The claus reaction: VI. The reaction mechanism over a Sn-Mo oxide catalyst

Article abstract of DOI:10.1023/A:1023312730777

Based on the comparison of reactant conversions in pulses on the stationary surface of the catalyst, the Claus reaction is found to occur via a stepwise mechanism. The nature of interaction of the SO₂ and H₂S molecules with the catalyst surface was studied by FTIR and UV-VIS spectroscopy and the reactivity of the adsorbed species was studied in situ. The intermediate adsorbed reactant species are determined. A scheme of the reaction mechanism over the Sn-Mo oxide catalyst is discussed.

Full text of DOI:10.1023/A:1023312730777

Kinetics and Catalysis, Vol. 44, No. 2, 2003, pp. 260–267. Translated from Kinetika i Kataliz, Vol. 44, No. 2, 2003, pp. 280–289.
Original Russian Text Copyright © 2003 by Davydov, Shepot’ko.
The Claus Reaction:
VI. The Reaction Mechanism over a Sn–Mo Oxide Catalyst
A. A. Davydov* and M. L. Shepot’ko**
*
Department of Chemical Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
*
* Novosibirsk State University, Novosibirsk, 630090 Russia
Received April 30, 1999
Abstract—Based on the comparison of reactant conversions in pulses on the stationary surface of the catalyst,
the Claus reaction is found to occur via a stepwise mechanism. The nature of interaction of the SO and H S
2
2
molecules with the catalyst surface was studied by FTIR and UV–VIS spectroscopy and the reactivity of the
adsorbed species was studied in situ. The intermediate adsorbed reactant species are determined. A scheme of
the reaction mechanism over the Sn–Mo oxide catalyst is discussed.
INTRODUCTION
and this indicates that the polymolybdenum com-
pounds are not bulk. When this polymolybdenum com-
pound is reduced, the support becomes “open” and
accessible to adsorption.
Catalytic tests. The catalytic activity of samples in
the Claus reaction was tested in a steel–glass flow-cir-
culation setup held under a constant temperature of
Earlier [1–5] one of us showed that the Sn–Mo
oxide catalyst (Sn : Mo = 9 : 1), which is a solid solu-
tion of molybdenum ions in SnO , is highly efficient
2
and promising for the Claus reaction:
SO + H S
S + H O.
x 2
2
2
4
03 K. Experiments were performed at 523 K and H2S
It is important that the catalyst was stable even in the
presence of oxygen in the reaction mixture, whereas
well-known catalysts for this reaction, for example,
Al O , are deactivated in the oxygen medium or in the
and SO concentrations of 0.5 and 0.25 vol %, respec-
2
tively. The partial pressure of sulfur was 6.7 Pa. The cir-
culation factor was 130 at a linear flow velocity of
2
3
8
m/s. Variation of the circulation factor showed that
presence of oxygen admixtures.
the effect of external diffusion on the rate was negligi-
ble. Samples with particle sizes of 0.25–0.5 mm were
studied. Variation of the particle size from 0.25 to
In this work, we attempted to elucidate the mecha-
nism of this commercially important reaction using cat-
alytic tests, UV–VIS and precision FTIR spectroscopy,
including dynamic in situ measurements.
3
0
.0 mm showed that, for the particles with the size
.25–0.5 mm, the reaction occurred in the kinetic
region. The specific reaction rate was expressed as the
number of the H S molecules based on the unit of the
catalyst surface area (cm ). Before each run, the sam-
2
EXPERIMENTAL
2
Catalyst. The Sn–Mo oxide catalyst (9 : 1) prepared
ples were activated by heating at 673 K in a helium flow
for 2 h. The catalytic activities in the Claus reaction
were compared at the following concentrations of reac-
by coprecipitation had a specific surface area of
2
~
120 m /g after drying for 10 h at 373 K followed by
calcination in air for 4 h at 723 K and represented a
solid solution of molybdenum ions in the SnO lattice
2
(
according to XRD). The surface of this solid solution,
Mo=O
O
according to the data of X-ray electron spectroscopy, is
enriched in molybdenum ions (Sn : Mo = 4 : 1) [6, 7].
As IR-spectroscopic studies showed, a compound
formed on the surface of the solid solution, as is a poly-
molybdenum (possibly heteropolymolybdenum) sys-
tem with a typical spectrum (Fig. 1) [8]. As follows
from further consideration below and findings reported
in [7, 9], this system mostly contains two-dimensional
compounds of six-coordinated molybdenum ions
bound to the support surface. Nearly all Mo=O bonds
are accessible to the adsorption of at least water and
Mo Mo
1
100
700
ν, cm–
1
Fig. 1. IR spectrum of the Sn–Mo oxide catalyst in the
NH . The absorption band with a maximum at
3
region of inherent vibrations recorded in KBr pellets with
–
1
–1
~
1010 cm shifts to 950 cm upon such interactions,
compensation by the SnO sample.
2
0
023-1584/03/4402-0260$25.00 © 2003 MAIK “Nauka /Interperiodica”

THE CLAUS REACTION: VI. THE REACTION MECHANISM
261
tants in the cycle (vol %): H S, 0.5 and SO , 0.2. To
H2S
2
2
compare the reaction rates over various oxide systems,
the testing conditions were chosen to be similar to those
described in [10] and used in the study of the catalytic
X
= 63%
X
= 48%
X
= 47%
H2S
H2S
SO2
activity of simple metal oxides in H S oxidation with
H2S
2
oxygen and sulfur dioxide.
