Ru添加对Co/SiO2费托合成催化剂性能的影响

摘要

用初湿含浸法制备了不同Ru添加量的Co/SiO2模型催化剂,然后进行N2物理吸附、XRD、H2-TPD、DRIFTS等表征和微分固定床费托( F-T)反应评价。 F-T反应结果表明,催化剂中添加Ru后,CO转化率显著提高,TOF值增大,CO2和CH4选择性降低,烯/烷比( O/P)降低。 FT-IR表征说明,催化剂添加Ru后Co-O键的强度减弱,相对应的H2-TPR也表明,催化剂的还原度得到显著提高。还原后的催化剂XRD结果进一步证实,加入Ru后,催化剂无钴氧化物被检出,并且当Ru添加量为0.5%(质量分数)时催化剂中金属钴主要以六方密堆(hcp)形式存在。 CO-DRIFTS结果显示,Ru的加入使CO的吸收峰发生红移,即Ru促进了CO的解离。 H2-TPD结果则表明,随着Ru添加量的增加,催化剂表面COads/ Cos 和CO*/ Cos 增大,这是CH4选择性降低的主要原因。%Three Co/SiO2 catalysts doped with different amounts of Ru were prepared by incipient wetness impregnation. These catalysts were characterized by N2 physisorption, XRD, H2-TPD, DRIFTS, etc.; their catalytic performance in Fischer-Tropsch ( F-T) synthesis was investigated in a micro fixed-bed reactor. The F-T reaction results showed that the Co/SiO2 catalysts doped with Ru exhibit higher CO conversion, higher turnover frequency ( TOF) , lower selectivity to CO2 and CH4 , as well as lower ratio of olefin to paraffin, in comparison with undoped Co/SiO2 . FT-IR spectra indicated that the Co-O bond in the as-prepared catalyst is weakened by the addition of Ru, which facilitates the reduction of the Co/SiO2 catalysts;such results are also supported by the H2-TPR profiles and XRD patterns of the reduced catalysts. The main cobalt phase in the reduced catalyst with 0. 5% (by weight) of Ru is in a hexagonal close packing (hcp) structure. CO-DRIFTS results revealed that the peak of linearly adsorbed CO is red-shifted by the addition of Ru, suggesting an improvement on the dissociation of adsorbed CO. CO-TPD results showed that the ratio of COads/Cos and CO*/Cos on catalysts surface is increased by the addition of Ru, which may contribute to the decrease of the selectivity to CH4 in F-T synthesis.