Catalysts for the Water−Gas Shift Reaction - American Chemical Society

J. Phys. Chem. C 2008, 112, 2121-2128

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In Situ Time-Resolved Characterization of Ni-MoO2 Catalysts for the Water-Gas Shift Reaction Wen Wen,† Jean E. Calderon,‡ Joaquin L. Brito,§ Nebojsa Marinkovic,| Jonathan C. Hanson,† and Jose´ A. Rodriguez*,† Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973, Department of Chemistry, UniVersity of Puerto Rico, Cayey, Puerto Rico 00736, Centro de Quı´mica, Instituto Venezolano de InVestigaciones Cientificas, Apdo. 20632, Caracas 1020-A, Venezuela, and Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: October 5, 2007; In Final Form: October 31, 2007

Active catalysts for the water-gas shift (WGS, CO + H2O f H2 + CO2) reaction were synthesized from nickel molybdates (β-NiMoO4 and nH2O‚NiMoO4) as precursors, and their structural transformations were monitored using in situ time-resolved X-ray diffraction and X-ray absorption near-edge spectroscopy. In general, the nickel molybdates were not stable and underwent partial reduction in the presence of CO or CO/H2O mixtures at high temperatures. The interaction of β-NiMoO4 with the WGS reactants at 500 °C led to the formation of a mixture of Ni (∼24 nm particle size) and MoO2 (∼10 nm particle size). These Ni-MoO2 systems displayed good catalytic activity at 350, 400, and 500 °C. At 350 and 400 °C, catalytic tests revealed that the Ni-MoO2 system was much more active than isolated Ni (some activity) or isolated MoO2 (negligible activity). Thus, cooperative interactions between the admetal and oxide support were probably responsible for the high WGS activity of Ni-MoO2. In a second synthetic approach, the NiMoO4 hydrate was reduced to a mixture of metallic Ni, NiO, and amorphous molybdenum oxide by direct reaction with H2 gas at 350 °C. In the first pass of the water-gas shift reaction, MoO2 appeared gradually at 500 °C with a concurrent increase of the catalytic activity. For these catalysts, the particle size of Ni (∼4 nm) was much smaller than that of the MoO2 (∼13 nm). These systems were found to be much more active WGS catalysts than CuMoO2, which in turn is superior to commercial low-temperature Cu-ZnO catalysts.

Introduction Because of a global increase in fuel demand and raise in oil prices, alternative energy sources are widely explored.1 Of great interest are fuel cell vehicles that could reform hydrocarbons to hydrogen on-board. The CO generated as a byproduct in the reforming process could poison the electrocatalysts used in fuel cells and decrease their performance.2,3 The water-gas shift reaction (WGS, CO + H2O f H2 + CO2) provides an efficient way for reducing the CO content. In the past, several mixed-metal oxides have been tested as catalyst precursors for the WGS.4 In principle, the combination of two metals in an oxide matrix can produce materials with novel chemical and physical properties that can lead to a superior performance in technological applications. One way of producing new WGS catalysts is by reducing a mixed metal oxide using either H2 or CO gas streams. Often, one of the metal ions is reduced to nanoclusters and attached to the surface of a metal oxide support.4-6 During the reduction process, oxygen vacancies could be produced, which are critical in the mechanism for the WGS reaction.7 Various metal clusters could be involved in the formation of active sites for the WGS, especially noble metals such as Au,7-10 Pt,11-16 Pd,13,15,17,18 etc. Of special * To whom correspondence should be addressed. E-mail. rodrigez@ bnl.gov. Fax: (631) 344-5815. † Brookhaven National Laboratory. ‡ University of Puerto Rico. § Instituto Venezolano de Investigaciones Cientificas. | University of Delaware.

