Pollutant Trapping Studies 2i-Spontaneous and Resonance Raman Studies of the Surface Reaction of Sulphur Dioxide on Zeolites

Peter Tsai and Ralph P. Cooney Department of Chemistry, University of Newcastle, Newcastle, New South Wales, Australia 2308

Raman spectra of SOz adsorbed on zeolites X, Y and A are reported together with a normal coordinate analysis of the various spectra of the adsorbed molecular SOz. The data indicate that crystal-like SOz is adsorbed on cation sites XI and 1x1 of the zeolitic oxide framework. Internal SOz force constants quantifying adsorbate distortion are developed and compared with the frequency difference parameter, A(va - vl). Raman spectral data also indicate the formation of several decomposition products of SOz, generated by surface reactions, which may include S;, S,, S,, S,O, SO:- and SO:-. Of these the lines attributed to S;, S1, S4 and S,O exhibit resonance enhancement.

INTRODUCTION EXPERIMENTAL

Sulphur dioxide is an irritating and potentially toxic component of polluted air of industrialized and urban areas. Most of the sulphur dioxide found in the atmosphere has been generated by combustion of coal and oil products containing up to 5 % ~ u l p h u r . ~ Since these energy sources are used in many millions of tons per year, SOz released into the atmosphere also amounts to millions of tons. At low concentrations it is not harmful to humans or animals except under smog conditions. In the case of plants, concentrations of 1-2 ppm may prove damaging.4 Sulphur dioxide may be removed from flue gases by a variety of wet and dry processes'.' including absorption in liquids (e.g. DMSO) and adsorption on solid substrates (e.g. activated carbon). It can be recovered',' as S03 , elemental sulphur or H2S.

There have been several IR studies of the adsorption of SOz on oxide but the present Raman study of SOz sorbed on zeolites is the first of its kind. Hydrogen-bonded SOz, chemisorbed and physisorbed SOz, sulphites and sulphates are mentioned among the products of surface reaction^.^-^ Of direct relevance to the present study is the Raman investigation" of de- composition products formed from SO2 subjected to discharges and trapped in matrices. Under such condi- tions the lower oxides of sulphur (i.e. the ratio 0 : S < 2 : l ) , which are usually unstable and generally decom- pose to sulphur and stable oxides or polymers, are stabilized and have been trapped at low temperatures. From the complex vibrational spectrum, Hopkins et al. lo

have identified products which include SO3, SZO, S3, S4, O3 and a poly(su1phur)oxide.

The present investigation has been undertaken with a view to understanding in a more complete fashion the adsorption of SOz on zeolites. In addition, a set of surface reaction products has been detected which is closely related to that reported for the radiofrequenc and microwave decomposition of SO2 in the gas phase.

Spectrosc. 9, 33-38 (1980).

1 4

18

t For paper 1 in this series see R. P. Cooney and P. Tsai, J. Raman

The alkali cation-exchan ed zeolites were prepared as described previously. The pretreatment procedures of the zeolites and the Raman cell used were similar to previous studies' '-13 and involved an activation temperature of 500 "C. Sulphur dioxide (BDH Chem- icals Ltd, Poole, UK) was dried over P205 before being admitted to the adsorbents. The IR spectrum of the gas showed no evidence of impurity. Sulphur dioxide gas of pressure ca. 600mmHg was admitted to the zeolite samples at room temperature.

All Raman spectra were recorded at room tempera- ture on a Cary 81 laser Raman spectrometer coupled to a Coherent Radiation model 52G argon ion laser. The spectrometer was used (as in Part 1 of this study) in a 90" mode with the laser beam unfocused. Plasma radiation was removed by interference filters (Baird-Atomic). The laser was used in the light mode in order to minimize power fluctuations. Laser power of 50-200mW was incident on the surface and a spectral bandpass of 6 cm-' was typically used. Intense lines are considered accurate to better than *2 cm-'.

The normal coordinate analysis programs and the various precautionary test calculations employed have been described previ0us1y.l~ As a further check, the calculation of force constants of SO2 produced a result in excellent accord with those given by Polo and W i l ~ o n ' ~ when using the same input frequencies and force field as those authors. The calculations for adsorbed SOz were based on those of Polo and W i l ~ o n ' ~ as the initial (zero-order) set.

1 1 - 2

RESULTS AND DISCUSSION

The Raman spectra of sulphur dioxide adsorbed on zeolites are given in Table 1. Figure 1 shows the spec- trum of the SO;? adsorption complex on CsX. All the zeolite samples turned yellow after the adsorption of s02.

