Physicochemical properties of hydrogel template-synthesized copper(II) oxide-modified clay influencing its catalytic activity in toluene combustion

Article abstract of DOI:10.1039/c6ra16278a

CuO-modified montmorillonite was synthesized by the template-assisted route. Poly(acrylic acid) was intercalated into the interlayer gallery of natural clay. Subsequently, various amounts of Cu²⁺ cations were introduced into the prepared hydrogel-clay composite using adsorption in different volumes of aqueous Cu(NO₃)₂ solution at constant pH = 6. The resulting materials were finally calcined at 550 °C (chosen using the results of TGA-IR analyses) to transform them into thermally stable oxide systems. The changes in the structural properties of the clay during the progressive modification were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Moreover, porosity, reducibility and surface composition of the calcined materials were determined by means of low-temperature N₂ adsorption, temperature-programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS). It was shown that a high concentration of CuO nanoparticles, which were well-dispersed and protected against sintering between montmorillonite grains, and weak interaction between CuO and the clay support (resulting in easy reducibility of CuO) were the most important features influencing the catalytic activity of the synthesized materials in the total oxidation of volatile organic compounds (VOCs).

Full text of DOI:10.1039/c6ra16278a

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PAPER
Physicochemical properties of hydrogel template-
synthesized copper(II) oxide-modified clay
influencing its catalytic activity in toluene
combustion
Cite this: RSC Adv., 2016, 6, 100373
a
a
a
a
a
Piotr Natkanski, Anna Rokicinska, Anna Wach, Marek Drozdek, Barbara Dudek,
´
´
b
a
Lidia Litynska-Dobrzynska and Piotr Kustrowski*
´
´
´
CuO-modified montmorillonite was synthesized by the template-assisted route. Poly(acrylic acid) was
2+
intercalated into the interlayer gallery of natural clay. Subsequently, various amounts of Cu cations
were introduced into the prepared hydrogel–clay composite using adsorption in different volumes of
aqueous Cu(NO
3
)
2
solution at constant pH ¼ 6. The resulting materials were finally calcined at 550 ꢀ
C
(chosen using the results of TGA-IR analyses) to transform them into thermally stable oxide systems. The
changes in the structural properties of the clay during the progressive modification were studied by X-
ray diffraction (XRD) and transmission electron microscopy (TEM). Moreover, porosity, reducibility and
surface composition of the calcined materials were determined by means of low-temperature N
2
adsorption, temperature-programmed reduction (H -TPR) and X-ray photoelectron spectroscopy (XPS).
2
Received 23rd June 2016
Accepted 6th October 2016
It was shown that a high concentration of CuO nanoparticles, which were well-dispersed and protected
against sintering between montmorillonite grains, and weak interaction between CuO and the clay
support (resulting in easy reducibility of CuO) were the most important features influencing the catalytic
activity of the synthesized materials in the total oxidation of volatile organic compounds (VOCs).
DOI: 10.1039/c6ra16278a
www.rsc.org/advances
using the templating technique, which includes following steps:
Introduction
(i) the formation of micelle structure by a template intercalated
Natural clay minerals, due to their relatively low price and in the clay interlayers, (ii) polymerization of a silica source on
specic properties resulting from a layered structure, have the micelle structures, (iii) removal of the template by calcina-
attracted great attention in many various elds including tion or extraction resulting in the exposition of interlamellar
1
catalysis. However, the catalytic application of clays demands silica pillars.
the opening of their pore structure, which can be achieved, for
example, by separation of adjacent layers by oxide nanoparticles been studied as catalysts in a wide range of reactions, including
e.g. SiO , Al O , TiO , Fe O , ZrO ) introduced into the inter- air pollution control (mainly in selective catalytic reduction of
The noble and transition metal-modied pillared clays have
(
2
2
3
2
2
3
2
2,4,5,8,10,13
layer space. Such a strategy of modication of the layered NOx).
Nevertheless, some examples of using of clay
structure is used in the formation of pillared interlayered clays catalysts in the total oxidation of volatile organic compounds
2–12
14
(PILCs) and porous clay heterostructures (PCHs).
