Rutile Catalysts: Effect of Support Dimensions on the Gold

Oct 3, 2007 - To analyze the effect of the support crystallite size on the activity of gold catalysts for CO oxidation, gold was supported on rutile w...
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J. Phys. Chem. C 2007, 111, 15210-15216

Au/Rutile Catalysts: Effect of Support Dimensions on the Gold Crystallite Size and the Catalytic Activity for CO Oxidation Xim Bokhimi,*,† Rodolfo Zanella,‡ and Antonio Morales† Institute of Physics, UniVersidad Nacional Auto´ noma de Me´ xico, A. P. 20-364, 01000, Me´ xico D. F., Mexico, and 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 ReceiVed: June 15, 2007; In Final Form: August 7, 2007

To analyze the effect of the support crystallite size on the activity of gold catalysts for CO oxidation, gold was supported on rutile with crystallite sizes between 10 and 76 nm. The catalysts were characterized by X-ray powder diffraction, refinement of the crystalline structures, transmission electron microscopy, and nitrogen adsorption and were tested for the oxidation of CO at temperatures below 200 °C. Rietveld refinement provided the crystallite dimensions of the support, its morphology, its percentage of microstrain, and its specific area, which was similar to that obtained from nitrogen adsorption experiments. This specific area together with the microstrain determined the size of the gold crystallites: As these two parameter decreased with increasing support crystallite size, the gold crystallite dimensions increased. The specific area of the gold crystallites varied between 54.7 m2/g for the largest crystallites to 108.2 m2/g for the smallest crystallites, and the respective conversion of CO into CO2 at 50 °C changed from 1 to 99. Therefore, the change in surface area alone cannot explain the changes in the catalytic activity. The gold crystallite morphology corresponded to cubooctahedra with extra layers of atoms on the surface parallel to the (111) planes. The smallest gold crystallite was modeled with the gold atoms treated as hard spheres; it contained a total of 893 gold atoms, 468 of them on the surface, with coordination numbers of 5 and 7-11.

Introduction The catalytic activity of many systems is very sensitive to the microstructure of the active phase. For example, when gold is supported on titania, it is active only when the gold particles have dimensions of a few nanometers.1-5 When gold catalyzes the oxidation of CO, catalysts with smaller the gold particles are more active.5-8 It is believed that, in addition to the particle dimension, its morphology and the interaction between the gold atoms and the defects on the support are crucial for the high catalytic activity.9 Among these factors, the morphology is a very important parameter,10-12 because it is believed that the active sites for activation of the oxygen molecules are associated with the gold atoms at the crystallite edges and with those in direct contact with the support.13 Gold nanoparticles can have different morphologies:14,15 Theoretical and experimental studies have revealed that very small gold particles can have morphologies different from those based on the fcc crystalline structure.16-18 When the gold particles are very small, the most stable gold morphologies correspond to cubooctahedra, truncated cubooctahedra, and Mackay icosahedra.11,12,16,19-22 When they are large, the cubooctahedral morphology, which is derived from a cubic crystalline structure centered on the faces, is the most stable.23-25 DFT calculations, however, show that, when gold is supported on Mg(OH)2, the most active gold particles have icosahedral symmetry.26 In a series of gold catalysts supported on titania with large variations in activity, the gold particles, however, had the same * Corresponding author. E-mail: [email protected]. † Institute of Physics. ‡ Centro de Ciencias Aplicadas y Desarrollo Tecnolo ´ gico.

morphology: they were cubooctahedra.10 In addition to this fact, theoretical27 and experimental28 results indicate that gold particles are stable when their morphology corresponds to truncated cubooctahedra that are based on the fcc crystalline structure of metallic gold. For the specific reaction of CO oxidation catalyzed with gold supported on titania, the active gold particles should be smaller than 5 nm.29 There is no clear explanation of this behavior: Probably, the number of active sites increases as the size decreases, there is a dramatic change in particle morphology, or a change in the electronic structure occurs. To find the origin of the change in activity with the particle size, it is necessary to obtain more detail about gold particles in this size range. Because the catalytic properties of gold supported on oxides also depend on the dimensions of the supporting particle,30,31 it is possible that variations in this dimension give rise to gold particles with different sizes. If this were so, then the observed change in the activity of these catalysts with the dimension of the support particle would be caused by the change in the gold particle size. To contribute to the answer of the above questions, we prepared a series of gold catalysts supported on rutile having different crystallite dimensions. The characterization included the simultaneous study of both the support and the gold particles. When gold particles with dimensions smaller that 5 nm are observed by electron microscope to determine their morphology, it becomes undetermined, because the strong interaction between the electron beam of the microscope and the electrons of the particles breaks down the Au-Au bonds, fusing the particle for short times, which changes the gold particle morphology. This change depends on the observation time and the beam intensity.32

