Effects of modified SiO2 on H2 and CO adsorption and hydrogenation of iron-based catalysts
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摘要: 采用溶胶凝胶方法通过掺杂修饰剂M（M=Mn、Zn、Zr、Sr）制备出改性SiO2载体，再用浸渍法将Fe元素负载于该载体上制成系列催化剂。采用X射线衍射（XRD）、氮气物理吸附-脱附、X射线光电子能谱（XPS）等手段表征了催化剂的织构性质、晶相组成和电子性质。利用程序升温手段研究了催化剂的H2还原吸附性质和CO加氢性能。借助动力学分析方法研究了催化剂与H之间的相互作用。结果表明，少量掺杂的修饰剂对催化剂的Fe物相组成以及表面Fe物种电子状态基本没有影响，但降低了催化剂的比表面积以及活性相分散度，削弱了对H2的吸附能力，降低催化剂的H2脱附活化能。Zn、Zr的掺杂抑制了催化剂的还原，而Mn、Sr的掺杂却促进催化剂的还原。Mn、Zn、Zr的掺杂抑制催化剂表面CO的解离吸附，Sr则促进CO的解离吸附，Mn、Zn、Zr、Sr均促进低温区间C-C耦合和加氢反应，其中，Mn、Zr促进加氢的作用更显著。Abstract: Metal-modified SiO2 supports were prepared by sol-gel method with Mn, Zn, Zr, and Sr cations doping, and then iron-based catalysts supported on modified SiO2 were prepared by the impregnation method. The iron-based catalysts were characterized by XRD, N2 adsorption, and XPS. The reduction adsorption property of H2 and hydrogenation property of CO were studied by temperature programmed methods. The interaction between catalyst and H was studied by kinetic analysis. The results indicate that metal modification has no influence on phase composition of Fe and surface electronic state of Fe species. However, the modification reduces surface area of catalyst and dispersion of active phase, weakens adsorption ability of H2 and lowers H2 desorption activation energy of the catalyst. Zn and Zr doping restrain reduction of catalysts, while Mn and Sr doping promotes reduction of catalysts. The doping of Mn, Zn and Zr inhibits adsorption of CO on the catalyst surface, while Sr promotes dissociation and adsorption of CO. Mn, Zn, Zr and Sr promote C-C coupling and hydrogenation reaction. Among them, Mn and Zr are more prominent.
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Key words:
- modified silica /
- modifier /
- iron-based catalyst /
- H2 adsorption /
- kinetic analysis
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图 4 催化剂的H2-TPD谱图
Figure 4 H2-TPD patterns of prepared catalysts
(a): Fe/SiO2; (b): Fe/Mn-SiO2; (c): Fe/Zn-SiO2; (d): Fe/Zr-SiO2; (e): Fe/Sr-SiO2; (f): 10 ℃/min
图 1 催化剂的氧化态(a)以及还原态(b)的XRD谱图
Figure 1 X-ray diffraction patterns of (a) fresh catalysts and (b) reduced catalysts
图 5 催化剂的H2脱附动力学分析曲线
Figure 5 H2 desorption kinetic analysis profiles of prepared catalysts
图 6 Fe/SiO2催化剂的CO-TPH谱图
Figure 6 CO-TPH patterns of Fe/SiO2 catalysts
(a): CH4; (b): C2H6; (c): H2O; (d): CO; (e): CO2
表 1 不同催化剂的织构参数和晶粒粒径
Table 1 Texture parameters and particle size of catalysts
Sample Surface area A/(m2·g-1) Pore volume v/(cm3·g-1) Average pore size d/nm Crystallite size d/nma Fe/SiO2 525 0.67 5.1 8.8 Fe/Mn-SiO2 483 0.80 6.6 9.9 Fe/Zn-SiO2 462 0.63 5.5 9.4 Fe/Zr-SiO2 477 0.54 4.5 9.3 Fe/Sr-SiO2 362 0.76 8.4 9.8 a: determined by XRD of the fresh catalysts with the diffraction line of 2θ at 41.6° for α-Fe2O3 
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表 2 催化剂表面存在的部分基元反应
Table 2 A partial elementary reactions on the surface of catalyst
Surface reaction of catalyst Reaction equation H2 dissociation adsorption H2+2M→2M-H CO molecular adsorption CO+M→M-CO CO dissociation adsorption 2M+CO→M-C+M-O Formation and hydrogenation of methine groups M-C+ M-H→M-CH+M M-CH+ M-H→M-CH2+M M-CH2+ M-H→M-CH3+M M-CH3+ M-H→CH4+2M Hydroxylation M-O+ M-H→M-OH+M Formation and hydrogenation of aldehyde group (alcohol species) M-CO+ M-H→M-COH+M M-CO+ M-H→M-CHO+M M-COH+ M-H→M-CHOH+M M-CHO+ M-H→M-CHOH+M Formation of C2 alkanes M-CH3+ M=CH2→M-CH2-CH3+M M=CH2+ M=CH2→M=CH-CH3+M M-CH2-CH3+M-H→CH3-CH3+2M M=CH-CH3+M-H→CH3-CH3+2M
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