SO2
SO2
Chromatographic analyses of H S, é , SO , and
2
2
2
ç é in both initial and final mixtures were carried out
2
using Porapak Q with helium as a carrier gas.
H O
The conversions of sulfur compounds to elemental
sulfur were calculated according to the equation
2
H2O
1
2
3
X = ([H S] – [H S] )
S
2
0
2
c
(1)
Fig. 2. Chromatograms and calculated conversions of SO
2
+
([SO ] – [SO ] )/[H S] + [SO ] ,
2
0
2
c
2
0
2
c
and H S on the stationary surface of the Sn–Mo oxide cata-
2
lyst at combined (1) and separate (2, H S; 3, SO ) pulse
2
2
where [H S] and [SO ] are the initial concentrations
feed of reactants. [H S] = 0.175 vol %, [SO ] = 0.09 vol %;
2
0
2 0
2 2
of the gases and [H S] and [SO ] are their concentra-
T = 523 K. The portion of H2S converted in the pulse is 14%
2
c
2 c
of the monolayer.
tions in the reaction cycle (mol/l). The conversions of
SO and H S were determined similarly.
2
2
The pulse technique in a flow system was used to
spectrum of a support and with the use of an attachment
for diffuse reflectance measurements. Measurements
were carried out in a special sealed quartz cell that
allowed sample training at temperatures up to 1273 K
in an atmosphere of various gases and in a vacuum,
as well as adsorption and desorption of gases and
vapors under various conditions. Spectra were recorded
at 293 K.
study the reaction mechanism. The conversions of both
reactants (SO and H S) on a stationary catalyst surface
2
2
were estimated by the pulses of mixtures with the fol-
lowing compositions: (1) H S + SO + He; (2) H S +
2
2
2
He; and (3) SO + He. The steady state of the catalyst
2
was reached by the treatment of the catalyst with the
reaction mixture in a flow system for 10 h. The small
H S and SO concentrations in the H S + He and SO +
2
2
2
2
He pulses were chosen to minimize changes to the
steady state. The portions of H S converted in the pulse
RESULTS AND DISCUSSION
2
were, as a rule, less than 1% of the monolayer even at
the maximum possible conversion. The number of mol-
ecules in each pulse was chosen to be much less than
1
. Determination of Reaction Stages
It is known [14] that a pulse technique with a chro-
matographic analysis of the reactants provides informa-
tion on whether the process occurs via a concerted
mechanism or through the separate interaction of reac-
tants with the catalyst. The mechanism with separate
interaction takes place when the interaction rates (con-
versions) of reactants with a stationary catalyst surface
are the same and equal to the reaction rate and the con-
versions of all reactants are close to those under sepa-
rate interaction. The concerted mechanism takes place
that of the surface atoms. Hence, the H S and SO con-
2
2
centrations in the H S + He and SO + He pulses were
2
2
low. They were preliminarily calculated taking into
account the weight of the catalyst in the reactor. There-
fore, the amount of surface atoms (in percent of a
monolayer) interacted with H S (SO ) (Fig. 2) was cal-
2
2
culated as the difference between the amounts of the
reaction product in the adsorption layer and in the reac-
tant pulse.
To study the IR spectra of adsorbed molecules, cat- when the interaction rates (conversions) of reactants
alyst samples were pressed into pellets with a density of and their mixtures are different, that is, the presence of
2
6
0–100 mg/cm and then placed in a special cell that both reactants is necessary for the reaction to occur effi-
allowed sample treatment at high temperatures (up to ciently.
73 K), as well as adsorption and desorption of gases at
different temperatures. The cell was connected to a vac-
7
Comparison of the H S and SO conversions over a
2
2
steady Sn–Mo oxide catalyst under both a combined
and separate feed of H S and SO is illustrated by
–
5
uum setup enabling a vacuum of up to 10 Torr. The IR
spectra were recorded on a FTS Nicolet instrument.
The concentration of the adsorption sites was estimated
from the IR spectral absorbance of the adsorbed probe
molecules such as CO, NH , and CO . Extinctions for
2
2
Fig. 2. As can be seen, the conversions of the reactants
in the combined and separate pulses are close (for
example, for H S they are 63 and 48%, respectively)
2
3
2
despite the high portion of the H S monolayer con-
2
these molecules were taken from the literature [11–13].
verted in the pulse. Over catalysts on which the con-
The UV–VIS spectra were measured on Shi- certed mechanism is effective (Al O and TiO ), the
2
3
2
madzu UV-300 and UV-160 spectrometers in the trans- conversions of the reactant (for example, H S) in the
2
mission and reflection modes with compensation of the combined and separated pulses differs substantially
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

2
62
DAVYDOV, SHEPOT’KO
lyst has been found by ESR and X-ray electron spec-
troscopies [6].
