One-Dimensional Ceria as Catalyst for the Low-Temperature

Dec 7, 2009 - Center for Functional Nanomaterials, BrookhaVen National ... of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973, and ...
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J. Phys. Chem. C 2009, 113, 21949–21955

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One-Dimensional Ceria as Catalyst for the Low-Temperature Water-Gas Shift Reaction Wei-Qiang Han,*,† Wen Wen,†,| Jonathan C. Hanson,‡ Xiaowei Teng,†,⊥ Nebojsa Marinkovic,§ and Jose´ A. Rodriguez‡ Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York 11973, Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973, and Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: July 14, 2009; ReVised Manuscript ReceiVed: NoVember 3, 2009

Synchrotron-based in situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to study pure ceria and Pd-loaded ceria nanotubes and nanorods (1D-ceria) as catalysts for the water-gas shift (WGS) reaction. While bulk ceria is very poor as WGS catalysts, pure 1D-ceria displayed catalytic activity at a temperature as low as 300 °C. The reduction of the pure 1D-ceria in pure hydrogen started at 150 °C, which is a much lower temperature than those previously reported for the reduction of 3D ceria nanoparticles. This low reduction temperature reflects the novel morphology of the oxide systems and may be responsible for the low-temperature WGS catalytic activity seen for the 1D-ceria. Pd-loaded 1D ceria displayed significant WGS activity starting at 200 °C. During pretreatment in H2, the ceria lattice parameter increased significantly around 60 °C, which indicates that Pd-oxygen interactions may facilitate the reduction of Pd-loaded 1Dceria. Pd and ceria both participate in the formation of the active sites for the catalytic reactions. The lowtemperature hydrogen pretreatment results in higher WGS activity for Pd-loaded 1D-ceria. Introduction The water-gas shift (WGS) reaction, which is typically applied to generate H2 through the reaction of a gas mixture of CO and H2O (CO + H2O f H2+CO2, ∆H°298 ) -41.1 kJ/ mol), is a catalytic process of well-known industrial importance.1,2 Based on thermodynamic and kinetic considerations, a high conversion of carbon monoxide has been obtained with a twobed operation at low (180-250 °C) and high-temperatures (350-500 °C). Cu/ZnO/Al2O3 and Fe2O3-Cr2O3 catalysts are commercial low- and high-temperature WGS catalysts, respectively, but they are not suitable for fuel-cell applications, mainly due to the lengthy and complex activation steps required before usage and their pyrophoric nature.3,4 Thus, it is critical to find efficient and safe catalysts for the WGS reaction. Recently, cerium oxide (CeO2) has been of great interest, particularly used to reduce the emissions of CO, NOx, and hydrocarbons from automobile exhaust,5 to abate soot formation in diesel fuels,6 and to minimize the CO content in fuel-cells.7 The key to these applications is that CeO2 can easily produce oxygen vacancies in an oxygen-deficient environment, shifting some Ce4+ to Ce3+ ions in the stable fluorite structure.8,9 Oxygen vacancies in ceria are considered to play an essential role in catalytic reactions.10-12 Ceria-supported noble-metal catalysts, especially Pd-loaded ceria, exhibit very interesting properties for the WGS reaction.13-18 Herein, we present the use of both pure and Pd-loaded onedimensional (1D) ceria, which is a mixture of nanotubes and nanorods, as catalysts for the WGS reaction at low-temperature. * To whom correspondence should be addressed. E-mail: [email protected]. † Center for Functional Nanomaterials, Brookhaven National Laboratory. ‡ Department of Chemistry, Brookhaven National Laboratory. § University of Delaware. | Present address: Experimental Division, Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, P. R. China, 201204. ⊥ Present address: Department of Chemical Engineering, University of New Hampshire, Durham, NH 03824.

