DFT Investigation of Hydrogenation and Dehydrogenation Reactions on Binary Metal Alloys: Effect of Surface Ensembles and Composition

Meeting Program – March 2015

Fuat E Celik
Department of Chemical and Biomolecular Engineering
Rutgers, The State University of New Jersey

Fuat Celik
Abstract
In supported metal catalysts, 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). In this talk we will explore two examples of binary alloys where surface composition and geometry play an important role in determining the selectivity of the catalyst through density functional theory (DFT).

In the first example, 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. We investigate the potential energy surfaces from ethane along the desired pathway to ethene, and along the undesired pathways towards surface carbon/coke. The effect of Pt/Sn ratio and surface geometry is investigated. 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.

In the second example, we evaluate Au/Ni near-surface alloys as potential oxygen reduction catalysts for the direct synthesis of hydrogen peroxide from O2 and H2, thereby avoiding the current anthraquinone process. While Au may have higher O-H bond formation activity, it is a poor O2-dissociation catalyst, and likewise Ni is very effective at O2-dissociation but not oxygen hydrogenation. Alloying Au with Ni(111) lowers H2 dissociation barrier while keeping the O2 dissociation barrier large relative to O2 hydrogenation. Desorption of H2O2 is similarly competitive with H2O2 dissociation on alloy surfaces. However, the selectivity for the OOH radical remains a challenge, with barrierless O-O bond dissociation and large (1.3 eV) hydrogenation barriers. We further investigate how the Au/Ni surface may rearrange itself to regenerate three-fold hollows of Ni atoms in the presence of strongly adsorbing surface species.