Aqueous-Phase Hydrodechlorination of Trichloroethylene on Ir Catalysts Supported on SBA-3 Materials

Article abstract of DOI:10.1002/cctc.201800873

This work investigates the catalytic properties of silica and aluminosilicate SBA-3 supported iridium catalysts in aqueous phase hydrodechlorination of trichloroethylene. SBA-3, AlSBA-3, Ir/SBA-3 and Ir/AlSBA-3 were thoroughly characterized by XRD, BET, TPR-H₂, H₂ chemisorption, TPD-NH₃ and investigated in catalytic removal of trichloroethylene from water in batch reactor under mild operation conditions. Uniformly dispersed small iridium nanoparticles (～3 nm) was found to be active in catalytic purification of water from chloroorganic compound. The studies showed that the differences in catalytic behavior of Ir/SBA-3 and Ir/AlSBA-3 depended on the properties of the support. The best catalytic properties in purification of water from trichloroethylene was obtained for Ir supported on more hydrophilic and ordered AlSBA-3 with acid sites.

Full text of DOI:10.1002/cctc.201800873

Accepted Article
Title: Aqueous-phase hydrodechlorination of trichloroethylene on Ir
catalysts supported on SBA-3 materials
Authors: Ewa Janiszewska, Michał Zieliński, Monika Kot, Emil
Kowalewski, and Anna Śrębowata
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ChemCatChem
10.1002/cctc.201800873
FULL PAPER
Aqueous-phase hydrodechlorination of trichloroethylene on Ir
catalysts supported on SBA-3 materials
[
a]
[a]
[a]
[b]
[b]
Ewa Janiszewska *, Michał Zieliński , Monika Kot , Emil Kowalewski , Anna Śrębowata
Abstract: This work investigates the catalytic properties of silica and
aluminosilicate SBA-3 supported iridium catalysts in aqueous phase
hydrodechlorination of trichloroethylene. SBA-3, AlSBA-3, Ir/SBA-3
and Ir/AlSBA-3 were thoroughly characterized by XRD, BET, TPR-
often used catalysts.[5] However, due to its hydrogenolytic
properties Pd is one of the most used catalyst in aqueous phase
hydrodechlorination of trichloroethylene.[6] Nevertheless, in
search of efficiency and stability in TCE hydrodechlorination,
both noble and non-noble metals in form of monometallic and
2 2 3
H , H chemisorption, TPD-NH and investigated in catalytic removal
of trichloroethylene from water in batch reactor under mild operation
conditions. Uniformly dispersed small iridium nanoparticles (~3 nm)
was found to be active in catalytic purification of water from
chloroorganic compound. The studies showed that the differences in
catalytic behavior of Ir/SBA-3 and Ir/AlSBA-3 depended on the
properties of the support. The best catalytic properties in purification
of water from trichloroethylene was obtained for Ir supported on
more hydrophilic and ordered AlSBA-3 with acid sites.
bimetallic
catalysts
are
used
in
aqueous
phase
hydrodechlorination of TCE, e.g. Rh, Ni-Fe, Ni.[9]
[
7]
[8]
From the platinum metal group, iridium catalysts have been
used in selective hydrogenation of α,β-unsaturated aldehydes,[
toluene hydrogenation,[11] naphthenic ring opening reactions,[12]
and selective reduction of nitrogen oxide (NO).[13] It has been
also employed in the commercial process of acetic acid
10]
TM
[14]
manufacture
(the
Cativa
process)
and
in
[15]
hydrodesulphurization processes. There are some information
in the US patents issued in the end of twentieth century on
activity of supported iridium catalysts in the removal of chlorine
from organic compounds. It was shown that iridium was active in
Introduction
catalytic oxidation of different aqueous organic contaminants
with low molecular weight including trichloroethylene.[16] It also
Ordered mesoporous materials have attracted considerable
attention during last decades due to their unique textural
properties such as high surface area and large pore volume with
a narrow pore size distribution.[1] Investigations on synthesis and
properties of the mesoporous molecular sieves develop
enormously due to the potential application of such materials as
catalysts, catalyst supports, absorbents and host materials.[2]
Many efforts have been made to design mesoporous materials
for use in heterogeneous catalyst systems. Their mesoporosity
has many advantages in catalysis, e.g. may reduce the extent of
deactivation due to coke formation and plugging that are
frequently observed in the microporous catalyst systems
showed activity in bimetallic catalysts (e.g iridium-copper) in gas
phase hydrogenation of 1,2-dichloropropane with high selectivity
propene.[17]
to
Monometallic
iridium
applications
in
hydrodechlorination reactions are rare, although its high
resistance to HCl attack and stability under severe reaction
conditions makes iridium attractive alternative for another metals.
