present experi-

mental conditions. At high CO concentration, the coverage of

CO molecules on the catalyst surface is very high, which

blocks most of the active sites and masks the competition

with steam. Further, the maximum in the reaction rate can be

understood as follows. If we assume formate/carbonate is an

active intermediate during the reaction, then the rate of

decomposition of these species depend on the concentration

of steam in the feed [56]. At high concentration of CO, there

are not enough H2O molecules in the feed to accelerate the

Fig. 9 e Normalized CO, CO2 and H2 concentrations in WGS reaction (a) Zr0.95Ru0.05O2Ld and (b) 5% Ru/ZrO2 (impregnated)

catalysts, respectively. The dotted line shows the equilibrium conversion.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 3 13969

decomposition. Therefore, this is the primary reason for the

decrease in the reaction rate at higher concentration of CO. On

the other hand, Fig. 11(b) shows a continuous increase in the

reaction rate with concentration of H2O without maximum in

the reaction rate. This shows that there are enough water

molecules in the feed, which facilitate the decomposition of

the formate/carbonate. This information was used to propose

the reaction mechanism over Zr0.95Ru0.05O2�d catalyst.

3.4. Kinetic model for WGS reaction

Numerous experimental and theoretical studies have been

performed to explore the reaction mechanism over various

catalysts. However, the exact nature of active intermediates is

still a matter of debate [2,15,28,57]. Most investigators agree

that the noble metal supported reducible metal oxide cata-

lysts are bi-functional with CO adsorption on noble metal and

H2O activation over the support [58]. In this regard, two

completely different mechanistic pathways namely, “redox”

or regenerative mechanism, and “associative” or adsorptive

mechanism have been proposed [2,9]. In redox mechanism,

CO adsorbs on the noble metal and reacts with the lattice

Fig. 10 e (a) Variation of fractional conversion of CO with W/FCOreaction.

oxygen of the support at metal-support interface to form CO2.

This results in the reduction of support with creation of oxide

vacancies, which would be rejuvenated by the oxygen pro-

vided by H2O, producing H2 [2]. The role of dispersed metallic

phase is not only restricted to the adsorption of CO but also

increases the reducibility of the support with the creation of

new active sites for activation of H2O [29].

On the other hand, the associativemechanism involves the

formation of surface carbon containing intermediate, e.g.

formate or carbonate from CO and OH which further de-

composes to H2 and CO2 [2,9]. The noble metals not only

facilitate the generation of the bridging OH group active sites

but also accelerate surface formate decomposition [59]. The

dissociative adsorption of H2O is the common step in both the

mechanisms [2]. The partial reduction of support increases

the rate of reaction for both the mechanisms [60]. For

example, in the formate mechanism, the reduced ceria sur-

face increases the concentration of bridging OH group active

sites. In the redox mechanism, reduced ceria (oxide vacancy)

is directly involved in the direct activation of H2O molecules

[54,61]. The stability of carbon containing intermediate de-

termines the reactionmechanism [57]. The nature of the oxide

(b) rate of reaction as function of temperature for WGS

Table 2 e Reaction rates and activation energies for WGSreaction on various supported catalysts.

Catalyst Rate(mmol/g/s)

(�C)

Activationenergy(kJ/mol)

Reference

Au/Fe2O3 0.022(100) 52 [50]

Au/TiO2 0.3(100) 46 [50]

Pd/CeO2 5.6(180) 48 [51]

Ce0.78Ti0.2Pt0.02O2�d 7.54(280) 56.48 [52]

Ce0.98Pt0.02O2�d 4.5(220) 52 [52]

Ce0.65Fe0.33Pt0.02O2�d 4.05(275) 50.62 [53]

Au/CeO2 0.2(100) 34 [54]

Ti0.84Pt0.01Fe0.15O2�d 2.74(280) 63 [55]

Ti0.73Pd0.02Fe0.25O2�d 4.36(240) 42 [55]

