Theoretical Prediction of MXene-based Single-atom Catalysts M-Ti2CO2 (M=Cu, Fe, Co, Ni) Applied in the Electroreduction of CO2 for Methanol Production

The single-atom catalyst has unique advantages such as high atomic utilization, high catalytic activity, and low raw material cost, which has attracted widespread attention and research in the field of catalysis. Various materials like graphene, metal oxides and alloys have been applied as the support to construct single-atom catalysts. Stable coordination of metal single atoms is essential for the synthesis of single-atom catalysts. A stable coordination environment can maintain the isolated state of single atoms and avoid aggregation, ensuring stable long-term catalytic performance. As an emerging type of two-dimensional material, MXene materials with rich surface functional groups such as -F, -OH and -O groups are conducive to anchoring single atoms. Strong metal-support interactions between metal single atom and surface functional groups enhance the stability of MXene-based single-atom catalysts. The electron property of metal single atom can also be easily tailored by changing the type of MXene support. Additionally, excellent electrical conductivity of MXene makes MXene-based single-atom catalysts highly potential electrocatalysts. Nevertheless, few exprolations of the application of MXene-based single-atom catalysts in electrocatalytic reduction of carbon dioxide have been made. Based on density functional theory (DFT), this study explored the potential of MXene-based single-atom catalysts in the preparation of renewable methanol fuel by electroreduction of carbon dioxide. Electrocatalytic performance of MXene-based single-atom catalysts towards methanol were investigated theoretically. As one of the early-studied typical MXenes, monolayer Ti2CO2 was selected as support to anchor transition metal single atoms such as Cu, Fe, Co, and Ni. Ti2CO2 with O atoms located on face-centered cubic (fcc) sites was found to be more thermodynamically stable. Stable adsorption sites of transition metal single atoms were identified as hollow sites of carbon atoms via comparing formation energies, cohesive energies and dissolution potentials of various M-Ti2CO2 (M = Cu, Fe, Co, Ni) atomic models. By calculating the adsorption energy of reaction intermediates and analyzing the thermodynamic tendency of reaction pathways, catalytic activity and product selectivity of MXene-based single-atom catalysts were predicted. Reaction pathways from carbon dixode towards C1 products (carbon monoxide, formic acid, methane and methanol) on M-Ti2CO2 catalysts were calculated, using binding energies and reaction Gibbs free energies as key criteria. Theoretical calculations revealed that Cu-Ti2CO2 catalyst exhibits high activity and selectivity towards methanol product with low theoretical limiting potential of -0.46 V, which indicates that Cu-Ti2CO2 has great potential as electrocatalysts in methanol fuel production. Compared to other transition metal (Fe, Co and Ni), Ti2CO2 supported Cu single atom has more moderate binding energies of reaction intermediates and exhibits greater tendency towards methanol product, which enhances the selectivity of methanol among C1 products. The rate-limiting reaction step towards methanaol on Cu-Ti2CO2 is the hydrogenation-dehydration of *HOCHO while the hydrogenation of *CH2O to OCH3 determines the selectivity of methanol product. Analysis on partial density of states of single metal atom in M-Ti2CO2 indicates that Cu single atom possesses high electron density near the Fermi level, which strengthens the binding of key reaction intermediates like *OCH3 on the surface of single-atom catalyst, thus promoting the catalytic activity and selectivity towards methanol product.