J. Phys. Chem. C 2007, 111, 2525-2532
2525
Crystallite Size and Morphology of the Phases in Au/TiO2 and Au/Ce-TiO2 Catalysts Xim Bokhimi*,† and Rodolfo Zanella‡ Instituto de Fı´sica, 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: October 9, 2006; In Final Form: December 8, 2006
Au/TiO2 and Au/Ce-TiO2 catalysts with 5 wt % Au and 12 wt % cerium were prepared with the sol gel technique, and characterized using X-ray powder diffraction and transmission electron microscopy. The crystallography, crystallite size, and morphology of all of the phases in the catalysts, including the metallic gold phase, were obtained by refining the crystalline structures with the Rietveld method. The gold crystallite size and morphology, which was based on truncated cuboctahedra, depended on the titania polymorph used as the support: when the support was anatase the gold crystallites had their smallest dimension (1 nm) perpendicular to the (111) planes of gold’s crystalline structure, whereas for brookite and rutile this dimension (1.73 for brookite and 1.54 for rutile) was perpendicular to the (200) planes. The large amount of structural defects of the support served as particle pinning centers of the gold crystallites, hindering the diffusion that produces larger crystallites. From the refinement, the contribution of the metallic gold to the X-ray diffraction pattern of the catalyst was extracted. Cerium doping decreased anatase and brookite crystallite size and increased the number of structural defects because cerium atoms incorporated into their crystalline structures.
1. Introduction Highly dispersed and nanostructured gold supported on metal oxides is very interesting because of its potential use in many catalytic reactions of industrial processes and environment care.1-4 The remarkable catalytic properties of supported gold were first reported by Haruta et al.5 for the oxidation of CO at subambient temperatures. In the search of improving the catalytic properties, gold has been supported on different oxides: for example, TiO2,6-11 Al2O3,12,13 Fe2O3,14,15 Co3O4,14 CeO2,16-18 SiO2,19,20 ZrO28, and NiO.14 Supported gold is also a good catalyst in various other reactions: in selective hydrogenation,21,22 in water gas shift,23,25 in the reduction of NO with hydrocarbons,26 in the epoxidation of propylene,27 in CO and CO2 hydrogenation,28 in reactions involving halogens,29 and in the oxidation of volatile organic compounds.30,31 Gold supported on titania is one of the most studied catalysts for CO oxidation because it has a high activity at low temperatures; Haruta and co-workers observed that this activity depended strongly of the gold particle size.32 Because titania has three different polymorphs at room temperature (anatase, brookite, and rutile), Yan et al.33 studied the effects of the crystalline structure of the titania polymorph on the properties of the gold catalyst; the polymorphs’ syntheses were performed via sonication and hydrothermal treatment. In this case, the catalysts with gold supported on brookite had the highest catalytic activity; the gold particles supported on this titania phase, however, were smaller than those in the other titania polymorphs; therefore, it is difficult to discern if the larger catalytic activity was due to the smaller size of the gold particles or the brookite atom distribution because some authors 6,34 report * Corresponding author. E-mail:
[email protected]. † Instituto de Fı´sica. ‡ Centro de Ciencias Aplicadas y Desarrollo Tecnolo ´ gico.
