Effect of calcination temperature on the physicochemical and catalytic properties of FeSO4/SiO2 in hydrogen sulfide oxidation

Article abstract of DOI:10.1134/S0023158411060206

The effect of calcination temperature on the state of the active component of iron-containing catalysts prepared by the impregnation of silica gel with a solution of FeSO₄ and on their catalytic properties in selective H₂S oxidation to sulfur was studied. With the use of thermal analysis, XPS, and Moessbauer spectros- copy, it was found that an X-ray amorphous iron-containing compound of complex composition was formed on the catalyst surface after thermal treatment in the temperature range of 400-500°C. This compound contained Fe³⁺ cations in three nonequivalent positions characteristic of various oxy and hydroxy sulfates and oxide and sulfate groups as anions. Calcination at 600°C led to the almost complete removal of sulfate groups; as a result, the formation of an oxide structure came into play, and it was completed by the production of finely dispersed iron oxide in the ε-Fe₂O₃ modification (the average particle size of 3.2 nm) after treatment at 900°C. As the calcination temperature was increased from 500 to 700°C, an increase in the catalyst activity in hydrogen sulfide selective oxidation was observed because of a change in the state of the active component. A comparative study of the samples by temperature-programmed sulfidation made it possible to establish that an increase in the calcination temperature leads to an increase in the stability of the iron-containing catalysts to the action of a reaction atmosphere. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S0023158411060206

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2011, Vol. 52, No. 6, pp. 896–906. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © M.A. Shuvaeva, I.V. Delii, O.N. Mart’yanov, O.A. Bayukov, E.I. Osetrov, A.A. Saraev, V.V. Kaichev, N.S. Sakaeva, G.A. Bukhtiyarova, 2011, published in
Kinetika i Kataliz, 2011, Vol. 52, No. 6, pp. 920–930.
Effect of Calcination Temperature on the Physicochemical
and Catalytic Properties of FeSO4/SiO2
in Hydrogen Sulfide Oxidation
,
d
, d
M. A. Shuvaevaa , I. V. Deliia , O. N. Mart’yanova, O. A. Bayukovb, E. I. Osetrovc,
A. A. Saraeva, V. V. Kaicheva , N. S. Sakaevae, and G. A. Bukhtiyarovaa
,
d
a Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
b Kirenskii Institute of Physics, Siberian Branch, Russian Academy of Sciences, Krasnoyarsk, 660036 Russia
c Siberian Federal University, Krasnoyarsk, 660041 Russia
d Novosibirsk State University, Novosibirsk, 630090 Russia
e OAO Katalizator, Novosibirsk, 630058 Russia
eꢀmail: mas@catalysis.ru
Received May 24, 2011
Abstract—The effect of calcination temperature on the state of the active component of ironꢀcontaining catꢀ
alysts prepared by the impregnation of silica gel with a solution of FeSO4 and on their catalytic properties in
selective H2S oxidation to sulfur was studied. With the use of thermal analysis, XPS, and Mössbauer spectrosꢀ
copy, it was found that an Xꢀray amorphous ironꢀcontaining compound of complex composition was formed
on the catalyst surface after thermal treatment in the temperature range of 400–500
tained Fe3+ cations in three nonequivalent positions characteristic of various oxy and hydroxy sulfates and
oxide and sulfate groups as anions. Calcination at 600 C led to the almost complete removal of sulfate groups;
as a result, the formation of an oxide structure came into play, and it was completed by the production of finely
dispersed iron oxide in the ꢀFe2O3 modification (the average particle size of 3.2 nm) after treatment at
900 C. As the calcination temperature was increased from 500 to 700 C, an increase in the catalyst activity
°
C. This compound conꢀ
°
ε
°
°
in hydrogen sulfide selective oxidation was observed because of a change in the state of the active component.
A comparative study of the samples by temperatureꢀprogrammed sulfidation made it possible to establish that
an increase in the calcination temperature leads to an increase in the stability of the ironꢀcontaining catalysts
to the action of a reaction atmosphere.
DOI: 10.1134/S0023158411060206
INTRODUCTION
tion of the waste gas of the Claus process. This
approach was implemented in several industrial proꢀ
The amount of hydrogen sulfide obtained at oil
refineries in the processes of hydrotreating continuꢀ
ously increases because more and more rigid requireꢀ
ments are imposed on the sulfur content of petroleum
refining products (motor fuels) and the fraction of highꢀ
sulfur oils in the total volume of petroleum refining is
increased. At present, the Claus process is widely used
for the removal of hydrogen sulfide from acid gases. The
main reaction of this process is the oxidation of hydroꢀ
gen sulfide to sulfur with sulfurous anhydride
®
cesses, among which the Superclaus process develꢀ
oped as a result of the joint efforts of specialists from
Comprimo, GasTec, and the University of Utrecht
(the Netherlands) became the most widely used proꢀ
cess in actual practice [3–6]. The selective oxidation
of hydrogen sulfide to sulfur (reaction (II)) is irreversꢀ
ible; however, a number of side reactions can occur on
the catalyst surface to decrease the yield of sulfur.
