(144e) Density Functional Theory Investigation of Ethene Dehydrogenation and Coke Formation on Binary Metal Alloys: Effect of Surface Ensembles and Composition

In supported metal catalysis, the tradeoff between activity and selectivity presents an important challenge for catalyst design. By allowing two dissimilar metals, we can attempt to tune the selectivity of the catalyst by enhancing bond-formation and desorption rates through the addition of a less-reactive element, while maintain high bond dissociation activity from the more active metal. The resulting catalyst properties depend strongly on the catalyst composition and ratio of the two metals (electronic effect), but may also depend on the local structure of surface ensembles of the alloy components (geometric effect). Through density functional theory (DFT), we have examined the effect of platinum tin alloy structure and composition on the kinetics and thermodynamics of dehydrogenation and coke formation pathways during light alkane dehydrogenation. 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 alleviated by alloying with Sn and other main group elements. We aim to understand how the structure and composition of these alloys affect their ability to suppress coke formation. As compared to pure Pt, bond scission is more difficult on the alloys and desorption is more facile, and both effects are enhanced as three-fold hollow sites consisting of only Pt atoms are eliminated at higher Sn coverage. On Pt(111), the formation of atomic carbon is thermodynamically favorable and kinetically competitive with ethene formation. As the Sn loading increases, carbon formation becomes less kinetically and thermodynamically competitive with ethene formation and at high tin coverage cannot be considered likely as the source of coke.