Ce0.85Si0.1Ru0.05O2�d 1.64(220) 51 [24]

Ce0.85Fe0.1Ru0.05O2�d 1.4(220) 56 [24]

Zr0.95Ru0.05O2�d 3.51(220) 32 Present study

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 313970

support has also a significant influence on the reaction

mechanism and the metal-support interface plays an impor-

tant role in governing the rate of decomposition of surface

carbon containing intermediate [62]. This originates from

either a direct or indirect involvement of the support [58]. For

example, noble metal supported CeO2 catalyst shows much

higher WGS activity in comparison with noble metal sup-

ported Al2O3 supported catalysts and this increase in the ac-

tivity is attributed to the reducibility and the oxygen storage

capacity of the ceria support [57]. Therefore, it is concluded

that the support directly participates in activating H2O mole-

cules. The support indirectly also influences the shape, size

and dispersion of the metal nanoparticles [63]. It also changes

the electronic state of metal nanoparticles and can stabilize

various ionic metal species [64].

Despite numerous experimental and theoretical studies,

the chemical nature of active carbon containing intermediate

and its true site location have not been clearly understood [2].

The initial transient rate of reaction of adsorbed formate with

water and adsorbed CO with water was quantified using a

novel SSITKA coupled with in situ DRIFTS and mass spec-

trometry experiments. It was observed that the formate is an

inactive or spectator species for the steady state WGS

Fig. 11 e (a) The effect of concentration of CO on the formation of

of H2O concentration on the formation of CO2 at a constant CCO

reaction [58,65]. The effects of reaction temperature and

support composition were also studied and it was found that

the reaction intermediates strongly depend on reaction

temperature, support composition, and Pt particle size

[2,65,66]. The reactivity of the intermediates formed during

the reaction was also investigated using in-situ DRIFTS/

SSITKA techniques. It was found that the rate of CO2 for-

mation is much higher than the rate of decomposition of

formats indicating that the observed formates are not the

active reaction intermediates [67].

It is important to note that the contribution of these

pathways to the overall rate of reaction is strongly dependent

on the experimental conditions [15]. However, the applica-

bility of the redox mechanism is restricted to the metal sup-

ported on “reducible” support. For non reducible supports

such as ZrO2, Al2O3 the utilization of lattice oxygen at low

temperature is unlikely during the reaction and it has been

reported in many studies that reaction mainly goes through

the formate mechanism [68]. It has been observed that the

WGS reaction precedes through a carboxylate (OCOH) inter-

mediate formed by direct oxidation of CO by hydroxyl groups

over Pt/Al2O3 catalyst [69]. In another study, two kinds of

formate namely active and inactive were identified over Pt/

Al2O3 [59,70]. The microkinetic modeling using DFT was

showed that the COOH mediated reaction pathway is

preferred over the redox pathway over Cu/ZrO2 catalyst and

the enhancement in electrostatic interaction between Cu ions

and adsorbates such as O and OH species was identified [71].

In another study, the observed differences in WGS activity

for different Cu supported ZrO2 polymorphs were explained

on the basis of different stability of the formate species on

these supports [72]. In-situ FTIR was used to investigate the

reactivity of mono and multi coordinated OH groups on Pt/

ZrO2 catalyst and it was found that mono coordinated OH

groups are active for formate formation while multi coordi-

nated OH groups and Pt are needed for formate decomposition

[73]. Therefore, based on the literature and our experimental

observations, a plausible reaction mechanism was proposed

to describe the reaction kinetics.

COþM! K1 COM (8)

CO2 at a constant CH2O [ 8.173 10L7 mol/cm3 and (b) effect

[ 11.2 3 10L7 mol/cm3, respectively for WGS reaction.

