A transient kinetic analysis of the evolution of a reducible metal oxide towards catalyzing nonoxidative alkanol dehydrogenation

Abstract

Quantitative analyses of four sets of aerobic/anaerobic ethanol conversion transients point to the evolution of a native high-surface-area cerium oxide surface that effects the reduction half of the ethanol oxidation turnover to catalyzing, exclusively, nonoxidative ethanol dehydrogenation upon complete surface reduction. Aerobic–anaerobic switches at 498 K lead to new steady state dehydrogenation 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 ethanol dehydrogenation turnovers owing their provenance to surface reduction. Implausibly high vacancy densities obtained from analysis of oxygen imbalances when using acetaldehyde and CO2 formation rates, unlike those obtained when using water and CO2 formation rates, point to the catalytic origin of at least part of the acetaldehyde formed during both aerobic–anaerobic switches and anaerobic induction periods. Normalized water molar flow rates, used as a measure of the relative contributions of catalytic and stoichiometric routes to acetaldehyde formation, evince a transition from stoichiometric ethanol oxidation to catalytic ethanol dehydrogenation upon progressive surface reduction. High-temperature hydrogen pretreatments can be used to manipulate both the initial contribution of ethanol dehydrogenation to overall acetaldehyde formation as well as the fractional surface reduction that ethanol, rather than molecular hydrogen, effectuates. Alpha hydrogen-free titrants such as phenol, on the other hand, can be used to titrate sites contributing to catalytic alkanol dehydrogenation without altering the prevalence of stoichiometric routes responsible for site creation in the first place. The combination of transient experiments used herein capture, with clarity, the evolution in catalytic function of high-surface-area cerium oxide toward ethanol upon progressive reduction that originates from its stoichiometric conversion over a native, fully oxidized surface. Not unimportantly, the results also point to an avenue for the water-free synthesis of alkanals over reducible metal oxides.