2
1
IR spectroscopic data for adsorbed probe molecules
such as CO, ëé , and NH indicate that the polymolyb-
2
3
2
1
denum compound formed upon oxidation on the sur-
face of the solid solution of molybdenum oxide in tin
dioxide is characterized by the presence of Brønsted
+
4
–
1
(
1
δs(NH
) = 1445 cm ) [15] and strong Lewis (δ (NH ) =
9
50 1050 1600 1800
3200
3600
s
3
–1
–1
ν, cm–
1
260 cm , δ (NH ) = 1610 cm ) acid sites, which are
as 3
6
+
5+
most likely Mo ions, because Mo ions are located
predominantly in the bulk of the solid solution [7]. This
compound covers the whole SnO surface (Mo/SnO ).
The surface OH groups of SnO , the Sn cations detect-
able by adsorption of NH (NH (δ (NH ) = 1240 cm )
Fig. 3. IR spectra of the Sn–Mo oxide catalyst (1) oxidized
at 723 K and (2) after interaction with water at 293 K.
2
2
4
+
2
–1
3
3
s
3
(
for example, over TiO they are 95 and 23%, respec-
2
2
–
and CO, and basic O ions detectable by CO adsorp-
tively). This is evidence that the process over an Sn–Mo
oxide catalyst occurs via the mechanism of the sepa-
rated interaction of the reactants with the catalyst. Note
2
tion were not found in the IR spectra. The polymolyb-
denum compound in the oxidized state has only weak
basic sites, which cannot stabilize CO as carbonates.
that the reaction product, H O, is completely evolved at
2
2
6
+
the stage of the interaction with H S. Hence, we can The molybdenum cations in the highest (Mo ) oxida-
suggest that over an Sn–Mo oxide catalyst, the Claus tion degree (hard acid) do not coordinate molecules of
2
process occurs via the stepwise mechanism.
soft bases, such as CO (the ionization potential (IP) is
4.1 eV), NO (9.25 eV), and C H (9.73 eV), but coor-
1
3
6
To elucidate the stages of this mechanism in detail
and the nature of activation of the reacting molecules at
a molecular level and to determine the nature and prop-
erties of the adsorption sites on the catalyst surface,
spectroscopic experiments were performed.
6
+
dinate a hard base–NH . We assume that the Mo ions
3
are probably “immersed” in the oxygen environment,
that is, are not accessible to weak electron-donor mole-
cules (CO, C H , and NO), whereas the molecules of
3
6
the strong electron-donor NH capable of forming a
3
strong bond can “extract” Mo cations from the oxygen
environment to the surface to form complexes. Hence,
this phenomenon is the so-called structural nonrigidity
of the surface [16].
2
. IR-Spectroscopic Study of the Interaction
of H S with SO
2
2
Nature of adsorption sites. Because the surface sites
determine the adsorbed species of reactants and the
routes of their transformations, let us first consider the
nature of these sites. The nature of the surface sites on
initial oxidized Sn–Mo oxide catalysts is determined by
a polymolybdenum compound formed on the surface of
the solid solution of molybdenum oxide in tin dioxide.
In the IR spectrum of the sample preliminarily dehy-
drated at 723 K and oxidized in oxygen at 723 K, the
absorption band at ~1010 cm is seen at frequencies
higher than those of the proper absorption of the sample
Fig. 3, spectrum 1). This absorption band is undoubt-
edly due to the vibration of the åÓ =é bond in the
light of data analyzed in [9] and was not found in the
spectrum of the initial sample in air. Only its shift to a
low-frequently region (960–970 cm ) is seen, where
the vibration of the åÓ=é bond in the hydrated poly-
molybdenum compound is observed (Fig. 1). The vari-
ations in the spectrum of the sample are due to the inter-
After reduction in hydrogen at 573–673 K, a portion
of the surface of SnO (solid solution of Mo ions in
2
SnO ) becomes open. As a result, the absorption bands
2
–
1
in the 1200–1700 cm region characterizing carbon-
ates appear in the IR spectrum of adsorbed ëé , and the
2
–
1
absorption band at 1240 cm is observed in the spectrum
of adsorbed ammonia and is due to ammonia coordi-
4+
nated to the Sn ions. Reduced molybdenum ions on the
–1
SnO surface are characterized by δ (NH ) = 1190 cm
–
1
2
s
3
4
+
–1
5+
(
most likely Mo ) and 1210 cm (åÓ ). It is typical
of the reduced polymolybdenum compound that
(
4
+
molybdenum ions in the Mo state appear, which coor-
6+
4
+
dinate CO to form the åÓ (CO) complex (absorption
–
1
bands at 2180 cm ) and NO (four absorption bands are
found, which, according to [17], indicate the formation
–
1
4+
5+
of åÓ (NO) and åÓ (NO) dinitrosyl complexes).
The Brønsted acid sites are not typical of this catalyst.
2
Interaction with H S. The intense interaction of the
2
6
+
action of the åÓ =é groups with water molecules, as oxidized surface of the Sn–Mo oxide catalyst with H2S
can be seen from changes in spectrum 1 (Fig. 3) after occurs at room temperature and is accompanied by a
water adsorption (Fig. 3, spectrum 2). The absorption substantial decrease in the intensity of the absorption
–
1
–1
bands in the region of lower frequencies, 800–1000 cm , band at ~1010 cm up to the complete disappearance
characterize the vibrations of the Sn–O–Mo, Mo–O–Mo, of this band and other absorption bands in the 800–
5
+
5+
–1
and probably Mo =O bonds. The presence of the Mo
1000 cm region, which characterize the Mo–O bonds.
