(645d) Structure and Site Evolution of Molybdenum Carbide Catalysts upon Exposure to Oxygen
Self-supporting molybdenum carbide (Mo2C) has been reported to possess acidic, basic, and metallic active sites that can catalyze dehydration, carbonyl condensation, and hydrogenation reactions. The complex and tunable nature of these carbidic formulations was investigated using isopropanol (IPA) as a probe reaction at ambient pressure and low temperature (120 – 160˚C). We report the ability to decrease the metallic and alkaline site densities while concurrently increasing acid site densities with an in situ O2 co-feed. Brønsted acid site densities could be reversibly modified by a factor of ~30 via O2 co-feed. IPA dehydration occurred over Brønsted acid active sites with an activation energy of 93 kJ mol-1 via an E2 elimination mechanism with a β-hydrogen scission rate determining step as identified using the kinetic isotope effect. IPA turnover frequency (TOF) at 142˚C was a constant 0.1 s-1 over a 80-fold range of site densities as measured via in situ 2,6-di-tert-butylpyridine titration experiments. Bulk 2-5 nm orthorhombic Mo2C crystallites were unaffected by O2 treatment as evinced by X-ray diffraction and transmission electron microscopy.