First-principles analysis of the ammonia decomposition reaction on high entropy alloy catalysts

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. In this study, we systematically develop a model of high entropy alloy catalysts, randomly ordered bimetallic alloys, and subsequently extend these ideas to incorporate multiple elements. Additionally, We develop tools to efficiently sample different binding sites and to investigate the free energy landscape for simple 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 HEAs. We determine the binding energies of various reaction intermediates on many randomly sampled arrangements of the HEA surfaces using DFT. We deduce that the rate-determining step for the ammonia decomposition reaction is recombinative nitrogen desorption. This conclusion remains constant across the different considered surface arrangements. The results form a strong basis for further studies and the development of high entropy alloy catalysts for ammonia decomposition.