H 2 S adsorption on chromium, chromia, and gold/chromia surfaces: Photoemission studies

Abstract

The reaction of H 2 S with chromium, chromia, and Au/chromia films grown on a Pt͑111͒ crystal has been investigated using synchrotron-based high-resolution photoemission spectroscopy. At 300 K, H 2 S completely decomposes on polycrystalline chromium producing a chemisorbed layer of S that attenuates the Cr 3d valence features. No evidence was found for the formation of CrS x species. The dissociation of H 2 S on Cr 3 O 4 and Cr 2 O 3 films at room temperature produces a decrease of 0.3-0.8 eV in the work function of the surface and significant binding-energy shifts ͑0.2-0.6 eV͒ in the Cr 3 p core levels and Cr 3d features in the valence region. The rate of dissociation of H 2 S increases following the sequence: Cr 2 O 3 ϽCr 3 O 4 ϽCr. For chromium, the density of states near the Fermi level is large, and these states offer a better match in energy for electron acceptor or donor interactions with the frontier orbitals of H 2 S than the valence and conduction bands of the chromium oxides. This leads to a large dissociation probability for H 2 S on the metal, and a low dissociation probability for the molecule on the oxides. In the case of Cr 3 O 4 and Cr 2 O 3 , there is a correlation between the size of the band gap in the oxide and its reactivity toward H 2 S. The uptake of sulfur by the oxides significantly increases when they are ''promoted'' with gold. The Au/Cr 2 O 3 surfaces exhibit a unique electronic structure in the valence region and a larger ability to dissociate H 2 S than polycrystalline Au or pure Cr 2 O 3 . The results of ab initio SCF calculations for the adsorption of H 2 S on AuCr 4 O 6 and AuCr 10 O 15 clusters show a shift of electrons from the gold toward the oxide unit that enhances the strength of the Au(6s)↔H 2 S(5a 1 ,2b 1 ) bonding interactions and facilitates the decomposition of the molecule.

Adsorption and dissociation of H2S on Mo2C(001) surface-A first-principle study

“…The adsorption energy of sulfur atom on Mo 2 C(001) surface is weaker than that on Mo(100) of 185.85 kcal/mol, while much stronger than that on Fe(100) of 66.74 kcal/mol [14]. These results are also consistent with the experiment observations that S atoms remain on the surface even for the temperatures in excess of 800 K [55][56][57]. The binding energy between S and Mo 2 C(001) surface is much stronger than that SH/Mo 2 C(001) and H 2 S/Mo 2 C(001) (77.20 kcal/mol, 26.12 kcal/mol, respectively) at their most stable adsorption sites, indicating that the thermodymic driving force is large for H 2 S dissociation on Mo 2 C(001) surface.…”

“…The bonding of the H 2 S to the surface involves mainly the interaction of the sulfur lone-pair orbital with the surface Mo atom. The transfer of electrons from the surfaces into the LUMO of H 2 S is minimal due to the high energy of this molecular orbital [55]. While the electrons transferred from the sulfur's lone-pair orbital to the surface are larger than those to the surface.…”

Surface chemistry of catalysis by gold

“…Collaborative density functional theory calculations revealed that SO2 dissociation was greatly enhanced on gold when the particle is located above an oxygen vacancy. Rodriguez and co-workers found further evidence of support effects in their comparison of SO2 adsorption on Au/TiO2 as compared to Au/MgO (361). In the case of MgO as a support, no dissociation of SO2 was observed, although the desorption temperature was much higher than that of SO2 from an Au single crystal.…”

WSe 2 -contact metal interface chemistry and band alignment under high vacuum and ultra high vacuum deposition conditions

“…The 0.53 eV electron Schottky barrier measured in this work is in relatively close agreement with previous DFT predictions of electron Schottky barrier magnitude for Au contacts to monolayer (0.58 eV) and bilayer (0.66 eV) WSe 2 [8] [31] and Cr x Se y (Φ Cr2Se3 = 5.15 eV) [68] present at the Cr-WSe 2 interface could result in a similar FL position. Although no Cr x O y is detected following Cr deposition under UHV conditions, the electronic structure of chromium oxide varies substantially with composition [68][69][70] and therefore is likely to appreciably affect the FL position when Cr is deposited under HV conditions. Recently, n-type conduction has been reported for Cr contacts (HV) to multi-layer WSe 2 -based devices in contrast with the band alignment obtained in this work between WSe 2 and Cr deposited under UHV conditions [71].…”