Photocatalysis

In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals: •OH) able to undergo secondary reactions. Its practical application was made possible by the discovery of water electrolysis by means of titanium dioxide (TiO2). In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals: •OH) able to undergo secondary reactions. Its practical application was made possible by the discovery of water electrolysis by means of titanium dioxide (TiO2). The earliest mention of photocatalysis dates back to 1911, when German chemist Dr. Alexander Eibner integrated the concept in his research of the illumination of zinc oxide (ZnO) on the bleaching of the dark blue pigment, Prussian blue. Around this time, Bruner and Kozak published an article discussing the deterioration of oxalic acid in the presence of uranyl salts under illumination, while in 1913, Landau published an article explaining the phenomenon of photocatalysis. Their contributions led to the development of actinometric measurements, measurements that provide the basis of determining photon flux in photochemical reactions. After a brief stint of lack of research on photocatalysis, in 1921, Baly et. al used ferric hydroxides and colloidal uranium salts as catalysts for the creation of formaldehyde under light in the visible spectrum. However, it wasn’t until 1938, when Doodeve and Kitchener discovered that TiO2, a highly-stable and non-toxic oxide, in the presence of oxygen, could act as a photosensitizer for bleaching dyes, as ultraviolet light absorbed by TiO2 led to the production of active oxygen species on its surface, resulting in the blotching of organic chemicals via photooxidation. This would actually mark the first observation of the fundamental characteristics of heterogeneous photocatalysis. Research in photocatalysis subsided for over 25 years due to lack of interest and absence of practical applications. However, in 1964, V.N. Filimonov investigated isopropanol photooxidation from ZnO and TiO2; at around the same time, Kato and Mashio, Doerffler and Hauffe, and Ikekawa et al. (1965) explored oxidation/photooxidation of carbon dioxide and organic solvents from ZnO radiance. A few years later, in 1970, Formenti et. al and Tanaka and Blyholde observed the oxidation of various alkenes and the photocatalytic decay of nitrous oxide (N2O), respectively. However, a breakthrough in photocatalysis research occurred in 1972, when Akira Fujishima and Kenichi Honda discovered electrochemical photolysis of water occurring between connected TiO2 and platinum electrodes, in which ultraviolet light was absorbed by the former electrode, and electrons would flow from the platinum electrode (anode; site of oxidation reaction) to the TiO2 electrode (cathode; site of reduction reaction); with hydrogen production occurring at the cathode. This was one of the first instances in which hydrogen production could come from a clean and cost-effective source, as the majority of hydrogen production back then – and still today – came/comes from natural gas reforming and gasification. Fujishima’s and Honda’s findings have lead to other advancements in photocatalysis; in 1977, Nozik discovered that the incorporation of a noble metal in the electrochemical photolysis process, such as platinum and gold, among others, could increase photoactivity, and that an external potential wasn’t required. Future research conducted by Wagner and Somorjai (1980) and Sakata and Kawai (1981) delineated the production of hydrogen on the surface of strontium titanate (SrTiO3) via photogeneration, and the generation of hydrogen and methane from the illumination of TiO2 and PtO2 in ethanol, respectively. Research and development in photocatalysis, especially in electrochemical photolysis of water, continues today, but so far, nothing has been developed for commercial purposes. In 2017, Chu et al. assessed the future of electrochemical photolysis of water, discussing its major challenge of developing a cost-effective, energy-efficient photoelectrochemical (PEC) tandem cell, which would, “mimic natural photosynthesis.” In homogeneous photocatalysis, the reactants and the photocatalysts exist in the same phase. The most commonly used homogeneous photocatalysts include ozone and photo-Fenton systems (Fe+ and Fe+/H2O2). The reactive species is the •OH which is used for different purposes. The mechanism of hydroxyl radical production by ozone can follow two paths. Similarly, the Fenton system produces hydroxyl radicals by the following mechanism In photo-Fenton type processes, additional sources of OH radicals should be considered: through photolysis of H2O2, and through reduction of Fe3+ ions under UV light: