Synthesis and Reactivity of Dimethyl Gold Complexes Supported on

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Langmuir 2005, 21, 3675-3683

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Synthesis and Reactivity of Dimethyl Gold Complexes Supported on MgO: Characterization by Infrared and X-ray Absorption Spectroscopies Javier Guzman,† Bruce G. Anderson,*,‡ C. P. Vinod,‡ Kanaparthi Ramesh,‡ J. W. Niemantsverdriet,‡ and Bruce C. Gates*,† Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, and Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received December 1, 2004 Dimethyl gold complexes bonded to partially dehydroxylated MgO powder calcined at 673 K were synthesized by adsorption of Au(CH3)2(acac) (acac is C5H7O2) from n-pentane solution. The synthesis and subsequent decomposition of the complexes by treatment in He or H2 were characterized with diffuse reflectance Fourier transform infrared (DRIFT), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The XANES results identify Au(III) in the supported complexes, and the EXAFS and DRIFTS data indicate mononuclear dimethyl gold complexes as the predominant surface gold species, consistent with the lack of Au-Au contributions in the EXAFS spectrum and the presence of νas(CH3) and νs(CH3) bands in the IR spectrum. EXAFS data show that each complex is bonded to two oxygen atoms of the MgO surface at an Au-O distance of 2.16 Å. The DRIFT spectra show that reaction of Au(CH3)2(acac) with MgO at room temperature also formed Mg(acac)2 and H(acac) species on the support. Treatment of the dimethyl gold complexes in He or H2 at increasing temperatures varying from 373 to 573 K removed CH3 ligands and caused aggregation forming zerovalent gold nanoclusters of increasing size, ultimately with an average diameter of about 30 Å. Analysis of the gas-phase products during the genesis of the gold clusters indicated formation of CH4 (consistent with removal of CH3 groups) and CO2 at 473-573 K, associated with decomposition of the organic ligands derived from acac species. O2 and CO2 were also formed in the decomposition of ubiquitous carbonates present on the surface of the MgO support.

Introduction Solid catalysts typically consist of metal, metal oxide, or metal sulfide particles stably dispersed on the internal surfaces of porous metal oxide supports. The small sizes of the particles ensure that a large fraction of the atoms in them are present at surfaces where they are accessible to reactants. Many important supported catalysts contain metals, and among those receiving the most attention recently are supported gold. Although gold has been widely investigated as a catalyst for CO oxidation at low temperatures1,2 and for selective oxidation of propene to give propene oxide,3,4 the synthesis chemistry and the structures of the gold species have not been determined precisely. The opportunities for elucidating this chemistry are favored when the supported gold is prepared from gold complexes containing ligands that are highly reactive and readily removed. We recently communicated the synthesis of a family of MgO-supported gold nanoclusters formed from Au(CH3)2(acac) (acac is C5H7O2);5 here we provide new data and a full report of the chemistry of Au(CH3)2(acac) on MgO. The objectives of this research were to understand the chemistry of the reaction of Au(CH3)2(acac) with partially * To whom correspondence should be addressed. E-mail: [email protected] (B. C. Gates) and B. G. [email protected] (B. G. Anderson). † University of California. ‡ Eindhoven University of Technology. (1) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (2) Lin, S. D.; Bollinger, M.; Vannice, M. A. Catal. Lett. 1993, 17, 245. (3) Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566. (4) Stangland, E. E.; Stavens, K. B.; Andres, R. P.; Delgass, W. N. J. Catal. 2000, 191, 332. (5) Guzman, J.; Gates, B. C.Nano Lett. 2001, 1, 689.

