Identification of a Selectivity Descriptor for Propane Dehydrogenation through Density Functional and Microkinetic Analysis on Pure Pd and Pd Alloys

Abstract

First principles periodic density functional theory (DFT) calculations, in conjunction with detailed microkinetic modeling and experimental characterization, are employed to elucidate the structure sensitivity and identify key selectivity descriptors for nonoxidative propane dehydrogenation (PDH) on intermetallic alloys. A comprehensive theoretical treatment of 1:1 PdIn surfaces demonstrates that the Pd-terminated steps have 5 orders of magnitude higher rates than do the (110) terraces, with nearly complete selectivity to propylene formation. Pure Pd steps and terraces, in contrast, have considerably lower propylene selectivity and higher coverages of adsorbed intermediates, suggesting that Pd may experience more coking and reduced lifetimes compared to the alloys. A degree of rate and selectivity control analysis on the optimized microkinetic model demonstrates that propane C–H bond scission to yield 1-propyl is the most kinetically relevant step for propylene formation, while the C–C bond breaking barriers are important for byproduct formation. From these analyses, a simplified rate expression is derived for the step surface of the alloy, leading to the identification of a selectivity descriptor expressed in terms of effective free energy barriers of the rate controlling transition states, propane C–H bond breaking, and propyne C–C bond breaking. This descriptor is subsequently generalized to evaluate the propylene production selectivities for a series of Pd-containing alloys. The results show enhanced agreement with experimentally measured selectivity trends compared to traditional selectivity descriptors, suggesting a general strategy for identification of highly selective, nonoxidative PDH alloy catalysts.