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P. TSAI AND R. P. COONEY

Table 1. Raman spectra (Afi, cm-') of the adsorbed species in the systems zeolites + SO,

NaX c s x NaA NaY Assignment suggested

Normal (spontaneous) 508 w ~ , ~ 512 w ~ , ~ v z ( S 0 2 ) Raman scatter 533 sh 533 sh 533 w 528 sh Y * ( S 0 2 )

979 m 980 w so:- 1030 w s0:-

Il5l 1 154" b 1146

1315

1151 s

1324w

1153vs

1332 m

Resonance Raman 508 ma.b 51 2 ma,b 506 ma.b s, scatter' 684 w 686 w 683 w 684 w s4

723 w 727 w 724 w 727 w s 2

852 vw 855 vw 849 vw 850 v w S,O?

a Residual intensity after partial desorption (30 min evacuation) of SO2 associated with other residual SO2 fundamentals

Residual coincident intensity after complete desorption of SO2 attributed to S,. This feature appears to exhibit some

Detected only with 457.9 nm Ar' as exciting line.

(v,. v3).

enhancement with the longer wavelength exciting lines (514 nm Ar+).

Raman lines assignable to the fundamental vibra- tional modes of SO2 were observed on all the zeolite samples (Table 1). The SOz molecule belongs to sym- metry point group CzV and has three Raman- and IR- active fundamentals: 2A1 +Bz. The Raman spectrum of sulphur dioxide in the gas, li uid and solid phases has been previously reported.l5-'' In the Raman spectra of adsorbed SO2 the fundamental frequencies were shifted from the gas phase values by the adsorption interactions with the surface. A comparison of the stretching frequencies v1 (A1) and v 3 (B2) with those for SO2 in the gas, liquid and solid indicates that the adsorbed SOz on zeolite surfaces is in a quasi-crystalline state. This is particularly evident for the antisymmetric stretching mode ( v g ) which is detected in the range 1315-1332 cm-' for the adsorbed phase on the zeolite surfaces and changes from 1362 (gas)15 to 1334 (liquid)I6 to 1324 (solid)" cm-' depending on phase. In this respect SOz is similar to a ~ e t y l e n e ' ~ and bromine" which also appear to form crystal-like adsorbed phases on zeolite surfaces.

1600 14CO 1200 1000 800 600 400 Frequency (cm-')

Figure 1. Raman spectrum of sulphur dioxide adsorbed on zeolite CsX. Exciting line457.9 nm Ar+, incident power80 mW, slitwidth 8 cm-'.

In the 650-850 cm-' region three weaker lines appeared in the Raman spectrum when 457.9 nm Ar+ was used as exciting line. These lines did not appear when longer wavelength exciting lines were employed. These observations are indicative of resonance phenomena. Other weaker features (ca. 980 and 1030 cm-') did not exhibit exciting line sensitivity and are therefore interpreted as normal (spontaneous) Raman spectra (Table 1).

Sulphur dioxide exhibits various modes of coordina- tion: bonding via the sulphur atom in a trigonal pyramidal configuration (e.g. (CH3)3NS02),'8 bonding via the sulphur atom in a tri onal planar configuration (e.g. [RU(NH~)~(SO~)CI]CI) ' and bonding via the oxy- gen atom in a non-linear OSO configuration (e.g. F5SbOS0).20 From the vibrational spectra of these types of complexes, Byler and Shriver'l have observed that the SO stretching vibrations ( v l and v3) always shifted to lower frequencies relative to the values for liquid SO2 while the bending mode ( v3) increased in frequency. The S- and 0- modes of bonding may be distinguished by the diagnostic nature of the frequency separation A = (v3 - v1). Values of A greater than 190 cm-' are con- sidered indicative of the 0- mode of bonding while values less than 190cm-' are indicative of S- coor- dination.'l

8

Adsorbed molecular sulphur dioxide

The preferential adsorption of the weak electron donor, SO2, on the cation sites rather than the negatively charged oxide framework is suggested by the displace- ment of the vl and v3 frequencies (and particularly the latter) with the change in zeolitic cation from NaX to CsX. The mode of attachment of SOz to these surfaces has been inferred from the A criteria of Byler and Shriver.'l All the values of A for adsorbed SO2 in the present study (see Table 2) were found to be less than 190 cm-' and so have been interpreted in terms of S- coordination.

