Nondeactivating Nanosized Ionic Catalysts for Water−Gas Shift

Ind. Eng. Chem. Res. 2009, 48, 6535–6543

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Nondeactivating Nanosized Ionic Catalysts for Water-Gas Shift Reaction Sudhanshu Sharma,† Parag A. Deshpande,‡ M. S. Hegde,*,† and Giridhar Madras†,‡ Solid State and Structural Chemistry Unit and Department of Chemical Engineering, Indian Institute of Science, Bangalore-560012, India

The water-gas shift reaction (WGS) is an important reaction to produce hydrogen. In this study, we have synthesized nanosized catalysts where Pt ion is substituted in the +2 state in TiO2, CeO2, and Ce1-xTixO2-δ. These catalysts have been characterized by X-ray diffraction and X-ray photoelectron spectroscopy (XPS), and it has been shown that Pt2+ in these reducible oxides result in solid solutions like Ti0.99Pt0.01O2-δ, Ce0.83Ti0.15Pt0.02O2-δ, and Ce0.98Pt0.02O2-δ. These catalysts were tested for the water gas shift reaction both in the presence and absence of hydrogen. It was shown that Ti0.99Pt0.01O2-δ exhibited higher catalytic activity than Ce0.83Ti0.15Pt0.02O2-δ and Ce0.98Pt0.02O2-δ. Further, experiments were conducted to determine the deactivation of these catalysts. There was no sintering of Pt and no carbonate formation; therefore, the catalyst did not deactivate even after prolonged reaction. There was no carbonate formation because of the highly acidic nature of Ti4+ ions in the catalysts. Introduction Currently, 80% of the energy demands are met by fossil fuels. However, the use of fossil fuel results in green house gas emissions and causes air pollution. In this regard, hydrogen is a possible alternative fuel to address energy demands. However, 96% of hydrogen produced is by steam reforming of fossil fuels1,2 and, thus, biomass could be an alternative to produce hydrogen.3,4 Hydrogen from biomass is environmentally friendly because carbon dioxide released from the process will be reutilized during photosynthesis, resulting in net zero CO2 emission.5 There are currently two types of processes that can be used for converting biomass to hydrogen,6 namely, the thermochemical and biochemical processes. Among the thermochemical processes, gasification can be considered as the most efficient7 and biomass gasification is less dependent on feedstock cost.8 However, synthesis gas produced from gasification results in a mixture of CO and CO2. But the performance of Pt electrode in a fuel cell system reduces significantly9 in presence of CO. Due to the growing demand of polymer electrolyte for a fuel cell (PEFC) for small-scale power sources, production of hydrogen with less than 10 ppm of CO is of importance. The water-gas shift (WGS) reaction (CO + H2O f CO2 + H2, ∆H ) -41.1 kJ mol-1) is a process to convert CO to H2. It is a reversible exothermic reaction that is assisted by a catalyst. Industrially, WGS is carried out in two stages to overcome the thermodynamic limitation for the reaction at higher temperatures and to achieve almost complete CO conversion. The two steps involve a high-temperature (350-500 °C) shift (HTS) over Fe2O3/Cr2O3 catalyst, followed by a low-temperature (200-250 °C) shift (LTS) over a Cu/ZnO/Al2O3 catalyst.10 Current industrial catalysts for the WGS (mixtures of Fe-Cr or Zn-Al-Cu oxides) are pyrophoric and normally require lengthy and complex activation steps before usage.11 These catalysts are also unsuitable for applications like fuel cells, which require fast start ups and should be nonpyrophoric. To provide lowtemperature WGS activity, Pt-group metals (PM), Au, or Cu are added in amounts that vary from 1 to 10 wt %. A support * To whom correspondence should be addressed. Phone/Fax: 9180-22932614. E-mail: [email protected]. † Solid State and Structural Chemistry Unit. ‡ Department of Chemical Engineering.