that the rate of the CO oxidation reaction increases markedly when the size of the gold particles decreases. Although it is known that factors such as gold particle size, synthesis method, preparation conditions, and support type determine the reactivity of the gold catalysts,2,4,35,36 the nature of the active sites and the reaction mechanism for CO oxidation is still in debate.4 A particular controversial point of discussion is the role of the support to supply oxygen for the catalytic reaction.6,13,34,37-39 For example, when a transition metal oxide (the cation can have different oxidation states) is used as the support, such as TiO2, the catalytic performance of the gold catalyst for CO oxidation is improved because it is believed that the support supplies reactive oxygen to the gold particles.37 Many of the above open questions can be answered if the atom distribution of the support particles, as well as their crystallite size and morphology, and their crystallite defects were known. This information helps us understand the origin of the gold crystallite size and morphology and their relationship to the catalytic properties.4,25,36 These gold properties can be obtained through the quantitative analysis of the X-ray diffraction patterns of the catalysts, via the refinement with the Rietveld method of all of the crystalline structures in the catalyst,40,41 and the analysis of the micrographs of the catalysts obtained with transmission electron microscopy. To stabilize the gold particles, which is a key point for the use of these catalysts in industrial applications,42 the titania supports were doped with cerium, because it can be dissolved in the titania polymorphs,43 stabilizing the oxides by hindering the thermal sintering of their particles.44,45 Besides, cerium has an itinerant valence that favors the releasing-uptaking of atomic oxygen to and from the environment,46 which is important in the catalytic reactions of oxidation.47 In this work, the crystallography and microstructure of the phases in the gold catalysts prepared on titania polymorphs via deposition-precipitation of gold through DP urea were ob-
10.1021/jp066635n CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007
2526 J. Phys. Chem. C, Vol. 111, No. 6, 2007 tained.48 Gold was supported on all three titania polymorphs pure and doped with 12 wt % ceria. The crystallography, structural defects, as well as the crystallite morphology and size of the phases in the catalysts were studied in detail by using X-ray powder diffraction, refinement of their crystalline structures, and transmission electron microscopy. The effect of the titania polymorph on the gold crystallite morphology was also analyzed. 2. Experimental 2.1. Synthesis of the Supports. Anatase. At room temperature, to 130 mol of water was added, drop by drop, 5 mol of nitric acid [HNO3; J. T. Baker, 66.5%] while the solution was stirred for 1 h. After that, 1.0 mol of titanium butoxide [TiBt; Aldrich, 97%] was added, drop by drop, to the above nitric acid solution, while stirring it for 30 min. Then, this solution was heated to 90 °C and maintained there for 15 h in reflux to generate a precipitate that was eventually dried at 60 °C in air. Ce-Anatase. At room temperature, to 130 mol of water was added, drop by drop, 5 mol of nitric acid while the solution was stirred; thereafter, 0.05 mol of cerium nitrate [Ce(NO3)‚ 6H2O; Aldrich, 99%] was dissolved in this solution. To it, 0.95 mol of titanium butoxide was added, drop by drop, while stirring the solution for 30 min; then, this solution was annealed to 90 °C and maintained there for 15 h in reflux to generate a precipitate that was dried at 60 °C in air. Brookite. At room temperature, 130 mol of distilled water was mixed with 9.2 mol of hydrochloric acid [HCl; J. T. Baker, 36.8%] and stirred for 1 h. Thereafter, 1.0 mol of titanium butoxide was added to the above solution, drop by drop, and maintained under stirring for 30 min. This solution was poured into the vessel of an autoclave, which was heated to 100 °C and maintained there for 15 h. The generated precipitate was dried at 60 °C in air. Ce-Brookite. At room temperature, to 130 mol of distilled water was added, drop by drop, 9.2 mol of hydrochloric acid, while stirring the solution for 1 h; then, 0.05 mol of cerium nitrate was dissolved in this solution. Thereafter, 0.95 mol of titanium butoxide was added to the above solution, drop by drop, while stirring the solution for 30 min. The final solution was poured into the vessel of an autoclave, which was heated to 100 °C, and maintained there for 15 h. The generated precipitate was dried at 60 °C in air. Rutile. At room temperature, to 130 mol of water was added, drop by drop, 27 mol of nitric acid while the solution was stirred for 1 h. After that, 1.0 mol of titanium butoxide was added, drop by drop, to the above nitric acid solution, while stirring it for 30 min. This solution was heated to 90 °C and maintained at this temperature for 15 h in