Study On The Catalytic Performance Of High-Surface-Area Ce-Zr Modified Alumina Supported Catalysts For Methane Combustion And Automotive Exhaust Purification

The common requirements for catalyst materials in methane combustion and automotive exhaust purification are higher low-temperature activity and high-temperature stability. Alumina-supported catalysts are undoubtedly the most studied systems. However it is frequently stated that alumina phase transition (γ→α) causing a drastic decrease in the surface area, and the sintering of the metals contribute to the thermal deactivation of the catalysts. So preparation of high surface area alumina resisting high temperature is vital to high temperature applications.In this work, the SiO₂ modified alumina supports for high-temperature catalysts are synthesized by sol-gel methods, and dried by supercritical drying in alcohol. The Ce_xZr_1-x/Al₂O₃ and Ce_xZr_1-x-Al supports are synthesized by impregnation and sol-gel methods respectively, the former is dried by simple evaporation and the latter is dried by supercritical drying in alcohol. The Pd/Ce_xZr_1-x/Al₂O₃ and Pd/Ce_xZr_1-x-Al catalysts are tested for methane combustion and exhaust pollution destruction separately. Effects of alkaline earths, transition metals and rare earths on the methane combustion over Pd/Ce_0.2Zr_0.8/Al₂O₃ catalysts have been studied. At the same time, bulk composition and redox behavior over these catalysts are characterized by some experiments. Some specific conclusions are drawn from this work as follows:1 The silicon modified alumina with high thermal stability are prepared from aluminium inorganic salts (NaAlO₂, Al(NO₃)₃·9H₂O, pseudoboehmite) and water soluble sodium silicate. The BET surface area of 10%SiO₂-Al₂O₃ prepared from NaAlO₂ and water soluble sodium silicate is 233m²/g after calcinations at 1100℃ for 4h. The crystalline structure of alumina remains 5-alumina phase. The TEM images of the samples show that the materials consist of fibrous particles. The results of ²⁷Al and ²⁹Si MAS NMR show that the addition of Si suppress the transformation of tetrahedral Al to octahedral Al.2 Catalytic properties of methane combustion over different supports (Al₂O₃, SiO₂-Al₂O₃, Ce_0.2Zr_0.8/Al₂O₃ and Ce_0.2Zr_0.8/SiO₂-Al₂O₃) supported Pd catalysts havebeen studied. The results show that the addition of Ce-Zr improves the thermal stability of Pd/Al2O3or Pd/SiO2-Al2O3 catalysts. The Pd/Ce0.2Zro.8/Al203 exhibits the highest activity and thermal stability for methane oxidation, and the T90% only increases 7°C after calcined at 1100 "C.3 The effects of Ce/Zr, pretreatment temperature of support, Ce-Zr loading and Pd loading on the methane combustion over Pd/CexZri_x/Al2O3 catalysts have been studied separately. The results show that the best condition is that Ce/Zr is 1:4, pretreatment temperature is 900 °C, Ce-Zr loading is higher than 18wt%. The catalytic activity and thermal stability of the Pd/Ceo.2Zro.8/Al203 catalyst increases with the improvement of Pd loading.4 Effects of alkaline earths (Mg, Ca, Sr and Ba) on the methane combustion over Pd/Ceo.2Zro.8/Al203 catalyst have been studied. The results show that the addition of alkaline earths increases the catalytic activity of Pd/Ceo.2Zro.8/Al203 catalyst under lower reaction temperature conditions and increases the reducibility of PdO dispersed on Ce-Zr-Al-rich grains. The Pd/Ce-Zr-Ca/AhOs exhibits the highest catalytic activity. At the same time, the addition of Ca to Pd/Ceo.2Zro.8/Al2C>3 catalyst inhibits the site growth and decomposition of PdO particles and improves the reduction-reoxidation properties of the active PdO species, which increases the thermal stability of the Pd/Ceo.2Zro.8/Al203 catalyst.5 Effects of transition metals (Cr, Mn, Fe, Co and Ni) on the methane combustion over Pd/Ceo.2Zro.8/Al203 catalysts have been studied. The results show that the addition of Cr, Ni and Fe obviously improves the activity of Pd/CeoaZroVAkOs catalyst for methane combustion and the addition of Mn and Co decreases the activity of Pd/Ceo.2Zro.8/Al203 catalyst. The Pd/Ce-Zr-Ni/Al2O3 has the highest catalytic activity among all catalysts. The addition of Ni improves the thermal stability of PdO, which increases the thermal stability of the Pd/Ceo^Zro.s/AbOs catalyst.6 Effects of rare earths (La, Pr, Nd, Sm and Y) on the methane combustion over Pd/Ceo^ZroVAbOs catalysts have been studied. The results show that the addition of Y, Sm or Nd obviously improves the activity of Pd/Ceo.2Zr0.8/Al203 catalyst for methane combustion, the addition of La deteriorates the activity ofcatalyst, and the addition of Pr has no obvious effect on the activity of Pd/Ceo^Zro.s/AbOs catalyst. The addition of rare earths improves the thermal stability of PdO, which increases the thermal stability of the Pd/Ceo.2Zro.8/Al203 catalyst. The Pd/Ce-Zr-Y/Al2O3 exhibits the highest catalytic activity.7 Catalytic properties of automotive exhaust purification over different supports (A12O3, Ce0.2Zro.8/Al203, SiO2-Al2O3, Ceo.2Zro.8/Si02-Al203 and Ceo.2Zr0.8-Al203)supported Pd catalysts have been studied. The results show that the Pd/Ceo.2Zro.8-Al203 catalyst exhibits the highest catalytic activity, thermal stability and oxygen-storage capacity.8 The CexZri.x-Al mixed oxides are prepared by Sol-gel and supercritical drying technique. The BET surface areas of CexZri.x-Al (x = 0.75 ) are higher than 120m2/g.9 Automotive exhaust purification over Pd/CexZri.x-Al catalysts have been studied. The Pd/Ceo 75Zro.25-Al catalyst exhibits the highest catalytic activity* thermal stability and oxygen-storage capacity. The catalytic activity is related with the reducibility of PdO dispersed on Ce-Zr-Al-rich grains and the capacity of oxygen desorption under lower temperature.