钠助剂对Fe/SiO2催化剂结构及其催化甲烷氧化偶联反应性能的影响

摘要

Silica-supported iron was taken as a probe to explore the effect of sodium promoter on the structure of Fe/SiO2 and its catalytic performance in OCM reaction using XRD, XANES and EXAFS techniques. XRD results indicate that SiO2 support still remains its amorphous feature in 1wt%Fe/SiO2 after calcined at 800℃, while a sample promoted with sodium and calcined at 800℃, i.e. 1wt%Na-1wt%Fe/SiO2, is transformed to α-cristobalite structure.EXAFS analyses reveal that the total coordination number of the nearest oxygen neighbors is 5.0 at an average Fe-O bond length of 0.194 nm for Fe/SiO2, whereas only 4.1 at a greatly shortened Fe-O bond length of 0.189 nm for Na-Fe/SiO2. XANES results show that the active species of the catalysts gradually migrated from octahedral (Oh) site to tetrahedral (Td) one due to the participation of sodium promoter. In spite of the conversion of CH4 decreased over Na-Fe/SiO2 instead of Fe/SiO2, the selectivity to C2 hydrocarbons promoted ca. 42%, suggesting that a mixed majorityTd/minority-Oh surface coordinate geometry structure effectively suppressed the deep oxidation of CH4 and remarkably enhanced the selectivity to C2 hydrocarbons.%以二氧化硅担载铁氧化物(1wt％Fe/SiO2)为探针,结合XRD、XANES以及EXAFS分析结果首次从表面配位几何的观点讨论了钠助剂对Fe/SiO2催化剂结构及其甲烷氧化偶联反应性能所产生的影响.XRD分析表明,Fe/SiO2经800℃焙烧后仍保持无定形特征,添加钠助剂并经800℃焙烧后,Na-Fe/SiO2已转化为α-方石英结构.Fe/SiO2的EXAFS拟合结果显示,铁中心被5个近邻氧环绕,平均Fe-O键长为0.194 nm.与Fe/SiO2相比,Na-Fe/SiO2的径向结构函数谱中最强峰的相对强度提高了1/4,表明形成了一个更加密集的Fe-O壳层.拟合结果表明围绕中心铁原子0.189 nm处平均有4.1个氧近邻.Fe K-边XANES谱表明钠的参与使催化剂的活性组分由配位不饱和的八面体位逐渐向四配位四面体位迁移.甲烷氧化偶联反应结果显示,添加钠助剂后甲烷转化率有所下降,但是C2烃的选择性提高了近42个百分点,Cox的选择性则明显下降.说明钠的引入使催化剂的表面结构发生变化,所形成的多数四面体/少数八面体混合结构有效地抑制了甲烷的深度氧化,乙烯乙烷的选择性明显提高.