Redox Behavior of Gold Species in Zeolite NaY - American Chemical

Nov 9, 2006 - Mihail Mihaylov,*,†,‡ Bruce C. Gates,§ Juan C. Fierro-Gonzalez,§ Konstantin Hadjiivanov,‡ and. Helmut Kno1zinger†. Department ...
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J. Phys. Chem. C 2007, 111, 2548-2556

Redox Behavior of Gold Species in Zeolite NaY: Characterization by Infrared Spectroscopy of Adsorbed CO Mihail Mihaylov,*,†,‡ Bruce C. Gates,§ Juan C. Fierro-Gonzalez,§ Konstantin Hadjiivanov,‡ and Helmut Kno1 zinger† Department Chemie und Biochemie, Physikalische Chemie, LMU Mu¨nchen, Butenandtstrasse 5-13 (Haus E), 81377 Mu¨nchen, Germany, Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, and Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: September 14, 2006; In Final Form: NoVember 9, 2006

The state of gold on a Au/NaY zeolite sample prepared from Au(CH3)2(C5H7O2) and subjected to various treatments was monitored by infrared spectroscopy of adsorbed CO as a probe molecule. Spectra of CO adsorbed on a sample exposed to air and evacuated at room temperature indicate predominantly zerovalent gold nanoclusters (Auδ+-CO band at 2147 cm-1, associated with positively charged sites on the metal clusters). The spectra also indicate a small number of Au3+ sites in cation positions (2207 cm-1) and other Au3+ sites (2183 cm-1). However, the majority of the deposited gold did not form any carbonyl species. Evacuation at 423 K resulted in autoreduction of the Au3+ ions. At 473 K, the Auδ+ sites were converted into Au0 (Au0CO band at 2122 cm-1). New bands at 2086 and 2036 cm-1, assigned to negatively charged gold carbonyls, were produced after CO adsorption on samples evacuated at temperatures g473 K. Interaction of the autoreduced sample with O2, even at ambient temperature, led to oxidation of some zerovalent gold to Auδ+ (2127 cm-1). With samples reoxidized at higher temperatures, the Auδ+-CO band was observed between 2132 and 2152 cm-1. When gold was reoxidized with an NO + O2 mixture, an Au+-CO band at ca. 2190 cm-1 was dominant in the carbonyl spectra. When CO was adsorbed at 85 K, the 2190 cm-1 band shifted at high coverage to 2185 cm-1. Results of CO-H2O coadsorption experiments indicate a static interaction of the Au+-CO species with CO molecules adsorbed on Na+ ions; in the presence of water, the Au+-CO band maximum was coverage independent because the Na+ ions were blocked by water molecules.

Introduction Since the discovery that highly dispersed supported gold is a highly active catalyst in the low-temperature oxidation of CO,1,2 the interest in gold catalysts has grown enormously. A major remaining challenge in the understanding of these catalysts is the identification of the gold species at various stages of catalyst treatment and under catalytic reaction conditions. A number of physical methods have been applied extensively in the characterization of supported gold catalysts, including X-ray diffraction (XRD), transmission electron microscopy, extended X-ray absorption fine structure spectroscopy, X-ray photoelectron spectroscopy, and Mo¨ssbauer spectroscopy. One of the techniques that can provide answers to a number of the remaining questions is infrared (IR) spectroscopy of adsorbed CO. CO is an appealing probe molecule, not only because CO is a reactant in CO oxidation, but also because νCO spectra provide detailed information about the structure, coordination, and oxidation states of the accessible gold sites.3-5 A difficulty with IR spectroscopy of absorbed CO for characterization of supported gold is that at present opinions differ strongly regarding the assignments of gold carbonyl bands. There is near unanimity in the literature in the judgment that metallic gold can be monitored by carbonyl bands between 2132 * Corresponding author. E-mail: [email protected]. † LMU Mu ¨ nchen. ‡ Bulgarian Academy of Sciences. § University of California.