like CeO2 is important because of high dispersion of metals and oxygen storage capacity.12-14 Catalysts like ionic Pt and Au in 10-20% La3+-doped CeO2 have shown high activity toward WGS reaction.15,16 However, this catalyst gets deactivated due to surface carbonate formation17 at long times. There are reports on Pt metal supported on TiO2 as WGS catalyst in the literature.18-22 However, the catalyst deactivates due to loss of metal dispersion16 or sintering of Pt metal.19 Further, due to the cost of Pt, high loading of Pt will be economically prohibitive.11 However, ceria containing only trace amounts of Pt can be active as well as economical. We have been working extensively on Pt ion substituted CeO2 catalyst in the form of Ce1-xPtxO2-δ solid solution.23 In this catalyst, Pt is primarily in the +2 oxidation state and it is substituted at the Ce site. Pt2+ ions were found to be the active adsorption sites. The activity was found to be enhanced by replacing CeO2 with CeO2-TiO2 solid solution in a particular stoichiometry with a formula Ce1-x-yTixPtyO2-δ. Typically, rates were found to be significantly higher for Ce1-x-yTixPtyO2-δ than Ce1-xPtxO2-δ toward CO oxidation, NOx reduction, and threeway catalysis.23 Pt2+ ions in CeO2 and TiO2 having the formulae Ti1-yPtyO2-δ and Ce1-yPtyO2-δ (y ) 0.01-0.02) have been used for NOx reduction,24,25 respectively. In this study, we have synthesized three ionic catalysts, namely, Ti0.99Pt0.01O2-δ, Ce0.78Ti0.20Pt0.02O2-δ, and Ce0.98Pt0.02O2-δ, and used them for the WGS reaction for the first time. The activity of these catalysts at various temperatures, at different conditions, was determined and compared to other catalysts in the literature. The deactivation of these catalysts was studied and compared with that of 20% La-doped Ce0.98Pt0.02O2-δ (Ce0.78La0.20Pt0.02O2-δ) by conducting the reaction for 22 h using daily startup and shutdown conditions and for 100 h for one catalyst. It was found that catalyst containing La substituted in ceria with Pt ion deactivated the most due to surface carbonate formation, and no carbonate species were observed with Ti0.99Pt0.01O2-δ. Thus, in this study, we show that Ti0.99Pt0.01O2-δ is a highly active nondeactivating catalyst for the water-gas shift reaction. Experimental Section Ti0.99Pt0.01O2-δ, Ce0.98Pt0.02O2-δ,26 and Ce0.83Ti0.15Pt0.02O2-δ23 were prepared by the single-step solution combustion method.

10.1021/ie900335k CCC: $40.75  2009 American Chemical Society Published on Web 05/22/2009

