Autoreduction of copper on silica and iron-functionalized silica nanoparticles with interparticle mesoporosity

Article abstract of DOI:10.1002/cctc.201300463

Copper-supported catalysts are of industrial importance in many catalytic processes. Mesoporous silica materials are of particular interest as green heterogeneous catalyst supports. In the present study we demonstrate the nature and reduction properties of copper oxide species, which are influenced by the peculiarity of the silica nanoparticles with interparticle mesoporosity (KIL family) and the presence of a second metal (iron) in the silica matrix. The copper-containing KIL-2 and FeKIL-2 samples are prepared by incipient wetness impregnation. The reduction of copper oxide species is easier on the FeKIL-2 supported sample in comparison to its KIL-2 supported analogue, whereas the copper-containing KIL-2 sample shows higher catalytic activity in total toluene oxidation. The presence of iron in the FeKIL-2 structure leads to autoreduction of copper followed by the redispersion and oxidation of metallic copper in the reaction medium; this results in the formation of different types of finely dispersed copper oxide species (<100 nm). The later species possess lower catalytic activity in toluene oxidation in comparison to species that are 100 nm in size and formed on KIL-2. Copyright

Full text of DOI:10.1002/cctc.201300463

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DOI: 10.1002/cctc.201300463
Autoreduction of Copper on Silica and Iron-Functionalized
Silica Nanoparticles with Interparticle Mesoporosity
[
a]
[b]
[b]
[b]
[a]
Margarita Popova, Alenka Risti c´ , Matja zˇ Mazaj, Darja Mau cˇ ec, Momtchil Dimitrov,
[b]
and Nata sˇ a Novak Tu sˇ ar*
Copper-supported catalysts are of industrial importance in
many catalytic processes. Mesoporous silica materials are of
particular interest as green heterogeneous catalyst supports. In
the present study we demonstrate the nature and reduction
properties of copper oxide species, which are influenced by
the peculiarity of the silica nanoparticles with interparticle
mesoporosity (KIL family) and the presence of a second metal
FeKIL-2 supported sample in comparison to its KIL-2 supported
analogue, whereas the copper-containing KIL-2 sample shows
higher catalytic activity in total toluene oxidation. The pres-
ence of iron in the FeKIL-2 structure leads to autoreduction of
copper followed by the redispersion and oxidation of metallic
copper in the reaction medium; this results in the formation of
different types of finely dispersed copper oxide species
(<100 nm). The later species possess lower catalytic activity in
toluene oxidation in comparison to species that are 100 nm in
size and formed on KIL-2.
(iron) in the silica matrix. The copper-containing KIL-2 and
FeKIL-2 samples are prepared by incipient wetness impregna-
tion. The reduction of copper oxide species is easier on the
Introduction
[
17–20]
Copper-supported catalysts are of industrial importance in
many important processes: catalytic oxidation of volatile or-
ganic compounds (VOCs) as an air-cleaning method, the syn-
thesis of cyclohexanone from cyclohexanol, the steam reform-
ing of methanol, the synthesis of methanol, the selective cata-
neous catalyst supports.
The interparticle mesoporosity of
the new KIL family of silicates gives a new opportunity for the
[21]
preparation of highly active catalysts. The KIL family is relat-
ed to mesostructured silicate families with textural or interpar-
[21]
ticle porosities (HMS, MSU, NBS). Our recent results clearly
demonstrate that silicates with interparticle mesoporosity are
highly promising catalysts for the decomposition of organic
[
1–8]
lytic reduction of nitrogen oxide, and so forth.
