Highly Active Noble-Metal-Free Copper Hydroxyapatite Catalysts for the Total Oxidation of Toluene

Article abstract of DOI:10.1002/cctc.201601714

Hydroxyapatite (Hap) supported Cu materials prepared by the wet impregnation method were designed as noble-metal-free catalysts for the total oxidation of toluene. Cu/Hap materials with different Cu loadings (2.5–20 wt %) calcined at 400 °C were characterized by using inductively coupled plasma optical emission spectroscopy, N₂ physisorption, XRD, Raman spectroscopy, IR spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion MS. The tenorite CuO phase was detected in all materials, and the libethenite Cu₂(PO₄)OH phase was observed for the sample with 20 wt % Cu. The presence of libethenite was accompanied by the formation of Ca₂CO₃H⁺ ions at the Hap surface. Residual NO₃ ^? species that interact with Cu and Ca were also found, and their amount increased with the increase of the Cu content in the sample. Interestingly, the specific activity in the total oxidation of toluene increased with the decrease of the Cu content in the catalyst. The rate per mole of Cu was increased by 10 times if the Cu content was decreased by four times. This noticeable result could be related to the presence of acid sites with a moderate strength as well as finely dispersed CuO species on the Hap, which allow the activation of toluene molecules and their oxidation through a redox mechanism. Moreover, Cu2.5 wt %/Hap showed a remarkably stable catalytic performance for 45 h time-on-stream, which evidences that this material has a high potential for applications in the removal of volatile organic compounds.

