(602d) Carbonate Dimorphism, and the Interpretation of Rates of (Non)-Stoichiometric Oxygen-Driven Mars-Van Krevelen Redox Cycles

The nature of non-stoichiometric domains on bulk metal oxide surfaces, and control over their density and catalytic function, are recurring themes in oxidation catalysis. Bulk nickel oxide catalyzed ethane oxidation is a prototypical example in this regard, with both the purported importance of surface non-stoichiometry as well as uncertainty as to its precise mechanistic function being the subject of numerous prior publications [1]. Despite its widespread importance in bulk oxide catalysis, we note that to date no characterization technique exists that enables the rigorous quantification of the entirety of surface non-stoichiometric oxygen. We hypothesized, based off of carbonate-mediated inhibition of ethane oxidation rates, that CO₂ exhibits stronger, irreversible binding onto non-stoichiometric oxygen, in contrast with its weaker binding onto stoichiometric oxygen. An analysis of the Dual-site Langmuirian nature of CO₂ adsorption isotherms, in concert with infrared spectroscopy data that identifies the two sites as stoichiometric and non-stoichiometric oxygen, reveals that non-stoichiometric oxygen-derived carbonates are the only rate inhibiting species present under reaction conditions. Transient ethane oxidation rates that evolve concomitantly with CO₂ coverage specifically over non-stoichiometric oxygen provide in part for the first time definitive evidence of their involvement in Mars-van Krevelen cycles producing ethene (Figure 1), in contrast with prior claims attributing to non-stoichiometric oxygen solely deep oxidation. The dimorphic nature of carbonates elucidated and exploited herein throws light on the basis for improvements in ethene selectivity that can be achieved through doping with niobium [2], and could prove highly useful in deciphering the mechanistic function of (non)-stoichiometric oxygen within Mars-van Krevelen cycles prevalent in a range of mixed metal oxide catalyzed partial oxidation reactions.

References: [1] Skoufa et al.; J. Catal. 2015, 322, 118–129 [2] Heracleous et al.; J. Catal. 2010, 270 (1), 67–75.