Hydrogen-Assisted\nDissociation of N<sub>2</sub>: Prevalence\nand Consequences for Ammonia Synthesis on Supported Ru Catalysts

Understanding the roles of coadsorbates on crowded catalytic\nsurfaces\nis critical to optimizing industrial catalysts that are generally\nemployed under high-pressure conditions. For ammonia synthesis from\nN<sub>2</sub> hydrogenation (i.e., the Haber–Bosch process),\nit is well-known that supported Ru catalysts tend to be highly covered\nby atomic H species, while the impact of these H species on N<sub>2</sub> activation is still under controversy. Herein, kinetic assessment,\nisotopic labeling experiments, and <i>in situ</i> spectroscopic\ncharacterization were combined to investigate the mechanism of ammonia\nsynthesis on Ru/CeO<sub>2</sub> catalysts with their structure tuned\nvia thermal treatments. Our experimental approaches reveal that the\ndominant H* surface species limit the availability of vacant Ru sites\nfor the widely proposed direct N<sub>2</sub> dissociation route but\ninstead lead to the prevalence of the H-assisted N<sub>2</sub> dissociation\nroute with the N–N cleavage in N<sub>2</sub>H* intermediates\nas a kinetically relevant step. Effects of Ru particle size and Ru–CeO<sub>2</sub> interaction on the catalytic activity were kinetically deconvoluted\nin accordance with this H-assisted mechanism, unveiling their decisive\ninfluences on intrinsic activity and surface coverage, respectively.\nDriven by these fundamental insights gained from the working conditions,\nsuperior ammonia formation rates were achieved for supported Ru catalysts\nvia optimizing Ru particle size and metal–support interaction\ncollaboratively.