Al2O3 Catalyst

CO adsorption and oxidation over Au/Al2O3 were studied by infrared (IR) spectroscopy at 150 K. Two types of CO adsorption were found on the support, A...
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J. Phys. Chem. B 2001, 105, 3017-3022

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Infrared Study of CO Adsorption and Oxidation over Au/Al2O3 Catalyst at 150 K Jifei Jia, Junko N. Kondo, Kazunari Domen,* and Kenzi Tamaru Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: August 29, 2000; In Final Form: December 11, 2000

CO adsorption and oxidation over Au/Al2O3 were studied by infrared (IR) spectroscopy at 150 K. Two types of CO adsorption were found on the support, Al2O3, with the existence of gaseous CO (266 Pa). An additional band of adsorbed CO at 2110 cm-1 was found on Au/Al2O3 at 150 K in the presence of gaseous CO and was assigned to CO adsorbed on ultrafine gold particles dispersed on Al2O3 support. After evacuation at the same temperature, the band of CO adsorbed on the ultrafine gold particles decreased in intensity and shifted to 2118 cm-1, whereas the CO adsorbed on Al2O3 disappeared. The band at 2118 cm-1 was assigned to CO irreversibly adsorbed on the ultrafine gold particles at 150 K. CO oxidation by O2 took place on Au/Al2O3 in the presence of CO and O2 gas mixture even at 150 K, whereas CO irreversibly adsorbed on Au/Al2O3 and could not be oxidized by the exposure to O2 at the same temperature. Therefore, the reversibly adsorbed CO on the ultrafine gold particles was found to be the active CO species for CO oxidation at 150 K. By spectral analysis, the reversibly adsorbed CO was found to show a band at 2108 cm-1. The ultrafine gold particles were oxidized by O2 treatment at 573 K, becoming inert for CO oxidation at 150 K, but were easily rereduced at 573 K by evacuation treatment. The recovered Au/Al2O3 also catalyzed CO oxidation at 150 K.

Introduction Gold has long been known to be far less catalytically active than other transition metals. However, Bond and other researchers have reported some pioneering works to indicate that gold could catalyze hydrogenation, hydrogen exchange, hydrocracking, and carbon monoxide oxidation reactions.1-10 In the past decade, Haruta and other researchers have discovered that highly dispersed gold particles were very active for various kinds of catalytic reactions such as low-temperature combustion, selective oxidation of hydrocarbons, selective hydrogenation of unsaturated hydrocarbons, reduction of nitrogen oxides, water gas shift reaction, and so on.11-28 The nature of ultrafine gold particles dispersed on supports has been studied by means of various approaches.12-14,29-37 CO oxidation reaction is usually used as a characteristic reaction to test the nature of gold nanoparticles.11-14,34-43 Among them, ultrafine gold particles dispersed on reducible metal oxides, such as TiO2, Fe2O3, Co3O4, and NiO, were found to be active for low-temperature CO oxidation even at 200 K.11,12,30 However, the active sites and the reaction mechanism of CO oxidation over supported gold catalysts are still being debated.12,18,34-38 We have reported that Au/Al2O3 catalyst was not only active for CO oxidation but also effective and selective for hydrogenation of acetylene.28 In this paper, following the former studies, we have studied CO adsorption and oxidation at 150 K to gain a deeper insight on the nature of Au/Al2O3 catalyst through IR characterization. Experimental Section Au/Al2O3 catalyst with a gold loading of 10.0 wt % was prepared by a deposition-precipitation method. Chloroauric acid * To whom correspondence should be addressed. E-mail: kdomen@ res.titech.ac.jp. Fax: +81-45-924-5282.

