Photolysis : Decomposition of a substance into simpler units, due to the absorption of light. Dissociation of water into hydrogen and oxygen by means of light energy; It is supposed to be part of photosynthesis. Photolysis is the breakdown of chemical bonds by radiant energy. Photolysis or photolysis, photodissociation, or photodecomposition is the dissociation of complex organic molecules by the effect of light, and is defined as the interaction of one or more photons with a target molecule. It is the process on which photosynthesis is based. Photodissociation is not limited to the visible spectrum. Any photon with enough energy can affect the chemical bonds of a chemical compound. Since photonic energy is inversely proportional to its wavelength, radiationelectromagnetic with visible or greater energy, such as ultraviolet radiation, x-rays and gamma rays are usually involved in such reactions
What is the photolysis of water?
It consists of the rupture of the chemical bonds of the water due to radiant energy. The dissociation of complex organic molecules by the effect of light is called photolysis, and is defined as the interaction of one or more photons with a target molecule. It is the process on which photosynthesis is based. Photolysis of water is the breakdown of the H2O molecule by the oxidative power of the p680 + ion breaks the H2O molecule, in 1 e- which is accepted by the p680 + chlorophyll, into 2 H + protons that are accepted by the coenzyme NADP to obtain NADPH2, and molecular O2 that is released into the atmosphere periodically renewing it.
Renewable hydrogen production by photolysis of water with sunlight
One of the most attractive methods of sustainable hydrogen production, without CO2 emissions, is through the use of solar radiation. The interest lies in the enormous potential of abundant, albeit diluted, energy, such as solar energy. Estimates indicate that for one hour the Earth receives energy from the Sun on the order of 14 TW (1 TW = 1018 J / s), an amount equivalent to the energy consumed on the planet over the course of a year. This energy must be fixed, converted and finally stored in the form of a versatile carrier such as hydrogen. While solar energy can be used to produce heat, electricity, or hydrogen, the use of sunlight to break the molecule of water and generating hydrogen is a process of great interest in that it is not subject to the limitations associated with the thermal breakdown of the water molecule or with the conversion of solar energy to electricity that can be used in a second stage to carry out the electrolysis of water.
The production of hydrogen (and oxygen) by photocatalytic decomposition of water by sunlight, and specifically with the part of the solar spectrum corresponding to the visible region (λ = 420-670 nm) has been studied intensively in the last decade. From the thermodynamic point of view, the water decomposition reaction is a highly endothermic process, with a large and positive change in Gibbs free energy (ΔG0 = + 237.2 kJ / mol).
Since pure water does not absorb sunlight, the breakdown process of the water molecule into its components (H2 and O2) requires the incorporation of a photocatalyst capable of absorbing radiation to subsequently carry out the reaction. The dissociation of water in H2 and O2 on semiconductors using sunlight began to be studied in detail starting in 1972 when the first device was built that allowed breaking the water molecule [1]. The most widely used photocatalyst for this purpose has been TiO2.
This system has a high activity in the reaction but requires the use of ultraviolet light. This peculiarity seriously limits the application of TiO2 in the dissociation reaction of water. H2O + hn → H2 + ½ O2 (1)
The efficiency of this process is mainly determined by the photo-physical properties and the morphology of the photocatalyst used. The developments made have been notable in this field over the last few years, even considering that the maximum efficiency achieved (close to 2.5%) is still far from practical application. According to the state of the art of this technology, the commercial application of the production of hydrogen through the photon energy of the visible spectrum requires important developments in science and engineering to achieve stable and active photocatalysts in the dissociation reaction. In a recent review,
