Editing Hydrogen (section)

===Industrial routes===
Nearly all of the world's current supply of hydrogen gas ({{chem2|H2}}) is created from fossil fuels.<ref>{{cite news |last1=Reed |first1=Stanley |last2=Ewing |first2=Jack |date=13 July 2021 |title=Hydrogen Is One Answer to Climate Change. Getting It Is the Hard Part |url=https://www.nytimes.com/2021/07/13/business/hydrogen-climate-change.html |work=The New York Times}}</ref><ref name="rosenow-2022">{{cite journal |last1=Rosenow |first1=Jan |date=27 September 2022 |title=Is heating homes with hydrogen all but a pipe dream? An evidence review |journal=Joule |volume=6 |issue=10 |pages=2225–2228 |bibcode=2022Joule...6.2225R |doi=10.1016/j.joule.2022.08.015 |s2cid=252584593 |doi-access=free}}  Article in press.</ref>{{rp|1}} Many methods exist for producing H<sub>2</sub>, but three dominate commercially: steam reforming often coupled to water-gas shift, partial oxidation of hydrocarbons, and water electrolysis.<ref name=KO/>

====Steam reforming====
[[File:SMR+WGS-1.png|thumb|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 reforming|steam methane reforming]] (SMR), the reaction of water and methane.<ref name="rotech">{{cite book |last1=Press |first1=Roman J. |url=https://archive.org/details/introductiontohy0000unse/page/249/mode/2up |title=Introduction to Hydrogen Technology |last2=Santhanam |first2=K. S. V. |last3=Miri |first3=Massoud J. |last4=Bailey |first4=Alla V. |last5=Takacs |first5=Gerald A. |publisher=John Wiley & Sons |year=2008 |isbn=978-0-471-77985-8 |pages=249 |url-access=registration}}</ref><ref name="Oxtoby">{{cite book
| first=D. W.|last=Oxtoby|date=2002
| title=Principles of Modern Chemistry
| edition=5th|publisher=Thomson Brooks/Cole
| isbn=978-0-03-035373-4}}</ref> Thus, at high temperature (1000–1400&nbsp;K, 700–1100&nbsp;°C or 1300–2000&nbsp;°F), steam (water vapor) reacts with [[methane]] to yield [[carbon monoxide]] and {{chem2|H2}}.

:{{chem2|CH4 + H2O → CO + 3 H2}}
Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide.<ref name="Bonheure-2021">{{Cite web |last1=Bonheure |first1=Mike |last2=Vandewalle |first2=Laurien A. |last3=Marin |first3=Guy B. |last4=Van Geem |first4=Kevin M. |date=March 2021 |title=Dream or Reality? Electrification of the Chemical Process Industries |url=https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |url-status=live |archive-url=https://web.archive.org/web/20210717132733/https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |archive-date=17 July 2021 |access-date=6 July 2021 |website=CEP Magazine |publisher=[[American Institute of Chemical Engineers]]}}</ref> The production of natural gas feedstock also produces emissions such as [[Gas venting|vented]] and [[Fugitive gas emissions|fugitive]] [[methane]], which further contributes to the overall carbon footprint of hydrogen.<ref name="Griffiths-20212">{{Cite journal |last1=Griffiths |first1=Steve |last2=Sovacool |first2=Benjamin K. |last3=Kim |first3=Jinsoo |last4=Bazilian |first4=Morgan |last5=Uratani |first5=Joao M. |display-authors=4 |date=2021 |title=Industrial decarbonization via hydrogen: A critical and systematic review of developments, socio-technical systems and policy options |url=https://www.sciencedirect.com/science/article/pii/S2214629621003017?dgcid=coauthor |url-status=live |journal=[[Energy Research & Social Science]] |volume=80 |page=39 |bibcode=2021ERSS...8002208G |doi=10.1016/j.erss.2021.102208 |issn=2214-6296 |archive-url=https://web.archive.org/web/20211016205152/https://www.sciencedirect.com/science/article/abs/pii/S2214629621003017?dgcid=coauthor |archive-date=16 October 2021 |access-date=11 September 2021}}</ref>

