【作者】 罗春容；

【导师】 翁维正； 万惠霖；

【作者基本信息】 厦门大学 ， 催化化学， 2008， 博士

【摘要】 本文以探明SiO₂负载的Rh、Ru、Ir催化剂上甲烷部分氧化（POM）制合成气的反应机理及其差异本质以及焙烧温度对Rh/Al₂O₃催化剂POM反应性能的影响为主要目的。首先采用低温N₂吸附、H₂（或O₂、CO）化学吸附、XRD、XPS、程序升温（H₂-TPR、O₂-TPD、TPSR）等实验技术对相关催化剂进行了表征,并对催化剂的POM反应性能进行了详细考察。在此基础上采用原位时间分辨红外光谱、原位拉曼光谱、CH₄脉冲、瞬时产物分析技术等对各贵金属催化剂上POM反应的初级产物、反应条件下活性金属的价态和催化剂表面的碳物种以及催化剂的亲氧能力进行了考察。实验结果表明,在还原态Rh/SiO₂上,CO是POM反应的初级产物,因而反应以直接氧化机理为主;而在还原态的Ru/SiO₂上,CO₂是POM反应的初级产物,因而反应以燃烧-重整机理为主;在新还原的Ir/SiO₂上,CO是POM反应的初级产物,而在稳定反应条件下CO生成的主要途径可能不同于新鲜的催化剂表面。在POM反应条件下,SiO₂负载的贵金属催化剂床层前部均未检测到活性金属氧化物的拉曼谱峰,在Rh/SiO₂和Ir/SiO₂上还可观察到由甲烷解离所生成的碳物种,表明大多数的活性金属处于还原状态。以含微量O₂的He为载气的CH₄脉冲反应结果表明在相同的实验条件下Ru较Rh和Ir更易于被氧化。上述结果表明造成SiO₂负载的贵金属催化剂上POM反应机理差异的本质可能与Rh、Ru和Ir等对氧亲合力及M-O键能的高低以及反应条件下催化剂上生成的积碳物种等有关。Rh/Al₂O₃催化剂上POM反应的初级产物与焙烧温度有关,在600℃焙烧的Rh/Al₂O₃上,CO是POM反应的初级产物;而在900℃焙烧的Rh/Al₂O₃上,CO₂是POM反应的初级产物。造成600℃和900℃焙烧的Rh/Al₂O₃催化剂上POM反应机理差异的本质主要是不同温度焙烧的催化剂上Rh物种与Al₂O₃载体之间的相互作用不同,从而导致Rh氧化物的可还原能力以及催化剂表面的O^2-物种浓度不同。更多还原

【Abstract】 The present work focuses on the studies of the mechanism of the partial oxidation of methane （POM） to synthesis gas over SiO₂-supported Rh, Ru, Ir catalysts and the effect of calcination temperatures on the catalytic performance of Rh/Al₂O₃ for POM to synthesis gas. The catalysts were characterized by N₂ adsorption at low temperature, H₂ （or O₂,CO） chemisorption, XRD, XPS and temperature-programmed techniques （H₂-TPR, O₂-TPD, TPSR）. Catalytic performance of the catalysts for POM to synthesis gas was also investigated in detail. Comparative studies using in situ time-resolved FTIR spectroscopy, in situ microprobe Raman spectroscopy, pulsed reactions of CH4, temporal analysis of products techniques have been carried on the noble metal catalysts in order to elucidate the primary products of POM, the nature of active species under reaction atmosphere, deposited carbon species and oxygen affinity of the catalysts. Over H₂-reduced Rh/SiO₂ catalyst, CO is the primary product of the POM reaction, and direct oxidation of CH4 to synthesis gas is the dominant pathway for the POM reaction. Unlike Rh/SiO₂, over H₂-reduced Ru/SiO₂ catalyst, CO₂ is the primary product of POM reaction, and the dominant reaction scheme of the POM to synthesis gas is via combustion-reforming mechanism. CO is the primary product of POM reaction over the fresh H₂-reduced Ir/SiO₂ catalyst, while the mechanism of CO formation of CO under the steady state reaction conditions may be different from that on the fresh catalyst. No Raman band of metal oxide species are detected on the SiO₂-supported catalysts under the POM reaction conditions, however Raman band of carbon species can be observed over Rh/SiO₂ and Ir/SiO₂ catalysts. These results clearly indicated that most of the surface active species are in the metallic state under the POM reaction conditions. The results of pulsed reactions of CH₄ in which He containing a trace amount of O₂ was used as carrier gas indicated that Ru species is more easily oxidized than Rh and Ir species under the same reaction conditions. These results suggested that the significant difference in the mechanism of the POM reaction over SiO₂-supported Rh, Ru and Ir catalysts can be related to the difference in the surface concentration of O^2- species over the catalysts under the reaction conditions. The primary product of the POM reaction over Rh/Al₂O₃ is closely related to the calcination temperatures of the catalysts. CO is the primary product over Rh/Al₂O₃ calcined at 600℃, while CO₂ is the primary product over Rh/Al₂O₃ calcined at 900℃. Based on the results of TPR and XPS characterizations, the difference in the POM performance of the Rh/Al₂O₃ catalysts calcined at 600 and 900℃may have resulted from the formation of Rh species of different redox property, which affects the concentration of O^2- species on the surface of the catalysts under the reaction conditions.更多还原