Artificial photosynthesis
[11] In a lecture that was later published in Science,[12] he proposed a switch from the use of fossil fuels to radiant energy provided by the sun and captured by technical photochemistry devices.
[13] During the late 1960s, Akira Fujishima discovered the photocatalytic properties of titanium dioxide, the so-called Honda-Fujishima effect, which could be used for hydrolysis.
The higher photovoltage available from the multijunction thin film device with visible light was a major advance over previous photolysis attempts with UV or other single junction semiconductor photoelectrodes.
[19][20][21] Some concepts for artificial photosynthesis consist of distinct components,[22] which are inspired by natural photosynthesis:[23][24] These processes could be replicated by a triad assembly, which could oxidize water at one catalyst, reduce protons at another, and have a photosensitizer molecule to power the whole system[25] Some catalysts for solar fuel cells are envisioned to produce hydrogen.
Those listed below, which includes both oxidizer and reducers, are not practical, but illustrative: [36] Similar to natural photosynthesis, such artificial leaves can use a tandem of light absorbers for overall water splitting or CO2 reduction.
These integrated systems can be assembled on lightweight, flexible substrates, resulting in floating devices resembling lotus leaves.
[23][41] Research centers were established across the globe,[42] including Sweden,[24] U.S.,[43] with the aim of finding a cost-effective method to produce fuels using only sunlight, water, and carbon-dioxide as inputs.
[44] Japan,[45] This was confirmed with the establishment of the KAITEKI Institute later that year, with carbon dioxide reduction through artificial photosynthesis as one of the main goals.
Its formation involves only the transference of two electrons to two protons: The hydrogenase enzymes effect this conversion[23][49][50] Dirhodium photocatalyst[51] and cobalt catalysts.
In nature, the oxygen-evolving complex performs this reaction by accumulating reducing equivalents (electrons) in a manganese-calcium cluster within photosystem II (PS II), then delivering them to water molecules, with the resulting production of molecular oxygen and protons: Without a catalyst (natural or artificial), this reaction is very endothermic, requiring high temperatures (at least 2500 K).
[55] Some ruthenium complexes, such as the dinuclear μ-oxo-bridged "blue dimer" (the first of its kind to be synthesized), are capable of light-driven water oxidation, thanks to being able to form high valence states.
This complexes and other molecular catalysts still attract researchers in the field, having different advantages such as clear structure, active site, and easy to study mechanism.
Oxides are easier to obtain than molecular catalysts, especially those from relatively abundant transition metals (cobalt and manganese), but suffer from low turnover frequency and slow electron transfer properties, and their mechanism of action is hard to decipher and, therefore, to adjust.
Gion Calzaferri (2009) describes one such antenna that uses zeolite L as a host for organic dyes, to mimic plant's light collecting systems.
RuBisCO is a rather slow catalyst compared to the vast majority of other enzymes, incorporating only a few molecules of carbon dioxide into ribulose-1,5-bisphosphate per minute, but does so at atmospheric pressure and in mild, biological conditions.
[61] The resulting product is further reduced and eventually used in the synthesis of glucose, which in turn is a precursor to more complex carbohydrates, such as cellulose and starch.
[66] Some solar cells are capable of splitting water into oxygen and hydrogen, approximately ten times more efficient than natural photosynthesis.
[67][68] Sun Catalytix, the startup based on the artificial leaf, stated that it will not be scaling up the prototype as the device offers few savings over other ways to make hydrogen from sunlight.
However, other energy-demanding metabolic pathways can compete with the necessary electrons for proton reduction, decreasing the efficiency of the overall process; also, these hydrogenases are very sensitive to oxygen.
[72] Researchers have achieved controlled growth of diverse foods in the dark via solar energy and electrocatalysis-based artificial photosynthesis.