Thus, the formation of sulfurous anhydride can occur
due to reaction (III), which is the reverse of the Claus
reaction, the oxidation of the resulting sulfur by oxyꢀ
gen in a gas phase or on the catalyst surface (reaction
(IV)), and a parallel reaction of H2S oxidation to sulꢀ
furous anhydride (reaction (V)):
3
n
(I)
2H2S + SO2 → 2H2O + Sn,
where n = 1–8.
However, the waste gas of Claus plants contains a
considerable amount of hydrogen sulfide because
reaction (I) is reversible; moreover, this waste gas is
also characterized by a high water vapor content (to
30 vol %) [1, 2].
The selective oxidation of hydrogen sulfide to sulꢀ
fur with molecular oxygen is a method for the purificaꢀ
1
2
1
n
(II)
H2S + O2 → Sn + H2O,
3
n
(III)
(IV)
Sn + 2H2O → 2H2S + SO2,
Sliq + O2 → SO2,
896

EFFECT OF CALCINATION TEMPERATURE ON THE PHYSICOCHEMICAL
897
devoted to the effects of the nature of an active comꢀ
ponent and various modifying additives, the effects of
heat treatment conditions on the catalytic properties
and the stability to sulfidation of ironꢀcontaining catꢀ
alysts for the selective oxidation of hydrogen sulfide
have not been adequately studied. Earlier, Bukhtiꢀ
yarova et al. [31] found that an increase in the calcinaꢀ
3
2
(V)
H2S + O2 → SO2 + H2O.
To reach a maximum yield of sulfur, it is necessary
to avoid the occurrence of side reactions leading to the
formation of sulfurous anhydride. The contribution of
reaction (III) to the formation SO2 is decreased by the
use of supports that are inactive in the Claus reaction,
tion temperature (Tc) of catalysts prepared by the
for example, ꢀAl2O3 or SiO2 [7, 8]. Published data on
α
impregnation of silica gel with a solution of iron
nitrate led to both an increase in their stability to sulfiꢀ
dation and a decrease in the catalytic activity caused
the effect of supports on the catalytic properties of
ironꢀcontaining catalysts in the oxidation reaction of
hydrogen sulfide with oxygen confirmed that the use
of SiO2 as a support resulted in higher selectivity, as
by the agglomeration of the resulting particles of ꢀ
α
Fe2O3. We failed to find out other publications conꢀ
cerning this subject matter.
compared with that of TiO2 and
θ
ꢀAl2O3, which
exhibited noticeable activity in the Claus reaction [9].
To decrease the contribution of reaction (IV) to the
formation of SO2, wideꢀpore supports are used for the
preparation of catalysts for the selective oxidation of
hydrogen sulfide to sulfur. It was demonstrated experꢀ
imentally that a support dominated by mesopores
ensures higher activity and selectivity than a support
dominated by micropores [10].
The processes of the formation of an active phase in
supported Feꢀcontaining catalysts prepared with the
use of Fe(II) sulfate are not clearly understood; howꢀ
ever, interest in this compound has quickened in the
past few years [32–36]. On the one hand, the use of
FeSO4 affords catalysts that are more effective
[32, 34]; on the other hand, this compound is a waste
product of the manufacture of titanium dioxide and it
should be utilized.
The formation of an active component, which
selectively oxidizes hydrogen sulfide to sulfur in the
temperature range of 200–320 C, plays a primary role
°
The aim of this work was to study the effect of calꢀ
cination temperature on the state of the active compoꢀ
nent and the catalytic properties of supported ironꢀ
containing catalysts obtained with the use of Fe(II)
sulfate in the reaction of hydrogen sulfide oxidation by
oxygen. The study of the comparative stability of samꢀ
ples to the action of hydrogen sulfide was performed by
temperatureꢀprogrammed sulfidation (TPS), which is
widely used for investigating the formation of an active
sulfide phase of hydrotreating catalysts [37].
in the achievement of a high yield of sulfur. The upper
temperature limit is restricted by the possibility of a
homogeneous reaction of the oxidation of hydrogen
sulfide to SO2, whereas the lower limit is restricted by
a decrease in the rate of oxidation due to the condenꢀ
sation of sulfur on the catalyst surface [11]. It is well
known that bulk and supported systems containing
Fe(III) compounds are most frequently tested as cataꢀ
lysts for the oxidation of hydrogen sulfide to sulfur in
the waste gas of the Claus process [5–9]. The inorꢀ
ganic (Fe(III) nitrate, Fe(III) chloride, Fe(III) sulꢀ
fate, and Fe(II) sulfate) [12–14] and organic salts of
iron (gluconate, citrate, ethylenediaminetetraacetate,
and acetylacetonate) [15, 16] were used as the precurꢀ
sors of an active component. In a number of publicaꢀ
tions, it was shown that the catalysts containing the
oxide and sulfate compounds of iron provided the
selective oxidation of hydrogen sulfide to sulfur
[5, 17–24]; the activity was increased by decreasing
the particle sizes of the active phase. However, as a
result of the interaction of an active component with
hydrogen sulfide, iron disulfide can be formed, in
presence of which the oxidation of hydrogen sulfide
occurs to sulfurous anhydride, decreasing the reaction
selectivity and the yield of sulfur [25, 26]. Conseꢀ
quently, along with high catalytic activity, a requireꢀ
ment imposed on ironꢀcontaining catalysts for the
selective oxidation of hydrogen sulfide to sulfur is the
stability of the active phase to sulfidation.