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In brief, for the preparation of Ti0.99Pt0.01O2-δ, titanyl nitrate [TiO(NO3)2], tetraammineplatinum nitrate [(NH3)4Pt(NO3)2], and glycine in the molar ratio of 0.99:0.01:1.5 were dissolved in water to make a clear solution. The solution was heated in a muffle furnace at 400 °C, and nanocrystalline powder of Ti0.99Pt0.01O2-δ was obtained after combustion. In this study, (NH3)4Pt(NO3)2 was used and this resulted in Pt being substituted mostly in the Pt2+ state. However, if H2PtCl6 is used as the precursor, Pt is primarily substituted in the Pt4+ state.27 This can be attributed to the different oxidation states of Pt, which is +4 in H2PtCl6 and +2 in (NH3)4Pt(NO3)2. Ce0.98Pt0.02O2-δ, Ce0.83Ti0.15Pt0.02O2-δ, and Ce0.78La0.20Pt0.02O2-δ were also prepared similarly. The catalysts Ti0.99Pt0.01O2-δ, Ce0.83Ti0.15Pt0.02O2-δ, Ce0.98Pt0.02O2-δ, and Ce0.78La0.20Pt0.02O2-δ are denoted as I, II, III, and IV, respectively. The actual weights of Pt in the catalysts are 23, 22.6, 31, and 22.5 mg/g in catalysts I, II, III, and IV, respectively. Ti0.99Pt0.01O2-δ corresponds to around 2.5 wt % of Pt substituted in titania. To compare the activity of metal impregnation with that of substitution, we also prepared 2.5 wt % Pt metal impregnated over titania (catalyst V). This was done by wet impregnation of the aqueous solution of H2PtCl6 over combustion synthesized TiO2 followed by slowly adding hydrazine hydrate to reduce H2PtCl6. The solution was continuously sonicated during the reduction. Finally, the catalyst was repeatedly washed by water and alcohol. Powder X-ray diffraction patterns of the catalysts were recorded on a Philips X’Pert diffractometer at a scan rate of 0.12° min-1 with a 0.02° step size in the 2θ range of 10° and 100°. The refinement was done using the Fullprof-fp2k program. X-ray photoelectron spectra (XPS) of the catalysts were recorded on a Thermo Fisher Scientific Multilab 2000 machine with Al KR radiation (1486.6 eV). The binding energies reported here are with reference to graphite at 284.5 eV and they are accurate within (0.1 eV. TEM of powders were carried out using a JEOL JEM-200 CX transmission electron microscope operated at 200 kV. The surface areas of catalysts I, II, and III are determined by N2 desorption technique (NOVA-1000 ver. 3.70) and the areas are 29, 33, and 14 m2 g-1, respectively. Catalytic studies were carried out with 100 mg of catalyst in a 4 mm diameter and 30 cm long quartz tube microreactor with 2 cm catalyst bed length. A mixture of 2 vol % CO with or without 50% of H2 was used. The flow rate of N2 was adjusted such that the total flow was 100 cm3/min. This corresponds to a space velocity of 21 500 h-1. Deactivation experiments were carried out with the feed gas containing 2% CO, 25% H2, and the rest N2 to make the total flow of 50 cm3/min. The flow rate of water for the experiments was maintained at 3 mL/min. Gases were analyzed using a gas chromatograph (ProGC, Mayura Analytical Pvt. Ltd.) equipped with flame ionization (FID) and thermal conductivity (TCD) detectors. Catalysts were analyzed before and after 22 h of reaction by XRD and XPS. The experiments were conducted in triplicate, and the errors in the conversions reported are less than (2%. Results and Discussion Rietveld refined powder X-ray diffraction patterns (XRD) of TiO2 and Ti0.99Pt0.01O2-δ are shown in Figure 1a,b. The lattice parameters are a ) b ) 3.7843(9) Å and c ) 9.5088(22) Å for TiO2, and for Ti0.99Pt0.01O2-δ they are a ) b ) 3.7928(21) Å and c ) 9.5063(61) Å. Overall, there is an increase in the cell volume by ∼0.5%, indicating the substitution of Pt ion (RPt2+ ) 0.80 Å) for Ti ion (RTi4+ ) 0.67 Å) in TiO2. Crystallite size of Ti0.99Pt0.01O2-δ estimated from Sherrer formula is ∼7 nm.

Figure 1. Rietveld-refined XRD patterns of (a) TiO2 and (b) Ti0.99Pt0.01O2-δ and (c) TEM picture of Ti0.99Pt0.01O2-δ.

TEM images (Figure 1c) of as-prepared Pt/TiO2 shows fine crystallites with homogeneous size distribution with the mean size of 8-10 nm, which agrees well with the XRD measurements. No Pt particles are observed. Powder X-ray diffraction patterns of catalysts I-III before (as prepared) the reaction are given in Figure 2a-c. Diffraction lines for CeO2 in fluorite structure are observed for catalysts II and III. However, Pt(111) peaks are not discernible. This confirms that Pt ions are substituted in the lattices of Ce1-xTixO2-δ and CeO2 for Ce4+ in catalysts II and III (Figure 2b,c). Substitution of Pt ion in Ce1-xLaxO2-δ has also been reported earlier.28 Detailed studies on the structure of Ce0.98Pt0.02O2-δ, Ce0.83Ti0.15Pt0.02O2-δ, and Ce0.78La0.20Pt0.02O2-δ by XRD, XPS, TEM, and EXAFS have shown that Pt ion is

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Figure 3. XPS of (a) Pt(4f) core level region in Pt metal nanoparticles and in catalyst I and (b) Ti(2p) core level region in catalyst I showing the shifted Pt(4f) region before and after the reaction.

