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J. Phys. Chem. C 2008, 112, 5900-5910
Preparation of Rhodium/CexPr1-xO2 Catalysts: A Nanostructural and Nanoanalytical Investigation of Surface Modifications by Transmission and Scanning-Transmission Electron Microscopy Marı´a P. Rodrı´guez-Luque,† Juan C. Herna´ ndez,† Marı´a P. Yeste,† Serafin Bernal,† Miguel A. Cauqui,† Jose´ M. Pintado,† Jose´ A. Pe´ rez-Omil,† Odile Ste´ phan,‡ Jose´ J. Calvino,† and Susana Trasobares*,† Departamento de Ciencia de los Materiales e Ingenierı´a Metalu´ rgica y Quı´mica Inorga´ nica, Facultad de Ciencias, UniVersidad de Cadiz, Campus Rio San Pedro, Puerto Real, 11510-Ca´ diz, Spain, and Laboratoire de Physique des Solides, UniVersite´ Paris Sud, 91495 Orsay, France ReceiVed: October 26, 2007; In Final Form: January 28, 2008
By using a combination of transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) techniques, we have followed in detail the structural and chemical changes occurring on the very surface layers of powder CexPr1-xO2 mixed oxides, used as supports, during the preparation of rhodiumsupported catalyst by wet impregnation techniques. Results demonstrate the occurrence of Pr3+ leaching out of the Ce-Pr solid solution during the impregnation step, which promotes a severe spatial redistribution of lanthanide cations at the surface of the mixed oxide. A highly heterogeneous surface composition is reached after the metal deposition. Mobilization of the support also affects the features of the final surface structure of the metal-supported nanoparticles, which become decorated by patches of support material. All these facts seem crucial to properly understand the redox and catalytic behavior of this kind of materials.
Introduction The incorporation of praseodymium within the fluorite host lattice of CeO2 provides new materials with interesting optical, electrical, and chemical properties.1-8 A variety of promising uses are currently envisaged for these Ce-Pr mixed oxides spanning from nontoxic red pigments,1,8 optoionic smart windows,2,9 cathode material for solid oxide fuel cells,3,10 oxygen separation membranes,11 surface effect gas sensors,6,7 and of course to catalysts. The improvements in high-temperature sintering behavior as well as in the low-temperature redox response observed after insertion of praseodymium, i.e., increases in oxygen storage capacity (OSC), are crucial factors in the case of applications related to catalysis.12-16 In this field, both bare Ce-Pr mixed oxides as well as different supported metal catalysts have been investigated. Thus, CO, CH3OH, and CH4 combustion has been reported to be enhanced on CexPr1-xO2-d solid solutions;17 N2O catalytic decomposition has been studied on Rh-loaded Pr-doped CeO2.18,19 Syngas generation from hydrocarbons seems also to be promoted on Pt/CePrOx systems,20-23 whereas Pd-loaded cerium-praseodymium mixed oxides have been fruitfully tested in hydrocarbon trap devices.12,24 Metal loading onto these Ce-Pr mixed oxides is usually accomplished by wet impregnation techniques,12,18,20,22-25 using aqueous solutions of some salt of the corresponding metal. The impregnation is followed by a drying step in an oven, under air, at temperatures above 373 K. The solutions employed for impregnation are commonly highly acidic, due to the presence of the metal in a high positive oxidation state, typically from * Author to whom correspondence should be addressed. Phone: + 34 956 016286, fax: + 34 956 016288, e-mail:
[email protected]. † Universidad de Cadiz. ‡ Universite ´ Paris Sud.
+2 up to +4. Therefore, during this step, the Ce-Pr supports become in contact with an acidic liquid layer, usually at room temperature. This contact is further prolonged at higher temperatures during the drying stage. Pure praseodymium oxide, PrO2-x, is well-known to suffer leaching of Pr3+ species present in the fluorite lattice when treated even with very mild acidic solutions.26-28 In fact this leaching process is used to prepare Pr3+-free oxides, i.e., oxides containing strictly praseodymium in +4 oxidation state. Taking this well-established feature of the chemical behavior of PrO2-x into account and the fact that the structure of the Ce-Pr mixed oxides remains the fluorite type, it seems reasonable to expect that the deposition of the metal component onto Ce-Pr mixed oxides could also imply the occurrence of some leaching of Pr3+ present in these mixed oxides. The presence of cerium in the mixed oxides will surely modify the behavior of praseodymium toward the leaching process, but in principle it should not be completely ruled out. Leached praseodymium species would come into solution and further redeposit onto the support, jointly with the metal component, during the drying step. Hence, the routine procedure used in the synthesis of metal catalyst supported on Ce-Pr mixed oxides could involve the mobilization of, at least, Pr3+ species. Such mobilization could profoundly affect not only the surface structure and composition of the starting support oxide but also the structural and chemical properties of the metallicsupported phase. Thus, Chun et al. have reported strong perturbations in the temperature programmed reduction (TPR) profiles of Ce-Pr-supported Pd catalysts with increasing Pr content of the support.24 Likewise, Sadykov et al. attribute the enhancement in the catalytic behavior in the syngas generation from methane on Pt/CePrOx systems to specific interactions between Pt and the support.20,23 Very recently, Bellie´re et al.29
10.1021/jp710363j CCC: $40.75 © 2008 American Chemical Society Published on Web 03/25/2008
Rh/CePrOx Catalyst Preparation: An (S)TEM Investigation have reported an in-depth study on the surface composition of Ce-La mixed oxides in a wide range of compositions. The authors concluded about a lanthanum (La3+) enrichment at the very first surface layers, a few nanometers wide, with respect to the nominal Ce-La mixed oxide composition. Given the very close chemical characteristics of La3+ and Pr3+ ions, quite similar results could reasonably be expected for Ce-Pr mixed oxides. Just from this viewpoint, it is clear that the detailed understanding of the surface chemistry properties of Ce-Pr mixed oxides, among them their catalytic performance, requires a detailed investigation of structure and composition at the very first surface layers. With the background information reported above it also seems clear that, in the case of supported catalysts, the investigation of the effects onto these features of the deposition of the metal component seems imperative to precisely determine the real state