hydroxylated MgO and to determine the structures and compositions of the products on this support surface. We report diffuse reflectance Fourier transform infrared (DRIFT), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectra characterizing the surface species formed by reaction of Au(CH3)2(acac) with MgO. The data give evidence of (a) ligand exchange of the acac ligand of Au(CH3)2(acac) with support oxygen and anchoring of Au(III) complexes to the support surface, (b) hydrogen-bonding interactions between support OH groups and surface Mg(acac)2 and H(acac) species formed from Au(CH3)2(acac), (c) the reactivity of CH3 groups bonded to Au and their subsequent removal as methane, (d) decomposition of surface Mg(acac)2 and H(acac) species by formation of CO2 and traces of acetate ligands on the surface, and (e) structural and electronic changes of the supported gold during He treatment. Experimental Section Materials. He (Matheson, 99.999%) was purified by passage through traps to remove traces of O2 and moisture. H2 was supplied by Matheson (99.999%) or generated by electrolysis of water in a Balston generator (99.99%) and purified by passage through traps. D2 was supplied by Hoek Loos (99.9%) and was used as received. The MgO support (EM Science, 97%) was calcined in O2 at 673 K for 2 h (BET surface area approximately 60 m2 g-1), isolated, and stored in a N2-filled glovebox until it was used. n-Pentane solvent (Fisher, 99%) was dried and purified by refluxing over sodium benzophenone ketyl and deoxygenated by sparging of N2. The precursor Au(CH3)2(acac) (Strem, 98%) was purified by sublimation after it was received; this compound is temperature-sensitive and requires refrigeration. The reference compounds Mg(acac)2 (magnesium acetylacetonate; Strem 99%)

10.1021/la0470434 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/11/2005

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and Hacac (acetylacetone or 2,4-pentanedione; Aldrich 99%; caution! this compound is toxic and explosive) were used as supplied. Sample Preparation. The synthesis and sample handling of the MgO-supported gold complexes were carried out as before with exclusion of air and moisture on a double-manifold Schlenk vacuum line and in a glovebox purged with N2 that was recirculated through traps containing particles of supported Cu and zeolite 4A for removal of O2 and moisture, respectively. The sample was prepared by slurrying Au(CH3)2(acac) in dried and deoxygenated n-pentane with partially dehydroxylated MgO powder. The slurry was stirred for 1 day and the solvent removed by evacuation (pressure 450 K. Consistent with these observations, aggregation and autoreduction of supported metals (e.g., Pd clusters supported on zeolite Y24 and copper-exchanged zeolite ZSM-525) have been shown to occur with increasing (22) Benfiled, R. E.; Grandjean, D.; Kro¨ll, M.; Pugin, R.; Sawitowski, T.; Schmid, G. J. Phys. Chem. B 2001, 105, 1961. (23) Salama, T. M.; Shido, T.; Ohnishi, R.; Ichikawa, M. J. Phys. Chem. 1996, 100, 3688. (24) Vogel, W.; Kno¨zinger, H.; Carvill, B. T.; Sachtler, W. M. H.; Zhang, Z. C. J. Phys. Chem. B 1998, 102, 1750. (25) Hu, S.; Reimer, J. A.; Bell, A. T. J. Phys. Chem. B 1997, 101, 1869.

