Adsorption and surface reactions of H 2S on clean and S-covered pt(111)

Abstract

The adsorption and decomposition of H 2S on the clean and S-covered Pt(111) surface has been characterized with a powerful combination of surface spectroscopies including: high-resolution electron energy-loss spectroscopy (HREELS), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). On the clean Pt(111) surface, a mixture of atomic sulfur, atomic hydrogen, two types of sulfhydryl (SH) groups and irreversibly bound H 2S species forms in the first layer at 110 K after a saturation H 2S exposure. On the (2 × 2)-S covered surface, an overlayer containing only one type of SH species and atomic hydrogen is found at 110 K after a saturation H 2S exposure. A physisorbed H 2S species is also present on both these surfaces. The decomposition of adsorbed H 2S on the clean and (2 × 2)-S covered surfaces is limited by the available H-adsorption sites; this in combination with H 2S island growth leads to the mixture of surface species on the clean surface at 110 K. On the ( 3 × 3 ) R30°- S covered surface, only a single physisorbed H 2S species is found that desorbs at ⪅ 125 K after a large H 2S exposure at 110 K. Upon heating the clean Pt surface saturated with H 2S, the two types of SH groups and the irreversibly bound H 2S species decompose at 185 K and form a (2 × 2) atomic sulfur lattice. The SH layer adsorbed on the (2 × 2)-S surface reacts through two competitive reaction pathways during heating: SH + H recombination at 200 K to form H 2S and SH decomposition at 210 K. The atomic sulfur deposited by SH decomposition at 210 K leads to the formation of a ( 3 × 3 ) R30°- S layer. The adsorbed SH in excess of the amount necessary to produce the ( 3 × 3 ) R30°- S layer is removed by reaction to form H 2S which desorbs in a reaction-limited peak.