Coordination Number Engineering of Zn Single-Atom Sites for Enhanced Transfer Hydrogenation Performance

Abstract

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 metal–organic framework-mediated pyrolysis method for direct construction of a sequence of heterogeneous zinc SACs embedded within a nitrogen-doped porous carbon support (denoted as Zn–N–C–T, T stands for the carbothermal treatment temperature) with different Zn–Nx coordination numbers (x = 2, 3, 4) and Zn loadings (ranging from 0.8 to 6.8% by weight) for transfer hydrogenation reactions, using hydrazine hydrate as a hydrogen source under environmental benign conditions. It is found that the catalytic activity of Zn–N–C–T is closely related to the coordination number of Zn single atomic sites, among which the under-coordinated Zn–N3 with defect and asymmetric electron distribution in Zn–N–C–1223 exhibits the best catalytic activity for the transformation of nitroarene to arylamine, 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.

Keywords: Metal-organic framework, single-atom catalysts, Nitrogen coordination number, Transfer hydrogenation, electron transfer