Theoretical investigation on hydrogenation of dinitrogen triggered by singly dispersed bimetallic sites

Abstract

The conversion of molecular dinitrogen into ammonia is one of the most important chemical processes, which involves the inert NN bond activation and multi-step hydrogenation. To meet the requirements of complicated catalytic processes, single-cluster catalysts (SCCs) have emerged as a promising platform to trigger synergistic effects. Herein, we present a systematic investigation on the stability and activity of a series of singly dispersed bimetallic catalysts, M₁Co_n/CoO_x SCCs, for the hydrogenation of dinitrogen by first-principles calculations. Our results indicate that late transition metals are prone to form stable singly dispersed bimetallic clusters by doping the O vacancy on CoO surfaces. Due to the low oxidation state of the doped metals in the stable M₁Co_n/CoO_x SCCs, the chemisorption of N₂ is enhanced by the multiple “pull–push effect” of the isolated bimetallic sites. Subsequently, an improved electron descriptor (μ) is proposed to effectively estimate the correlation between the favorable bridging N₂ adsorption energies and the essential electronic characteristics of M₁Co_n/CoO_x SCCs, in which the influence of the metal d orbital electrons and the intrinsic 1st ionization energy are considered. We focus on the first hydrogenation in the associative mechanism on the multicenter of M₁Co_n/CoO_x SCCs, and the Pd₁Co₄/CoO_x SCC is proposed to exhibit superior charge buffer capacity towards thermal dinitrogen hydrogenation. Furthermore, a superimposed evaluation strategy associates the activity of nitrogen fixation on SCCs with certain intrinsic features of multi-active sites at both static and kinetic states. The present work not only provides a potential candidate for the thermal N₂-to-NH₃ conversion, but also sheds insights into the rational design of heterogeneous catalysts for thermal nitrogen fixation.