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.

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(2 x 2) structure into a (square root(3) x square root(3))R30 degrees structure and then into a p(2 x 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 square root(3) x square root(3))R30 degrees 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(2 x 1) phase.