ETD Collection

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Author: search.filters.author.Mokoena, Lebohang VivaciousHas files: Yes

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    Effect of gold nanoparticles on the activity of perovskites for CO oxidation
    (2011-11-18) Mokoena, Lebohang Vivacious
    Gold has for many years been regarded as being inert and catalytically inactive compared to the PGMs (platinum group metals). However, in the past decade it has attracted a lot of interest as both a heterogeneous and a homogenous catalyst and has been shown to catalyse a wide range of reactions e.g. oxidation, hydrogenation and reduction among others. Highly dispersed gold nanoparticles on metal oxides, like titanium oxide (Degussa, P25) have predominantly been studied because they yield some of the most active and stable catalysts. Modification of the catalysts and/or supports has been shown to affect their catalytic properties. Likewise, perovskites, which can be manipulated by partial substitution, are reported to be active supports for CO oxidation, but only at high temperatures with no activity shown for temperatures below 200°C. In this study, these perovskites were investigated at low temperatures (below 100°C) with improved activity found upon gold deposition. The presence of gold nanoparticles therefore significantly enhanced the catalytic activity, while the support itself was suspected to be involved in the reaction mechanism. A series of perovskites of the type ABO3 (LaMnO3, LaFeO3, LaCoO3 and LaCuO3) were prepared using the citrate method, while the gold was deposited on them using the deposition-precipitation method. The supports were calcined at different temperatures for optimisation. The catalysts were tested for carbon monoxide oxidation and the active catalysts characterised by XRF, XPS, XRD, Raman spectroscopy and BET surface area measurements. With the support calcined at 800ºC, the best catalyst was then modified and compared with the unmodified catalyst. The 1-wt%Au supported on LaFeO3 was found to give the best catalytic performance. This support was then modified with various weight loadings of calcium to determine the effect of calcium on the catalytic activity. Calcium-doped materials showed decreased surface area, poorer crystallinity and a drop in catalytic activity relative to the Au-LaFeO3 which indicated the best results for CO oxidation. In addition, Au-LaFeO3 showed online stability over 21 hours. Calcining the support improved the incorporation of gold nanoparticles into the perovskite lattice, resulting in superior catalytic activity. Nevertheless, at higher calcination temperatures, the catalytic activity of Au-CaTiO3 was depressed while that of Au-LaFeO3 was enhanced. The activity of perovskites increased upon gold deposition. XPS, revealed that in the active catalysts, both cationic and metallic gold co-existed, whilst in the inactive catalysts the gold existed predominantly either as cationic or metallic gold.