| The rapid expansion of global modernization and industrialization has made the atmospheric environment on which human beings depend increasingly polluted.Volatile organic compounds(VOCs)and carbon monoxide(CO)in typical atmospheric pollutants are abundant in industrial exhaust and automobile exhaust,which are extremely harmful to humans and the environment.The removal of VOCs and CO is currently mainly catalytic degradation technology,and catalysts are the core components.The widely used noble metal catalyst resources are scarce and expensive,which seriously hinders the development of atmospheric environmental pollution control technology,so it is urgent to research and develop high-performance non-noble metals catalysts to replace them.Copper-cerium non-noble metal catalysts show great potential in the field of air pollution control,but the active site and its exchange mechanism in redox reaction are not clear,the lack of effective active site control strategies and methods to improve the long-term temperature stability of high temperature have seriously affected the development and application of copper-cerium catalysts.Based on copper-cerium-based composite oxides,this article in-depth research on the identification of active sites and their dynamic exchange mechanism,active site stability and its enhancement strategy,and applies to catalytic of VOCs and COs,in order to provide valuable guidance for design and development of non-noble metal catalysts with both low-temperature activity and high-temperature stability,and the contents and conclusions are as follows:(1)Copper-cerium model catalyst was designed and synthesized to explore the active site,and it was revealed in detail as Cu+-Ov-Ce3+at the copper-cerium interface by characterization methods such as XPS,H2-TPR,O2-TPD,in-situ Raman and in-situ DRIFTS,and named it asymmetric oxygen vacancy(ASOv).The results show that the oxygen vacancy(Ov)in ASOv has a strong adsorption-activation ability for oxygen in the gas phase,and the catalytic activity is better than that in the symmetric oxygen vacancy(SOv),in which Cu+ adsorbe CO,and the fast electron transfer rate between Cu+ and Ce3+in the asymmetric coordination site is one of the reasons for the better catalytic activity.The dynamic exchange behavior of ASOv during the reaction process was Cu+-Ov-Ce3+?Cu2+-O2--Ce4+by in-situ Raman,in-situ DRIFTS and in-situ XPS,and the O2 in the environment was adsorbed on ASOv as reactive oxygen species(O2-),and ASOv recovered as the adsorption-conversion oxygen species were gradually consumed by pollutants.The unique redox equilibrium in copper-cerium catalysts can continuously produce new ASOv.The quantification of in-situ technology confirmed that in the oxygen-containing reaction,13%ASOv was first converted to Cu2+-O2--Ce4+species,followed by complete recovery of ASOv in the presence of CO,and during the CO oxidation process,the dynamic exchange of ASOv under the conditions of T50 and T100 reaction remained balance.(2)Based on the previous chapter,a series of copper-cerium composite oxides were synthesized by changing the precipitation method,Cu/Ce atomic ratio and precipitation pH value to explore the optimal ASOv regulation strategy.Through Raman,XPS,H2-TPR,O2-TPD and in-situ DRIFTS,a relative quantification method for ASOv was established,and the concentration of the active site of copper-cerium composite oxide ASOv after different strategies was accurately compared and studied.The influence rule of the above regulatory strategies on ASOv and the promoting mechanism of catalytic degradation of CO and VOCs(toluene and ethyl acetate)were revealed.Compared with the deposition precipitation method,the CuCeX catalyst prepared by co-precipitation can produce more copper-cerium interface to form more ASOv.The highest concentration of ASOv was the highest when the copper-cerium atomic ratio was 1:3,which showed better catalytic activity.The concentration of ASOv can be precisely adjusted by the pH of the precipitate,and studies have shown that as the pH increases,the concentration of ASOv first increases and then decreases.The CuCe3-11 catalyst synthesized by co-precipitation with a copper-cerium atomic ratio of 1:3 and pH=11 reached a maximum ASOv concentration at the copper-cerium interface,showing excellent catalytic activity,and the complete conversion temperatures of CO(WHSV=60 000 mL gcat-1 h-1,10000 ppm)and toluene(WHSV=60 000 mL gcat-1 h-1,10 000 ppm)were 101℃ and 242℃,respectively.At the same time,the lowest reaction activation energy(CO:Ea=29.10 KJ/mol,toluene:Ea=84.91 KJ/mol)was exhibited.(3)On the basis of the previous chapter,the precursors of copper-cerium composite oxides after ASOv regulation were selected,and their effects on the formation and stability of ASOv active sites were studied by different calcination temperatures,and the optimal formation conditions were explored.The effects of calcination temperature on active site formation and concentration were studied by TG-DTG,H2-TPR,O2-TPD,CO-TPD,Raman and in-situ XRD.The results show that from the formation of the active site at 300℃ to the concentration at 900℃,there is a trend of first increasing and then decreasing.400℃ is the optimal formation temperature and exhibits the maximum ASOv concentration while exhibiting optimal catalytic activity.It was further found that temperature had a great influence on the sintering resistance and high temperature stability of ASOv,so catalysts with different temperature gradients were prepared to explore and improve the stability and sintering resistance.The temperature gradient effectively stabilizes the copper-cerium interface and increases ASOv concentration.Compared with 600℃ sample,the 300-600 catalyst has significantly improved activity and excellent stability,and activity remains stable after 1700h of reaction under the conditions of low conversion,high temperature and pollutant transformation.(4)The Cu-Ce catalyst that has been studied and prepared has excellent performance in CO oxidation compared with precious metals,but the improvement of catalytic performance for VOCs pollutants such as toluene is not obvious.Therefore,based on the previous chapter,ternary composite oxides were prepared by co-precipitation to further compound Co elements,and the main active sites of CuCoCe were explored by XRD,Raman,XPS,HRTEM,H2-TPR and O2-TPD technologies.The results showed that the interface of copper-cobalt and cobalt-cerium existed in CuCoCe catalysts,but the main active site was ASOv at the copper-cerium interface,and the addition of cobalt reduced the particle size of CuO and CeO2 species,promoted dispersion,and increased the ASOv concentration.In the catalytic degradation reaction of CO and toluene,the catalytic activity of Cu-Ce catalyst was significantly improved.Complete conversion of toluene and CO was completed at 226℃ and 94℃.In addition,a small amount of cobalt is added to form a large number of Co2+species,which helps to improve the activity and stability of toluene catalytic degradation.At the same time,it is found that it is almost difficult for CoCe and CoCu to form asymmetric oxygen vacancies,and the interaction between metals and metals is also weaker than that of CuCe and CuCoCe.In summary,the active sites of copper-cerium based composite oxides and their dynamic exchange mechanisms in situ reactions were further studied and revealed,and the quantification methods of active sites in static and dynamic environments were established.A series of simple and effective active site regulation strategies were proposed.The factors influencing the stability of active sites and the strategies for improving the stability of high temperature resistance as well as the effects and promotion of ternary complex on active sites were studied.Performance of the prepared catalyst was significantly improved for VOCs and CO.The series of methods and strategies established in this study provide a valuable theoretical reference for the application of non-noble catalysts in the field of heterogeneous catalysis. |
PDF Full Text Request