Surface structure and reactivity of Pd(100) during CO oxidation near ambient pressures

The surface structure of Pd(100) during CO oxidation was measured using a combination of a flow reactor and in situ surface X-ray diffraction coupled to a large-area 2-dimensional detector. The surface structure was measured for P(O2)/P(CO) ratios between 0.6 and 10 at a fixed total gas pressure of 200 mbar and a fixed CO pressure of 10 +/- 1 mbar. In conjunction with the surface structure the reactivity of the surface was also determined. For all P(O2)/P(CO) ratios the surface was found to oxidize above a certain temperature. Three different types of oxides were observed: the (root 5 x root 5)R27 degrees surface oxide, an epitaxial layer of bulk-like PdO, and a non-epitaxial layer of bulk-like PdO. As soon as an oxide was present the reactivity of the surface was found to be mass transfer limited by the flux of CO molecules reaching the surface.