Hydrogen gas was first produced artificially in the 17thcentury by the reaction of acids with metals. Henry Cavendish, in1766–1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned: this is the origin of hydrogen's name, which means 'water-former' (from Ancient Greek: ὕδωρ, romanized: húdōr, lit.'water', and γεννάω, gennáō, 'I bring forth'). Understanding the colors of light absorbed and emitted by hydrogen was a crucial part of the development of quantum mechanics.
Hydrogen, typically nonmetallic except under extreme pressure, readily forms covalent bonds with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in acid–base reactions, which mainly involve proton exchange among soluble molecules. In ionic compounds, hydrogen can take the form of either a negatively-charged anion, where it is known as hydride, or as a positively-charged cation, H+, hydron. Although tightly bonded to water molecules, hydrons strongly affect the behavior of aqueous solutions, as reflected in the importance of pH. Hydride, on the other hand, is rarely observed because it tends to deprotonate solvents, yieldingH2.[13]
Nearly all hydrogen production is done by transforming fossil fuels, particularly steam reforming of natural gas. It can also be produced from water or saline by electrolysis, but this process is more expensive. Its main industrial uses include fossil fuel processing and ammonia production for fertilizer. Emerging uses for hydrogen include the use of fuel cells to generate electricity.
High-precision values for the hydrogen atom energy levels are required for definitions of physical constants. Quantum calculations have identified nine contributions to the energy levels. The eigenvalue from the Dirac equation is the largest contribution. Other terms include relativistic recoil, the self-energy, and the vacuum polarization terms.[18]
Nomenclature
The standards organization for chemical names, IUPAC, gives general names when the context assumes natural isotope abundance or ignores the isotope. These general names are hydrogen for the neutral atom, hydron for the positive cation, H+, hydride for the negative cation, H-. The name proton is often used for the positive cation but this strictly correct only for the cation of the dominant isotope 1 H.[19]
Isotopes
The three naturally-occurring isotopes of hydrogen: hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium)
Hydrogen has three naturally-occurring isotopes, denoted 1 H, 2 H and3 H. Other, highly-unstable nuclides(4 H to 7 H) have been synthesized in laboratories but not observed in nature.[20][21]
1 H is the most common hydrogen isotope, with an abundance of >99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium.[22] It is the only stable isotope with no neutrons (see diproton for a discussion of why others do not exist).[23]
2 H, the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in the nucleus. Nearly all deuterium nuclei in the universe are thought to have been produced in Big Bang nucleosynthesis, and have endured since then.[24]: 24.2 Deuterium is not radioactive, and is not a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for 1 H-NMR spectroscopy.[25] Heavy water is used as a neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusion.[26]
3 H is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through beta decay with a half-life of 12.32years.[27] It is radioactive enough to be used in luminous paint to enhance the visibility of data displays, such as for painting the hands and dial-markers of watches. The watch glass prevents the small amount of radiation from escaping the case.[28] Small amounts of tritium are produced naturally by cosmic rays striking atmospheric gases; tritium has also been released in nuclear weapons tests.[29] It is used in nuclear fusion,[30] as a tracer in isotope geochemistry,[31] and in specialized self-powered lighting devices.[32] Tritium has also been used in chemical and biological labeling experiments as a radiolabel.[33]
Unique among the elements, distinct names are assigned to hydrogen's isotopes in common use. During the early study of radioactivity, heavy radioisotopes were given their own names, but these are mostly no longer used. The symbols D andT (instead of 2 H and 3 H) are sometimes used for deuterium and tritium, but the symbolP was already used for phosphorus and thus was not available for protium.[34] In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry(IUPAC) allows any of D, T, 2 H, and 3 H to be used, though 2 H and 3 H are preferred.[35]
Under standard conditions, hydrogen is a gas of diatomic molecules with the formulaH2, officially called "dihydrogen",[39]: 308 but also called "molecular hydrogen",[40] or simply hydrogen. Dihydrogen is a colorless, odorless, flammable gas.[40]
Combustion
[ video — the proxy does not render media ]
Combustion of hydrogen with the oxygen in the air. When the bottom cap is removed, allowing air to enter, hydrogen in the container rises and burns as it mixes with the air.
