J. Phys. Chem. C 2008, 112, 12463–12467
12463
Au/Rutile Catalysts: Effect of the Activation Air Flux on the Catalytic Activity for CO Oxidation and the Gold Crystallite Size Xim Bokhimi,†,* Rodolfo Zanella,‡ and Antonio Morales† UniVersidad Nacional Auto´noma de Me´xico, Instituto de Física, A.P. 20-364, 01000, Me´xico D. F., Mexico, and Centro de Ciencias Aplicadas y Desarrollo Tecnologico, A.P. 70-186, 04510 Me´xico D. F., Mexico ReceiVed: January 30, 2008; ReVised Manuscript ReceiVed: April 17, 2008
Gold was supported on nanocrystalline rutile and activated with an air flux between 150 and 850 mL/min to catalyze the oxidation of CO. The corresponding catalysts were characterized by X-ray powder diffraction and refinement of the crystalline structures. The catalytic activity depended on the air flux and had a maximum at 600 mL/min; the activity difference was caused by the variation of the gold crystallite size and morphology with the air flux. The catalyst with the maximum activity had an average gold crystallite with the morphology of a cuboctahedron with depressed Au(100) faces. The quantitative analysis also provided the surface area, volume, specific area, and dimensions and morphology of the average crystallites. An analysis of the dependence of these parameters on the air flux showed that they were not fundamental parameters explaining the catalytic activity. The quantitative analysis also provided the number of gold atoms in the average gold crystallites: 96, 59, 56, and 21 for the samples prepared at 150, 300, 600, and 850 mL/min. It is discussed that, as in microscopic large gold crystallites, the surface properties of the supported gold crystallites could be determined by the reordering of the atom distribution on Au(100) crystallite faces, caused by the relativistic effects on electrons in 5d orbitals. Introduction Gold nanoparticles supported on titania have been extensively studied as catalysts for the oxidation of CO at temperatures below 200 °C.1–6 Their catalytic activity increases as the gold particle dimensions decrease below 5 nm.1,3,7–15 Some authors have reported an activity maximum for gold particle dimensions of 3 nm.5,16,17 Several models have been extensively discussed to explain the origin of this catalytic activity. For instance, it is claimed that, on crystallite surface, the low-coordinated Au atoms are the active ones,3,18–25 because their number increases as crystallite size decreases.21,26–28 In another model, the electronic structure of gold particles is considered essential to explain the activity, because it shows a metal-insulator transition when gold crystallite size decreases.5,29,30 Other models involve indirect effects of the support on gold particles, for example, the transfer of charge31–33 or the induction of strain21 that produces the lattice deformation (microstrain) of the gold particles, which enhances their surface reactivity.34–36 Steps, edges, and tensile-strains all enhance the adsorption and dissociation of oxygen molecules and could be the cause of the high catalytic activity observed for gold catalysts having small thin Au particles.21 Because the active gold particles should have dimensions smaller than 5 nm, only those synthesis methods that can produce such small particles are of interest.8,37 One example is the deposition-precipitation method with NaOH (DP NaOH).38,39 In this case, the gold particle size and the deposited gold concentration depend on the pH of the gold precursor solution. For some other synthesis methods,40,41 in addition to the pH value, the synthesis time and temperature are also important.
