Oxidation of Sulphur Dioxide using Micro- and Nanoparticles of various Iron Oxides

Article abstract of DOI:10.1002/zaac.202100091

Sulphuric acid is the chemical with highest production rates in the world. At present, it is mainly synthesized using vanadium pentoxide as catalyst, which determines the applied production process particularly in terms of gas pre-treatment and heat management. For processes, which cannot be run with vanadium pentoxide, alternative catalysts are required to make different SO₂ qualities accessible to sulphuric acid production. Ferric oxides are a very promising alternative, since they combine higher thermal with improved chemical stability. Within this study, various ferric oxides were examined with regard to conversion rates and structural changes during application. Effects of crystal structure, particle size as well as thermal treatment and the influence of precipitation conditions were studied. Although conversion rates are very promising, it has become apparent, though, that these materials cannot compete with vanadium pentoxide in terms of conversion rate as well as long-time stability, yet. Nevertheless, from the results of this study, it is clear that high potential lies in focused catalyst optimisation.

Full text of DOI:10.1002/zaac.202100091

Title: Oxidation of Sulphur Dioxide using Micro- and Nanoparticles of
various Iron Oxides
Authors: Lydia Reichelt, Sebastian Hippmann, Vyacheslav Nikolaevich
Brichkin, and Martin Bertau
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Z. anorg. allg. Chem. 10.1002/zaac.202100091
Link to VoR: https://doi.org/10.1002/zaac.202100091

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
Oxidation of Sulphur Dioxide using Micro- and Nanoparticles of
various Iron Oxides
Lydia Reichelt[a,b], Sebastian Hippmann , Vyacheslav Nikolayevich Brichkin and Martin Bertau*
[a]
[c]
[a]
Abstract: Sulphuric acid is the chemical with highest production rates
in the world. At present, it is mainly synthesized using vanadium
pentoxide as catalyst, which determines the applied production
process particularly in terms of gas pre-treatment and heat
management. For processes, which cannot be run with vanadium
terms of temporal fluctuations as well as sulphur dioxide
concentration.[3,5] Thus, sulphur dioxide containing gases with
constant or temporal fluctuating sulphur dioxide concentrations in
the range of 2 - 12 vol.% can be converted with these
processes.[2,3,5–7] Higher concentrated gases need to be diluted or
incompletely converted for the purpose of limiting adiabatic
temperature increase in the course of the reaction, since the
maximum temperature is determined by the catalyst.[6,8–10] In
contrast to that, application of lower sulphur dioxide
concentrations requires additional heating, as heat production
during reaction is insufficient for autothermal operation.[
Furthermore, utilisation of sulphur dioxide containing exhaust
gases necessitates the implementation of additional process
steps prior to conversion for separating water, catalyst poisons,
such as fluorine or chlorine, as well as arsenic, which affects
recycling.[3–5] Altogether, execution of contact processes is
essentially determined by operating temperature range and
chemical properties of the vanadium pentoxide catalyst.[
pentoxide, alternative catalysts are required to make different SO
2
qualities accessible to sulphuric acid production. Ferric oxides are a
very promising alternative, since they combine higher thermal with
improved chemical stability. Within this study, various ferric oxides
were examined with regard to conversion rates and structural
changes during application. Effects of crystal structure, particle size
as well as thermal treatment and the influence of precipitation
conditions were studied. Although conversion rates are very
promising, it has become apparent, though, that these materials
cannot compete with vanadium pentoxide in terms of conversion
rate as well as long-time stability, yet. Nevertheless, from the results
of this study, it is clear that high potential lies in focused catalyst
optimisation.
3]
2–8,11,12,13]
as well as the need for recycling of spent catalyst, since it is toxic
[4,14],
and a complete recycling is mandatory [4,5]
.
As the use of exhaust gases for sulphuric acid production
Introduction
becomes more and more important, research on alternative
catalysts has been intensified in recent years.[
6–9,13–21]
Promising
The oxidation of sulphur dioxide to sulphur trioxide is one of the
most often applied catalytic processes, since it is the key step for
sulphuric acid production. Sulphuric acid is the most important
chemical in the world as well as the one with the highest
production rate. The global sulphuric acid market size reached a
volume of 284.4 million metric tons in 2020 and is likely to reach
a volume of 311 million metric tons by 2026.[1] As a base chemical
it finds manifold applications, spanning from metallurgic
processes to chemical syntheses.[2–4]
At present, the contact and double contact processes using
vanadium pentoxide catalysts are widely used for sulphur dioxide
oxidation. The applied process is chosen in dependence on the
sulphur dioxide source used and its characteristic properties in
materials should combine a wider operating temperature range
with improved stability against poisoning. Thus higher
concentrated gases typical for modern metallurgic processes
applying oxygen could be used directly , while other exhaust
gases could be applied in a process with an improved number of
process steps.
Ferric oxides are very promising materials, since their catalytic
activity for sulphur dioxide oxidation is sufficient for applications
in industrial processes and its durability against poisoning is
known for decades.[3,5,12,16] Additionally, they possess a higher
[6]
thermal stability in comparison to vanadium pentoxide based
[6,15,17,18]
catalysts, which enables their application up to 800 °C.
However, maximum conversion is achieved at temperatures in
the range of 550 – 760 °C and often without reaching
thermodynamic equilibrium.[6,8,14,22,23] Thus conversion rarely
reaches levels achieved with vanadium pentoxide and patented
processes mostly combine catalyst layers of vanadium pentoxide
and ferric oxide.[8,9,15]
[a]
Dr. L. Reichelt, Dr. S. Hippmann and Prof. Dr. M. Bertau
Corresponding Author)
(
Freiberg University of Mining and Technology
Institute of Chemical Technology,
Leipziger Straße 29, 09599 Freiberg, Germany
E-mail: martin.bertau@chemie.tu-freiberg.de
sebastian.hippmann@chemie.tu-freiberg.de
Dr. L. Reichelt
PFARR Stanztechnik GmbH
Am kleinen Sand 1, 36419 Buttlar, Germany
E-mail: L.Reichelt@freenet.de
However, recent research has shown that catalytic activity as well
as onset temperature of SO oxidation of ferric oxides can be
2
improved by reducing the particle size, although only gases with
sulphur dioxide concentrations in the ppm-range where
examined.[ Additionally, investigations on the application of
[
[
b]
c]
14]
sulphated ferric oxides showed that these materials possess
improved catalytic activity in acid-base catalysis.[24] Moreover, the
incorporation of sulphate into the catalyst precursor is promising,
since sulphate coordinated by the ferric oxide lattice can be seen
as preformed catalytic active site for sulphur dioxide coordination.
Prof. Dr. V.N. Brichkin
Saint-Petersburg Mining University
Metallurgy Department
2, 21st Line, St Petersburg 199106, Russia
E-mail: Brichkin_VN•@pers.spmi.ru
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
Therefore, this study deals with the application of sulphated ferric
oxides for sulphur dioxide oxidation, which is a continuation of
former investigations concerning reproducible production of
sulphated ferric oxides using a microjet mixer [25] and research on
the influence of precipitation parameters [25] as well as calcination
temperature [26] on chemical and structural properties. However,
for better classification of observed effects and dependencies as
well as identification of influencing parameters, various
commercially available ferric oxide micro- and nanoparticles
flow rate and catalyst loading. Thus, investigations on various
catalyst loadings were done in this study resulting in space
velocities in the range of 350 to 750 h-1 applying 10 vol.% SO
at
2
650 °C. However, these experiments did not show any significant
effects on conversion, which is the reason why these parameters
were not varied in consecutive experiments.
