| Volatile organic compounds (VOCs) is one of the main pollutants in atmosphere. Catalytic oxidation of VOCs has been recognized as one of the most promising VOCs removal technologies, which has the characteristics of high efficiency, low energy consumption, limited by-products pollution, etc.. The development of high performance catalysts is crucial to this method. Compared with metal oxide catalysts, composite metal oxides have exhibited better catalytic performance and application prospect. In view of this, a novel preparation route, named as continuous super (sub) critical-water syntheses (CHFS) was utilized for the preparation of high performance composite metal oxide catalysts. The effects of CHFS route on the properties of the products and their VOCs catalytic performances were extensively explored. We expected that the work conducted herein could provide experimental and theoretical basis to the development and application of CHFS technology and the promotion of advanced catalysts for VOCs removal.We have firstly synthesized Ce0.6Zr0.3La0.05Y0.05O2/AlOOH (CZA) composite support by using the CHFS route. The resulted product was directly compared with that prepared from conventional co-precipitation route. The effects of varied preparation routes on the morphology and thermo-stability of the products were investigated. Experimental results indicated that as compared with the co-precipitation route, the CHFS route made catalyst had a homogenously dispersed Ce0.6Zr0.3La0.05Y0.05O2 over the alumina support, leading to a more effective anti-sintering for the support. Thereafter, this composite support was utilized for toluene catalytic oxidation application where the active species, Pd, was loaded by using either CHFS route or wet impregnation route for comparison. It was found that the CHFS route had yielded a remarkable dispersion of Pd species over the CZA support, as well as a strong interaction with the support. This had led to a superior performance in toluene oxidation for the catalyst, where 90% toluene (i.e. T90) could be converted at the temperature as low as 145 ℃.Secondly, considering the advantage of CHFS route in the preparation of superior composite catalyst, another composite perovskite, LaCoO3/MgO, was also synthesized. Experimental results indicated that compared with citrate route, the CHFS route had yielded a "support dispersion" feature for the LaCoO3/MgO composite catalyst, which had not only ensured the functioning of MgO as sintering barriers, but also provided sufficient contacting between LaCoO3 and MgO support, leading to the Mg2+ ions being partly substituted into the B site of perovskites lattice. As such, the LaCoO3/MgO had showed remarkable catalytic performances in toluene (T90= 240 ℃) and methane (T90=521 ℃) oxidations. Further modification over the A site (La3+) of LaCoO3/MgO composite catalyst had indicated that the co-substitution of Ca2+ and Mg2+ had a positive effect on its suprafacial oxidation process (e.g. toluene oxidation) but showed somehow opposite effect on its intrafacial reaction process (e.g. methane oxidation).Finally, considering the syntheses of perovskites by using CHFS route generally following a subsequent calcination process, which has led to a low surface area and unsatisfied catalytic performance for the product, we had modified the reactor of CHFS route and successfully achieved one-step syntheses of LaMnO3 perovskite. The resulted product was prepared within a short time at ca. 7 s with the surface area as high as 88.3 m2/g with, which showed a better performance in toluene oxidation than that prepared by citrate route. |
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