摘要
本文采用循环氧化还原法,脉冲反应及TPD等实验手段对钙钛矿型氧化物掺杂SrTiO_3体系上甲烷氧化偶联反应中的活性氧物种的作用进行了研究.结果表明,催化剂由于掺杂产生的未被充分还原的氧物种(O^(α-)(0<α<2)是活化甲烷并促使其发生偶联反应的主要的活性中心,而表面晶格氧(O_L^(2-))则主要使甲烷深度氧化;消耗掉的未被充分还原的氧或晶格氧可以在高温下氧气氛中氧化复原.对于掺杂的SrTiO_3样品,体相中的氧在惰性气氛或还原气氛中可以向表面扩散.吸附氧可能不直接与甲烷作用而主要通过催化剂表面活性氧物种(O^(α-)或O_L^(2-))而起作用.
Acceptor-doped SrTiO3 perovskites were active and selective catalysts for oxidative coupling of methane. To understand the process of methane activation more deeply, effects of oxygen species on these catalysts were investigateed by means of cyclic redox method, pulse reaction and TPD-MS.
From O2-TPD results, it can be seen that the acceptor-doped SrTiO3 samples have three oxygen desorption peaks, which can be attributed to the adsorbed oxygen, incompletely reduced oxygen (OLa )(0<α<2) and lattice oxygen (OL2-). On the lattice of doped sample SrTi0.8 Mg0.2 O3.8 or SrTi0.9 Li0.1 O3.8, the form of surface oxyen could be mainly the incompletely reduced oxygen , while on the lattice of SrTiO3 sample, mainly lattice oxygen. Cyclic redox reaction results suggested that over these acceptor-doped SrTiO3 catalysts, incompletely reduced oxygen is the main active site for methane coupling and surface lattice oxygen for methane combustion preferably. These results also indicated that ethane was probably the primary product and ethylene the secondary one. The consumed incompletely reduced oxygen or lattice oxygen can be restored when catalysts are exposed to oxygen at high temperature. As incompletely reduced oxygen has higher energy than lattice oxygen, it might be more mobile, so when the incompletely reduced oxygen on the surface of acceptor doped samples was consumed, the oxygen in the bulk of catalysts could migrate onto the surface in inert gas or reductive gas. Adsorbed oxygen does not activate methane directly, but it has the same effects on oxidative coupling of methane by the formation of surface active oxygen species (Oa or OL2 ). For doped SrTiO3 samples, adsorbed oxygen increases the amount of C2 formation, but for undoped sample adsorbed oxygen increases the amount of CO2 formation.
出处
《分子催化》
EI
CAS
CSCD
1993年第1期16-22,共7页
Journal of Molecular Catalysis(China)
基金
国家自然科学基金资助课题
关键词
甲烷
氧化
掺杂
催化剂
活性
Methane, Oxidative coupling, Stronsium titanate, TPD, Oxygen species.
