(477g) First-Principles Analysis of the Ammonia Decomposition Reaction on High Entropy Alloycatalysts

The development of periodic Density Functional Theory (DFT) calculations, combined with advanced synthesis techniques, has accelerated the understanding and development of multimetallic alloy catalysts. Recently, a new class of materials, known as high entropy alloys (HEAs), has opened up additional catalyst design possibilities in the alloy space. HEAs are comprised of many component elements, with completely mixed atomic structures, leading to potentially millions of unique chemical environments around active sites. These materials have attractive properties for catalysis, including enhanced stability due to entropic effects, as well as highly tunable active site structures that could be exploited to optimize catalytic activity and selectivity. Nevertheless, the huge materials space cannot be navigated exhaustively using first-principles methods, and the scaling theories and models proposed for traditional alloys may not be directly translatable to HEAs.

In this study, we develop a simple structural model of disordered alloy catalysts, and we introduce workflows to efficiently sample different binding sites and investigate free energy landscapes for adsorbates on these sampled sites. To illustrate this approach, we choose the ammonia decomposition reaction as a probe reaction and Co-Mo as a model catalyst, based on the promising activity demonstrated experimentally for this chemistry on Co-Mo-based HEA’s. We determine the binding energies of various reaction intermediates on many randomly sampled arrangements of the HEA surfaces using DFT. We assess the limitations of linear scaling relationships for these disordered systems, and we complement these scaling relationships with nonlinear machine learning-based strategies to more accurately estimate adsorption energetics. We additionally evaluate thermodynamics and kinetics of elementary steps in ammonia decomposition and deduce that the rate-determining step on disordered CoMo alloys is recombinative nitrogen desorption. The results form a strong basis for further studies and the development of high entropy alloy catalysts for ammonia decomposition and related reactions.