Coordination number engineering of Zn single-atom sites for enhanced transfer hydrogenation performance

Metal single-atom catalysts (SACs) have great potential to replace traditional nanoparticle catalysts in practical applications, however, the task-specific construction of SACs containing metal-nitrogen (M-N) moiety with tunable M-N coordination number remains a colossal challenging issue. Herein, we report a facile strategy to modulate the s-band of Zn by constructing N-coordinated Zn SACs (denoted as Zn-N-C-T, T stands for the py-rolysis temperature) with tunable nitrogen coordination number (denoted as Zn-Nx, x = 2, 3, and 4) and Zn loadings (ranging from 0.8 to 6.8% by weight) via a metal-organic framework-mediated pyrolysis method. The catalytic activity of Zn-N-C-T is found to be closely related to the coordination number of Zn single atomic sites for the transfer hydrogenation of nitroarene to arylamine, using hydrazine hydrate as a hydrogen source under environmental benign conditions, among which the under-coordinated Zn-N3 with defect and asymmetric electron distribution in Zn-N-C-1223 exhibits the best catalytic activity, followed by lower-coordinated Zn-N2 with defect in Zn-N-C-1273 and saturated coordination Zn-N4 in Zn-N-C-1123. Experimental results and theoretical analysis uncover that appropriately lowering the coordination number increases the electron density of Zn single atoms and simultaneously introduces H-acceptor sites, which cooperatively contribute to the enhancement of the performance for transfer hydrogenation.