and 2098 cm-1.6-34 Negatively charged carbonyls have been proposed in the frequency range 2055-1950 cm-1.15,16 Bands in the 2155-2130 cm-1 region usually have been attributed to CO on positively polarized gold particles affected by oxygen in the vicinity.6-24,35,36 As a rule, bands in the 2200-2150 cm-1 region have been assigned to Au+-CO species.6-14,26-33,35 Among these, the bands at higher wavenumbers (>2170 cm-1) are associated with gold cations in zeolites.14,27-31 In most cases, Au3+ ions have not been considered to be possible adsorption sites because they are coordinatively saturated8,11,23 or immediately reduced by CO.6 However, in a recent paper39 we proposed that bands located at wavenumbers > 2200 cm-1 characterize (di)carbonyls of Au3+ ions located in cation positions. In other positions in zeolites, Au3+ ions form carbonyls that absorb at lower frequencies (down to 2170 cm-1).38,39 Some authors33 have suggested that bands characterizing Au3+-CO complexes can be found at considerably lower frequencies, namely, 2139 cm-1. The goal of the work reported here was to characterize CO adsorption on a Au/NaY zeolite sample prepared from the precursor Au3+(CH3)2(C5H7O2) (C5H7O2 is acetylacetonate, abbreviated as acac). Such samples are highly moisture sensitive, and in previous work we made efforts to minimize contact of the samples with air during the whole characterization procedure, starting with synthesis and including pressing of a wafer and its transfer to an IR cell, followed by IR measurements.39 In this work we report on the results with a sample treated in a conventional way to establish the effect of contacting of the

10.1021/jp066005l CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007

Redox Behavior of Au Species in Zeolite NaY

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2549 was supplied by Aldrich. NO (>99.0%) was supplied by Messer Griesheim GmbH. Before use, CO was further purified by passage through an Oxisorb cartridge. X-ray Diffractometry. X-ray diffractograms were registered on a Stoe Stadi P powder diffractometer, with Cu-KR1 radiation (λ ) 1.5406 Å). The average crystallite size of gold was calculated by using the Au(111) line width in the Scherrer equation with Gaussian line shape approximation. Instrumental peak broadening was determined by using a lanthanum hexaboride line standard reference material (Aldrich). Results

Figure 1. IR spectra of the as-prepared Au/NaY zeolite sample in the form of a wafer (a); after evacuation at ambient temperature overnight (b); after evacuation for 1 h at 423 (c), 523 (d), 623 (e), and 723 K (f); after interaction with O2 for 1 h at 723 K (g); and after interaction with a NO + O2 mixture for 1 h at 573 K, followed by evacuation at 473 K (h).

sample with air (as would be convenient for industrial application) on the final state of the supported gold. Experimental Methods Sample Preparation. The sample was prepared by adsorption of Au(CH3)2(acac) on zeolite NaY powder at 298 K and 105 Pa. The synthesis was carried out under anaerobic and anhydrous conditions, as reported elsewhere.38 The NaY zeolite (W. R. Grace and Co., Si/Al atomic ratio ) 2.6) was treated in flowing O2 at 573 K for 4 h and then was evacuated at the same temperature for 16 h. The zeolite-supported gold samples were prepared by slurrying Au(CH3)2(acac) (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 < 0.1 Pa) for 1 day. The sample was stored in the dark in a sealed glass ampule. The gold content in the sample was 1.0 wt %. The activity of this sample for CO oxidation catalysis has been reported.38 IR Spectroscopy. The IR cell 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. To prepare self-supporting wafers for the IR experiments, the sample was briefly exposed to air. The sample in the cell was subjected to various treatments as described below. 12CO (>99.997%) used in the IR experiments was supplied by Linde AG; 13C-labeled CO (99 atom% 13C, 2180 cm-1 were detected after adsorption of CO on a sample evacuated at 423 K (Figure 3, spectrum d). This result indicates a complete reduction of the accessible Au3+ ions at this temperature. The principal carbonyl band was registered at 2129 cm-1, suggesting conversion of the Auδ+ sites into metallic ones. This band was detected at 2122 cm-1 with a sample evacuated at 473 K (Figure 3, spectrum e), which could be explained by further reduction of the Auδ+ sites or by a change in the state of metallic gold. Figure 4 shows the evolution of the carbonyl bands during evacuation of the sample that had been activated at 473 K.

Redox Behavior of Au Species in Zeolite NaY

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Figure 4. IR spectra of CO adsorbed at room temperature on the Au/ NaY zeolite sample evacuated at 473 K for 1 h. Equilibrium CO pressure: 1.1 kPa (a), 570 (b), 280 (c), 140 (d), 70 (e), and 30 Pa (f), and evolution of the spectra in dynamic vacuum (g, h).

