EPA-650/3-74-006
August 1974
Ecological Research Series
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EPA-650/3-74-006
STRUCTURE AND REACTIVITY
OF ADSORBED OXIDES OF SULFUR
by
J . H . Lunsford
Department of Chemistry
Texas A & M University
College Station, Texas 77843
Grant No. 801136
ROAP No. 21AJX-2
Program Element No. 1A1008
EPA Project Officer: Jack L. Durham
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
August 1974
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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ABSTRACT
The purpose of the research reported here was to determine
the structure and reactivity of adsorbed oxides of sulfur and
other small sulfur-containing molecules. The molecules that
were studied include the anion radicals of sulfur dioxide ,
sulfur trioxide, a triatomic sulfur species and carbonyl
sulfide. Diamagnetic sulfite and sulfate ions, as well as
covalently bonded oxides of sulfur, were also studied. The
adsorbent was a high surface area magnesium oxide powder.
Electron paramagnetic resonance and infrared spectroscopy
were used to characterize the surface species.
The results show that electronegative molecules such as
sulfur dioxide adsorb either by an electron transfer from the
solid, forming a negative radical ion, or by reacting with the
oxide ions of the lattice. At room temperature the adsorbed
sulfur dioxide anion radical may be oxidized with molecular
oxygen to the sulfur trioxide anion radical; whereas, at
elevated temperatures the sulfite ions may be oxidized. In
contrast, the anion radical of carbonyl sulfide is very
unstable and dissociates, presumably to elemental sulfur and
carbon monoxide, at low temperatures. Elemental sulfur reacts
with partially hydroxylated magnesium oxide, forming the
11
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triatomic negative ion, hydrogen sulfide, and other unidentified
products.
This report was submitted in fulfillment of Grant Number
R-801136 by the Texas A&M Research Foundation, under the sponsor-
ship of the Environmental Protection Agency. Work was completed
as of July 1973.
111
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TABLE OF CONTENTS
Page
Abstract ii
Table of Contents iv
List of Figures v
List of Tables vi
Acknowledgments vii
Sections
I. Conclusions 1
II. Recommendations 3
III. Introduction 4
IV. Materials and Methods 9
V. Results 14
VI. Discussion 48
VII. References 73
VIII. List of Publications 78
IX. Glossary of Terms, Abbreviations, and Symbols 79
iv
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LIST OF FIGURES
No. Page
1. Infrared cell 10
2. The development of the SC>2 spectrum from the S-center on
MgO. 15
3. SC>2 on MgO pretreated at 800°. 16
4. 33S02 on MgO pretreated at 800°. 21
5. S16O17O~ on MgO pretreated at 800°. 22
6. EPR spectrum of 303 on MgO. 25
7. EPR spectra of COS", CS^ on MgO, and CO^ on MgO. 27
8. EPR spectrum of sulfur radicals following the reaction of
sulfur with partially hydroxylated magnesium oxide. 29
9. X-band EPR spectrum of 83 on MgO. 30
10. Amplification of hyperfine lines from 25.5% 33S. 33
11. Q-band EPR spectrum of 83 on MgO. 34
12. Simulated X-band spectrum for 25.5% 33S enrichment. 35
13. The infrared spectra of MgO at several dehydration
temperatures. 37
14. The infrared spectra of SO2 and its reaction products
adsorbed on MgO. 38
15. The infrared spectra of sulfite species on MgO before
and after reaction with O2• 44
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LIST OF TABLES
No. Paqe
1. Principal g values for SC>2 adsorbed on MgO and in other
matrices. 18
2. Principal values of the 33S and 17O hyperfine tensors
(gauss) of SO;? adsorbed on MgO and in other matrices. 19
3. EPR parameters for SO3. 24
4. Principal value of the g and hyperfine tensors
for 33S~. 31
5. Frequencies of the observed ir bands and their assign-
ment. 39
6. Isotropic and anisotropic parts of the 33S and *^O
hyperfine tensors (gauss). 49
7. 2b}" MO coefficients and spin densities on S and O
for SO2 on MgO. 50
8. Comparison of hyperfine couplings, spin densities,
and bond angles for 303 in different environments. 55
9. Experimental and calculated percentages for 33S
isotope combinations in S S S~. 63
aba
VI
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ACKNOWLEDGMENTS
The efforts of Drs. Robert Schoonheydt, Dennis Johnson and
Younes Ben Taarit as postdoctoral fellows associated with this
project are gratefully acknowledged. Miss Mei Jan Lin, a grad-
uate research assistant, has also contributed significantly to
this work.
VI1
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SECTION I
CONCLUSIONS
Results from this research have demonstrated that S02 reacts
with the surface of magnesium oxide, forming a number of differ-
ent surface ions. If the surface has a reducing character, the
thermally stable SO2 ion is formed. In high concentrations on
the surface this ion reacts with molecular oxygen to produce
the 803 ion, which may be an intermediate in the catalytic
oxidation of SO2 to 803.
Exposure of magnesium oxide to S02 also results in the
formation of two types of S0^~ ions which are hydrogen-bonded
to surface hydroxyl groups. These two species correspond to
two different active sites on the surface of MgO. The number
of sites, as reflected by the intensity ratio of the bands
2_
of the two S03 species, depends on the pretreatment of the
2_
MgO catalyst. At temperatures above 500°C, S03 is unstable
and decomposes. As a consequence, after degassing at 800°C
under vacuum, residual bands are present which are attributed
to a SO^~ surface species. When the adsorbed So|~ ion is
heated in O2, a complex infrared spectrum is observed which
corresponds to a mixture of bidentate and chelate sulfato
complexes along with strongly bonded 803.
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Other radical anions such as COS" and CS2 are formed upon
reacting the corresponding neutral molecule with a reducing
magnesium oxide surface. These surface ions are thermally
unstable at room temperature, and the COS" ion dissociates,
yielding CO and presumably elemental sulfur.
Partially hydroxylated magnesium oxide reacts with
elemental sulfur to form polymeric radicals at temperatures
around 200°C and a triatomic sulfur ion, 83, at 400°C. The
latter reaction is accompanied by the production of H2S,
which desorbs from the surface. The 83 ion reacts with
molecular oxygen at room temperature, yielding an unidentified
diamagnetic product.
It is clear from these studies that sulfur and its oxides
undergo numerous chemical changes on the surface of magnesium
oxide, which may be taken as a typical basic surface. The
chemistry of these systems may be conveniently studied by
epr and ir techniques. Although these results have been
obtained under ideal conditions which do not duplicate the
atmosphere, the spectra of potentially important species
have been identified, and the work may now be extended to
more realistic conditions.
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SECTION II
RECOMMENDATIONS
A comprehensive understanding of the effects of aerosols
on animal and human physiology will require detailed information
on the structure and reactivity of surface species. The
concentration of sulfur oxides is usually reported simply as
"sulfates"; however, the results of this work and similar
studies clearly demonstrate that many different sulfur oxides
may be stabilized on a model surface. The types of sulfur
oxides which exist on aerosols in a real atmosphere have
yet to be conclusively identified, although one may be
reasonably sure that other forms of sulfur than as sulfate
ions will be found.
It is recommended, therefore, that the work reported
here be extended in two directions. First, the work on pure
metal oxides should be continued, but the adsorbents should
more closely correspond to the composition of aerosols in the
earth's atmosphere and any irradiation of the sample should
simulate the solar spectrum. Second, the project should be
expanded to include studies of collected atmospheric aerosols
which have varying sulfur content. The samples should be studied
as received and after exposure in the laboratory to atmospheres
containing different levels of SO2. Results from such studies
should elucidate some of the complex chemistry which must be
taking place at the surface of the aerosol samples.
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SECTION III
INTRODUCTION
GENERAL
The overall objective of the research was to determine the
structure and reactivity of adsorbed oxides of sulfur and other
small sulfur-containing molecules. Magnesium oxide was used
as a representative metal oxide surface. The surface species
were identified by electron paramagnetic resonance (epr) and
infrared (ir) spectroscopy . The scope of the study included
paramagnetic ions SO;?, SO^, 83 and COS" as well as the dia-
magnetic ions SOf and SO^. Other covalently bonded oxides
of sulfur were also detected.
SO2 AND 303 ON MAGNESIUM OXIDE
Sulfur dioxide is a very electronegative molecule which
readily forms the SO2 ion upon adsorption, provided electrons
at a sufficient potential are available at the surface. Mashc-
henko , Pariiskii, and Kazanskii reported the formation of
SO;> as well as diamagnetic species , on partially reduced
by adsorption of S02 • These authors also presented evidence
that the reaction of SO2 with 02 on the TiO2 surface leads to
02 and vice versa through intermediate surface sulf ates , but
they strongly emphasized that the reaction of SO^ with 02
leads mainly to diamagnetic species.
It was our intention to extend these studies to activated
MgO where it is possible to trap electrons on the surface of
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a suitably prepared sample. Upon adsorption of SC>2 the electron
is transferred to the sorbed SC>2 molecule to form SO^- The
characteristic molecular parameters of the SC>2 on the surface
of MgO have been determined by evaluating the g tensor and
the 33S and 0 hyperfine tensors. These values are compared
with those obtained from SOj trapped in solid matrices. Also,
the thermal stability of SO;? has been investigated, as well
as the effects of different pretreatments of the MgO catalyst.
The results are interpreted in terms of the molecular orbital
of the odd electron in SOJ and the nature of the adsorption
sites on the MgO surface.
Reactions involving the SOJ ion on surfaces have been
studied in an effort to determine the importance of this
species in the catalytic oxidation and reduction of S02- It
will be subsequently shown that under proper conditions the
SOj ion reacts with molecular oxygen to form the adsorbed 863
ion. The epr spectrum of SO^ has been well characterized by
Chantry and coworkers. In their studies the ion was formed
upon y-irradiation of a number of salts such as sodium dithio-
nate, which has an unusually long S-S bond. From the 33S
hyperfine structure it was possible to compare 803 with the
2- _
isoelectronic ions PO% and 0103. More recently Lind and
4
Kewley also observed the spectrum of 803 upon -y-irradiation
of taurine. This natural amino acid in its zwitterion form
(H3N+-CH2-CH2SO3) undergoes homolysis of the C-S bond, thus
generating the 303 ion.
COS" AND CS2 ON MAGNESIUM OXIDE
Like SO2/ COS and CS2 are able to accept trapped electrons
from the magnesium oxide surface, forming the respective radical
-------
anions. Since COS is an intermediate in the catalytic reduction
of SO2 by CO, information regarding the stability of COS~
on surfaces was of interest. In a recent mass spectrometric
5
study MacNeil and Thynne observed only the decomposition
products for COSaat electron energies less than 1 eV, but
no COS" was detected; hence the ion must be unstable in the
gas phase. On the other hand COj is stable on the surface of
MgO . Although the molecule ions COj and CS^ have been studied
in detail by epr spectroscopy, the spectrum of the isoelectron
COS~ ion had not previously been reported.
83 ON MAGNESIUM OXIDE
Elemental sulfur and sulfur ions play an important role
in the behavior of catalysts for desulfurization reactions
and the reduction of SO2 over metal oxides. The state of the
sulfur on the surface has been studied, mainly by epr spectro-
scopy; but the spectra are usually lacking in hyperfine structure
and it is difficult to make a positive identification of the
surface radicals. The sulfur ions S~, Sj and S^ have been
identified in single crystals on the basis of their g tensors
and hyperfine structure. The oxygen ions O~, Oj and Oj are
easily formed on a magnesium oxide surface, and it seems
reasonable that the analogous sulfur ions could be stabilized
on the surface. However, the lower vapor pressure of sulfur
and a tendency to produce polynuclear molecules make it
difficult to produce S~ and S^ on MgO. Evidence is presented
here for the formation of 83 upon reacting elemental sulfur
with a partially hydroxylated magnesium oxide surface. The
sulfur-33 hyperfine splitting was used to identify the
molecule and to establish its structure.