Detailed examination of the spectra of SOz on zeolites NaX and CsX reveals that each of the three profiles of

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POLLUTANT TRAPPING STUDIES-2

the fundamentals of SO2 incorporates two components, i.e. there are two closely spaced sets of SOz fundamen- tals. In a progressive desorption study, one set of SO2 fundamentals was easily removed by evacuation for 30 min (Tables 1 and 2). The double set of fundamentals has been interpreted in terms of two cation sites of adsorption, viz. sites I1 and I11 of the X framework. For both NaX and CsX, the first set of fundamentals to disappear on evacuation had A values closer to those of the gas phase m ~ l e c u l e ' ~ (see Table 2) than the second set. Therefore the second set is clearly associated with SO2 molecules more strongly coordinated to the surface. Also the first set (particularly v2 and v3) for the NaX case more closely resembles the single set of fundamentals observed for NaY + SO2. As NaY has cations in sites I1 but not in sites 111,'1~22 the first set of fundamentals to disappear in the NaX case is assigned to SO2 on sites I1 and the second set (representing more firmly held SOz) is assigned to SO2 on the more coordinatively unsaturated sites 111. This assignment is supported by the relative magnitudes of the frequency separations, A = (v3 - vl). The A value for the first set of fundamentals to disap- pear on evacuation (173 cm-') is closer to the liquid (189-190 cm-') and gas values (211 cm-') than the A value for the more firmly held SO2 (163 cm-'). This result is also in accord with the assignment of the latter spectrum to more firmly held molecules on the more coordinatively unsaturated sites, i.e. sites I11 of the X framework.

Results for NaA are consistent with single-site adsorption as only one set of SO2 fundamentals was detected. The complete assignments of the various spectra of adsorbed SO2 are summarized in Table 2.

Table 2. Observed and calculated frequencies (AC, cm-') for adserbed sulphur dioxide

Zeolite

NaX (S II)

NaX (S Ill)

c s x (S 11)

c s x (S 111)

NaA

NaY (S II)

Gas"

a See Ref. 15.

Vibration

"1

"2

v3

"1

"2

v3

"1

u2

"3

V1

v2

"3

"1

"2

v3

"1

"2

v3

"1

"2

"3

Observed

1151 533

1324

1154 508

1317

1146 533

1315

1143 51 2

1296

1151 533

1324

1153 528

1332

1151 518

1362

Calculated

1151.1 532.9

1324.1

11 54.0 508.1

1317.1

1 146.0 533.0

1315.1

1143.0 512.0

1296.0

1151.1 532.9

1324.1

11 53.0 527.9

1332.1

1151.0 517.9

1362.0

% Error d

0.01 0.01 0.01

0.00 0.01 0.01

0.00 0.00 0.01

0.00 0.00 0.00

0.00 0.01 0.01

0.00 0.01 0.01

0.00 0.03 0.00

Normal coordinate analyses for physisorbed SOz

Normal coordinate calculations were carried out for physisorbed SOz to determine whether the A criterion of Byler and Shriver" was a reliable parameter of the strength of the adsorption interaction (in addition to its use in elucidating the coordination configuration of SOz). All three SO2 fundamentals were detected for NaX (two sites), CsX (two sites), NaA (one site) and NaY (one site). These were the input frequency parameters in a series of calculations based on a simple Czv configuration with the S-0 distance set at 0.1432 nm and a bond angle of 119.5".

Five force constants were included initially of which two were diagonal force constants and three interaction force constants. The diagonal force constants were a S-0 stretching force constant (fr) and a bending force constant (fa). The interaction force constants included a stretch-stretch interaction force constant (frr) and a stretch-bend interaction force constant ( f r a ) . A non- bonded Urey-Bradley force constant ( F ( 0 . ..O)) was also included but was usually constrained at zero. The interaction between the zeolitic cation and SO2 was not included because of the failure to detect a frequency assignable to the interaction. Tables 2 and 3 summarize the results of the calculation: the force constants and calculated frequencies for SO2 on the various zeolites and in the gas phase. A close frequency fit was obtained in all cases with identical constraints and assumptions maintained throughout. The potential energy dis- tribution confirms the 'purity' of the stretching frequen- cies, with fr determining ca. 96% and ca. 100% of the symmetric ( vl) and antisymmetric ( v3) stretching frequencies respectively. Also, fu determines 97% of the bending mode frequency ( ~ 2 ) . As fr largely determines the values of both v1 and v3, it is not immediately clear that A( = v3 - vl ) is necessarily the most direct parameter of adsorption interaction. Therefore the correlation of fi with A has been examined (see Fig. 2).

