采用CO加氢反应、静态化学吸附、程序升温还原、CO程序升温脱附和程序升温表面反应等技术研究了助剂Li对Rh-Mn/SiO_2催化剂上CO加氢合成碳二含氧化合物性能的影响.结果表明,Li的加入及其负载量的增加抑制了烃类,特别是CH_4的生成,而对碳二及碳二以上烃类的选择性影响较小.Li的加入还提高了碳二含氧化合物的选择性,主要是乙酸的选择性,但同时降低了Rh基催化剂的CO加氢活性.表征结果表明,Li的加入既降低了催化剂解离CO的能力,又减少了催化剂上CO解离活性位的数量,从而降低了Rh基催化剂上CO加氢的速控步骤--CO解离反应的速率.Li负载量对Rh-Mn/SiO_2催化剂上H_2和CO的化学吸附量影响较小,这表明并非所有的Li都和Rh发生了相互作用,而是有相当一部分Li只是分散在载体SiO_2上,并没有与Rh发生接触.
The influence of Li loading on the catalytic activity of Li-promoted Rh-Mn/SiO_2 catalyst for the synthesis of C_2-oxygenates through CO hydrogenation was investigated. The catalyst samples were characterized by volumetric chemisorption, temperature-programmed reduction, temperature-programmed desorption of CO, and temperature-programmed surface reaction. The addition of Li significantly suppressed the formation of CH_4 on the Rh-based catalyst, while the selectivity for higher hydrocarbons was little affected. The Li promoter largely enhanced the selectivity for C_2-oxygenates, especially that of acetic acid. However, the activity of the Rh-based catalyst for CO hydrogenation gradually decreased with the increase in Li loading. It was suggested that the role of Li was to decrease the CO dissociation ability and to reduce the number of active sites for CO dissociation. The uptakes of H_2 and CO on the Li-promoted Rh-based catalyst were almost the same as those on the Rh-Mn/SiO_2 catalyst, which implied that most of Li was positioned on the support, and only a small part of the added Li was positioned on the top of Rh particles.
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