Ceria nanotubes have outer and inner surfaces and thus can have a relatively large concentration of oxygen vacancies, which is essential for a better catalytic activity.10-12 Furthermore, the 1D character of these systems lessens the aggregation compared to 3D ceria nanoparticles and thus distributes the loaded-metals uniformly. We will focus on their applications as low-temperature shift catalysts. To our best knowledge, there is no report for using Pd-loaded 1D-ceria as a catalyst for the WGS reaction. From previous studies, it is known that different crystal faces of pure ceria and metals on ceria affect the WGS activity.19 Related effects will be described for our 1D ceria. Experimental Method Synthesis. The 1D-ceria was synthesized by a hydrothermal method with two successive stages: precipitation and aging. During the precipitation stage, an aqueous ammonia hydroxide was added into a cerium nitrate (Ce(NO3)3.6H2O) solution at 100 °C and followed by the aging, where the solution was quickly cooled to 0 °C inside a refrigerator.20 For the Pd-loaded sample, Pd was deposited on the 1D-ceria through a dropwise addition of palladium nitrate (Pd(NO3)2.xH2O) (1 wt %) into a suspension of 1D ceria in an aqueous solution. The solution was stirred for one day and then was washed with ethanol and distilled water. There is no way to measure the actual concentration of Pd in the samples. The cited value corresponds to the weight percentage of Pd added to the precursors’ solution. Electron Microscopy. Transmission electron microscopy (TEM) experiments were carried out with a 300 KV JEOL3000F electron microscope operated at 300 KV, equipped with a field emission gun and a high-resolution objective lens, as well as an energy-dispersive X-ray spectrometry (EDS). X-ray Diffraction. In situ time-resolved X-ray diffraction (XRD) experiments were performed at beamline X7B (λ ) 0.922 Å) of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory.21-24 Samples in the amount of 1-2 mg were loaded into a 1-mm sapphire capillary tube,

10.1021/jp9066444  2009 American Chemical Society Published on Web 12/07/2009

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which was attached to a flow system. A small resistance heater was wrapped around the capillary and the temperature was monitored with a chromel-alumel thermocouple which was placed in the capillary near the sample. A MAR345 image detector was used to record twodimensional X-ray patterns, and the powder rings were integrated with the FIT2D code.25 X-ray Absorption near-Edge Spectroscopy. The in situ Ce LIII-edge X-ray absorption near-edge spectra (XANES) were collected at the NSLS using beamline X19A in the “fluorescenceyield mode” with a PIPS (Passivated-implanted planar silicon) detector. The monochromator was a boomerang type flat crystal.22 During the measurements, the photon flux was detuned by 40% in order to avoid the disturbance of the higher order harmonics. The in situ Pd K-edge data were collected at beamline X18B of the NSLS in the transmission mode. A Si (1 1 1) channel cut crystal was used as the monochromator. The incident (I0), transmission (It), and reference (Iref) chambers are all filled with Ar gas for data acquisition. The sample was placed between the I0 and It chambers and a Pd reference foil was placed between the It and Iref chambers. At the Pd-K edge, it is not necessary to detune the monochromator. The XANES data were analyzed using the Athena module of the IFEFFIT program.26 Water-Gas Shift Reaction. The water-gas shift reaction was carried out isothermally in the flow cell at 200, 250, 300, and 350 °C. The sample was held at each temperature for 3 h for equilibration. The H2O vs CO vapor ratio was ∼ 0.35 by purging the 5% CO/He gas through a water bubbler at a flow rate of 10 mL/min. The water bubbler was at ambient temperature. The products from time-resolved XRD and XAFS experiments were measured with a 0-100 amu quadruple mass spectrometer (QMS, Stanford Research Systems). A portion of the exit gas flow passed through a leak valve and into the QMS vacuum chamber. This provided the relative pressure of the products. QMS signals at mass-to-charge ratios of 2 (H2), 4 (He), 18 (H2O), 28 (CO), and 44 (CO2) were monitored during the experiments, and these were recorded at the same time by an online computer. Temperature Programmed Reduction (Oxidation). A similar set up to that used for the WGS reaction was employed for the temperature programmed reduction (oxidation). Pure H2 (5% O2 in He) was used for the reduction (oxidation) process. The temperature ramp rate was ∼2 °C/min. Surface Area Analysis. BET (Brunauer, Emmett, and Teller) surface areas were measured by using a Mettler Toledo AB204 system. Samples were degassed under flowing UHP grade nitrogen for 2 h at a temperature of 200 °C and their surface area measured. Nitrogen gas adsorption measurements were taken at relative pressures of 0.05, 0.1, 0.15, 0.2, and 0.25 of Po using a Micromeritics Gemini 2360 instrument. The free space in the analysis tube was measured by the Helium method. The five pressure points were used to calculate the BET surface area. Results and Discussion TEM image of the pure 1D-ceria shows polycrystalline ceria nanorods and nanotubes (∼80%) together with ceria nanoparticles (∼20%) that had a diameter similar as that of the 1Dceria (Figure 1a). Mostly 1D-ceria are nanorods whose diameters typically range from 5-20 nms. Figures 1, panels b and c, are high-resolution TEM images of a ceria nanotube and a nanorod, respectively. In Figure 1b, the selected area diffraction patterns (obtained by fast Fourier transform (FFT) techniques) in the