interest are metal clusters involving more abundant and thus inexpensive metals, such as Cu10,16 and Ni.16,19 Recently, a novel, nonexpensive, and highly active Cu-MoO2 catalyst was synthesized by partial reduction of a CuMoO4 precursor with CO or H2 at 200-250 °C.20 During the reduction process the diffraction pattern of the CuMoO4 collapsed, and copper metal lines were observed on an amorphous material background that was assigned to molybdenum oxides. In the first pass of the WGS reaction, diffraction lines for Cu6Mo5O18 and MoO2 appeared around 350 °C, and Cu6Mo5O18 was further transformed to Cu-MoO2 at higher temperature. Good WGS catalytic activity was observed with relatively stable plateaus in product formation at 350, 400, and 500 °C. The interfacial interactions between Cu clusters and MoO2 increased the WGS catalytic activities at 350 and 400 °C. In this work, we will explore the potential of nickel molybdates (β-NiMoO4 and nH2O‚NiMoO4) as precursors of catalysts for the WGS reaction. At atmospheric pressure, NiMoO4 has two isomorphs, i.e., R-NiMoO4 and β-NiMoO4 (see Figure 1, C2/m space group).21-24 In these two isomorphs Ni ions are in an octahedral coordination, while the Mo ions are either in an octahedral (R-NiMoO4) or in a tetrahedral coordination (β-NiMoO4). The R isomorph is stable at room temperature, and the β predominates at temperatures above 400 °C.22 The reduction and reoxidation of Ni molybdates have been studied extensively.6,21,23,25 No intermediates such as Ni2Mo3O8 or NiMoOx were found during the reduction process in 5% H2/ He,6,23,25,26 and the reduced catalyst could be transformed back to a nickel molybdate in a subsequent oxidation process.23 When

10.1021/jp709771c CCC: $40.75 © 2008 American Chemical Society Published on Web 01/19/2008

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Figure 1. Primitive cell for β-NiMoO4. The red spheres represent oxygen atoms. The metal atoms are represented by light (Mo) or dark (Ni) blue spheres.

heated in He, the nH2O‚NiMoO4 hydrate transforms directly into β-NiMoO4 around 400 °C.23,27 Because of the different electronic and structural properties of the R- and β-NiMoO4 isomorphs,6 different catalytic activities are also observed.6,22,27 β-NiMoO4 is interesting as a precursor for WGS catalysts because it combines two inexpensive metals, provides a fixed Ni/Mo ratio, and can be stable in the temperature range (200500 °C)22,23 in which the catalytic process is frequently performed.2,4,5 We have used in situ time-resolved X-ray diffraction (TRXRD) and X-ray absorption near-edge spectroscopy (XANES) to study the stability and behavior of β-NiMoO4 and nH2O‚ NiMoO4 in the presence of CO or CO/H2O mixtures at high temperatures. Nanostructured Ni-MoO2 is produced by partial reduction of the mixed-metal oxides. This system is found to be a much more active WGS catalyst than Cu-MoO2, which in turn is superior to commercial low-temperature Cu-ZnO catalysts.20 Experimental The in situ time-resolved XRD patterns were obtained at the beam line X7B (λ ) 0.922 Å) of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory.7,11,28-33 LaB6 was used to calibrate the wavelength, and the raw data were integrated using the Fit2d code. A Mar345 image plate detector was used for fast data acquisition. The Rietveld refinement was performed using EXPGUI/GSAS program.34 The particle size was determined using the Scherrer’s equation,35 where the diffraction peaks of MoO2 and Ni were fitted and the fwhm was obtained. A beamline broadening factor of 0.1 was used during the calculation of the particle size.35 The Mo LIII-edge XANES spectra were obtained at beam line X19A of the NSLS at Brookhaven National Laboratory where a boomerang-type flat crystal monochromator was used to obtain monochromatic X-ray photons with a typical energy resolution of 0.5 eV.6,22,23 The XANES spectra were measured in the fluorescence mode, using a PIPS (passivated implanted planar silicon) fluorescence detector. During the measurements, the photons flux was detuned by 70%, to minimize the disturbance from higher order harmonics. The Ni K-edge XANES spectra

