Editing Haber process (section)

== Catalysts ==
[[File:Ammoniak-Reaktor 1913 Oppau (retuschiert).jpg|thumb|upright|First reactor at the Oppau plant in 1913]]
[[File:Heterogene Katalyse.svg|thumb|upright|Profiles of the active components of heterogeneous catalysts; the top right figure shows the profile of a shell catalyst.]]
The Haber–Bosch process relies on catalysts to accelerate N<sub>2</sub> hydrogenation. The catalysts are [[heterogeneous catalyst|heterogeneous]] solids that interact with gaseous reagents.<ref name="mittasch">{{Cite journal |last=Mittasch |first=Alwin |year=1926 |title=Bemerkungen zur Katalyse |journal=Berichte der Deutschen Chemischen Gesellschaft (A and B Series) |volume=59 |pages=13–36 |doi=10.1002/cber.19260590103}}</ref>

The catalyst typically consists of finely divided iron bound to an iron oxide [[catalyst support|carrier]] containing [[Promoter (catalysis)|promoters]] possibly including [[aluminium oxide]], [[potassium oxide]], [[calcium oxide]], potassium hydroxide,<ref>{{Cite web |title=3.1 Ammonia synthesis |url=http://resources.schoolscience.co.uk/ICI/14-16/catalysis/catsch3pg2.html |url-status=live |archive-url=https://web.archive.org/web/20200706200813/http://resources.schoolscience.co.uk/ICI/14-16/catalysis/catsch3pg2.html |archive-date=6 July 2020 |website=resources.schoolscience.co.uk}}</ref> molybdenum,<ref>{{Cite book |last=Rock |first=Peter A. |url={{google books |plainurl=y |id=TLJoF9kizrAC|page=317}} |title=Chemical Thermodynamics |date=19 June 2013 |isbn=978-1-891389-32-0 |page=317|publisher=University Science Books }}</ref> and [[magnesium oxide]].

=== Iron-based catalysts ===
The iron catalyst is obtained from finely ground iron powder, which is usually obtained by reduction of high-purity [[magnetite]] (Fe<sub>3</sub>O<sub>4</sub>). The pulverized iron is oxidized to give magnetite or [[wüstite]] (FeO, ferrous oxide) particles of a specific size. The magnetite (or wüstite) particles are then partially reduced, removing some of the [[oxygen]]. The resulting catalyst particles consist of a core of magnetite, encased in a shell of [[wüstite]], which in turn is surrounded by an outer shell of metallic iron. The catalyst maintains most of its bulk volume during the reduction, resulting in a highly porous high-surface-area material, which enhances its catalytic effectiveness. Minor components include [[Calcium oxide|calcium]] and [[aluminium oxide]]s, which support the iron catalyst and help it maintain its surface area. These oxides of Ca, Al, K, and Si are unreactive to reduction by hydrogen.<ref name="Appl" />

The production of the catalyst requires a particular melting process in which used [[raw material]]s must be free of [[Catalyst poisoning|catalyst poisons]] and the promoter aggregates must be evenly distributed in the magnetite melt. Rapid cooling of the magnetite, which has an initial temperature of about 3500&nbsp;°C, produces the desired precursor. Unfortunately, the rapid cooling ultimately forms a catalyst of reduced abrasion resistance. Despite this disadvantage, the method of rapid cooling is often employed.<ref name="Appl" />

The reduction of the precursor magnetite to α-iron is carried out directly in the production plant with [[synthesis gas]]. The reduction of the magnetite proceeds via the formation of [[wüstite]] (FeO) so that particles with a core of magnetite become surrounded by a shell of wüstite. The further reduction of magnetite and wüstite leads to the formation of α-iron, which forms together with the promoters the outer shell.<ref name="max">{{Ullmann|author=Max Appl|title=Ammonia|year=2006|doi=10.1002/14356007.a02_143.pub2}}</ref> The involved processes are complex and depend on the reduction temperature: At lower temperatures, wüstite [[disproportionates]] into an iron phase and a magnetite phase; at higher temperatures, the reduction of the wüstite and magnetite to iron dominates.<ref name="jozwiak">{{Cite journal |last1=Jozwiak |first1=W. K. |last2=Kaczmarek |first2=E. |year=2007 |title=Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres |journal=Applied Catalysis A: General |volume=326 |issue=1 |pages=17–27 |doi=10.1016/j.apcata.2007.03.021|bibcode=2007AppCA.326...17J }}</ref>

The α-iron forms primary [[crystallite]]s with a diameter of about 30 nanometers. These crystallites form a bimodal pore system with pore diameters of about 10 nanometers (produced by the reduction of the magnetite phase) and of 25 to 50 nanometers (produced by the reduction of the wüstite phase).<ref name="max" /> With the exception of [[cobalt oxide]], the promoters are not reduced.

