3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Synthesis and characterisation of gold-rhodium nanocatalysts and their catalytic activity on carbon monoxide oxidation(2016-05-10) Rikhotso, Rirhandzu ShamaineGold nanoparticles are ideally suited as catalysts for selected low temperature reactions such as CO oxidation for catalytic convertors in the motor industry due to their high activity. But they are prone to sintering at high temperatures. Platinum-group-metal based catalysts are efficient at elevated temperatures and generally inactive at lower temperatures. This study explored the CO oxidation efficiency of gold nanoparticles and of a combination of gold and rhodium nanoparticles. Variables such as pH, loading concentration and type of support were varied to control the final properties of the Au based catalysts. Possible bimetallic systems of gold and rhodium were explored for wider temperature range activity than gold alone. All catalysts were characterised using Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-Ray Diffraction (XRD). Activity was measured using a temperature controlled, custom-built reactor linked to a gas chromatograph. The conditions yielding the smallest gold nanoparticles were established by adding 5, 8 or 10 wt.% loadings of chloroauric acid to aqueous suspensions of either TiO2 or SiO2 at pH 5, 7 or 9 and at 70-75 °C over 60 minutes. Each preparation was sealed in parafilm, aged in the dark at room temperature for 3 days, vacuum-filtered and subsequently calcined at 300 °C. Gold nanoparticles were smallest when deposited onto TiO2 instead of SiO2, at pH 7 and at a loading of 5 wt. %. A combination of gold and rhodium catalysts were subsequently prepared using these conditions, with the simultaneous addition of rhodium at 1, 3, 5 or 10 wt. % loading. Hydrolysis of gold is highly dependent upon pH, resulting in the synthesis of smaller particles under alkaline conditions. Catalytic activity of samples analysed at 70 and 150 °C was highest for gold nanoparticles below 5 nm, in agreement with previous studies. In the proposed bimetallic catalysts, it was difficult to distinguish gold and rhodium nanoparticles in TEM images, although EDS confirmed their combined presence on the TiO2 support. Particle sizes remained below 5 nm, appearing monodispersed on the TiO2 support except at 10 % rhodium loading where some nanoparticle aggregation was observed. CO oxidation activity showed an apparent temperaturedependent shift in the optimal rhodium loading. Au-TiO2 catalysts with a 5% loading showed the highest activity up to 350 °C for a period of 10 hours and the catalyst deactivated due to sintering. At 150 and 200 °C the Au/Rh-TiO2 catalyst remained active for more than 12 hours. It was concluded that the inclusion of rhodium is a potentially-favourable method for stabilising the activity of gold catalysts.Item Investigating the effect of gold-palladium bimetallic nanoparticles on TiO2 and the catalytic activity in CO oxidation(2013-04-29) Ntholeng, NthabisengIn recent years, studies have shown that supported Au catalysts have high activity for CO oxidation at low or ambient temperatures. However, the activity of these catalysts is dependent on a lot of synthesis conditions as reproducibility of small sized gold particles is hard. In this study supported Au catalysts were prepared via deposition precipitation-method (DP). The small sized Au particles were supported on TiO2 (P25). The suitable synthesis conditions such as pH, aging, metal loading and catalyst pre-treatment were investigated in order to obtain optimum synthesis conditions. The catalysts were characterized with TEM, XRD, and HRTEM. It was found that 3.7 nm Au particles were best synthesized when Au metal loading is 3% at pH 8 and aged for 72 h. The suitable calcination temperature was 200 °C. It was found that the Au particle size was 4.5 nm when Au was supported on SiO2 thus making TiO2 a suitable support. Bimetallic catalyst was synthesized via DP where Pd metal was incorporated as the second metal. It was found that the type of bimetallic formed was heterostructed where both metals where separately attached on the support. The interatomic distance measured from HRTEM results confirmed that both metal were individually attached on the support. XRD results showed that there was no Au-Pd alloy phase or PdO confirming that the Pd metal on the support was indeed in metallic form. Carbon monoxide (CO) oxidation reactions were undertaken in a tubular glass flow reactor. The monometallic Au catalyst showed superior activity at 200 °C with almost 100% CO conversion. It was also observed that the activity of these catalyst decreased as temperature increased. The CO-TPD studies showed that as temperature increased there was a low CO adsorption due to a decrease in adsorption sites. Varying Pd composition in the bimetallic catalyst did not enhance catalytic activity. However, 25Au75Pd catalyst showed a better conversion as compared to other Au-Pd catalysts. Temperatures studies on bimetallic catalysts showed that as temperature increased there was a decrease in activity. The observed decrease could be attributed to catalyst formation of large particle aggregates. It was also assumed that the low activity was due to how these catalysts were prepared as there was no surfactant utilized during preparation.