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J. Phys. Chem. C 2009, 113, 12465–12475

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Pretreatment-Induced Nanostructural Evolution in CeO2-, Sm2O3-, and CeO2/ Sm2O3-Supported Pd Catalysts for Intermediate-Temperature Methanol Fuel Cells R. T. Baker,*,† L. M. Go´mez-Sainero,‡ and I. S. Metcalfe§ EaStChem, School of Chemistry, UniVersity of St Andrews, North Haugh, St Andrews, Fife, KY16 9ST, U.K., Seccioń de Ingenierıá Quı´mica, Facultad de Ciencias, UniVersidad Autońoma de Madrid, Cantoblanco, 28049 Madrid, Spain, School of Chemical Engineering and AdVanced Materials, Newcastle UniVersity, Newcastle upon Tyne NE1 7RU, U.K. ReceiVed: August 22, 2008; ReVised Manuscript ReceiVed: May 13, 2009

Catalysts of 2 wt % Pd supported on CeO2, Sm2O3, and CeO2/Sm2O3 (in a 4:1 molar ratio) were studied in detail by high-resolution transmission electron microscopy and elemental mapping, both as-prepared and after undergoing one of three reduction pretreatments of increasing severity. The evolution in the nanostructure, composition, and disposition of the phases were studied in detail as a function of starting composition and of pretreatment conditions. The trends observed were compared with the trends in activity and selectivity of the catalysts for hydrogen generation from methanol fuel, with a view to their application in direct methanol intermediate-temperature solid oxide fuel cells. The catalyst preparation conditions had a dramatic effect on the Sm-containing materials but not on the Pd/CeO2. The Pd/CeO2/Sm2O3 catalyst was seen to develop a beneficial hierarchical structure in which Pd particles were supported on fine Sm2O3 crystallites, which were in turn supported on larger CeO2 particles. This effect may be useful for the deliberate design of such nanostructured catalysts. 1. Introduction Methanol may become an important fuel in the future for use as a convenient energy vector for efficient and clean power generation in electric motor vehicles, stationary applications, and mobile and portable electronic devices. Methanol has several advantages over other fuels, leading to discussion of the Methanol Economy.1 Because it is a liquid under ambient conditions, it has a high volumetric energy density and it can be much more easily and efficiently stored, transported, and handled than gaseous and more volatile fuels like hydrogen, methane, and higher hydrocarbons. Desulfurization of methanol at point of use is not required and large tonnages of methanol are already transported globally and routinely in the chemicals sector. Most methanol is currently obtained from fossil fuels but the possibility of its derivation from biomass sources is increasingly attractive. Methanol can be easily converted to H2 by decomposition, oxidative reforming,2 and steam reforming reactions. It can be externally reformed to provide a H2-rich gas feed, for a fuel cell system, for example. However, this elevates the cost of the overall process. Internal reforming of methanol - that is, within the anode compartment of the fuel cell - is a more desirable option. Among the many fuel-cell designs, solid oxide fuel cells (SOFCs) are potentially well suited to run on hydrocarbon and oxyhydrocarbon fuels because of their relatively high operating temperatures and the fact that they are constructed entirely of solid ceramic components, which gives them greater fuel flexibility. However, SOFCs usually operate at temperatures of around 900 °C. Lowering the temperature below about 700 °C would allow integration with conventional balance-of-plant * To whom correspondence should be addressed. E-mail: rtb5@ st-andrews.ac.uk. Tel: +44 1334 463899; Fax: +44 1334 463808. † University of St Andrews. ‡ Universidad Autońoma de Madrid. § Newcastle University.

materials, such as engineering steels.3 Furthermore, by reducing the operating temperature further, to around 500 °C, problems of slow start-up, caused by the time required to heat the cell to its operating temperature, in electric motor vehicle applications would become manageable.4 Several catalyst systems have been investigated for the internal steam reforming of methanol in the anode chamber of SOFCs. Ni-YSZ (yttria stabilized zirconia) cermets are widely used as anode materials in SOFCs and some authors have evaluated Ni-based systems for use with methanol fuels. Laosiripojona et al.5 reported serious catalyst deactivation of Ni/YSZ catalysts during steam reforming of methane, methanol, and ethanol, which they demonstrated to be caused by deposition of carbon on the catalyst. For methanol reforming, it was necessary to increase the operating temperature to 1000 °C to avoid carbon deposition. Sasaki et al.6 successfully demonstrated internal steam reforming of C1-C4 alcohols over Ni-YSZ and Ni-SSZ (scandia stabilized zirconia) catalysts at 1000 °C. Methanol gave similar cell performance to simulated reforming gas at this temperature. However, on decreasing the operating temperature, methanol conversion fell, increasing the concentration of unreacted methanol and byproducts and reducing the H2 yield. The authors recommended that more active catalyst systems be developed to increase reaction rates at such temperatures. The operation of SOFCs with direct methanol feeds at reduced temperature using Ni-ZrO2 cermets has been found to be limited by slow internal reforming of methanol to H2 at the SOFC anode by other authors. In one study, an intermediate temperature (IT-) SOFC with such an anode composition was evaluated in direct methanol operation. Power densities fell by half compared to the same cell with a H2 feed or with externally steam-reformed methanol.7,8 The poorer performance was attributed to the lower concentration of H2 to which the anode was exposed because of the inefficiency of the internal reforming of methanol over the Ni-containing anode.

10.1021/jp8075194 CCC: $40.75  2009 American Chemical Society Published on Web 06/11/2009

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Cu-based catalysts are very active and selective for conventional methanol reforming.9-11 However, a major problem here is deactivation of the catalyst - at temperatures as low as 300 °C12 - because of coarsening of the Cu particles. Nevertheless, Kim et al.13 and Brett et al.14 both employed Cu anode materials supported on CeO2-based supports in direct methanol SOFCs (with no or limited steam addition). These groups showed reasonable to good cell performance, depending on catalyst composition, but both groups also reported significant catalyst deactivation, which they attributed to sintering of the CeO2 additive, because of the very low oxygen partial pressures,13 or to coarsening of the Cu during operation at 600 °C.14 Brett et al. concluded that the Cu-based catalysts were effective for methanol decomposition and reforming but were poor electrocatalysts for H2 oxidation. Therefore, other catalyst systems are sought for this application. Transition metals such as Pd and Pt are known to be good catalysts for reforming and to be more durable under operating conditions than Cu catalysts.1,12,15-17 Pd has a cost advantage over Pt. Furthermore, Pd is also used in methanol synthesis,18 the oxidative reforming of methanol,2 and as a hydrocarbon oxidation catalyst in automotive exhaust catalysts. For these reasons, we used Pd in our SOFC anode catalysts in the present work. As in the above studies,13,14 we also used a CeO2-based oxide to support the metallic function. In a working SOFC, this would supply O2- ions from the electrolyte to the site of H2 oxidation. Ceria-samaria (CS) mixtures have shown good performance in SOFC anodes. Uchida et al.19 used CS anodes with and without a highly dispersed Ru loading to enhance the activity of the catalyst in a medium-temperature SOFC operating under H2 and CH4 fuels. More recently, a direct methanol high-temperature SOFC with a range of loadings of CS in the electrodes was evaluated.20 The CS-metal cermets used gave rise to a higher open-circuit potential than Ni-YSZ. Furthermore, Feng at al21 used CS as the support for a study of Cu-, Ni-, and Co-based anode catalysts in a low-temperature direct methanol SOFC. Therefore, it was of interest to study the catalytic behavior of Pd/CS and related catalysts, with a view to their use in methanolfueled IT-SOFC systems. In a previous study, our group22 investigated the catalytic activity and selectivity of three catalyst compositions for the generation of hydrogen from methanol via reforming and decomposition reactions. The aim was to determine the suitability of these materials for application as anodes in direct methanol IT-SOFCs. The catalysts all contained a 2 wt % loading of Pd supported on CeO2, Sm2O3, or (CeO2)0.8(Sm2O3)0.2 (denoted CS here) oxide supports. Full experimental details and results are presented elsewhere.22 In the current work, the same catalyst samples were used as starting materials for a detailed study of the evolution of the catalyst nanostructure as induced by a range of increasingly severe reductive pretreatments. Changes in the structure, composition, and phase of these catalyst compositions were investigated using high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) with the aim of explaining the changes in their catalytic activity in greater depth. HRTEM is proving to be an invaluable tool for the analysis of the composition, structure, and disposition of the phases present in heterogeneous catalysts on length scales down to the atomic scale.23 2. Experimental Section The catalyst samples had been prepared in a previous study22 and consisted of 2 wt % Pd supported on CeO2, Sm2O3, and

