A First-Principles Study and Microkinetic Simulation of Ammonia Decomposition on CoIrNiRhRu High-Entropy Alloy Surfaces

Hydrogen is a high-density and clean energy source, and ammonia is a hydrogen carrier. Catalytic ammonia decomposition (AD), as a means of hydrogen generation, has received much attention. High-entropy alloy (HEA) has been studied as a promising catalyst for AD. Despite many studies, the reaction network and mechanism of AD on HEA surfaces are still not well understood. Herein, we combine density functional theory calculations with microkinetic modeling to investigate the network of the AD process, including both dehydrogenation and N-containing species coupling on the two sites (site 1 and site 2), which refer to small areas centered on the two most favorable positions for NH3 adsorption at the (111) surface of CoIrNiRhRu HEA, a catalyst showing good performance for AD at high temperatures experimentally. It is found that site 2 is much less active than site 1. On site 1, the adsorption energy of atomic N is the catalytic activity descriptor, with the rate-controlling step being N* + N* = N2* + * and the dominant pathway being NH3* → NH2* → NH* → N*→ N2*. N2 is selectively produced on both sites beyond 700 K. The (111) surface of CoIrNiRhRu HEA has many different sites that are composed of different element combinations and feature different adsorption and kinetic properties. Linear Ridge Regression (RR) machine learning models were constructed to correlate well the site composition with the N1 species’ adsorption energies. Analysis indicates that the impact of an element on the adsorption energy and reaction kinetics can be both positive and negative depending on the position. Transition state scaling (TSS) relations were also established for the dehydrogenation and coupling reactions, and their extensibility, as well as that of the Ridge Regression models, was tested. A more active site or local structure on the CoIrNiRhRu HEA (111) surface was designed based on the RR models and the TSS relations. The activity of this designed site is 71(3840) times as high as that of site 1(site 2) at 773 K. The present paper not only presents a comprehensive understanding of AD on CoIrNiRhRu HEA surfaces but also provides a theoretical scheme to optimize and design low-temperature, highly active HEA catalysts for AD reaction, which deserves generalization for solving similar problems.