Some discussion on its use can be found in the publications
F,[18]
concerning the gas phase hydrodechlorination of CCl
3
[19]
.[20] Until now, not a lot of attention has been
CCl
2
F
2
and CCl
4
paid on the application of iridium in the aqueous-phase
hydrodechlorination process. The application of clay-supported
CeO and Ir in the aqueous phase hydrodechlorination of 4-
2
(
zeolites).[3] The possibility of controlling the pore size by
adjusting the preparation conditions make ordered mesoporous
materials well suited as catalysts or catalyst supports. Moreover,
high surface area of ordered mesoporous materials used as
supports can be exploited to create highly dispersed active
species on their surface. All of these properties make them an
excellent materials in an aqueous – phase hydrodechlorination
of chloroorganic compounds.[4]
chlorophenol (4-CP) and 4-chloroaniline (4-CA) showed its
activity. However, the partial coverage of Ir crystallites by cerium
oxide, significantly decreased the activity of this material.[21] This
phenomenon clearly indicates, that the activity of supported
catalysts depends on the composition of the catalyst, which may
modify the properties of the active phase. Physical as well
chemical properties of the support, its acidity or reducibility and
Besides of the support, selection of the appropriate active
phase plays the crucial role in catalytic properties of
metal/support systems. In the case of hydrodechlorination
processes, noble metals such as Pd, Pt and Rh are the most
the extent of interaction with the active metal, play important
roles in the complex chemistry of supported metal catalysts.[22]
Therefore, in the present work, we describe the preparation
of new mesoporous catalytic materials containing highly
0
dispersed reduced particles of Ir on silica and aluminosilicate
SBA-3 supports. Their catalytic activity was investigated in the
aqueous-phase hydrodechlorination of trichloroethylene as a
model reaction. To the best of our knowledge, no data are
available on the use of iridium catalysts in the aqueous-phase
hydrodechlorination of trichloroethylene. The information on
[
[
a]
b]
Dr. E. Janiszewska*, Dr. M. Zieliński, M. Kot
Faculty of Chemistry,
Adam Mickiewicz University in Poznań,
Umultowska 89B, 61-614 Poznań (Poland)
*
E-mail: eszym@amu.edu.pl
E. Kowalewski, Dr. A. Śrębowata
Institute of Physical Chemistry,
Polish Academy of Sciences,
using
ordered
mesoporous
SBA-3
as
support
of
hydrodechlorination catalyst is also nonexistent, therefore our
investigations seem to be fully justifiable.
Kasprzaka 44/52, 01-224 Warszawa (Poland)
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Results and Discussion
catalysts indicate that the hexagonally ordered structure of SBA-
3
was maintained after the impregnation and reduction
The pore ordering of the samples was confirmed by low
angle XRD analysis (Fig.1A). Both supports (SBA-3 and AlSBA-
procedures. Incorporation of Ir into the AlSBA-3 matrix did not
change the ordering of the support (similar intensity of the
reflections), whereas some decrease in intensity and broadening
after Ir deposition on SBA-3 was observed. It suggested some
loss of the ordering of the pore structure. The reflections position
slightly shifts to lower angles and the unit cell parameter (a )
o
decreases for iridium catalysts in comparison to the pristine
supports (Table 1).
3) and iridium catalysts showed the typical characteristic
patterns of the hexagonal phase of SBA-3 structure, matching
well those reported in the literature.[23] The XRD patterns of
AlSBA-3 support showed well-resolved sharp reflections of
higher intensity in comparison to intensity of reflections of
siliceous counterpart. It indicated better ordering of AlSBA-3
sample than that of pure silica SBA-3. XRD patterns of iridium
Table 1. Characterization of supports (calcined at 550 oC) and iridium catalysts reduced at 400 C.
o
Physical characterization of
iridium catalysts
Hydrogen chemisorption data for
Average Ir particle size,
nm
c)
Total
number
of acid
Ir/support catalysts
[100]a),
a
0
a),
d
Sample
Total
pore
Average
pore
diameter,
nm
Volume adsorbed,
Dispersion,
%
Surface
area,
m g
nm
nm
sitesb)
3
-1
TEMe)
,
cm ·g
CHEMd)
-1
volume,
μmolg
2
-1
3
-1
cm g
H
-
t
H
irr
H
-
r
D
irr
before
after
SBA-3
1292
1293
0.72
0.71
2.2
2.2
3.29
3.16
3.80
3.65
0
0
-
-
-
-
-
Ir/SBA-3
0.49
0.27
0.22
46
2.4
3.1
3.6
AlSBA-3
1316
1271
0.83
0.80
2.5
2.5
3.24
3.14
3.74
3.63
25
26
-
-
-
-
-
-
-
Ir/AlSBA-3
0.53
0.24
0.29
42
2.7
2.8
3.4
a)
b)
c)
d
[100] interplanar spacing obtained from XRD; a
Number of acid sites determined by TPD-NH
– total adsorbed hydrogen, H – reversibly adsorbed hydrogen, Hirr – hydrogen adsorbed irreversibly, Dirr – dispersion calculated from irreversibly adsorbed
chemisorption measurements are specified in Experimental Section.
0
– lattice parameter (a
method.
0
= 2d[100]/√3)
3
H
t
r
hydrogen. The conditions of H
2
d)
Mean size of metal particles calculated from the amount of irreversibly chemisorbed hydrogen (Hirr).
Mean size of metal particles calculated from TEM – before and after hydrodechlorination of trichloroethylene.
e)
particle size of iridium is very small (below 3 nm), XRD analysis
cannot detect them. XRD patterns of the iridium catalysts in the
wide angle range (Fig. 1B) show only a very broad reflection in
the 2Θ range of 15-30º, which is always attributed to the
0
amorphous silica. No metallic iridium (Ir ) was observed by XRD
(
absence of reflections at 40.6° and 47.3°), suggesting the
presence of very small, well dispersed metal crystallites (under 3
nm). Similarly, Silvennoinen at al.[ showed lack of reflections
24]
from metallic iridium in the case of very small metal crystallites
~2 nm), whereas Haneda et al.[25] did not record Ir reflection for
0
(
1
wt.% Ir/SiO
2
catalysts if the iridium dispersion was above 40%.