Baker et al. CS, as well as samples of the support materials alone. Briefly, the preparation employed CeO2 and Sm2O3 powders of 99.9% purity (Aldrich) to be used as supports directly and to prepare the CS material. This was done by milling the powders together in a CeO2:Sm2O3 molar ratio of 4:1 and calcining the milled powder at 800 °C for 2 h in air. Pd impregnation was carried out by the incipient wetness technique from an aqueous solution of H2PdCl4 (pH ∼1). After impregnation, the catalyst samples were dried and calcined in air at a final temperature of 400 °C for 2 h. Further preparative details can be found elsewhere.22 The catalyst samples were examined using HRTEM. The instrument used was a JEOL JEM 2011 (a modified 2010 unit) operating at 200 keV with a LaB6 filament and having a structural resolution of 0.194 nm. The instrument was equipped with a Gatan CCD camera and an ISIS X-ray spectrometer for high spatial resolution elemental analysis by EDS. All samples were prepared on 3 mm Cu grids coated with holey carbon film. As-prepared samples of the catalyst support materials, CeO2 and Sm2O3, and the Pd/CS catalyst, were studied in the TEM. Each of the catalysts, Pd/CeO2, Pd/Sm2O3, and Pd/CS, were subjected to two different pretreatment procedures and the resulting six samples were also studied by HRTEM. The two Sm-containing catalyst samples were also studied after they had been subjected to a more severe pretreatment. The pretreatments involved sequential reduction in flowing pure, dry H2 for 1 h, purging in flowing dry N2 for 1 h and passivation. During the passivation step, the sample tube was cooled in an acetone ice bath and the gas flow switched from N2 to dry synthetic air. The sample tube was allowed to warm slowly to ambient temperature. The purpose of the passivation step was to allow any reoxidation of the oxide supports to proceed slowly and in a controlled manner while also avoiding oxidation of the metallic Pd particles so maintaining the Pd in its reduced state. All flow rates were 60 mL/min (STP). The three pretreatments - mild, moderate, and severe - differed only in the temperatures of reduction and of purging, which were, respectively, and in order of increasing severity of pretreatment: 150 and 500 °C (henceforth identified with the suffix A); both at 500 °C (suffix B); and 500 and 900 °C (suffix C). The purging step at 500 °C was intended to allow the removal of the adsorbed water and hydroxyl species, formed during the reduction step, from the catalyst surfaces. In pretreatment C, the sintering behavior of the catalyst materials was to be investigated under extreme conditions so a purging temperature of 900 °C was used. The purpose of these pretreatments was to cover the range of prereduction conditions experienced by the samples prior to the catalytic activity tests described in our previous article.22 In this way, it was possible to investigate the evolution in the nanostructure of the catalyst materials as a function of pretreatment - by employing HRTEM - and to relate these structural trends to the trends in catalytic activity observed in our previous work.22 To provide a direct comparison with the catalytic activity results of an individual catalyst composition studied in the previous work of the authors,22 an additional sample of fresh Pd/CS was prepared by reduction in flowing 5%H2/Ar for 1 h at 300 °C (without purging in inert). The sample was allowed to cool and was passivated and prepared for inspection in the TEM as described above. This sample was named Pd/CSR300. HRTEM images obtained with the CCD camera were analyzed using the Digital Micrograph software. Digital diffraction patterns (DDPs) were obtained from these images by performing a Fourier Transform on selected regions of interest. The Inorganic Crystal Structure Database (part of the Chemical

Intermediate-Temperature Methanol Fuel Cells

Figure 1. HRTEM images of samples of (a) the CeO2 support viewed tilted away from the [110] zone axis and (b) the Sm2O3 support viewed along a high index zone axis close to the [110] zone axis (text). DDPs are inset for each and spots labeled with Miller indices.

Database Service24) and the electron diffraction pattern analysis and image simulation software, EMS,25-27 were used to index the diffraction patterns to known crystal phases. The EDS instrument was used to collect X-ray spectra from points in the sample and from wider regions of interest (ROI). Energy ranges in the EDS spectrum that corresponded uniquely to each of the elements Pd, O, Sm, and Ce were identified and X-rays emitted in these ranges were collected to build up the respective elemental maps. XRD patterns of the starting CeO2 and Sm2O3 support materials are presented elsewhere.22 The XRD pattern of the CeO2 showed the expected cubic structure with space group Fm3m j (225) and a ) 5.41 Å.28 The pattern for the Sm2O3 starting material was interpreted as a mixture of the monoclinic phase with space group C2/m (12) and a ) 14.18 Å, b ) 3.63 Å, c ) 8.85 Å, R ) γ ) 90°, β ) 99.97°29 and a cubic phase of space group Ia3j (206)30,31 with a ) 10.93 Å. No new diffraction lines were observed in XRD on mixing the CeO2 and Sm2O3 starting materials and calcining either at 800 °C for 2 h or at 1000 °C for 2 h. Therefore, no new phases were obvious in the (CeO2)0.8(Sm2O3)0.2 material. However, mixed oxides of Ce and Sm, SmxCe1-xO2-δ, over the composition range, x ) 0.1-0.4, are reported32 to have the same crystal structure, Fm3m j , and very similar unit cell dimensions to pure CeO2. Therefore, the existence of some samarium cerium oxide could not be discounted on the basis of the XRD data. For the same reasons, it is unrealistic to expect to distinguish any of these mixed oxides from pure CeO2 by electron diffraction in the TEM. It should be noted that a further ambient pressure cubic phase of Sm2O3 - with space group I213 (199) - has been reported in an electron diffraction study (with a ) 10.93 Å),33 and another report (with a ) 10.85 Å).34 Metallic Pd was expected to have a cubic crystal structure, also with space group Fm3m j , and a ) 3.89 Å.35 Electron diffraction patterns obtained in the TEM were compared with these crystal structures for identification and indexing. 3. Results 3.1. Support Materials. The images in Figure 1 are of the two support materials, CeO2 and Sm2O3, and demonstrate that these starting materials were highly crystalline. DDPs were obtained from the images and are inset in the figure. The crystal in part a of Figure 1 was indexed to cubic CeO2 (space group Fm3m j ) viewed along a direction tilted slightly away from the [110] zone axis. This is in agreement with the XRD pattern recorded for this material.22 Some but not all of the diffraction spots observed in the DDP of the crystal in part b of Figure 1 can be indexed to the [335] zone axis of cubic Sm2O3.

