Editing Haber process (section)

== Mechanism ==
=== Elementary steps ===
The mechanism of ammonia synthesis contains the following seven [[elementary step]]s:

# transport of the reactants from the gas phase through the boundary layer to the surface of the catalyst.
# pore diffusion to the reaction center
# [[adsorption]] of reactants
# reaction
# [[desorption]] of product
# transport of the product through the pore system back to the surface
# transport of the product into the gas phase

Transport and diffusion (the first and last two steps) are fast compared to adsorption, reaction, and desorption because of the shell structure of the catalyst. It is known from various investigations that the [[rate-determining step]] of the ammonia synthesis is the [[Dissociation (chemistry)|dissociation]] of nitrogen.<ref name="Appl" /> In contrast, exchange reactions between [[hydrogen]] and [[deuterium]] on the Haber–Bosch catalysts still take place at temperatures of {{convert|-196|C}} at a measurable rate; the exchange between deuterium and hydrogen on the ammonia molecule also takes place at room temperature. Since the adsorption of both molecules is rapid, it cannot determine the speed of ammonia synthesis.<ref name="moore">{{Cite book |last1=Moore |first1=Walter J. |title=Physikalische Chemie |last2=Hummel |first2=Dieter O. |date=1983 |publisher=Walter de Gruyter |isbn=978-3-11-008554-9 |location=Berlin |page=604}}</ref>

In addition to the reaction conditions, the adsorption of nitrogen on the catalyst surface depends on the microscopic structure of the catalyst surface. Iron has different crystal surfaces, whose reactivity is very different. The Fe(111) and Fe(211) surfaces have by far the highest activity. The explanation for this is that only these surfaces have so-called C7 sites – these are iron atoms with seven closest neighbours.<ref name="Appl" />

The dissociative adsorption of nitrogen on the surface follows the following scheme, where S* symbolizes an iron atom on the surface of the catalyst:<ref name="max" />

: N<sub>2</sub> → S<sup>*</sup>–N<sub>2</sub> (γ-species) → S*–N<sub>2</sub>–S<sup>*</sup> (α-species) → 2 S*–N (β-species, ''surface nitride'')

The adsorption of nitrogen is similar to the chemisorption of carbon monoxide. On a Fe(111) surface, the adsorption of nitrogen first leads to an adsorbed γ-species with an adsorption energy of 24 kJmol<sup>−1</sup> and an N-N stretch vibration of 2100&nbsp;cm<sup>−1</sup>. Since the nitrogen is [[Isoelectronicity|isoelectronic]] to carbon monoxide, it adsorbs in an on-end configuration in which the molecule is bound perpendicular to the metal surface at one nitrogen atom.<ref name="Ertl1982adsorption" /><ref name="Ertl1982potassium">{{Cite journal |last1=Lee |first1=S. B. |last2=Weiss |first2=M. |year=1982 |title=Adsorption of nitrogen on potassium promoted Fe(111) and (100) surfaces |journal=Surface Science |language=en |volume=114 |issue=2–3 |pages=527–545 |bibcode=1982SurSc.114..527E |doi=10.1016/0039-6028(82)90703-8}}</ref><ref name="Appl" /> This has been confirmed by [[photoelectron spectroscopy]].<ref name="ErtlSolidSurfaces">{{Cite book |last=Ertl |first=Gerhard |url=https://archive.org/details/reactionsatsolid00ertl_180 |title=Reactions at Solid Surfaces |publisher=John Wiley & Sons |year=2010 |isbn=978-0-470-26101-9 |page=[https://archive.org/details/reactionsatsolid00ertl_180/page/n135 123] |language=en |url-access=limited}}</ref>

[[Ab initio quantum chemistry methods|Ab-initio-MO calculations]] have shown that, in addition to the [[Σ bond|σ binding]] of the free electron pair of nitrogen to the metal, there is a [[Π bond|π binding]] from the [[d orbitals]] of the metal to the π* orbitals of nitrogen, which strengthens the iron-nitrogen bond. The nitrogen in the α state is more strongly bound with 31 kJmol<sup>−1</sup>. The resulting N–N bond weakening could be experimentally confirmed by a reduction of the wave numbers of the N–N stretching oscillation to 1490&nbsp;cm<sup>−1</sup>.<ref name="Ertl1982potassium" />

Further heating of the Fe(111) area covered by α-N<sub>2</sub> leads to both [[desorption]] and the emergence of a new band at 450&nbsp;cm<sup>−1</sup>. This represents a metal-nitrogen oscillation, the β state. A comparison with vibration spectra of complex compounds allows the conclusion that the N<sub>2</sub> molecule is bound "side-on", with an N atom in contact with a C7 site. This structure is called "surface nitride". The surface nitride is very strongly bound to the surface.<ref name="ErtlSolidSurfaces" /> Hydrogen atoms (H<sub>ads</sub>), which are very mobile on the catalyst surface, quickly combine with it.

