The reign of the energy and greenhouse gas-intensive Haber–Bosch process continues as “king of the industrial ammonia synthesis castle”. Since its development more than a century ago at BASF in 1913, there have been many attempts by challengers to disrupt this robust technology through electrochemistry and photochemistry, seeking milder temperature and pressure experimental conditions as the modus operandi. A common thread in all post-Haber–Bosch attempts for a more eco-friendly ammonia synthesis is to drive the process with renewable electricity, whether thermo-, electro- or photochemically enabled.
In an eye-opening recent report it was demonstrated that the ammonia synthesis process could be powered mechanochemically in a simple ball mill laboratory experiment under the seemingly very gentle conditions of 45 °C and 1 atmosphere. The mechanochemical ammonia yield reported was found to be significantly higher than the Haber–Bosch.
The catalyst was earth abundant iron powder, the balls in the mill were made of hardened steel. It was run in a chemical looping mode. The endothermic N2 dissociation step was carried out under one set of ball mill conditions, and the exothermic H2 reduction step on another, in a strategy designed to circumvent the constraints of the thermodynamic equilibrium law or kinetic scaling law in catalytic ammonia synthesis.
Mechanical energy from iron powder-ball collisions provide the local heating and pressure and active sites needed for the nitrogen dissociation and the desorption of strongly adsorbed hydrogenated nitrogen intermediates. The ease and accessibility of this genre of mechanochemistry are elegant in their simplicity and impressive in terms of their technological ramifications.
Seems almost too good to be true, and indeed the devil could be in the details. The mass averaged NH3 yield indicates that mechanochemical chemical looping is of the same order as most photocatalytic and electrocatalytic ammonia syntheses, far below that reported for state-of-the-art thermocatalytic Ni/LaN and thermochemical looping Ni-BaH2 materials.
The fact that the industrial Haber–Bosch synthesis loop for N2 + 3H2 = 2NH3 is exothermic also poses challenges to the scale-up of the mechanochemical pathway in terms of the associated thermodynamics. This means not only the thermal management would be a challenge to the ball-milling ammonia synthesis process, namely, reactor cooling is required for an exothermic reaction requiring additional energy input, but also the energy efficiency is lower compared to that of the Haber–Bosch process. In fact, it would be astonishing to see a challenger to the high energy efficiency of around 70% for the Haber-Bosch process. The efficiency challenge in turn results in a capital investment problem as the major cost of producing ammonia is the energy cost.
The most eco-friendly way forward for a sustainable ammonia process through mechanochemistry, or indeed photochemistry or electrochemistry, is to power the processes with renewable energy. An envisioned application scenario for these green ammonia pathways integrates stand-alone, compartmentalized, self-contained, small ammonia synthesis modules powered by solar or wind electricity for distributed fertilizer production on farms, instead of massive centralized plants; the future for sustainable agriculture.
Article co-authored by Chengliang Mao
Reference: Gao-Feng Han, Mechanochemistry for ammonia synthesis under mild conditions, Nature Nanotechnology (2020), DOI: 10.1038/s41565-020-00809-9