Fig. 12 e The fitted rate of WGS reaction to kinetic model as function of concentration of (a) CO and (b) H2O.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 3 13971

H2OþM! K2H2OM (9)

H2OM þ S/k3OHS þHM (10)

COM þOHS/k4CO2 þHM (11)

HM þHM/k5H2 þ 2M (12)

M and S represent the vacant Ru metal and support site,

respectively. The adsorption of CO and H2O over Ru sites is

represented by Equation (8) and Equation (9), respectively. The

competitive adsorption of CO and H2O over the catalyst is

justified based on the fact that the rate of reaction shows a

maximum at higher concentration of CO. Therefore, it is ex-

pected that there is an optimum coverage of CO and H2O over

the catalyst surface. Equation (10) depicts the spillover of

adsorbed H2O species to the support with the formation of

hydroxyl groups. The formation of OH species over ZrO2 and

their participation in WGS reaction has been well reported in

the literature [73]. The formation of surface intermediate

HOCO from interaction between adsorbed CO molecule and

OH group and the subsequent decomposition with release of

CO2 and H2 is depicted by Equations (11) and (12). It has been

proposed that the decomposition of formate species is the rate

limiting step [56]. Therefore, while deriving rate expression, it

was explicitly assumed that decomposition of formate species

(Equation (11)) is the rate limiting step. Thus

rate ¼ K1234CCOCH2O�1þ K1CCO þ K2CH2O

�2 (13)

At constant concentration of H2O, the above equation can

be written as

�CCO

rate

�1=2

¼ DA1=2

þ BA1=2

� CCO ¼ Eþ F� CCO (14)

where, A ¼ K1234CH2O, B ¼ K1 and D ¼ ð1þ K2CH2OÞThe left side of Equation (14) with concentration of CO was

plotted and the values of E and F were determined from the

intercept and slope, respectively. A similar fit was also

obtained with the variation concentration of H2O and values

of K1, K2 and K34 were determined (see Fig. 12). The values for

K1 (cm3/mol), K2 (cm

3/mol) and K34 (mol/g s) are 3.32 � 105exp

(5733/RT), 6.52� 105exp (8796/RT) and 4.8� 106exp (�4153/RT),respectively.

4. Conclusions

Highly active and stable Zr0.95Ru0.05O2�d catalyst was devel-

oped for CH4 steam reforming and WGS reactions. The

detailed kinetics was studied for a wide range of experimental

conditions including effect of temperature, catalyst loading

and concentration of product/reactants concentration on the

reaction rates. The rates of reaction were first order in CH4

concentration and independent on concentrations of CO and

H2O. This indicates the adsorptive dissociation of CH4 is the

rate determining step. Further, the present catalyst was also

stable toward coke deposition even at much lower inlet steam

to carbon ratio. The high activity and selectivity (100%) toward

H2 production for WGS reaction is really promising. A lack of

CO methanation activity over present catalyst is also

encouraging in the context of development of Ru based cata-

lysts forWGS reaction and this is attributed to the ionic nature

of Ru species. Therefore, the present catalystmay be a suitable

alterative to noble metals (Rh, Pt, and Au) based catalysts for

the production of hydrogen.

Acknowledgments

Authors gratefully acknowledge the Gas Authority of India

Limited for financial assistance.

r e f e r e n c e s

[1] Brown LF. A comparative study of fuels for on boardhydrogen production for fuel cell powered automobiles. Int JHydrogen Energy 2001;26:381e97.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 313972

[2] Kalamaras CM, Gonzalez ID, Navarro RM, Fierro JLG,Efstathiou AM. Effects of reaction temperature and supportcomposition on the mechanism of water gas shift reactionover supported-Pt catalysts. J Phys Chem C2011;115:11595e610.

[3] Son IH, Lee SJ, Soon A, Roh H-S, Lee H. Steam treatment onNi/g-Al2O3 for enhanced carbon resistance in combinedsteam and carbon dioxide reforming of methane. Appl CatalB 2013;134:103e9.

[4] Jakobsen JG, Jørgensen TL, Chorkendorff I, Sehested J. Steamand CO2 reforming of methane over a Ru/ZrO2 catalyst. ApplCatal A 2010;377:158e66.

