Reduction of CuO in H2: in situ time-resolved XRD studies

CuO is used as a catalyst or catalyst precursor in many chemical reactions that involve hydrogen as a reactant or product. A systematic study of the reaction of H2 with pure powders and films of CuO was carried out using in situ time-resolved X-ray diffraction (XRD) and surface science techniques. Oxide reduction was observed at atmospheric H2 pressures and elevated temperatures (150–300 8C), but only after an induction period. High temperature or H2 pressure and a large concentration of defects in the oxide substrate lead to a decrease in the magnitude of the induction time. Under normal process conditions, in situ time-resolved XRD shows that Cu1þ is not a stable intermediate in the reduction of CuO. Instead of a sequential reduction (CuO)Cu4O3 )Cu2O)Cu), a direct CuO)Cu transformation occurs. To facilitate the generation of Cu1þ in a catalytic process one can limit the supply of H2 or mix this molecule with molecules that can act as oxidant agents (O2, H2O). The behavior of CuO-based catalysts in the synthesis of methanol and methanol steam reforming is discussed in the light of these results.