EXPERIMENTAL
KSKG silica gel (specific surface area, 287 m2/g;
average pore diameter, 141 Å; pore volume,
0.81 cm3/g) (OOO KhromAnalit) and FeSO4
7H2O
⋅
(Acros Organics) were used for the preparation of catꢀ
alysts. The silica gel (0.25–0.5 mm) was incipient wetꢀ
ness impregnated with an aqueous solution of Fe(II)
sulfate and dried at room temperature for a day and
then at 110
FeSO4/SiO2 were calcined at different temperatures
(400, 500, 600, 700, and 900 ) in air for 4 h. The
°С for 4 h. Individual samples of
°С
resulting samples are referred to as 4FS(400),
4FS(500), 4FS(600), etc. Numbers in parentheses
correspond to sample calcination temperatures, and
the first number approximately corresponds to the
concentration of iron (wt %). According to chemical
analysis data, the iron content of the 4FS(700) sample
was 3.8 wt %.
To increase the resistance of catalysts based on iron
oxide to the action of a reaction atmosphere, the use of
The Xꢀray diffraction patterns of the samples were
various modifying additives, for example, Ce [27], Cr measured on an XTRA diffractometer (Switzerland)
[28], and Sn [29] oxides or more complex Feꢀcontainꢀ using Cu radiation (wavelength = 1.5418 Å) at a
scanning step of 0.05° and an accumulation time of
proposed. Although a wide range of publications were 3 s per point.
K
λ
α
ing compounds such as Fe–Ni phosphates [30], was
2θ
KINETICS AND CATALYSIS Vol. 52
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2011

898
SHUVAEVA et al.
The XPS spectra were recorded on a SPECS elecꢀ O2, 1.0 vol %; water vapor, 30 vol %; and the balance
tron spectrometer (Germany). The spectrometer was He. The catalytic activity of the samples was characꢀ
equipped with a PHOIBOSꢀ150 hemispherical anaꢀ terized by the reaction rate of hydrogen sulfide oxidaꢀ
lyzer, a nineꢀchannel detector of electrons, and an tion and by the selectivity of hydrogen sulfide converꢀ
XRꢀ50 source of characteristic Xꢀray radiation with an sion into sulfur at the following concentrations of
Al/Mg dual anode. Nonmonochromated Al
K radiaꢀ reagents in the circulation loop: H2S, 1.0 vol % and O2,
α
tion (h = 1486.6 eV) was used for measuring the specꢀ 0.6 vol % (the degree of hydrogen sulfide conversion
ν
tra. The catalyst samples were fixed in a sample holder was 50%). The concentrations of reagents at the reactor
with the aid of doubleꢀsided adhesive tape. The scale inlet and outlet were determined with the use of a therꢀ
of binding energies (BE) was calibrated by the internal mal conductivity detector (TCD) after the separation of
standard method using the Si 2p line silicon that is a components on a chromatographic column 3 m in
component of the support (BE = 103.5 eV). The relaꢀ length packed with a modified support of Polysorbꢀ1 +
tive concentrations of elements in the analysis zone 25% PEG15M [24].
were determined based on the integrated intensities of
XPS lines taking into account the photoionization
cross sections of the corresponding terms [38]. The
decomposition of spectra into individual components
was used for a detailed analysis. Accordingly, an experꢀ
imental curve was decomposed into a number of lines
corresponding to the photoemission of electrons from
atoms in different chemical environments after the
Shirley background subtraction. Data processing was
performed using the CasaXPS program package.
The comparative resistance of the catalysts to sulfiꢀ
dation was studied in a flow quartz reactor with the use
of a gas mixture containing 2 vol % H2S in helium at a
flow rate of 1.8 l/h. Before the sulfidation, a flow of
helium was passed through the reactor for removing
air and then a sulfidizing mixture was passed for 0.5 h.
Thereafter, the temperature was increased to 500
°
С at
a heating rate of 8 /min. The concentration of H2S
°
С
in the flow was monitored with the use of a gas chroꢀ
matograph with a TCD; the samples were taken at regꢀ
ular 1ꢀmin intervals. The results of the experiments
were represented as the temperature dependence of
hydrogen sulfide concentration at the reactor outlet—
the TPS curves.
Thermal analysis was performed on a NETZSCH
STA 449CꢀJupiter thermoanalytical system. The TG,
DTG, and DSC curves were recorded over a temperaꢀ
ture range from 25 to 600
°
С in a flow of air at a heating
rate of 10 /min. For the analysis, 20 mg of a test
°
С
sample was loaded in a corundum crucible, and calꢀ
cined silicon oxide was used as a reference sample.
RESULTS AND DISCUSSION
Physicochemical Properties
The Mössbauer spectroscopic studies were perꢀ
formed at room temperature with a Co57(Cr) source on
powder absorbents 0.5 mm in thickness. Isomeric
chemical shifts were measured with reference to iron
The samples obtained were found Xꢀray amorꢀ
phous regardless of calcination temperature. In conꢀ
nection with this, thermal analysis, XPS, and Mössꢀ
bauer spectroscopy were used for studying the formaꢀ
tion of the active component of catalysts in the course
of thermal treatment.
metal ( ꢀFe). The procedure used for processing the
α
spectra and obtaining the corresponding parameters
was described in detail earlier [39].
