Origin of the Activity Trend in the Oxidative Dehydrogenation of Ethanol over VOx /CeO2

Supported vanadia (VO_x ) is a versatile catalyst for various redox processes where ceria-supported VO_x have shown to be particularly active in the oxidative dehydrogenation (ODH) of alcohols. In this work, we clarify the origin of the volcano-shaped ethanol ODH activity trend for VO_x /CeO_x catalysts using operando quick V K- and Ce L₃ - edge XAS experiments performed under transient conditions. We quantitatively demonstrate that both vanadium and cerium are synergistically involved in alcohol ODH. The concentration of reversible Ce⁴⁺ /Ce³⁺ species was identified as the main descriptor of the alcohol ODH activity. The activity drop in the volcano plot, observed at above ca. 3 V nm^-2 surface loading (ca. 30 % of VO_x monolayer coverage), is related to the formation of spectator V⁴⁺ and Ce³⁺ species, which were identified here for the first time. These results might prove to be helpful for the rational optimization of VO_x /CeO₂ catalysts and the refinement of the theoretical models.

Theoretical studies on the monomeric vanadium oxides supported by ceria: the atomic structures and oxidative dehydrogenation activities

Structures and activities of CeO₂(111) supported monomeric VO_x with different O contents have been systematically studied by using first principles calculations.

Size-dependent catalytic activity of supported vanadium oxide species: oxidative dehydrogenation of propane

Possible reaction pathways for the oxidative dehydrogenation of propane by vanadium oxide catalysts supported on silica are examined by density functional theory. Monomeric and dimeric vanadium oxide species are both considered and modeled by vanadyl-substituted silsesquioxanes. The reaction proceeds in two subsequent steps. In a first step, hydrogen abstraction from propane by a vanadyl (O═V) group yields a propyl radical bound to a HOV(IV) surface site. Propene is formed by a second hydrogen abstraction, either at the same vanadia site or at a different one. V(V)/V(IV) redox cycles are preferred over V(V)/V(III) cycles. Under the assumption of fast reoxidation, microkinetic simulations show that the first step is rate-determining and yields Arrhenius barriers that are lower for dimers (114 kJ/mol at 750 K) than for monomers (124 kJ/mol). The rate constants predicted for a mixture of monomers and dimers are 14% larger (750 K) than for monomers only, although the increase remains within experimental uncertainty limits. Direct calculations of energy barriers also yield lower values for dimeric species than for monomeric ones. Reactivity descriptors indicate that this trend will continue also for larger oligomers. The size distribution of oligomeric species is predicted to be rather statistical. This, together with the small increase in the rate constants, explains that turnover frequencies observed for submonolayer coverages of vanadia on silica do not vary with the loading within the experimental uncertainty limits.