10.1021/jp0746547 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/03/2007

Au/Rutile Catalysts When the particles are monocrystalline, an alternative method for analyzing their dimensions and morphology is X-ray diffraction.33,34 In this case, the number of X-ray photons interacting with each particle is reduced and insufficient to alter the particle properties. Because gold particles are monocrystalline when supported on titania, the conditions for this analysis with X-ray diffraction are fulfilled. Because the support was crystalline and its crystalline structure was known, it was possible to refine its crystalline structure and microstructure as well as those of the gold phase. Therefore, the refinement provided information about the dimensions and morphologies of the average crystallites of both the supports and the active gold phases. This study also allowed us to obtain some other properties of the gold and rutile crystallites, including their mass densities, specific areas, volumes, masses, numbers of gold atoms per crystallite, unit cell dimensions, and microstrains caused by the variation of these dimensions; as well as the atom positions in the respective unit cells. Most of this information is new in the literature and shows the strong capability of the X-ray diffraction technique to obtain it, by using a conventional X-ray diffractometer and the technique of crystal structure refinement by the Rietveld method. Experimental Section Synthesis of the Catalysts. Details about the preparation of the support and the gold deposition are reported elsewhere.34 After the synthesis of a fresh support sample, the support was divided into five parts that were heated at different temperatures (i.e., 300, 400, 500, 600, and 700 °C) for 14 h at a heating rate of 2 °C/min to obtain supports with different crystallite sizes. Characterization Techniques. X-ray Powder Diffraction. The X-ray powder diffraction patterns of the catalysts were measured at room temperature in air in a Theta-Theta Bruker D-8 Advance diffractometer that had a Bragg-Brentano geometry, Cu KR radiation, a graphite secondary-beam monochromator, and a scintillation detector. The diffraction intensity as a function of the angle 2θ was measured between 20 and 110°, with a 2θ step of 0.02° and a counting time of 10 s per point. The crystalline structures were refined via the Rietveld method using the FullProf code.39 Crystallite morphology was modeled using spherical harmonics as base functions;40 assuming crystallite microstrain to be isotropic; and modeling the background with 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 and tables show the variation in the last figure of a number. When they correspond to Rietveld refined parameters, their values are estimates not of the probable error in the analysis as a whole but only of the minimum possible probable errors based on their normal distribution.41 The data for the crystallite morphologies obtained from Rietveld refinement were used to produce images of the average crystallites using POV-Ray software, version 3.6.42 TetGen software (version 1.4)43 was used not only to generate a tetrahedral mesh of the crystallite bulk44 that was used to calculate the crystallite volume, but also a three-dimensional boundary conforming Delaunay triangular mesh of the crystallite surface, which was used to calculate the surface area of the crystallites.45 Electron Microscopy. The catalysts were analyzed by transmission electron microscopy (TEM) in a JEOL JEM-2010F microscope. The catalyst powder was dispersed in ethanol before being placed in a copper grid covered with Formvar.

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Figure 1. Rietveld refinement plot of the gold catalyst made with the support prepared at 600 °C. In the upper curve, dots correspond to the experimental data, and the continuous line corresponds to the calculated diffraction pattern. The lower curve represents the difference between the experimental and calculated data. The upper marks correspond to rutile (RF ) 0.011), and the lower ones correspond to metallic gold (RF ) 0.013).

