SEMINAR

Conventional reactions are mostly driven by heat, light, and electricity to overcome activation barriers. They are named as thermochemistry, photochemistry, and electrochemistry, respectively. Likewise, chemical reactions induced by mechanical actions, such as abrasion, friction, cracking, colliding, and so on, are defined as mechanochemistry. The most representative tool for operating mechanochemistry is ball-milling, which can offer a new avenue for sustainable materials synthesis, including (1) single atom catalysts (SACs) from bulk metal balls, (2) methane from char coals and hydrogen, and (3) ammonia from nitrogen and hydrogen. Various SACs can be produced by a top-down mechanochemical abrasion method, in which the bulk metal balls (single atom precursors) are directly atomized onto different substrates, such as carbon frameworks, metal oxides, carbides, and nitrides.¹ Carbon frameworks, such as char coals in the presence of hydrogen, can also be efficiently converted into methane (carbon hydrogasification) via mechanochemical ball-milling.² The rate of carbon hydrogasification is four orders of magnitude higher than the conventional thermochemical method. Furthermore, ammonia has mainly been produced by the Haber-Bosch process over 110 years. However, it cannot be performed under mild conditions (near ambient temperature and pressure) for sustainable applications, because of thermodynamic reasons. We have discovered a new mechanochemical method for the ammonia synthesis from nitrogen and hydrogen in the presence of iron (Fe) under mild conditions (45 °C and 1 bar).³ With this new process with potassium (K) promoter, the final concentration of ammonia have reached as high as 94.5 vol%,⁴ which is nearly 4 times higher than the state-of-art Haber-Bosch process (~25 vol%) under harsh conditions (450 °C and 200 bar). The difference between conventional chemistry and mechanochemistry is likely to be their catalytic actions. For example, the catalytic role for the conventional chemistry is lowering activation barrier, while that for mechanochemistry is raising initial state. In addition, in the view point of catalyst, conventional chemistry is a static process, whereas mechanochemistry is a dynamic process. Mechanochemically induced high-density defects by violent dynamic mechanical actions are responsible for repeatedly maintaining high initial state to efficiently drive chemical changes.⁵

1. Han, et al., Nature Nanotechnology 17 (2022), 403-407.
2. Han, et al., Angew. Chem. Int. Ed. 61 (2022), e202117851.
3. Han, et al., Nature Nanotechnology 16 (2021), 325-330.
4. Kim & Han, et al., Nature Communications 14 (2023), 2319.
5. Han, et al., Science Advances 5 (2019), eaax8275.