Electronic Structure of Discrete Pseudotetrahedral Oxovanadium Centers Dispersed in a Silica Xerogel Matrix: Implications for Catalysis and Photocatalysis

Abstract

The electronic structure of psuedotetrahedral oxovanadium groups (-O3V=O) dispersed in a silica xerogel matrix, is determined on the basis of a spectroscopic investigation. From this investigation it was found that the highest occupied molecular orbital of this species is a nonbonding a2 symmetry orbital localized on the basal plane ligands. The first excited state is assigned to an E symmetry triplet resulting from a one-electron promotion from this a2 nonbonding orbital to an e symmetry antibonding orbital of the terminal V=O group. On the basis of this orbital description, the long-lived, vibronically structured emission at 549 nm is assigned to a 3E 'AI transition from the e antibonding orbital back down to the a2 nonbonding orbital [(a2)'(e*)'] [(ad2(e*)]. The vibronic progression in the emission band at 977 & 10 cm-I, previously assigned to the terminal V=O stretch, is reassigned to a V-0 stretch involving the basal plane oxygens, consistent with the orbital assignment. Contrary to previous descriptions, excitations involving x x* type transitions localized on the terminal V=O group lie at higher energy. The first well-resolved singlet band at 290 nm is of AI symmetry and has a resolved vibronic progression which corresponds to the terminal V=O stretch. This band is assigned to a 'AI 'AI transition involving a [(e)4(a2)2(e*)] [(e)3(a2)2(e*)'] one-electron promotion which can qualitatively be described as a x-x* V=O transition. The electronic structure of the pseudotetrahedral oxovanadium group established in this study differs dramatically from the conventionally accepted model which localizes the ground and first excited state on the terminal V=O group. This new description, however, is completely consistent with observed photochemical processes and, unlike the previous model, provides a coherent explanation of how factors such as the nature of the substrate directly affect the oxovanadium center. Vanadium oxide dispersed on metal and semimetal oxide supports catalyze a number of important reactions. Catalysts of this type have been used for the selective catalytic oxidation (SCO) of a number of substrates, including aliphatic and aromatic hydrocarbons, and for the selective catalytic reduction (SCR) of nitric oxide with ammonia.' In addition, a number of photochemically induced transformations such as the photooxidation of CO, the photoisomerization of butene, and the photopolymerization of acetylene have also been ~ b s e r v e d . ~ , ~ The reactivity and selectivity of these catalyst systems is highly dependent on the substrate on which the oxovanadium is dispersed and in some cases on the specific crystallographic phase of that substrate! Increasingly, studies of these catalysts have suggested that substrates with monolayer or submonolayer oxovanadium coverage form, in many cases, the most reactive catalysts. This heightened reactivity is attributed to the presence + University of Califomia, Riverside. California Institute of Technology. 5 Current address: Dept. of Chemistry, Florida State University, Tal@ Abstract published in Advance ACS Abstracts, February 1, 1995. (1) (a) Oyama, S . T. Res. Chem. Inter. 1991, 15, 165. (b) Bond, G. C.; Tahir, S. F. Appl. Catal. 1991, 71, 1. (2) (a) Anpo, M.; Tanahashi, I.; Kubokawa J . Phys. Chem. 1980, 84, 3440. (b) Anpo, M.; Sunamoto, M.; Che, M. J . Phys. Chem. 1989, 93, 1187. (3) Stiegman, A. E.; Eckert, H ; Plett, G.; Anderson, M.; Yavrouian, A. Chem. Mater. 1993, 5, 1592. (4) Cristiani, C.; Forzatti, P.; Busca, G. J . Catal. 1989, 116, 586 and references therein. lahassee, FL 32306. 0002-7863/95/1517-2618$09.00/0 of discrete oxovanadium centers in a pseudotetrahedral coordination environment which are bound to the substrate.