Fundamentals and Applications of Mechanocatalytic Processes
Mechanochemical processes use mechanical collisions in a ball mill or similar device to drive chemical reactions. The collisions can create transient surface sites with extraordinary catalytic activity and hot spots that are characterized by rapid local temperature rises followed by dissipation of heat to the environment. In addition, mechanical forces can create intimate contact between two solids, so that the conversion of a solid reactant over a solid catalyst becomes viable. The formation of hot spots is analyzed in a fundamental study of the conversion of CaCO3 to CaO . Based on models for the impact of the milling ball and heat dissipation, each collision can be modeled as a transient batch reactor. The rates of CO2 formation in a flow-through milling vessels are determined at different milling frequencies to validate the model. The impact of a 20 mm steel ball with a net velocity of 4.5 m/s results in a hot spot temperature of above 800 °C. These dynamic environments can be used for ammonia synthesis from the elements . During milling in a mixture of N2 and H2, titanium metal is converted into TiN. Additional collisions lead to the formation of reactive nitride species. During the decay of the hot spot, the system passes through a regime in which hydrogenation of reactive nitrides to ammonia is thermodynamically and kinetically feasible.
The ability to convert solid feedstock opens new possibilities for converting lignin  and waste plastics . For example, the depolymerization of poly(ethylene terephthalate) (PET) occurs readily when the polymer is milled with NaOH . After an initial period, in which monomers are produced at a constant rate, the reaction mixture is converted into a wax that coats the milling balls or is pressed into the sites of the milling vessel. After wax formation, the remaining polymers are converted much faster. The reaction kinetics are explained with a modified shrinking core model. For depolymerization of poly(ethylene) we demonstrate a process, in which the polymer is partially oxidized in random positions of the backbone to facilitate cleavage of C-C bonds 
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