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Can mechanical agitation catalyze reactions? It appears so (thanks to Mithoron for the pointer!).

We observed the following: We were using 10-micron hBN powder to impact-plate copper bearings (e.g., by running them together in a vibratory tumbler, and other similar processes). Even though the mechanical processes ran in a standard atmosphere and did not exceed 60C we consistently observed that ammonia was released. As a control: I have run the same copper with all sorts of other media through the same mechanical processes and never detected ammonia.

Our conclusion is that the BN is decomposing during the mechanical processing. But hBN is very thermally and chemically stable: hBN doesn't react with Cu below 1000C!

So is it plausible that mechanical abrasion at this low a temperature would break the B-N bond? If so, what subsequent reactions (again, in a standard atmosphere) might cause the free N to form $\ce{NH3}$ in detectable amounts, instead of just forming $\ce{N2}%edit$?

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  • $\begingroup$ en.wikipedia.org/wiki/Mechanochemistry $\endgroup$
    – Mithoron
    Aug 10, 2015 at 19:28
  • $\begingroup$ How did you determine that the product was ammonia? How much was produced? Where might the hydrogen have come from? $\endgroup$
    – ericksonla
    Aug 10, 2015 at 19:57
  • $\begingroup$ @ericksonla: That the product was ammonia was deduced entirely by smell and mucosal irritation on opening the processing vessels. One of the core parts of this question is exactly your last: Where could the hydrogen come from? E.g., does breaking the B-N bond release something other than heat that might ionize water vapor? I don't know enough to begin making more than wild guesses like that. $\endgroup$
    – feetwet
    Aug 10, 2015 at 20:08
  • $\begingroup$ I'd guess it is highly unlikely you stumbled upon a low pressure low temperature nitrogen to synthesize a super industrially important chemical from water. What about something like a nitrile o-ring thats getting hotter than your bulk system and decomposing? Many nitrogen compounds smell substantially similar to ammonia and nitrile is chemically more similar than BN. $\endgroup$
    – ericksonla
    Aug 10, 2015 at 20:35
  • $\begingroup$ @ericksonla - Note that I have plenty of control runs in the mechanical processes. The only time the ammonia (or something that smells and irritates like it) is produced is when BN powder is introduced. Also, I just recalled further evidence that it is in fact ammonia: We happened to leave one of the plastic (I think HDPE) processing vessels closed for a week or so after running it. When we opened it up all of the copper was tarnished a dark color. $\endgroup$
    – feetwet
    Aug 10, 2015 at 20:57

2 Answers 2

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Fine hexagonal boron nitride powder is hygroscopic, so this implies there might be some water in your system. According to Reaction of Hexagonal Boron Nitride Nano-crystals under Mild Hydrothermal Conditions "The reaction between water and hBN nano-crystals starts at very low pressure and temperature, i.e. 220 C and 1.0 MPa [~10 atmospheres]."

Those conditions are likely to be met in a ball-mill or vibratory tumbler, at least momentarily at the points of impact (in a similar way, cavitation can heat water to luminescence!).

A simple test would be to preheat the tumbler and its contents to 150 or 200 C for an hour or two, then seal it airtight before running the tumbling cycle. If there's no ammonia odor, then it indicates that adsorbed water in the hBN had been reacting with it, perhaps catalyzed by the copper bearings, as well as by impact heating and pressure.

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  • $\begingroup$ Let us suppose the hBN is somewhat hydrated. And let us assume that locally high temp and pressure can be created in the tumblers. I agree that both are reasonable assumptions. Then what is the reaction stipulated? All I get from that paper is that hBN crystals deteriorate to cBN. (NB: I actually did bake the hBN before several runs to remove moisture, but not certain if I baked long enough to fully dehydrate, and didn't control or note other variables well enough to establish correlation of that with absence of ammonia production.) $\endgroup$
    – feetwet
    Aug 10, 2015 at 22:14
  • $\begingroup$ Though there might be other products, a simple displacement is $\ce{2BN + 3H2O -> B2O3 + 2 NH3}$. See webqc.org/balance.php?reaction=BN+%2B+H2O+%3D+B2O3+%2B+NH3+ $\endgroup$
    – DrMoishe Pippik
    Aug 10, 2015 at 22:46
  • $\begingroup$ I can see that equation "balances," and the left side is consistent with our hypothesis. But what would motivate the double replacement? The hypothesis is that mechanical action can break the B-N bond of some BN molecules. For that reaction to occur the water molecules also have to decompose. Is there any reason to expect or believe that actually happens, other than that it's plausible and consistent with the original observation? E.g., is the right hand side more stable, or lower energy? Or does B have a greater affinity for O than does H? Or anything like that? $\endgroup$
    – feetwet
    Aug 11, 2015 at 0:24
  • $\begingroup$ See Reaction of Hexagonal Boron Nitride Nano-crystals under Mild Hydrothermal Conditions $\endgroup$
    – DrMoishe Pippik
    Aug 11, 2015 at 21:58
  • $\begingroup$ I just re-read the paper. As I understand it, the paper suggests that "mild hydrothermal conditions" can transform hBN to cBN. The only other compound mentioned in the paper is ammonium borate, which is produced at 1200C and 5.6 GPa. Am I missing something else that applies to my questions? $\endgroup$
    – feetwet
    Aug 11, 2015 at 22:30
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The classic action of water on a nitride is the formation of ammonia. For example, with magnesium nitride:

$$\ce{Mg3N2(s) + 6 H2O(l) → 3 Mg(OH)2(aq) + 2 NH3(g) }$$

Water vapor may be entering the system, and friction is adding heat. The latter factors may be fostering the expected chemical reaction, resulting in the formation of ammonia gas.

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