N-2 adsorption on high-entropy alloy surfaces: unveiling the role of local environments

Developing highly active catalysts to electrochemically reduce N-2 to NH3 under ambient conditions is challenging but bears the promise of using ammonia as a potential energy vector in sustainable energy technology. One of the scientific challenges concerns the inertness of N-2 emanating from the highly stable triple bonds and the lack of dipole moments, making N-2 fixation on catalytic surfaces difficult. Another critical challenge is that electrons are more prone to reduce hydrogen than N-2 at the surface, forming a scaling relationship where the reduction ability of the catalyst most often benefits hydrogen reduction instead of nitrogen reduction. Here we show that high-entropy alloys (HEA) - a new class of catalysts with vast compositional and structural possibilities, can enhance N-2 fixation. More specifically, we investigate the role of the local environment in the first and second solvation shell of the adsorbing elements in the bond strength between the dinitrogen molecules and the HEA surfaces. Density functional theory using a Bayesian error estimation functional and vdW interactions is employed to clarify the properties dictating the local bonding. The results show that although the main property calibrating the N-2 bond strength is the d-band centers of the adsorbing elements, the value of the d-band centers of the adsorbing elements is further regulated by their local environment, mainly from the elements in the first solvation shell due to electron donor-acceptor interactions. Therefore, there exists a first solvation shell effect of the adsorbing elements on the bond strength between N-2 molecules and the surface of HEAs. The results show that apart from the direct active site, the indirect relation adds further modulation abilities where the local interactions with a breath of metallic elements could be used in HEAs to engineer specific surface environments. This is utilized here to form a strategy for delivering higher bond strength with the N-2 molecules, mitigating the fixation issue. The analysis is corroborated by correlation analysis of the properties affecting the interaction, thus forming a solid framework of the model, easily extendable to other chemical reactions and surface interaction problems.