Adsorption and dehydrogenation of C2–C6n-alkanes over a Pt catalyst: a theoretical study on the size effects of alkane molecules and Pt substrates

Abstract

Adsorption and dehydrogenation of C₂–C₆n-alkanes are investigated on a Pt substrate using density functional theory (DFT) calculations, and the size effects of alkane molecules and Pt substrates are discussed in detail. The Pt(111) surface and Pt₅₅ cluster are chosen to represent large and small Pt nanoparticles, respectively. The C₂–C₆ straight-chain alkanes show no site preference on Pt(111) drifting over the surface, but prefer to locate along the edge sites of Pt₅₅. Our results suggest that a linear relationship holds for the adsorption energies of n-alkanes against the chain length on Pt(111), in accordance with the experimental observations. Pt₅₅ also exhibits a similar linear relationship for n-alkanes but with larger adsorption energies due to the low-coordinated Pt atoms at the edge site. For the two-step dehydrogenation from alkanes to alkenes, the first dehydrogenation reaction is the rate-determining step (RDS) on Pt(111), and a larger size of alkane molecule will lead to a lower dehydrogenation activity. While on Pt₅₅, no RDS is present and the dehydrogenation activity oscillates slightly as the chain length of n-alkane increases. Generally, Pt₅₅ involves lower energy barriers for most dehydrogenation steps compared to Pt(111), indicating that small Pt particles with more low-coordinated Pt atoms are more active towards alkane dehydrogenation. In addition, a clear BEP relationship is identified for all the dehydrogenation reactions of C₂–C₆n-alkanes on Pt substrates, and this linear relationship is independent of the particle size of the Pt substrate and the chain length of alkanes.