Surface Reactions and Catalytic Activities for Small Alcohols over LaMnO₃(100) and La_0.7Sr_0.3MnO₃(100): Dehydrogenation, Dehydration, and Oxidation

Abstract

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 (CO x ). 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.

Ethanol Conversion over La0.7Sr0.3MnO3–x(100): Autocatalysis, Adjacent O-Vacancies, Disproportionation, and Dehydrogenation

“…In some earlier steady-state studies of ethanol oxidation over LaMnO 3 -based catalysts, , ethene production was not reported. However, in our recent study and in this study, ethene production is noticeable over a La 0.7 Sr 0.3 MnO 3– x catalyst both during PE-TPR (above) and during continuous exposure (Section ). A relevant question is whether ethene is formed as a product via acetaldehyde formation or separately from acetaldehyde formation.…”

“…Based on this temperature range, the 400–430 K ethanol is ascribed to being produced from intermediates in O-vacanciessuch as by recombination of alkoxies in O-vacancies with surface hydrogens or more complex chemistry (this has been observed over weakly reduced V 2 O 3 (0001) , and CeO 2 (111)). The observation of some products over the oxidized surface in this temperature rangealthough representing much less than a monolayerindicates that there are either intrinsic O-vacancies/defects even after oxidation or that some O-vacancies are formed in situ during PE-TPR . This is not surprising as pre-oxidized perovskites are expected to have some O-vacancies at the start of the experiment due to thermodynamics and other considerations ,− and because it is known that some O-vacancies can be formed by reactions even before 300 K .…”

“…This trend can be rationalized if the aldehyde production is from a disproportionation reaction that requires neighboring ethoxies in O-vacancies (a previously unknown pathway), while the ethene production is from a β-dehydrogenation route involving single ethoxies in O-vacancies (eqs and ), respectively. These reactions are the critical steps for acetaldehyde and ethene production and fit into a larger set of reactions elucidated over the same surface recently …”

CheKiPEUQ Intro 1: Bayesian Parameter Estimation Considering Uncertainty or Error from both Experiments and Theory**

“…Temperature programmed reactions (including temperature programmed desorption) are transient kinetics experiments and are often utilized to try to gain insight into the mechanisms and kinetics of surface reactions. Studies investigating transient kinetics through use of differential equations to simulate the rates and concentrations across time can be useful for obtaining kinetic parameters for complex reaction networks (including for cases where there are coverage dependent activation energies) [8,83–97] …”

A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols

“…Perovskites with ABO 3 structure have been employed as catalysts in the context of ethanol oxidation [87c,d,g,h] . Partial substitution of Mn with Ni into the solid solution of LaMnO 3 perovskites generates Ni−O−Mn bridging lattice oxygen sites.…”