–1
ions in the oxidized sample of the Sn–Mo oxide cata- Simultaneously, the absorption band at ~950 cm and
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

THE CLAUS REACTION: VI. THE REACTION MECHANISM
263
–1
the new absorption bands at 1120 and 1620 cm appear
Fig. 4, spectrum 3). Taking into account a marked
(
increase in the absorption in the region of ν(éç) =
1
2
–
1
3
200–3800 cm upon the H S admission onto the sam-
2
–
1
ple of the Sn–Mo oxide catalyst, the band at 1620 cm
most likely belongs to the bending vibrations (δ) of the
water molecules formed upon the interaction of hydro-
gen sulfide with the oxidized catalyst surface. Special
experiments on water adsorption on both oxidized and
reduced Sn–Mo oxide catalyst confirmed this assign-
ment.
3
Note that the removal of water molecules from the
catalyst surface by deep evacuation at room tempera-
4
–1
ture (the adsorption band at 1620 cm disappears from
the spectrum, indicating the desorption of all water
molecules within the accuracy limit of the method)
–1
does not restore the absorption band at ~1010 cm in
the spectrum (this region of the spectrum practically
does not change upon evacuation). In our opinion, this
indicates that Mo=O groups participate in the reaction
with hydrogen sulfide. It is obvious that a shift of the
Mo=O absorption band is not due to the ligand effect of
the water molecules formed during the reaction. It is
1
200
1100
1000
900
ν, cm–
1
most likely that, during the interaction of H S with the
Fig. 4. IR spectra of (1) initial Sn–Mo oxide catalyst oxi-
dized at 723 K; (2) after treatment with water at 293 K;
3) after reduction with H at 523 K followed by evacuation
at 673 K; and (4) after treatment with H S (10 Torr) at
2
Sn–Mo oxide catalyst, direct exchange of the sulfur
atom of the hydrogen sulfide molecule with the oxygen
atom on the surface of the oxide occurs to form water
and a sulfide ion:
(
2
2
2
98 K for 30 min.
2
–
2–
H S + (é )
ç é + (S ).
2
2
catalyst after the interaction with H S (Fig. 4, spectrum 4)
2
Such a reaction, first of all, occurs with the participation is due to the vibrations of ν(S=é) bonds formed by the
of the Mo=O and possibly Mo–O–Mo groups. This oxidation of a small portion of the hydrogen sulfide
reaction causes the disappearance of the absorption
molecules on the surface of the catalyst under study.
The structure corresponding to this absorption band
does not decompose upon evacuation; therefore, irre-
versible adsorption takes place, which is typical of the
S=O bonds [20]. It is difficult to describe this adsorbed
species in more detail. Hence, the findings of the stud-
ies show that, in contrast to the acid–base catalysts
–
1
band at 1010 cm from the spectrum upon the hydro-
gen sulfide adsorption. (It is difficult to follow the other
absorption bands belonging to the polymolybdenum
compound, although the intensities of all the bands
characterizing the Mo–O bonds decrease.) The forma-
tion of molybdenum sulfide in this two-dimensional
phase of the polymolybdenum compound is hardly
probable since, according to the quantitative data
obtained, not even all the oxygen atoms of the polymo-
lybdenum compound are replaced by sulfur. The for-
(Al O or TiO ) on which dissociation mostly occurs
2 3 2
via only one S–H bond of hydrogen sulfide, on a redox
Sn–Mo oxide catalyst both S–H bonds dissociate with
2–
the incorporation (substitution) of S ions into the cat-
mation of the åÓS phase found in [15] may be due to
2
alyst and the evolution of water.
the destruction of the solid solution because of severe
conditions of the catalyst treatment with hydrogen sul-
The participation of surface sites in the H S adsorp-
2
fide employed in that work, as well as to the formation tion. The treatment of the oxidized catalyst with hydro-
of åÓS and the involvement of all molybdenum ions. gen sulfide at 293 K causes a significant decrease in the
2
Note that the interaction of hydrogen sulfide with the concentrations of both Lewis and Brønsted acid sites,
surface of the Sn–Mo oxide catalyst at moderate tempera- as follows from the corresponding changes in the spec-
tures does not lead to spectral evidence for H S adsorption. trum of adsorbed ammonia. After the interaction at
2
When H S is adsorbed on oxides (e.g., Al O [18, 19]) the 573 K, the concentrations of acid sites decrease
2
2
3
absorption bands usually appear in the region of stretching
1
0 times. Such changes in the site distribution confirm
–
1
vibrations (νSç) ~ 2500–2600 cm . In contrast to Al O ,
2
3
the above scheme for the H S interaction with the cata-
2
these bands were not found in the case of the Sn–Mo
lyst surface. These changes may be due to the reduction
of the polymolybdenum compound upon interaction
with hydrogen sulfide, when the compound loses the
oxide catalyst apparently because of complete H S dis-
2
sociation on the surface of this catalyst.
