(308e) First Principles Analysis of Coke Formation on Pt-Based Catalysts for Propane Dehydrogenation

Coking is a major cause of deactivation for heterogeneous catalysts used in hydrocarbon conversion reactions. In this broad context, coke deposition on heterogeneous catalysts for non-oxidative alkane dehydrogenation reactions has attracted particular interest, as these catalysts are highly susceptible to carbon deposition. For example, coke formation during propane dehydrogenation (PDH) on Pt- or Pd-based alloys can significantly affect both the stability of the catalysts and the selectivity to propylene, the desired product. In spite of these significant challenges, however, relatively little mechanistic information about coke formation is available, and such information could facilitate the development of more coke-tolerant catalysts.

In this study, density functional theory (DFT) simulations are utilized to provide molecular-level insights into the nucleation and growth of graphene-like coke models on Pt catalysts. On both smooth Pt(111) terraces and Pt(332) steps, a detailed analysis is performed of the energies of carbon structures at a wide range of coverages. On terraces, it is found that carbon forms isolated C atoms at very low coverages, develops chains at intermediate coverages, and yields a mixture of five- and six-membered rings at still higher coverages. On steps, carbon atoms nucleate as dimers at the step bridge sites, form chains at intermediate coverages, and ultimately grow as five- and six-membered rings. Chemical potentials derived from these energetics indicate that isolated carbon atoms and chains on Pt terraces yield significant nucleation barriers for graphene/coke growth, while on steps, the carbon atoms are stabilized, lowering the barrier to graphene/coke formation. These thermodynamic insights are augmented by kinetic barrier calculations which suggest that barriers for adding carbon atoms to hexagonal rings are relatively high. Finally, the results are interpreted in the context of density of states (DOS) analyses, and avenues for modeling of coke growth on Pt alloys and oxide-supported Pt nanoparticles are discussed.