[5] Xu J, Yeung CM, Ni J, Meunier F, Acerbi N, Fowles M, et al.Methane steam reforming for hydrogen production usinglow water ratios without carbon formation over ceria coatedNi catalysts. Appl Catal A 2008;345:119e27.

[6] Jones G, Jakobsen JG, Shim SS, Kleis J, Andersson MP,Rossmeisl J, et al. First principles calculations andexperimental insight into methane steam reforming overtransition metal catalysts. J Catal 2008;259:147e60.

[7] Wei J, Iglesia E. Reaction pathways and site requirements forthe activation and chemical conversion of methane on Ru-based catalysts. J Phys Chem B 2004;108:7253e62.

[8] Rostrupnielsen J, Hansen JB. CO2 reforming of methane overtransition metals. J Catal 1993;144:38e49.

[9] Ratnasamy C, Wagner JP. Water gas shift catalysis. Catal RevSci Eng 2009;51:325e440.

[10] Choung SY, Ferrandon M, Krause T. Pt-Re bimetallicsupported on CeO2-ZrO2 mixed oxides as water gas shiftcatalysts. Catal Today 2005;99:257e62.

[11] Chen W-H, Syu Y-J. Hydrogen production from water gasshift reaction in a high gravity (Higee) environment using arotating packed bed. Int J Hydrogen Energy 2010;35:10179e89.

[12] Azzam K, Babich I, Seshan K, Mojet B, Lefferts L. Stable andefficient Pt-Re/TiO2 catalysts for the water gas shift: on theeffect of rhenium. ChemCatChem 2012;4:1e9.

[13] Zhai Y, Pierre D, Si R, Deng W, Ferrin P, Nilekar AU, et al.Alkali-stabilized Pt-OHx species catalyze low-temperaturewater gas shift reactions. Science 2010;329:1633e6.

[14] Deng W, Carpenter C, Yi N, Flytzani-Stephanopoulos M.Comparison of the activity of Au/CeO2 and Au/Fe2O3

catalysts for the CO oxidation and the water gas shiftreactions. Top Catal 2007;44:199e208.

[15] Burch R. Gold catalysts for pure hydrogen production in thewater gas shift reaction: activity, structure and reactionmechanism. Phys Chem Chem Phys 2006;8:5483e500.

[16] Azzam K, Babich I, Seshan K, Lefferts L. Single stage watergas shift conversion over Pt/TiO2 problem of catalystdeactivation. Appl Catal A 2008;338:66e71.

[17] Azzam K, Babich I, Seshan K, Lefferts L. A bifunctionalcatalyst for the single-stage water gas shift reaction in fuelcell applications. Part 2. roles of the support and promoter oncatalyst activity and stability. J Catal 2007;251:163e71.

[18] Xie H, Lu J, Shekhar M, Elam JW, Delgass WN, Ribeiro FH,et al. Synthesis of Na-stabilized nonporous t-ZrO2 supportsand Pt/t-ZrO2 catalysts and application to water gas shiftreaction. ACS Catal 2012;3:61e73.

[19] Deshpande PA, Hegde MS, Madras G. Pd and Pt ions as highlyactive sites for the water gas shift reaction over combustionsynthesized zirconia and zirconia-modified ceria. Appl CatalB 2010;96:83e93.

[20] Hwang K-R, Ihm S-K, Park S-C, Park J-S. Pt/ZrO2 catalyst for asingle-stage water-gas shift reaction: Ti addition effect. Int JHydrogen Energy 2013;38:6044e51.

[21] Ceron ML, Herrera B, Araya P, Gracia F, Toro-Labbe A.Influence of the monoclinic and tetragonal zirconia phaseson the water gas shift reaction. A theoretical study. J MolModel 2013:1e7.

[22] Rhodes MD, Bell AT. The effects of zirconia morphology onmethanol synthesis from CO and H2 over Cu/ZrO2 catalysts:part I. Steady-state studies. J Catal 2005;233:198e209.