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SULFITE AND SULFATE IONS ON MAGNESIUM OXIDE
Sulfur dioxide also forms several stable diamagnetic
species upon adsorption at metal and metal oxide surfaces.
On porous silica glass SO2 displaces the 3750 cm"1 band
of the free hydroxyl groups to a lower wave number by 115 cm"1.
On Fe and Ni surface SO2 adsorbs chemically to form a SO^
surface species, which is believed to be an intermediate in
P
the catalytic oxidation of S02 to 803 . Such a strong
9
chemical change is not always observed. Habgood and coworkers
studied the adsorption of t^S and SO2 on y-alumina, NaOH-doped
y-alumina, NaY and HY zeolites. Except for NaY, where no OH
groups were present in measureable amounts, S02 adsorbed on
these samples through strong hydrogen bonding. After heating
up to 400° under dynamic vacuum the adsorbed SO2 molecules
on Y~alumina and on NaOH-doped y-alumina transformed to a
chemisorbed species. A sulfate-like structure involving
two lattice oxygens and the adsorbed S02 molecule was
suggested as an explanation. However, the Si-O and Al-0
adsorption bands in the 700-1200 cm"1 region obscure the
S-0 vibrations of this SO^ species and no unambiguous proof
could be advanced by these authors.
The infrared spectrum of SO2 on CaO and MgO has recently
been studied by Low and coworkers ' . They observed that
the main interaction of SO2 with degassed CaO is irreversible
and leads to the formation of a surface sulfite; however,
reversible chemisorption occurs at high degrees of surface
coverage. On heating the sample under vacuum at 550°C a
new spectrum attributed to a surface sulfite and polymerization
are thought to occur. Heating in oxygen resulted in a spectrum
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which resembles that of bulk CaSOit. On MgO Low proposed
that the adsorption of S02 resulted in the formation of
monodentate- and bidentate-like surface sulfites which
could only be removed by degassing at high temperatures.
A reversible chemisorption occurred at high degrees of
surface coverage. SC>2 was then physically adsorbed on top
of the more tightly bound species. Heating in oxygen con-
verted the adsorbed material to species resembling bulk
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SECTION IV
MATERIALS AND METHODS
The MgO used in these experiments was a reagent grade
powder supplied by Mallinckrodt Chemical Works. The powder
was boiled in distilled water for 2-3 hr, during which time
it was converted mainly to Mg(OH)2. The slurry was dried
at 100°C until a paste was obtained. For the epr experiments
the MgO/Mg(OH)2 paste was extruded into pellets, approxi-
mately 1 mm in diameter, with a hypodermic syringe and then
dried at 100°C. For the ir experiments a small amount of the
paste was spread over a 2 x 1 cm platinum grid. The film
obtained in this manner was fitted in the quartz side-arm of the
infrared cell depicted in Figure 1. One sample was studied
without the platinum grid in order to demonstrate that the
platinum did not influence the reactions. Because of the
platinum grid it is difficult to give an accurate sample
density; however, a value of 20 mg/cm2 of MgO may be used
for comparison purposes.
Pretreatment of the sample varied from one type of
experiment to another. In the formation of SO2 ions, the
MgO/Mg(OH)2 was heated under vacuum to 500° or 800°C for
5 to 7 hr, which converted the Mg(OH)2 to a high surface
area MgO. The activated MgO was placed in contact with
very pure H2 and the sample was uv-irradiated with a low-
-------
Quartz side-arm
AgCI windows
Figure 1. Infrared cell with quartz side-arm and AgCI windows,
10
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pressure mercury vapor lamp (A = 2537A). The MgO became
blue due to electrons trapped at surface centers (S-centers).
Ultraviolet irradiation at liquid N2 temperatures produced
more S-centers in a much shorter period of time than at
room temperature. Alternately, the uv irradiation was
performed after the SO2 adsorption. In this case no
hydrogen was used.
For the formation of 803, the MgO/Mg(OH)2 was degassed
for 2 hr at 350°C and for 1 hr at either 450 or 850°C. The S
centers, formed in the same manner as before,were subsequently
allowed to react with purified N2O or SO2 producing O~ or SOJ
on the surface. Alternately, the activated MgO was irradiated
at room temperature in the presence of N2O which yielded
adsorbed 03 as well as excess 02 and N2.
The molecule ions CO2 and COS" were formed by first
producing S centers on activated MgO and then adsorbing either
COS or C02 at -135°C or -78°C. The samples were rapidly
cooled to -196°C and maintained at that temperature.
For the formation of 83 the MgO/Mg(OH)2 was degassed
under vacuum at 300°C for 2 hr and 450°C for 1 hr. Elemental
sulfur was kept in a 4 mm quartz sidearm of the reactor.
After the MgO had been degassed, it was tapped into the quartz
sidearm on top of the sulfur, and the sidearm was heated,
usually to 400°C. Generally the sample had to be heated
16 hr at 400°C for the maximum S^ spectrum to appear.
Typical pretreatments for the ir studies consisted in
evacuating the sample in the sidearm of the ir cell to
1 x 10~5 Torr and subsequently heating it to 300, 500 or 800°C
for 3-7 hr. After cooling the film was allowed to glide
11
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between the two AgCl windows. Magnesium oxide is not transparent
below 700 cm"1. Spectra were taken at 25°C under vacuum in
the regions 700-1700 cm"1 and 3100-3800 cm"1 after each pre-
treatment. About 3 Torr of pure SO2 was allowed into the cell.
Spectra were taken in the same wavenumber regions soon after
the SO2 introduction and at several stages of the subsequent
desorption-reaction process. In this latter procedure the
sample was heated under vacuum up to 800°C in intervals of
200°C. The sample was equilibrated for 2 hr at each 200°
interval, cooled, and the spectrum was recorded.
The reactivity of the adsorbed species towards O2 was
also investigated. For this purpose SO2 was adsorbed and
subsequently degassed at 500°C for 2 hr under vacuum. The
spectrum was taken from 700-1500 cm"1 and 3100-3800 cm"1 at
room temperature. This was followed by the introduction of
about 150 Torr O2. The sample was then heated for 2 hr in
the O2 atmosphere at 200, 400, 600 and 800°C. After each
step the spectra were recorded in the same wavenumber ranges
at room temperature without previous evacuation of the excess O2.
The anhydrous S02 used in these experiments was obtained
from a commercial source, and was purified several times by the
freeze-pump technique prior to use. Sulfur enriched with 25
and 44% ^3g an(j oxygen enriched with 47% O were used to
produce enriched 33gQ^ ancj s ^O^O, respectively. Suitable
amounts of oxygen and sulfur were allowed to react at 450°C
for 1-4 hr.
The epr spectra were obtained with a Varian Model V-4500
or E-6S spectrometer. Spectra were recorded at both X-band
and Q-band microwave frequencies. The g tensors were evaluated
12
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by using a phosphorus-doped silicon standard which had a g
value of 1.9987.
All infrared spectra were recorded with a Beckman IR-12
spectrophotometer in the absorbance mode. The estimated error
in the assignment of wave numbers is ±5 cm~* for sharp bands
and ±10 cm"1 for broad bands.
13
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SECTION V
RESULTS
SO2 ON MAGNESIUM OXIDE
When S02 was allowed to absorb on previously uv-irradiated
MgO, which had been degassed at 500°, the spectra of Figure 2
were obtained. This figure shows a decrease in intensity of the
S-center with a simultaneous increase in the SOj spectrum. It
is also apparent that two SO;? species are formed on the surface
of MgO. The species with the highest g values, SO^ (A) ,
appeared first followed by the S02 with slightly lower g values,
SOJ (B). With increased S02 adsorption the SO2 (B) becomes the
predominant species. When all the sites are saturated, the ratio
SO2 (B):SO2 (A) approximately equals two.
When MgO was pretreated at 800° , qualitatively the same
results were obtained. The difference being the final ratio
of S02 (B):SO2 (A): as shown in Figure 3 the ratio for this
case is less than unity if one considers the doubly inrtegrated
signal.
A sample, previously saturated with SO2, undergoes
exactly the reverse behavior when heated under vacuum. Although
both S02 species were stable under a dynamic vacuum of 10~5 Torr
at room temperature, the SOJ (B) species disappeared far more
rapidly than the SO^(A) species with increasing temperature.
Essentially no S02~(B) is apparent following the treatment at
14
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H
g=2.000
g=2.000
Figure 2. The development of the S02 spectrum from the S-center
on MgO pretreated at 500°. (a) Original S-center;
(b), (c), (d), increase in intensity of SO^ signal
with concomitant decrease in intensity of S-center
signal; (e) final SC>2 spectrum.
15
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eyy(A)=2.0103
= 2.OO91
=2.OO59
=2.OO29
Figure 3. SO2 on MgO pretreated at 800°.
16
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200° for 2 hr under 1 x 10 5 Torr. A temperature of 400° was
necessary to remove S02 (A).
Not all of the adsorbed SO2 is converted into S02.
Analogous to the results on Ti02 the majority of the SC>2
molecules adsorb in a nonparamagnetic form. Indeed, after
the MgO was degassed for several hours at 500° under vacuum
in order to remove completely the SO2 species, and subsequently
submitted to uv irradiation, also under vacuum, the S02 spectra
reappeared. When the irradiation was performed under a H2
atmosphere, only the S02 (A) species appeared. The existence
of nonparamagnetic SO2 was further shown by infrared spectro-
scopy.
The ratio SO2 (A) is also influenced by whether the uv
irradiation is applied before or after the S02 adsorption.
If no S-centers were produced prior to the S02 adsorption, but
uv irradiation was applied after the SO2 was adsorbed on MgO,
the formation of SOj (B) was strongly favored, even when MgO
was previously pretreated at 800°. SO2 (A) can only be seen
as a shoulder on the low-field side of SO2 (B). Moreover, SO2
adsorbed on MgO without uv irratiation, either before or
after the adsorption, gave rise to small amounts of SO2. The
amounts increase with increasing pretreatment temperature with
both SO2 species being present. It was thought that this
electron transfer from the solid to the SO2 may create V-type
centers; however, attempts to detect them failed.
SO2 can also be adsorbed on low-surface area MgO after
uv irradiation. Both S02 species are present with a SO2 (B):
SO2 (A) ratio equal to one-half.
17
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Table 1 summarizes the g values for SO2 adsorbed on MgO.
Table 1: PRINCIPAL g VALUES FOR SOj ADSORBED
ON MGO AND IN OTHER MATRICES
S02 (A)3
S02 (B)3
SO2 on T1021
K2S20512
Na2S0^3
Na2S205^
KC115
KBr15
YY
2.0097
2.0078
2.005
2.012
2.0218
2.0102
2.0100
2.0100
zz
2.0052
2.0053
2.001
2.0057
2.0076
2.0057
2.0071
2.0075
2.
2.
2.
2.
2.
2.
2.
2.
XX
0028
0014
001
0019
0069
0024
0025
0050
Every g value is an average of seven independent measure-
ments .
They are compared with the values obtained for SO;> in other
environments.
Sulfur-33 and Oxygen-17 Hyperfine Splittings
The 33S (nuclear spin 3/2) hyperfine lines for either of
the two S02 species could be determined by a suitable choice
18
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of the experimental conditions according to the results reported
above. By pretreatment of MgO at 500° spectra were obtained
with SC>2 (B) predominantly present. Heating MgO at 800° gave
SO2 (B) and SC>2 (A) in approximately equal amounts. Upon
subsequent degassing at 200° only SOj (A) remained.
