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(1 H -pyrazole) (3,5-H 2 L) with copper(II) and nickel(II) acetates under solvothermal conditions led to the four mixed-metal metal–organic frameworks (MIXMOFs) [Cu x Ni 1– x (3,5-L)] (Cu x Ni 1– 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 m 2 /g range. Despite the low specific surface areas, their CO 2 adsorption capacity at ambient temperature and pressure (highest: Cu 0.05 Ni 0.95 and Cu 0.2 Ni 0.8; 5.6 wt % CO 2 ) and isosteric heat of adsorption (highest: Cu 0.2 Ni 0.8; Q st = 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/KCl sat: besides H 2 from the hydrogen evolution (HER) side reaction, CO and CH 4 were the main reduction products observed under the applied conditions. Cu 0.05 Ni 0.95 showed the best performance with an overall [CO + CH 4 ] 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 Cu 0.05 Ni 0.95 mainly produces CO, Cu 0.5 Ni 0.5 mostly generates CH 4 . The preferential CO 2 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···Ni II is observed, but the progressive increase of the copper content in the MIXMOF equals the metal–gas distances with simultaneous M II ···O═C═O···M II activation by two nearby metal ions and a bridging CO 2 coordination mode. The analysis of the spent catalyst revealed partial formation of metal nanoparticles under the applied strongly reducing conditions.