The iridium catalyst supported on AlSBA-3 (Fig. 1B) showed
extra reflections at 2Θ of 48.3 and 50.8° which can originate
from some extra-framework aluminium species. In our previous
work we showed that aluminosilicate SBA-3 synthesized with
high aluminum content (Si/Al=10) indicated some amount of
extra-framework alumina that probably are sintered during
impregnation and reduction procedures and became visible in
XRD pattern.[26]
Figure 1. Low- (A) and wide- (B) angle XRD patterns of supports and catalysts.
The Ir contents in fresh Ir/SBA-3 and Ir/AlSBA-3 catalysts
are the same as the intended ones (ICP-OES analysis). Such a
good agreement between the actual and target contents results
from the fact that the catalysts were obtained by the
impregnation method with cyclohexane solution of tetrairidium
dodecacarbonyl followed by solvent evaporation to dryness.
XRD patterns in the wide angle range (Fig. 1B) were
recorded in order to determine the crystallite size of iridium.
However, one should remember that XRD technique is not
sensitive if the amount of phase is too low (less than 5 wt.%) as
was in the case of investigated catalysts. Moreover, if the
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The TEM analysis confirmed the well-ordered hexagonal
pore system of catalysts (Fig. 2A,C), which is in accordance with
the results of low angle XRD. The ordering of the catalysts is
also preserved after hydrodechlorination of trichloroethylene
process (Fig. 2B,D).
A) Ir/SBA-3 before reaction
B) Ir/SBA-3 after reaction
C) Ir/AlSBA-3 before reaction
D) Ir/AlSBA-3 after reaction
Figure 2. TEM images and histograms (insets) of iridium particle size distribution of catalysts before (A and C) and after (B and D) hydrodechlorination of
trichloroethylene.
The pure silica and aluminosilicate SBA-3 supports show
almost similar nitrogen adsorption-desorpion isotherms of type
IV(b) characteristic for mesoporous materials (Fig. 3A).[27] As the
relative pressure increases isotherms of the supports show a
capillary condensation of nitrogen within uniform mesopores.
This step for SBA-3 is less prominent and shifted to lower p/p . It
indicates that silica SBA-3 possesses smaller mesopores.[28] The
nitrogen adsorption-desorpion isotherms and pore size
distributions of the catalysts are similar to those of supports.
0
0
sharp step, in the range of p/p = 0.15-0.3, characteristic of
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This indicates that the texture of the supports is not altered by
the impregnation and heat treatment. The nitrogen adsorption-
desorpion isotherms of presented supports and catalysts are
similar to those presented in literature for SBA-3 materials.
These isotherms show no hysteresis loop between adsorption
and desorption branches, similar to that reported in the
literature.[29] Steep slope in the initial part of the isotherms
implies the presence of micropores in investigated
materials.[29a,30]
Incorporation of aluminum into the amorphous walls of
mesoporous SBA-3 generate some acidity of the sample (TPD-
NH
3
analysis, Table 1). The TPD-NH
3
profile (Fig. S1,
Supporting Information) showed one broad peak in the
temperature range of 200-500 ºC with maximum at 300 ºC. It
corresponds to the desorption of NH from the weak, medium
3
and strong acid sites of the material with great majority of
medium acid sites.[ Our previous studies (FT-IR studies of
adsorbed pyridine) have shown that the acidity originates mostly
34]
from Lewis acid sites.[26] The TPD-NH
analysis indicated that
3
the introduction of Ir particles on the surface of silica and
aluminosilicate SBA-3 did not influence their total acidity (Table
1
). It can be explained by relatively low Ir content and using of
inert tetrairidium dodecacarbonyl as iridium precursor.[ The
increase of total acidity of the supported noble metal catalysts in
comparison to the acidity of the supports have been reported in
the literature only in the case of using metal chlorides or
chlorometallic acids as a metal precursor. It was shown that
introduction of chlorides produced new acid sites by the
replacement of OH groups by Cl and the displacement of water
coordinated to surface atoms.[36]
35]
Figure 3. N
2
adsorption–desorption isotherms (A) and pore size distributions
(B) of supports and catalysts.
Textural properties calculated by the
N
2
adsorption-
desorption isotherms are listed in Table 1. The impregnation
with iridium precursor of siliceous support does not change its
textural properties. The iridium catalyst on SBA-3 and pristine
SBA-3 support have similar surface areas and pore size and
only the pore volume is slightly lower in the case of the Ir
catalyst. The introduction of iridium on the surface of AlSBA-3
leads to slight decrease of surface area (~3.4%) and pore
volume of the catalyst but maintains the pore diameter. Probably
in the case of both supports, use the inert tetrairidium
dodecacarbonyl that decomposes directly into metallic iridium
and carbon monoxide allows to introduce iridium phase on the
supports in mild conditions without changing the textural
properties. Another explanation of unchanged textural properties
can be the introduction of relatively small amount of metallic
phase on the supports with high surface areas. The unchanged
textural properties of the catalyst with small amount of the active
phase in comparison to these properties of the support were
presented in literature.[31] Usually the decrease of surface area
or pore diameter was observed in the case of high metal loading
on the supports (>5 wt. %)[32] or use the acidic metal precursor
Figure 4. TPR-H
Ir (CO)12
2
profiles of dried iridium samples and iridium precursor -
4
.