PILCs are (VOCs) can be also found in the literature. VOCs, including
prepared by exchanging the charge-compensating cations various aliphatic/aromatic hydrocarbons and their derivatives,
present in the interlamellar space of clay with large oligomeric are emitted from both natural and anthropogenic sources. The
metal hydroxycations formed by hydrolysis of chosen metal processing and incomplete burning of fossil fuels as well as the
salts. Upon calcination, these polyoxocations undergo dehy- use of solvents are considered as the main anthropogenic
dration followed by dehydroxylation, and are converted into the sources. VOCs present in the atmosphere contribute to photo-
corresponding metal oxide clusters, so called pillars, which link chemical reactions, which give products responsible for the
15
permanently the clay layers. On the other hand, PCHs based on smog phenomenon and the greenhouse effect. On the other
the clay layers pillared by a silica framework are synthesized hand, the increasing concentration of VOCs (containing many
toxic and carcinogenic chemicals) has a harmful effect on the
human and animal health. Methods for the reduction of VOCs
Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krak ´o w, Poland. emission based on recovery (e.g. membrane separation,
a
E-mail: piotr.kustrowski@uj.edu.pl
condensation, sorption) as well as destruction operations (e.g.
thermal, catalytic and photocatalytic oxidation) have been
b
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Reymonta
2
5, 30-059 Krak ´o w, Poland
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16–18
ꢀ
However, in the case of low concen- heated to 65 C and kept at this temperature for 3 h to ensure
therefore developed.
tration emission sources, the catalytic combustion seems to be the complete polymerization process. The synthesized
ꢀ
the most effective approach to the VOCs elimination.
It was found that noble metals (especially Pt and Pd) sup- grinded to ne powder (<125 mm).
ported on clays exhibit very high catalytic activity in the total In the next step, the obtained composite (MT–PAA) was
oxidation of various organic compounds (e.g. aromatic hydro- modied by adsorption of Cu ions from an aqueous of cop-
composite was shredded in a cutting mill, dried at 60 C and
2
+
18–20
carbons and ketones).
A signicant enhancement in the per(II) nitrate solution (0.01 M). MT–PAA (2.0 g) was added to
VOCs conversion was observed for Pd deposited on Al
2
O
3
-pil- various volumes of Cu(NO
3
)
2
solution (100, 200, 300, 400, 500,
ꢀ
lared clays modied additionally with cerium(IV) oxide, which 600 or 700 mL) and kept at a constant temperature (30 C) and
stabilizes the active phase at high oxidation state and good pH ¼ 6 (maintained by an addition of 0.1 M NaOH) on
21–24
dispersion.
It should however kept in mind that the noble a magnetic stirrer (500 rpm). Aer 24 h, the material was
metal-containing materials are expensive and characterized by separated from the solution by centrifugation (3000 rpm, 10
a limited resistance to poisoning. Therefore, signicantly min) and washed with deionized water. The washing operation
2
+
cheaper pillared clays modied with transition metal oxides was repeated three times. Finally, the Cu -modied samples
3
,25–33
ꢀ
(
e.g. Cr, Mn, Ce, Co, Cu, Ni, Fe) have been oen studied.
Li were dried in air at 65 C overnight, and calcined in a tube
ꢁ
1
ꢀ
et al. showed that ne dispersion of active phase nanoparticles furnace in an air ow (33 mL min ) at 550 C for 8 h with
supported by mesoporosity nature of the modied material a heating rate of 1 C min in order to remove the hydrogel
provide a good catalytic behavior in the VOCs combustion.
ꢀ
ꢁ1
34
template. The studied materials were designated as MT–PAA
Recently, a novel method of synthesis of clay catalysts based CuY, where Y represents the volume (mL) of 0.01 M Cu(NO3)2
on using of clay–polymer composites, which is not limited to solution used during the modication.
the introduction of metals forming stable oligomeric hydrox-
ycations (PILCs) or condensed species on the interlamellar
35,36
Characterization
micelles (PCHs), has been proposed.
In the rst step of this
procedure, hydrophilic monomers are intercalated into a clay X-ray diffraction (XRD) patterns were recorded at room
˚
gallery and subsequently in situ polymerized forming a clay– temperature with a Cu anode (l ¼ 1.54184 A) on a X-ray powder
ꢀ
hydrogel composite, which exhibits enhanced swelling and diffractometer (D2 Phaser, Bruker) in two ranges of 2q ¼ 1–50
35,37
ꢀ
ꢀ
metal cation adsorption capacities.
Then, the obtained and 33–41 with a step size of 0.02 and two scan rates equal to 1
composite is modied with transition metal cations by and 5 second per a step, respectively.
adsorption performed at controlled conditions inuencing the
TGA-IR experiments were performed using a TA Instruments
content and distribution of introduced species. Finally, the SDT Q600 thermoanalyzer coupled via a TGA-IR (Thermo
modied sample is calcined to remove the hydrogel template, Scientic) equipment with a FT-IR spectrometer (Nicolet 6700,
and the oxide catalyst containing a highly dispersed active Thermo Scientic). About 20 mg of a sample was heated from 30
ꢀ
ꢀ
ꢁ1
phase is formed.