Figure 1. Pure 1D ceria sample: (a) A low-magnification TEM image, (b) a high-magnification TEM image of a ceria NT, (c) a highmagnification TEM image of a ceria NR, and (d) a 3D plot of in situ time-resolved XRD patterns collected during the hydrogen reduction process.

upper right corner correspond to cubic ceria. The incident electron beam direction is along 〈110〉, i.e., the exposed crystal plane is (110). The axis of the ceria nanotube is along the 〈110〉 direction. Two kinds of lattice fringe directions attributed to (111) and (200) are observed, which have a respective inter-

Water-Gas Shift Reaction planar spacing of 3.1 and 2.7 Å. Figure 1c shows the ceria nanorod has the same crystalline features as the nanotube. For the 1D-ceria, the preferred exposed crystal planes for both nanorods and nanotubes are {110} and {100}, which is similar to that of the nanorods reported by Mai et al.27 The BET surface area of the 1D-ceria is 59 m2/g. Before the WGS reaction, the 1D-ceria was pretreated in pure H2 up to 400 °C for activation.22,24 The in situ time-resolved XRD patterns (Figure 1d) showed that the cubic-fluorite structure was retained and peak widths were nearly constant during the reduction process although there were significant lattice parameter changes from thermal expansion and partial reduction of the cerium oxide: from 5.43 Å at 25 °C to 5.47 Å at 400 °C. The reduction of pure 1D-ceria in H2 started at 150 °C, which is a much lower temperature than those previously reported for bulk or 3D ceria nanoparticles (i.e., nanoparticles with no preferred growth in any direction).10,22 Once the catalyst was cooled to ambient temperature, the gas was switched to 5% CO/He passing through a water bubbler. After equilibration, the WGS reaction was carried out isothermally at 200, 250, 300, and 350 °C, with a holding time of 4 h at each temperature. Figure 2a displays the H2 and CO2 relative pressure as a function of time. Bulk and 3D-ceria NPs exhibit a negligible catalytic activity for the WGS reaction.10,22 This is not the case for 1Dceria. In Figure 2a, the catalytic activity was increased with increasing temperature, becoming significant at 300 °C. A series of in situ XRD patterns connected during the WGS reaction showed no obvious changes however further analysis (see below) revealed changes in the amount of oxygen vacancies in the ceria structure and in the cell dimensions. It has been proposed28 that ceria participates in the WGS reaction when the oxygen vacancies formed by CO reduction facilitate the breakdown of the H2O to form H2 and O2- ions. The amount of oxygen vacancies can be determined under WGS conditions and H2 reduction conditions, from the lattice parameters of the ceria.22,28-30 This is because the cell expands when cerium is reduced from Ce4+ to Ce3+. However, the cell also expands because of thermal expansion; consequently one can only compare relative oxidation at a chosen temperature from lattice parameters. Figure 2b is the TEM image of the 1D ceria (∼60%) after the WGS reaction. Compared to the prereaction sample, the 1D-ceria are still crystalline and the preferred exposed crystal planes are still {110} and {100}, though the amount of 1D ceria slightly decreased. Figure 3 shows the lattice parameter of the ceria determined from the in situ diffraction during WGS and H2 reduction conditions as a function of temperature. In this figure we see that at 350 °C the cell expands more in the H2 reduction conditions than under WGS conditions. This shows that the presence of the H2O together with the CO partly reoxidizes the ceria. This is consistent with the hypothesis that the reaction of H2O at the O vacancy sites produces adsorbed O and H2 gas.22,27 To further enhance the catalytic activities at low-temperature, we loaded Pd (1% in weight) on the 1D-ceria. Examination with TEM of the Pd-loaded 1D-ceria sample did not find any isolated Pd0 or Pd-ion nanoparticles. EDS also did not detect a Pd signal. This indicates that there was no Pd-rich area. Figure 4a displays the in situ XRD patterns of the Pd-loaded 1D-ceria collected during the reduction process with pure H2 up to 200 °C. It was observed that the cubic-fluorite structure of ceria was stable during the reduction process, similar to the pure 1D-ceria. At room temperature, no other diffraction peaks were observed except for those of the 1D-ceria. Around 60 °C, the diffraction peaks of Pd started to appear. However only Pd (111) was

