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| Occurrence and Production of Hydrogen |
| H2 is produced in chemistry and biology laboratories, often as a by-product of other reactions. |
 Laboratory syntheses
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| In the laboratory, H2 is usually prepared by the reaction of acids on metals such as zinc. |
Zn + 2H+  Zn2+ + H2 |
| Aluminum produces H2 upon treatment with acids but also with base: |
| 2Al + 6H2O 2Al(OH)3 + 3H2
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| The electrolysis of water is a simple method of producing hydrogen, although the resulting hydrogen necessarily has less energy content than was required to produce it. A low voltage current is run through the water, and gaseous oxygen forms at the anode while gaseous hydrogen forms at the cathode. Typically the cathode is made from platinum or another inert metal when producing hydrogen for storage. If, however, the gas is to be burnt on site, oxygen is desirable to assist the combustion, and so both electrodes would be made from inert metals. (Iron, for instance, would oxidize, and thus decrease the amount of oxygen given off.) |
| 2H2O(aq) 2H2(g) + O2(g)
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| Industrial syntheses
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| Hydrogen can be prepared in several different ways but the economically most important processes involve removal of hydrogen from hydrocarbons. Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (700 - 1100°C), steam (water vapour) reacts with methane (CH4) to yield carbon monoxide (CO) and H2. |
| CH4 + H2O CO + 3H2 |
| CH4 C + 2H2 |
| CO + H2O CO2 + H2
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| CH4 + 0.5O2 CO + 2H2
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| C + H2O CO + H2 |
| Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the Haber process for the production of ammonia (the world's fifth most produced industrial compound), hydrogen is generated from natural gas. |
| Hydrogen is also produced in usable quantities as a co-product of the major petrochemical processes of steam cracking and reforming. Electrolysis of brine to yield chlorine also produces hydrogen as a co-product. |
| Biological syntheses
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| H2 is a product of some types of anaerobic metabolism and is produced by several micro-organisms, usually via reactions catalyzed by iron- or nickel-containing enzymes called hydrogenases. These enzymes catalyze the reversible redox reaction between H2 and its component two protons and two electrons. Evolution of hydrogen gas occurs in the transfer of reducing equivalents produced during pyruvate fermentation to water. |
| Water splitting, in which water is decomposed into its component protons, electrons, and oxygen, occurs in the light reactions in all photosynthetic organisms. Some such organisms - including the alga Chlamydomonas reinhardtii and cyanobacteria - have evolved a second step in the dark reactions in which protons and electrons are reduced to form H2 gas by specialized hydrogenases in the chloroplast. Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H2 gas even in the presence of oxygen. |
| Other rarer but mechanistically interesting routes to H2 production also exist in nature. Nitrogenase produces approximately one equivalent of H2 for each equivalent of N2 reduced to ammonia. Some phosphatases reduce phosphite to H2. |