Wen et al. were obtained at beam line X18B of the NSLS in the transmission mode. A channel cut Si (111) crystal was utilized to detune the photon flux by 20%. The gas used to fill the I0 and Ir ion chambers was N2, and the gas for the transmission chamber was a 25% Ar/N2 mixture for good photon absorption. The XANES data were analyzed using the Athena module of the IFEFFIT program.36 The NiMoO4 hydrate was synthesized by coprecipitation from aqueous solutions of Ni nitrate and ammonium heptamolybdate.21,27,37-40 An amount of 1-2 mg of sample was loaded into a sapphire capillary tube, which was attached to a flow reaction cell as reported in previous works.7,11,28 Swagelok fittings (1/16 in.) with Vespel ferrules were used to connect the capillary. During the reaction, the temperature was monitored via a chromel-alumel thermocouple, which was inserted straight into the sapphire tube near the oxide sample. A small resistance heater was wrapped around the capillary and used to heat the sample. Two gas systems, such as pure H2 and 5% CO in He, were used in the reduction process. The WGS reaction was carried out isothermally at 350, 400, and 500 °C, with a flow of 5% CO/He gas mixture through a water bubbler at a rate of ∼10 mL/min. At each temperature, the WGS reaction was monitored for a period of at least 3 h. The relative ratio of water vapor pressure to CO in the feed gas mixture was adjusted to be around 0.35. The concentrations of the residual gases were measured with a 0-200 amu quadrupole mass spectrometer while in situ diffraction patterns were collected. Because of the very small amount of catalyst (1-2 mg) in the microreactor, our detection setup was able to detect the products (CO2, H2) only at temperatures above 300 °C when the WGS rate was fast, but the reaction could be taking place also below 300 °C.28 In some experiments, the nickel molybdate precursor was pretreated in H2 or CO at elevated temperatures (360 or 700 °C). The sample was exposed to a stream of pure H2 or a 5% CO/He mixture. A heating rate of 3.75 °C/min was used to increase the sample temperature to 360 or 700 °C. For reduction of the nickel molybdate in 5% CO/He, the catalyst was further held at 700 °C for 3 h. The major changes in the structure of the sample were monitored by in situ time-resolved XRD. Results and Discussion The section is organized as follows. First, we will study the WGS reaction on the nH2O‚NiMoO4 and β-NiMoO4 precursors, following their structural transformations with in situ timeresolved XRD. This will be followed by studies in which the nickel molybdates are pretreated in H2 or CO before the WGS reaction in an attempt to reduce the precursors and generate large amounts of the active phase. In the final part of the article, we will discuss the electronic and structural properties of the synthesized catalysts as determined from XANES and XRD. A. WGS Reaction on nH2O‚NiMoO4 and β-NiMoO4. Figure 2 displays in situ XRD patterns collected while heating the NiMoO4 hydrate under WGS reaction conditions from 25 to 350 °C and then holding the temperature at 350, 400, and 500 °C. No WGS activity was observed under these conditions. The nH2O‚NiMoO4 hydrate was fully transformed to β-NiMoO4 around 350 °C and remained stable up to 500 °C. The observed transformation is consistent with data reported in the literature where the phase transformation starts around 250 °C when the NiMoO4 hydrate is heated in He environment.23 The similar tetrahedral coordination environment in β-NiMoO4 and the nH2O‚NiMoO4 facilitates such structural change.23 The intermediates found during the WGS reaction on CuMoO4, which

Time-Resolved Characterization of Ni-MoO2 Catalysts

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Figure 2. In situ time-resolved XRD patterns collected during the WGS reaction process using a nH2O‚NiMoO4 as a catalyst. Initially the sample was heated from 25 to 350 °C. The WGS reaction was carried out at 350, 400, and 500 °C. At each temperature, the catalyst was held for 3 h.

