Rutile-Supported Ir, Au, and Ir−Au Catalysts for CO Oxidation - The

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Rutile-Supported Ir, Au, and Ir-Au Catalysts for CO Oxidation Xim Bokhimi,*,† Rodolfo Zanella,‡ and Carlos Angeles-Chavez†,§ Instituto de Fı´sica, UniVersidad Nacional Auto´noma de Me´xico, A.P. 20-364, 01000 Me´xico D. F., Mexico, Centro de Ciencias Aplicadas y Desarrollo Tecnolo´gico, UniVersidad Nacional Auto´noma de Me´xico, A.P. 70-186, 04510 Me´xico D. F., Mexico, and Instituto Mexicano del Petro´leo, Eje Central No. 152, 07730 Me´xico D. F., Mexico ReceiVed: April 5, 2010; ReVised Manuscript ReceiVed: June 25, 2010

Ir/rutile, Au/rutile, and Ir-Au/rutile catalysts were synthesized at 80 °C and activated at 300, 400, or 500 °C in hydrogen. These catalysts were analyzed with X-ray powder diffraction and refinement of the crystalline structures as well as with high-resolution electron microscopy and infrared spectroscopy. They were used to catalyze the oxidation of CO: Ir/rutile catalysts started their catalytic activity above 50 °C, whereas Au/rutile and Ir-Au/rutile catalysts were active for this reaction even below room temperature. The catalysts containing both iridium and gold were the most active; these catalysts consisted of small and large metallic crystallites supported on rutile. The small crystallites, which contained iridium or iridium and gold, had the morphology of a cuboctahedron and dimensions near 1 nm; the large ones contained only gold. The small crystallites, based on iridium, were very stable because iridium oxide has the same crystalline structure as the support, rutile. In contrast, the local atom distribution of gold oxide is incompatible with the crystalline structure of rutile; therefore, the metallic gold crystallites grew differently: they grew on the defects of the support, which produced crystallites with dimensions between 2 and 3 nm. The improvement of the catalytic activity for the oxidation of CO of Ir-Au/rutile catalysts, in comparison with Au/rutile catalysts, could be due to a synergetic effect caused by the combination of gold and iridium in the small particles. Time aging of these catalysts showed that they adsorbed carbonates on active sites, which decreased their catalytic activity. This activity, however, was recovered after removing the carbonates by heating the catalyst in air and then in hydrogen at 300 °C. Introduction Nanocrystalline gold supported on oxides has been intensively studied since the discovery that it is catalytically active when its particle dimensions are a few nanometers;1-5 in these studies, special interest is taken in its capacity to catalyze the oxidation of CO at room temperature.6,7 These gold catalysts, however, are not stable because their catalytic activity decreases with time8-10 due to either the sintering of gold particles8,10,11 or the adsorption of carbonates on catalytic active sites.12,13 This lack of stability has hindered their wide use in commercial applications. Many attempts have been made to stabilize these gold catalysts by adding a second metal. This has been done in analogy to the well-known improvement of the catalytic properties of other metallic catalysts, not based on gold, when a second metal is added to them.14-16 The addition of a second metal could change the electronic properties of gold particles,17 or it could change their local atom distribution by forming core-shell structures between gold and the added metal.18,19 Iridium is one of the metals that has been added to gold catalysts; its addition improves the catalytic activity for the oxidation of CO and stabilizes the catalyst.15 Iridium alone supported on titania also catalyzes the oxidation of CO, but its catalytic activity starts at temperatures higher than room temperature.15,20 Iridium atoms donate electrons more * To whom correspondence should be addressed. Tel: +525556225079. Fax: +525556225008. E-mail: [email protected]. † Instituto de Fı´sica. ‡ Centro de Ciencias Aplicadas y Desarrollo Tecnolo´gico. § Instituto Mexicano del Petro´leo.

