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Creating a good ammonia catalyst from an unreactive metal

Creating a good ammonia catalyst from an unreactive metal

10 May 2024

The presence of lanthanum near a cobalt catalyst quenches the catalyst’s spin and lowers the activation energy required to split molecular nitrogen.

A manufacturing plant in Donaldsonville, Louisiana.
An ammonia production facility in Donaldsonville, Louisiana. Credit: CF Industries

Large manufacturing plants, such as the one shown above, can make hundreds of thousands of tons of ammonia each year using the Haber–Bosch process. At least 80% of that yield serves as crop fertilizer. Its synthesis consumes nearly 2% of the global energy output and, in the process, produces 500 million tons of carbon dioxide.

The synthesis is so energy intensive largely because the Haber–Bosch process requires brute force—high temperature (up to 650 °C) and high pressure (200 atmospheres)—to split apart the triply bonded nitrogen atoms, which then react with hydrogen over an iron catalyst. Iron is effective and inexpensive as an industrial catalyst. But chemical engineers have been searching for decades for alternatives to catalyze the reaction in milder, environmentally friendlier conditions.

Diagram of the mechanism for a cobalt catalyst with added lanthanum.
A spin-density depletion region (blue cloud) is pictured at the adsorption site of a transition state (TS*) of molecular nitrogen on lanthanum-promoted cobalt. The depletion region signifies the loss of cobalt’s magnetic moment. Once N2 dissociates, three individual hydrogen atoms (not shown) bond to each N atom as it diffuses along the surface until ammonia forms. Credit: Adapted from K. Zhang et al., Science 383, 1357 (2024)

For several years, researchers have been aware of alternatives to iron, such as nickel and cobalt. But both of those metals are normally unreactive with nitrogen and can be turned into effective catalysts only in the proximity of particular additives, known as promoters. A collaboration between Technical University of Denmark researchers, led by theorist Jens Nørskov and experimentalist Ib Chorkendorff, now propose a mechanism by which cobalt becomes a particularly good catalyst of ammonia in the presence of lanthanum.

According to the researchers’ model, lanthanum quenches cobalt’s magnetic moment, as shown in the diagram. The suppression of the local magnetism, in turn, lowers the activation energy for the dissociation of molecular nitrogen atop cobalt atoms and thus catalyzes their reaction with hydrogen. The team used pressure-cell experiments to demonstrate that the lanthanum-promoted cobalt catalyst outperforms the traditional iron-mediated ammonia synthesis by a factor of two—at least at atmospheric pressure and 350 °C, the conditions at which Ang Cao (Nørskov's former postdoc) predicted are optimal for lanthanum-promoted cobalt.

From her calculations, Cao predicted that step edges on cobalt crystals should be the most active sites for ammonia synthesis. In one set of experiments, Ke Zhang (Chorkendorff’s postdoc) evaporated lanthanum onto stepped-cobalt single crystals. In another, he produced cobalt nanoparticles that were then deposited onto lanthanum nitride thin films. By comparing the catalytic activities on the different surfaces, Zhang confirmed cobalt step edges as the sites where lanthanum atoms preferentially adsorb and where ammonia synthesis takes place.

Nørskov, Chorkendorff, and their colleagues don’t envision the lanthanum-promoted cobalt catalyst as an immediate replacement of the industrial Haber–Bosch ingredients. For one thing, cobalt is more expensive than iron, and lanthanum is more expensive than potassium oxide—the current promoter of choice in today’s industrial facilities. But the researchers haven’t considered every rare-earth element, and there may be others that are less expensive than lanthanum and just as effective. (K. Zhang et al., Science 383, 1357, 2024.)

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