Dimethyl Gold Complexes on MgO

treatment temperature in He or vacuum, and the process has been suggested to involve a charge-transfer transition from the support to the metal and the participation of hydroxyl groups of the support.24,25 The EXAFS data (Table 4) show how the Au-Au firstshell coordination number increased from 4.0 to 9.4 as the sample was treated in He at increasing temperatures, indicating that the average cluster diameter gradually increased to about 30 Å, as determined from models26 that relate the average cluster size to the EXAFS first-shell metal-metal coordination number.27 Similarly, it has been suggested that Mg(OH)2-supported gold catalysts aggregate and change in structure during thermal treatment and aging.28,29 DRIFTS and Mass Spectrometry Characterizing Surface Species and Their Products during Thermal Treatment. To determine the thermal stability of the surface species and characterize the reactions that occurred as the temperature was increased, DRIFT spectra were recorded at intervals as the sample made from Au(CH3)2acac on MgO was heated in He flowing at 10 mL/ min. Between the selected measurement temperatures, the sample temperature was increased linearly at a rate of 5 K/min. Figure 6A shows the spectra recorded in the C-H stretching region at temperatures between 298 and 573 K. No visible change was observed at temperatures up to 353 K. At higher temperatures the bands at 2953 and 2910 cm-1 decreased in intensity, most clearly at temperatures exceeding ca. 473 K. To quantify the changes approximately, the spectrum measured at each temperature was deconvoluted. The spectrum of Au(CH3)2(acac)/MgO recorded at room temperature was curve resolved into 10 component peaks. The peak maxima and the full widths at half-maximum (fwhm) of each component peak in the deconvoluted spectrum were then optimized by nonlinear least-squares regression analysis with respect to the measured spectrum. The resultant optimized peak maxima are listed in Table 2. Each of the other spectra was then subjected to a similar analysis; however, the peak maxima of the 10 component peaks were constrained to the values obtained from the analysis of the room-temperature spectrum. (The fwhm of each component band was similar (within 5 cm-1) at each temperature, suggesting that the process of fixing the peak positions did not force any unrealistic constraints on the data sets.) Each resultant band was then integrated. Figure 7 shows the normalized integrated intensity ratios of the bands at 2953 and 2910 cm-1 and of those at 2953 and 2852 cm-1. The peak intensities of the former pair of bands decreased in tandem, and they are therefore suggested to be indicative of a common surface species (this point is discussed below). Some of each species remained following the experiment (after 14 h in flowing He at 573 K). The relative intensity changes of several selected mass fragments measured in the off-gas during the experiment are shown in Figure 8; data are shown for water, O2, methane, and CO2 (m/e ) 18, 32, 15, and 44, respectively). As methanol formation was also possible and would also produce a peak at m/e 32, its parent peak (m/e ) 31) was also monitored. The data show that only trace amounts (26) (a) Kip, B. J.; Duivenvoorden, F. B. M.; Koningsberger, D. C.; Prins, R. J. Catal. 1987, 105, 26. (b) Jentys, A. Phys. Chem. Chem. Phys. 1999, 1, 4059. (27) The largest clusters are characterized by a second-shell Au-Au coordination number of 3.4, indicating that they are three-dimensional. (28) Vogel, W.; Cunningham, D. A. H.; Tanaka, K.; Haruta, M. Catal. Lett. 1996, 40, 175. (29) Cunningham, D. A. H.; Vogel, W.; Kageyama, H.; Tsubota, S.; Haruta, M. J. Catal. 1998, 177, 1.

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Figure 6. IR spectra characterizing products of Au(CH3)2(acac)/ MgO during thermal decomposition in He at various stated temperatures between 298 and 573 K: (A) C-H stretching region; (B) fingerprint region; and (C) O-H stretching region.

of methanol were formed at any of the temperatures investigated (the amount of methanol was about 103 times less than the amount of methane). At temperatures below ca. 373 K, only water was evolved. At temperatures between 373 and 473 K, a large amount of water was produced, along with methane and small amounts of O2 and CO2. At temperatures exceeding approximately 473 K, large amounts of water were produced along with methane and CO2; O2 was consumed during this period. Figure 6B shows the measured spectral changes in the IR fingerprint region recorded during the treatment sequence mentioned immediately above. Only little change was observed in the intensities of these sharp bands at temperatures below 373 K. At 473 K each band had decreased considerably in intensity, but no new bands

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Figure 7. Dependence of ratio of intensities of pairs of IR bands at various temperatures (normalized to their intensity ratio at 298 K). Circles represent intensity ratios of 2953 cm-1 band to 2910 cm-1 band. Squares represent intensity ratios of 2953 cm-1 band to 2852 cm-1 band.

