Quasi-equilibrium predictions of water desorption kinetics from rapidly- heated metal oxide surfaces

K Leung and LJ Criscenti and AC Robinson, JOURNAL OF PHYSICS-CONDENSED MATTER, 32, 335101 (2020).

DOI: 10.1088/1361-648X/ab85f6

Controlling sub-microsecond desorption of water and other impurities from electrode surfaces at high heating rates is crucial for pulsed power applications. Despite the short time scales involved, quasi- equilibrium ideas based on transition state theory (TST) and Arrhenius temperature dependence have been widely applied to fit desorption activation free energies. In this work, we apply molecular dynamics (MD) simulations in conjunction with equilibrium potential-of-mean-force (PMF) techniques to directly compute the activation free energies (Delta G*) associated with desorption of intact water molecules from Fe2O3 and Cr2O3 (0001) surfaces. The desorption free energy profiles are diffuse, without maxima, and have substantial dependences on temperature and surface water coverage. Incorporating the predicted Delta G* into an analytical form gives rate equations that are in reasonable agreement with non-equilibrium molecular dynamics desorption simulations. We also show that different Delta G* analytical functional forms which give similar predictions at a particular heating rate can yield desorption times that differ by up to a factor of four or more when the ramp rate is extrapolated by 8 orders of magnitude. This highlights the importance of constructing a physically-motivated Delta G* functional form to predict fast desorption kinetics.

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