involved gradual reduction with the increase of temperature, were not observed here.20 This seems to indicate that β-NiMoO4 is more stable than CuMoO4 under WGS reaction conditions.20 However, the β-NiMoO4 formed in the experiments of Figure 2 might contain adsorbed water molecules that block the adsorption sites for CO molecules and subsequent reduction. When the NiMoO4 hydrate was heated in temperatureprogrammed desorption experiments, three H2O desorption peaks were observed around 120-160, 280-350, and 490530 °C.23 In another set of experiments, the NiMoO4 hydrate was transformed to β-NiMoO4 by heating it in He at 550 °C to induce the desorption of any water remaining from the nH2O‚ NiMoO4 compound. The temperature was allowed to decrease to around 300 °C, which is well above the transformation temperature from β-NiMoO4 to R-NiMoO4 (140-180 °C).23 At 300 °C, the gas was switched to CO/H2O, and the temperature was held at 300 °C for 1 h before the WGS reaction. Figure 3a displays the in situ XRD patterns collected during the WGS reaction process. The β-NiMoO4 sample remained unchanged at 350 and 400 °C but was transformed to Ni and MoO2 around 500 °C. Figure 3b displays the WGS catalytic activity, showing that the catalysts were activated at 500 °C as the structural transformation occurred. No WGS catalytic activity was observed around 350 and 400 °C, but the catalytic activity rose as the concentration of Ni and MoO2 increased at 500 °C. Figure 4 displays the H2 and CO2 concentration during the second pass of the WGS reaction. Good WGS catalytic activity was observed at 350, 400, and 500 °C, while the structure of the catalyst remained unchanged. The catalyst after the WGS reaction was named catalyst I, which will be discussed in more detail later. B. Preactivation of nH2O‚NiMoO4 and β-NiMoO4 in H2. Looking at the β-NiMoO4 f Ni-MoO2 transformation in Figure 3, one wonders what induces this reduction process and if H2 or CO could be used to preactivate the mixed-metal oxides. Figure 5 displays in situ XRD patterns collected during reduction with pure hydrogen of the NiMoO4 hydrate. The starting material shows a broad diffraction peak around the 2θ angle of 6.47°.23,41 At 350 °C, the diffraction peaks of the nH2O‚NiMoO4 disappeared, and those of metallic Ni increased gradually. The reduced system consisted of a mixture of NiO, amorphous

Figure 3. (a) In situ time-resolved XRD patterns collected during the WGS reaction process. (b) The concentrations of H2 and CO2 during the first pass of the WGS reaction process. In the initial step, the nH2O‚ NiMoO4 was ramped to 500 °C in helium. The temperature was allowed to decrease to 300 °C before the gas was switched to a mixture of CO/H2O. The WGS reaction was carried out at 350, 400, and 500 °C. The catalyst was held at each temperature for 3 h.

Figure 4. Concentrations of H2 and CO2 during the second pass of the WGS reaction over the catalyst synthesized in Figure 3a. The NiMoO2 catalyst was held at the indicated temperatures (350, 400, and 500 °C) for 3 h.

molybdenum oxide, and metallic Ni, as indexed in Figure 5b. A similar diffraction pattern was observed during the reduction of R-NiMoO4 under milder conditions, pointing to the generation of a mixture of Ni4Mo, Ni, and NiO.6,25,39 Similar to the

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Figure 5. (a) In situ time-resolved XRD patterns collected during the reduction process in H2 of nH2O‚NiMoO4. The temperature was ramped to 360 °C (3.75 °C/min). (b) XRD pattern of the reduced catalyst.

reduction of R-NiMoO4, no intermediates such as NiMoOx or Ni2Mo3O842,43 were observed during the reduction process of nH2O‚NiMoO4.6,25,26 It is also noted that, under hydrogen, the NiMoO4 hydrate is transformed without the appearance of β-NiMoO4. Figure 6a shows in situ XRD patterns collected during the first pass of the WGS reaction using the catalyst precursor synthesized in Figure 5. The material remains unchanged at 350 and 400 °C, while a gradual increase of the MoO2 concentration was observed around 500 °C. The final product consisted of a mixture of Ni and MoO2. No intermediates were observed, which is different from the WGS process using catalyst synthesized via reduction of CuMoO4, where Cu6Mo5O18 appeared around 350 °C before it was fully converted to Cu plus MoO2.20 Figure 6b displays the corresponding H2 and CO2 concentration during the first pass of the WGS reaction. A very small WGS catalytic activity was observed around 350 and 400 °C, which was mainly due to the metallic Ni. The catalytic activity increased when the reaction was held at 500 °C for 3 h, while the concentration of MoO2 also increased. Figure 7 shows the H2 and CO2 concentration during the second pass of the WGS reaction, where stable catalytic activity was observed at 350, 400, and 500 °C. In test experiments, we found that, at 350 and 400 °C, the Ni-MoO2 system was much more active than isolated Ni (some activity) or isolated MoO2 (negligible activity).

