(586f) Quantifying the Role of Subsurface Hydrogen on the H2-D2 Exchange Activity of AgXPd1-X Alloys

Exploration and validation of adsorption models is extremely important for heterogeneous catalysis since they give insight into how molecules interact with catalyst surfaces. Ag_xPd_1-x alloys are commonly used as solid permeation membranes for the purification of hydrogen from gas streams. While H₂ adsorbs dissociatively on Pd with a negligible energy barrier, adsorption of H₂ onto Ag surfaces is predicted to be endothermic. Nonetheless, alloying Ag with Pd yields hydrogen purification membranes and hydrogen catalysts that are superior to pure Pd. Recently, the importance of subsurface hydrogen (H’) in metallic catalysts has been explored. Some computational studies have shown that among the transition metals, Pd is the only one in which subsurface H’ atoms are more energetically favorable than gaseous H₂. Therefore, it is critical to consider the contribution of subsurface H’ when modelling the kinetics of catalytic reactions on Pd and Pd-based alloys. The kinetics of H₂-D₂ exchange have been independently measured over 90 Ag_xPd_1-x alloys that were all contained on a composition spread alloy film (CSAF) with a continuous composition range. Experiments were performed using a microreactor array that allowed measurement of the reaction rate at all alloy compositions across a temperature range of 333 – 593 K and at varying partial pressures of H₂ and D₂. Four different adsorption mechanisms have been considered: a Langmuir-Hinshelwood model, a breakthrough model invoking direct reaction between surface H(D) and subsurface H’(D’), and two models invoking activation of H(D) by H’(D’). Kinetic parameters for each model have been fit to the data. Results of parameter fitting show that the pre-exponents and energy barriers vary smoothly with alloy composition, and that even dilute amounts of Ag increase the adsorption energy barrier significantly. Quantification of kinetic parameters and adsorption model analysis for H₂ interaction with Ag_xPd_1-x alloys greatly aids the design of multicomponent catalysts.