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 & PRIME;-bis(1H-pyrazole) (3,5-H2L) with copper(II) and nickel(II)acetatesunder solvothermal conditions led to the four mixed-metal metal-organicframeworks (MIXMOFs) [Cu x Ni1-x (3,5-L)] (Cu x Ni1-x , x = 0.05, 0.1, 0.2, 0.5), whichwere thoroughly characterized in the solid state. The textural analysisunveiled their macroporous nature, with BET specific surface areasfalling in the 140-240 m(2)/g range. Despite the lowspecific surface areas, their CO2 adsorption capacity atambient temperature and pressure (highest: Cu0.05Ni0.95 and Cu0.2Ni0.8; 5.6 wt % CO2) and isosteric heat of adsorption (highest: Cu0.2Ni0.8; Q ( st ) = 26.2 kJ/mol) are reasonably high. All of the MIXMOFs were testedas heterogeneous catalysts in carbon dioxide electrochemical reduction(CO2RR) in acetonitrile solution at variable potential. The best resultswere obtained at E = -1.5 V vs Ag/AgCl/KClsat: besides H-2 from the hydrogen evolution (HER)side reaction, CO and CH4 were the main reduction productsobserved under the applied conditions. Cu0.05Ni0.95 showed the best performance with an overall [CO + CH4] conversion of & SIM;200 ppm and a Faradaic efficiency of & SIM;52%.CO2RR product selectivity seems to be correlated to the most abundantmetal 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 tothe metal centers. For low copper loading, a prevalent end-on interactionof the type O C O & BULL;& BULL;& BULL;Ni-II isobserved, but the progressive increase of the copper content in theMIXMOF equals the metal-gas distances with simultaneous M-II & BULL;& BULL;& BULL;O C O & BULL;& BULL;& BULL;M-II activation by two nearby metal ions and a bridging CO2 coordination mode. The analysis of the spent catalyst revealedpartial formation of metal nanoparticles under the applied stronglyreducing conditions.