of the exposed surfaces in the final metal-supported catalyst. To our knowledge the actual surface structure of this family of catalysts, and more precisely the effects of the deposition of the metal phase, have not been previously investigated. Electron microscopy techniques are especially well suited to investigate the structure and composition of catalyst powders at the size scale required to discriminate the characteristics of the very first outer layers at the surface from those of the bulk and, in parallel, to investigate the interactions between the metal and the support.14,30 In this paper, we have used a variety of transmission and scanning-transmission electron microscopy techniques to reveal the composition and structure of two Ce-Pr mixed oxides, Ce0.8Pr0.2O2-x and Ce0.5Pr0.5O2-x′ (the x and x′ values would account for the oxygen vacancies related to residual Pr3+), both at the surface and bulk levels as well as the changes produced by the preparation of Rh-loaded catalysts by wet impregnation using solutions of Rh(NO3)3 as the metal precursor. By using atomic-scale resolution structural and analytical techniques, we have addressed the following questions: (1) Does Pr-leaching actually take place when Pr is incorporated into the host ceria structure? Or, on the contrary, is this process inhibited in the mixed oxide phase? (2) Does the wet impregnation procedure induce any modification of the surface structure or composition of the mixed oxide? (3) Do these eventual modifications affect somehow the nanostructural properties of the supported metal phase? The results we have included here provide not only a novel, highly accurate picture of the real structure of this type of systems but also some of the keys to understand previous observations reported above. Experimental Section Two different Ce-Pr mixed oxides with Ce/Pr molar ratios 80/20 and 50/50 were prepared by coprecipitation with ammonia from aqueous solutions containing the appropriate molar ratios of Ce(NO3)3 and Pr Ce(NO3)3. The precipitates were repeatedly washed with distilled water, dried in air at 383 K, and calcined at 873 K during 4 h. The BET surface areas of the resulting oxides were 7 and 43 m2/g for the 80/20 and 50/50 samples, respectively. XRD data confirmed a fluorite type structure for the two mixed oxides which can be considered as PrO2-x-CeO2 solid solutions. Given that in these mixed oxides, as it is also the case of pure praseodymia, there is always present a residual amount of Pr3+ whose value may depend on Ce/Pr molar ratio and BET surface area, we will refer to our oxide samples as Ce0.8Pr0.2O2-x and Ce0.5Pr0.5O2-x′. Rhodium was deposited onto these supports by incipient wetness impregnation using aqueous solutions of rhodium nitrate. Impregnation was followed by
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5901 drying in air in an oven overnight at 393 K. The metal loading, as determined by ICP-MS, was found to be 3% for both catalysts. Samples for scanning electron microscopy (SEM) microscopy were prepared, depositing a small amount of the powders onto the SEM sample holder using carbon paste. Sputtering was not used in order to avoid any modification of the surface topology of the samples. Secondary electrons SEM images were recorded working at 2 kV in a Sirion FEI instrument. X-ray energy dispersive spectroscopy X-EDS in the SEM was recorded using an EDAX analytical system. For TEM and STEM studies, samples were prepared by depositing small amounts of the powders directly onto holedcarbon coated Cu grids. Excess powder was removed from the grids by gentle blowing with a nozzle. Samples for TEM X-EDS experiments were prepared by ultramicrotomy. In this case the powders were embedded into an epoxy resin and sliced at room temperature in a Leica Ultracut UCT using an ultra-35° Diatome diamond knife. High-resolution electron microscopy images were recorded in a JEOL2010F instrument working at 200 kV. The structural resolution of this instrument is 0.19 nm. Operation of this equipment in TEM-EDS mode allows forming electron probes with diameters down to 1 nm. TEM X-EDS spectra were recorded using an Oxford INCA Energy TEM 200 system, which allows an energy resolution of 0.13 keV. Electron energy-loss spectra (EELS) were recorded in the VGHB501-dedicated STEM at Orsay, working at 100 kV and using a 15 mrad convergence semiangle and a 24 mrad collection angle. The spectrum imaging (SI) mode was used,31 recording 64 × 64 spectra collections, scanning onto the sample a 0.5 nm beam with a current in the order of 0.1-1 nA. The high angle annular dark field scattered intensity signal (HAADF) was also collected at each point of the scan. This approach allows us to correlate nanoanalytical with structural information of the region under study. The above-mentioned instrument allows recording electron energy loss spectra, with high signal-to-noise ratios in the energy loss region of interest. In this study, we used acquisition times as small as 10 ms. Thus, recording 64 × 64 SI data requires only 45 s. Such small total acquisition time allows avoiding sample drift due to charging effects which could eventually limit the spatial resolution of the measurements. At the same time, sample irradiation problems are also minimized under these conditions. To determine the concentration of lanthanide ion species in solution, two different techniques were used: ultraviolet-visible (UV-vis) absorption and inductively coupled plasma mass spectrometry (ICP-MS). The former is used to measure Pr3+ (aq) and the latter to estimate the total concentration of cerium species, including Ce4+ (aq) which does not absorb in the UVvis range. These measurements were performed using a PE 552 spectrophotometer and a Thermo Elemental SerieX7 instrument. Results and Discussion Direct Chemical Evidence of Lanthanide Ions Leaching during Impregnation. In order to simulate the effects of an acidic attack during the impregnation with the metal precursor solution, the mixed oxides were subjected to a treatment with an aqueous solution of HNO3 of pH ) 1. For this experiment, 0.5 g of each oxide was immersed, independently, into a vial containing 25 mL of the acidic solution and kept there, during 24 h, at room temperature with no shaking. After exposure to this medium, the supernatant solution was carefully separated and filtered to remove any solid residue in suspension. The presence of lanthanide ion species in the filtered supernatant
5902 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Figure 1. UV-vis spectra recorded from HNO3 solutions (pH ) 1) after contact with the mixed oxides.
