SiO2 - The

ARTICLE pubs.acs.org/JPCC

Well-Defined Negatively Charged Gold Carbonyls on Au/SiO2 K. Chakarova,† M. Mihaylov,† S. Ivanova,‡ M.A. Centeno,‡ and K. Hadjiivanov*,† † ‡

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Departamento de Química Inorganica e Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Avda. Americo Vespucio, 49, 41092 Sevilla, Spain

bS Supporting Information ABSTRACT: A Au/SiO2 sample was prepared by ammonia-assisted grafting using HAuCl4 as a gold precursor. Gold on the sample evacuated at 673 K is essentially in metallic form: adsorption of CO at 100 K results in formation of Au0CO species (IR band at 2122 cm1 shifting to 2103 cm1 at high coverage). Coadsorption of CO and O2 even at ambient temperature leads to creation of Auδ+ sites and oxidation of CO. On the contrary, increase of the contact time between CO and the sample leads to a gradual reduction of Au0 to Auδ species. The process is slightly favored by the presence of water and strongly enhanced in the presence of hydrogen. Back oxidation of Auδ to Au0 and to Auδ+ occurs in the presence of oxygen. The Auδ sites strongly adsorb CO and form different interconverting carbonyls observed in the 20802050 cm1 region. On the basis of adsorption of CO13CO and CO13C18O isotopic mixtures, it is concluded that all AuδCO species are linear, and probably ordered structures are formed. Intensity transfer phenomena are nicely monitored during adsorption of CO isotopic mixtures. The eventual role of negatively charged gold in catalysis is discussed.

1. INTRODUCTION During the last two decades, supported gold catalysts have been a very topical subject of investigations.1 The enormous interest was provoked by the discovery that, when highly dispersed, gold is a very active catalyst in the low-temperature CO oxidation.2 The interest has been periodically fomented by reports showing activity of gold in other important catalytic processes, e.g., preferential oxidation (PROX);3 watergas shift reaction;4 reduction of nitrogen oxides with propene, carbon monoxide, or hydrogen;5 epoxidation of propene;6 synthesis of hydrogen peroxide,7 of vinyl chloride,8 and of vinyl acetate;9 selective oxidation of alcohols;10 oxidation of volatile organic compounds;11 and carboncarbon bond coupling reactions.12 FTIR investigations of CO adsorption on gold catalysts are important not only because CO is a reactant in many goldcatalyzed reactions but also because it is practically the only IR probe molecule giving valuable information on the state of the supported gold species. Recently, we have summarized the achievements of the use of CO as an IR probe for this purpose.13 Briefly, CO can form carbonyls with metallic, cationic, and anionic gold species. Surface carbonyls of Au3+ are easily decomposed and observed at frequencies above ca. 2170 cm1. Au+CO species absorb in the overlapping region 21902150 cm1 but are characterized by a significantly higher stability because of the synergism between the σ- and π-bonds between the cation and CO. Indeed, bulk carbonyls are known only with Au+. CO binds to the surface of metal gold particles relatively weakly and only to particular sites (e.g., step sites). The CO stretching frequency in this case is typically in the 21302090 cm1 region. The socalled Auδ+ sites form carbonyls that are characterized by frequencies and stabilities intermediate between those typical of r 2011 American Chemical Society

Au+CO and Au0CO species. It is proposed that the Auδ+ sites represent Au+ ions on the surface of gold particles: as a result of charge transfer from the metal they are with reduced positive charge.14 AuδCO species are more stable than Au0CO and are characterized by carbonyl stretching bands at low frequencies (20851950 cm1). According to some authors,15 gold nanoclusters can acquire a negative charge as a result of interaction with surface defects, e.g., oxygen vacancies (F centers) on reduced supports. However, by analogy with the Auδ+ sites, one can suppose that the Auδ species represent Au ions located on metal particles.13 A problem when studying the gold catalyst by IR spectroscopy of adsorbed CO is that, in many cases, it is difficult to distinguish the carbonyls of the support from these of gold. In this sense, for model studies, gold supported on silica is very appropriate because the only carbonyl bands formed with the support are those of SiOH 3 3 3 CO species (ca. 2155 cm1) and physically adsorbed CO (ca. 2139 cm1),16 and these bands are observed at low temperatures only and are easily removed by evacuation. There are many reports17 indicating that, when supported on silica, gold is not catalytically active. This could restrict the interest to CO adsorption on Au/SiO2. Different hypotheses have been proposed to explain the low activity of Au/SiO2, mainly including the reducibility of the support.18 Silica is a nonreducible and inherently “inert” support which can not supply reactive oxygen for CO oxidation. However, according to other authors,1,19 the low activity is simply due to the fact that usually Received: July 24, 2011 Revised: September 18, 2011 Published: September 19, 2011 21273

