This paper comes under the heading of “early days, but possibly of great interest”. It demonstrates room-temperature synthesis of ammonia from nitrogen gas using a samarium/molybdenum system, and chemists of all sorts will sit up and that news and say “Hold it. Ammonia is the Haber-Bosch process, isn’t it?”
That it is. And that’s a reaction that keeps over half the human race alive, through its use in fertilizer production. This is a prime example of one of the absolute foundation stones of modern existence that most people are completely unaware of. All over the world, there are huge reactors running at high pressures and temperatures, full of carefully-worked-out iron-based catalysts, that are pulling nitrogen and hydrogen in and pushing ammonia out at a rate (worldwide) of nearly 300 tons per minute. Until Haber worked out that reaction (and Bosch improved it for industrial scale), the only creatures on Earth that could do that were certain types of bacteria. In 1909, we became the second. Note that those bacteria are also (and still) keeping large swathes of the human race alive, since they’re the nitrogen-fixers found around the roots of legumes. And if either of these things stopped working, the bacteria or our industrial ammonia plants, we’d be looking at mass famines within months with effects on the human population not seen since the Black Death. Both going down at once would have a decent shot at collapsing our civilization.
The Haber process has a reputation of being pretty severe, what with those temperatures (>400 C) and pressures (around 400 atmospheres), but as this commentary on the new paper makes clear, it’s really a pretty good setup. Thermodynamically, it makes very efficient use of its energy input and wastes very little on side processes. The problem is the size of those inputs: running a Haber-Bosch plant is an energy-intensive undertaking, and it’s even more so when you consider the energy that has to go into making the hydrogen starting material as well. A new catalytic route that lowered the energy barriers involved would be very desirable from a power-consumption and carbon-emissions view.
This new reaction doesn’t use hydrogen gas at all – rather, the H atoms in the ammonia product come from water, with the samarium iodide allowing it to serve as such a source. Under normal conditions that’s not such an easy reaction – about 111 kcal/mole for O-H dissociation in bulk water – but in hydrated samarium iodide the O-H bond strength is knocked down to 26 kcal/mole. It also looks like other OH-containing molecules (such as ethylene glycol) can also serve. So the samarium end of the reaction furnishes the hydrogen, and the molybdenum end activates the nitrogen (indeed, it’s an important element in the bacterial nitrogenase enzymes as well). The reaction turns over at greater than 100x per minute, which may not sound too impressive compared to an enzymatic process, but that actually approaches the ammonia production of nitrogenase, because it’s obviously not an easy reaction under any conditions.
What it doesn’t approach is the throughput of the Haber-Bosch, of course, and as it stands the reaction isn’t really suitable for industrial scaleup. But it’s of potential importance, as mentioned, because of the non-hydrogen-gas aspect. This is new way to get ammonia to form, and it already works at least 10x better than any other artificial system that’s not a Haber-Bosch variant, and this at atmospheric pressure and room temperature. Thermodynamically, it still has a way to go when you add up the total energy involved versus ammonia production, but this looks very much like an area to pursue. The Haber plants are going to be with us for a while to come (they’d better), but there could be better ways. . .