During the reduction of the iron oxide with synthesis gas, water vapor is formed. This water vapor must be considered for high catalyst quality as contact with the finely divided iron would lead to premature aging of the catalyst through [[Recrystallization (metallurgy)|recrystallization]], especially in conjunction with high temperatures. The [[vapor pressure]] of the water in the gas mixture produced during catalyst formation is thus kept as low as possible, target values are below 3 gm<sup>−3</sup>. For this reason, the reduction is carried out at high gas exchange, low pressure, and low temperatures. The [[Exothermic reaction|exothermic]] nature of the ammonia formation ensures a gradual increase in temperature.<ref name="Appl" />

The reduction of fresh, fully oxidized catalyst or precursor to full production capacity takes four to ten days.<ref name="Appl" /> The wüstite phase is reduced faster and at lower temperatures than the [[magnetite]] phase (Fe<sub>3</sub>O<sub>4</sub>). After detailed kinetic, microscopic, and [[X-ray spectroscopy|X-ray spectroscopic]] investigations it was shown that wüstite reacts first to metallic iron. This leads to a gradient of iron(II) ions, whereby these diffuse from the magnetite through the wüstite to the particle surface and precipitate there as iron nuclei. A high-activity novel catalyst based on this phenomenon was discovered in the 1980s at the [[Zhejiang University of Technology]] and commercialized by 2003.<ref>{{Cite journal |last1=Liu |first1=Huazhang |last2=Han |first2=Wenfeng |last3=Huo |first3=Chao |last4=Cen |first4=Yaqing |date=2020-09-15 |title=Development and application of wüstite-based ammonia synthesis catalysts |url=https://www.sciencedirect.com/science/article/abs/pii/S0920586119305966 |journal=Catalysis Today |series=SI: Energy and the Environment |volume=355 |pages=110–127 |doi=10.1016/j.cattod.2019.10.031 |issn=0920-5861}}</ref>

Pre-reduced, stabilized catalysts occupy a significant [[market share]]. They are delivered showing the fully developed pore structure, but have been oxidized again on the surface after manufacture and are therefore no longer [[Pyrophoricity|pyrophoric]]. The reactivation of such pre-reduced catalysts requires only 30 to 40 hours instead of several days. In addition to the short start-up time, they have other advantages such as higher water resistance and lower weight.<ref name="Appl" />
{| class="wikitable centered"
!Typical catalyst composition<ref name="ertl1">{{Cite journal |last=Ertl |first=Gerhard |year=1983 |title=Zum Mechanismus der Ammoniak-Synthese |journal=Nachrichten aus Chemie, Technik und Laboratorium |language=de |volume=31 |issue=3 |pages=178–182 |doi=10.1002/nadc.19830310307}}</ref>
! Iron (%)
! Potassium (%)
! Aluminium (%)
! Calcium (%)
! Oxygen (%)
|-
|align="left" | Volume composition
| 40.5
| {{0}}0.35
| {{0}}2.0
| 1.7
| 53.2
|-
|align="left" | Surface composition before reduction
| {{0}}8.6
| 36.1
| 10.7
| 4.7
| 40.0
|-
|align="left" | Surface composition after reduction
| 11.0
| 27.0
| 17.0
| 4.0
| 41.0
|}

=== Catalysts other than iron ===
Many efforts have been made to improve the Haber–Bosch process. Many metals were tested as catalysts. The requirement for suitability is the [[dissociative adsorption]] of [[nitrogen]] (i. e. the nitrogen molecule must be split into nitrogen atoms upon adsorption). If the binding of the nitrogen is too strong, the catalyst is blocked and the catalytic ability is reduced (self-poisoning). The elements in the [[periodic table]] to the left of the [[iron group]] show such strong bonds. Further, the formation of surface nitrides makes, for example, chromium catalysts ineffective. Metals to the right of the iron group, in contrast, adsorb nitrogen too weakly for ammonia synthesis. Haber initially used catalysts based on [[osmium]] and [[uranium]]. Uranium reacts to its nitride during catalysis, while osmium oxide is rare.<ref name="Bowker">{{Cite book |last=Bowker |first=Michael |title=The Chemical Physics of Solid Surfaces |date=1993 |publisher=Elsevier |isbn=978-0-444-81468-5 |editor-last=King |editor-first=D. A. |volume=6: ''Coadsorption, promoters and poisons'' |pages=225–268 |chapter=Chapter 7 |editor-last2=Woodruff |editor-first2=D. P.}}</ref>

