Photocatalytic water splitting
Photocatalytic water splitting is a process that uses photocatalysis for the dissociation of water into hydrogen and oxygen. The inputs are light energy and water. The process, still more hypothetical than practical, depends on catalyst. Photocatalytic water splitting is inspired by photosynthesis, which converts water and carbon dioxide into oxygen and carbohydrates. Water splitting using solar radiation has not been commercialized.
Photocatalytic water splitting is achieved by irradiation of an aqueous dispersion of particles of photocatalysts. They are unlike Photoelectrochemical cell. Hydrogen fuel production using water and light, instead of petroleum, is an important renewable energy strategy.
Concepts
Two mole of is split into and using light in the process shown below.A photon with an energy greater than is needed to generate an electron–hole pairs, which react with water on the surface of the photocatalyst. The photocatalyst must have a bandgap large enough to split water; in practice, losses from material internal resistance and the overpotential of the water splitting reaction increase the required bandgap energy to to drive water splitting.
The process of water-splitting is a highly endothermic process. Water splitting occurs naturally in photosynthesis when the energy of four photons is absorbed and converted into chemical energy through a complex biochemical pathway.
O–H bond homolysis in water requires energy of . Infrared light has sufficient energy to mediate water splitting because it technically has enough energy for the net reaction. However, it does not have enough energy to mediate the elementary reactions leading to the various intermediates involved in water splitting. Nature overcomes this challenge by absorbing four visible photons. In the laboratory, this challenge is typically overcome by coupling the hydrogen production reaction with a sacrificial reductant other than water.
Materials used in photocatalytic water splitting fulfill the band requirements and typically have dopants and/or co-catalysts added to optimize their performance. A sample semiconductor with the proper band structure is titanium dioxide and is typically used with a co-catalyst such as platinum to increase the rate of production. A major problem in photocatalytic water splitting is photocatalyst decomposition and corrosion.
Method of evaluation
Photocatalysts must conform to several key principles in order to be considered effective at water splitting. A key principle is that and evolution should occur in a stoichiometric 2:1 ratio; significant deviation could be due to a flaw in the experimental setup and/or a side reaction, neither of which indicate a reliable photocatalyst for water splitting. The prime measure of photocatalyst effectiveness is quantum yield, which is:To assist in comparison, the rate of gas evolution can also be used. A photocatalyst that has a high quantum yield and gives a high rate of gas evolution is a better catalyst.
The other important factor for a photocatalyst is the range of light that is effective for operation. For example, a photocatalyst is more desirable to use visible photons than UV photons.
Photocatalysts
The efficiency of solar-to-hydrogen of photocatalytic water splitting, however, has remained very low.Gallium-indium nitride
A STH efficiency of 9.2% indium.is another catalyst solely activated by UV and above light. It does not have the performance or quantum yield of :La. However, it can split water without the assistance of co-catalysts and gives a quantum yield of 6.5%, along with a water splitting rate of 1.21 mmol/h. This ability is due to the pillared structure of the photocatalyst, which involves pillars connected by triangle units. Loading with NiO did not assist the photocatalyst due to the highly active evolution sites.
()()
had the highest quantum yield in visible light for visible light-based photocatalysts that do not utilize sacrificial reagents as of October 2008. The photocatalyst featured a quantum yield of 5.9% and a water splitting rate of 0.4 mmol/h. Tuning the catalyst was done by increasing calcination temperatures for the final step in synthesizing the catalyst. Temperatures up to 600 °C helped to reduce the number of defects, while temperatures above 700 °C destroyed the local structure around zinc atoms and were thus undesirable. The treatment ultimately reduced the amount of surface Zn and O defects, which normally function as recombination sites, thus limiting photocatalytic activity. The catalyst was then loaded with at a rate of 2.5 wt% Rh and 2 wt% Cr for better performance.Molecular catalysts
Proton reduction catalysts based on earth-abundant elements carry out one side of the water-splitting half-reaction.A mole of octahedral nickel complex, 2+ produced 308,000 moles of hydrogen over 60 hours of electrolysis with an applied potential of -1.25 V vs. standard hydrogen electrode.
Ru with three 2,2'-bipyridine ligands is a common compound for photosensitization used for photocatalytic oxidative transformations like water splitting. However, the bipyridine degrades due to the strongly oxidative conditions which causes the concentration of Ru32+ to diminish. Measurements of the degradation is difficult with UV-Vis spectroscopy but MALDI MS can be used instead.
Cobalt-based photocatalysts have been reported, including tris cobalt, compounds of cobalt ligated to certain cyclic polyamines, and some cobaloximes.