easily than gold atoms do; therefore, if, during the addition of iridium to gold catalysts, iridium atoms interacted with gold particles, they could change the ability of gold to donate electrons. In bulk, however, iridium and gold have a very limited miscibility.21 This fact hinders the possible coexistence of iridium and gold atoms in the same crystalline structure. A recent study of supported Au-Ir catalysts proposes that gold atoms could progressively cover the surface of iridium particles24 because metallic gold has a lower surface free energy (1410 erg cm-2) than metallic iridium (3000 erg cm-2) does. Inspired by the above suggestion, Akita et al.23 deposited gold and iridium simultaneously on a rutile single crystal to produce metallic gold and iridium oxide particles. The gold particles were deposited on the top of IrO2 pillars that were in direct contact with the support. The authors claim that the crystalline IrO2 pillars were formed from a Au-Ir complex by oxidation of Ir during the heat treatment of the samples in air. Density functional theory calculations of Liu et al.24 in the Au/IrO2-TiO2 system show that the presence of iridium oxide produces an active Au/IrO2 interface that hinders the sintering of gold particles, which suggests that Au atoms bond stronger to iridium oxide than to titanium oxide. Recently, we have studied the catalytic activity and stability of a series of Ir and Au-Ir catalysts supported on P25 (a mixture of 80 wt % anatase and 20 wt % rutile) during the oxidation of CO.15 We observed that iridium atoms covered titania particles with a thin layer of IrO2 clusters deposited preferentially on rutile. When the catalysts were reduced in hydrogen, the iridium oxide particles dispersed homogeneously on both anatase and rutile and were present even after reducing the catalysts

10.1021/jp103053e  2010 American Chemical Society Published on Web 07/30/2010

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at 300 °C in hydrogen. The most active Au-Ir catalysts were those prepared by sequential deposition: first depositing iridium and then gold. These Au-Ir catalysts had a higher catalytic activity for the oxidation of CO than those containing only gold and were more stable. Besides CO oxidation, supported Au-Ir catalysts have been used to catalyze other chemical reactions. For example, in the MCP hydrogenolysis,25 the addition of Au enhanced the chemisorptive and catalytic properties of bimetallic Ir-Au/γAl2O3 catalysts compared to the catalysts containing only iridium.25 In these catalysts, the preparation method also determined the catalyst stability. Okumura et al.26 report that Au catalysts in combination with Ir increase their catalytic activity for the decomposition of dioxins at temperatures below 200 °C. Conte et al.27 studied the hydrochlorination of acetylene using supported gold-based bimetallic catalysts, including the Au-Ir system: the activity of gold by the addition of iridium or any other metal, however, was not improved. In the present work, catalysts with iridium and gold, with rutile as a support, were prepared using the depositionprecipitation with urea technique. They were activated in hydrogen at 300, 400, or 500 °C; their catalytic activity for the oxidation of CO was measured at temperatures below 300 °C. Catalysts were characterized with X-ray powder diffraction, refinement of the crystalline structures, high-resolution electron microscopy, energy-dispersive X-ray spectroscopy, and infrared spectroscopy. This characterization provided information about the concentration, crystallite size and morphology of the different phases as well as about the elemental composition of the catalysts and the adsorbed species in the catalytic active sites. Experimental Section Catalyst Preparation. Details about the synthesis of the support (rutile) are reported elsewhere,8,28-30 which was annealed in air at 300 °C for 24 h before the catalyst sythesis. HAuCl4 · 3H2O (Aldrich) and IrCl4 · xH2O (Aldrich) were the respective gold and iridium precursors. Catalyst synthesis was done in darkness to avoid the transformation of the gold precursor. The synthesis of Au/rutile catalysts (4 wt % Au) was performed by deposition-precipitation with urea following the previously reported procedure.3,31,32 Ir/rutile catalysts (4 wt % Ir) were also prepared by deposition-precipitation with urea. For that, 1 g of rutile was mixed with an aqueous solution (50 mL) containing IrCl4 (4.2 × 10-3 M) and urea (0.42 M) to produce a dispersion, which had an initial pH of 4. This suspension was heated to 80 °C for 16 h, stirring it vigorously; the decomposition of the urea slowly increased the suspension pH from 4 to 7.5. After this heating, the suspension was centrifuged to separate the solids, which were washed with water to generate a new suspension that was also centrifuged; this procedure was repeated four more times. Eventually, the solid was dried in a vacuum for 2 h at 100 °C. The catalysts’ activation was done in a U reactor with a fritted plate 1.5 cm in diameter; hydrogen was flowed for 2 h on the solid at 1 mL min-1 per mg of the sample. The activation temperatures were 300, 400, or 500 °C at a heating rate of 2 °C/min. After activation, catalysts were stored at room temperature in a vacuum and in darkness to prevent their alteration.31 Bimetallic Ir-Au/rutile catalysts (4 wt % Ir, 4 wt % Au) were prepared by the sequential deposition method reported previously.15 In this approach, iridium was first deposited on rutile by the deposition-precipitation with urea method described above for the synthesis of Ir/rutile catalysts. The