[23] Xu W, Si R, Senanayake SD, Llorca J, Idriss H, Stacchiola D,et al. In situ studies of CeO2 supported Pt, Ru, and Pt-Ru alloycatalysts for the water gas shift reaction: active phases andreaction intermediates. J Catal 2012;291:117e26.

[24] Shinde VM, Madras G. Synthesis of nanosizedCe0.85M0.1Ru0.05O2�d (M ¼ Si, Fe) solid solution exhibiting highCO oxidation and water gas shift activity. Appl Catal B2013;138e139:51e61.

[25] Duarte R, Nachtegaal M, Bueno J, Bokhoven JV.Understanding the effect of Sm2O3 and CeO2 promoters onthe structure and activity of Rh/Al2O3 catalysts in methanesteam reforming. J Catal 2012;296:86e98.

[26] Wang Y, Zhai Y, Pierre D, Flytzani-Stephanopoulos M. Silicaencapsulated platinum catalysts for the low temperaturewater gas shift reaction. Appl Catal B 2012;342:342e50.

[27] Fu Q, Saltsburg H, Flytzani-Stephanopoulos M. Activenonmetallic Au and Pt species on ceria-based water gas shiftcatalysts. Science 2003;301:935e8.

[28] Fu Q, Deng W, Saltsburg H, Flytzani-Stephanopoulos M.Activity and stability of low content gold cerium oxidecatalysts for the water gas shift reaction. Appl Catal B2005;56:57e68.

[29] Deng W, Frenkel AI, Si R, Flytzani-Stephanopoulos M.Reaction-relevant gold structures in the low temperaturewater gas shift reaction on Au-CeO2. J Phys Chem C2008;112:12834e40.

[30] Ligthart D, Van Santen R, Hensen E. Influence of particle sizeon the activity and stability in steam methane reforming ofsupported Rh nanoparticles. J Catal 2011;280:206e20.

[31] Hegde MS, Madras G, Patil K. Noble metal ionic catalysts. AccChem Res 2009;42:704e12.

[32] Shinde VM, Madras G. Nanostructured Pd modified Ni/CeO2

catalyst for water gas shift and catalytic hydrogencombustion reaction. Appl Catal B 2013;132:28e38.

[33] Baidya T, Gupta A, Deshpandey PA, Madras G, Hegde MS.High oxygen storage capacity and high rates of CO oxidationand NO reduction catalytic properties of Ce1�xSnxO2 andCe0.78Sn0.2Pd0.02O2�d. J Phys Chem C 2009;113:4059e68.

[34] Shinde VM, Madras G. Water gas shift reaction over multi-component ceria catalysts. Appl Catal B2012;123e124:364e78.

[35] Morterra C, Cerrato G, Meligrana G, Signoretto M, Pinna F,Strukul G. Catalytic activity and some related spectralfeatures of yttria-stabilised cubic sulfated zirconia. Catal Lett2001;73:113e9.

[36] Singh P, Hegde MS. Ce1�xRuxO2�d (x ¼ 0.05, 0.10): a new highoxygen storage material and Pt, Pd-free three way catalyst.Chem Mater 2009;21:3337e45.

[37] Guo X, Sun Y-Q, Cui K. Darkening of zirconia: a problemarising from oxygen sensors in practice. Sens Actuators B1996;31:139e45.

[38] Wei J, Iglesia E. Structural requirements and reactionpathways in methane activation and chemical conversioncatalyzed by rhodium. J Catal 2004;225:116e27.

[39] Rostrup-Nielsen JR, Sehested J, Nørskov JK. Hydrogen andsynthesis gas by steam and CO2 reforming. Adv Catal2002;47:65e139.

[40] Wei J, Iglesia E. Isotopic and kinetic assessment of themechanism of reactions of CH4 with CO2 or H2O to formsynthesis gas and carbon on nickel catalysts. J Catal2004;224:370e83.