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12467 Alternatively, all spots can be included if it is accepted that the pattern is a slightly distorted version of the pattern expected when viewed along the [111] zone axes of cubic Sm2O3. On balance, the pattern can best be explained as the cubic I213 Sm2O3 phase viewed along a high index zone axis close to the [111] zone axis. Lattice images of the known monoclinic phase (space group C2/m) were not observed in the TEM images. However, as discussed above, the XRD pattern of this sample indicates the presence of both cubic and monoclinic Sm2O3 phases.22 The CS support was also found to consist entirely of regular crystallites. Mixed oxides of Sm and Ce have been reported,32 which have a very similar crystallography to pure CeO2. However, in this material, interplanar spacings indicative of both Sm2O3 and CeO2 crystal phases were measured, implying that formation of mixed oxides was not widespread, in agreement with the XRD results. In summary, all three supports were highly crystalline, as would be expected. In the following sections, we will consider the HRTEM results obtained for the CeO2-, Sm2O3-, and CS-supported Pd catalyst samples prepared by impregnation of aqueous H2PdCl4 solution, drying, and calcination at 400 °C.22 These samples were studied in the TEM after undergoing the three reductive pretreatments, A, B, and C, described above. In addition, the Pd-CS sample was studied in its as-prepared state. 3.2. Pd-CeO2. After the mildest of the reductive pretreatments, the sample, Pd/CeO2-A, exhibited crystalline support particles with small, well-dispersed Pd particles on the surface of some of these, as seen in part a of Figure 2. Examples of such Pd particles are indicated. In parts b and c of Figure 2, the Pd particles have regular shapes consistent with the truncated cuboctahedron previously described in HRTEM studies of supported metal nanoparticles.36 Furthermore, the particles are generally orientated so that their planes are aligned with those of the support oxide. In part b of Figure 2, the DDP of the upper Pd particle is inset in the figure. The planes identified from the DDP are indicated in the image. One set of (111) planes of the Pd particle is seen to be aligned in an epitaxial arrangement with the corresponding (111) planes of the underlying CeO2 support particle. The corresponding interplanar spacings are 2.25 and 3.12 Å, respectively. In part c of Figure 2, DDPs were obtained for the lower Pd particle alone, the nearby oxide material alone, and for the Pd particle and the oxide support material in its immediate vicinity together. Both the Pd and the CeO2 support can be indexed as viewed along their respective [110] zone axes. Although the DDPs are rather noisy because of the small size of the regions sampled, it is still clear that the Pd and CeO2 systems are aligned with each other along their respective (002) planes. The corresponding interplanar spacings are 1.95 and 2.71 Å, respectively. Most of the Pd particles imaged in this sample had diameters on the order of 2-3 nm. A slightly larger particle, with diameter of about 5 nm, is shown in part d of Figure 3. Again, this lies in the [110] zone axis and the DDP is inset in the image. The additional spots in the DDP are derived from the oxide support material, which lay below the Pd particle and not in contact with it. When the same sample was subjected to pretreatment B, an evolution is noted in the general form of the Pd particles. The majority of the Pd particles were slightly larger than for pretreatment A, at 3-4 nm. Two particles of CeO2 supporting numerous Pd particles of such dimensions are seen in part e of Figure 2 and at higher magnification, with resolution of some

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Figure 2. HRTEM images of Pd/CeO2-A: (a) Intermediate magnification image showing the small size and the distribution of the Pd particles on the CeO2 support. (b) Pd particle with (111) planes aligned with those of the CeO2 support. DDP of the Pd particle alone is inset and the corresponding planes are indicated on the image. (c) Pd particle with (002) planes aligned with those of the CeO2 support. DDPs are of Pd particle only (left), of oxide support only (right), and of Pd particle and associated region of support (center). (d) A slightly larger Pd particle with DDP inset. Pd particles in (b), (c), and (d) are viewed in the [110] zone axis. HRTEM images of Pd/CeO2-B: (e) Two particles of CeO2 support covered by many small Pd particles and (f) a detail of this with some of the Pd planes resolved. (g) Two larger (∼8 nm) supported Pd particles with the DDP of one inset. (h) A higher magnification image of the same particle showing all planes viewed along the [110] zone axis.

Pd planes, in part f of Figure 2. The particles appear to maintain a regular morphology. Occasional larger particles were observed. The particles in part g of Figure 2 are about 8 nm in diameter. The lower of the two is seen at higher magnification in part h of Figure 2. This particle was imaged along the [110] zone axis and gave rise to an excellent DDP in which all spots are well resolved, including those assigned to the (22j0) planes that have an interplanar spacing in Pd of 1.38 Å.

Baker et al.

Figure 3. HRTEM images of Pd/Sm2O3-A: (a) Intermediate magnification image showing the rounded, irregular nature of the support particles. (b) and (c) Typical support particles showing lack of longrange order. Inset DDPs show semiamorphous nature of these particles. (d) Pd particle in a region of crystalline support material. The (002) planes of the Pd particle are aligned with planes of the underlying support as indicated by the corresponding spots in the inset DDP being collinear. HRTEM images of Pd/Sm2O3-B showing the widespread crystallinity of the support: (e) Support crystal occupying almost all of image and giving rise to the DDP inset. This was indexed to cubic Sm2O3 viewed along the [515] zone axis. (f) Crystal of approximately 8 nm diameter (ringed) indexed here to monoclinic Sm2O3 viewed along the [1j3j4] zone axis (DDP inset). (g) Small crystal of support material giving rise to a clear DDP (inset), which was indexed to cubic Sm2O3 viewed along the [103] zone axis. (h) Support crystal giving a partial DDP, which was indexed to cubic Sm2O3 viewed along the [112] zone axis.

