| Abstract
| - This study provides kinetic and isotopic evidence for the identity and kinetic relevance of elementary steps in CH3OH−H2O reactions on monofunctional Cu-based catalysts. H2, CO2, and CO were the only products formed in these reactions, which proceed via kinetically relevant C−H bond activation in chemisorbed methoxides. All other elementary steps are in quasi-equilibrium, consistent with the effects of reactants and products on rates, with isotopic tracing and kinetic isotope evidence, and with the equilibrated nature of water gas shift reactions at all reaction conditions. Turnover rates increased with CH3OH pressure but did not depend on CO, CO2, or H2O concentrations. H2 inhibited CH3OH−H2O reactions by decreasing the concentration of surface methoxide intermediates via quasi-equilibrated CH3OH dissociation steps. Isotopic scrambling was complete among hydroxyl groups in methanol and water, but undetectable among hydroxyl and methyl groups in methanol, consistent with quasi-equilibrated O−H activation and irreversible C−H activation steps. Deuterium substitution at methyl groups in methanol gave normal kinetic isotope effects, whereas substitution at hydroxyl groups in methanol or water led to weaker isotope effects consistent with their thermodynamic origin from quasi-equilibrated O−H activation steps. These mechanistic conclusions are consistent with detailed kinetic data on both large and small Cu clusters and with reforming pathways requiring only Cu surfaces to complete catalytic turnovers. CH3OH−H2O turnover rates increased weakly with Cu dispersion as Cu cluster size decreased from 30 to 5 nm, suggesting that these reactions are insensitive to structure when CH3OH−H2O reaction occurs on Cu via monofunctional pathways limited by H-abstraction from methoxide intermediates. This structure insensitivity may reflect the titration of low-coordination surfaces with strongly held and unreactive forms of adsorbed intermediates, causing turnovers to occur preferentially on low-index surfaces for clusters of varying size.
|
