(766c) Hydrogen-Release Kinetics From Pd(100): The Influence of H In the Surface, Subsurface, and Bulk States

Understanding the interactions of hydrogen atoms on the
surface and within the subsurface regions of Pd is critical to the development
of advanced energy technologies for hydrogenstorage and
separations, as well as catalytic processes.  While many of the physical,
chemical, and electronic properties of the H₂-Pd system are known,
the kinetics and thermodynamics during absorption into the bulk, transport back
to the Pd surface, and desorption at low temperature remain unclear.  In this work,
the H₂ release kinetics from Pd were measured and modeled over a
range of exposure pressures and temperatures using temperature programmed
desorption.  To simulate the observed kinetic behaviors, a continuum-based
model was extended from other previously published work (M. Mavrikakis, et
al., J. Chem. Phys 105, 8398, 1996) to include activation barriers
for desorption and transport that are dependent on H concentration.  The use of
concentration dependent barriers improves the model's ability to predict the
experimental trends across temperatures ranging from 100 ? 600 K.  With the
model, the transition and intermediate states that control the net release rate
are also identified using a transient version of the degree of rate control (C.
Stegelmann, et al., JACS 131, 13563 (2009)).  It is shown
that many states are involved in the release process depending on the
temperature and hydrogen distribution in the Pd system, so that the net
activation barrier is defined by either a single reaction step or by states
that are at distant locations in the reaction coordinate.