reflux. Eventually, the generated precipitate was dried at 60 °C in air. Ce-Rutile. At room temperature, to 130 mol of water was added, drop by drop, 27 mol of nitric acid while the mixture was stirred; thereafter, 0.05 mol of cerium nitrate was dissolved in this solution. Then, to it was added, drop by drop, 0.95 mol of titanium butoxide, while stirring the solution for 30 min. The final solution was heated to 90 °C and maintained there for 15 h in reflux to generate a precipitate that was dried at 60 °C in air. 2.2. Gold Deposition on the Supports. Before preparation of the gold catalyst, the corresponding titanium oxide support was heated at 2 °C/min to 300 °C and maintained there during 4 h in flowing (100 mL/min) ultrapure air (Praxair). The catalysts were prepared in the dark by depositionprecipitation with urea (DP Urea).48,49 The gold precursor,
Bokhimi and Zanella HAuCl4 (4.2 10-3 M), and the urea (0.42 M) were dissolved in 60 mL of distilled water to generate a solution with a pH of 2.0. Then, 1 g of the titania powder was added to this solution under constant stirring; after that, the suspension temperature was increased to 80 °C and maintained there for 16 h. The urea decomposition at this temperature produced a gradual rise in pH from 2 to 7.48 The amount of gold in solution corresponded to a maximum gold loading of 5 wt % of the catalyst. After the gold deposition, the solids were separated from the suspension through centrifugation at 5000 rpm for 12 min. Then the separated solids were washed by suspending them in 100 mL of distilled water, stirring the suspension for 10 min at 40 °C, and centrifugating it to obtain a washed solid; this operation was repeated several times. The final washed solids were dried in a vacuum at 100 °C for 2 h, and stored in an evacuated desiccator that maintained the solid in the dark. Before characterization, the dried samples were heated at 2 °C/min to 200 °C and maintained at this temperature during 4 h in flowing (100 mL/min) ultrapure air because with this treatment the catalytic activity of the gold catalysts for CO oxidation is maximal.6 The thermal treatment of the impregnated samples leads to the decomposition of Au (III) complexes into gold metal particles and can be performed with reducing gases, for example, H2, or oxidizing gases, for example, air. The supported gold precursor thermally decomposes in air into Au0 because the Au3+ species are unstable (∆Hf (Au2O3) ) 19.3 kJ mol-1).50 Finally, samples were stored at room temperature in a desiccator that maintained the catalysts in the dark to prevent any alteration by light.51 2.3. Characterization Techniques. X-ray Powder Diffraction. The X-ray diffraction patterns of the catalysts were measured in a θ-θ Bruker D-8 Advance diffractometer having the 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 by use of the Fullprof code.52 Crystallite morphology was modeled by using spherical harmonics as base functions,53 while the background was modeled with a polynomial function that, in addition to the constant, linear, quadratic, and cubic terms in 2θ, also had the terms (1/ 2θ) and (1/2θ)2. The standard deviations given in parentheses in the text and tables show the last figure variation of a number. When they correspond to Rietveld refined parameters, their values are not estimates of the probable error in the analysis as a whole, but only of the minimum possible probable errors based on their normal distribution.54 Electron Microscopy. Catalysts were analyzed with transmission electron microscopy (TEM) in a Jeol JEM-2010F microscope. Scanning electron microscopy was performed in a JEOL JSM-5900-LV microscope that had an energy dispersive X-ray spectroscopy (EDS) system of Oxford-ISI. 3. Results and Discussion The supports of the gold catalysts were a mixture of titania polymorphs. For each catalyst, the main phase had a minimal concentration of 62 wt % (Table 1) and was used to identify the support; for example, in the catalyst “Au/anatase” the main phase was anatase with a concentration of 80(1) wt % (Table 1). Phase concentrations were determined by refining the structure of the crystalline phases in the catalyst using the Rietveld method.55 These phases were anatase, brookite, rutile, ceria, and
Au/TiO2 and Au/Ce-TiO2 Catalysts
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2527
Figure 1. Rietveld refinement plot of the Au/brookite catalyst. In the upper curve crosses correspond to the experimental data and the continuous line corresponds to those calculated; the lower curve is the difference of both data. Upper marks correspond to metallic gold (RF ) 0.0026), the next ones down correspond to brookite (RF ) 0.0061), the next ones down correspond to anatase (RF ) 0.0057), and the lowest marks correspond to rutile (RF ) 0.0079).
Figure 2. X-ray diffraction patterns of the catalysts supported on anatase. Miller indices in the upper curve correspond to anatase, and the index in the lower pattern corresponds to metallic gold.