Bokhimi et al. corresponding samples were dried at 80 °C and calcined in air at 400 °C for 2 h, at a heating rate of 2 °C/min, before gold was deposited by the deposition-precipitation with urea method. After gold deposition, the corresponding samples were washed and dried at 80 °C in a vacuum. The catalysts’ activation was done in a U reactor with a fritted plate 1.5 cm in diameter; hydrogen was flowed for 2 h on the solid at 1 mL min-1 per mg of the sample. The activation temperatures were 300, 400, or 500 °C at a heating rate of 2 °C/min. Characterization Techniques. Catalytic ActiWity. The CO + O2 reaction was studied in a RIG 150 LT microreactor system, from room temperature to 300 °C in a flow reactor at atmospheric pressure. Before measuring the catalytic activity, 40 mg of the catalyst was activated at 300, 400, or 500 °C for 2 h in flowing hydrogen, at a heating rate of 2 °C/min; thereafter, catalysts were cooled to room temperature in the same atmosphere. During the reaction, the reactant gas contained 1% CO, 1% O2, and 98% N2 and flowed through the catalyst at 100 mL/min. Exit gases were analyzed online with a gas chromatograph from Agilent Technologies, model 6890N, with flame ionization and thermal conductivity detectors. X-ray Powder Diffraction. The X-ray powder diffraction patterns of the catalysts were measured in air at room temperature with a Bruker D-8 Advance diffractometer with the Bragg-Brentano θ-θ geometry, Cu KR radiation, a Ni 0.5% Cu-Kβ filter in the secondary beam, and a one-dimensional position-sensitive silicon strip detector (Bruker, Lynxeye).33 The diffraction intensity as a function of 2θ angles was measured between 20° and 130°, with a 2θ step of 0.019447°, for 264 s per point. Crystalline structures were refined using the Rietveld method implemented in the TOPAS code, academic version 4.1.34 Crystallite size and morphology were modeled in reciprocal space with a symmetrized harmonics expansion.35 Lattice deformations were assumed anisotropic and modeled with a multidimensional distribution of lattice metrics.36 The background model was a polynomial function that, in addition to the constant, linear, quadratic, and cubic terms in 2θ, also included the terms (1/2θ) and (1/2θ)2.The standard deviations, given in parentheses in the text, show the variation in the last digit of a number; when they correspond to Rietveld refined parameters, the values are not estimates of the probable error in the analysis as a whole, but only of the minimum possible probable errors based on a normal distribution.37 Transmission Electron Microscopy. Catalysts were analyzed with an analytical transmission electron microscope from Jeol, model JEM 2010-F, equipped with a Z-contrast annular detector. Digital image processing (including Fourier transform) was made by using DigitalMicrograph code, version 3.7.0, from Gatan Inc. Chemical Analysis. Average Au and Ir weight concentrations in the catalysts after their activation were determined with energy-dispersive X-ray spectroscopy using two different energy-dispersive spectrometers: The first one had an OxfordISIS silicon drift detector integrated to a JEOL model JSM5900-LV scanning electron microscope; in this case, atoms were excited with electrons. The second spectrometer was an S2RANGER from Bruker with an X-ray tube of 50 W with Pd as anode and an X-Flash silicon drift detector; this spectrometer was previously calibrated for different gold concentrations. In this case, the sample was totally illuminated (a pressed powder disk 6 mm in diameter). Measurements with the electron microscope were performed in 50 different areas of the sample to have a representative value; the spot size was 10 µm. The measured concentration values in both spectrometers were

Rutile-Supported Ir, Au, and Ir-Au Catalysts

Figure 1. TEM bright-field images of the catalyst support, rutile.

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Figure 3. CO conversion of Ir/rutile catalysts activated in hydrogen at different temperatures.

Figure 2. Rietveld refinement plot of the catalyst support, rutile. In the upper curve, dots correspond to the measured diffraction pattern, whereas the continuous line corresponds to the calculated pattern. The lower curve is the difference between calculated and measured diffraction patterns. Marks correspond to rutile.

similar and differed in less than 15%. Because the atomic number of iridium and gold is high, the concentration values obtained with the second X-ray spectrometer were more reliable. Infrared Spectroscopy. Experiments were carried out in a Nicolet Nexus 670 spectrophotometer. In each experiment, 0.005 g of the catalysts was mixed with 0.1 g of KBr previously dried. The KBr + catalyst sample was pressed in a pellet and analyzed in IR transmission mode. The spectra were collected under an air atmosphere at room temperature from 128 scans with a resolution of 4 cm-1.

Figure 4. X-ray diffraction patterns of Ir/rutile catalysts activated in hydrogen at different temperatures. Miller indices correspond to rutile.

