Do metals react with ammonia gas (not liquid phase)?

Question

In my chemistry notes I have two chemical equations as follows:

$$ \begin{align} \ce{2 NH3(g) + 6 Li(s) &-> 2 Li3N(s) + 3 H2(g)}\tag{R1} \\ \ce{2 NH3(g) + 2 Na(s) &-> 2 NaNH2 + H2(g)}\tag{R2} \end{align} $$

Before digging into the reasons for different products in two equations, when I searched in the internet to find the clear picture of what are the metals (like alkali metals, alkaline earth metals, transition metals) that can react with ammonia in gaseous form, I could only find metals dissolving in liquid ammonia.

I would be grateful if someone could tell that metals don't react with gaseous ammonia, or, if it reacts, what are the metals those react with gaseous ammonia? And are there any conditions needed for the reaction (like heat)?

Answer 1

Wikipedia reports that magnesium nitride can be prepared:

By passing dry nitrogen over heated magnesium:

${\displaystyle{\begin{matrix}{}\\{\ce {{3Mg} + N2->[{\ce{800^{\circ }C}}]Mg3N2}}\\{}\end{matrix}}}$

or ammonia:

${\displaystyle{\begin{matrix}{}\\{\ce{{3Mg} + 2NH3->[{\ce{700^{\circ }C}}]{Mg3N2} + 3H2}}\\{}\end{matrix}}}$

No references are cited.

Answer 2

As to the question does gaseous NH3 directly react with metals, other than complex formations, a more accurate short answer is likely no. A better answer is a reaction may proceed from heat (or other energy sources, like light) induced breakdown products of ammonia interacting with the metal.

A review of the literature suggests that if ammonia is heated to high temperature or subject to photodissociation, then a reported radicalization may occur per a source, Journal of Chemical Physics: 'Dissociation of NH3 to NH2 + H ':

$\ce{NH3(g) + Heat/UV light/ArF laser -> •NH2 + •H}$

The laser reference is provided here. A thermal reference relating to the heating of ammonia on a Tungsten metal surface states:

When NH2 forms by the dissociation of NH3 with the release of H, the bond becomes covalent...

Thus, it is possible in the presence of say a metal M:

$\ce{M -> M+ + e-}$

$\ce{•NH2 + e- -> NH2-}$

For a net reaction:

$\ce{M + •NH2 -> MNH2}$

Note, the so-called amino radical can be created at near room temperatures as well, as per Wikipedia, to quote a reaction between ammonia and the hydroxyl radical:

$\ce{NH3 + •OH → •NH2 + H2O}$

And, may I suggest a related, but slower radical reaction, as a possibility with say surface absorbed •H on Aluminum (discussed on a prior thread), placed in a NONE aqueous medium containing ammonia (in relative excess to the hydrogen radical):

$\ce{NH3 + •H → •NH2 + H2}$

Source: See Reaction [10] in this 2008 article:'Photochemical Behavior of Ammonia in Aqueous Suspension of TiO2' by Ki-Min Bark, available online.

$\ce{•NH2 + •H → •NH + H2}$

Added source: See ebook. And also possible an unwanted reverse reaction like:

$\ce{•NH2 + •H → NH3}$

Nevertheless, in the presence of select metals some possible creation (per reaction scheme outlined above) of corresponding salts of interest with ammonia. For example, per this 2000 source with magnesium:

MgNH2 could be produced in interstellar/circumstellar gas from the association reaction of Mg+ + NH3, as predicted by theory.

It should be further noted commencing with the formation of $\ce{NH4+}$ salt from ammonia, the following reaction pathway can lead to other salts:

$\ce{NH4+ ⇆ H+ + NH3}$

$\ce{H+ + e- ⇆ •H}$

$\ce{NH3 + •H → •NH2 + H2}$

An example includes $\ce{ZnNH2}$ whose surface formation on ZnS in the presence of ammonia is noted. From a thesis by Peng Huang (available online), 'CHITOSAN IN DIFFERENTIAL FLOTATION OF BASE METAL', to quote:

Due to the cationization effect, ZnNH3+ represents ZnNH2, indicating that strong interaction occurred between –NH2 and zinc on sphalerite surface. This hypothesis was further supported by the XPS results (i.e., amine group NH2 was the dominant species...

My final example is a bit explosive, I am referring to Silver nitride (Ag3N) whose formation is reportedly accompanied with secondary products of Silver amide (AgNH2) and the imide (Ag2NH). As a clue to the reaction mechanics, I quote a source noting the conditions favoring Ag+ creation:

Elemental silver ionises rapidly when exposed to an aqueous environment such as wound exudate.[66]

So, assuming the silver is an alloy (and not 100% Ag) in the presence of aqueous ammonia, to cite a source:

"Ammonia precipitates Ag+ as brown/black oxide.

$\ce{2 Ag+(aq) + 2 NH3(aq) + H2O(l) -> Ag2O(s) [brown/black] + 2 NH4+(aq)}$

In excess, ammonia dissolves the precipitate, forming a diamine silver complex.

$\ce{Ag2O(s) + 4 NH3(aq) + H2O(l) -> 2 [Ag(NH3)2]+(aq) + 2 OH−(aq)}$ "

Unfortunately, on standing with water evaporation and, I would argue, likely exposure to oxygen and light, a reverse reaction reforming Silver oxide and NH4+. More recently, a 2011 article noting that the action of light on Ag2O forms an Ag/Ag2O pair that is powerful photocatalst. The introduction of electron holes and e- likely attacks the ammonium ion as I detailed above leading to AgNH2 and beyond.

For the general interaction of the amino radical with organics, a good review can be found here.