3.3. Pd-Sm2O3. The Pd/Sm2O3-A sample exhibited a completely different structure from that of the corresponding CeO2-supported sample, as shown in Figure 3. The support material was found to be largely semiamorphous, showing very little long-range order. The rounded and irregular morphology of the support material is shown in part a of Figure 3 and the

Intermediate-Temperature Methanol Fuel Cells lack of crystalline order is demonstrated by the images and the DDPs in parts b and c of Figure 3. The DDPs consist of diffuse rings rather than resolved spots. The interpretation of this, for an image taken at such high magnification, is that certain ranges of interplanar distances were represented but that no crystallites of significant size were present. The curving surface of the particles, inconsistent with long-range crystalline order, is also clear. Very few Pd particles were identified compared to the equivalent Pd/CeO2-A sample. When they were present, this was almost always in association with regions of atypical, crystalline support material, as shown in part d of Figure 3. This particle was identified as Pd from the DDP (inset) and is viewed along the [110] zone axis. From the DDP, it can be determined that the (002) planes of the Pd are aligned with planes of the underlying oxide support that have an interplanar spacing of 2.8 Å. A great change occurs on moving to the next sample in the series, Pd/Sm2O3-B. Whereas after pretreatment A the support material imaged was almost entirely semiamorphous, after pretreatment B the support had become almost entirely crystalline. Four images of crystalline support particles are presented in parts e-h of Figure 3. All of these crystals were aligned in such a way that they give rise to clear DDPs and these are inset in the images with the assignments of the diffraction spots indicated. The crystal which occupies most of the image in part e of Figure 3 was indexed to the cubic Sm2O3 structure viewed along the [515] zone axis. (Sm2O3 crystallites which can be indexed equally to both IA3 and I213 space groups will be described here as cubic. For crystallites where this is not the case, the distinction is made in the text. Miller indices are identical for the two systems IA3 and I213 in all DDPs presented here as cubic Sm2O3.) Interplanar spacings of 7.43, 2.11, and 2.11 Å were measured and assigned as the (1j01), (051j), and (1j5j0) planes, respectively. Certain spots in the DDP of the image that do not fit this pattern were identified as coming from a different small particle also present in the image. The small crystal in part f of Figure 3 could not be indexed exclusively to either one of the cubic or monoclinic Sm2O3 systems. The pattern was consistent with the cubic structure viewed along the [314] zone axis. However, the best fit was to the monoclinic system viewed along the [1j3j4] zone axis. The corresponding assignments are shown on the DDP in the figure and relate to measurements of 3.27, 3.05, and 2.89 Å, assigned as (111), (401), and (31j0), respectively. The crystal ringed in part g of Figure 3 gave rise to a clear DDP, which can be indexed to the cubic Sm2O3 viewed along the [103] zone axis. The spots labeled as (020) and (301j) correspond to interplanar spacings measured as 5.14 and 3.16 Å, respectively. Interestingly, the diffraction pattern is better explained by crystallography based on the I213 space group than the IA3 space group because the (301j) planes are not expected to be visible in the latter system, even when dynamical diffraction effects are accounted for. The crystal in part h of Figure 3 gives rise to a DDP, which, although not complete, can be indexed again to cubic Sm2O3, this time viewed along the [112] zone axis. The Miller indices (1j10) and (2j2j2) correspond to measurements of 7.57 and 3.18 Å, respectively. Another important change that occurs on going from Pd/ Sm2O3-A to Pd/Sm2O3-B is that identifiable particles of Pd become more common. Part a of Figure 4 shows a region of crystalline support in the latter sample. Although the crystal planes are visible, these are distorted and appear to contain dark spots of about 1 nm diameter. A Pd particle of diameter less than 2 nm lying at the interface between this region and an adjacent support particle is shown in part b of Figure 4. Despite

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Figure 4. HRTEM images of Pd/Sm2O3-B: (a) A region of distorted crystalline support material. (b) A Pd particle of less than 2 nm diameter seen in planar view adjacent to the region shown in (a). The particle is identified as Pd from the DDP inset. (c) A Pd particle of around 7 nm diameter seen in profile view. (d) A twinned Pd particle seen in planar view. In the inset DDP, the spots corresponding to one of the two microdomains viewed along the [110] zone axis are indicated. Spots resulting from the planes of the other microdomain, indexed as though viewed along the [1j1j0] zone axis, are labeled 1j11′ and 002′. The planes labeled (1j11j) are common to both microdomains. HRTEM images of Pd/Sm2O3-C: (a). A Pd particle of 18 nm decorated with support material and viewed along the [110] zone axis. The particle was identified as Pd from the DDP inset. (b) A Pd particle of around 36 nm identified by EDX.

its small size and the fact that it is imaged in planar view, the particle is in zone and all three major planes can be identified from the DDP. These results indicate that, during pretreatment B, Pd particles emerged form the semiamorphous phase as it became more crystalline. Slightly larger Pd particles are shown in parts c and d of Figure 4. The particle in the latter image is particularly interesting because it is twinned. That is, it contains two microdomains whose structures in this case share one set of visible {111} planes and are inter-related by a rotation through 60°. The spots in the DDP, which belong to the two superimposed patterns arising from the two microdomains, confirm this. Again, this would be consistent with the emergence of the Pd particle from the Sm-containing semiamorphous phase during pretreatment B because the twinning of the particle may be the result of growth in a constrained space within the Smcontaining phase. These findings are consistent with the simultaneous crystallization of small Pd particles and of Sm2O3 from the semiamorphous phase, which had been observed in the Pd/Sm2O3-A

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Figure 5. (a). HRTEM image of as-prepared Pd/CS showing the typical semiamorphous layer. (b) HRTEM image of as-prepared Pd/CS showing the region used to record an elemental map. The distributions of O, Pd, Ce, and Sm within this region are shown individually. (c) HRTEM image of Pd/CS-A showing that the semiamorphous phase is still evident. (d) HRTEM image of Pd/CS-A showing the region used to record an elemental map. The distributions of O, Pd, Ce, and Sm within this region are shown individually at the right of this figure.

sample, after the mild pretreatment. The semiamorphous phase, therefore, would seem to be formed from the crystalline Sm2O3 starting material during the Pd impregnation step. Dissolution of the Sm2O3 in the highly acidic conditions of Pd impregnation (pH ∼1) would be followed by reprecipitation of a semiamorphous phase containing Sm, O, and Pd. During the pretreatments, particularly the more severe ones, the semiamorphous phase would begin to crystallize to form the recognizable crystalline particles of Pd and of Sm2O3 seen in the HRTEM images. After the most severe pretreatment, sample Pd/Sm2O3-C contained some much larger Pd particles. The particle in part e of Figure 4 was identified from its DDP (inset) and the particle in part f of Figure 4 from the EDS spectrum of the region occupied by the particle, which showed a large Pd peak. Interestingly, the particle in part e of Figure 4 appears to be associated with support material. This may be evidence for the continuation of the segregation of the original semiamorphous phase into crystalline Pd and Sm2O3 phases. 3.4. Pd-CS. Figure 5 presents data for the as-prepared Pd/ CS catalyst. This sample was characterized by a ubiquitous semiamorphous layer similar to that observed in the Pd/ Sm2O3-A sample. Part a of Figure 5 presents a typical image of this. The semiamorphous phase in this sample appeared to form a relatively thick (for catalytic purposes) layer on the surface of what appeared to be more regular particles. Elemental maps were collected for the ROI indicated in part b of Figure

