Selective Catalytic Reduction Of No On Transition Metal Oxide Surfaces: A First-principles Study

Nitrogen oxides（NO_X）from pollutant emissions have been threatening human health and living environment in the past decades.Selective catalytic reduction（SCR）of NO is an effective way to remove NO_x.CO-SCR is particularly attractive because it simultaneously eliminates two pollutants and H₂-SCR is considered to be one promising environmentally benign de-NOx technology.In spite of highly catalytic activity of the noble metal catalysts including Rh,Pd,and Pt,high cost and poor stability limit their practical applications.On the other hand,transition metal oxides were found to play a very important role in catalysis areas due to their lower cost and good reactivity,and have thus attracted considerable attention in the field of air pollution control.In this paper,based on density functional theory plus the on-site Coulomb repulsion（DFT+U）method,we provided a deep insight in mechanisms of NO reduction on transition metal oxide and supported noble metal surfaces.The possible reaction pathways for the formation of N-containing products were identified and the role of the surface oxygen vacancies in the reaction process have been investigated.1.The detailed reaction mechanisms for NO reduction with CO on the Co₃O₄（110）-B and CoO（110）surfaces were investigated by the present study.The surface oxygen vacancies are first generated by CO oxidation through the Mars–van Krevelen mechanism on both surfaces and then replenished by adsorbing NO molecule to endure the catalytic cycle.Possible reaction pathways for N₂ and N₂O formation are considered,and whole reaction mechanism is identified through our calculations.It is shown that the reaction pathway involving the co-adsorption structure ONNO_L（Path 1 and Path 1′）is identified for N₂O formation on the two surfaces.The formation of N₂ proceeds via the O_LNNO_L dimer intermediate（Path 3）followed by two almost barrierless N-O bond scissions on Co₃O₄（110）-B,while on the CoO（110）surface,the pathway involving the reaction of the NCO intermediate with NO（Path 5′）is most favorable.Based on the DFT results,microkinetic analysis was performed to estimate the relative product selectivity of the N₂O and N₂ under the experimental conditions.Results were compared with experimental reports.2.The reaction mechanisms of NO reduction by H₂ on the catalysts with singly dispersed bimetallic sites MCo₄（M=Pt and Rh）anchored on Co₃O₄ were investigated by using DFT+U methods with periodic slab models.Adsorption and dissociation of NO,possible intermediates and reaction pathways for N-containing products in this process were identified on the Pt（Rh）Co₄/Co₃O₄（110）-B surfaces.It is found that the surface oxygen vacancy(O_vac)generated by the hydrogenation of the lattice oxygen（O_L）plays a key role in the catalytic reduction of NO,and O_vac and bimetallic sites MCo₄ are the active sites responsible for NO dissociation and N₂ formation.The presence of H atoms has a larger promotion effect for NO dissociation on RhCo₄/Co₃O₄ than on PtCo₄/Co₃O₄,producing major intermediates NH and N,respectively;accordingly,different N₂ formation pathways were identified on MCo₄/Co₃O₄（110）-B.The hydrogenation of O or OH is the rate-controlling step in whole catalytic cycle for N₂ formation,and the energy barrier for this step is almost identical,suggesting that PtCo₄/Co₃O₄（110）-B and RhCo₄/Co₃O₄（110）-B have similar reaction activity.The products of N₂O and NH₃ are energetically unfavorable to be formed on both surfaces because of the large energy barriers.The present results show that the Co₃O₄-based catalysts consisting of singly dispersed bimetallic sites MCo₄have highly catalytic activity and selectivity toward N₂,which is consistent with the experimental observations.3.We report here a theoretical investigation of the catalytic mechanism of NO reduction with H₂ on the SAC catalyst Pt₁/FeO_x surface base on density functional theory plus the on-site Coulomb repulsion（DFT+U）calculation.The presence of H₂in reactant gases can produce the single O vacancy or two O vacancies on the surface by abstracting the lattice O_L and replenished by adsorbing NO molecule,which play an important role in the catalytic process.The possible reaction pathways are considered on the surface with the single O vacancy or two O vacancies,and then a complete catalytic cycle is identified through our calculations.The results show that the formation of N₂O proceeds via the ONNO_L intermediate on the surface with the single O vacancy and two O vacancies.The formation of N₂ comes from the further dissociation of N₂O*.At low temperature,the reactions of Ov-1+NNO→Ov-1+N₂O（g）and Ov-2+NNO→N₂+O_L occur on the single O vacancy surface,whereas the formation of Ov-1 is more favorable than the Ov-2,and therefore the main product is N₂O.At high temperatures,the formation of N₂（g）is thermodynamically more favorable on the surface of the single or two O vacancies.Thus,N₂O is the main product at low temperatures,and N₂ is the only product at high temperatures.When H₂ is excessive,the intermediate Ov+ONNO_L reacts with H₂ to prompt the selectivity of N₂.Results are basically agreement with experimental observation.