A Density Functional Theory (DFT) Modeling Study of NO Reduction by CO over Graphene-Supported Single-Atom Ni Catalysts in the Presence of CO₂, SO₂, O₂, and H₂O

The mechanisms of NO reduction by CO over nitrogen-doped graphene (N-graphene)-supported single-atom Ni catalysts in the presence of O₂, H₂O, CO₂, and SO₂ have been studied via density functional theory (DFT) modeling. The catalyst is represented by a single Ni atom bonded to four N atoms on N-graphene. Several alternative reaction pathways, including adsorption of NO on the Ni site, direct reduction of NO by CO, decomposition of NO to N₂O followed by reduction of N₂O to N₂, formation of active oxygen radical O*, and reduction of O* by CO, were hypothesized and the energy barrier corresponding to each of the reaction steps was calculated using DFT. The most probable pathway was found to be that NO adsorbed on the Ni site decomposes via the Langmuir-Hinshelwood mechanism to form N₂O and subsequently N₂, leaving an active oxygen radical (O*) on the surface, which is then reduced by CO. The large adsorption energy of NO on the Ni site results in strong resistance to CO₂, SO₂, O₂, and water vapor. The activation energy of N₂O reduction to N₂ was found to be larger than those of NO decomposition to N₂O and active oxygen radical reduction by CO, illustrating that the step of N₂O reduced to N₂ is the rate-controlling step.