Mechanistic Insights into Selective CO₂ Conversion via RWGS on Transition Metal Phosphides: A DFT Study

Selective conversion of CO₂ to CO via the reverse water gas shift (RWGS) reaction is an attractive CO₂ conversion process, which may be integrated with many industrial catalytic processes such as Fischer–Tropsch synthesis to generate added value products. The development of active and cost friendly catalysts is of paramount importance. Among the available catalyst materials, transition metal phosphides (TMPs) such as MoP and Ni₂P have remained unexplored in the context of the RWGS reaction. In the present work, we have employed density functional theory (DFT) to first investigate the stability and geometries of selected RWGS intermediates on the MoP (0001) surface, in comparison to the Ni₂P (0001) surface. Higher adsorption energies and Bader charges are observed on MoP (0001), indicating better stability of intermediates on the MoP (0001) surface. Furthermore, mechanistic investigation using potential energy surface (PES) profiles showcased that both MoP and Ni₂P were active toward RWGS reaction with the direct path (CO₂* → CO* + O*) favorable on MoP (0001), whereas the COOH-mediated path (CO₂* + H* → COOH*) favors Ni₂P (0001) for product (CO and H₂O) gas generation. Additionally, PES profiles of initial steps to CO activation revealed that direct CO decomposition to C* and O* is favored only on MoP (0001), while H-assisted CO activation is more favorable on Ni₂P (0001) but could also occur on MoP (0001). Furthermore, our DFT calculations also ascertained the possibility of methane formation on Ni₂P (0001) during the RWGS process, while MoP (0001) remained more selective toward CO generation.