Precious-Metal-Free Mo-MXene Catalyst Enabling Facile Ammonia Synthesis Via Dual Sites Bridged by H-Spillover.

Abstract

To date, NH₃ synthesis under mild conditions is largely confined to precious Ru catalysts, while nonprecious metal (NPM) catalysts are confronted with the challenge of low catalytic activity due to the inverse relationship between the N₂ dissociation barrier and NH_x (x = 1-3) desorption energy. Herein, we demonstrate NPM (Co, Ni, and Re)-mediated Mo₂CT_x MXene (where T_x denotes the OH group) to achieve efficient NH₃ synthesis under mild conditions. In particular, the NH₃ synthesis rate over Re/Mo₂CT_x and Ni/Mo₂CT_x can reach 22.4 and 21.5 mmol g^-1 h^-1 at 400 °C and 1 MPa, respectively, higher than that of NPM-based catalysts and Cs-Ru/MgO ever reported. Experimental and theoretical studies reveal that Mo⁴⁺ over Mo₂CT_x has a strong ability for N₂ activation; thus, the rate-determining step is shifted from conventional N₂ dissociation to NH₂* formation. NPM is mainly responsible for H₂ activation, and the high reactivity of spillover hydrogen and electron transfer from NPM to the N-rich Mo₂CT_x surface can efficiently facilitate nitrogen hydrogenation and the subsequent desorption of NH₃. With the synergistic effect of the dual active sites bridged by H-spillover, the NPM-mediated Mo₂CT_x catalysts circumvent the major obstacle, making NH₃ synthesis under mild conditions efficient.