Electron paramagnetic resonance evidence for the formation


Electron paramagnetic resonance evidence for the formation of sulfur trioxide(-) ion by the oxidation of sulfur dioxide(-) ion on magnesium oxide ...
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Formation of SO3- by the Oxidation of S02- on MgO

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Electron Paramagnetic Resonance Evidence for the Formation of SO3- by the Oxidation of SO2- on MgO Y. Ben Taarit' and J. H. Lunsford* Department of Chemisfry, Texas A & M University, College Station, Texas 77843 (Received December 15, 19721 Publication costs assisted by the Environmental Protection Agency

Molecular oxygen reacts with a high concentration of SO2- ions on MgO forming stable SO3- radicals. The epr spectra of 33s03-is characterized by g = 2.0034 f 0.0003 with a , = 147 f 2 G and a 1 = 102 ZZI 2 G. The sp hybridization on the sulfur atom indicates a n O S 0 bond angle of 112". At 25" the ion appears to librate in a plane which includes the symmetry axis.

Introduction

were formed by uv irradiation (2537 A) of the MgO which was in an atmosphere of pure hydrogen. The S centers were subsequently allowed to react with purified N2O or SO2 producing 0- or SOz- on the surface.4-9 Alternately, the activated MgO was irradiated at room temperature for 72 hr in the presence of N2O which yielded adsorbed 0 3 as well as excess 0 2 and N2.10J1 Sulfur-33 enriched SO2 was prepared by reacting -3 mg of sulfur containing 25% 33s with an excess of pure 0 2 a t 450" for 1 hr. The SO2 was purified by the conventional freeze-pump technique prior to each adsorption step. The epr spectra were recorded a t X band either a t rooin temperature or at 77°K using a Model E-6S Varian spectrometer. The g values were determined relative to a phosphorous-doped silicon standard with g = 1.9987.

Reactions involving the SO2- ion on surfaces have been studied in an effort to determine the importance of this species in the catalytic oxidation and reduction of SOz. In the present work the reaction between adsorbed SOzions and molecular oxygen to form SO3- on MgO has been investigated. Che,2 as well as Kazanskii and coworker^,^ has detected by means of electron paramagnetic resonance (epr) spectroscopy the formation of S02- on TiO2, but no evidence is given for subsequent reactions leading to SOs-. To the contrary, it was reported that SO2- reacted with 0 2 over TiO2, forming 0 2 - plus diamagnetic ions.3 The SOz- ion has now been observed on a number of other reducing surfaces.4,5 Sulfur dioxide reacts with MgO a t room temperature to Results form sulfite and sulfinatq complexes, as indicated by their Upon addition of excess molecular oxygen to adsorbed infrared spectra.6 At elevated temperatures the infrared SOz- it was observed that the spectrum of SO2- deevidence suggests that the sulfite ions react with molecucreased and a nearly isotropic line with g = 2.0034 f lar oxygen to form bidentate sulfato complexes in addition 0.0003 was formed. Evidence will subsequently be given to strongly held S03.6 If trapped electrons are available at for the assignment of the new spectrum to the SOB- ion. the surface of the MgO, SO2 also reacts to form the stable The formation of SO3- was not reversible, i.e., the origiSOz- ion.4 The epr spectra of SOP- enriched with sulfurnal S02- signal could not be restored by evacuating the 33 and oxygen-17 have been used to demonstrate that the sample a t room temperature for over 4 hr. It should be binding forces between this ion and the surface are purely noted, however, that some SO2- remained unreacted as electrostatic. Provided the concentration of the S02- ion indicated by the g,, component which could still be dieis sufficiently high, it will readily react with molecular tected, though it was greatly reduced in amplitude. oxygen to form SO3- . The rate and extent of the reaction increased with an The epr spectrum of so3- has been well characterized increased concentration of the initial S02- ion. The SOsby Chantry and coworkers.7 In their studies the ion was ion could be generated from SO2- produced from S cenformed upon y-irradiation of a number of salts such as soters; however, the reaction was more extensive if SO2 a a s dium dithionate, which has a n unusually long S-S bond. (1) On leave from lnstitut de Recherches s u r la Catalyse, C.N R.S , \/11From the 335; hyperfine structure it was possible to comleurbanne, France. pare SO3- with the isoelectronic ions Po32- and c103-. (2) M. Che, Thesis, Universlty of Lyon, 1968. 13) A. I. Mashchenko. G. P. Pariiskii. and V. B. Kazanskii. Kinet. Kat,/.. More recently Lind and Kewleys also observed the spec9, 151 (1968). trum of SO3- upon y-irradiation of taurine. This natural (4) R. A. Schoonheydt and J. H. Lunsford, J. Phys. Chem., 76, 323 118771 amino acid in its zwitterion form ( H ~ N + - C H Z - C H ~ S O ~ - ) (5) V. M. Vorotyntsev, V . A. Shvets, and V. B. Kazanskii, Kinet. Katai., undergoes homolysis of the C-S bond, thus generating the 12, 1249 (1971). so3- ion. (6) A. J. Goodsel, M. J. D. Low, and N. Takezawa, Environ. Sci. Tech., I

.