Nitrogen Adsorption. N2 isotherms were measured at -196 °C in an Autosorb-1 apparatus from Quantachrome Instruments. Prior to the measurements, samples were heated at 200 °C for 12 h in a vacuum to eliminate the adsorbed gases. Specific surface areas were calculated using the BrunauerEmmet-Teller (BET) method for relative pressures P/Po between 0.1 and 0.3. Results and Discussion Gold catalysts were supported on rutile synthesized by embedding the titanium precursor in a very acidic solution generated with hydrochloric or nitric acid.34 The acidic solution had a H/Ti atom ratio of 9.2 when it was prepared with hydrochloric acid and 27 when it was prepared with nitric acid. Because the gold catalysts were used to catalyze the oxidation of CO and because Cl ions can play a role in the catalytic process,6,46,47 in the present study, we discarded the rutile prepared with hydrochloric acid rather than using it as a catalyst support, thereby avoiding any erroneous interpretation of the catalytic results. The main interest of the present study was to analyze the effect of the crystallite size of the catalyst support on the catalytic properties. To vary this size, the fresh rutile was heated in air at temperatures between 300 and 700 °C, which gave rise to rutile crystallites with dimensions between 10 and 76 nm. To obtain the crystallite size, morphology, strains, and crystallography of the crystalline phases in the catalysts, these phases were refined using the Rietveld method. This refinement was done for all of the catalysts, and it was done for the pure support, before deposition of the active phase, for only some of them; we observed that the gold deposition and activation did not produce any changes in the support properties. For the Rietveld refinement, the crystalline structure of rutile was modeled with a tetragonal unit cell with the atoms distributed according to space group P42/mnm. The basis of the unit cell contained only two atoms: one titanium atom at the relative coordinates (0.0, 0.0, 0.0) and one oxygen atom at the relative coordinates (x, x, 0.0). The initial value of x used for the refinement was 0.3. Gold was modeled with a cubic unit cell that had a basis of only one gold atom at the relative coordinates (0.0, 0.0, 0.0) and the symmetry of space group Fm3m. Figure 1 shows a typical Rietveld refinement plot. To obtain information about the size and morphology of the crystallites using the Rietveld refinement, the model for the crystallites assumed that they all had the same form and

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Bokhimi et al. TABLE 1: Rutile Average Crystallite: Sizes Perpendicular to the (200) and (002) Planes (R(200) and R(002), Respectively), Areas, Volumes, and Area-to-Volume Ratios, A/V, as a Function of the Support Synthesis Temperature T (°C)

R(200) (nm)

R(002) (nm)

area (nm2)

volume (nm3)

A/V (nm-1)

300 400 500 600 700

10.2 13.6 23.8 43.6 61.2

19.0 32.6 32.8 52.2 75.4

1455 2914 3803 9855 19471

4259 12245 19162 82080 228807

0.342 0.238 0.198 0.120 0.085

TABLE 2: Rutile: BET Specific Surface Areas (BRAs), Specific Areas Obtained from the Rietveld Refinement (RRAs), and BET Specific Surface Areas Compared to the Value for the Catalyst with the Support Prepared at 700 °C Figure 2. Rutile average crystallites obtained from Rietveld refinement, for the gold catalysts with supports (rutile) prepared between 300 and 700 °C.

Figure 3. TEM micrographs: (A,B) pure rutile synthesized at 300 °C, (C,D) gold catalyst with the support (rutile) synthesized at 700 °C.

dimension. Because this model does not generally correspond to the real situation in the sample, the average crystallite dimensions and morphology were affected by the corresponding real distributions in the sample; therefore, in the specialized literature, the obtained size and morphology for the crystallites are said to be apparent.48 The calculated parameters, however, were a first good approximation of the true values. When these distributions are not large, the obtained average dimension and morphology of the crystallites correspond to those of the actual crystallites. The rutile crystallites were anisotropic (Figure 2). Those with the smallest dimension were nearly cuboids that slowly transformed into cylinders as the dimensions of the crystallite grew with the synthesis temperature. The TEM micrograph of the catalyst with the rutile prepared at 700 °C (Figure 3C) shows that the rutile crystallites observed with the electron microscope had a similar morphology and dimension to those obtained from the Rietveld refinement. The largest dimension of the rutile crystallites was along the c axis. To have an idea of the crystallite dimensions, their thicknesses perpendicular to the