[41] Halabi M, De Croon M, Van der Schaaf J, Cobden P,Schouten J. Intrinsic kinetics of low temperature catalyticmethane steam reforming and water gas shift over Rh/CeaZr1�aO2 catalyst. Appl Catal A 2010;389:80e91.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 3 9 6 1e1 3 9 7 3 13973

[42] Au C-T, Ng C-F, Liao M-S. Methane dissociation and syngasformation on Ru, Os, Rh, Ir, Pd, Pt, Cu, Ag, and Au: atheoretical study. J Catal 1999;185:12e22.

[43] Blaylock DW, Ogura T, Green WH, Beran GJ. Computationalinvestigation of thermochemistry and kinetics of steammethane reforming on Ni (111) under realistic conditions. JPhys Chem C 2009;113:4898e908.

[44] Wang S-G, Liao X-Y, Hu J, Cao D-B, Li Y-W, Wang J, et al.Kinetic aspect of CO2 reforming of CH4 on Ni (111): a densityfunctional theory calculation. Surf Sci 2007;601:1271e84.

[45] Andersson M, Abild-Pedersen F, Remediakis I, Bligaard T,Jones G, Engbæk J, et al. Structure sensitivity of themethanation reaction: H2-induced CO dissociation on nickelsurfaces. J Catal 2008;255:6e19.

[46] ErdThelyi A, Fodor K, Solymosi F. Reaction of CH4 with CO2

and H2O over supported Ir catalyst. Stud Surf Sci Catal1997;107:525e30.

[47] Shustorovich E, Bell AT. Oxygen-assisted cleavage of O-H, N-H, and C-H bonds on transition metal surfaces: bondorderconservation Morsepotential analysis. Surf Sci1992;268:397e405.

[48] Wang Y, Chin Y-H, Rozmiarek RT, Johnson BR, Gao Y,Watson J, et al. Highly active and stable Rh/MgOAl2O3

catalysts for methane steam reforming. Catal Today2004;98:575e81.

[49] Rhodes C, Hutchings G, Ward A. Water gas shift reaction:finding the mechanistic boundary. Catal Today1995;23:43e58.

[50] Sakurai H, Ueda A, Kobayashi T, Haruta M. Low temperaturewater gas shift reaction over gold depositedon TiO2. ChemCommun 1997:271e2.

[51] Gorte R, Zhao S. Studies of the water gas shift reaction withceria supported precious metals. Catal Today 2005;104:18e24.

[52] Sharma S, Deshpande PA, Hegde MS, Madras G.Nondeactivating nanosized ionic catalysts for water gas shiftreaction. Ind Eng Chem Res 2009;48:6535e43.

[53] Mahadevaiah N, Singh P, Bhaskar DM, Parida SK, Hegde MS.Ce0.67Fe0.33O2�d and Ce0.65Fe0.33Pt0.02O2�d: new water gas shift(WGS) catalysts. Appl Catal B 2011;108:117e26.

[54] Leppelt R, Schumacher B, Plzak V, Kinne M, Behm R. Kineticsand mechanism of the low temperature water gas shiftreaction on Au/CeO2 catalysts in an idealized reactionatmosphere. J Catal 2006;244:137e52.

[55] Shinde VM, Madras G. Low temperature CO oxidation andwater gas shift reaction over Pt/Pd substituted in Fe/TiO2

catalysts. Int J Hydrogen Energy 2012;37:18798e814.[56] Shido T, Iwasawa Y. Reactant promoted reaction mechanism

for water gas shift reaction on Rh-doped CeO2. J Catal1993;141:71e81.

[57] Azzam K, Babich I, Seshan K, Lefferts L. Bifunctionalcatalysts for single stage water gas shift reaction in fuel cellapplications: part 1. Effect of the support on the reactionsequence. J Catal 2007;251:153e62.

[58] Kalamaras CM, Panagiotopoulou P, Kondarides DI,Efstathiou AM. Kinetic and mechanistic studies of the watergas shift reaction on Pt/TiO2 catalyst. J Catal 2009;264:117e29.