Their catalyt-
ic properties depend on the oxidation and the coordination
states of the copper ions. Particular interest has been paid to
the preparation of a copper catalyst in a specific oxidation
state with copper(I) prevailing over copper(II). The copper(II)
ions that are supported on microporous aluminosilicate zeolite
ZSM-5 are well known to be thermally reduced to copper(I)
under vacuum or in an inert gas flow at high temperature (the
reducing agent is the Brønsted acid sites, which are formed
[22]
pollutants in a liquid phase. The conditions for the formation
of catalytically active metal oxides and their transformations
during the reaction as a result of KIL-support peculiarity are
important topics that have not been investigated yet. The
nature and dispersion and reduction properties of copper
oxide species can be influenced by the peculiarity of the nano-
sized KIL structure and the presence of a second metal in the
silica matrix (to form Brønsted acid sites as a reducing agent in
[
9]
because of the presence of aluminum). The thermal autore-
duction process can be explained by the dehydrogenation of
Cu(II)-(OH) species and the bridging extra-lattice oxygen
atoms, which are desorbed as oxygen molecules; this is ac-
[23,24]
the structure). Our previous investigations
showed that
the modification of mesoporous silica with two metals simulta-
neously leads to high metal oxide dispersion, which positively
influences their catalytic behavior in toluene oxidation. The
modification by copper and chromium also results in a high
copper dispersion of CuCr/SBA-15 materials and stabilizes the
formed copper oxide species in copper chromates and
bichromates.
[
9–16]
companied by reduction of copper(II) to copper(I) ions.
copper(II) species dispersed on aluminosilicates that have
The
[
12]
a lower Si/Al ratio are prevented from autoreduction. During
the last few decades, mesoporous silica materials, with their
uniform mesoporous channel structure and high specific sur-
face area, have been of particular interest as green heteroge-
In this study, we demonstrate for the first time the nature
and reduction properties of copper oxide species that are in-
fluenced by the peculiarity of the silica nanoparticles with in-
terparticle mesoporosity (KIL-2 structure from KIL family) and
the presence of a second metal (iron) in the silica matrix. The
catalytic behavior of copper-modified KIL-2 and FeKIL-2 sam-
ples, pretreated in different conditions, is studied in total tolu-
ene oxidation (oxidation of toluene as a model air pollutant to
CO and H O).
[a] M. Popova, M. Dimitrov
Institute of Organic Chemistry with Centre of Phytochemistry
Bulgarian Academy of Sciences
Sofia (Bulgaria)
[
b] A. Risti c´ , M. Mazaj, D. Mau cˇ ec, Prof. Dr. N. N. Tu sˇ ar
National Institute of Chemistry
Ljubljana (Slovenia)
E-mail: natasa.novak.tusar@ki.si
2
2
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face area, total pore volume, pore size distribution as well as
surface properties. The shape of the isotherm obtained from
these adsorption measurements is important because it can
reveal the kind of porosity (micro, meso, or macroporosity)
present in the sample. Nitrogen adsorption isotherms for KIL-2,
FeKIL-2 and Cu/FeKIL-2 materials are shown in Figure 1b,
whereas structural parameters determined on the basis of
these isotherms are listed in Table 1. KIL-2 and FeKIL-2 samples
Results and Discussion
Physicochemical characterization of the samples
XRD patterns show reflections that are typical of copper oxide
for Cu/KIL-2 (Figure 1a), but not for the Cu/FeKIL-2 sample,
probably because of the formation of small copper oxide
nanoparticles. Copper oxide with a particle size of approxi-
mately 100 nm is determined for Cu/KIL-2 (Figure 1a) with cal-
culations from the Scherrer equation, which is based on the se-
lected diffraction peaks of the corresponding XRD pattern.
Gas physisorption is extensively used in the characterization
of porous solids, particularly for evaluation of their specific sur-
Table 1. Structural parameters of the catalysts determined from nitrogen
adsorption isotherms.
[a]
[b]
[c]
[d]
Sample
Copper oxide
wt%]
S
BET
2
V
T
V
meso
3
w
BJH
ꢀ1
3
ꢀ1
ꢀ1
[
[m g
]
[cm g
]
[cm g
]
[nm]
KIL-2
–
6
–
6
545
359
556
407
1.480
0.732
1.459
1.105
1.392
0.648
1.326
1.004
19.9
14.8
21.6
16.1
Cu/KIL-2
FeKIL-2
Cu/FeKIL-2
[
a] BET surface area. [b] Total pore volume evaluated from the adsorption
isotherm at the relative pressure of ꢁ0.98. [c] Primary mesopore volume
evaluated by using the a -plot method. [d] Mesopore diameters at the
s
maximum of the BJH pore size distribution.
