【作者】 李扬；

【导师】 王野；

【作者基本信息】 厦门大学 ， 物理化学， 2007， 硕士

【摘要】 本学位论文主要研究了Cu/SBA-15和Fe/SBA-15催化剂上的甲烷选择氧化制甲醛反应。论文结合催化性能和催化剂表征的研究,探究了催化活性中心的本质,同时通过细致的脉冲法研究考察了这两个催化剂上的反应过程,提出了可能的反应机理。论文首先考察了高分散于SBA-15中的不同过渡金属（过渡金属与硅摩尔比为1/13200）催化剂在甲烷选择氧化反应中的催化性能,发现Cu/SBA-15和Fe/SBA-15上甲醛的收率和甲醛生成的转化频率（TOF）都明显高于SBA-15负载其它过渡金属的催化剂。对催化剂中Cu含量进行优化的实验表明,当Cu含量为0.008 wt%时有着最佳的催化性能,并且在其上得到了5.6 s^-1的甲醛生成TOF,该值是迄今为止所报道的最高值。针对Fe/SBA-15的研究表明,相同反应条件下,当Fe含量为0.05 wt%时有着最高的甲醛收率（1.9%）。结合反应结果和XRD表征表明,在催化剂中Cu含量和Fe含量分别大于1.0 wt%和4.6 wt%时形成的CuO和Fe2O3微晶或小簇会导致深度氧化。这与MoO_x/SBA-15催化剂上所形成的MoO3小簇对应于较高的甲醛生成活性不同。本论文认为SBA-15中高度分散乃至孤立的Cu^II和Fe^III可能是促进甲烷选择氧化生成甲醛的关键。针对0.008 wt% Cu/SBA-15和0.05 wt% Fe/SBA-15上的脉冲实验研究表明,甲烷可以与其中的晶格氧反应,但产物为CO_x,并没有甲醛生成。气相分子氧的参与是生成甲醛的必要条件。对Cu/SBA-15施以多次（CH₄ + O₂）脉冲时,甲烷转化率和甲醛选择性随着脉冲次数的增加而增加,并最终趋于稳定。并且随着脉冲中P（O₂）/P（CH₄）的减小,催化剂达到稳定甲烷转化率和甲醛选择性所需的脉冲次数（诱导期）缩短。在（CH₄ + O₂）脉冲中引入少量的H2,可以大幅缩短这一诱导期。在Fe/SBA-15上在较低的P（O₂）/P（CH₄）下进行多次（CH₄ + O₂）脉冲反应也可以观察到类似诱导期。EPR的表征证实了Cu^II和Fe^III能够被甲烷所还原。EPR表征同时表明不论是在流动反应还是在脉冲反应中,反应后催化剂中均有部分Cu^II和Fe^III被还原,这说明在反应过程催化剂表面可能经历了Cu^II向Cu^I或Fe^III向Fe^II的还原过程。根据上述结果本论文推测,在反应过程中产生的还原态的Cu^I或Fe^II可能参与了氧分子的活化,生成促使甲烷转化为甲醛的活性氧物种。更多还原

【Abstract】 This dissertation focuses on the studies of the selective oxidation of methane to formaldehyde over Cu/SBA-15 and Fe/SBA-15 catalysts. The nature of the active sites has been elucidated through the correlation of catalytic behaviors and characterizations of these catalysts. Kinetic measurements and pulse studies have been exploited to study the reaction mechanisms over these two kinds of catalysts.The comparison of the catalytic performances of SBA-15-supported various transition metal species （the mole ratio of transition metal to silicon is 1/13200） showed that HCHO yield and turnover frequency （TOF） for HCHO formation over the Cu/SBA-15 and Fe/SBA-15 catalysts were higher than other transition metal-containing catalysts. The optimization for the Cu/SBA-15 catalysts with different copper contents revealed that the best catalytic performance was obtained over the sample with a Cu content of 0.008 wt%. The TOF for HCHO formation could reach to 5.6 s^-1 over this catalyst, which was remarkably higher than those reported to date. For the series of Fe/SBA-15 catalysts with different Fe contents, the highest HCHO yield （1.9%） was gained over the sample with an Fe content of 0.05 wt%. As the Cu and Fe contents were increased to 1.0 wt% and 4.6 wt%, respectively, CuO and Fe₂O₃ crystallites were observed from XRD, and these crystallites could catalyzed the deep oxidation to COx. This was different from the result observed over the MoOx/SBA-15 catalyst, where the MoO3 clusters enhanced the HCHO formation. We conclude that the highly dispersed copper and iron sites, especially the isolated CuII and FeIII sites, are responsible for the selective oxidation of CH4 to HCHO.The reaction of the lattice oxygen with CH4 pulses over the 0.008 wt% Cu/SBA-15 and 0.05 wt% Fe/SBA-15 suggested that the lattice oxygen was not responsible for HCHO formation. The pulse reaction using （CH₄ + O₂） pulses over Cu/SBA-15 could produce HCHO, and we found that both CH4 conversion and HCHO selectivity increased significantly with the pulse numbers at the initial stage. It was of interest that the pulse number to reach the constant CH4 conversion and HCHO selectivity （induction period） decreases with decreasing the P（O₂）/P（CH₄） in the pulse, or with introducing a small amount of H2 into the （CH₄ + O₂） pulse. The same phenomenon was also observed over the 0.05 wt% Fe/SBA-15 catalyst if the P（O₂）/P（CH₄） in the pulse was sufficiently low. EPR characterizations confirmed that the Cu^II and Fe^III in the catalysts could be reduced by CH₄. EPR studies also demonstrated that a part of CuII and FeIII sites underwent reductions during the flow reactions and the pulse reactions. On the basis of the results described above, we speculate that the reduced Cu or Fe sites (i.e. Cu^I or Fe^II) may participate in the activation of molecular oxygen, forming an active oxygen species for the conversion of CH4 to HCHO.更多还原