Figure 5. IR spectra of CO (equilibrium pressure of 1.1 kPa) adsorbed at room temperature on the Au/NaY zeolite sample subjected to various pretreatments: sample evacuated at 573 K for 1 h (a) and sample reoxidized with O2 (for details see text) at room temperature (b), 373 (c), 473 (d), and 573 K (e).

Analysis of the spectra show that the species represented by the 2122 cm-1 band are less stable than the species represented by a band at 2147 cm-1. This result is consistent with the loss of the positive charge of the gold sites as a result of reduction of the gold during heating. The maximum of the band at 2122 cm-1 is coverage independent. There are numerous reports pointing out that the Au0-CO bands are strongly red-shifted as a result of increasing coverage.7,9-21,34 Boccuzzi et al.20 elegantly demonstrated that this effect is a consequence of lateral interactions of the CO on gold with CO molecules adsorbed on the support (in Bocuzzi’s case, TiO2). In our experiments, only a weak band assigned to CO adsorbed on the support (Na+-CO, 2171 cm-1) was observed. As a result, no lateral interactions can occur, and the Au0-CO band maximum is coverage-independent. Thus, our results are in agreement with the proposition of Boccuzzi et al.20 New, weak bands at ca. 2086 (shoulder) and 2036 cm-1 were detected when CO was adsorbed on samples evacuated at temperatures higher than 473 K (Figure 3, spectra f-j). Measurements made at decreasing coverages showed that the maxima of these bands are also coverage independent (spectra not shown). The band at 2036 cm-1 was most intense for samples evacuated at 523-573 K (Figure 3, spectra f,g), whereas the band at 2086 cm-1 was most intense for samples evacuated at 623-673 K (Figure 3, spectra h,i). These two new bands are best assigned to negatively charged gold carbonyls.15,16 Indeed, in this case the frequency is strongly red-shifted with respect to that of gas-phase CO because of the facilitated π-backdonation. Such species have been observed only for reduced gold samples.15,16 Evidently, in our case reduction caused by residual organic species took place. The gradual elimination of the organic residues is evident in the background spectra (Figure 1, spectra b-f) and consistent with the gradual increase in intensity of the Au0-CO band with increasing temperature of evacuation in the range of 573-723 K (Figure 3, spectra g-j). CO Adsorption on Au/NaY Zeolite Oxidized with O2 at Various Temperatures. Another pellet was evacuated at 573 K. The accessible gold was in metallic form as evidenced by the Au0-CO band at 2122 cm-1, which appeared after CO adsorption (Figure 5, spectrum a). A computer deconvolution of this band indicates that it consists of four components (at

2129, 2121, 2109, and 2086 cm-1), suggesting some heterogeneity of the metal sites (Supporting Information, Figure S5, spectrum a). After adsorption of CO, the sample was treated with O2 at various temperatures, and then (to prevent any autoreduction) O2 was removed by evacuation at room temperature. Adsorption of CO on a sample reoxidized at room temperature produced a band at 2127 cm-1 (Figure 5, spectrum b). It again consisted of four components, at 2133, 2126, 2117, and 2094 cm-1 (i.e., all bands had been blue-shifted, Supporting Information, Figure S5, spectrum b). With a sample reoxidized at 373-473 K, a component at 2138 cm-1 developed, matching the major one characterizing the sample reoxidized at 573 K (Figure 5, spectra c-e) (for details see Supporting Information, Figure S5, spectra c,d). The results can be understood as follows: the metallic sites active in CO adsorption are oxidized as a result of contacting of the sample with O2. Only a small part of the gold atoms on the metal cluster surfaces can adsorb CO (the so-called defect sites).16,19,22,25,32 Boccuzzi et al.17 reasoned that at low-temperature dissociation of oxygen is not probable, and they proposed formation of Auδ+-O2- sites. Their interpretation appears to be probable for our sample when it was oxidized at room temperature (Auδ+-CO bands at 2133-2126 cm-1). When the contacting with O2 was carried out at higher temperatures (373573 K), dissociation of oxygen took place, and the more ionic Auδ+-O2-, bond is reflected in a higher frequency of the respective carbonyl band (2138 cm-1). With a sample reoxidized at 723 K, the band was observed at 2152 cm-1. We suppose that under these conditions more metallic gold sites on the gold cluster surfaces were oxidized. Thus, we infer that the number of Auδ+ sites per metal cluster increased and the ability of the metal to share the positive charge decreased. As a result, the effective charge of the Auδ+ sites increased, as reflected in the increase of the frequency of the respective Auδ+-CO species. We emphasize, however, that even at 723 K, the reoxidation failed to produce isolated Au+ ions. This phenomenon can be explained by the fact that high temperatures favor autoreduction of gold and sintering, even in the presence of O2 (calcination of gold-containing samples generally leads to formation of metal