TABLE 1: Concentration of Lanthanide Species in the HNO3 Solutions (pH ) 1) after Contact with the Two Mixed Oxides support
[Ce] mg/l ICP
[Pr] mg/l UV-vis
Ce0.5Pr0.5O2 Ce0.8Pr0.2O2
166 ( 1 119 ( 1
3820 ( 33 991 ( 4
TABLE 2: Number of Ln4+ Cations Per Square Nanometer (Surface Density) on Different Crystallographic Facets of the Mixed Oxides facet (hkl)
number of Ln4+/nm2
111 100 110
7.9 6.8 4.8
liquid was followed by UV-vis spectroscopy, Figure 1. The three narrow absorption peaks in the 400 nm - 500 nm range as well as one at about 590 nm very closely match the major absorption bands of the Pr3+ ion (3H4 electronic state), which confirms the presence of Pr3+(aq) in the liquid phase.32 This can be considered direct evidence of the occurrence of leaching of Pr3+ out of the structure of the mixed oxides under the conditions employed in routine wet impregnation procedures. Given that the exposure conditions of both oxides were the same, Figure 1 suggests that the leaching process is more intense in the 50/50 mixed oxide. Table 1 summarizes the analytical measurements obtained from the two mixed oxide powders. The concentration of Pr3+ species in the HNO3 solution which came in contact with the Ce0.5Pr0.5O2-x′ sample is about fourfold that present in the liquid which contacted the Ce0.8Pr0.2O2-x mixed oxide. The higher molar ratio of praseodymium and the higher specific surface area of the former help to explain this trend. To detect and quantify dissolved cerium species (either Ce4+ or Ce3+) in the acid solution, ICP-MS measurement was
Rodrı´guez-Luque et al. performed, Table 1. According to our results, the leaching process does not only affect praseodymium but also cerium though the amount of dissolved cerium is much lower than that of praseodymium. If we consider the molar ratios of Ce and Pr in the oxides investigated, we can draw a conclusion about the preferential leaching of praseodymium. Table 2 shows the number of lanthanide ions per square nanometer expected for the most abundant, lower Miller indices, crystal surfaces of these mixed oxides. These numbers have been calculated taking into account an MO2 oxide with fluorite structure and cell parameters as those of the two oxides under study. Taking into account these calculations, the corresponding molar ratios, and the BET surface area values and considering, in a first approximation, a completely homogeneous distribution of both lanthanide elements in the oxide, it can be concluded that the amount of Pr3+ detected by UV-vis would involve the attack of not only the first surface layer but also of sites deeper down to the third layer. Though praseodymium is the most affected cation, it seems that a much smaller fraction of cerium in these layers is also leached out. The presence of absorption bands in the UV region, in the range 210-250 nm, indicate that a fraction of cerium brought into solution by the acid attack corresponds to Ce3+ species.32 Effects of leaching on Texture and Surface Composition of the Oxides. Once the occurrence of Pr3+ leaching in the case of Ce/Pr mixed oxides was confirmed, we concentrated on determining the textural, structural, and compositional effects such phenomena could involve. For this purpose, a detailed characterization study of the oxide samples before and after impregnation with Rh(NO3)3 was run. Analysis of Textural Changes by SEM Imaging. Figure 2, which shows SEM images at the same magnification of the 80/ 20 mixed oxide before (a) and after (b) impregnation, illustrates the effects of leaching on the texture. According to these images, the size of the polycrystalline aggregates in the impregnated oxide is smaller, the fraction of small size crystallites increases, and at the same time, changes in the pore size distribution also seem to occur. Such modifications could be explained by the dissolution of a small fraction of the surface of the crystallites during the impregnation and further reprecipitation of the dissolved matter during the drying step. In any case, SEM images suggest slight textural changes after the deposition of the metallic component. Effects of Leaching on Surface Composition. To monitor chemical composition changes linked to impregnation, three different techniques have been used: SEM-XEDS, TEM-XEDS, and STEM-EELS. With this combination of techniques different spatial scales can be explored. Thus, SEM X-EDS would account for compositional changes that could occur at the
Figure 2. SEM images of Ce0.8Pr0.2O2-x before (a) and after (b) impregnation Rh (3%)/Ce0.8Pr0.2O2-x.