(HAuCl4, Wako Pure Chemical Industries, Ltd., purity 99%) was first dissolved in distilled water. After the pH of the HAuCl4 aqueous solution was adjusted to 7.5 by adding 1 M NaOH solution, Al2O3 powder (Surface area: 100 m2‚g-1, Aluminum oxide C of Aerosil Company Ltd.) was suspended and kept at 343 K for 1 h. The suspension was washed with distilled water repeatedly, dried, and then calcined in air at 623 K for 4 h. The particle sizes of gold and Al2O3 of the catalyst were found to be 4 and 30 nm in average, respectively, which was estimated by transmission electron microscopy by JEOL HIGHTECH Co. Ltd. and Tanaka Precious Metal Co.28 About 20 mg of each of the samples, Al2O3 and Au/Al2O3, was pressed into a self-supported IR disk, whish was 20 mm in diameter and 1 mm in thickness. The disk was placed in the center of a quartz IR cell. The cell was connected to a Pyrex closed gas-circulation system. The temperature was controlled in a range from 150 to 1073 K by liquid nitrogen flow and an electronic heater. The temperature, measured by a thermocouple from outside the cell, was corrected by comparison with that of the direct measurements from the thermocouple attached to the disk inside the cell. Pretreatment of the samples was performed by evacuation at 573 K for 0.5 h. The pressure after evacuation was of the order of 10-1-10-2 Pa. The sample was cooled to 150 K by liquid nitrogen, and CO gas was then introduced to the IR cell at a pressure of 266 Pa at the same temperature. All of the gases used in IR measurement were of a purity of 99.99% and further refined by passing through a liquid N2 trap. IR spectra were recorded on a JASCO 7000 FTIR spectrometer with a liquid nitrogen cooled MCT detector and were averaged with 64 scans at 4 cm-1 resolution. IR spectra of adsorbed species such as CO were obtained by subtracting a spectrum of Au/Al2O3 or Al2O3 from that with surface species measured at the same temperature.

10.1021/jp003112l CCC: $20.00 © 2001 American Chemical Society Published on Web 03/23/2001

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Figure 2. IR spectra of CO (266 Pa) adsorbed on Au/Al2O3 at 150 K. (a) before and (b) after evacuation.

Figure 1. A. IR spectra of CO (266 Pa) adsorbed on Al2O3 at 150 K. (a) before and (b) after evacuation. B. IR spectra of CO oxidation over Al2O3 at 150 K. (a) CO (266 Pa) adsorbed on Al2O3 at 150 K. (b) (a) + O2 (1330 Pa) 1 min (c) (a) + O2 (1330 Pa) 20 min.

Results and Discussion 1. Study of CO Adsorption over Al2O3 at 150 K. The study of CO adsorption on Al2O3 at 150 K was done before the examination on Au/Al2O3 catalyst because CO could adsorb not only on supported ultrafine gold particles but also on Al2O3.34,44 IR spectrum of CO (266 Pa) adsorbed on Al2O3 at 150 K is shown in Figure 1A(a). Two bands of adsorbed CO species were observed with the existence of CO in gas phase. Among them, the band at 2156 cm-1 is assigned to CO adsorbed on the surface OH groups, whereas the band at 2190 cm-1 is assigned to CO adsorbed on the Lewis acid sites of Al2O3 surface.44 Two negative bands in the range between 3750 and 3650 cm-1 appeared due to the shift of the isolated to the hydrogen-bonded OH groups with a broad band at 3570 cm-1. After evacuation at the same temperature as shown in Figure 1A(b), both bands disappeared. Therefore, the adsorption of CO on Al2O3 was found to be totally reversible at 150 K. The possibility of CO oxidation over Al2O3 at 150 K was excluded as shown in Figure 1B. CO (266 Pa) was introduced on Al2O3 at 150 K (Figure 1B(a)). Then, O2 (1330 Pa) was added to the system at the same temperature (Figure 1B(b)). The introduction of O2 did not affect the peaks of adsorbed CO, and no CO2 (band at about 2346 cm-1) produced even 20 min later (Figure 1B(c)). It was, therefore, confirmed that Al2O3 did not have activity for CO oxidation at 150 K. 2. Study of CO Adsorption and Oxidation over Au/Al2O3 at 150 K. IR spectrum of CO adsorbed on Au/Al2O3 at 150 K is shown in Figure 2 (a). A band at 2110 cm-1 appeared in addition to those on Al2O3. This band can be assigned to CO