Synthesis atmosphere and the activation temperature of gold particles also play a role in the final gold particle dimensions.38,42–44 For example, when gold catalysts are prepared with the deposition-precipitation method, the gold particle size grows with activation temperature. Synthesis heating rate also alters the catalyst performance,12 because decreasing the rate diminishes gold particle dimensions. During the synthesis of gold catalysts, the ratio between the air flux and catalyst weight also affects the final gold particle size.44 In this case, the average gold particle size decreases and the size distribution becomes narrower when the flux is increased or the catalyst weight is decreased. This behavior can be related to the elimination of the water and remaining chlorides produced in the catalysts during their synthesis.44 Chloride ions favor the sintering of gold particles.8,45 To continue studying the effect on gold catalyst properties of the air flux used to activate the gold during catalyst synthesis, in the present work, we prepared gold catalysts with only one titania polymorph, rutile, as the support, and varied the air flux, maintaining a constant catalyst weight. For each air flux, measurement of the catalytic activity for CO oxidation was performed simultaneously with the analysis of the crystallography and the crystallite dimensions and morphology of the phases in the catalyst. The methods used were X-ray powder diffraction together with the refinement of the crystalline structures with Rietveld method,19,46 which provided enough information on gold and rutile particles to reckon their average crystallite areas, volumes, masses, specific areas, and the number of atoms in the average crystallite. These physical parameters were analyzed to search an explanation for the dependence of the catalytic activity on air flux. Experimental Details
* Corresponding author. E-mail:
[email protected]. † Universidad Nacional Auto ´ noma de Me´xico. ‡ Centro de Ciencias Aplicadas y Desarrollo Tecnolo ´ gico.
Synthesis. Support (Rutile). At room temperature, 27 mol of nitric acid was added dropwise to 130 mol of water. Then,
10.1021/jp800893b CCC: $40.75 2008 American Chemical Society Published on Web 07/19/2008
12464 J. Phys. Chem. C, Vol. 112, No. 32, 2008 1.0 mol of titanium butoxide was also added dropwise to obtain a new solution that was refluxed at 90 °C for 15 h to precipitate rutile. This precipitate was dried in air at 60 °C and annealed at 500 °C. Gold Catalyst. HAuCl4 · 3H2O from Aldrich was used as the gold precursor to prepare catalysts with a gold concentration of 5 wt %. Before catalyst preparation, the support was dried in air at 100 °C for 24 h. Synthesis was performed in the absence of light because light decomposes and reduces this gold precursor. Gold was added to the support by the deposition-precipitation method with urea (DP urea) as follows:41,46,47 the gold precursor (4.2 × 10-3 M) and urea (0.42 M) were dissolved in 63 mL of distilled water to produce a solution with a pH of 2.4. Then, rutile powder (1.0 g) was added to this solution to generate a suspension that was heated at 80 °C for 16 h; the decomposition of urea gradually increased the pH of the solution from 2.4 to 7.41 After the above procedure, the dispersion was centrifuged to obtain a condensate that was dispersed in water, stirred at 50 °C for 10 min, and centrifuged; this last procedure was repeated four more times. The final condensate was dried under vacuum for 2 h at 100 °C and divided into four equal parts (0.6 g). Each part was activated as catalyst at 200 °C in flowing air44 for 2 h at a fixed flux (150, 300, 600, or 850 mL/min). Characterization Techniques. 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 graphite secondary-beam monochromator, and a punctual scintillation detector. The diffraction intensity as a function of 2θ angle was measured between 20° and 110°, with a 2θ step of 0.02°, for 10 s per point. Crystalline structures were refined using the Rietveld method and the FullProf code.48 Crystallite size and morphology were modeled in reciprocal space with a symmetrized harmonics expansion.49 For rutile, lattice deformations were assumed anisotropic and modeled with a multidimensional distribution of lattice metrics.50 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 Crystallite size distribution was simulated using the model in FullProf code that corresponds to an isotropic microstrain contribution to the Lorentzian part of the peak profile. The standard deviations given in parentheses in text and tables 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.51 The crystallite morphology data obtained from the Rietveld refinement were used to generate images, areas, and volumes of the average crystallites. Because the data correspond to spatial positions on crystallite surface, they were used to generate a mesh of triangles that allowed us to calculate the corresponding surface area and to render a three-dimensional image of the average crystallite using Medit software, Release 2.3b (Feb. 2005).52 The connection of these triangles with the center of the crystallite generated a mesh of tetrahedra used to reckon crystallite volume. X-ray diffraction analysis also provided the mass density of the different crystalline phases, which, together with the corresponding volume and surface area, was used to calculate the specific surface area and the number of atoms in the average crystallite. Catalytic ActiWity Measurements. CO + O2 reaction was studied between 0 and 200 °C in a flow reactor at a pressure of 1 atm. Typically 0.04 g of the catalyst was calcined in flowing
Bokhimi et al.