Baker et al. 5 for O, Pd, Ce, and Sm. These are presented on the right of the figure. They show that O was distributed relatively evenly across the ROI and that Sm and Pd are present wherever there is O. However, the distribution of Ce is not even, there being a high concentration deep inside the particle and a layer containing very little Ce at the surface. This layer is approximately 20 nm thick in this case. Therefore, it appears that the semiamorphous layer contains Sm, Pd, and O but much less Ce and that this layer coats the Ce-rich particles. The Pd/CS-A sample was found to retain a large amount of semiamorphous phase, such as the material imaged in part c of Figure 5. At slightly lower magnification, an agglomerate is shown in part d of Figure 5. The distributions of O, Pd, Ce, and Sm within the ROI indicated are included to the left of these images. Again, it is evident that the O, Pd, and Sm are closely associated, whereas the Ce is concentrated along the backbone of the agglomerate. The Sm, on the contrary, is evenly distributed across the agglomerate and is present around the edges where the Ce is essentially absent, for example, above the arrows in the figure. Only a small number of Pd particles were observed in this sample. Typically, these appeared to be either in association with crystalline oxide material, as in the image in part a of Figure 6, or, more rarely, to occur on support material of low long-range order and high Sm content, as in part b of Figure 6. Here, the core of the support particle is high in Ce (EDX peak area ratio: Ce/Sm ) 5.7), whereas the surface on which the Pd particle sits is rich in Sm (Ce/Sm ) 0.62). (It should be noted that the Ce/Sm ratios are indicative only since they are not calibrated values and, because of the teardrop shape of the measurement volume, some contribution to the spectra from the surroundings of the ROI is inevitable.) Furthermore, there does not appear to be an epitaxial relationship between this particle and the support, such as that seen in the corresponding Pd/CeO2-A sample. Although the support is largely coated with semiamorphous material, some particles of high crystallinity were observed. However, in all cases these were best indexed to CeO2 viewed in common zone axes such as [110] and [001], as in parts c and d of Figure 6, respectively. Crystalline Sm2O3 was therefore not identified in this sample. However, it should be remembered here that CeO2 and solid solutions of formula Ce1-xSmxO2-x/2 (at least for 0 e x g 0.432) would be indistinguishable by electron diffraction (above). Examination of the Pd/CS-B sample provides further evidence for the close association of Pd particles and the Smcontaining phase. The region in part e of Figure 6 contains several Pd particles. Determination of the elemental composition of different areas of the parent agglomerate (part f of Figure 6) showed that the core of the agglomerate consisted of large, regular crystalline particles that were Ce-rich. Around the edge of these existed a layer of much smaller particles, also crystalline, but which were rich in Sm. The Pd particles were concentrated in this layer. This is completely consistent with the formation of a semiamorphous Sm- and Pd-containing layer, as described above for the Sm2O3-supported samples. Furthermore, Pd particles were found to be generally more numerous in this sample than in the Pd/CS-A, as was also seen to be the case for the Pd/Sm2O3 sample. Part g and h of Figure 6 show individual Pd particles. The first is seen in profile view and is supported on a crystalline particle of which one set of planes is resolved and has a spacing of 6.7 Å. This could, therefore, be indexed to monoclinic Sm2O3 (most likely, the (200) planes) rather than cubic Sm2O3 or CeO2. Inset in the figure are DDPs of (i) the Pd particle alone, viewed in the [110] zone axis, (ii) the oxide support only; and (iii) the particle and the adjacent



Figure 7. TEM images of Pd/CSR300 sample. (a, b) Comparison of elemental composition at indicated positions at edges and at centers of particles. Compositions are expressed as atom % of Pd/Ce/Sm. (c, d) images of individual Pd particles with inset DDPs. (e) Pd particles on the support. Particles with visible planes consistent with the Pd structure are arrowed. (f) Distribution of many small particles over support surface.

Figure 6. HRTEM images of Pd/CS-A (a) Pd particle viewed along [110] zone axis. (b) Pd particles supported on low-order, Sm-rich support material. (c) Crystalline support material indexed to CeO2 viewed along the [110] zone axis. (d) Crystalline support material indexed to CeO2 viewed along the [001] zone axis. HRTEM images of Pd/CS-B: (e) Higher magnification image of the region indicated in (f) showing several Pd particles with the direction of visible (111) planes indicated with arrows. (f) Agglomerate in which core particles are Ce-rich, whereas the surface layer is Sm-rich. Pd particles occur mainly in the latter. (g) Pd particles on crystalline Sm2O3 support. The DDP inset shows the epitaxial alignment between the (002) planes of the Pd particle and the visible planes of the support oxide. (h) Pd particle in planar view. DDP confirms the Pd crystal structure, viewed along [110] zone axis, and that the particle is slightly misaligned with support material.

oxide support. The latter demonstrates that one set of (111) planes of the Pd particle are aligned with the visible planes of the Sm2O3 support because the corresponding spots are collinear. This might be expected if the Pd and Sm2O3 support crystallized from the semiamorphous phase simultaneously, as postulated here. The second particle, in part h of Figure 6, is in planar view and, as seen in the DDP, was slightly misaligned with

respect to the underlying support but was lying on a less-ordered surface than the particle in part g of Figure 6. In TEM images, the Pd/CSR300 sample was seen to contain many relatively large, regular particles. In elemental analysis using EDX in the TEM, these particles generally gave very low Sm and Pd signals and very high Ce signals when analyzed by directing the EDX beam at their centers, whereas when the EDX spot was positioned at the particle edge, higher values of both Sm and Pd together were typically observed. Such results are presented in parts a and b of Figure 7. This confirmed the existence of a Sm- and Pd-containing surface layer covering the CeO2 particles as was observed in fresh Pd/CS and Pd/CS-A. This layer is, of course, expected to cover the whole surface of the CeO2 particle but is most evident when analyzed in profile at the edge of the particle. Whereas very few Pd particles were observed in either the fresh Pd/CS or Pd/CS-A, significant numbers of Pd particles were seen in Pd/CSR300. Those shown in parts c and d of Figure 7 could be unambiguously identified because their DDPs gave complete patterns, which were indexed to Pd viewed along the [110] zone axis. Parts e and f of Figure 7 show large numbers of particles distributed across the surface of the support. Where crystal planes are visible in these particles, these are nearly always consistent with the Pd structure and usually match the (111) spacing of 2.25 Å. Figure 8 shows TEM images that contain

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Figure 8. TEM images of Pd/CSR300 sample. (a) Support particle with apparent surface layer containing small particles. (b) Highmagnification image of area shown in (a). Particles with visible planes consistent with Pd are arrowed. (c) Support particle edge showing Pd particles (some arrowed) and apparent surface layer at left of image. (d) Support particle with apparent surface layer in profile at left side (arrowed).

similar particles. Again, where crystal planes are visible in these particles the majority of spacings are consistent with the particles being of Pd. Importantly, the images in this figure also show what appears to be a surface layer. This appears as an irregular coating in the images. This is consistent with the observation of the amorphous surface layers with high Pd and Sm content in the Pd/CS and Pd/CS-A samples. However, PdSCR300 differs from these materials in the important respect that it contains a much larger number of what can be assumed to be mainly Pd nanoparticles at the support surface. After the most severe pretreatment, the Pd/CS-C sample contained several relatively large Pd particles, such as the 60 nm diameter particle in part a of Figure 9. These are evidence of further segregation of Pd from the Sm-containing phase and subsequent sintering to form the larger particles. Another change observed in this sample was that crystals of Sm2O3 could be easily identified. These were exclusively indexed to the cubic Sm2O3 lattice. The example in part b of Figure 9 was indexed to cubic Sm2O3 viewed along the [001] zone axis. In addition, the crystallite was identified as an oxide of samarium by EDX. A further example of cubic Sm2O3, which can be assigned to the I213 phase viewed along the [111] zone axis, is shown in part c of Figure 9 and in an expanded image in part d of Figure 9. The DDP includes three sets of planes with interplanar spacings of 7.6 Å and interplanar angles of 60°, which can be assigned as the (1j10), (011j), and (11j0) planes. These results indicate that the Sm-containing material, which was originally semiamorphous after the Pd impregnation step, became highly crystalline during the reductive pretreatments, especially the most severe one. During this process, Pd also emerged from the semiamorphous phase to form small crystalline Pd particles that combine to form larger particles as a function of the severity of the pretreatment. Part e of Figure 9 shows an agglomerate in the Pd/CS-C sample and corresponding elemental maps of O, Pd, Ce, and

