Effect of Sr Substitution in LaMnO₃(100) on Catalytic Conversion of Acetic Acid to Ketene and Combustion-Like Products

The chemistry of acetic acid reactions was studied over La_1–xSr_xMnO₃(100) surfaces (x = 0, 0.3, or 0.7). Two types of temperature-programmed reaction (TPR) experiments were performed: pre-exposure TPRs as well as continuous-exposure TPRs (CE-TPRs). The main products observed in both types of experiments were CO, CO₂, H₂O, acetaldehyde, and ketene (ethenone). In acetic acid CE-TPR experiments with 1:1 codosing of oxygen, the surfaces showed ketene production in the order of LaMnO₃(100) > La_0.7Sr_0.3MnO₃(100) > La_0.3Sr_0.7MnO₃(100). The presence of oxygen increased the absolute amount of ketene produced. The results are consistent with ketene being formed by a dehydration mechanism, with the pathway suppressed if there are too many surface oxygen vacancies. The effect of Sr substitution can be explained as increasing the ease of (or initial quantity of) oxygen vacancies, thereby decreasing the amount of surface oxygens which ultimately inhibits both ketene formation and formation of the combustion-like products CO, CO₂, and H₂O. Our interpretation is that both of these routes are catalyzed by sites (or site ensembles) that include surface oxygens, which is consistent with other published studies.