Preparation And Properties Of Aluminum-based Composite Oxide DOC Catalysts

Diesel vehicles are widely used in transportation,industry and agriculture,and defense construction because of their low fuel consumption,high adaptability,good thermal stability and high thermal efficiency.The continuous increase in diesel vehicle ownership has led to the generation of a large number of pollutants,mainly carbon monoxide(CO),hydrocarbons(HC),nitrogen oxides(NO_x)and particulate matter(PM),which pose serious risks to our health and the environment.Diesel oxidation catalytic converters(DOC)mainly purify CO and HC in exhaust gas into CO₂ and H₂O which are not harmful to air,and play a crucial role in diesel after-treatment technology.The low temperature of the diesel vehicle engine at the cold start leads to incomplete fuel combustion,resulting in a large number of pollutants,especially SO₂ in the exhaust gas can lead to catalyst poisoning,so DOC catalysts need to have a good low-temperature fast-starting ability and sulfur resistance.CuO/?-Al₂O₃ catalysts have received attention in the catalytic oxidation of CO,NO,and HC,but the catalysts have poor low-temperature activity,anti-toxicity,and poor dispersion,so the?-Al₂O₃ carrier needs to be modified.Ce O₂ has excellent oxygen storage and oxygen release properties,mainly in the form of Ce⁴⁺/Ce³⁺interconversion and easy to produce oxygen vacancies.Zr O₂ has high thermal stability.The introduction of M(M=Ce,Zr)can promote the dispersion of active components,improve the reduction and adsorption ability of catalysts,enhance the adsorption and activation of oxygen to form lattice oxygen,improve oxygen migration ability,and thus improve the low-temperature activity of catalysts.In this paper,different elements M are introduced to form MO_x-Al₂O₃ composite oxides,aiming to obtain CuO/Al_xM_1-xO_y catalysts with good low-temperature catalytic oxidation performance,good sulfur resistance and good thermal stability.In this thesis,Al_xCe_1-xO_y carriers(denoted as A_xC_y-H and A_xC_y-C,respectively,x/y represents Al/Ce molar ratio)were synthesized by hydrothermal and co-precipitation methods,and Al_xZr_1-xO_y carriers(denoted as A_xZ_y-H)were prepared by hydrothermal method and subsequently loaded with the active component CuO to obtain CuO/Al_xM_1-xO_y catalysts.The relationship between the structure and catalytic activity of DOC catalysts was analyzed by XRD,BET and TEM techniques.The main studies were conducted on the following:(1)Firstly,the effects of carrier hydrothermal temperature,hydrothermal time,catalyst roasting temperature and loading on the catalytic oxidation performance of CO were investigated.Then the effect of the Al/Ce molar ratio on the catalytic oxidation performance of CO and C₃H₈ was investigated.When the hydrothermal temperature was 180°C,the hydrothermal time was 24 h,the roasting temperature was 650°C,the CuO loading was 15 wt.%,and the Al/Ce molar ratio was 1/1,the CuO/A₁C₁-H catalysts had the best activity,and the T₅₀ and T₉₀ of CO were 67°C and 112°C,respectively(T₅₀ and T₉₀ are the temperatures corresponding to 50%and 90%conversion,respectively).The T₅₀ and T₉₀ of C₃H₈ were 325°C and 416°C,respectively.For the preparation of a series of CuO/A_xC_y-C catalysts by co-precipitation,the effects of different Al/Ce molar ratios on the oxidation performance of CO and C₃H₈ were investigated.The CuO/A₃C₁-C catalysts showed the best activity when the Al/Ce molar ratio was 3/1,with T₅₀ and T₉₀ of 90°C and 177°C for CO,and T₅₀ and T₉₀ of 376°C and 479°C for C₃H₈,respectively.The better activity of the catalysts prepared by hydrothermal method was attributed to the ability of the active components to be more uniformly distributed on the carrier surface with larger pore capacity and pore size.(2)A series of CuO/Al_xZr_1-xO_y-H catalysts were prepared by hydrothermal and impregnation methods.The catalyst preparation conditions were optimized using CO oxidation reaction as a probe:hydrothermal temperature of 180°C,hydrothermal time of 24 h,roasting temperature of 550°C,and CuO loading of 10 wt.%.Then,the effects of different Al/Ce molar ratios on the oxidation performance of Co and C₃H₈ were investigated.When the Al/Zr molar ratio was 1/1,CuO/A₁Z₁-H catalyst had the best activity.The T₅₀ and T₉₀ of CO are 152?and 195?,respectively,and the T₅₀ and T₉₀of C₃H₈ are 347?and 412?,respectively.(3)The sulfur resistance and thermal stability performance of CuO/Al₂O₃-H,CuO/A₁Z₁-H and CuO/Zr O₂-H catalysts were tested to investigate the effect of 200 ppm SO₂ on the catalysts'activity in oxidizing CO and C₃H₈.After passing 3 h of SO₂,the CuO/A₁Z₁-H catalyst showed better sulfur resistance compared with the single carrier catalyst.The conversion of CO decreased from 100%to 71.13%and that of C₃H₈decreased from 100%to 94.12%.In addition,the stability tests of CO and C₃H₈ were carried out at 240?and 480?for the three catalysts,respectively,and the activity of all catalysts remained almost unchanged after 12 h of continuous reaction,indicating that the catalysts have good thermal stability.The results of BET analysis showed that the catalysts prepared by hydrothermal method had larger pore size and pore capacity,and the larger pore size was favorable for the passage of gas molecules,thus the catalysts prepared by hydrothermal method had better low-temperature activity.the results of XPS analysis showed that the CuO/A₁C₁-H catalyst had the highest content of Ce³⁺,which indicated that there were more oxygen vacancies on the surface of CuO/A₁C₁-H,which was favorable for oxygen adsorption and The H₂-TPR results indicated that the introduction of M changed the interaction between the active component and the carrier,thus lowering the reduction temperature and enhancing the reduction ability of the catalyst.The introduction of M was found to improve the dispersion of the active component by TEM,thus enhancing the low-temperature activity of the catalyst.