Thomas B. Stewart

Laboratory studies of heterogeneous reactions of SO2

Abstract The rates of removal of gaseous SO2 over solids commonly found in urban aerosols were measured in the laboratory. A tubular flow reactor, in which the walls of an inner, concentric cylinder were coated with the solid of interest, was used in these studies. Analysis of the data, using models that specifically accounted for mass transport in the laboratory system, yielded collision efficiencies or the fraction of gas-solid collisions that are effective in removing SO2. Experimentally measured collision efficiencies for fresh solid coatings range from As the time of SO2 exposure increased we found that the rates of SO2 removal gradually diminished until, with prolonged exposure, the solids completely lost their ability to remove this species from the gas phase. The relative humidity of the reaction mixture was found to be important in determining the total amount, but not the initial rate, of SO2 uptake, with SO2 uptake increasing at higher humidities. Overall, selected solids removed up to several tenths of a gram of SO2 per gram solid from humidified reaction mixtures. Further reaction could be induced by exposure to small amounts of ammonia. The saturation type of behavior observed on prolonged exposure to SO2 led to the suspicion that fly ash materials examined in this study, as received, may already have undergone substantial reaction with SO2, before or during collection. Further experiments on these materials, involving washing those as received materials with distilled water to remove soluble sulfates, supported this contention. Of the six fly ash materials examined, initial collision efficiencies for four of these materials were increased by factors ranging from 2 to > 300 by the water pretreatment. The other two materials exhibited high initial collision efficiencies (~10−4) that were unaffected, to within experimental error, by the water washing. Atmospheric projection of results from this study suggests that freshly emitted aerosols can be quite effective in converting gaseous SO2 to particulate sulfate. The capacity limited nature of the reactions suggests that these processes will be most important at or near emission sources, although further, non-source interactions can be induced by atmospheric ammonia.

Laboratory measurement of SO2 deposition velocities on selected building materials and soils

Abstract Measurements of SO 2 deposition velocities have been carried out in the laboratory utilizing a cylindrical flow reactor. Analysis of data from these experiments was carried out using models that specifically account for diffusive transport in the system. Consequently, the resulting deposition velocities were independent of diffusion processes and represent the maximum removal rates that would be encountered in the environment under turbulent atmospheric conditions. The measured values range from 0.04 cm s −1 for asphalt to 2.5 cm s −1 for cement, and were found to be independent of SO 2 and oxygen concentrations, as well as relative humidity and total pressure. Prolonged exposure to SO 2 eventually destroyed the ability of the various solids to remove this species. Overall capacities were found to increase significantly at moderate relative humidities yielding values of 0.4–2.8 g SO 2 m −2 of solid in moist systems. Several experiments indicated that the reactivity of a solid subjected to prolonged SO 2 exposures could be restored by washing the surface with distilled water or exposing the spent solid to ammonia. Some implications of these findings with regard to the environment are discussed.

Chemical effects of Ne+ bombardment on the MoS2(0001) surface studied by high-resolution photoelectron spectroscopy

The effect of 1 keV Ne+ bombardment on the clean MoS2(0001)-1 × 1 surface with fluences between 4 × 1014 and 4 × 1016 Ne+/cm2 was studied using high-resolution photoelectron spectroscopy excited with synchrotron radiation. Spectra of the Mo 3d and S 2p core levels were measured with photon energies that ensured that the kinetic energy of the photoelectrons was the same, resulting in the same depth being probed for both core levels. For lower fluences (i.e., ≲2 × 1015 Ne+/cm2), S vacancy defect formation occurs in the MoS2 lattice, with the concurrent formation of a small amount (< 10%) of dispersed elemental molybdenum [Mo(0)]. For fluences greater than ∼l × 1016 Ne+/cm2, the Mo(0) is the predominant species in the surface region, while the remaining species consist of amorphous MoS2−x and polysulfide species. Valence band spectra taken with photon energies of 152 and 225 eV were consistent with the core level results. The movement of the valence band maximum toward the Fermi level indicated the formation of a metallic surface region. Annealing the sample to temperatures up to 1000 K resulted in the formation of metallic Mo coexisting, in approximately equal amounts, with reformed MoS2 in a surface with no long-range order as determined by LEED. Finally, a qualitative depth distribution of the chemical species present after Ne+ bombardment was determined by varying the photon energies used for the core level spectra. The results indicate that the preferential sputtering of sulfur over molybdenum occurs predominantly through a mechanism involving chemical bonding effects, specifically, through the preferential emission of polysulfide ions over other species in the bombarded region.

