Comparative studies of manganese-doped copper-based catalysts: the promoter effect of Mn on methanol synthesis

A Mn-promoted Cu/ZnO/Al2O3 catalyst with 5–10% improvement in methanol yield as compared with the undoped Cu/ZnO/Al2O3 catalyst under identical conditions has been synthesized. In order to understand the promoter effect of Mn on the methanol production, Cu/Al2O3 and Cu/SiO2 catalysts with and without various amounts of Mn were also prepared and comparatively investigated by microreactor, N2O titration, TPR (Temperature Programmed Reduction) and XPS (X-ray Photoelectron Spectroscopy). Experimental results show that these Mn-promoted catalysts have higher catalytic activity, larger surface Cu concentration, and elevated Cu reduction temperature than their Mn-undoped counterparts. XPS study gives evidence that in the reduced or reacted catalysts, the interaction between Mn and Cu occurs, resulting in the reduction of Mn4+ to Mn2+ or Mn3+, as well as the oxidation of Cu0 and Cu+ to higher oxidation states. This interaction may be responsible for the harder Cu reduction process, better Cu dispersion, higher active center concentration, and thus enhanced methanol yield.