J . Phys. Chem. 1990, 94. 3261-3265
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Nonaqueous Heterogeneous Oxidation of Sulfur Dioxide Judith Ann Halstead,*,+Roger Armstrong,* Bruce Pohlman,§Scott Sibley, and Robert Maier* Department of Chemistry and Physics, Skidmore College, Saratoga Springs, New York 12866, Department of Chemistry and Physics, Russell Sage College, Troy, New York 12180, and Department of Chemistry, Williams College, Williamstown, Massachusetts 01 267 (Received: September 29, 1989: In Final Form: February 16, 1990)
The nonaqueous oxidation of SO2 on various solid catalysts was investigated in the presence of oxygen and ozone. The amount of product formed was investigated as a function of catalyst mass and the duration of experiment. For alumina catalysts a surface saturation value of approximately 0.5 mg of S042-/m2of alumina was found for a variety of different aluminas independent of surface area, surface pH, and water content. This result corresponds to a monolayer of S042- if a surface area of 0.3 nm2/S042-is assumed. For an activated carbon catalyst a surface saturation value of 0.25 mg of S04*-/mZof carbon was found under wet nonaqueous conditions. For the activated carbon catalysts, unlike the alumina catalysts, the presence of at least a few layers of readily desorbable water is a significant variable for the oxidation of sulfur dioxide.
Introduction The study of reactions occurring at solid/gas interfaces is a subject both of fundamental theoretical importance and also an arca of increasing interest to atmospheric scientists. At the 1989 Second International Conference on Chemical Kinetics, heterogeneous reactions at solid/gas and liquid/gas interfaces received considerable attention in session discussions and invited papers.'-) The heterogeneous oxidation of SO2 at a solid/gas interface has been suggested as a potentially important pathway in the oxidation of SO2 in the atmosphere4" and has recently received increased attention by laboratory investigators. Studies of the nonaqueous heterogeneous oxidation of S02.at a solid/gas interface have focused mainly on reactions of SO2 with O2 on a carbon or soot surface7-Is with fewer investigations into the effect of fly ash and metal oxides.16-21 Novakov22has stated that "Soot catalyzed SO2 oxidation can proceed by two mechanisms: a "dry" mechanism, in the presence of water, and a "wet" mcchanism, when the soot particles are covered by a liquid water laycr". We prefer the terms "nonaqueous" and "aqueous" since the nonaqueous mechanism does proceed in the presence of water. (Scc Discussion and Conclusions section for further comments on the use of the terms aqueous and nonaqueous.) Previous investigators have stated that a dominant factor in the SO2 oxidation capacity at the catalyst is the surface pH and that this is true for a wide range of materials.12~'6~20 There is gcncral agreement that surface saturation occurs during the noand that the naqueous heterogeneous oxidation of SO>12.13,16*20-22 surfacc saturation value is independent of SO2 concentration.8*12*13.16921 However, the reported surface saturation capacities and thc reported effects of gas-phase relative humidity on this reaction vary widely. In the current work we have investigated the oxidation of SO2 on alumina, activated carbon, and other catalysts in the presence of oxygen, ozone, and water. For the alumina catalyst, the effects of surface acidity, surface area, water content. and supplier were investigated. The effect of water contcnt was also investigated with the activated carbon catalyst. In addition to the possible relevance of heterogeneous reactions to atmospheric chemistry, the role of solid/gas interfaces in chemical rcactions is a subject of theoretical interest. While the oxidation of SO2 by ozone to form SO3 is thermodynamically favorcd, it has not been observed in the gas phase.23 This is a likcly result of the fact that this reaction is forbidden by the Wigner rules for electronic spin conservation. It seems reasonable that rcstrictions like spin conservation may not apply in the an* Corresponding author.
'Skidmore College. 8
Russell Sage College. Undergraduate Research Assistants, Williams College. Undergraduate Research Assistant, Skidmore College.
0022-3654/90/2094-326l$02.50/0
isotropic environment of gas/solid interfaces particularly if an unpaired electron from the catalytic surface is involved.
Experimental Section A gaseous mixture of SO2 and oxidant was passed through four parallel plug-flow reactors, each containing a preweighed amount of catalyst. Each reactor consisted of a detachable catalyst chamber in which the solid catalyst was configured perpendicular to the flow direction, and a calibrated capillary outlet. The gas mixture used for the experiments represented in Figures 1 and 2 and Table I 1 had a mean composition of 290 f 20 ppm SO2, 640 f 65 ppm 03,2% N2, and 98% 02.The amount of nitrogen was determined from the flows through two calibrated rotameters. The average flow rate through each reactor was 13.5 f 0.8 cm3/min and the average flow rate through the entire system was 603 f 35 cm3/min with the remainder diverted through an analytical loop for SOz or O3 analysis. Under these conditions, 16.7 f 1 pmol of SO2 impinged on the catalyst per liter flow of gas mixturc. Experiment duration varied from I to 60 h. For thc high surface area catalysts (Figure 3 and Table 111) surface ( I ) Anderson, J. G.;Brune, W. H.; Twhey, D. W. Presented at the Second International Conference on Chemical Kinetics, NIST, 1989. (2) Tolbert, M. A.; Quinlan, M. A.; Golden, D. M. Presented at the Second International Conference on Chemical Kinetics, NIST, 1989. (3) Kolb, C. E.; Worsnop, D. R.; Zahniser, M. S. Presented at the Second International Conference on Chemical Kinetics, NIST. 1989. (4) Finlayson-Pitts, B. J.; Pitts, J. N. Atmospheric Chemistry: Fundanrentals and Experimen!al Techniques; Wiley: New York, 1986. ( 5 ) Baldwin, A. C. In!. J . Chem. Kinet. 1982, 14, 269. (6) Novakov, T.; Chang, S. G.;Hauker, B. Science 1974, /86,259. (7) Goto, S.; Morita, M. Chem. Eng. Commun. 1987,60, 253. (8) Harrison, R. M.; Pio, C. A. Atmos. Enuiron. 1983, 17, 1261. (9) Cofer, W . R. I l l ; Schryer, D. R.; Rogowski, R. S. Atmos. Enuiron. 1984, 18, 243. (IO) Cofer, W. R. I l l ; Schryer, D. R.; Rogowski, R. S. Atmos. Enoiron. 1981, 15, 1281. ( I I ) Cofer, W. R. I l l ; Schryer, D. R.; Rogowski, R. S. Atmos. Enoiron. 1980, 14, 571. ( I 2) Dlugi, R.; Jordan, S.;Lindemann, E. J . Aerosol Sci. 1981,12, 1985. (13) Britton, L. G.;Clarke, A. G. Atmos. Enoiron. 1980, 14, 829. ( 14) Tartarelli, R.; Davini. P.; Morelli, F.; Corsi, P. Atmos. Enuiron. 1978, 12. 283. (I 5 ) Otake. T.; Tone, S.; Yokota, Y.; Yoshimura, K. J. Chem. Eng. 1971, 4. 155.