2552 J. Phys. Chem. C, Vol. 111, No. 6, 2007

Figure 6. IR spectra of CO adsorbed at room temperature on the Au/ NaY zeolite sample reoxidized with NO + O2 (see text for details). Equilibrium CO pressure of 1.1 kPa, followed by evacuations for 10 min at room temperature (a), 323 (b), 373 (c), and 423 K (d).

clusters or particles). We stress that we have no data indicating at which temperature the reoxidation occurs. In our experiments, the pellet was heated in O2 at the designated temperature, then cooled in the same atmosphere (i.e., the sample was in contact with O2 at all temperatures below the designated one, down to the ambient temperature). CO Adsorption on Au/NaY Zeolite Oxidized with a Mixture of NO + O2. In attempts to perform more efficient oxidation of gold, we treated the pellet in a mixture of NO + O2. This mixture is a more effective oxidizing agent than O2 itself because of the ability of NO2 to dissociate and provide atomic oxygen.43 The sample was treated in a mixture of NO + O2 (13.3 kPa NO and 26.6 kPa O2 initial pressures) for 1 h at 573 K and then was evacuated at 473 K. As a result, bands at 1425, 1376, and 1273 cm-1 were observed (Figure 1, spectrum h). Similar bands have been reported after coadsorption of NO and O2 on zeolite NaY and assigned to symmetric-like nitrates (1415 and 1373 cm-1) and bidentate nitrites (1270 cm-1).44 Subsequent CO adsorption (followed by evacuation) resulted in the appearance of an intense band at 2188 cm-1 (Figure 6, spectrum a). A weak Auδ+-CO band at 2138 cm-1 also was observed. We note that the 13CO satellite of the 2188 cm-1 band also contributed to the absorbance at 2138 cm-1. Evacuation of the sample at 323 K barely affected the bands (Figure 6, spectrum b). However, after evacuation at 373 K, the band at 2188 cm-1 had strongly decreased in intensity and the band at 2138 cm-1 was essentially absent (Figure 6, spectrum c). The band at 2188 cm-1 disappeared after evacuation at 423 K (Figure 6, spectrum d). The high thermal stability of the corresponding species is an unambiguous indication that they are Au+CO.9-12,27,29-31,35,37 Indeed, the only known bulk carbonyls of gold are those of Au+.45 The stability of these species is attributed to the simultaneous formation of σ- and π-bonds and the synergism between them. The spectra registered under CO equilibrium pressure (not shown) also give evidence of the presence of a band at 2207 cm-1, assigned to Au3+-CO species. These results thus demonstrate that some fraction of the gold had been oxidized to Au3+. Additional experiments (details are omitted for brevity) showed the following phenomena:

Mihaylov et al.

Figure 7. IR spectra of CO adsorbed at 85 K on the Au/NaY zeolite sample reoxidized with NO + O2 (see text for details). Equilibrium CO pressure of ca. 100 Pa (a) and evolution of the spectra during evacuation at 85 K and elevated temperatures (b-e). Zoomed spectra in the range 2200-2175 cm-1 are shown in the inset.

