Mo/γ-Al_2O_3催化剂中金属-载体相互作用位点的研究

STUDY ON METAL-SUPPORT INTERACTION SITES OVER Mo/γ-Al_2O_3 CATALYSTS

载体表面性质直接影响金属-载体相互作用的强弱,进而对加氢催化剂的结构和性能产生重要影响。为深入认识载体表面性质对金属-载体相互作用的影响机制,以γ-Al_2O_3为研究对象,首先通过对其进行不同温度(20～180℃)的水热处理,调变了其表面各种位点(碱性、中性、酸性羟基及Al-CUS)的相对浓度,然后借助IR-OH和Py-IR表征手段对这些位点相对浓度的变化进行了分析,并采用Mo平衡吸附法对金属-载体相互作用强弱进行了量化分析。分析结果表明,随水热处理温度的升高,载体表面碱性羟基和酸性羟基的浓度均先增加后减少,且均在100℃水热处理时浓度最大,但后者的浓度在水热处理温度大于100℃后变化较小;中性羟基的浓度随水热处理温度升高总体上呈减少的趋势;Al-CUS浓度则随水热处理温度的升高逐渐降低。Mo平衡吸附量分析结果表明,随水热处理温度的升高,金属-载体相互作用先增强后减弱。通过将载体表面各位点的相对浓度与金属-载体相互作用进行关联,结果发现,碱性羟基是Mo金属-载体相互作用的关键位点。

Metal-support interaction imposes a great influence on the structure and catalytic activity of the hydrogenation catalysts,and meanwhile it directly depends on the surface properties of the support.In this article,for deeply understanding the influencing mechanism of the support properties on the interaction,the supportγ-Al 2O 3 was first hydrothermally treated under different temperatures(from 20℃to 180℃)and the relative concentration of sites on support surface(including basic hydroxyl groups,neutral hydroxyl groups,acidic hydroxyl groups and coordinatively unsaturated sites)were modified.The obtained samples were then characterized by IR-OH and Py-IR,and the strength of metal-support interaction was evaluated according to the equilibrium adsorption contents of MoO 3 on the support surface.IR-OH and Py-IR analysis results indicate that,with the increase of the hydrothermal temperature,the relative concentration of either basic hydroxyl groups or acidic hydroxyl groups first increases and then decreases,and reaches the maximum at 100℃.While the relative concentration of acidic hydroxyl groups changes after 100℃,and in general the relatire concentration of neutral hydroxyl groups shows a decreasing trend.In addition,the relative concentration of Al-CUS gradually decreases.Meanwhile,according to the results of equilibrium adsorption contents of MoO 3,it can be seen that,with the increase of temperature,the metal-support interaction first increases and then decreases.Based on the above analysis results,the suggestion that the metal-support interaction is mainly originated from the basic hydroxyl groups on the alumina surface can be given.