Results Support, Rutile. Rutile particles were polycrystalline elongated bars (Figure 1A) with rutile crystallites oriented along their c lattice parameter and parallel to the bar length axis (Figure 1B); average crystallite dimensions were 10 nm perpendicular to the particle length axis and 30 nm along this axis. The particles had many defects (Figure 1B), which deformed the lattice of the rutile crystallites. The anisotropy of the rutile crystallite morphology produced X-ray diffraction peaks with different widths (Figure 2). The microstructure of the particles did not change significantly when they were annealed between 300 and 500 °C. Ir/Rutile Catalysts. These catalysts started catalyzing the oxidation of CO at 50 °C and reached a conversion of 100% above 200 °C (Figure 3). Their X-ray diffraction patterns did not show any phase containing iridium (Figure 4), but the corresponding EDS analysis in the electron microscope showed that they contained 2.2 wt % iridium; the nominal concentration was 4 wt %. The micrographs showed that iridium was dispersed on the support as small metallic iridium crystallites with dimensions of the order of 1 nm (Figure 5), a lattice parameter of 0.394 nm, and the morphology of a cuboctahedron (Figure 6). The crystallite dimension perpendicular to (200) planes was 1.182 nm, whereas the one perpendicular to (111) planes was 1.32 nm.

Figure 5. TEM micrographs of Ir/rutile catalysts taken in Z-contrast.

Au/Rutile Catalysts. They catalyzed the oxidation of CO at room temperature and lower temperatures (Figure 7). Gold was supported on rutile as single metallic crystallites with dimensions between 2 and 3 nm (Figure 8), which produced a clear contribution to the X-ray powder diffraction patterns (Figure 9). The refinement of the phases in these catalysts gave a gold concentration of 4.5 wt %. The micrographs show that gold crystallites grew on rutile lattice defects (Figures 10 and 11). Ir-Au/Rutile Catalysts. For the oxidation of CO, these catalysts were catalytically more active than those of Au/rutile and Ir/rutile (Figure 12), showing a synergetic effect at room temperature. In the X-ray diffraction patterns of Ir-Au/rutile catalysts, only the peaks corresponding to rutile and metallic gold were observed (Figure 13); the X-ray fluorescence analysis,

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Figure 6. TEM high-resolution image of an Ir/rutile catalyst. The small crystallite (A) corresponds to metallic iridium oriented along its [1-10] zone axis; the large crystallite (B) corresponds to the support, rutile, oriented along its [111] zone axis.

Bokhimi et al.

Figure 8. TEM high-resolution bright-field image of a Au/rutile catalyst. The small crystallites correspond to metallic gold, whereas the large ones correspond to the support, rutile. The isolated gold crystallites were oriented along their [1-10] zone axis.

Figure 7. CO conversion of Au/rutile catalysts activated in hydrogen at different temperatures.

however, showed that they contained iridium (2 wt %), gold (4 wt %), and titanium oxide. The micrographs showed that these catalysts were made of supported small and large particles. The small particles had dimensions around 1 nm (as in the Ir/rutile catalysts), whereas the large ones were between 2 and 3 nm. Most of the small particles contained only iridium and were identified as metallic iridium; the rest of them contained iridium and gold (Figure 14). The large particles corresponded to metallic gold. The catalytic activity of these catalysts for the oxidation of CO decreased with time (Figure 15). The X-ray diffraction analysis and the micrographs showed that, after six months of

Figure 9. X-ray diffraction patterns of Au/rutile catalysts activated in hydrogen at different temperatures. Miller indices in the upper curve correspond to rutile.

aging, the size of the small particles containing only iridium or iridium and gold did not change, whereas the size of those containing only gold increased to nearly 4 nm (Figure 16). The decrease of catalytic activity with aging was produced by the adsorption of carbonates on active sites (Figure 17). These carbonates were removed by heating the catalyst in air at 300 °C

Rutile-Supported Ir, Au, and Ir-Au Catalysts

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Figure 13. X-ray diffraction patterns of Ir-Au/rutile catalysts activated in hydrogen at different temperatures. Miller indices in the upper curve correspond to rutile. Figure 10. TEM high-resolution bright-field image of a Au/rutile catalyst.

Figure 11. TEM bright-field (A) and Z-contrast (B) images of a Au/ rutile catalyst. The large particles correspond to the support (rutile), whereas the small particles correspond to metallic gold. The Z-contrast image shows clearly how gold crystallites grew preferentially along rutile particle borders.