Figure 8. Measured mass spectral responses of selected mass fragments recorded during the thermal decomposition of the sample formed from Au(CH3)2(acac) on MgO in flowing He. Mass fragments are for water, O2, methane, and CO2 (m/e ) 18, 15, 32, and 44), respectively.

had appeared. At 573 K (after 14 h), broad, relatively featureless bands remained, centered around ca. 1550, 1450, 1350, and 1250 cm-1. Changes in the hydroxyl region as a function of temperature are shown in Figure 6C. The sharp bands at 3767 and 3700 cm-1 showed little change at temperatures up to 353 K. The former peak had broadened somewhat at 473 K. A low-frequency shoulder appeared on the higherfrequency peak, which shifted to 3759 cm-1. However, the total intensities of these peaks were not greatly altered at 473 K. After a 14-h treatment in He at 573 K, the band at 3700 cm-1 had vanished. The intensity of the highfrequency peak was still relatively unaltered, but its maximum had shifted to 3754 cm-1, and considerable asymmetry became apparent. Similarly, DRIFTS experiments were performed with the sample in flowing H2 (10 mL/min) instead of He. The results are virtually identical to those described above. Additional experiments were performed with flowing D2/

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Figure 9. IR spectra characterizing the O-H and C-H stretching regions of the sample formed from Au(CH3)2(acac) and MgO during deuteration reaction in D2 (1 mL/min) and He (10 mL/min) at temperatures between 298 and 353 K.

Figure 10. IR spectra characterizing the O-H and C-H stretching regions of the sample formed from Au(CH3)2(acac) and MgO (a) measured at room temperature in He and (b) following 60 min of deuteration in D2 (1 mL/min) and He (10 mL/min) at 573 K.

He (1 mL of D2/min, 10 mL of He/min). Figure 9 shows changes in the O-H and C-H stretching regions observed at temperatures between 298 and 353 K; the spectrum measured at 298 K with the sample in He is shown for comparison. The spectra indicate no observable change in the bands of the C-H region and no changes in the intense O-H peaks at 3770 and 3700 cm-1 as the temperature increased to 353 K. However, a gradual increase in a broad band at ca. 2700 cm-1 occurred with a slight, concomitant decrease in the broad band at ca. 3500 cm-1. Figure 10 shows the IR spectrum measured following 60 min of exposure of the sample to flowing D2 in He at 573 K. Two intense bands, at 2770 and 2723 cm-1, appeared at the expense of the O-H bands at 3755 and 3690 cm-1. Discussion Reaction of Au(CH3)2(acac) with MgO. Metal complexes with acetylacetonate ligands have been used frequently as precursors of oxide-supported metal species, because these ligands are reactive enough to be removed

