Mechanical energy is a clean and abundant energy source in natural environments and plays an important role in the transition toward sustainability. Today, it is used as an energy input for chemical transformations such as biomass valorization and polymer depolymerization^[1,2]. In fact, the use of mechanical forces to induce chemical reactions has been recognized for more than a century as an important subdiscipline of chemistry: mechanochemistry^[3-6]. Similarly, tribochemistry specifically concerns chemical reactions in solids induced by frictional forces^[7,8]. Given the central role of catalysis in chemistry, mechanocatalysis^[9-11] and tribocatalysis^[12,13] have naturally emerged as branches of mechanochemistry and tribochemistry, respectively.

Mechanical energy is particularly effective in initiating reactions between solids. Because biomass and polymers are often insoluble in common solvents, mechanochemistry and mechanocatalysis have been widely applied to biomass valorization and polymer depolymerization^[1,2,14-17]. Research on tribochemistry and tribocatalysis, by contrast, has largely focused on reducing friction and even achieving superlubricity between solids^[18-20]. To date, therefore, mechanochemistry and tribochemistry have primarily addressed mechanically induced reactions between solid-state reactants, whereas the use of mechanical energy to drive chemical reactions in solutions remains comparatively under-explored. It should also be noted that, perhaps due to the difficulty of scaling up, ultrasound is generally not considered a mechanical energy source within traditional mechanochemistry^[5]; accordingly, ultrasound-induced reactions in solutions are not regarded as part of the field. Low-frequency mechanically stimulated reactions in solutions emerged much later. In 2019, tribocatalytic degradation of organic dyes was first reported using Ba_0.75Sr_0.25TiO₃ (BST) nanoparticles suspended in aqueous solutions and stimulated by magnetic stirring^[21]. In that system, BST nanoparticles absorbed mechanical energy through friction and converted it into chemical energy that drove dye degradation in the surrounding aqueous solution. This work established a new form of low-frequency, mechanically driven heterogeneous catalysis in solutions. In subsequent years, tribocatalytic performance has improved markedly, and important mechanistic insights have also emerged, broadening both the theoretical and practical scope of mechanochemistry. To clarify the development in recent years, this mini-review summarizes the key milestones in the evolution of magnetic stirring-stimulated tribocatalysis toward efficient heterogeneous catalysis in aqueous solutions.

Mechanochemical reactions are commonly conducted in closed milling devices, where transient “hot spots” above 1,000 K can form at impact sites^[22-24]. Because intermediate processes are difficult to monitor^[25-27], these devices are often treated as “black boxes”^[28,29], and mechanistic understanding of mechanochemistry, including mechanocatalysis, remains limited. In superlubricity-related tribocatalysis, the in situ formation of tribofilms under shear stress is highly sensitive to sliding speed, load, and humidity, further complicating mechanistic interpretation^[30-32]. Figure 1 summarizes the annual numbers of publications on mechanocatalysis and tribocatalysis identified in Web of Science. Although both terms have been used for more than 30 years, annual publication numbers have remained low, especially when compared with photocatalysis, reflecting the challenges associated with understanding conventional mechanocatalysis and tribocatalysis.

By contrast, the history of tribocatalysis shows a pronounced turning point in 2019, after which publication numbers rose rapidly. As discussed below, this change coincides with the emergence of heterogeneous tribocatalysis in aqueous solutions. This development not only extended the scope of mechanochemistry to low-frequency friction-driven reactions in solutions, but also provided valuable mechanistic insights into tribocatalysis.

Unlike semiconductors, for which tribocatalysis was first identified in aqueous systems, metallic materials were already known to exhibit mechanocatalytic activity^[72,73]. In particular, certain metals can mechanically catalyze reactions between solids when used as milling balls or milling vessels in ball-milling processes, an approach termed direct mechanocatalysis^[10,74]. Soon after tribocatalysis was identified in semiconductor materials in aqueous solutions, analogous tribocatalytic behavior was also reported for metals^[75].

Magnetic stirring with magnetic rotary disks has been instrumental in revealing the tribocatalysis of metals in aqueous solutions. Following the design of the PTFE magnetic rotary disk shown in Figure 4, metallic magnetic rotary disks were fabricated. As shown in Figure 12, four Ti disks (10 mm in diameter) were mounted on a larger PTFE disk (35 mm in diameter), with 18 magnets installed inside using the same design as the PTFE magnetic rotary disk^[76]. When placed in a beaker on a conventional magnetic stirrer, the resulting Ti magnetic rotary disk rotates against the beaker bottom, producing dynamic friction at the Ti-bottom interface. Replacing the Ti disks with other metals, such as Cu, Ni, Ag, or Pb, readily produces corresponding metallic rotary disks and allows analogous metal-bottom friction pairs to be investigated^[75].

