STRUCTURE, COMPOSITION, THERMAL-STABILITY AND ELECTROCHEMICAL REACTIVITY OF HS-(AQ)-DERIVED SPECIES CHEMISORBED AT PD(111) ELECTRODE SURFACES

The structure, composition, thermal stability, and electrochemical reactivity of the chemisorbed layer formed when a clean and ordered Pd(111) electrode is exposed to a dilute solution of Na2S buffered at pH 10 have been studied. Under these adsorption conditions, the predominant solution species is the bisulfide ion, HS(aq)-. Experimental measurements were based upon low-energy electron diffraction, Auger electron spectroscopy, and thermal desorption mass spectrometry in conjunction with classical electrochemical methods. The bisulfide ion is oxidatively chemisorbed onto Pd(111) as zerovalent sulfur which forms a highly ordered structure upon slight heating, Pd(111)(square root 7 x square root 7) R19.1-degrees-S, and coverage GAMMA = 1.07 +/- 0.06 nmol cm2. This particular adlattice is thermally stable up to 1100 K, the highest temperature investigated, or up to 900 K in the presence of 10(-6) Torr of O2(g). On the other hand, it is easily oxidized electrochemically to aqueous SO4(2-) ions. Application of negative potentials has no effect either on the structure or the composition of this chemisorbed layer. However, if copious amounts of electrogenerated H2(g) are present at the applied negative potentials, all the sulfur is hydrogenatively desorbed from the surface, most likely back to HS(aq)- species.