Surface Chemistry of Supported Chromium Oxide Catalysts

A series of 1% CrO3 supported on different oxide supports (Al2O3, TiO2, ZrO2, Nb2O5, SiO2, and MgO) was structurally characterized by Raman spectroscopy and chemically probed by methanol oxidation. The Raman results reveal that the polymeric chromium oxide species possessing two terminal CrO bonds preferentially exist on the Al2O3, TiO2, ZrO2, and Nb2O5 supports, and isolated chromate species with two CrO bonds predominate on the SiO2 support. Nonstoichiometnc compounds such as Mgx(CrO4)y are present on the MgO support due to the strong acid-base interaction of CrO2−4 with MgO. The methanol oxidation reaction reveals that the reactivity of the surface chromium oxide species was primarily controlled by the specific oxide support which acts as a ligand, and not the molecular structure of the surface chromium oxide species. The selectivity of the supported chromium oxide catalysts is influenced by the acid-base characteristics of the oxide supports.