Temperature programmed reduction with hydrogen was
(
e.g. chlorides or chlorometallic acids).[33] For these reasons the
used to determine temperature needed to remove ligands of
iridium precursor from the catalyst surface. Studies of TPR-H2
were carried out on the iridium precursor-impregnated samples
dried at 100 oC. No reduction signals were seen in TPR-H2
profiles of the supports (data not shown), which proved that the
textural properties of the presented catalysts are similar to the
properties of the supports. The unchanged surface area and
pore diameter as well the decrease of pore volume for catalysts
suggests the deposition of iridium particles inside the pores.
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supports were irreducible. Figure 4 presents the reduction
higher binding energies for Ir/AlSBA-3 catalyst. It also confirms
stronger interaction of iridium species with AlSBA-3 support.[39]
An important parameter characterizing metallic catalysts is
the dispersion of the metal. The dispersion and size of iridium
2
profiles of the Ir/support-D samples and TPR-H profile of the
active phase precursor - Ir (CO)12 supported on a quartz sand.
4
The reduction profile of tetrairidium dodecacarbonyl showed the
signals testifying to the occurrence of decarbonylation process
with the maxima at 130 and 270 oC, and to the reaction of
hydrogen with carbonyl ligands in methanation of CO with the
particles were determined by chemisorption of H
2
on the
reduced catalysts. The iridium catalysts prepared from both
supports showed high dispersion of metal calculated from
irreversibly adsorbed hydrogen (Table 1). The analysis indicated
particle size smaller than 3 nm in the case of both catalysts.
These results are in good agreement with XRD data. The high
dispersion and small particle size explain the lack of reflections
from metallic iridium in XRD patterns. The Ir/SBA-3 exhibit
slightly higher dispersion (46%) and lower particle size (2.4 nm)
when compared to iridium catalyst on AlSBA-3 (42%, 2.7 nm).
Higher dispersion of iridium particles on SBA-3 could be due to
easier reduction of iridium species on this support what was
maximum at 397 °C.[11d] The TPR-H
profiles of Ir/SBA-3-D and
2
Ir/AlSBA-3-D samples exhibit weak first two signals originating
as in the case of precursor from decarbonylation process and a
more pronounced peak above 400 °C. The high temperature
signals (270 and 397 oC – for precursor) are shifted to higher
temperatures for iridium precursor supported on mesoporous
materials. The fact that the reduction of catalysts takes place
mainly at higher temperatures indicates that iridium species
strongly interact with the supports making their reduction
process more difficult.[37] The TPR-H
profile of Ir/AlSBA-3-D
confirmed by H
Ir/AlSBA-3 catalyst results from specific adsorption of Ir
-TPR analysis.[40] Lower dispersion of iridium on
(CO)12
2
2
showed mainly high temperature signal at 413 °C with negligible
shoulder in temperature range of 330-350 °C and very low signal
at 130 °C. It indicated stronger metal–support interactions on
AlSBA-3 in comparison to interactions of Ir precursor with SBA-3
4
on AlSBA-3 surface. Probably, iridium carbonyl is adsorbed on
the AlSBA-3 surface with the involvement of the acidic centres
leading to the stronger interaction of iridium with the support.
Strong interaction of iridium carbonyl with the AlSBA-3 support
2 2
support. The TPR-H profiles of catalysts after activation (H
reduction, 400 °C, 2 h) showed no reduction signals (data not
shown) that proved the metallic state of iridium in the
investigated catalysts.
was confirmed by TPR-H
2
studies.
The TEM micrographs of the catalysts indicated
a
homogeneous dispersion of the nanoparticles on the surface of
supports for fresh as well for used catalysts (Fig. 2). The mean
particle size estimated from TEM images is in good agreement
with particle size calculated by hydrogen chemisorption
technique for iridium catalyst supported on AlSBA-3 and slightly
differs for Ir/SBA-3. The lower mean particle size obtained by H
2
chemisorption studies for the latter sample results from the fact
that this technique allows to detect even single metal atoms
present on the surface of the support, whereas in TEM
photographs it is difficult to notice very small particles and the
mean size is calculated only from visible, larger particles. The
size distribution diagram for Ir/AlSBA-3 (inset in Figure 2C)
exhibit a narrow size distribution with majority of particle with
diameter of 3 nm. The iridium particle size distribution for Ir/SBA-
3
is broader with predominance of particle with diameter of 2-3
nm and some contribution of bigger ones (inset in Figure 2A).
The TEM analysis indicated an increase of around 20% in metal
particle size after reaction caused by some metal aggregation.
Catalytic properties of Ir/SBA-3 and Ir/AlSBA-3 materials
have been investigated in aqueous phase hydrodechlorination of
trichloroethylene. Based on the literature data concerning the
beneficial role of water on Pd/C activity in hydrodechlorination of
Figure 5. X-ray photoelectron spectra (Ir 4f core levels) for Ir/SBA-3 (A) and
Ir/AlSBA-3 (B) samples.
XPS analyses were performed for reduced Ir/SBA-3 and
Ir/AlSBA-3 samples (Fig. 5). The peak corresponding to Ir 4f7/2
has been analyzed. Two types of binding energies are found for
halides,[41]
aromatic
the
catalytic
aqueous
phase
hydrodechlorination of trichloroethylene was performed without
additional organic chemicals. Therefore the effect of organic
solvent on HDC rate and products distribution was eliminated.