According to theoretical and practical studies, improvements over pure iron are limited. The activity of iron catalysts is increased by the inclusion of cobalt.<ref>{{Cite journal |last1=Tavasoli |first1=Ahmad |last2=Trépanier |first2=Mariane |last3=Malek Abbaslou |first3=Reza M. |last4=Dalai |first4=Ajay K. |last5=Abatzoglou |first5=Nicolas |date=1 December 2009 |title=Fischer–Tropsch synthesis on mono- and bimetallic Co and Fe catalysts supported on carbon nanotubes |url=https://www.sciencedirect.com/science/article/pii/S0378382009002069 |journal=Fuel Processing Technology |language=en |volume=90 |issue=12 |pages=1486–1494 |doi=10.1016/j.fuproc.2009.07.007 |bibcode=2009FuPrT..90.1486T |issn=0378-3820}}</ref>

==== Ruthenium ====
[[Ruthenium]] forms highly active catalysts. Allowing milder operating pressures and temperatures, Ru-based materials are referred to as second-generation catalysts. Such catalysts are prepared by the decomposition of [[triruthenium dodecacarbonyl]] on [[graphite]].<ref name="Appl" /> A drawback of activated-carbon-supported ruthenium-based catalysts is the methanation of the support in the presence of hydrogen. Their activity is strongly dependent on the catalyst carrier and the promoters. A wide range of substances can be used as carriers, including [[carbon]], [[magnesium oxide]], [[aluminium oxide]], [[zeolite]]s, [[spinel]]s, and [[boron nitride]].<ref name="YouZhixiong">{{Cite journal |last1=You |first1=Zhixiong |last2=Inazu |first2=Koji |last3=Aika |first3=Ken-ichi |last4=Baba |first4=Toshihide |date=October 2007 |title=Electronic and structural promotion of barium hexaaluminate as a ruthenium catalyst support for ammonia synthesis |journal=Journal of Catalysis |volume=251 |issue=2 |pages=321–331 |doi=10.1016/j.jcat.2007.08.006}}</ref>

Ruthenium-activated carbon-based catalysts have been used industrially in the KBR Advanced Ammonia Process (KAAP) since 1992.<ref name="rosowski">{{Cite journal |last1=Rosowski |first1=F. |last2=Hornung |first2=A. |last3=Hinrichsen |first3=O. |last4=Herein |first4=D. |last5=Muhler |first5=M. |date=April 1997 |title=Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, characterization, and power-law kinetics |journal=Applied Catalysis A: General |volume=151 |issue=2 |pages=443–460 |doi=10.1016/S0926-860X(96)00304-3|bibcode=1997AppCA.151..443R }}</ref> The carbon carrier is partially degraded to [[methane]]; however, this can be mitigated by a special treatment of the carbon at 1500&nbsp;°C, thus prolonging the catalyst lifetime. In addition, the finely dispersed carbon poses a risk of explosion. For these reasons and due to its low [[acid]]ity, magnesium oxide has proven to be a good choice of carrier. Carriers with acidic properties extract electrons from ruthenium, make it less reactive, and have the undesirable effect of binding ammonia to the surface.<ref name="YouZhixiong" />

=== Catalyst poisons ===
[[Catalyst poisoning|Catalyst poisons]] lower catalyst activity. They are usually impurities in the [[synthesis gas]]. Permanent poisons cause irreversible loss of catalytic activity, while temporary poisons lower the activity while present. [[Sulfur]] compounds, [[phosphorus]] compounds, [[arsenic]] compounds, and [[chlorine]] compounds are permanent poisons. Oxygenic compounds like water, [[carbon monoxide]], [[carbon dioxide]], and [[oxygen]] are temporary poisons.<ref name="Appl" /><ref>{{Citation |last=Højlund Nielsen |first=P. E. |title=Poisoning of Ammonia Synthesis Catalysts |url=https://doi.org/10.1007/978-3-642-79197-0_5 |work=Ammonia: Catalysis and Manufacture |pages=191–198 |year=1995 |editor-last=Nielsen |editor-first=Anders |access-date=30 July 2022 |place=Berlin, Heidelberg |publisher=Springer |language=en |doi=10.1007/978-3-642-79197-0_5 |isbn=978-3-642-79197-0}}</ref>

Although chemically inert components of the synthesis gas mixture such as [[noble gas]]es or [[methane]] are not strictly poisons, they accumulate through the recycling of the process gases and thus lower the partial pressure of the reactants, which in turn slows conversion.<ref name="roempp">{{Cite book |last=Falbe |first=Jürgen |title=Römpp-Lexikon Chemie (H–L) |date=1997 |publisher=Georg Thieme Verlag |isbn=978-3-13-107830-8 |pages=1644–1646}}</ref>