Figure 9. HRTEM images of Pd/CS-C. (a) A large Pd particle (arrowed) with the DDP obtained from a higher magnification image of the top edge showing that the particle is viewed along the [110] zone axis. (b) Crystallite of cubic Sm2O3 viewed along the [001] zone axis with DDP inset. (c) Crystallite of cubic Sm2O3 (space group I213) viewed along the [111] zone axis with DDP inset. (d) Expanded image of the crystallite viewed in (c). (e) HRTEM image of an agglomerate in Pd/CS-C and EDX maps of the distribution of the elements O, Pd, Ce, and Sm across the agglomerate.

Sm. Again, and even after the most severe pretreatment, the presence of Sm across the whole agglomeration and of Ce mainly at the center is confirmed. This is consistent with the formation of a Sm-containing surface layer as described above. It has been seen that this catalyst appeared to contain the same semiamorphous Sm-O-Pd phase as the Pd/Sm2O3 catalyst and that this was also formed during the acidic Pd impregnation step. The main difference is that, in this case, this phase initially formed a thin surface coating over the CeO2 particles rather than being the majority phase, as it was in the as-prepared Pd/Sm2O3 catalyst. 4. Discussion The nanostructural findings described in the previous section show that the behavior of the Sm-containing catalysts was unusual and very different from the more conventional nanostructural trends seen in the CeO2-supported catalyst. The latter exhibited small, well-dispersed Pd catalysts even after lowtemperature reduction in pretreatment A. The main effect of higher temperature reduction was to cause a small increase in the average Pd particle size. In the Pd/Sm2O3 and Pd/CS samples, however, the nanostructural evolution of the support



TABLE 1: Redox and Catalytic Performance Data for Pd/CeO2, Pd/Sm2O3, and Pd/CS Catalysts, Summarized from Previous Work22 Catalytic Activityc catalyst Pd/CeO2 Pd/Sm2O3 Pd/CS

reduction peaksa(°C) 87 (s) 107 (w) 460 (s, b) 107 (w) 430 (s, b)

D400b (%)

Tredd (°C)

reaction temperature Trxn (°C)

methanol conversion (%)

H2 productivity (molh-1g-1)

turnover frequency (h-1)

10.0 0.5

400 400 500 300 400

400 400 400 400 400

14.6 0 2.0 25.7 71.9

0.09 0 0.01 0.16 0.46

2500

20.5

6000

from TPR spectra in 5% H2/Ar: s ) strong, w ) weak, b ) broad. Pd dispersion by CO chemisorption after sample reduction in pure H2 for 1 h at 400 °C. c Experimental conditions: total pressure, 1 atm.; mass of catalyst, 0.1 g; total gas flow rate, 50 mL min-1; molar methanol concentration, 15%; molar ratio H2O:CH3OH, 1.2. d Prereduction temperature of catalyst in 20% H2/He for 1 h. a

has a very significant effect on the availability of the Pd active phase. Dissolution of the Sm2O3 in the Pd impregnation step and its subsequent precipitation as an amorphous phase initially masked the Pd from the gas phase. A similar phenomenon has been reported by Bernal et al.37 who compared the structures of Rh/CeO2 and Rh/Sm2O3. During reduction in pretreatment B, however, the Sm phase gave rise to a thin layer of small Sm2O3 crystallites and the Pd particles emerged and were concentrated mainly on the Sm2O3 surface. In the hightemperature pretreatment C, significant sintering of the Pd particles was seen to occur. The Pd/CeO2, Pd/Sm2O3, and Pd/CS starting materials employed in the current work were characterized in a previous study.22 The specific surface areas (SSAs) were measured using the BET technique. Redox behavior was investigated using temperature-programmed reduction (TPR) and the Pd dispersion in the samples was determined by CO chemisorption. SSAs, TPR results, and Pd dispersion data are collected together in Table 1 and can be compared directly with the TEM data from the current study. In catalytic experiments, the methanol conversions and hydrogen productivities were evaluated under isothermal conditions at a number of reaction temperatures in the range 300-500 °C after in situ reduction in 20% H2/He at temperatures, Tred, of 300, 400, or 500 °C for 1 h. A mixture of CH3OH:H2O in a 1.2:1 molar ratio was diluted in a He carrier gas and was supplied to the reactor. Again, some of these results are summarized in Table 1. The pretreatments for the TEM study were chosen to represent a wide range of conditions and were affected by the need to provide stable samples for transfer to, and study in, the microscope. Therefore, except for Pd/CSR300, these pretreatments do not correspond directly with those of the catalytic study mentioned. However, HRTEM gives us the advantage of being able to visualize nanostructural phenomena directly and the trends observed in these nanostructures can be usefully related to the trends observed in the catalytic behavior of these materials. 4.1. Pd/CeO2. The nanostructure of the Pd/CeO2 catalyst samples corresponds very well to the redox and activity data presented in Table 1. Only one, sharp reduction peak was seen in the TPR of this catalyst at the very low temperature of 87 °C. The small Pd particles dispersed across the surface of the CeO2 support observed by HRTEM are consistent with such a rapid, low-temperature reduction of the catalyst. After reduction at 400 °C, the dispersion value, D400, of 10.0%, was reasonably high and the catalyst exhibited moderate methanol conversion and H2 productivity. This is also in good agreement with the nanostructural evolution observed in the HRTEM images in which the Pd particles were seen to have undergone some sintering after pretreatment B but remained nanoparticular and well dispersed across the support. In short, this material behaved