dx.doi.org/10.1021/jp2070562 | J. Phys. Chem. C 2011, 115, 21273–21282

The Journal of Physical Chemistry C

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gold is poorly dispersed on silica. Recently, preparation techniques leading to high metal dispersion of gold on silica have been proposed, and a high CO oxidation activity of such samples was reported.20 In addition, small gold particles on silica showed surprisingly high stability against annealing in oxygen-containing atmosphere up to 773 K which was attributed to a strong bond between gold and defects on the silica surface.21 In this way, for example, the Au/SiO2 catalysts can be easily regenerated. In this paper, we report the results of a careful study of CO adsorption on a Au/SiO2 sample which was characterized by a high gold dispersion and was active in CO oxidation at relatively low temperatures. We have shown that in the presence of CO metallic gold can be reduced to Au. The latter species, situated on the metal gold particles, form a variety of interconverting carbonyl species.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Au/SiO2 was prepared using an earlier described procedure for gold deposition.22 Briefly, SiO2 (fumed silica, 204 m2 g1) was suspended in diluted aqua solution of HAuCl4 and agitated at 348 K for 1 h. Then, the solution was cooled to room temperature, and NH4OH was added under stirring. After that, the precipitate was filtered, washed thoroughly with water, dried overnight at rt, and finally calcined at 673 K for 2 h. The gold content in the sample was 1.84 wt %. 2.2. Techniques. For the chemical analysis, the Au/SiO2 sample was dissolved in a mixture of HNO3 and HCl acids and then analyzed by Inductively Coupled PlasmaAtomic Emission Spectrometry (ICPAES) using a Jobin Yvon apparatus (JY Ultima 2, France) operating at 40.68 MHz. Powder XRD patterns were registered at room temperature with a Bruker D8 Advance diffractometer using Cu Kα radiation and a SolX detector. Analysis of the line broadening was performed by using TOPAS 4.2 software. The TEM photographs were taken with a JEOL 2100 microscope operating at 200 kV. The specimens were prepared by grinding the samples in agate mortar and dispersing in methanol by ultrasonic treatment for 6 min. A droplet of suspension was dispersed on holey carbon films on Cu grids. FTIR spectra were registered with a Nicolet Avatar 360 spectrometer at a spectral resolution of 2 cm1 and accumulating up to 128 scans. Self-supporting wafers were prepared from the sample powder and treated directly in a purpose-made IR cell. The latter was connected to a vacuum-adsorption apparatus with a residual pressure below 103 Pa. Prior to the IR experiments, the sample was activated by 10 min heating at 573 K in 30 kPa O2 followed by 2 h evacuation at 673 K. This activation procedure was similar to the activation before the catalytic test. Carbon monoxide (99.997%) was supplied by Linde. Labeled carbon monoxide, 13C18O (13C isotopic purity of 99% and 18O isotopic purity of 95%), was provided by Cambridge Isotope Laboratories, Inc. Before adsorption, CO and the CO isotopic mixtures were passed through a liquid nitrogen trap.

3. RESULTS AND DISCUSSION 3.1. Basic Characterization of the Sample. The details on the catalytic activity of the material and the characterization by different techniques will be reported separately. Briefly, the sample demonstrated some activity in CO oxidation even at ambient temperature (CO conversion of about 10%). The XRD

Figure 1. FTIR spectra of CO adsorbed on activated AuSiO2. Equilibrium CO pressure of 200 Pa at 100 K (a) and evolution of the spectra under dynamic vacuum at 100 K (bj) and at increasing temperatures (ko) up to 298 K. The spectra are background and CO gas-phase corrected. Some spectra are shifted along the y-axis for better visualization.

patterns of the sample before and after the IR experiments are shown in Figure S1 from the Supporting Information. It is seen that both diffractograms are very similar. The mean diameter of gold particles of the fresh sample, calculated on the basis of the fwhm of the (111), (200), (220), and (311) peaks, was 5.2 nm. It remained practically the same after the IR experiments (5.0 nm). These results indicate that no substantial changes in the gold particle size have occurred. The same conclusions were made analyzing the TEM pictures where particles of about 5 nm were detected with the two samples. 3.2. Adsorption of CO on Activated Sample. Initially CO adsorption was studied at low temperature to suppress eventual reactive adsorption. Introduction of CO (200 Pa equilibrium pressure) to the activated sample at 100 K leads to the appearance of three carbonyl bands, at 2157, 2137, and 2103 cm1 (Figure 1, spectrum a). The bands at 2157 and 2137 cm1 are due to CO attached to silanol groups and physically adsorbed CO, respectively,16 and easily disappear during evacuation (Figure 1, spectra b, c). The band at 2103 cm1 is assigned to Au0CO species.13,14,16,2331 This band decreases in intensity during evacuation at 100 K (Figure 1, spectra bj) and at increasing temperatures (Figure 1, spectra ko) and is simultaneously gradually shifted to 2122 cm1 at very low coverage (Figure 1, spectrum o). Similar behavior, although not typical of CO adsorbed on metals, is well documented for gold catalysts14,2330 and is due to the restricted back π-donation from the metal to CO.23 Thus, the results evidence that the activation procedure applied has led to reduction of the deposited gold to metal. The next set of experiments was aimed at IR monitoring the activity of the sample in CO oxidation and the eventual change in the gold oxidation state. Introduction of CO (1 kPa equilibrium pressure) to the sample at ambient temperature led to the appearance of a Au0CO band at 2110 cm1 (Figure 2, spectrum a). 21274

dx.doi.org/10.1021/jp2070562 |J. Phys. Chem. C 2011, 115, 21273–21282


Figure 2. FTIR spectra of CO adsorbed on activated AuSiO2. Equilibrium CO pressure of 1 kPa at 298 K (a) and after introduction of O2 (1 kPa initial equilibrium pressure) to the system and keeping the sample in this atmosphere for 10 min at 298 (b), 373 (c), 473 (d), 573 (e), and 673 K (f). The spectra are background and gas-phase corrected.