This reaction is favored at low pressures, Nonetheless, conducted at high pressures (2.0&nbsp;MPa, 20&nbsp;atm or 600&nbsp;[[inHg]]) because high-pressure {{chem2|H2}} 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. [[Hydrocarbon]]s 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:
:{{chem2|CH4 → C + 2 H2}}

Therefore, steam reforming typically employs an excess of {{chem2|H2O}}. 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:<ref name="Oxtoby" />
:{{chem2|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.<ref>{{cite web| last=Funderburg| first=E.| title=Why Are Nitrogen Prices So High?| publisher=The Samuel Roberts Noble Foundation| date=2008| url=http://www.noble.org/Ag/Soils/NitrogenPrices/Index.htm| access-date=11 March 2008| archive-url=https://web.archive.org/web/20010509065844/http://www.noble.org/ag/Soils/NitrogenPrices/Index.htm| archive-date=9 May 2001| df=dmy-all}}</ref>

====Partial oxidation of hydrocarbons====
Other methods for CO and {{chem2|H2}} production include partial oxidation of hydrocarbons:<ref name="uigi"/>

:{{chem2|2 CH4 + O2 → 2 CO + 4 H2}}

Although less important commercially, coal can serve as a prelude to the shift reaction above:<ref name="Oxtoby" />

:{{chem2|C + H2O → CO + H2}}

Olefin production units may produce substantial quantities of byproduct hydrogen particularly from cracking light feedstocks like [[ethane]] or [[propane]].<ref>{{Cite journal |last=Hannula |first=Ilkka |date=2015 |title=Co-production of synthetic fuels and district heat from biomass residues, carbon dioxide and electricity: Performance and cost analysis |url=http://dx.doi.org/10.1016/j.biombioe.2015.01.006 |journal=Biomass and Bioenergy |volume=74 |pages=26–46 |doi=10.1016/j.biombioe.2015.01.006 |bibcode=2015BmBe...74...26H |issn=0961-9534}}</ref>

====Water electrolysis ====
[[File:Hydrogen production via Electrolysis.png|thumb|Inputs and outputs of the electrolysis of water production of hydrogen]]
[[Electrolysis of water]] is a conceptually simple method of producing hydrogen.  
:{{chem2|2 H2O(l) → 2 H2(g) + O2(g)}}
Commercial [[electrolyzer]]s use [[nickel]]-based catalysts in strongly alkaline solution. Platinum is a better catalyst but is expensive.<ref>{{cite journal |doi=10.1038/ncomms5695 |title=Nanoscale nickel oxide/Nickel heterostructures for active hydrogen evolution electrocatalysis |date=2014 |last1=Gong |first1=Ming |last2=Zhou |first2=Wu |last3=Tsai |first3=Mon-Che |last4=Zhou |first4=Jigang |last5=Guan |first5=Mingyun |last6=Lin |first6=Meng-Chang |last7=Zhang |first7=Bo |last8=Hu |first8=Yongfeng |last9=Wang |first9=Di-Yan |last10=Yang |first10=Jiang |last11=Pennycook |first11=Stephen J. |last12=Hwang |first12=Bing-Joe |last13=Dai |first13=Hongjie |journal=Nature Communications |volume=5 |page=4695 |pmid=25146255 |bibcode=2014NatCo...5.4695G |s2cid=205329127 |doi-access=free }}</ref> The hydrogen created through electrolysis using renewable energy is commonly referred to as "[[green hydrogen]]".<ref name="RoyalSociety-2021">{{Cite web |date=June 2021 |title=The role of hydrogen and ammonia in meeting the net zero challenge |url=https://royalsociety.org/-/media/policy/projects/climate-change-science-solutions/climate-science-solutions-hydrogen-ammonia.pdf |website=The Royal Society}}</ref>