T (°C)

BRA (m2/g)

RRA (m2/g)

BRA/BRA(700 °C)

300 400 500 600 700

51.7 32.5 22.0 12.0 8.2

80.4 56.0 46.7 28.3 20.0

6.30 3.96 2.68 1.46 1.00

(200) and (002) planes are given, denoted as R(200) and R(002), respectively (Table 1). These dimensions varied from R(200) ) 10.2 nm and R(002) ) 19 nm for the rutile prepared at 300 °C to R(200) ) 61.2 nm and R(002) ) 75.4 nm for that prepared at 700 °C. The rutile crystallites of the support annealed at 300 °C (Figure 3A,B) had diameters similar to the smallest crystallite dimension (Table 1 and Figure 2) and formed bundles parallel to the particle length. In this bundle formation, the entire area of the individual particles was available for impregnation with the gold solution, because the interaction between the rutile particles was weak. The specific surface area for this catalyst, as obtained with the BET method (Table 2), was 51.7 m2/g, whereas the corresponding value obtained from the Rietveld refinement was 80.4 m2/g; these values are of the same order of magnitude. If the anisotropic strain and the size distribution were taken into account, the calculated average crystallite sizes would be larger (these two effects cause broadening of the diffraction peaks) and consequently smaller specific surface areas. Another explanation for the difference between the measured specific surface area and that obtained from the Rietveld refinement could be caused by the assumption that rutile crystallites were isolated from each other. The micrographs of these catalysts, however, show that the crystallites were aligned and sintered along the c axis (Figure 3B), forming a fiber. Therefore, not all of the area of each individual crystallite in the rutile particle was available to adsorb the nitrogen molecules used in the experiments to determine the BET specific surface area. The bundle formation of the rutile particles favored their sintering as the sample was annealed at temperatures higher than 300 °C. This sintering produced crystallites with the dimension perpendicular to the particle length growing faster than that along the length (Table 1) and decreased the specific surface area of the support (Table 2). Consequently, the sintering of the support particles played a significant role in the size of the gold particles generated during the deposition of the gold on the support and its activation to produce the corresponding gold catalyst. The crystallites of the different supports are shown in Figure 2; they all have the same scale. This, together with Table 1, make evident that the crystallites of the support grew with the synthesis temperature.

Au/Rutile Catalysts

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TABLE 3: Rutile: Microstrain R〈E〉, Percentage of Titanium Vacancies, and Lattice Parameters a and c as Functions of the Support Synthesis Temperature T (°C)

R〈〉 (%)

Ti vacancies

300 400 500 600 700

0.385 0.281 0.222 0.147 0.083

8.8 9.6 7.2 8.8 8.0

a (nm)

c (nm)

0.46010(2) 0.45977(1) 0.45964(1) 0.45954(1) 0.45944(1)

0.29575(1) 0.29577(1) 0.29586(1) 0.29591(1) 0.29591(1)

