CO oxidation on nanoporous gold: A combined TPD and XPS study of active catalysts

Disks of nanoporous gold (np-Au), produced by leaching of silver from AgAu alloy and prepared as active catalysts for CO oxidation in a continuous-flow reactor, were investigated in detail by x-ray photoelectron spectroscopy and temperature-programmed desorption spectroscopy in ultra-high vacuum. Np-Au exhibits several oxygen species on and in the surface: Chemisorbed oxygen (Oact), probably generated at residual silver sites at the surface, is readily available after np-Au preparation and consumed by CO oxidation. It can be replenished on activated np-Au by exposure to O2. In addition, strongly bound oxygen, probably at subsurface sites, is present as a major species and not consumed by CO oxidation. Pronounced CO desorption at temperatures above 200 K observed after exposing np-Au to CO at 105 K indicates an additional, more stable type of CO binding sites on np-Au as compared to pure gold. Only CO at these binding sites is consumed by oxidation reaction with Oact. It is proposed that the presence of strongly bound subsurface oxygen stabilizes CO adsorption on np-Au, thereby being as crucial for the observed catalytic activity of np-Au as residual silver.