甲烷化反应过程中硫化的钼催化剂的拉曼光谱研究

STUDY ON THE SURFACE STRUCTURAL CHANGES OF SUPPORTED MOLYBDENUM CATALYSTS USED IN METHANATION BY RAMAN SPECTROSCOPY

本文研究了以γ-Al_2O_3、ZrO_2、CeO_2及La_2O_3为载体的硫化态钼催化剂在甲烷化反应过程中的激光拉曼光谱,发现在反应初期,催化剂表面氧硫钼结构变化较大,其活性下降也较快.在反应温度下,催化剂表面上的-S-键松动,部分硫原子升华,并发生一系列氧硫交换,形成体相MoS_2.甲烷化催化剂活性达到稳定后,则催化剂表面上的主要变化是MoS_2.晶粒不断长大.由于载体效应的影响.各个催化剂上表面结构的不同,与其达到活性稳定的时间有着直接的联系.另外,在反应过程中,催化剂表面上均无积碳形成.提出了硫化态钼催化剂在甲烷化反应过程中的表面结构变化模型.

The changes of surface structure of sulfided MoO3/ γ-A12O3, MoO3/ ZrO2, MoO3/CeO2 and MoO3/La2O3 catalysts used in methanation were investigated by laser Raman spectroscopy.The Raman spectra of the fresh catalyst and catalysts used for 5 and 10 h were obtained. For the exact assignment of the experimental results, the Raman spec.tra of several standard substances such as MoS2, MoS3, MoO2,MoO3 and supports were used.The strongest Raman bands are 280,818 and 994 cm-1 for MoO3,378 and 406 cm-1 for MoS2. For sulfided samples of the four catalysts, the Raman spectra show weak bands at 378, 406 cm-1 which indicate the formation of MoS2.The broad bands at the region of 780-920 cm-1 are assigned to surface oxysulfide molybdenum structure. The weak bands at 530-570 cm-1 in the Raman spectra of Mo/γ-Al2O3 and Mo/ZrO2 show the stretching vibration of S-S bond for some complex sulfur compounds on the surface.The sharp band at 994 cm-1 in the spectra of three samples (except Mo/ CeO2)is assigned to Mo = O bond stretching.This is probably due to the substitution of S atoms by O atoms after exposure of samples to air. After methanation reaction for 5 and 10 h, the tendency of changes for the four catalysts were the same. The intensities of the characteristic Raman scattering of MoS2(380 and 406 cm-1) increases.This means that the degree of aggregation forMoS2 is raised.Meanwhile, the Raman scattering of surface oxysulfide molybdenum structure (760-920 cm-1) and surface Mo = O bond stretching (992 cm-1) are weakened.Thus, it seems to indicate that the surface S-bonds are weakened and some element sulfur is released at reaction temperature.No carbon deposit was found on the catalysts after reaction. According to the results of Raman scattering, it is demonstrated that the surface structures composed of O, S and Mo atoms are destroyed gradually in reaction, especially at the initial period of reaction, and this is directly related to the decrease in.catalytic activity.A series of exchanges for O-S atoms takes place and the MoS2 microcrystals grow up step by step in reaction, so the number of active centre for the reaction is decreased.Because of the influence of supports, the stability of catalysts in reaction is dependent on the surface structure for these catalysts with different supports. A model for the changes of surface structure of sulfided molybdenum catalysts in the reaction is proposed.For tetrahedral Mo, the scheme is shown on p. 127 A similar process is also suggested for the octahedral Mo.