Hydrogen gas is highly flammable, reacting with oxygen in air to produce liquid water:
Hydrogen gas forms explosive mixtures with air in concentrations from 4%–74%[42] and with chlorine at 5%–95%. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).[43] In a high-pressure hydrogen leak, the shock wave from the leak itself can heat air to the autoignition temperature, leading to flaming and possibly explosion.[44]
Hydrogen flames emit faint blue and ultraviolet light.[45]Flame detectors are used to detect hydrogen fires as they are nearly invisible to the naked eye in daylight.[46][47]
Spin isomers
Molecular H2 exists as two nuclear isomers that differ in the spin states of their nuclei.[48] In the orthohydrogen form, the spins of the two nuclei are parallel, forming a spin triplet state having a total molecular spin ; in the parahydrogen form the spins are antiparallel and form a spin singlet state having spin . The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form.[49] The ortho form is an excited state, having higher energy than the para form by 1.455 kJ/mol,[50] and it converts to the para form over the course of several minutes when cooled to low temperature.[51] The thermal properties of these isomers differ because each has distinct rotational quantum states.[52]
The ortho-to-para ratio in H2 is an important consideration in the liquefaction and storage of liquid hydrogen: the conversion from ortho to para is exothermic, and produces sufficient heat to evaporate most of the liquid if the conversion to parahydrogen does not occur during the cooling process.[53]Catalysts for the ortho-para interconversion, such as ferric oxide and activated carbon compounds, are therefore used during hydrogen cooling to avoid this loss of liquid.[54]
Liquid hydrogen becomes solid hydrogen at standard pressure below hydrogen's melting point of 14.01 K (−259.14 °C; −434.45 °F). Distinct solid phases exist, known as PhaseI through PhaseV, each exhibiting a characteristic molecular arrangement.[58] Liquid and solid phases can exist in combination at the triple point; this mixture is known as slush hydrogen.[59]
Metallic hydrogen, a phase obtained at extremely high pressures (in excess of 400 million Pa (58,000 psi)), is an electrical conductor. It is believed to exist deep within giant planets like Jupiter.[58][60]
When ionized, hydrogen becomes a plasma. This is the form in which hydrogen exists within stars.[61]
Thermal and physical properties
Thermal and physical properties of hydrogen (H2) at atmospheric pressure[62][63]
In 1671, Irish scientist Robert Boyle discovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.[64][65]
Boyle did not note that the gas was flammable, but hydrogen would play a key role in overturning the phlogiston theory of combustion.[66]
In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance "phlogiston"[67][68] and further finding in1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.[11][10]
Antoine Lavoisier, who identified the element that came to be known as hydrogen
In 1783, Antoine Lavoisier identified the element that came to be known as hydrogen[69] when he and Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned.[10]Lavoisier produced hydrogen for his experiments on mass conservation by treating metallic iron with a stream of water through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:
Fe + H2O → FeO + H2
2 Fe + 3 H2O → Fe2O3 + 3 H2
3 Fe + 4 H2O → Fe3O4 + 4 H2
Many metals react similarly with water, leading to the production of hydrogen.[70] In some situations, this H2-producing process is problematic, for instance in the case of zirconium cladding on nuclear fuel rods.[71]
One of the first quantum effects to be explicitly noticed, although not understood at the time, was James Clerk Maxwell's observation that the specific heat capacity of H2 unaccountably departs from that of a diatomic gas below room temperature, and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H2 because of its low mass. These widely-spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.[73]
Hydrogen emission spectrum lines in the four visible lines of the Balmer series
Because of its simple atomic structure, consisting only of a proton and an electron, the hydrogen atom, together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of atomic structure.[75] The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, in which the electron "orbits" the proton, like how Earth orbits the Sun. However, the electron and proton are held together by electrostatic attraction, while planets and celestial objects are held by gravity. Due to the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.[76]
Hydrogen's unique position as the only neutral atom for which the Schrödinger equation can be directly solved, has significantly contributed to the understanding of quantum mechanics through the exploration of its energetics.[77] Furthermore, study of the corresponding simplicity of the hydrogen molecule and the corresponding cation, H+2, brought understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.[78]
