Surface chemistry of supported chromium oxide catalysts

A series of 1% CrO[sub 3] supported on different oxide supports (Al[sub 2]O[sub 3], TiO[sub 2], ZrO[sub 2], Nb[sub 2]O[sub 5], SiO[sub 2], 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 Cr[double bond]O bonds preferentially exist on the Al[sub 2]O[sub 3], TiO[sub 2], ZrO[sub 2], and Nb[sub 2]O[sub 5] supports, and isolated chromate species with two Cr[double bond]O bonds predominate on the SiO[sub 2] support. Nonstoichiometric compounds such as Mg[sub x](CrO[sub 4])[sub y] are present on the MgO support due to the strong acid-base interaction of CrO[sup 2[minus]][sub 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. 33 refs., 2 figs., 1 tab.