Table 2: PRINCIPAL VALUES OF THE 33S AND l70
HYPERFINE TENSORS (GAUSS) OF SO2 ADSORBED ON
MGO AND IN OTHER MATRICES
33S
S02 (A)
S02 (B)
K2S2°5
15
KC1
15
KBr
SOg (A)
SO2 (B)
K2S20512
KC115
KBr15
g g g
xx yy zz
59 ± 1 (6.6 ± 2)a 9.4 ± 1
55 ± 1 (6.6 ± 2)S 9.4 ± 1
58 ±0.5 4±4 4±4
52.5 ±1 8.6 ± 1 7.1 ± 1
54.3±1 7.1+1 7.1±1
17 0
g g g
xx yy zz
36 ± 1 (3 ± 3)a (3 ± 3)a
29 ± 1 (3 ± 3)a (3 ± 3)a
30 ± 0.5 3 ± 3 3 ± 3
... ... ...
• •• ••• •»•
Estimated from other work. '
19
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Comparison of the different spectra obtained in this way enabled
us to distinguish between the 33S hyperfine splitting constants
of SC>2 (B) and SO^ (A) as indicated in Figure 4 and summarized
in Table 2.
The lines corresponding to the x direction are clearly
resolved for 33SC>2 (A) and 33SO2 (B) ; however, the hyperfine
lines corresponding to the y and z directions overlap with the
33SO2 spectrum. Of the four lines separated by a and centered
zz
around g only the two extreme lines may be observed as small
peaks on the tails of the 32S02 spectrum. Prom the separation
between these two lines we calculated a = 9.4 ± 1 G. This
zz
value should be regarded as a maximum limit to a . The four
' zz
lines centered around a could not be resolved from the SOo
yy
spectrum. We adopted, therefore, the arithmetic mean of the
values of a reported by other authors: ' a = 6.6 ± 2 G.
yy yy
A nearly axially symmetric 33S hyperfine tensor is obtained
12, 15
in agreement with previous work.
The experimental procedure used to obtain the 17O (nuclear
spin 5/2) hyperfine tensor is the same as for 33S02- Due to
the larger number of lines and the overlap with the S16C>2
spectrum, a and a could not be calculated from the
yy zz 12
experimental spectra. Therefore we adopted Reuveni's value
of 3 ± 3 G. The spectra are given in Figure 5 and the 17O
hyperfine-splitting constants in Table 2.
303 ON MAGNESIUM OXIDE
Upon addition of excess molecular oxygen to adsorbed S02
it was observed that the spectrum of SOj decreased and a
nearly isotropic line with g = 2.0034 ± 0.0003 was formed
Evidence will subsequently be given for the assignment of the
20
-------
(a)
xx
'XX
Figure 4.
(a) 33SC>2 on MgO pretreated at 800°. Both "S02 (A)
and 33SC>2 (B) are present.
3 3c(~t~
(b) 33SC>2 on MgO pretreated
at 800°, followed by subsequent degassing at 200°,
only 33SC>2 (A) is clearly present.
21
-------
o
x
x
ID
o
O
O
CO
•O
0)
0)
H
4J
(1)
G
O
O
ID
r-*
w
-------
new spectrum to the 803 ion. The formation of SO^ was not
reversible, i.e., the original SC>2 signal could not be
restored by evacuating the sample at room temperature for
over 4 hr. It should be noted, however, that some SO^
remained unreacted as indicated by the g component which
yy
could still be detected, though it was greatly reduced in
amplitude.
The rate and extent of the reaction increased with an
increased concentration of the initial SO^ ion. The 803
ion could be generated from SC>2 produced from S centers;
however, the reaction was more extensive if SC>2 was first
allowed to react with the ozonide ion to form a much higher
SC>2 concentration. Upon addition of SC>2, the symmetric
line at g = 2.0034 appeared directly. In the latter-case
no additional 02 was needed since it is photochemically
produced from N20. When excess oxygen formed during the
uv irradiation of N2O was removed prior to absorption of
SC>2, only SC>2 was observed and no oxidation occurred unless
02 was added back to the system.
The oxidation of SC>2 to 803 was difficult on samples
degassed at 850°. This may be related to a decrease in the
concentration of one type of SO^ ion, or to a decrease in
the concentration of certain diamagnetic ions as will be
discussed in a subsequent section.
By using S02 enriched to 25% in 33S(I = 3/2) four
distinct hyperfine lines were observed. These lines in
addition to the 32S spectrum are depicted in Figures 6a
3 4
and 6b. As noted by other investigators ' the outer
lines are shifted about 3 G downfield and the inner lines
23
-------
about 8 G downfield with respect to the 32SO3 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 6a was recorded with the sample at
77°K, whereas the spectrum of Figure 6b was recorded with the
sample at room temperature. In both cases it was relatively
easy to saturate the spin system with microwave power. From
the shape of the hyperfine lines of Figure 6 the values of
a I I and ai were determined. The principal values of the
hyperfine tensors are given in Table 3.
SO 3
SO3
SOs
SO3
Table 3:
on MgO (77°K)
on MgO (25°)
in K2CH2(S03)2
in taurine4*
EPR PARAMETERS FOR 303
g a
2.0034 ± 0.0003 147
2.0034 ± 0.0003 102
2.0036 ± 0.0007 153
2.0035 ± 0.0004 135
,G ai,G
± 2 102 ± 2
± 2 121 ± 2
±1 112+1
±4 99 ± 4
Under conditions of high modulation amplitude and gain it
was even possible to detect the hyperfine structure from SO^
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 SO2.
24
-------
5OG
X330
.9
.2.0034
X1
a i'
X85 X19
X1
(a)
(b)
Figure 6. EPR spectrum of SO^ on MgO: (a) recorded at 77°K,
(b) recorded at 25°.
25
-------
It is perhaps worthwhile to mention that O did not react
with SC>2 to form SO^. This result was rather surprising in
view of the observations that O~ reacts with 02 forming O^,
with CO forming COj, with C2Hit forming C^^O", and with C02
forming 003. In this experiment O~ was first produced from
N20 and S02 was condensed above the sample which was at 77°K.
The sample was allowed to warm progressively to higher tempera-
tures and the resulting spectra indicated that O~ reacted
with SO2 forming SQ2. The same electron transfer step was
detected upon treating SO2 with adsorbed O2.
COS" AND CS5 ON MAGNESIUM OXIDE
The spectrum of COS" is shown in Figure 7a along with the
principal g-values. By analogy with the CO2 molecule ion,
the z direction is taken as the symmetry axis and the x
direction is taken perpendicular to the plane of the molecule.
The low-field spectrum was formed independently from the COS"
spectrum and it had different saturation characteristics.
This spectrum is attributed to a sulphur radical produced
by the dissociation of COS" at certain sites. The COS" species
was unstable above -135°C, and at room temperature the
characteristic spectrum of a CO complex on MgO was observed
The spectrum of CS2 is shown in Figure 7b. The g-values
are nearly indentical to those reported for CSj formed by
17
reacting CS2 with alkali metals . Again, another spectrum
was observed at lower fields. The spectrum of CS2 grew
slowly at 77°K even though the color change of the MgO
indicated a rapid transfer of the electrons at -78°C. A
similar phenomenon was also observed for COS", but not for
CO2 at room temperature. It is conceivable that dianions are
26
-------
a)
g =2.0049
Figure 7. EPR spectra of (a) COS on MgO; (b) CSJ on MgO and
(c) CC>2 on MgO. The spectra of other species are
represented by dashed lines.
27
-------
first formed and that these slowly dissociate to form the
singly charged ion.
For comparison purposes the spectrum of CO2 on MgO is
shown in Figure 7c.
83 ON MAGNESIUM OXIDE
Exposing the magnesium oxide to sulfur vapor at temperatures
from 25 to 375°C resulted in a series of complex spectra, which
are apparently due to polymeric sulfur radicals. As an example,
the spectrum following reaction at 375°C is shown in Figure 8.
Even for sulfur enriched to 25.5% with 33S no hyperfine structure
was observed, apart from that assigned to 83. The absence
of hyperfine structure appears to be a typical of such sulfur
1R
radicals formed on high surface area solids.
Following reaction at temperatures near 400°C a new
species strongly dominated the spectrum. The transformation
at this temperature was accompanied by the evolution of H2S
from the surface, or D2S if Mg(OD)2 was used to prepare the
sample. Magnesium oxide did not react with pure H2S to form
the paramagnetic species. The resulting spectrum from non-
labeled sulfur is shown in Figure 9a. It may be described
in terms of an apparent axial symmetry with g> = 2.043 and
giI = 2.004. A small residual peak from the previous spectrum
may be observed at g = 2.052. There is also evidence from
the Q-band spectra that a weak parallel component may be
present at g i i = 2.003. The spectrum recorded at room
temperature was a broad symmetric signal with g = 2.032. It
should be noted that this is essentially equal to —[2(2.043) +
2.004], which is the g value expected for complete motional
averaging. A weak maximum at g = 2.048 and a minimum at g =
2.003 were also observed.
28
-------
10G
g|(=2.004
Figure 8. EPR spectrum of sulfur radicals following the reaction
of sulfur with partially hydroxylated magnesr.ura oxide
at 375°C.
29
-------
Lr 2.043
Figure 9. X-band EPR spectrum of S^ on MgO: (a) 32S; (b) sulfur
enriched with 9.8% 33S; (c) sulfur enriched with
25.5% 33S.
30
-------
Line broadening with molecular oxygen was used to determine
whether the radicals were indeed on the surface. Upon exposure
of the sample to oxygen at room temperature and then
evacuation, the primary spectrum disappeared irreversibly
and a weak spectrum similar to that of Figure 8 was observed.
When the oxygen was admitted to the original sample at 77°K
the spectrum again disappeared, but it reappeared following
evacuation of the sample as it was warmed to room temperature.
The spectra detected with the sample in the presence of
oxygen establish that the weak signal at g = 2.052 and the
related minimum at g = 2.003 are due to a sulfur radical
that is not on the surface; whereas, the primary paramagnetic
species is on the surface.
The reaction of sulfur enriched to 9.8% in 33S (natural
abundance is 0.74%) yielded the spectrum shown in Figure 9b.
Two hyperfine quartets with an intensity ratio of about 2:1
are centered on the field corresponding to gii• Several
components of these quartets could also be detected in the
spectrum of nonlabeled sulfur at high gain. Hyperfine lines
Table 4:
g
±0.0
PRINCIPAL VALUE OF THE g
TENSORS (G) FOR 33S~
gi a i (a) a
01 ±0.001 ±1 G ±1
S3/MgO 2.004 2.043 20.7 39
Sg/KCl15 2.0026 2.0319a 19. 3b 51
Sg/KCl19
... 19 53
AND HYPERFINE
•
(b) a, (a)
G ±1 G
.6 3.7
.7 1.4a
<10
a,(b)
±1 G
7.5
10. ia
<10
xx and yy components.
The hyperfine tensor and the g tensor are not diagonalized
along the same coordinates.
31
-------
in the perpendicular region of the spectrum are likewise
apparent. The hyperfine lines are even more pronounced
following the reaction with sulfur enriched to 25.5% 33S
[Figure 9c]. At high gain secondary and teriary splittings
may be observed (Figure 10) in addition to two quartets.
The Q-band spectrum of Figure 11 is consistent with the
X-band spectrum of Figure 9c.