Figure 3. IR spectra of irreversible CO oxidation over Au/Al2O3 at 150 K. (a) evacuation of CO (266 Pa) adsorbed on Au/Al2O3 at 150 K. (b) (a) + O2 (1330 Pa) 1 min (c) (a) + O2 (1330 Pa) 20 min.

adsorbed on ultrafine gold particles dispersed on Al2O3 support with on-top structure.34-36 After evacuating CO at the same temperature as shown in Figure 2(b), the band at 2154, 2186, and 3574 cm-1 disappeared due to the CO desorption from Al2O3. The band of CO on the ultrafine gold particles decreased in intensity and shifted to 2118 cm-1 after evacuation. The band at 2118 cm-1 remaining after evacuation is thus assigned to irreversible CO adsorbed on the ultrafine gold particles. Accordingly, CO adsorbed on the ultrafine gold particles (band at 2110 cm-1) at 150 K could be divided to irreversibly adsorbed CO (band at 2118 cm-1) and reversibly adsorbed CO that disappears after evacuation. After evacuation of gaseous CO at 150 K, O2 (1330 Pa) was introduced at the same temperature to observe the possibility of oxidation of irreversible CO (Figure 3). The introduction of O2 did not change the band of adsorbed CO (Figure 3(b)), and no CO2 produced even after 20 min (Figure 3(c)). From this result, it became clear that the irreversibly adsorbed CO was not related to the active species for CO oxidation at 150 K on Au/Al2O3 catalyst. Next, CO oxidation with the coexistence of CO and O2 in gas phase was studied over Au/Al2O3 catalyst at 150 K (Figure 4). CO (266 Pa) was introduced at 150 K (Figure 4(a)), followed by addition of O2 (1330 Pa) at the same temperature (Figure 4(b)). The formation of CO2 was immediately noticed by the appearance of a new band at 2346 cm-1 (asymmetric O-C-O stretching).43 The introduction of O2 did not decrease the band

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Figure 4. IR spectra of CO oxidation over Au/Al2O3 at 150 K. (a) CO (266 Pa) adsorbed on Au/Al2O3 at 150 K. (b) (a) + O2 (1330 Pa) 1 min (c) (a) + O2 (1330 Pa) 20 min.

Figure 6. IR spectra of different CO species adsorbed on surface of ultrafine gold particles of Au/Al2O3 catalyst when CO (266 Pa) absorbed at 150 K. (a) totally adsorbed CO. (b) irreversibly adsorbed CO. (c) reversibly adsorbed CO.

Figure 5. IR spectra of CO2 adsorbed at 150 K on (a) Au/Al2O3 and (b) Al2O3.

of adsorbed CO in intensity because the amount of consumed CO was constantly supplied from gas phase. Twenty min after the introduction of O2 (Figure 4(c)), the amount of adsorbed CO2 increased greatly. It is, therefore, demonstrated that CO is oxidized by O2 over Au/Al2O3 catalyst even at 150 K. From results in Figures 3 and 4, gaseous CO was found to be indispensable for CO oxidation on Au/Al2O3 catalyst. In the other words, the reversibly adsorbed CO on the ultrafine gold particles is identified as the active species. No CO2 was detected in gas phase during the course of CO oxidation over Au/Al2O3 catalyst at 150 K. Therefore, the observed CO2 species did not desorb from the catalyst at 150 K. In Figure 4(c), the intensity of the 2154 cm-1 band (CO on OH groups) decreased even under the supply of molecules from gas phase, whereas the other two bands in CO stretching region stayed unchanged. It is also noticed in the same spectrum that the spectral appearance of OH stretching region (3800-3400 cm-1) obviously changed. Therefore, CO2 was supposed to adsorb on OH groups of the Al2O3 support, prohibiting the CO adsorption to the same sites. CO2 adsorption on Au/Al2O3 and Al2O3 were also studied at 150 K (Figure 5). Almost the same spectra were observed on Au/Al2O3 (Figure 5(a)) and Al2O3 (Figure 5(b)), regardless of