In the presented paper, the clay–hydrogel composite based min ). 2D FT-IR spectral maps of gaseous products evolved
to 1000 C at a heating rate of 10 C min in owing air (20 mL
ꢁ
1
ꢁ1
on natural montmorillonite and poly(acrylic acid) was synthe- during decomposition were recorded with resolution of 4 cm
2
+
sized and modied by sorption of various amounts of Cu
cations. The subsequent calcination resulted in the formation
collecting 8 scans for each spectrum.
Temperature-programmed reduction (H -TPR) experiments
2
of thermally stable oxide-type catalysts for the total oxidation of were conducted using the same thermoanalyzer. The measure-
chosen aromatic volatile organic compounds. We investigated ments were carried out using 10 mg of a sample placed in
the relationship between various physicochemical properties of a corundum crucible in a ow of mixture containing 10 vol% H
2
ꢁ
1
the synthesized materials (including surface and bulk compo- in argon (60 mL min ). The sample was heated from 30 to
ꢀ
ꢀ
ꢁ1
sition, structure, texture, and reducibility) and their catalytic 1000 C at a rate of 10 C min
.
activity in the above mentioned reaction.
N2 adsorption isotherms were collected using a Micro-
meritics ASAP 2020 instrument. Prior to the measurements,
samples were degassed at 350 C for 6 h under vacuum.
X-ray uorescence (XRF) analyses were carried out on an ARL
Quant'x analyzer (Thermo Scientic) for 300 mg of each sample
ꢀ
Experimental
Synthesis
The clay–hydrogel composite was synthesized by the in situ in a powder form.
polymerization method. The mixture containing natural
X-ray photoelectron spectroscopy (XPS) measurements were
Wyoming montmorillonite (10 wt%) and monomers (10 wt%) – performed on a Prevac spectrometer equipped with a hemi-
0
acrylic acid (AA, Arkema) and N,N -methylenebisacrylamide spherical VG Scienta R3000 analyzer. Samples were introduced
ꢁ
7
(
NMBA, Sigma-Aldrich) at the NMBA/AA molar ratio of 1 : 100 – through a load lock (where pressure better than 3 ꢂ 10 mbar
in water was agitated with a mechanical stirrer (400 rpm) at was achieved) into an ultrahigh vacuum analytical chamber
ꢁ9
room temperature to obtain homogeneous suspension. Then, with the base pressure of 5 ꢂ 10 mbar. The XPS experiments
oxygen was removed from the slurry by purging with argon for were carried out using a monochromatized aluminum source
1
0 min, and ammonium persulfate (APS, Sigma-Aldrich) was AlKa (E ¼ 1486.6 eV) and a low energy electron ood gun
added at the APS/AA molar ratio of 0.01. The suspension was (FS40A-PS) to compensate charge on the surface of
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nonconductive samples. Photoelectron peaks were recorded
with constant pass energy of 100 eV for the survey and high
resolution spectra. The scale of the binding energy value was
adjusted by the position of Si 2p peak at 103.0 eV. The
composition and chemical surrounding of sample surface were
investigated on the basis of the areas and binding energies of
Cu 2p, Si 2p, Al 2p, O 1s, Na 1s, Mg 2p and C 1s photoelectron
peaks. Fitting of the high resolution spectra was provided
through Casa XPS soware.
Transmission electron microscope (Tecnai G2 F20) equipped
with a eld emission gun operating at 200 kV was used. Before
TEM analyses, samples in a powder form were placed on
a nickel grid.
Catalytic tests
The obtained oxide materials were tested as catalysts in the total
oxidation of VOCs (benzene, toluene, o-xylene, m-xylene and p-
xylene). Prior to a catalytic run, a catalyst (0.1 g with a sieved
particle fraction of 160–315 mm) was placed in a central position
of quartz ow microreactor on a quartz wool plug and outgassed
ꢀ
ꢁ1
at 500 C for 30 min in the ow of air (100 mL min ). Then, the
ꢀ
reactor was cooled down to 200 C and VOCs dosing started by
passing air through a saturator (kept at a selected temperature)
to achieve the VOCs vapor concentration of 1000 ppm in the
reaction mixture. The catalytic tests were carried out at 200, 250,
Fig. 1 XRD patterns of raw montmorillonite and MT–PAA composite
before and after deposition of Cu cations.
