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 hydrogen storage 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.