Redox Dynamics of Active VO xSites Promoted by TiO xduring Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS

Abstract

Titania-supported vanadia (VOx/TiO2) catalysts exhibit outstanding catalytic in a number of selective oxidation and reduction processes. In spite of numerous investigations, the nature of redox transformations of vanadium and titanium involved in various catalytic processes remains difficult to detect and correlate to the rate of products formation. In this work, we studied the redox dynamics of active sites in a bilayered 5% V2O5/15% TiO2/SiO2catalyst (consisting of submonolayer VOxspecies anchored onto a TiOxmonolayer, which in turn is supported on SiO2) during the oxidative dehydrogenation of ethanol. The VOxspecies in 5% V2O5/15% TiO2/SiO2show high selectivity to acetaldehyde and an ca. 40 times higher acetaldehyde formation rate in comparison to VOxspecies supported on SiO2with a similar density. Operando time-resolved V and Ti K-edge X-ray absorption near-edge spectroscopy, coupled with a transient experimental strategy, quantitatively showed that the formation of acetaldehyde over 5% V2O5/15% TiO2/SiO2is kinetically coupled to the formation of a V4+intermediate, while the formation of V3+is delayed and 10-70 times slower. The low-coordinated nature of various redox states of VOxspecies (V5+, V4+, and V3+) in the 5% V2O5/15% TiO2/SiO2catalyst is confirmed using the extensive database of V K-edge XANES spectra of standards and specially synthesized molecular crystals. Much weaker redox activity of the Ti4+/Ti3+couple was also detected; however, it was found to not be kinetically coupled to the rate-determining step of ethanol oxidation. Thus, the promoter effect of TiOxis rather complex. TiOxspecies might be involved in a fast electron transport between VOxspecies and might affect the electronic structure of VOx, thereby promoting their reducibility. This study demonstrates the high potential of element-specific operando X-ray absorption spectroscopy for uncovering complex catalytic mechanisms involving the redox kinetics of various metal oxides.