Experimental Study Showed a Potential Means of Producing Green Ammonia Sustainably

Ammonia output is estimated to be over 200 megatons per year. This makes it the world's second-most-produced chemical, after only sulphuric acid.

The results of a combined experimental and computational investigation for a novel group of catalysts that produce ammonia under moderate circumstances seem promising.

Ammonia Production

Although there are various ways to make ammonia, the Haber-Bosch method is the most common, accounting for roughly 90% of overall output. In any event, Haber-Bosch and other industrial-scale manufacturing methods necessitate high temperatures (above 400°C) and high pressures (more than 150 bar). Certain conditions are required to break the strong bonds in nitrogen and react with hydrogen to generate ammonia (NH3).

These processes, which account for around 1% of world energy use, rely primarily on fossil fuels. As a result, ammonia is the most greenhouse gas-intensive chemical process on the planet, accounting for nearly 5% of total world CO2 emissions. Furthermore, ammonia demand is predicted to rise in the future, owing to its use in synthetic fertilizers required to feed a growing worldwide population.

"The production of ammonia, which is currently made in some of the world's largest factories, is one of the major challenges on the climate, energy, and food fronts," says Professor Tejs Vegge of DTU Energy and the VILLUM Center for the Science for Sustainable Fuels and Chemicals. "The only efficient way to make ammonia is under high temperatures and high pressure, using a carbon-based feedstock," he adds (V-Sustain). He collaborated with Professor Ping Chen of the Chinese Academy of Sciences' Dalian Institute of Chemical Physics (DICP).

"Nature is quite adept at producing ammonia at ambient pressure and temperatures in enzymes like nitrogenase," Tejs Vegge adds, "but the process is incredibly sluggish and impractical to scale up to the commercial output."

Figuring Out a Sustainable Option

For decades, scientists have been attempting to develop new and more environmentally friendly methods of producing ammonia. Tejs Vegge and his DTU colleagues, Dr. Jaysree Pan and Associate Professor Heine A. Hansen, have proposed a possible game-changer with a novel family of complex metal hydride catalysts that allowed them to achieve the sought mild-condition ammonia synthesis in collaboration with the DICP team. Their technology, they hope, might open the door for new and more sustainable ammonia manufacturing methods. Their findings were reported in the journal Nature Catalysis.

They can make ammonia at temperatures as low as 300°C and pressures as low as 1 bar with their method. The practical application of these catalysts shows promise in terms of small-scale ammonia synthesis using renewable energy. Catalysts working at pressures of roughly 50 bar and temperatures below 400°C would be required in such systems.

"We believe our research is unique in that this new class of catalysts lies somewhere between biological and industrial processes, with elements of both. It has something from the human, artificial, heterogeneous catalysis and something from enzymatic and homogeneous catalysis. It's an entirely new way of making ammonia, and we're combining the best of both worlds to lower the temperature and pressure."

Complex Chemical Composition

Essentially, their complex metal hydride catalysts (Li4RuH6 and Ba2RuH6) may catalyze the synthesis of ammonia from hydrogen (H2) and nitrogen (N2) (N2). Multiple ruthenium hydride complexes, [RuH6]4-rich in electrons and hydrogen, are used to reduce nitrogen. Between the center and the nitrogen, the hydrogen carries electrons and protons. The alkaline metals lithium and barium (Li/Ba) stabilize the chemical intermediates simultaneously. On the other hand, the process is very dynamic, with numerous elements of the complex serving several purposes. It took years to perform the computations alone.

"Everything is different from what we've seen before; for example, ruthenium is a well-known component in ammonia catalysis, but it's present in an other form and behaves differently. It's surrounded by hydrogen atoms and forms a hydride complex, allowing it to transfer hydrogen in a novel way. You could think of this catalyst as a symphony orchestra, where every part has to work together to make it work. The fascinating part

"As a scientist, finding a genuinely new mechanism that opens a door into a new world is very satisfying. However, it may also open up new possibilities for ammonia production to take place in a less energy-intensive way. The large factories of today are needed to make the production profitable. Our catalysts or similar concepts may enable production in smaller, decentralized factories.

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