Novel amino-functionalized Ni(II)-based MOFs for efficiently photocatalytic reduction of CO2 to CO with superior selectivity under visible-light illumination

Metal-organic frameworks (MOFs) have aroused ever-growing scientific interest for photocatalytic CO2 reduction reaction (CO2RR). Delicate design and synthesis of MOFs with open metal sites (OMSs) is the key to activate CO2 and even stabilize the reaction intermediates of CO2RR, further determine the superior selectivity. Herein, two amino-functionalized MOFs (Ni-Bpyb and Ni-Bipy) with open Ni(II) sites were constructed from 2′-amino-5′-(4-carboxyphenyl)-[1,1′:3′,1″-terphenyl]-4,4″-dicarboxylic acid (NH2-BTBH3) and different lengths of linear N-donor linkers, 1,4-bis(4-pyridyl) benzene (Bpyb) or 4,4′-bipyridine (Bipy). Under visible-light irradiation, Ni-Bpyb shows more excellent CO yield of 1326.7 μmol·g−1·h−1 with impressive apparent quantum efficiency (AQE, 2.12 %@420 nm) and selectivity (98.80 %). Significantly, this AQE value is at the high end among MOFs-based photocatalysts for CO2RR. The reaction mechanism has been carefully investigated by experimental and theoretical calculations results. Photoelectric experiments demonstrate that Ni-Bpyb shows higher electron–hole separation efficiency compared to Ni-Bipy. Theoretical calculations validate that the open Ni(II) sites of Ni-Bpyb possess lower energy barriers of CO2 adsorption and reducing CO2* to COOH*, as well as better stabilizes COOH* intermediate than those of Ni-Bipy. These merits synergistically contribute to the superior performance and selectivity of Ni-Bpyb. This work might give a guidance for the design of efficient MOFs-based photocatalysts with excellent selectivity for CO2RR.