Structure and site evolution of molybdenum carbide catalysts upon exposure to oxygen

Acid site densities could be reversibly tuned by a factor of ∼30 using an O₂ co-feed, which reversibly creates Brønsted acid sites on the carbide surface without altering the bulk crystal structure of 2-5 nm Mo₂C crystallites. Unimolecular isopropanol (IPA) dehydration at 415 K, a probe reaction, occurred on Brønsted acid sites of these oxygen-modified carbides with an intrinsic activation energy of 93 ± 1.3 kJ mole^-1 via an E₂ elimination mechanism with a kinetically-relevant step of β-hydrogen scission. Site densities were estimated via in situ 2,6-di-tert-butylpyridine (DTBP) titration and used to calculate a turnover frequency (TOF) of 0.1 s^-1, which was independent of site density. Oxygen co-processing allows for facile in situ tunability of acidic and metallic sites on highly oxophilic metal carbides.