The effect of H2 treatment at 423–573 K on the structure and synergistic activity of Pd–Cu alloy catalysts for low-temperature CO oxidation

Abstract

The effect of H2 treatment at 423–573K on the structure of Cu/Pd/Al2O3 catalysts for low-temperature CO oxidation was studied by means of XRD, TEM, EDS, FFT, DRIFTS, and XPS methods. The formation of 5nm alloy particles in the Cu/Pd catalyst reduced at 423K was detected. The reaction on Cu/Pd, Pd, and Cu catalyst reduced at 423K starts at 275, 400, and 475K, respectively. The activation energy for CO oxidation on Cu/Pd, Pd, and Cu catalyst is 30, 90, and 110kJ/mol, respectively. The enhanced activity of Cu/Pd catalyst is explained by oxidation on Pd0–MOx interfaces (M=Pd, Cu). The key steps of this process are: oxidation of CO molecules strongly bound to Pd0 by lattice oxygen of MOx; fast desorption of CO2 molecules; replenishment of oxygen vacancies in MOx by oxygen molecules from reaction media. As a result, a substantial part of “blocked” Pd0 sites become available for adsorption and dissociation of O2 molecules. This is favorable for low-temperature oxidation via Langmuir–Hinshelwood mechanism. The increase in the reduction temperature results in a separation of alloy particles into metallic particles. As a result, Cu/Pd catalysts reduced at high temperatures are characterized with a decreased content of Pd0–MOx interfaces; an essential amount of “blocked” Pd0 sites and a decreased light-off activity.