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Figure 2. Pure 1D ceria sample (a) H2 and CO2 relative pressure during the WGS reaction; (b) TEM image of the sample after WGS reaction.

observed due to the low concentration of the Pd as well as the broad dispersion of Pd on the 1D-ceria system. Combining both the XRD and TEM results, it thus can be concluded that the Pd existed in the starting sample as noncrystalline palladium species with size smaller than 1 nm (not observable by high-resolution TEM). In addition, the cerium oxide cell dimension was still 5.449 Å (see Table 1) when the sample was cooled under H2 flow to room temperature. This could be accounted for by stabilization of the reduced structure of the ceria in a flow of pure hydrogen or by storage of hydrogen in the ceria lattice.31 DFT calculations and experimental measurements of ceria cell dimension expansion from storage of hydrogen in the ceria lattice suggest that changes of 0.02 to 0.04 Å are possible under these conditions.32 After the catalyst was cooled to ambient temperature, the gas was switched to 5% CO/He flowing through a water bubbler. After stable reactant conditions were achieved at room temperature, the WGS reaction was carried out isothermally at 200, 250, 300, and 350 °C, with a holding time of 3 h at each temperature. Figure 4b displays the resulting H2 and CO2 relative pressure as a function of time. Some WGS activity was observed at 200 °C and the catalytic activity increased at the other three

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Figure 3. Lattice parameter of the ceria determine from the in situ diffraction during WGS and H2 reduction conditions as a function of temperature, which show relative cell expansion of H2 vs WGS.

temperatures ranges. The corresponding in situ time-resolved XRD patterns displayed structures of cubic ceria and metallic Pd with small cell changes. Under identical experimental conditions, the WGS reaction was repeated for the same sample in the two additional passes. Good catalytic activity was still observed for these two passes. The XRD patterns of the Pd-loaded 1D-ceria sample after WGS runs in CO/He/H2O showed the Pd (111) shifted to lower two theta angle after the first pass of the WGS. The details of the lattice parameters of the Pd and ceria are displayed in Table 1. It was found that the lattice parameters of Pd decreased after each pass of the WGS reaction. However, after H2 reduction (RT, H2 flow), the lattice parameter of Pd is 4.035 Å, which is much larger than that of the measured Pd lattice parameter of 3.889 Å (powder diffraction file 46-1043). Since the lattice parameter of PdH0.76 is 4.02 Å (powder diffraction file 19-0951), it is likely that the Pd is in the hydride form. The in situ timeresolved XRD pattern of Pd-loaded 1D-ceria during the pretreatment in pure hydrogen displays a shift of the diffraction peak of Pd (111) to lower two theta angle as soon as the temperature was decreased to ambient temperature. This suggests that PdHx is formed during cooling.33 Figure 4c is the lattice parameters of the ceria determined from the in situ diffraction during WGS first pass and second pass, and we observe that the cell parameters are less in the second pass. This could be correlated to the activity decrease after each pass, since the decrease of the ceria lattice parameter indicates less extensive reduction and thus fewer oxygen vacancies as in the succeeding WGS reaction. Lattice constants were determined by a Rietveld analysis using the GSAS (General Structure Analysis System) program.34-36 The full series of powder patterns was refined using the sequential refinement feature in GSAS for the usual structural parameters. In situ XANES measurements were used to study the oxidation state of the Pd and Ce in the 1D-ceria during the catalytic process. The Pd K-edge XANES spectra of the Pdloaded 1D-ceria sample are displayed in Figure 5a. The absorption edge of the starting material is at a higher energy position compared with that of the Pd foil, and this suggests that the Pd is ionic. This much larger white-line intensity, also suggests that the Pd is ionic in the starting material. After purging at room temperature in H2 for 3 h, the Pd K-edge spectra

