Insight into the mechanism of CO2 and CO methanation over Cu(100) and Co-modified Cu(100) surfaces: A DFT study

Density functional theory calculations were carried out to investigate the mechanism of CO2 and CO methanation over pure Cu(100) and Co-Cu bimetallic catalysts. The most favorable pathways for the CO2 and CO hydrogenation were obtained. For the Cu(100) surface, the barriers of the rate-limiting step for the HCOO* and CO* hydrogenation were 122.52 kJ/mol and 106.14 kJ/mol. Because the barrier (77.34 kJ/mol) for the H2CO* hydrogenation is more than the desorption energy of 54.80 kJ/mol, H2CO gas was the main product from the hydrogenation of CO2 and CO on a pure Cu(100) surface. For the Co-4/Cu(100) surface, the optimal pathways for the CO2 and CO methanation were the same as those on the Cu(100) surface. The rate-limiting step for CO2 and CO methanation is the H2COO* (barrier of 103.57 kJ/mol) and H2CO* hydrogenation (barrier of 107.80 kJ/mol). Compared to the mechanism of CO2 and CO over Cu(100), the Co dopant can modify the rate-limiting step and decrease the activation barrier. Particularly, the barrier for the H2COH decomposition was changed from 100.96 kJ/mol to 69.81 kJ/mol (CO2 pathway) and 61.26 kJ/mol (CO pathway). Furthermore, the co-adsorbed OH* group affects the hydrogenation pathway of some intermediates rather than electronic structures.