In this work, recent progress on graphene/metal oxide composites as advanced materials for HgCl2 and CO2 capture was investigated. Density Functional Theory calculations were used to understand the effects of temperature on the adsorption ability of HgCl2 and water vapor on CO2 adsorption on CaO (001) with reinforced carbon-based nanostructures using B3LYP functional. Understanding the mechanism by which mercury and CO2 adsorb on graphene/CaO (g-CaO) is crucial to the design and fabrication of effective capture technologies. The results obtained from the optimized geometries and frequencies of the proposed cluster site structures predicted that with respect to molecular binding the system possesses unusually large HgCl2 (0.1- 0.4 HgCl2 g/g sorbent) and CO2 (0.2-0.6 CO2 g/g sorbent) uptake capacities. The HgCl2 and CO2 were found to be stable on the surface as a result of the topology and a strong interaction with the g-CaO system; these results strongly suggest the potential of CaO-doped carbon materials for HgCl2 and CO2 capture applications, the functional gives reliable answers compared to available experimental data.