exhibit adsorption isotherms typical for KIL-2 and FeKIL-2 ma-
[
30–32]
terials,
that is, with relatively narrow hysteresis loops of
[26]
type IV. The FeKIL-2 sample undergoes a capillary condensa-
tion step at higher relative pressures than the KIL-2 sample,
which indicates that larger mesopores are present in the
[32]
FeKIL-2 material compared to the KIL-2 support. It can be
clearly observed that the presence of copper oxide nanoparti-
cles in KIL-2 and FeKIL-2 supports leads to a marked change in
the shape of the hysteresis loop. The hysteresis loop of cop-
per(II) oxide loading samples is widening, less intensive and
closing down at lower p/p values compared to the original
0
supports, which indicates that the pores are partially narrowed
with copper oxide nanoparticles. The loading of copper oxide
nanoparticles into KIL-2 and FeKIL-2 supports also leads to
a decreased specific surface area, pore volume and pore diam-
eter (Table 1). Therefore, it can be concluded that the copper
oxide nanoparticles are dispersed inside the KIL-2 and FeKIL-2
matrices.
Pore size distribution of the different mesoporous materials
was determined by using the Barrett–Joyner–Halenda (BJH)
[28]
model, which is widely used for this type of samples. Al-
[29]
though this model often underestimates pore sizes, it is ap-
propriate for comparative purposes. The pore size distributions
determined from the adsorption isotherms are displayed in
Figure 2b (inset). It can be observed that the maxima charac-
teristics of the mesopores of KIL-2 and FeKIL-2 supports are
more intense and broader in comparison to their copper oxide
loaded samples. They show average pore diameters of 19.9
and 21.6 nm, respectively. The maxima characteristics of
copper oxide impregnated materials are shifted to lower pore
size values (14.8 nm for Cu/KIL-2 and 16.1 nm for Cu/FeKIL-2).
A TEM micrograph of the Cu/KIL-2 sample shows a porous
texture with interparticle porosity typical for KIL-2 silica materi-
Figure 1. a) XRD patterns with a 01-080-1916 reference for copper oxide,
b) nitrogen physisorption data, and c) UV/Vis spectra for the studied sam-
ples.
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The observations at higher magnification did not reveal the
presence of any smaller metal oxide nanoparticles that could
be located within the voids of KIL-2 matrix(Figure 2d).
TEM analysis of the Cu/FeKIL-2 sample shows similar textural
properties as for the Cu/KIL-2 sample. The FeKIL-2 silica matrix
is inhomogenously covered with the copper oxide phase, how-
ever individual metal oxide nanoparticles are somewhat small-
er (100 nm) and seem to be agglomerated into larger clusters
(
Figure 2e). Similarly, as in the case of the Cu/KIL-2 sample, the
observations at higher magnification do not reveal the pres-
ence of any smaller metal oxide nanoparticles (Figure 2 f).
Diffuse reflectance UV/Vis spectroscopy was used to investi-
gate the nature of the iron and copper ions in the Cu/KIL and
Cu/FeKIL-2 samples (Figure 1c). The UV/Vis spectrum of the
FeKIL-2 sample reveals two peaks at l=240 and 285 nm. The
same peaks can be found in the UV/Vis spectrum of the Cu/
FeKIL-2 sample. These peaks are assigned to the charge trans-
fer between the iron(III) ions and the oxygen of the FeꢀOꢀSi in
[33]
the network of FeKIL-2, which indicates the presence of tet-
[34]
rahedrally and octahedrally coordinated iron(III) species. The
peak at l=240 nm can be attributed to copper(II) ions, and
oxygen charge transfer transitions in Cu/KIL-2 and Cu/FeKIL-2
Figure 2. TEM micrographs of a) Cu/KIL-2 and b) Cu/KIL-2 with copper oxide
crystallites (dark spots) located on the surface of the KIL-2 silica matrix,
c) SAED pattern of Cu/KIL-2 with selected reflections with the corresponding
marked crystal lattices, d) close-up TEM micrograph of Cu/KIL-2, e) TEM mi-
crograph of Cu/FeKIL-2 with copper oxide crystallites (dark spots) located on
the surface of the FeKIL-2 silica matrix, f) close-up TEM image of Cu/FeKIL-2.