Figure 1. X-ray diffraction patterns of the catalysts activated at different air fluxes. The Miller indices on the upper diffraction pattern correspond to rutile, and those on the lower pattern correspond to metallic gold. Because the intensity of the peaks associated with metallic gold was very low, these peaks are not clearly visible, but their positions are indicted by arrows.
air for 2 h before the catalytic measurements. The reactant gas mixture, 1 vol % CO plus 1 vol % O2 with a balance of N2, flowed on the catalyst at 100 mL/min. Exit gases were analyzed with an online gas chromatograph (Agilent Technologies 6890N) equipped with flame ionization and thermal conductivity detectors. Results and Discussion Gold catalysts were prepared with rutile synthesized at 90 °C from a nitric acid solution with a H/Ti atom ratio of 27 and annealed in air at 500 °C for 12 h. The nominal gold concentration was 5 wt %, but the corresponding values obtained from the Rietveld refinement were 4.4(1), 4.8(1), 4.9(1), and 5.4(2) wt % for the catalysts prepared at air fluxes of 150, 300, 600, and 850 mL/min, respectively. These concentration values are reasonably acceptable and confirm the reliability of the Rietveld technique for determining the gold concentration in these catalysts, despite the small gold crystallite dimensions. Because all of the catalysts contained only rutile and metallic goldphases,bothofwhichhaveverywell-knowncrystallographies,46,53 the different contributions (microstructure, phase concentration, etc.) to their respective diffraction patterns (Figure 1) were modeled easily. Of particular interest was the modeling of the corresponding microstructure (crystallite size and lattice deformation), which was responsible for broadening of the diffraction peaks. The unit cell of rutile was modeled with the tetragonal symmetry described by the space group P42/mnm, and a basis containing 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) with an initial x value of 0.3. The cubic unit cell of metallic gold was modeled with the symmetry described by the space group Fm3m and a basis containing only one gold atom at the relative coordinates (0.0, 0.0, 0.0). The model for the lattice deformation and crystallite morphology of rutile considered their anisotropy. The gold crystallite morphology was also modeled considering anisotropy. However, the gold lattice deformations could not be modeled; so, they were not taken into account, which of course affected the obtained crystallite dimensions. Figure 2 shows representative images of the average crystallite of rutile in real space (left) and its microstrain distribution in reciprocal space (right); the crystallite morphology was similar
Au/Rutile Catalysts
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Figure 2. Au/rutile catalyst activated at 150 mL/min: (left) average rutile crystallite image in real space and (right) its maximum microstrain in reciprocal space.
Figure 3. Rietveld refinement plot of the catalyst activated at 150 mL/min. The points and the upper solid line correspond to the experimental and calculated data, respectively; the lower solid line is the difference between them. Upper marks correspond to rutile, whereas the lower ones correspond to metallic gold.
Figure 4. X-ray diffraction patterns associated with gold in the catalysts for different air fluxes. The patterns were obtained from the calculated diffraction patterns of the catalyst obtained from the Rietveld refinement. The Miller indices on the upper diffraction pattern correspond to metallic gold.