the presence of ultrafine gold particles. The band at 2346 cm-1 is thus assigned to CO2 adsorbed on Al2O3 support. The band at 3800-3400 cm-1 is then attributed to OH groups being hydrogen-bonded with CO2. Comparing the spectra in Figures 4 and 5, it is confirmed that CO2 migrated to the OH groups on Al2O3 after being produced on the ultrafine gold particles, and that CO adsorption on the OH groups was prohibited during the course of CO oxidation. Although the active (reversible) CO always coexist with the inert (irreversible) CO in IR spectra, we attempted to separately observe the IR band of the active species. IR band of the active CO was obtained by spectral treatment. Because the band of reversible CO is accompanied by those of CO on Al2O3, the subtraction of spectrum of CO on Al2O3 from that of CO on Au/Al2O3 was carried out. As a result, a mixed band of reversible and irreversible CO was observed (Figure 6(a)). A negative band due to CO on Lewis acid sites appeared because more Lewis acid sites existed on Al2O3 than on Au/Al2O3 by comparison with Bronsted acid sites. The band at 2118 cm-1 in Figure 6(b) represents the irreversible CO measured after evacuation, and subtraction of this from Figure 6(a) produces the IR spectrum of the active CO (Figure 6(c)). It is indicated, therefore, the band due to reversible CO appears at lower frequency, which is responsible for lowering the peak position when measured in the presence of CO in gas phase. Among the reports for highly dispersed gold catalysts, Al2O3 was not considered to be a good support to give high activity to ultrafine gold particles for catalyzing CO oxidation because of its irreducibility.11-42 Therefore, it is valuable to note that Au/Al2O3 is able not only to absorb CO but also to catalyze CO oxidation even at 150 K, much lower temperature than that reported previously.11

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Figure 7. A. IR spectra of CO (266 Pa) adsorbed on oxidized Au/ Al2O3 (with the O2 1330 Pa treatment at 573 K) at 150 K. (a) before and (b) after evacuation. B.IR spectra of CO oxidation over oxidized Au/Al2O3 catalyst at 150 K. (a) CO (266 Pa) adsorbed on oxidized Au/Al2O3 catalyst at 150 K. (b) (a) + O2 (1330 Pa) 1 min (c) (a) + O2 (1330 Pa) 20 min.

Al2O3, different from reducible metal oxide such as TiO2, Fe2O3, Co3O4, and NiO, does not adsorb O2. Therefore, the activation of both CO and O2 is considered to occur on the ultrafine gold particles themselves. The active sites of the ultrafine gold particles seem to adsorb both CO and O2 reversibly at 150 K simultaneously, in contract to the bulk gold which does not adsorb O2. 3. Study of CO Adsorption and Oxidation over Oxidized Au/Al2O3 Catalyst. The effect of O2 or O adsorption to the adsorbed CO species on the ultrafine gold particles was investigated from 150 to 573 K. After the normal pretreatment, O2 (1330 Pa) was added to Au/Al2O3 catalyst at 573, 295, or 150 K for 10 min. Then the catalyst was cooled, and the O2 was evacuated immediately. After the catalyst was cooled to 150 K, CO adsorption and oxidation were carried out again. The O2 treatment at 150 and 295 K did not change Au/Al2O3 with respect to CO adsorption and oxidation at 150 K. IR spectra of adsorbed CO on the catalyst after O2 treatment at 573 K are shown in Figure 7 A. Two bands attributed to adsorbed CO species and one broad band of CO-bonded OH groups were observed in the presence of CO in gas phase (Figure 7A(a)). No peak at about 2110 cm-1 was observed in contrast to Au/Al2O3 catalyst without the O2 treatment. It is noted that the intensity of the band at 2154 cm-1 was very strong, which remained even after evacuation (Figure 7A(b)). Therefore, the band at 2152 cm-1 in Figure 7A(b) is assigned to CO adsorbed on the oxidized ultrafine gold particles’ surface irreversibly.34,35 This band overlapped with CO on OH groups and increased

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Figure 8. A. IR spectra of CO (266 Pa) adsorbed on rereduced Au/ Al2O3 (with the O2 1330 Pa treatment at 573 K) at 150 K. (a) before and (b) after evacuation. B. IR spectra of CO oxidation over rereduced Au/Al2O3 catalyst at 150 K. (a) CO (266 Pa) adsorbed on rereduced Au/Al2O3 catalyst at 150 K. (b) (a) + O2 (1330 Pa) 1 min (c) (a) + O2 (1330 Pa) 20 min.

