Carbon Monoxide Oxidation Promoted by a Highly Active Strained PdO Layer at the Surface of Au30Pd70(110)

The evolution of the Au30Pd70(110) surface was studied by coupling grazing incidence X-ray diffraction and mass spectrometry under oxygen-rich conditions at moderate temperatures (300 to 470 K). This allows us to correlate the depth profile of its structure to its catalytic properties for carbon monoxide (CO) oxidation. Under increasing pressure from ultrahigh vacuum up to 100 mbar, both oxygen and CO induce Pd segregation, even at room temperature. However, in pure oxygen the surface is reorganized with a (1 × 2) missing row reconstruction, whereas in pure CO it is strongly roughened. When oxygen pressure is increased a phase corresponding to the initial step of the oxidation with oxygen dissolution in the subsurface region appears at first. Then, from about 400 K onward, an oxidized thin Pd layer (≤1 nm) is formed growing in the [100]PdO direction. This PdO phase is strained and does not coincide with the P42/mmc structure usually observed for this oxide under ambient conditions. It is more probably con...