Figure 12. CO conversion of Ir-Au/rutile catalysts activated in hydrogen at different temperatures.

and then in hydrogen at the same temperature, which recovered its activity to catalyze the oxidation of CO (Figure 15). This heat treatment did not change the dimensions of the small particles but increased the dimensions of the large ones (Figure 18). Discussion The atom distribution in the crystalline phases of Ir-Au/rutile catalysts is analyzed in detail to understand their catalytic

Figure 14. Microanalysis of different particles of an Ir-Au/rutile catalyst. Large particles contained only gold (A). Small particles contain only iridium (B) or iridium and gold (C).

Figure 15. CO conversion of the Ir-Au/rutile catalyst activated in hydrogen at 300 °C: (A) fresh catalyst, (B) aged at room temperature for 7 months, (C) after heating the aged catalyst in air at 300 °C and then in hydrogen also at 300 °C.

activity for the oxidation of CO. This understanding will be easier if the corresponding properties of Ir/rutile and Au/rutile catalysts are studied first. Previous studies of gold supported on titania,29,30 to catalyze the oxidation CO, showed that, during the activation of the catalyst in air or hydrogen at 200 °C, the produced gold phase was metallic gold, independent of the heating atmosphere.

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Bokhimi et al.

Figure 16. Contribution of the large metallic gold particles to the X-ray diffraction pattern for the fresh and aged catalysts and the one heated after aging. This information was obtained after refining the crystalline structures of rutile and metallic gold. Figure 18. TEM micrograph, taken in Z-contrast, of an aged Ir-Au/ rutile catalyst after it was heated in air at 300 °C and then in hydrogen at the same temperature.

Figure 17. FTIR spectra of an Ir-Au/rutile catalyst: (A) fresh, (B) aged for 7 months, (C) aged catalyst after it was heated in air at 300 °C and then in hydrogen at the same temperature.

In contrast to this, when iridium is supported on titanium oxide, the formed iridium phase depends on the atmosphere used in the activation. For example, when iridium was supported on P25 and activated in air or in hydrogen, the supported iridium particles were large enough to be identified with X-ray powder diffraction and the crystalline phases could be refined using the Rietveld method. The refinement showed that the activation of the iridium catalysts in air produced iridium oxide (IrO2) as the iridium phase, with an average crystallite size of 6 nm, whereas the activation in hydrogen produced metallic iridium with an average crystallite dimension of 2.8 nm. The gold-iridium catalyst activated in hydrogen had a larger activity for the oxidation of CO.15 Therefore, after this experience, for the present work, we decided to activate all of the catalysts in hydrogen. To interpret more easily the observed results, in the present work, the support used to prepare the catalysts was pure rutile synthesized at 80 °C by precipitation and annealed at 300 °C in air before catalyst impregnation. This produced rutile particles with many defects (Figure 1B) that deformed the lattice of the rutile crystallites. This deformation as well as the links between crystallites functioned as pinning centers for gold atoms during their deposition. At the temperatures of catalyst activation, 300, 400, and 500 °C, the rutile crystallite size and lattice deformations did

not change notoriously, which made the catalyst properties independent of this temperature. In the following paragraphs, we will detail the properties of Ir/rutile catalysts with an iridium concentration of 2.2 wt % (its nominal value was 4.0 wt %) and activated in hydrogen. For all of the activation temperatures, these catalysts started catalyzing the oxidation of CO at 50 °C, reaching a conversion of 100% above 200 °C (Figure 3); pure rutile alone did not catalyze this reaction at these temperatures. In these catalysts, the iridium particles were so small that their contribution to the X-ray diffraction patterns could not be distinguished from the background (Figure 4); only the peaks corresponding to rutile were observed. Even after refining the rutile crystalline structure, it was impossible to detect any residues in the diffraction pattern that could be clearly identified with an iridium phase. These results suggested that iridium atoms were highly dispersed on the support. Because the atomic number of iridium is 77, that of titanium is 22, and that of oxygen is 8, the Ir/rutile catalysts gave good images of iridium phases when they were observed with the transmission electron microscope in Z-contrast mode. The corresponding micrographs showed that iridium was highly dispersed on the rutile surface, forming very small iridium particles (Figure 5). The analysis of the bright-field high-resolution images of these iridium particles showed that they corresponded to single crystals of metallic iridium with crystallite dimensions of the order of 1 nm, a lattice parameter of a ) 0.394 nm, and the morphology of a cuboctahedron (Figure 6). This metallic iridium was also observed by X-ray powder diffraction when iridium was supported on P25 and activated in hydrogen; this was possible because iridium crystallites were large enough to be detected with this technique (the distance between the faces of the average crystallite parallel to the (111) planes was 2.8 nm). The micrographs showed that the iridium crystallite dimension defined by the faces parallel to the (200) planes was 1.182 nm, whereas the one defined by the faces parallel to the (111) planes was 1.32 nm; these dimensions were smaller than those obtained