Figure 1 shows the TG, DTG, and DSC curves for
the 4FS(110) sample. The DTG curve exhibited two
weight loss regions: the first of them is caused by the
The study of the samples by highꢀresolution transꢀ
mission electron microscopy (TEM) were performed
with the use of a JEMꢀ2010 transmission electron
microscope (JEOL, Japan) with an accelerating voltꢀ
age of 200 kV and a resolution of 0.14 nm. The partiꢀ
cles of catalysts were applied to copper gauze by disꢀ
persing a suspension of the sample in alcohol with the
use of an ultrasonic disperser.
removal of adsorbed water (at 70–120
°
С), and the secꢀ
ond (at 580–700 ) is caused by the decomposition of
°
С
iron sulfates. The experimental results are consistent
with published data. Thus, the decomposition temperꢀ
ature of bulk Fe2(SO4)3 in a flow of air is 490–550
[40]. The decomposition of bulk FeSO4 begins at
300 and passes through the formation and decomꢀ
°С
2−
O4
The concentrations of Fe and
S
in the samples
°С
were determined by inductively coupled plasma
atomic emission spectrometry on an OPTIMA 4300
DV spectrometer from Perkin Elmer.
position of complex oxy and hydroxy sulfate comꢀ
pounds—FeOHSO4 and Fe2O(SO4)2. According to
published data, the decomposition of the latter begins
The catalytic properties were studied in a flow cirꢀ
culation reactor at atmospheric pressure and a temꢀ
at 505
°
С
and becomes complete with the formation of
C [41, 42].
α
ꢀFe2O3 after calcination at 830
°
perature of 250 С with the use of a catalyst fraction of
°
The [
2−] : [Fe] ratio was 0.96, as calculated from
S
O4
0.25–0.5 mm. An additional test for the circulation
ratio of the reaction mixture and the effect of particle
sizes on the reaction rate showed that the reaction
occurred in the kinetic region under the selected conꢀ
sample weight losses upon the removal of sulfate ions.
This result is consistent with chemical analysis data for the
4FS(110) sample, according to which the [
S
2−] : [Fe]
O4
ditions. The volume fractions of reagents in the initial ratio is ~1.01. This ratio decreased to 0.05 after calciꢀ
reaction mixture were the following: H2S, 2.0 vol %; nation at 700 ; this is indicative of the almost comꢀ
°С
KINETICS AND CATALYSIS Vol. 52
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EFFECT OF CALCINATION TEMPERATURE ON THE PHYSICOCHEMICAL
DTG (%/min) TG, %
0.1
899
0.1
100
0.0
0.0
98
–0.1
–0.1
96
–0.2
–0.2
94
–0.3
–0.3
92
–0.4
–0.4
–0.5
90
88
–0.5
–0.6
0
200 400 600 800 1000
Temperature, °C
Fig. 1. Thermal analysis curves for the FS(110) sample.
(а)
(b)
710.0
710.4
S 2s
Fe 2p
233.3
1
2
1
2
3
4
3
4
5
6
5
6
700 705 710 715 720 725 730 735 740
Binding energy, eV
218 222 226 230 234 238 242
Binding energy, eV
Fig. 2. XPS spectra of 4FS samples calcined at temperatures of (1) 110, (2) 400, (3) 500, (4) 600, (5) 700, and (6) 900°C.
plete decomposition of iron sulfate on the catalyst surꢀ the most intense traditionally used S 2
face.
In this work, we used XPS to obtain information on
p
line because of
the overlapping of the latter (BE ~160–169 eV) with a
line due to the support–Si 2 (BE ~154.5 eV). Note
that the Fe 2 spectra of the test catalysts are a doublet
of Fe 2 3/2–Fe 2 1/2 with a 2 : 1 ratio between the inteꢀ
grated line intensities (Fig. 2a). As a rule, both the
position of the main Fe 2 3/2 line and the shape of the
line was chosen for analysis in place of Fe 2 spectrum, namely, the intensities and relative
s
p
the state of the active component in 4FS samples with
different calcination temperatures. Figure 2 shows the
p
p
Fe 2
4FS(500), 4FS(600), 4FS(700), and 4FS(900) samꢀ
ples. The S 2
p and S 2s XPS spectra of the 4FS(110), 4FS(400),
p
s
p
KINETICS AND CATALYSIS Vol. 52
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2011

900
SHUVAEVA et al.
