jp5121146_si_001.pdf (513.66 kB)
Shear-Induced Mechanochemistry: Pushing Molecules Around
journal contribution
posted on 2015-04-02, 00:00 authored by Heather
L. Adams, Michael T. Garvey, Uma Shantini Ramasamy, Zhijiang Ye, Ashlie Martini, Wilfred T. TysoeThe molecular mechanisms by which
mechanical energy accelerates
a chemical reaction at sliding solid–solid interfaces are not
well understood because of the experimental difficulties in monitoring
chemical processes and their rates, and in controlling parameters
such as interfacial temperature. These issues are addressed by measuring
the shear-induced rate of methane formation from the decomposition
of adsorbed methyl thiolate species on copper in ultrahigh vacuum
(UHV), where the frictional heating is negligible. The effect of a
constant force F on the energy profile for thiolate
decomposition from density functional theory calculations is modeled
by superimposing a linear potential, V(x) = −Fx. This enables the change in activation
barrier to be calculated as a function of force. The mechanically
induced reaction rate is measured by sliding a ball over a methyl
thiolate-covered copper surface from the methane yield measured by
a mass spectrometer placed in the UHV chamber. Molecular dynamics
simulations reveal that a wide distribution of forces are exerted
on the thiolates and comparing the measured methyl thiolate decomposition
rate with the rate calculated by assuming a wide force distribution
reproduces the experimental data. This reveals that only a small proportion
of the adsorbed thiolates experience sufficiently high forces to reduce
the activation barrier to reproduce the experimentally measured rate
constant.