Artificial Photosynthesis: Using Polluted Water For Energy?

Artificial photosynthesis is a chemical process that mimics the natural process of photosynthesis. It involves using natural sunlight to convert carbon dioxide and water into fuels, such as hydrogen and carbon-based compounds. This approach has the potential to address the limitations of fossil fuels and photovoltaic cells by providing a clean, storable, and carbon-neutral energy source. While artificial photosynthesis has been studied for decades, recent advancements have brought it closer to practical realisation. One of the key challenges in this field is finding stable and efficient catalysts that can split water molecules without corroding. Researchers have explored various materials, including metals, metal oxides, and dye-sensitised titanium dioxide, with some success. The goal is to develop a cost-effective and stable system that can efficiently convert sunlight, water, and carbon dioxide into usable fuels, offering a promising path towards a fossil-fuel-free future.

The use of manganese as a catalyst in artificial photosynthesis

Manganese is a catalyst found in the photosynthetic core of plants. A single atom of manganese triggers the natural process that uses sunlight to split water. Using manganese in an artificial system is a biomimetic approach — it directly mimics the biology found in plants.

Manganese oxide is a good catalyst for water oxidation. It has been used in a simple and low-cost hydrothermal procedure to create a composite with silica aerogel. This composite is a good manganese-based catalyst with a turnover frequency of ~0.3 and 0.5 (mmol O2/(mol Mn·s)) in the presence of Ru(bpy) 3+3 and cerium(IV) ammonium nitrate, respectively.

Manganese is also used in combination with calcium in nature's water-splitting catalyst. The Mn4O5Ca cofactor converts into an 'activated' form prior to the O-O bond formation step. This activated state, which represents an all MnIV complex, is similar to the structure observed by X-ray crystallography but requires the coordination of an additional water molecule.

The Mn4Ca cluster in photosynthesis catalyses the oxidation of two water molecules into O2 and protons. This is a more complex chemical reaction than proton reduction. The exact structure of the oxygen-evolving complex has been hard to determine experimentally. As of 2011, the most detailed model was from a 1.9 Å resolution crystal structure of photosystem II.

The trouble with manganese is that it is somewhat unstable and it doesn't dissolve in water, making a manganese-based system inefficient and impractical. However, manganese oxide materials already show reasonable, if somewhat slow, water-splitting capacity normalised to manganese content.

Overall, manganese is a promising catalyst for artificial photosynthesis, but further research is needed to improve its stability and solubility in water.

The use of dye-sensitised titanium dioxide as a catalyst in artificial photosynthesis

Dye-sensitised titanium dioxide (TiO2) is a stable metal that can act as an efficient catalyst in artificial photosynthesis. It is used in a dye-sensitised solar cell, also known as a Graetzel cell, which has been around since the 1990s.

In a Graetzel cell, the TiO2 is suspended in a layer of dye particles that capture sunlight and then expose it to the TiO2 to start the reaction. The dye-sensitised solar cells are constructed using TiO2 nanoparticles and green natural dyes. The average particle size of the synthesized TiO2 nanoparticles was measured to be about 151.6 nm. The TiO2 nanoparticles were characterised optically with a UV-visible and X-ray fluorescence spectrophotometer and structurally using X-ray diffraction (XRD).

The dye-sensitised solar cells based on TiO2 nanoparticles have shown promising results in terms of power conversion efficiency. The power conversion efficiencies of solar cells were calculated to be 2.44%, 3.94%, and 7.67% with TiO2 synthesized via flame spray pyrolysis, hydrothermal sol-gel method, and reference Titania electrode, respectively.

The use of dye-sensitised TiO2 as a catalyst in artificial photosynthesis offers advantages such as stability and efficiency. However, there are also challenges, including the complexity of molecular geometry and the stability of the catalyst. Overall, dye-sensitised TiO2 shows potential as a catalyst in artificial photosynthesis, but further research and development are needed to optimise its performance and efficiency.

The use of cobalt oxide as a catalyst in artificial photosynthesis

Cobalt oxide is a promising catalyst for artificial photosynthesis (AP), a process that aims to replicate natural photosynthesis to convert solar energy, water, and carbon dioxide into high-energy fuels. AP has been widely studied, but a detailed understanding of the catalysts used is still lacking, hindering the development of efficient AP systems.

Cobalt oxide, specifically Co4O4 cubane, has emerged as a promising catalyst for AP due to its ability to orchestrate critical bond-breaking and bond-making steps during fuel generation. By converting the unstable Co4O4 cubane into a metal-organic framework (MOF), researchers have significantly improved its stability and gained valuable insights into its mechanism. This stabilization technique allows for a more detailed examination of the catalyst's behavior and presents a meaningful step forward in the quest to optimize materials for AP and address the global energy crisis.