[[Electrolysis]] of [[brine]] to yield [[chlorine]]<ref>{{cite web| last=Lees| first=A.| title=Chemicals from salt| publisher=BBC|date=2007|url=http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml|access-date=11 March 2008|archive-url = https://web.archive.org/web/20071026052022/http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml |archive-date = 26 October 2007}}</ref> also produces high purity hydrogen as a co-product, which is used for a variety of transformations such as [[hydrogenation]]s.<ref>{{Cite book |last1=Schmittinger |first1=Peter |chapter=Chlorine |date=2006-01-15 |title=Ullmann's Encyclopedia of Industrial Chemistry |place=Weinheim, Germany |publisher=Wiley-VCH Verlag GmbH & Co. KGaA |language=en |doi=10.1002/14356007.a06_399.pub2 |isbn=978-3-527-30673-2 |last2=Florkiewicz |first2=Thomas |last3=Curlin |first3=L. Calvert |last4=Lüke |first4=Benno |last5=Scannell |first5=Robert |last6=Navin |first6=Thomas |last7=Zelfel |first7=Erich |last8=Bartsch |first8=Rüdiger}}</ref>

The [[electrolysis]] process is more expensive than producing hydrogen from methane without [[carbon capture and storage]].<ref name="Evans-2020">{{Cite web |last1=Evans |first1=Simon |last2=Gabbatiss |first2=Josh |date=30 November 2020 |title=In-depth Q&A: Does the world need hydrogen to solve climate change? |url=https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |url-status=live |archive-url=https://web.archive.org/web/20201201155033/https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |archive-date=1 December 2020 |access-date=1 December 2020 |website=[[Carbon Brief]]}}</ref>

Innovation in [[Electrolysis of water|hydrogen electrolyzers]] could make large-scale production of hydrogen from electricity more cost-competitive.<ref>{{Cite book|author1-link=International Energy Agency|last1=IEA|title=Net Zero by 2050: A Roadmap for the Global Energy Sector|year=2021|url=https://iea.blob.core.windows.net/assets/ad0d4830-bd7e-47b6-838c-40d115733c13/NetZeroby2050-ARoadmapfortheGlobalEnergySector.pdf|archive-date=23 May 2021|archive-url=https://web.archive.org/web/20210523155010/https://iea.blob.core.windows.net/assets/ad0d4830-bd7e-47b6-838c-40d115733c13/NetZeroby2050-ARoadmapfortheGlobalEnergySector.pdf|url-status=live
|pages=15, 75–76}}</ref>

==== Methane pyrolysis ====

Hydrogen can be produced by [[pyrolysis]] of natural gas (methane), producing hydrogen gas and solid carbon with the aid a catalyst and 74 kJ/mol input heat:
:{{chem2|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.<ref>{{Cite journal |last1=Rasul |first1=M. G. |last2=Hazrat |first2=M. A |last3=Sattar |first3=M. A. |last4=Jahirul |first4=M. I. |last5=Shearer |first5=M. J. |date=2022-11-15 |title=The future of hydrogen: Challenges on production, storage and applications |url=https://linkinghub.elsevier.com/retrieve/pii/S0196890422011049 |journal=Energy Conversion and Management |volume=272 |pages=116326 |doi=10.1016/j.enconman.2022.116326 |bibcode=2022ECM...27216326R |issn=0196-8904}}</ref>{{rp|17}}<ref>{{cite journal |last1=Schneider |first1=Stefan |title=State of the Art of Hydrogen Production via Pyrolysis of Natural Gas |journal=ChemBioEng Reviews |year=2020 |volume=7 |issue=5 |pages=150–158 |publisher=Wiley Online Library |doi=10.1002/cben.202000014 |doi-access=free }}</ref>

