Titanocene Electrocatalysts for Ammonia Synthesis.

[NNadir]

One of the most intriguing stories in history - possibly the most Faustian story in science other than that of Robert Oppenheimer - is the tale of Fritz Haber, the Nobel Laureate who developed the Haber-Bosch process for nitrogen fixation to make ammonia. (Bosch was the chemical engineer who was able to design and build the high temperature/high pressure reactors in which the reaction, the reaction of 3 moles of hydrogen gas with one mole of nitrogen gas to make two moles of ammonia.

If you have food on your table, it's because of the Haber-Bosch process, the only real "green revolution" was the one that took place in the 1950's, the industrialization of fertilizers containing ammonium nitrate and ammonium hydrogen phosphates.

Haber was an interesting person because not only did he make food readily available for the masses (for the first time by the way), he also made it possible for the German War machine to fight World War I, since before World War I, all of the world's gunpowder manufacture depended on the importation of salt peter, potassium nitrate, which was mined in Chile. (Ammonia can be oxidized readily to nitrate.) Denying access to salt peter and not food (although food shortages, and not gunpowder shortages were the result) was a primary motivation for the British blockade of Germany. Haber, a fervent German nationalist almost to the point of fascism although he was, in fact, purely descended from Jews, also drove the introduction of gas warfare in the First World War, and after that war, the allies couldn't decide whether to celebrate his Nobel Prize or to try him as a war criminal.

Because of his Jewish heritage, he was expelled from Germany in 1938 and died shortly thereafter in Swiss exile.

The Haber Bosch process is still practiced today, although in most places the dangerous coal that was used to generate hydrogen has been displaced by dangerous natural gas reformation.

The story is ably told in great detail by Vaclav Smil - anyone who wants to know about energy should read the delightfully sarcastic Smil - in his important scientific and engineering book, Enriching the Earth.

(Smil is at his most amusing when he makes fun of the anti-nuke idiot Amory Lovins.)

The Haber-Bosch process is still being practiced today, albeit more efficiently, with profound environmental consequences, but is highly energy intensive, consuming something in the neighborhood of 3% of the world's energy. Were it abandoned, we would require about half of the world's current population to starve to death as a result.

Because the reaction requires such extreme conditions, it would be nice to have a milder reaction to make ammonia, and I came across a possible one in a recent issue of ACS Sustainable Chemistry and Engineering, this paper: Electrochemical Ammonia Synthesis Mediated by Titanocene Dichloride in Aqueous Electrolytes under Ambient Conditions (ACS Sustainable Chemistry & Engineering 2017 5 (11), 9662-9666)

Some excerpts from the introduction:

Renewable energy production and supply rates are rising worldwide as serious attempts to combat greenhouse gas emissions caused by the depletion of fossil fuels are pursued to mitigate catastrophic climate change; concomitantly, relevant research and development are actively being explored.1 Renewable energy requires energy carriers or storage systems because of regional and temporal variabilities. Recently, the use of ammonia (17.6 wt % H2) as a renewable-energy carrier has drawn considerable research interest in terms of storing and converting renewable energy, the so-called “power-to-gas technology”.2 As a hydrogen reservoir containing 17.6 wt % H2, ammonia is a noncarbon fuel that releases only water and nitrogen during combustion. Ammonia has a higher energy density per volume (NH3 HHV: 13.6 GJ·m−3) than that of hydrogen and is much easier to store and transport than hydrogen3,4 because it is liquid below 10 bar at room temperature. Furthermore, more than 150 million tons per year of ammonia are currently consumed globally;5 thus, infrastructure to support ammonia based technologies...

I don't agree with very much of what's been written here, particularly the idea of ammonia fuel, but no matter. What is important is the conditions under the reaction takes place at ambient conditions. Some comments by the authors:

Transition metals with strong reducing abilities for nitrogen are located in groups 4−6 of the periodic table, and the ammonia yield decreases in moving from the left to right in each row (e.g., Ti > V > Cr; Zr > Nb > Mo). Among them, Ti exhibits high activity for nitrogen.18 Judicious choice of coordinating ligand in these catalysts can lead to enhanced capacity of the metal to bind to nitrogen, thereby affecting the activity the metal toward nitrogen. These ligands include cyclopentadienyl,20 acetylacetonates,21 and phosphine complexes. 19 Bayer and Schurig studied the chemical synthesis of ammonia using titanium compounds such as titanocene dichloride (Cp2TiCl2), cyclopentadienyltitanium(IV) trichloride (Cp2TiCl3), zirconocene dichloride (Cp2ZnCl2), and titanium tetrachloride (TiCl4), as well as various alkali and alkaline earth metals (Li, Na, K, Mg, Ca, La, Cs, and Na).