Wen et al.

Figure 6. (a) In situ time-resolved XRD patterns collected during the first pass of the WGS reaction process using the Ni-MoO2 catalyst synthesized in Figure 5. (b) The production of H2 and CO2 during the first pass of the WGS reaction process using the catalyst synthesized in Figure 5.

Figure 7. The production of H2 and CO2 during the second pass of the WGS reaction over the Ni-MoO2 catalyst synthesized in Figure 5 and then used in the experiments of Figure 6. The catalyst was held at each temperature for 3 h.

Thus, cooperative interactions between the admetal and oxide support are probably responsible for the high WGS activity of Ni-MoO2. The catalyst prepared in this process was named catalyst II and its electronic properties will be examined in detail later with X-ray absorption fine structure.

Time-Resolved Characterization of Ni-MoO2 Catalysts

Figure 8. (a) In situ time-resolved XRD patterns collected during the reduction of β-NiMoO4 in 5% CO/He. (b) XRD patterns typical of the phase transitions occurring during the reduction process. The nH2O‚ NiMoO4 was transformed into β-NiMoO4 by ramping to 500 °C in helium. The temperature was allowed to decrease to 300 °C before the gas was switched to 5% CO/He and then the temperature was ramped up to 700 °C.

C. Preactivation of the nH2O‚NiMoO4 and β-NiMoO4 in CO. Of the two reactants in the WGS, CO is the only one that has the ability to reduce NiMoO4 since water can be classified as a weak oxidant agent. It is worthwhile to examine the interaction of CO with the catalyst precursors. To the best of our knowledge no systematic study has been reported studying the reduction and structural transformations of nickel molybdates in the presence of CO at elevated temperatures. Figure 8 shows diffraction patterns collected after exposing β-NiMoO4 to CO. In a first preliminary step, the NiMoO4 hydrate was rapidly heated in He from room temperature to 500 °C to induce the formation of β-NiMoO4.23 The temperature was allowed to cool down to 300 °C, where the transformation from β-NiMoO4 to R-NiMoO4 does not occur.23 Then, the temperature was ramped from 300 to 700 °C in 5% CO/He at a rate of 3.5 °C/min. In the final step, the sample was held at 700 °C for 3 h. In the in situ XRD patterns of Figure 8a, the phases of Ni and MoO2 start to appear around 450 °C, and their concentration increases afterward. Around 600 °C, the catalysts were fully converted to a mixture of MoO2 and Ni, while an R-Mo2C phase44 begins to form around 640 °C. Nickel carbides could also be present since they have a diffraction pattern similar to that of pure nickel.45 The final product probably consists of

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Figure 9. (a) In situ time-resolved XRD patterns collected during the reduction of the nH2O‚NiMoO4 in 5% CO/He. (b) XRD patterns typical of the phase transitions occurring during the reduction process. The nH2O‚NiMoO4 was reduced in 5% CO/He, by ramping to 700 °C in 3 h. The temperature was held at 700 °C for another 3 h.

a mixture of Ni and Mo carbides on the surface, with Ni and MoO2 in the core of the particles. The details of the most important XRD patterns for the different phases are displayed in Figure 8b. The WGS activity of the catalyst formed at 700 °C in Figure 8 was tested. Some catalytic activity was observed at 500 °C, and no activity existed at 350 or 400 °C. The in situ XRD patterns collected under WGS conditions indicated that the structure of the Ni/R-Mo2C/MoO2 composite remained unchanged. The catalyst after the water-gas shift reaction at 500 °C was named catalyst III, and its electronic properties will be discussed in detail below. Figure 9 displays in situ XRD patterns collected during the reduction of nH2O‚NiMoO4 in 5% CO/He. The NiMoO4 hydrate transformed into β-NiMoO4, while no other phases was observed around 498 °C. β-NiMoO4 was reduced readily, and the sample became Ni-MoOx around 540 °C. As the temperature was further increased to 700 °C, R-Mo2C started to form, and this carbide formation was fully observed after holding at 700 °C for 3 h. The final product consisted of a mixture of R-Mo2C, MoO2, and Ni. The WGS activity of this system was tested at

