Formation of sulfur dioxide anions on alumina

Formation of sulfur dioxide anions on alumina

JOURNAL OF CI\TALYSIS 50, 181-18R Formation (1977) of Sulfur Dioxide Anions on Alumina Bcforc exposure to SOZ, each treated sample showed a smal...

185KB Sizes 0 Downloads 10 Views

JOURNAL

OF CI\TALYSIS

50,

181-18R

Formation

(1977)

of Sulfur Dioxide Anions on Alumina Bcforc exposure to SOZ, each treated sample showed a small peak at q = 2.00, but the intensity was negligibly small compared to that described below for sulfur dioxide anions. After exposure to sulfur dioxide, the sample showed a strong ESR signal with axially symmetric g values of g,, = 2.0075 and g1 = 2.0023. These g values agree with those for SOZ- on zeolites (9-11). The amount of SOZ- increased with the adsorption time until 40 min and then remained constant when samples evacuated at 500°C for 5 hr were exposed to SOZ at 8 mm Hg and various temperatures. The effect of adsorption temperature on the amount of SOZ- was examined. The sample calcined at 500°C was exposed to SOZ vapor for 40 min, and then the ESR measurements were carried out at room temperature. The result is shown in Fig. 1. The amount of radical increases with adsorption temperature, goes through a maximum at 24O”C, and decreases with further elevation of adsorption temperature. The effect of the pretreatment temperature on the SOZ- formation was investigated by the same procedure ; samples were evacuated at various temperatures, 2401150%, for 5 hr and were exposed to SOZ at 240°C for 40 min. The results are shown in Fig. 2. The amount of radical increases considerably when the temperature is raised from 500 to 700°C. It reaches a maximum of 4.8 X lOI spins/g around 700°C and decreases above this temperature. Approximately 5.0 X 1Olgmolecules/g of SOa adsorb on the alumina at 240°C. Thus, about 10% of the sulfur dioxide

Alumina is well known to have both electron-acceptor and electron-donor sites, which are capable of converting adsorbed molecules into the corresponding radical cations and anions, respectively. Electron spin resonance studies of the formation and properties of adsorbed radicals (1-11) have given valuable information on the surface properties. The adsorbates used so far were aromatic hydrocarbons (I), aromatic amines (.2), nitrobenzenes (s-5), and so on (6’-8), which are solid or liquid at room temperature. Recently, it was reported that sulfur dioxide, a simple gaseous molecule, forms its anion radical when adsorbed on decationated zeolites (9-11). The present study deals with the formation of sulfur dioxide anions (SOZ-) over alumina,. The conditions of the formation of SW and the thermal stability on alumina have been studied. Alumina sol from Nissan Kagaku was boiled in deionized water for several hours and was dried at 100°C. A 50-mg sample was then placed in an ESR sample tube with an inner diameter of 5 mm and heated at 120°C for 1 hr and then at 240-1150°C for 5 hr under high vacuum. The sample was then exposed to sulfur dioxide vapor at 8 mm Hg (1.1 X lo3 Pa) at lOO-500°C for 10-60 min. ESR measurements were carried out at room temperature wit.h a JEOL-BE-1X spectrometer with lOO-kHz magnetic modulation. Radical concentrations were estimated by comparison with the standard solution of l,l-diphenyl-2picrylhydrazyl in benzene. 181 Copyright All rights

0 1977 by Academic Press, Inc. of reproduction in any form reserved.

ISSN

0021-9517

182

NOTES

Adsorption

temp.

%

Fm. 1. Effect of SO2 adsorption temperature on the formation of SOP- radicals on alumina; pretreatment temperature : 500°C.

molecules were converted to their anion radicals. The surface area determined by the BET method increased with the pretreatment temperature, passing through a maximum of 184 m2/g at 500°C and decreasing at higher t,emperatures to 9 m2/g at 1150°C. However, the amount of radical divided by the surface area still increases with the pretreatment temperature up to 700°C and remains ra.ther constant, though it decreases slightly at higher temperatures (see Fig. 2). The maximum value was 3.6 X 1012 spins/cm2, obtained at 700°C. Therefore, the increase in the amount of radical with pretreatment temperature (500-700°C) is mainly ascribed to the

formation of electron-donor sites on alumina by heating in vacua, and the considerable decrease above 800°C is due mainly to the decrease in the surface area caused by the irreversible change of the alumina phase from r-Al203 to a-A1203. Indeed, X-ray diffraction of the sample heated at 1200°C showed the pattern of ~A1203. These results are qualitatively consistent with the effects of pretreatment temperature on the formation of trinitrobenzene (4) or tetracyanoethylene (6, 7) anions on alumina. The surface site responsible for the charge transfer is probably the oxide-ion defect produced by the dehydration of surface hydroxyl groups (7). The thermal stability of SOZ- was examined by evacuating the system at various temperatures. At room temperature the radicals were ent.irely stable for 24 hr under high vacuum, and at 200°C they were still stable with the almost same intensity for 400 min. But, above 35O”C, the ESR signal decreased with the evacuation time. The decay rate of SO2- on alumina follows approximately secondorder kinet,ics, with an activation energy of about 170 kJ/mol. ACKNOWLEDGMENT We wish to thank Professor T. Keii for helpful discussions.

N

I

REFERENCES 1. Flockhart, B. D., Scott, J. A. N., and Pink, R. C., Trans. Faraday Sot. 62, 730 (1966). 8. Flockhart, B. D., Mollan, P. A. F., and Pink, R. C., J. Chem. Sot. Faraday Trans. I 71, 1192 (1975). S. Flockhart, B. D., Leith, I. R., and Pink, R. C., J. Catal. 9, 45 (1967). 4. Flockhart, B. D., Leith, I. R., and Pink, R. C., Activation

Temp.

‘C

FIG. 2. Effect of pretreatment temperature on the formation of SOS- radicals on alumina. a: (0) number of SOZ- radicals divided by the weight of alumina (-) ;zb: ( l ) number of SOZ- radicals divided by the surface area of alumina (- - -).

Trans.

Faraday

Sot.

66, 469

(1970).

6. Flockhart,, B. D., Naccache, C., Scott, J. A. N., and Pink, R. C., Chem. Commun., 238 (1965). 6. Naccache, C., Kodratoff, Y., Pink, R. C., and Imelik, B., J. Chem. Phys. 63, 341 (1966). 7. Flockhart, B. D., Leith, I. R., and Pink, R. C., Trans. Faraday Sot. 65, 542 (1969).

183

NOTES 8. Bodrikov, I., Khulbe, K. C., and Mann, R. S., J. Catal. 43, 339 (1976). 9. Tokunaga, H., Ono, Y., and Keii, T., Bull. Chem. Sot. Japan 4.5, 3362 (1973). 10. Ono, Y., Tokunaga, H., and Keii, T., J. Phys. Chem. 79, 752 (1975). 11. Ono, Y., Kaneko, M., Kogo, K., Takayanagi, H., and Keii, T., J. Chem. Sot. Faraday Trans. I 72, 2150 (1976).

HIROYUKI SHUN-ICHI

YOSHIO ON0 TAKAGIWA FUKUZUMI

Department of Chemical Engineering Tokyo Institute of Technology Ookayama, Meguro-ku, Tokyo, Japan Received

December

29, 1976