Zundel- and Eigen-like Surface Hydrated Protons on Pt(111)

The interaction between adsorbed water and hydrogen on metallic surfaces is important for fundamental understanding of heterogeneous catalysis and electrode surface reactions in acidic environment. Here, we explore a long-standing question of whether hydronium ion can exist or not on a Pt surface coadsorbed with atomic hydrogen and water. Studies based on mass spectrometry and infrared spectroscopy show clear evidence that hydrogen atoms are converted into hydrated protons on a Pt(111) surface. The preferential structures of hydrated protons are identified as multiply hydrated $H_5O_2{^+}$ and $H_7O_3{^+}$ species rather than as hydronium ions. The multiply hydrated protons may be regarded as two dimensional zundel ($H_5O_2{^+}$) and Eigen cation ($H_7O_3{^+}$) in water-metal interface. These surface-bound hydrated protons may be key surface intermediates of the electrochemical interconversion between adsorbed hydrogen atoms and solvated protons.