TABLE 1: Rietveld Analysis of the Catalysts: Concentrations in Weight Percent of the Titania Polymorphs, Ceria, and Metallic Gold catalyst
anatase
brookite
rutile
Au/anatase Au/Ce-anatase Au/brookite Au/Ce-brookite Au/rutile Au/Ce-rutile
80(1) 74(2) 18.7(8) 17.1(7)
11(1) 16(2) 75(1) 62(2)
5.6(6) 2.0(3) 0.8(1) 8(2) 97(2) 97(2)
ceria
7.7(8)
rutile; cerium oxide, however, did not appear as one of the crystalline phases (Table 1). Cerium doping affected the crystallography of the anatase phase as well as the textural properties of the catalyst (Table 2). The anatase unit cell expanded a little, because the oxygen atom was displaced along the z coordinate (Table 2), indicating that cerium atoms were dissolved in the anatase lattice substituting for titanium atoms. This was confirmed by the EDS analysis of the catalyst (Table 3), which registered a cerium concentration of 12.0 wt % cerium (the cerium nominal concentration), which must be in the titania polymorphs (mainly in anatase) lattices because the X-ray diffraction analysis showed that the catalysts contained only the crystalline phases of these polymorphs. Anatase crystallites had dimensions between 4 and 8 nm, with its longest dimension along the c axis (Table 2, Figure 3A). Cerium doping diminished these dimensions (Figure 3C) but did not affect the crystallite morphology (Table 2). The dimension decrease was caused by the large number of defects in the anatase crystallites produced by the substitution of some titanium atoms by cerium atoms, which hindered crystallite growth. The anatase crystallites of the anatase-based gold catalysts had their smallest dimension in a direction perpendicular to (101) planes; this dimension was 3.5 nm for the catalyst with cerium and 4.1 nm for that without cerium. Anatase crystallites had large microstrains: 3.40% in the catalysts without cerium and 3.84% in those doped with cerium; the increase of microstrains was caused by the defects produced by the substitution of some titanium atoms by cerium atoms in the anatase lattice. These microstrains determined the size of the deposited gold particles and could also be important in the catalytic properties of the catalyst.36 The micrographs of these catalysts show that the precipitated gold particles were uniformly distributed on the anatase crystallites (Figure 3B and D); the particle size distribution varied between 1.5 and 5.5 nm for the non-doped samples and between 1.5 and 4.5 nm for the cerium-doped samples (Table 3). The micrographs also reveal the presence of a large amount of defects between the anatase crystallites (Figure 3A and B), which could work as particle pinning centers of the
gold 4.1(1) 4.8(2) 5.1(1) 4.6(1) 3.1(1) 3.1(1)
metallic gold, depending on the specific gold catalyst. The crystallography information used as the initial approximation for the titania polymorphs and ceria was that reported in the literature for them.56,57 Anatase was modeled with a tetragonal unit cell with its symmetry described by the space group I41/amd; brookite was modeled with an orthorhombic unit cell having the symmetry of the space group Pbca; and rutile was modeled with a tetragonal unit cell that had the symmetry given by the space group P42/mnm. Ceria was modeled with a cubic unit cell with a symmetry described by the space group Fm3m; the base consisted of one cerium atom at the relative fractional coordinates (0.0, 0.0, 0.0) and one oxygen atom at the relative fractional coordinates (0.25, 0.25, 0.25). Metallic gold was modeled with a cubic unit cell that had the symmetry of the space group Fm3m and a base with only one gold atom at the fractional coordinates (0.0, 0.0, 0.0). Figure 1 shows a typical Rietveld refinement plot. Although the supports of the gold catalysts based on anatase were prepared with hydrochloric and nitric acids, only those prepared with nitric acid were studied because chlorine ions could affect the catalytic properties.13 This would simplify the interpretation of any results to correlate the atom distribution and microstructure of the catalysts with their catalytic properties. Two different catalysts were prepared with anatase as the main phase (Figure 2): one where the support did not contain cerium, which had 80(1) wt % anatase, and another where the support was doped with cerium, which had an anatase concentration of 74(2) wt %. The cerium doping of the catalyst support increased the concentration of brookite and decreased that of
TABLE 2: Anatase-Rich Catalysts: Anatase Lattice Parameters (in nm) and the Relative z Coordinate of the Oxygen Atom, Average Crystallite Size A (in nm) Perpendicular to the Planes (101), (004), and (200), and the Average Isotropic Microstrain catalyst
a
c
zo
A(101)
A(004)
A(200)
(%)
Au/anatase Au/Ce-anatase
0.37950(3) 0.38025(4)
0.94827(8) 0.9487(1)
0.0853(2) 0.0861(3)
4.1 3.5
7.6 6.3
4.5 3.7
3.40 3.84
2528 J. Phys. Chem. C, Vol. 111, No. 6, 2007
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Figure 4. X-ray diffraction patterns of the catalysts supported on brookite. The Miller indices in the upper curve correspond to anatase, and those on the lower curve correspond to metallic gold.