In our opinion, the appearance of the absorption Brønsted sites, as mentioned above (a phenomenon
–
1
band at ~1120 cm in the spectrum of the Sn–Mo oxide usually observed during the reduction of the surface
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

2
64
DAVYDOV, SHEPOT’KO
to these absorption bands decomposes due to desorp-
tion at room temperature. Irreversible adsorption,
2
which is characterized by the absorption bands at ~1080
–1
and 1100–1140 cm (Fig. 6, spectrum 2), is observed
simultaneously. This adsorbed species does not decom-
pose upon desorption up to high temperatures, and the
–
1
absorption bands at ~1080 and 1100–1140 cm ,
according to the earlier reported data on the SO2
adsorption over oxides [22–26], most probably charac-
terize ν(S=O) in both the sulfite and sulfate structures.
Note that the concentrations of these compounds on the
surface of the Sn–Mo oxide catalyst are not so high as
that on MgO [25, 26] or Al O [22–24], and this is
2
3
undoubtedly due to the lower basicity of oxygen on the
surface of the Sn–Mo oxide catalyst compared to MgO
and Al O . The appearance of the ëé absorption
1
2
3
2
bands in the IR spectrum of the oxidized Sn–Mo oxide
catalyst also indicates the lower basicity of oxygen in
the polymolybdenum compound. As mentioned above,
ëé does not interact with the oxidized surface of this
2
ν, cm–1
catalyst at 293 K, unlike Al O and MgO, on which
2
3
1
000
1400
such interaction readily occurs under these conditions
to form carbonates [13]. In the case of the Sn–Mo oxide
catalyst, the interaction with the surface oxygen result-
ing in the formation of sulfite–sulfate structures is
apparently due to the presence of the strong oxidation
Fig. 5. IR spectra of ammonia adsorbed on the oxidized sur-
face of the Sn–Mo oxide catalyst (1) before and (2) after
interaction with H S.
2
2
3
–
sites participating in the formation of the surface SO
Mo species [11]). Another reason is that the formation
of new “rigid” Mo–S bonds in the structures similar to
MÓS (more “rigid” than the Mo–O bonds in åÓé )
2
–
and SO groups. Certainly, it is necessary to take into
4
2
3
account the stronger acidic properties of SO compared
2
makes the incorporation of ammonia into the coordina-
tion sphere of the molybdenum ions impossible. In the
to ëé2.
spectra of NH adsorbed on this catalyst, the absorption
According to the reported UV–VIS and IR spectro-
3
bands of ammonia coordinated by both reduced molyb- scopic data [22–26] on the SO adsorption on oxides,
2
–1
6+
denum ions (δ (NH ) = 1190, 1210 cm ) and åÓ ions the following species can be formed upon the reversible
s
3
–
1
(
δ(NH ) = 1260 cm ) were not found. At the same SO
2
adsorption:
3
4
+
time, the Sn ions coordinate ammonia molecules (the
–
1
SO2
absorption band δ ~ 1240 cm ) (Fig. 5).
s
O O
S
O O
S
O O
Note that few Brønsted sites remain on the surface
of this catalyst even upon high-temperature and
(
OH)
S
O S O
Men+
I
O2– Men+
IV
Me
II
O H
III
repeated treatment with H S (Fig. 5, spectrum 2). How-
2
ever, the presence of the Brønsted sites in the samples
can be due to both the molybdenum ions in the highest
oxidation states, as in the case of the oxidized sample,
V
Difficulties in the spectral identification of these com-
pounds are due to their close thermal stability and sym-
metry, which determine the proximity of their spectral
characteristics. Let us examine the possibility of the
and to the appearance of the Brønsted sites on the SnO
2
2
–
surface due to its modification with ions SO . The
4
ions are probably formed in a small amount on the sur- formation of each of these species, taking into account
face of the polymolybdenum compound and migrate to the nature and properties of the surface sites of this cat-
the support upon reduction, thus modifying the support alyst. The existence of basic hydroxyl groups on the
and giving rise to the formation of proton centers, catalyst surface is improbable. Clear absorption bands
2
4
–
ν(éç) have not been found in the IR spectrum [7]. As
a result, the formation of species II can be ruled out
with a high probability. Compound III can be formed
and contributes to the absorption bands, which charac-
which were found, for example, in TiO doped by SO
ions [21].
2
Interaction with SO . After SO adsorption at room
temperature on the oxidized surface of the Sn–Mo terize the reversible adsorption. However, this type of
oxide catalyst, the absorption bands at 1140 and interaction cannot be preferred, because only the block-
340 cm appear in the spectrum, which characterize ing of the surface basic sites by CDCl substantially
reversible adsorption, since the structure corresponding decreased the fraction of the reversible adsorption,
2
2
–
1
1
3
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

THE CLAUS REACTION: VI. THE REACTION MECHANISM
265
whereas the treatment of the catalyst surface with
+
NaOH resulting in the removal of the ç sites slightly
affected the fraction of the reversible adsorbed species.
Note that the reversibly adsorbed species do not
markedly affect the Mo=O bonds, since the absorption
1
–
1
bands in the 900–1050 cm region and especially that
–
1
3+
at ~1010 cm characterizing (åÓ=é) remain almost
unchanged during the SO2 adsorption–desorption
cycles at room temperature. These data most probably
2
3
indicate that SO does not interact directly with either
2
the oxygen or molybdenum ions forming these pairs.