The band maximum is somewhat shifted as compared to the lowtemperature experiments because of the lower CO coverage at ambient temperature. Then, O2 (1 kPa) was added to the system. As a result, two shoulders of the main band, at 2129 and 2119 cm1, were immediately formed (see the inset in Figure 2). Similar behavior was reported with other supported gold catalysts, e.g., Au/TiO214a and Au/La2O3,26 and attributed to formation of Auδ+CO carbonyls. As already noted, the Auδ+ species are proposed to be Au+ cations on the surface of metal particles. Due to electron transfer from the bulk of the metal particle, a decrease of their positive charge occurs, and the respective carbonyls are observed at frequencies intermediate between those typical of Au0CO and Au+CO complexes formed with isolated cations.13 The heterogeneity of the Auδ+CO carbonyls could be explained by different charge balancing anions, e.g., O2 and O2. Increase of the interaction temperature up to 573 K leads to an additional increase in the intensity of the Auδ+CO bands (Figure 2, spectra be). Simultaneously, CO2 is produced. It was well detected after interaction at 373 K by a band at 2341 cm1 due to adsorbed CO2 (Figure S2 from the Supporting Information, spectrum a). This band increased in intensity after interaction at 473 and 573 K (Figure S2 from the Supporting Information, spectra b and c, respectively). At the same time, the CO concentration in the gas phase decreases. This contributes to the decrease in intensity of the Au0CO band. After interaction at 673 K, all CO has been oxidized, and no surface carbonyls are detected in the spectrum (Figure 2, spectrum f). An intense band in the 24002250 cm1 region (due to adsorbed and gaseous CO2) was registered (Figure S2 from the Supporting Information, spectrum d). Subsequent evacuation of the sample at 673 K followed by CO adsorption produced a carbonyl band at 2114 cm1 having a weak high-frequency shoulder around 2127 cm1 (Figure 3, spectrum a). The results indicate that, once formed, the Auδ+

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Figure 3. FTIR spectra of CO (1 kPa equilibrium pressure) adsorbed at 298 K on reactivated Au/SiO2 (a) and after reduction of the sample in this CO atmosphere for 1 h at 673 K (b). Increase of the CO equilibrium pressure to 2 kPa (c), evacuation and introduction again of CO (2 kPa) and after 1 h (d) and 20 h contact with CO (e). The spectra are background and CO gas-phase corrected. Some spectra are shifted along the y-axis for better visualization.

sites are not readily converted to metal by evacuation at 673 K. Moreover, it seems that the stable cationic species have been produced after the interaction of the sample with the CO + O2 mixture because gold was essentially in metallic form on the sample treated first in oxygen and then in vacuum. In an attempt to remove the oxidized gold sites, we have reduced the sample with CO (1 kPa) at 673 K. Indeed, as a result, a symmetric carbonyl band at 2111 cm1, due to Au0CO species, was formed (Figure 3, spectrum b). In addition, a very weak feature centered at 2055 cm1 was also seen. After several CO adsorption/desorption experiments (initial CO equilibrium pressure of 2 kPa), a weak band around 2075 cm1 was formed (Figure 3, spectrum c) and increased in intensity with repeating the experiments (Figure 3, spectrum d). Allowing the sample to stay in the CO atmosphere for 20 h resulted in a strong increase in intensity of the band (observed at 2079 cm1) and almost full disappearance of the Au0CO band (Figure 3, spectrum e). The bands at 2055 and 2075 cm1 could be assigned to negatively charged gold carbonyls.13,2731 Due to the enhanced back π-donation, the CO extinction coefficient is expected to be higher as compared to the Au0CO species.13,16 This explains the high intensity of the band at 2075 cm1. It is of interest to follow the formation of negatively charged gold species at different conditions. The interaction of CO (1 kPa equilibrium pressure) with the activated sample is illustrated in the upper part of Figure 4. Even at room temperature, a weak band of negatively charged gold carbonyls appeared and rose in intensity with the contact time (Figure 4, spectra bd). Note, however, that in this case the maximum is located at 2057 cm1. Simultaneously, the band at 2116 cm1 also developed and gradually shifted to 2113 cm1 at the expense of a high-frequency 21275



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Figure 4. FTIR spectra of CO (1 kPa equilibrium pressure) adsorbed on AuSiO2 at 298 K (a, a0 ) and evolution of the spectra in the CO atmosphere after 10 (b, b0 ), 30 (c, c0 ), and 60 min (d, d0 ), and after 10 min heating the sample in the presence of CO at 373 (e, e0 ), 473 (f, f0 ), 573 (g, g0 ), and 673 K (h, h0 ). Panel A: reactivated sample. Panel B: reactivated sample pretreated with H2O (200 Pa, 10 min, followed by evacuation at 298 K). The spectra are background and CO gas-phase corrected.

