VARIABLE-ENERGY PHOTOELECTRON SPECTROSCOPIC STUDIES OF H2S CHEMISORPTION ON CU2O AND ZNO SINGLE-CRYSTAL SURFACES - HS- BONDING TO COPPER(I) AND ZINC(II) SITES RELATED TO CATALYTIC POISONING

Adsorption of H2S on the Cu2O(111), ZnO(0001), and ZnO(1010BAR) surfaces has been investigated using variable-energy photoelectron spectroscopy. At room temperature, all surfaces react to form sulfide. At low temperature (140 K) and low H2S coverages (0-0.3 L), H2S chemisorbed on the Cu2O(111) surface is found to completely dissociate, forming sulfide and hydroxide. Under these conditions, the sulfide S 3p photoemission band is observed at 10 eV below the vacuum level, and the S 2p3/2 and 2p1/2 core levels lie at 166.4 and 167.6 eV, respectively. At intermediate coverages (0.3-3.4 L), partial deprotonation of H2S yields HS- on the Cu2O(111) surface, resulting in three valence band photoemission peaks at 17.0 (Cu-S-H-sigma), 13.8 (Cu-S pseudo-sigma), and 10.3 eV (Cu-S-pi). At high coverages (> 3.4 L), H2S is molecularly adsorbed, producing valence band peaks at 13.9 (1b2), 12.0 (2a1), and 9.4 eV (1b1) and S 2p core features at 168.4 (2p3/2) and 169.6 eV (2p1/2). Alternatively, higher H2S exposures corresponding to an order of magnitude lower sticking probability are required for chemisorption on the ZnO surfaces at low temperature (140 K), and only coordinated HS- is observed. Thus, the reactivity of H2S on ZnO is fundamentally different than on Cu2O. The valence band photoemission spectrum of this system exhibits peaks at 18.2 (Zn-S-H-sigma), 13.8 (Zn-S pseudo-sigma), and 11.8 eV (Zn-S-pi), while the S 2p levels are found at 167.0 and 168.2 eV. Studies of the stabilized HS- intermediate on the two surfaces demonstrate significant variation in their electronic structures; a greater energy splitting is observed between the M-S pi and M-S pseudo-sigma valence peaks for HS- on Cu(I), indicating a stronger metal ion-SH- bonding interaction. In addition, variable-energy PES shows the presence of more metal 4s character in the Cu(I)-SH- sigma-bonding orbitals relative to the corresponding Zn(II)-SH- levels. These results show that the bonding is stronger and reactivity is higher for H2S on Cu(I) than on Zn(II) sites. The electronic origin of these differences in bonding and reactivity and its relation to catalytic poisoning are addressed on a molecular level.