Dimethyl Gold Complexes on MgO

readily in subsequent treatments, sometimes giving simple, site-isolated metal species on the support. Examples of the precursors include VO(acac)2, used for preparation of vanadium oxide on SiO2,30 Fe(acac)3 for iron oxide on ZrO2,31 and Cr(acac)3 for chromium oxide on the mesoporous MCM-41.32 Often it has been assumed that reaction occurs between the precursor and support OH groups by ligand exchange to form Hacac, which remains on the support and typically can be removed by calcination in air at temperatures between 573 and 673 K. An investigation of the reactions of a family of transition metal acac compounds with alumina33 provided further insight into the surface chemistry, leading to the conclusion that the acac complexes reacted not only with OH groups but also with coordinatively unsaturated metal ions in the support surface; when basic hydroxyl groups were present, bidentate complexes including Al(acac)3 and Hacac were formed on the surface. Consistent with this pattern, our group reported results showing that the precursor used in the present work, Au(CH3)2(acac), reacts with both coordinatively unsaturated Al ions and OH groups on partially dehydroxylated γ-Al2O3, forming a gold species with the support playing the role of a bidentate ligand, Au(CH3)2{OAl}2 (the braces denote species terminating the bulk γ-Al2O3).34 The data reported here for the reaction of Au(CH3)2(acac) with MgO (specifically, a comparison of the IR spectra of the species formed on MgO with that of MgO itself (Figures 1-3)) indicate the formation of new surface species, as follows: First, following reaction, the O-H stretching region (Figure 1) was characterized by an increase in intensity of the 3769-cm-1 band and the appearance of an intense new peak at 3700 cm-1 and a weaker band at 3744 cm-1. These indicate the generation of one-, three-, and four-coordinated OH groups on MgO, as suggested by the results of a recent IR investigation of MgO nanoparticles (Table 1).35 Both the 3762- and 3744cm-1 bands characterizing the nanoparticles were assigned to isolated, singly coordinated OH groups, with a peak at 3702 cm-1 being assigned to isolated, three-coordinate OH groups. Thus, we infer that the reaction of Au(CH3)2(acac) with the MgO surface gave OH groups, some of them present at defect sites. The assignment of these two peaks to isolated OH groups was confirmed by deuteration of the Au(CH3)2(acac) on MgO at 573 K. The peak at 3770 cm-1 shifted to 2770 cm-1 and that at 3700 cm-1 shifted to 2723 cm-1 (Figure 10). These isotopic shifts (with a frequency ratio of the bands of ca. 0.73) are in good agreement with the expectation (1/x2 ) 0.71). The spectra of Figure 6c provide evidence of the thermal stability of these surface OH groups: both bands showed only little change upon treatment at temperatures up to 353 K. However, following the 14-h treatment in He at 573 K, the 3700-cm-1 band had vanished, and the intensity of the higher-frequency peak remained relatively unaltered, although its maximum had shifted to 3754 cm-1 (30) van der Voort, P.; White, M. G.; Vansant, E. F. Langmuir 1998, 14, 106. (31) van der Voort, P.; van Welzenis, R.; de Ridder, M.; Brongersma, H. H.; Baltes, M.; Mathieu, M.; van de Ven, P. C.; Vansant, E. F. Langmuir 2002, 18, 4420. (32) Weckhuysen, B. M.; Rao, R. R.; Pelgrims, J.; Schoonheydt, R. A.; Bodart, P.; Debras, G.; Collart, O.; van der Voort, P.; Vansant, E. F. Chem. Eur. J. 2000, 6, 2960. (33) van Veen, J. A. R.; Jonkers, G.; Hesselink, W. H. J. Chem. Soc., Faraday Trans 1 1989, 85, 389. (34) Guzman, J.; Gates, B. C. Langmuir 2003, 19, 3897. (35) Diwald, O.; Sterrer, M.; Kno¨zinger, E. Phys. Chem. Chem. Phys. 2002, 4, 2811.

