Computational surface Pourbaix diagrams to unravel cathodic hydride formation on defective palladium surfaces

Defects, both intrinsic and hydrogen-induced, are commonplace in electrochemical processes, particularly in catalysis where hydrogen can penetrate the catalysts and influence chemical reactions. Palladium (Pd), renowned for its high hydrogen permeability, forms defects upon exposure to hydrogen. Herein, we investigate various defective Pd-surfaces containing missing row, vacancy, and adatom defects, and their interplay with hydrogen atoms to enhance our understanding of Pd-based catalysts during hydrogenation reactions or with Pd as a cathode. Low-index defective surfaces and various hydrogen (H) coverages are explored to construct surface Pourbaix diagrams (SPD) and study their H-termination at specific pH and potential. The SPDs show increased hydrogen adsorption upon lowering the electrode potential. The stability of defective surfaces follows Pd'(110) > Pd'(100) > Pd'(111), in contrast to the stability trend observed for pristine surfaces, Pd(111) > Pd(100) > Pd (110). This reversal is attributed to the tendency of 'less stable' open surfaces, such as Pd(110), to reconstruct, or be stabilized by hydrogen. Our study emphasizes the importance of hydrogen sublayers in stabilizing H-covered defective surfaces, which facilitates the formation of Pd vacancies in the sublayers. Our work is essential to advance catalysis and surface science, as it provides valuable insights into material restructuring under electrocatalytic environments.