(586d) Active Site Requirement for Ethylene Hydrogenation on Intermetallics

Ethylene hydrogenation is a useful and convenient probe reaction, as it involves simple molecules and proceeds at ambient temperature. Numerous studies have used the Horiuti-Polanyi mechanism to construct kinetic models that reconcile ethylene and hydrogen reaction orders to an elementary mechanism. However, building quantitative connection between experimental and computational kinetics results on extended surfaces is a significant difficulty, which originates from the lack of physical basis of active site, continuum coverage dependence of elementary reaction energetics and side reaction of ethylene hydrogenation on extended surfaces.

In this talk, we will present the use of intermetallics to define the active site requirements for ethylene hydrogenation from both computational and experimental perspectives. The Pd-Zn γ-brass phase is used to control active site nuclearity, which exposes only Pd₁ monomers for Pd₈Zn₄₄ and includes Pd₃ trimers for Pd₉Zn₄₃^. A number of previous studies have investigated the kinetics of ethylene hydrogenation on late transition metal surfaces, and have required the use of a “special” site on which H* can adsorb without competition from ethylene within a Langmuir-Hinshelwood framework. The molecular level definition of such sites is elusive, arising from the complex combination of mixed coverages and difference in adsorbate size between ethylene and hydrogen. The well-defined, isolated sites on Pd-Zn γ-brass intermetallics allow us to directly consider all possible ethylene, ethyl, and H co-adsorption geometries with DFT, and to include all possible configurations explicitly in a microkinetic model. We determine that Pd₃ trimer sites are covered with ethylene and ethyl species together under reaction conditions, with H₂ dissociative adsorption at the remaining middle Pd atom the rate limiting step. Isotopic distributions during deuterogenation allows us to reconcile fully the reaction potential energy surface. H addition to ethyl is slower than the reversible ethylene to ethyl step, allowing H/D scrambling in the eventual ethane product