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Transient studies on the oxidation of CO over Au/TiO{sub 2} have been performed at 243-363 K using a time-resolved fast-scanning Fourier transform infrared (FTIR) spectrometer coupled with a quadrupole mass spectrometer. Different CO-O{sub 2} interactions were performed by stepping CO into a steady-state He flow, O{sub 2} into a steady-state CO flow, CO into a steady-state O{sub 2} flow, and CO/O{sub 2} into an He flow. The CO oxidation reaction occurs at all temperature ranges investigated whether CO is preadsorbed or O{sub 2} is preadsorbed, indicating that there is no competition between CO and O{sub 2} for adsorption at a temperature range of 243-363 K (noncompetitive adsorption). It suggests that CO and O{sub 2} adsorption occurs on two different types of sites. When there is no O{sub 2} present in step feed (i.e., step CO into He), CO immediately builds up on the surface (rapid adsorption of CO) and desorbs slowly after cutting off CO flow. However, when O{sub 2} is present in the step feed (i.e., step CO/O{sub 2} into He), adsorbed CO reacts with adsorbed oxygen immediately (fast surface reaction), whereas CO{sub 2} product desorption monitored by mass spectroscopy appears to be very slow. Slow desorption of CO{submore » 2} product is claimed to be the rate-limiting step of CO oxidation over Au/TiO{sub 2} at a temperature range of 243-273 K.« less |
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