Figure 5. FTIR spectra of CO (1 kPa equilibrium pressure) adsorbed on the reactivated AuSiO2 sample at 298 K (a), subsequent introduction of H2 (500 Pa initial pressure) (b), and evolution of the spectra after allowing the sample to stay in the CO + H2 mixture for 10 (c), 30 (d), and 60 min (e) at 298 K and after 10 min heating at 373 (f), 473 (g), 573 (h), and 673 K (i) in the same atmosphere. The spectra are background and CO gas-phase corrected.

component associated with positively charged gold. These results indicate reduction of gold species. Heating the sample in the CO atmosphere for 10 min at 373 K resulted in a slight increase in intensity of the band at 2057 cm1 (Figure 4, spectrum e). At the same time, the Au0CO band (2113 cm1) increased in intensity, while the higher-frequency shoulder of the band (due to Auδ+CO species) decreased. After interaction of the sample with CO at 473 K, the spectrum changed dramatically (Figure 4, spectrum f). The Au0CO band (settled at 2116 cm1) strongly decreased in intensity and almost lost its higher-frequency component (Auδ+CO). At the same time, an intense band at 2068 cm1 with a shoulder at 2033 cm1 was detected. These changes were even more pronounced at higher interaction temperatures (Figure 4, spectra g, h). After interaction at 673 K, the residual Au0CO band was detected at 2119 cm1 and the band due to negatively charged carbonyls at 2074 cm1. We also note that, once produced, the negatively charged gold species are more readily formed which indicates a memory effect. A weak negative band in the OH region at 3745 cm1 was also detected after CO interaction with the sample at high temperatures (spectra not shown) which suggested participation of the OH groups in the reduction process. To check for the eventual role of water we have repeated the same experiment with a “wet” surface. For that purpose, before CO adsorption, water was introduced to the sample and evacuated at ambient temperature. The existence of some water residual to evacuation was monitored by a band at 1630 cm1. The results on CO adsorption on the sample thus treated are presented on the lower part of Figure 4. It is evident that residual water has a slight promoting effect on the formation of negatively charged gold carbonyls even at ambient temperature and 373 K. The effect is well pronounced

after heating the sample in CO at 473 K (compare spectra f and f0 ). However, it fades after an additional increase of the interaction temperature. Note also that the Auδ+ sites are more easily reduced on the wet surface. A possible explanation is that water reacted with CO thus producing hydrogen which facilitates gold reduction. To check for the effect of hydrogen on the formation of negatively charged gold, we performed analogous experiments with a reactivated sample, but after the adsorption of CO, hydrogen was added to the system. The spectra recorded are presented in Figure 5 and clearly show a remarkable positive effect of hydrogen in the appearance of negatively charged gold carbonyls. Moreover, in these experiments the band assigned to Au0CO species (detected at 2128 cm1) acquired a negligible intensity, while the band of negatively charged gold was detected at 2079 cm1. Analysis of the spectra in the OH region (see Figure S3 from the Supporting Information, spectrum a) indicates that initially some silanol groups are consumed. With time, water is produced, as suggested by the broad band centered at 3490 cm1 (Figure S3 from the Supporting Information, spectra b, c). It was also of interest to obtain information on the interaction between the negatively charged gold carbonyls with oxygen. For that purpose, CO was initially adsorbed on a sample reduced by a CO + H2 mixture at 673 K. An intense band at 2077 cm1 and a weak one at 2119 cm1 were detected (Figure 6, spectrum a). Then small doses of oxygen were successively added into the system. As a result, the band at 2077 cm1 was gradually eroded, and the band at 2119 cm1 rose in intensity with a simultaneous shift of its maximum to 2112 cm1 (Figure 6, spectra bo). During the reaction, a shoulder at 2126 cm1 (indicating the presence of positively charged gold sites) also developed. The band 21276



Figure 6. FTIR spectra of CO (200 Pa equilibrium pressure) adsorbed on a sample reduced in a CO + H2 mixture (a) and evolution of the spectra after successive introduction of small doses of O2 (bo). The spectra are background and CO gas-phase corrected.

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Figure 8. FTIR spectra of CO adsorbed on CO-reduced Au/SiO2. Adsorption of CO (200 Pa) at 100 K followed by evacuation at 100 K (ag) and at increasing temperatures (hs) up to 298 K (t).

The next experiments were designed to check the stability of the negatively charged gold carbonyls. For that purpose the sample was reduced with CO at 473 K and the spectrum under 1 kPa equilibrium pressure initially recorded. It contained a Au0CO band at 2117 cm1 and a more intense and tailed one of anionic gold carbonyls at 2075 cm1 (Figure 7, spectrum a). Then the CO coverage was gradually decreased by progressive evacuation. As a result, the band at 2117 cm1 quickly decreased in intensity and practically disappeared. The band at 2075 cm1 also decreased in intensity, but more slowly. Simultaneously, its maximum shifted to lower frequencies. At the same time, a new band, at 2057 cm1, appeared and developed during the coverage decrease. The rise of this band was not connected with the band at 2117 cm1 but followed the changes of the band at 2075 cm1. Thus, the results indicate conversion between one type of species (B, characterized by a band at ca. 2075 cm1) to another type (A, displaying a band at 2057 cm1) BfA

Figure 7. FTIR spectra of CO (1 kPa equilibrium pressure) adsorbed on CO-reduced AuSiO2 (a) and evolution of the spectra under dynamic vacuum (bo). The spectra are background and CO gas-phase corrected.

at 2077 cm1 shifted to lower frequencies with the intensity decrease and was finally settled at 2058 cm1. Analysis of the gasphase CO spectrum indicated that the CO partial pressure remained practically the same. The results evidence a fast oxidation of negatively charged gold by molecular oxygen. As a result, Auδ+, Au0, and Auδ sites can coexist on the sample. Note, however, that even in excess of oxygen some Auδ sites remained on the sample.