Figure 4. Pd-loaded 1D-ceria sample: (a) A 3D plot of in situ timeresolved XRD patterns collected during the reduction process. (b) H2 and CO2 relative pressure during the 1st pass of the WGS reaction using the catalyst of Pd-loaded ceria. The catalyst was at first ramped to 200 °C in H2. (c) The lattice parameters of the ceria determined from the in situ diffraction during WGS first pass and second pass.

of 1D Pd-ceria resembled that of the Pd foil, indicating a reduction of the Pd species even at ambient temperature, although the spectrum still displayed slightly larger white-line intensity compared with that of the Pd foil. This indicated that

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TABLE 1: Detailed Analysis of the XRD Patterns for Pd-Loaded CeO2 Catalyst lattice parameter (Å) sample

CeO2 CeO2 From 400 peak

starting material 5.423 after H2 reduce At RT in H2 5.449 before 1st WGS 5.427 before 2nd WGS 5.435 before 3rd WGS 5.433

5.427 5.432

Pd

4.035 3.948 3.938

the Pd ions are partially reduced. Pd hydride could also be formed that has a similar XANES pattern compared with that of the Pd foil.37 However, Pd hydride displays a slightly larger white-line intensity in the Pd-K edge XANES as well as some shift of peaks ii and iii to relatively lower energy value. For a similar cluster of Pd or Pd hydride, it is observed that (Er - Eb)R2 ) constant, where R is the interatomic distance, Er is the energy of resonance, and Eb is the energy of a bound state at threshold.38,39 Thus, this shift essentially reflects the increase of the Pd-Pd bond distance due to the increase of the lattice parameter during hydride formation process, which is confirmed by the XRD pattern. Also shown in Figure 5a, at 300 °C, the Pd K-edge spectrum of the catalyst is very similar to that of the Pd foil, pointing to a complete reduction of Pd to a metallic form. Figure 5b is the Ce LIII-edge XANES spectrum of the Pd-loaded 1D-ceria sample during the reduction process. The Ce LIII-edge shifted to a lower energy position in pure hydrogen atmosphere at ambient temperature. This is consistent with the results of the Pd K-edge XANES, where the sample of Pd-loaded 1D-ceria could be partially reduced at ambient temperature. At 60 °C, the Ce LIII-edge shifted to an even lower position, which indicates the further reduction of the ceria. At 200 °C, the shift of the Ce LIII-edge does not shift much more. It is also noted that there is a shoulder peak at 5727.4 eV in pristine Pd-ceria catalyst (highlighted by the arrow), pointing to the existence of some Ce3+ ions. The intensity of this shoulder increased gradually as the temperature was raised, indicating an increase of the Ce3+ ions concentration. In CO/H2O, the Ce LIII-edge spectrum of the Pd-loaded 1D ceria was similar to that in H2 atmosphere at ambient temperature, indicating a slight oxidation of the catalyst after fully reduced in H2 at 200 °C (Figure 5c). During the WGS reaction, the Ce LIII-edge position shifted to lower energy at 350 °C, with the observation of intensity increase at 5727.4 eV, pointing to the reduction of ceria in the Pd-ceria catalyst under the WGS reaction condition. To understand the interaction between the Pd-loaded ceria with the pure 1D-ceria support further, we compare the ceria lattice parameter changes of pure 1D-ceria, Pd-loaded 1D-ceria, and 3D-nano ceria during the hydrogen reduction process (Figure 6). For pure 1D-ceria, the lattice parameter of ceria barely changed at the beginning of the ramping process and increased after the onset temperature of 150 °C. At 350 °C, the lattice expanded ∼0.04 Å compared with that at 150 °C, which is the consequence of Ce4+ reduction to Ce3+ ions, since the radius of the Ce3+ (1.14 Å) is much larger than that of the Ce4+ (0.97 Å). This reduction temperature was much lower than that reported by TPR for ceria nanoparticles (490 °C).40,41 The special 1D-feature, which increased the effective surface area by reducing the aggregation problem of nanoparticles and also benefiting from the double surfaces of nanotubes, contributed to the decrease of the reduction temperature, which is probably responsible for the low-temperature catalytic activity. During the hydrogen reduction process, a dramatic increase of the ceria lattice parameter (0.025 Å) was observed for Pd-

