J. Phys. Chem. B 2000, 104, 7439-7448
7439
Interaction of SO2 with MgO(100) and Cu/MgO(100): Decomposition Reactions and the Formation of SO3 and SO4 Jose´ A. Rodriguez,* Tomas Jirsak, Andrea Freitag, and John Z. Larese Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973
Amitesh Maiti Molecular Simulations Inc., 9685 Scranton Road, San Diego, California 92121 ReceiVed: March 1, 2000; In Final Form: May 25, 2000
Synchrotron-based high-resolution photoemission and first-principles density functional calculations (DFTGGA) were used to study the interaction of SO2 with clean and modified (OH, Oδ-, O vacancies, or Cu adatoms present) MgO(100) surfaces. The reaction of the molecule with pure and hydroxylated powders of MgO was investigated using X-ray absorption near-edge spectroscopy (XANES). At 100 K, the main product of the adsorption of sulfur dioxide on MgO(100) is sulfite (SO2,gas + Olattice f SO3,ads). No evidence is found for bonding of SO2 to Mg sites of the surface or decomposition of the molecule. DFT calculations show that a η3-S,O,O adsorption configuration leads to a SO3-like species, and this is much more stable than configurations which involve bonding to only Mg sites or formation of SO4. On a flat MgO(100) substrate, the formation of SO4 is not energetically viable. A SO3 f SO4 transformation is observed at temperatures between 150 and 450 K with a substantial reconstruction of the oxide surface. From 450 to 650 K, the adsorbed SO3/SO4 species decompose and SO2 desorbs back into gas phase. The presence of OH groups and Oδ- (δ < 2) species on MgO favors the formation of SO4 at the expense of SO3. On the other hand, the creation of O vacancies in MgO(100) by ion sputtering leads to decomposition of SO2. The chemistry of SO2 on Cu/MgO(100) surfaces is rich. At 150 K, the SO2 molecule chemisorbs intact on the supported Cu particles and forms SO3 on the oxide substrate. Heating to room temperature induces full decomposition of SO2 and the formation of SO4. The Cu adatoms facilitate the decomposition of SO2 by providing electronic states that are very efficient for interactions with the lowest unoccupied molecular orbital (S-O antibonding) of the molecule.
I. Introduction In our industrial society, sulfur dioxide (SO2) is frequently produced as a result of burning sulfur-containing impurities present in coals and fuels derived from petroleum.1,2 In the atmosphere SO2 contributes to air pollution and smog1. It is a major contributor to acid rainfall and constitutes a serious health hazard.1 This toxic gas can cause severe irritation in the skin, eyes, mucous membranes, and respiratory system. There is a clear need to mitigate the negative effects of SO2.1,3 Over the past 30 years several processes have been proposed and developed for the removal of SO2 from exhaust systems (DeSOx operations).2-4 There is still no universally acceptable solution to this problem.3,4 The implementation of new and more stringent regulations for the control of environmental pollution has encouraged the search for more efficient DeSOx operations.2,3 Metal oxides offer a potential route for the removal or destruction of SO2.2,4 They can be useful as “throw-away” sorbents or can be combined with metals to generate catalysts for the reduction of sulfur dioxide to elemental sulfur (2CO + SO2 f 2CO2 + Ssolid or 2H2S + SO2 f 2H2O + 3Ssolid). Magnesium oxide is a well-known sorbent in industrial processes.4,5 On the other hand, copper supported on zeolites,6 ceria,7 and alumina8 is a catalyst for the destruction of SO2. It is not clear how these interesting systems operate at a * Corresponding author. E-mail:
[email protected].