2+
ꢀ
2
75, 300, 325, 350, 400, 450 and 500 C. The reactor was kept at
ꢀ
ꢁ1
each temperature (reached at a heating rate of 10 C min ) for
0 min with three 25 minutes lasting analyses of reaction
deposition results in a decrease in the distance between the clay
layers, which depends on the volume of copper(II) nitrate solu-
tion used. For the sample modied in the solution containing
the lowest Cu amount (100 mL) the d(001) value is 1.23 nm,
whilst for the material obtained by the contact with the highest
8
products conducted using a Bruker 450-GC gas chromatograph
equipped with three columns (molecular sieve 5A for separation
of O , N and CO, Porapak S – CH , CO and H O, and Chro-
2
2
4
2
2
mosorb WAW-DMCS – aromatic compounds), a thermal
conductivity detector, two ame ionization detectors as well as
a methaniser.
Cu(NO
3 2
) solution volume (700 mL) the d-spacing increases to
1
.68 nm, and is only slightly lower than for MT–PAA. Obviously,
2
+
Cu cations migrate into the interlayer space of composite and
interact with COO groups of PAA. Crosslinking of PAA chains
ꢁ
Results and discussion
Structure and thermal stability of Cu -modied MT–PAA
by transition metal ions cannot be excluded. This effect could
explain the decrease in the d001 value observed aer the Cu
2
+
2
+
deposition. On the other hand, the introduction of MT–PAA
composite in the raising volume of copper(II) nitrate solution
results in more effective swelling of modied material and
growing basal spacing is observed. As is shown by the XRF
composite
The XRD patterns of MT, MT–PAA and MT–PAA CuY materials
aer adsorption of Cu ions are shown in Fig. 1. In the case of
raw clay (MT), the diffraction lines at 9.1, 19.9, 27.9 and 35.2
are attributed to (001), (111), (003) and (211) planes, respectively
2
+
ꢀ
3 2
analysis (Table 1), higher volumes of Cu(NO ) solution enable
2
+
to introduce more Cu cations into the MT–PAA structure. It
should however be kept in mind that a part of PAA macromol-
ecules is most likely deposited on the external clay surface. This
[PDF 00-060-0315]. The interlayer distance (d001) for this
sample, calculated according to the Bragg's equation (nl ¼
2
0
d sin q) based on the position of (001) peak, is equal to
.98 nm. The determined value of d001 parameter is typical of
2
+
hydrogel also participates in adsorption of Cu cations, which
thereby remain outside of the MT interlayers.
dried natural montmorillonite, in which sodium and calcium
cations are located in the ditrigonal cavities between the adja-
cent layers of clay. Moreover, in the studied mineral small
amounts of crystalline impurities, mainly in the form of quartz,
The thermal stability of the MT–PAA composite before and
2+
aer the introduction of Cu cations was studied by TGA. The
gaseous products evolved during the measurements were addi-
tionally analyzed by FT-IR. The recorded TG, DTG and DTA curves
and corresponding 2D FT-IR spectral maps for the MT–PAA, MT–
PAA Cu100 and MT–PAA Cu500 samples are presented in Fig. 2.
For the unmodied composite, the decomposition of poly-
38
ꢀ
are present as conrmed by the (101) diffraction line at 26.6
[PDF 00-001-0649]. An increase in the d(001) value to 1.76 nm is
observed aer the successful intercalation of poly(acrylic acid)
into the interlayer gallery of MT (MT–PAA sample). What is
surprising, the further modication of MT–PAA by the Cu
ꢀ
mer begins at temperature as low as 180 C just aer the
2
+
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Table 1 Cu content as well as textural, structural and redox properties of the calcined materials
N
2
adsorption
XRD
H
2
-TPR
2
ꢁ1
3
ꢁ1
ꢀ
Sample
Content of Cu [wt%]
S
BET [m g
]
V
total [cm g
]
DCuO [nm]
T
red [ C]
H2,consum./H2,theor.
MT
—
3.8
8.8
15.5
19.3
22.2
27.2
28.7
31.0
23.5
28.2
37.4
42.5
50.8
43.4
55.4
0.09
—
19
15
15
14
12
13
14
—
—
MT–PAA Cu100 calc
MT–PAA Cu200 calc
MT–PAA Cu300 calc
MT–PAA Cu400 calc
MT–PAA Cu500 calc
MT–PAA Cu600 calc
MT–PAA Cu700 calc
0.135
0.158
0.209
0.194
0.220
0.164
0.190
267
253
250
250
241
289
305
1.10
0.97
0.90
0.79
0.83
0.75
0.81
ꢀ
dehydration step. Up to 350 C the emission of gaseous, organic decomposition at this step. Nevertheless, Chen et al., who
decomposition products containing carboxyl groups is man- studied the thermal degradation of acrylate–methacrylate
ꢁ
1
ꢁ1
ifested by the FT-IR bands in the ranges of 1760–1780 cm and copolymers by TG-FT-IR, suggested that the band at 1767 cm
ꢁ1
ꢀ
135–1190 cm , assigned to the vibrations of C]O and C–O, observed above 310 C can also correspond to the formation of
respectively. Acrylic acid can be the main product of acid anhydride. Simultaneously, above 260 C the oxidation of
1
39
ꢀ
Fig. 2 TG, DTG and DTA curves as well as FTIR maps recorded during decomposition of MT–PAA, MT-PAA Cu100 and MT-PAA Cu500 in flowing
air.
