Integrated Carbon Dioxide\nCapture by Amines and Conversion\nto Methane on Single-Atom Nickel Catalysts

Direct electrochemical reduction of carbon dioxide (CO<sub>2</sub>) capture species, i.e., carbamate and (bi)carbonate, can\nbe promising\nfor CO<sub>2</sub> capture and conversion from point-source, where\nthe energetically demanding stripping step is bypassed. Here, we describe\na class of atomically dispersed nickel (Ni) catalysts electrodeposited\non various electrode surfaces that are shown to directly convert captured\nCO<sub>2</sub> to methane (CH<sub>4</sub>). A detailed study employing\nX-ray photoelectron spectroscopy (XPS) and electron microscopy (EM)\nindicate that highly dispersed Ni atoms are uniquely active for converting\ncapture species to CH<sub>4</sub>, and the activity of single-atom\nNi is confirmed using control experiments with a molecularly defined\nNi phthalocyanine catalyst supported on carbon nanotubes. Comparing\nthe kinetics of various capture solutions obtained from hydroxide,\nammonia, primary, secondary, and tertiary amines provide evidence\nthat carbamate, rather than (bi)carbonate and/or dissolved CO<sub>2</sub>, is primarily responsible for CH<sub>4</sub> production.\nThis conclusion is supported by <sup>13</sup>C nuclear magnetic resonance\n(NMR) spectroscopy of capture solutions as well as control experiments\ncomparing reaction selectivity with and without CO<sub>2</sub> purging.\nThese findings are understood with the help of density functional\ntheory (DFT) calculations showing that single-atom nickel (Ni) dispersed\non gold (Au) is active for the direct reduction of carbamate, producing\nCH<sub>4</sub> as the primary product. This is the first example of\ndirect electrochemical conversion of carbamate to CH<sub>4</sub>,\nand the mechanism of this process provides new insight on the potential\nfor integrated capture and conversion of CO<sub>2</sub> directly to\nhydrocarbons.