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Figure 11. Rietveld analysis of a diffraction pattern for catalyst II.

Figure 10. (a) Ex situ Mo LIII-edge XANES spectra of catalysts I and III and some Mo standards. (b) Ex situ Ni K-edge XANES spectra of catalysts I, II, and some Ni standards.

350, 400, and 500 °C, and a minor generation of H2 and CO2 was seen only at the highest temperature. In summary, catalysts I-III have been synthesized following different approaches. Very different WGS catalytic activities were observed. To aid in the understanding of these catalysts, the corresponding XANES spectra were obtained and are discussed in the next section. D. XANES Measurements. The oxidation state for Mo in catalysts I cannot be deduced directly from its XRD pattern, because these data show significant deviations from the MoO2 pattern and the Ni/MoO2 phase ratio indicates that a substantial portion of the Mo present in the NiMoO4 precursor is not accounted for. In principle, all the catalysts could contain a MoOx species, which has a short-range order and does not appear in XRD. Figure 10a displays ex situ Mo LIII-edge XANES spectra for catalysts I and III, a Mo foil, MoO2, and MoO3. It is clear that the spectrum of catalyst I is quite similar to that of the MoO2 standard. The second derivatives of the Mo LIII-edge XANES spectra display a similar edge position around 2521.5 eV for catalyst I and MoO2, while the edge position for MoO3 is near 2523.2 eV. On the other hand, the Mo LIII-edge spectrum of catalyst III resembles that of the Mo foil, with an edge position of 2520.7 eV, which is mainly due to the metallic state of Mo atoms in molybdenum carbide.46-48 The XANES spectrum of MoO3 exhibits a doublet, with peaks located at 2524.9 and 2527.5 eV, respectively, mainly due to

the ligand field splitting of the Mo 4d orbitals in an octahedral environment.49-51 The intensity of the lower energy peak (2524.9 eV) is larger compared with that of the higher energy one, pointing to an octahedral coordination environment around the Mo ions.23,49 These features of MoO3 were not seen in any of the catalysts. The Ni K-edge XANES spectra for catalyst I, catalyst II, the pure NiMoO4 hydrate, and a Ni foil are shown in Figure 10b. Because of the +2 oxidation state of the Ni ions, the XANES spectrum of the NiMoO4 hydrate displays a shift of the edge position to higher photon energy compared with that of the Ni foil. A pre-edge feature is also observed, which is due to the transition from the Ni 1s orbitals to Ni 3d orbitals and is dipole forbidden and quadrupole allowed. The Ni K-edge XANES spectra of catalyst I and II resemble that of the Ni foil, while a slightly higher whiteline intensity is observed, pointing to a possible existence of Ni-O interaction between the interface of the Ni and MoO2. E. Structure vs Activity of the Catalysts. Figure 11 displays the Rietveld analysis for catalyst II (carried out using the EXPGUI code), which contains Ni and MoO2. The Ni phase was refined to a space group of fm3m symmetry with lattice parameters of a ) b ) c ) 3.529 Å. The MoO2 was refined to a space group of p21/c symmetry with lattice parameters of a ) 5.632 Å, b ) 4.865 Å, and c ) 5.564 Å. In catalyst II the intensity of the MoO2(-1 1 1) diffraction peak (around the 2θ angle of 15°) is weaker than expected if compared with standard MoO2.52 In addition, the Rietveld refinement of the XRD pattern for catalyst II points to a Ni/MoO2 ratio of 1/0.36, which is much larger than the ratio 1 expected after using a nH2O‚ NiMoO4 precursor. Thus, a very large fraction of the MoO2 present in catalyst II is amorphous or has short range order. In fact, the Rietveld refinements for catalyst II show some variations in the Ni/MoO2 ratio depending on the passes for the WGS. The XRD patterns of MoO2 and catalyst I-III are compared in the top panel of Figure 12. Since MoO2 is not stable under ambient conditions and usually forms a thin layer of MoO3, the XRD pattern of MoO2 was obtained in situ after two passes of the WGS reaction. The diffraction lines for catalyst III are consistent with a mixture of Ni, MoO2, and R-Mo2C phases. As in the case of catalyst II, catalyst I contains only Ni and MoO2. For catalyst I, the peak intensity of the MoO2 (-1 1 1) diffraction is even smaller than in catalyst II, and the diffraction lines of nickel clearly dominate the XRD pattern. The variations in the intensity of the MoO2 (-1 1 1) diffraction in catalysts I and II could be due to the effect of a substantial concentration of O vacancies that may lead to the formation of “stacking