concentration of 7.7(8) wt % of the catalyst and coexisted with the other titania polymorphs. Because the total concentration of cerium, measured in these catalysts by EDS analysis, was 12.2 wt % (Table 3), and, according to the X-ray diffraction analysis, part of the cerium was segregated on the brookite particle surface as CeO2, the rest of it should be dissolved in the brookite lattice. This result indicates that cerium was more soluble in the lattice of anatase than in that of brookite. The cerium doping of the catalysts based on brookite slightly decreased the average crystallite size of brookite in all directions (Table 4) because the substitution of some titanium by cerium atoms increased the number of defects in the lattice, hindering brookite’s crystallite growth. Brookite crystallites were thin plates with their smallest dimension perpendicular to (210) planes (Table 4 and Figure 5A). The cerium doping also increased the percentage of the average brookite’s crystallite microstrain (Figure 5C) from 1.78% for the catalyst without cerium to 2.61% in that with cerium. As was discussed for the catalysts based on anatase, in the present samples the microstrains could function as particle pinning centers for the formation of the metallic gold crystallites, as well as for hindering the diffusion that produces larger gold particles. With the cerium doping, brookite lattice dimensions changed slightly only along the a axis (Table 4). The deposited metallic gold crystallites in the brookite-based catalysts were uniformly distributed on the surface of the support particles (Figure 5B and D and Table 3). Details about the morphology and size of the gold crystallites will be analyzed in the last part of the present section. The supports for the gold catalysts based on rutile were prepared with hydrochloric and nitric acid. Only those prepared with nitric acid will be discussed because in this case it is easier to find correlations between the crystallography and microstructure of the crystalline phases in the catalyst with the catalyst macroscopic properties, for example, with their catalytic properties. The catalysts prepared with hydrochloric acid could contain chlorine ions that alter the catalytic properties. The support of these gold catalysts only contained the titania polymorph rutile (Table 1), with crystallites that had a lower
Figure 3. TEM micrographs of the gold catalysts supported on anatase: (A and B) Au/anatase; (C and D) Au/Ce-anatase.