d Y . - L

173

With the exception of CsX (site II)+S02 there appears to be a reasonable correlation (Fig. 2) between f r and A. As it represents a refinement of the observed spectral data, it appears likely that fr is a somewhat more reliable parameter of adsorption interaction. Neverthe-

,I )

less, as a conveniently measured parameter, the A parameter of Byler and Shriver*' is a useful qualitative monitor of adsomtion interaction in addition to its

163

function as a criterion of adsorption geometry of SO*. 169

Table 3. Force constants" for adsorbed sulphur dioxide 153 assuming C,, symmetry

fr fa f,, fr,

(102Nm-') (10-''Nm) I1O2Nm ') 110-'N) 173

NaX (S 11) 9.58 1.77 0.12 0.00 NaX (S Ill) 9.46 1.64 0.10 -0.17 c s x (S 11) 9.57 1.74 0.23 0.16

179 c s x (S Ill) 9.53 1.59 0.46 0.37 NaA 9.58 1.77 0.12 0.00 NaY 9.61 1.75 0.03 -0.07 Gasb 10.05 1.63 0.03 0.32

a F(O...O), the non-bonded Urey-Bradley force constant, was constrained at zero.

21 1

Frequencies used in the calculations are from Ref. 14.

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P. TSAI AND R. P. COONEY

230

210

I

>

a I70

150

L

*CsX ( b ) /

Figure2.CorrelationoftheS-Oforceconstant(f,, lo2 N m-')and the stretching frequency difference parameter ( A = v3- I.,) of adsorbed S02: (a) before pumping; (b) after pumping for 30 min.

Resonance Raman spectra of decomposition products

Intensity changes which were observed on evacuation of the zeolite X + SO2 samples proceeded in three stages: the zeolite X (site II)+SO;? spectrum decreased in intensity first, the zeolite X (site 111) + SO2 spectrum then decreased in intensity, and finally a number of residual lines remained even after extended evacuation. One of these residual lines (508 cm-' for NaX + SO2 and 5 12 cm-' for CsX + SO4 was revealed as being virtually coincident with v2 of SO2 on sites I11 (e.g. 508 cm-' for NaX) which had previously decreased in intensity together with the much more intense v1 (e.g. 1154 cm-' for NaX). When vl was no longer detectable, significant residual intensity was still observed at 508 cm-' for NaX which did not arise from the zeolitelitself. The presence of a band in this region (506 cm- ) was more clearly evident in the case of NaY where only one set of site-based SO2 fundamentals was detected and there was no overlaying v2(S02) line associated with sites 111.

On detailed examination of the entire spectrum the band at 508 cm-' does not appear to be associated with any other line(s) in the spectrum. The frequency is too low to be assigned to S-0 stretching vibrations, and an OSO bending mode is not likely to be the most intense

feature in a Raman spectrum. The frequency is, however, central in the range of Raman-active S-S stretching frequencies (see Table 4). The species asso- ciated with the 508cm-' line in the spectrum of the NaX + SO2 system is therefore most likely to be a simple homonuclear sulphur molecule. The small but significant cation sensitivity shift in the frequency between the NaX system (508cm-') and CsX (512 cm-') suggests that the species may be associated with a zeolitic cation site and therefore argues against its assignment to a cationic sulphur species (S:'), as does the absence of highly acid and oxidizing conditions associated with such species.30 The chemical alternatives are neutral or anionic sulphur species. An examination of the data4' summarized in Table 4 indicates that only one of these species, viz. S;, has a Raman line adjacent to the line at ca. 510cm-'. Intense Raman lines in the region 523-555 cm-' have been p r e v i ~ u s l y ~ ~ - ~ ~ assign- ed to S; species. Further adsorption interactions have in past studies"-'3 generally led to an incremental decrease in the key adsorbate frequencies. This prefer- red assignment to S; rather than S3 or S i is also suppor- ted by the absence of a blue-excited resonance enhancement effect which would be expected for the latter species by virtue of the existence of associated absorption bands at 400-410 nm whereas S5 absorb^^"^^ at longer wavelengths (590 nm). The observation of an apparent twofold enhancement of the band ca. 510 cm-' as the exciting line is changed from 457.9 nm to 514.5 nm is in accord with this assignment. In another aluminosilicate matrix (ultramarine) S exhibits a resonance spectrum34 with longer wavelength exciting lines.

