Effect of Tin Coverage on Selectivity for Ethane Dehydrogenation over Platinum-Tin Alloys

Periodic, self-consistent generalized gradient approximation (GGA PW-91) density functional theory (DFT) was applied to study the effect of tin coverage in platinum-tin alloys in the dehydrogenation of ethane. Light alkane dehydrogenation to olefins can add significant value to hydrocarbon processes that generate ethane and propane by converting low value commodity fuels to high-value chemical and polymer precursors. Supported Pt catalysts are known to be active but show significant coke formation and deactivation, which can be reduced by alloying with Sn. Model surfaces of substitutional surface alloys of Pt and Sn were constructed with 1/4 and 1/2 of a monolayer of Pt atoms in the surface of fcc Pt(111) replaced by Sn atoms. Potential energy surfaces for all C-1 and C-2 derivatives from C-H and C-C bond cleavage from ethane were computed with kinetic reaction barriers obtained using the climbing-image nudged elastic band method. While the presence of Sn weakened the binding energies of all species, the effect for low Sn coverage was purely an electronic effect as binding geometries were unchanged relative to those on Pt(111). At higher Sn coverage, the elimination of 3-fold hollow sites consisting of Pt-atom nearest neighbors resulted in geometric changes in binding geometries and larger effects on the reaction and activation energies than the purely electronic effect observed at lower Sn coverage. The experimentally observed reduction in deactivation with H-2 cofeed arose due to competition between H atoms and ethene for binding sites whereby ethene was forced to desorb from the surface at moderate hydrogen coverages. Pathways to the formation of atomic carbon were compared on the alloys, and while atomic carbon may play a role in coke formation on pure Pt, it is unlikely to be relevant in coke formation over the alloys.