Molecular chemisorption at well-defined Pd(111) electrode surfaces: hydroquinone sulfonate studied by UHV-EC-STM

The chemisorption of 2,5-dihydroxybenzenesulfonate (hydroquinone sulfonate, HQS) from dilute aqueous solutions, onto a well-defined Pd(111) electrode surface has been studied by a combination of Auger electron spectroscopy, high-resolution electron energy loss spectroscopy, scanning tunneling microscopy and electrochemistry. The results indicate that HQS is oxidatively chemisorbed and exists on the surface as benzoquinone sulfonate (BQS); it is oriented essentially parallel to the surface with a slight tilt due to the bulky SO₃⁻ group. The H⁺ counter-ions in the ionic adlayer can be exchanged reversibly and quantitatively by K⁺ or Cs⁺ ions without changes in the structure and coverage of the surface-anchored species.