100
(
pigments, ferric oxide nanoparticles and microparticles
synthesized for research application) were examined, too.
5 vol.%
80
60
40
20
0
10 vol.%
15 vol.%
20 vol.%
28.5 vol.%
Results and Discussion
General trends
Former investigations on the reaction of ferric oxides with sulphur
dioxide covered a wide temperature [14,27,28] as well as a wide
concentration range [8,14,18,23,29]
, and results differed largely.
Therefore, experiments with ferric oxide microparticles were done
first for the purpose of identifying promising reaction conditions as
well as general trends in physical and chemical changes of the
used catalyst during the catalytic reaction. The investigated
parameters comprised temperatures from 450 to 650 °C and
sulphur dioxide concentrations in the range of 5.0 - 28.5 vol.%.
These ranges were chosen, since they cover conditions applied
for catalysis with vanadium pentoxide enabling comparison of the
results as well as conditions relevant in exhaust gas treatment.
4
50
500
550
600
650
Temperature [°C]
Figure 1. Dependence of thermodynamically (dashed line) and experimentally
determined (solid line) sulphur dioxide conversion rates from temperature and
sulphur dioxide concentration.
A comparison of conversion rates determined in this work with
those from earlier studies (Table 1) showed that they differ to a
large extent due to different composition and production
processes. Hematite microparticles studied in this work showed
intermediate catalytic activity.
2
The value of 28.5 vol.% SO resulted from stoichiometric values
of sulphur dioxide and oxygen concentration in the reactant gas
stream.
From Figure 1 can be seen, that within the temperature range up
to 600 °C conversion increased continuously. This dependency
was observed before and is caused by temperature dependent
kinetics.[14] However, with further ascending temperature,
concentration dependent behaviour was found to result in a
maximum conversion between 600 and 650 °C. Thus, the highest
conversion rate was found at a lower reaction temperature, when
the reactant concentration was increased. In fact, only
Table 1. Comparison of conversion rates with literature reports
Catalyst
c(SO
2
)/
T/
Conversio
n rate/ %
Ref.
vol.%
°C
α-Fe
α-Fe
α-Fe
2
2
2
O
O
O
3
3
3
5
450
450
450
450
640
22.5 ± 2.5
14.2 ± 0.8
5 - 7
This work
This work
[16]
10
7.4
7.4
7
Sn25,9Fe74,1
O
x
18
[16]
2
experiments with 5 vol.% SO gave the thermodynamically limited
yield within the examined temperature range, while
thermodynamic equilibrium is expected to be reached below
Catalyst synthesized from
dust containing Fe, Cu, Si,
Zn, Pb
56
[7]
7
2
50 °C for the application of 10 and 15 vol.% SO . As the
Fe
Iron silicate
α-Fe
2
O
3
supported on SiO
2
20
20
20
600
600
600
72
[15]
thermodynamic equilibrium lies rather on the reactant side in this
temperature range, resulting possible conversion rates are lower
than yields reached at 650 °C. Finally, experiments with 20 and
77
[15]
2
O
3
46.2 ± 3.9
This work
2
8.5 vol.% SO
2
were characterised by a decreased conversion at
These differences are a result of varying synthesis methods as
well as reactant gas and catalyst compositions. Additionally,
catalytic conversion achieved with ferric oxide is lower than that
of vanadium pentoxide, which is known to reach thermodynamic
equilibrium conversion, as long as sulphur dioxide concentration
is between 2 - 12 vol.%.
In order to investigate the effect of sulphur dioxide oxidation on
the catalyst, the ferric oxides used were analysed before and after
the reaction for their specific surface area as well as their
chemical composition. In general, XRD analyses showed that
650 °C, without reaching thermodynamic equilibrium conversion
at all. This is in agreement with preceding studies showing both
results: experiments with conversion levels always below
thermodynamic equilibrium [22,23] as well as studies in which the
thermodynamic equilibrium was reached [6,14,22]. Interestingly,
investigations on Pt/Pd catalysts for sulphur dioxide oxidation
examined by Koutsopoulos et al. [19,20] also showed maximum
conversion degrees without reaching thermodynamic equilibrium.
What they concluded was that the position of observed maximum
conversion depended on reaction conditions including volume
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
after conversion the catalysts were composed of hematite, α-
Fe O , and ferric sulphate, Fe (SO ) (Figure S1 and S2).
2 3 2 4 3
The formation of ferric sulphate is a function of sulphur dioxide
concentration and temperature dependent positions of reaction
equilibria. In addition, an increasing degree of sulphation with
increasing temperature is a result of improved kinetics of reaction
(1), until above 500 °C both ferric sulphate decomposition (see
Figure S4) (2) and the reverse reaction of (1) gain in importance,
which in turn cause the degree of sulphation to decline. These
effects are superimposed by the desulphation during cooling,
which is more pronounced if the reaction was carried out at higher
temperatures, which favours the decomposition of iron sulphate
and leads to a higher reaction velocity. Thus, catalysts treated at
650 °C with sulphur dioxide concentrations in the range of 5 to
However, the quantity of sulphate incorporated into the catalyst
materials was never the same as under reaction conditions, since
ferric sulphate was partially decomposed to ferric oxide and
sulphur trioxide during cooling down as a result of changing gas
atmosphere at elevated temperature. But as all experiments were
conducted following the same cooling procedure, a comparison of
the materials treated at the same reaction temperature is feasible,
and some conclusions on experiments at differing temperatures
can be drawn as well. Results of these analyses are outlined in
Figure 2 and Figure 3.
10 vol.% did not contain any sulphate after cooling, although
intermediary formation of ferric sulphate is essential for catalysis.
However, sulphate was determined in materials treated at higher
concentrations. This is a result of reactions (1) and (2): With
increasing sulphur dioxide concentration in the reactant gas the
sulphation ratio of the ferric oxide catalyst during the catalytic
reaction grows, too. As all materials were cooled down in the
same manner, the reaction velocity of ferric sulphate
decomposition (2) is independent from ferric sulphate ratio. At
high sulphation ratios, reaction velocity is not high enough for
complete decomposition of ferric sulphate during cooling down,
which is why sulphate can be detected in these used materials.
50
40
30
20
10
0
0
5
1
1
2
2
vol.%
vol.%
0 vol.%
5 vol.%
0 vol.%
8.5 vol.%
450
500
550
600
650
Temperature [°C]
6
0
5
1
vol.%
vol.%
0 vol.%
Figure 2. Temperature dependent sulphation grade of ferric oxide
5
4
3
2
1
15 vol.%
20 vol.%
microparticles in dependence on used sulphur dioxide concentration.