Taking into account the facts that CO, NO, and ë ç do
1400
1300
1200
1100
ν, cm
3
6
not interact with molybdenum cations in this oxidized
–1
6
+
catalyst (Mo ) and that the C H (IP = 9.73 eV) and
3
6
NO (IP = 10.2 eV) molecules are stronger donors than
SO (IP = 12.34 eV), it is unlikely that SO interacts
Fig. 6. IR spectra of SO adsorbed on the oxidized surface
2
2
2
of the Sn–Mo oxide catalyst after adsorption of SO (1) 1 and
2
with the molybdenum ions. Meanwhile, reversible SO2
adsorption is observed. Therefore, we can suggest that
(2) 10 Torr followed by desorption (3). T = 298 K.
the most probable interaction in this case is SO adsorp-
2
2–
–1
tion on the surface O basic sites in the Mo–O–Mo poly- 1340 cm , as was found for the oxidized catalyst. This
molybdenum compound and/or in the Mo–O–Sn frag- absorption band 1330 cm probably characterizes the
–
1
2
–
ments to form the donor–acceptor complex IV. As men-
tioned above, hydroxyl groups are absent from the
surface of this catalyst, the molybdenum cations are
inaccessible because of the low donor ability of SO2,
and the basicity of oxygen in the polymolybdenum
complex of SO with the surface O sites, which, as
2
2–
mentioned above, are more basic than the O ions of
the polymolybdenum compound. It is obvious from the
IR spectroscopic characteristics of SO on MgO
2
–1
(
in complex I, ν ~ 1320 cm [26]) that the ν value for
3
3
compound is insufficient to form sulfites. In this case, it
is clear that the interaction of the surface oxygen with
this complex decreases with an increase in the basicity
of the surface oxygen. The fact that SO adsorption
2
SO stops at the stage of complex IV formation. Taking
2
2
–
2–
into account the weak basicity of the polymolybdenum resulting in the SO and SO4 structures does not
3
compound, one can assume that this adsorption is
affect Mo=O groups can be explained by (i) the small
reversible and dominating. In our opinion, it is this spe-
concentration of strong oxidation sites in the catalyst
cies that is characterized by the absorption bands at
2
–
–
1
under study, that is, low concentrations of SO and
~
1340 and 1140 cm (ν and ν are somewhat shifted
3
3
1
2
–
relative to a free SO molecule). The positions of these
2
SO ions, and (ii) possible stabilization of such com-
4
bands in the case of the Sn–Mo oxide catalyst are prac-
tically independent of the surface coverage with
pounds by tin ions, that is, by the formation of such
structures with the bridging oxygen atoms (in Mo–O–Sn
or Sn–O–Sn fragments). These oxygen atoms are more
basic than those in the Mo–O–Mo groups participating
in the reversible adsorption, and this fact probably
determines sulfite formation according to the scheme
adsorbed SO . Taking into account the amphoteric
2
properties of SO (this molecule can act as both an elec-
2
tron-donor and electron-acceptor), the formation of sta-
ble SO complexes with pyridine, trimethylamine, etc.,
2
and the IR spectroscopic characteristics of these com-
plexes [27], the conclusion that complex IV is formed
seems to be reasonable.
O O
S
O O
S
.
O
The data on SO interaction with the sulfided sur-
O
O
2
+SO2
O
O
O
O
O
face of the Sn–Mo oxide catalyst are in good agreement
Mo
Mo
Sn
Mo
Mo
Sn
with the above scheme for SO interaction with the oxi-
2
6+
It is obvious that a portion of molybdenum ions (Mo ,
dized surface. As shown above, the interaction with
–1
the absorption band of Mo=O at ~1010 cm ) should be
H S results in the reduction of the molybdenum ions
2
5+
reduced to at least Mo (the absorption band of
and their complete blocking for adsorption of even such
5
+
–1
åÓ =é at ~950 cm ). However, the concentration of
strong electron-donor ligands as ammonia, as well as in
the contact points for the polymolybdenum compound
4+
the appearance of Sn ions. The absorption band at
with the SO surface, which are accessible to the inter-
2
–
1
~
Sn
1325 cm is probably due to the formation of the
action with SnO , is small. Moreover, the absorption
2
4
+
Sé complexes at a small amount of SO
2
2
bands of Mo=O bonds superimpose on the absorption
5
+
(
1.8 µmol) adsorbed on this catalyst. Note that the posi- bands of Sné and the solid solution of Mo in SnO .
2
2
tion of this absorption band depends on the coverage. As a result, all these facts may slightly affect the inten-
However, even with complete coverage (9 µmol), this sities of the corresponding absorption bands in the
absorption band is observed at 1330 cm rather than at spectra.
–
1
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

2
66
DAVYDOV, SHEPOT’KO
Interaction between SO and adsorbed H S. As time is ~1013 mol H S cm s at 523 K; that is, it
–2
–1
2
2
2
mentioned above, the interaction of H S with the sur- exceeds the steady-state reaction rate on this catalyst.