ð1Þ

Evacuation at 373 K led to a strong decrease in intensity of the band at 2057 cm1, and the maximum was settled at 2040 cm1 (spectrum not shown). To obtain information on the carbonyl species formed at even higher coverage, we studied low-temperature CO adsorption. The spectrum registered after adsorption of CO at 100 K followed by a brief evacuation (to remove most of the weak adsorption forms) contains bands at 2157 (CO attached to silanol groups) and 2133 cm1 (physically adsorbed CO) and two bands of roughly equal intensity at 2117 and 2083 cm1 (Figure 8, spectrum a). The latter band has a pronounced shoulder at 2071 cm1. Subsequent evacuation leads to a decrease in intensity of the band at 2117 cm1 and a shift of its maximum to 2123 cm1 (Figure 8, spectra bg). Simultaneously, the band at 2083 cm1 increases in intensity and is shifted to 2079 cm1. 21277



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Figure 9. FTIR spectra of a CO13CO isotopic mixture (molar ratio of ca. 4:5) adsorbed at 100 K on reduced Au/SiO2. Equilibrium pressure of 1 kPa (a) and evolution of the spectra during evacuation at 100 K (bl). The spectra are background and gas-phase corrected.

Further evacuation leads to a picture similar to that described for the experiments at ambient temperature. The band at 2123 cm1 quickly disappears, and the band at 2079 cm1 decreases in intensity with a simultaneous red shift (Figure 8, spectra gt). Finally, a band at 2057 cm1 is recorded (Figure 8, spectrum t). However, in these experiments an intermediate band at 2068 cm1 is well discernible (see Figure 8, spectrum m). It seems that the 2068 cm1-intermediate species are not stable and are “frozen” during the low-temperature experiments. Therefore, the above proposed conversion (eq 1) is more complicated, and we can propose an improved scheme for coverage decrease at low temperature B f A0 f A

ð2Þ

where A0 denotes the 2068 cm1 species. 3.3. Adsorption of CO Isotopic Mixtures. To obtain more information on the nature of the carbonyls formed with negatively charged gold species, we have studied coadsorption of two isotopic mixtures: CO13CO and CO13C18O. The former mixture is widely used by many authors16 and ensures a separation of the individual carbonyl bands by ca. 50 cm1. There are only few works reporting the use of CO13C18O mixtures.32 However, in this case the separation of the bands is much higher, about 100 cm1. The spectra registered after adsorption of a CO13CO isotopic mixture (molar ratio of ca. 4:5) at 100 K are shown in Figure 9. Under 1 kPa total equilibrium pressure, five intense bands are well discernible: SiOHCO at 2157 cm1, SiOH 13 CO at 2109 cm1, physically adsorbed CO at 2137 cm1 (shoulder at 2134 cm1), and 13CO at 2090 cm1 (shoulder at 2087 cm1),16 as well as a band at 2074 cm1 already assigned to CO adsorbed on negatively charged gold sites (Figure 9, spectrum a). The intensity ratio between the CO and 13CO bands for the former four peaks is consistent with the isotopic ratio in the gas mixture. However, surprisingly, no band due to 13CO

adsorbed on negatively charged gold (expected around 2028 cm1) was clearly discernible even with the second derivatives of the spectra (Figure S4 from the Supporting Information). Only a weak feature centered on ca. 2010 cm1 was detected. This is explained by strong intensity transfer phenomena (see below). Evacuation at 100 K leads to a gradual decrease in intensity of the forms due to physically adsorbed CO (13CO) and CO (13CO) attached to silanol groups (Figure 9, spectra bi). The gold carbonyl bands are hardly affected. They are changed at higher evacuation temperatures. However, to avoid any effect of the temperature background changes on the spectra (in fact more affecting the 13C18O region), we followed the CO coverage changes at ambient temperature. Adsorption of the CO13CO isotopic mixture (1 kPa total pressure) at 298 K on a reduced Au/SiO2 sample leads to the appearance of one band at 2068 cm1 having a shoulder at 2016 cm1 (Figure 10, spectrum a). Subsequent evacuation leads to a gradual decrease in intensity of the band at 2068 cm1 and development of new bands at 2051 and 2002 cm1 (Figure 10, spectra ai). A weak band at 2016 cm1 initially develops and then disappears (see the inset in Figure 10). The final intensity ratio between the bands at 2051 and 2002 cm1 is ca. 2:1. In all cases, the band at 2016 cm1 is much less intense than the band at 2068 cm1. Analogous experiments were performed with a CO13C18O (1: 1) isotopic mixture, and selected spectra are shown in Figure 11. The main principal differences between the spectra recorded with a CO13C16O mixture are: • At low coverages the intensities of the CO (2046 cm1) and 13 18 C O (1952 cm1) bands are almost the same (Figure 11, spectrum m). • A band at 1963 cm1 is more pronounced than the band at 2016 cm1 registered in the presence of 13CO. This band is detected even at full coverage. • The shift of the maxima of the CO bands to lower frequencies is more important as compared to the case of coadsorption of CO and 13CO.

