Redox Dynamics of Active VO_x Sites Promoted by TiO_x during Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS

Abstract

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.

Beware of beam damage under reaction conditions: X-ray induced photochemical reduction of supported VOx catalysts during in situ XAS experiments

“…The pre-edge intensity, besides, depends on the coordination number of vanadium and decreases in the following order: tetrahedral–pentahedra–octahedra (for details refer to ref. 36 and to the ESI,† Section 3.1). In this work, for the rapid qualitative analysis of the oxidation state of vanadium, we decided to use two descriptors: the pre-edge height and the edge position.…”

“…We estimated the concentration of oxygen traces in the operando cell of ca. 0.02 vol% under our working conditions based on the residual (background) transformation of ethanol into acetaldehyde over a 5 wt% V 2 O 5 /15 wt% TiO 2 /SiO 2 catalyst of 1 × 10 −6 mol AcH min −1 measured in an ethanol–helium flow of 50 mL min −1 at 160–210 °C using 15–20 mg of the catalyst and considering a complete transformation of oxygen traces into acetaldehyde 36 At 120 °C, catalytic ethanol oxidation starts (Fig. 1b), which competes with re-oxidation, and starting from 190 °C, chemical vanadium reduction by ethanol prevails over all other processes.…”

“…The majority of experiments described in this work were performed using a model bilayered 5 wt% V 2 O 5 /15 wt% TiO 2 / SiO 2 catalyst, 44 which we used in our previous work which was focused on the redox activity of vanadium and titanium during thermocatalytic ODH of ethanol. 36 This catalyst has close to one monolayer titanium coverage and was prepared as an analog of a widely used V 2 O 5 /TiO 2 catalyst. The latter one demonstrates outstanding activity in a variety of redox catalytic reactions, [45][46][47][48] however, it could not be efficiently investigated by timeresolved V K-edge XAS due to almost full X-ray absorption by titanium (the Ti K-edge, 4966 eV, is located just below the V K-edge, 5465 eV).…”

“…41 Vanadium reduction in these publications was associated with the loss of lattice oxygen, which is enhanced by ultra-high vacuum, 43 whereas the presence of low levels of oxygen in the gas phase prevented or slowed down this process. 42 In our previous work, using operando quick XAS, 36 we showed that the rate of ethanol oxidation to acetaldehyde over a bilayered 5 wt% V 2 O 5 /15 wt% TiO 2 /SiO 2 catalyst (consisting of sub-monolayer VO x species anchored onto a TiO x monolayer supported on SiO 2 ) is kinetically coupled to the V 5+ -V 4+ redox process. The effect of the X-ray beam on the kinetic data at the catalysis relevant temperatures (above 150 1C) was tested and excluded.…”

“…30,31 The mechanisms of heterogeneous catalytic reactions of selective oxidation and reduction over vanadium-based catalysts are still being revised by modern in situ and operando X-ray-based techniques, such as XPS and XAS. [32][33][34][35][36][37][38][39][40] There are several publications, which describe X-ray-induced vanadium reduction during XPS experiments. 26,[41][42][43] It was suggested that the electron-hole pair generation or Auger decay may lead to holes in the valence band, which from a physical point of view are similar to bond breaking.…”