Reaction Pathways and Microkinetic Modeling of n-Butane Oxidation to 1-Butanol on Cu, Cu3Pd, Pd, Ag3Pd, and PdZn (111) Surfaces
Jiazhou Zhu, Rachel B. Getman
Index: 10.1021/acs.iecr.8b00589
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Abstract
Density functional theory (DFT) calculations and microkinetic modeling are used to model reactions in the oxidation of n-butane to 1-butanol, 1-butanal, and 1-butene over pure metal and metal alloy (111) surfaces. Specifically, catalytic thermodynamic and kinetic energies are calculated with DFT, and linear scaling relationships are developed that link these values to simpler “descriptors” of catalytic activity. The scaling relationships are used in microkinetic modeling to identify the optimal descriptor values, which maximize the rate and selectivity to 1-butanol. Degree of rate control (DRC) analysis is performed to reveal the catalytic intermediates and transition states that have the greatest influence on the rate. The Cu3Pd(111) and Ag3Pd(111) surfaces are found to be the most active for n-butane oxidation to 1-butanol, with Cu3Pd additionally exhibiting high selectivity for 1-butanol. Achieving high activity and selectivity toward 1-butanol is found to require a precise balance of the catalyst affinity for OH* and O*, with catalysts that bind these species too strongly garnering large coverages of O*, which block active sites and inhibit the rate of n-butane conversion, and catalysts that bind these species too weakly promoting dehydrogenation of C4 species, as this process supplies H atoms that can convert OH* and O* to the more-stable H2O*. Catalytic affinity for C* is also found to have a significant impact on selectivity toward 1-butanol, since the formation energy of C* on catalyst surfaces is found to correlate to catalytic ability to break C–H bonds, with catalysts that bind C* too strongly tending to overdehydrogenate C4 species. The reaction C4H9* + O* ↔ C4H9O* + * is found to be rate-controlling on those catalysts that are most active for 1-butanol production.