In the Fe 2
600 and 700
and a shake up satellite are shifted to the regions of
710.1–710.4 and 713–714 eV, respectively; in combiꢀ
nation with a decrease in the S 2s line intensity, this is
indicative of the decomposition of iron sulfate. This
conclusion is consistent with the results of chemical
p
С
spectra of samples 4 and 5 calcined at
, respectively, the main Fe 2p3/2 peak
°
900°C
700°C
600°C
500°C
analysis. The temperature of 500 С corresponds to the
°
onset of the decomposition of iron sulfate on the cataꢀ
lyst surface. The sulfur content of the sample calcined
at 900 C was lower than the limit of detection by XPS;
°
thus, we can conclude that the complete decomposiꢀ
tion of iron sulfate with the formation of iron oxide
occurred. In the sample calcined at 700 С, the
[Fe] : [Si] ratio was 0.010, whereas it was 0.012 in the
4FS(900) sample. A small change in the value of
[Fe] : [Si] with increasing
°
400°C
110°C
Tc suggests that the agglomꢀ
eration of the active phase did not occur. In the Fe 2
p
spectrum of the 4FS(900) sample, the main Fe 2p3/2
peak and a shake up satellite were observed in regions
of 710.4 and 713.3 eV, respectively. Note that this
binding energy and the position and intensity of the
shake up satellite are not characteristic of iron oxide in
–5
0
5
V,
mm/s
the modifications of
α
ꢀFe2O3 and ꢀFe2O3. For these
γ
Fig. 3. Mössbauer spectra of 4FS* catalysts calcined at difꢀ
ferent temperatures.
compounds, the observed binding energy was 711 eV;
in this case, a clearly pronounced shake up satellite
was present in the spectrum at 719 eV [44, 47].
Mössbauer spectroscopy was used for the more
detailed characterization of ironꢀcontaining comꢀ
pounds formed on the catalyst surface after heat treatꢀ
ment at different temperatures. To increase the sensiꢀ
tivity of this technique, the samples were prepared
with the use of Fe(II) sulfate obtained from iron metal
with a high concentration of the 57Fe isotope (99.5%)
in accordance with a previously described procedure
positions of shake up satellite lines caused by the manꢀ
ifestation of manyꢀelectron processes, are used for the
identification of the state of iron. The position and
intensity of shake up satellite lines depend on the
chemical state of iron. Thus, for instance, in the specꢀ
tra of FeSO4, FeO, Fe2(SO4)3, ꢀFe2O3, and FeOOH,
α
the shake up satellite lines are located 4.3, 5.65, 5.7,
8.05, and 8.5 eV higher on the scale of binding energies
with respect to the main Fe 2p3/2 and Fe 2p1/2 lines
[43]. At the same time, shake up satellite lines were
[48]. The preparation of the FeSO4 H2O compound
⋅
was confirmed by XRD analysis. The resulting catalyst
samples are referred to as 4FS*(400), 4FS*(500),
4FS*(600), etc. Numbers in parentheses correspond
to sample calcination temperatures, and the first numꢀ
ber approximately corresponds to the concentration of
iron (wt %). According to chemical analysis data, the
iron content of the 4FS*(700) sample was 3.4 wt %.
absent from the spectra of iron metal, FeS2, and Fe3O4
In the Fe 2 3/2 spectra of the 4FS(110) sample dried
at 110 and 4FS samples calcined at 400 and 500
.
p
°С
°С,
an intense peak characteristic of Fe(III) compounds
occurred at 711.0 0.05 eV. A less intense peak at
714.7 eV corresponds to a shake up satellite. In accorꢀ
dance with published data for FeO, Fe2O3, and
Fe2(SO4)3, the Fe 2p3/2 binding energies lie in the
ranges of 709.5–710.2, 710.95–711.2, and 713.25–
713.5 eV, respectively [43–45]. Taking into account the
chemical composition of the catalyst, we assume that
iron compounds formed on the surface of silica gel after
Figure 3 shows the Mössbauer spectra of 4FS*
samples measured at room temperature. The spectra
of the samples calcined at 500
pole doublets characteristic of the paramagnetic state
of a substance. After calcination at 600 , a magnetiꢀ
cally ordered phase was formed on the surface of silica
gel and its concentration increased with Tc
°С or lower are quadruꢀ
°С
.
calcination at temperatures of no higher than 500 С
°
contain sulfate groups. Indeed, the [S] : [Fe] atomic
ratio for these catalysts lies in the range of 1.1–1.6. In
this case, an intense narrow line with BE = 233.3 eV,
Table 1 summarizes the interpretation of the specꢀ
tra. In the samples calcined at temperatures of no
higher than 500 С, three nonequivalent positions of
°
which belongs to sulfate groups, is observed in the S 2
s
iron, Fe1, Fe2, and Fe3, were detected; they differed
in the value of quadrupole splitting or the degree of
distortion of a local environment. Comparing the
Mössbauer parameters of these states with the paramꢀ
eters of iron sulfate derivatives [49–53], we can conꢀ
2−
spectra. For sulfur in the state S6+
(
groups), the
SO4
binding energies fall within the range of
232.0–237.7 eV [14, 44, 46]. Thus, for instance, the
S 2 binding energy for FeSO4 7H2O is 232.0 eV [47].