(16) Dlugi, R.; Gusten, H. Aimos. Enuiron. 1983, 17, 1765. ( I 7) Davis, S. M.; Lunsford, J. H. J . Colloid Interface Sci. 1978,65,352. (18) Davis, S. M.; Lunsford, J. H. J . Enoiron. Sci. Heolth 1976,12, 735. (19) Haury, G.;Jordan, S.; Hofmann, C. A m o s . Enoiron. 1978,IZ.281. (20) Judeikis, H. S.;Stewart, T. B.; Wren, A. G. Atmos. Enuiron. 1978. I2. 1633. ( 2 1 ) Chun, C. K.; Quon, J. E. Enoiron. Sci. Technol. 1973,7 , 532. (22) Novakov, T. I n Heterogeneous Atmospheric Chemistry; American Geophysical Union: Washington, DC, 1982. (23) Calvert, J . G.;Stockwell, W. R. SO2,NO2 Oxidation Mechanisms: Aimusphrric Conriderations: Calvert, J. G..Ed.; Butterworth: Boston, 1984.
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saturation could not be reached in a reasonable time under these conditions. Longer experiments (up to 5 days) were conducted at higher SO2concentrations (200 f 5 pmol/L or 0.34%)and faster flows (18-300 cm3/min) through each reactor. For the experiments involving alumina catalysts, the 0, concentration was 0.5%. N o 0, was used for the faster flow experiments with activated carbon catalyst. All experiments were done at ambient C and 142 f 4 kPa. temperature, 19 f 2 ' After exposure to the gas mixture, the catalyst chamber and its contents were rinsed with several portions of doubly deionized water (DD), totaling 100 mL. The filtered recovery solution was analyzed for SO?-with a Dionex Series 40oOi ion chromatograph. I f substantial quantities of SO2remained on the catalyst unoxidized, with prompt analysis, we would expect to observe a SO,2peak in addition to the S042-peak. Product recovery solutions analyzed did not display SO,2- peaks whenever the catalyst was activated carbon or whenever the ozone was present if an alumina catalyst was used. Small quantities of SO3' were observed on occasion when the catalyst was an alumina and no ozone was present. Significant quantities of SO,2-were observed only for the few experiments conducted in the absence of added O2 (in a S 0 2 / N 2 mixture). Ozone analysis was carried out by classical titration of iodine formed in the aqueous KI "ozone traps", using standardized thiosulfate titrant and starch indicator. On-line collection and assays for 0, were carried out with no flow of SO2into the system. Similar titrations were performed on samples of catalyst which werc promptly immersed in aqueous KI after on-line exposure to an 03/02 flow at conditions comparable to those in actual experimential oxidation runs, but with SO2 absent. The latter titrations established that little or no 0,remained adsorbed on the catalyst when it was removed from the catalyst chamber and immersed in an aqueous solution. This result indicates that oxidation of SO2 by 0,did not take place as part of the analysis procedure. It has been established that uncatalyzed aqueous phase oxidation of SO2is too slow to take place to a significant degree under our analytical condition^.^^ Sources and some characteristics of the catalysts used are given in Table I . Surface area determinations except those of the Pt and Pd catalysts were done by Gary Moniz of Physical Sciences, Inc., using the N2 adsorption BET technique. The Pt and Pd surface area values were obtained from product sheets from the commercial vendors. Preparation and Analysis of Wet and Dry Samples. As purchased, Darco activated carbon-wet was gravimetrically determined to be 21% w / w water, corresponding to appoximately I layer of adsorbed water. Hereafter, activated carbon-wet, as received from the supplier, will be referred to as "carbon ( 2 1% watcr)". Samples were prepared which were 53% and 66% w/w water with the rcmainder Darco activated carbon. I f each water molcculc occupies25 m2 these moistened samples have approximately 4 and 8 layers of adsorbed water, respectively. Subsequent drying of subsamples confirmed that the water was uniformly distributed. Average weight losses of 51 f 0.3% and 66 f 0.34, respectively, were observed when the 53% and 66% w / w DD watcr were dried at 1 IO O C for 3-4 days. All six high surface area alumina catalysts were purchased as Brockman Activity I , indicating 0% water.26 Drying at 1 IO O C for 24 h confirmed that each of these catalysts had