[59] Olympiou GG, Kalamaras CM, Zeinalipour-Yazdi CD,Efstathiou AM. Mechanistic aspects of the water gas shift

reaction on alumina-supported noble metal catalysts: in situDRIFTS and SSITKA-mass spectrometry studies. Catal Today2007;127:304e18.

[60] Panagiotopoulou P, Christodoulakis A, Kondarides D,Boghosian S. Particle size effects on the reducibility oftitanium dioxide and its relation to the water gas shiftactivity of Pt/TiO2 catalysts. J Catal 2006;240:114e25.

[61] Kalamaras CM, Petallidou KC, Efstathiou AM. The effect ofLa3þ-doping of CeO2support on the water gas shift reactionmechanism and kinetics over Pt/Ce1�xLaxO2�d. Appl Catal B2013;136:225e38.

[62] Rodriguez JA, Evans J, Graciani Js, Park J-B, Liu P, Hrbek J,et al. High water gas shift activity in TiO2 (110) supported Cuand Au nanoparticles: role of the oxide and metal particlesize. J Phys Chem C 2009;113:7364e70.

[63] Shekhar M, Wang J, Lee W-S, Williams WD, Kim SM,Stach EA, et al. Size and support effects for the water gasshift catalysis over gold nanoparticles supported on modelAl2O3 and TiO2. J Am Chem Soc 2012;134:4700e8.

[64] Si R, Raitano J, Yi N, Zhang L, Chan S-W, Flytzani-Stephanopoulos M. Structure sensitivity of the lowtemperature water gas shift reaction on CueCeO2 catalysts.Catal Today 2012;180:68e80.

[65] Kalamaras C, Dionysiou D, Efstathiou A. Mechanistic studiesof the water gas shift reaction over Pt/Cex Zr1�x O2 catalysts:the effect of Pt particle size and Zr dopant. ACS Catal2012;2:2729e42.

[66] Kalamaras CM, Americanou S, Efstathiou AM. “Redox” vs“associative formate witheOH group regeneration” WGSreaction mechanism on Pt/CeO2: effect of platinum particlesize. J Catal 2011;279:287e300.

[67] Meunier F, Reid D, Goguet A, Shekhtman S, Hardacre C,Burch R, et al. Quantitative analysis of the reactivity offormate species seen by DRIFTS over a Au/Ce(La)O2 water gasshift catalyst: first unambiguous evidence of the minorityrole of formates as reaction intermediates. J Catal2007;247:277e87.

[68] Flaherty DW, Yu W-Y, Pozun ZD, Henkelman G, Mullins CB.Mechanism for the water gas shift reaction onmonofunctional platinum and cause of catalyst deactivation.J Catal 2011;282:278e88.

[69] Grabow LC, Gokhale AA, Evans ST, Dumesic JA,Mavrikakis M. Mechanism of the water gas shift reaction onPt: first principles, experiments, andmicrokinetic modeling. JPhys Chem C 2008;112:4608e17.

[70] Kalamaras CM, Olympiou GG, Efstathiou AM. The water gasshift reaction on Pt/g-Al2O3 catalyst: operando SSITKA-DRIFTS mass spectroscopy studies. Catal Today2008;138:228e34.

[71] Tang Q-L, Liu Z-P. Identification of the Active Cu phase in thewater gas shift reaction over Cu/ZrO2 from first principles. JPhys Chem C 2010;114:8423e30.

[72] Aguila G, Guerrero S, Araya P. Influence of the crystallinestructure of ZrO2 on the activity of Cu/ZrO2 catalysts on thewater gas shift reaction. Catal Commun 2008;9:2550e4.

[73] Graf P, De Vlieger D, Mojet B, Lefferts L. New insights inreactivity of hydroxyl groups in water gas shift reaction onPt/ZrO2. J Catal 2009;262:181e7.