(779i) First Principles Light Alkane Dehydrogenation on Pt:Main Group Alloys and the Effect of Hydrogen Spectators

The effects of alloying platinum with transition metals and main group elements on the kinetics and thermodynamics of dehydrogenation and coke formation pathways during light alkane dehydrogenation have been studied using density functional theory. Due to the recent shale boom, light alkane prices have dropped significantly; ethane is now the same price as methane and is flared in some areas. Light alkane dehydrogenation to olefins can add significant value to and reduce greenhouse gas emissions from hydrocarbon processes that generate alkanes by converting low value commodity fuels to high-value chemical and polymer precursors.

Supported Pt catalysts are known to be active for light alkane dehydrogenation, but the high temperatures required by these endothermic reactions leads to significant coke formation and deactivation. Numerous Pt alloys have been experimentally shown to decrease coke formation and deactivation, including with Sn, Ga, In, Cu, Au, and Re. In addition, low amounts of hydrogen feed have also been experimentally shown to increase selectivity of ethylene over coke. Using periodic density functional theory we have studied the dehydrogenation for an extensive number of alloys and under hydrogen coadsorption. We show that hydrogen spectators decrease the ethylene binding energy by competing with adsorption locations, thus making it easier to desorb. Hydrogen spectators also increase the ethylene dehydrogenation activation energy barrier making it harder to continue dehydrogenating to carbonaceous species providing support to hydrogen feed increasing selectivity. In addition we have calculated potential energy surfaces on numerous alloys to better understand the reduction in surface carbon formation during ethane dehydrogenation. At low concentrations, the main group elements are the only species that decrease the binding energy of carbon, and presumably carbonaceous species in general. These elements affect Pt purely though electronic effects at low alloy ratios while at higher concentrations geometric effects also play an increasingly important role. The electronic effects of Bi in Pt₃Bi/Pt suggest higher ethylene selectivity than either Pt₃Sn/Pt or Pt₃In/Pt, but at higher alloy concentrations, the geometric effects of In in PtIn/Pt suggest an higher selectivity than PtBi/Pt.