Because H2 has only 7% the density of air, it was once widely used as a lifting gas in balloons and airships.[79] The first hydrogen-filled balloon was invented by Jacques Charles in1783. Hydrogen provided the lift for the first reliable form of air-travel following the1852 invention of the first hydrogen-lifted airship by Henri Giffard. German count Ferdinand vonZeppelin promoted the idea of rigid airships lifted by hydrogen that later were called Zeppelins, the first of which had its maiden flight in1900.[10] Regularly-scheduled flights started in1910 and by the outbreak of World WarI in August1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships in the form of blimps were used as observation platforms and bombers during World WarII, especially on the USEastern seaboard.[80]
The first non-stop transatlantic crossing was made by the British airshipR34 in1919 and regular passenger service resumed in the1920s. Hydrogen was used in the Hindenburg, which caught fire over New Jersey on 6May 1937.[10] The hydrogen that filled the airship was ignited, possibly by static electricity, and burst into flames.[81] Following this disaster, commercial hydrogen airship travel ceased. Hydrogen is still used, in preference to non-flammable but more expensive helium, as a lifting gas for weather balloons.[82]
H2 is relatively unreactive. The thermodynamic basis of this low reactivity is the very strong H–H bond, with a bond dissociation energy of 435.7 kJ/mol.[83] It does form coordination complexes called dihydrogen complexes. These species provide insights into the early steps in the interactions of hydrogen with metal catalysts. According to neutron diffraction, the metal and two Hatoms form a triangle in these complexes. The H-H bond remains intact but is elongated. They are acidic.[84]
Although exotic on Earth, the H+3ion is common in the universe. It is a triangular species, like the aforementioned dihydrogen complexes. It is known as protonated molecular hydrogen or the trihydrogen cation.[85]
Hydrogen reacts with chlorine to produceHCl, and with bromine to produceHBr, via a chain reaction. The reaction requires initiation. For example, in the case of Br2, the dibromine molecule is split apart: Br2 + (UV light) → 2Br•. Propagating reactions consume hydrogen molecules and produceHBr, as well as Brand Hatoms:
Hydrogen can exist in both +1 and −1oxidation states, forming compounds through ionic and covalent bonding. The element is part of a wide range of substances, including water, hydrocarbons, and numerous other organic compounds.[88] The H+ion—commonly referred to as a proton due to its single proton and absence of electrons—is central to acid–base chemistry, although the proton does not move freely. In the Brønsted–Lowry framework, acids are defined by their ability to donate H+ions to bases.[89]
Hydrogen forms a vast variety of compounds with carbon, known as hydrocarbons, and an even greater diversity with other elements (heteroatoms), giving rise to the broad class of organic compounds often associated with living organisms.[88]
Hydrogen compounds with hydrogen in the oxidation state−1 are known as hydrides, which are usually formed between hydrogen and metals. The hydrides can be ionic (aka saline), covalent, or metallic. With heating, H2 reacts efficiently with the alkali and alkaline earth metals to give the ionic hydrides of the formulasMH and MH2, respectively. These salt-like crystalline compounds have high melting points and all react with water to liberate hydrogen. Covalent hydrides include boranes and polymeric aluminium hydride. Transition metals form metal hydrides via continuous dissolution of hydrogen into the metal.[90] A well-known hydride is lithium aluminium hydride: the [AlH4]−anion carries hydridic centers firmly attached to the Al(III).[91] Perhaps the most extensive series of hydrides are the boranes, compounds consisting only of boron and hydrogen.[92]
Hydrides can bond to these electropositive elements not only as a terminal ligand but also as bridging ligands. In diborane(B2H6), four hydrogen atoms are terminal, while two bridge between the two boron atoms.[27]
Hydrogen bonding
When bonded to a more electronegative element, particularly fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a lone pair like oxygen or nitrogen. This phenomenon, called hydrogen bonding, is critical to the stability of many biological molecules.[93]: 375 [94] Hydrogen bonding alters molecule structures, viscosity, solubility, melting and boiling points, and even protein folding dynamics.[95]
Protons and acids
An "A-T base pair" in DNA illustrating how hydrogen bonds are critical to the genetic code. The drawing illustrates that in many chemical depictions, C-H bonds are not always shown explicitly, an indication of their pervasiveness.
In water, hydrogen bonding plays an important role in reaction thermodynamics. A hydrogen bond can shift over to proton transfer.
