(677h) A Transient Kinetic Analysis of the Evolution of Cerium Oxide Towards Catalyzing Non-Oxidative Alkanol Dehydrogenation

Cerium oxide has been studied extensively as a catalytic material and also been applied commercially as an oxygen storage component in three-way catalysis. Even though ceria’s propensity to undergo rapid reduction-oxidation cycles has been believed as the key to its catalytic ability, individual functionality of these oxidized and reduced surfaces, effect of controlling these functionalities on catalytic properties, and any plausible shift in reaction pathways remain obscure. We’ve conducted four distinct types of transient kinetic experiments of ethanol conversion over ceria to decipher the evolution of the surface, from effecting the reduction half of ethanol oxidation turnover, towards ethanol dehydrogenation. Aerobic-anaerobic switches lead to new steady-state rates, rather than termination of catalytic acetaldehyde formation. Concurrent termination of oxygen imbalances (reflecting ceria reduction) and induction periods (reflecting active site creation) in anaerobic experiments point to dehydrogenation turnovers owing their provenance to surface reduction. Implausibly high vacancy densities obtained using acetaldehyde and CO₂ formation rates, unlike using water and CO₂ formation rates, point to the catalytic origin of at least part of acetaldehyde formed. Normalized water molar flowrates, used as a measure of the relative contributions of oxidative and non-oxidative routes, evince a transition from non-catalytic oxidation to catalytic dehydrogenation upon progressive surface reduction. High-temperature hydrogen pretreatments are used to manipulate the fractional surface reduction of starting materials, and impact both initial contribution of dehydrogenation turnover and fractional surface reduction that ethanol effectuates. Alpha hydrogen-free titrants like phenol, differently, can be used to selectively titrate sites catalyzing dehydrogenation turnovers without altering the prevalence of non-catalytic routes. Combination of transient experiments used herein capture, with unprecedented clarity, the evolution in catalytic function of ceria towards ethanol dehydrogenation upon progressive reduction that originates from its stoichiometric conversion. Not unimportantly, the results also point to an avenue for water-free alkanals production over reducible metal oxides.