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and considerable asymmetry was present (indicated by a low-frequency shoulder). It appears that that at this temperature the defect sites corresponding to isolated, three-coordinate hydroxyl groups were either reconstructed into one-coordinate sites or that migration of the OH group occurred from one type of site to the other. Reaction of MgO with Au(CH3)2(acac) also led to an increase in intensity of the broad absorption between 3675 and 3200 cm-1. Deuteration at only 353 K caused a partial shift of this absorption to about 2700 cm-1 (Figure 9). The position, breadth, isotopic shift, and apparently weak bond strength support the identification of the surface species as hydrogen bonded. The complex IR spectra in the C-H stretching region (Figure 4), with peaks at 3080, 2954, 2909, 2852, and 2821 cm-1 and shoulders at 2992 and 2883 cm-1, were interpreted on the basis of the spectra of Mg(acac)2, Hacac supported on MgO (Hacac/MgO), and Au(CH3)2(acac)36 (Table 2). Both Mg(acac)2 and Hacac are planar bidentate species, and their spectra include three intense peaks (2993, 2970, and 2920 cm-1) and (2992, 2965, and 2920 cm-1), respectively. These bands are assigned to the three distinct C-H stretching frequencies of the two methyl groups in a bidentate acac complex.37,38 The IR spectrum of Mg(acac)2 includes bands at 2866 and 2836 cm-1. Our IR spectrum of the species formed from Au(CH3)2(acac) on MgO similarly includes intense peaks at 2953 and 2910 cm-1 (Figure 4). Comparison with the spectra of Mg(acac)2 and Hacac (Figure 4) shows that these two peaks are not present. Rather, only unresolved shoulders appear, at ca. 2992, 2970, and 2920 cm-1. As shown in Table 2, Miles et al.36 reported a spectrum for Au(CH3)2(acac) different from ours. These authors observed bands at 2990, 2915, and 2813 cm-1. Curiously, they reported no band at 2953 cm-1. Hence they assigned the bands at 2990 and 2813 cm-1 to the asymmetric and symmetric stretching modes of the Au-CH3 bond. Thus, they reported observing only one of the three methyl acac stretching modes (at 2915 cm-1). To ascertain whether the band at 2910 cm-1 was attributable to the acac complex or associated with the methyl group on the gold, we plotted the normalized integrated intensity ratios of the 2953- and 2910-cm-1 pair and the 2953- and 2854-cm-1 pair as a function of temperature (Figure 7). Within experimental error, both ratios are close to 1. The ratio representing the former pair of bands changed in the same direction (increased), whereas the ratio representing the latter pair increased and then decreased sharply with increasing temperature. Hence it seems more reasonable that the latter pair belong to the same species than the former. Furthermore, by comparison with the three acac spectra measured in our investigation, it seems more appropriate that the bands at 2953 and 2910 cm-1 be assigned to the asymmetric and symmetric stretching modes of the Au-CH3 bond. We are aware that the symmetric methyl stretching mode normally occurs at frequencies below 2860 cm-1 in hydrocarbons, but this does not necessarily have to hold for organometallic complexes. Although we were not able to locate IR data for other dimethyl gold complexes, our assignment is supported by the fact that the asymmetric and symmetric stretching modes of the Sn-CH3 bond in SnCl2(CH3)2 have been reported at 2990 and 2920 cm-1, (36) Miles, M. G.; Glass, G. E.; Tobias, R. S. J. Am. Chem. Soc. 1966, 88, 5738. (37) Nakamoto, K.; Martell, A. E. J. Phys. Chem. 1960, 32, 588. (38) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Complexes, Part B, 5th ed.; Wiley: New York, 1997.

