Surface Sciencr and Environmental Catalysis

SURFACE SCIENCE AND ENVIRONMENTAL CATALYSIS

Metal and oxide catalysts play an important role in oil refining and reducing harmful emissions in motor vehicles. But these catalysts are deactivated by minute quantities of sulfur that are impurities in petroleum-derived feedstocks and fuels.
We are using x-rays and ultraviolet light at the Laboratory's National Synchrotron Light Source to examine the effects of sulfur on the structural, electronic and chemical properties of metal and oxide catalysts. Using the fundamental knowledge gained in these experiments, researchers may eventually be able to design catalysts that are sulfur-resistant and/or remove sulfur from crude oil. Preventing the so-called sulfur poisoning by either of these methods would save the chemical industry millions of dollars annually. It would also be a boon for the environment, as sulfur oxide pollutants produced during the burning of fuel would be reduced or eliminated. {Review article: “Interaction of Sulfur with Well-Defined Metal Oxide Surfaces: Unraveling the Mysteries behind Catalyst Poisoning.” Rodriguez, J. A., and Hrbek, J. Accts. Chem. Res. 32, 719-728 (1999)}

In a recent high-resolution photoemission study at the U7A beam line we investigated the adsorption and chemical reaction of SO2 with a Ru(001) surface. Six different sulfur species present on the metal surface were identified and their stabilities/reactivities evaluated, as summarized in Figure 1.
(T. Jirsak, J.A. Rodriguez, S.Chaturvedi and J. Hrbek, Surface Science 418 (1998) 8 )

While studying the S interaction with a series of Mo based bimetallic catalysts we discovered that the second metal can promote sulfidation of the Mo support. In absence of the second metal on the Mo surface, the chemisorbed S does not form molybdenum sulfide. The extent of promotion depends on the metal used (Figure 2A), and most importantly, this parameter correlates with theactivities of industrial catalysts used in the hydrodesulfurization of dibenzothiophene (Figure 2B).
(M. Kuhn, J.A. Rodriguez and J. Hrbek, Surf. Sci. 365 (1996) 53)
TiO2 is employed on a large industrial scale for the removal of H2S in the Claus process. Little is known about the elementary steps of the original Claus reaction (H2S + 1/2O2 6 H2O + 1/nSn) and its latter modification, where one third of the H2S is first burned (H2S + 3/2O2 6 SO2 + H2O) and SO2 reacts catalytically with the rest of the H2S (2H2S + SO2 6 2H2O + 3/nSn). Recently, we have carried out a detail investigation of the adsorption of sulfur on a TiO2(110) single crystal. On this surface sulfur can interact with several adsorption sites: titanium, in-plane oxygen, bridging oxygen, and vacancies in the bridging oxygen rows. Figure 3 shows S 2p photoemission spectra acquired after dosing S2 to a TiO2(110) surface at 300 K.

Fig.3 S 2p photoemission spectra for the adsorption of S2 on TiO2(110) [J.A. Rodriguez, J. Hrbek, J. Dvorak ,T. Jirsak and A. Maiti, Chem. Phys. Lett., submitted]
When similar experiments were done with a commercial source of X-rays, only a broad (~ 6 eV) peak was observed in the S 2p region. In experiments at beamline U7A of the NSLS, we were able to identify four different types of sulfur species on the TiO2(110) surface. For the smallest dose of S2 in Figure 3, a doublet is observed with the S 2p3/2 features at ~ 161.8 eV. This peak position corresponds to S atoms bonded to O vacancies in the bridging oxygen rows of the TiO2(110) surface. Additional dosing of S2 leads to a drastic change in the line shape of the S 2p spectrum. The resulting spectrum (a) is well-fitted by a set of four doublets with 2p3/2 components at 161.6 (s1), 162.8 (s2), 163.3 (s3) and 167 eV (s4). s1 is assigned to S atoms on O vacancies. The difference in binding energy between s2 and s3 is only ~ 0.3 eV, and we assign these features to S atoms bonded to the Ti rows of the surface. The s4 matches well S 2p binding energies observed for SOx species, and could be attributed to SO2 or SO3 (more likely) groups on the oxide. The amount of SOx species present is very small and probably generates near steps or defect sites of the surface. Most atoms in the bridging oxygen rows of the surface are not reactive enough and, therefore, no substantial amounts of SOx species are formed. New doses of S2 induce the growth of the s3 peak, which becomes dominant. The S 2p spectrum obtained after saturation of the surface with sulfur at 300 K (b in Figure 2) was well fitted by a set of four doublets (s1, s2, s3, s4 species). The s2, s3, and s4 species were not strongly bound to the surface and desorbed at temperatures below 500 K. On the other hand, the S bonded to the oxygen vacancies (s1) remained on the oxide up to temperatures as high as 800 K.
H2S and S2 mainly interact with the metal centers of TiO2. In contrast, SO2 preferentially reacts with the O centers forming SO3 and SO4 species. Figure 4 shows a S K-edge spectrum acquired after adsorbing SO2 on a TiO2(110) surface at 300 K.

Fig 4. S K-edge XANES spectra for the adsorption of SO2 on TiO2(110),MgO(100) and ZnO(0001)-O
No signal is seen for chemisorbed SO2 on the Ti cations, and the typical peak for SO4 appears near 2482 eV. For the adsorption of SO2 on MgO and ZnO, oxides also used frequently as sorbents/catalysts in desulfurization operations, one again finds SO4 or SO3 on the surface and no dissociation of the adsorbate. On these oxides, the Mg-SO2 and Zn-SO2 interactions are too weak to lead to substantial bonding or dissociation of the molecule. However, the Mg2+ and Zn2+ cations interact relatively well with H2S and S, as do the metal cations of TiO2.
Supported by the US Department of Energy (contract No. DE-AC02-98CH10886), Office of Basic Energy Sciences, Chemical Science Division