The areas, volumes, and specific areas of the support crystallites were obtained by using the data generated with the Rietveld refinement. The area of each representative crystallite was obtained by modeling the crystallite surface with a mesh of triangles and adding the corresponding areas of all of these triangles. As an indication of the number of associated nodes and triangles of the mesh, note that, for the analysis of the support prepared at 700 °C, the number of nodes was 64753, and the number of generated triangles was 26780. The large number of triangles in the mesh indicates that the area of the mesh was a very good approximation to the crystallite area. In a similar way, the crystallite volume was modeled with a mesh of tetrahedra. The volume was obtained by adding the corresponding volumes of the individual tetrahedra. For the support prepared at 700 °C, the number of tetrahedra in the mesh was 450494. Table 1 provides a summary of the areas and volumes of the different rutile crystallites, as well as the area-to-volume ratios, A/V. This ratio is related to the specific surface area of the phase through the relationship As ) (A/V)/F, where F is the crystallite mass density (4.25 g/cm3 for rutile). The specific areas of the supports as a function of the support synthesis temperature are reported in Table 2, together with the respective specific surface areas obtained from nitrogen adsorption experiments and calculated using the BET approximation.49 For all of the supports, the specific area obtained from the Rietveld refinement was larger than that obtained with the nitrogen adsorption experiments (Table 2). This is because the crystallites were interacting with each other and the common area of the crystallites involved in this interaction was not available for the adsorption of nitrogen. Although the BET and Rietveld areas were slightly different, they exhibited the same variation with the synthesis temperature of the support: they decreased as this temperature increased, which will be crucial for interpreting the observed changes in the catalytic properties of the respective catalysts for the oxidation of CO. Rutile crystallites had microstrains in a percentage that decreased with the synthesis temperature (Table 3): At 300 °C the average microstrain was 0.385%, whereas at 700 °C, it was only 0.083%. This microstrain could also play a role in the catalytic properties of the catalysts, but probably, it could be more important for their stability. The crystallites were also deficient in the cation titanium (Table 3). This deficiency, however, did not correlate with the evolution of the microstrain with temperature and was probably not the main cause of the observed microstrain. The lattice parameters of the rutile crystallites varied with the synthesis temperature (Table 3): The lattice parameter a decreased by 0.14% from 300 to 700 °C, whereas the lattice parameter c increased by only 0.05%. These changes in lattice parameters could be the main cause for the observed changes in crystallite microstrain with annealing temperature. When gold was deposited on the rutile support to prepare the catalyst, the dimensions of the gold crystallites increased

TABLE 4: Gold Average Crystallite: Dimensions Perpendicular to the (200), (220), and (111) Planes, G(200), G(220), and G(111), Respectively, and Lattice Parameter a as Functions of the Synthesis Temperature of the Support T (°C)

G(200) (nm)

G(220) (nm)

G(111) (nm)

a (nm)

300 400 500 600 700

1.95 2.80 3.05 4.13 4.46

3.41 4.05 3.79 5.21 5.63

3.28 4.23 4.43 6.43 6.81

0.40655(8) 0.40725(5) 0.40708(3) 0.40742(2) 0.40757(3)

as the dimensions of the support crystallite increased (Tables 2 and 4 and Figure 4). This can have the following explanation: All of the catalysts were prepared with the same gold concentration, 5 wt %. Because the specific area of the support available for gold deposition decreased as the synthesis temperature of the support increased, the gold concentration for the deposition per gram of support increased. For example, this area was 6.3 times larger for the catalyst with the support annealed at 300 °C than for that annealed at 700 °C (Table 2). This means that the gold concentration for the catalyst with the rutile prepared at 700 °C was equivalent to a concentration of 6.3 × 5 ) 31.5 wt %, referred to the gold concentration of 5 wt % for the catalyst with the rutile synthesized at 300 °C. If all of the particle pinning centers, which hinder gold atoms mobility, were occupied, then a larger gold concentration would imply the formation of larger gold crystallites; these pinning centers are normally associated with defects in the support crystallites. In the present case, we observed that the microstrain, which is associated with local defects in the crystalline structure of the support, decreased as the synthesis temperature of the rutile support increased (Table 3), so the number of particle pinning centers decreased. This observation, together with the increase of gold available for deposition on the support surface as the support synthesis temperature was increased, explains the increase of the gold crystallite size with the synthesis temperature of the support. As the dimensions of the gold crystallite decreased, the unit cell contracted (Table 4). The lattice parameter of the gold prepared with the support synthesized at 300 °C was reduced by 0.025% compared to its value for the catalyst with the support prepared at 700 °C. The decrease of the gold crystallite dimensions also caused an increase of the specific area, which changed from 53.5 m2/g for the gold deposited on the rutile synthesized at 700 °C to 108.2 m2/g for the gold crystallite

Figure 4. Average gold crystallites obtained from the Rietveld refinement, for gold catalysts with rutile supports prepared at temperatures between 300 and 700 °C.

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Bokhimi et al.