28.5 vol.%
As can be seen in Figure 2, the ferric sulphate ratio in treated
ferric oxide catalysts depended strongly on temperature as well
as sulphur dioxide concentration. Iron oxides can interact with
sulphur dioxide in many ways. Thus, both adsorption and direct
catalytic conversion of SO
dominates depends strongly on the reaction temperature. The
adsorption of SO with the formation of iron sulphate (equation 1)
2
are possible. Which mechanism
2
450
500
550
600
650
occurs mainly below the decomposition temperature of iron
sulphate (equation 2, T>600 °C; see Figure S3), while the direct
catalytic conversion predominates at higher temperatures and
Temperature [°C]
Figure 3. Dependence of specific surface area of ferric oxide microparticles on
iron sulphate can no longer be detected at envious SO
2
temperature and treatment with sulphur dioxide.
concentrations. With increasing temperature sulphation increased
until a maximum was reached between 500 and 600 °C. This
maximum depended on reactant concentration and was reached
at higher temperatures with increasing sulphur dioxide
concentration. Expectedly, ferric sulphate ratio increased at
higher sulphur dioxide concentration. These observations are a
result of the chemical reactions involved:
Based on these conclusions the relationship between reaction
conditions and observed specific surface area was examined
(
Figure S6-S10). As shown in Figure 3, specific surface area
decreased continuously with increasing temperature in the
absence of sulphur dioxide, which is an outcome of thermally
induced sintering. Characterisation of catalyst materials collected
after catalytic conversion of sulphur dioxide did not give a clear
indication, though, but comparison of sulphation with measured
specific surface area visualized in Figure 4 shows that there is a
connection between both parameters, as an increasing specific
surface area is linked to decreasing sulphation. This relationship
becomes clear, when it is considered that reaction (1) occurs at
first with highly energetic ferric ions of the crystalline lattice. These
ions are characterised by low coordination and are mainly situated
Fe
2
O
3
+ 1.5 O
2
+ 3 SO
(SO
2
⇌ Fe
⇌ Fe
2
(SO
4
)
3
(1)
(2)
Fe
2
4
)
3
2
O
3
+ 3 SO
3
Resulting in the over-all chemical equation
SO + 0.5 O ⇌ SO
2
2
3
(3)
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
at corners and edges.[30] In the course of sulphation these
crystalline imperfections are smoothened and pores are filled with
formed ferric sulphate, which causes the observed decrease in
surface area.
at 650 °C compared to what had been expected from the changes
in sulphur dioxide oxidation equilibrium.
The reason is a higher reaction velocity of ferric sulphate
decomposition marked by decreasing ferric sulphate ratios
measured after application when going from 600 to 650 °C. For
example, the ferric sulphate ratio of ferric oxide treated with 20
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
450 °C
500 °C
550 °C
600 °C
650 °C
45
2
vol.% SO decreased from 40.65 ± 3.91 % to 8.52 ± 3.34 % in this
temperature range. As stated before, this observation was
40
35
30
25
20
accompanied by an increase of specific surface area from 2.14 ±
2
0
.07 to 3.62 ± 0.16 m /g, causing the over-all effective reaction
15
10
5
2
velocity to decrease from 2.05 ± 0.24 to 1.11 ± 0.08 µmol/m s.
0
0
5
10 15 20 25 30
0
5
10 15 20 25 30
0
5
10 15 20 25 30
0
5
10 15 20 25 30
0
5
10 15 20 25 30
Sulphur dioxide concentration [vol.%]
Figure 4. Connection between measured specific surface area and ferric
3
.0x10-3
450 °C
00 °C
550 °C
600 °C
sulphate ratio of used ferric oxide materials.
5
.5x10-3
2
As a consequence of the significant changes in specific surface
area SKat during reaction, the effective reaction velocity reff, which
is dependent from specific surface area, is defined according to
650 °C
2.0x10-3
cSO2 decreases
1
.5x10-3
equation (4) including conversion rate USO2 initial sulphur
̇
dioxide concentration c
, volume flow rate 푉 and the mass
1.0x10-3
0,SO2
of applied catalyst mKat
:
5
.0x10-4
USO2 ∙ c0,SO2 ∙ V
̇
reff=
(4)
mKat ∙ SKat
0.0
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Conversion [%]
Using this equation, the interplay between effective reaction
velocity, conversion as well as reaction temperature and reactant
concentration is graphically represented in Figure 5 and Figure 6.
It shows that the effective reaction velocity depended on
temperature as well as sulphur dioxide concentration, whereby
the effect of reactant concentration was most pronounced at
Figure 5. Dependence of the effective reaction velocity on the conversion and
the applied temperature.
600 °C. The increased sulphur dioxide partial pressure, on the
one hand, promotes the decomposition of iron sulphate to set the
thermodynamic equilibrium (equation 3) and, on the other hand,
increases the formation of iron sulphate according to equation 1.
Up to 600 °C reaction rates were found to increase, which is
mainly owed to decreasing effects of kinetic hindrance. Lower
reaction rates found at 650 °C are a consequence of a declining
conversion with increasing temperature as the thermodynamic
equilibrium of sulphur dioxide oxidation (3) is shifted to favour the
decomposition of sulphur trioxide. In fact, this decrease in
effective reaction velocity was more pronounced than the
observed decrease in conversion rate as specific surface area
increased from 600 to 650 °C due to reduced sulphation of the
spent catalyst.
.0x10-3
3
2
2
1
1
5
5 vol.%
10 vol.%
15 vol.%
.5x10-3
20 vol.%
28.5 vol.%
.0x10-3
.5x10-3
.0x10-3
.0x10-4
0.0
450
500
550
600
650
Temperature [°C]
Considering these analyses, the results shown in Figure 1 can be
interpreted in a way that decreasing conversion with increasing
sulphur dioxide concentration at low temperatures is owed to a
higher degree of catalyst sulphation. Since sulphation causes
catalytically active sites blocked, it is the interaction of reaction (1)
and (2), which determine catalytic activity with increasing
temperature. It is apparent that experimentally determined
Figure 6. Dependence of effective reaction velocity of ferric oxide microparticles
on applied temperature and sulphur dioxide concentration.
Based on these results further investigations were done using
2
10 vol.% SO at 600 °C. These reaction conditions bear the
potential for identification of materials with better catalytic
properties and maximum conversion rates within the range of
conditions applied for catalytic conversion with vanadium
conversion rates at 550 °C and 5 vol.% SO
those at 600 °C and 10 vol.% SO , although effective reaction
rates varied significantly. As can also be seen from Figure 1,
conversion decreased less pronounced for 20 and 28.5 vol.% SO
2
were very similar to
2
2 5
pentoxide (V O ).
