Deactivation of Pd particles supported on Nb 2O 5/Cu 3Au(1 0 0): SFG and TPD studies from UHV to 100 mbar

Abstract

Structural changes that occur on Pd–Nb 2O 5/Cu 3Au(1 0 0) model catalysts upon thermal annealing were followed by sum frequency generation (SFG) and temperature-programmed desorption (TPD) using CO as probe molecule. SFG experiments were performed both under ultrahigh vacuum and mbar pressure. Heating the catalyst to temperatures above 300 K lead to an irreversible 50% decrease in the CO adsorption capacity and modified the remaining adsorption sites. Alterations of the phase between resonant and non-resonant SFG signals upon annealing indicate a change in the electronic structure of the surface, which excludes Pd sintering or migration of Nb 2O 5 over Pd particles to cause the observed effect and rather suggests the formation of “mixed Pd–NbO x ” sites. The same changes in surface properties also occur during CO hydrogenation at 1 bar and high temperature, pointing to an involvement of “mixed Pd–NbO x ” sites in catalytic reactions.