Theoretical Study On Carbon-Based Single/Double Atom Catalysts For The NH₃-SCR Reaction

Nitrogen oxides（NO_x）from fossil fuel combustion are considered as a key contributor to anthropogenic emissions since it leads to environmental problems such as acid rain,photochemical smog and greenhouse effect.Therefore,controlling the emission of NO_x is of great significance.Selective catalytic reduction of NOx with NH3（NH3-SCR）is a dominant technology to reduce NO_x（deNO_x）.However,there are two shortcomings for commercial deNO_x catalysts（vanadium-titanium-based metal oxides）,such as poor low-temperature efficiency and toxicity.Thus,developing novel environmentally friendly low-temperature SCR catalysts is the key to reducing NOx in coal-fired power plants.Due to the high catalytic activity and selectivity of single/dual-atom catalysts,they are expected to overcome the problem of insufficient low-temperature catalytic activity.Based on density functional theory,the carbon-based single-atom Fe doped with four N atoms by double vacancy（Fe₁-N₄）catalyst and the carbon-based double-atom Fe-Fe coordinated with six N atoms（Fe₂-N₆）catalyst were selected as research targets.The research can not only prove the feasibility of two catalysts to catalytic promote the NH₃-SCR reaction at low temperature,but also provide new insights to design novel catalysts for NH₃-SCR.First,the optimization calculations were carried out on Fe₁-N₄ catalyst and Fe₂-N₆catalyst,and the thermal stability and structural stability of the catalysts were verified through binding energy and ab initio molecular dynamics（AIMD）simulation.Various reaction gases were adsorbed on the optimized catalysts,and their adsorption energy and charge transfer characteristics were studied to clarify the adsorption type and bonding mechanism of each gas.Subsequently,the reaction pathways of NH₃-SCR reaction over the two catalysts were systematically studied by tuning the adsorption order of reaction gases.The reaction path analysis was conducted to explore the dominant reaction pathway of the NH₃-SCR reaction over the two catalysts.In addition,thermodynamic analysis,kinetic analysis and microkinetic modeling were selected to further analyze each reaction path,revealing the effect of temperature on the NH₃-SCR reaction,the speed of the NH₃-SCR reaction on the catalysts and the optimal temperature windows of two catalysts.The results demonstrate that the NH₃-SCR reaction on Fe₁-N₄ the catalyst contains seven possible reaction pathways.Various intermediates such as N₂H,NH₂NO and NHNO are found during the reaction.The dominant reaction pathway of NH₃-SCR reaction over the Fe₁-N₄ catalyst is a three-step process including NO oxidation,NO₂ reduction,and NHNO decomposition.The energy barrier of the rate-determining step is 0.99 eV,which exhibits higher catalytic activity than other NH₃-SCR catalysts.Simultaneously,the fast oxidation of NO on Fe₁-N₄ catalyst contributes to the reaction of“Fast SCR”,which significantly promotes the reaction rate of NH₃-SCR reaction.The optimal temperature window of Fe₁-N₄ catalyst is 300～430 K.The NH₃-SCR reaction over the Fe₂-N₆ catalyst includes four possible reaction paths,of which the dominant reaction pathway includes two reaction steps,consisting of the“Fast SCR”reaction and HONO formation.The rate-determining step of the reaction is the formation of HONO with an energy barrier of 1.0eV,which is lower than that of other NH₃-SCR catalysts and has higher catalytic activity.The existence of the“Fast SCR”reaction also indicates that the NH₃-SCR reaction over the Fe₂-N₆ catalyst has a faster reaction rate.Additionally,the Fe₂-N₆ catalyst exhibits outstanding low-temperature activity in the temperature window of 300～500 K.Therefore,based on the reaction energy barrier and optimal operating temperature windows,the two catalysts have potential applications in the low-temperature NH₃-SCR reaction.