2
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
Application of precipitated ferric oxides
In order to find an explanation for these results, analyses
concerning chemical and physical properties were done. They
showed that under reaction conditions any tested material was
converted into hematite and ferric sulphate. A deviating catalytic
activity is therefore not necessarily influenced by the particle
crystallinity. However, analyses of ferric sulphate ratio, specific
surface area and diameter of formed hematite crystallites showed
some differences. As summarized in Table 2, analyses of
Former studies had shown that chemical as well as physical
properties of ferric oxides were strongly effected by precipitation
conditions and subsequent treatment.[25,26] These variations were
studied for a potential impact on catalytic activity in sulphur
dioxide oxidation. For the purpose of verifying such dependences,
experiments with ferric oxides synthesized in our laboratory were
conducted. In the following, influences of thermal treatment,
mixing conditions during precipitation and aging were investigated. materials treated at 300 and 500 °C were found to have very
Finally, the choice of applied reaction temperature was checked
and the material showing highest conversion was used for several
successive experiments.
similar specific surface areas and hematite crystallite diameters
after the catalytic reaction, while the degree of sulphation
depended on the applied precursor. The decrease of specific
surface area was a characteristic property of each individual iron
oxide, a detailed study of which had been reported before.[6] In
general, the catalytic conversion reaction caused an increase of
sulphation and crystallisation, whereas specific surface area
decreased. Upon comparing the materials annealed at 900 °C it
became evident that during this pre-treatment ferric oxides were
formed, the properties of which were not significantly affected by
the catalytic reaction. This is in agreement with former studies
Influence of annealing temperature
From our former study concerning precipitation of ferric sulphate
with ammonia solution it was known that 2-line-ferrihydrite and
schwertmannite were formed as
precipitation.[25] Samples of both materials were annealed at 300,
00 and 900 °C for 8 h in a subsequent study.[26] These samples
a function of pH during
5
were chosen for investigation of catalytic activity, too. However,
materials treated at 300 and 500 °C are likely to undergo
physicochemical alterations during heating-up to the reaction
temperature of 600 °C. This expectation was evidenced by
analyses of specific surface areas of annealed ferrihydrites
showing a significant decrease of specific surface area between
[
27,31]
and a result of high annealing temperature which causes the
development of ferric oxides with very low specific energy. Hence,
resulting catalytic activity is much lower than that of ferric oxides
treated at lower temperatures. Remarkably, catalytic conversion
still depended on the used precursor, which was in connection
with differing specific surface areas and structures. This shows
that differences in particle structure observed before [ had an
influence on the catalytic conversion and achievable product
yields, even after treatment at 900 °C.
2
2
5
00 (52.6ꢀ±ꢀ0.6ꢀm /g) and 600 °C (29.5ꢀ±ꢀ0.4ꢀm /g).
26]
Considering this fact, it was expected that the catalytic activity
does not differ significantly for these materials, while ferric oxides
treated at 900 °C should show noticeable differences. Figure 7
displays the results of catalytic oxidation revealing that these
expectations were right. They clearly confirm that sintering during
annealing at temperatures above 800 °C affected catalytic
activity.[27,31] Interestingly, obtained conversion rates depended
on the used precursor, which is very important for catalyst
development. Thus, catalytic activity of treated schwertmannite
was higher than that of formerly tested microparticles (section 3.1)
as well as ferrihydrite.
Table 2. Analyses of fresh and spent ferric oxide
Annealing
Ferric sulphate
ratio/ wt.%
Specific surface
Hematite crystallite
diameter/ nm
2
temperature
area/ m /g
/
°C
Fresh
Used
Fresh
Used
Fresh
14 ± 2
Used
2-
300
500
900
300
500
3.5 ± 0.3
49.6
± 0.7
49.3
± 0.9
0 ±
155 ± 2
1.7 ±
0.1
37 ± 5
Line-
ferri
hydri
te
3.3 ± 0.2
0 ± 0.1
53 ± 1
<1
1.2 ±
0.1
37 ± 9
96 ± 1
-
39 ± 17
90 ± 1
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
Ferric oxide microparticles
-Line-ferrihydrite
Schwertmannite
<1
2
0.1
Sch
wert-
man
nite
28.0 ± 1.0
28.7 ± 1.4
0 ± 0.1
57.2
± 0.4
58.3
± 0.9
0 ±
26 ± 1
20 ± 1
1.3 ±
0.1
45 ± 22
39 ± 10
82 ± 5
1.4 ±
0.1
18 ± 7
96 ± 1
9
00
2.2 ±
0.2
2.4 ±
0.2
0.1
Influence of precipitation conditions
According to the results of the former section, the influence of
precipitation conditions on the catalytic activity of ferrihydrites was
investigated using materials annealed at 500 °C. For these
purposes, samples were selected whose precipitation conditions
differed with respect to ageing before separation from the liquid
medium, reactant concentration, reaction temperature, mixing
rate, application of aged ferrous sulphate solution and mixing
regime. Experimentally observed conversion rates of all materials
90
300
500
900
Temperature of annealing [°C]
Figure 7. Dependence of sulphur dioxide conversion on annealing temperature;
reaction temperature: 600 °C, feed gas composition: 10 vol.% SO , 18 vol.% O
and 72 vol.% N
2
2
2
.
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
separated directly after precipitation were in the same range of
35 ± 4 to 42 ± 4 %. However, ferrihydrite aged for one week in its
70
3.5x10-3
Ferric oxide microparticles
Schwertmannite
mother liquor was more active (55 ± 3 %). This is due to changes
in the crystalline structure of the precipitated ferrihydrite within this
period resulting in an improved stability of the specific surface
area during the catalytic reaction. Experiments with
schwertmannite revealed the same relationship.
3.0x10-3
60
50
40
30
20
10
0
2.5x10-3
2.0x10-3
1.5x10-3
1.0x10-3
5.0x10-4
0.0
Influence of reaction temperature
After demonstrating that schwertmannite annealed at 500 °C was
catalytically more active than ferric oxide microparticles and
ferrihydrite at 600 °C it was important to find out whether this
observation was correct for the entire temperature range of
450
500
550
600
650
450 - 650 °C. Thus, additional experiments with 10 vol.% SO
2
Temperature [°C]
were conducted. The results shown in Figure 8 proved that the
choice of 600 °C was right, since this was the only temperature,
where significant differences in catalytic activity were observed.
Additionally, experiments at this temperature were characterised
by highest differences of almost a factor of four in effective
reaction velocity, which were due to the low specific surface area
of treated schwertmannite and improved conversion. Decreasing
reaction velocity of schwertmannite with further increasing
reaction temperature was the result of an increasing specific
Figure 8. Dependence of conversion (solid line) and effective reaction velocity
dotted line) on applied ferric oxide; feed gas composition: 10 vol.% SO
8 vol.% O and 72 vol.% N
(
2
,
1
2
2
.
Figure 9 illustrates, how conversion rates decreased from
experiment to experiment. It was found, too, that conversion rates
decreased less with each experiment, but no steady state was
observed. This progress differed from the study by Wingen et al.
2
surface area from 1.4 ± 0.1 to 16 ± 1 m /g since ferric sulphate
decomposition occurred at high rates. Hence ferric sulphate ratio
measured after the catalytic conversion decreased from 58 ± 1 to
[6]
who described catalytic conversion to pronouncedly decrease
from first to second usage, while further application of their
supported catalyst caused only minor changes. However, in
accordance to this report, increasing sulphation during multiple
application was observed. The different behaviour is attributed to
the application of a volume catalyst, the progressing deactivation
of which is majorly results from blocking catalytically active sites
by sulphation. Interestingly, specific surface area of spent
schwertmannite decreased already within the first application to a
value, which remained constant throughout the following
experiments.