==== Thermochemical ====
[[Water splitting]] is the process by which water is decomposed into its components.  Relevant to the biological scenario is this simple equation:
:{{chem2|2 H2O  ->  4 H+ + O2 + 4e-}}
The reaction occurs in the [[Light-dependent reactions|light reactions]] in all [[photosynthetic]] organisms. A few organisms, including the alga ''[[Chlamydomonas reinhardtii]]'' and [[cyanobacteria]], have evolved a second step in the [[dark reaction]]s in which protons and electrons are reduced to form {{chem2|H2}} gas by specialized hydrogenases in the [[chloroplast]].<ref>{{cite journal|last1=Kruse|first1=O.|last2=Rupprecht|first2=J.|last3=Bader|first3=K.|last4=Thomas-Hall|first4=S.|last5=Schenk|first5=P. M.|last6=Finazzi|first6=G.|last7=Hankamer|first7=B.|title=Improved photobiological H<sub>2</sub> production in engineered green algal cells|journal=The Journal of Biological Chemistry|date=2005|volume=280|issue=40|pages=34170–7|doi=10.1074/jbc.M503840200|pmid=16100118|s2cid=5373909|url=http://espace.library.uq.edu.au/view/UQ:75490/UQ75490_OA.pdf|access-date=24 August 2019|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015735/https://espace.library.uq.edu.au/data/UQ_75490/UQ75490_OA.pdf?Expires=1611885542&Key-Pair-Id=APKAJKNBJ4MJBJNC6NLQ&Signature=Qmpjq4YH0rwOJNqiSZ5M7-E5cYH~Dm2B-4kasb1eH66pVWPlvPNRj7TfcTKR1lDhF0--bkJdtE~yrSWwcZAA8FzxAA3MXY99mHTIOxyD3s73Dai1bwrLNuOkibXTVo6WbY5RKv7JAhXJ2sUV~TDIphC4Qikr0AWk5z-dwdY997n0NzcdTlqr0sn5n9WsOari3pJ0wRuL0w6Ged~HhrQ6ClrheilhtRo43U6HuaATFKEAuUM682rv4gvRCEVR1ljVOW0jwruB0SAJszTOZAbqNtb3V0SJh0x7wI8~ZZrp-XYqqzLDsWOB9w3ttyGSpLjcE2LvI7ty5vUljlfBGbnnLg__|url-status=live|doi-access=free}}</ref>

Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to more efficiently generate {{chem2|H2}} gas even in the presence of oxygen.<ref>{{cite web
|first1= Hamilton O.
|last1= Smith
|last2= Xu
|first2= Qing
|date= 2005
|url= http://www.hydrogen.energy.gov/pdfs/progress05/iv_e_6_smith.pdf
|title= IV.E.6 Hydrogen from Water in a Novel Recombinant Oxygen-Tolerant Cyanobacteria System
|work= FY2005 Progress Report
|publisher= United States Department of Energy
|access-date= 6 August 2016
|archive-url= https://web.archive.org/web/20161229231756/https://www.hydrogen.energy.gov/pdfs/progress05/iv_e_6_smith.pdf
|archive-date= 29 December 2016
|url-status= live
}}</ref> Efforts have also been undertaken with genetically modified [[Biological hydrogen production (Algae)|alga in a bioreactor]].<ref>{{cite news| last=Williams| first=C.| title=Pond life: the future of energy| work=Science| publisher=The Register| date=24 February 2006| url=https://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| access-date=24 March 2008| archive-url=https://web.archive.org/web/20110509143055/http://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| archive-date=9 May 2011| url-status=live}}</ref>

Relevant to the thermal water-splitting scenario is this simple equation:
:{{chem2|2 H2O  ->  2 H2 + O2}}
More than 200 thermochemical cycles can be used for [[water splitting]]. Many of these cycles such as the [[iron oxide cycle]], [[cerium(IV) oxide–cerium(III) oxide cycle]], [[zinc zinc-oxide cycle]], [[sulfur-iodine cycle]], [[copper-chlorine cycle]] and [[hybrid sulfur cycle]] have been evaluated for their commercial potential to produce hydrogen and oxygen from water and heat without using electricity.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|title=Development of solar-powered thermochemical production of hydrogen from water|first1=Al|last1=Weimer|date=25 May 2005|publisher=Solar Thermochemical Hydrogen Generation Project|access-date=21 December 2008|archive-url=https://web.archive.org/web/20070417134156/http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|archive-date=17 April 2007|url-status=live}}</ref> A number of labs (including in [[France]], [[Germany]], [[Greece]], [[Japan]], and the [[United States]]) are developing thermochemical methods to produce hydrogen from solar energy and water.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|title=Development of Solar-Powered Thermochemical Production of Hydrogen from Water, DOE Hydrogen Program, 2007|author=Perret, R.|access-date=17 May 2008|archive-url=https://web.archive.org/web/20100527212241/http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|archive-date=27 May 2010}}</ref>