Catalysis of Organic..

Miller and Chuang 1492(a)235921751692155214421357(b)2Adsorbance1.51250 o C200 o C150 o CAuCl 3HAuCl 4F CO2(mmol/min)1.51AuCl 30.5100 o C0.550 o CHAuCl 40227525 o C195016251300050 100 150 200 250Wavenumber (cm -1 )Temperature (C)Figure 2. (a) IR Spectra Temperature Programmed Reaction on AuCl 3 andHAuCl 4 -Catalyst, (b) Rate of formation of CO 2 on AuCl 3 and HAuCl 4 -Catalyst.In situ infrared spectra of CO oxidation on both catalysts as a function oftemperature, shown in Fig. 2(a) reveal that AuCl 3 catalysts (Au/TiO 2 prepared fromAuCl 3 precursor) is more active than HAuCl 4 (Au/TiO 2 prepared from HAuCl 4 ) asindicated by CO 2 intensity. Furthermore, AuCl 3 catalyst gave higher carbonate andcarboxylate intensities in the 1350 - 1560 cm -1 region than HAuCl 4 . The negativeband at 1692 cm -1 is a result of the incomplete cancellation of the IR backgroundbefore and during CO oxidation. Comparison of CO 2 formation rate in Fig. 2(b)shows AuCl 3 catalyst exhibit low temperature CO oxidation activity. Rate of CO 2formation over AuCl 3 is more sensitive to temperature than over HAuCl 4 .Fig. 3 (a) shows CO adsorbed on HAuCl 4 catalyst as linear CO on Au 0 site at 2110cm -1 ; Fig 3(b) shows weakly adsorbed linear CO on AuCl 3 at 2180 cm -1 on Ti 4+ (1),CO adsorbed as linear CO on Au + at 2122 cm -1 . 2046 and 2004 cm -1 bands resemblethose reported in the literature on gold electrodes during electro-oxidation of CO atnegative potentials. (11, 12) The intensity of these adsorbed CO decreased uponswitching of the CO/He flow to the He flow. Removal of gaseous CO by switchingof the flow from CO/He to the pure He allowed the direct observation of the contourof the adsorbed CO band and their binding strength to the catalyst surface. Althoughthe 2046 and 2004 cm -1 bands were strongly bonded on the Au 0 surface, they werenot observed during CO oxidation in Fig. 2 (a). The difference in IR spectra betweenCO adsorption (Fig 3) and CO oxidation (Fig, 2(a)) suggests that the Au/TiO 2catalyst surface state varies with its gaseous environment.