Due to the fact that the particle size and mixing rate are two
major factors that can control mass transfer rates during
catalytic process,[42] using Ir catalysts with small particles sizes
and high mixing rates (1000 rpm) allowed to lower the likelihood
of mass transfer limitation.[42c]
Control test confirmed that the chloroorganic compound
removal did not proceed in the absence of the SBA-3, AlSBA-3,
both catalysts. The core electrons of iridium (4f7/2 signal) at
δ+
~
61.8 eV is assigned to Ir-O (Ir ) and BE of 60.7 eV
[
38]
δ+
corresponds to metallic iridium. The presence of Ir is caused
by oxidation of Ir0 as a result of contact with air during
transferring the samples to XPS cell. The contribution of iridium
oxide for iridium catalyst on AlSBA-3 support is lower (21%) than
for Ir/SBA-3 (33%). It indicates less susceptibility to oxidation of
0
Ir on AlSBA-3 due to its stronger interaction with support. The
position of the peaks of both iridium forms is slightly shifted to
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Ir/SBA-3 and Ir/AlSBA-3 (Fig. 6). Chromatographic analysis of
the reaction mixture formed during TCE removal in the presence
of supports (SBA-3 and AlSBA-3) showed only the decrease of
TCE concentration without formation of any products. The
unchanged pH value during catalytic test (Fig. 7) confirmed that
hydrodechlorination on pure supports did not occur. It indicates
that the decrease of TCE concentration is caused by its
adsorption on supports. Although both of the supports have
shown similar efficiency of trichloroethylene sorption from water,
slightly better results were obtained for AlSBA-3 material with
better ordered structure (Fig. 6).
hydrodechlorination of TCE. It means that very small iridium
nanoparticles (Table 1) are active in water purification. Our
results are in line with earlier studies concerning the relationship
between size of metals (e.g. Pd, Pt, Au nanoparticles) and their
activity in the aqueous phase hydrodechlorination of TCE
confirming activity of metal in nanoparticle forms.[45] Although
iridium dispersion is similar in both samples, Ir/AlSBA-3 has
shown higher activity than Ir/SBA-3 (Fig. 6, Table 2). After 60
minutes of reaction almost 80 % of TCE is removed from water
for Ir/AlSBA-3, in contrast to Ir/SBA-3 where ~ 50 % of TCE
conversion is observed (Fig. 6A). Moreover, the carrying out the
reaction on Ir/AlSBA-3 and Ir/SBA-3 for a further 30 minutes led
to 93 and 66 % of trichloroethylene conversion, respectively.
The selectivity of the products depends on the used catalyst.
However, in both cases the formation of mono- and di-
chlorinated compounds (if any) was under the limit of ECD
detection (Fig. S2, S3, Supporting Information). The iridium
catalyst on SBA-3 support show selectivity to ethene (53 %) and
ethane with some contribution of methane (6 %). The Ir/AlSBA-3
catalyst show higher selectivity to ethene (74 %) without
formation of methane. The differences between the catalysts
behaviour concerning selectivity towards products could be
related with the presence of mainly Lewis acid active sites which
The sorption phenomenon could be also related with surface
hydrophobicity of the supports. Hydrophobicity index calculated
on the basis of the amount of water desorbed at 150 °C and
4
00 °C for both iridium catalysts indicate that both catalysts are
characterized by hydrophobic nature of their surface.[
Received results indicate, however slightly higher hydrophilicity
of Ir/AlSBA-3 catalyst (0.79) than Ir/SBA-3 catalyst (0.87). It is
caused by higher hydrophilicity of Al containing SBA-3 support in
comparison to pure siliceous SBA-3. It is in agreement with
literature data and could be explained by the presence of a
strong electrostatic field created on framework anion sites of
AlSBA-3 support.[44]
43]
A comparative study between iridium catalysts have shown
activity for both samples in aqueous phase hydrodechlorination
of trichloroethylene indicating catalytic activity of iridium in
could
influent
the
mechanism
of
aqueous
phase
hydrodechlorination of trichloroethylene on Ir/AlSBA-3 catalyst.
A
B
Figure 6. TCE removal (A) and kinetics (B) of aqueous phase TCE hydrodechlorination.
Due to the lack of the significant differences in the dispersion of
the active phase (Table 1), the acidity, hydrophobicity and
especially the ordering of the structure are believed to play the
crucial role in catalytic behaviour of Ir/SBA-3 and Ir/AlSBA-3.
Based on the XRD results (Fig. 1A) AlSBA-3 support possesses
better ordered structure than SBA-3. In accordance with the
literature data, this specific surface feature could improve activity
of this catalyst in aqueous phase hydrodechlorination of the
chloroorganic compounds.[4,5b,9a] It was shown that the
application of ordered materials as the supports for active phase
leads to formation of uniformly dispersed, resistant for
agglomeration and leaching, rather small metal nanoparticles.
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Due to the predominant amount of hydrogen in reaction
mixture, aqueous phase hydrodechlorination of TCE over
Ir/SBA-3 and Ir/AlSBA-3 is assumed to be pseudo-first-order.