Figure 5. Pd-loaded 1D-ceria sample: (a) Pd K-edge XANES spectra during the reduction process, (b) Ce LIII-edge XANES spectra during the reduction process, and (c) Ce LIII-edges XANES spectra during the WGS reaction.

loaded 1D-ceria when the temperature was about 60 °C, and concomitantly, metallic Pd appeared. This suggested that the interactions between Pd cluster and the 1D-ceria might facilitate the reduction of Pd-loaded 1D-ceria, which was consistent with the literature that Au (or Cu) can activate the surface of ceria and decrease the ceria reduction temperature.42

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Figure 6. Ceria lattice parameter changes during the temperature ramping of 1D-ceria, Pd-loaded 1D-ceria, and 3D-nanoceria.

The Pd-loaded CeO2 could be reduced at much lower temperatures (∼100 °C lower) compared with those for pure CeO2, where a large amount of oxygen vacancies could be produced to participate in the formation of the active sites for WGS reaction. As observed from Figure 6, pure CeO2 was significantly reduced at 300 °C, which leads to the good catalytic activity observed in Figure 2a, mainly due to the 1D nature of the CeO2, whose preferred exposed crystal planes are {110} and {100}. The {110}/{100} dominated surface structures are more reactive for CO oxidation than the {111}-dominated one.27 To investigate the effect of pretreatments of hydrogen to the catalysts, we repeated the activity measurements combined with subsequent TEM measurements. The relative WGS catalytic activity at 250 °C for samples of Pd-loaded (1%) 1D-ceria pretreated in H2 up to 400 and 200 °C are 1.83% and 2.96%, respectively. For Pd-loaded 1D-ceria samples, the one pretreated in H2 at 200 °C has a much higher activity than that treated in H2 at 400 °C. For the Pd-loaded ceria samples, the percentages of 1D-nanostructures remaining after WGS reaction were 25% for both samples pretreated in hydrogen at 200 and 400 °C. The preferred exposed crystal planes for these remaining 1Dcerias are still {110} and {100} for both samples pretreated in hydrogen at 200 and 400 °C (Figure 7, panels a and c). For both samples, a large amount of 1D-ceria nanostructures were broken down into nanoparticles after a couple of redox cycles. TEM image shows that the NPs of the sample pretreated in the hydrogen atmosphere at 200 °C after WGS reaction are often polyhedras with clear crystalline facets, and the preferred exposed surfaces are still {100} and {110} (Figure 7b). Many NPs of the sample pretreated in the hydrogen atmosphere at 400 °C after WGS reaction do not have clear crystalline facets and preferred crystalline orientation (Figure 7d). Besides {100} and {110}, there are many other exposed surfaces, such as (111), which is a nonfavored surface for CO oxidation.27 This might explain why the 1D-ceria sample pretreated in hydrogen at 200 °C has a better WGS activity. Zhou et al. reported that the ceria surface undergos structure evolution under redox conditions since the surface area of NPs shrinks from 89 to only 35 m2/g after a single redox cycle at 873 K, which hints that the change of crystallographic orientations of ceria is occuring.43 Another factor for the decreasing of the surface area could be aggregation of NPs after the reaction. In the present work, a higher reducing temperature introduces more changes of crystallographic faces to those that are not favored for the activity. Figure 8 shows a

Figure 7. TEM images of Pd-loaded 1D-ceria reduced pretreated in hydrogen at 200 °C after WGS reaction: (a) nanorods and (b) nanoparticles. TEM images of Pd-loaded 1D-ceria pretreated in hydrogen at 400 °C after WGS reaction (c) nanorods and (d) nanoparticles.