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the PAA backbone intensies and the formation of huge
amounts of carbon dioxide (identied by the bands at 670–750
ꢁ
1
cm attributed to the bending vibrations, and at 2250–2400
ꢁ
1
cm – the asymmetric stretching vibrations of CO ) and water
2
ꢁ1
ꢁ1
(
the bands at 3450–3750 cm
sponding to the symmetric and asymmetric stretching vibra-
tions and the bending vibrations of H O, respectively) together
with traces of carbon monoxide (the bands at 2110 and 2200
and 1250–2000 cm
corre-
2
ꢁ
1
39,40
cm related to the stretching vibrations of CO) is detected.
Both the DTG curve and the FT-IR map exhibit an additional,
high-temperature step of the sample decomposition observed
ꢀ
in the range of 575–710 C, which is related to two processes
running simultaneously: (i) the combustion of carbon deposit
formed due to incomplete oxidation of the polymer present in
the clay interlayers, and (ii) dehydroxylation of MT layers. These
two independent transformations are reected in the FT-IR map
as the intensive bands characteristic of carbon dioxide (at 670–
ꢁ
1
ꢁ1
7
50 cm and 2250–2400 cm ) and the weak absorption peaks
ꢁ1
attributed to water vapour (at 3450–3750 cm and 1250–2000
ꢁ
1
cm ).
2
+
For the Cu -adsorbed composites, thermal decomposition
of polymer matrix occurs at signicantly lower temperatures
compared to MT–PAA. The decomposition process undergoes in
ꢀ
the temperature range of 200–525 C with the emission of
carbon dioxide and water solely. It could be discerned that the
temperature of complete oxidation of polymer template strongly
ꢀ
depends on the Cu content and decreases from 575 C for MT–
ꢀ
ꢀ
PAA to 525 C and 430 C for MT–PAA Cu100 and MT–PAA
Cu500, respectively. This accelerating effect of Cu species in the
removal of the organic template causes the transformation of
2
+
Cu -modied composites into the nal oxide-type catalysts
ꢀ
possible at the temperature as low as 550 C. Therefore, this
temperature was selected as appropriate to eliminate hydrogel
from the prepared precursors avoiding the destruction of the
montmorillonite structure by the dehydroxylation of layers.
Structural, textural and surface properties of calcined MT–
CuO materials
In the XRD patterns of the calcined copper-containing samples
(Fig. 3A) the diffraction lines typical of the montmorillonite
structure are still present. Nevertheless, a shi of the (001) peak
ꢀ
to higher 2q values (8.9–9.2 ), compared to that position detected
for the samples before the thermal treatment, is observed.
Moreover, a gradual increase in the intensity of (001) diffraction
line in relation to the intensity of (111) reection is found at the materials measured in the 2q ranges of 1–50 (A) and 33–41 (B).
Fig. 3 XRD patterns of calcined MT–PAA and Cu-containing MT–PAA
ꢀ
ꢀ
increasing Cu loading. Obviously, the highly ordered layered clay
structure remains aer the performed modication.
partial dehydroxylation of clay layers and the penetration of
octahedral vacancies by some Cu ions.
+
2+
2+
The behavior of various cations (e.g. Li , Cu , Cd ) in the
clay interlayers upon thermal treatment has been previously
studied by many researchers. Zhu et al. observed the collapse of
layered structure of Cu-modied montmorillonite already
2+
42–44
In the case of the studied materials, calcination performed at
ꢀ
5
50 C results in the complete elimination of hydrogel template.
2+
In a consequence, a signicant part of Cu ions locates in the
interlamellar ditrigonal cavities or even inltrates the MT
layers, especially in the samples with the relatively high
amounts of Cu introduced into the PAA chains. Thus, the
negative charge of MT layers is partly neutralized by Cu
ꢀ
41
slightly above 200 C. On the other hand, the EPR and DTA
study of the Cu-modied montmorillonite calcined at different
ꢀ
2+
temperatures revealed that below 400 C Cu cations are
2+
located mainly in the ditrigonal cavities in the Si–O sublayers,
2+
ꢀ
whereas an increase of temperature above 460 C leads to the
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cations leading to the stabilization of the layered structure. On
2
+
the other hand, at the increasing amount of Cu ions they are
mainly adsorbed by the hydrogel located outside of the MT
galleries. Aer the thermal treatment the introduced species
form CuO nanoparticles highly dispersed on the external
surface of MT. The presence of crystalline CuO phase in the
calcined samples is conrmed by three distinct diffraction lines
ꢀ
ꢀ
ꢀ
at 35.6 , 38.8 and 48.8 (Fig. 3A and B), which correspond to
(
ꢁ111), (111), (ꢁ202) planes, respectively [AMCSD 0018812].
