掺杂CaTiO_3体系上甲烷氧化偶联反应的研究

OXIDATIVE COUPLING OF METHANE OVER ACCEPTOR-DOPED CaTiO_3

受主杂质Ｍ（Ｌｉ＋、Ｍｇ２＋、Ａｌ３＋）部分替代ＣａＴｉＯ３中Ｔｉ４＋形成的ＣａＴｉ０．９Ｍ０．１Ｏ３－δ具有稳定的类似ＣａＴｉＯ３的正交晶格结构．掺杂元素的价态变化与样品的ｐ型导电性、活泼氧物种的形成及甲烷氧化偶联（ＯＣＭ）的活性之间有着规律性的依赖关系，即随掺杂元素价态降低，Ｐ型导电性增强，活泼氧物种的浓度增加，Ｃ２选择性和收率也相应提高．其中ＣａＴｉ０．９Ｌｉ０．１Ｏ３－δ的Ｐ型电导最大，它的ＯＣＭ性能也最好，其甲烷转化率为３５．７％，Ｃ２选择性５３．９％，Ｃ２收率１９．２％．ＴＰＲ和ＸＰＳ结果证明掺杂可引起活泼氧物种的形成，活泼氧物种的浓度和它的迁移能力与掺杂元素的价态有关．纯ＣＨ４与催化剂表面晶格氧物种的反应表明，催化剂表面上通常的晶格氧物种Ｏ２－有利于甲烷的完全氧化生成ＣＯｘ，而掺杂引起的活泼氧物种Ｏδ－（０＜δ＜２）有利于甲烷的偶联生成Ｃ２烃．ＣａＴｉ０．９Ｌｉ０．１Ｏ３－δ在１５０ｈ内ＯＣＭ的反应性能稳定，说明杂质Ｌｉ占据晶格中Ｔｉ位置后，可使Ｌｉ的流失速率大大减慢．

The partial substitution of acceptor impurities (Al3+, Mg2+, Li+)for Ti4+ in perovskite SrTiO3 could result in a significant improvementof C2 selectivity for oxidative coupling of methane (OCM) and a good relationship between the C,selectivity and the p-type semiconductivity ofthe doped SrTiO3 has been found. It was found in this paper that partialsubstitution of acceptor impurities (Al3+, Mg2+, Li+ ) for Ti4+ inCaTiO3 could also result in a significant improvement both for C2 selectivity of OCM and for p-type semiconductivity. The highest C2 yield wasobserved for CaTi0.9 Li0.1 O3-δ catalyst which gave CH, conversion 35.7 %with a C2 hydrocarbons selectivity of 53. 9% at 750℃. XPS and TPRmeasurements showed that non-fully-reduced oxygen species Oδ- (o<δ<2) existed on doped CaTiO3, which was created by interaction between lattice oxygen and h' in valence band of the p-type semiconductor. CH4 reaction with lattice oxygen of catalyst showed that lattice oxygen O2- was responsible for complete oxidation of CH4 and non-fully-reduced oxygen species Oδ- was responsible for oxidative coupling of CH4.Li in CaTi0.9 Li0.1 O3-δ was more stable than that in Li-doped metal oxidecatalysts, such as Li/MgO and Li/TiO2.