Conclusion

The aim of the project was to synthesise visible light active material by starting with a UV active material, and modifying to induce visible light absorption, and then confirming the ability of this material to undergo photocatalysis reactions by performing photocatalysis reactions under strict protocols, to prove the visible light response of the photocatalyst. Niobium oxide was chosen as the UV active material, and two forms of Nb2O5 were formed: mesoporous and nanoparticulate. The nanoparticulate material, synthesised through a precipitation method, was successfully scaled up from the 1 g scale through to 25 g scale, then finally 200 g scale, with characterisation of the particles at each stage of the scale up comparable in terms of the physical properties of the material and the photocatalytic properties. The ease of scale up indicates that this material could be scaled up still further to produce kilogram quantities of a UV active photocatalyst capable of degrading model dyes such as methyl orange, and producing detectable quantities of hydrogen from a methanol and water solution without any further modification. The simplicity of this precipitation route also leads the possibility of using this method for synthesising other transition metal oxides such as tantalum oxide (Ta2O5),[1] a UV photocatalyst in it’s own right, and a starting material for visible light active TaON.[2-3] This niobium oxide material was modified with platinum group metals, in order to investigate further the effect PGMs have on photocatalysts, and a survey concentrating on platinum, rhodium and palladium was carried out, with material synthesised and characterised using notably dispersion and TEM techniques to characterise the PGM on the surface of the niobium oxide. The hydrogen generation results indicate that even a low weight percentage of PGM added to the niobium oxide (0.01%) show increased performance compared to unpromoted niobium oxide, which is consistent with Ohtani’s observation of a single particle of platinum required per particle of semiconductor in his study of platinum on titanium dioxide.[4] _{Whilst preparing a material with both an}

oxidation and reduction co-catalyst was only briefly looked at, protocols and characterisation of the PGM added material synthesised can be followed to study both material with oxidation co-catalyst added, and material with both oxidation and reduction co-catalysts added, in order to synthesise material capable of splitting pure water.

Finally, niobium oxide material was modified by depositing chromium (III) oxide by an impregnation method, to induce charge transfer between the chromium (III) on the surface, and the niobium (V) in the parent niobium oxide. The diffuse reflectance of this composite material was studied, and manipulated to display just the interaction between the chromium (III) and niobium (V), and from this the required energy of the interaction could be calculated, and found to be in the visible region of the spectrum. This interaction was found to be able to degrade the model dye methyl orange when illuminated with visible light, with an action spectrum confirming the chromium (III) / niobium (V) interaction was responsible for the degradation of methyl orange, as opposed to a dye degradation reaction. As noted, the efficiencies for this reaction are low, compared to world leader visible light photocatalysts such as Domen’s GaN/ZnO,[5] however the protocol of identifying the interaction between two metals in composite material, determining the energy of the transitions involved, and demonstrating that this interaction is responsible for organic degradation under visible light is ideal for composite material involving two metal oxides, leading to further combinations of metal oxides being explored using comparable protocols.

As shown in the introduction, the field of research in solar energy, and particularly photocatalysis is a dynamic area, with energy and environmental concerns driving the need for utilizing renewable energy such as solar energy. For commercial considerations, efficiency is key, with single figure quantum efficiencies for even the most efficient visible light catalysts limiting commercial development of photocatalysts. Identifying such visible light catalysts can also be problematic, with techniques needed to prove the photocatalyst

in the reactions is utilizing the visible light absorbed. This thesis, in particular when characterising the visible light active chromium – niobium composites, aimed to develop protocols that can definitively identify visible light interactions. By utilizing these techniques, proven visible light material such as the chromium – niobium composites can be demonstrated, with the knowledge gained used to produce new photocatalysts with the aim of improving efficiency, and developing a functional highly efficient visible light photocatalyst capable of addressing the energy and environmental issues addressed. [1] T. Sreethawong, S. Ngamsinlapasathian, Y. Suzuki, S. Yoshikawa, J. Mol. Catal.

A: Chem. 2005, 235, 1-11.

[2] G. Hitoki, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi, K. Domen, Chem. Commun. 2002, 1698-1699.

[3] K. Maeda, M. Higashi, D. Lu, R. Abe, K. Domen, J. Am. Chem. Soc. 2010, 132, 5858-5868.

[4] B. Ohtani, K. Iwai, S.-i. Nishimoto, S. Sato, J. Phys. Chem. B 1997, 101, 3349- 3359.

[5] K. Maeda, K. Teramura, D. L. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Nature 2006, 440, 295-295.