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Comment by msandford | Hacker News Reader

ATP is required to make the reaction happen. My suspicion is that it's energy net-negative. Sure you make some electricity but how much does it cost to make ATP? Probably more than you get out in electricity. Not that there's a fast and easy way to turn electricity into ATP. https://www.reddit.com/r/askscience/comments/46ax8r/can_elec...

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Just from a conservation of energy perspective it has to be net-negative. There is going to be something fed into entropy and that something had to come from somewhere.

I think this isn't interesting for terrestrial usage but for space travel where ATP can be used as an efficient chemical energy store.

Theoretically, ATP could be harvested from plants or bacteria. They could also be modified to produce the ammonia directly via this reaction. That wouldn't actually reduce the energy cost, but it could reduce the economic cost by making it logistically easier to produce. Maybe instead of making ammonia in complicated chemical plants, we could "grow" it on farms.

I mean, sure, you can do that. But it's kind of silly. "Grow" ammonia feedstock on farms to then turn into fertilizer to use at the farm to grow food?

Maybe you should just plant legumes instead and let them fix the nitrogen straight into the soil where you want to grow food. It's not sexy since it's 19th century technology (or older) but it works. And it requires very little in the way of logistics or infrastructure meaning that it's resilient.

https://en.wikipedia.org/wiki/Nitrogen_fixation

Oh you city slickers... This has been a thing for centuries and it works pretty well. Getting big industry in the middle of the process would be new but the process is old.

https://en.wikipedia.org/wiki/Green_manure

IIRC, ATP is a short lived energy carrier in cells. Instead of harvesting it from cells, it's probably easier to turn that around and use the existing ATP mechanism in cells to generate ammonia.

All of the components involved are being extracted from existing cells. There are already plenty of nitrogen fixing bacteria, the sources of their nitrogenase and hydrogenase enzymes.

This is maybe novel in that they got the nitrogenases working in vitro. But from a world changing technology perspective this is a very tiny step compared to re-engineering the enzymes to work off of a different non-biological energy source than ATP.

Of course it is energy net negative. There is an energy cost to reducing nitrogen to ammonia. Enzymes aren't magic and nothing changes the energy cost equation of the products vs the reagants.

Of course it is not an energy negative. The production of ammonia from hydrogen and nitrogen gas is an exothermic process. At room temperature it's even a spontaneous process.

The reason we need these complicated processes is the massive activation energy of the reaction, making it extremely slow uncatalyzed.

These are professional chemists, publishing in a serious journal. Did you really think they didn't take into account the basic energy balance of their equation?

I'd like to remind you that the process we are discussing is currently responsible for 1% of the world's energy consumption annually. That's clearly a "net-negative" cost. Feel free to review the Haber-Bosch process to confirm.

Ammonia has a negative enthalpy of formation of about -46 kJ/mol [0]. If you read the wikipedia page of the Haber process [1] it explicitly states the reaction is exothermic.

The reason the Haber process is so costly is because it uses very high pressures and temperatures to overcome the activation energy, even with the best catalysts we have. Not because creating ammonia from nitrogen and hydrogen needs energy. It's in effect wasted energy.

Enzymes can't change the cost equation, but luckily it's in our favor. If they can reduce the activation energy without wasting a ton of energy, it can be a net energy producer.

0: https://en.wikipedia.org/wiki/Ammonia 1: https://en.wikipedia.org/wiki/Haber_process

Yes, it can be a net energy producer given a source of hydrogen, but hydrogen is typically not available for free. See my other comment.

Nobody is saying hydrogen is free. Nor will the production of ammonia from compounds we have in abundance (i.e. water, air and rock) ever produce energy, as it's a high energy compound, releasing quite a bit of energy when combusted. But that is absolutely not what this is about.

However, I was refuting the GP's wrong assertion that the energy cost equation of this reaction shows that it cannot be a net energy producer.

It makes sense to optimize this step, because the energy equation shows the energy is just wasted as heat, as it's not contained in the compound itself.

I agree 100%. Thanks for the clarification!

Not sure where you get a pure hydrogen source on this planet without using some energy. I'd like to own such a source. Sort of like owning an oil well.

That's moving the goalposts. The point of this is to reduce the energy cost of making ammonia, assuming you already have hydrogen gas available.

The article is not purporting to have conceived of a new energy source, only to have found a way to recuperate some of the cost of making ammonia, by extracting some energy from a step that ought to be a net energy producer.

I was thinking that producing the hydrogen feed stock would be most of the energy used in the process. This site [1] states that, using the current global feed stock mix of light carbon and heavy carbon sources, about half the energy used in production is feed stock energy and half is added energy. Maybe an industrial chemist could estimate how much energy this new tech could save if it could be used at the scale needed?

[1] http://ietd.iipnetwork.org/content/ammonia

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