Characterization of supported vanadium oxide catalysts by low temperature oxygen chemisorption technique: II. The V2O5/SiO2 system

Abstract

Low Temperature Oxygen Chemisorption (LTOC) has been applied to characterize a series of silica-supported V-oxide catalysts containing up to 5 μmol V m−2. LTOC data, coupled with ancillary information obtained from ESR, IR, XRD, ESCA and surface area measurements, have thrown light on the nature of the dispersed vanadia which has been found to consist of two distinct highly dispersed V-oxide phases. Based on the experimental results, a model of the surface structure of the dispersed phases has been proposed. It has been envisioned that a “monolayer” phase containing, in small part, isolated (VO4)4− species and in great part, a dimeric V5+ oxide species, both in tetrahedral coordination, is formed at lower vanadia loading region, while at loadings beyond the so-called “monolayer” coverage a second phase consisting of only V5+-oxide in square pyramidal or octahedral surroundings (i.e., a two dimensional structure akin to that of bulk V2O5) is formed preferentially on the top of the “monolayer” phase. The completion of the “monolayer” and the beginning of the formation of the “post monolayer” phases have been clearly identified by a maximum in the oxygen uptake capacity of the samples.