Rutile-Supported Ir, Au, and Ir-Au Catalysts by X-ray diffraction for the iridium catalyst supported on P25. The cuboctahedron morphology of the crystallite and their dimensions corresponded to an iridium crystallite with 147 iridium atoms, which is one of the magic numbers associated with crystallites having this type of morphology.38 It is important to remark that the cuboctahedra having the magic numbers correspond to very stable atom distributions because their corresponding internal energies are minimal. The iridium crystallites having the morphology of a cuboctahedron and 147 atoms had 3 shells of iridium atoms, with 92 of them on the shell that corresponds to the surface: 12 atoms were at the corners, 48 at the edges, 8 on the faces parallel to the (111) planes, and 24 on the faces parallel to the (200) planes. It is important to notice that, in the metallic crystallites of 5d elements, the crystallite faces parallel to the (200) planes have an atomic ordering reconstruction caused by the contraction of the 6s orbital and the corresponding expansion of 5d orbital.39 This contraction is produced by the relativistic speeds of the associated electrons to the 6s orbital when they move near the atom’s nucleus.40 During the surface reconstruction, the square lattice on the faces parallel to (200) planes reconstruct to a nearly hexagonal lattice, which, at crystallite edges, will be incompatible with the hexagonal lattice on the faces parallel to (111) planes, producing a large atom disorder. Therefore, this surface reconstruction could be one of the possible origins to explain the observed catalytic activity of the metallic iridium crystallites for the oxidation of CO at 50 °C. Because these relativistic effects are larger for gold atoms, the corresponding surface reconstruction in metallic gold crystallites could produce a larger disorder on crystallite edges, which could explain the larger catalytic activity reported for metallic gold crystallites during the oxidation of CO.29 In the following paragraphs, we will present some arguments to explain the origin of the small metallic iridium particles in the Ir/rutile catalysts, as well as another possible origin of their catalytic activity for the oxidation of CO. Iridium oxidizes, forming IrO2 with the rutile-type crystalline structure. The lattice parameter, a, of IrO2 is 1.4% smaller than the one of rutile, whereas the lattice parameter c differs about 5%. These differences of the lattice parameters between IrO2 and TiO2 (rutile) suggest that iridium particles formed mainly on the rutile crystallite faces perpendicular to the rutile [001] direction. This suggestion was confirmed in the high-resolution images of the iridium crystallites supported on rutile (Figure 6), where iridium crystallites grew on the rutile surface parallel to the (1-10) planes of rutile, which were perpendicular to this direction. Because of the compatibility of the crystallite structures of IrO2 and rutile, during the iridium deposition on rutile, the first iridium atoms that arrived at the rutile crystallite surface occupied, with a high probability, the atom positions that corresponded to titanium atoms because these are the most stable site positions for Ir atoms on this surface. This substitution deformed the initial Ti-O-Ti-O local environment on the corresponding rutile face, which transformed into Ti-O-Ir-O because the Ir-O bond lengths (0.1976 and 0.1978 nm) were slightly larger than the corresponding Ti-O bond lengths (0.1962 and 0.1963 nm). This deformation restricted the growth of any Ir-O-Ir-O... island on the rutile surface to only a few unit cells of rutile. Because of the compatibility of rutile and IrO2 crystalline structures, this island was very stable and, consequently, the particle that will grow on it will be fixed firmly to the support. If, during iridium deposition, the environment contained oxygen atoms, the formed iridium phase will be