Figure 2. XRD patterns before and after the 22 h of WGS reaction for (a) catalyst I, (b) catalyst II, and (c) catalyst III.

substituted for Ce4+ ion and Pt2+ is the major constituent (78%) in these compounds.23,26,29,30 XPS of Pt(4f) and Ti(2p) of catalyst I before and after the reaction is shown in Figure 3a,b. Pt(4f) peaks are broad and shifted to higher binding energy compared to Pt(4f) peaks in the platinum metal nanoparticles, suggesting Pt in multiple oxidation states (Figure 3a). Taking into consideration the peak positions of Pt2+ and Pt4+, 4f(7/2,5/2) spin orbit splitting value of 3.2 eV, and the full width at half maxima (fwhm), Pt(4f) peaks are resolved into sets of 4f7/2, 4f5/2 spin orbit doublets for Pt2+ and Pt4+ states. Accordingly, Pt(4f7/2, 4f5/2) peaks at 72.5 and 75.8 eV represent Pt in the +2 state and 74.6 and 77.8 eV are

due to the Pt4+ state, while 71.1 eV corresponds to the Pt in the metallic state. From the area under each Pt state, Pt2+ and Pt4+ are in the ratio of 0.78:0.22. XPS of Ti(2p) from catalyst I shows two peaks with binding energies at 458.6 and 464.5 eV corresponding to Ti(2p3/2) and Ti(2p1/2) showing Ti in the +4 state (Figure 3b). Similarly, the deconvoluted spectrum of Pt(4f7/2,5/2) core level in catalyst II is shown in Figure 4a. From the area under each Pt state, Pt2+ and Pt4+ are in the ratio of 0.80:0.20. The Ce(3d) core level spectra of catalyst II is shown in Figure 4b. Binding energies of Ce4+(3d5/2) at 882.6 eV (A) along with its satellites at 889.6 (A′) and 898.7 eV (A′′) and Ce4+(3d3/2) at 901.1 eV (B) along with its satellites at 908.1 (B′) and 917.1 eV (B′′) in catalyst II are characteristics of CeO2. Ti(2p) core level spectra in catalyst II is in +4 state as observed from Figure 4c. Similarly, Pt(4f) core level spectrum in catalyst III (Figure 5a) shows Pt in multiple valence states, and from the area under each Pt state, Pt0, Pt2+, and Pt4+ are in the ratio of 0.10:0.75:0.15. Accordingly, Ce(3d) core level spectrum in catalyst III (Figure 5b) shows Ce in the +4 state. Thus, in catalysts I-III, Pt is present mainly in the Pt2+ state. XPS of Ce0.78La0.20Pt0.02O2-δ before and after the reaction is given in Figure 6a-c. Deconvoluted Pt(4f) spectrum (Figure

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Figure 5. XPS of (a) Pt(4f) and (b) Ce(3d) core level region before and after the 22 h of WGS reaction in catalyst III.

Figure 4. XPS of (a) Pt(4f), (b) Ce(3d), and (c) Ti(2p) core level region before and after the 22 h of WGS reaction in catalyst II.

6a) in as-prepared Ce0.78La0.20Pt0.02O2-δ gives sets of spin-orbit doublets similar to that of other catalysts. The percent ratio calculated from the area under the curve for Pt0:Pt2+:Pt4+ is 6:74:20, respectively. Therefore, also in the case of Ce0.78La0.20Pt0.02O2-δ, Pt2+ is the major constituent. Ce(3d) again shows characteristics of Ce4+ in Ce0.78La0.20Pt0.02O2-δ (Figure 6b). The La(3d) core level spectra of catalyst IV is shown in Figure 6c. La(3d) spectrum gives characteristics of La3+ (as

