The synthesis and study of some metal catalysts supported on modified MCM-41

Mokhonoana, Malose Peter
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The main aim of this thesis has been to study the way in which Fe(III) and Co(II) incorporation into Si-MCM-41 synthesis gels affects the properties of the unmodified material. Another aim was to investigate the influence of these hetero-atoms on the dispersion and particle size distribution as well as the catalytic activity of supported Au nanoparticles in the CO oxidation reaction. Si-MCM-41 has been successfully synthesized in this work using mixtures containing CTAB as a structure-directing agent (SDA) and water-glass as a SiO2 source. Replacement of water-glass with pre-calcined Si-MCM-41 for SiO2 source in the secondary synthesis step has produced Si-MCM-41 with improved structural properties (XRD, HRTEM and Raman spectroscopy), including restructured and more crystalline pore walls (Raman spectroscopy). The conventional shortcomings of Si-MCM-41 as a support for catalyticallyactive (transition) metal components such as low hydrothermal stability, low PZC, lack of cation exchange capacity and no reducibility have been partially addressed by modification with Fe(III) and Co(II). The premodification was achieved both during framework synthesis and after synthesis by the incipient wetness impregnation (IWI) method. As opposed to the one-pot synthesis of metal-containing derivatives, the IWI method gave materials with high metal loadings and maximal retention of the properties of pristine Si-MCM-41. On the other hand, metal incorporation during synthesis to a loading of ~8.8 wt% using aqueous solutions of metal precursors showed some collapse of the mesostructure. Consequently methods were sought to incorporate this amount of metal (and up to double, i.e., 16 wt%) with maximal retention of the MCM-41 characteristics. These methods included (i) using Si-MCM-41 as a SiO2 source, (ii) dissolving the metal precursors in an acid solution before inclusion into the synthesis gel, and (iii) using freshly precipitated alkali slurries of the metal precursors. The first method produced a highly ordered 16wt% Fe-MCM-41 material with excellent reducibility (TPR showed three well-resolved peaks) and pore-wall structure (Raman spectroscopy). Like the aqueous route, the acid-mediated metal incorporation route did not produce ordered materials at metal contents of ~16 wt%. The base precipitate route produced highly ordered composite materials up to 16 wt% metal content, with characteristics similar to those of Si-MCM-41 (XRD, BET and HRTEM), although some metal phases were observed as a separate phase on the SiO2 surface. Thus, metal-containing MCM-41 materials could be obtained with conservation of MCM-41 mesoporosity. Raman spectroscopic studies have shown that the effect of transition metal incorporation in MCM-41-type materials is to strengthen the pore walls (shift of Si-O-Si peaks to higher frequencies), while TPR studies revealed that the essentially neutral framework of Si-MCM-41 could be rendered reducible by transition metal incorporation. Gold-containing mesoporous nanocomposites were prepared by both direct synthesis and post-synthetically. Catalysts prepared by direct hydrothermal synthesis were always accompanied by formation of large Au particles because of the need to calcine the materials at 500 oC in order to remove the occluded surfactant template. The presence of transition metal components in Me-MCM-41 (Me = Fe and Co) has been found to play a significant role in the particle size distribution and also the dispersion of Au nanoparticles when these materials were used as supports. In general, a base metal-containing support was found to produce smaller Au nanoparticles than the corresponding siliceous support. It has been proposed that the transition metal components serve as anchoring or nucleation sites for the Au nanoparticles, which are likely to sinter during calcination. The anchoring sites thus retard the surface mobility of Au at calcination temperatures above their TTammann. The use of the Au/Me-MCM-41 materials as catalysts in the CO oxidation reaction has led to the following observations: (i) catalyst on metal-containing supports showed better activity than those on Si-MCM-41, probably due to the induced reducibility in metal-MCM-41, (ii) catalysts prepared by direct synthesis showed inferior activity owing to large Au particles, (iii) increasing Au content improves the catalytic performance, (iv) increasing the Fe content of the support at constant Au improves the catalytic performance, and (v) changing the base metal component of the support from Fe to Co led to a significant improvement in catalytic activity. The similarity of the apparent activation energies (Ea) for the 5 wt% Au-containing 5 wt% Fe- and 5 wt% Co-MCM-41 suggested that the difference in catalytic activity is associated with the number of active sites possessed by each catalyst system. The observed order of catalytic activity of these 5 wt% Au-containing systems in terms of the support type is: Co-MCM-41 > Fe-MCM-41 > Si-MCM-41. This was further supported by the average Au particle size, which, in terms of the support, followed the order Co-MCM-41 < Fe-MCM-41 < Si-MCM-41. Thus, metal-support interactions between Au and MCM-41 have been enhanced by introducing Fe(III) and Co(II), which also induced framework charge, ion exchange capacity (IEC) and reducibility in the neutral siliceous support.
PhD thesis - Faculty of Science
Fe(III), Co(II), Si-MCM-41, synthesis gels, hetero-atoms, dispersion, particle size distribution, catalytic activity, Au nanoparticles