ESCA研究CH_4-O_2在铁和锰表面上的化学行为

Abstract

[中文摘要]利用ESCA研究比较了CH_4—O_2在过渡金属铁和锰表面上的化学行为,结果表明:(1)在室温和高真空(P=10~(-5)Pa)下,甲烷与金属(铁和锰)表面和预先氧化的金属(Fe_2O3和MnO)表面作用,均未观察到有任何化学反应。但当甲烷与氧共吸附时,金属表面上就可检测到碳物种生成(CH_x,CH_xO,Carbide等),说明过渡金属铁和锰表面上的过渡态氧能够使甲烷脱氢活化;(2)合理地选择反应气CH_4/O_2比值。对有效且有选择地进行甲烷转化十分重要。CH_4/O_2比值太低,易使在表面生成的碳物种深度氧化;CH_4/O_2比值太高,表面生成碳物种的速率(甲烷转化率)大为减小;(3)在设计和选择甲烷氧化偶联反应催化剂时,应考虑催化剂表面上金属离子对氧和碳的键合强度因素。[英文摘要]The development of a better control over the selective oxidation of methane requires an understanding of some mechanistic questions, such as how is the C-H bond of methane activated, what are the surface intermediates and what is the active oxygen species for methane activation? The answers to the above questions will help the development of processes for selective oxidation of methane. In this paper, the chemical interactions of CH4 -O2 on iron and manganese surfaces have been studied by Photoelectron Spectroscopy.The chemisorption of methane was not observed at low pressure (10-5Pa)and room temperature on Fe and Mn surfaces and on their preoxidised surfaces (Fe2O3 and MnO).However,when the atomically clean iron and manganese surfaces were exposed to the mixed-gas of methane and dioxygen,the chemical dissociative adsorption of methane was observed on the exposed-surfaces,indicating that the surface transient oxygen,most probably O-(s),produced on the metallic surfaces of Fe and Mn is responsible for the cleavage of C-H bond in methane molecule:CH4(g)+O-(s)→CHx(a)+OH(a) (1)Increasing the sample temperature would result in partial oxidation and even total oxidation of CHx species by labile lattice oxygen O2- on the surface:CH4(g