Surface Reactions and Catalytic Activities for Small Alcohols over LaMnO₃(100) and La₀.₇Sr₀.₃MnO₃(100): Dehydrogenation, Dehydration, and Oxidation

The surface chemistry and catalytic activity of small alcohols (methanol, ethanol, and propan-2-ol) were studied over LaMnO₃(100) and La₀.₇Sr₀.₃MnO₃(100) thin films. The observed C-containing products corresponded to products typically associated with dehydrogenation (methanal, ethanal, and propanone), dehydration (ethene and propene), and oxidation (COₓ). No coupling products were observed. Two types of temperature-programmed reaction experiments (TPR) were performed: pre-exposure TPR (PE-TPR) experiments in which the reactant is preadsorbed and the temperature is then ramped, as well as continuous exposure TPR (CE-TPR) experiments during which the reactants are continuously introduced and the temperature is ramped. Mechanistic pathways are proposed based on the results and literature, including removal of lattice oxygens by surface hydrogens, organic intermediates adsorbed in oxygen vacancies, C–H bond breaking by alkoxy disproportionation reactions, and direct C–H bond breaking. The vacancy related pathways are suppressed by the Sr substitution. The data are interpreted to indicate that Sr substitution causes vacancies to bind adsorbates less strongly, resulting in decreased catalytic activity for the vacancy-mediated reactions in the study. Sr substitution increases the ratio of alkene production to aldehyde/ketone production. Catalytic turnover frequencies are reported for the C-containing products for the temperatures of 650 and 750 K for both surfaces.