Rh/CePrOx Catalyst Preparation: An (S)TEM Investigation TABLE 3: Lanthanide Element Percentages in the Ce0.8Pr0.2O2-x Mixed Oxide before and after Impregnation As Determined from SEM-XEDS Measurements sample
% Pr (M)
%Ce (M)
Ce0.8Pr0.2O2 Rh/Ce0.8Pr0.2O2
19.3 19.9
80.7 80.1
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5903 TABLE 4: Results of TEM X-EDS Analysis in Spot Mode on the Bare and Metal-supported Ce0.8Pr0.2O2-x Mixed Oxide sample
position
%Ce (M)
% Pr (M)
Ce0.8Pr0.2O2
surface bulk surface bulk
79.8 79.7 89.8 79.3
20.2 20.3 10.2 20.7
Rh/Ce0.8Pr0.2O2
largest, micron, scale. TEM X-EDS has been used to detect changes that could affect volumes a few nanometers (typically 2-3 nm) in size. Finally, STEM-EELS has allowed chemical composition analysis with subnanometer spatial resolution, i.e., at the finest scale. Chemical composition analysis by SEM X-EDS, Table 3, reveals no net change, at the micron scale, after impregnation. In both samples, the Ce/Pr molar ratios are close to the nominal values and no significant variation is detected after the addition of Rh on either of the oxides. The exact rhodium loading, according to this data, is close to that determined by ICP. To check for eventual compositional changes of the oxides at a finer spatial scale, TEM X-EDS measurements were performed, Figure 3. To avoid the overlap of crystallites and to work on, as much as possible, constant-thickness specimens, the mixed oxide powders were subjected to ultramicrotomy prior to the X-EDS measurements. Thus, all TEM X-EDS spectra were obtained from ultrathin, 20 nm thick, oxide sections. As marked in Figure 3a, a small electron probe, about 2 nm in diameter, was focused on two different types of locations of these sections: (Spot 1) sites, just at the border of the crystallites, and (Spot 2) sites, far from the border of the crystallites, i.e., a
distance, with a value a number of times bigger than the size of the electron probe, away from the border. The former should provide composition details of the outer layers of the oxide whereas in the latter, though some contribution from the surface could in some case be included, the composition of the bulk of the crystallites should be dominant. Note how, Table 4, in the oxide section, prior to impregnation, both types of sites give rise to quite similar analytical results, with compositions very close to the nominal values. As it can be seen, though the peaks (LR, Lβ1,β2, and Lγ) of Ce and Pr are close in the energy range (some tenth of keV), they can still be resolved and adequately quantified. The fact that the Pr/Ce LR lines intensity ratio does not seem to modify when going from the surface to bulk positions is in good agreement with the results of the quantitative analysis. Figure 4 displays the results obtained on the 3%Rh/ Ce0.8Pr0.2O2-x catalyst. In this case, one can observe that the Pr/Ce LR lines intensity ratios change between the surface and bulk locations. In these spectra in particular, the intensity of the Pr peaks decreases at the surface, thus resulting in a significant deviation of the Pr% whereas the values at the bulk remain
Figure 3. TEM X-EDS analysis of the surface and bulk area of a Ce0.8Pr0.2O2-x crystal. (a) TEM image of the analyzed crystal and (b) EDS spectra corresponding to Spots 1 and 2.
Figure 4. TEM X-EDS analysis of the surface and bulk area of a 3%Rh/Ce0.8Pr0.2O2-x crystal. (a) TEM image of the analyzed crystal. (b) EDS spectra from Spots 1 and 2.
5904 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Figure 5. TEM-EDX surface and bulk analysis data corresponding to Ce0.8Pr0.2O2-x and 3%Rh/Ce0.8Pr0.2O2-x samples.
very close to the nominal values, see Table 4. Analysis on other surface sites shows, as expected, deviations in the opposite direction, i.e., to Pr% values higher than 20%, thus suggesting the occurrence of a Pr redistribution in the near-surface regions. A more general view of the TEM X-EDS study on the samples based on the Ce0.8Pr0.2O2-x oxide is presented in Figure 5. Each point in this figure corresponds to the quantification of Ce and Pr percentages on one spot of the sample. Points lying on the two lower lines correspond to measurements on the bare oxide, i.e., before impregnation. Measurements after impregnation, i.e., those obtained on the Rh/Ce0.8Pr0.2O2-x catalyst, are shown on the upper lines. In each case, values obtained on the surface have been plotted separately from those corresponding to the bulk. Two aspects are worth comment: (1) All the values of Pr% measured on the bare oxide, either on the surface or the bulk, are very close to each other, in the range 18-22%, and centered around the nominal composition. The deviation range is very narrow both in the bulk and the surface. (2) Much larger Pr% are observed in the rhodium catalyst, i.e., after impregnation. In this sample the composition window widens, most of
Rodrı´guez-Luque et al. all in the surface locations. This reflects a highly heterogeneous surface with respect to lanthanide elements distribution. Both Pr-rich and Pr-depleted spots were detected. These results point out two very important ideas. First the impregnation with the rhodium solution gives rise to severe lanthanide element redistribution at the surface of the oxide. Second, at the nanometer scale the actual surface of the Rh catalyst becomes rather inhomogeneous in terms of composition, becoming patched in regions of differing Pr/Ce molar ratios. As mentioned above, to have a sub-nanometer scale description about the distribution of the lanthanide elements in these materials, the samples were analyzed by high spatial resolution electron energy-loss spectroscopy in STEM mode (STEMEELS). To obtain precise maps of the distribution of the elements in the oxides before and after impregnation, the spectrum-line (SL) and spectrum-imaging (SI) modes were used. In our experiments, lines made up of 64 acquisition points or images made up of 64 × 64 points were obtained. Figure 6 illustrates a 64 × 64 SI over a 3%Rh/Ce0.5Pr0.5O2-x′. At each point of the scan, a full EELS spectrum is acquired simultaneously with the HAADF signal (Figure 6a). To avoid information superposition and at the same time maintaining a high spatial resolution, the distance between successive points was 0.47 nm. From the spectrum image, a set of data from A to B (sequence of 26 spectra) was directly extracted, Figure 6b and 6c. The gray-scale images of the spectra extracted from the SI (Figure 6b) show the O-K, CeM4,5, and Pr M4,5 signal evolution along the A-B line. A drop in intensity of the Ce signal is clearly seen at the end of the line. Figure 6c shows a 3D representation of a zoom-in view in the Ce-Pr energy loss region. The quantitative analysis of the spectra collections allows for the study of changes in the element distribution that can be correlated with the HAADF signal. In the case of these mixed oxides the detection and quantification of the lanthanide elements can be done by analyzing their M4 and M5 lines, which appear in the energy loss range 870-970 eV. The lines corresponding to the Ce species (Ce3+, Ce4+) are clearly separated from those of Pr species (Pr3+, Pr4+),
Figure 6. 64 × 64 Spectrum image over a 3%Rh/Ce0.5Pr0.5O2-x′ crystal display in (a) HAADF signal acquired simultaneously with the EELS spectra. (b and c) Collection of 26 spectra (as image or as 3D-representation, respectively) extracted directly from the A-B line in the SI. The grayscale images of the spectra collection b shows the O-K, CeM4,5, and Pr M4,5 signal evolution along the A-B line. (c) 3D representation of a zoom in the Ce-Pr energy loss region.