11 ± 7 % while crystallinity of formed hematite increased. Since
no significant temperature dependent changes in catalyst
composition and specific surface area were noted at
temperatures up to 600 °C, lower reaction rates observed in this
range are a consequence of decreased conversion and kinetic
effects. Thus, the deviating conversion rates detected at 600 °C
were a result of kinetic effects as well as relative proportions of
occurring reactions (1) and (2). This is evidenced by measured
sulphation ratios of treated catalysts, which showed that
sulphation of ferric oxide decreased already at 600 °C while
sulphation of schwertmannite was stabile up to 600 °C and only
decreased at even higher temperatures. These deviations in
reaction behaviour are probably caused by precursor dependent
differing catalytic active site structures.
7
0
5
70
65
60
55
50
45
Effective reaction velocity
Conversion
Regression: y = 64.48 * x-
Ferric sulphate ratio
6
0.16
60
55
50
45
Reuse of schwertmannite
When aiming at substituting vanadium pentoxide in industrial
sulphuric acid plants besides catalytic activity also long-term
stability is an important criterion. Former investigations by Wingen
et al. [6] had proven that ferric oxides deposited on a catalyst
support meet this requirement. Prior to long-term studies, though,
this study focused on the behaviour of nanostructured ferric oxide
.5x10-3
1.5x10-3
1.0x10-3
1
1
5
.0x10-3
.0x10-4
5.0x10-4
0.0
0.0
[
25,26]
synthesized in microjet mixers
with the objective of
1
2
3
4
5
6
7
Experiment
identifying influencing parameters on catalytic activity, before
catalysts based on supported nanostructured ferric oxides are
developed. These latter in turn will be investigated in a follow-up
study in long-term experiments. Therefore, in order to simulate
the effects taking impact on catalyst lifetime aged schwertmannite
Figure 9. Effective reaction velocity, conversion and ferric sulphate ratio of
multiple application of schwertmannite; reaction temperature: 600 °C, feed gas
2 2 2
composition: 10 vol.% SO , 18 vol.% O and 72 vol.% N .
(
annealed at 500 °C) was subjected to multiple short-term
experiments, since repeated heating and cooling down is known
to stress catalyst materials.
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
Application of commercially available ferric oxides
nanoparticles
possessed an average catalytic activity, while performance of
schwertmannite was above-average. However, the highest
sulphur trioxide yield obtained within this study was reached with
For a better classification of the obtained conversion rates with
ferrihydrite and schwertmannite, commercially available products
were tested. For these purposes, pigments and ferric oxide
nanoparticles synthesized for research applications were
examined. The experiments showed additional parameters taking
effect on the catalytic activity of ferric oxides.
goethite Bayoxide E F 20 (SO
2
conversion: 72,5 %).
a)
b)
Catalytic activity of commercially available ferric oxide products
Investigations on catalytic activity of pigments and ferric oxide
nanoparticles were done with nine different samples. Conversion
rates and effective reaction rates achieved with these materials
are shown together with the best results of the former sections in
Figure 10. Characteristic properties of the applied pigments are
summarised in Table 3.
c)
d)
Table 3.
Crystal phase (XRD) and physical properties of applied ferric
oxide pigments
Pigment
crystal phase
Primary particle
size [nm]
Particle shape
Bayferrox 110
Bayferrox 180
Bayferrox 415
Bayoxide E III
Bayoxide E 8611
Bayoxide E F 20
Hematite
Hematite
Goethite
Hematite
Magnetite
Goethite
90
spherical
spherical
prismatic
spherical
spherical
acicular
700
Figure 11. SEM analyses of ferric oxides after sulphur dioxide conversion at
200 x 300
magnification 20,000 x. a) Bayferrox 110 (hematite); b) Bayferrox 415
b)
200
(goethite); c) Bayoxide E 8611 (magnetite); d) Bayoxide E F 20 (goethite).
500
100 x 600
It is known from literature reports that a correlation exists between
conversion rate and the crystal structure of the precursor.[
Expectedly the same applied to the catalyst materials investigated
in this study. What was striking however was the finding that all
tested materials were converted into hematite and ferric sulphate
under process conditions. In other words, initial crystal phase
affects the reaction outcome, although the catalyst material itself
is converted into hematite during application. Additionally, SEM
analyses (Figure 11) of four samples of various initial crystal
structure showed that particle shapes differed largely after
application. Thus, differing ratios of exposed crystal surfaces are
likely to influence the experimentally observed catalytic
conversion rates.
32]
d)
90
80
70
60
50
40
30
20
Thermodynamic equilibrium (V2O5)
Effective reaction velocity
Conversion
3
2
1
.0x10-3
.0x10-3
.0x10-3
0.0
0
1
0
18
5
41
III
id
s
e
te
ni
an
After all, the catalytic power of the investigated ferric oxides can
be ordered as:
goethite > maghemite, schwertmannite > 2-line-ferrihydrite,
hematite, magnetite
1
86
1
20
nm
nm
nm
le
it
dr
1
E
F
tic
e
E
10
it
40
0
10
y
rox
er
rox
fer
rox
fer
id
E
-
-
ar
rih
ox
e
ide
ox
7
20
e,
it
rop
ic
-Fer
er
hw
Sc
t
m
f
y
y
y
y
e,
Ba
Ba
Ba
Ba
yox
y
it
e,
at
m
ne
m
e
it
Ba
Ba
he
ag
m
Hem
Li
he
at
em
2-
M
ag
M
H
At this point however, there emerged a discrepancy between the
findings of Fu et al. [33] and Zhang et al. [34], who identified
Figure 10. Effective reaction velocity and conversion rates of various ferric
oxides at 600 °C. The dotted line marks thermodynamic equilibrium conversion
opposing results concerning catalytic activity of hematite and
goethite. Catalytic activities of the oxides give no homogeneous
picture so far. This situation gives reason to assume that it may
be crystalline defects, the role of which has to be considered
thoroughly in order to explain the results satisfactorily. This issue
will be matter of forthcoming investigations.
Additionally, particle size influences surface structure as well as
catalytic conversion.[30,35] Experiments with hematite were
characterised by decreasing conversion with increasing particle
diameter. Accordingly, experiments with hematite, 100 nm
as achievable with vanadium pentoxide; feed gas composition: 10 vol.% SO
2
,
1
8 vol.% O and 72 vol.% N
2
2
As shown in Figure 10, conversion rates in the range of 20 to 70 %
were obtained with ferric oxides without optimisation. These
results have to be assessed in view of the thermodynamic
2 5
equilibrium (reached by V O ) according to which conversion
cannot exceed 79 %.
Thus, ferric oxides appear highly promising for the catalytic
conversion. It has to be noted, though, that typical conversion
rates ranged between 30 - 50 %. Therefore, ferrihydrite
(
IoLiTec) and Bayferrox 110 (90 - 100 nm) yielded comparable
conversion rates of 45.9 ± 4.6 and 46.2 ± 1.7 %, while Bayoxide
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
E III (200 nm) gave only 37.2 ± 2.0 %. This behaviour was already
known before for the oxidation of sulphur dioxide [14,31], carbon
monoxide and alcohols [36]. It was explained by an increasing
number of catalytically active surface sites with decreasing
particle diameter the structure of which is similar to the
maghemite crystal lattice.[30,35] In fact, maghemite was more
active than hematite within this study.
