Understanding and Modifying the Scaling Relations for Ammonia Synthesis on Dilute Metal Alloys: From Single-Atom Alloys to Dimer Alloys
Yining Zhang, Sha Li, Chao Sun, Ping Wang, Yijun Yang, Yi Ding, Xi Wang, Jiannian Yao
Abstract
Industrial ammonia synthesis through the Haber–Bosch process operated under harsh reaction conditions leaves ample room for improvement through material design. Designing a catalyst with high activity and low cost is considered as the key to enable large-scale operation under mild conditions. In this work, dilute metal alloys are studied using density functional theory (DFT) calculations and microkinetic modeling to investigate their catalytic performance for ammonia synthesis. Thermochemical scaling relations developed between reaction intermediates and Brønsted–Evans–Polanyi (BEP) relations developed for *N2 dissociation and *NHx hydrogenation form the basis of microkinetic simulations of reaction rates. A degree of rate control analysis shows that the overall reaction is rate-controlled by either *N2 dissociation or *NH2 hydrogenation, resulting in a volcano plot for single-atom alloys (SAAs) with Nb-doped Ag(111) SAA siting at the volcano peak. The BEP relationship for N2 dissociation derived on dimer alloys is closer to the ideal limit in comparison to that obtained on SAAs, leading to higher activities of dimer alloys for ammonia synthesis. Among the dimer alloys, Mo2/Ag(111) is not only more active than the commercial Ru catalysts but also very stable under real reaction conditions and could potentially be used in industrial processes.