Rh/CePrOx Catalyst Preparation: An (S)TEM Investigation
Figure 7. Spectrum line analysis across a Ce0.8Pr0.2O2-x crystal display on the HAADF images (a). Pr/Ce ratio (b) obtained after treating the spectra acquired across the line indicated on the HAADF image.
allowing the simultaneous and reliable analysis of both elements. It is also well-known that study of the fine structure of these lines leads to further information concerning the oxidation state of these elements.33 The overlap of the Pr-M4,5 edge, especially that of the Pr-M5 one with the extended region of the Ce-M4, and the lack of precisely determined Ce and Pr M4,5 cross-sections, makes quantification a delicate task. Here, the Ce/Pr molar ratio has been measured from the EELS data by determining the integrated intensity in the Ce-M5 and Pr-M4 ionization edges using an energy window of 14 eV. Before integration, a power law model has been used to remove the background before Ce-M5 and Pr-M5 edges.34 The window width used for this background subtraction operation was around 10 eV. A Hartree-Slater model for estimating the M-edge cross-sections was used.34 Figure 7 illustrates the use of the SL mode to monitor the main compositional features of the bare mixed oxides. Figure 7a shows the HAADF image of one crystallite of the Ce0.8Pr0.2O2-x mixed oxide. The black line drawn on this image indicates the path along which a collection of 64 EELS spectra was acquired. The whole set of spectra covers a total length of 50 nm, corresponding to a spatial increment between spectra of roughly 0.78 nm, a value slightly larger than the probe size. From the collection of spectra, the Pr/Ce ratio was estimated at each point using the quantification procedures explained above. Figure 7b displays the variation of Ce/Pr molar ratio from the surface of the crystallite to the bulk. Our results suggest the presence of praseodymium at the surface at higher concentrations than those in the bulk. The Pr atomic percent (%Pr) at the
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5905 first atomic layers, down to ca. 2 nm, is in the range 28%37%, above the nominal value. This segregation of Pr at the surface is compensated by a value of Pr lower than the nominal (Pr/Ce ) 0.25) in the bulk which maintains rather homogeneous. Though the precise values reported here could be affected by small errors due to the intrinsic difficulties of quantification, they can be considered a clear clue about some Pr enrichment at the surface of the mixed oxide crystallites. These results are also in good agreement with those obtained by Belliere et al.29 on Ce-La mixed oxides. In that case a La3+ enrichment was observed at the surface in the Ce-rich composition range of these mixed oxides. After impregnation, the actual surface of the mixed oxide crystallites suffers severe modifications, specially in the case of the 3% Rh/Ce0.5Pr0.5O2-x system. Figure 8 illustrates first the changes detected by STEM-EELS in the 3% Rh/ Ce0.8Pr0.2O2-x catalyst. Thus, Figure 8a shows the bright field STEM image of one of the crystallites analyzed in this sample. The grainy aspect of the contrasts is very likely due to the presence of Rh nanoparticles onto the mixed oxide support crystallite. Again, the line drawn on the image indicates the path of a SL analysis. The variation of the HAADF signal along the line, Figure 8b, describes the crystal thickness variation. Note that in this case, the Ce and Pr amounts change dramatically and quickly from point to point in a complex manner, Figure 8c. In this crystal in particular much lower Pr contents are observed on the outer parts of the crystallite (area 2, Figure c) whereas high Pr signals are observed at the middle (area 1, Figure c). The spectra shown in Figure 8d illustrate these two situations. In the spectrum corresponding to the middle position of the SL collection, which according to the HAADF signal corresponds to the thick part of the crystal, contents of Pr up to 85% could be detected (spectrum 1). On the contrary, the spectrum obtained from thin parts at the outer region (spectrum 2) shows a much lower Pr%. This result, in good agreement with STEM X-EDS, points out a redistribution of the lanthanide elements during impregnation. Leaching of Pr3+, and possibly of small quantities of Ce3+ also, and further redeposition during drying is the basis of this highly heterogeneous lanthanide element distribution. STEM-EELS SI data obtained on the 3% Rh/Ce0.5Pr0.5O2-x′ catalyst support further this idea, Figure 9. In particular, Figure 9a shows the HAADF signal recorded on a crystallite representative of this sample. Figures 9b and 9c correspond, respectively, to the Ce and Pr maps estimated from the
Figure 8. (a) Spectrum line analysis across the Rh/Ce0.8Pr0.2O2-x. display on the bright field image. (b) HAADF signal profile acquired across the crystal display on the inset figure. (c) Ce and Pr profile across the crystal. (d) EELS spectrum corresponding to position 1 (bulk) and position 2 (close to the surface).
