The area of methanol synthesis and utilization has been attracting research interests due to its positive impact on the environment and also from energy perspectives. Methanol synthesis from CO<sub>2</sub> hydrogenation not only produces methanol which is a key platform chemical and a clean fuel, but can also recycle CO<sub>2</sub> which is one of the major greenhouse gases causing global warming. As a mobile energy carrier (particularly as a hydrogen carrier), methanol is a versatile molecule which is able to generate H<sub>2</sub> via its decomposition. Catalysis plays a decisive role in the success of both methanol synthesis from CO<sub>2</sub> hydrogenation and its reverse decomposition reaction. Pd/Ga<sub>2</sub>O<sub>3</sub> binary catalyst has recently been identified as an active catalyst for the methanol synthesis reaction. In this thesis, it is reported the shape effect of Pd promoted Ga<sub>2</sub>O<sub>3</sub> for this reaction. The catalytic H<sub>2</sub> evolution from methanol photodecomposition has also been studied over these catalysts. Three shapes of Ga<sub>2</sub>O</sub>3</sub> nanomaterials (i.e. rod and plate β-Ga<sub>2</sub>O</sub>3</sub>, and particle γ-Ga<sub>2</sub>O<sub>3</sub>) have been synthesized, followed by doping with Pd metal to form corresponding Pd/Ga<sub>2</sub>O<sub>3</sub> nanocatalysts. It was found that a (002) polar Ga2O3 surface which was dominantly presented on the plate form was unstable, giving a higher degree of oxygen defects and mobile electrons in the conduction band than the other non-polar (111) and (110) surfaces of the rod form. It was shown that a significantly stronger metal support interaction was found between the (002) polar Ga<sub>2</sub>O<sub>3</sub> on the plate form and Pd, which gave higher methanol yield and selectivity. For methanol photodecomposition, it was found that, for pure Ga<sub>2</sub>O<sub>3</sub> catalysts of different shapes, the plate form with a highest degree of defects (unstable polar surface) could encourage a non-radiative catalytic recombination of electron and hole pairs upon irradiation, hence giving a highest photocatalytic activity for H<sub>2</sub> production. Once Pd was introduced onto these oxide surfaces, it was noted that there was a fast and readily electron transfer from the conduction band of Ga<sub>2</sub>O<sub>3</sub> to Pd due to the formation of a Schottky junction between the two materials. This produces metal sites for hydrogen production and further enhances the rate of the photocatalytic reaction over the radiative recombination of excitons. However, it was also found that at higher Pd content (>1%), the significantly shortened exciton lifetimes reduce the catalytic rate hence giving an overall volcanic response of activity to increasing Pd content for each shape of Ga<sub>2</sub>O<sub>3</sub>. At the higher Pd content, the plate form appeared to sustain a longer lifetime for photocatalysis compared to the other forms at the equivalent Pd loading.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:588467 |
| Date | January 2013 |
| Creators | Zhou, Xiwen |
| Contributors | Tsang, S. C. Edman |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:ed45a832-d0d5-4f1d-8c14-aa54df10e8cb |
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