Catalysts consisting of metal particles supported on reducible oxides exhibit promising activity and selectivity for a variety of current and emerging industrial processes. Enhanced catalytic activity can arise from direct contact between the support and the metal or from metal-induced promoter effects on the oxide. Discovering the source of enhanced catalytic activity and selectivity is challenging, with conflicting arguments often presented based on indirect evidence. Here, we separate the metal from the support by a controlled distance while maintaining the ability to promote defects via the use of carbon nanotube hydrogen highways. As illustrative cases, we use this approach to show that the selective transformation of furfural to methylfuran over Pd/TiO2 occurs at the Pd-TiO2 interface while anisole conversion to phenol and cresol over Cu/TiO2 is facilitated by exposed Ti3+ cations on the support. This approach can be used to clarify many conflicting arguments in the literature.

Selective activation of renewable carboxylic acids on promoted molybdenum oxides to form alcohols and aldehydes is reported. A combination of reaction kinetics, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) reveals that the activity scales with the concentration of Mo5+ active sites and is a strong function of surface hydrogen coverage. The addition of a very small loading (0.05 wt %) of Pt drastically increases rates of selective deoxygenation at lower temperatures (<350 °C) but diminishes rates at elevated temperatures due to over-reduction of the support. Here, it is reported that the incorporation of Pt clusters on MoO3 decreases the apparent activation barrier for acid conversion by over 32 kJ/mol, which highlights the significant role of site regeneration facilitated by hydrogen splitting and spillover. Our findings suggest that the rate-determining step for converting pentanoic acid shifts upon introducing Pt clusters from formation/regeneration of oxygen vacancies to H addition to the carbonyl carbon.

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