TABLE 5: Gold Average Crystallite: Areas, Volumes, Area-to-Volume Ratios (GA/V), Specific Areas (GAs), and CO Conversions into CO2 at 50 °C for the Respective Catalyst

300 400 500 600 700

area (nm2)

volume (nm3)

GA/V (nm-1)

GAs (m2/g)

CO conversion (%)

31.36 47.98 49.09 108.08 118.81

15.01 28.52 30.01 99.88 115.05

2.09 1.68 1.63 1.08 1.03

108.2 87.2 84.8 56.1 53.5

99 93 63 14 1

deposited on the rutile synthesized at 300 °C (Table 5). The change in specific surface area of these gold crystallites certainly had an effect on their catalytic properties for the oxidation of CO, but it cannot explain the large observed change in catalytic activity as a function of the support synthesis temperature (Table 5 and Figure 5). As the specific area of the gold crystallites changed from 108.2 m2/g for the catalyst with the support prepared at 300 °C to 53.5 m2/g for that prepared at 700 °C, the corresponding conversion of CO into CO2 at 50 °C changed from 99 to 1%. This dramatic change in the activity could be more associated with the change in the number of gold atoms in the sites of low coordination. This point of view, however, is very simple; therefore, it would be much better to obtain an approximation of the electronic density on the crystallite surface as well as the chemical reactivity. After the identification of these sites, it would be possible to find which of them could be responsible for the observed catalytic activity for the oxidation of CO; under the present conditions, any proposition about these sites would be merely speculative. The observed crystallites contain only two faces, one parallel to the (111) plane and the other one parallel to the (200) plane. A detailed analysis of the crystallites shows that almost all of the sites with lower coordination were at the edges of the faces parallel to the (111) planes; therefore, the catalytic reaction would probably occur at the edges of these faces. The Rietveld refinement gave not only the dimensions of the gold particles, but also their morphology (Figure 4). This morphology was based on a cuboctahedron that had extra layers of atoms on the (111) planes; as the crystallites grew, the edges show also a depression in its middle. In the present work, we give only a rough description of all these crystallites; a more detailed discussion will be presented in future publications, together with the corresponding electronic structures. Because the largest catalytic activity for the oxidation of CO (Figure 5) was obtained for the catalyst with the support prepared at 300 °C, which also had the smallest gold crystallite size, we give an atomistic model, based on hard spheres, of the corresponding gold crystallite (Figure 6). This model, at least, gives information about the sites with different coordination numbers on the crystallite surface. To obtain the crystallite model based on hard spheres, we constructed a model using the crystallography obtained from the Rietveld refinement. We started by analyzing the experimental distances obtained from the Rietveld refinement for the gold crystallite (Table 4). Because the distances perpendicular to the (111) and (200) planes corresponded to a truncated cube and because we did not know how large the truncation was, these distances were not taken as the reference for the model. Instead, we used as a reference the distance perpendicular to the (220) planes to construct the cube, which was 3.42 nm (Table 4). Because the lattice parameter of the unit cell associated with this crystallite was 0.40655(8) nm, the number of unit cells that gave a value of approximately 3.42 nm for the distance perpendicular to the (220) planes was 6 (6.0 × 0.40655 × x2

Figure 5. Catalytic activities of gold catalysts with rutile supports prepared at temperatures between 300 and 700 °C.

Figure 6. Average gold crystallite of the catalyst with the rutile prepared at 300 °C and the corresponding model made with hard spheres.