Since the (ꢁ111) reection is overlapped by the (211) reection
attributed to MT, the average crystallite size of CuO (D ) in the
CuO
studied samples was calculated according to the Scherrer
equation based on the (111) line broadening at half the
maximum intensity (FWHM) (Table 1). The DCuO values below
2
0 nm may suggest that poly(acrylic acid), acting as the com-
2
+
plexing agent of Cu cations, separates them and prevents the
formation of larger metal oxide crystals during the thermal
treatment.
The presence of nanocrystalline CuO phase was additionally
conrmed by the TEM measurements (Fig. 4). The collected
images clearly show the existence of CuO crystallites with the
average particle size corresponding to the DCuO values calcu-
lated from the XRD data, highly dispersed between the MT
particles, which evidently protect them against agglomeration.
The textural parameters of the calcined materials were
determined by means of N adsorption–desorption measure-
2
ments at 77 K. The specic surface area (SBET) and total pore
volume (Vtotal) calculated based on the BET and single point
methods, respectively, are demonstrated in Table 1. The ob-
tained results show a slight growth in the specic surface area
and total pore volume with the increasing Cu content. These
textural changes should be attributed to the appearance small
grains of CuO, which located between MT particles, forming
interparticle porosity.
The surface composition and the state of the Cu species in
the catalysts were studied by X-ray photoelectron spectroscopy.
The XPS spectrum of the Cu 2p3/2 region was deconvoluted into
two peaks at 933.2 ꢃ 0.1 eV (peak I) and 935.1 ꢃ 0.1 eV (peak II)
Fig. 4 TEM images of calcined MT–PAA Cu400: (A) bright field mode,
(B) HR image, and (C) SAED pattern.
(cf. Fig. 5), that suggest the existence of at least two chemical
2
+
environments of Cu cations in the MT–CuO materials. More-
sublayers, and are also supported by differences in the catalyst
composition determined by XRF (Table 1) and XPS (Table 2),
which show slightly lower Cu contents on the surface than in
over, the presence of intense satellite peaks (at 941.7 eV and
9
44.0 eV) and the lack of peak at about 932.6 eV conrm an
43,45,46
absence or a presence of negligible amounts of Cu O.
The
2
41,43
the bulk.
peak at the binding energy of 933.2 ꢃ 0.1 eV is characteristic of
2
H -TPR experiments were carried out to recognize reduc-
2
+
43,47
Cu in the CuO phase.
The binding energy of another tted
ibility of the copper species formed in the thermally treated
materials. As presented in Fig. 6, the proles recorded for the
studied samples show one main reduction peak, which nishes
2
+
peak (935.1 ꢃ 0.1 eV) indicates the existence of Cu ions con-
nected to the –OH groups, which were formed on the surface of
CuO particles during an exposure of the samples to a humidi-
ꢀ
2+
at temperature below 340 C. Obviously, Cu species distrib-
45
2+

ed atmosphere. On the other hand, the interaction of Cu
uted in the modied materials are reduced to metallic Cu under
cations with –OH groups from the clay layers is also possible. As
shown in Table 2, the increasing participation of peak I (at 933.2
eV) in relation to peak II (at 935.1 eV) as well as the increasing
the applied H -TPR process conditions in two overlapping
2
32,48
processes:
2 3
Al/Si atomic ratio, attributed to the deposition of Al O particles
on the external surface of MT, are observed with the raising Cu
2
CuO + H
2 2 2
/ Cu O + H O
content in the MT–PAA CuY samples. These ndings conrm
Cu O + H / 2Cu + H O.