The chemical interaction of Mn with the MoS2(0001) surface studied by high‐resolution photoelectron spectroscopy

The interface produced by vapor deposition of Mn on the MoS2(0001) surface has been studied in situ by high‐resolution photoelectron spectroscopy using synchrotron radiation. The evolution of the Mo 3d, Mn 3p, and S 2p core levels and of the valence‐band spectra during growth of thin films (10–58 A) is consistent with partial conversion of the Mn overlayer to MnS via the overall reaction 2Mn+MoS2→2MnS+Mo. The persistence of the substrate components of the Mo 3d and S 2p spectra for thicknesses >35 A are consistent with the Volmer–Weber growth mode. Annealing a 58‐A film to 770 K resulted in an overlayer film consisting mostly of MnS coexisting with some metallic Mn. Analysis of the Mo 3d core levels indicates the production of a MoS2(0001) surface with S vacancy defects. Annealing to temperatures between 850 and 1040 K drove the reaction to completion (as shown by the valence band and Mn 3p core level spectra). Annealing of the sample to 1130 K resulted in uncovering the MoS2(0001) surface due to breakup ...

Measurements of spatial reactant and product concentrations in a flow reactor using laser‐induced fluorescence

A system capable of measuring heterogeneous and homogeneous reactions inside cylindrical flow reactors is described. In this system, the laser‐excited fluorescence of reactant gas molecules is used to measure the spatial concentration of these molecules down the length of a cylinder coated with a suitable catalyst. The laser beam can be placed either on axis or off axis of the cylinder to measure the gas distribution. From the spatial concentration and certain experimental parameters, the rate constant can be determined. This technique is more versatile than a gas sampling probe because the gas flow is not disturbed. Also, an instantaneous reading of the relative gas concentration and a high degree of sensitivity are obtained. This technique is applicable to most gases, but, for this study, it was used to examine NO2 solid reactions in polluted atmospheres.

Aromatic Solute Molecular Ion and Triplet State Formation in Polystyrene and Polymethylmethacrylate

Solutions of naphthalene‐d8 and N,N,N′,N′ tetramethyl‐p‐phenylenediamine (TMPD), singly and in combination, in polystyrene (PS) and polymethylmethacrylate (PMMA) were irradiated by 1.9‐MeV electrons at 86°K. The irradiation pulse had a half‐width of 0.7 sec. Examination of the absorption spectra of samples immediately after the irradiation pulse indicated that solute triplet state molecules and positive ion molecules were produced, with TMPD as the solute in both polymer hosts. With naphthalene, positive ions were observed only in PMMA, whereas triplet state molecules were observed in both polymers. Analysis of the data from the mixed solute samples indicates that the presence of 0.05M TMPD captures all of the mobile positive charge, even when the naphthalene concentration is as high as 0.2M. Surprisingly, the production of naphthalene triplet state molecules is only slightly affected by the presence of TMPD. Charge migration or delocalization over appreciable distances occurs in both polymers. The radiat...

Cylindrical flow reactor for the study of heterogeneous reactions of possible importance in polluted atmospheres

This paper discusses the construction of a cylinder flow reactor used for studying heterogeneous reactions, with particular reference to reactions involving NOx. This system incorporates methods of gas analysis such as optical absorption (NO2), chemiluminescent (NO or O3), gas chromatography (N2 or O2) and mass spectrometry (SO2, etc). The system was constructed such that experimental conditions could be varied over a large pressure range from 1 to 700 Torr with a relative humidity variation of 0–90%.

Heterogeneous Conversion of SO2 to Sulfate

Abstract Laboratory studies were conducted to measure the rates of interaction of SO2 with solids that are of interest in urban aerosols. Results from these studies indicate that, for most of the solids examined, SO2 removal occurs by capacity limited reactions that convert the SO2 to adsorbed sulfate. Atmospheric projections of the laboratory results suggest that these reactions will be most important at or near the emission source.

Positive Displacement Gas Circulating Pump

The design and operation of a simple gas circulating pump is described. This pump incorporates a piston with an O‐ring seal and hollow glass check valve and is capable of circulating gases over a large pressure range, 769–1 Torr. The pump could conceivably operate at lower pressure by changing the check valves. During pumping operations the leak rate was less than 1 μ (for 400–500 cycles). The maximum pumping speed is about 110 cm3/sec although greater pumping rates can be obtained from larger pumps. The pump is designed so that it can be easily disassembled for cleaning.