Surface Chemistry at the Solid-Solid Interface; Selectivity and Activity in Mechanochemical Reactions on Surfaces

Abstract

This paper summarizes the concepts that underpin the way in which forces exerted on chemical systems can either accelerate their rates or induce reaction pathways that cannot be accessed thermally. This was first described in 1935 by Evans and Polanyi who showed how hydrostatic pressure could accelerate the rates of chemical reactions using a thermodynamic analysis of transition-state theory to demonstrate that the reaction rate increased exponentially with pressure, and depends on a so-called activation volume. This is typically ∼17 Å3/molecule and indicates that pressures on the order of GPa′s are required to accelerate the rates of chemical reactions, which commonly occur at solid-solid interfaces such as found in a ball mill. We describe how surface mechanochemical reaction mechanisms are studied using the example of the shear-induced decomposition of carboxylates on copper. They thermally decompose on heating to ∼650 K to evolve carbon dioxide and deposit a hydrocarbon on the surface but shear stresses accelerate the rate of this process so that it occurs at room temperature. However, it is also found that forces parallel to the COO plane induce a non-thermal reaction pathway to evolve carbon monoxide and adsorbed oxygen on the surface. This reaction pathway can be quenched by using adsorbed benzoate species because the larger area of the aryl ring accentuates the tilt of the COO plane towards the surface to increase the rate of the thermal reaction.