Catalytic CO oxidation reaction on a Pd(100) single-crystal surface under several hundred mTorr pressure conditions has been studied by ambient pressure X-ray photoelectron spectroscopy and mass spectroscopy. In-situ observation of the reaction reveals that two reaction pathways switch over alternatively depending on the surface temperature. At lower temperatures, the Pd(100) surface is covered by CO molecules and the CO 2 formation rate is low, indicating CO poisoning. At higher temperatures above 190 °C, an O-Pd-O trilayer surface oxide phase is formed on the surface and the CO 2 formation rate drastically increases. It is likely that the enhanced rate of CO 2 formation is associated with an active oxygen species that is located at the surface of the trilayer oxide.
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