Au-TiO2 catalysts supported on carbon nanostructures for CO removal reactions
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Awareness of the climatic changes due to emission of CO2 and other greenhouse gases to the atmosphere has made it necessary to find more environmentally friendly methods for producing energy. Catalysis plays a significant role in environmental protection and clean energy production. The transformation from a carbon based to a hydrogen based economy requires new developments in catalysis. In this PhD thesis, Au/TiO 2 catalysts supported on carbon nanofibres and carbon nanofibres grown on a carbon felt to obtain a structured catalyst have been studied. The Au was prepared by deposition-precipitation or deposition from a Au colloid solution. TiO2 has been prepared by hydrolysis of TiCl4 and from Ti[OCH(CH3)2]4 using a sol-gel impregnation technique. The samples contain 1-5 wt.% Au and 10-20wt.% TiO2. The catalytic activity has been demonstrated in the water-gas shift reaction, CO oxidation and selective CO oxidation in the presence of hydrogen. A comparison between the two Au deposition methods has been performed for different supports; TiO 2, carbon nanofibres and TiO2 supported on carbon nanofibres. Small particles of Au could easily be obtained on TiO2 by both methods. However, deposition-precipitation gave larger Au (>50 nm) particles when carbon nanofibres were present. To obtain a high dispersion of Au it was necessary to tailor the Au particle size prior to deposition in the presence of carbon nanofibres. However, a selective deposition of Au on TiO2 can be achieved and both methods produce Au in the metallic state. The particle size of TiO2 remained unchanged after deposition on carbon nanofibres. Measurements of the catalytic activity in the water-gas shift reaction revealed that both Au and TiO2 need to be present, indicating that the active sites are either on the Au-TiO2 interface or that the reaction follows a bifunctional mechanism. The activity measurements also demonstrated that the activity is related to the preparation conditions. The turnover frequency is about one order of magnitude higher for Au/TiO2 prepared by deposition-precipitation compared to the colloid method. The Au particle size obtained after thermal treatment depends on the support, and hence the interaction between the Au particles and the support. In situ X-ray absorption spectroscopy revealed that the Au particles in contact with the support carry a small positive charge due to changes in the electronic structure. An increase in the Au δ+ was detected during the water-gas shift reaction. The particle size of Au remained constant during the water-gas shift reaction. Improved stability was observed for catalysts supported on carbon nanofibres because of stabilisation of the TiO2 particles and hence preventing subsequent sintering of the Au particles. The reaction rate was found to depend on the coordination number with an apparent maximum close to eight which corresponds to a particle size of approximately 3 nm. Carbon nanofibres were grown on a commercial carbon felt to obtain a structured support for the active phase (Au/TiO 2). The carbon structure may not only be used as a support, but also to provide heat for the reaction by the Joule effect. A concept for a compact internally heated structured reactor for CO oxidation was demonstrated. The internal heating offer a stable reactor system with rapid temperature response at relatively low energy input. Comparison of the internal and external heating, show a higher CO conversion in the low-temperature range with internal heating. The catalyst system show promising stability at 250°C. For Au particles of comparable sizes a higher CO oxidation activity is obtained when the TiO 2 structure is more crystalline. This does not apply for the oxidation of hydrogen which seems unaffected by the structure of the support. However, thermal treatment after Au deposition from the Au colloid solution introduces a stronger Ausupport interaction. The catalyst activity decreases more rapidly during CO oxidation than selective CO oxidation and the activity can be restored during the selective CO oxidation.