The experimental hyperfine constants are listed in
Table 4. The parallel hyperfine splitting is obvious from
the spectra; however, the smaller splitting in the perpen-
dicular region is less well resolved. Computer-simulated
spectra were used to establish the optimum set of values
and the best spectrum is depicted in Figure 12. The low-
field component at H is too weak to be a primary hyperfine
3.
line. Furthermore, it increased more than the parallel
components when the 33S content was changed from 9.8 to
25.5%. This line and the other low-field structure must
be associated with still another paramagnetic sulfur species.
INFRARED RESULTS FOR S02 ON MAGNESIUM OXIDE
The ir Spectrum of MgO Following Pretreatment at 250-800°
Spectra of MgO at several stages of dehydration were
recorded in the regions 1100-1700 cm"1 and 3000-3800 cm"1.
They are displayed in Figure 13. Heating the sample for
2 hr at 250° resulted in a huge band in the region 3600-
3670 cm"1 with a shoulder at 3390 cm"1 (Figure 13a). In
the 1100-1700 cm"1 region bands were observed at 1265, 1350,
1410, 1435, 1490 and 1535 cm"1. Following dehydration at
300° the 1535 and 1435 cm"1 bands disappeared (Figure 13b).
In the OH stretching region the most intense band appeared
32
-------
CO
m
<*>
in
in
E
2
M-l
Ul
Q)
C
•H
i (0
0 id
TI
C C
O O
•H O
0)
U)
Cn
C
-H
IS
O
(0
O
H
0)
Cn
•H
33
-------
g
lH-
r-H
3
CO
0)
£
EH
C
o
|m
W
df
m
•P in
O (N
W -O
0)
•O ^3
C O
(0 -H
^3 H
(1)
3
Q)
34
-------
O)
CM
CO
m
ro
O
m
ro
00
N
CM
CM
m
CM
m
CM
CM
m
Q
_i
LU
CM
o
CM
UJ
c
(D
O
•H
M
(I)
co
m
ro
tfP
in
m
(N
o
M-l
-P
U
0)
td
X!
X
T3
-------
at 3750 cm l with a shoulder at 3725 cm *. The band in the
3600-3670 cm"1 region was strongly reduced in intensity and
three components at 3660, 3615 and 3530 cm""1 may be resolved.
The 3390 cm"1 band remained unchanged.
At dehydration temperatures above 300° all these bands
decreased rapidly in intensity. At 400° only bands at 1265
and 1500 cm"1 remained in the" 1100-1700 cm"1 region (Figure 13c)
Bands occurred also at 3750 cm"1and 3615 cm"1 , although
strongly reduced in intensity. At 500°, only the 3750 cm"1
band was visible in the spectrum (Figure 13d), and at 800°
no infrared bands were observed in the regions investigated
(Figure 13e). Table 5a summarizes the OH bands observed in
the 3000-3800 cm"1 region.
The ir Spectrum of SO 7 Adsorbed on MgO at Room Temperature
Beside the bands due to gaseous SC>2 at 1340 and 1150 cm" 1,
a broad band was observed between 800 and 1100 cm" 1 after
adsorption of SO 2- The 3750 cm"1 band disappeared but a new
broad band developed around 3655 cm"*. The 3725-3730 cm" 1
shoulder remained unaltered. Bands were always present at
3575 and around 3450 cm" 1 but were not always clearly resolved
because of their broadness. These features were independent
of the pretreatment temperature, except for the 800° pre-
treatment where no detectable amounts of OH groups were left
on the MgO surface. Figure 14a gives an example of the
spectra obtained following removal of S02 by evacuating at
room temperature for 20 min. The MgO sample had been pre-
treated at 500°.
The Desorption and Reaction of SO? at Elevated Temperatures
Figures 14b-e give typical spectra from the desorption
and reaction of SO2 with the surface upon heating the sample
36
-------
0)
c i
o
at
cm
Figure 13. The infrared spectra of MgO at several dehydration
temperatures: A. after 2 hr heating at 250°;
B. after 2 hr heating at 300°; C. after 2 hr
heating at 400°; D. after 2 hr heating at 500°;
E. after 2 hr heating at 800°.
37
-------
cm •
Figure 14. The infrared spectra of SO2 and its reaction products
adsorbed on MgO after degassing in vacuo (MgO pre-
treated at 500°): A. at room temperature for 20 min;
B. at 220° for 1.5 hr; C. at 420° for 2 hr; D. at
600° for 2 hr; E. at 800° for 2 hr.
38
-------
Table 5: FREQUENCIES OF THE OBSERVED IR BANDS AND
THEIR ASSIGNMENT
A. OH band spectrum of MgO
Pretreat-
ment tem-
perature Frequency
°C (cm"1) Assignment
300 3750 Free, accessible OH
3725 Partially inaccessible OH
3615 Hydrogen bonded OH
3660 OH group in Mg(OH)2
3530 OH group in Mg(OH)2
3390 OH group in Mg(OH)2
B. Bands formed during desorption of SO2 under vacuum
Degassing
temper-
ature
°C
300-500
Frequency
(cm'1)
895
925
975
1040
1070
Assignment
S032" (A)
S032~ (A)
SO32~ (A) , S032~
S032" (B)
S0o2~ (B)
(B)
39
-------
Table 5 (Continued)
Degassing
temper-
ature Frequency
°C (cm ) Assignment
800 860 SO^2"
1020 SO^2"
1070 SO,/"
C. Bands formed during reaction of adsorbed SC>2 with
Reaction
tempera-
ture
°C
400
Frequency
(cm"1) Assignment
3740-3750 Free OH
3640 Mg-OH
803
3430 Mg-OH
S03
1345
1060 3
1240 0/0
/ \ / '
/ \ /
1120 M S^ (1)
1060 ^^ V°
980
40
-------
Table 5 (Continued)
1290
1175
io15
800 1390
1360
s /r
0 0
910
1195
M
1120 X0 ,0
x
S
1045 ,o' x'0
M'
980
1050 S°3
1220 / \ •'
1//U M S (3)
\ / \\
1170 0 0
1050
950
41
-------
to increasingly higher temperatures under dynamic vacuum.
After heating 1.5 hr at 220° under vacuum no changes occurred
in the 800-1100 cm"1 region. The bands in the OH stretching
region were better resolved and occurred at 3400, 3600, 3670
and 3730 cm"1 (Figure 14b). The latter was increased some-
what in intensity. Further heating for 2 hr at 420° restored
the 3740-3750 cm"1 band, while the other bands in the OH
region disappeared (Figure 14c) . The 800-1100 cm"1 region was
completely altered. Five intense bands were resolved: two
partially overlapping bands at 895 and 925 cm"1 , a band at
975 cm"1 and a band at 1040 cm"1 with a shoulder at 1070 cm"1.
Up to 500° this five-band spectrum was unaltered.
Above 500° all the bands decreased strongly in intensity
as seen in Figure 14d. At 800° no OH bands were left in the
3300-3800 cm"1 region and three broad bands at 860, 1020, and
1070 cm"1 remained in the 800-1100 cm"1 region, regardless
of the pretreatment of the MgO sample. A summary of the bands
observed under various conditions and their assignments, as
explained in the discussion, are given in Table 5b.
A sample pretreated at 330° behaved essentially in the
same manner. Its OH bands at 3750 and 3615 cm"1 were
shifted to lower frequencies at 3655, 3590 and 3400 cm"1.
Desorption of S02 at 300° for 2 hr restored the two original
OH bands. At the same time the five-band spectrum appeared
in the 800-1100 cm region with an additional weak band at
955 cm-1. The latter disappeared after degassing at 500°.
For MgO pretreated at 800° the same five-band spectrum
occurred in the 800-1100 cm"1 region at only 300°, although
strongly reduced in intensity with respect to the samples
42
-------
pretreated at 330 and 500°. This reduction in intensity is
probably due to a loss in surface area. Some overlapping
occurred with the bands at 1020, 1070 and 860 cm"1. As a
consequence, the band at 1040 cm"1 was not resolved. The
bands at 1070 and 1020 cm"1 appeared more intense than
the bands at 975, 925 and 895 cm"1. This is the reverse of
the behavior observed for the samples pretreated at 330 and 500°
The infrared spectrum of SO2 adsorbed and reacted even at 300°
on MgO pretreated at 800° has the characteristic features of
SO2 reacted at 800° on MgO pretreated at 500°.
The Reactivity of Adsorbed SO? with O?
After desorption of S02 under vacuum at 500° the residual
adsorbed SO2 species were allowed to react with O2 at 200°
and at increasingly higher temperatures up to 800°. The
equilibration time at each temperature was 2 hr. Figure
15 displays the spectra obtained for MgO pretreated at 500°.
Very little change was observed in the spectra after
2 hr heating at 200°, except for the fact that in the 1100-
1400 cm"1 region (Figure 15b) bands began to appear at the
expense of the bands in the 800-1100 cm"1 region. This
was clearly evident after the 400° treatment: new bands
occurred at 1345, 1290, 1240, 1195, 1175 and 1120 cm"1
(Figure 15c). Weak bands appeared at 910 and 980 cm"1
while the 1040 cm"1 band with its shoulder at 1070 cm"1
was replaced by a triplet at 1015, 1045 and 1060 cm"1.
Drastic changes occurred also in the OH stretching region
where two new bands appeared at 3640 and 3430 cm"1 besides
the 3750 cm"1 band.
43
-------
Figure 15.
The infrared spectra of suifite species on H,0 before
ana after reaction ^0,= A. sulfite on «0 after
degassin, at 500" in vacuo,- B. heating 2 hr in excess
0, at 200-, C. after 2 hr in excess 02 at 400°;
„. after2hrinexcess02at600°;E. after 2 hr xn
excess 02 at 800°.
44
-------
Upon reaction at 600° the bands in the 3100-3800 cm"1
region decreased in intensity and became broader (Figure 15d).
The 3640 and 3430 cm"1 bands were shifted to 3650 and 3470 cm"1,
respectively. A new band was evident at 3570 cm"1. The 800-
1500 cm region was completely changed. The 1290 cm band
was reduced in intensity, and the four bands in the 1100-
1250 cm"1 region, originally present after heating at 400°,
were replaced by two strong bands at 1220 and 1170 cm"1.
The triplet around 1050 cm"1 occurred at 1015, 1055, and
1100 cm"1. A weak band at 955 cm"1 was the only one remain-
ing at the lower wave numbers.
After 2 hr in oxygen at 800° the OH band spectrum disappear-
ed completely. A broad band was observed around 1050 cm"1
(Figure 15e). It is probably composed of three components,
equivalent to the triplet described above. Also, the 1220
and 1170 cm"1 bands became broader. Two new weak broad bands
appeared around 1360 and 1390 cm"1. Table 5c summarizes the
observed frequencies and their assignment as explained in the
discussion section.
In order to obtain bands with detectable intensity for
MgO pretreated at 800° , the SO2 was not degassed at 500°
under dynamic vacuum prior to heating in excess O^' No bands
were observed in the 3100-3800 cm"1 region. After heating 2 hr
at 400° in excess 02 a spectrum was obtained similar to that
shown in Figure 15e with the exception of the 1360 and 1390 cm"1
bands. Also the band envelopes at 1220 cm"1 and 1170 cm"1
were more intense than the band envelope at 1050 cm"1. For
MgO pretreated at 500° the reverse intensity ratio was
observed.
45
-------
The ir Spectra of MgSOg and MgSO^ on MgO
By way of comparison Figure 16 shows the spectra of MgO
doped with solutions of MgSC>3 and MgBO^ and subsequently
degassed at 500°. The MgSC>3 spectrum was not well resolved;
however, it resembles qualitatively the spectrum obtained by
heating SO2 on MgO under vacuum at 500°. A broad adsorption
occurred in the region 890-925 cm"1 together with bands at
980 cm"1 and at 1040 cm"1. The latter had a shoulder at
1080 cm"1. In addition three bands occurred around 1180,
1220, and 1265 cm"1.