published S 2
s
s
⋅
KINETICS AND CATALYSIS Vol. 52
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2011

EFFECT OF CALCINATION TEMPERATURE ON THE PHYSICOCHEMICAL
901
Table 1. Mössbauer parameters of FS* catalysts calcined at different temperatures
IS 0.005,
mm/s
QS 0.03,
mm/s
W
mm/s
0.05,
H
5, kOe
A
0.03
0.38
Position
FSꢀ110
°
C
0.419
0.426
0.420
1.157
1.187
1.245
0.411
0.424
0.425
0.401
0.410
0.398
0.412
0.507
0.300
0.376
0.336
0.325
0.349
0.404
0.374
0.423
0.344
0.334
0.314
0.385
0.374
0.352
0.389
0.233
0.383
0.311
0.362
–
–
0.49
0.80
1.13
1.61
2.14
2.59
0.48
0.80
1.13
0.52
0.90
1.34
0
0.33
0.31
Fe1 – Fe2O(SO4)2
⋅
x
H2O
0.34
0.17
0.04
0.06
0.02
0.44
0.37
0.19
0.37
0.45
0.17
0.05
0.04
0.08
0.15
0.16
0.44
0.07
0.03
0.06
0.24
0.13
0.14
0.37
0.03
0.09
0.05
0.22
0.29
0.07
0.17
0.11
Fe2 – FeOHSO4
Fe3 – FeOHSO4
Fe2+ in Fe1
Fe2+ in Fe2
Fe2+ in Fe3
Fe1
⋅
2H2O
–
0.30
–
0.36
–
0.37
–
0.23
FSꢀ400
FSꢀ500
FSꢀ600
°C
°C
°C
–
0.34
–
0.32
Fe2
–
0.33
Fe3
–
0.33
Fe1
–
0.38
Fe2
–
0.42
Fe3
442
398
362
231
–
0.31–1.07
0.81–0.99
2.18
B1 in
B2 in
B2 in
ε
ε
ε
ꢀFe2O3
ꢀFe2O3
ꢀFe2O3
0
0
0
2.27–2.76
0.43
A in εꢀFe2O3
0.55
0.91
1.55
0
Fe1* – oxide
–
0.58
Fe2* – oxide
–
0.56
Fe3* – oxide
B1
FSꢀ700
°
C
461
432
389
248
–
0.41
0
0.77
B2
0
1.83
B2
0.26
0.62
0.93
1.92
–0.45
–0.71
–0.21
0
0.58–1.96
0.48
A
Fe1*
Fe2*
Fe3*
B1
–
0.67
–
0.49
FSꢀ900
°
C
441
415
382
249
–
0.19–0.48
0.18–0.69
0.50–1.23
0.72–1.53
0.37
B2
B2
A
0.74
0.80
1.76
Fe1*
Fe2*
Fe3*
–
0.84
–
0.89
Note: IS is the isomeric chemical shift relative to
absorption line width (the widths of internal and external lines, which demonstrate the magnetic nonuniformity of given positions
in a crystal are specified for the sextets), and is the fractional position population.
α
ꢀFe, H is the hyperfine field on the iron nucleus, QS is quadrupole splitting, W is the
A
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902
SHUVAEVA et al.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
ε
ꢀFe2O3
Fe1
Fe2
Fe3
0
200
400
600
800
1000
Tc, °C
Fig. 4. Dependence of the relative concentration of ironꢀcontaining phases in 4FS* catalysts on calcination temperature.
clude that the local environments of the Fe1, Fe2, and with the characteristic values of quadrupole splitting,
Fe3 positions are close to the environments in the remain in the oxide phase; that is, octahedrons, which
were present in hydroxo sulfates, serve as the building
blocks of oxide phases. The quadrupole splitting of
~0.6 mm/s (Fe1 positions) is usually inherent in the
octahedrons of oxide compounds with the cubic packꢀ
ing of oxygen atoms, in which the adjacent octaheꢀ
drons have a shared edge, and this structure has tetraꢀ
hedral along with octahedral interstitial sites. The quaꢀ
drupole splitting of ~0.9 mm/s (Fe2 positions) is
characteristic of oxides with the hexagonal packing of
oxygen atoms, in which the octahedrons are faceꢀ
shared. We attribute Fe3 positions with large quadruꢀ
pole splitting (QS ~1.3–1.7 mm/s) to interblock iron,
possibly, with a distorted octahedral coordination.
Fe O
2
⋅
)
H2O, FeOHSO4 H2O, and FeOHSO4
⋅
SO
(
4
2
compounds, respectively. These designations are used
in Table 1. The Mössbauer procedure provides inforꢀ
mation on the local characteristics of the positions
rather than the phase composition. In principle, the
Fe1–Fe3 positions can be related to positions in a
compound of a complex composition with a weakly
ordered distribution of hydroxyl, oxygen, and sulfate
groups.
Judging by the values of isomeric chemical shifts,
iron in a trivalent state was present in all of the samples
other than FS*(110). In accordance with the values of
quadrupole splitting, three positions of Fe2+ detected
in FS*(110) can be attributed to the states Fe1–Fe3.
After calcination at 400
In the range of 110–500
the three states Fe1–Fe3 smoothly decreased to indiꢀ
cate that iron compounds gradually lost crystal water.