J. Phys. Chem. C, Vol. 114, No. 33, 2010 14107 iridium oxide, which will not produce extra lattice deformation. But, if the iridium deposition is done in a reducing environment, the formed iridium phase will be metallic iridium; this will produce an additional lattice deformation on the surface in contact with the rutile surface. During iridium deposition, iridium atoms reached the rutile surface at random, producing a high dispersion of the small Ir-O-Ir-O... islands that will serve as seeds for growing metallic Ir particles. As it will be described in the following paragraphs, this particle growing mechanism is different than the one followed by gold particles. In contrast to Ir/rutile catalysts, at 50 °C, Au/rutile catalysts promoted the oxidation of CO with a conversion of 100% (Figure 7). In Au/rutile catalysts, for all activation temperatures, gold particles were single crystals of metallic gold (Figure 8), which were large enough (between 2 and 3 nm) to be detected with X-ray powder diffraction (Figure 9). The refinement of the phases in these catalysts gave gold concentrations near 4.5 wt %, which were similar to the nominal concentration of 4.0 wt %. Gold oxide, Au2O3, has a crystalline structure that differs totally from the crystalline structure of rutile. Au-O bonds form a planar polyhedron41 that is incompatible with the structure of rutile, which is based on (TiO6)8- octahedra sharing their edges.42 Therefore, the formation of gold crystallites should follow a different mechanism from the one described above for iridium particles. Besides, the formation of Au2O3 requires the presence of oxygen, a high temperature, and a high pressure;43 therefore, under the deposition conditions of gold on rutile to synthesize Au/rutile catalysts, the formation of seeds to get this oxide was impossible. This also explains why the activation of the Au/rutile catalyst in air or in hydrogen results, in both cases, in the formation of metallic gold. High-resolution images of the Au/rutile catalysts show that gold particles grew on the defects of the rutile lattice (Figures 10 and 11). Two kinds of defects are observed in these images: those between rutile crystallites and the sharp borders defined by the morphology of the rutile particles. The Z-contrast micrographs (Figure 11B) showed that gold crystallites grew preferentially along the sharp edges of the rutile bars. The mobility of the gold particles on rutile as a support then depended on the defects on it, on the characteristics of the sharp borders of the rutile particles, and on the strength of the interaction between the defects and the seeds of the gold particles. This gives a good idea of the origin of the difference in stability of iridium and gold particles supported on rutile. When both Ir and Au were sequentially supported together on rutile, their catalytic activity for the oxidation of CO was better than that of Ir/rutile and Au/rutile catalysts (Figure 12). The X-ray diffraction patterns of the Ir-Au/rutile catalysts showed the peaks of only two phases: rutile and metallic gold (Figure 13). In contrast to this, the chemical analysis of the catalysts showed that they contained 2 wt % iridium and 4 wt % gold (the nominal concentrations were 4 wt % Ir and 4 wt % Au). This means that, in these catalysts, iridium particles were so small that, in the respective X-ray powder diffraction patterns, their contribution could not be distinguished from the background; this behavior was similar to the one observed in Ir/ rutile catalysts. The high-resolution micrographs of Ir-Au/rutile catalysts showed that iridium particles had dimensions of the order of 1 nm. As in Ir/rutile catalysts, the bright-field high-resolution images of the particles in Ir-Au/rutile catalysts showed that many of the iridium particles corresponded to metallic iridium

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with the morphology and dimensions of a cuboctahedra having 147 iridium atoms. The transmission electron microscope used for the present work had also the capability of detecting the punctual atomic composition in spatial regions of the order of 0.5 nm in diameter. Using this capability, the chemical analysis of the particles in the Ir-Au/rutile catalysts showed that some of the small particles contained only iridium and some others contained iridium and gold (Figure 14); this chemical analysis also showed that all large particles contained only gold. According to equilibrium binary-phase diagrams, Ir is soluble in Au at levels up to 2 atom % in the bulk.21,44,45 For very small particles of about 1 nm, as the one synthesized here, the solubility can increase, and a diluted alloy of iridium and gold can be expected. The above results show that Ir-Au/rutile catalysts were composed of three active phases: metallic iridium, metallic gold, and metallic iridium containing gold. For the oxidation of CO, the catalytic activity of the metallic pure iridium particles was smaller than the one of pure gold particles; therefore, below 50 °C, during the oxidation of CO, only the particles containing gold should participate. Ir-Au/rutile catalysts, however, were more active than Au/ rutile catalysts. This suggests that the iridium particles containing gold should be more active than pure gold particles. The synthesis conditions used to generate Ir-Au/rutile catalysts were appropriate for the deposition of gold on the iridium particles’ surface, as reported in the literature for these catalysts.15,22-24 The iridium particle surface corresponds to the third shell of the 147-atom iridium particles and contains 92 of all atoms. The other two inner shells of these iridium particles are not flexible enough to relax the Ir-Ir bond lengths (0.2745 nm) to make them compatible with the larger Au-Au (0.2885 nm) bond lengths. The proposition in the above paragraph that the catalytically active gold atoms were those interacting with iridium atoms was reinforced by the results obtained when the Ir-Au/rutile catalysts activated at 300 °C were again characterized after their aging for seven months at room temperature. Their activity for the oxidation of CO decreased after this aging time (Figure 15): the fresh catalyst had a conversion of 50% at 10 °C, whereas the aged one reached this conversion at 60 °C. The X-ray diffraction analysis and the micrographs of the aged catalyst showed that the small particles in the catalyst did not grow, but the large particles, containing only gold, however, increased their dimensions from 2.07(4) to 3.64(7) nm (Figure 16). A gold catalyst with a gold particle size of 3.64 nm is almost inactive to catalyze the oxidation of CO at 60 °C.8 This result suggested that the observed catalysts deactivation was not related to an increase of gold crystallite size. Therefore, we searched for other possible factors causing this deactivation, for example, the adsorption of species on the active sites. The adsorption of carbonates on catalytic active sites has been evoked as a cause of the gold catalysts’ deactivation.12,13 To know if the observed deactivation of the aged catalyst could be related to the adsorption of carbonates, the fresh and aged (for 7 months) Ir-Au/rutile catalysts were analyzed by infrared spectroscopy in transmission mode (Figure 17, spectra A and B). In the fresh catalyst (Figure 17, spectrum A), there were not resolved bands in the carboxylate region of 1400-1800 cm-1, which characterizes the presence of adsorbed carbonates.46,47 Figure 17, spectrum B, shows that the aged catalysts adsorbed carbonates; therefore, several bands appear in the carbonate region, the biggest one appearing at 1710 cm-1. The bands appearing around 1700 cm-1 have been associated with the