) 3.45). Therefore, the starting crystallite for our analysis was a cube with a side of 2.4393 nm, which was 6 times the value of the lattice parameter; this cube contained 1099 gold atoms, as counted with the CARINE code used for the simulation.50 To reproduce the morphology of the crystallite obtained through the Rietveld refinement, this cube was cut perpendicular to the (111) planes. First, the eight vertices were eliminated; this gave rise to a distance perpendicular to (111) planes of 3.77 nm, which was larger than the experimental value of 3.28 nm (Table 4). Therefore, a new layer of gold atoms (six per layer) parallel to the (111) planes was cut, which eliminated 6 × 8 ) 48 additional atoms. With this cut, the distance perpendicular to the (111) planes in the model crystallite was 3.38 nm, which was near the experimental value of 3.28 nm. In this way, we adjusted the distances perpendicular to the (111) planes of the model to those of the experimental crystallite. The next step was to cut the model crystallite to adjust the distance perpendicular to the (200) planes of the experimental crystallite, which was 1.95 nm. In the crystallite model, the initial distance between the surfaces parallel to the (200) planes was 2.4393 nm, which is larger than the experimental value. To decrease this distance, we eliminated all of the atoms of the last layer of these surfaces, except those atoms that belonged to the crystallite surface parallel to the (111) plane. This gave a final distance perpendicular to the (200) planes of 2.04 nm, which is near the corresponding distance, 1.95 nm, obtained from experiment. The atoms eliminated from the faces parallel to the (200) planes explain the origin of the apparent holes observed in the surfaces of the crystallites obtained from the Rietveld refinement; the number of eliminated atoms in this operation was 25 × 6 ) 150. The total number of atoms eliminated from the initial cube to construct the model crystallite was 8 + 48 + 150 ) 206. Then, the model crystallite was formed from 1099 - 206 ) 893 atoms. This number of atoms is equal to the number of gold atoms, 893.5, obtained from the crystallite generated with the refinement and calculated in the following way: The volume and mass density (19.4633 g/cm3) of the crystallite were obtained from X-ray measurements, and then

Au/Rutile Catalysts the mass of the crystallite was calculated to obtain 2.92 × 10-19 g. Dividing this mass by the atomic weight of gold, 196.966 amu, gives the number of gold atoms in the crystallite. Figure 6 shows that the model crystallite based on hard spheres reproduced very well the crystallite size and morphology obtained from the Rietveld refinement. The morphology corresponds to a cuboctahedron with extra layers of atoms on the (111) planes. This morphology was similar to that observed with the electron microscope (Figure 3D); it is worth mentioning that, in this micrograph, most of the gold crystallites were oriented with their (111) faces parallel to the rutile crystallite surface; some of them, however, were oriented as reported previously34 with the (220) planes parallel to the rutile surface. Of the 893 atoms forming the gold crystallite, 468 were on the surface (52.4% of the total number of atoms), and the rest were in the bulk. The atoms on the surface had different coordination numbers, namely, 11, 10, 9, 8, 7, and 5, and of them, only 36 had the coordination number 5. Because most of the gold atoms with low coordination numbers are on (111) planes, these were probably the most reactive planes. At the moment, however, it is not possible to say which of these atoms on the surface with different coordination numbers were responsible for the observed catalytic properties of the gold crystallites. It is important to note, however, that most of the atoms with low coordination were on the edges of the surfaces parallel to (111) planes; therefore, these planes were probably the most active for the catalytic reaction. Conclusions We have prepared catalysts of gold supported on rutile that had different crystallite sizes obtained by annealing of the support between 300 and 700 °C before depositing the gold. In the catalysts, the rutile and gold phases were characterized by refining their crystalline structure with the Rietveld method. This gave the variations of the crystallographic parameters and the microstructure of the rutile and gold phases in the catalysts. The refinement provided the dimensions, morphologies, and specific areas of the corresponding average crystallites. The gold crystallite size depended on the crystallite size of the support: larger support crystallites gave rise to larger gold crystallites. This was mainly caused by the decrease of the specific surface area of the support as its synthesis temperature was increased. The decreasing of the average microstrain of the support crystallites with synthesis temperature was also important for the growth of the gold crystallites. Although the catalytic oxidation of CO increased as the gold crystallite size decreased, the variation in CO conversion at 50 °C was much larger than the variation in the specific area of the gold catalysts. Therefore, the gold crystallite size and specific surface area are not good parameters to explain the observed variation in CO conversion. The above results suggest, however, that the change in catalytic activity could be associated with the variation in the number of surface gold atoms with low coordination numbers as a function of the gold crystallite dimensions. Because the gold crystallite surface has atoms with different coordination numbers, the electronic structure of the surface should be analyzed in detail before suggesting which of these atoms catalyze the oxidation of CO. Acknowledgment. We thank M. Aguilar, A. Go´mez, and L. Rendo´n for technical support and the Laboratorio Central de Microscopı´a of the Instituto de Fı´sica of the Universidad Nacional Auto´noma de Me´xico for the electron microscopy facilities. This work was financially supported by the “Proyecto

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