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

• O2 treatment of Mo2C catalysts suppresses metallic/alkaline catalytic activity.
• O2 co-feed reversibly alters Brønsted acid site density on Mo2C by a factor of ∼30.
• O∗–Mo2C Brønsted acid sites dehydrate IPA with a rate-determining step of β-hydrogen scission.
• Bulk 2–5 nm orthorhombic β-Mo2C crystallites were not affected by O2 exposure.

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.

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