shown in Figure 6c) with characteristic satellites centered 4 eV below the main peak. The spectrum is similar to La in the +3 oxidation state, as observed for pure La2O3. Having established the structures of the catalysts, these catalysts were tested for catalytic activity. CO conversion for the reaction of CO with H2O in presence of catalysts I (Ti0.99Pt0.01O2-δ), II (Ce0.83Ti0.15Pt0.02O2-δ), and III (Ce0.98Pt0.02O2-δ) and 2.5% Pt metal in TiO2 (catalyst V) toward CO + H2O is shown in Figure 7a. Over ∼99.9% CO conversion occurs at ∼245 and 310 °C in presence of catalysts I and II, respectively. However, with catalyst III, ∼99.5% conversion is attained only at ∼380 °C. The catalyst with 2.5% Pt metal impregnated in TiO2 could convert only 80% at 350 °C. The reaction was also studied with hydrogen in presence of these three catalysts. After adding 50 vol % of H2 in the reaction mixture of CO + H2O, the conversion profiles for catalysts I, II, and III shift toward higher temperatures (Figure 7b). Nearly 98% CO conversion is observed at ∼310 and 360 °C for catalysts I and II, respectively. CH4 was not detectable in presence of catalysts I and II up to 350 °C. However, only ∼95% conversion occurs at ∼400 °C in the presence of catalyst III. Further ∼80 ppm of CH4 is observed in the reaction mixture of CO

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Figure 7. CO conversion for the reaction of CO (2 vol %) with H2O (a) without H2 and (b) with 50% H2 for catalysts I, II, III, and V.

Figure 6. XPS of (a) Pt(4f), (b) Ce(3d), and (c) La(3d) core level region before and after the 22 h of WGS reaction in catalyst IV.

+ H2O + H2 above 300 °C in the presence of catalyst III. CH4 formation increases up to 500 ppm at 350 °C. In presence of 2.5% Pt metal in TiO2, a maximum of only 72% conversion is obtained at 325 °C.

The reaction rates for catalysts I-III toward CO (2%) + H2O (30%) and CO(2%) + H2O (30%) + H2 (50%) were estimated by conducting the reaction in differential mode and by plotting fractional conversion with W/F (F ) flow rate of reactive component, W ) weight of the catalyst). The reaction rate was obtained from the slope of the linear portion of the plot of fractional conversion (of less than 10%) with W/F (Figures S1-S3, Supporting Information). The variation of reaction rates with temperature for CO + H2O and CO + H2O + H2 reaction is given in parts a and b of Figures 8, respectively. Without feed hydrogen, the rates at 220 °C over catalysts I, II, and III are 14, 4.5, and 3.5 µmol g-1 s-1, respectively. Thus, the reaction with catalyst I shows the highest rates, which are approximately 3 and 4 times higher than those of catalysts II and III. Catalyst I also shows much higher rates compared to catalyst II and III in presence of feed hydrogen. At 280 °C, the reaction rates are 13, 7.5, and 6.5 µmoles g-1 s-1, respectively. This indicates that the catalyst I shows the highest rates for both the reactions (with or without feed H2). The Arrhenius plots for catalysts I-III for the CO + H2O + H2 reaction are shown in the inset of Figure 8b. The activation energies are 10, 13.5, and 15 kcal mol-1 for catalysts I, II, and III, respectively. This indicates that catalyst I is the most active (exhibits the highest rate of reaction) with the lowest activation energy. Deactivation of the catalyst is an important parameter in determining the usefulness of the catalyst. To check for deactivation, experiments are normally carried out using the start up and shutdown condition (daily start and shut down, DSS). For example, the DSS for the WGS reaction was studied for

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Figure 10. C(1s) spectra for (a) catalyst I; (b) catalyst II; (c) catalyst III, and (d) catalyst IV before (solid lines) and after (scattered points) 22 h of WGS reaction.

Figure 8. Variation of rates with temperature for the reaction CO (2 vol %) + H2O (a) without H2 and (b) with 50% H2. The Arrhenius plots are shown in the inset.

