Insights into the rates and kinetic relevance of steps mediating redox turnovers are often limited in clarity and rigor by the heterogeneity of site speciation characteristic of most traditional synthetic heterogenous catalytic systems. Metal-organic framework materials (MOFs) offer an opportunity to investigate catalytic properties of well-defined multinuclear sites, the structure of which persist upon subjection to gas phase turnovers. In this study, we elucidate the density and identity of active sites, reaction kinetics, and mechanism of CO oxidation over mixed-valence oxo-bridged trimers in MIL-100 that each carry one M2+ and two M3+ open-metal sites. Steady state and transient kinetics experiments evidence steps constituting exclusively the oxidation half cycle to be kinetically relevant, with activation energies reflecting standard state free energy differences between Fe(IV)=O intermediates and gas phase nitrogen, and the bare Fe2+ surface and gas phase N2O. We show that CO oxidation turnover frequencies and apparent activation energies can be systematically tuned through manipulation of second sphere coordination environment, with both quantities decreasing monotonically with increasing average trimer electron density (Figure 1). Rate/activation energy trends can be rationalized using the average charge on each trimer determined by the coverage and identity of bound oxygenates, with larger alkyl chain lengths leading to higher electron densities and hence lower rates/activation energies at constant oxygenate coverage. Overall, the results provide clarity as to the effect of second sphere coordination environment on the rates of redox steps occurring over multinuclear metal clusters, and point to the utility of metal organic framework materials in addressing questions hitherto rendered intractable due to heterogeneity in active site speciation.

Active sites, kinetics, and second sphere coordination effects for CO oxidation over mixed-valence oxo-bridged trimers

ACS Fall 2021

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