MgSO^ gave a better resolved spectrum. A triplet was
observed at 1055, 1040 and 1015 cm"1. Two weak bands were
observed at 950 and 980 cm"1. Bands of medium intensity
occurred at 1338, 1260, 1215 and 1195 cm"1. A shoulder was
observed at 1215 cm"1.
46
-------
980
1400
Figure 16. A. Infrared spectrum of MgO doped with Mg
solution and subsequently degassed at 500° for
2 hr. B. Infrared spectrum of MgO doped with a
Mg S03 solution and subsequently degassed at 500c
for 2 hr.
47
-------
SECTION VI
DISCUSSION
S02 ON MAGNESIUM OXIDE
The qualitative agreement between the esr spectra obtained
with S02 adsorbed on MgO before or after uv irradiation of
the solid, the similarity of our g values with those of other
authors (Table 1) , and the hyperfine-splitting data unambig-
uously reveal that our esr signal is due to SO2- Moreover,
the two different signals indicate two slightly different
adsorption sites on MgO.
Characterization of the SO~ Molecule
SO2 has 19 valence electrons. The odd electron occupies
a 2bj" molecular orbital according to Walsh's diagram.
12 21
Reuveni and Dinse have given an explicit wave function
for this 2bi" MO neglecting the sulfur 3d-orbital contribution
+ 2p ) (1)
where 1//2 is a normalization constant.
The molecular orbital occupied by the odd electron is
thus a linear combination of atomic px orbitals , perpendicular
12
to the plane of the molecule. According to Reuveni, et al. ,
one expects the following characteristics for the SO2 molecule
and its g tensor and hyperfine-splitting tensors : (a) two
magnetically equivalent oxygens, (b) axially symmetric 17O
and 33S hyperfine tensors with their unique components parallel
48
= CjS(3px) + _ 0(2p
-------
to each other and perpendicular to the molecular plane, and
(c) along this unique hyperfine direction the g value should
be close to the free-electron value. The maximum g value
should be along the O-O direction, which is the y direction.
Sulfur and Oxygen Hyperfine Interactions
The 33S and 17O hyperfine tensor may be resolved into an
isotropic part (A. ) and a traceless anisotropic part (A) as
described in Table 6. The choice of the sign is made so as
12
to be in agreement with A. for SO? in solution where
ISO Z
A. for 170 = 8.96 ± 0.05 G and A. for 33S = 14.67 ± 0.05 G.
ISO ISO
For the pure 3s orbital of 33S the coupling is 970 G
and for the pure 2s orbital of 17O a value of 590 G is
3 22
reported. ' Comparing with our values, we conclude that
the spin density in the 3s orbital of sulfur and 2s orbital
of 0 is between 1 and 2%. This number is negligibly small
and can be accounted for entirely by spin polarization. The
wave function adopted in the previous section can therefore
be considered as a good representation of the molecular orbital
of the odd electron, at least in the LCAO scheme.
Table 6: ISOTROPIC AND ANISOTROPIC PARTS OF THE 33S
AND 170 HYPERFINE TENSORS (GAUSS)
A.
iso
33S02
33S02
S16Q17
S16Q17
(A)
(B)
0~ (A)
0~ (B)
14.3
13
10
7.7
± 2
± 2
± 4
± 4
A
XX
44.7
42
26
21.3
± 2
± 2
± 4
± 4
-20.
-19.
A
yy
9 ±
6 ±
-13 ±
-10.
7 ±
A
zz
3
3
5
5
-23.
-22.
7
4
-13
-10.
7
± 2
± 2
+ 5
± 5
49
-------
The coeffiecients b^ and c^ of the 2b}" molecular orbital
may then be calculated by comparing the experimental values of
A with the theoretical values for an odd electron in a p
xx
orbital on 33S and 17O, respectively. The theoretical value
of A for 33S was calculated with the aid of the numerical
xx
value, < r~3 > = 3.41 x 1025 cm~3 , reported by Dinse and
91 9"")
Mobius. Using the value of < r~3 > = 3.36 x 1025 cm"3
the theoretical value of A for 170 was also calculated.
xx
Table 6 lists the spin densities on S(3p ) and O(2p 1 + 2p „)
X 5C •*• X t-
together with the total spin density calculated in this manner.
It must be remarked that the spin density on the oxygen reported
here is higher than for SC>2 trapped in a solid matrix, while
the values for 33S are nearly the same. This difference,
however, may only be due to the approximations made during
the calculation.
Table 7: 2b1" MO COEFFICIENTS AND SPIN DENSITIES
ON S AND O FOR SO^ ON MGO
SO2 (A) SO2 (B)
cl 0.86 0.84
bj 0.52 0.48
C!2 0.75 0.71
b}2 0.27 0.23
cj2 + b!2 1.02 0.94
50
-------
The g Tensor
The dominant contribution to the g shift comes from
mixing the 2b1" orbital with the 3aj' and 2b2' orbitals. '
The wave functions of these orbitals are
b2
') = c2S(3p ) + c^SOs) + O(2p + 2p „) +
z ^ - zi z^
b2
0(2pyl
and
= c3S(3p ) + —— 0(2p - 2p
Y zi
b5
0(2p , + 2p .) (3)
Exact values for the different coefficients occurring in these
wave functions are not available. We assume, as Reuveni did, 03
b3 = 0. The ratio c2/Ci+ can be obtained from the bond angle of
S02 with the aid of Coulson's relation between the degree of
hybridization and bond angle. The bond angle of SO2 is not
available. We adopted, therefore, the angle of SO2 in the
•^ 24
3Bi state as calculated by Brand and coworkers. This value
is 126.2° and gives 02/04 = 1.71.
Assuming with Reuveni that c22 = 2b22 and that b^ = b5,
and applying the normalization condition of the wave functions
ifj(3a}) and i|j(2b2) including overlap, one obtains the following
values for the coefficients: c^ = 0.36, c2 = 0.63, b2 = 0.40,
and b^ = b5 = 1//2. Then the deviations from the free-electron
g value are
51
-------
g
, 0 (4)
= 2(clc2Xs + blb2X0) (C1C2 + blb2)
yy E(2b1") - E(2b2') (5)
Ag = 2b12b52A0
ZZ EUbj") - E(2b2') (6)
where X = 386 cm"1 is the spin-orbit constant for S and A0 =
157 cm"1 is the spin-orbit coupling constant for O. The
energy differences in the denominators have been estimated by
Dinse and Mobius.
EOa]/) - E(2b1") = 34,500 crrT1 (7)
E(2b2') - E(2b1") = 38,500 cm"1 (8)
We obtain then for SO? (A) Ag = 0, Ag = 0.0011, and Ag
* xx zz yy
0.0140; whereas, for SO2 (B) Ag = 0, Ag = 0.0009, and
XX ZZ
Ag = 0.0136.
yy
The agreement with experiment is only qualitative in
the sense that we predict the right sequence of Ag shifts.
No conclusion about the bond angle of S02 can be obtained.
We adopted the 126.2° bond angle because we feel that the S02
bond angle is closer to that of the 3Bj excited state of S02
than to that of the ground state of S02 . A widening of the
bond angle of SC>2 with respect to SO2 is not unreasonable
when one considers Walsh's diagram. The 2bi" molecular
orbital increases only slightly in energy in going from 90
to 180° bond angel. The 4a^ ' orbital which is the highest
filled orbital of SO2 has a much greater energy increase in
going from 90° bond angle to the linear form. A bond angle
close to 90° is therefore more likely.
52
-------
The Nature of the Adsorption Sites
When MgO is degassed and subsequently uv irradiated in a
H2 atmosphere, two different epr signals are developed, corre-
sponding to a pretreatment temperature of 500 and 800°,
respectively. The center obtained after the 500° pretreatment
has an axially symmetric g tensor and is believed to be an
electron trapped at an oxygen ion vacancy. The center after
the 800° pretreatment was first ascribed to an electron
2
trapped at an anion-cation vacancy pair by Lunsford and Jayne.
24
Nelson and coworkers, however, showed that this epr signal
involves a hydrogen hyperfine splitting and that two centers
are present.
The existence of two different paramagnetic centers is
clearly confirmed in this work by the presence of two different
types of SC>2. Moreover, each SO^ species retains its same
asymmetric g tensor whatever the pretreatment of MgO. This
suggests that only the relative number of the two adsorption
sites changes with changing pretreatment temperatures.
We think that it is reasonable to ascribe both the
electron trapping centers and the SC>2 adsorption sites to
oxygen ion vacancies at the surface. As a working model one
may consider that the sites giving rise to SO^ (B) are
oxygen ion vacancies on the edges of the microcrystals: the
centers forming SC>2 (A) are then oxygen ion vacancies on
the flat surface of the microcrystals. The change of their
relative number with increasing pretreatment temperature is
in agreement with this assignment; that is, one can imagine
that the decrease in surface area at 800° is due to a smoothing
out of the surfaces of MgO crystallites, thus resulting in a
53
-------
decrease in the number of edges present on the nonideal crystal
surfaces. The number of sites (a maximum of 10 /g) can easily
be accounted for in this manner.
No explicit proof can be offered to show the orientation of
SOJ with respect to the surface. The odd electron is entirely
located on the SO;? ion as suggested by our calculations. Since
it plays no role in any kind of covalent bond, the bonding
forces are purely electrostatic. The oxygen ion vacancies have
a positive character and are likely to attract the more electro-
negative oxygen atoms instead of the sulfur atom. An analogous
17 25
orientation has been proposed by Bennett and coworkers '
for CC>2 and CS2 bonded to alkali metals.
803 ON MAGNESIUM OXIDE
g Tensor
According to a Walsh diagram for AB3 type molecules, 503
with 25 valence electrons should be pyramidal with C3V symmetry.
The filling of the A^ ground state should be according to the
sequence
... (Ie)lf(5a1)2(2e)tt(3e)1+(la2)2(6a1)1
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 2Aj ground state to the 2E state, either
as
... (Ie)tf(5a1)2(2e)3(3e)if(la2)2(6a1)2
or as ... (Ie)lt(5a1)2(2e)tf (3e)3(la2)2(6a1)2.
For the parallel direction g shifts would be due to excitation
from the ground state to the 2A2 state
... (Ie)4(5a1)2(2e)lt(3e)t+(la2)1(6a1)2
54
-------
In terms of energy the 6a} orbital lies well separated from the
2e, 3e, and Ia2 orbitals; hence, only a small departure of the
principal g values from the free electron value is expected.
One would predict that gii and gi would be slightly greater
than 2.0023, which is consistent with the observed value of
2.0034.
Hyperfine Tensor
The hyperfine structure of Figure 6a is in good agreement
with that reported for the 863 ion, thus confirming the
identification of the ion on magnesium oxide. The tensor
has nearly cylindrical symmetry, which is expected for the
pyramidal 803 ion. The direction of the unique axis is normal
to the plane formed by the three oxygen atoms.
Table 8: COMPARISON OF HYPERFINE COUPLINGS, SPIN DENSITIES, AND
BOND ANGLES FOR 303 IN DIFFERENT ENVIRONMENTS
803 on 803 in 803 in
MgO K2CH2(SO3)2 taurine
Isotropic coupling,
A , G 117 126 111
ISO
Anisotropic coupling,
2B, G 30 27 24
Spin density
3s
3p
030 angle, degree
0.12
0.51
112
0.13
0.46
111
0.11
0.41
111
55
-------
The hyperfine tensor may be resolved into its isotropic
part, A. , and aniotropic part, 2g, in the usual manner.
ISO
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 A. = 970 G for a pure 3s orbital of 33S and 23 = 59 G
ISO ^
for a pure 3p orbital were used. Results for SO^ in different
environments are given in Table 8.