Cobalt oxide catalysts play a crucial role in water oxidation, a critical step in AP. Water oxidation involves breaking down water molecules into molecular oxygen and protons. In nature, this process occurs in photosystem II (PS II), where a manganese-calcium cluster accumulates reducing equivalents (electrons) and delivers them to water molecules. The exact mechanism of water oxidation within the cluster is not fully understood, but bio-inspired manganese and manganese-calcium complexes, such as [Mn4O4] cubane-type clusters, have been synthesized with catalytic activity.

Cobalt oxide catalysts have also been explored for carbon dioxide reduction, another essential aspect of AP. Carbon dioxide reduction aims to convert carbon dioxide, a greenhouse gas, into valuable products like carbon monoxide or organic compounds. Cobalt quaterpyridine, a molecular catalyst with an earth-abundant metal, has shown high efficiency in this process, operating at low overpotentials in aqueous neutral conditions. When coupled with a p-type Cu(In,Ga)Se2 semi-conductor, the cobalt quaterpyridine catalyst achieved high selectivity for carbon monoxide production with good stability.

In conclusion, cobalt oxide catalysts, particularly Co4O4 cubane and cobalt quaterpyridine, have demonstrated great potential in AP. By enhancing our understanding of these catalysts and their role in water oxidation and carbon dioxide reduction, researchers are making significant strides toward developing efficient and sustainable energy solutions through AP.

The use of ruthenium complexes as catalysts in artificial photosynthesis

Ruthenium complexes are an important class of catalysts with multiple applications, including in artificial photosynthesis. In artificial photosynthesis, ruthenium complexes can be used as both photosensitisers and catalysts.

Ruthenium complexes as photosensitisers

Ruthenium polypyridine complexes, such as tris(bipyridine)ruthenium(II) and its derivatives, have been used extensively in hydrogen photoproduction due to their efficient visible light absorption. Other noble metal-containing complexes used include those with platinum, rhodium and iridium.

Ruthenium complexes as catalysts

Ruthenium complexes can be used as catalysts in both homogeneous and heterogeneous systems. They are particularly useful in homogeneous systems, where they can be used to catalyse a wide range of chemical transformations, including hydrogenation and transfer hydrogenation, oxidation and hydroxylation, and C–C, C–X and N–X bond formation.

Ruthenium complexes have been explored for their water oxidation catalytic activity, with mononuclear and binuclear ruthenium complexes investigated. The influence of monodentate and polydentate ligands on the course of water oxidation has been studied.

Ruthenium complexes have also been used as catalysts in the reduction of water, where solar hydrogen production was coupled to photoinitiated electron collection in polyazine chromophores.

Supramolecular dyads

Supramolecular dyads with photosensitisers and catalysts covalently bound together have been developed. These include covalently linked photosensitiser-catalyst assemblies, which have been grafted onto solid electrodes.

The use of metal-organic frameworks as catalysts in artificial photosynthesis

Metal-organic frameworks (MOFs) are a class of compounds made up of metal ions held together by organic linking molecules. They are used in artificial photosynthesis as catalysts, enhancing the performance of photoelectrochemical (PEC) systems. MOFs can be used as photosensitizers, mediators, cocatalysts, and precursors.

MOFs can enhance and broaden the light absorption of semiconductors, which is a challenge for many inorganic semiconductors, such as TiO2, that have wide bandgaps and do not absorb visible light. Aminated Ti-based MOFs, for example, can be used to form a core-shell hybrid nanostructure on the surface of TiO2 nanowires, significantly improving PEC water oxidation performance under visible light.

MOFs can also act as mediators to steer charge dynamics, promoting charge separation and transfer. In a TiO2/MIL-125-NH2 heterojunction, the photogenerated electrons are quickly injected into the conduction band of TiO2, while the holes in the HOMO level of MOFs participate in water oxidation for oxygen evolution. MOFs can also be used as co-catalysts, providing efficient active sites for the adsorption and activation of reactants. For example, cobalt-containing ZIF9 can be integrated with Co3O4 nanowires for PEC CO2 reduction, where the CO2 molecules are adsorbed and activated through O-Co binding.

MOFs can also be used as precursors to construct various functional structured photoelectrodes, such as a ternary Co3O4/TiO2/Si heterojunction photoanode for PEC water oxidation. The derivatives of MOFs can expose abundant active sites for catalytic reactions and provide a long optical pathway for more efficient light absorption.

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