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respectively.39 Further evidence supporting this assignment is discussed below. IR spectra of the fingerprint region representing Mg(acac)2, Hacac/MgO, Au(CH3)2(acac), Au(CH3)2(acac)/MgO, and CuII(acac)2 are summarized in Table 3. The calculated frequencies reported for CuII(acac)2 are the results of a normal coordinate analysis of the planar model, CuII(acac).37 This C2v model has 21 normal modes of vibration, all of which are IR active. The most important modes for distinguishing the bidentate acac complexes are those involving either the C-O ring stretching or the C-C-C ring stretching modes. Although these modes are diagnostic for these species, there is still not complete agreement as to which bands represent which modes. In the spectrum of CuII(acac)2, these bands appear at ca. 1580 and 1520 cm-1; the frequencies depend on the coordinating metal ion. They have been observed for MgII(acac)2 at 1623 and at 1528 cm-1. Miles et al.36 reported a strong band at 1590 cm-1 for Au(CH3)2(acac). A comparison of the reported spectra with those measured in our investigation for the species formed from Au(CH3)2(acac) on MgO and for Hacac/ MgO and those of other bidentate acac species strongly suggests that reaction of Au(CH3)2(acac) with MgO involves the displacement of the acac ligands and that the gold precursor reacts with the MgO surface to form both Hacac and MgIIacac (associated with the broad peak with maximum at 1610 cm-1) (Figure 3). The presence of these different surface acac complexes is further evidenced by the splitting of the bands at ca. 1477 and 1411 cm-1. The higher-frequency pair of bands is assigned to a combination mode of the C-C stretching and the C-H bending mode. The other pair is assigned to a deformation mode of the acac methyl group (Table 3). EXAFS and XANES Evidence of Formation of Au(III) Complexes on MgO. The X-ray absorption spectra identify the species formed by adsorption of Au(CH3)2(acac) on MgO as AuIII(CH3)2{OMg}2, consistent with previous reports,5,11,40 the lack of Au-Au first- and second-shell contributions in the EXAFS spectrum, and the IR data. No Au-Au contributions were found at distances typical of Au-Au bonds (e.g., 2.88 Å), consistent with the inference that the supported species were site isolated and mononuclear. The EXAFS data show that an Au atom in the surface complex was bonded to two O atoms, inferred to be part of the MgO surface (a bidentate ligand), because the Au-O distance was found to be R ) 2.16 Å (vs 2.08 Å in Au(CH3)2(acac)17). This distance is typical of M-O bonding distances in other zeolite- and oxide-supported mononuclear group 8 metal complexes.41 The data also show a weak Au-C contribution (N ) 2.0) at a distance of 2.04 Å (vs 2.05 Å in Au(CH3)2(acac)17) that confirms the bonding of two carbon atoms to the gold center. The fact that the XANES data characterizing the supported gold complex are virtually identical to those characterizing crystalline Au(CH3)2(acac) (Figure 6)42 indicates that the gold retained its oxidation state of +III after reaction of the precursor with the MgO. Treatment of Au(CH3)2(acac)/MgO: Genesis of Gold Clusters. In the treatment of the sample formed from Au(CH3)2(acac) and MgO in He as the temperature increased, the IR spectra (Figure 6) show that all the C-H containing species (methyl groups of the acac and the methyl ligands on gold) were stable at temperatures up to 353 K. This conclusion is supported by mass spectro(39) Davydov, A. Molecular Spectroscopy of Oxide Catalyst Surfaces; Sheppard, N. T., Ed.; Wiley: Chichester, 2003. (40) Guzman, J.; Gates, B. C. Angew. Chem., Int. Ed. 2003, 42, 690. (41) Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1992, 14, 271. (42) Guzman, J.; Gates, B. C. J. Phys. Chem. B 2003, 107, 2242.

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scopic analysis of the off-gases, as only water was evolved up to this temperature. At temperatures between 373 and 473 K, a large amount of water was produced, inferred to have arisen from physisorbed water and dehydroxylation (condensation) of weakly bound surface OH species. As shown in Figure 6, at ca. 473 K the bands at 2953 and 2910 cm-1 (assigned above to the asymmetric and the symmetric stretching modes of the Au-CH3 species) began to decrease in intensity. The assignment of these bands to a single species is supported by the fact that the measured normalized integrated intensity ratio of these two bands remained close to unity with increasing temperature. By contrast, the intensity of the band at 2854 cm-1, assigned to a combination band of the acac ligand, decreased somewhat more slowly (Figure 7). The IR spectrum of MgII(acac)2 also includes bands at 2866 and 2836 cm-1 (Table 2). Examination of the fingerprint region of this spectrum (Table 3) shows that a combination of a number of vibrations would produce bands in the 2800-2900 cm-1 region. For example, the combination of {υs(C-C) + δ(C-H)} + δs(CH3) would result in a band at 2986 cm-1. A similar combination would produce a band at 2851 cm-1 for Au(CH3)2(acac). Some small loss of methyl groups of acac was also observed, as evidenced by the slight reduction in the intensity of the band at 2992 cm-1. Demethylation occurred with the evolution of methane and trace amounts of methanol, which were detected in the off-gas. The small amounts of O2 and CO2 evolved were probably formed by the decomposition of weakly bound carbonate species that are normally present on the surface of MgO (and formed originally from CO2 in the atmosphere). At temperatures between 473 and 573 K, large amounts of water were produced along with methane and CO2. Surface carbonate decomposition and dehydroxylation, which are known to occur on MgO at these temperatures, is inferred to have been responsible for the production of CO2, O2, and water. The O2 thus formed was consumed during this period to oxidize the acac ligands during the He treatment at these temperatures. These results are in agreement with those of Haruta et al.,43 who deposited Au(CH3)2(acac) onto the surface of silica and monitored the removal of the resultant bidentate acac complex during calcination in air by thermogravimetric differential thermal analysis (TG-DTA). They reported that, in air, removal of these ligands began at about 375 K, showing an endothermic peak at 400 K, assigned to decomposition of the precursor. At temperatures between 500 and 550 K, an exothermic peak was observed and assigned to oxidation of the ligands. The authors concluded that calcination temperatures above 573 K were needed for total removal of the acac ligands. Our results agree well with this conclusion, as we observed that, following thermal decomposition in He to 573 K, organic ligands were still present in addition to some surface acetates and carbonates. Reduction and Aggregation of Gold Species. The results demonstrate the transformation of supported mononuclear gold complexes, modeled as AuIII(CH3)2{MgO}2, into clusters of supported Au(III) oxide, which, upon treatment in He at increasing temperatures, were converted into metallic gold clusters, ultimately with an average diameter of about 30 Å. The evidence for this conclusion is as follows: The EXAFS results demonstrate that as the initially prepared sample was treated (Table 4), increasingly large (43) Okumura, M.; Tsubota, S.; Haruta, M. J. Mol. Catal. A 2003, 199, 73.

