In situ Infrared Study of Catalytic Oxidation over Au/TiO2 Catalysts

Abstract

Abstract In situ infrared study of catalytic CO oxidation over Au/TiO2 shows that the catalyst prepared from AuCl3 exhibits higher activity than those prepared from HAuCl4. The high activity of Au appears to be related to the presence of reduced and oxidized Au sites as well as carbonate/carboxylate intermediates during CO oxidation. Addition of H2O2 further promotes the oxidation reaction on Au/TiO2 catalysts. Introduction Supported Au catalysts have been extensively studied because of their unique activities for the low temperature oxidation of CO and epoxidation of propylene (15). The activity and selectivity of Au catalysts have been found to be very sensitive to the methods of catalyst preparation (i.e., choice of precursors and support materials, impregnation versus precipitation, calcination temperature, and reduction conditions) as well as reaction conditions (temperature, reactant concentration, pressure). (6-8) High CO oxidation activity was observed on Au crystallites with 2-4 nm in diameter supported on oxides prepared from precipitation-deposition. (9) A number of studies have revealed that Au0 and Au+3 play an important role in the low temperature CO oxidation. (3, 10) While Au0 is essential for the catalyst activity, the Au0 alone is not active for the reaction. The mechanism of CO oxidation on supported Au continues to be a subject of extensive interest to the catalysis community. The reaction pathway, reactivity of the active sites, and the nature of adsorbed intermediates constitute the catalytic reaction mechanism. Our study has been focused on the investigation of the nature of adsorbed intermediates under reaction conditions. We report the results of in situ infrared study of CO and ethanol oxidation on Au/TiO2 catalysts. This study revealed the high activity of Au/TiO2 is related to the presence of reduced Au and oxidized Au sites which may promote the formation of carbonate/carboxylate intermediates during CO oxidation. Experimental Section Two 1% Au/TiO2 catalysts, designated as HAuCl4 and AuCl3 were prepared by deposition-precipitation of HAuCl4 (Aldrich) and AuCl3 (Alfa Asar) onto Degussa-P25 TiO2, respectively (16, 17). The specific procedure involves (i) adding NaOH solution in an appropriated amount of aqueous solution (150ml) of AuCl3 or HAuCl4-4H2O with 2 g TiO2 at 343 K to adjust the mixture to pH = 7, (ii) washing the resulting solid five times with warm distilled water, (iii) centrifuging to remove Na+ and Cl-ions, (iv) drying the sample at 353 K for 12 h, and then (v) calcining the sample at 673 K for 5 h. The experimental apparatus is explained elsewhere (18) but briefly described here. The experimental apparatus consists of (i) a gas flow system with a four port and six port valve, (ii) a DRIFTS (Diffuse Reflectance Infrared Spectroscopy) reactor, (iii) an analysis section with Mass Spectrometer (MS). The CO oxidation was performed from 298 K to 523 K. The reactor temperature was varied at a rate of 10 K/min. The gas species consists of He/CO (90/10 Vol%), He/CO/O2 (72/14/14 Vol%), He/CO/H2O2/O2 (72/13.3/13.3/1.4 Vol%), He/O2 (86/14 Vol%), He/H2O2/O2 (84/2/14 Vol%), He/CH3CH2OH (83/17 Vol%), and He/CH3CH2OH/O2 (72/14/14 Vol%), He/CH3CH2OH/H2O2/O2 (75/8/2/15 Vol%) at a total flow rate of 35 cm3/min; it takes 13 s for the gases to reach the DRIFTS reactor and 27 s to reach the MS from the four port valve. CH3CH2OH and H2O2 species added by flowing He through a saturator. Transient IR Spectra were collected by a Digilab FTS4000 FT-IR. The effluent gases of the DRIFTS reactor were monitored by a Pfeiffer OmnistarTM Mass Spectrometer. Results and Discussion Fig. 1(a) shows both Au catalysts give very similar XRD patterns; Fig. 1(b) shows both catalysts exhibit a UV peak in the 500-600 nm region; the Au particle size was determined to be 88 nm for HAuCl4 and 86 nm for AuCl3 by XRD.