Redox Dynamics of Active VO x Sites Promoted by TiO x during Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS.

Titania-supported vanadia (VO _x /TiO ₂ ) 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% V ₂ O ₅ /15% TiO ₂ /SiO ₂ catalyst (consisting of submonolayer VO _x species anchored onto a TiO _x monolayer, which in turn is supported on SiO ₂ ) during the oxidative dehydrogenation of ethanol. The VO _x species in 5% V ₂ O ₅ /15% TiO ₂ /SiO ₂ show high selectivity to acetaldehyde and an ca. 40 times higher acetaldehyde formation rate in comparison to VO _x species supported on SiO ₂ with 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% V ₂ O ₅ /15% TiO ₂ /SiO ₂ is kinetically coupled to the formation of a V ⁴⁺ intermediate, while the formation of V ³⁺ is delayed and 10-70 times slower. The low-coordinated nature of various redox states of VO _x species (V ⁵⁺ , V ⁴⁺ , and V ³⁺ ) in the 5% V ₂ O ₅ /15% TiO ₂ /SiO ₂ catalyst 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 Ti ⁴⁺ /Ti ³⁺ 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 TiO _x is rather complex. TiO _x species might be involved in a fast electron transport between VO _x species and might affect the electronic structure of VO _x , 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.
Competing Interests: The authors declare no competing financial interest.
(© 2022 The Authors. Published by American Chemical Society.)