合成气制C2含氧化合物Rh-Mn/SiO2催化剂上CO吸附的红外光谱研究

摘要

采用浸渍法制备了一系列不同Mn含量的Rh-xMn/SiO2(x为Mn/Rh=0～3)催化剂,在对催化剂进行透射电镜、X射线衍射表征和CO加氢制C2含氧化合物催化性能评价的基础上,采用红外光谱技术分别考察了50和280℃下CO在催化剂上的吸附,以及50～280℃内合成气在Rh/SiO2和Rh-0.5Mn/SiO2催化剂上的程序升温反应,并在真空和CO气氛中考察了Rh-3Mn/SiO2催化剂上孪生CO吸附物种随温度的变化情况.结果表明,在CO气氛中,当温度升至200℃时,孪生CO吸附物种消失,表明在反应温度下可能不存在Rh+,即Rh+可能不是CO插入的活性位;Mn的添加有助于倾斜式CO吸附物种的生成;在合成气反应中Mn 通过促进高位能甲酰基中间态的生成,提高了催化剂氢助解离CO的能力.这与适量添加Mn使催化剂上CO转化率提高的结果一致.同时,Mn的添加还有助于削弱线式CO吸附物种的Rh-C键,使其更易于在表面上迁移,进而有利于CO的插入,提高C2含氧化合物的选择性.%The Rh-xMn/SiO2 (x is Mn/Rh atomic ratio = 0-3) catalyst samples were prepared by conventional co-impregnation. The samples were characterized by transmission electron microscopy, X-ray diffraction and catalytic performance evaluation using CO hydrogenation to C2-oxygenates. CO adsorption behavior over Rh-xMn/SiO2 catalyst under CO and syngas atmosphere at 50 and 280 ℃, temperature-programmed reaction of CO with H2 on the Rh/SiO2 and Rh-0.5Mn/SiO2 catalyst samples between 50 and 280 ℃, and the change of the gem-dicarbonyl species with increasing temperature under CO atmosphere and vacuum were also studied by infrared spectroscopy. The results showed that, when the temperature was higher than 200 ℃, the gem-dicarbonyl species was undetectable on the catalyst in the presence of CO, implying that Rh+ site is unlikely the active site for CO insertion. The addition of Mn promoter resulted in the formation of tilted CO. The addition of Mn could be also in favor of weakening the Rh-C bond in the linear CO adspecies, which facilitated the linear CO migration on the catalyst surface and enhanced the rate of CO insertion into CHx. This is consistent with the results from the experiment to improve C2-oxygenates selectivity with addition of Mn into Rh/SiO2 catalyst. The formyl adspecies was detected by infrared spectroscopy in the experiment of temperature-programmed reaction of syngas over Rh-0.5Mn/SiO2 catalyst. This result suggests that addition of Mn to rnRh/SiO2 is helpful to stabilize formyl species and this will be favorable to enhance the rate of hydrogen-assisted CO dissociation during the rnhydrogenation of CO to C2-oxygenates.