Figure 9. CO conversion with time at 300 °C for catalysts I (solid triangle), II (solid circle), III (solid rectangle), and IV (solid star).

10-12 h17 and it was concluded that DSS is a severe test for the deactivation, especially for the WGS reaction. Similarly, other studies have been reported in the literature where DSS has been used to study the deactivation during WGS.11,30-32 Thus, experiments were carried out for all the catalysts in the startup and shutdown condition of the reactor for 22 h to check if the catalysts deactivate. The reaction with catalyst IV (Ce0.78La0.20Pt0.02O2-δ) was also carried out under the same experimental conditions for comparison. CO conversion with respect to time at 300 °C for catalysts I-IV toward CO + H2O + H2 is shown in Figure 9. The average CO conversion in the

initial stages for catalysts I, II, III, and IV are 97, 35, 25, and 23%, respectively, thus showing that catalyst I is the most active one. After 5 h of operation, the reaction is stopped and restarted for the next 5 h. In this period, the average CO conversions for catalysts I, II, III, and IV are 96, 36, 25, and 23%, respectively. Similarly, after 12 h of WGS reaction, there is over 97% CO conversion for catalyst I, 35% CO conversion with catalyst II, and 25% CO conversion with catalysts III and IV. After 22 h of operation, catalyst IV deactivates and % CO conversion reduces from 23% to 12%. However, there is no significant deactivation in catalysts I-III. The conversions over catalysts I, II, and III after 22 h of reaction are 96, 34, and 23%, similar to the conversion observed initially. Thus, there is negligible deactivation in Pt2+-based ionic catalysts I-III. In our previous study33 we have carried out the reaction for 100 h using a similar catalyst (Ce0.98Pt0.02O2-δ) for the partial oxidation of methane. Similarly, the CO conversion for the reaction CO + H2O + H2 at 300 °C was examined in presence of catalyst I for 100 h. The conversion remained constant at 96% throughout the reaction. This also confirms that catalyst I does not deactivate during the WGS reaction. To determine the reasons behind the deactivation of the catalysts, XRD patterns after 22 h of reaction for all the catalysts I-III were recorded. There was no change in the crystal structure of catalysts I except for a little rutile impurity after the reaction (Figure 2a). The rutile impurity appears because rutile is the stable phase of titania at high temperatures.34 As is obvious from CO conversion in Figure 9, this small rutile impurity has not hampered the catalytic activity of catalyst I. XRDs of catalysts II and III after 20 h of water-gas shift reaction can be seen in Figure 2b,c. XRD of catalyst IV also did not show any detectable difference after 22 h reaction (not shown). Therefore, structures of these catalysts are stable during the long run of the experiment. The XPS of catalysts I-IV before and after 22 h of reaction were investigated. The Pt(4f) spectrum of catalyst I does not

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Table 1. Comparison of the Present Work and the Literature in Terms of Reactivity and Kinetic Parameters compound

% CO

0.5%Pt/CeO2 0.5% Pt/Ce0.5Ti0.5O2 0.5%Pt/TiO2

3

0.5%Pt/TiO2

3

% H2O

% H2

% CO2

% CH4

T50

7.5

max conv and Tmax (°C)

rate (µmol g-1s-1) at listed temp

Ea (kcal/mol)