TABLE 3: EDS-TEM Analysis: Gold and Cerium Concentrations in Weight Percent and the Average, Lower, and Upper Limits of Gold Particle Size (in nm) Obtained from the Micrographs
catalyst
Au
Au/anatase Au/Ce-anatase Au/brookite Au/Ce-brookite Au/rutile Au/Ce-rutile
5.8 5.6 5.9 5.5 5.0 4.9
Ce
Au particle size average
Au particle size lower limit
Au particle size upper limit
3.4(9) 2.9(7) 3.3(9) 2.6(6) 3.5(9) 2.8(7)
1.5 1.5 1.5 1.5 1.5 1.5
5.5 4.5 5.5 4.5 5.5 4.5
12.0 12.2 0.0
gold particles hindering their diffusion, which would produce larger gold particles. The above results describe a possible mechanism to explain the origin of the small crystallite size of gold in these catalysts based on anatase. The titania catalysts rich in brookite could not be prepared with nitric acid, only with hydrochloric acid, which could be inconvenient because the catalyst could contain chlorine ions participating in the catalytic activity. The catalysts with brookite as the main phase (Figure 4) contained anatase as a secondary phase with concentrations of 18.7(1) wt % for the catalyst without cerium and 17.1(7) for that with cerium. The large amount of this secondary phase could mask the catalytic properties of these catalysts because these properties cannot be interpreted as coming only from the gold supported on brookite. In these catalysts, rutile was only an impurity phase that would be irrelevant during the interpretation of any measured macroscopic property. The cerium doping of the catalyst produced a small expansion of the brookite unit cell along the a axis (Table 4). In the brookite-based catalyst, however, very small ceria crystallites (ca. 1 nm in diameter) were segregated on the surface of the brookite crystallites in a
TABLE 4: Brookite-Rich Catalysts: Brookite Average Crystallite Size B (in nm) Perpendicular to the Planes (210), (020), and (002), Lattice Parameters (in nm), and the Average Isotropic Microstrain catalyst
a
b
c
B(210)
B(020)
B(002)
(%)
Au/brookite Au/Ce-brookite
0.9177(1) 0.9189(2)
0.54505(4) 0.54512(8)
0.51486(3) 0.51481(8)
5.9 5.3
12.7 12.4
12.6 10.4
1.78 2.61
Au/TiO2 and Au/Ce-TiO2 Catalysts
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2529
Figure 5. TEM micrographs of the gold catalysts supported on brookite: (A and B) Au/brookite; (C and D) Au/Ce-brookite.
Figure 6. X-ray diffraction patterns of the catalysts supported on rutile. The Miller indices in upper curve correspond to rutile, and those on the lower curve correspond to metallic gold.
TABLE 5: Rutile-Rich Catalysts: Rutile Average Crystallite Size R (in nm) Perpendicular to the Planes (110), (200), and (002), the Lattice Parameters (in nm), and the Average Isotropic Microstrain catalyst
a
c
Au/rutile 0.46023(3) 0.29570(2) Au/Ce-rutile 0.46028(3) 0.29571(2)
R(110) R(200) R(002) (%) 6.2 5.2
3.4 2.6
14.4 15.9
1.68 1.60
microstrain (Table 5) than the anatase (Table 2) and the brookite (Table 4) crystallites reported above. This, as will be evident in the following paragraphs, affected the gold crystallite size. Doping the support based on rutile with cerium had practically no effect on the crystallography of rutile and on the catalyst texture: the unit cell dimensions did not change, neither did the average isotropic microstrain. In the rutile crystallites, which were fibers with their length dimension along the c axis (Table 5 and Figure 6), cerium doping decreased only the crystallites cross section and enlarged their length slightly. The EDS analysis of these catalysts (Table 3) showed that they did not contain cerium, which explains why the crystallography of the support was not affected by the assumed doping. The observed
Figure 7. TEM micrographs of the gold catalysts supported on rutile: (A and B) Au/Ce-rutile; (C) Au/rutile; and (D) Au/Ce-rutile.
change in the rutile crystallite morphology of the “doped” Au/ CeO2-rutile catalyst can be explained as an effect produced during the synthesis of rutile by the presence of the cerium salt in the solution. It is worth commenting that the length of the rutile crystallites was about 15 nm, but the length of the rutile particles was more than 10 times larger (Figure 7A and B). Because a crystallite corresponds to a region where atoms are ordered with translational symmetry, the difference in lengths of the particles and the crystallites indicated that each particle had many crystallites; it was polycrystalline. The high-resolution electron microscopy analysis showed that all of the crystallites in a particle, however, were oriented with their c axis parallel to the particle length. The boundary between these crystallites was full of defects (Figure 7A and B) and served as particle pinning centers of the gold particles; these centers hindered the diffusion of the metallic gold crystallites deposited on the rutile particles, preventing the formation of larger gold particles. Because the boundary between the crystallites was along the rutile particle length, the gold particles arranged along this length producing a one-dimensional structure (Figure 7D). After refining the crystalline structures in the gold catalyst, it was possible to extract the contribution of the gold crystallites to the X-ray diffraction pattern of the catalyst (Figure 8). For example, the catalyst “Au/brookite” contained four phases (Table 1): 75(1) wt % brookite, 18.7(8) wt % anatase, 5.1(1) wt % gold, and 0.8(1) wt % rutile. Figure 8 shows the contribution of each phase to the X-ray diffraction pattern of the sample. Although the weight concentration of the gold particles to the catalyst was only 5.1(1) wt %, its contribution to the intensity of the diffraction pattern was large because gold atoms have more electrons than titanium and oxygen atoms, and the electrons are the particles that scatter the X-rays. The above results show that the X-ray powder diffraction technique together with the Rietveld refinement of the phases in the gold catalysts is a very useful technique for the characterization of the phases present in the catalyst. Figure 9 gives the contributions of the metallic gold phase to the X-ray diffraction patterns of the catalysts studied in the present work. The gold contribution to the X-ray diffraction
2530 J. Phys. Chem. C, Vol. 111, No. 6, 2007
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Figure 10. (A) TEM micrograph, taken along the [111] zone axis, of a gold crystallite supported on rutile. (B) Simulation of the crystalline structure of metallic gold; (111) planes are parallel to the sheet surface; the (220) and (-220) planes are indicated by putting the Miller indices parallel to the respective planes. Figure 8. Phases contribution (Table 1) to the diffraction pattern of the “Au/brookite” catalyst; they were extracted from the Rietveld refinement.