• Oxidation of gold also occurred when the coadsorption of NO + O2 was performed at ambient temperature. In this set of experiments, bands assigned to Au+-CO species (2188 cm-1) and various Auδ+-CO species in low concentration (2156 and 2138 cm-1) were seen after CO adsorption (Supporting Information, Figure S6, spectrum a). • Even NO itself at room temperature was sufficient for substantial oxidation of gold, as shown by the carbonyl band at 2188 cm-1 produced after CO adsorption on a sample that had been treated with NO at room temperature (Supporting Information, Figure S6, spectrum b). Low-Temperature CO Adsorption on Au/NaY Zeolite Oxidized with a Mixture of NO + O2. Cations in zeolites are generally characterized by low coordination numbers and can, as a result, adsorb two or more small molecules each. Thus, Cu+ ions in CuY zeolite are able to accommodate up to three CO molecules.46 To determine whether the Au+ ions formed by reoxidation of metallic gold on Au/NaY zeolite could form geminal di or tricarbonyls, we investigated the adsorption of CO at low temperatures. The experiments were carried out with an Au/NaY zeolite sample reoxidized by a NO + O2 mixture at 573 K and evacuated at 473 K. Exposure of the sample to CO, followed by cooling to 85 K, caused the appearance of a series of bands at the following wavenumbers: 2204, 2185, 2162, and 2141 cm-1 (Figure 7, spectrum a). The band at 2141 cm-1 disappeared very easily during evacuation, even at 100 K (Figure 7, spectra b,c). There is a consensus that this band arises from physically adsorbed CO.4 The band at 2162 cm-1 characterizes Na+(CO)2 species.42 During evacuation, they lose one CO ligand, and the resultant Na+-CO species42 were monitored by a band at 2173 cm-1 (Figure 7, spectra c,d). The band at 2185 cm-1 was gradually blue-shifted and ultimately converted into a band at 2191 cm-1 (Figure 7, inset). The somewhat higher frequency of this band, compared with those observed in the room-temperature experiments, is attributed to a temperature effect. The band at 2204 cm-1, assigned to the dicarbonyl Au3+(CO)2, was converted into a less intense band at 2208 cm-1, assigned to the Au3+-CO species. The behavior of the bands at 2191 and 2185 cm-1 could be explained by interconversion of mono and dicarbonyl species. As mentioned above, such a conversion can be expected with cations in zeolites. However, there are two phenomena that impeach such an assignment: • The band at 2185 cm-1 is expected with a ca. two times higher intensity than the band at 2191 cm-1 because of the

Redox Behavior of Au Species in Zeolite NaY

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Figure 8. IR spectra of 12CO and 13CO adsorbed at 85 K on the Au/ NaY zeolite sample reoxidized with NO + O2 (see text for details). Equilibrium CO pressure of ca. 100 Pa (a) and evolution of the spectra during evacuation at 85 K and elevated temperatures (b-f).

Figure 9. IR spectra of CO adsorbed at 85 K on H2O-precovered Au/ NaY zeolite sample reoxidized with NO + O2 (see text for details). Equilibrium CO pressure of ca. 100 Pa followed by progressive evacuation at 85 K (a-d).

doubling of the number of absorbing CO molecules. In fact, the σ-component of the Au+-CO bond is expected to decrease after adsorption of a second CO molecule. As a result of the σ-π synergism, one can also expect a decrease in the π-component of the bond. It is known that the extinction coefficient of adsorbed CO is enhanced with the strengthening of the π-bond.4 Hence, some reduction of the C-O extinction in the hypothetical dicarbonyls, as compared to the monocarbonyls, could be expected but should be insignificant. • It was established that the 2191 cm-1 band characterizes stable monocarbonyls. Therefore, the dicarbonyls should be characterized by two bands arising from the symmetric and antisymmetric modes. The absence of splitting of the CO stretching modes of dicarbonyls is reported only when the respective monocarbonyl species are unstable and the M-CO bond is essentially electrostatic.42 In this case, however, the CO extinction coefficient should remain almost unchanged after adsorption of the second CO molecule. Another possible explanation for the appearance of one band for dicarbonyl species is that the angle between the two CO ligands is 180o, which should be reflected in a vanishing intensity of the symmetric modes, but this is not likely for surface species. Low-Temperature 12CO-13CO Coadsorption on Au/NaY Zeolite Oxidized with a Mixture of NO + O2. To accept or reject unambiguously the hypothesis of formation of dicarbonyls, we investigated the coadsorption of 12CO and 13CO at a molar ratio of 1:1. If the species under consideration were dicarbonyls, bands assigned to mixed ligand species, namely, Au+(12CO)(13CO), should have been observed. The respective 12CO mode is expected at a higher wavenumber than in the case of 12CO adsorption only.47 The results of the 12CO-13CO coadsorption experiments are presented in Figure 8. The bands in the 12CO region are essentially the same as those observed when only 12CO was adsorbed. These results clearly demonstrate that no dicarbonyl species were formed on the Au+ ions even at low temperature. Another conclusion following from the results of the 12CO13CO coadsorption experiments is the lack of dipole-dipole coupling between the CO molecules adsorbed on the Au+ ions. It is known4 that the shift of the carbonyl bands with increasing coverage is caused by the sum of two effects: (i) a static shift or lateral interaction (leading to a red shift of the carbonyl band) and (ii) a dynamic shift or dipole-dipole interaction (leading to a blue shift of the carbonyl band). For the dipole-dipole interaction to occur, the adsorbed molecules must vibrate with the same intrinsic frequency. Therefore, dilution of 12CO with