The OSO bond angle for this ion was calculated from the
equation
~
where A 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 .
Obviously the crystalline environment does not greatly distort
the 803 ion.
The spectrum of 803 in Figure 6b is unique to MgO. It
appears to suggest that a i > a i , 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 8 for 303 in quite different environments. A
more plausible explanation of the spectrum observed at room
temperature is evident if one assumes motion in a plane which
includes the threefold symmetry axis. A libration such as
depicted in Figure 17, would have the effect of averaging ai i
and one a i component giving a i ' . The other a i component
would become a i i ' . The numerical values of a i ' and a i i ' given
in Table 8 are in agreement with this model. Such motion could
56
-------
Fig-lire 17. Schematic representation of SO^ motion on the edge
of a MgO crystallite.
58
-------
easily occur on edges of the MgO crystallites which are approxi-
o
mately 100 A on a side. Similar motion for S^ on MgO was
detected even at 77°K.
Reaction Mechanism
The epr results reported here clearly establish that SC>2
may be oxidized to SO^ with molecular oxygen. Furthermore,
the ease and extent of oxidation depend on the concentration
of SC>2 ions. This concentration effect may be interpreted in
terms of a concerted process involving an intermediate such
as S02 . . .O-O. . .SOp , although the line width of the epr
spectrum indicates that the paramagnetic ions are separated
o
from one another by a distance greater than 5 A. It is more
reasonable to assume that an intermediate such as SOp • - .O-O. . .SC>32~
is formed, which then results in 303 and a diamagnetic SO^2" ion.
Experiments currently in progress on other types of surfaces may
help to resolve this mechanistic problem.
COS" AND CS2 ON MAGNESIUM OXIDE
It is of interest to examine the g-tensor shifts for the series
COJ, COS" and CS^ . The largest deviation from the free electron
g-value, Ag , is associated with a mixing of the 2bj excited
state into the 4a^ ground state. For CO2 the 4a^ orbital can
be described as
^l>(4al) = cl 4>C(s) + c2^C(p )
z
+ c3 i//0(p + p ,) + ck O(s + s1) , (10)
and the 2bj orbital as
C(p ) + d3 i];0(p + p ). (11)
2C A. vs.
The g shift is given by
59
-------
Ag = -2(c2d2 A + 2c3d3A )(c2d2 + 2c3d3)/AE, (12)
where A and A are the one-electron spin-orbit coupling
90
parameters (A = 29 cm"1, A = 151 cm"1, A = 382 cm"1 ).
CO S „_
Using the hyperfine data of Ovenall and Whiffen for
CO2, one may calculate that cj = 0.389 and c2 = -0.586. The
coefficient c3 can be estimated from the normalization
condition
cf + c2 + 2c3 + 2ci+ + 2c\ = 1. (13)
2 2
Since c^ + GS is expected to be nearly zero, a value of
c3 = 0.5 results. Using d2 = 0.75 and d3 = -0.47 [7] and
ry *j
AE = 29,400 cm"1 , eq. (3) leads to a value of Ag = -0.0052,
yy
in close agreement with Ag (obs) = -0.0049.
For CS2 the hyperfine data of Bennett et al. indicate
2
that cj = 0.280 and c2 = -0.650. The observation that cj +
2
c2 are of equal magnitude for CO2 and CS2 is surprising, in
view of the difference in electronegativities of 0 and S.
It follows that c3 is equal to 0.5. Using Bennett's
estimation of AE = 10,000 cm, and the same d2 and d3 as
above, one calculates that Ag = -0.037, which compares
favorably to Ag (obs) = -0.039. It is of course necessary
yy
to substitute A for A in eq. (1).
Theoretical calculations are more difficult on COS"
because there is no hyperfine data presently available.
22 _
Since cf + c2 are nearly identical for CO2 and CS2, it seems
reasonable to take the average of the coefficients for those
two ions as an estimate for COS". This assumption results
in cj = 0.334, c2 = -0.618, and c3 = 0.5. The second term
60
-------
in eq. (10) becomes c3d3(A + A ). Taking AE = 19,700 cm
w O
(averaging AE's from COp and Csp and d2 and d3 as before,
Ag = -0.013, in fair agreement with Ag (obs) = -0.016.
yy yy
Currently, work is underway to determine hyperfine data for
COS" and CS2 on MgO.
83 ON MAGNESIUM OXIDE
The hyperfine structure provides convincing evidence
that the principal sulfur species on MgO after the reaction
at 400°C is a triatomic sulfur molecule. The 2:1 ratio for
the two main quartets indicates that there are twice as many
atoms of one type as another type. The presence of tertiary
splitting confirms that at least three sulfur atoms are
present. Furthermore, assuming the radical is an aba type
molecule, the percentages of each labeled atom are in good
agreement with the calculated values (Table 9). These
percentages were determined from the relative amplitudes of
the hyperfine lines.
These observations suggest that the paramagnetic sulfur
is either a 19-valence-electron 83 ion or a 17-valence-
electron S* ion. The 83 ion has been extensively studied
in alkali halide single crystals by Schneider et al.
19
and by Suwalski and Seidel. The g and hyperfine tensors
for the ion in KC1 are reported in Table 4. Here, the z
direction is perpendicular to the plane of the molecule
and y bisects the S S, S angle. If one assumes that the ion
a b a ^
on the surface is librating in the xy plane, the g tensor
and the hyperfine constants are reasonably close to those
reported for 83.
Although there is no reported spectrum for 8^, the
17-electron NO2 and CO2 molecules have been extensively
61
-------
investigated on MgO and in other media. They are generally
characterized by one g value, which is less than g . Moreover,
the wavefunction of the molecular orbital containing the
unpaired electron has considerable s character on the central
atom. Both the g and the hyperfine tensors of the sulfur
ion are in disagreement with these observations for 17-electron
radicals.
Additional support for S^ is apparent if one considers
the spin density on each sulfur atom. Neglecting any d
orbital contribution, the odd electron would occupy the
2bj MO, which has the form
|2b!> = -ci(2p b) + cp(2p a + 2p a )/2. (14)
z z z
The coefficients cj and c2 may be evaluated by resolving the
experimental hyperfine tensor into its isotropic and aniso-
tropic components:
-7.5
-7.5
39.6
=8.2+
-15.7
-15.7
31.4
(15)
and
-3.7
-3.7
20.7
=4.4+
-8.1
-8.1
16.3
(16)
Here, the negative signs for the perpendicular components
have been chosen in agreement with the results for SO^ and
62
-------
SeO2- For these ions the isotropic term was evaluated
directly; hence, the relative signs of the elements in the
experimental hyperfine tensor could easily be deduced.
Using a theoretical value of 97 G for the isotropic interaction
Table 9: EXPERIMENTAL AND CALCULATED PERCENTAGES
FOR 33S ISOTOPE COMBINATIONS IN S S S~.
aba
25% 33S 10% 33<
Exptl Calc Exptl Calc
33S
a
33sK
b
33S
ab
33S ,
aba
30.4 28.4 15.2 16
14.7 14.2 6.7 8
9.5 9.7 1.7 1
0.9 1.7 ... 0
.0
.0
.8
.1
in a pure 3s orbital and 59 G for 28 in a pure 3p orbital
[23 = 4/5y y 3r>] one maY calculate that
G Ii ^
cj2 = 0.533
and
c22 = 0.55. (17)
Using the hyperfine data of Schneider et al, one obtains
cj2 = 0.65 and c22 = 0.44. These results indicate that the
electron is completely localized on the 83 ion. In addition
it appears that the spin density on the central atom is less
28
for 83 on MgO than in KC1. Schlick has reported values
63
-------
of cj2 = 0.473 and c22 = 0.528 for O§ in KC103. A Huckle
molecular orbital treatment yields cj2 = c22 = 0.5.
In view of the lack of motion for ions such as COo,
SO2, and 03 on MgO it is a bit surprising that 83 is
librating at 77°K. On the other hand, the larger size of
the S^ ion may prevent it from fitting into a more restricted
position such as an oxide ion vacancy. Schneider et al. observed
motional averaging of S02 and SeO^ in KC1 and KBr at room
temperature. This phenomenon was explained by the hopping
of the paramagnetic molecule among its twelve equivalent
eauilibrium sites in the cubic crystal. Studies on the
hindered motion of NC>2 in zeolites have demonstrated that
libration of 45° from an equilibrium position (90° total
angle) is sufficient to completely average any anisotropy.
Such motion in the xy plane of the molecule is reasonable
if the S^ ions are located on the edges or corners of the
o
MgO crystallites, which are approximately 100 A on a side.
At room temperature it appears that the ions are also
experiencing motion in the xz and yz planes.
The origin of the reducing electron is uncertain, although
there is ample evidence that such electrons are available on
the surface of MgO. Curiously, higher purity MgO has a
29
greater capacity for reducing O2 to Oj; therefore, it is
unlikely that impurity ions are directly involved. It has
been argued that the electron transfer takes place at corner
oxide ions, where there is maximum instability.
The formation of 83 on the partially hydroxylated MgO
may involve more than a simple electron transfer step. The
simultaneous formation of 83, the loss of the sulfur polymer,
64
-------
and the formation of H2S at 400°C suggest that a reaction
such as
S + 4OH" -> 2H2S + 283- + SO= + S
may occur. The absence of any paramagnetic S or S
following the reaction at 400°C is evidence that the sulfur
either completely reacts or that it desorbs. Actually,
very weak hyperfine lines due to S^ were observed at
temperatures as low as 200°C; however, at these tempera-
tures the spectrum was strongly dominated by the S
radical.
INFRARED STUDY OF S02 ON MAGNESIUM OXIDE
The Hydroxyl Band Spectrum of MgO
Imelik and coworkers reported that physisorbed water
was desorbed from MgO at 150° under dynamic vacuum. This
treatment also eliminated the 1640 cm"1 deformation band of
water. Therefore, the bands in the 1100-1700 cm region
reported in Figure 13 can not be ascribed to water. We
rather believe that they are due to carbonate species on the
surface. The frequencies fall in the same range as those
32
reported in the literature for carbonate species.
•3 Q
Anderson observed bands at 3750, 3710 and 3620 cm"1
which are ascribed to three types of OH groups on the MgO
surface. The 3750 cm"1 band is attributed to isolated OH
groups pointing out of the surface; whereas, the 3620 cm"1
band is assigned to OH groups with their oxygen linked to
two Mg ions. The band at 3710 cm"1 only appeared after
rehydration of MgO and is acribed to an intermediate product
34
between MgO and Mg(OH)2- Lisachenko et al. disagree with
65
-------
this interpretation of the 3620 and 3710 cm 1 bands. They
interpret the 3620 cm"1 band as due to hydrogen bonded
hydroxyl groups and the 3710 cm"1 band as due to hydroxyl
groups with a free proton but with their oxygen hydrogen
bonded to a neighboring OH group:
H-0 . . . H-0
I 1
-M- -M-
3710 cm"1 3620 cm"1
After heating at 300° we reported three additional bands at
3660, 3530, and 3390 cm"1. These may be ascribed to preferred
geometric arrangements for other hydrogen bonded OH groups.
33 34
We agree with previous investigators ' concerning the
assignment of the 3750 cm"1 band to isolated hydroxyl groups.
However, as indicated in Figure 13, the 3710 cm"1 band,
which occurs at 3725 cm in our case, is already present
during the initial dehydration process in contradiction with
Anderson's work. Moreover, our adsorption data with SO2
revealed that a similar band (3730 cm"1) is present after
adsorption. This suggests that the band is due to OH groups
in the bulk MgO or that they are screened off from the S02
molecules by inaccessible pores.