The authors state:

in this study, experiments were conducted into the chemical and electrochemical synthesis of ammonia in the 0 to −2 V applied voltage range using various organic solvents with Li electrolytes and the Cp2TiCl2 catalyst, which has a high nitrogen-activation capacity. Furthermore, the ammonia synthesis rates and faradaic e_ciencies were compared in terms of the applied voltage. In addition, the rate-limiting step in the proposed ammonia-synthesis mechanism using Cp2TiCl2 was examined by density functional theory (DFT) calculations

There's a lot of discussion of the experiments conducted but what is relevant here is the conclusion:

We theoretically and experimentally demonstrated that chemical and electrochemical ammonia synthesis in Li-based aqueous electrolytes containing Cp2TiCl2 is feasible using various solvents including water, methanol, and THF. In the theoretical study, DFT calculations reveal that the nitrogenreduction reaction in a Li-based aqueous electrolyte containing the Cp2TiCl2 catalyst prefers to occur via the Cp2TiClN2 intermediate due to its relatively low _G, rather than the Cp2TiN2N2 intermediate. According to the DFT calculations, the activation barrier for the electrochemical ammoniasynthesis reaction is about 0.7 eV, which is lower than the Tafel-type reaction barrier (*H + *NHx _ *NHx+1, 1 eV) of most transition metal catalysts in the literature.28−30 Hence, the DFT results in this study suggest that Cp2TiCl2 significantly lowers the activation barrier for the protonation

Good news, if true and scalable, since it makes it simpler to utilize the only truly sustainable form of carbon free energy, nuclear energy, to displace the role of dangerous natural gas in this important, if problematic, industrial chemistry. (THF can be obtained from corn cobs, and methanol is accessible by the direct hydrogenation of carbon dioxide and, indeed, the electrochemical reduction of carbon dioxide.)

Have a nice afternoon.

[eppur_se_muova]

1. I have been hoping to work on the problem of using NH3 as an energy storage medium --

Much more easily storable and transportable than H2, as pointed out. Of course you can't burn it directly unless you want to destroy the world with NOx (though that's how the X-15 did it) but it should be fine in fuel cells -- only output would be N2 + H2O. But you'd really need to find a more efficient ammonia synthesis, which of course everybody and his brother is hoping to discover -- it's the modern chemist's equivalent of the search for the Philosopher's Stone, only one based on facts, not mysticism.

Of course, this is actually quite a bit outside my specialty, and I'm sidelined anyway -- so probably have to watch someone else collect those laurels.

[NNadir]

2. I've gone around in a few places on this ammonia as a fuel proposal.

It will, of course, work, if - a huge caveat - ammonia can be made by climate change gas free means.

This said, I am strongly opposed to the idea of ammonia as a fuel, because the environmental and safety issues associated with distributed use of ammonia are too dire.

As is the case with dangerous natural gas - which is by the way the chief starting material for ammonia synthesis right now - it is inevitable that some will escape, or leak catastrophically in some places.

We need ammonia for food, but current demand is on the order of a few hundred million tons. The environmental fate of ammonia includes the microbial generation of nitrous oxide, which is currently a major contributor to ozone depletion. Concentrations are rising, and it's expected to displace residual CFC's as the primary ozone depleting agent this century. Other elements of the environmental fate include nitrates, nitric acid, and other nitrogen oxides. Ammonia and ammonium salts are already responsible for the destruction of major bodies of water through eutrophication. Of course an ammonia leak in an ammonia fueled automotive accident could kill or at least blind anyone near it.

I shudder to think what would be involved in billions of tons of ammonia as a fuel.

If we must store hydrogen as a small simple molecule, DME, dimethyl ether is vastly superior to ammonia, and in fact, any other molecule I can imagine. It's a wonder fuel, and it's insane that we are not well on the way to phasing it in.. It has a high critical temperature and is easily liquified, burns cleanly lacking a carbon-carbon bond and has an atmospheric half life of about 5 days. With simple modifications it works in diesel engines, can displace so called "natural" gas in all applications, as well as LPG. It is an excellent refrigerant, albeit a flammable one. It is non-toxic (except for a mild anesthetic effect) and it can be made by direct hydrogenation of carbon dioxide.