The low-temperature reduction of Pd-doped transition metal oxide surfaces with hydrogen

The reaction of hydrogen with a series of polyvalent metal oxides (Fe2O3, WO3, MoO3, V2O5, Sb2O3, PbO2, Cr2O3, NiO, CuO, Co3O4, MnO2, PdO, Ag2O) was investigated at low temperatures (77-320 K) and pressures (0.001-0.7 kPa). Pd-doped (0.1-0.5 wt.%) transition metal oxides can be reduced by hydrogen at 77-320 K whereas the onset of the reduction of the pure oxides occurs at temperatures higher than 500-700 K. It is shown that oxides, possessing a low stability of the metal-oxygen bond, a significant oxygen-diffusion coefficient from the oxide volume, and a large specific surface area are promising low-temperature vacuum hydrogen getters. Two temperature regions with different kinetics have been found for the most active oxides (Co3O4, CuO, MnO2): 77-210 and 220-320 K. At 77-180 K, the chemical interaction between hydrogen and the Pd-doped oxides proceeds with an apparent zero activation energy according to the equation P = In tau. At 195-320 K, a first-order reaction is found. XPS studies of the low-temperature reduction catalysts show Pd on oxides. A possible reaction mechanism is discussed. It is supposed that proton spillover into the lattice is critical for the low-temperature oxide reduction. A electron tunnel mechanism is probable too. (C) 2002 Elsevier Science B.V. All rights reserved.