原位红外光谱法研究中孔炭限域钌纳米催化剂的形成机理

doi:10.1016/S1872-2067(17)62958-9

摘要/Abstract

摘要：

炭载金属纳米催化剂广泛应用于精细化学品加氢反应及燃料电池等许多领域.炭载体因具有较高的表面积、易于调控的表面化学官能团以及特有的耐酸耐碱等性质而经常用作负载型金属催化剂的载体.但是相对于氧化物载体，炭载体表面较为惰性，与金属纳米粒子的相互作用较弱，采用后引入金属前体，如沉淀-沉积法和浸渍法等方法制备的催化剂，在液相和高温反应条件下，金属纳米粒子易流失和烧结.因此制备高稳定性的炭负载金属纳米催化剂仍是多相催化剂制备领域的一个重要课题.随着新型炭材料的出现及纳米孔材料制备科学的发展，极大丰富和推动了炭载金属催化剂制备方法的发展.近年来，通过炭热还原法即在制备中孔炭的过程中引入金属前体，一步制备炭载金属催化剂已经成为炭载金属催化剂的一个新的制备方法.此法制备的催化剂通常具有金属纳米粒子分散均匀、炭和金属活性中心之间的作用力强、热稳定性好、炭载体对负载金属纳米粒子具有限域作用等诸多优点，而且在诸多催化反应中具有优异的催化性能.例如本课题组曾以RuCl₃/SBA-15为硬模板，采用原位碳热还原法制备了Ru-OMC催化剂，它在液相苯环加氢、合成氨及费托合成反应中均具有优异的催化性能及稳定性，但是对于中孔炭中均匀分散的钌纳米颗粒形成的机理尚不清楚.
基于此，本文采用原位的红外光谱结合热重表征技术对sucrose-RuCl₃/SBA-15炭化过程钌物种的形成过程及机理进行了研究，探讨了蔗糖在炭化过程中对高分散钌纳米颗粒形成过程的稳定机制.研究发现，尽管经历了高达850℃的高温炭热处理，所得Ru-OMC催化剂中钌纳米粒子仍然可以均匀分散，钌粒径在1-2nm之间.同时，由于这种方法中钌前体预先负载在SBA-15载体表面，在炭化过程中，钌纳米粒子可以均匀地分散在模板氧化硅和形成的炭骨架之间的界面上，去除氧化硅模板后，钌纳米粒子可以更多的暴露在中孔炭的孔道内侧，因而具备更好的催化剂性能.通过对sucrose-RuCl₃/SBA-15炭化过程中原位红外光谱表征发现，Ru³⁺在炭化过程中逐步被还原，并和具有含氧官能团的炭前体形成类金属羰基配合物Ru （CO）_x.这种配合物的生成可以有效抑制钌纳米粒子在热处理过程的迁移乃至长大，因而对得到均匀分散的钌纳米粒子具有至关重要的作用.同时Ru （CO）_x周围刚性的氧化硅模板和碳骨架可以有效地防止钌纳米粒子在高温处理过程中烧结和团聚.对sucrose-RuCl₃/SBA-15炭化中间体的X射线光电子能谱表征进一步证明了Ru³⁺在350℃之前即可被还原，钌的3p轨道结合能发生了位移，说明钌和炭载体之间具有较强的相互作用.该结果可为炭载贵金属催化剂的调控制备及高活性纳米催化剂的形成机理研究提供一定的参考.

关键词: 钌纳米粒子, 红外光谱, 原位, 镶嵌式钌炭催化剂,中孔炭

Abstract:

The carbonization process of a sucrose-RuCl₃/SBA-15 composite towards a Ru-containing ordered mesoporous carbon (Ru-OMC) catalyst was studied by in situ temperature-programmed infrared spectroscopy to identify the stabilization role of organic carbon precursors during the formation of highly dispersed Ru nanoparticles. The results show that the formation of metal carbonyl species results in the formation of homogeneously distributed Ru nanoparticles, and the rigid silica support and carbon matrix around the Ru(CO)_x complex can significantly avoid the sintering and agglomeration of Ru metal particles during elevated temperature thermal treatment. These results ultimately demonstrate that sucrose plays important roles in the formation of homogeneously distributed Ru nanoparticles in a porous carbon matrix.

Key words: Ruthenium nanoparticles, Infrared spectroscope, in-situ, Ru-containing ordered mesoporous carbon, Mesoporous carbon

蓝国钧, 周亚萍, 沈行加, 唐浩东, 李瑛. 原位红外光谱法研究中孔炭限域钌纳米催化剂的形成机理[J]. 催化学报, 2018, 39(1): 146-156.

Guojun Lan, Yaping Zhou, Hangjia Shen, Haodong Tang, Ying Li. Formation mechanism of highly dispersed semi-embedded ruthenium nanoparticles in porous carbon matrix determined by in situ temperature-programmed infrared spectroscopy[J]. Chinese Journal of Catalysis, 2018, 39(1): 146-156.

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