Infrared spectroscopically detected surface imides (NH<sub>ad</sub>), surface amides (NH<sub>2,ad</sub>) and surface ammoniacates (NH<sub>3,ad</sub>) are formed, the latter decay under NH<sub>3</sub> release ([[desorption]]).<ref name="Holleman" /> The individual molecules were identified or assigned by [[X-ray photoelectron spectroscopy]] (XPS), [[High resolution electron energy loss spectroscopy|high-resolution electron energy loss spectroscopy]] (HREELS) and [[Ir Spectroscopy]].

:[[File:KatNH3.svg|thumb|upright=1.2|Drawn reaction scheme]]

On the basis of these experimental findings, the [[reaction mechanism]] is believed to involve the following steps (see also figure):<ref>{{Cite web |last1=Wennerström |first1=Håkan |last2=Lidin |first2=Sven |title=Scientific Background on the Nobel Prize in Chemistry 2007 Chemical Processes on Solid Surfaces |url=https://www.nobelprize.org/uploads/2018/06/advanced-chemistryprize2007.pdf |access-date=17 September 2015 |publisher=Nobel Foundation}}</ref>
# N<sub>2</sub> (g) → N<sub>2</sub> (adsorbed)
# N<sub>2</sub> (adsorbed) → 2&nbsp;N (adsorbed)
# H<sub>2</sub> (g) → H<sub>2</sub> (adsorbed)
# H<sub>2</sub> (adsorbed) → 2&nbsp;H (adsorbed)
# N (adsorbed) + 3&nbsp;H (adsorbed) → NH<sub>3</sub> (adsorbed)
# NH<sub>3</sub> (adsorbed) → NH<sub>3</sub> (g)

Reaction 5 occurs in three steps, forming NH, NH<sub>2</sub>, and then NH<sub>3</sub>. Experimental evidence points to reaction 2 as being slow, [[rate-determining step]]. This is not unexpected, since that step breaks the nitrogen triple bond, the strongest of the bonds broken in the process.

As with all Haber–Bosch catalysts, nitrogen dissociation is the rate-determining step for ruthenium-activated carbon catalysts. The active center for ruthenium is a so-called B5 site, a 5-fold coordinated position on the Ru(0001) surface where two ruthenium atoms form a step edge with three ruthenium atoms on the Ru(0001) surface.<ref name="gavnholt">{{Cite journal |last1=Gavnholt |first1=Jeppe |last2=Schiøtz |first2=Jakob |year=2008 |title=Structure and reactivity of ruthenium nanoparticles |url=https://backend.orbit.dtu.dk/ws/files/4788727/Jeppe.pdf |journal=Physical Review B |volume=77 |issue=3 |pages=035404 |bibcode=2008PhRvB..77c5404G |doi=10.1103/PhysRevB.77.035404 |s2cid=49236953}}</ref> The number of B5 sites depends on the size and shape of the ruthenium particles, the ruthenium precursor and the amount of ruthenium used.<ref name="YouZhixiong" /> The reinforcing effect of the basic carrier used in the ruthenium catalyst is similar to the promoter effect of alkali metals used in the iron catalyst.<ref name="YouZhixiong" />

=== Energy diagram ===
[[File:Potential energy diagram for ammonia synthesis.svg|thumb|upright=1.7|[[Energy profile (chemistry)|Energy diagram]] ]]

An [[Energy profile (chemistry)|energy diagram]] can be created based on the [[Enthalpy of Reaction]] of the individual steps. The energy diagram can be used to compare homogeneous and heterogeneous reactions: Due to the high [[activation energy]] of the dissociation of nitrogen, the homogeneous gas phase reaction is not realizable. The catalyst avoids this problem as the energy gain resulting from the binding of nitrogen atoms to the catalyst surface overcompensates for the necessary dissociation energy so that the reaction is finally exothermic. Nevertheless, the dissociative adsorption of nitrogen remains the rate-determining step: not because of the activation energy, but mainly because of the unfavorable [[pre-exponential factor]] of the rate constant. Although [[hydrogenation]] is endothermic, this energy can easily be applied by the reaction temperature (about 700 K).<ref name="Appl" />
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