From our data it is impossible to decide whether
Anderson's interpretation or Lisachenko's is correct con-
cerning the 3620 cm"1 OH groups. The broadness of the
bands and the fact that these OH groups are more easily
dehydrated than the 3750 cm"1 OH groups are experimental
results in favor of Lisachenko's interpretation.
66
-------
The Species Formed during the Adsorption and Desorption of SO2
under Vacuum
No SO2 bands around 1360 cm"1 and 1150 cm""1 are left in
the spectrum after degassing the excess SO2 at room temperature;
hence, no physisorbed SO2 remains on the surface. At the
same time the 3740-3750 cm"1 OH band does not reappear. To
restore this 3750 cm band it is necessary to heat the
sample to 300°.
The complex OH band spectrum of Figure 14a and b contains
the 3725-3730 cm"1 band, attributed to inaccessible OH groups,
and three bands around 3670, 3600 and 3400 cm"1. The latter
can only come from the original 3750 cm"1 band. The shifts
to three lower frequencies could indicate three different
strengths of interaction with only one sulfur species adsorbed
on three different sites. It is, however, more probable that
three different sulfur species are present on the surface.
Each of them interacts with OH groups. This results in
three different frequency shifts according to the type and
strength of interaction in each individual case.
The broad band between 800-1100 cm"1 (Figure 14a and b)
does not allow an identification of the possible sulfur species
on the surface at low temperatures. It is possible that SO2
is present in the form of a strongly complexed surface species.
In sulfinato complexes, where SO2 is linked with one oxygen
or both oxygens to the complexing ion, the free S-O stretching
vibration absorbs in the range 1100-1050 cm"1 while the v
^ as
(S-O-M) vibration adsorbs at still lower wave numbers. The
67
-------
difference between the two frequencies is 100-200 cm 1 for
sulfinato, O-complexes and 10-80 cm"1 for sulfinato O, 0'-
complexes . These frequencies fall in the broad adsorption
range of 800-1100 cm"1, but the existence of such complexed
SC>2 cannot be verified from our experiments.
At 300° the 3750 cm"1 band is completely restored,
which indicates that no OH groups remain hydrogen bonded
or react with the sulfur species. At the same time the
broad 800-1100 cm""1 absorption goes over to the five-band
spectrum shown in Figure 14c. The most reasonable explana-
tion is that certain surface species of SC>2 are desorbed and
others react with the MgO surface.
The spectrum of Figure 14c agrees qualitatively with
that of MgO doped with MgSO3 (Figure 16b). In the latter
spectrum bands occur also at 1220 and 1265 cm"1 which are
not present in the degassed MgO samples of Figure 14c.
The origin of these bands is not known. In the 800-1100 cm"1
2_
region the free SO 3 has a nondegenerate vibration vj at
-i _i36
961 cm : and a doubly degenerate vibration V3 at 1010 cm
If the C3v symmetry is lowered to C the 1010 cm"1 band
splits into two bands. Splitting of the degenerate v3
O "7 *3 Q
vibration of S03 occurs into sulfito complexes ' . If
the SO^~ ion is bonded with its S atom to the surface the C3V
symmetry is preserved. If bonding occurs through O the
symmetry can be lowered to C for unidentate complexes while
S
it is always C in bidentate complexes. Furthermore, coor-
S
dination through sulfur shifts the SO stretching bands to
higher frequencies, whereas coordination through oxygen
shifts them to lower frequencies relative to the frequencies
68
-------
f >_, f .36
of the free ion
In sulfito bidentate complexes Baldwin observed the \>\
at 988 cm"1 and the two bands of V3 in the regions 1093-
1070 cm"1 and 1036-1042 cm"1. These frequencies agree
reasonably well with the 975 cm"1, 1040 and 1070 cm"1
bands observed in our spectra (Figure 14c). This observa-
tion together with the similiarly to the spectrum of MgSC>3
i-\
doped MgO is strong evidence for the existence of a 303" sur-
face species on MgO. We believe also that the pair of
bands at 925 cm"1 and 895 cm"1 belongs to the nearly degenerate
2_
V3 (E) vibration of a second 863 species. The latter species
is less stable than the former as can be seen from Figure 14d.
Following this reasoning the 1040-1070 cm-1 pair of bands
2
is assigned to a S0§~ species coordinated through its sulfur
r\
while the 925-895 cm"1 pair of bands is assigned to a 803"
species coordinated through an oxygen atom.
Also some samples have a weak 955 cm"1 band which
completely disappears at 500°. The origin of this unstable
band is unknown.
2
The 800° pretreated MgO has only weak 803 bands at the
same frequencies as those shown in Figure 14c. This indicates
2_
the presence of very small amounts of 803 . One reason for
this decrease is the loss in surface area due to the 800°
pretreatment with respect to the 500° pretreatment. It
2_
also appears that the 803 may react to form other species
since sulfite salts are unstable above 500°. The spectra
of the SO2 adsorbed on MgO pretreated at 800° predominantly
show the features of the spectrum of S02 on MgO pretreated
69
-------
at 500° and subsequently degassed at 800°, i.e., two broad
bands at 1070 and 1020 cm"1 with a peak around 860 cm"1 .
The most reasonable explanation for these bands is the forma-
P
tion of some SO^. species. Blyholder and Cagle also give
three frequencies at 865, 955 and 1055 cm"-*- for a surface
S0i+ species on Fe. Our frequencies on MgO fall in approximately
the same range. A comparison of these frequencies with those
of sulfato complexes indicates that a monodentate complex is
probably formed on the surface
A comparison of the spectra of SO2 adsorbed on MgO
and CaO reveals significant differences, although the over-
all reactions are similar. At low coverages it appears
that SO? reacts with CaO at 25° to form a pyramidal sulfite
This assignment by Low and coworkers is based on the obser-
vation of bands at 973, 925, and 632 cm"1. The bands at 973
and 927 cm"1 are in agreement with two of those observed
2 —
for 803 (A) on MgO. The rather complex spectrum obtained
by degassing the CaO-SO2 sample to 500° has been attributed
to a surface sulfate; whereas, we attribute a somewhat
similar spectrum to the formation of two sulfite species.
Following treatment of the samples at 750-800° a much more
complex spectrum was observed on CaO than on MgO. The
2-
resulting spectra were assigned to species such as 8203 ,
2- 2- 2-
S205, S308, etc., for CaO and SO^ for MgO.
Reaction of Adsorbed SO? with O?
It is apparent from the spectra of Figure 15 that the
sulfito groups on MgO react with oxygen to form new surface
species. At least eleven bands are apparent in the spectrum
of Figure 15c and other weak bands may be hidden beneath the
70
-------
dominant ones. As the temperature is raised to 600 and 800°
some of the bands diminish while new bands appear. Although
the situation is complex, one may make reasonable assignments
concerning the origin of the bands.
The oxidation products are probably sulfato complexes.
The free sulfate ion has T symmetry. Its v3 and v^
vibrations are infrared active at 1105 and 613 cm"-'- ,
respectively . When the ion is bonded through one oxygen
the forbidden Vj vibration is allowed at 970 cm"1 and
the triply degenerate v3 vibration splits into two bands.
2-
Por a bidentate complex where SO^ is bonded with two
oxygens, the symmetry becomes C2V with three bands from the
split v3 vibration in addition to the one band from the i
vibration. Two types of bidentate complexes are possible
one is a chelating complex and the other is a bridging
complex. Typically the v3 vibrational frequencies of the
chelating complex (1230, 1120, 1060, and 960 cm"1) are
higher than those of the bridging complex (1160, 1100, 1030
and 970 cm"1)36.
As indicated in Table 5c, the bands observed following
reaction of the sulfito complex with oxygen at 400, 600
and 800° may be divided into three chelating and one
bridging complex. Since the thermal treatment at 800°
produces a drastic modification of the surface, it is not
surprising that the surface complexes are different at
800° compared to 400°. A bridging complex and a sulfato
complex denoted by Cj were the first to decrease and a new
complex appeared following reaction at 600°. The spectrum
of a second chelating complex, Cj/ was also weaker at this
71
-------
temperature. Such concomitant behavior of certain bands was
used to make the assignments. At 600° a new chelating complex,
€3, appeared which was the dominant species following reaction
at 800°.
At all three reaction temperatures bands appeared in the
1345-1390 cm""-' region with a possible counterpart in the
1050-1060 cm"1 region. Free SO3 exhibits bands at 1391 and
1068 cm"1; hence, it is reasonable to assume that the similar
bands on MgO are due to strongly adsorbed 803 which is in
equilibrium with the sulfato complexes. The hydroxyl bands
at 3640 and 3430 cm"1 may be shifted and the OH~ groups
stabilized by interactions with the strongly bonded 803.
The assignment of the bands to 863 is supported by the
observation that the 1360 and 1390 cm"1 bands did not appear
on MgO pretreated at 800°, and the counterpart of the 803
spectrum at 1050 cm"1 became less important than the pair
of bands at 1170 and 1220 cm"1. Pretreatment at 800°
must destroy the sites responsible for the formation of the
803 species.
Low and coworkers likewise studied the reaction of
adsorbed sulfur species with oxygen at elevated temperatures.
They observed that the sulfate band pattern produced by
degassing the sample at 750° became more like that of an
ionic sulfate upon reaction with oxygen. They concluded
that small patches or microcrystals of CaCO^ are formed on
the CaO surface. The spectrum of Figure 16c is probably due,
in part, to the formation of corresponding microcrystals on
MgO. A comparison of this spectrum with those of Figure 15
C-E shows that similarities exist; however, the presence of
distinctly different bands indicates that more than one type
of sulfate species is formed upon reaction with oxygen.
72
-------
SECTION VII
REFERENCES
1. Mashchenko, A. I., G. B. Pariiskii, and V. B. Kazanskii.
Radicals Formed during Chemisorption of Electron-Acceptor
Molecules on the Surface of n-Semiconductors. III. Forma-
tion of Radicals during Chemisorption of Sulfur Dioxide
on the Surface of Reduced Titanium Dioxide and Their
Reaction with Oxygen. Kinet. Katal. (Novosibirsk). 9_:
151-157, January 1968.
2. Lunsford, J. H., and J. P. Jayne. An Electron Paramagnetic
Resonance Study of Surface Defects on Magnesium Oxide.
J. Phys. Chem., 70: 3464-3469, November 1966.
3. Chantry, G. W., A. Horsfield, J. R. Morton, J. R. Rowlands,
and D. H. Whiffen. The Optical and Electron Resonance
Spectra of 303. Mol. Phys. (London). _5(3) : 233-239,
May 1962.
4. Lind, G. and R. Kewley. Electron Spin Resonance of y~
Irradiated Taurine. Canad. J. Chem. (Ottawa). 50: 43-49,
January 1972.
5. MacNeil, K. A. G. and J. C. J. Thynne. Negative Ion
Formation in Carbon Disulfide and Carbonyl Sulfide.
J. Phys. Chem. 73: 2960-2964, September 1969.
6. Lunsford, J. H. and J. P. Jayne. Formation of COj
Radical Ions When CO2 is Adsorbed on Irradiated Magnesium
Oxide. J. Phys. Chem. 69: 2182-2184, July 1965.
7. Crawford, V. Infrared Spectra of Adsorbed Gases. Quarterly
Revs. 14: 378-401, 1960.
8. Blyholder, G. D. and G. W. Cagle. Infrared Spectra of
Hydrogen Sulfide, Carbon Disulfide, Sulfur Dioxide,
Methanethiol, and Ethanethiol Adsorbed on Iron and
Nickel. Environ. Sci. Technol. 5: 158-161, February 1971.