b

as a classical supported metal heterogeneous catalyst and can serve as a comparator for the behavior of the Sm-containing samples. 4.2. Pd/Sm2O3. Both the nanostructure of the Pd/Sm2O3 material, as viewed in the electron microscope, and its catalytic properties, presented in Table 1, were very different from those of the CeO2-supported catalyst. In the TPR spectrum, two peaks were seen rather than one. There is a small peak at low temperature and the main, broad peak has its maximum at 460 °C. Pd dispersion after reduction at 400 °C is very low, at 0.5%, and the material shows no catalytic activity after prereduction at this same temperature. These data can be explained by the unusual nanostructure seen in HRTEM. After pretreatment A, the Pd appears to have remained trapped within the semiamorphous Sm-Pd-O phase. Hardly any Pd particles are seen. The small, sharp, low-temperature reduction peak in the TPR spectrum can be attributed to the few accessible particles present at the surface of the semiamorphous material. The main TPR peak must then be attributed to the reduction of the main portion of the Pd species. Because these were dispersed within the bulk of the semiamorphous phase, it is consistent that this peak is broad. During this reduction feature, Pd particles crystallized out of the semiamorphous phase, giving rise to the nanostructures seen after pretreatment B. In light of the very low dispersion value (D400, ) 0.5%), this process was not complete after reduction at 400 °C. This also explains the inactivity of this catalyst after prereduction at 400 °C. After reduction at 500 °C, the TEM data after treatment B indicate that the crystallization of the Pd nanoparticles was more advanced and, indeed, some limited catalytic activity was seen after prereduction at this temperature (Table 1). The reason why the catalyst was not more active is clearly that a large proportion of the Pd particles, while small, remained too deep within the agglomerations of Sm2O3 crystals (also newly formed) to be accessible to the gas phase. After pretreatment C, the material is seen to have completely crystallized into highly ordered Sm2O3 particles and Pd particles. The latter had undergone quite severe sintering. Although the active phase, the Pd, was no longer obscured by the masking semiamorphous phase after pretreatment C, the increased size of the Pd particles corresponds to low specific surface area of the Pd and poor catalytic activity would be expected. 4.3. Pd/CS. It has been seen that the Pd/CS catalyst appeared to contain the same semiamorphous Sm-O-Pd phase as the Pd/Sm2O3 catalyst but that, in this case, this phase initially formed a thin surface coating over the CeO2 particles rather than being the majority phase, as it was in the as-prepared Pd/ Sm2O3 material. This morphology is reflected in the TPR peak positions. These are qualitatively similar to the situation for the Pd/Sm2O3 with a small low-temperature peak, which can be

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related to the reduction of the small fraction of Pd species present at the surface of the semiamorphous phase, and a large, broad peak at much higher temperatures, related to the reduction of Pd species within the semiamorphous surface layer. The maximum of this feature for Pd/CS occurred at a lower temperature than for Pd/Sm2O3. This can be related to the larger accessible surface area of this phase in the former because of the semiamorphous phase being supported on the CeO2 phase, as observed directly in the TEM images. The same structural phenomenon also explains the dramatically higher value of D400, 20.5%, in the Pd/CS than in Pd/Sm2O3 and the high methanol conversion and H2 productivity observed in Pd/CS after reduction at 400 °C. The sensitivity of the catalyst nanostructure and catalytic behavior is manifested in the dramatic appearance of the Pd nanoparticles on comparing the HRTEM images of Pd/ CS-A and Pd/CS-B and in the dramatic increase in methanol conversion and H2 productivity observed in this catalyst when the prereduction temperature was changed from 300 to 400 °C. After pretreatment Pd/CS-C, however, the Pd particles in Pd/ CS show significant sintering. Therefore, the Pd/CS catalyst after reduction at 400-500 °C corresponds approximately to the best nanostructure and the best catalytic performance. It is interesting to note the hierarchical structure of this ‘optimum’ catalyst material. Very fine nanoparticles of Pd are held predominantly on the surfaces of small Sm2O3 crystals, which are supported in turn on the larger CeO2 particles, so maintaining a high Pd dispersion and excellent contact with the gas phase (parts e and f of Figure 6). This structure is a result of both the preparation procedure and the thermochemical history of the sample. However, as can be deduced from the TOF results from our previous article (Table 1), the higher activity of the Pd/CS catalyst cannot be exclusively attributed to the high Pd dispersion. Another advantage of this nanostructural arrangement is that the Pd particles are supported mainly on the Sm2O3. Sm2O3 is reported to be a better support than CeO2 in catalysts where activation of hydrocarbons is involved,.1,38 As has been described by other authors,39-42 the Sm2O3 provides basic active centers in the vicinity of the Pd particles that favor the activation of hydrocarbons like methane or methanol by promoting the adsorption of reactants and favoring the surface diffusion of reaction intermediates, thus giving rise to a higher TOF per exposed Pd atom than for the CeO2-supported catalyst. Therefore, the three-component configuration combines a good Pd dispersion (which is not the case in Pd/Sm2O3) with the presence of basic centers on the support, which favor spillover of reactants. In the catalytic tests in our previous work,22 the Pd/CS sample reduced at 300 °C showed high activity, which was, nevertheless, lower than that for the same composition reduced at 400 °C. The TEM analysis of the Pd/CSR300 sample showed that it was different in nanostructure from both the Pd/CS-A material and the Pd/CS-B material, and that it can be considered to be intermediate between the two. That is, it has relatively good coverage of Pd particles but, because the preparation temperature was lower than that for Pd/CS-B, it does not appear to contain well-defined Sm-rich crystallites at the surface. It is logical, therefore, that in this sample, a partial segregation of the initial amorphous phase into Pd nanoparticles and Sm-rich oxide had occurred. This explanation agrees well with the TPR data for Pd/CS in the previous work22 in which the broad peak associated with the segregation of the amorphous phase had already just started at 300 °C, although it did not reach its maximum until above 400 °C. In addition, TPR is a transient technique and so reduction processes occur in TPR at slightly

Baker et al. higher temperatures than they would under steady-state conditions. The result of this is that holding the Pd/CS at 300 °C for 1 h in H2/Ar can be expected to have caused a significant degree of segregation of the amorphous phase into Pd nanoparticles and Sm-rich oxide. This led to a catalyst with activity intermediate between those of Pd/CS-A and Pd/CS-B. The absence of well-defined Sm-rich crystallites in Pd/CSR300 can be attributed to the lower pretreatment temperature and shorter pretreatment time than those employed to prepare Pd/CS-B. The omission of a purging step in the former case is also a factor here. It is evident, nevertheless, that the Pd nanoparticles in Pd/CSR300 are intimately associated with the Sm-containing phase and that this phase forms a layer on the Ce-rich particles. Thus, a similar enhancement in catalytic activity to that seen in Pd/CS-B is expected. This is entirely consistent with the fact that the Pd/CS sample reduced at 300 °C showed a higher catalytic activity than Pd/CeO2 reduced at 400 °C.22 5. Conclusions 1. All three support materials were found to be highly crystalline before the Pd impregnation step. 2. The Pd/CeO2 was found to be activated at very low temperatures, corresponding approximately to pretreatment A, after which the Pd particles were numerous, small, generally aligned epitaxially with the support, and of regular morphology. Both the Pd and the CeO2 phases were highly crystalline. With increasing severity of pretreatment, slight sintering of the Pd particles occurred. The redox, activity, and nanostructural data agreed that this was a typical supported metal catalyst. 3. In the catalysts with Sm-containing supports, a semiamorphous phase was formed during the acidic Pd impregnation step. The Pd appeared to be intimately associated within this phase. This did not occur in the Pd/CeO2 material. Virtually no Pd particles were seen in the as-prepared Pd/Sm2O3 and Pd/CS samples. 4. As pretreatment temperature increased, the Pd began to emerge from the semiamorphous phase, forming metallic nanoparticles, as the semiamorphous phase itself began to crystallize. 5. The Pd nanoparticles observed in HRTEM in samples with Sm-containing supports were often not aligned with the support and sometimes exhibited twinning or decoration. These factors are indicative of the above process. 6. Both the nanostructure and the catalytic activity of the Pd/ CSR300 sample can be considered to be intermediate between those of the Pd/CS-A and the Pd/CS-B samples. 7. After the severe pretreatment of the Sm-containing samples, the Pd particles began to sinter and the occurrence of identifiable Sm2O3 particles increased. 8. By far the most active catalyst was shown to have a hierarchical structure in which Pd nanoparticles were concentrated at the surfaces of fine Sm2O3 crystallites and these in turn were supported on larger CeO2 particles. In addition to this advantageous nanostructure, there was evidence from the turnover frequencies that Pd on Sm2O3 was also more active than Pd on CeO2 for the target reaction. 9. These findings indicate that desirable hierarchical catalyst nanostructures might be designed by careful control of experimental conditions, for example by applying strongly acidic conditions in the metal impregnation step, as described here. Acknowledgment. Electron microscopy was carried out at the Electron Microscope Facility at the University of St Andrews. Crystallographic data were obtained using the Chemi-