2
2
2
2
+
the migration of Cu cations into the ditrigonal cavities and/or
3
+
2+
the partial replacement of the Al cations by Cu ions in the
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Fig. 6 Temperature-programmed reduction profiles of calcined Cu-
containing MT–PAA materials.
mass loss is mainly attributed to the dehydroxylation of
2
+
aluminosilicate support. However, the reduction of Cu ions
Fig. 5 Cu 2p XPS spectra of selected calcined Cu-containing MT–PAA located in the clay layers and the interlayer ditrigonal cavities at
materials.
so high temperatures cannot be also excluded. The values of
H2,consum./H2,theor. ratio between the real amount of consumed
ꢀ
H in the temperature range up to 340 C and the theoretical
2
Only weak shoulder at lower temperatures appears, which
2
+
one (Table 1), calculated assuming the reduction of CuO to
metallic Cu, conrm this supposition. The increase in the
amount of introduced Cu results in the decreasing H2,consum.
2,theor. values exhibiting the existence of part of Cu species
which are very highly stabilized in the structure of the formed
catalysts.
can be attributed to the reduction of surface Cu . Nevertheless,
an inuence of Cu(NO solution volume used during the
modication on the position of H -TPR maximum (Tred) is
3 2
)
/
2
2
+
H
clearly observed for the MT–PAA CuY materials (Table 1). This
ꢀ
peak is centered at 267 C for MT–PAA Cu100, and subsequently
shis to lower temperatures with the increasing Cu content up
ꢀ
to MT–PAA Cu500 (Tred ¼ 241 C). The further increase in the Cu
2
+
loading results in harder reducibility of Cu species attributed
to stronger interaction of formed CuO nanoparticles with the
MT platelets.
Catalytic activity of calcined MT–CuO materials in toluene
combustion
In the collected H
2
-TPR proles, the broad peak in the The Cu-containing clay catalysts were synthesized to be used in
ꢀ
temperature range of 550–750 C is additionally identied. This the total oxidation of volatile organic compounds.
Table 2 Surface composition of the calcined materials determined by XPS
2+
2+
ꢁ
Cu in CuO
peak I)
Cu –OH
Atomic
ratio
(
(peak II)
Sum
Sample
BE [eV]
% at.
BE [eV]
% at.
Peak I/peak II
% at.
Isat/Imp
Cu/Si
Al/Si
Na/Si
MT
—
—
—
—
—
—
—
—
0.36
0.45
0.48
0.53
0.54
0.75
0.71
0.72
0.08
0.80
0.58
0.40
0.31
0.23
0.15
0.09
MT–PAA Cu100 calc
MT–PAA Cu200 calc
MT–PAA Cu300 calc
MT–PAA Cu400 calc
MT–PAA Cu500 calc
MT–PAA Cu600 calc
MT–PAA Cu700 calc
933.3
933.1
933.3
933.2
933.1
933.1
933.1
0.89
1.47
2.01
3.10
4.77
4.54
4.16
935.1
935.0
935.2
935.2
935.2
935.2
935.2
0.76
1.36
1.99
2.56
3.17
3.33
2.91
1.17
1.08
1.01
1.21
1.50
1.36
1.43
1.65
2.83
4.00
5.66
7.94
7.87
7.07
0.43
0.45
0.55
0.54
0.44
0.48
0.45
0.04
0.07
0.10
0.13
0.18
0.18
0.15
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In Fig. 7A the conversion of toluene achieved over the catalytic performance and the Cu content (especially the surface
ꢀ
studied materials in the temperature range of 200–550 C are concentration of Cu species determined by XPS) is found.
shown. In addition, T50 (the reaction temperature required for Nevertheless, it should be kept in mind that dispersion of CuO
50% conversion of toluene) is used to evaluate the catalytic phase also plays an important role inuencing the catalytic
activity of the catalysts (Fig. 7B). It should be noted that carbon activity. For all synthesized catalysts, the mean crystallite size of
dioxide and water were the main products formed during the CuO is very similar and mainly nanoparticles lower than 20 nm
toluene combustion in the presence of investigated samples. are identied. However, at the higher CuO concentrations, the
Only traces of benzene, a by-product formed with selectivity interaction between CuO active phase and aluminosilicate
lower than 0.05%, were detected.
support becomes too strong. In a consequence, the materials
It is clear that the presence of CuO species signicantly with the highest Cu loadings show harder reducibility and lower
enhanced the catalytic activity of MT. The value of T decreases activity (due to the most probable Mars–van Krevelen mecha-
5
0
ꢀ
with the increasing Cu content from 427 C (MT–PAA Cu100) to nism involving lattice oxygen), compared to the materials con-
ꢀ
3
10 C (MT–PAA Cu500), and then grows slightly for the mate- taining relatively lower Cu amounts. Moreover, it should be kept
ꢀ
2+
rials with the highest Cu loadings (MT–PAA Cu600 – 313 C and in mind that Cu species built-in the interlayers and sublayers
ꢀ
MT–PAA Cu700 – 317 C). Thus, correlation between the are catalytically inactive being inaccessible for the reacting
molecules.