Two effects were observed in the Mössbauer spectra
after the calcination of catalysts at 600 С. First, a magꢀ
The magnetically ordered phase, which resulted
°
С
,
Fe2+ was oxidized to Fe3+
, the chemical shifts of all of
.
from calcination at 600 С, manifested itself as sextets
°
in the Mössbauer spectra. In this phase, four nonꢀ
equivalent positions of iron were identified, whose
Mössbauer parameters are close to the parameters of
°С
the ꢀFe2O3 oxide [54, 55]. Positions A relate to the
ε
tetrahedral positions, whereas positions B1–B3 relate
to three nonequivalent octahedral positions of this
°
netically ordered phase appeared, in which four nonꢀ
equivalent iron positions were revealed (A, B1–B3). Secꢀ
ond, the chemical shifts of the states Fe1–Fe3 sharply
decreased to the values characteristic of Fe3+ in an
octahedral coordination to oxygen. The observing
changes in the spectra suggest that the catalyst loses
oxide. The fraction of the
rial increased with c, and a cation redistribution over
the positions occurs.
εꢀFe2O3 phase in the mateꢀ
T
Figure 4 shows changes in the relative concentraꢀ
tions of ironꢀcontaining phases in the catalyst with
increasing
ꢀFe2O3 phase, whose structure has the cubic packing
neous formation of new oxide lattices. The question of of oxygen atoms, is formed at a temperature of 600
Tc. According to experimental data, the
sulfate groups in the course of calcination at 600
and hydroxo sulfates decompose with the simultaꢀ
°С
ε
°С
the type of these lattices is still an open question based on Fe1* positions, which have predominantly
because the Mössbauer procedure makes it possible to the cubic packing of oxygens. Because of this, the
determine only the local characteristics of positions. Fe2* phase population with the predominantly hexagꢀ
Three nonequivalent positions of iron, Fe1*–Fe3* onal packing of oxygen atoms increases. Recall that
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EFFECT OF CALCINATION TEMPERATURE ON THE PHYSICOCHEMICAL
903
Intensity
the phase of FeOHS H2O is the precursor of the
⋅
O4
Fe2* phase. As
T
c is further increased, the concentraꢀ
250
200
150
100
50
tion of the
εꢀFe2O3 phase increases.
To support the conclusion on the formation of the
ꢀFe2O3 phase, we prepared a series of samples with a
ε
0
higher iron content of 5.65 wt % for the sample calꢀ
cined at a temperature of 900 (6FS). The Xꢀray difꢀ
fraction patterns of 6FS samples calcined at 400–
700 do not contain reflections, whereas the Xꢀray
10
20
30
40
50
60
70
θ
°С
2
°С
Fig. 5. Xꢀray diffraction pattern of the 6FS(900) sample.
diffraction pattern of the 6FS(900) sample exhibits a
number of the clearly pronounced peaks, which correꢀ
spond to a wellꢀcrystallized phase (Fig. 5). In accorꢀ
dance with published data, the observed reflections at
2θ
= 27.636, 29.949, 32.901, 35.215, 36.691, 40.111,
45.646, 52.941, 60.973, and 63.372 belong to the
ꢀFe2O3 phase, which was described recently [54, 55].
°
ε
Note that no traces of the
observed in this case.
αꢀFe2O3 phase were
The TEM study of the 4FS(900) sample made it
possible to establish that, after calcination at 900 С,
°
iron oxide particles in a highly dispersed state with the
average particle size of 3.2 nm were present on the surꢀ
face of silica gel (Fig. 6).
Thus, the set of the experimental results allows us
to conclude that a compound with a complex compoꢀ
sition is formed upon the impregnation of silica gel
with a solution of Fe(II) sulfate and thermal treatment
20 nm
Fig. 6. Electron micrograph of the 4FS(900) sample.
at 400–500 С. The components of this compound are
°
oxide, sulfate, and hydroxide groups as the anions, and
the cations of iron (Fe3+) are located in three nonꢀ
equivalent positions characteristic of various oxy and
1.3 as
activity insignificantly decreased after treatment at
higher temperatures. As a rule, an increase in c leads
to the agglomeration of the active component of supꢀ
ported catalysts and, as a result, to a decrease in the
activity [56]. The increase in the catalyst activity
T
c was increased from 500 to 700
°
С, whereas the
hydroxy sulfates: Fe O
⋅
)
2
H O, FeOHS
O4
2
⋅
SO
(
2
4
T
2H2O, and FeOHSO4. Calcination at 600 С leads to
°
the almost complete removal of sulfate groups. As a
result, the formation of an oxide structure comes into
play; after calcination at 900
formation of finely dispersed iron oxide (the average
particle size of 3.2 nm) with the structure of ꢀFe2O3
The Fe 2 XPS spectrum of iron oxide with the
structure of ꢀFe2O3 has characteristic properties that
can be used for the identification of this phase: the
positions of the main Fe 2 3/2 peak and a shake up satꢀ
°С, it completes by the
observed in this work as
Tc was increased from 500 to
700 can be explained by the fact that sulfate groups
°С
ε
.
are removed in this temperature range. The experiꢀ
mental result suggests that the activity of the formed
oxide was higher than that of the initial compound,
p
ε
p
ellite are observed at 710.4 and 713.3 eV, respectively.
This binding energy and the position and intensity of
the shake up satellite are uncharacteristic of iron oxide
Table 2. Catalytic activity of 4FS samples calcined at difꢀ
ferent temperatures in the reaction of H2S oxidation with
oxygen
in the
α
ꢀFe2O3 and
γ
ꢀFe2O3 modifications.