As shown in Figure 13 surfaces of nanoparticles underwent the
strongest decline, while this effect was less pronounced with all
ferric oxide pigments. Decreasing surface stability with increasing
specific surface area was caused by ascending potential energy
favouring crystallisation and sintering. However, it emerged, too,
that from surface area as well as surface stability one cannot
conclude on the outcome of the catalytic conversion. For instance,
Bayferrox 180 displayed a very stable surface area, but poor
conversion, while Bayferrox 110 got a high specific surface area
after use, but sulphur dioxide conversion was intermediate.
Measuring the degree of ferric oxide sulphation (Figure 14)
confirmed that the applied materials did not contain any sulphate.
Additionally, comparison with conversion rates of Figure 10
demonstrate that if there was no sulphate detectable in the spent
catalyst this material was characterised by a low catalytic activity
(e. g. Bayferrox 180), too. However, detection of sulphate was not
a measure for the catalytic activity since Bayoxide E F 20 and
hematite, 100 nm were characterised by comparable sulphation
but circa 20 % differing conversion.
In contrast to hematite, conversion increased from 37.9 ± 6.0 to
66.3 ± 2.9 % when maghemite with a particle size of 20 - 40 nm
was used instead of a smaller fraction (7 - 10 nm). As SEM
analyses (Figure 12) of maghemite samples after transformation
into hematite showed that smaller maghemite particles gave
larger hematite particles, this experimental result confirms the
formerly stated relationship, too. This particular transformation
behaviour was noticed before by Feitknecht and Mannweiler [37]
.
They concluded from microscopic analyses that during
transformation of maghemite (15 nm) up to 100 primary particles
stuck together to form an only hematite particle while bigger
particles were transformed one by one.
a)
b)
60
50
40
30
20
10
fresh
spent
Figure 12. SEM analyses of maghemite annealed at 600 °C (magnification
00,000). a) 7 - 10 nm, b) 20 - 40 nm.
0.1
.0
0
1
0
0
8
5
III
d
1
0
m
m
m
s
e
te
ri
ite
1
1
1
2
n
n
n
1
1
4
E
6
F
articl
ann
x
rox
rox
e
8
10
40
100
yd
ih
rr
rro
d
E
E
r
r
xi
yo
e
e
-
-
,
p
ertm
e
e
e
d
7
h
0
atite
o
Subsequent to sulphur dioxide oxidation, spent catalyst materials
were analysed for their degree of sulphation as well as changes
in specific surfaces, in order to retrieve further information on the
observed trends.
yf
yf
yf
xi
ite,
e, 2
i
cr
a
a
a
a
xi
yo
ne-Fe chw
Li
S
B
B
B
B
yo
it
m
a
a
hem
m
em
it
e
t
B
B
e
H
-
g
a
2
a
g
m
e
M
a
M
H
Figure 14. Analyses of ferric sulphate ratios of fresh and spent ferric oxide
catalysts; reaction temperature: 600 °C, feed gas composition: 10 vol.% SO
8 vol.% O and 72 vol.% N
2
,
1
2
2
.
100
8
6
4
0
0
0
fresh
used
Reuse of goethite Bayoxide E F 20
Best conversion rates within this study were achieved with
Bayoxide E F 20 (72.5 %; V : 79 %), which was the reason why
20
15
10
5
0
2
O
5
this material was investigated for repeated use. It was expected
that this catalyst should be characterised by a rather low decrease
in conversion rate from experiment to experiment since its surface
structure was found more stable than this of schwertmannite. In
fact, this assumption held true (Figure 15). Even after five
experiments, conversion exceeded 65 % and the loss in activity
was very low, while effective reaction velocity increased. However,
the specific surface area declined continuously from the first
application, while the degree of sulphation increased. Thus,
specific surface area did not reach a minimum within the first
application as found for schwertmannite. This indicates that
smaller changes during the first application are not due to a higher
stability but slower progress of surface reorganisation.
0
0
8
5
III
1
0
m
m
m
s
te
e
ite
1
1
1
2
n
n
n
e
ri
yd
1
1
4
E
6
F
icl
art
ann
x
rox
rox
e
a
8
10
40
100
rro
d
E
B
E
ih
rr
r
r
xi
yo
e
e
-
-
,
p
o
rtm
e
e
e
d
d
e, 7
, 20
atite
yf
yf
yf
xi
xi
yo
icr
ne-Fe
a
a
a
a
it
e
w
ch
B
B
B
B
yo
it
m
a
hem
em
tite
Li
-
2
B
e
m
H
S
g
h
a
a
g
m
e
M
a
M
H
Figure 13. Analyses of specific surface area of fresh and spent ferric oxide
catalysts; reaction temperature: 600 °C, feed gas composition: 10 vol.% SO
8 vol.% O and 72 vol.% N
2
,
1
2
2
.
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
Experimental Section
7
7
6
6
5
0
5
0
80
70
60
Effective reaction velocity
Conversion
Regression: y = 72.61 * x
Ferric sulphate ratio
Materials
-
0.06
Ferric oxide (<5 µm) was purchased from Sigma-Aldrich Chemie
GmbH, Munich, Germany, and maghemite (7 – 10 nm;
50
20 – 40 nm) and hematite (100 nm) where obtained from IoLiTec
40
Ionic Liquids Technologies GmbH, Heilbronn, Germany.
Bayferrox 110 (hematite), Bayferrox 180 (hematite), Bayferrox
30
415 (goethite), Bayoxide E III (hematite), Bayoxide E 8611
20
-
4
4
4
4
(magnetite) and Bayoxide E F 20 (goethite) were provided by
Lanxess Deutschland GmbH, Germany. Aluminium oxide beads
ProCerox Al 99 with a diameter of 1.0 – 1.4 mm were obtained
from ProCeram GmbH, Willmenrod, Germany. Standards and all
other chemicals were purchased from VWR International GmbH,
Darmstadt, Germany, as well as Carl Roth GmbH & Co. KG,
Karlsruhe, Germany. All chemicals had a minimum purity of
8
6
4
2
x10
x10
x10
x10
-
-
-
10
0
0
1
2
3
4
5
Experiment
Figure 15. Effective reaction velocity, conversion and ferric sulphate ratio of
multiple application of goethite Bayoxide E F 20; reaction temperature: 600 °C,
>99 %, while ferric oxide content of examined pigments ranged
2 2 2
feed gas composition: 10 vol.% SO , 18 vol.% O and 72 vol.% N .
from 95 to >99 %.
Helium, synthetic air (20 vol.% O , 80 vol.% N ) and nitrogen (5.0)
2 2
were purchased from Praxair GmbH, Düsseldorf, Germany,
sulphur dioxide (3.8) was obtained from Air Liquide Deutschland
GmbH, Düsseldorf, Germany. Compressed air (oil-free, dew point
Conclusions
3
°C) was produced in-house.
Sulphur dioxide oxidation experiments with ferric oxides were
conducted at temperatures in the range of 450 - 650 °C and
Equipment
5
2
- 28.5 vol.% SO applying ferric oxide microparticles as well as
Mass-flow controllers (MFC) were the product of MKS
Instruments Deutschland GmbH, Germany. IR-spectroscopy was
done by using a Nicolet iS10 FT/IR spectrometer by Thermo
Fisher Scientific Inc., Waltham, MA, USA. A gas flow cell
equipped with silicon windows for IR measurements was
produced by PIKE Technologies, Fitchburg, WI, USA. Heating
regulation was done using a controller LR 316 by JUMO GmbH &
Co. KG, Leipzig, Germany. X-Ray analyses were conducted with
an X-ray diffractometer D5000 by Siemens AG, Munich, Germany.