Figure 9. Contribution of gold to the X-ray diffraction pattern of the catalysts, as extracted from the Rietveld refinement. The Miller indices correspond to metallic gold.
TABLE 6: Gold Concentration, Average Crystallite Size G (in nm) in Directions Perpendicular to the (111), (200), (220), and (311) Planes, and Lattice Parameter a catalyst
conc. (wt %)
G(111)
G(200)
G(220)
G(311)
a (nm)
Au/anatase Au/Ce-anatase Au/brookite Au/Ce-brookite Au/rutile Au/Ce-rutile
4.1(1) 4.8(2) 5.1(1) 4.6(1) 3.1(1) 3.1(1)
1.3 1.0 2.2 3.7 2.5 3.9
1.7 1.4 1.7 2.8 1.5 2.1
1.9 1.2 1.9 3.4 2.4 3.3
1.7 1.2 1.9 3.2 1.9 2.8
0.4081(1) 0.4100(1) 0.40683(6) 0.40761(4) 0.40702(5) 0.40742(4)
patterns produced broader diffraction peaks when it was supported on anatase and anatase doped with cerium than when it was supported on the other titania polymorphs. The overlapping of all of the gold contributions shows that the peak positions of the patterns with the broader peaks, which correspond to smaller crystallite sizes, were shifted to lower angles. This means that their unit cell dimensions were larger (Table 6), indicating the weakening of the interaction between the gold atoms when the gold crystallite size decreased. By refining the crystalline structures, it was possible to make a quantitative analysis of the phases in the catalysts (Table 6). For example, the obtained gold concentration with the refinement in all catalysts was near the nominal value (5 wt %); the catalyst with rutile as the support had a lower gold concentration, probably because in these catalysts the gold-support interaction was weaker, as suggested by the fact that the gold crystallite
size was larger on this titania polymorph than on anatase or brookite. This claim, however, should be tested by studying this interaction as a function of the support’s crystallite size. The gold concentrations determined from the Rietveld refinement were smaller than those determined by EDS with the electron microscope (Table 3); they, however, were very similar. From the micrographs (Figures 3B, 3D, 5B, 5D, 7C, and 7D), it was also possible to get information about the average particle size of the gold particles and their distribution (Table 3): Their average values were slightly larger than those obtained from the Rietveld refinement (Table 6), this was probably because the particles were not single crystals; and the average crystallite size obtained from the Rietveld refinement analysis corresponds to the regions in the particle where atoms are ordered with translational symmetry. The Rietveld refinement shows that the gold crystallites morphology was anisotropic and depended on the support (Table 6). Because the atom distribution in metallic gold is cubic, the supported crystallites were truncated cuboctahedra (Figure 10). In the micrograph of Figure 10, where the gold particles were grown on rutile, the (220) planes of the gold crystallite were parallel to the support surface. It is necessary to perform more experiments in order to know if for all of the catalyst supports it is the most probable growing orientation of the metallic gold crystallites. The gold crystallite size and anisotropy depended on the titania polymorph used as the support: the smallest gold crystallites were obtained on anatase, and the largest were obtained on rutile (Table 6). The crystallite anisotropy can be estimated by the rate of the crystallite dimension perpendicular to the (111) planes of gold (the most dense planes), G(111), and the dimension perpendicular to the respective (200) planes, G(200). This rate, G(111)/G(200), was smaller than 1 for the gold supported on anatase and larger than 1 for the gold supported on brookite and rutile (Table 6). When gold was supported on anatase the rate was 0.765, which decreased to 0.714 when anatase was doped with cerium. Because in this last case the crystallite dimensions were smaller, in general this rate probably increases as the crystallite dimension of the support increases. The answer to this question can be obtained after performing systematic studies on catalysts where the support has different crystallite sizes but the same crystalline structure. When gold was supported on brookite or rutile, the smallest dimension of the gold crystallites was perpendicular to the (200) planes of gold (Table 6); therefore, the rate that quantifies the anisotropy was larger than 1, indicating preferential growth of the gold crystallites along the (111) planes. This could be the
