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

[摘要] 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 unmodifiedmaterial. Another aim was to investigate the influence of these hetero-atomson the dispersion and particle size distribution as well as the catalytic activity ofsupported Au nanoparticles in the CO oxidation reaction.Si-MCM-41 has been successfully synthesized in this work using mixtures containingCTAB as a structure-directing agent (SDA) and water-glass as a SiO2source. Replacement of water-glass with pre-calcined Si-MCM-41 for SiO2 sourcein the secondary synthesis step has produced Si-MCM-41 with improved structuralproperties (XRD, HRTEM and Raman spectroscopy), including restructuredand 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 addressedby modification with Fe(III) and Co(II). The premodification was achieved bothduring framework synthesis and after synthesis by the incipient wetness impregnation(IWI) method. As opposed to the one-pot synthesis of metal-containingderivatives, the IWI method gave materials with high metal loadings and maximalretention of the properties of pristine Si-MCM-41. On the other hand, metalincorporation during synthesis to a loading of ~8.8 wt% using aqueous solutionsof metal precursors showed some collapse of the mesostructure. Consequentlymethods 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 methodsincluded (i) using Si-MCM-41 as a SiO2 source, (ii) dissolving the metal precursorsin an acid solution before inclusion into the synthesis gel, and (iii) usingfreshly precipitated alkali slurries of the metal precursors. The first methodproduced 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 routedid not produce ordered materials at metal contents of ~16 wt%. The base precipitateroute produced highly ordered composite materials up to 16 wt% metalcontent, with characteristics similar to those of Si-MCM-41 (XRD, BET andHRTEM), although some metal phases were observed as a separate phase on theSiO2 surface. Thus, metal-containing MCM-41 materials could be obtained withconservation of MCM-41 mesoporosity. Raman spectroscopic studies have shownthat the effect of transition metal incorporation in MCM-41-type materials is tostrengthen the pore walls (shift of Si-O-Si peaks to higher frequencies), whileTPR studies revealed that the essentially neutral framework of Si-MCM-41 couldbe rendered reducible by transition metal incorporation.Gold-containing mesoporous nanocomposites were prepared by both direct synthesisand post-synthetically. Catalysts prepared by direct hydrothermal synthesiswere always accompanied by formation of large Au particles because of the needto calcine the materials at 500 oC in order to remove the occluded surfactanttemplate. The presence of transition metal components in Me-MCM-41 (Me = Feand Co) has been found to play a significant role in the particle size distributionand also the dispersion of Au nanoparticles when these materials were used assupports. In general, a base metal-containing support was found to producesmaller Au nanoparticles than the corresponding siliceous support. It has beenproposed that the transition metal components serve as anchoring or nucleationsites for the Au nanoparticles, which are likely to sinter during calcination. Theanchoring sites thus retard the surface mobility of Au at calcination temperaturesabove their TTammann.The use of the Au/Me-MCM-41 materials as catalysts in the CO oxidationreaction has led to the following observations: (i) catalyst on metal-containingsupports showed better activity than those on Si-MCM-41, probably due to theinduced reducibility in metal-MCM-41, (ii) catalysts prepared by direct synthesisshowed inferior activity owing to large Au particles, (iii) increasing Au contentimproves the catalytic performance, (iv) increasing the Fe content of the support at constant Au improves the catalytic performance, and (v) changing the basemetal component of the support from Fe to Co led to a significant improvement incatalytic activity. The similarity of the apparent activation energies (Ea) for the 5wt% Au-containing 5 wt% Fe- and 5 wt% Co-MCM-41 suggested that thedifference in catalytic activity is associated with the number of active sitespossessed by each catalyst system. The observed order of catalytic activity ofthese 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 Auparticle size, which, in terms of the support, followed the order Co-MCM-41

[发布机构] University of the Witwatersrand