Atomistic insights into the reformation of CH₄with CO₂on metal-free gC₃N₄: Unraveling the reaction mechanisms using first-principles DFT calculations

Photocatalytic dry reformation of methane (DRM) is an appealing alternative to transform CO2 into precious syngas for the Fischer-Tropsch synthesis while simultaneously reducing greenhouse gas emissions. However, the reaction mechanisms of DRM over photocatalysts have not been fully explored to date. In this work, two-dimensional graphitic carbon nitride (gC3N4) nanosheets are taken as a case study to shed light on their behaviors under the multistep reaction of DRM through first-principles calculations. The results show that gC3N4 is a promising candidate for DRM due to its suitable electronic band structure to drive the redox reaction and its ability to facilitate the adsorption of reactants (CO2 + CH4) and the desorption of products (CO + H2). The systematic Gibbs free energy calculations identified the possible reaction pathways for the reforming of CH4 to syngas using CO2. We observed that the H atoms from CH4 dissociation are more likely to form H2 since the Gibbs free pathway indicates that the main contributor of CO formation is the direct reduction of CO2 to CO rather than the oxidation of CH4 to CO due to the large activation barrier required for the formation of the CH2O intermediate. Overall, our work sheds light on the mechanism underlying the photocatalytic dry reforming of CH4 over gC3N4 nanosheets.