Generally, the chemistry of VPO-based catalysts was controlled by the properties of surface defects. Therefore, the effect of isolated vacancies on the reactivity of vanadyl pyrophosphate, the main component of VPO catalysts, seems to be a natural starting point for a theoretical analysis to understand better the real material. Here, density functional calculations were made of the electronic structure of different clusters which modeled the defective (VO)2P2O7(100) surface at which a neutral O monovacancy was incorporated. This complemented previous investigations of the energetics and a Löwdin charge analysis of the same type of defect. The electronic structure of the defective (100) surface was discussed by means of the total and atomic (partial) density of states to show modifications of the electronic states caused by the differently located vacancies. A theoretical estimation of the density of states reveals evidence for the presence of color center-induced levels for high-coordinated vacant sites and their absence in the case of low-coordinated vacancies. It was shown that only in the case of high-coordinated O vacancies the band gap was reduced relative to the clean surface. Specific features of the density of states projected onto different cationic and anionic surface sites agreed with experimental evidence on their implications for catalytic performance.

Chemical Nature of Point Defects at the (VO)2P2O7(100) Surface. A.Haras, M.Witko, D.R.Salahub, H.A.Duarte: Surface Science, 2003, 538[3], 160-70