CO2 Capture and Conversion to C1 Chemicals with Mixed-Metal Copper/Nickel Bis(amino)bipyrazolate Metal-Organic Frameworks

The reaction of 3,5-diamino-4,4?-bis(1H-pyrazole) (3,5-H2L) with copper(II) and nickel(II) acetates under solvothermal conditions led to the four mixed-metal metal-organic frameworks (MIXMOFs) [CuxNi1-x(3,5-L)] (CuxNi1-x, x = 0.05, 0.1, 0.2, 0.5), which were thoroughly characterized in the solid state. The textural analysis unveiled their macroporous nature, with BET specific surface areas falling in the 140-240 m2/g range. Despite the low specific surface areas, their CO2 adsorption capacity at ambient temperature and pressure (highest: Cu0.05Ni0.95 and Cu0.2Ni0.8; 5.6 wt % CO2) and isosteric heat of adsorption (highest: Cu0.2Ni0.8; Qst = 26.2 kJ/mol) are reasonably high. All of the MIXMOFs were tested as heterogeneous catalysts in carbon dioxide electrochemical reduction (CO2RR) in acetonitrile solution at variable potential. The best results were obtained at E = -1.5 V vs Ag/AgCl/KClsat: besides H2 from the hydrogen evolution (HER) side reaction, CO and CH4 were the main reduction products observed under the applied conditions. Cu0.05Ni0.95 showed the best performance with an overall [CO + CH4] conversion of ~200 ppm and a Faradaic efficiency of ~52%. CO2RR product selectivity seems to be correlated to the most abundant metal ion in the catalyst: while the Ni-richest phase Cu0.05Ni0.95 mainly produces CO, Cu0.5Ni0.5 mostly generates CH4. The preferential CO2 adsorption sites determined through GCMC simulations are close to the metal centers. For low copper loading, a prevalent end-on interaction of the type O=C=O···NiII is observed, but the progressive increase of the copper content in the MIXMOF equals the metal-gas distances with simultaneous MII···O=C=O···MII activation by two nearby metal ions and a bridging CO2 coordination mode. The analysis of the spent catalyst revealed partial formation of metal nanoparticles under the applied strongly reducing conditions.