摘要
研究了添加Li的La_2O_3和ZrO_2两种催化剂对甲烷氧化偶联催化性能的影响,并用XRD,LRS和XPS等方法对催化剂进行了表征。结果表明,Li^+的加入提高了催化剂的C_2选择性,但两类催化剂上Li^+的添加作用机制不同。La_2O_2加入Li^+达40mol％时,样品可获得34.4％的甲烷转化率和60.4％的C_2选择性。Li^+在La_2O_3表面分散,表面O/(Li+La)比随Li^+添加量增加而下降。催化剂对C_2选择性的提高可能是因为Li^+覆盖了La_2O_3表面部分完全氧化中心所致。ZrO_2中添加Li^+有Li_2ZrO_3生成,75mol％Li-ZrO_2仅检测到Li_2ZrO_3晶相,该样品可得34％甲烷转化率和63％C_2选择性。样品的XRD及XPS研究表明,随Li^+加入量增加,表面Li/Zr比接近2:1,没有Li的富集,样品表面存在两种氧物种,其O_1s结合能分别为530.3和531.9eV。对75mol％Li—ZrO_2样品,后者占95％,该表面氧物种可能与C_2选择性提高有关。
Two series of Li added La2O3 and ZrO2 catalysts have been studied. XRD, LRS and XPS are used for catalyst characterization. The results show that the addition of Li to the oxides results in the improvement of C2 selectivity. However, the roles of Li in the two catalysts are different.
All the catalysts were prepared by impregnating metal oxides with aqueous solution of LiOH or Li2CO3, followed by drying and calcining in air. The catalytic reactions were carried out in a quartz fixed bed reactor with on-line G.C..
For Li-La2O3 catalysts, with Li + content increasing, the O2 conversion decreases and the C2H4 selectivity increases while the CH4 conversion and the C2H6 selectivity change slightly in the whole Li + content range studied. When the Li+ content reaches 40 mol%, a 34.4% CH4 conversion with 60.4% C2 selectivity is obtained. With space velocity decreasing or reaction temperature increasing, the O2 and CH4 conversion increase and the C2 (mainly C2H6 ) selectivity decreases. CH4/O2 ratio also has great influence on the catalytic activity and selectivity. Only La2O3 phase is detected by XRD for all Li-La2O3
samples. LRS studies show that when the Li + content reaches 40 mol% the characteristic peaks of Li2CO3 appear. XPS results show that the atomic ratio of Li/La on the surface is much greater than that in the bulk. All these results indicate that Li+ is highly dispersed on the La2O3 surface. Since with Li+ amount increasing, the surface atomic ratio of O/Li + La and the O2 conversion decrease but the C2 selectivity increases,we suggest that Li+ may cover some surface oxygen centers and suppress the complete oxidation.
For Li-ZrO2 catalysts, as Li+ content changes from 50 mol% to 75 mol%, the O2 conversion decreases dramatically and C2 selectivity increases steeply. A 34% CH4 conversion and 63% C2 selectivity is obtained for the 75 mol% Li-ZrO2. The O2 and CH4 conversion increase with the space velocity, while the C2 selectivity remains nearly unchanged. The XRD results show that a new compound Li2ZrO3 is formed when Li+ is added to ZrO2, only Li2ZrO3 phase is detected in 75 mol% Li-ZrO2 catalyst. XPS results reveal that for 75 mol% Li-ZrO2 catalyst the surface atomic ratio of Li/Zr is around 2, there is no enrichment of Li on the surface. The surface atomic ratio of O/(Li + Zr) increases with the addition of Li and is much greater than the stoichiometric value. Two species of oxygen with Ols binding energy of 530.3 eV and 531.9 eV are found on the catalyst surface. The relative amount of the latter one is about 95% for 75 mol% Li-ZrO2. This species of oxygen may be responsible for the high C2 selectivity of the catalyst.
出处
《分子催化》
EI
CAS
CSCD
1991年第2期131-138,共8页
Journal of Molecular Catalysis(China)
基金
国家自然科学基金
