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含氯挥发有机物(CVOCs)广泛用于化工原料以及有机溶剂,由于其毒性大,难降解,直接排放可引起严重的空气污染问题,采用催化燃烧的技术可以实现CVOCs高效净化,其关键在于高活性和高稳定性的催化剂.CVOCs净化催化剂主要有负载型贵金属催化剂、(复合)氧化物催化剂和复合分子筛催化剂.我们以具有高稳定性的LaMnO_3钙钛矿为研究对象,主要考察了不同制备方法对于氯乙烯催化燃烧性能的影响;并通过XRD,Raman,N_2-吸附脱附,O_2-TPD,H_2-TPR,ICP-AES,XPS等表征方法研究催化剂的结构和物化性能.性能评价结果表明,MnO_2虽具有良好的催化性能,但LaMnO_3催化剂则具有更好的循环稳定性.同时,制备方法对LaMnO_3催化剂上氯乙烯催化燃烧的性能有显著的影响,其活性高低的顺序为:溶胶凝-胶法(SG)>共沉淀法(CP)>硬模版剂法(HT)>水热法(HM),其中LaMnO_3-SG催化剂在182℃时氯乙烯的转化率即可达到90%.XPS结果表明,不同的制备方法导致LaMnO_3催化剂表面La和Mn的富集程度不同,并显著影响了催化剂表面Mn离子的价态、分布和氧空穴的数量.其中,LaMnO_3-SG催化剂具有最高的表面Mn~(4+)浓度,其对应的氯乙烯催化燃烧活性最高.而对于LaMnO_3-HM催化剂,La(OH)3的生成导致其具有最高的表面La/Mn比(2.29)和最低的表面Mn~(4+)浓度.由XPS计算氧空穴浓度可知,LaMnO_3-SG催化剂氧空穴浓度(1.03)远高于LaMnO_3-HM催化剂表面的氧空穴浓度(0.07),进而LaMnO_3-SG在O_2-TPD中表现出更高的O_2脱附量.进一步分析可知Mn~(4+)离子浓度与氧空穴浓度成正相关的关系,即:Mn~(4+)离子浓度越高,则表面氧空穴浓度越高.而催化剂表面氧空穴浓度越高,则有利于氧在催化剂表面的吸附和活化,从而使得催化剂表面氧物种的浓度增加,这与O_2-TPD结果一致.同时,制备方法对催化剂氧化还原性能也有显著的影响,由H_2-TPR所得催化剂的耗氢量顺序为:LaMnO_3-SG>LaMnO_3-CP>LaMnO_3-HT>LaMnO_3-HM,这与它们催化活性的顺序一致.结合XPS和H_2-TPR结果可知,催化剂表面Mn~(4+)/Mn~(3+)比例高,则催化剂的氧化还原能力也越强.以上分析表明,LaMnO_3催化剂的催化活性与催化剂表面Mn~(4+)浓度和氧空穴数量相关.具有较高的Mn~(4+)浓度有利于氯乙烯在催化剂表面吸附;而氧空穴数量的增加有利于氧在催化剂表面的吸附和活化,从而提高氯乙烯催化燃烧的反应性能.
Chlorinated volatile organic compounds (CVOCs) are widely used in chemical raw materials and organic solvents. Because of their toxicity, they are difficult to degrade, and direct emissions can cause serious air pollution problems. Using catalytic combustion technology to achieve efficient purification of CVOCs, the key is high activity And high stability catalyst.CVOCs purification catalyst mainly includes supported noble metal catalyst, (composite) oxide catalyst and composite molecular sieve catalyst.We take LaMnO_3 perovskite with high stability as the research object, mainly studied the different preparation methods for And the catalytic combustion properties of vinyl chloride were investigated. The structure and physicochemical properties of the catalyst were characterized by XRD, Raman, N 2 -adsorption and desorption, O 2 -TPD, H 2 -TPR, ICP-AES and XPS. LaMnO_3catalyst has better cyclic stability, while the preparation method has a significant effect on the catalytic combustion of vinyl chloride on LaMnO_3catalyst.The order of activity is: sol-gel method (SG)> Coprecipitation method (CP)> Hard template method (HT)> Hydrothermal method (HM), in which the conversion of vinyl chloride at 182 ℃ is up to LaMnO_3-SG catalyst 90% .XPS results show that different preparation methods lead to different enrichment of La and Mn on the surface of LaMnO_3 catalyst, and significantly affect the valence, distribution and the number of oxygen vacancies of Mn ions on the surface of the catalyst. Among them, LaMnO_3-SG catalyst Has the highest Mn (4+) concentration on the surface, which corresponds to the highest catalytic activity of vinyl chloride combustion. For LaMnO_3-HM catalyst, La (OH) 3 has the highest surface La / Mn ratio The concentration of oxygen vacancies in LaMnO_3-SG catalyst (1.03) is much higher than that of the oxygen vacancies in LaMnO_3-HM catalyst (0.07), and then the molar ratio of LaMnO_3 -SG showed higher O_2 desorption amount in O_2-TPD.Further analysis showed that there was a positive correlation between Mn 4+ ion concentration and oxygen hole concentration, that is, the higher the Mn 4+ ion concentration , The higher the concentration of oxygen vacancies on the surface, while the higher the concentration of oxygen vacancies on the surface of the catalyst is, the higher the concentration of oxygen vacancies is, the better the adsorption and activation of oxygen on the surface of the catalyst, which leads to the increase of oxygen species concentration on the catalyst surface. At the same time, the preparation method also had a significant impact on the redox performance of the catalyst. From the H 2 -TPR The order of hydrogen consumption of the catalysts is: LaMnO_3-SG> LaMnO_3-CP> LaMnO_3-HT> LaMnO_3-HM, which is consistent with the order of their catalytic activities. Combined with the results of XPS and H_2- ) / Mn ~ (3+) ratio, the stronger the redox ability of the catalyst.The above analysis shows that the catalytic activity of LaMnO_3 catalyst is related to the concentration of Mn 4+ and the number of oxygen vacancies on the catalyst surface, Mn 4+ concentration is favorable for the adsorption of vinyl chloride on the catalyst surface. The increase of the number of oxygen vacancies is favorable for the adsorption and activation of oxygen on the surface of the catalyst, thus improving the catalytic performance of vinyl chloride combustion.