Intermediate-Temperature Methanol Fuel Cells cal Database Service and electron diffraction patterns were interpreted using the EMS software. L.M.G.S. acknowledges the support of the European Commission through a Marie Curie Research Training Fellowship (c/n: ERK5-CT-1999-50003). References and Notes (1) Olah. G. A.; Goeppert, G.; Surya Prakesh, G. K. In Beyond Oil and Gas: The Methanol Economy, Wiley-VCH: New York, 2006. (2) Liu, S.; Takahashi, K.; Eguchi, H.; Uematsu, A. Catal. Today 2007, 129, 287. (3) Steele. B. C. H. British Ceramic Proceedings; Institute of Materials: London, 1997. (4) Yamamoto, O. Electrochim. Acta 2000, 45, 2423. (5) Laosiripojana, N.; Assabumrungrat, S. J. Power Sources 2007, 163, 943. (6) Sasaki, K.; Watanabe, K.; Teraoka, Y. J. Electrochem. Soc. 2004, 151 (7), A965. (7) Sahibzada, M.; Steele, B. C. H.; Zheng, K.; Rudkin, R. A.; Metcalfe, I. S. Catal. Today 1997, 38, 1. (8) Sahibzada, M.; Steele, B. C. H.; Hellgardt, K.; Barth, D.; Effendi, A.; Mantzavinos, D.; Metcalfe, I. S. Chem. Eng. Sci. 2000, 55 (16), 3077. (9) Kobayashi, H.; Takezawa, N.; Minochi, C. Chem. Lett. 1976, 12, 1347. (10) Peppley, B. A.; Amphlet, J. C.; Kearns, L. M.; Mann, R. F. Appl. Catal., A 1999, 179 (1-2), 21. (11) Liu, Y. Y.; Hayakawa, T.; Suzuki, K; Hamakawa, S; Tsunoda, T; Ishii, T; Kumagai, M. Appl. Catal., A 2002, 223 (1-2), 137. (12) Iwasa, N.; Mayanagi, T.; Nomura, W.; Arai, M.; Takezawa, N. Appl. Catal., A 2003, 248, 153. (13) Kim, T.; Ahn, K.; Vohs, J. M.; Gorte, R. J. J. Power Sources 2007, 164, 42. (14) Brett, D. J. L.; Atkinson, A.; Cumming, D.; Ramirez-Cabrera, E.; Rudkin, R.; Brandon, N. P. Chem. Eng. Sci. 2005, 60, 5649. (15) Iwasa, N.; Masuda, S.; Ogawa, N.; Takezawa, N. Appl. Catal., A 1995, 125 (1), 145. (16) Karim, A.; Conant, T.; Datye, A. J. Catal. 2006, 243 (2), 420. (17) Tibiletti, D.; de Graaf, E. A. B.; Teh, S. P.; Rothenberg, G.; Farrusseng, D.; Mirodatos, C. J. Catal. 2004, 225, 489. (18) Sahibzada, M.; Chadwick, D.; Metcalfe, I. S. Catal. Today 1996, 29 (1-4), 367. (19) Uchida, H.; Osuga, T.; Watanabe, W. J. Electrochem. Soc. 1999, 146, 1677.

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12475 (20) Tseng, L. K.; Huang, T. J. J. Chin. Inst. Chem. Eng. 2000, 31, 493. (21) Feng, B.; Wang, C. Y.; Zhu, B. Electrochem. Solid State Lett. 2006, 9 (2), A80. (22) Go´mez-Sainero, L. M.; Baker, R. T.; Metcalfe, I. S.; Sahibzada, M.; Concepcioń, P.; Lo´pez-Nieto, J. M. Appl. Catal., A 2005, 294, 177. (23) Baker, R. T.; Bernal, S.; Calvino, J. J.; Pe´rez-Omil, J. A.; Lo´pezCartes, C. Nanotechnology in Catalysis; Zhou, B., Hermans, S., Somorjai, G. A., Eds; Kluwer Academic/Plenum: New York, 2004, p 403. (24) http://cds8.dl.ac.uk/cds/cds.html. (25) Spycher, A.; Stadelmann, P.; Buffat, P.; Fleuli, M. J. J. Electron Microscopy Technique 1988, 10 (4), 369. (26) Stadelmann, P.; Buffat, P. HelVetica Physica Acta 1982, 55 (2), 132. (27) Buffat, P.; Stadelmann, P. J. de Microscopie et de Spectroscopie Electroniques 1981, 6 (3), A8. (28) Whitfield, H. J.; Roman, D.; Palmer, A. R. J. Inorg. Nucl. Chem. 1966, 28, 2817. (29) Cromer, D. T. J. Phys. Chem. 1957, 61, 753. (30) Bartos, A.; Lieb, K. P.; Uhrmacher, M.; Wiarda, D. Acta Crystallogr. B 1993, 49, 165. (31) Saiki, A.; Ishizawa, N.; Mizutani, N.; Kato, M. Acta Crystallogr. B 1984, 40, 76. (32) Brauer, G.; Gradinger, H. Z. Anorg. Allg. Chem. 1954, 276, 209. (33) Zav’yalova, A. A.; Imamov, R. M.; Ragimli, N. A.; Semiletov, S. A. Kristallografiya 1976, 21, 727. (34) Zachariasen, W. H. Norsk Geologisk Tidsskift 1927, 9, 310. (35) Loebich, Eg. O.; Raub, C. J. Journal of the Less-Common Metals 1977, 55, 67. King, H. W.; Manchester, F. D. J. Phys. F 1978, 8, 15. (36) Bernal, S.; Botana, F. J.; Calvino, J. J.; Cifredo, G. A.; Garcia, R. Ultramicroscopy 1998, 72 (3-4), 135. (37) Bernal, S.; Botana, F. J.; Calvino, J. J.; Lopez-Cartes, C.; PerezOmil, J. A.; Rodriguez-Izquierdo, J. M. Ultramicroscopy 1990, 34, 60. (38) Choudhary, V. R.; Prabhakar, B; Rajput, A. M.; Mamman, A. S. Fuel 1998, 77, 1477. (39) Inui, T. J. Alloys Compd. 1993, 193, 47. (40) Lin, C. H.; Campbell, K. D.; Wang, J. X.; Lunsford, J. H. J. Phys. Chem. 1986, 90, 534. (41) Lin, C. H.; Campbell, K. D.; Wang, J. X.; Lunsford, J. H. J. Am. Chem. Soc. 1985, 107, 5062. (42) Choudhary, V. R.; Rane, V. H. J. Catal. 1991, 130, 411.

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