Five types of metallic magnetic rotary disks - Cu, Ni, Ti, Ag, and Pb - were separately rotated in reactors containing H₂O and CO₂^[75]. Although H₂, CO, and CH₄ were detected in all cases, H₂ production from Cu, Ni, and Ti exceeded that from Ag and Pb by more than two orders of magnitude. Accordingly, Cu, Ni, and Ti were classified as tribocatalytically active, whereas Ag and Pb were regarded as inert. The tribocatalytic conversion of H₂O and CO₂ can also be strongly regulated by the material coating the reactor bottom. For a Ni magnetic rotary disk, for example, Figure 13 shows the gas products obtained with different coatings^[77]. A glass bottom gave the highest absolute yields of H₂ and CH₄, whereas a Ni-coated bottom produced the highest CH₄/H₂ and C₂H₆/H₂ ratios; C₂H₄ was detected only with a Ti-coated bottom. These results reveal substantial potential for using low-frequency friction with metallic materials to convert H₂O and CO₂ into flammable gases.

In one study of five metals (Cu, Ni, Ti, Ag, and Pb), three - Cu, Ni, and Ti - were found capable of mechanically catalyzing the conversion of H₂O and CO₂^[75]. The apparent generality of this behavior motivated a relatively simple mechanism for metal tribocatalysis. Taking a Ni magnetic rotary disk in water as an example^[77], magnetic stirring generates dynamic friction between the Ni disk and the reactor bottom, whether glass [Figure 14A] or Ni [Figure 14B]. Because the friction surfaces are microscopically rough, water can become trapped in surface micro-holes and compressed through the elastic deformation of Ni. Under the resulting elevated local pressure, Ni, as well as Cu and Ti^[75], can catalyze the conversion of H₂O and CO₂ trapped in these micro-holes. By contrast, Ag and Pb exhibit no catalytic activity under the same conditions, indicating intrinsic differences among metals in their catalytic behavior at high pressure. Importantly, this mechanism provides a second pathway - distinct from semiconductor tribocatalysis - by which friction can convert mechanical energy into chemical energy without relying on bulk heating or bond breaking in the solid. This absorption of frictional energy through elastic deformation lays the foundation for metal tribocatalysis, although it still requires more direct quantitative verification of the interfacial pressure and deformation.

In photocatalytic and electrocatalytic conversion of H₂O and CO₂, the catalysts are typically nanostructured materials, and their long-term stability remains a major barrier to practical application. By contrast, the ability of bulk metals to catalyze the conversion of H₂O and CO₂ into flammable gases under low-frequency friction^[75,77] suggests that metal tribocatalysis could provide a robust approach for large-scale H₂O and CO₂ conversion. This area therefore warrants much broader investigation, particularly for the conversion of H₂O and CO₂.

It is useful to compare tribocatalysis in semiconductors and metallic materials. First, semiconductors absorb frictional energy by exciting electron-hole pairs, whereas metallic materials absorb it through elastic deformation. Second, semiconductor tribocatalysis arises from the combined action of electrons, holes, and the reactive radicals they generate, whereas metallic-material tribocatalysis exploits the intrinsic catalytic activity of the metal under elevated local pressure. These two classes of tribocatalysis therefore differ fundamentally in mechanism. In terms of applications, semiconductor tribocatalysis is highly versatile and may be applicable to many processes conventionally addressed by photocatalysis, whereas metallic-material tribocatalysis appears particularly promising for H₂O and CO₂ conversion^[75-77]. Because tribocatalysis is application-oriented, selecting the appropriate material is of primary importance.

For convenient comparison, Table 1 summarizes several representative systems for which this mini-review has collected sufficient information on device, mechanical-input conditions, performance, and reaction time.

Since tribocatalytic degradation of organic dyes was first reported in 2019, the field has expanded rapidly, generating numerous studies of environmentally relevant reactions driven by mechanical energy. Beyond organic-pollutant degradation and the conversion of CO₂ and H₂O, proposed applications now include N₂ fixation^[78], H₂O₂ synthesis^[79], Li-ion battery recovery^[80], uranium extraction^[81], and bacterial sterilization^[82]. At the same time, important mechanistic insights have emerged into the direct conversion of mechanical energy into chemical energy through friction in solutions, and several broadly effective strategies for enhancing and regulating tribocatalysis have been established. Tribocatalysis is therefore expected to continue developing as an important tool for sustainable technologies.

Despite the growing number of studies on this emerging form of heterogeneous catalysis in aqueous solutions, many investigations have not yet incorporated the most effective design principles established in the field. For example, conventional magnetic stirring with a commercial PTFE magnetic rod is still widely used for tribocatalytic degradation of organic pollutants, even though its degradation efficiency is substantially lower than that achieved with the PTFE magnetic rotary disk introduced in 2022^[38]. Similarly, coatings on vessel bottoms have been shown to strongly influence tribocatalysis in many systems^[65], yet they remain unexplored in most current studies. Because tribocatalysis fundamentally depends on dynamic friction, future studies should continue to optimize the friction interface, while also improving the quantitative characterization of mechanical-energy input and standardizing reporting of device geometry and operating conditions. More direct mechanistic measurements, targeting organic pollutants beyond model dyes, evaluating energy efficiency, and scaling up will all be important for practical development.