Editing Haber process (section)

=== 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|url-access=subscription }}</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" />