(509bb) On the Evolution of Ceria Surfaces Towards Catalyzing Non-Oxidative Alkanol Dehydrogenation - a Transient Kinetic Analysis

Despite the fact of Ce⁴⁺ to Ce³⁺ interconversion facilitated via oxygen vacancy formation has been believed as the origin of the catalytic ability of ceria in different redox chemistries, the individual functionality of these oxidized and reduced surfaces, the effect of controlling one of these functionalities to the overall catalytic turnover, and any plausible shift in the reaction pathway and/or the overall reaction all remain obscure. In this study, we’ve conducted four distinct types of transient kinetic experiments of ethanol conversion over bulk cerium oxide to decipher the evolution of the surface, via reduction half of the oxidation turnovers, towards non-oxidative dehydrogenation. Aerobic-anaerobic switches at 498 K lead to new steady state dehydrogenation rates which implies the existence of both oxidative and non-oxidative turnovers over ceria. Concurrent termination of oxygen imbalances (reflecting ceria reduction) and induction periods (reflecting active site creation) in anaerobic experiments point to ethanol dehydrogenation turnovers owing their provenance to surface reduction. Implausible vacancy densities obtained from analysis of oxygen imbalances using acetaldehyde and CO₂ formation rates, unlike using water and CO₂ formation rates, point to the catalytic origin of at least part of the acetaldehyde formed during induction periods. Normalized water molar-flow rates, used as a measure of the relative contributions of catalytic and non-catalytic routes to acetaldehyde formation in absence of air, evince a transition from non-catalytic oxidation to catalytic dehydrogenation upon progressive surface reduction. Initial contribution of oxidative dehydrogenation can be suppressed by high-temperature hydrogen pretreatments implying the involvement of lattice oxygen in this stoichiometric event whereas phenol, an alpha hydrogen-free titrant, can selectively titrate sites contributing to catalytic ethanol dehydrogenation without altering the non-catalytic routes responsible for creating these sites in the first place. Not unimportantly, the results also point to an avenue for the water-free production of alkanals over reducible metal oxides.