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 (2×2) structure into a (3×3) R30° structure and then into a p (2×1) structure with increasing CO coverage. These three O phases exhibit distinctly different reactivities. (1) The p (2×2) phase does not react with CO unless the surface temperature is sufficiently high (<290 K). (2) In the (3×3) R30° 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 (2×1) phase.
ASJC Scopus subject areas