[35]
are observed as well. No contribution at greater than l=
00 nm is observed in FeKIL-2, which excludes the occurrence
3
of an oligonuclear iron complex and iron oxide particles in this
sample. A broad absorption peak from l=400 to 800 nm can
be observed in the Cu/KIL-2 and Cu/FeKIL-2 UV/Vis spectra,
which owes to d–d transitions of copper(II) with the octahedral
als (Figure 2a). Agglomerated silica nanoparticles with an esti-
mated size of 10 nm form voids and cavities with a broad size
distribution. The porous silica matrix contains individual crys-
tallites of copper oxide. The relatively large thickness of the
crystallites makes the TEM observation somewhat difficult,
however an approximate size of 100 nm could be estimated
[35]
environment in copper oxide particles.
The additional
shoulder at approximately l=340 to 360 nm can be seen in
Cu/FeKIL-2, but not in the Cu/KIL-2 sample, which corresponds
[
35]
to the presence of [CuꢀOꢀCu] surface species. The differen-
n
ces in the nature of copper species, which are formed on the
modified samples, can be ascribed to the presence of iron in
the silica matrix.
(Figure 2b). This is in agreement with the calculations from the
Scherrer equation, which is based on the selected diffraction
peaks of the corresponding XRD pattern.
More information on the oxidation state of copper oxide
species and their reducibility was obtained by temperature-
programmed oxidation–reduction thermogravimetric analysis
(TPO/TPR-TG) experiments (Figure 3). Pretreatment in different
media (air and argon) was applied before the TPR experiments.
The samples showed different behavior during the pretreat-
ment procedure, which depended on the pretreatment media
and the used support (Figure 3).
The presence of the monoclinic copper oxide phase (Cc, a=
4.69 ꢁ, b=3.43 ꢁ, c=5.13 ꢁ, b=99.658) in the CuKIL-2 sample,
which is already indicated by XRD analysis, is finally confirmed
by the selected area electron diffraction method (SAED). The
SAED pattern of the copper oxide crystallites with the selected
maxima and their corresponding crystal planes are shown in
Figure 2c, whereas a comparison of measured and referenced
d values of selected crystal planes are shown in Table 2.
There were no significant changes in the weight of both
samples Cu/KIL-2 and Cu/FeKIL-2 during the pretreatment in
air. A low intensity peak is registered at 870 K for both sam-
ples; this is probably due to silica matrix dehydroxylation. In
contrast, the pretreatment in argon results in significant
weight changes, which correspond to 100% reduction of cop-
per(II) ions to metallic copper on the FeKIL-2 sample. Reduc-
tion peaks at 595 and 770 K are registered for Cu/FeKIL-2 pre-
treated in argon. For the first time the copper autoreduction
process is determined on mesoporous materials. We ascribe
the copper autoreduction process to the influence of iron(III)
ions and the formation of weak Brønsted acid sites in the
sample as a result of iron incorporation into the silica structure.
The predominant formation of copper(II) ions on this sample is
The sizes of the copper oxide crystallites indicate that they
are located exclusively on the surface of the KIL-2 silica matrix.
Table 2. Comparison of measured and referenced d values of corre-
sponding crystal planes.
[
a]
Crystallographic plane
Measured
nm]
PDF (No. 01-080-1917)
[nm]
[
d
1
d
2
d
3
[110]
[002]
[202]
2.79
2.56
1.60
2.75
2.52
1.58
[a] Powder diffraction file.
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Figure 3. TPO/TPR-DTG profiles of a,b) Cu/FeKIL-2 and c,d) Cu/KIL-2 samples.
due to the presence of Brønsted acid sites in the iron-contain-
ing KIL-2 support and their interaction with the copper precur-
sor during the impregnation process. These copper(II) species
can be easily reduced to metallic copper during evacuation at
high temperature or under an inert atmosphere [Eq. (1)].