4. DISCUSSION 4.1. Formation and Properties of Negatively Charged Gold Species. To the best of our knowledge, the first well-

documented IR observation of negatively charged gold carbonyls with supported gold was reported by Bocuzzi et al.27 The authors detected, after CO adsorption on reduced Au/TiO2 and Au/ Fe2O3 catalysts, bands in the 20551990 cm1 region and assigned them to negatively charged species. Thereafter, AuδCO carbonyls were reported with different samples mainly after reduction.2831 Very recently, Bianchi et al.33 reported carbonyl bands around 2070 cm1 that were formed after contact of the Au/Al2O3 catalyst with CO. The bands developed with time and in the presence of relatively high CO equilibrium pressures at the expense of the Au0CO complexes observed immediately after CO adsorption. The new species were characterized by a higher heat of adsorption as compared to the normal Au0CO species. The authors assigned the bands around 2070 cm1 to a second type of Au0CO carbonyls produced as a result of surface reconstruction. Although reconstruction indeed seems to occur, we do not agree with the assignment of the band to Au0CO complexes. Although bands at such low frequencies can be observed with metallic gold, they are very unstable. For instance, pressures of 10100 Torr are necessary to detect gold carbonyls 21278



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Figure 10. FTIR spectra of the CO13CO isotopic mixture (molar ratio of ca. 4:5) adsorbed at 298 K on reduced Au/SiO2: 1 kPa total equilibrium pressure (a) and evolution of the spectra during evacuation (bm). The spectra are background and gas-phase corrected.

Figure 11. FTIR spectra of the CO13C18O isotopic mixture (molar ratio of ca. 1:1) adsorbed at 298 K on reduced Au/SiO2: 1 kPa total equilibrium pressure (a) and evolution of the spectra during evacuation (bm). The spectra are background and gas-phase corrected.

on the Au(111) face.34 On the contrary, due to enhanced back π-donation (which is very restricted for CO adsorbed on metallic gold23), the anionic gold carbonyls are stable and appear at low stretching frequencies. It is proposed15,27,28 that negatively charged gold clusters are produced as a result of electron transfer from the reduced support to very small gold clusters. However, our experiments impeach such a point of view. At first, silica is an inert and nonreducible support and therefore is not able to transmit electrons. Second, we observed that practically all of the gold sites able to absorb CO can be converted to negatively charged sites. Therefore, we propose that the negatively charged gold carbonyls are formed with Au anions situated on the surface of the metal particles. Most probably, H+ cations act as charge balancers. We have observed that the Auδ sites are slowly formed in the presence of CO only, but the reaction rate strongly increased in the presence of hydrogen. Therefore, one could expect a high concentration of Auδ sites in catalytic reactions occurring in reductive media (e.g., WGS, PROX). Indeed, Daly et al.35 have detected carbonyl bands in the 20701950 cm1 region in operando studies of the watergas shift reaction over a Au/CeZrO4 catalyst. The authors concluded that the corresponding species were responsible for the catalyst deactivation. One can speculate whether they play the same role in the PROX reaction or are catalytically active sites. We have also observed that, although easily interacting with O2, some Auδ sites remained on the surface in the presence of oxygen. Thus, the possibility for these sites to play a role in reactions occurring in oxidative media cannot be excluded. In particular, the Auδ species could be important for oxygen activation because it could be able to transfer electrons to dioxygen. 4.2. Assignment of the IR Bands. As already discussed, the IR carbonyl bands we detected below 2090 cm1 are generally associated with negatively charged gold sites. However, different

carbonyl bands were detected which need a more precise assignment. Some authors have reported even lower frequencies for AuδCO species, down to 1950 cm1.27,35 This can be rationalized by the above-made assumption that Auδ sites are located on the surface of the gold particles and transfer electrons to the bulk. Thus, the frequency of the respective carbonyls should depend on the density of the Auδ sites and the dimension of the metal gold particles. Our spectra clearly show conversion between different species. Boccucci et al.27 have detected similar conversion with reduced TiO2- and Fe2O3-supported gold catalysts. However, the maxima they found were at lower wavenumbers (2055 and 1990 cm1). The authors proposed conversion between bridging and linear forms of adsorbed CO. We do not support similar assignment because of several reasons. First, the wavenumber we observed for the lower-frequency band (2054 cm1) is too high for bridging species. Second, the extinction coefficient of bridging carbonyls, due to rehybridization of CO, should be much smaller than that of linear carbonyls, which was not observed. The spectra presented in Figure 7 (CO adsorbed at 298 K) show conversion of the band at 2075 cm1 into a band at 2057 cm1. These spectra are very similar to published spectra describing conversion of dicarbonyl to monocarbonyl species.36 In addition, the spectra recorded at low temperature (Figure 8) could suggest even formation of tricarbonyls (band at 2117 cm1). In fact, dicarbonyls should display two CO modes, symmetric and antisymmetric. However, due to metal-adsorbate selection rules, the antisymmetric modes could not be IR active.37 Thus, dicarbonyls can be characterized by one IR mode only. In fact, Yang et al.38 have proposed a band at 2060 cm1 to correspond to the symmetric modes of gold dicarbonyl species. Note, however, that selection rules could not be valid for very small metal particles. 21279


