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J. Phys. Chem. B 1999, 103, 11308-11317
Chemisorption and Reaction of Sulfur Dioxide with Oxidized and Reduced Ceria Surfaces S. H. Overbury,* D. R. Mullins, D. R. Huntley,† and Lj. Kundakovic Oak Ridge National Laboratory, Oak Ridge, Tennessee ReceiVed: June 30, 1999; In Final Form: October 11, 1999
The interaction of SO2 with highly oriented ceria films is studied as a function of coverage, temperature, and degree of oxidation of the ceria. Soft X-ray photoemission (SXPS) is used to analyze the oxidation state of the ceria and the evolution of the SO2-derived species, while thermal desorption spectroscopy is used to analyze the desorption products. The principle interaction is chemisorption of SO2 at oxygen anions, which reversibly desorbs as SO2 over a broad temperature range from 200 to 600 K. This interaction is characterized as a Lewis acid-base interaction and occurs with approximately equal facility if the surface is hydroxylated or sulfided. On reduced ceria the adsorption is more heterogeneous, suggesting a variety of local bonding environments, including a small, distinguishable amount of adsorption at Ce3+ sites. A significant portion of the chemisorbed SO2 converts to sulfide above 300 K, and the resulting sulfide then equilibrates between bulk and surface sites above 600 K. Adsorption at a sulfide site leads to a 1.5 eV shift in the sulfide S 2p state toward higher binding energy. Thermal conversion of chemisorbed SO2 to sulfate is not observed. The results are compared with previous studies of NO adsorption on ceria films and with similar studies of SO2 chemisorption on titania.
1. Introduction The presence of S in most liquid and solid fuels automatically makes the control and conversion of SO2 and its effect upon emission control catalysts a problem of significant interest in mobile and stationary sources. The search for sulfur tolerant emission control systems in automobiles is driven by a need to maintain long-term efficiency in current three-way catalysts, the problems of H2S emissions, the high sensitivity of potential lean burn catalysts toward SO2, and problems of stable, deactivating sulfate formation on NO adsorbers.1 In current three-way convertors, ceria is added mainly for oxygen storage,2 but its function and activity is affected by reaction with SO2, leading to decreased oxygen storage capacity and H2S emissions.3,4 Ceria, doped ceria, and transition metal promoted ceria have also been shown to be effective catalysts for SO2 removal by reduction with CO and methane, suggesting applications for flue gas desulfurization in stationary sources.5,6 Although many of these problems have been circumvented, the details of the mechanisms of the relevant reactions are still not always clear. The interaction of SO2 with a reducible oxide under changing redox conditions can be complex. In addition to chemisorbing, SO2 has been reported to act either as a reductant or as an oxidant on ceria.5,7 Simultaneous redox processes may occur among SO2, the oxide, and other reactants. The behavior of SO2 may be expected to depend on the oxidation state or the presence of defects at the oxide surface. Previous results have indicated ceria is an effective adsorbent forming reversibly adsorbed sulfite.7 However, a residual amount of sulfur remains at high temperatures that is associated with the formation of sulfate. Evidence for this is derived from XPS, * To whom correspondence should be addressed. Address: Oak Ridge National Laboratory, P.O. Box 2008, Bldg. 4500 N, MS 6201, Oak Ridge, TN 37831-6201. E-mail:
[email protected]. Phone: 865 574 5040. Fax: 865 576 5235. † Current address: Saginaw Valley State University, Department of Chemistry, University Center, MI, 48710-0001.
FTIR, and thermal desorption spectroscopy (TPD).4,7 Interpretation of these results has been confused somewhat by questions of the stability of Ce(SO4)2, which has been reported to decompose at 468 K,4,8 although differential thermal analysis indicates that it is stable to at least 1273 K.9 It is claimed that SO2 reduces ceria at 773 K to form Ce2(SO4)3, which decomposes around 1173 K.10 Evidence of ceria reduction has been reported by UV-visible diffuse reflectance7 and Ce XPS measurements.10,11 We have applied soft X-ray photoelectron spectroscopy (SXPS) and thermal desorption spectroscopy (TPD) to study the adsorption of SO2 on a highly oriented CeO2(111). Our techniques for growing the oxide permits an ability to continuously alter the ratio of Ce3+/Ce4+ at the surface. Adsorption and reaction of SO2 is then studied as a function of this parameter as well as temperature and coverage. The use of a synchrotron source for photoemission studies allows a high level of sensitivity and resolution, enabling detection of small changes in the oxidation state of the sulfur species and quantitative measurement of the Ce3+/Ce4+ ratio. TPD permits correlation of desorbed species with changes in the oxidation state and surface species present upon the surface. Interestingly, we find that although it is difficult to induce sulfate formation or associated ceria reduction under any conditions, reduced ceria is readily oxidized by SO2, generating ceria oxysulfide. Possible causes for this unexpected result are discussed. 2. Experimental Methods These experiments were performed in two different ultrahigh vacuum (UHV) chambers. The soft X-ray photoemission experiments were conducted at the National Synchrotron Light Source at beamline X-1B using photon energies ranging from 290 to 600 eV but nominally 295 eV for the S 2p photoemission spectra. The end-station chamber contained a VSW EA125 electrostatic analyzer operated in the constant pass energy mode for photoemission measurements. Sample dosing of SO2 was
10.1021/jp992240a CCC: $18.00 © 1999 American Chemical Society Published on Web 12/04/1999
Chemisorption and Reaction of Sulfur Dioxide performed using a directed doser configured to enhance exposure of the front surface compared to the back and sides of the sample. The dosing rate was controlled by a laser-drilled effusion aperture and adjusted by the pressure in a gas ballast behind the aperture. A second independent and nearly identical directed doser was used for dosing H2O. A second UHV system was used for temperature-programmed desorption (TPD) measurements, performed using a 3 K/s linear heating rate and using 70 eV electron impact ionization for mass spectrometric detection. This system was equipped with XPS capabilities to permit characterization of the oxidation state of the surface utilizing the Ce 3d core levels. SO2 dosing was performed using a gas doser similar to that in the first UHV system. Achieving reproducible exposures of SO2 or H2O was problematic in both apparatuses because of adsorption within the dosing lines, introducing large errors in the exposures. Most measurements reported below are for large exposures (10-100 L) believed to yield “saturation” coverages. Ceria surfaces used were ultrathin films of ceria (
