Integrated Carbon Dioxide Capture by Amines and Conversion to Methane on Single-Atom Nickel Catalysts

Direct electrochemical reduction of carbon dioxide (CO2) capture species, i.e., carbamate and (bi)­carbonate, can be promising for CO2 capture and conversion from point-source, where the energetically demanding stripping step is bypassed. Here, we describe a class of atomically dispersed nickel (Ni) catalysts electrodeposited on various electrode surfaces that are shown to directly convert captured CO2 to methane (CH4). A detailed study employing X-ray photoelectron spectroscopy (XPS) and electron microscopy (EM) indicate that highly dispersed Ni atoms are uniquely active for converting capture species to CH4, and the activity of single-atom Ni is confirmed using control experiments with a molecularly defined Ni phthalocyanine catalyst supported on carbon nanotubes. Comparing the kinetics of various capture solutions obtained from hydroxide, ammonia, primary, secondary, and tertiary amines provide evidence that carbamate, rather than (bi)­carbonate and/or dissolved CO2, is primarily responsible for CH4 production. This conclusion is supported by 13C nuclear magnetic resonance (NMR) spectroscopy of capture solutions as well as control experiments comparing reaction selectivity with and without CO2 purging. These findings are understood with the help of density functional theory (DFT) calculations showing that single-atom nickel (Ni) dispersed on gold (Au) is active for the direct reduction of carbamate, producing CH4 as the primary product. This is the first example of direct electrochemical conversion of carbamate to CH4, and the mechanism of this process provides new insight on the potential for integrated capture and conversion of CO2 directly to hydrocarbons.