Making oxygen from water

Using energy from sunlight to make oxygen and the compounds of life is of fundamental importance - and very difficult. It is done by photosystem II, which is a protein complex found in plants and cyanobacteria. 

This is an immensely complex system with about 100 cofactors and more than 1300 molecules of water, but at its core is the oxygen-evolving complex, and it is the structure of this that we are concerned with here. 

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Oxygen may be produced by electrolysis of water. In such electrolytic procedure, small amounts of H2SO4 or NaOH may be added to water. Electrolysis methods, however, are not used as much commercially as are air bquefaction processes which cost less. However, in making hydrogen from water by electrolysis, oxygen is obtained as a by-product. 

The above model makes four critical assump-hons (i) the proportions of oxygen from water and O2 are the same in soils as observed in laboratory cultures (ii) there are no fractionations resulting from the incorporation of oxygen from water or O2 during nitrihcation (iii) the of water used by the microbes is idenhcal to that of the bulk soil water and (iv) the 6 0 of the O2 used by the microbes is identical to that of atmospheric O2. 

When a water-based fluid makes contact with a flame or a hot surface its water component evaporates and forms a steam blanket that displaces oxygen from around the hot area, and this obviates the risk of fire. Water-based products all contain at least 35% water. Because water can be lost by evaporation, they should not be subjected to operating temperatures above about 60°C (140°F). Table 52.8 shows a comparison of oil and FR fluids. 

The oxygen that reacts with the sulfur dioxide comes from water (H20) that is also present in Venus atmosphere. When the sun s high-energy ultraviolet (UV) rays hit a water molecule, it dissociates (breaks down) into hydrogen and oxygen—the elements that make up water. 

As seen in reaction (6.5.3) photogenerated holes are consumed, making electron-hole separation more effective as needed for efficient water splitting. The evolution of CO2 and O2 from reaction (6.5.6) can promote desorption of oxygen from the photocatalyst surface, inhibiting the formation of H2O through the backward reaction of H2 and O2. The desorbed CO2 dissolves in aqueous suspension, and is then converted to HCOs to complete a cycle. The mechanism is still not fully understood, with the addition of the same amount of different carbonates, see Table 6.2, showing very different results [99]. Moreover, the amount of metal deposited in the host semiconductor is also a critical factor that determines the catalytic efficiency, see Fig. 6.7. 

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