FeKIL-2 supported sample in comparison to its KIL-2 supported
analogue. It is probable that the formation of different types
of copper oxide species on the two applied supports and the
influence of iron in FeKIL-2 are responsible for the different re-
duction behavior. This assumption is also supported by diffuse
reflectance UV/Vis data, in which the formation of different
copper species is registered.
2þ
þ
2þ
þ
0
Cu ! CuðOHÞ ! ½CuꢀOꢀCuꢂ ! Cu ! Cu
ð1Þ
[
13]
Kuroda et al also found that the presence of Brønsted acid
sites in the sample that has a high Si/Al ratio is a necessary
condition for the formation of copper(I) ions. The reduction
profiles of the copper-containing catalysts depend on the sup-
port material used and the pretreatment procedure. The
copper-containing KIL-2 and FeKIL-2 samples pretreated in air
possess clearly defined low-temperature reduction peaks at
Catalytic activity for total toluene oxidation
The catalytic activities versus the temperature and versus the
time on a stream of copper-containing materials that were pre-
treated in an inert gas or air at 723 K in total toluene oxidation
are presented in Figure 4. The pretreatment in different media
(air or argon) does not lead to a different catalytic activity with
a temperature increase for Cu/FeKIL-2 (Figure 4b). It is the op-
posite for Cu/KIL-2, the increase in the temperature leads to
a higher catalytic activity in toluene oxidation for this sample
after the pretreatment in air at 773 K (Figure 4a). The use of
KIL-2 as a support has a positive effect on the catalytic per-
formances of the Cu/KIL-2 sample. The Cu/FeKIL-2 catalyst
shows stable catalytic activity at 653 K after pretreatment in
different reaction media (air and argon) (Figure 4d), whereas
the Cu/KIL-2 sample shows stable catalytic activity only in the
pretreatment with air, and a trend towards deactivation in the
pretreatment with argon (Figure 4c). The physicochemical
characteristics (UV/Vis and TPR data) of the catalysts show the
5
30 and 440 K, respectively, which can be attributed to cop-
per(II) to metallic copper reduction. The extent of reduction is
9.33% for Cu/KIL-2 and 100% for Cu/FeKIL-2. The narrow re-
8
duction peak with a maximum at 430 K, registered for Cu/KIL-2
after pretreatment in argon (Figure 3d), is related to the more
homogenous distribution of copper oxide species on the sup-
ports. There are no peaks in the TPR-TG profile of the Cu/KIL-2
sample after pretreatment in argon at 773 K (Figure 3c), which
is due to total copper autoreduction during the pretreatment
under an inert atmosphere.
By summarizing the above observations we can conclude
that the reduction of copper oxide species is easier on the
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reaction. Redispersion of the
copper oxide phase during the
reaction on both samples (Cu/
KIL-2 and Cu/FeKIL-2) was ob-
served after pretreatment in
inert gas. The copper autoreduc-
tion in argon on FeKIL-2 results
in the formation of more finely
dispersed copper(I) and copper
(
II) species (not detectable by
XRD, Figure 5) in the oxidative
reaction medium, which are less
active. The pretreatment in air of
Cu/KIL-2 results in the formation
of copper oxide that is 100 nm
in size; this possesses the high-
est catalytic activity in toluene
oxidation but lower stability
compared to the Cu/FeKIL-2 and
Cu/KIL-2 samples pretreated in
argon. Our experiments show
that finely dispersed copper spe-
cies are less active, and the for-
mation of nanosized crystalline
copper oxide is responsible for
the high catalytic activity in total
toluene oxidation. The Cu/KIL-2
and Cu/FeKIL-2 catalysts after
Figure 4. Catalytic activity versus temperature on a) Cu/KIL-2 and b) Cu/FeKIL-2 samples and versus time on
stream at 653 K on c) Cu/KIL-2 and d) Cu/FeKIL-2 samples after pretreatment in argon or air.
formation of finely dispersed copper oxide species on both
the pretreatment in argon keep their catalytic activity in 3 re-
action cycles. A trend to deactivation is observed only for Cu/
KIL-2 after pretreatment in air most probably because of the
agglomeration of the formed copper oxide on the KIL-2 sup-
port after pretreatment (Figure 5a(2)). The pretreatment in air
of the Cu/FeKIL-2 catalyst doesn’t have a negative influence on
its catalytic stability, which can be related to the formation and
stabilization of finely dispersed copper(II) oxide and copper(I)
oxide species.