Figure 8 shows the kinetic data according the linearized pseudo-
first-order rate equation. So the reaction kinetic can be ascribed
0
as ln(C/C ) = - kt, where k is the apparent first-order rate
-1
constant (s ), and t is the reaction time (s). The initial reaction
rates TOF and reaction rate constants values, are
r
0
,
k
presented in Table 2. To the best of our knowledge there are no
available literature data concerning the application of iridium
catalysts
in
aqueous
phase
hydrodechlorination
of
trichloroethylene. It makes impossible the direct comparison our
studies with results obtained by other groups. However, it should
be stressed here that activity of Ir/SBA-3 and Ir/AlSBA-3 in TCE
hydrodechlorination looks promising. It is comparable or better
than the activity of other catalysts supported on mesoporous
material in similar conditions.[4] The k value obtained for
Ir/AlSBA-3 is approximately two times higher than that obtained
for Ir/SBA-3. Due to the fact that the amounts of iridium active
sites are similar in both catalysts, the differences in catalytic
behaviour of iridium samples are probably related with the
properties of the surface. The higher TOF value obtained for
Ir/AlSBA-3 than for Ir/SBA-3, both with similar iridium dispersion,
confirm our supposition that catalytic properties of these
materials depends on the acidity, hydrophobicity and ordering of
the supports.
Figure 7. pH of the reaction mixture during catalytic tests.
Only slight changes in average iridium particles size for spent
catalysts in comparison with fresh samples indicate that ordering
structure of SBA-3 and AlSBA-3 can affect the properties of
iridium catalysts. On the other hand, our results are opposite to
studies of Munoz et al.[46] showing that the catalysts with higher
surface areas and lower ordered structures showed higher HDC
rates than the materials with higher ordered structure in
hydrodechlorination of 4-chlorophenol. This may suggest that
the phenomenon of the relationship between the ordering of the
material and the activity is not trivial problem and could depend
on many factors such as the kind of the support and metal
particles size, and kind of the substrate. The activity can be also
influenced by hydrophobicity of the catalysts. Based on the
literature data[47] hydrophobicity plays beneficial role in
protection of active phase from poisoning and leaching and
helps the kinetics. Hence, the properties of Ir/SBA-3 and
Ir/AlSBA-3 in removal of TCE from water could be strongly
related with the hydrophobicity of these materials. Additionally,
the hydrophobic nature of Ir/AlSBA-3 and Ir/SBA-3 acts as a
benign protection layer to restrict both the agglomeration or
redispersion or leaching effect, as it was confirmed by TEM
analysis for spent catalysts (Fig. 2).[47]
0
Figure 8. Plot of ln(C/C ) versus reaction time for hydrodechlorination of TCE
on Ir/SBA-3 and Ir/AlSBA-3.
Table 2. The reaction rate constant k, initial reaction rate, TOF values and
selectivity towards products for aqueous phase hydrodechlorination of
trichloroethylene on iridium catalysts.
The XRF analysis of water portions after reaction did not
show any amounts of iridium within the limits of the detection.
Therefore, the effect of active phase leaching from the catalysts
to reaction mixture in the case of Ir/SBA-3 and Ir/AlSBA-3 is
believed to be negligible. To confirm this conjecture, the
additionally experiments were done. The fresh catalysts were
stirred separately in water for 15 min at reaction temperature
and then the TCE hydrodechlorination tests were performed
over water separated from the catalysts.
Initial
reaction
rate,
Reaction rate
constant
Selectivity, %
a)
TOF
s
Catalyst
-
1
-
0 – 90 min,
mmolmin
-
1
s
CH
4
C
2
H
6
2 4
C H
1
-1
g
Ir
-
-
2
2
-
3
3
1.79·10
0.02
Ir/SBA-3
0.36
0.84
2.48·10
5.86·10
6
-
41
26
53
74
±
-
3.77·10
0.06
Ir/AlSBA-3
±
a)
TOF calculated according to Molina et. al.[48]
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ChemCatChem
10.1002/cctc.201800873
FULL PAPER
Catalytic properties of Ir/SBA-3 and Ir/AlSBA-3 have been
evaluated in aqueous phase hydrodechlorination of
trichloroethylene. Our results clearly show that iridium supported
catalysts are active in hydrodechlorination of TCE. The catalytic
performance of iridium catalysts depends on their acidic,
hydrophobic and textural properties. More hydrophilic AlSBA-3
supported iridium catalyst containing acidic sites was found to
be more active than Ir/SBA-3.
Experimental Section
Supports and catalysts preparation
Pure silica (symbol SBA-3) and aluminosilicate (symbol AlSBA-3)
supports were prepared by hydrothermal synthesis performed under
acidic conditions according to our own procedure. Tetraethyl orthosilicate
(
TEOS
-
Aldrich, 98%) was used as
a
silica source and
Aldrich, 95%) as
cetyltrimethylammonium bromide (CTMABr
-
a
surfactant. The silica SBA-3 sample was synthesized according to
following procedure. First, CTMABr (8.3 g) was dissolved in 165 cm3 of
3
deionized water followed by an addition of TEOS dropwise (23 cm ).
3
After stirring for 15 minutes, 38% HCl was added (34 cm , POCh - Polish
Chemicals Reagents, Poland) with continuous stirring for next 30 min.
After this, the additional 36 cm3 of hydrochloric acid were added, which
resulted in precipitation of white gel. The mixture was stirred for next 1.5
h at room temperature. Then, aging of the gel was carried out at room
temperature for 8 days in static conditions. The resulting solid was
filtered, washed, dried and calcined at 550 °C for 8 h in air.
The synthesis of aluminosilicate SBA-3 (AlSBA-3) was done according to
the above procedure with minor modification. The source of aluminum
(
dropwise to the CTABr solution before introducing the TEOS to the
mixture. The Si/Al molar ratio was 10. After introducing two parts of HCl,
the pH of the suspension was increased up to 2.2 with aqueous ammonia
Figure 9. Three cycles of TCE hydrodechlorination in aqueous phase on
Ir/SBA-3 (A) and Ir/AlSBA-3 (B).