the band at 2154 cm-1 in intensity in Figure 7A(a). The band position of adsorbed CO on the oxidized ultrafine gold particles is higher than that of the adsorbed CO on the ultrafine gold particles because of the lack of the back-donation. Next, possibility of CO oxidation over oxidized Au/Al2O3 catalyst was examined at 150 K (Figure 7B). The production of CO2 was not observed (2400-2300 cm-1) nor did the whole spectral range change. The results indicated that the oxidized ultrafine gold particles did not have activity for CO oxidation at 150 K. After oxidation treatment at 573 K, Au/Al2O3 catalyst was evacuated at the same temperature for 2 h. Then, CO (266 Pa) adsorption and oxidation over the catalyst at 150 K (Figure 8) were carried out again. The IR spectra showed no difference from those of fresh catalyst. Therefore, the ultrafine gold particles that were oxidized by O2 treatment were rereduced by evacuation at 573 K. The treatment with H2 (1330 Pa) at 573 K of the catalyst played the same role as evacuating the catalyst at 573 K for 2 h. 4. Study of CO Adsorption and Oxidation at 150 K over Au/Al2O3 Catalyst with the Average Gold Particle Size of 330 nm. To see the effect of the particle size of gold, we synthesized an Au/Al2O3 catalyst with the average gold particle size of 330 nm. The catalyst was prepared by calcining the Au/ Al2O3 with the average gold particle size of 4 nm in air at 1273 K for 12 h. The TEM results indicated that gold particles were almost separately existent from Al2O3 particles. The average gold particle size increased to 330 nm, whereas the average

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Figure 9. A. IR spectra of CO (266 Pa) adsorption on Au (330 nm)/Al2O3 catalyst at 150 K. (a) before and (b) after evacuation. B.IR spectra of CO oxidation over Au (330 nm)/Al2O3 catalyst at 150 K. (a) CO (266 Pa) adsorbed on the Au/Al2O3 catalyst at 150 K. (b) (a) + O2 (1330 Pa) 1 min (c) (a) + O2 (1330 Pa) 20 min.

particle size of Al2O3 remained unchanged (not shown for the sake of brevity). IR spectra of CO adsorption at 150 K over Au (330 nm)/Al2O3 catalyst are shown in Figure 9A. Two bands were observed at 2178 and 2156 cm-1, whereas that at 2110 cm-1 (see Figure 2(a)) assigned to CO on the ultrafine gold particles was absent in Figure 9 A (a). After evacuation (Figure 9A(b)), all of the bands disappeared. Therefore, the observed bands were assigned to CO on Al2O3. In contrast to the untrafine gold particles, it was found that the gold particles did not adsorb CO in the present experimental condition when the particle size became as large as 330 nm. Although CO on Au was not observed, CO oxidation ability of Au (330 nm)/Al2O3 was evaluated. IR spectra of CO oxidation at 150 K over Au (330 nm)/Al2O3 catalyst are shown in Figure 9B. It is clear that the gold particles lose their ability for CO oxidation when they become large particles. Therefore, it was concluded that gold could catalyze CO oxidation at 150 K only when its particle size is as small as mere nanometers. Conclusion 1. Two kinds of adsorbed CO on ultrafine gold particles of Au/Al2O3 catalyst, reversible CO and irreversible CO, were observed at 150 K. 2. The ultrafine gold particles catalyzed the oxidation of reversible CO at 150 K, but did not convert the irreversible CO to CO2 at the same temperature. 3. The ultrafine gold particles were easily oxidized and then rereduced at 573 K by O2 treatment and then evacuation treatment. 4. The oxidized ultrafine gold particles had no activity for CO oxidation at 150 K. 5. After calcining Au/Al2O3 catalyst at 1273 K, the gold particles dispersed on Al2O3 did not adsorb CO and did not have CO oxidation activity anymore. Acknowledgment. One of the authors (J. J.) was able to join the research group by obtaining support from the Japan Society for the Promotion of Science, to which our thanks are due.

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