13CO

should lead to a decrease of the dipole-dipole interaction between the 12CO molecules. Because the positions of the 12CO carbonyl bands were essentially the same in the 12CO and 12CO-13CO adsorption experiments, we conclude that no significant dipole-dipole interaction occurs between the adsorbed molecules, which suggests that the adsorption sites are isolated. Low-Temperature 12CO Adsorption on Au/NaY Zeolite Sample Partially Covered by Water. The spectra suggest that the red shift of the band at 2191 cm-1 could be associated with the formation of (di)carbonyl species of sodium. Indeed, lateral interactions of adsorbed CO molecules with CO adsorbed on other sites are known. It has been reported that the band assigned to Ca(CO)3 adducts (2188 cm-1), produced after low-temperature CO adsorption on a CaNaY zeolite, is gradually red-shifted up to 3 cm-1 in parallel with the formation of Na+(CO)x species.48 The strong lateral interactions of Au0-CO species with CO molecules adsorbed on the support are also well documented.20 To test the above-mentioned hypothesis, we investigated lowtemperature CO adsorption on a sample partially covered by water. We expected to block the Na+ sites by water, thus preventing formation of sodium carbonyls. Initially, CO and water vapor were coadsorbed at room temperature, and then the sample was cooled to 85 K. As a result, bands at 2187.5 and 2154 cm-1 appeared in the spectrum (Figure 9, spectrum a). The latter had pronounced shoulders at 2170 and 2132 cm-1. The strong band at 2154 cm-1 is assigned to CO bonded to OH groups of water.3,4 The shoulder at 2132 cm-1 characterizes physically adsorbed CO. The band at 2170 cm-1 is assigned to carbonyls formed with the participation of sodium ions. Because the intensity of the latter band is low, it is inferred that most Na+ sites were blocked by water molecules. The band at 2187.5 cm-1 is assigned to Au+-CO species, the frequency being somewhat red-shifted relative to that of the spectrum characterizing the “dry” sample. This comparison indicates that water molecules, adsorbed on the support, slightly modified the properties of the Au+ ions. Evacuation of the sample resulted in a fast disappearance of the band assigned to hydrogen-bonded CO (2154 cm-1) and the shoulders (Figure 9, spectra b-d). However, the Au+-CO band was hardly affected. The results demonstrate that the observed red shift of the Au+-CO band (from 2191 to 2185 cm-1) during the low-temperature experiments is indeed a consequence of lateral interactions with CO molecules adsorbed