TABLE 1: Rutile: Crystallite Dimensions Perpendicular to the (200) and (002) Planes (R(200) and R(002), Respectively) and Maximum Microstrain (〈ε〉) along the Reciprocal a* and c* Axes, as a Function of the Air Flux flux (mL/min)
R(200) (nm)
R(002) (nm)
R〈ε〉a* (%)
R〈ε〉c* (%)
150 300 600 850
16.3 15.6 15.3 16.4
30.3 34.4 34.1 38.5
0.41 0.39 0.38 0.40
0.29 0.32 0.31 0.34
to that observed by transmission electron microscopy.19 To illustrate the good fit between the experimental and calculated diffraction patterns, a typical Rietveld refinement plot is shown in Figure 3; it corresponds to the catalyst prepared at an air flux of 150 mL/min. The microstrain of rutile was slightly anisotropic (Figure 2 and Table 1), being 13-20% smaller along the c* reciprocal lattice parameter than along the a* reciprocal lattice parameter. This anisotropy decreased as the air flux was increased. The microstrain strength, however, was almost flux-independent (Table 1). It is interesting to note that increasing the air flux favored the crystallization of rutile along the c axis (Table 1). Because gold crystallites were extremely small, they generated very broad diffraction peaks. This made it impossible to model the corresponding lattice deformations, although they could probably be obtained from X-ray diffraction patterns generated with a synchrotron X-ray source, which has high beam intensity. Figure 4 shows the contribution of the gold particles to the X-ray diffraction patterns of the different catalysts. This contribution was extracted (after refinenement of the crystallite structures of rutile and metallic gold) from the models of the diffraction patterns reported in Figure 1. As the air flux increased, the diffraction peaks associated with gold became wider, indicating a decrease of the gold crystallite dimensions (Figure 5). Table 2 summarizes the quantitative analyses of the gold phase: lattice parameter a was almost flux independent. It is interesting to note that the gold crystallite morphologies generated at air fluxes of 300 and 600 mL/min approached
Figure 5. Average gold crystallites of the catalysts for the different air fluxes. The number in parenthesis correspond to the number of gold atoms.
TABLE 2: Metallic Gold: Crystallite Dimensions Perpendicular to the (111), (200), (220), and (311) Planes (G(111), G(200), G(220), G(311), Respectively) and Lattice Parameter a as Functions of the Air Flux flux (mL/min)
G(111) (nm)
G(200) (nm)
G(220) (nm)
G(311) (nm)
a (nm)
150 300 600 850
1.66 1.38 1.33 0.93
1.15 0.86 0.87 0.52
1.55 1.44 1.41 1.14
1.36 1.11 1.10 0.74
0.40769(7) 0.40783(8) 0.40784(8) 0.4089(1)
cuboctahedral (Figure 5) with a depression in Au(100) faces (these are the crystallite faces parallel to the (200) planes of metallic gold). The gold crystallite generated at 150 mL/min had the morphology of a truncated cube; this means that atoms layers grew more easily along the (111) planes. These crystallites also showed the depression in the Au(100) faces, but it was smaller than in the cuboctahedral crystallites obtained at 600 mL/min. In the gold crystallites generated at 850 mL/min, both the Au(100) and the Au(111) (these crystallite faces are parallel to the (111) planes of metallic gold) faces show a depression, being larger in the Au(100) faces. This morphology suggests that these crystallites could be composed of eight tetrahedra sharing their edges and with one of their faces parallel to the Au(111) faces. The gold catalysts were tested for the oxidation of CO at temperatures between 0 and 200 °C (Figure 6). For all reaction
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Figure 6. Catalytic activity of the gold catalysts during the oxidation of CO as a function of the air flux.
TABLE 3: Metallic Gold: Area (A), Volume (V), Area-to-Volume Ratio (A/V), Specific Area (SA), and Number of Atoms (Na) of the Average Gold Crystallite as Functions of the Air Flux flux (mL/min)
A (nm2)
V (nm3)
A/V (nm-1)
SA (m2/g)
Na
150 300 600 850
7.22 5.60 5.35 3.20
1.63 1.00 0.95 0.36
4.4 5.6 5.61 8.90
229.2 290.5 291.2 465.6
96 59 56 21