Under the Brønsted–Lowry acid–base theory, acids are proton donors, while bases are proton acceptors.[96]: 28
A bare proton(H+) essentially cannot exist in anything other than a vacuum. Otherwise it attaches to other atoms, ions, or molecules. Even chemical species as inert as methane can be protonated. The term "proton" is used loosely and metaphorically to refer to solvated hydrogen cations attached to other solvated chemical species; it is denoted"H+" without any implication that any single protons exist freely in solution as a species. To avoid the implication of the naked proton in solution, acidic aqueous solutions are sometimes considered to contain the "hydronium ion"([H3O]+), or still more accurately, [H9O4]+.[97] Other oxonium ions are found when water is in acidic solution with other solvents.[98]
The concentration of these solvated protons determines the pH of a solution, a logarithmic scale that reflects its acidity or basicity. Lower pHvalues indicate higher concentrations of hydronium ions, corresponding to more acidic conditions.[99]
In astrophysics, neutral hydrogen in the interstellar medium is called HI and ionized hydrogen is called HII.[103] Radiation from stars ionizes HI to HII, creating spheres of ionized HII around stars. In the chronology of the universe neutral hydrogen dominated until the birth of stars during the era of reionization, which then produced bubbles of ionized hydrogen that grew and merged over hundreds of millions of years.[104]
These are the source of the 21-centimeter hydrogen line, at 1420 MHz, that is detected in order to probe primordial hydrogen. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmologicalbaryonic density of the universe up to a redshift of z = 4.[105]
Protonated molecular hydrogen(H+3) is found in the interstellar medium, where it is generated by ionization of molecular hydrogen by cosmic rays. This ion has also been observed in the upper atmosphere of Jupiter. The ion is long-lived in outer space due to the low temperature and density. H+3 is one of the most abundant ions in the universe, and it plays a notable role in the chemistry of the interstellar medium.[107] Neutral triatomic hydrogenH3 can exist only in an excited form and is unstable.[108]
Terrestrial
Hydrogen is the third most abundant element on the Earth's surface,[109] mostly existing within chemical compounds such as hydrocarbons and water.[27] Elemental hydrogen is normally in the form of a gas, H2, at standard conditions. It is present in a very low concentration in Earth's atmosphere (around 0.53 parts per million on a molar basis[110]) because of its light weight, which enables it to escape the atmosphere more rapidly than heavier gases. Despite its low concentration in the atmosphere, terrestrial hydrogen is sufficiently abundant to support the metabolism of several varieties of bacteria.[111]
Large underground deposits of hydrogen gas have been discovered in several countries including Mali, France and Australia.[112] As of 2024, it is uncertain how much underground hydrogen can be extracted economically.[112]
Production and storage
Industrial routes
Nearly all of the world's current supply of hydrogen gas(H2) is produced from fossil fuels.[113][114]: 1 Many methods exist for producing H2, but three dominate commercially: steam reforming often coupled to water-gas shift, partial oxidation of hydrocarbons, and water electrolysis.[115]
Steam reforming
Inputs and outputs of steam reforming (SMR) and water gas shift (WGS) reaction of natural gas, a process used in hydrogen production
Hydrogen is mainly produced by steam methane reforming(SMR), the reaction of water and methane.[116][117] Thus, at high temperature (1,000–1,400 K [730–1,130 °C; 1,340–2,060 °F]), steam (water vapor) reacts with methane to yield carbon monoxide andH2.