Physisorption experiments were done using an ASAP 2020 by
Micromeritics GmbH, Aachen, Germany. Scanning electron
microscopy (SEM) was performed on a JSM 7001F by Jeol
nanoparticles. Best yields were obtained at 600 - 650 °C with
most pronounced differences in catalytic activity at 600 °C.
Thermodynamic equilibrium was reached with hematite
microparticles only in the case of 5 vol.% SO
2
at 650 °C
(
71.4 ± 4.6 %). Experiments with various ferric oxides were
2
conducted with 10 vol.% SO at 600 °C showing that sulphur
dioxide conversion rates in the range of 13.6 ± 2.1 to 72.8 ± 1.0 %
are achievable without optimisation of the applied catalyst. The
thermodynamic equilibrium (79.3 %), which is typically
established, when vanadium pentoxide catalysts are used, was
reached under neither of the tested conditions. Highest sulphur
dioxide conversion was obtained with goethite material Bayoxide
E F 20, while above-average catalytic activity was found for
schwertmannite. Precipitation conditions did not significantly
affect catalytic activity of ferrihydrite and schwertmannite, while
preceding ageing in mother liquor promoted catalysis.
Experiments with hematite and maghemite demonstrated that the
catalytic activity depends on particle size as well.
Germany
GmbH,
Freising,
Germany.
DTA/TG-MS/IR
e
measurements were done on a STAR System by Mettler Toledo,
Columbus, OH, USA, equipped with
a quadrupole mass
spectrometer (ThermoStarTM by Pfeiffer Vacuum, Aßlar,
Germany) and an IR spectrometer (Nicolet iS10 by Thermo Fisher
Scientific Inc. Waltham, MA, USA).
Analyses of spent catalysts showed that in the course of catalytic
reaction all catalysts were transformed into hematite. Additionally,
the materials took up sulphate while specific surface area got lost.
Insufficient sulphation during application caused low conversion
rates, which was especially found for materials treated at high
temperatures, e.g. 900 °C, before application.
Experimental setup
For detailed experimental conditions of ferric oxide precipitation
the reader is kindly referred to [31]. All self-synthesized materials
were annealed in a tube furnace for 8 hours at 300, 500 or 900 °C
with overflowing compressed air at a volume flow rate of 10 L/h.
Sulphur dioxide oxidation experiments were conducted using the
experimental setup shown in Figure 16. It essentially consists of
three Mass-Flo Controller (MFC) for gas dosing, a heated tube
reactor and a FT/IR spectrometer equipped with a gas flow cell
with silicon windows and an inner volume of 38.5 mL for
continuous sulphur dioxide quantification. Wash bottles filled with
concentrated sulphuric acid and 1 M sodium hydroxide solution
were used for absorption of sulphur trioxide as well as sulphur
In summary, these results are very promising. They show that it
is possible to develop a ferric oxide catalyst bearing the potential
of competing with vanadium pentoxide. However, conversion rate
as well as long-term stability still have to be improved.
Experimental results of this study as well as former investigations
[
6,16]
show promising strategies to meet these challenges.
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
dioxide. The reactor (inner diameter: 25 mm) was made from
fused silica and was equipped with a glass drip as supporting
surface for the catalyst layer. The latter consisted of a mixture of
peak at 2 = 33 ° using the Scherrer equation (K = 0.89). Particle
structure investigations were done by physisorption of N .
2
Samples were prepared by outgassing at 90 °C for 6 h. Scanning
electron microscopy (SEM) measurements were done at 3 and
5 kV. Samples were prepared on carbon tabs and metallized with
gold. Sulphate content of the ferric oxides were estimated by
analysis of mass loss during thermal decomposition. Therefore
DTA/TG-MS/IR measurements were done. The temperature
programme was: After holding the temperature for 10 min at
10 g catalyst with 20 g aluminium oxide beads arranged between
two layers of glass wool. The reactor was additionally equipped
with a glass capillary reaching into the catalyst layer. This was
used for measurement of reaction temperature with
a
thermocouple (type S) as well as regulation of heating. All
connections were made of steel or PTFE/ETFE.
25 °C it was heated with 20 K/min to 400 °C. Then, temperature
was increased with 10 K/min to 800 °C, which was hold for
additional 30 min, followed by further heating up to 1,000 °C at a
rate of 10 K/min.
Acknowledgements
Financial support by the German Research Foundation (DFG,
grant number INST 267/125-1 FUGG) is gratefully acknowledged.
The authors thank Lanxess Deutschland GmbH and ProCeram
UG for supplying material samples. Regina Moßig is owed thanks
for X-ray analyses and Anja Obst for SEM analyses.
Keywords: Sulphur dioxide • Ferric oxide • Nanoparticles •
Catalytic oxidation • catalyst
Figure 16. Experimental setup
[
1]
EMR, "Sulphuric Acid Market – Share, Price, Trends, Outlook, Report
and Forecast, 2021-2026", can be found under
Experimental procedure: While heating up to the reaction
temperature, 200 sccm of synthetic air were led through the
reactor and were afterwards diluted with 2 sLm compressed air.
This mixture passed the FT/IR spectrometer for baseline
measurement. After reaching the desired reaction temperature,
the experiment started by adding sulphur dioxide (5.0 - 28.5
vol.%) to the synthetic air stream. Reactant concentration was
adjusted by lowering volume flow rate of synthetic air to keep the
volume flow rate through the reactor constant, so the oxygen
concentration varied between 14.3 - 19.0 vol.%. Each experiment
was carried out for 5 hours, after which heating was stopped and
https://www.expertmarketresearch.com/reports/sulphuric-acid-market-
report 2020.
C. H. Bartholomew, R. J. Farrauto, Fundamentals of Industrial Catalytic
Processes, 2nd ed., Wiley, Hoboken, N.J 2006.
European Commission, "Integrated Pollution Prevention and Control.
Reference Document on Best Available Techniques for the Manufacture
of Large Volume Inorganic Chemicals - Ammonia, Acids and Fertilisers",
[
[
2]
3]
can
be
found
under
https://eippcb.jrc.ec.europa.eu/sites/default/files/2019-11/lvic-
s_bref_0907.pdf 2007.
[
[
4]
5]
F. Näumann, M. Schulz, in: Handbook of Heterogeneous Catalysis (Eds.:
G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp), Wiley-VCH Verlag GmbH
& Co. KGaA, Weinheim, Germany 2008, 2623–2635.
European Sulphuric Acid Association, European Fertilizer Manufacturer
Association, "Best Available Techniques Reference Document on the
2
00 sccm synthetic air were fed into the reactor for 1 hour,
Production
http://files.gamta.lt/aaa/Tipk/tipk/4_kiti%20GPGB/54.pdf 1999.
A. Wingen, N. Anastasievič, A. Hollnagel, D. Werner, F. Schüth, J. Catal.
000, 193, 248–254.
V. I. Yavor, I. D. Ivanova, V. E. Soroko, Zh. Prikl. Khim. 1984, 57, 2115–
118.
