Modified TiO2 Photocatalysts for the Degradation of Organic Pollutants and H2 generation via Solar Energy Conversion.

Abstract

Titanium dioxide (TiO2) is one of the most widely used and studied semiconductor photocatalysts due to its stability, low toxicity and natural abundance. However, its wide band gap of 3.2 eV limits its photoactivity to the ultraviolet (UV) region of the electromagnetic spectrum. In order to take advantage of the freely available and sustainable energy from the Sun it is necessary to modify the physical properties of TiO2 to sensitise its surface to visible light excitation. In this thesis, a new photocatalyst was synthesised using a photochemical reduction procedure to deposit nanostructured Pd metal onto TiO2. The catalyst was systematically optimised to improve its photocatalytic activity. The new photocatalyst consisting of Pd nanoparticles of 2 – 4 nm in diameter deposited on 20 nm TiO2 displays an 8% enhancement in solar light harvesting capability compared to the unmodified TiO2. This increased photo-response of the modified photocatalyst corresponded to a reaction half-life for rhodamine b decolourisation of 0.5 min compared to the 9.4 min for pristine TiO2 under identical reaction conditions. The improvement in the visible light activity is due to a significant red-shift in the absorption profile of the catalyst, which is associated with the visible light active local surface plasmon resonance of the Pd nanostructures supported on TiO2. This overall red-shift in the light harvesting potential of the catalyst leads to photocatalytic activity for photo-excitations up to 600 nm. The modified TiO2 was also tested in the ambient temperature, photo-induced reforming of methanol to produce hydrogen gas (H2). The highest rate of H2 generation, which was measured at 5820 μmol/hour/gram of catalyst, corresponded to a solar to hydrogen conversion efficiency of 0.95%. This value represents a 14% increase in the rate of H2 production of similar systems and is among the highest obtained in the literature for TiO2 modified with platinum group metals (PGMs).