\ . - . - I .

Experimental Section The magnesium oxide pellets were prepared in the manner as previously reported.4 The samples were degassed for 2 hr a t 350" and for l hr a t either 450 or 850". The final pressure in the vacuum system was 10-5 Torr. Trapped electrons a t the surface, known as S centers,

(7) (8) (9) (10) (11)

6, 268 (1972); R. A. Schoonheydt and J. H. Lunsford, J. Catai., 26, 261 (1972). G. W. Chantry, A. Horsfield, J. R. Morton, J. R. Rowlands. and D. H. Whiffen, Mol. Phys., 5, 233 (1962). G. Lind and R. Kewley, Can. J. Chem., 50, 43 (1972). N.-B. Wong and J. H. Lunsford, J. Chem. Phys., 55,3008 (1971). N.-B. Wong and J. H. Lunsford, J. Chem. Phys., 56, 2665 (1972); Y . Ben Taarit and J. H. Lunsford. Proc. Int. Congr. Catai., Cith, 1972 (1972). R. J. Cvetanovik, J. Chem. Phvs.. 23, 1203, 1208, 1375 (1955). The Journal of Physical Chemistry, Voi. 77, No. 11, 1973

den Taarit and J. H . Lunsford

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TABLE I : Epr Parameters for SOs-

2.0034 f 0.0003 SO3- on MgO (77'K) SO3- on MgO (25") 2.0034 f 0.0003 S03- in K Z C H Z ( S O ~ ) ~2.0036 ~ f 0.0007 SO3- in taurineb 2.0035 f 0.0004 a

x1

1-

all'

I A

Reference 7 .

147 f 2 102 f 2 102 I 2 121 d= 2 153 f 1 112 f 1 135 f 4 99 I 4

* Reference 8.

tem with microwave power. From the shape of the hyperfine lines of Figure 1 the values of a , and aLwere determined. The principal values of the hyperfine tensors are given in Table I. Under conditions of high modulation amplitude and gain it was even possible to detect the hyperfine structure from SO3- containing natural sulfur, which has only 0.74% sulfur-33. This means that the presence of this ion may be confirmed without resorting to the use of enriched

soz.

It is perhaps worthwhile to mention that 0- did not react with SO2 to form SO3-. This result was rather surprising in view of the observations that 0- reacts with 0 2 forming &-,lo with CO forming CO2-,12 with CzH4 forming C Z H ~ O -and , ~ ~with COz forming C03-.13 In this experiment 0- was first produced from N 2 0 and SO2 was then condensed above the sample which was a t 77°K. The sample was allowed to warm progressively to higher temperatures and the resulting spectra indicated that 0 reacted with SO2 forming SOz-. The same electron transfer step was detected upon treating SO2 with adsorbed 02-.

x1

Figure 1. Epr spectrum of SO3- on MgO: (a) recorded at 77"K,

( b ) recorded at 25".

first allowed to react with the ozonide ion to form a much higher SOz- concentration. Upon addition of S O z , the symmetric line a t g = 2.0034 appeared directly. In the latter case no additional 0 2 was needed since it is photochemically produced from NzO.11 When excess oxygen formed during the uv irradiation of N 2 0 was removed prior to adsorption of SOZ, only SO*- was observed and no oxidation occurred unless 0 2 was added back to the system. The oxidation of SOz- to SO3- was difficult on samples degassed at 850". This may be related to a decrease in the concentration of one type of SOzor to a decrease in the concentration of certain diamagnetic ions6 as will be discussed in a subsequent section. By using SO2 enriched to 25% in 33S(I = 3/2) four distinct hyperfine lines were observed. These lines in addition to the 3 2 s spectrum are depicted in Figures l a and l b . As noted by other investigators7ss the outer lines are shifted about 3 G downfield and the inner lines about 8 G downfield with respect to the 32s03-line. This shift in the hyperfine components is predicted by second-order terms in the spin Hamiltonian. The shape of the hyperfine spectra varied significantly with changes in the sample temperature. The spectrum of Figure l a was recorded with the sample a t 77"K, whereas the spectrum of Figure l b was recorded with the sample a t room temperature. In both cases it was relatively easy to saturate the spin sysThe Journal of Physical Chemistry, Vol. 77, No. 1 7 , 7973

Discussion g Tensor. According to a Walsh diagram7 for AB3 type molecules, So3- with 25 valence electrons should be pyramidal with CaV symmetry. The filling of the 2A1 ground state should be according to the sequence14

. . . (le)4(5 a1)2(2 e)4(3e)4(la2)2(6all1 The g tensor should reflect the axial symmetry of the anion. The g shifts for the perpendicular direction would be due to the excitation from the 2A1 ground state to the ZE state, either as