The Journal of Physical Chemistry C There are several important arguments against the formation of dicarbonilic (and polycarbonilic) structures. At first, adsorption of isotopic mixtures should lead to formation of mixedligand species (i.e., Auδ(CO)(13CO) or Auδ(CO)(13C18O)). These should display a single 12C16O mode between the symmetric and antisymmetric modes of the dicarbonyls. In addition, the symmetric modes of dicarbonyls with two 12C16O ligands should be observed. Looking to the spectra presented in Figure 10, the band at 2068 cm1 might be assigned to dicarbonyls with different CO ligands. However, no band of Au(CO)2 species was detected. The same situation was achieved using the isotopic mixture containing 13C18O (Figure 11). Another important argument against the formation of polycarbonyls is the fact that, in part of the spectra registered at high CO equilibrium pressure, the band of negatively charged gold carbonyls was detected at 2057 cm1 instead of 2075 cm1 (see, for example, Figure 4, spectra ae). These spectra concern cases where the Auδ sites are in a low concentration. The results indicate that no two CO molecules can be simultaneously adsorbed on one Auδ site. Evidently, when the sites are “diluted”, the interaction between the adsorbed CO molecules is weak. On the contrary, when the sites are of high density, the adsorbed CO molecules strongly interact, thus producing the band at 2075 2079 cm1. Therefore, we conclude that all bands in the 2090 2050 cm1 region are due to linear carbonyls. At this stage we can speculate that different ordered structures are formed, thus giving rise to different carbonyl bands. Here, we shall discuss the information on the reduction mechanism that can be derived from the IR spectra. When the negatively charged sites were initially formed, the maximum of the respective carbonyl band was detected at 2075 cm1 (Figure 3, spectra c, d). This observation indicates that the Auδ sites, although in low concentration, were of high density. Therefore, we conclude that in this case only a restricted number of gold particles were covered by negatively charged sites. Before discussing the spectra of adsorbed isotopic mixtures, let us now briefly recall some peculiarities of the spectral performance of CO adsorbed on metal surfaces.37 In fact, the negative charge of metal enhances these peculiarities. • At first, when CO coverage increases, the individual carbonyl band is gradually shifted to higher frequencies. This is due to dipoledipole coupling between the CO molecules. Although no such behavior was observed with CO on metal gold (due to restricted π-back-donation23), it was detected for the negatively charged species. • Simultaneously with the blue shift of the band maximum, the CO extinction coefficient decreases. Therefore, the intensities of the bands are not proportional to the surface concentration and could be used only for estimations. • Intensity transfer occurs from bands situated at lower wavenumbers to higher-wavenumber bands. According to the classic review of Hollins,37 intensity transfer occurs with adsorbates on metals. It is restricted when the maxima of the carbonyl bands are separated by 100 cm1 or more. Also, the effect depends on the density of the adsorbed CO molecules. An important thing is that the intensity gained by one band is equal to the intensity lost by another. Turning back to our results, one can expect a restricted intensity transfer from the 13C18O to the CO bands, while the effect should be more pronounced with 13CO and CO bands.

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However, the spectra of coadsorbed isotopic mixtures clearly show important intensity transfer phenomena in all cases. Consider first the spectra at low coverage. It is evident that the intensity transfer from the band at 1952 cm1 to the band at 2046 cm1 observed after coadsorption of CO and 13C18O is restricted (Figure 11). This is consistent with the expectations based on the large difference in the wavenumbers of the two bands. However, at higher coverages, when bands at 2061 and 1963 cm1 dominate in the spectra, an important intensity transfer occurs: the integral intensity of the bands around 2061 cm1 is more than two times higher than the integral intensity of the bands around 1963 cm1. With the spectra of coadsorbed CO and 13CO even at low coverage, a substantial intensity transfer occurs: the band at 2002 cm1 is ca. two times less intense than the band at 2051 cm1 (Figure 10). With coverage increase, the intensity transfer becomes much more important, and the band at 2016 cm1 (counterpart of the band at 2068 cm1) is hardly observable only in few spectra (see the inset in Figure 10). It is also normal to expect that the intensity transfer will occur with the bands observed after adsorption of CO alone. We can expect important transfer from the band at 2058 cm1 to the band at 2079 cm1 (Figure 5), but unfortunately, we cannot give any quantitative values. Also, it seems that important intensity transfer occurs from the band at 2079 cm1 to the band around 2120 cm1 (Au0CO). This phenomenon can explain the spectra presented in the upper part of Figure 8. The transfer becomes important at low temperature and in the presence of CO in the gas phase which ensures a high CO coverage. An important factor affecting CO stretching frequency when CO adsorbed on metals is the dipole coupling between the adsorbed molecules. As already mentioned, this leads to an increase of the CO stretching frequency with coverage increase. However, dipole coupling can be reduced by changing the frequency of some of the molecules, i.e., using isotope mixtures. Indeed, the spectra we recorded clearly show a decrease of the dipole coupling when isotopic mixtures are adsorbed. Thus, at low coverage, the principal carbonyl band was detected at 2057 cm1 after adsorption of CO. The band shifted to 2050 cm1 after coadsorption of CO and 13CO and to 2046 cm1 after coadsorption of CO and 13C18O. Looking at Figures 4 and 7 one can conclude that CO adsorbed on isolated Auδ sites vibrates at 2057 cm1. However, based on the thermal desorption experiments, it seems this frequency is at 2040 cm1. These observations can be rationalized by the assumption that, with the increase of the site density, they become more electronegative because of the enrichment of the gold particles with electrons. Increase of the coverage leads to an increase of the CO stretching frequency (due to dynamic coupling) to 2057 cm1, a value almost coinciding with the maximum of the carbonyl band when the sites were of very low density. This assumption explains why the band is approaching 2040 cm1 when isotopic mixtures are adsorbed and the dynamic coupling decreases.

