Mechanism of CO oxidation reaction on O-covered Pd(111)surfaces studied with fast x-ray photoelectron spectroscopy:Change of reaction path accompanying phase transition of O domains

摘要

We studied the mechanism of CO oxidation on O-precovered Pd(111)surfaces by means of fast x-ray photoelectron spectroscopy(XPS).The oxygen overlayer is compressed upon CO coadsorption from a p(2X2)structure into a(~(1/2)3X ~(1/2)3)R30 deg structure and then into a p(2X 1)structure with increasing CO coverage.These three O phases exhibit distinctly different reactivities.(1)The p(2 X 2)phase does not react with CO unless the surface temperature is sufficiently high(<290 K).(2)In the(~(1/2)3 X ~(1/2)3)R30 deg phase,the reaction occurs exclusively at island peripheries.CO molecules in a high-density phase formed under CO exposure react with oxygen atoms,leading to quite a small apparent activation energy.(3)The reaction proceeds uniformly over the islands in the p(2X 1)phase.