73
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9. Deo, A. V., I. G. Dalla Lana, and H. W. Habgood. Infrared
Studies of the Adsorption and Surface Reactions of Hydrogen
Sulfide and Sulfur Dioxide on Some Aluminas and Zeolites.
J. Catal. _21(3) : 270-281, June 1971.
10. Low, M. J. D., A. J. Goodel, and N. Takezawa. Reactions
of Gaseous Pollutants with Solids I. Infrared Study of
the Sorption of SC>2 on CaO. Environ. Sci. Tech. _5_: 1191-
1195, December 1971.
11. Goodsel, A. J., M. J. D. Low, and N. Takezawa. Reactions
of Gaseous Pollutants with Solids. II. Infrared Study of
Sorption of SO2 on MgO. Environ. Sci. Tech. 6_: 268-273,
March 1972.
12. Renveni, A., Z. Luz, and B. L. Silver. ESR of SO^-
J. Chem. Phys. 53; 4619-4623, December 1970.
13. Hariharan, N. and J. Sobhanadri. Electron Spin Resonance
in X-Irradiated Sodium Sulphate. Mol. Phys. (London).
r?(5): 507-516, November 1969.
14. de Lisle, J. M., and R. M. Golding. ESR Study of X-
Irradiated Sodium Thiosulfate Single Crystals. J. Chem.
Phys. £3_(9) : 3298-3303, November 1965.
15. Schneider, J. , B. Dischler, and A. Raiiber. Electron Spin
Resonance of Sulfur and Selenium Radicals in Alkali Halides.
Phys. Status Solidi, 13_(1) : 141-157, 1966.
16. Lunsford, J. H. and J. P. Jayne. Study of CO Radicals on
Magnesium Oxide with Electron Paramagnetic Resonance
Techniques. J. Chem. Phys. 44: 1492-1496, February 1966.
17. Bennett, J. E., B. Mile, and A. Thomas. Electron Spin
Resonance Spectrum of CS^ Radical Ion at 77°K. Trans.
Faraday Soc. (London) e>3_(530) : 262-273, February 1967.
18. Dudzik, Z. , and K. F. Preston. Paramagnetism and Catalytic
Activity of Sulfur-Impregnated Zeolites. J. Colloid and
Interface Sci. 26: 374-378, March 1968.
74
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19. Suwalski, J. and H. Seidel. ENDOR Investigation of the
S^ Sulphur Centre in KCl and NaCl. Phys. Status Solidi,
13_(1) : 159-168, 1966.
20. Walsh, J. The Electron Orbitals, Shapes, and Spectra of
Polyatomic Molecules. Part II. Non-hydride AB2 and BAG
Molecules. J. Chem. Soc. 2266-2288, August 1953.
21. Dinse, K. P., and K. Mobius. E.P.R. Studies of Electro-
lytically Generated SO^. Z. Naturforsch (Berlin). A23;
695-702, May 1968.
22. Luz, Z. , A. Reuveni, R. W. Holmberg, and B. L. Silver.
ESR of 170-Labeled Nitrogen Dioxide Trapped in a Single
Crystal of Sodium Nitrite. J. Chem. Phys. 5^(9): 4017-
4024, November 1969.
23. Atkins, P. W., and M. C. R. Symons. The Structure of
Inorganic Radicals. Amsterdam, Elsevier, 1967. 273 p.
24. Nelson, R. L., A. J. Tench, and B. J. Hamsworth. Chemi-
sorption on Some Alkalin Earth Oxides. I. Surface
Centres and Fast Irreversible Oxygen Adsorption on
Irradiate MgO, CaO and SrO. Trans. Faraday Soc. (London).
^3_(534) : 1427-1446, June 1967.
25. Bennett, J. E., B. Mile, and A. Thomas. Electron Spin
Resonance Spectrum of the CO^ Radical Ion at 77°K.
Trans. Faraday Soc. (London). jxL(515) : 2357-2364,
Novmeber 1965.
26. Ovenall, D. W., and D. H. Whiffen. Electron Spin
Resonance and Structure of the CO;? Radical Ion. Mol. Phys.
(London). j4(2) : 135-144, March 1961.
27. Chantry, G. W., and D. H, Whiffen. Electron Absorption Spectra
of CO2 Trapped in y-Irradiated Crystalline Sodium Formate.
Mol. Phys. (London). 5(2): 189-194, March 1962.
75
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28. Schlick, S. ESR Spectrum of 05 Trapped in a Single Crystal
of Potassium Chlorate. J. Chem. Phys. 56: 654-661,
January 1972.
29. Derouane, E. G. and V. Indovina. ESR of 0^ Species
Adsorbed on Thermally Activated MgO Powders. Chem. Phys.
Letters (Amsterdam). L4(4): 455-459, June 1972.
30. Baird, M. J. and J. H. Lunsford. Catalytic Sites for the
Isomerization of 1-Butene Over Magnesium Oxide. J. Catal.
26: 440-450, August 1972.
31. Faure, M., J. Fraissard, and B. Imelik. The Surface
Properties of Magnesium Oxide. I. Constitution Water.
Bull. Soc. Chim. Fr. (Paris). 2287-2293, July 1967.
32. Evans, J. V., and T. L. Whateley. Infra-Red Study of
Adsorption of Carbon Dioxide and Water on Magnesium Oxide.
Trans. Faraday Soc. (London). 63(539): 2769-2777,
November 1967.
33. Anderson, P. J., R. F. Horlock and J. F. Oliver. Inter-
action of Water with the Magnesium Oxide Surface. Trans.
Faraday Soc. (London). 61(516): 2754-2762, December 1965.
34. Lisachenko, A. A., and V. N. Filimonov. Infrared Spectro-
scopic Study of Surface Hydroxyls and the Chemisorption
of Hydrogen on Magnesium Oxide. Dokl. Akad. Nauk SSSR
(Moscow) 177: 391-394, November 1967.
35. Vitzthum, G. and E. Lindner. Sulfinato Complexes. Angew.
Chem. Int. Ed. (Weinheim) 1£: 315-326, May 1971.
36. Nakamoto, K. Infrared Spectra of Inorganic and Coordina-
tion Compounds. New York, Wiley, 1970. pp. 94-97,
174-177.
37. Baldwin, M. E. Sulphitobis(ethylenediamine)cobalt(III)
Complexes. Chem. Soc. 3123-3128, July 1961.
76
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38. Newman, G., and D. B. Powell. The Infra-Red Spectra and
Structures of Metal-Sulfite Compounds. Spectrochim. Acta.
(Belfast). 19: 213-224, January 1963.
77
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SECTION VIII
LIST OF PUBLICATIONS
1. "An Electron Paramagnetic Resonance Study of SO;? on
Magnesium Oxide", R. A. Schoonheydt and J. H. Lunsford,
J. Phys. Chem., 76, 323-328 (1972).
2. "Infrared Spectroscopic Investigation of the Adsorption
and Reactions of SO2 on MgO", R. A. Schoonheydt and J. H.
Lunsford, J. Catal., 26, 261-265 (1972).
3. "The EPR Spectra of COS" and CSJ on Magnesium Oxide",
M. J. Lin, D. P. Johnson and J. H. Lunsford, Chem. Phys.
Letters, 15, 412-414 (1972).
4. "Electron Paramagnetic Resonance Study of 83 Formed on
Magnesium Oxide", J. H. Lunsford and D. P. Johnson, J.
Chem. Phys., _58, 2079-2083 (1973).
5. "Electron Paramagnetic Resonance Evidence for the Formation
of 303 by the Oxidation of SO;> on MgO" , Y. Ben Taarit and
J. H. Lunsford, J. Phys. Chem., 77, 1365-1367 (1973).
78
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SECTION IX
GLOSSARY OF TERMS, ABBREVIATIONS, AND SYMBOLS
A. isotropic hyperfine splitting.
a
, A hyperfine coupling parallel to a symmetry axis.
a i , A i hyperfine coupling perpendicular to a symmetry
-L -L axis.
a , a , a hyperfine coupling along the x, y, and z direction,
xx yy zz .
respectively.
bi 9 coefficient of an atomic orbital.
' . . .
C}f9 coefficient of an atomic orbital.
c0 , c symmetry notation for molecules.
(le) , (5aj)... sequence of molecular orbitals .
E energy of a molecular orbital.
g g factor.
g i g component for the magnetic field parallel to
1 the axis of symmetry.
gi g component for the magnetic field perpendicular
-^ to the axis of symmetry.
g , g , g g component for the magnetic field alona the x, y,
xx yv zz . _ . . . . . ,
and z direction, respectively.
G gauss.
79
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p , p , p atomic wave functions with unit orbital angular
*x y z
* momentum.
Q-band 30-35 GHz microwave region.
r radius.
s atomic wave function with zero orbital angular
momentum.
x-band 9-10 GHz microwave region.
2B anisotropic hyperfine coupling.
v magnetogyric ratio of the electron.
e
Y magnetogyric ratio of the nucleus.
n
Vj 2 vibrational frequencies.
bond angle.
0 wave function.
80
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TECHNICAL REPORT DATA
(/'lease read Instructions on the reverse before < <
'li lin\;l
EPA-650/3-74-006
Structure and Reactivity of Adsorbed Oxides
of Sulfur
5. REPORT DATE
Approval: October 1974
3 RECIPIENT'S ACCESSION-NO.
6. PERFORMING ORGANIZATION CODE
7 AUTHOR(S)
Jack H. Lunsford
8. PERFORMING ORGANIZATION REPORT NO.
9 PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Chemistry
Texas A&M University
College Station, Texas 77843
10. PROGRAM ELEMENT NO.
1A1008
11. CONTRACT/GRANT NO
Grant 801136
12 SPONSORING AGENCY NAME AND ADDRESS
U. S. Environmental Protection Agency
National Environmental Research Center
Research Triangle Park, North Carolina 27711
13. TYPE OF RE PORT AND PERIOD COVERED
Final: 1 May 70 to 30 Apr.73,
14. SPONSORING AGENCY CODE
15 SUPPLEMENTARY NOTES
16 ABSTRACT
The purpose of the research reported here was to determine the structure
and reactivity of adsorbed oxides of sulfur and other small sulfur-containing
molecules. The molecules that were studied include the anion radicals of sulfur
dioxide, sulfur trioxide, a triatomic sulfur species and carbonyl sulfide. Dia-
magnetic sulfite and sulfate ions, as well as covalently bonded oxides of sulfur,
were also studied. The adsorbent was a high surface area magnesium oxide powder.
Electron paramagnetic resonance and infrared spectroscopv were used to characterize
the surface species.
The results show that electronegative molecules such as sulfur dioxide adsorb
either by an electron transfer from the solid, forming a negative radical ion, or
by reacting with the oxide ions of the lattice. At room temperature the adsorbed
sulfur dioxide anion radical may be oxidized with molecular oxygen to the sulfur
trioxide anion radical; whereas, at elevated temperatures the sulfite ions may be
oxidized. In contrast, the anion radical of carbonyl sulfide is very unstable and
dissociates, presumably to elemental sulfur and carbon monoxide, at low temperatures,
Elemental sulfur reacts with partially hydroxylated magnesium oxide , forming the
triatomic negative ion, hydrogen sulfide, and other unidentified products.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Sulfur Compounds: Sulfur dioxide, Sulfur
trioxide, Sulfate, Sulfides, Sulfite;
Magnesium oxide; Spectrometry: Infrared;
Electron paramagnetic resonance; Catalysis;
Surface properties; Chemical reactions:
Reduction, Oxidation.
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Held/Group
13. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (This Report)
Not Classified
21. NO. OF PAGES
88
20. SECURITY CLASS (Thispage)
Not Classified
22. PRICE
EPA Form 2220-1 (9-73)
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