Au/TiO2 and Au/Ce-TiO2 Catalysts origin of the larger size of the metallic gold crystallites on brookite and rutile in comparison with their size on anatase. This notorious difference in gold crystallite morphology can be used to distinguish the titania polymorph on which the metallic gold particles were supported. In recent studies, W. Yan et al.33 reported that when gold is supported on the different titania polymorphs the smallest gold particles are formed on brookite. The present results (Table 6), however, show clearly that the smallest gold crystallite sizes were obtained when gold was deposited on anatase. The difference between both results could be a consequence of the microstructure of the used support, for example, its crystallite size or the number of defects in it, which was not considered by W. Yan et al. The doping of the Au/anatase catalyst with cerium reduced the crystallite dimensions of the gold particles (Table 6), whereas in the Au/brookite and Au/rutile catalyst, this doping increased the gold crystallite size. Because all cerium atoms were dissolved in anatase, they produced many particle pinning centers where metallic gold particles were formed, decreasing with that the size of the deposited particles. When brookite was doped with cerium, only a part of it incorporated into its lattice; the other part was segregated on the crystallite surface as cerium oxide with a very small crystallite size. Although the brookite crystallite size decreased with the doping (Table 4), the number of the particle pinning centers available for the formation of the gold crystallites was reduced because some of them were occupied by the segregated cerium oxide particles. When the support based on rutile was doped, cerium atoms neither incorporated into the rutile lattice nor segregated on the rutile crystallite surface. Therefore, the effect of cerium on the gold crystallite size should be different as in brookite and anatase. The only observed effect of cerium in the Au/Ce-rutile catalyst was an increase in the rutile crystallite dimensions, probably produced by the presence of cerium in the solution used to prepare the support. The increase of the rutile crystallite size decreased the number of defects in the crystallite (Table 5), which was according to the reported evolution of microstrains in rutile as a function its crystallite size.37 This reduction of microstrains also decreased the number of particle pinning centers for the formation of the gold particles; therefore, gold particles diffused easily, producing larger gold crystallites than those in the rutile catalysts without cerium. 4. Conclusions By using X-ray powder diffraction and refining the crystalline structures of the phases in the gold catalysts supported on titania polymorphs, the crystallography, crystallite size, and morphology of all of the phases were obtained, with an emphasis on those of metallic gold. The study shows that the gold crystallite size and morphology depends on the titania polymorph used as the support. When the support was anatase, the metallic gold crystallites had their smallest dimension perpendicular to the (111) planes of gold crystalline structure, whereas for brookite and rutile this dimension was perpendicular to the (200) planes. From the refinement, it was possible to extract the contribution of gold to the intensity of the X-ray diffraction pattern of the catalysts. When anatase and brookite were doped with cerium, these atoms incorporated into the crystalline structure, decreasing the size of their crystallites and increasing their structural defects, which served as particle pinning centers of the gold crystallites, hindering their diffusion on the surface to produce larger gold crystallites.
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