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The viability of alkane oxidative dehydrogenation (ODH) processes specifically, and catalytic partial oxidation reactions more generally, are oftentimes limited by the formation of undesired deep oxidation products such as CO and CO2. The forced dynamic operation (FDO) of catalytic reactors has been proposed as a means for enhancing desired olefin or oxygenate selectivity and yield over those of CO and CO2, but an elucidation of the precise mechanistic bases for the dynamic enhancement observed continues to remain evasive. In this work, we provide an explanation of the extent of dynamic enhancement noted during ethane ODH over supported MoOx catalysts but not VOx ones─an explanation grounded in a quantitative analysis of the density and reactivity of chemisorbed and lattice oxygen species on these two classes of catalysts. Supported vanadia catalysts, unlike molybdena ones, carry oxygen species with similar reducibilities, resulting in highly contrasting trends in dynamic and steady state ODH properties for the two catalysts. Whereas in the case of VOx/Al2O3, oxygen speciation affects the nature of the hydrocarbon activated (ethane or ethylene), in the case of MoOx/Al2O3, it affects the type of product formed (ethylene or COx). Metal oxide loading is shown to be a key parameter impacting dynamic enhancement, with the FDO enhancement of higher loading molybdena samples converging toward that of the vanadia catalyst. The preferential depletion of chemisorbed oxygens is revealed to be a key determinant of the extent of dynamic enhancement, with an asymmetry in modeled O*/OL ratios under dynamic conditions relative to SS ones helping rationalize the effect that modulation frequency has on FDO enhancement. Collectively, the results presented here establish a quantitative, molecular-level basis for dynamic enhancement noted during the ODH of ethane, and point to considerations relating to the reactivity of chemisorbed and lattice oxygens as well as their dynamic and steady state ratios as levers for mitigating side-product formation through FDO. 

The precise effect of oxide understoichiometry on bulk oxide catalytic properties continues to remain a subject of intense investigation. Of specific interest in this regard is the role of oxygen vacancies present on bulk ceria catalysts that have recently been reported to represent a more cost‐effective alternative to the more toxic and expensive catalysts used industrially for the selective hydrogenation of acetylene to ethylene. Contrasting claims as to the effect of surface reduction on hydrogenation rates exist in the open literature, with vacancy formation attributed, in separate studies, either a favorable or a deleterious role in effecting hydrogenation turnovers. We report here the non‐monotonic behavior of ethene hydrogenation rates that subsumes both of these trends as a function of degree of surface reduction over a sufficiently large range of pre‐reduction temperatures. Steady state transient kinetic and isotopic exchange data combined with in‐situ titration experiments suggest that this non‐monotonic trend can be attributed not to a change in either the kinetic relevance of specific elementary steps or the hydrogenation mechanism, but rather to site requirements that stipulate the need for two distinct, proximal sites. We also show that the sensitivity of hydrogenation rates to surface reduction can be altered by varying ceria surface termination, with the more open (110) and (100) surfaces exhibiting a less asymmetric effect of surface reduction on ethene hydrogenation rates.