2554 J. Phys. Chem. C, Vol. 111, No. 6, 2007 on the support. However, the reason why the effect of adsorbed CO is stronger than the effect of adsorbed water remains to be explained. XRD Analysis. To obtain information about the gold particle size, we investigated the samples by XRD directly after the treatment in the IR cell. No metal gold particles were detected with the fresh sample that was evacuated only at ambient temperature (Supporting Information, Figure S7, pattern a). This result suggests that no gold particles with diameters larger than 5 nm (the instrumental detection limit) were present in the sample. Furthermore, after reoxidation of the sample at room temperature by a mixture of NO + O2, metal particles were not detectable (Supporting Information, Figure S7, pattern b). Note that Au+ ions were detected on the sample thus treated by using CO as a probe molecule. Moreover, metal particles were not evident on a sample evacuated at 573 K, suggesting that even after this pretreatment the metal particles, if present at all, remained small (Supporting Information, Figure S7, pattern b). Consistent with this inference, we have observed X-ray absorption spectra showing that treatment of these samples in flowing helium at temperatures ranging from 373 to 573 K leads to the formation of gold clusters that are smaller than the R-cages of the zeolite.49 However, relatively large gold particles (average diameter 26 nm) were detected on the sample evacuated at 723 K followed by oxidation with a NO + O2 mixture at 573 K (Supporting Information, Figure S7, pattern d). The results demonstrate that this high-temperature treatment leads to agglomeration of gold, most probably outside the zeolite pores, and that the reoxidation even with a mixture of NO + O2 is not sufficient to destroy the relatively large gold particles. Discussion Evolution of Gold Species on Activated Samples. The results of the present investigation, compared with our earlier investigations with the “air-avoided” sample,39 show that the gold speciation in a Au/NaY zeolite prepared from the precursor Au3+(CH3)2(C5H7O2) is strongly affected by the presence of air and/or water. It has been shown that gold is in the form of Au3+ species on the as-prepared “air-avoided” sample.39 However, even a short exposure to air leads to a fast hydrolysis of the supported precursor with the evolution of methane. Hydrolysis with participation of the acac ligand also occurs, as evidenced by the appearance of acetylacetone. However, the products of the latter reaction remain adsorbed on the sample, thus probably facilitating reduction of gold. Indeed, testing of the sample exposed to air indicated in addition to the Au3+ species a large number of Auδ+ sites that represent oxidized sites on gold nanoclusters. In contrast, we have not detected bands at wavenumbers < 2170 cm-1 with an “air-avoided” sample,39 indicating that no reduction of gold had occurred in it. We also have concluded that a large amount of gold was not detected by CO as a probe molecule in our samples, suggesting that many gold sites might be blocked. On the one hand, we suggest that the products of hydrolysis of acac ligands probably blocked some sites. However, this cannot be the only explanation of the phenomenon, because a comparable amount of organic species was detected on the “air-avoided” sample when a significantly larger fraction of accessible gold has been detected.39 The logical explanation is that (most of) the sites must have been blocked by water molecules. Indeed, the spectra presented in Figure 1 clearly demonstrate that some water was present on the sample even after evacuation at 523 K.

Mihaylov et al. Water is a strong electrostatic Lewis base50 and can thus block electrostatic Lewis acid sites. Among gold species, the only electrostatic Lewis acids are Au3+ ions. Indeed, it has been demonstrated that Au+ ions and Au0 atoms form carbonyl complexes even in the presence of water11 (i.e., H2O is not able to prevent CO adsorption). This result is explained by the fact that CO is a more favorable ligand for Au+ and for Au0 than water, because it can act both as a σ-donor and π-acceptor. Small amounts of water enhance the CO oxidation activity of supported gold catalysts.13,51 On the basis of our results, we are not able to draw any definite conclusions regarding the possible influence of water; however, an important observation is that water does not inhibit the formation of Au+-CO and Auδ+-CO complexes. Therefore, we conclude that exposure of the sample to air led to blocking of a large fraction of Au3+ sites by water molecules, in addition to reduction of some of them. We already have noted that the blocking of Au3+ ions by water is the main reason why these cations are usually not detected with CO as a probe.11 Evacuation at elevated temperatures, intended to liberate these sites, leads to autoreduction of the Au3+ species. Indeed, Au2O3 is thermodynamically stable up to ca. 400 K.19 In line with this point, we observed that evacuation of the sample at elevated temperatures (373-423 K) led to reduction initially of the Au3+ sites in cation positions in the zeolite and then of the other Au3+ ions. We cannot draw definite conclusions about whether autoreduction takes place in our case, because of the presence of some organic ad-species. However, it is well documented that simple heating of gold catalysts, even in the presence of oxygen, leads to formation of metallic particles.9,10,13,15,17,18,20,21,22,32,52 On the basis of our results, we infer that Au+ ions on metal clusters (Auδ+ sites) are autoreduced at temperatures of approximately 473 K. Evidently, the surface organic residues play a role in the formation of negatively charged gold species. Indeed gold is the most electronegative metal and forms Au- ions.51 We emphasize that negatively charged carbonyls are known for other metals, such as cobalt and platinum (illustrated by the so-called Chini-clusters).4 Oxidation of Metallic Gold. There is no consensus regarding the nature of the catalytically active gold sites in the CO oxidation reaction. Many authors have inferred that the reaction proceeds on metallic gold surfaces with sound arguments supporting this view (e.g., ref 23). On the other hand, the participation of cationic gold species in the reaction is an interpretation that is gaining in acceptance and is also supported by many experimental observations (e.g., ref 38). In any case, the characterization of cationic sites and the determination of conditions for easy reoxidation of gold are important for understanding catalysis by supported gold. The results of our experiments show that O2 easily oxidizes some surface metal gold atoms but fails to produce isolated cationic sites. It seems that the interaction of Au0/NaY zeolite with O2 at 298-573 K leads to oxidation of the unsaturated (or “defect”) sites that also act as CO adsorption sites. This view is supported by the comparable intensities of the Au0-CO and Auδ+-CO bands observed before and after interaction of the sample with O2, respectively. We believe that the main achievement in this work is the observation that metallic gold can be easily reoxidized to give isolated cationic sites by a mixture of NO + O2 and even by NO itself. It is well known that the ability of metallic gold to be oxidized depends on the particle size: small clusters are more readily oxidized than larger particles. Because it was already