CH4 + H2O → CO + 3 H2
Producing one tonne of hydrogen through this process emits 6.6–9.3tonnes of carbon dioxide.[118] The production of natural gas feedstock also produces emissions such as vented and fugitive methane, which further contributes to the overall carbon footprint of hydrogen.[119]
This reaction is favored at low pressures but is nonetheless conducted at high pressures(2.0 MPa [20 atm; 590 inHg]) because high-pressureH2 is the most marketable product, and pressure swing adsorption(PSA) purification systems work better at higher pressures. The product mixture is known as "synthesis gas" because it is often used directly for the production of methanol and many other compounds. Hydrocarbons other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly-optimized technology is the formation of coke or carbon:
CH4 → C + 2 H2
Therefore, steam reforming typically employs an excess ofH2O. Additional hydrogen can be recovered from the steam by using carbon monoxide through the water gas shift reaction(WGS). This process requires an iron oxide catalyst:[117]
CO + H2O → CO2 + H2
Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the Haber process for ammonia production, hydrogen is generated from natural gas.[120]
Partial oxidation of hydrocarbons
Other methods for CO and H2 production include partial oxidation of hydrocarbons:[48]
2 CH4 + O2 → 2 CO + 4 H2
Although less important commercially, coal can serve as a prelude to the above shift reaction:[117]
C + H2O → CO + H2
Olefin production units may produce substantial quantities of byproduct hydrogen, particularly from cracking light feedstocks like ethane or propane.[121]
Water electrolysis
Inputs and outputs of the electrolysis of water production of hydrogen
Commercial electrolyzers use nickel-based catalysts in strongly alkaline solution. Platinum is a better catalyst but is expensive.[122] The hydrogen created through electrolysis using renewable energy is commonly referred to as "green hydrogen".[123]
Innovation in hydrogen electrolyzers could make large-scale production of hydrogen from electricity more cost-competitive.[127]
Methane pyrolysis
Hydrogen can be produced by pyrolysis of natural gas (methane), producing hydrogen gas and solid carbon with the aid of a catalyst and 74 kJ/mol input heat:
CH4(g) → C(s) + 2 H2(g) (ΔH° = 74 kJ/mol)
The carbon may be sold as a manufacturing feedstock or fuel, or landfilled. This route could have a lower carbon footprint than existing hydrogen production processes, but mechanisms for removing the carbon and preventing it from reacting with the catalyst remain obstacles for industrial-scale use.[128]: 17 [129]
Thermochemical
Water splitting is the process by which water is decomposed into its components. Relevant to the biological scenario is this equation:
Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to more efficiently generate H2gas even in the presence of oxygen.[131] Efforts have also been undertaken with genetically‐modified alga in a bioreactor.[132]
Relevant to the thermal water-splitting scenario is this simple equation:
H2 is produced in organisms by enzymes called hydrogenases. This process allows the host organism to use fermentation as a source of energy.[135] These same enzymes also can oxidizeH2, such that the host organisms can subsist by reducing oxidized substrates using electrons extracted fromH2.[136]
Some bacteria such as Mycobacterium smegmatis can use the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking. Their hydrogenases feature small channels that exclude oxygen from the active site, permitting the reaction to occur even though the hydrogen concentration is very low and the oxygen concentration is as in normal air.[110][139]
Confirming the existence of hydrogenase‐employing microbes in the human gut, H2 occurs in human breath. The concentration in the breath of fasting people at rest is typically under 5 parts per million(ppm), but can reach 50 ppm when people with intestinal disorders consume molecules they cannot absorb during diagnostic hydrogen breath tests.[140]
Hydrogen is also often a by-product of other reactions. Many metals react with water to produceH2, but the rate of hydrogen evolution depends on the metal, the pH, and the presence of alloying agents. Most often, hydrogen evolution is induced by acids. The alkali and alkaline earth metals as well as aluminium, zinc, manganese, and iron, react readily with aqueous acids.[99]
Zn + 2 H+ → Zn2+ + H2
Many metals, such as aluminium, are slow to react with water because they form passivated oxide coatings. An alloy of aluminium and gallium, however, does react with water. In high-pH solutions, aluminium can react with H2:[99]
2 Al + 6 H2O + 2 OH− → 2 [Al(OH)4]− + 3 H2
Storage
If H2 is to be used as an energy source, its storage is important. It dissolves only poorly in solvents. For example, at room temperature and 0.1 millipascals (9.9×10−10atm), 0.05 moles of hydrogen dissolve into one kilogram (2.2 lb) of diethyl ether.[90] H2 can be stored in compressed form, although compressing costs energy. Liquefaction is impractical given hydrogen's low critical temperature. In contrast, ammonia and many hydrocarbons can be liquified at room temperature under pressure. For these reasons, hydrogen carriers—materials that reversibly bindH2—have attracted much attention. The key question is then the weight percent of H2-equivalents within the carrier material. For example, hydrogen can be reversibly absorbed into many rare earths and transition metals[145] and is soluble in both nanocrystalline and amorphous metals.[146] Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice.[147] These properties may be useful when hydrogen is purified by passage through hot palladium disks, but the gas's high solubility is also a metallurgical problem, contributing to the embrittlement of many metals,[148] complicating the design of pipelines and storage tanks.[149]