g99.5%, 300 g99.5%, 300 g99.5%, 300

18

300 °C 9.0

10

Ti0.99Pt0.01O2-δ Ce0.98Ti0.20Pt0.02O2-δ Ce0.98Pt0.02O2-δ

250 290

ref

∼99.8%, 360 ∼99.8%, 300

220 °C 14.0 4.5

>99.5% >99.5% >99.5%

350 °C 1.0 1.5 2

12

22 this work

With Hydrogen 1.5%Pt10%La/CeO2 2.7% Pt10%La/CeO2 3.7% Pt10%La/CeO2

11

26

26

7

290 270 260

2% Pt/CeO2 5%Pt/CeO2 MEs-5%Pt/CeO2

7.68

25

23

7.68

0.77

1.18Pt/CeO2 1.13%Pt1 V/CeO2 1.15%Pt6 V/CeO2 1.09%Pt12 V/CeO2 1.08%Pt18 V/CeO2

6

60

16

1.6

0.4

0.52%Pt/6.6%CeTiO2 0.56%Pt/TiO2 0.5%Pt/CeO2

4.4

30

28

8.7

0.1

1%Pt/CeO2

7

22

37

8.5

Ti0.99Pt0.01O2-δ Ce0.98Ti0.20Pt0.02O2-δ Ce0.98Pt0.02O2-δ

2

30

50

show any significant change after 22 h of WGS reaction and Pt is mostly in the +2 state (∼78%) (Figure 3a). After 22 h of WGS reaction, Ti remains in the +4 state without any change. In the case of catalyst II, Pt(4f) does not show any significant change (Figure 4a). Ce(3d) in catalyst II (Figure 4b) does show 10% Ce4+ reduction to give Ce3+ after 22 h of WGS reaction. This reduction is an indication of the utilization of lattice oxygen in the reducing atmosphere of the hydrogen rich stream. Ti also remained in the +4 state before and after the reaction in catalyst II (Figure 4c), as seen from the binding energies at 458.6 and 464.5 eV. Pt(4f) core level spectrum of III does not show any noticeable change before and after the reaction, and Pt is mostly in the +2 state (Figure 5a). Ce(3d) spectra after the reaction shows partial reduction of Ce4+ to Ce3+ to the extent of 20% (Figure 5b). However, the extent of Ce4+ reduction in the case of II is less than that of III. From the Pt(4f) spectrum of catalyst IV, one can clearly observe the oxidation of Pt2+ to the Pt4+ state (Figure 6a). Ce(3d) and La(3d) spectra remain in the +4 and +3 state before and after the reaction (Figure 6 b,c). Importantly, Ce(3d) core level spectra do not show any reduction. This means that lattice oxygen in this case is not as activated as in the case of cerium-containing catalysts II and III. The surface concentrations35 of Pt in each of the catalysts before and after the reaction were estimated. As XPS probes the surface of the material, the surface concentration can be estimated by the ratio of intensities of the metal component and the support in the material from the XPS spectra. The percentage values are 4.4 and 4.9 for catalyst I, 3.8 and 3.4 for catalyst II,

18 18 18

58.6%, 500 53.2%, 500 62.5%, 500

37

300 °C 1.89 2.70 3.81 3.33 2.20 260 250 290

38

70%, 300 g90%, 300 80%, 300

20

200 °C 0.59 250 285 300

97%, 310 98%, 355 95%, 400

15

280 °C 12.90 7.54 6.62

18

39

10 13.5 15

this work

and 1.1 and 1.0 for catalyst III. Thus, variation in the surface concentration is not significant. C(1s) spectra are investigated before and after 22 h of reactor operation (Figure 10). The graphitic carbon peak at 284.5 eV is seen for all the catalysts. Catalyst I does not show any significant indication of surface carbonate at 289 eV. Catalyst II shows a very small carbonate peak at 289 eV after the reaction. Catalyst III shows a broad peak at 289.5 eV due to C(1s) from carbonate. The carbonate peak at 289 eV in catalyst IV is of high intensity compared to that of catalyst III. The formation of surface carbonate has also been verified by FTIR spectroscopy (not shown), which shows that catalysts I and II do not show any indication of carbonate formation. Catalyst III shows a small peak showing small surface carbonate. On the other hand, catalyst IV shows a significant broad peak of carbonate in this range. Hence, XPS and FTIR studies confirm that during the WGS reaction, surface carbonate formation is the highest in catalyst IV and the least in catalyst II. Catalyst I does not show any indication of surface carbonate formation. In the case of catalyst IV (Ce0.78La0.20Pt0.02O2-δ), acidic CO2 forms CO32- ions on the surface because La3+ ion is more basic than the Ce4+ ion. In catalysts II and III, significant amounts of Ce3+ states are observed that are comparatively more basic than Ce4+ and, therefore, provide sites for CO32- formation. Comparatively, catalyst II shows less CO32- formation than catalyst III. Ti4+ is much more acidic than Ce4+ ions, and thus, catalyst I does not deactivate due to surface carbonate formation. According to Fajan’s rule,36 the charge to radius ratio of La3+, Ce3+, Ce4+, Ti3+, and Ti4+ ions, respectively, are 25.8, 24.2,