Calcination temperature, Oxidation reaction rate
mol H2S g–1 s–1
w,
°
C
Catalytic Properties
500
600
700
900
9.51
1.22
1.26
1.15
×
×
×
×
10–6
10–5
10–5
10–5
Table 2 summarizes the steadyꢀstate reaction rates
of H2S oxidation obtained as a result of studying the
activity of catalyst samples. The selectivity of hydroꢀ
gen sulfide conversion into sulfur was 100% on all of
the synthesized samples under the reaction condiꢀ
tions. As follows from the above data, the reaction rate
of hydrogen sulfide oxidation increased by a factor of
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904
SHUVAEVA et al.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
500°C
700°C
900°C
100
200
300
c, °C
400
500
T
Fig. 7. TPS curves for 4FS samples calcined at 500, 700, and 900°C.
which contained sulfate groups in its composition. A spectroscopy. This study allowed us to reveal the reguꢀ
further increase in
T
c to 900 С did not lead to a conꢀ
°
larities of the conversion of surface compounds
formed on the silica gel surface. After thermal treatꢀ
siderable decrease in the activity; this was likely a conꢀ
sequence of the high thermal stability of the formed
phase. The stability of the samples to agglomeration
was confirmed by XPS and TEM data.
ment in the temperature range of 400–500 С, a comꢀ
°
pound containing oxygen and sulfate groups as the
anions was present on the surface of silica gel. Accordꢀ
ing to Mössbauer spectroscopic data, the cations of
iron (Fe3+) in these compounds are located in three
nonequivalent positions characteristic of oxy and
Figure 7 shows the results of the study of the stabilꢀ
ity of 4FS samples calcined at different temperatures
to sulfidation. The comparative analysis of the TPS
curves shows that a maximum decrease in the concenꢀ
tration of H2S, which reflects the absorption of hydroꢀ
gen sulfide by the catalyst due to the conversion of the
iron oxide phase into iron disulfide, shifted toward
hydroxy sulfates of different composition: Fe O
⋅
)
SO
(
2
4
2
H O, FeOHS 2H2O, and FeOHSO4. Calcination at
⋅
O4
2
600°С
leads to the almost complete removal of sulfate
groups; as a result, the formation of iron oxide with the
structure of ꢀFe2O3 comes into play to be complete
after calcination at 900 . The average particle size of
the resulting ꢀFe2O3 particles is 3.2 nm. An increase
in c from 500 to 700 leads to an increase in the catꢀ
higher temperatures with increasing the Tc of the samꢀ
ε
ples. The TPS curves also exhibited two peaks of
hydrogen sulfide absorption upon the interaction of
hydrogen sulfide with the 4FS(500) and 4FS(700)
samples. This shape of the TPS curves, which was
repeatedly reproduced for the samples with different
iron contents, suggests the presence of particles with
different stability to sulfidation. The hypothesis on the
heterogeneous phase composition of the compounds
formed upon the thermal treatment of the catalysts in
°С
ε
T
°С
alyst activity in the reaction of the selective oxidation
of hydrogen sulfide, which can be explained by a
higher activity of iron oxide, as compared with sulfate
compounds. A further increase in the calcination temꢀ
perature to 900
decrease in activity, which is explained by the high staꢀ
bility of the formed ꢀFe2O3 particles to agglomeraꢀ
°С leads to only an insignificant
the temperature range of 500–700
able explanation. An increase in Tc to 900
°С
is the most probꢀ
leads to
°С
ε
an increase in the stability of the sample to sulfidation:
the maximum of hydrogen sulfide absorption shifts
toward higher temperatures. The TPS curve exhibits
one peak, which suggests the uniform composition of
the resulting ironꢀcontaining compounds.
tion. With increasing the temperature of calcination,
an increase in the stability of the ironꢀcontaining catꢀ
alysts to the action of a reaction atmosphere is
observed in the entire test range of calcination temperꢀ
atures.
CONCLUSIONS
ACKNOWLEDGMENTS
The study of catalyst samples prepared by the
impregnation of silica gel with a solution of Fe(II) sulꢀ
fate and calcined at different temperatures was perꢀ
We are grateful to A.V. Ishchenko for his assistance
in the characterization of samples by transmission
formed by thermal analysis, XPS, and Mössbauer electron microscopy.
KINETICS AND CATALYSIS Vol. 52
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EFFECT OF CALCINATION TEMPERATURE ON THE PHYSICOCHEMICAL
905
23. Ismagilov, Z.R., Kerzhentsev, M.A., Khairulin, S.R.,
Karakutz, V.N., and Ismagilov, F.R., HTI Q., Winter
1994/95, p. 59.
This work was supported by the Presidium of the
Russian Academy of Sciences within the framework of
Program no. 27 “Fundamental Studies on Nanotechꢀ
nologies and Nanomaterials” (project no. 46).
24. Bukhtiyarova, G.A., Sakaeva, N.S., Varnek, V.A., Burꢀ
gina, E.B., Plyasova, L.M., Bukhtiyarov, V.I.,
Kaichev, V.V., and Zolotovskii, B.P., Khim. Interes. Ust.
Razv., 1999, vol. 7, no. 4, p. 359.
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