temperatures, the catalyst activated at 600 mL/min had the highest conversion rate, whereas that one prepared with the highest flux (850 mL/min) had the lowest activity. The catalysts with gold crystallites having the morphology of a cuboctahedron (Figure 5) had higher catalytic activities. Because the X-ray diffraction patterns of the catalysts were measured at the same time as their catalytic activities (in two different instruments), the information on the crystallography and microstructure obtained from these patterns correspond to the catalyst conditions at reaction time. The catalysis and X-ray diffraction results suggest the existence of an optimal gold crystallite size for which the catalytic activity has a maximum (Table 2 and Figures 5 and 6). This crystallite should have dimensions similar to those obtained for the catalyst prepared at the flow rate of 600 mL/ min (Table 2): 1.66, 1.15, 1.55, and 1.36 nm for the directions perpendicular to the (111), (200), (220), and (311) crystallographic planes of metallic gold, respectively. These results reinforce the claim that the catalytic activity for the oxidation of CO has a maximum at a specific gold crystallite size.5,16,17 The average gold crystallite dimensions derived from the Rietveld refinement were used to calculate the crystallite areas, volumes, area-to-volume ratios, specific areas, and numbers of gold atoms (Table 3). From these data, it can be seen that changes in the area-to-volume ratio or the crystallite specific area, which, qualitatively, are normally used to explain the catalytic activity changes when the crystallite size of the metallic active phase decreases, cannot explain the observed maximum of the catalytic activity.54,55 Recently it is reported that the physical properties of isolated gold clusters with less than 100 atoms depend on their number of atoms.56 Because this is the region of atom numbers obtained in the present work for gold crystallites, we will try finding a possible correlation between these numbers and the catalytic activity of gold for the oxidation of CO. The number of gold atoms, 56 and 59, of the average gold crystallites of the more active catalysts (Table 3 and Figure 6) are similar to the number of gold atoms, 58, reported for isolated anion gold clusters with a low symmetry and a stable disordered atom distribution,56 which also is the atomic core in gold clusters with 59 through 64 atoms.56
The coincidence of these numbers obtained from two totally different experiments suggests that the real gold crystallites or clusters supported on rutile could have properties similar to those reported for the isolated Au58 clusters,56 which depend on the relativistic effects, characteristic of compounds made of heavy atoms,57 that affect the valence electrons of gold. Several gold properties are produced by relativistic effects.57 In particular, the atom ordering on the most external layer of gold crystallite Au(100) face.58 In large gold crystallites, the atom distribution on this face does not have the expected square symmetry characteristic of face-centered cubic (fcc) lattices; instead of that, gold atoms redistribute on this face forming an contracted hexagonal close packing.58 The atom reordering is caused by the expansion of the 5d atomic gold orbitals,59 induced by the screening of the interaction between 5d electrons and atomic nucleous. The screening is produced by contraction of the inner electron orbitals when the respective electrons descend to the neighborhood of gold atomic nucleous moving to radial velocities near light velocity, which enhances electron mass.57 The relativistic effect is also important when gold compounds have only a few gold atoms, for example, in gold complexes used as homogeneous catalysis.60 The last two paragraphs suggest that the relativistic effects on valence electrons should also be important for supported gold clusters or crystallites of any size. This means that, even for the small crystallites described in this work, the atom distribution on Au(100) faces should transform as it does in large gold crystallites, in a similar way as it does in free-of-support gold clusters with 58 atoms.56 If in the supported metallic gold crystallites reported in the present work the atomic reordering on the Au(100) face of the crystallite is present (as a consequence of the relativistic effects), the match between these faces and the Au(111) faces would produce a lot of defects, that could be responsible of the observed catalytic activity. The depression in the Au(100) faces of the gold crystallites (Figure 5) could be a consequence of this atom reordering. In the average gold crystallites with 96 atoms (prepared at 150 mL/min), these depressions are less pronounced (Figure 5); here the area of the Au(100) faces is larger than in the Au(111) faces, because the crystallite has the morphology of a truncated cube. Therefore, the number of atoms associated with the interface between Au(100) and Au(111) faces is smaller than in the crystallites with 56 and 59 atoms. This reduces the number of defects that are the probable active sites of the catalysts. The morphology of the average gold particles with 21 atoms prepared at 850 mL/min (Figure 5) can be modeled with eight