[19]
supports, which determine their high catalytic activities. Kim
also found that the presence of highly-dispersed copper spe-
cies had a positive effect on the catalytic activity in toluene ox-
idation. The catalytic activity of bulk copper oxide is presented
in Figure 4 as a reference. All supported samples show much
higher catalytic activity in comparison to bulk copper oxide.
The easier oxygen release is an essential step in the oxidation
[19]
reaction according to the Mars-van-Krevelen mechanism and
in this relationship the reduction
properties of the samples can be
used for the interpretation of
their
catalytic
performance.
However, the higher extent of
copper reduction at lower tem-
perature observed for Cu/FeKIL-
2
(Figure 3) does not lead to
higher catalytic activity in tolu-
ene oxidation. We did not find
a
direct relationship between
the reduction properties and the
catalytic activity of the catalysts,
which could be assumed from
the mechanistic point of view.
Additional XRD experiments of
the spent catalysts (Figure 5)
were applied to indicate the
changes of the oxidation state of
copper oxide species during the Figure 5. XRD images of a) spent Cu/KIL-2 and b) Cu/FeKIL-2 samples after pretreatment in argon (1) and air (2).
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On the basis of the presented catalytic data we conclude
that copper supported on KIL-2 pretreated in air is the better
catalyst compared to the copper-supported FeKIL-2 sample
with a Fe/Si molar ratio of 0.01 for total toluene oxidation. The
presence of iron in the KIL-2 structure leads to autoreduction
of copper followed by the redispersion and oxidation of metal-
lic copper in the reaction medium; this results in the formation
of finely dispersed copper oxide clusters. The latter species
possess low catalytic activity in toluene oxidation.
Nitrogen physisorption measurements were performed at 77 K by
using an ASAP 2020 Micromeritics volumetric adsorption analyzer.
Before the adsorption analysis, the samples were outgassed under
vacuum for 2 h at 473 K in the port of the adsorption analyzer. Ni-
trogen adsorption measurements were measured at 77 K on a Tris-
tar 3000 Micromeritics volumetric adsorption analyzer. Before the
adsorption analysis, the samples were outgassed under vacuum for
2
h at 473 K in the port of the adsorption analyzer. The BET specific
[25]
surface area was calculated from adsorption data in the relative
adsorption data in the relative pressure range from 0.05 to 0.2. The
total pore volume was estimated on the basis of the amount ad-
sorbed at a relative pressure of 0.98. The mesopore volume V
was determined by using the a -plot method from the adsorp-
s
[27]
meso
[26]
Conclusions
tion data in the range of the standard reduced adsorption from 2.1
The copper-containing KIL-2 and FeKIL-2 samples with inter-
particle mesoporosity are prepared by incipient wetness im-
pregnation. The relationship between the type of copper oxide
species generated and the stabilized copper oxide species, the
KIL-2 support and the presence of iron as a second metal are
studied. The type of the generated and stabilized copper oxide
species depends on the presence of iron in the KIL-2 support
and the pretreatment procedure. The formation of copper
oxide nanoparticles that are smaller than 100 nm in size and
are 100 nm in size for FeKIL-2 and KIL-2 respectively, is regis-
tered. The copper-containing KIL-2 sample shows higher cata-
lytic activity in total toluene oxidation if compared to copper-
containing FeKIL-2, whereas copper autoreduction is easier on
FeKIL-2. The presence of iron in the KIL-2 structure leads to the
redispersion and oxidation of metallic copper in the reaction
medium, which results in the formation of finely dispersed
copper oxide clusters (<100 nm). Further investigations on the
optimal relationship between iron content and the particle size
of copper oxide species for optimal catalytic activity of the
supported catalysts are in progress.
to 3.0. In the a -plot calculations, a macroporous silica material Li-
s
2
ꢀ1
Chrospher Si-1000 (S =22.1 m g ) was used as a reference ad-
BET
[19]
sorbent. The pore size distributions (PSDs) were calculated from
nitrogen adsorption data by using an algorithm based on the BJH
[27]
method. The maxima on the PSD were considered as the pri-
mary mesopore diameters for the given samples.