Bokhimi et al. asymmetric stretching frequency of a side-on bent CO2carboxylate species adsorbed on Ti3+ sites, in close contact with the gold small clusters.48 The CO2- carboxylate species is an intermediate in the formation of surface carbonate and bicarbonate species.49 The presence of carbonate species in gold catalysts has been related either to unreactive spectator species that block the active sites1,11,50,51 or to intermediate products that might represent a key step in the catalytic oxidation of carbon monoxide.52-54 As the catalyst was not under reaction conditions during the FTIR analysis, the observed carbonate species must be related to species that block the active sites. Assuming that this was the cause of the catalysts’ deactivation, the aged catalysts were heated at 300 °C in air and then in hydrogen at the same temperature to eliminate the carbonates. After this heat treatment, the aged catalysts recovered their capacity to catalyze the oxidation of CO (Figure 15) because the sample thermally treated in air and then in hydrogen desorbed the carbonates (Figure 17, spectrum C). The X-ray diffraction analysis and the micrographs (Figure 18) of the reactivated catalysts showed that the dimensions of the small particles did not change, whereas the large particles containing only gold increased their dimensions; from the Rietveld refinement, the obtained average gold crystallite size was 6.3(3) nm (Figure 16), which was too large to be catalytically active for the oxidation of CO.8 The FTIR study and the electron microscopy and X-ray diffraction results show that the deactivation of Ir-Au/rutile catalysts was mainly due to the adsorption of carbonates that blocked the active sites. Conclusions Ir/rutile, Au/rutile, and Ir-Au/rutile catalysts catalyzed the oxidation of CO. Ir/rutile catalysts started their catalytic activity above 50 °C, whereas Au/rutile catalysts and Ir-Au/rutile catalysts at this temperature showed a conversion of CO into CO2 of 100%. The most active catalysts were those containing both iridium and gold. Because iridium oxide has the same crystalline structure as rutile, and the bond lengths Ir-O and Ti-O were similar, iridium atoms fixed firmly to the catalyst support (rutile) when they were deposited on it. The difference of lattice parameters between IrO2 and rutile limited the size of the grown iridium particles, which were metallic iridium cuboctahedral crystallites having 147 atoms (one of the magic numbers associated with this geometry). Gold crystallites in the Au/rutile catalyst had a completely different growing mechanism because the local environment of the gold oxide was incompatible with the crystalline structure of rutile; they grew on the defects in the support, which was formed by polycrystalline particles with the morphology of an elongated rod and having the rutile crystallites oriented with their c lattice parameter parallel to the rod length. The Ir-Au/rutile catalysts contained three different particle types: small metallic iridium crystallites having 147 atoms and dimensions of the order of 1 nm, metallic gold crystallites with dimensions between 2 and 3 nm, and small iridium particles containing gold and having dimensions of the order of 1 nm. These last particles were probably responsible for the enhancement of the catalytic activity of the Ir-Au/rutile catalysts for the oxidation of CO. Ir-Au/rutile catalysts decreased their activity with aging time at room temperature caused by the adsorption of carbonates, which were eliminated by heating the catalysts at 300 °C in air and then in hydrogen. After this, the catalysts recovered their activity to catalyze the oxidation of CO.

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