Synthesis And Application Of Doped Ordered Mesoporous Alumina With High Thermal Stability

With pollution of automobile exhaust intensified and the improvement of exhaust emission standard, the catalysts of automobile exhaust purification were required to have a longer service life and higher catalytic activity. Currently, the main carrier of catalyst was ?-Al₂O₃. ?-Al₂O₃ possesses desirable surface acidity and basicity, high surface area, which make it widely applied as catalyst supports. The working temperature in practical application of catalysts is 1000 ?, which makes ?-Al₂O₃ phase transition, sintering and the ?-Al₂O₃ generated. The specific surface area is significantly reduced, the active metal component on the surface gathered and sintering. The catalyst activity is reduced, which can not meet the emission requirements. Ordered mesoporous alumina ?OMA? possesses many desirable textural properties, including highly uniform channels and high surface area, which make it widely applied in catalyst and adsorbent. The OMA with large pore sizes and high surface area will reduce the condensation effect of hydroxy between adjacent particles. At present, OMA was obtained mainly through nanocasting pathway and sol-gel method. The sol-gel method is more convenient than nanocasting method. Hence, OMA with high surface area, large pore sizes and high thermal stability was successfully synthesized in this paper via a sol-gel method by doping elements and pore-enlarging agents. Taking as-synthesized OMA as carrier, the catalysts supported by Pd were obtained by incipient wetness impregnation. The catalytic activity for gas mixture HCx, CO and NOx was investigated. The main research work is as follows:1. In the sol-gel system with P123 as template agent and HCl as acidity regulator, improved synthesis method to synthesize Si-doped OMA, then investigate the effect of silicon content and adding way of aluminum source on the stability of OMA. Optimizing synthesis conditions to synthesize OMA with large pore size, high specific surface area and high thermal stability.2. B-doped OMA was synthesized via sol-gel method by adding HBO₃ and the effect of boron content on the pore structure of OMA were also investigated. On this basis of it, study the impact of different pore-enlarging agent on the pore structure and thermal stability of OMA by adding different pore-enlarging agents.3. Mg-doped OMA was synthesized via sol-gel method by adding Mg?NO₃?₂·6H₂O and the effect of boron content on the pore structure and thermal stability of OMA were also investigated. Comparing the differences of silicon-doped and boron-doped OMA with Mg-doped OMA, provide new thoughts to design synthesis of OMA with large pore sizes and high thermal stability.4. Taking as-synthesized OMA as carrier, the catalysts with high thermal stability supported by Pd were obtained by a process of thermal incipient wetness impregnation. And the catalytic activity for gas mixture including HCx, CO and NOx was investigated.Through the above research content, the research results are as follows:?1? Employing the acetic acid easily removed at lower temperature as interfacial protector, ordered mesoporous silicon-doped y-alumina with high thermal stability was synthesized through a modified sol-gel process by adding aluminum isopropoxide in batches to control the hydrolytic and condensation rate of Al³⁺. Si-OMA-20 still maintained ordered mesoporous structure with a large BET of 139.8 m² g^-1 even the calcination temperature up to 1100 ?. Taking as-synthesized OMA as carrier, the catalysts with high thermal stability supported by Pd were obtained by incipient wetness impregnation, which shows highly dispersed nano-particles. The catalytic activity of which is higher than Pd/CA.?2? The addition of suitable amounts of boric acid can also improve the ordering and specific surface area of B-doped OMA. When the boron content is 8 wt%, B-OMA-8 still maintained high thermal stability with a large specific surface area of 139.8 m² g^-1 and narrow pore size distribution when the calcination temperature was 1100 ?. Meanwhile, the addition of urea and ammonium bicarbonate could increase the specific surface area and pore volume of the boron-doped OMA. In the preparation of catalysts with B-doped OMA adding urea as carrier, Pd/B-OMA-8 possesses more excellent catalytic activity.?3? The Mg-doped OMA was synthesized by doping magnesium in the sol-gel system. The addition of suitable amount of magnesium can improve the thermal stability and specific surface area of Mg-doped OMA. When calcined at 1000 ?, it possesses narrow pore size distribution with specific surface area of 208.3 m² g^-1. The complete transformation temperature (T_100%) of reaction gases for all Mg-doped catalysts were lower than Si-doped and B-doped OMA. This can be due to the surface acidity and basicity of carriers, which will be further studied in the future.