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41.2, 44.7, and 66.1. Accordingly, the acidity of these ions increases in the order La3+ < Ce3+ < Ce4+ < Ti3+ < Ti4+. Indeed, the tendency to form carbonate decreased with La3+ > Ce3+ > Ce4+ > Ti3+ > Ti4+ (Figure 10). Therefore, Ti0.99Pt0.01O2-δ (catalyst I) is the most active nondeactivating catalyst. Even if Ti4+ is partially reduced to Ti3+ state, Ti3+ ion acidity is higher than that of Ce4+ ions, and catalyst I is not prone to deactivation by CO32- formation on the surface. In catalysts where Pt metal is dispersed over TiO2, Pt gets sintered during the prolonged WGS reaction.19 In the case of catalyst I (Ti0.99Pt0.01O2-δ), sintering is avoided and platinum remains in the ionic state after the prolonged WGS reaction (Figure 2a). The activation energies and reaction rates of these catalysts are compared with those of the other catalysts reported in the literature and presented in Table 1. Except for the catalysts reported in this study, Pt is shown to be present as metal in the other catalysts presented in Table 1. It is possible that a part of Pt can be present as Pt ions, but this has not been supported by XPS studies. The activation energy is the lowest with catalyst I compared to other catalysts used in previous studies. Although, studies have been reported on Pt metal supported over TiO2 for WGS reaction,18,20,22 significant deactivation was found due to sintering of Pt metal.19 As discussed earlier, Pt is in ionic form in catalysts I-III, and thus, sintering is completely avoided. Conclusions We have shown that Pt2+ ions substituted TiO2 shows the highest activity, lowest activation energy and it is not deactivated due to carbonate formation because Ti4+ is the most acidic ion. Rates with Ti0.99Pt0.01O2-δ are more than 3 times higher compared to Pt ion substituted in Ce1-xTixO2 or CeO2 catalysts for the same amount of Pt. La ion substituted ceria support gets deactivated due to more basic La3+ ion which is prone to react with acidic CO2 forming carbonate. Ti ion substitution in CeO2 has decreased the tendency of carbonate formation and thus improved the stability and activity of the catalyst. Acknowledgment The authors acknowledge ICAR and the Department of Science and Technology, Government of India, for financial support. Supporting Information Available: Figures S1-S3, showing the CO conversion with variation in the weight of catalysts I, II, and III, respectively, for (a) CO + H2O and (b) CO + H2O + H2. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Logan, B. E. Extracting hydrogen and electricity from renewable resources. EnViron. Sci. Technol. 2004, 160. (2) Spivey, J. J. Catalysis in the development of clean energy technologies. Catal. Today 2005, 100, 171–180. (3) Chaudhari, S. T.; Dalai, A. K.; Bakhshi, N. N. Production of hydrogen and/or syngas (H2 + CO) via steam gasification of biomassderived chars. Energy Fuels 2003, 17, 1062. (4) Haryanto, A.; Fernando, S.; Adhikari, S. Ultrahigh temperature water gas shift catalysts to increase hydrogen yield from biomass gasification. Catal. Today 2007, 129, 269–274. (5) Haryanto, A.; Fernando, S.; Adhikari, S. Producing sustainable hydrogen from biomass gasification with water gas shift reaction. ACS National Meeting, Atlanta, April 26-28, 2006. (6) Ni, M.; Leung, D.; Leung, M.; Sumathy, K. An overview of hydrogen production from biomass. Fuel Process. Technol. 2006, 87, 461.

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ReceiVed for reView March 4, 2009 ReVised manuscript receiVed April 16, 2009 Accepted May 2, 2009 IE900335K