Figure 8. Schematic image of the evolution of a nanorod (a) breaking down into faceted NPs and (b) subsequently changing to nonfaceted NPs.

schematic image of the evolution of a nanorod breaking down into faceted NPs and subsequently changing to nonfaceted NPs. Various methods have been used to prepare special ceria morphologies that have enhanced reducibility.27,44-47 Yu et al. prepared ceria nanocrystals with spherical, wire, and tapole shapes from a nonhydrolytic sol-gel reaction of cerium(III) nitrate and diphenyl in the presence of surfactants.44 Natile et al. made ceria NPs by two different synthetic routes: precipitation from a basic solution (sizes around 8-15 nm) and microwave-assisted heating hydrolysis (size around (3.3-4.0 nm).43 They found that ceria NPs made by the later method are more reduced than those made by the former method. The methanol oxidation is also favored on the ceria NPs prepared by the later method because of the high specific area and higher presence of active sites of Ce(III) cations.45 Zr4+ and La3+ doped porous ceria NPs with a high BET surface area of 160 m2/g were prepared and exhibited a photovoltaic response, which was directly derived from the nanometric particle size; normal ceria does not show this response.46 Porous hollow microspheres of ceria with high surface area (144.1 m2/g) were synthesized by a carbon sphere template method.47 XPS shows that the atomic ratio of O and Ce of the porous microspheres is about 6.3, i.e., a high Ce(III) ratio.47 All of the above results show ceria with high surface area can increase the Ce(III) ratio which leads to

Water-Gas Shift Reaction high reducibility. Because of having both outer and inner surfaces, nanotubes have a much higher surface area compared to nanorods and nanoparticles with the same diameters. Techniques to make high yield ceria nanotubes with sustainable stability during the WGS reaction is still challenging and worthy of further efforts. Conclusion As a summary, the high catalytic activity of pure and Pdloaded 1D-ceria has been observed in the WGS reaction at lowtemperature. Both CeO2 and Pd play very important roles during the WGS reaction process. While CeO2 could be easily reduced at low temperature to produce oxygen vacancies, Pd can activate the reduction of CeO2. It seems that the WGS reaction showed herein follows the redox mechanism, where CO reduces ceria and forms oxygen vacancies and the H2O molecule reacts with the oxygen vacancies of the ceria. The changes of lattice parameter for both ceria and Pd play an important role for the activity. The hydrogen pretreatment temperature plays one of the key roles for the catalytic activity of pure 1D-ceria and Pd loaded 1D-ceria. The special 1D feature could efficiently increase the catalytic activity of ceria because the effective surface area can be increased by reducing the aggregation problem of nanoparticles and also benefiting from the double surfaces of nanotubes, which decrease the reduction temperature of CeO2. In addition, the increased activity can be correlated with the crystal faces present in the different nano crystalline morphologies. Acknowledgment. This work is supported by the U.S. DOE under Contract DE-AC02-98CH10886 and Laboratory Directed Research and Development Fund of Brookhaven National Laboratory. This research is carried out at X7B, X18B, and X19A of the National Synchrotron Light Source, which is supported and maintained by DOE under the Contracts DEFG05-89ER45384 and DE-AC02-76CH00016. References and Notes (1) Twigg, M. V. Catalyst Handbook, 2nd ed.; Wolfe: U.K., 1989. (2) Panagiotopoulou, P.; Kondarides, D. I. J. Catal. 2004, 225, 327. (3) Spencer, M. S. Top Catal. 1999, 8, 259. (4) Burch, R. Phys. Chem. Chem. Phys. 2006, 8, 5483. (5) Trovarelli, A. Catalysis by Ceria and Related Materials; Imperial College Press: London, 2002. (6) Summers, J. C.; Van Houtte, S.; Psaras, D. Appl. Catal., B 1996, 10, 139. (7) Shao, Z.; Haile, S. M.; Ahn, J.; Ronney, P. D.; Zhan, Z.; Barnett, S. A. Nature 2005, 435, 795. (8) Tsunekawa, S.; Ishikawa, K.; Li, Z. Q.; Kawazoe, Y.; Kasuya, A. Phys. ReV. Lett. 2000, 85, 3440. (9) Zhang, F.; Chan, S. W.; Spanier, J. E.; Apak, E.; Jin, Q.; Robinson, R. D. Appl. Phys. Lett. 2002, 80, 127. (10) Trovarelli, A. Catal. ReV. Sci. Eng. 1996, 38, 439. (11) Zhu, T.; Kundakovic, L.; Dreher, A.; Flytzani-Stephanopoulos, M. Catal. Today 1999, 50, 381. (12) Cordatos, H.; Ford, D.; Gorte, R. J. Phys. Chem. 1996, 100, 18128.

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