Study On The Preparation And Characteristics Of Catalyst For NO Oxidation And Reduction

At present,selective catalytic oxidation（SCO）combined with absorption process and selective catalytic reduction（SCR）process are two main technologies for NO_x elimination.Catalysts are the core of the above two de-NO_x technologies.The methods of compounding different transition metal oxides and modification have been used by researchers to improve the catalytic activity and property,and some valuable results were achieved.The modification of metal doping and reduction treatment can promote the formation of surface oxygen vacancy and increases the specific surface area,distribution of active species and redox properties,and then enhance the catalytic activity of catalyst.However,it needs a further investigation to broaden the active temperature window of the catalyst,construct the structure-activity relationship between structural characteristics and activity,and explore its industrial application.The SCO catalyst of cobalt cerium composite metal oxide x Co₃O₄/Ce_0.85Co_0.15-3xO_2-δwas prepared by the sol-gel method.The activity test results show that the NO oxidation conversion of x Co₃O₄/Ce_0.85Co_0.15-3xO_2-δcatalyst can reach 60.2%at 300℃.The characterization results of Raman and XPS indicated that x Co₃O₄/Ce_0.85Co_0.15-3xO_2-δcatalyst is composed of Ce_0.85Co_0.15-3xO_2-δsolid solution and Co₃O₄ species.It is found that the NO conversion activity decreased by 25%after removing Co₃O₄ species by hydrochloric acid treatment.For x Co₃O₄/Ce_0.85Co_0.15-3xO_2-δcatalysts,free Co₃O₄species provide NO adsorption sites and Ce_0.85Co_0.15-3xO_2-δsolid solution promotes the activation of O₂,which promote the NO oxidation reaction over x Co₃O₄/Ce_0.85Co_0.15-3xO_2-δcatalysts.The polyethylene glycol modified TiO₂ supported CrO_x catalyst（CrO_x/y PEG-TiO₂）for the catalytic oxidation of NO was prepared by the impregnation method.Through XRD,BET,Raman,XPS,H₂-TPR,O₂-TPD,NO-TPD and in-situ DRIFTS characterization analysis,the relationship between the structure and activity of the catalyst was investigated.The results show that polyethylene glycol modification can promote the interaction between CrO_x and TiO₂.Among the prepared CrO_x/y PEG-TiO₂（y=0,30,50,80）catalysts,the interaction between CrO_x and TiO₂ in CrO_x/50PEG-TiO₂ is the strongest,and the higher amount of surface Cr³⁺ions can promote the formation of chemically adsorbed oxygen.At the same time,there was a certain amount of Cr⁶⁺on the surface of CrO_x/50PEG-TiO₂.Owing to the synergistic effect between CrO_x and chemisorption oxygen,CrO_x/50PEG-TiO₂ catalyst showed the highest catalytic activity,the NO oxidation conversion reached 55%at 350℃.In addition,CrO_x/50PEG-TiO₂ has good stability in a wide reaction temperature range of225～425℃.A series of flat-plate V₂O₅-Mo O₃/TiO₂（VMo Ti）catalysts were prepared to investigate the effects of calcination temperature on their denitrification performance and mechanical properties.The test results indicated that the NH₃-SCR denitration activity of VMo Ti catalyst calcined at 500℃was highest.The catalyst structure and surface properties were characterized and analyzed by XRD,XRF,BET,Raman,XPS,H₂-TPR and NH₃-TPD.It was found that with the increase of the calcination temperature,the polymer vanadate on the catalyst surface increased accordingly.In addition,the ratio of V⁴⁺/V⁵⁺,(V⁴⁺+V³⁺)/V⁵⁺and O_α/（O_α+O_β）of the catalyst also increased,which improves the denitrification efficiency of the catalyst at low temperature（<370°C）.At the same time,when the calcination temperature exceeds 560℃,it will cause the catalyst to form a large amount of N₂O during the reaction in the high temperature section（>370°C）,and then decrease the activity.The lower or higher calcination temperature will cause the decrease of mechanical strength of the catalyst.Because of the narrow active temperature range（300～400℃）of the commercial VTi catalyst used for the selective catalytic reduction of NO with NH₃ and it cannot match the variable flue gas temperature（250～400℃）,V₂O₅ supported on reduced TiO₂（VTi R）was prepared by wet-impregnation method.NH₃-SCR activity test results indicated that VTi R catalyst displayed a high NO removal efficiency(T₈₀=255°C)than VTi catalyst(prepared by same method using commercial TiO₂,T₈₀=296°C).Through XRD,BET,XPS,H₂-TPR and NH₃-TPD analysis,it is found that VTi R exhibits a larger specific surface area,richer V⁴⁺ions and surface oxygen vacancies,more highly dispersed V₂O₅ and faster V⁵⁺/V⁴⁺redox cycle.The above characteristics all promote the adsorption and activation of NO,O₂ and NH₃ on the surface of the catalyst,thereby promoteing the NH₃-SCR denitration performance of the catalyst.Finally,the study on the industrial preparation and industrial application of flat-plate V₂O₅-Mo O₃/Ti R（VMo Ti R）catalyst was carried out.The formulation of the paste in the preparation of the flat-plate VMo Ti R catalyst and the calcination process of the shaped catalyst was optimized to obtain the flat-plate VMo Ti R catalyst.The denitrification performance on the laboratory pilot platform and the industrial application platform was tested,and investigated the catalytic activity,ammonia escape and SO₂/SO₃ conversion rate during aging test.The results show that the flat-plate VMo Ti R catalyst has good activity and stability at medium and low temperature,which meets the national standards for product application and the requirements of industrial denitration applications.