microscopic or molecular level. Thus, it is necessary to establish the relative importance of the oxideTSO2 and copperTSO2 interactions and identify ways to facilitate or promote the dissociation of S-O bonds. The Cu/MgO(100) system has been the subject of detailed studies in the past9-12 and, in this respect, is ideal for investigating the interaction of SO2 with copper supported on an oxide. In this article we study the adsorption of SO2 on well-defined MgO(100) and Cu/MgO(100) surfaces using synchrotron-based high-resolution photoemission and firstprinciples density-functional (DFT) calculations. X-ray absorption near-edge spectroscopy (XANES) is used to examine the chemistry of SO2 on (100)-oriented powders of MgO. Cu(111)13 and most transition-metal surfaces14-16 are able to dissociate sulfur dioxide (SO2,gas f Sads + 2Oads) at room temperature, and some SO3/SO4 species can be formed as a result of the reaction of SO2 with oxygen adatoms (SO2,gas + nOads f SO2+n,ads, n ) 1, 2). In principle, the chemistry of SO2 on a metal oxide can be very complex, because the molecule can interact with the metal (production of chemisorbed SO2) and/or oxygen centers (formation of SO3 or SO4).14,17 A few studies have been published examining the interaction of SO2 with well-defined surfaces of oxides: TiO2(110),18,19 TiO2(100),20 Ti2O3(101h2),21 Fe2O3(0001),22 V2O3(10ih2),23 V2O5(001),24 NiO(100),25 ZnO(0001h),26 and CeO2(111).27 The chemistry obserVed is strongly substrate dependent.20,26 Formation of SO3 and SO4 species has been found on TiO2(110), TiO2(100), and Fe2O3(0001).19,20,22 Only SO3 has been detected
10.1021/jp000806l CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000
7440 J. Phys. Chem. B, Vol. 104, No. 31, 2000 on ZnO(0001h) and CeO2(111).26,27 On Ti2O3(101h2), sulfur dioxide decomposes on the metal centers and oxidizes them to a mixture of TiS2 and TiO2.21 All these oxide systems are semiconductors17,28 and not very ionic compounds.17,29,30 On the other hand, magnesium oxide is an insulator17,28 and a highly ionic compound.30,31 Due to this difference, a detailed study of the behavior of SO2 on MgO(100) is worthwhile. II. Experimental and Theoretical Methods II.1. Work with MgO(100) and Metallic Mg. The photoemission experiments were performed in a conventional ultrahigh-vacuum (UHV) chamber (base pressure < 8 × 10-10 Torr) located at the U7A beamline of the National Synchrotron Light Source (NSLS).12,26 This UHV system is equipped with optics for low-energy electron diffraction (LEED), a quadrupole mass spectrometer for thermal desorption mass spectroscopy (TDS), an X-ray source (dual anode with Al or Mg KR radiation), and a hemispherical electron energy analyzer with multichannel detection for photoemission studies. The S 2p spectra reported in section III were obtained using the beam from the synchrotron with a photon energy of 260 eV. The binding energy of these spectra was calibrated by the position of the Fermi edge in the valence region. Al KR radiation (1486.6 eV) was used to take Mg 1s and 2p, O 1s, Cu 2p, and S 2p data in X-ray photoelectron spectroscopy (XPS). The binding energy scale in XPS was calibrated by setting the Cu 2p3/2 peak of a thick Cu multilayer at a binding energy of 932.5 eV.12 MgO(100) thin films were grown on a clean Mo(100) substrate using procedures described in detail elsewhere.32,33 Mg was vapor deposited onto the Mo(100) surface at 550-600 K in a 1 × 10-6 Torr O2 atmosphere, followed by heating to 1200 K in 1 × 10-6 Torr O2. The Mg doser consisted of a Mg ribbon (99.998% purity) placed on a Ta crucible which was heated resistively. The oxidation of Mg was monitored using XPS.12,34 A (1 × 1) LEED pattern from the MgO(100) films was observed, indicating epitaxial growth with respect to the Mo(100) substrate.32,33 From the relative intensities of the Mg 1s and Mo 3d XPS signals,34,35 we estimate that the oxide films used in this work have a thickness of ∼25 Å. Furthermore, after examining the MgO films in the synchrotron with a photon energy of 380 eV (i.e., only two to three layers near the surface were probed in the photoemission experiments), no signal was seen for the Mo 3d levels, indicating that the films grew continuously and did not expose the Mo substrate. This was valid even after briefly sputtering some of the MgO(100) films with Ar+ ions to produce oxygen vancancies. Previous studies have shown that this type of oxide film is very useful for examining the surface chemistry of small molecules on MgO.12,35-37 Experiments were also carried out with polycrystalline films (>10 ML in thickness) of pure metallic Mg prepared by vapor depositing the metal on the Mo(100) substrate under a UHV (