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gold clusters formed. Thus, we infer that the initially supported gold species migrated readily on the MgO surface. It has been suggested44,45 that the density of OH groups on the MgO influences the aggregation of supported metal clusters, possibly by affecting the strength of the metal-support interaction. The results suggest that the OH groups on our MgO (which was calcined at 673 K and had a significant fraction of its surface covered by OH groups) facilitate the aggregation of the gold clusters. The XANES results provide confirming evidence of a gradual reduction of the supported gold complexes during treatment in He at increasing temperatures, ultimately indicating the conversion of all the gold to the zerovalent state. Conclusions The DRIFTS, XANES, and EXAFS data demonstrate that AuIII(CH3)2(acac) reacts at room temperature with the surface of MgO to form AuIII(CH3)2{MgO}2 and (acac){Mg} species by ligand exchange of the acac of Au(CH3)2(acac) with support oxygen and anchoring of a site-isolated Au(III) complex on the support. The data give evidence of (a) hydrogen-bonding interactions between surface Mg(acac)2 and H(acac) species and support OH groups, (b) stability and reactivity of CH3 groups bonded to Au with their subsequent removal as methane, (c) decomposition of surface Mg(acac)2 and H(acac) species by oxidation to (44) Triantafillou, N. D.; Gates, B. C. J. Phys. Chem. 1994, 98, 8431. (45) Kawi, S.; Gates, B. C. Inorg. Chem. 1992, 31, 2939.

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form CO2 and traces of acetate ligands on the surface, and (d) the structural and electronic changes of the supported gold during He treatment. The results provide one of the most complete accounts of the reaction of a metal acetylacetonate complex with an oxide surface, including characterization of the resultant surface organic, organometallic, metal oxide, and metallic species. Acknowledgment. This research was supported by the U.S. Department of Energy, Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences, Contract Nos. FG02-87ER13790 and FG0204ER15513. A National Science Foundation IGERT grant (Contract No. DGE-9972741) and the Schuit Institute of Catalysis supported J. Guzman’s travel to the Schuit Institute of Catalysis. We acknowledge the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences, under Contract No. DE-AC02-98CH10886, and the staff of beamline X-18B. Supporting Information Available: A table of crystallographic data characterizing the reference compounds and Fourier transform ranges used in EXAFS data analysis and figures of experimental EXAFS functions characterizing the sample formed by the reaction of Au(CH3)2(acac) with MgO following different treatments. This material is available free of charge via the Internet at http://pubs.acs.org. LA0470434