The mesostructure characteristics and presence of copper oxide
was investigated by using high-resolution transmission electron
microscopy (HRTEM) on a 200 kV field-emission gun (FEG) micro-
scope JEOL JEM 2100. Dispersed samples in ethanol were placed
on a carbon-coated Cu grid. The specimens were additionally
coated with carbon to prevent excessive charging of the samples
under the electron beam.
The TPO/TPR-TG investigations were performed in a Setaram TG92
instrument. Prior to the TPR experiments the samples (40 mg)
were treated in situ in an air or argon flow (temperature increase
ꢀ1
rate of 5 Kmin ) up to 773 K, followed by a hold-up of 1 h. In
a typical TPR measurement, the sample was heated in a flow of
3
ꢀ1
5
0 vol% H in Ar (flow rate of 100 cm min ) up to 773 K at
2
ꢀ
1
5 Kmin and a final hold-up of 1 h.
Diffuse reflectance spectra of the samples in the UV/Vis region
were registered by using a Jasco V-650 UV/Vis spectrophotometer
equipped with an integrated sphere. All spectra were recorded
under ambient conditions.
Experimental Section
Catalyst preparation
We prepared porous silica with interparticle mesoporosity, which is
denoted as KIL-2 and iron-modified KIL-2 with Fe/Si molar ratios of
Catalytic activity tests
0
.01, which is denoted as FeKIL-2, under procedures already report-
Prior to the catalytic test the samples were pretreated for 1 h in air
or argon up to 723 K Toluene oxidation was studied at atmospher-
.
[21,22]
ed in the literature.
ic pressure by using a fixed-bed flow reactor, air as carrier gas and
the sample (30 mg, particle size 0.2–0.8 mm) diluted with glass
beads (60 mg) of the same diameter, which was previously
checked to be inactive. The air stream passed through a saturator
filled with toluene and equilibrated at 273 K (ptoluene =0.9 kPa). The
activity was determined in the temperature interval of 550–750 K
An incipient wetness impregnation technique with copper nitrate
was applied for loading of 6 wt% copper oxides and the samples
were designated as Cu/FeKIL-2 and Cu/KIL-2.
Characterization
ꢀ1
at a weight hourly space velocity of 1.2 h . On-line analysis of the
X-ray diffractograms were recorded on a PANanalytical X’Pert PRO
high-resolution diffractometer by using CuKa radiation (l=
reaction products was performed by using high pressure gas chro-
matography (HPGC) with a PLOT Q capillary column (25 m). The
turnover frequency (TOF) was calculated as the converted number
of toluene molecules per metal atoms per second.
1
.5406 ꢁ) in the 2q range from 0.5 to 58 (100 s per step of 0.0348)
and from 5 to 608 (100 s per step of 0.0168) for the samples and
from 10 to 908 (100 s per step 0.0168) for the sample holder by
using a fully opened X’Celerator detector.
Acknowledgements
Morphology and surface properties of the samples were observed
by SEM on a Zeiss Supra 3VP microscope. Elemental analysis of all
samples was performed by the energy-dispersive X-ray spectrosco-
py (EDX) method with an INCA Energy system attached to a Zeiss
Supra 3VP microscope.
Financial support from the Bulgarian Scientific Fund (Bulgaria-
Slovenia bilateral project 01/6), the Slovenian Research Agency
(research program P1-0021, Slovenia-Bulgaria bilateral project BI-
ꢀ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 271 – 277 276

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Keywords: copper autoreduction · silica support · iron-silica
support · silica nanoparticles · interparticle mesoporosity
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Received: June 14, 2013
Revised: September 4, 2013
Published online on November 20, 2013
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 271 – 277 277