Mechanism of hydrogen adsorption/absorption at thin Pd layers on Au(1 1 1)

Hydrogen adsorption and absorption at thin palladium deposits of 0.8-10 monolayers (ML) on Au(1 1 1) was studied in 0.1 M H₂SO₄ and HClO₄ using cyclic voltammetry, ac voltammetry, and impedance spectroscopy in the absence and in the presence of poison, crystal violet. Hydrogen adsorption on palladium is more reversible in sulfuric acid than in perchloric acid but it occurs at potentials 30 mV more positive in latter. The charge-transfer resistance exhibits a minimum at ∼0.27 V versus RHE and decreases with increasing in Pd deposit thickness in both acids. Adsorption capacitance at 0.8 ML Pd reaches maximum at the same potential. At other deposits the pseudo-capacitance starts to increase at lower overpotentials indicating the beginning of absorption, even at 2 ML Pd. The double layer capacitance is similar for all the deposits in sulfuric acid and it has a sharp maximum at 0.27 V versus RHE. In perchloric acid a broad maximum is observed. Crystal violet inhibits hydrogen adsorption but makes hydrogen absorption more reversible. The results suggest a fast direct hydrogen absorption mechanism that proceeds in parallel with slower hydrogen adsorption and indirect absorption.