Isotopic and Kinetic Assessment of the Mechanism of CH₃OH−H₂O Catalysis on Supported Copper Clusters

Abstract

This study provides kinetic and isotopic evidence for the identity and kinetic relevance of elementary steps in CH 3 OH-H 2 O reactions on monofunctional Cu-based catalysts. H 2 , CO 2 , 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 CH 3 OH pressure but did not depend on CO, CO 2 , or H 2 O concentrations. H 2 inhibited CH 3 OH-H 2 O reactions by decreasing the concentration of surface methoxide intermediates via quasi-equilibrated CH 3 OH 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. CH 3 OH-H 2 O 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 CH 3 OH-H 2 O 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.

Reaction Path Analysis of CO2 Reduction to Methanol through Multisite Microkinetic Modelling over Cu/ZnO/Al2O3 Catalysts

“…decreased water production rate when a large fraction of CO was present, we also added the water gas shift reaction and determined its coefficients. In addition, the approach to equilibrium [53] was calculated and determined that it has insignificant effect on apparent reaction coefficient determination. All the comparisons between experimental and model data along with approach to equilibrium calculations can be found in Supporting Information in Section 5.…”

A Kinetic Isotope Effect in the Formation of Lanthanide Phosphate Nanocrystals

“… 1 , 2 Since a strong isotope effect is associated with forming or breaking of bonds involving the isotopically substituted atoms, it informs about which bond is formed or broken, elucidating the reaction pathway. 3 − 5 Previous works regarding biochemical processes isolated from organisms for in vitro laboratory studies have shown pronounced changes in the reaction rate upon changing the solvent from H 2 O to D 2 O. 6 Similar effects were also studied for hydrolysis reactions in organometallic systems and for the enzymatic oxidation of alcohols and amines.…”

Formation of Copper Nanoparticles in LTL Nanosized Zeolite: Kinetics Study

“…because of their high conductivity, catalytic properties, and much lower cost . Indeed, supported copper catalysts have been utilized successfully in a wide range of reactions, including selective catalytic reduction of NO x and SO 2 , , synthesis and decomposition of methanol, − direct dimethyl ether synthesis from syngas, , dehydrogenation of cyclohexanol, and acetone hydrogenation . However, compared to the extensive studies on gold and silver, reports on copper nanoparticles are still limited, primarily because of relatively low Cu 0 /Cu 2+ redox potential (+0.34 V), which causes spontaneous oxidation at ambient conditions. ,, It has been reported that aggregation and oxidation of Cu NPs can be avoided by using various protecting agents, such as polymers, organic ligands, or porous materials …”