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 (O-act), 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 O-2. 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 O-act. 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. (C) 2012 Elsevier B.V. All rights reserved.