抗烧结Rh-Sm_2O_3/SiO_2催化剂的制备和表征及其甲烷部分氧化制合成气性能

Preparation and Characterization of Sinter-Resistant Rh-Sm_2O_3/SiO_2 Catalyst and Its Performance for Partial Oxidation of Methane to Syngas

以乙酰丙酮铑(Rh(acac)_3)和乙酰丙酮钐(Sm(acac)_3)为前驱体,用浸渍法制备了Rh/SiO_2和Rh-Sm_2O_3/SiO_2催化剂。采用原位红外光谱、热重分析、低温N_2吸附、X射线粉末衍射、高分辨透射电子显微镜、H_2-程序升温还原和X射线光电子能谱等实验技术对催化剂的制备过程,比表面积和物相以及Rh与Sm_2O_3间的相互作用进行了表征,并以甲烷部分氧化制合成气为目标反应对催化剂的稳定性进行了考察。研究表明:以Rh(acac)_3和Sm(acac)_3为前驱体采用简单的浸渍法即可制备出Rh平均粒径为2.3 nm且具有良好抗烧结性能的Rh-Sm_2O_3/SiO_2催化剂。在浸渍过程中乙酰丙酮化合物通过与SiO_2表面羟基形成氢键而负载于载体表面。Sm(acac)_3在SiO_2表面的单层负载量(质量分数)约为31%,对应于Sm_2O_3的质量分数约为15%,只要Sm(acac)_3的质量分数低于这一阈值,均可保证分解后生成的Sm_2O_3以高分散形式负载于SiO_2上,且不会因高温(800°C)焙烧而团聚。高分散于SiO_2表面的Sm_2O_3与Rh之间存在强的相互作用,可显著提高Rh的分散度,防止其在高温反应条件下烧结,进而使低Rh负载量的催化剂表现出良好的甲烷部分氧化制合成气反应活性和稳定性。

Rh/SiO2 and Rh-Sm2O3/SiO2 catalysts were synthesized by the conventional impregnation method using rhodium acetylacetonate （Rh（acac）3） and samarium acetylacetonate （Sm（acac）3） as precursors. The preparation and catalytic properties, as well as the interaction between Rh and Sm2O3, were characterized in detail by in situ infrared spectroscopy （IR）, thermogravimetric analysis （TG）, N2 physisorption （Brunauer-Emmett-Teller （BET） method）, X-ray powder diffraction （XRD）, transmission electron microscopy （TEM）, temperature-programmed reduction （H2-TPR） and X-ray photoelectron spectroscopy （XPS）. The performance of the catalysts for the partial oxidation of methane （POM） to syngas was also investigated. The results showed that a sinter-resistant Rh-Sm2O3/SiO2 catalyst with an average Rh particle size of ~2.3 nm could be synthesized using the conventional impregnation method with Rh（acac）3 and Sm（acac）3 as precursors. The surface silanol groups of SiO2 acted as the centers to interact with M（acac）3 （M=Rh, Sm） molecules when SiO2 was impregnated in the M（acac）3 solution, leading to the formation of a hydrogen-bonded M（acac）3 layer on the SiO2 surface. In this experiment, the monolayer coverage of Sm（acac）3 on the SiO2 surface was equal to a Sm（acac）3 loading （mass fraction） of approximately 31%, which in turn corresponded to a Sm2O3 loading of approximately 15%. When a Sm（acac）3/SiO2 sample with Sm（acac）3 loading below 31% was heated in air to approximately 360℃, the monolayer Sm（acac）3 species decomposed into highly dispersed Sm2O3 species on the SiO2 surface, which displayed superior stability against sintering at high temperature. No aggregation of the Sm2O3 species was observed even when the sample was heated to 800℃ in air. The strong interaction between the highly dispersed Sm2O3 and Rh plays a key role in increasing the dispersion of Rh species in the catalyst and preventing the Rh species from sintering under high temperature conditions. This factor should also be responsible for the superior activity and stability of the Rh-Sm2O3/SiO2 catalyst with extremely low Rh loading for the catalytic partial oxidation of methane to syngas.