Production of hydrogen from oxidative steam reforming of methanol II. Catalytic activity and reaction mechanism on Cu/ZnO/Al2O3 hydrotalcite-derived catalysts

Cu/ZnO/Al2O3 catalysts derived from a hydrotalcite precursor were studied for oxidative steam reforming of methanol (OSRM), with O2:H2O:CH3OH molar ratios = 0.12:1.1:1. The results were compared with those obtained in nonoxidative steam reforming (SRM) and in partial oxidation (POM) of methanol. The catalysts were highly active for OSRM, giving total CH3OH conversion at 350–400°C with GHSV = 60000–120000 h−1. They also showed high selectivity, giving H2 yield of 2.5 mol per mole CH3OH, with CO concentration below 500 ppm. Besides the main products, CH2O and (CH3)2O were also observed at all temperatures, while some CH4 was produced at T >300°C. The catalytic activity increased with increasing heating rate of the catalyst precursor. High selectivity was observed also in SRM, but with lower conversion. On the other hand, POM appeared faster than SRM, but produced CO concentrations of a few percent at T >300°C, due to CH3OH decomposition. The production of CO was strongly reduced in the presence of water vapor. The reaction network was described in detail. The interaction methanol catalyst was studied by IR spectroscopy. IR spectra showed the presence of adsorbed methoxy groups, that were converted into formate groups at high temperature, and then decomposed into H2 and CO in the absence of O2 and/or H2O, while in their presence CO was probably oxidized to CO2 before desorption, due to the action of Cu(II) species.