Weak features were observed in all the spectra in the region 650-850cm-1 only when 457.9nm was employed as exciting line. This result has been inter- preted as a resonance Raman spectrum. Three lines were detected at 684, 723 and 852 cm-' on NaX with lines in similar positions for CsX, NaA and NaY (Table 1). These lines did not disappear on evacuation at 100 "C and therefore appear to be chemisorbed species on the zeolite surfaces.

The appearance of lines in this region of the Raman spectrum (Table 4) may again be attributed to homonuclear sulphur species beczuse of the absence of associated resonance-enhanced features in the SO stretching region (>800cm-'). Lines at ca. 980 and 1030 cm-' are non-resonance features and are more reasonably assigned (see later) to sulphite species.

The observed line at 723 cm-' is assigned to the species S2 as this is the only homonuclear sulphur speciesz3'& that has a stretchin fre uency in this region (Table 4). The line at 684 cm- may be assigned to the 4 9

-

Table 4. Survey of most intense Raman-active fundamentals (cm-') of homonuclear sulphur species

Chain Chain Singly charged Polysulphide

(Refs. 23-25) (Refs. 26-32) (Refs. 33-36) (Refs. 37-39) allotropes allotropes species ions

Cationic species (Ref. 401

S2: 718 SB: 583 S,: 668

S g : 471 S;: 590-612 S;-: 451 S p : 584 S7: 481 S;: 523-555 Si-: 466 Sg: 475 S:-: 434,485 s,2: 459 S$-: 432

s:-: 373

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POLLUTANT TRAPPING STUDIES-2

species S4. Meyer and S t r ~ y e r - H a n s e n ~ ~ have reported the IR s ectrum of S4 and found bands in the 636- 688 cm-' region which they assigned to stretching vibrations of this species.

Finally, the assignment of the very weak line at 852 cm-' is unclear. Hopkins et a1.l' observed a line in this region of the Raman spectra of the decomposition products when SO2 is subjected to microwave and radiofrequency discharges. In accord with their assign- ment the line at 852 cm-' is tentatively attributed to poly(su1phur oxide) species, S,O. Given the weakness of the line at 852cm-', the associated SO stretching frequency (-1100 may not be detectable. In any case, it seems likely that S20 , which is not detected in this study, may nevertheless be a key intermediate in the decomposition of SO;? on active sites into homonuclear sulphur species and sulphites (via refor- med SO2) as has been previously reported.45

Raman lines in the 950-1050 cm-' region

Two lines in this region were observed in some of the spectra (Table 1). These bands at ca. 980 and 1030 cm-' are very weak and do not disappear on evacuation. A survey of the literature5-* suggests that these lines can be attributed to sulphates and sulphites. The free sulphite ion4' belongs to the symmetry point group, C3u, which has its v1 (Al) frequency at 983 cm-' and a v3 ( E ) frequency at 947 cm-'. If the sulphite ion is bonded via the sulphur atom, the C3, symmetry would be preserved and this type of coordination shifts the S - 0 stretching

vibrations to higher f r e q ~ e n c i e s . ~ ~ On the other hand, if the sulphite ion is bonded via the oxygen atom, the symmetry would be lowered to C, resulting in shifts of the S - 0 stretching vibrations to lower frequencies relative to the free ion.46 If the assignment of the lines at ca. 980 and 1030 cm-' in the present study to sulphites is correct, then such sulphite surface species are prob- ably S - coordinated. However, it is also possible that some of the intensity at ca. 980cm-' arises from ~ u l p h a t e s ~ ~ and that some of the intensity at ca. 1030 cm-' arises47 from HSO;.

CONCLUSION

In contrast to Part 1 of this study, involving nitrogen dioxide adsorption on oxide surfaces, the adsorption of SO;? on zeolite surfaces involves predominantly the undecomposed molecular SO2. However, the partial decomposition that is observed involves the formation of molecules (e.g. S 2 ) of considerable interest in the chemistry of sulphur. Finally, the observed yellow colour of the zeolite tablets after adsorption probably arises from the combination of UV-blue (Sz) and red (S;) absorbing chromophores on the surface.

Acknowledgements

The authors are indebted to the Australian Research Grants Com- mittee for providing the spectroscopic equipment and one of the authors (P.T.) is grateful for a Commonwealth Postgraduate Award.

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Received 16 July 1979

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