Metallgesellschaft AG (L. Reh, H. Grimm, K. H. Dörr, K. Vydra),
DE2213580 [1973].
MG Technologies AG (N. Anastasijevic, D. Werner, M. Runkel, S.
Laibach, E. Winkler, A. Hollnagel), WO2001085611 [2001].
of
Sulphuric
acid",
can
be
found
under
whereupon gas flow of synthetic air was shut down and
compressed air flow rate was raised to 2.2 sLm for baseline
measurement. Conversion rates were determined from remaining
sulphur dioxide concentration in the exhaust gas during the last
hour of operation, when catalytic reaction had reached a steady
state.
Determination of sulphur dioxide conversion and sample
characterisation
Sulphur dioxide conversion was measured continuously by FT/IR
spectroscopy. For this purpose, extinction in the range of 1,362 to
[
6]
2
[7]
2
[
[
[
8]
9]
10] W. Becker, Sulphur 1982, 30–33.
[11] a) A. Urbanek, M. Trela, Catal. Rev.: Sci. Eng. 1980, 21, 73–133; b) O.
B. Lapina, B. S. Bal'zhinimaev, S. Boghosian, K. M. Eriksen, R.
Fehrmann, Catal. Today 1999, 51, 469–479.
[12] J. Loskyll, K. Stoewe, W. F. Maier, Sci. Technol. Adv. Mater. 2011, 12,
54101.
-1
-1
1,363 cm was evaluated applying a resolution of 0.121 cm . For
[
13] J. P. Dunn, H. G. Stenger Jr., I. E. Wachs, Catal. Today 1999, 53, 543–
56.
[14] Y. Shi, M. Fan, Ind. Eng. Chem. Res. 2007, 46, 80–86.
studies on crystal structure of the product an X-ray diffractometer
was used, which was equipped with a monochromator using Cu-
K
5
[
15] Metallgesellschaft AG (E. Winkler, G. Schmidt, A. Hollnagel, D. Werner,
N. Anastasijevic, F.-F. Schüth, A. Wingen), DE 19800800 [1999].
[16] J. Loskyll, K. Stöwe, W. F. Maier, ACS Comb. Sci. 2013, 15, 464–474.
α
radiation and operated at 40 kV and 40 mA. Hematite
crystallite sizes were calculated from half width of the hematite
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
[
[
17] (L. Cavalli, R. Nardini), US5108976 [1992].
18] H.-H. Tseng, M.-Y. Wey, Y.-S. Liang, K.-H. Chen, Carbon 2003, 41,
[28] a) G. Liu, S. Debnath, K. W. Paul, W. Han, D. B. Hausner, H.-A. Hosein,
F. M. Michel, J. B. Parise, D. L. Sparks, D. R. Strongin, Langmuir 2006,
22, 9313–9321; b) S. Seisel, T. Keil, R. Zellner, Z. Phys. Chem. 2009,
223, 1477–1495.
[29] a) G. Lunge, K. Reinhardt, Z. Angew. Chem. 1904, 17, 1041–1051; b) J.
P. Dunn, H. G. Stenger Jr., I. E. Wachs, J. Catal. 1999, 181, 233–243.
[30] M. Fernández-García, A. Martínez-Arias, J. C. Hanson, J. A. Rodriguez,
Chem. Rev. 2004, 104, 4063–4104.
1
079–1085.
19] S. Koutsopoulos, K. M. Eriksen, R. Fehrmann, J. Catal. 2006, 238, 270–
76.
20] S. Koutsopoulos, T. Johannessen, K. M. Eriksen, R. Fehrmann, J. Catal.
006, 238, 206–213.
21] N. Inoue, A. Matsumoto, T. Suzuki, S. Ozeki, K. Kaneko, Langmuir 1988,
, 774–776.
[
[
[
2
2
4
[31] R. Paudert, Chem. Tech. 1969, 21, 477–480.
[
[
22] B. Neumann, E. Goebel, Z. Elektrochem. 1928, 34, 734–740.
23] M. J. Zetlmeisl, K. J. McCarthy, D. F. Laurence, Ind. Eng. Chem. Prod.
Res. Dev. 1983, 22, 710–716.
[32] a) H. Fu, X. Wang, H. Wu, Y. Yin, J. Chen, J. Phys. Chem. C 2007, 111,
6077–6085; b) X. Zhang, G. Zhuang, J. Chen, Y. Wang, X. Wang, Z. An,
P. Zhang, J. Phys. Chem. B 2006, 110, 12588–12596.
[
24] a) A. S. C. Brown, J. S. J. Hargreaves, B. Rijniersce, Catal. Today 1998,
[33] H. Fu, X. Wang, H. Wu, Y. Yin, J. Chen, J. Phys. Chem. C 2007, 111,
6077–6085.
45, 47–54; b) A. S. C. Brown, J. S. J. Hargreaves, B. Rijniersce, Catal.
Lett. 1998, 53, 7–13; c) A. S. C. Brown, J. S. J. Hargreaves, B. Rijniersce,
Top. Catal. 2000, 11/12, 181–184; d) A. S. C. Brown, J. S. J. Hargreaves,
Green Chem. 1999, 1, 17–20; e) A. Kayo, T. Yamaguchi, K. Tanabe, J.
Catal. 1983, 83, 99–106; f) Z. Feng, J. Zhao, F. E. Huggins, G. P. Huffman,
J. Catal. 1993, 143, 510–519.
[34] X. Zhang, G. Zhuang, J. Chen, Y. Wang, X. Wang, Z. An, P. Zhang, J.
Phys. Chem. B 2006, 110, 12588–12596.
[35] I. V. Chernyshova, M. F. Hochella Jr, A. S. Madden, Phys. Chem. Chem.
Phys. 2007, 9, 1736–1750.
[36] X. Mou, X. Wei, Y. Li, W. Shen, CrystEngComm 2012, 14, 5107–5120.
[37] W. Feitknecht, U. Mannweiler, Helv. Chim. Acta 1967, 50, 570–581.
[
[
[
25] L. Reichelt, M. Bertau, Chem. Eng. Res. Des. 2015, 98, 70–80.
26] L. Reichelt, M. Bertau, Z. Anorg. Allg. Chem. 2015, 641, 1696–1700.
27] L. Wöhler, W. Plüddemann, P. Wöhler, Z. Phys. Chem. 1908, 641–677.
This article is protected by copyright. All rights reserved.

Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202100091
ACCEPTED ARTICLE
Entry for the Table of Contents
Layout 1:
FULL PAPER
Text for Table of Contents
L. Reichelt, S. Hippmann, V.N. Brichkin,
M. Bertau*
reaction
velocity
conversion
Page No. – Page No.
Oxidation of Sulphur Dioxide using
Micro-and Nanoparticles of various
Iron Oxides
precipitation
conditions
x
Fe Oy
sulphation
morphology
50 nm
specific
surface area
particle
size
thermal
treatment
Additional Author information for the electronic version of the article.
Prof. Dr. M. Bertau:
Dr. S. Hippmann: 0000-0003-1568-6140 (ORCID)
Prof. Dr. V.N. Brichkin: 0000-0002-1708-5624 (ORCID)
0000-0003-1236-1164 (ORCID)
This article is protected by copyright. All rights reserved.