CO<sub>2</sub> Capture and Conversion to C<sub>1</sub> Chemicals with Mixed-Metal Copper/Nickel Bis(amino)bipyrazolate Metal–Organic Frameworks
Patrizio Campitelli, Alessia Tombesi, Corrado Di Nicola, Claudio Pettinari, Anna Mauri, Simona Galli, Tongan Yan, Dahuan Liu, Jiaxin Duan, Subhadip Goswami, Giulia Tuci, Giuliano Giambastiani, Joseph T. Hupp, Andrea Rossin
Abstract
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.