2
face of the Sn–Mo oxide catalyst results in the forma- This indicates that the reaction can occur via this stage.
tion of the polymolybdenum hydroxysulfide compound The data obtained allow us to suggest that, in the pre-
2
–
catalytic stage, the complex S2–
SO can exist (at
in which the O ions are partially substituted for the
*
2
2
–
S ions. A portion of the molybdenum ions is reduced
to Mo , and the adsorption on the support (solid solu-
tion of Mo in Sné ), which is close to Sné in troscopy at room temperature, probably because of the
least, such an interaction resulting in sulfur formation
occurs), but it is impossible to observe it by IR spec-
4+
5
+
2
2
2
–
adsorption properties, becomes possible. Sulfur high rates of sulfur formation. This interaction of (S )
dioxide reacts with the sulfided catalyst surface after with SO , which is necessary for both surface oxidation
2
evacuation at 373–473 K at rather low temperatures and sulfur formation, becomes obvious in the experi-
(
293–372 K). It is obvious that water removal from the ments on the SO interaction with the surface reduced
2
catalyst surface becomes possible at ~523 K and this
with ç . In this case, the reoxidation of the catalyst by
SO was not observed up to 573 K, whereas the catalyst
treated with H S reacts with SO and its surface is reox-
idized even at room temperature. In our opinion, the lat-
2
surface becomes accessible to the interaction with SO2.
2
Correspondingly, the absorption bands at 1010 and
2
2
–
1
6+
9
80–990 cm
characterizing the åÓ =O and
6+
6+
åÓ −é–åÓ bonds appear in the IR spectra, and their ter is due to the fact that the surface reoxidation is facil-
intensities increase. Therefore, it is beyond question itated by the SO interaction with sulfur:
2
that this catalyst surface treated with H S is oxidized by
2
O
O
SO . Note that on both oxidized catalysts and those
2
S4+
reduced with H S, adsorption was found in the IR spec-
2
–
e
Me S2– Me
trum after the consumption of 1.8 µmol SO , whereas on
the sulfided catalyst, the absorption band at 1325 cm
appeared only after the adsorption of 18 µmol. These
2
–1
VI
findings, along with data on sulfur formation, undoubt- SO adsorption at 298 K on the catalyst reduced with
2
edly indicate that SO molecules are consumed for the H was observed in the IR spectra beginning from
2
2
surface oxidation and sulfur formation. This follows 2 µmol of adsorbed SO , since the SO molecules did
2 2
2
–
from an examination of the difference UV–VIS spectra not react with S .
of the sulfided Sn–Mo oxide catalyst before and after its
Interaction of adsorbed SO with H S. The dosed
2
2
interaction with SO . Absorption in the 300–400 nm adsorption of hydrogen sulfide on the sample contain-
2
region was found for these samples. According to the ing preliminarily adsorbed SO allowed us to show that
2
spectra of individual sulfur compounds [18], this the intensities of the absorption bands at 1330 and
–
1
absorption can only be due to the formation of sulfur 1140 cm corresponding to the reversible SO adsorp-
2
−
compounds. Hence, these data give evidence that S ions tion rapidly decrease and completely disappear upon
the adsorption of 3.3 µmol H S. Special experiments
are the active sites for the interaction with the SO mol-
ecules from the gas phase. As quantitative measure-
ments show, interaction with SO even at room temper-
ature continues until the complete consumption of
these sites. After this, the absorption band ν at
1325 cm characterizing the vibrations of SO mole-
2
2
showed that the disappearance of these absorption
bands is due to the interaction between SO and H S
2
2
2
rather than the displacement of SO from the surface.
2
UV–VIS spectra of the catalyst after this interaction
revealed the appearance of elemental sulfur on the sur-
face. Absorption in the 300–400 nm region typical of
3
–
1
~
2
cules reversibly adsorbed on the support surface
the S –S elemental sulfur and S was found in
these spectra [18], and this confirmed the explanation
suggested above for the change in the absorption bands
corresponding to the reversible SO adsorption.
Figure 7 presents data on the reactivity of various
species of adsorbed SO toward H S. As can be seen,
the reversibly adsorbed SO , which is characterized by
the absorption band at 1330 cm , is most reactive. The
2
8
polymer
appears in the spectra and grows to a certain limit
(
clearly as a result of the small number of adsorption
sites). The intensity of this absorption band does not
change in time, indicating that this adsorbed species
2
was not consumed. Special experiments on SO adsorp-
tion on SnO showed that this absorption band was also
found when the Sn
2
2
2
2
2
4+
SO and/or Sn–O
SO2
–1
2
complexes were formed. The formation of such com- evaluation of the rate of the interaction of this adsorbed
plexes probably determines the appearance of this species with H S showed that this species provides the
2
absorption band in the spectrum of the catalyst after occurrence of the Claus reaction at 523 K; that is, the
interaction with SO . As mentioned above, the catalyst initial interaction rate with H S is even higher than the
2
2
is reduced due to the interaction with H S, and the sup- reaction rate.
2
port surface becomes “open”, that is, accessible to
The Claus reaction mechanism. As shown above,
interaction. The initial rate of the interaction with S the reversibly adsorbed species, which is the donor-
2–
evaluated by a decrease in the SO concentration in acceptor complex of SO with the surface O ions, is
2
2
KINETICS AND CATALYSIS Vol. 44 No. 2 2003

THE CLAUS REACTION: VI. THE REACTION MECHANISM
267
Absorbance, cm–1
3. Kasumov, F.B. and Davydov, A.A., Kinet. Katal., 1991,
vol. 32, no. 5, p. 1193.
