Development Of Sio2 Tio2 Photocatalyst With Nanosized Tio2 Crystals

Author : Sy Eun Park
Publisher :
ISBN 13 :
Total Pages : 488 pages
Book Rating : 4.:/5 (893 download)

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Download or read book Development of Novel Titania-Based Photocatalysts for Solar H2 Production written by Sy Eun Park and published by . This book was released on 2014 with total page 488 pages. Available in PDF, EPUB and Kindle. Book excerpt: The development of a hydrogen (H2) economy hinges on the discovery of simple, low-cost and environmentally friendly technologies for H2 production. Photocatalytic water splitting using sunlight and semiconductor photocatalysts has the potential to deliver an unlimited supply of carbon free H2 gas. Semiconductor photocatalysts such as titania (TiO2) are active for photocatalytic water splitting and/or ethanol photodecomposition under UV light in the presence of certain metal co-catalysts (M). Considerable research effort is now being directed into the development of improved M/TiO2 photocatalysts for H2 production from water and biofuels. This MSc research project targeted the development of novel M/TiO2 (M=Pd, Pt, Au) photocatalysts for H2 production from biofuels (alcohol-water mixtures). This work utilised a range of different TiO2 supports (some we fabricated, others commercially available) and then systematically examined the effect of metal co-catalyst deposition and treatment conditions on the activity of resulting M/TiO2 photocatalysts. The project began with the synthesis of sub-micron sized monodisperse TiO2 colloids by the controlled hydrolysis of titanium (IV) tetraisopropoxide (TTIP) in an acetonitrile:methanol:H2O solvent mixture using dodecylamine (DDA) as the catalyst and structure directing agent. By varying the TTIP:H2O ratio, it was possible to produce batches of monodisperse amorphous TiO2 colloids of different diameter. These colloids readily self-assemble under gravitational sedimentation to give 3D TiO2 colloidal crystals with pseudo photonic band gaps (PBGs) at NIR wavelengths. Calcination of the TiO2 colloids at 500°C or hydrothermal treatment at 160°C in ethanol:H2O solutions, followed by calcination at 500°C, yielded low (~10 m2/g) and high surface area (~70 m2/g) anatase colloidal crystals, respectively. Calcination at 750°C yielded rutile colloidal crystals. The optical properties of the TiO2 colloidal crystals were examined by UV-Vis reflectance measurements, and were consistent with a modified Bragg's Law expression that considered both the refraction and diffraction of electromagnetic radiation in the 3D colloidal crystals. A series of Si1-xTixO2 inverse opal photonic crystals with 3-dimensionally ordered macroporous structures, solid volume fractions between 5-10% and pseudo PBGs at visible wavelengths were then successfully fabricated, using poly (methyl methacrylate) (PMMA) colloidal crystals as sacrificial templates. The structural and optical properties of the Si1- xTixO2 inverse opals (x=0-1) were systematically characterised using SEM, TEM, XRD, UVVis and N2 physisorption. The structural data revealed a progressive transition from amorphous SiO2 to nanocrystalline anatase as the TiO2 content in the Si1-xTixO2 inverse opals increased. The pseudo PBG for Bragg diffraction on fcc (111) planes red-shifted and broadened as the TiO2 content increased, consistent with the fact that anatase (n=2.2-2.3) has a higher refractive index than amorphous SiO2 (n=1.45). The TiO2 colloids, Si1-xTixO2 inverse opals and a commercially available TiO2 (Degussa P25 TiO2) were then functionalised with Pd, Pt or Au nanoparticles to give a series of M/TiO2 photocatalysts. The metal loadings were 0.5-4 wt.%. The activity of all the TiO2 photocatalysts for H2 production from ethanol-water mixtures (80 vol.% and 10 vol.%) under UV excitation increased dramatically after the addition of the noble metal, which is explained by the metal co-catalysts acting as sinks (electron acceptors) for electrons photoexcited in TiO2, thereby preventing electron-hole pair recombination. In TiO2, ethanol serves as a sacrificial hole acceptor (electron donor) and proton source in the reaction. H2 production tests were carried out at UV-fluxes comparable to that of sunlight. Highest rates of H2 production were observed in the M/P25 TiO2 system (1 wt.% Pt/P25 TiO2=56.4 mmol g-1 h-1, 1 wt.% Pd/P25 TiO2=50.4 mmol g-1 h-1, 2 wt.% Au/P25 TiO2=47.2 mmol g-1 h-1) with rates being strongly dependent on the catalyst preparation and pretreatment conditions which influences both co-catalyst particle size and dispersion over the TiO2 support. Lower rates of H2 production were seen in the Au/TiO2 colloid and Au/TiO2 inverse opal systems, which is explained in terms of phase composition of TiO2 present in these samples. Degussa P25 TiO2, which is a mixture of anatase and rutile (85 wt.% anatase; 15 wt.% rutile), is a superior photocatalyst support than pure anatase (of which both the TiO2 colloids and TiO2 inverse opals are composed) on account of the synergistic electron transfer processes that occur at the anatase/rutile interface. On the basis of the results presented, strategies for optimising the activity of M/TiO2 photocatalysts for solar H2 production from biofuels can be developed.