Among the studied samples, the calcined MT–PAA Cu500
material with an optimal content and high dispersion of easily
reducible CuO active phase appeared to be an ideal catalyst for
49
the toluene abatement. Kim showed that among various
transition metal oxides, CuO supported on g-Al was found to
2 3
O
be the most promising catalyst in the aromatic VOCs combus-
ꢀ
tion. Nevertheless, those results (T50 ꢄ 265 C) were achieved in
the presence of higher amount of catalyst (0.5 g) compared to
ꢀ
ꢀ
the present study (T50 ꢄ 310 C). The T50 values of 303–307 C
over 0.1 g of a catalyst at toluene concentration of 1000 ppm
were observed for the CuO–Al O systems containing more than
2
3
50
5
0 wt% of Cu. In order to increase in the catalytic activity of
CuO-based materials, they were promoted by other various
51–53
53,54
55
components (e.g. MnO
x
,
CeO
2
,
ZrO
2
). Thus, the main
advantage of the materials developed in this study is high
catalytic activity at a relatively low CuO content and absence of
expensive additives.
The activity of the MT–PAA Cu500 catalyst was tested in the
total oxidation of other aromatic VOCs (benzene, o-xylene, p-
xylene and m-xylene) as well. The collected results are presented
in Fig. 8. Only CO and H O were formed as the products of
2 2
these reactions. It was found that the activity with respect to the
VOC molecule follows the sequence: toluene > xylenes >
Fig. 7 Conversion of toluene (A) and values of T50 (B) determined over Fig. 8 Conversion of various aromatic volatile organic compounds
calcined MT–PAA and Cu-containing MT–PAA materials.
determined over calcined MT–PAA Cu500.
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49
benzene. The similar sequence was previously reported by Kim
9 C. B. Molina, A. H. Pizarro, J. A. Casas and J. J. Rodriguez,
for 5 wt% Cu/g-Al O . Obviously, benzene with the highest
Appl. Catal., B, 2014, 148–149, 330.
2
3
ionization potential is the most stable molecule, which 10 L. Chmielarz, P. Ku ´s trowski, Z. Piwowarska, B. Dudek, B. Gil
demands using higher temperatures to be oxidized.
and M. Michalik, Appl. Catal., B, 2009, 88, 331.
1 L. Chmielarz, B. Gil, P. Ku ´s trowski, Z. Piwowarska, B. Dudek
and M. Michalik, J. Solid State Chem., 2009, 182, 1094.
2 R. M. A. Saboya, J. A. Cecilia, C. Garc ´ı a-Sancho, F. Murilo,
T. Luna, E. Rodr ´ı guez-Castell ´o n and C. L. Cavalcante Jr,
Appl. Clay Sci., 2016, 124–125, 69.
1
1
Conclusion
The hydrogel template-assisted route, based on (i) the interca-
lation of poly(acrylic acid) into the interlayers of montmoril-
lonite, (ii) the subsequent modication of the formed 13 L. Chmielarz, Z. Piwowarska, P. Ku ´s trowski, A. W ˛e grzyn,
2
+
composite with Cu cations using adsorption in an aqueous
B. Gil, A. Kowalczyk, B. Dudek, R. Dziembaj and
solution of Cu(NO ) at constant pH ¼ 6, and (iii) nally calci-
M. Michalik, Appl. Clay Sci., 2011, 53, 164.
nation at elevated temperature of 550 C, allowed us to 14 S. Zuo, Q. Huang and R. Zhou, Pillared Clays and Related
3
2
ꢀ
synthesize CuO-containing clay catalysts. It was shown that the
increasing Cu content introduced into the material resulted in
Catalysts, ed. A. Gil, S. A. Korili, R. Trujillano and M. A.
Vicente, Springer-Verlag, New York, 2010, ch. 9.
a higher order of layered clay structure attributed to the 15 R. Atkinson, Atmos. Environ., 2000, 34, 2063.
2
+
migration of part of Cu cations into the clay interlayers and/or 16 A. Berenjian, N. Chan and H. J. Malmiri, Am. J. Biochem.
even incorporation of these ions in the montmorillonite sub-
Biotechnol., 2012, 8, 220.
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nanoparticles, highly dispersed between montmorillonite 13, 527.
grains, aer the thermal treatment. The produced MT–CuO 18 J.-J. Li, Z. Jiang, Z.-P. Hao, X.-Y. Xu and Y.-H. Zhuang, J. Mol.
composites showed the open pore structure and high stability of Catal. A: Chem., 2005, 225, 173.
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