Time-Resolved Characterization of Ni-MoO2 Catalysts

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Figure 12. Top panel: ex situ XRD patterns of (a) catalyst I, (b) catalyst II, (c) catalyst III. (d) MoO2. Bottom panels: Percentage of CO conversion during the WGS reaction over catalyst I, II, III, or Ni. The CO percentage conversion was averaged over a period of 3 h for each temperature. (a) At 350 °C; (b) at 400 °C.

TABLE 1: Intensity Ratio between Peaks -1 1 1 and -2 1 1 of MoO2 vs the Catalytic Activity 350 °C

400 °C

500 °C

name

I(-111)/ I(-211)

CO conversion percentage

I(-111)/ I(-211)

CO conversion percentage

I(-111)/ I(-211)

CO conversion percentage

catalyst I first trial second pass catalyst I second trial second pass catalyst II second trial second pass catalyst II second trial third pass

0.081 0.274 0.676 1.076

2.769 3.973 4.912 5.62

0.083 0.280 0.687 1.080

9.161 11.965 13.792 16.065

0.092 0.314 1.007 1.129

12.596 14.959 20.038 21.251

faults” and a systematic extinction of certain diffraction peaks. A detailed calculation of the intensity ratio between the MoO2 (-1 1 1) and (-2 1 1) peaks is displayed in Table 1. A correlation between the concentration of imperfections in the lattice of MoO2 and the activity of catalysts I and II is not straightforward, but catalyst II exhibits the biggest I(-111)/ I(-211) ratios and the highest catalytic activities. The CO percentage conversion for catalysts I-III and a Ni powder during the WGS reaction is displayed in the bottom panels of Figure 12. Among these systems, catalyst II displays the highest activity at both 350 and 400 °C. Catalyst I has a more significant CO conversion capability than the Ni powder. Catalyst III, which consists of Ni, MoO2, and R-Mo2C, shows almost no catalytic activity at 350 and 400 °C. At 350 and 400 °C, we found that MoO2 did not display any WGS activity,20 which indicates that a cooperative interaction between the nickel clusters and the molybdenum oxide support leads to the high WGS activity of catalyst II. A comparison between the very active catalyst II and Cu-MoO2 was also made, and catalyst II was found to be much more active than Cu-MoO2, which in turn is much active than commercial Cu-ZnO catalysts.10,20,53

TABLE 2: Particle Size Analysisa for the Synthesized Catalysts particle size (nm) name catalyst II second Trial catalyst I first Trial catalyst I second Trial Cu-MoO2 a

after first pass after second pass after third pass after first pass after second pass after first pass after second pass after first Pass

Ni (Cu)

MoO2

3.894 5.473 7.582 23.801 24.160 29.095 29.687 43.131 (Cu)

