Investigation On The Fabrication And Application In NO Reduction By CO Over Co-based Catalysts

The excessive emission of nitrogen oxides（NO_x）caused the atmosphere pollution such as acid rain,haze and photochemical smog,which is significant harmful to the ecological environment and human health.With the increasing vehicle ownership and the ratio of NO_x emission from vehicle,it is important for the control of NO_x emission from vehicle to develop efficiently deNO_x technologies.The pollutants of NO and CO in exhaust could be simultaneously eliminated with the presence of catalysts by through NO reduction by CO,which is low cost,high safety,and good operability.The development of catalysts with higher stability and satisfactory low-temperature catalytic activity is crucial for NO reduction by CO due to the characteristics of low temperature and the presence of oxygen and water in the rich-burn exhaust gas.The Co-based catalysts were applied in NO reduction by CO due to the outstanding oxygen mobility,redox properties and the more active sites obtained from Co₃+/Co²⁺ cycle.However,the low-temperature catalytic performance of Co-based catalysts is unsatisfactory,and the structure-activity relationship and performance enhancement mechanism is still confused.According to the shortcomings of Co-based catalysts,herein,the catalysts with superior low-temperature activity were developed.The structure and physicochemical properties of catalysts were deeply investigated.The structured-activity relationship,reaction routes and performance enhancement mechanism were revealed.The main achievements are as follows:A series of oxygen vacancy-rich porous Co₃O₄ nanosheets with diverse surface oxygen vacancy contents were synthesized by a facile solution reduction method in NaBH4 solution.Compared with Co₃O₄-p,the OV-Co₃O₄ catalysts exhibited better catalytic activity,and the best Co₃O₄-0.05 achieved 100%NO conversion at 175 ℃ in NO reduction by CO.The Co₃O₄-0.05 exhibited outstanding catalytic stability and resistance to high GHSV.The characterization results revealed that the efficient surface reduction leads to the presence of more surface oxygen vacancies,simultaneously maintaining the morphologies and crystal structures.The delocalized electrons transfer from previous oxygen 2p orbitals to surrounding Co sites could be promoted by higher content surface oxygen vacancies,leading to the decrease of Co valance in the OV-Co₃O₄ catalysts.The surface oxygen mobility and NO activation ability were distinctly enhanced by more surface oxygen vacancies and promoted delocalized electron transfer.The enhanced performance of OV-Co₃O₄ catalysts for NO reduction by CO resulted from the promoted formation and conversion of dinitrosyl species at lower temperatures（<225℃）and-NCO at higher temperatures（≥225℃）.The Co₃O₄-CoO catalysts with heterointerface were in situ prepared,the role of CoO and the interfacial effect between Co₃O₄ and CoO were investigated for NO reduction by CO.The CoO_x-350-t catalysts exhibited better catalytic performance and lower apparent active energy than single phase ones.The CoO_x-350-7 sample achieved the best catalytic performance,reaching 100%NO conversion at 150℃ and the apparent active energy is 54.2 kJ·mol^-1.The formation of heterointerface structure induced abundant oxygen vacancies.The reducibility and oxygen migration were consequently optimized.The electron transfer between Co₃O₄ and CoO via the heterointerface in Co₃O₄-CoO catalysts,and the interaction between Co₃O₄ and CoO not only optimized the reactants adsorption and activation on Co₃O₄-CoO catalyst,but also enhanced the electron donation ability from catalyst to NO molecules.Therefore,the stimulated dissociation of NO and the formation of-NCO could enhance the NO conversion to N₂ at lower and higher temperatures,respectively.The Ag doped Co₃O₄ porous nanosheets were prepared and applied into NO reduction with CO reaction.The xAgCo catalysts exhibited superior NO conversion and N₂ selectivity compared with Co₃O₄.The 0.02AgCo achieved the optimal catalytic activity,reaching 100%NO conversion at 175℃ and more than 90%N₂ selectivity at 200℃,and the apparent active energy is 34.0 kJ·mol^-1.The 0.02AgCo catalyst exhibited outstanding catalytic stability and high GHSV resistance.The doped Ag species promoted the formation of oxygen vacancies through weakening the Co-O bond strength.Thus the redox properties and surface oxygen mobility of xAgCo catalysts were enhanced.The excessive CO adsorption was inhibited and the reactivation of oxygen vacancies was promoted,which favors for the exposure of active sites.The more oxygen vacancies resisted the formation of stable nitrate species and promoted the electron donation ability to promote the dissociation and conversion of NO.The introduction of 5 vol%H₂O promoted the decomposition of accumulated carbonate species to the expose the active sites,thus enhanced the NO conversion.The presence of 10 vol%H₂O and O₂ inhibited NO conversion,owing to the competition reactions of CO oxidation and the coverage of reaction sites.A series of CuCe_xCo_1-xO_y catalysts were prepared for NO reduction with CO.The CuCe_0.2Co_0.8O_y catalysts in particular exhibited outstanding catalytic properties for NO reduction at low temperatures,achieving 100%NO reduction at 175℃.The addition of Ce promoted the dispersity of CuO species on the surface of CuCe_xCo_1-xO_y catalysts,which is conducive to the enhancement of the interaction between the highly dispersed Cu species and the support.The redox properties and the generation of more low-oxidation-state(Cu+,Co²⁺)of CuCe_xCo_1-xO_y catalysts were promoted by the redox equilibrium and electronic transfer between metal ions,which contribute to the regeneration of SOVs at low temperatures.In summary,the Co-based catalysts developed in this thesis exhibited excellent low-temperature（<225 ℃）catalytic performance and stability in NO reduction by CO reaction.Meanwhile,the analysis about structure-activity relationship,reaction routes and performance enhancement mechanism would be helpful for the further development of efficient catalysts in NO reduction by CO reaction,and provide experimental support and theoretical guidance for the application of catalysts into the NO_x emission control from vehicle.