Uptake, transport, and release of hydrogen from Pd(100)

a r t i c l e i n f o Understanding the interactions of hydrogen atoms with the surface and the subsurface regions of Pd is critical to the development of advanced energy technologies for hydrogen storage, hydrogen separations, and catalytic conversion processes. While many of the physical and chemical characteristics of the H 2 –Pd system are known, the kinetics and thermodynamics of H atom absorption into the bulk, transport from the bulk back to the surface, and desorption from the surface remain unclear. In this work, the kinetics of D 2 release from Pd following exposure to D 2 over a range of pressures and temperatures were measured using temperature programmed desorption. To accurately simulate the kinetics of D 2 release, the continuum-based model of Mavrikakis, et al. was extended to include activation barriers for desorption and transport that depend on D atom concentration in the surface, subsurface and bulk regions of the Pd. The use of concentration dependent barriers improves the ability of the model to predict the hydrogen uptake and release kinetics observed across temperatures ranging from 100 to 600 K. The ability of metallic Pd to dissociate H 2 and absorb H atoms has led to its use in a number of important technologies, including H 2 separation , H 2 storage, and heterogeneous catalysis. However, despite years of experimental and computational study of the Pd–H system, many details of the atomic level interactions between Pd and H remain unclear. Only a few studies have combined first-principles calculations and experimental data to develop kinetic and thermodynamic models of hydrogen uptake, transport, and release from Pd; even fewer models can predict the behavior of the Pd–H system over a wide range of temperatures and concentrations [1]. A significant challenge to the development of such models is a complete understanding of pathways and energetics for H penetration into bulk Pd. H-transport in solids is commonly described using a theoretical framework based on a set of mean-field reaction–diffusion equations. A historical perspective of the evolution of this approach is given by Pisarev et al. [2]. The most frequent application of this modeling scheme has been to describe H-permeation in separation membranes, storage devices, and nuclear reactor walls. Mean-field reaction–diffusion models customarily focus on either surface-limited or bulk-limited regimes depending on the temperature. For transport across a membrane, they stipulate equilibrium between H in the bulk metal and …