(378g) First Principles Analysis of Selectivity and Durability of Pt-Based Bimetallic Alloys for Light Alkane Dehydrogenation

Full exploitation of shale resources requires new catalytic techniques to efficiently convert the ethane and propane in shale gas to value-added products. Catalytic light alkane dehydrogenation, in turn, is receiving increasing interest due to the growing demand of light alkenes as key building blocks in petrochemical industry. Pt-based bi-metallic alloy nanoparticles have been shown to be effective for this chemistry and have exhibited high selectivity to light alkenes, and a key interest, in turn, is to examine the mechanistic details of the reaction on Pt and its alloy counterparts and explore the molecular-level properties that describe catalyst selectivity and coke deposition.¹ Herein, a detailed mechanistic study based on periodic density functional theory, graphitic coke nucleation theory, and microkinetic modeling, is performed to probe these details. The study begins with comparing a pure Pt catalyst and Pt₃Mn alloy in propane dehydrogenation. Kinetic measurements demonstrate that the Pt₃Mn catalyst exhibits reasonably higher selectivity to propylene than pure Pt and many other alloys.² A comprehensive free energy analysis, combined with preliminary microkinetic modeling, suggests that the alloy surface is more selective to propylene formation than pure Pt, which can be attributed to several key details: (1) the rate of propylene desorption is higher than the undesired deep-dehydrogenation on the alloy, and (2) pathways that are inherently non-selective to propylene, which lead to the formation of isomers, are hindered on the alloy surface. Finally, the coking/graphene island analysis demonstrates that the stability of graphene/Pt(111) interface is strongly moiré pattern-driven. The kinetic and coking analyses are incorporated into the comprehensive microkinetic study to identify rate- and selectivity-determining steps using degree of rate control techniques, where the insights are used to develop more accurate descriptors for catalyst screening.

¹J.J.H.B. Sattler, et al., Chem. Rev. 114 (2014) 10613

²Z. Wu, et al., JACS 140 (2018) 14870