Density Functional Theory Study on the Reduction of NO by CO Over Fe3O4 (111) Surface

Iron or its oxides (FexOy) commonly exist in the ash of fossil fuel and biomass, and are used as oxygen carriers in chemical looping process, and Fe3O4 is a main occurrence. The NO reduction mechanisms by CO over Fe3O4 (111) surface was investigated via density functional theory (DFT) calculations. An optimized unit cell of Fe3O4 was constructed. The interaction between molecules and cell surface was described by the calculated adsorption properties and electronic structures. Results showed that the most stable adsorption of NO/CO belongs to chemisorption and NO has higher adsorption energy than CO. NO can be absorbed onto Fe3O4 surface to form (NO)(2) dimer structure, which easily decomposes via a small energy barrier. (NO)(2) dimer mechanism is a possible pathway for the reduction of NO by CO over Fe3O4, following three steps: 2NO -> (NO)(2)*, (NO)(2)* -> N2O + O*, O* + CO* -> CO2. After the decomposition, the intermediate species N2O molecule and the remaining O atom adsorbed strongly on the Fe3O4 surface can be removed by CO. CO also promotes the gaseous decomposition of N2O. DFT results also showed O-2 will prevent NO reduction reaction. The calculated reaction rate constants further verify the existence of (NO)(2) dimer mechanism and the rate-limiting step is the removal of the surface O atom.