5906 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Figure 9. (a) HAADF image of a Rh/Ce0.5Pr0.5O2-x′ crystal. Areas 1-3 designate where the spectra have been extracted. (b and c) Ce and Pr map extracted from the 64 × 64 EELS spectra data set. (d) Pr/Ce map ratio extracted from the data set. (e) EELS spectra corresponding to points 1-3 displayed in HAADF image.
quantitative analysis of the whole 64 × 64 collection of spectra recorded in this SI experiment. Likewise Figure 9d shows the Pr/Ce molar ratio map. In these figures, a high intensity (white) pixel would correspond to a high concentration of the mapped element whereas a low intensity (black) pixel would allocate the points where the concentration of the element has a small value. By comparing the different maps in this figure it becomes clear that although there are regions where the Pr/Ce ratio is rather homogeneous, there are also specific points where large deviations occur. Thus, very rich Pr regions are observed at the upper right corner of the crystallite (region 1, Figure 9a) and at the bottom part (at the left of region 3, Figure 9a). Likewise a Pr depleted zone is observed at the bottom right (region 3, Figure 9a). The central region of the crystallite corresponds to a nominal concentration (region 2, Figure 9a). An alternative way to illustrate these compositional changes is shown in Figure 9e. In this case EELS spectra representative
Rodrı´guez-Luque et al. of three different areas of the same crystallite are shown for comparison. Each of these spectra has been obtained as the average of the spectra corresponding to a number of pixels selected in different positions of the crystal shown in Figure 9a. Spectrum number 2 was obtained from pixels in the central region of the Pr/Ce map (mark as 2 in Figure 9a) where a uniform, medium value pixel intensity is observed. The Pr/Ce ratio calculated from this average spectrum corresponds closely to the nominal value. The average of the spectra of pixels selected from the low-intensity region in the Pr/Ce map, marked as 3 in Figure 9d, results in spectrum number 3. In this case, the %Ce value is over the nominal one. Finally, spectrum number 1 is the average of spectra corresponding to the pixels situated on the white area (marked as 1 in Figure 9d) of the Pr/Ce map. Consequently this last spectrum shows dominant Pr M4,5 peaks and a high %Pr value. Note that these two rich zones are both on positions close to the border. Once more these data reflect an heterogeneous distribution of the lanthanide elements which is not related to a surface/bulk distribution at all, as it was initially the case in the bare oxides. The redistribution of praseodymium in the catalysts is so strong that regions with nearly pure PrOx in the form of isolated ensembles of nanocrystals could be detected in the 3% Rh/Ce0.5Pr0.5O2-x sample, inset Figure 10a. Both EELS, Figure 10a, and X-EDS spectra, Figure 10b, confirm this idea. These ensembles come very likely from recrystallization of leached Pr3+ without contact with the surface of the mixed oxide. All the experimental evidence presented above point to a strong mobilization of the lanthanide elements, preferentially of praseodymium, during the impregnation step. Hence, the picture of a well-defined surface composition has to be disregarded in these materials and substituted by a picture in which the surface is made up of patches of different composition, with Pr/Ce molar ratios spanning over a wide range around the nominal value. A final aspect to be considered, from the point of view of the support, is the oxidation state of the lanthanide elements in the supported metal catalysts. The analysis of the fine structure of the M4,5 edges of Ce and Pr in EELS spectra obtained in a STEM instrument allows studying the change of oxidation state of these elements from +4 to +3 with very high spatial resolution. Given the high sensitivity of Ce4+ and specially Pr4+ to suffer reduction and the potential reducing power of highenergy electron beams on metal oxide materials,33 electron beam irradiation effects are a major concern when trying to investigate distribution of oxidation states in these materials. In our case these studies have been conducted to minimize the influence of the beam, at the VG HB501-dedicated STEM at Orsay. This instrument allows both high-energy resolution, necessary to observe the fine structure (typically 0.7 eV on Ce and Pr M edges), and very short acquisition times (in the order of the
Figure 10. EELS (a) and EDS spectrum (b) corresponding to Pr crystals of 2 nm size indicated on the HAADF image (inset figure).
Rh/CePrOx Catalyst Preparation: An (S)TEM Investigation
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5907
Figure 11. Ce oxidation state variations in a Rh/Ce0.5Pr0.5O2-x′ sample. (a) EELS spectra corresponding to CeO2 and Ce2O3 (Ce4+ and Ce3+ oxidation states) from a reference sample. (b) EELS spectra from different areas on the studied sample showing a mixture of Ce3+ and Ce4+.
Figure 12. Metal particle decoration on Rh/Ce0.5Pr0.5O2-x′ system. (a) HREM image of a Rh decorated particle. EELS (b) and EDS (c) spectra corresponding to the catalytic support (1), metal particle (2), and metal particle surface (3).
ms) which opens the possibility of using spectra recording conditions with very low irradiation doses. In order to minimize the radiation damage the microscope was operated at 80 kV.
Under these conditions, the changes detected in the EELS signal of cerium can be considered reliable. However, in the case of praseodymium, which is easily reduced, the effects of the beam
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Figure 13. Graphical summary of the transformation taking place during Rh/CexPr1-xO2-x′ synthesis: Impregnation (1) Pr3+ preferential leaching (2), and drying (3) processes.