Redox Behavior of Au Species in Zeolite NaY established that the gold precursor is adsorbed in the supercages of the Y zeolite, one can expect that the size of the metal clusters should be restricted by the zeolite pores. This restriction seems to be one of the reasons for the easy reoxidation of gold. Indeed, the XRD results show that relatively large gold particles (26 nm in diameter) are highly inert and resistant to reoxidation. It is of interest to compare the results obtained in this work with results observed with other gold-containing samples. Some of us reported that CO adsorption on a fresh Au3+/TiO2 sample did not lead to formation of gold carbonyl species because of the coordinative saturation of the Au3+ ions.11 However, calcination of Au/TiO2 resulted in autoreduction of gold, and probing of the sample with CO indicated the creation of Auδ+ sites (carbonyl band at ca. 2140 cm-1).10 These sites were reduced to Au0 sites after evacuation of the sample at 673 K (metal particles with an average diameter of about 4 nm and the corresponding carbonyl band at 2106 cm-1). Reoxidation with a mixture of NO + O2 even at 673 K failed to produce isolated Au+ sites, but Auδ+ sites (carbonyl band at 2154 cm-1) were created. These results are generally consistent with the observations in this investigation and support the proposed assignments. It also was established that the Au+ species in Au/NaY zeolite, formed as a result of oxidation of gold by NOx, are not able to form polycarbonyls. This result suggests that the Au+ species were not located in SII positions. We suggest that these cations were located in supercages incorporating nitrate counterions (formed from NO) and coordinated to some framework oxygen ions. Investigations with a Au/HY zeolite sample (in which the proton can be easily exchanged by the produced Au+ ions) would provide a test of the suggestion. Regarding the possible effects of nitrates on the activity of gold catalysts, it recently has been reported that small amounts of nitrate increase the activity of Au/TiO2 catalysts for CO oxidation.53 In light of our observations, this increase could be explained by nitrate-induced enhancement of the reoxidation of gold. This suggestion is consistent with the idea of participation of cationic gold sites in the process. Conclusions Exposure of a Au3+/NaY zeolite sample (synthesized from Au3+(CH3)2(C5H7O2)) to air leads to hydrolysis of the precursor and reduction by evolved organics of part of the Au3+ species to give metallic gold. The surfaces of the metal clusters remain oxidized, however, as indicated by the presence of Auδ+ sites representing gold cations on the metal clusters. The Auδ+ sites are autoreduced to give Au0 at 473 K. This process is reversible, and interaction with O2 at elevated temperatures leads to conversion of the zerovalent sites back to Auδ+. The higher the temperature, the more the zerovalent gold sites are reoxidized and the higher the frequency of the bands characterizing the Auδ+-CO species formed after subsequent CO adsorption. Reoxidation of the gold with a mixture of NO + O2 is more efficient and results in the appearance of isolated Au+ ions. The latter are suggested to be bound to nitrate ions and to have only one coordinative vacancy each. Acknowledgment. This work was supported by the Alexander von Humboldt Foundation (AvH fellowship to MM) and a NATO Collaborative Linkage Grant (PST.CLG 980289). The work at the University of California was supported by the U.S. Department of Energy, Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences, Contract FG02-87ER13790. We thank Dr. J. Sauer for his assistance with the XRD measurements.

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