J. Phys. Chem. B 2006, 110, 7695-7701
7695
Formation of Nonclassical Carbonyls of Au3+ in Zeolite NaY: Characterization by Infrared Spectroscopy Mihail Y. Mihaylov,†,‡ Juan C. Fierro-Gonzalez,§ Helmut Kno1 zinger,‡ Bruce C. Gates,§ and Konstantin I. Hadjiivanov*,† Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, Department Chemie und Biochemie, Physikalische Chemie, LMU Mu¨nchen, Butenandtstrasse 5-13 (Haus E), 81377 Mu¨nchen, Germany, and Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: December 21, 2005; In Final Form: February 27, 2006
Adsorption of CO on gold supported in zeolite NaY at 85 K led to the formation of (i) various carbonyls and isocarbonyls typical of the zeolite and (ii) carbonyls formed at cationic gold sites (observed in the 21862171 cm-1 region). Analysis of the behavior of the bands allows their assignment to carbonyls of Au3+ ions. At temperatures higher than 220 K, CO adsorption led to the formation of a new type of Au3+-CO species (2207 cm-1). Once formed, these complexes could be transformed into the dicarbonyls Au3+(CO)2 when the sample was cooled to 85 K in the presence of CO. The results are explained by migration of Au3+ ions to more accessible positions within the zeolite at increasing temperatures. When a CO molecule is already adsorbed, it stabilizes the Au3+ ion in the new position, and a second CO molecule can be coordinated, thus forming a geminal species. These results are the first evidence of Au3+(CO)2 complexes.
1. Introduction Although bulk gold is unreactive, gold dispersed on various supports is catalytically highly active for reactions such as CO oxidation1,2 and the water gas shift3 and selective for reactions such as NO reduction,4 propene epoxidation,5 and synthesis of vinyl chloride.6 It has been suggested that gold nanoclusters are responsible for the catalytic properties,7-10 but the involvement of gold cations has also been demonstrated, and some catalysts contain both gold nanoclusters and cationic gold.11-15 It has been proposed that the sites for CO oxidation in catalysts containing gold nanoclusters consist of cationic gold located at the gold-support interface.7,11,16-20 Recently, the idea of participation of Au3+ ions in some reactions has gained popularity.11,20-25 Therefore, a detailed characterization of supported gold is of great importance. One of the most powerful techniques giving information about the oxidation and coordination states of accessible surface cationic and metallic sites is IR spectroscopy of probe molecules, and especially of CO.26,27 Although this technique has been used to characterize supported gold,20,21,25,28-54 the reported results do not give a consistent picture of the nature of the gold species. Analysis of the reported results20,21,25,28-54 shows that surface gold carbonyls have been registered in a wide spectral region, namely, 2204-1990 cm-1. Most authors25,27-32,40-43,45,46,48-54 agree that surface carbonyls on metallic gold are observed in the 2138-2090 cm-1 region. Bands in the 2145-2130 cm-1 region have usually been attributed to (i) CO on positively polarized gold particles29,38,40,41,46,47,50,52,54 (probably having oxygen in the vicinity40,52) or (ii) to Au+-CO species.20,25 As a rule, bands in the 2200-2150 cm-1 region have been assigned to Au+-CO species.31,32,38,40,42-45,51-54 Among these, the bands †
Bulgarian Academy of Sciences. LMU Mu¨nchen. § University of California, Davis. ‡
at the higher wavenumbers (>2170 cm-1) are associated with gold cations in zeolites.31,32,38,42-45 In most cases, Au3+ ions have not been considered to be possible adsorption sites. However, in recent investigations, some of us have proposed that bands located at approximately 2170 cm-1 characterize Au3+-CO species.20 According to others,25 bands characterizing Au3+-CO complexes can be found at a considerably lower frequency, namely, 2148 cm-1. Evidently, gold carbonyl chemistry needs a serious reevaluation, and well-defined samples need to be investigated to provide a basis for unambiguous conclusions. The structures of supported gold catalysts depend on the details of the preparations and of the reaction environments, such as the water content.33,34 Attempting to make structurally simple supported gold samples, some authors13-15,20,21,35-37 have used the reactive mononuclear complex Au3+(CH3)2(C5H7O2) as a precursor, characterizing its reactions with oxides and with a zeolite (NaY). This precursor yields cationic gold on the supports, sometimes in the absence of zerovalent gold;13 thus, samples prepared from Au3+(CH3)2(C5H7O2) permit investigation of the properties and reactivity of cationic gold. The aim of the work reported here was to characterize CO adsorption on a Au/zeolite NaY sample prepared from the precursor Au3+(CH3)2(C5H7O2). Because it has been reported that such samples are highly moisture-sensitive, we made efforts to minimize contact of the sample with air during the whole procedure, starting with synthesis, and including pressing of a wafer and its transfer to the IR cell and finishing with the IR measurements. This procedure allowed us to isolate carbonyls and dicarbonyls of Au3+ that have not been reported before. One of the complications arising from the use of CO as a probe of supported gold is that it can reduce gold cations easily,31,32 and CO-induced changes of gold nanoclusters are also possible. To avoid eventual reduction of cationic gold by
10.1021/jp057426q CCC: $30.25 © 2006 American Chemical Society Published on Web 03/31/2006
7696 J. Phys. Chem. B, Vol. 110, No. 15, 2006
Mihaylov et al.