cannot be completely ruled out. For this reason, we have concentrated our first investigations on the behavior of cerium. Figure 11 shows some EELS results concerning this point. EELS spectra in Figure 11a depict the aspect of the Ce-M4,5 edge in two reference compounds, CeO2 and Ce2O3. Note how the change in oxidation state from +3 to +4 introduces measurable changes in the signal. The change in the M4/M5 white line intensity ratio, the shift in energy position of these lines, and the disappearance of the small peak at the right of the two white lines are all qualitative indicators of the change in oxidation state. With these oxidation state fingerprints in mind, the information obtained on our catalysts can be properly interpreted (see Figure 11b). The spectra shown in this figure were recorded in different areas (1 nm size) of the 3%Rh/Ce0.5Pr0.5O2-x catalyst. Note that, in general, both Ce3+ and Ce4+ species seem to be present on the catalyst. On the other hand, it also seems clear that the relative amount of these two species varies from one position to another in the sample. The very small size, only a few nanometers, of the mixed oxide crystallites and the impossibility to avoid the overlap of these crystallites in this sample precludes the determination of the spatial distribution of the two different oxidation states, i.e., it is not possible to correlate the presence of one particular EELS signal with a bulk or surface position in the sample. More detailed SL experiments performed on the 3%Rh/Ce0.8Pr0.2O2-x catalyst, which presents much bigger crystallites but at the same time much less intense leaching effects, suggest that Ce3+ species are preferentially located at the surface of the crystallites, as expected. Summarizing, Ce is present in the catalysts in two different oxidation states, nonuniformly distributed over the sample and, very likely, preferentially in the +3 state at the surface of the crystallites. The Ce3+ species detected by UV spectroscopy in the impregnating solution is in good agreement with these EELS observations. Effects of Leaching onto the Metal Phase. Another point of interest refers to which extent the support mobilization phenomenon affects the supported metal phase. We have
investigated this point by HREM and nanoanalysis by STEMEELS and STEM X-EDS. Figure 12 summarizes the most relevant results of this study. As shown in Figure 12a, HREM images obtained on the catalysts after reduction in hydrogen at low temperature (473 K) indicate that in the supported metal catalysts, the metal nanoparticles become covered by layers made up of support material. The image intensity profile (not shown here), which was obtained from the image, indicate the presence of peaks at 0.22 nm, characteristic of Rh (111) planes, which are then followed, at the surface positions, by peaks at larger distances, 0.31 nm. These cannot be due to metallic Rh but are due to (111) planes of the mixed oxide. STEM-EELS and STEM-X-EDS analysis performed on these particles further confirm this idea, Figures 12b and 12c. The EELS spectrum which is labeled as 1 in Figure 12b was obtained in a position of the 3% Rh/Ce0.5Pr0.5O2-x sample inside the support crystallite, as confirmed by X-EDS (Figure 12c). Moving the electron probe to the interface between the metal particle and the support, point 2 of the sample, gives rise to an EELS spectrum in which the peaks of Ce and Pr, though with less intensity, are still present and a X-EDS spectrum where the presence of Rh is also detected. Finally placing the fine electron probe over the surface of the Rh nanoparticle, position 3, both the EELS and the X-EDS spectra contain the features characteristic of the support. Taking into account that the distance between the points of analysis is away from the support a few times the diameter of the electron probe, the contribution to these spectra of some signal coming from the underlying support crystallite can be ruled out. Hence, these results can be considered as clear evidence of the decoration of the metal nanocrystallites by patches of the support. The presence of these patches can be easily explained simply by considering that during the impregnation step the dissolved rhodium species mix with lanthanide cations, mostly Pr3+, leached out from the support as a consequence of the acidic nature of the impregnating Rh(NO3)3 solution. The liquid layer deposited over the surface of the attacked support crystallites contains both Rh3+
Rh/CePrOx Catalyst Preparation: An (S)TEM Investigation and lanthanide ion species. In the drying step both components deposit over the surface of these crystallites. Given that the mixture of the two types of ions (Rh3+, Pr3+) is present in the liquid phase, it seems unlikely that they separate completely during their deposition. The decoration effect by patches of support is much more intense in the catalyst based on the Ce0.5Pr0.5O2-x mixed oxide given that this is the one which suffers leaching more intensively, Table 1. The presence of these oxide layers on top of the metal surface crystallites could explain, for example, the modifications in the temperature programmed reduction profiles reported by Chun et al.24 on their 2%Pd/CePrOx catalysts. The steadily increasing temperature of reduction observed in these systems with the percentage of Pr present in the mixed oxide correlates with an increasing intensity of the leaching process. The more Pr is leached, the highest the degree of metal coverage can finally be. Likewise, the more effective stabilization of Rh oxide species in Rh/CePrOx, as compared to Rh/CeO2 catalysts under N2O decomposition reaction conditions,18 could also be related to the presence of these support overlayers. Borchert et al.25 did report a significant increase in the surface segregation tendency of Pr3+ after addition of 1.4%Pt which promotes significant changes in the reducibility of Ce-Pr mixed oxides by CH4. Both effects can be clearly related to the mobilization of Pr3+ due to leaching. The presence of Pt in unusual oxidation states (Pt+) and with increasing dispersion as the amount of Pr is increased in the formulation of the mixed oxide23,25 are also facts very likely related to the occurrence of such phenomena. Conclusions The whole set of results presented above addresses the different questions asked in Introduction. Thus, our results confirm the occurrence of an extraction of lanthanide elements, preferentially of Pr3+, out of the fluorite-type host structure of Ce-Pr mixed oxides during their impregnation with acidic aqueous solutions of metallic precursors. Though Pr3+ is the most selectively leached species, smaller amounts of cerium also finally evolve into the solution during the leaching process. The extent of this leaching increases with the percentage of praseodymium present in the mixed oxide as well as with its BET surface area. Leaching and further redeposition of the lanthanide ions during the drying step create a severe modification of the surface composition with respect to that observed on the bare oxide, just before impregnation. The redistribution of these elements, mainly Pr, gives rise to a compositionally patched surface made up of Pr-enriched down to Pr-depleted spots randomly distributed over mixed oxide unattacked cores. From a rather unique situation in the bare oxide, which can be modeled by crystallites of the mixed oxide consisting of a core of homogeneous composition surrounded by a 1-2 nm shell of Pr-enriched surface, impregnation with Rh(NO3)3 produces a transition to a quite different state, from the compositional point of view, in which a highly heterogeneous system is produced. The regular core-to-shell compositional change in the mixed oxide is not the dominant feature in the metal-supported catalyst. Instead, there is a high degree of compositional disorder at the very surface layers, which affects not just the depth but also the lateral coordinates. Spatial disorder also reflects on the distribution of the oxidation states of cerium. The surface of the supported metal nanoparticles is also affected by the deposition of patches of support material onto their surfaces. The extent of surface coverage is directly related to the intensity of the leaching process.
