Editing Phosgene (section)

== Production ==
Industrially, phosgene is produced by passing purified [[carbon monoxide]] and [[chlorine]] gas through a bed of porous [[activated carbon]], which serves as a [[catalyst]]:<ref name=Ullmann/>
:{{chem2|CO + Cl2 → COCl2}} (Δ''H''<sub>rxn</sub> = −107.6 kJ/mol)

This reaction is exothermic and is typically performed between 50 and 150&nbsp;°C. Above 200&nbsp;°C, phosgene reverts to carbon monoxide and chlorine, ''K''<sub>eq</sub>(300&nbsp;K) = 0.05. World production of this compound was estimated to be 2.74 million tonnes in 1989.<ref name=Ullmann/>

Phosgene is fairly simple to produce, but is listed as a [[List of Schedule 3 substances (CWC)|Schedule 3 substance]] under the [[Chemical Weapons Convention]]. As such, it is usually considered too dangerous to transport in [[Bulk cargo|bulk quantities]]. Instead, phosgene is usually produced and consumed within the same plant, as part of an "on demand" process. This involves maintaining equivalent rates of production and consumption, which keeps the amount of phosgene in the system at any one time fairly low, reducing the risks in the event of an accident. Some batch production does still take place, but efforts are made to reduce the amount of phosgene stored.<ref>{{cite journal |last=Gowland |first=Richard |title=Applying inherently safer concepts to a phosgene plant acquisition |journal=Process Safety Progress |date=1996 |volume=15 |issue=1 |pages=52–57 |s2cid=110707551 |doi=10.1002/prs.680150113}}</ref>

=== Inadvertent generation ===
====Atmospheric chemistry====
Simple [[organochloride]]s slowly convert into phosgene when exposed to [[ultraviolet]] (UV) irradiation in the presence of [[oxygen]].<ref name=Singh>{{cite journal |last=Singh |first=Hanwant Bir |title=Phosgene in the ambient air |journal=Nature |date=December 1976 |volume=264 |issue=5585 |pages=428–429 |pmid=1004568 |bibcode=1976Natur.264..428S |s2cid=4209599 |doi=10.1038/264428a0}}</ref> Before the discovery of the [[ozone hole]] in the late 1970s large quantities of organochlorides were routinely used by industry, which inevitably led to them entering the atmosphere. In the 1970-80s phosgene levels in the [[troposphere]] were around 20-30 [[Parts-per notation#Mass fraction vs. mole fraction vs. volume fraction|pptv]] (peak 60 pptv).<ref name=Singh /> These levels had not decreased significantly nearly 30 years later,<ref>{{cite journal |last1=Fu |first1=Dejian |last2=Boone |first2=Chris D. |last3=Bernath |first3=Peter F. |last4=Walker |first4=Kaley A. |last5=Nassar |first5=Ray |last6=Manney |first6=Gloria L. |last7=McLeod |first7=Sean D. |title=Global phosgene observations from the Atmospheric Chemistry Experiment (ACE) mission |journal=Geophysical Research Letters |date=14 September 2007 |volume=34 |issue=17 |pages=L17815 |doi=10.1029/2007GL029942|bibcode=2007GeoRL..3417815F |s2cid=44164908 |doi-access=free }}</ref> despite organochloride production becoming restricted under the [[Montreal Protocol]].

Phosgene in the troposphere can last up to about 70 days and is removed primarily by hydrolysis with ambient humidity or cloudwater.<ref>{{cite journal |last1=Kindler |first1=T.P. |last2=Chameides |first2=W.L. |last3=Wine |first3=P.H. |last4=Cunnold |first4=D.M. |last5=Alyea |first5=F.N. |last6=Franklin |first6=J.A. |title=The fate of atmospheric phosgene and the stratospheric chlorine loadings of its parent compounds: CCl 4, C 2 Cl 4, C 2 HCl 3, CH 3 CCl 3, and CHCl 3 |journal=Journal of Geophysical Research: Atmospheres |date=20 January 1995 |volume=100 |issue=D1 |pages=1235–1251 |doi=10.1029/94JD02518|bibcode=1995JGR...100.1235K }}</ref> Less than 1% makes it to the [[stratosphere]], where it is expected to have a lifetime of several years, since this layer is much drier and phosgene decomposes slowly through UV [[Photodissociation|photolysis]]. It plays a minor part in [[ozone depletion]].

====Combustion====
[[Carbon tetrachloride]] ({{chem2|CCl4}}) can turn into phosgene when exposed to heat in air. This was a problem as carbon tetrachloride is an effective fire suppressant and was formerly in widespread use in [[fire extinguisher]]s.<ref name="Burke">{{cite book |last=Burke |first=Robert |title=Fire Protection: Systems and Response |date=2007-11-06 |publisher=CRC Press |isbn=978-0-203-48499-9 |pages=209}}</ref> There are reports of fatalities caused by its use to fight fires in [[confined space]]s.<ref>{{cite journal |last1=Fieldner |first1=A. C. |last2=Katz |first2=S. H. |last3=Kinney |first3=S. P. |last4=Longfellow |first4=E. S. |date=1920-10-01 |title=Poisonous gases from carbon tetrachloride fire extinguishers |url=https://www.sciencedirect.com/science/article/pii/S0016003220914941 |access-date=2022-02-03 |journal=Journal of the Franklin Institute |volume=190 |issue=4 |pages=543–565 |language=en |doi=10.1016/S0016-0032(20)91494-1}}</ref> Carbon tetrachloride's generation of phosgene and its own toxicity mean it is no longer used for this purpose.<ref name="Burke" />

====Biologically====
Phosgene is also formed as a metabolite of [[chloroform]], likely via the action of [[cytochrome P-450]].<ref>{{cite journal |last1=Pohl |first1=Lance R. |last2=Bhooshan |first2=B. |last3=Whittaker |first3=Noel F. |last4=Krishna |first4=Gopal |title=Phosgene: A metabolite of chloroform |journal=Biochemical and Biophysical Research Communications |date=December 1977 |volume=79 |issue=3 |pages=684–691 |pmid=597296 |doi=10.1016/0006-291X(77)91166-4}}</ref>