Trends in C-H bond dehydrogenation energetics for small molecule conversion

Abstract

Low-temperature activation of C-H bonds and the conversion of C-H bond containing small molecules has remained a holy grail of chemical reactions over the past few decades. The design of materials to maximize product selectivity for wide–ranging energy and environmental applications is typically carried out by a creating of small library of materials. Optimal catalysts are identified by a series of measurements, and in most cases the underlying reaction mechanism is not well understood leading to difficulty in designing future catalysts. Systematic studies have to be carried out in order to investigate the catalyst surface under reaction conditions to probe the nature of reaction intermediates as well as the products of the reaction.

In this thesis, we studied the interaction of small molecules such as formaldehyde, methanol, methane, and propane with oxide surfaces to reveal trends in adsorption energies, product selectivity, and reaction rates. We achieve this by developing suitable design descriptors by studying the reaction mechanism in situ. We first generated a library of manganese oxide catalysts to probe the reaction mechanism for formaldehyde oxidation to CO2 at room temperature. We identified γ-MnOx to have one of the highest reaction rates for formaldehyde oxidation and show that catalytic activity can further be improved by the addition of water. We then show that room temperature selective methanol oxidation towards methyl formate and methane oxidation to CO2 can be realized by increasing the surface oxygen activity of iridium oxide-based catalysts. We further developed a rational design approach for perovskite oxides by tuning the surface O 2p-band center to selectively oxidize methanol to formaldehyde. Finally, we extended this descriptor-based approach for oxidative dehydrogenation of propane to propene. Thus, using a combination of kinetic measurements, surface sensitive in situ techniques, and theoretical calculations, we show how catalyst surface can be designed to optimize product selectivity.