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5909 Figure 13 sketches the major processes which, accordingly to the different nanoscale characterization results presented here, take place during the deposition of the metallic component. This figure depicts a realistic model of the structure of this type of catalyst, in a scenario which is in accordance with the wellestablished chemical properties of Ce-Pr mixed oxides but that had not been considered up to now to interpret their catalytic behavior. All the results presented also clarify another point of interest: the powerful capabilities of TEM/STEM techniques to elucidate the nanoscale features of supported metal catalysts and to investigate the solid-state chemistry processes which are involved in their synthesis. Acknowledgment. We acknowledge the financial support from the MEC/FEDER-EU (Project MAT2005-00333), the Junta de Andalucı´a (Groups FQM-110 and FQM-334) and the FranceSpain Acciones Integradas HF2005-0046. The authors acknowledge financial support from the European Union under the Framework 6 program under a contract for an Integrated Infrastructure Initiative. Reference 026019 ESTEEM.”. S.T. acknowledges the MEC Ramon y Cajal Fellowships Program (2003-05). The SCCYT of UCA is also acknowledged. References and Notes (1) Aruna, S. T.; Ghosh, S.; Patil, K. C. Int. J. Inorg. Mater. 2001, 3, 387. (2) Hartridge, A.; Krishna, M. G.; Bhattacharya, A. K. Mater. Sci. Eng., B 1999, 57, 173. (3) Nauer, M.; Christos, F. B.; C. H., S. J. Eur. Ceram. Soc. 1994, 14. (4) Olazcuaga, R.; Lepolles, G.; Elkira, A.; Leflem, G.; Maestro, P. J. Solid State Chem. 1987, 71, 570. (5) Secary, J. J.; Tong, T. W. Int. Commun. Heat Mass Transfer 1992, 19, 339. (6) Stefanik, T. S.; Teuller, H. L. J. Electroceram. 2004. (7) Stefanik, T. S.; Tuller, H. L. J. Eur. Ceram. Soc. 2001, 21, 1967. (8) Sulcova, P., Trojan, M. Thermochim. Acta 2003. (9) Catlow, C. R. A. Solid State Ionics 1984, 12, 67. (10) Durstine, R. T.; Blumenthal, R. N.; Keuch, T. F. J. Electrochem. Soc. 1979, 126, 264. (11) Shuk, P.; Greenblatt, M. Solid State Ionics 1999, 116, 217. (12) Logan, A. D.; Shelef, M. J. Mater. Res. 1994, 9, 468. (13) Sinev, M. Y.; Graham, G. W.; Haack, L. P.; Shelef, M. J. Mater. Res. 1996, 11, 1960. (14) Bernal, S.; Blanco, G.; Cauqui, M. A.; Martin, A.; Pintado, J. M.; Galtayries, A.; Sporken, R. Surf. Interface Anal. 2000, 30, 85. (15) Trovarelli, A. Catal. ReV. Sci. Eng. 1996, 38, 439. (16) Rossignol, S.; Gerard, F.; Mesnard, D.; Kappenstein, C.; Duprez, D. J. Mater. Chem. 2003. (17) Luo, M. F.; Yan, Z. L.; Jin, L. Y. J. Mol. Catal. A: Chem. 2006, 260, 157. (18) Bueno-Lopez, A.; Such-Basan˜ez, I.; Salinas-Martinez de Lecea, C. J. Catal. 2006, 244, 102. (19) Imamura, S.; Hamada, R.; Saito, Y.; Hashimoto, K.; Jindai, H. J. Mol. Catal. A: Chem. 1999, 139, 55. (20) Sadykov, V. A.; Pavlova, S. N.; Bunina, R. V.; Alikina, G. M.; Tikhov, S. F.; Kuznetsova, T. G.; Frolova, Y. V.; Lukashevich, A. I.; Snegurenko, O. I.; Sazonova, N. N.; Kazantseva, E. V.; Dyatlova, Y. N.; Bobrova, L. N.; Kuz’min, V. A.; Gogin, L. L.; Vostrikov, Z. Y.; Potapova, Y. V.; Muzykantov, V. S.; Paukshtis, E. A.; Burgina, E. B.; Rogov, V. A.; Sobyanin, V. A.; Parmon, V. N. Kinet. Catal. 2005, 46, 227. (21) Sadykov, V. A.; Voronin, V. I.; Petrov, A. N.; Frolova, Y. V.; Kriventsov, V. V.; Kochubei, D. I.; Zaikovskii, V. I.; Borchert, H.; Neophytides, S. Structure specificity of nanocrystalline praseodymia doped ceria. Materials Research Society Symposium Proceedings, 2005. (22) Sadykov, V. A.; Frolova, Y. V.; Alikina, G. M.; Lukashevich, A. I.; Muzykantov, V. S.; Rogov, V. A.; Moroz, E. M.; Zyuzin, D. A.; Ivanov, V. P.; Borchert, H.; Paukshtis, E. A.; Bukhtiyarov, V. I.; Kaichev, V. V.; Neophytides, S.; Kemnitz, E.; Scheurell, K. React. Kinet. Catal. Lett. 2005, 86, 21. (23) Sadykov, V. A.; Frolova, Y. V.; Alikina, G. M.; Lukashevich, A. I.; Neophytides, S. React. Kinet. Catal. Lett. 2005, 86, 29. (24) Chun, W.; Graham, G. W.; Lupescu, J. A.; McCabe, R. W.; Koranne, M. M.; Brezny, R. Catal. Lett. 2006, 106, 95.
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