12.917 14.495 15.509 10.145 10.467 11.996 12.532 14.041

Performed through the Scherrer’s equation.35

The particle sizes of catalyst I, catalyst II, as well as that of an active Cu-MoO2 catalyst are displayed in Table 2. For catalyst II, which is the most active catalyst among these three, the particle sizes of Ni and MoO2 are around 7 and 14 nm, respectively. However, for catalyst I and Cu-MoO2, the sizes of the metal particles are much larger compared with those of the MoO2 support. This variation of the particle size probably plays an important role in the catalytic activity. In fact, recent

2128 J. Phys. Chem. C, Vol. 112, No. 6, 2008 TEM studies confirm that catalyst II has the smaller particle size and largest metal dispersion for the systems listed in Table 2.54 Conclusions Studies of in situ time-resolved XRD indicate that β-NiMoO4 and nH2O‚NiMoO4 are not stable and undergo partial reduction in the presence of CO or CO/H2O mixtures at high temperatures. At 450-550 °C, a NiMoO4 f Ni + MoO2 transformation occurs. Additional exposure to CO at 650-700 °C leads to further reduction of the oxide and the formation of a complex mixture of R-Mo2C, MoO2, and Ni. Active catalysts for the WGS can be synthesized using the nickel molybdates as precursors. The interaction of β-NiMoO4 with the WGS reactants at 500 °C produces particles of Ni (∼24 nm in size) and MoO2 (∼10 nm in size). These Ni-MoO2 systems displayed good catalytic activity at 350, 400, and 500 °C. At 350 and 400 °C, catalytic tests revealed that the NiMoO2 system was much more active than isolated Ni (some activity) or isolated MoO2 (negligible activity). Thus, cooperative interactions between the admetal and oxide support were probably responsible for the high WGS activity of Ni-MoO2. In a second synthetic approach, the NiMoO4 hydrate was reduced to a mixture of metallic Ni, NiO, and amorphous molybdenum oxide by direct reaction with H2 gas at 350 °C. In the first pass of the WGS reaction, MoO2 appeared gradually at 500 °C with a substantial increase of the catalytic activity. For these catalysts, the particle size of Ni (∼4 nm) was much smaller than that of the MoO2 (∼13 nm). These systems were found to be much more active WGS catalysts than Cu-MoO2, which in turn is superior to commercial low-temperature CuZnO catalysts. Acknowledgment. This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Chemical Science Division (DE-AC02-98CH10886). The NSLS is supported by the Materials and Chemical Sciences Divisions of DOE. We are grateful to S. Khalid for his assistance with the Ni K-edge XANES measurements. References and Notes (1) Patt, J.; Moon, D. J.; Phillips, C.; Thompson, L. Catal. Lett. 2000, 65, 193. (2) Trimm, D. L. Appl. Catal. A 2005, 296, 1. (3) Sasaki, K.; Zhang, J.; Wang, J.; Uribe, F.; Adzic, R. R. Res. Chem. Interm. 2006, 32, 543. (4) Kung, H. H. Transition Metal Oxides: Surface Chemistry and Catalysis; Elsevier: New York, 1989. (5) Delmon, B. Handbook of Heterogeneous Catalysis; VCH-Wiley: New York, 1997; p 264. (6) Rodriguez, J. A.; Kim, J. Y.; Hanson, J. C.; Brito, J. L. Catal. Lett. 2002, 82, 103. (7) Wang, X.; Rodriguez, J. A.; Hanson, J. C.; Perez, M.; Evans, J. J. Chem. Phys. 2005, 123, 221101. (8) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2002, 298, 811. (9) Rodriguez, J. A.; Wang, X.; Liu, P.; Wen, W.; Hrbek, J.; Pe´rez, M.; Evans, J. Top. Catal. 2007, 44, 73. (10) Rodriguez, J. A.; Liu, P.; Hrbek, J.; Evans, J.; Pe´rez, M. Angew. Chem., Int. Ed. 2007, 46, 1329. (11) Chupas, P. J.; Ciraolo, M. F.; Hanson, J. C.; Grey, C. J. Am. Chem. Soc. 2001, 123, 1694. (12) Ricote, S.; Jacobs, G.; Milling, M.; Ji, Y. Y.; Patterson, P. M.; Davis, B. H. Appl. Catal. A 2006, 303, 35.

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