The most problematic aspect of metal hydrides for storage is their modest H2content, often on the order of1%. For this reason, there is interest in storage of H2 in compounds of low molecular weight. For example, ammonia borane (H3N−BH3) contains 19.8weight percent ofH2. The problem with this material is that after release of H2, the resulting boron nitride does not re-add H2: i.e., ammonia borane is an irreversible hydrogen carrier.[150] More attractive are hydrocarbons such as tetrahydroquinoline, which reversibly release someH2 when heated in the presence of a catalyst:[151]
C9H10NH ⇌ C9H7N + 2 H2
Applications
Hydrogen Ladder: Ranking of hydrogen applications and uses in the medium term, but analysts disagree[152]
Hydrogenation, the addition of H2 to various substrates, is done on a large scale. Hydrogenation of N2 produces ammonia by the Haber process:[153]
N2 + 3 H2 → 2 NH3
This process consumes a few percent of the energy budget in the entire industry and is the biggest consumer of hydrogen. The resulting ammonia is used extensively in fertilizer production; these fertilizers have become essential feedstocks in modern agriculture.[154] Hydrogenation is also used to convert unsaturated fats and oils to saturated fats and oils. The major application is the production of margarine. Methanol is produced by hydrogenation of carbon dioxide; the mixture of hydrogen and carbon dioxide used for this process is known as syngas. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. H2 is also used as a reducing agent for the conversion of some ores to the metals.[155][99]
Fuel
The potential for using hydrogen(H2) as a fuel has been widely discussed. Hydrogen can be used in fuel cells to produce electricity,[156] or burned to generate heat.[157] When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor.[157] When burned, hydrogen produces relatively little pollution at the point of combustion, but can lead to thermal formation of harmful nitrogen oxides.[157]
If hydrogen is produced with low or zero greenhouse gas emissions (green hydrogen), it can play a significant role in decarbonizing energy systems where there are challenges and limitations to replacing fossil fuels with direct use of electricity.[158][126]
Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals, thus contributing to the decarbonization of industry alongside other technologies, such as electric arc furnaces for steelmaking.[159] However, it is likely to play a larger role in providing industrial feedstock for cleaner production of ammonia and organic chemicals.[158] For example, in steelmaking, hydrogen could function as a clean fuel and also as a low-carbon catalyst, replacing coal-derived coke (carbon):[160]
2FeO + C → 2Fe + CO2
vs
FeO + H2 → Fe + H2O
Hydrogen used to decarbonize transportation is likely to find its largest applications in shipping, aviation and, to a lesser extent, heavy goods vehicles, through the use of hydrogen-derived synthetic fuels such as ammonia and methanol and fuel cell technology.[158] For light-duty vehicles including cars, hydrogen is far behind other alternative fuel vehicles, especially compared with the rate of adoption of battery electric vehicles, and may not play a significant role in future.[161]
A Space Shuttle Main Engine burns hydrogen with oxygen, producing a nearly invisible flame at full thrust.
Hydrogen produced when there is a surplus of variable renewable electricity could in principle be stored and later used to generate heat or to re-generate electricity.[163] It can be further transformed into synthetic fuels such as ammonia and methanol.[164] Disadvantages of hydrogen fuel include high costs of storage and distribution due to hydrogen's explosivity, its large volume compared to other fuels, and its tendency to embrittle materials.[119]
Leak detection: Pure or mixed with nitrogen (sometimes called forming gas), hydrogen is a tracer gas for detection of minute leaks. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries;[185] it also allows for leak testing in food packaging.[citation needed]
In hydrogen pipelines and steel storage vessels, hydrogen molecules are prone to reacting with metals, causing hydrogen embrittlement and leaks in the pipeline or storage vessel.[189] Since it is lighter than air, hydrogen does not easily accumulate to form a combustible gas mixture.[189] However, even without ignition sources, high-pressure hydrogen leakage may cause spontaneous combustion and detonation.[189]
Hydrogen is flammable when mixed even in small amounts with air. Ignition can occur at a volumetric ratio of hydrogen to air as low as 4%.[190] In approximately 70% of hydrogen ignition accidents, the ignition source cannot be determined.[189]
Hydrogen fire, while being extremely hot, is almost invisible to the human eye, and thus can lead to accidental burns.[47] Hydrogen is non-toxic,[191] but like most gases it can cause asphyxiation in the absence of adequate ventilation.[192]
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