Taming the Complexity of High Entropy Alloy for Catalysis using Multinary Intermetallics

With support from the Chemical Catalysis program in the Division of Chemistry, Robert Rioux and Michael Janik of the Pennsylvania State University will examine the reactivity of multinary alloys for selective hydrogenation catalysis. Catalysts made with many metal constituents, referred to as “high entropy alloys,” may offer new avenues for improved performance. However, understanding the multi-metal interactions is complex due to the large number of structures that form when mixing many metals together. This collaborative research team will pursue an effort to identify and quantify these complex interactions in catalysts with up to five different metals using a structurally defined alloy, called an intermetallic, to reduce the structural complexity. These alloys will contain from three to five different metals allowing the team to probe these complex metal interactions by systematically incorporating more metals into the alloy structure. Using a combined experimental and computational approach, the team aims to understand the origin(s) of the catalytic behavior of compositionally-diverse intermetallics for selective hydrogenation of alkynes and alkenes. Through this project, Drs. Rioux and Janik will help to train the next generation of catalytic scientists to solve fundamental and applied problems in chemical catalyst design and optimization. There will a significant participation of undergraduates in research as part of this projecting, leveraging established recruitment programs at Pennsylvania State University.The γ-brass intermetallic structure offers well-defined and controllable distribution of its atomic constituents among four symmetry inequivalent sites. Precise stoichiometric control during solid-state synthesis enables preparation of M8-11Zn44-41 systems that distribute M (Pd, Ni) atoms either isolated by all Zn nearest neighbors or in small M3 clusters. Rioux and Janik of the Pennsylvania State University will prepare, characterize, and examine the reactivity of ternary, quaternary, and quinary γ-brass systems generated by substituting some number of Pt, Ir, Cu, and/or Au atoms. Controlled introduction of compositional complexity is designed to enable the development of a quantitative catalysis science of high entropy materials through a combined, multi-faceted experimental and computational study. H2-D2 exchange and ethylene hydrogenation are highly sensitive to the composition of the trimer sites, while selective hydrogenation of 1,3-butadiene will probe selectivity on the trimer active site composition. Interpretation of the observed catalytic activity-selectivity will be aided by rigorous characterization of γ-brass intermetallic microstructure, crystal structure, and methods to characterize the trimer sites present on γ-brass HEIs. Computational efforts based on DFT- and cluster-expansion calculations will define stable bulk and surface trimer assemblies as a function of multinary γ-brass intermetallic composition. DFT calculations of elementary reaction energetics will inform microkinetic models to compare rates, with the hypothesis that rates on surfaces distributing an array of site compositions will be a simple sum of rates on individual sites due to site isolation in the Zn host.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.