Visible Light Photocatalyst for Water Purification and Self-Cleaning Coatings

Abstract

Pollution in water and atmosphere is a growing concern in the current society and demands short-term solutions. Conventional purification techniques like biological treatment, direct oxidation, etc. cannot eliminate low concentrations or some of the so-called emerging pollutants (hormones, detergents, pharmaceuticals, etc.) thus calling for the use of advanced oxidation processes (AOP), namely, techniques in which highly oxidising agents (mostly OH radicals) are produced to degrade organics compound, bacteria and viruses, reaching the complete mineralization or, at least, producing nontoxic by-products. One of these techniques is heterogeneous photocatalysis: redox reactions are developed on the surface of a solid catalyst that is activated by light. Nowadays the more employed and studied photocatalyst is titanium dioxide (TiO2). Its commercial applications are very diverse, including self-cleaning materials (cements, paints), antifogging materials (mirrors, glasses) and water and air purification devices. The operating limits of this effective photocatalyst are in the fact that it needs UV radiation to be activated, due to its relatively wide band gap (3.2 eV). This is a significant problem in economic terms, for high energy costs, in healthy terms, since UV radiation is dangerous for sight and skin, and it is a limit to the use of sunlight, as only 4 % of the solar spectrum that reaches the Earth surface is in the UV range. To overcome this problem, investigations are focused on TiO2 modifications to render it active with visible light (doping with metal and non-metal ions, coupling with other semiconductors, etc) as well as on the use of other semiconductors with narrower band gap. A ceramic material, lanthanum orthoferrite (LaFeO3), with ionic and electronic semiconductor properties, is synthesized in our laboratory since 2008 and it is studied for its application in the combustion cells. Knowing that it also works as photoconductor under visible light [3], it can be considered a good candidate to be used as visible light photocatalyst. Its effectiveness in the degradation of organic dyes and other molecules has been reported. After the successful degradation of organic compounds in the slurry tests carried up with the LaFeO3 powders, our group started to develop deposition systems in order to avoid filtration problems, such as loss of catalyst and costs, thus rendering the reuse of the photocatalyst easier to achieve a practical applicability. In the present thesis two deposition methods were studied: the preparation of a photocatalytic paint and the deposition of the catalyst over a ceramic foam by dip-coating process. In the first part of the work, LaFeO3 was added to a commercial water based paint. This innovative photocatalytic paint has been tested to prove two aspects: • Its self-cleaning performances, following the degradation of an azo dye under visible light irradiation. • The paint durability, executing accelerated ageing tests in a climatic chamber, and subsequently verifying visible alterations and possible diminution of its self-cleaning performances. In the second part, the objective was the application of the material for the wastewater treatment. In collaboration with the Universidad de Las Palmas de Gran Canaria a handy and resistant photocatalytic foam was developed, to avoid the vigorous continuous stirring required to keep powder in slurry and the necessary filtration after the reactions to reuse the photocatalyst. The support chosen was a commercial ceramic inert porous material (foam) and the deposition method was the dip-coating technique. To apply this coating technique high quantities of catalyst had to be used because it required the preparation of a very concentrate solution. For this reason and for a possible scaling up, the production rate of the photocatalyst in our laboratory had to be increased. The quality of the catalyst had been largely tested for low production rates. Theoretically the results of the synthesis must be the same; however, a complete characterization and determination of photocatalytic performances were carried out on three batches of catalyst prepared on a large scale. The photocatalytic foam and the different batches in slurry were tested in the degradation of 4-Nitrophenol. A comparison between the slurry test and the fixed bed test is presented.