(513cb) Density Functional Theory Analysis of Trends in Propane Dehydrogenation on Palladium Alloys

Alkane dehydrogenation has the potential to convert large amounts of ethane and propane available in the shale gas to more valuable alkenes and hydrogen. Bimetallic alloy catalysts have been used for this purpose, as they have been found to achieve higher selectivity than their pure metal counterparts. This improvement in performance has been attributed to the electronic and geometric effects due to alloying, but the exact mechanistic details remain poorly understood. In the present study, first-principles periodic density functional theory (DFT) calculations, in conjunction with microkinetic modelling, are used to elucidate the characteristics of nanoparticle alloys that underlie their selectivity and improvement in turnover rates. We focus, in particular, on PdIn alloys, for which the experimental results from our collaborators have shown to have high selectivity to propylene.

We performed an extensive thermodynamic and kinetic analysis on a series of steps and terraces on the Pd and PdIn surfaces. The results show that the barriers for dehydrogenation are 0.4-0.7 eV lower on the Pd-terminated step of PdIn as compared to the PdIn (110), suggesting that these are the active sites for the alloys. These energetics are then input into a microkinetic model, and a degree of rate control analysis has been performed to derive plausible activity, selectivity and stability descriptors. Further to test the validity of the computationally obtained selectivity descriptors, catalytic experiments were performed on a series of intermetallic Pd alloys (Pd₃Fe, Pd₃Mn, Pd₂Ga, and PdZn). The comparisons with experiments show that the selectivity descriptor could qualitatively distinguish between the highly selective 1:1 alloys, with the partly less selective 3:1 alloys which have 3-fold Pd ensembles. Moreover, a detailed transition state structure analysis shows how the promoter/heteroatom influences the C-H and C-C bond breaking barriers, which has implications for rational design of dehydrogenation catalysts.