3 3 2 2
3.86 g of Al(NO ) ·9H H O) was admitted
O (POCh) dissolved in 5 cm3
However, no reaction progress was observed and obtained
results were similar to control curve. It confirms no leaching of
iridium from catalysts. To check the stability of the iridium
catalysts, the same portions of Ir/SBA-3 and Ir/AlSBA-3 were
used three times in the hydrodechlorination of TCE. As it was
shown in Fig. 9, differences in TCE conversion obtained after
(
25%, POCh) in order to introduce aluminium atoms to the framework.
After calcination, SBA-3 and AlSBA-3 supports were modified with iridium
by means of impregnation, using the cyclohexane solution of tetrairidium
dodecacarbonyl (Ir (CO)12, 98%, Aldrich). The required quantity of
4
tetrairidium dodecacarbonyl was dissolved in cyclohexane and this
solution was added to the evacuated support (the amount of precursor
was calculated to achieve iridium loading of 1 wt%). Cyclohexane was
removed on a rotary evaporator. The impregnated samples were dried at
150 min of subsequent reactions could be related to catalysts
mass loss during filtration and drying between the cycles than to
the deactivation by the formation of deposits. It indicates high
stability of investigated iridium catalysts.
1
00 oC for 24 h and labeled as Ir/SBA-3-D and Ir/AlSBA-3-D (D stands
for dried catalyst).
Prior to the characterization each precursor-impregnated support was
3
-1
placed in a porcelain boat and reduced with H
2
(50 cm min , 99.99%,
Linde). At the beginning, the system was purged at room temperature
Conclusions
3
-1
with argon (150 cm min , 99.99%, Linde) which was later replaced with
3
-1
hydrogen (150 cm min ). After 30 minutes, the hydrogen flow was
Using of mesoporous SBA-3 materials of different
composition as supports for iridium catalyst allowed to obtain
catalyst with very small, well dispersed metal crystallites (~3 nm).
The unchanged surface area and pore diameter as well the
decrease of pore volume for catalyst in comparison to supports
indicated the deposition of iridium particles inside the pores. The
reducibility of iridium precursor depended on the chemical
composition of the support and was more difficult on acidic
aluminosilicate SBA-3. The differences in the reducibility did not
have a significant impact on the dispersion and particle size of
the iridium active phase.
3
-1
o
-1
decreased to 50 cm min and the temperature ramp (10 Cmin )
started. After reaching the setpoint (400 oC), the reduction of catalyst
precursor was carried out for 2 h. Then the catalyst was cooled to room
temperature in hydrogen flow. The last step consisted in flushing with
argon for 15 min and passivation with 5 vol.% O /He mixture. The
2
reduced catalysts were labelled as Ir/SBA-3 and Ir/AlSBA-3.
Supports and catalysts characterization
The X-ray powder diffraction was performed on a Bruker AXS D8
Advance diffractometer with Ni-filtered CuK radiation (λ=1.54056 Å)
over the 2Θ ranges of 2-8 and 10-60.
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ChemCatChem
10.1002/cctc.201800873
FULL PAPER
Textural characterization of supports calcined at 550 oC and catalysts
reduced at 400 oC was obtained by low-temperature (-196 oC) nitrogen
Energy dispersive X-ray fluorescence analysis (EDXRF) was carried out
using MiniPal 4 equipment from PANalytical Co., with an Rh-tube and
silicon drift detector (resolution 145 eV). EDXRF method was used to
gain information on the elemental composition of liquid samples after
catalytic experiments.
adsorption using
a
sorptometer ASAP 2010 manufactured by
Micromeritics. Prior to the measurements of adsorption-desorption
isotherms, the samples were outgassed at 300 oC for 4 h. The surface
area was determined by Brunauer-Emmett-Teller (BET) method,
whereas total pore volume and average pore diameter were calculated
using Barrett-Joyner-Halenda (BJH) method.
X-ray photoelectron spectra (XPS) were recorded on an Ultra High
Vacuum (UHV) System (Specs, Germany). The examined materials were
irradiated with
a monochromatic Al Kα radiation (1486.6ꢀeV). The
−
9
Transmission electron microscope (TEM) images were recorded on a
JEOL 2000 microscope operating at accelerating voltage of 80 kV or on a
Hitachi HT7700 operating at 100 kV (sample Ir/AlSBA-3 after reaction).
The results were used to estimate ordering of the samples and to
estimate the Ir particle size. The materials studied were deposited on
nickel grids coated with a carbon film. The average particle size was
calculated from 100 particles using following formula:[48]
operating pressure in the chamber was close to 2ꢀ×ꢀ10 mbar. Binding
energies were referenced to the C1s peak from the carbon surface
deposit at 284.6 eV.
The temperature-programmed desorption of ammonia measurements
3
(TPD-NH ) were carried out on Pulse ChemiSorb 2705 (Micromeritics)
instrument. In the typical experiment a sample (~100 mg) was heated in
He (99.999%, Linde) at the rate of 10 oCmin-1 up to 400 oC and then
maintained at this temperature for 1 h. Afterwards it was cooled down to
푛푖ꢀ푖3
∑
∑
푑 =
110
o
C and saturated with ammonia for 1 h. The physically adsorbed NH3
2
푛푖ꢀ
푖
o
was removed by purging with helium flow at 110 C for 1 h. The TPD-NH
analysis was carried in the range of 110 – 600 oC with a heating rate of
10 Cmin .
3
o
-1
where: d stands for particle size and n is the number of particles.
