甲醇选择氧化MoO_3-TiO_2催化剂的研究

A STUDY OF THE MoO_3-TiO_2 CATALYSTS FOR PARTIAL OXIDATION OF METHANOL

用XRD、LRS、ESR、XPS等方法对机械混合物焙烧法制备的两组MoO_3—TiO_2催化剂进行了表征,同时考察了催化剂的甲醇选择氧化活性和选择性。结果表明:MoO_3与TiO_2机械混合物在450℃焙烧过程中发生TiO_2中锐钛矿转变成金红石的相变,但若将TiO_2在空气中550℃预处理8h后再与MoO_3混合焙烧则未观察到相变的发生。在上述两组催化剂中都发生了MoO_3在TiO_2表面的分散。当MoO_3含量小于10wt％时,MoO_3晶相消失,钼以表面钼氧物种存在。MoO_3与TiO_2之间的强相互作用导致Mo(Ⅴ)的生成和pKa≤+3.3的表面酸位的产生。TiO_2晶型对催化剂表面结构及反应活性有明显影响,锐钛矿转变为金红石使最佳反应温度提高20～40℃,Mo在表面富集程度增大,而Mo(Ⅴ)和表面酸位浓度下降。表面Mo(Ⅴ)和高分散于表面的八面体配位钼氧物种可能是甲醇氧化的活性位。

Two groups of MoO3-TiO2 catalysts prepared by calcination of relevant mechanical mixtures have been characterized by XRD, LRS ESR and XPS techniques, the catalytic activity and selectivity for partial oxidation of methanol was also studied. It is shown that during 450℃ calcination of the mechanical mixture of MoO3 and TiO2, the anatase in TiO2 is transformed into rutile, while for the TiO2 pretreated in air at 550℃ for 8h before calcination, no phase transformation occurs. XPS and EDAX results reveal the enrichment of Mo on the surface for both two groups of catalysts. Raman spectra also confirm the dispersion of MoO3 on TiO2. When MoO3 content is lower than 10 wt%,the crystalline MoO3 phase disappears and MoO3 exists as a two dimensional surface molybdate species. The surface species are formed from the isolated tetrahedra MoO4 or close packed octahedra MoO6 depending on the MoO3 content. When MoO3 content is higher than 10 wt%, Raman spectra confirm the existence of MoO3 crystallites. This is consistent with the XRD results. A simple model is suggested to describe the dispersion of MoO3 on TiO2 surface as a function of MoO3 content. ESR results show that Mo ( V ) is formed after calcination of MoO3 and TiO2. It is believed that the electron transfer from TiO2 to MoO3 results in the valence change of Mo ( VI ) to Mo (V). Since anatase is more facile to donate electrons than rutile, Mo ( V ) concentration is much higher in group B samples. The strong interaction between MoO3 and TiO2 also results in the formation of surface acid sites with pKa≤+ 3.3.