CO, we initially investigated CO adsorption at a low temperature, ca. 85 K. 2. Experimental Section 2.1. Preparation of the Sample and of Self-Supporting Wafers. The supported sample was prepared by adsorption of Au3+(CH3)2(C5H7O2) at 298 K and 105 Pa on zeolite NaY powder. The synthesis and transfer of the zeolite-supported gold samples were carried out under anaerobic and anhydrous conditions, as reported elsewhere.20 The zeolite NaY powder (W. R. Grace and Co., Si:Al atomic radio ) 2.6) was treated in flowing O2 at 573 K for 4 h and then evacuated at the same temperature for 16 h. The zeolite-supported gold samples, containing 1 wt % Au, were prepared by slurring Au3+(CH3)2(C5H7O2) (Strem, 98%) in dried and deoxygenated n-pentane with the treated zeolite powder. The slurry was stirred for 48 h and the solvent removed by evacuation (pressure < 10-3 Torr) for 1 day. Because the gold precursor is moisture-sensitive, after preparation the sample was stored in the dark in a sealed glass ampule. The gold content in the sample thus prepared was 1 wt %. Preliminary EXAFS study20 has shown the presence of mononuclear gold complexes, as evidenced by the lack of detectable Au-Au contributions. To prepare self-supporting wafers for the IR experiments, the ampule containing the sample, the pressing accessories, and the sample compartment of the IR cell were placed in a glovebag filled with Ar (99.999, Messer Griesheim, GmbH). The Ar was purified by passage through an Oxisorb cartridge (Messer Griesheim, GmbH). The ampule in the glovebag was broken, and a wafer of the sample (ca. 10-15 mg cm-2) was prepared, placed into a sample holder, and transferred to the IR cell, all this in the Ar atmosphere. Carbon monoxide used in the IR experiments (>99.997%) was supplied by Linde AG. Before use it was additionally purified by passage through an Oxisorb cartridge. 2.2. IR Spectroscopy. The IR cell55 was designed to allow recording of spectra at temperatures between 85 K and ambient temperature. The cell was connected to a vacuum adsorption system with a residual pressure below 10-3 Pa. Spectra were recorded with a Bruker IFS-66 spectrometer at a spectral resolution of 2 cm-1; each spectrum was the average of 128 scans. The sample in the cell was subjected to various treatments, as described below. 3. Results 3.1. Characterization of the Initially Prepared Sample. The IR spectrum of the initially prepared sample, recorded in an Ar atmosphere, exhibited bands characterizing residual water, at 3695, 3280, and 1640 cm-1 (Figure 1, spectrum a). This result shows that the preparation procedure and the handling in the glovebag were not sufficient to completely avoid the presence of at least small concentrations of air/water vapor. However, the amount of water on the sample was minimized, as shown by a comparison of the spectrum of the sample with a spectrum of a sample that was intentionally brought in contact with air (Figure 1, spectrum b). In the latter case, a broad feature, indicative of adsorbed water, is evident in the 3800-3000 cm-1 region. Its integral intensity is much greater than the integral intensity of the feature at about 3280 cm-1 characterizing the sample that was not allowed to contact air. In addition, a series of bands originating from the methyl and acetylacetonate ligands of the precursor Au3+(CH3)2(C5H7O2): (2960, 2930, 2868, 1460, 1385 cm-1)36 was also observed (Figure 1, spectrum a). Two very weak bands, at 1397
Figure 1. IR spectra of (a) the as-prepared Au/zeolite NaY sample in the form of a wafer; (b) the sample after exposure to air; and (c) the as-prepared sample after evacuation at ambient temperature for 3 h.
and 1363 cm-1, could be assigned either to the precursor interacting with the support or to products of its partial hydrolysis. Note that the contact of the sample with air led to an almost complete disappearance of the C-H modes (2960, 2930, and 2868 cm-1) arising from the precursor (Figure 1, spectrum b), which indicates hydrolysis according to the following reaction:
Au(CH3)2(C5H7O2) + 3H2O f Au(OH)3 + 2CH4 + C5H8O2 In this case, some weak bands (at 1537, 1400, and 1363 cm-1), suggesting chemical transformation of the precursor (most probably of acetylacetonate ligands), were also recorded. These results indicate that our efforts to minimize sample contact with air were successful. Evacuation of the sample for 3 h at
