Raman, FTIR and theoretical evidence for dynamic structural rearrangements of vanadia/titania DeNOx catalysts

Abstract

Insight into the selective catalytic reduction (SCR) of NO by NH3 over vanadia/titania catalysts is obtained from a combination of Raman and FTIR spectroscopic investigations and density functional theory (DFT) calculations. Studies of the V–OH and V=O functional groups under different conditions coupled with calculations of the stability and mobility of H atoms provide evidence that dynamic structural rearrangements may occur during the SCR reaction. Hydrogen atoms are bonded more strongly to oxygen atoms that are coordinated to a single vanadium atom (V=O species), compared to bonding at oxygen atoms that are coordinated to multiple vanadium atoms (e.g., V–O–V species); and, activation energy barriers for hydrogen transfer from a V=O species to another V=O species and to a V–O–V species are estimated from DFT calculations to be 60 and 130 kJ/mol, respectively. This dynamic nature of hydrogen transfer between oxygen atoms having different coordination environments also appears to explain some of the spectroscopic changes observed for vanadia/titania catalysts having different vanadia loadings.