Dispersion and mean size of Ir particles were determined by static H
2
Measurements of temperature-programmed reduction with hydrogen
chemisorption method using a Micromeritics ASAP2010C sorptometer.
Prior to hydrogen chemisorption measurements, fresh dried catalysts
(TPR-H
instrument. Dried metal precursor-impregnated supports (50 mg) were
reduced in the flow of 10 vol.%. H – Ar (99.999%, Linde) at the flow rate
studies, quartz sand (Aldrich)
impregnated with metal precursors (Ir (CO)12) was used as a reference
material. The measurements were conducted in the temperature range
2
) were carried out on Pulse ChemiSorb 2705 (Micromeritics)
were reduced with H
2
at 400 oC for 2 h and then the catalyst samples
2
3
-1
were pretreated in situ to purify their surfaces from adsorbed gases. The
pretreatment consisted in the evacuation at room temperature for 15 min
of 30 cm min . In the TPR-H
2
a
4
o
and then at 350 C for 60 min, followed by reduction in hydrogen flow (40
3
-1
o
o
o
-1
cm min , 99.999%, Linde) at 350 C for 60 min and evacuation at 350
from 50 to 900 C at a linear temperature ramp of 10 Cmin . The TPR-
analysis were also performed for catalysts after activation (H , 400 °C,
2 h). All TPR-H profiles, presented in this work, have been normalized to
the same sample weight.
o
o
C for 120 min. Chemisorption of hydrogen was carried out at 35 C, and
the isotherms were determined using five different pressures in the range
of 12-40 kPa (H - total adsorbed hydrogen). Then the catalyst was
evacuated at 35 C for 30 min to remove reversibly adsorbed hydrogen
) and the same procedure was repeated. The difference between
H
2
2
2
t
o
(H
r
Surface hydrophilicity of the catalysts was measured by water adsorption.
For this purpose, the samples were moisturized in desiccator
adsorbed hydrogen extrapolated to zero pressure value for two isotherms
equals to the amount of hydrogen irreversibly bound (Hirr).
a
containing demineralized water at room temperature for 24h. The amount
of water adsorbed in the catalysts were determined by using
thermogravimetric analysis (Mettler). The value of HI (hydrophobicity
index) was calculated based on the loss of water at 150 °C over weight
loss at 400 °C determined by thermogravimetric analysis: [41]
2
-1
The metallic surface area (S), expressed in m gIr , was calculated from
[
49]
the following equation:
v  N na 100
m
A
m
S 
2
2414mwt
3
where v
m
- volume of adsorbed hydrogen expressed in cm , N
A
-
Weight loss up to 150 °C
퐻퐼 =
23
-1
Avogadro’s number (6.02210 mol ), n - chemisorption stoichiometry
Weight loss up to 400 °C
2
(
n=2), a
m
- surface area (m ) occupied by one iridium atom, m - sample
weight (g), wt (%) - metal loading.
where HI = 1 is representative for a very hydrophobic sample and HI = 0
The dispersion (D) of iridium was calculated from the formula:
is characteristic for a very hydrophilic sample.
S M
D 
Catalytic test
am  NA
The aqueous – phase hydrodechlorination tests in batch mode were
performed under atmospheric pressure at room temperature (25 C) in a
glass reactor equipped with a magnetic stirring bar, pH-meter and a
temperature controller. Every time, 350 cm3 of Milipore water was
saturated with hydrogen for 30 minutes before adding 20 µl of TCE
where S is metallic surface area, M is iridium atomic weight, N
Avogadro’s number and a is the surface covered by one iridium atom.
m
The average size of Ir particles (P, expressed in nm) were calculated
from the formula: [49]
A
is
o
(
about 8000 times higher concentration than accepted in drinking water).
6
000
P 
Prior to catalyst addition (0.1 g), the reaction mixture was stirred (1000
rpm) for 30 minutes to provide homogeneity. To check activity of used
sample in the repeat experiments, iridium catalyst was recovered by
filtration and then washed with distilled water and dried in the oven at
temperature 393 K for the subsequent catalytic run. The catalytic
procedure was repeated three times. The reaction products were
analyzed after 0, 2, 5, 10, 15, 20, 60, 90, 120 and 150 min of the reaction.
The concentration of trichloroethylene and products of TCE
S 
where S – the metallic surface area (S), expressed in m gIr-1; ρ – metal
2
-
3
density expressed in g∙cm .
Iridium content in fresh catalysts reduced at 400 oC for 2 h was
determined by inductively coupled plasma optical emission spectroscopy
ICP-OES) on a Varian Vista-MPX spectrometer.
(
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ChemCatChem
10.1002/cctc.201800873
FULL PAPER
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ChemCatChem
10.1002/cctc.201800873
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[
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ChemCatChem
10.1002/cctc.201800873
FULL PAPER
Efficient
preparation
of
new
Ewa Janiszewska*, Michał Zieliński,
Monika Kot, Emil Kowalewski, Anna
Śrębowata
mesoporous catalysts containing
highly dispersed nanoparticles of Ir0
on silica and aluminosilicate SBA-3
supports. The catalyst are active in
aqueous phase hydrodechlorination
of trichloroethylene. The results show
that Ir/AlSBA-3 catalyst containing
acidic sites was found to be more
active than Ir/SBA-3.
Page No. – Page No.
Aqueous-phase
hydrodechlorination of
trichloroethylene on Ir catalysts
supported on SBA-3 materials
This article is protected by copyright. All rights reserved.