TiO2-based catalysts for energy production and environmental protection

Abstract

In the next future commercial photocatalysis would become both technically and economically competitive to counterpart technologies, for example hydrogen production from hydrocarbon reforming, CO2 conversion by dry reforming and chemical synthesis, water treatment by adsorption, biological treatment or advanced oxidation processes (AOPs). Furthermore, compared to above industrial processes, air purification especially indoor air purification appears to be a promising field in which photocatalysis could potentially act as a commercialized technology, integrating with particulate matter removal technologies. Since its commercial production in the early twentieth century, titanium dioxide (TiO2) has been widely used as pigment and in sunscreens, paints, ointments, toothpaste, ecc. Moreover after its first application in water splitting by Fujishima and Honda in 1972, TiO2 has been heavily investigated in photocatalysis, solar cells, lithium ion battery electrodes, biomedical devices and intelligent coatings. However, there are still some intrinsic drawbacks that have limited the wide application of TiO2 in its many multidimensional nanostructure forms. Withal wide band gap, TiO2 (anatase: 3.2 eV, rutile: 3.0 eV) has a low utilization of the solar light spectrum. Furthermore, a fast recombination of photo-generated electron-hole pairs and a large over potential for water splitting leads to low photocatalytic efficiency. Therefore, in these years in order to improve the TiO2 photoefficiency, more efforts have been devoted to enlarging the effective photocatalytic surface, forming Schottky junctions or heterojunctions, and engineering the band structure to match particular energy levels with structural or chemical modifications. In this work three different approaches were used to modify the chemico-physical properties of TiO2 investigating the effects of these changes on the photocatalytic performance both in the photo-oxidation and photo reduction reactions either under UV than solar light irradiation. The first strategy was to add at the commercial TiO2 another oxide as CeO2 and noble metals as gold or silver to exploit their surface Plasmon resonance effect. In particular the Au/TiO2-CeO2 catalyst has showed good performance both in the photocatalytic water splitting than in the photo-oxidation of 2-propanol in the gas-phase. The enhanced charge carrier separation due to the presence of gold and the redox properties of cerium oxide were the key factors to increase the photoactivity of TiO2. The second approach was a structural modification of TiO2 with the introduction of Ti3+ and oxygen vacancies through laser irradiation. The remarkable increase of hydrogen production by photocatalytic water splitting was related to the presence of defects inside the crystalline structure of TiO2. The combinationon (third approach) of a TiO2 structural modification as the synthesis of inverse opal materials, and chemical modifications as the addition of a host component as BiVO4, CeO2, CuO or doping agent as N, W or Hf can be a promising strategy to enhance the titania photoactivity under solar light irradiation. The high performance of these catalysts was due to the peculiar porous backbone of inverse opal TiO2 that led to have a high light absorption inside the material and to exploit the photonic effects. Moreover, the presence of a photosensitizer as BiVO4 or the introduction of defects, eased by the presence of doping agents, further enhances the light absorption and the electron-hole charge separation of TiO2. The contemporaneous presence of structural and chemical modifications of titanium dioxide could be a promising approach to achieve an efficient use of solar energy applied to the TiO2-based photocatalysis for energy production and environmental protection.