The synthesis, modification and characterisation of metal incorporated H1 SIO2 mesoporous materials

Abstract

Two approaches have been utilised for the modification of Hi SiOi mesoporous materials in order to produce well-defined active sites for catalysis: (i) the incorporation of transition metal salts into the synthesis gel in order to form Hi M/Si02 materials and (ii) the post- synthesis modification with Pt(acac)2 and various germanium additives (i.e. GeBu4, GeBugH, GePh4) and their subsequent decomposition/reduction to 673K under 10% hydrogen in nitrogen or pure nitrogen. The decomposition/reduction of Pt(acac)2 supported on amorphous silica and alumina has also been examined. The first part of this thesis describes the synthesis and characterisation of Hi M/Si02 (M = V, Cr, Fe, Co, Zn, Ga, Ge, Sn). It has been shown that the incorporation of metal sites via this methodology does not typically lead to direct incorporation into the silicate framework. However, the chromium incorporated material contains chromium as discrete species, with no formation of any oxide. Furthermore, characterisation suggests that the germanium has been successfully incorporated into the silicate 6amework and a new one-pot methodology for the incorporation of metal sites (i.e. Sn) has also been developed. A new methodology for the in situ study of heterogeneous systems utilising Energy Dispersive EXAFS (EDE) has been used to investigate the decomposition/reduction of Pt(acac)2 on various supports. Characterisation has shown that support choice has little influence on the 10% H2 in N2 reduction procedure, although the desorption of carbonaceous fragments was influenced, with the temperature of complete carbon removal shown to be Hi Si02<Si02-Al203. However, decomposition of supported Pt(acac)2 in a N2 atmosphere was shown to be influenced by support choice and follows a two stage decomposition mechanism, reducing over a broader temperature range (80K) and forming naked platinum particles at a much higher temperature (ca. 546K). The choice of support is observed to play an important role in defining platinum particle morphology. Most interestingly, the formation of platinum clusters, morphologically defined by the size and shape of the mesopores of the Hi Si02 material, show novel reactivity towards CO with the simultaneous appearance of three IR bands that are related to the terminal adsorption on CO. The incorporation of GeBu4 into the Pt(acac)2/Hi Si02 system leads to a significant modification in the H2/N2 reduction mechanism, with the EDE and TPR both indicating an increase in the reduction temperature range (ca. lOOK) and an increase in the fmal temperature in platinum particle formation (ca. 60K). At higher temperatures, the presence of germanium-platinum interactions indicates alloy formation. Ex situ characterisation of this system shows that Ge incorporation leads to significant modification of the platinum clusters with the formation of Pt-Ge alloys. The DRIFTS spectrum aAer CO adsorption exhibits only two bands, indicating that the Pt sites analogous to flat platinum surfaces are blocked, and the other platinum adsorption sites, proposed to relate to the high aspect ratio clusters, are unaffected by incorporation of GeBu4. However, further ex situ studies conducted using other germanium additives show that the choice of additive has signiGcant consequences on the reduction products. GeBusH influences only the high aspect ratio clusters, GePh4 impedes all CO adsorption by the formation of large arrays of Pt-Ge alloys (GePts, Ge2Pt3) and prior incorporation of the germanium into the mesoporous material appears to have no significant inGuence on the reduction products at all.

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