Experimental Investigations of the Interaction of SO₂ with MgO

Abstract

High resolution adsorption isotherms, temperature programmed desorption (TPD), x-ray diffraction (XRD) and x-ray absorption near edge spectroscopy (XANES) methods were used to investigate the interaction of SO2 with high quality MgO powders. The results of these investigations indicate that when SO2 is deposited on MgO in monolayer quantities at temperatures near 100K both SOS and SO4 species form that are not removed by simply pumping on the pre-dosed samples at room temperature. TPD and XANES studies indicate that heating of pre-dosed MgO samples to temperatures above 350 "C is required for full removal of the SO3604 species. XANES measurements made as a function of film thickness indicate for coverages near monolayer completion that the SO4 species form first. IntroductionSulfur dioxide is one of the main pollutants released into the atmosphere as a result of volcanic activity, and the burning of sulfur bearing fossil fuels in automobile engines, industrial complexes, power plants and households. The subsequent interaction of the sulfur dioxide with air and atmospheric moisture results in the formation of "acid rain" leading to the corrosion of metals and degradation of stone buildings and statuary [ 11. To diminish the environmental effects of sulfur dioxide emissions one must either dissociate or remove the SO2 from the effluent. MgO and CaO are two materials that are widely used as commercial scrubbers in industry [2]. We describe our recent investigations using adsorption isotherms, TPD, XANES and XRD investigations of the interaction of SO2 with MgO.

Interaction of SO2 with MgO(100) and Cu/MgO(100): Decomposition Reactions and the Formation of SO3 and SO4

“…Studies using infrared spectroscopy show adsorbed SO 3 after exposing MgO powders to 1−5 Torr of SO 2 . 5a, This is similar to the behavior found above for the adsorption of large amounts of SO 2 on MgO(100). XANES showed the formation of SO 4 when MgO powders were exposed to very small doses of SO 2 , which is consistent with the data in Figure for SO 2 /Mg(100). In Figures and , one can see a SO 3 → SO 4 transformation at elevated temperatures and the qualitative trends in the two sets of experiments are similar.…”

“…In Figure , the S K-edge spectrum for Na 2 SO 4 exhibits a line shape similar to that found in the corresponding spectrum for MgSO 4 . The SO 4 2- ion belongs to the T d point group, and there are two main sets of unoccupied valence orbitals: a 1 * of S 3s character and t 2 * of S 3p character .…”

“…In Figures and , one can see a SO 3 → SO 4 transformation at elevated temperatures and the qualitative trends in the two sets of experiments are similar. Results of temperature-programmed desorption for SO 2 /MgO powders show desorption of SO 2 and SO 3 from 500 to 650 K. , …”

“…After exposing bulk powders of MgO to moderate pressures of SO 2 at 300−400 K, SO 3 appears as the main product (Figure and refs a, , ). A similar result is found for SO 2 adsorption on ZnO and BaO powders. , In contrast, under similar reaction conditions, SO 4 is formed on powders of “oxygen-rich oxides” such as CeO 2 , TiO 2 , Fe 2 O 3 , and V 2 O 5 .…”

TDDFT Calculations of NEXAFS Spectra of Model Systems for SO2 Adsorbed on the MgO (100) Surface

“…This system represents a model particularly suited for the calibration of the computational strategy because of the large number of experimental and theoretical investigations on the adsorption of SO 2 at oxide surfaces. The study of the complex chemistry of SO 2 on oxide surfaces − is of great importance because metal oxides offer a potential route for removing or destroying SO 2 , which is one of the most dangerous components of atmospheric pollution. MgO is frequently used in industry as scrubber to reduce SO 2 emission, and we have chosen it as the substrate for the adsorption because the MgO surface has been widely studied and well-characterized both experimentally − and computationally, − including several specific studies of SO 2 adsorption. , Spectroscopic techniques, such as infrared IR, , X-ray photoemission XPS, − and NEXAFS spectroscopies, − also give a substantial contribution to understand the molecular aspects of the interaction of SO 2 with the adsorption sites of the MgO surface and to characterize the SO x species formed after dosing SO 2 to the surface.…”