Hydrogen-Assisted Dissociation of N2: Prevalence and Consequences for Ammonia Synthesis on Supported Ru Catalysts

Understanding the roles of coadsorbates on crowded catalytic surfaces is critical to optimizing industrial catalysts that are generally employed under high-pressure conditions. For ammonia synthesis from N2 hydrogenation (i.e., the Haber–Bosch process), it is well-known that supported Ru catalysts tend to be highly covered by atomic H species, while the impact of these H species on N2 activation is still under controversy. Herein, kinetic assessment, isotopic labeling experiments, and in situ spectroscopic characterization were combined to investigate the mechanism of ammonia synthesis on Ru/CeO2 catalysts with their structure tuned via thermal treatments. Our experimental approaches reveal that the dominant H* surface species limit the availability of vacant Ru sites for the widely proposed direct N2 dissociation route but instead lead to the prevalence of the H-assisted N2 dissociation route with the N–N cleavage in N2H* intermediates as a kinetically relevant step. Effects of Ru particle size and Ru–CeO2 interaction on the catalytic activity were kinetically deconvoluted in accordance with this H-assisted mechanism, unveiling their decisive influences on intrinsic activity and surface coverage, respectively. Driven by these fundamental insights gained from the working conditions, superior ammonia formation rates were achieved for supported Ru catalysts via optimizing Ru particle size and metal–support interaction collaboratively.