The origin of the ligand effect in supported and bulk metal oxide catalysts: In situ infrared, Raman, UV-Vis DRS, and kinetic studies during methanol oxidation.

摘要

The strong influence of the oxide support upon the turn-over frequency (TOF) of methanol oxidation over supported metal oxide catalysts has been well documented in recent years, but has not yet been completely understood at a fundamental level. The present dissertation probes the mechanistic origin of this interesting phenomenon (e.g., adsorption equilibrium of methanol to surface methoxy species, rate-determining methoxy surface decomposition, and product desorption equilibrium) and also extends the investigation to include similar ligand effects in bulk metal oxide catalysts. Qualitative in situ Raman, IR, and UV-Vis DRS spectroscopy of supported vanadia catalysts during steady-state methanol oxidation demonstrated that the molecular structures of surface vanadia species are very sensitive to the coordination and H-bonding effects of the adsorbed surface methoxy species (the reactive surface intermediates). Hence, spectroscopic monitoring of the surface methoxy intermediates, instead of the surface metal oxide species, provides more reliable information about the surface reaction kinetics responsible for ligand effects in methanol oxidation.;Quantitative in situ infrared (IR) spectroscopy was used to determine the active surface site densities of metal oxide catalysts (methanol chemisorption techniques) and, also, the concentrations of adsorbed surface methoxy intermediates present on the metal oxide surfaces during steady-state methanol oxidation. The intrinsic TOR of bulk molybdates were shown to be relatively similar to those of model supported catalysts with the same co-cation (e.g., MoO3/NiO vs. NiMoO4)—possibly due to the formation of a “monolayer” of surface molybdenum oxide species on the surfaces of the bulk metal molybdates. Furthermore, quantification of the adsorbed, steady-state concentrations of surface methoxy intermediates using a novel in situ IR cell designed to operate as a fixed-bed catalytic reactor allowed for decoupling of the kinetic steps in the reaction mechanism. Comparisons of the relative values of the adsorption equilibrium constants, Kads, and the kinetic rate constants for the surface decomposition step, krds, indicate that the TOF clearly correlates with the rate constant for the surface decomposition step. More fundamentally, the electronegativity of the ligand (or support) cation determines the magnitude of krds via its influence on the hydride abstraction of methyl hydrogen by the active metal cation.