3
4. Kasumov, F.B. and Davydov, A.A., Kinet. Katal., 1991,
1
0
6
vol. 32, no. 6, p. 1425.
5
6
. Kasumov, F.B., Kinet. Katal., 1991, vol. 32, no. 6,
p. 1430.
. Allakhverdova, N.Kh., Kasumov, F.B., Adzhamov, K.Yu.,
and Alkhazov, E.G., Kinet. Katal., 1984, vol. 25, no. 3,
p. 684.
7
8
9
. Davydov, A.A., React. Kinet. Catal. Lett., 1982, vol. 19,
nos. 3–4, p. 377.
. Davydov, A.A. and Goncharova, O.I., Usp. Khim., 1993,
vol. 62, no. 2, p. 22.
. Davydov, A.A. and Goncharova, O.I., Zh. Prikl. Spek-
trosk., 1989, vol. 50, p. 76.
2
0
1
2
1
2
3
4
1
7
2
(
Amount of adsorbed H S molecules) × 10 , mol/m
2
10. Marshneva, V.I. and Mokrinskii, V.V., Kinet. Katal.,
988, vol. 29, p. 989.
1
Fig. 7. Changes in the absorbance of the absorption bands
11. Davydov, A.A., IR Spectroscopy of Adsorbed Species on
the Surface of Transition Metal Oxides, New York: Wil-
ley, 1990.
of adsorbed SO during interaction with H S at 293 K:
2
2
–
1
(
1) 1330, (2) 1080, and (3) 1620 cm .
1
1
1
1
1
2. Budneva, A.A., Paukshtis, E.A., and Davydov, A.A.,
React. Kinet. Lett., 1987, vol. 34, no. 1, p. 63.
the predominant adsorbed species on the oxidized sur-
face of the Sn–Mo oxide catalyst. Hydrogen sulfide
reacts with the surface of the Sn–Mo oxide catalyst
3. Davydov, A.A., Shepot’ko, M.L., and Budneva, A.A.,
Kinet. Katal., 1994, vol. 35, no. 2, p. 299.
4. Boreskov, G.K., Geterogennyi kataliz (Heterogeneous
2
–
to a greater extent to form new Mo–S bonds and
Catalysis), Moscow: Nauka, 1986.
2
–
water. The S groups that are stronger donors than
5. Davydov, A.A., Zh. Fiz. Khim., 1993, vol. 67, no. 9,
2
–
the O ions readily react with the reversibly adsorbed
p. 1900.
SO molecules and those from the gas phase to form
2
6. Pel’menshikov, A.G. and Zhidomirov, G.M., 7th Soviet-
Japanese Catalysis Seminar. Novosibirsk, 1983, p. 52.
elemental sulfur as a reaction product and to oxidize the
surface. The reaction easily occurs when the water that
2
–
17. Startsev, L.N., Kuznetsov, B.N., Mamaeva, E.K., Bud-
neva, A.A., Davydov, A.A., and Ermakov, Yu.I., in:
Katalizatory, soderzhashchie nanesennye kompleksy
is formed due to H S adsorption and blocks the S sites
2
is removed. Removing the water is not difficult and can
be done by increasing the reaction temperature above
(
Catalysts Containing Supported Complexes), Novosi-
3
73 K, when the water begins to desorb. Hence, the
birsk, 1977.
2
–
4+
redox electron transfer from S to S ions occurs dur-
1
8. Karge, H.G., Recent Development in Catalysis: Theory
and Practice, Yiswanasan, B. and Pillai, C.N., Eds.,
Paris: Technip, 1992.
2–
ing the interaction of S with SO . Simultaneously, the
2
catalyst surface is reoxidized by the oxygen of SO2
molecules. The energy released upon reoxidation can
be utilized for the whole reaction; that is, the coupling
1
2
2
9. Lavalley, J.-C., Saad, A.B., Tripp, C.P., and Morrow, B.A.,
J. Phys. Chem, 1986, vol. 90, p. 980.
2–
of stages takes place. Thus, the interaction of S ions
0. Nakamoto, K., Infrared and Raman Spectra of Inorganic
and Coordination Compounds, New York: Wiley, 1978.
with the SO molecules is obvious. The formation of
2
elemental sulfur is accompanied by the reoxidation of
the catalyst with the oxygen of sulfur dioxide. The data
obtained indicate that over the Sn–Mo oxide catalyst,
the Claus reaction occurs via the stepwise redox mech-
anism with coupled surface oxidation and sulfur reduc-
tion.
1. Khadzhiivanov, K.I. and Davydov, A.A., Kinet. Katal.,
1
988, vol. 29, no. 2, p. 460.
2
2
2. Chang, C.C., J. Catal., 1978, vol. 53, p. 374.
3. Karge, H.G. and Dalla, I.G., J. Phys. Chem., 1984,
vol. 88, p. 1538.
2
4. Datta, A., Cavell, R.G., Tower, R.W., and George, Z.M.,
J. Phys. Chem., 1985, vol. 89, p. 443.
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KINETICS AND CATALYSIS Vol. 44 No. 2 2003