Bulk mixed metal oxide compounds are employed as industrial oxidation catalysts for many reactions, but there is still debate in the heterogeneous catalysis literature about the nature of their catalytic active sites as well as their location in the catalyst that is responsible for the chemical transformations.
Lehigh Universitys department of chemical engineering (ChE) and department of material science (MatSci) teamed up to put this debate to rest.
Authors Kamalakanta Routray (ChE), Wu Zhou (MatSci), Christopher J. Kiely (MatSci), and Israel E. Wachs (ChE, Principal Investigator) used a multitude of characterization techniques to solve this long-standing debate with Infrared (IR) and Raman spectroscopy, High Resolution Transmission Electron Microscopy (HRTEM), Temperature Programmed Surface Reaction (TPSR) spectroscopy and steady-state kinetic analysis. Their results have been chosen for publication in the first issue of the new American Chemical Society: Catalysis peer-reviewed journal released for January 2011.
This article examines one model system, the bulk mixed metal oxide FeVO4, for the oxidation reaction of methanol (CH3OH) to formaldehyde (HCHO). For comparison, the systems of crystalline V2O5 and α-Fe2O3 phases and supported 4% V2O5/α-Fe2O3, possessing a two-dimensional surface VOx layer, were also studied in order to elucidate the contributions of pure phases and the surface vanadium phase to that of the bulk FeVO4 catalyst.
Raman spectroscopic analysis confirmed that the bulk FeVO4 catalyst is indeed of pure FeVO4 phase (not possessing extraneous V2O5 or α-Fe2O3 phases) and that the supported 4% V2O5/α-Fe2O3 catalyst only contains the vanadium oxide as an amorphous surface VOx monolayer on the bulk α-Fe2O3 support. The surface composition of all the samples was further probed with CH3OH adsorption and monitored with IR spectroscopy. The CH3OH-IR results revealed that the surfaces of V2O5, bulk FeVO4, and 4% V2O5/α-Fe2O3 all look the same, indicating that their surfaces consist of a surface VOx layer.
The presence of an enriched VOx layer of ~1 nm thickness on the surface of the bulk FeVO4 catalyst was directly confirmed with High Resolution Transmission Electron Microscopy (HRTEM). The presence of such an amorphous layer is usually quite difficult to detect by more conventional methods (XRD, X-ray Absorption Spectroscopy (XAS), solid state 51NMR spectroscopy, Electron Spin Resonance (ESR), Raman spectroscopy, etc.) that tend to be dominated by the signal from the bulk phase.
Spectrokinetic analyses definitively revealed that the catalytic active sites for methanol oxidation to formaldehyde and water by the bulk FeVO4 catalyst are the surface VOx species. The role of the bulk FeVO4 phase is simply to store the lattice oxygen that is used to reoxidize the surface VOx sites and the gas phase molecular O2 does not directly oxidize the reduced surface VOx sites (Mars-van Krevelen reaction mechanism).
This new insight is causing a paradigm shift in how catalytic reactions proceed by bulk mixed metal oxide catalysts that will require an examination of the previously accepted models for chemical transformations by bulk mixed metal oxide catalysts, says Dr. Wachs.
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The article is published in ACS: Catalysis 2011, 1, 54-66. dx.doi.org/10.1021/cs1000569