Adsorption and dehydrogenation of C-2-C(6)n-alkanes over a Pt catalyst: a theoretical study on the size effects of alkane molecules and Pt substrates

Adsorption and dehydrogenation of C-2-C(6)n-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 Pt(55)cluster are chosen to represent large and small Pt nanoparticles, respectively. The C-2-C(6)straight-chain alkanes show no site preference on Pt(111) drifting over the surface, but prefer to locate along the edge sites of Pt-55. Our results suggest that a linear relationship holds for the adsorption energies ofn-alkanes against the chain length on Pt(111), in accordance with the experimental observations. Pt(55)also exhibits a similar linear relationship forn-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 Pt-55, no RDS is present and the dehydrogenation activity oscillates slightly as the chain length ofn-alkane increases. Generally, Pt(55)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 C-2-C(6)n-alkanes on Pt substrates, and this linear relationship is independent of the particle size of the Pt substrate and the chain length of alkanes.