edge-sharing tetrahedra, with one of their faces parallel to the corresponding Au(111) face. The external vertices of these tetrahedra occupy the vertices (12 in total) of an associated cuboctahedron, whereas the internal vertices of the tetrahedra (8 in total) form a cube. If one assumes that in each tetrahedron vertex stays a gold atom, then, the total number of gold atoms in this crystallite is 20, which is near 21, the atom number of the average crystallite obtained from experiment. This 20-atom gold polyhedron, however, is completely different from those reported for free-of-support gold clusters with the same number of atoms.61 The above analysis shows that the 12 atoms of the external vertices of the tetrahedra construct the crystallite surface. Because these atoms are in ordered stable positions, they should be chemically inactive to catalyze the oxidation of CO (Figure 6). In the literature, it has also been suggested that the gold oxidation states Au1+ and Au3+ could play a key role in the catalytic reaction for CO oxidation.62 If this model were correct, then the observed variation of the catalytic activity reported in
Au/Rutile Catalysts Figure 6 would require a maximum in the gold valences to be reached as a function of the air flux used for the synthesis of the catalyst. Such an analysis would be interesting, but it is outside the scope of the work presented here. Conclusions The catalytic activity of gold supported on rutile for the oxidation of CO was determined as a function of the air flux used to activate the catalyst. When this flux was varied from 150 to 850 mL/min, the catalytic activity had a maximum at 600 mL/min. A detailed analysis of the crystallography and microstructure of the catalyst phases, which were obtained by refining their crystalline structures with the Rietveld method, showed that the air flux determined the gold crystallite size and morphology. In the more active catalysts, those prepared at 600 and 300 mL/min, the average gold crystallites had the morphology of a cuboctahedron with a depression in the Au(100) faces, and 56 and 59 gold atoms, respectively. These numbers are very near to the number 58 found for low symmetry stable gold clusters free-of-support. The depression of the Au(100) faces could indicate a reordering of the atoms from a square symmetry into a contracted hexagonal close-packing, produced by relativistic effects on the 5d electrons of gold. This reordering could produce a lot of defects at the interface between Au(100) and Au(111) faces, which would be responsible of the high catalytic activity for CO oxidation. The catalyst activated at the lowest air flux (150 mL/min) had an average gold crystallite with 96 atoms and the morphology of a truncated cube, indicating a preferential growth of (111) planes. The Au(100) faces of this average crystallite had a small depression, suggesting a reduced number of defects on crystallite surface, which explains the lower catalytic activity of this catalyst. The average gold crystallite associated to the catalyst prepared with the highest flux (850 mL/min) contained only 21 atoms and had a morphology that can be described with eight edge-sharing octahedra with gold atoms at their vertices in stable positions that make them chemically inactive to catalyze the oxidation of CO. Acknowledgment. We thank M. Aguilar for the technical help. This work was financially supported by the Proyecto Universitario de Nanotecnología Ambiental (PUNTA) of the Universidad Nacional Auto´noma de Mexico. R.Z. is indebted to DGAPA (IN106507) and CONACYT (55154) for financial support. References and Notes (1) Haruta, M. Catal. Today 1997, 36, 153. (2) Haruta, M. Chem. Rec. 2003, 3, 75. (3) Zanella, R.; Giorgio, S.; Shin, C. H.; Henry, C. R.; Louis, C. J. Catal. 2004, 222, 357. (4) Valden, M.; Goodman, W. Israel J. Chem. 1998, 38, 285. (5) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (6) Valden, M.; Pak, S.; Lai, X.; Goodman, D. W. Catal. Lett. 1998, 56, 7. (7) Haruta, M. Catal. SurV. Jpn. 1997, 61. (8) Haruta, M. CATTECH 2002, 6, 102. (9) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (10) Boccuzzi, F.; Chiorino, A.; Tsubota, S.; Haruta, M. J. Phys. Chem. 1996, 100, 3625. (11) Bamwenda, G. R.; Tsubota, S.; Nakamura, T.; Haruta, M. Catal. Lett. 1997, 44, 83. (12) Kozlov, A. I.; Kozlova, A. P.; Asakura, K.; Matsui, Y.; Kogure, T.; Shido, T.; Iwasawa, Y. J. Catal. 2000, 196, 56. (13) Schumacher, B.; Plzak, V.; Kinne, K.; Behm, R. J. Catal. Lett. 2003, 2003, 109. (14) Schwartz, V.; Mullins, D. R.; Yan, W.; Chen, B.; Dai, S.; Overbury, S. H. J. Phys Chem. B 2004, 108, 15782.
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