Surface Coverage Analysis Reveals Potential-Determining Heterolytic Reactions during Thermocatalytic Aerobic Glucose Oxidation
Minju Chung, Karl Albrecht, Joshua M. Terrian, William Thomas Broomhead, David W. Flaherty
Abstract
High Resolution Image Download MS PowerPoint Slide Electric fields that form spontaneously at catalytic solid–liquid interfaces reflect the kinetics of surface reactions, coverage of reactive intermediates, and the nature of the microenvironments that encompass active sites. The measurement of these fields via the electrode potential of the catalyst and their interpretation can reveal mechanistic features of reactions that are not accessible by other methods. Here, the aqueous phase aerobic oxidation of glucose over carbon-supported platinum nanoparticles provides a representative model to demonstrate these concepts. Coupled analysis of steady-state rates for glucose oxidation and in situ open-circuit potentiometry obtained across a wide range of reactant concentrations (0.05–1 M glucose, 22–2170 kPa O 2, 353 K) reveal the fundamental connections between the electrode potential of the catalyst ( E cat ) and the kinetics of surface reactions. The interpretation of these phenomena through networks of Faradaic and non-Faradaic elementary steps shows that differences in scaling relationships among E cat, rates, and reactant concentrations signify transitions among kinetic regimes and dominant surface intermediates. Redox reactions with high degrees of potential control (i.e., those with the greatest contribution to determining E cat ) largely involve prevalent surface intermediates implied by the analysis of product formation rates. However, deviations between experimental observations and predictions obtained from mixed-potential theory show contributions from homolytic reaction pathways that emerge in response to high barriers for comparable heterolytic processes. These analyses demonstrate that concurrent interpretations of E cat and steady-state kinetics yield a deeper understanding of chemical phenomena at charged solid–liquid interfaces and reveal mechanistic features not otherwise evident.