. . . (le)4(5a1)2(2e)3(3e)4(la2)'( 6a1)2 or a s

. . . (le)*(5a1)2(2e)4(3e)3(la,)2(6a,)2 For the parallel direction g shifts would be due to excitation from the ground state to the 2A2 state

. . (le)4(5al)2(2e)4((3e)4(laz)1(6al)2 In terms of energy the 6al orbital lies well separated from the 2e, 3e, and la2 orbitals; hence, only a small departure of the principal g values from the free electron value is expected. One would predict that g , and g, would be slightly greater than 2.0023, which is consistent with the observed value of 2.0034. Hyperfine Tensor. The hyperfine structure of Figure l a is in good agreement with that reported for the sos- ion, (12) C. Naccache, Chem. Phys. Lett., 3, 323 (1971). (13) A. J. Tench, T. Lawson, and J. F. J. Kibblewhite. J. Chem. SOC., Faraday Trans. 1, 68, 1169 (1972). (14) K. P. Dinse and K. Mobius, 2. Naturforsch. A, 23,695 (1968).

Formation of SO:,-by the Oxidation of SO?-on MgO

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TABLE I I: Comparison of Hyperfine Couplings, Spin Densities, and Bond Angles for SO3- in Different Environments SOa- on MgO

Isotropic coupling, Ais03 G Anisotropic coupling, 2P, G Spin density 3s

3P OS0 angle, degree

SO3- in K2CHz(S03)2

SO3taurine

117

126

111

30

27

24

0.12 0.51 112

0.13 0.46 111

in

0.1 1 0.41 111

thus confirming the identification of the ion on magnesium oxide. The tensor has nearly cylindrical symmetry, which is expected for the pyramidal SO3- ion. The direction of the unique axis is normal to the plane formed by the three oxygen atoms. The hyperfine tensor may be resolved into its isotropic part, Aim, and anisotropic part, ZP, in the usual manner. Upon assuming that the d orbital contribution to the wave function was negligible, the s and p character of the unpaired electron localized on the sulfur were calculated. Here, values of Ais0 = 970 G for a pure 3s orbital of 33s and 2/3 = 59 G for a pure 3p orbital were used.7J4 Results for so3- in different environments are given in Table I. The OS0 bond angle for this ion was calculated from the equation15

1.5 2x2

+3

2

where A2 is the ratio of the p to s character of the unpaired electron on the sulfur, This leads to a calculated bond angle of 112", which is very similar to that reported by Chantry, et al.7 Obviously the crystalline environment does not greatly distort the so3- ion. The spectrum of so3- in Figure l b is unique to MgO. It appears to suggest that a l > all, which yields a value for the 3p character of only 0.27. This drastic reduction in the 3p character is unlikely in view of the consistent parameters noted in Table I1 for SO3- in quite different environments. A more plausible explanation of the spectrum observed a t room temperature is evident if one assumes motion in a plane which includes the threefold symmetry axis. A libratnon such as depicted in Figure 2, would have the effect of averaging all and one a ~ c o m p o n e n tgiving a i . The otber a i component would become a,,'. The numerical va,lues of a i and all' given in Table I are in agreement with this model. Such motion could easily

Figure 2. Schematic representation of SOs- motion on the edge of a MgO crystallite. occur on edges of the MgO crystallites which are approximately 100 8, on a side.16 Similar motion for S3- on MgO was detected even a t 77"K.17 Reaction Mechanism. The epr results reported here clearly establish that SO2- may be oxidized to s03- with molecular oxygen. Furthermore, the ease and extent of ox.idation depend on the concentration of SOz- ions. This concentration effect may be interpreted in terms of a concerted process involving an intermediate such as SOz-...O-O-.SOz-,although the line width of the epr spectrum indicates that the paramagnetic ions are separated from one another by a distance greater than 5 A. I:t is more reasonable to assume that an intermediate such as S02-.-O-0...S032is formed, which then results in so3- and a diamagnetic SO42- ion. Experiments curren1;ly in progress on other types of surfaces may help to resolve this mechanistic problem.

Acknowledgment. The authors acknowledge the contributions of Dr. Robert Schoonheydt during the early pa:rt of this study. This investigation was supported by research Grant No. 801136, Air Pollution Control Office, Environmental Protection Agency. (15) p. W. Atkins and M. C. R. Symons, "The Structure of Inorganic Radicals," Elsevier, New York, N. Y., 1967. (16) P. J. Anderson and R. F. Horlock, Trans. Faraday SOC.,58, 1993 (1962);R. F. Horlock, P. L. Morgan, and P. J. Anderson, ibid., 5'3, 721 (1963). (17) J. H. Lunsford and D. P. Johnson,J. Chem. Phys., '58,2079(1973)

The Journal of Physical Chemistry, Vol. 77, No. 1 7 , 1973