5. CONCLUSIONS Evacuation of Au/SiO2 at 673 K leads to reduction of the deposited gold to metal. Auδ+ sites are created in the copresence of CO and O2 and are stable in vacuo at 673 K. When interacting with CO, gold is reduced to Auδ species. The process is strongly enhanced in the presence of hydrogen. The Auδ sites are easily 21280


The Journal of Physical Chemistry C oxidized to Au0 and even to Auδ+ in the presence of oxygen. Negatively charged Auδ sites strongly adsorb CO and form different interconverting linear carbonyls observed in the 2080 2050 cm1 region. The gold clusters with negatively charged species exhibit strongly metallic character, and intensity transfer phenomena occur.

’ ASSOCIATED CONTENT

bS

Supporting Information. XRD patterns (1 figure) and FTIR spectra (3 figures). This material is available free of charge via the Internet at http://pubs.acs.org.

’ ACKNOWLEDGMENT The work was supported by the Bulgarian Scientific Fund (grants DCVP 02/2 and DO 02-290). S.I. thanks the Spanish Ministerio de Educacion y Ciencia for her contract (Ramon y Cajal Programme). Thanks are also due to Dr. D. Kovacheva, Dr. D. Nihtianova, and Dr. N. Velichkova for their help with some experiments. ’ REFERENCES (1) (a) Meyer, R.; Lemire, C.; Shaikhutdinov, Sh.K.; Freund, H.-J. Gold Bull. 2004, 37, 72. (b) Haruta, M. CATTECH 2002, 6, 102.Bond, G. C.; Louis, C.; Thompson, D. T. Catalysis by Gold; Imperial College Press: London, 2006. (2) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (3) (a) Laguna, O. H.; Romero-Sarria, F.; Centeno, M. A.; Odriozola, J. A. J. Catal. 2010, 276, 360. (b) Laguna, O. H.; Centeno, M. A.; Arzamendi, G.; Gandía, L. M.; Romero-Sarria, F.; Odriozola, J. A. Catal. Today 2010, 157, 155. (4) Andreeva, D.; Idakeiv, V.; Tabakova, T.; Andreev, A.; Giovanoli, R. Appl. Catal., A 1996, 134, 275. (5) Ueda, A.; Haruta, M. Gold Bull. 1999, 32, 3. (6) Stangland, E. E.; Stavens, K. B.; Andres, R. P.; Delgass, W. N. J. Catal. 2000, 191, 332. (7) Landon, P.; Collier, P. J.; Papworth, A. J.; Kiely, C. J.; Hutchings, G. J. Chem. Commun. 2002, 18, 2058. (8) Nkosi, B.; Adams, M. D.; Coville, N. J.; Hutchings, G. J. J. Catal. 1991, 128, 378. (9) Provine, W. D.; Mills, P. L.; Lerou, J. J. Stud. Surf. Sci. Catal. 1996, 101, 191. (10) Prati, L.; Rossi, M. J. Catal. 1998, 176, 552. (11) Centeno, M. A.; Paulis, M.; Montes, M; Odriozola, J. A. Appl. Catal., A 2002, 234, 65. (12) Carrettin, S.; Guzman, J.; Corma, A. Angew. Chem., Int. Ed. 2005, 44, 2242. (13) Mihaylov, M.; Kn€ozinger, H.; Hadjiivanov, K.; Gates, B. C. Chem. Ing. Technol. 2007, 79, 795. (14) (a) Klimev, Hr.; Fajerwerg, K.; Chakarova, K.; Delannoy, L.; Louis, C.; Hadjiivanov, K. J. Mater. Sci. 2007, 42, 3299. (b) Venkov, Tz.; Klimev, Hr.; Centeno, M. A.; Odriozola, J. A.; Hadjiivanov, K. Catal. Commun. 2006, 7, 308. (c) Centeno, M. A.; Hadjiivanov, K.; Venkov, Tz.; Klimev, Hr.; Odriozola, J. A. J. Mol. Catal. A 2006, 252, 142. (d) Fierro-Gonzalez, J. C.; Gates, B. C. Catal. Today 2007, 122, 201. (e) Li, M.; Wu, Z.; Ma, Z.; Schwartz, V.; Mullins, D. R.; Dai, S.; Overbury, S. H. J. Catal. 2009, 266, 98. (15) (a) Vindigni, F.; Manzoli, M.; Chiorino, A.; Boccuzzi, F. Gold Bull. 2009, 42, 106. (b) Sterrer, M.; Yulikov, M.; Fischbach, E.; Heyde, M.; Rust, H.-P.; Pacchioni, G.; Risse, T.; Freund, H.-J. Angew Chem., Int. Ed. 2006, 45, 2630. (c) Yoon, B.; H€akkinen, H.; Landman, U.; W€orz, A. S.; Antonietti, J.-M.; Abbet, S.; Judai, K.; Heiz, U. Science 2005, 307, 403.

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