Full text of DOI:10.1002/cctc.201601714

Accepted Article
Title: Highly active noble metal-free copper-hydroxyapatite catalysts for
toluene total oxidation
Authors: Dayan Chlala, Jean-Marc Giraudon, Nicolas Nuns, Madona
Labaki, and Jean-François Lamonier
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10.1002/cctc.201601714
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Highly active noble metal-free copper-hydroxyapatite catalysts
for toluene total oxidation
Dayan Chlala,[a,b] Jean-Marc Giraudon,[a] Nicolas Nuns,[a] Madona Labaki,[b] and Jean-François
Lamonier*[a]
Abstract: Hydroxyapatite (Hap)
-
supported copper materials
One of the best efficient methods to reduce this harmful VOC in
environment is the catalytic total oxidation in air into carbon
dioxide and water. [1] This technology saves energy by shifting to
lower values the temperature needed for non-catalytic oxidation,
as well as it reduces the noxious byproducts formation.[2,3]
Although supported noble metal based catalysts show excellent
activities for the VOC total oxidation at low temperatures,[4,5] their
high cost limit their wide applications. Thus, the design and
development of economic noble metal-free catalysts with
comparable or superior activity and stability than noble metal or
noble metal oxide based catalysts is a major research area for the
sake of development of more sustainable industrial chemical
processes.
Transition metal oxides have been found to be active in VOC
oxidation. Moreover in comparison with noble metals these oxides
show lower cost, superior resistance to poisoning and higher
thermal stability.[6,7] However their activity at low temperature
must be improved in order to compete noble metal based
catalysts.
Copper oxides are considered to be active and selective in the
oxidation of toluene.[8,9] Wang[10] demonstrated that copper
deposited on alumina exhibited the best catalytic performance
with respect to the complete oxidation of toluene among the
different transition metals (Fe, Mn, Cr, Co, Mo and Ni). In addition,
Saqer et al.[11] found that copper oxide supported on γ-Al2O3
exhibits the highest catalytic activity in the oxidation of toluene
(about 100 % conversion at 350 °C) compared to the other
transition metal oxides (CeO2, CsO, ZrO2, MnO, V2O5, MgO,
Nd2O3 and Cr2O3). This result was linked to the better dispersion
of the active phase on this support.
Another key parameter that controls the activity, selectivity and
stability of the supported metal oxides materials is the support
composition. Deng et al.[12] found that by incorporation of
appropriate amount of ZrO2 in Cu-CeO2 systems, the oxygen
mobility of the support is enhanced and the toluene oxidation is
achieved at lower temperature. Carabineiro et al.[13] correlated the
total acidity of the catalysts and their activity in toluene oxidation,
highlighting the role of acidity in the VOC oxidation process. High
concentration of acid sites with moderate strength was favorable
for the catalytic activity.
prepared by the wet impregnation method were designed as noble
metal-free catalysts for toluene total oxidation. Cu/Hap with different
Cu loading (from 2.5 wt% to 20 wt%), calcined at 400 °C, were
characterized by inductively coupled plasma optical emission
spectroscopy, N2 physisorption, X-ray diffraction, Raman, infrared, X-
ray photoelectron and time-of-flight secondary ion mass
spectroscopies. Tenorite CuO phase was detected in all materials,
while additional libethenite Cu2(PO4)OH phase was observed for
sample with 20 wt% Cu. The presence of libethenite was
accompanied by the formation of Ca2CO3H+ ions at the hydroxyapatite
-
surface. Residual NO3 species in interaction with Cu and Ca were
also found, their amount increasing with Cu content in the sample.
Interestingly the specific activity in the toluene total oxidation
increased with the decrease in Cu content in the catalyst. The rate per
mole of copper was increased 10 times when the copper content was
4 times reduced. This noticeable result could be related to the
presence of acid sites with moderate strength as well as fine
dispersed CuO species on the hydroxyapatite, allowing the toluene
molecules activation and their oxidation through a redox mechanism.
Moreover Cu2.5wt%/Hap showed remarkable stable catalytic
performance for 45 h time-on-stream, providing evidence that this
material has a high potential for Volatile Organic Compound catalytic
removal application.
Introduction
Volatile Organic Compounds (VOC) are an important group of air
pollutants which particularly harm human health and environment.
Among them toluene is widely distributed in the environment. The
main source of release into the environment is related to the high
presence of toluene in gasoline. In this case, toluene is emitted
either directly during the vaporization of gasolines (petrol station,
transport and storage of fuels ...), or in the exhaust gases of
gasoline-powered vehicles (unburned, volatilization, etc.). Other
emissions come from the vapors of toluene used as a solvent,
production discharges and incineration discharges.
Hydroxyapatite Ca10(PO4)6(OH)2 (Hap) based materials, are safe,
non-toxic and inexpensive, and have attracted great attention in
several fields such as medical and biological applications,
applications in archeology, optics and in chemical applications.[14]
Furthermore, these compounds present a high chemical and
thermal stability, and have a structural flexibility. First, Hap
contains both acidic and basic sites with moderate strength [15] in
a single crystal lattice, hence it can be used in reactions requiring
bifunctional solid catalyst, such as the Friedel-Crafts reactions [16]
and the Guerbet reaction.[17] Chen et al.[18] have prepared
Manganese oxides loaded on various supports (SBA-15, MgO,
[a]
D. Chlala, J.-M. Giraudon, N. Nuns, J.-F. Lamonier
Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 –
UCCS – Unité de Catalyse et Chimie du Solide, F-59000 Lille,
France
E-mail: jean-francois.lamonier@univ-lille1.fr
[b]
D. Chlala, M. Labaki
Lebanese University, Laboratory of Physical Chemistry of Materials
(LCPM)/PR2N, Faculty of Sciences, Fanar, BP 90656, Jdeidet El
Metn, Lebanon
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201601714
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Al2O3, TiO2 and Hap) with different acid-base properties, to study
the direct imine formation by oxidative coupling of alcohols and
amines. They reported that hydroxyapatite (with amphoteric
nature) supported manganese oxides (MnOx/Hap) showed the
best activity and selectivity for this reaction compared to the inert
(SBA-15), basic (MgO) or acidic (Al2O3 and TiO2) support.
Furthermore Hap is recognized for its ability to easily exchange
calcium ions exposed on the surface. For example the doping of
*
*
*
+
*
+
*
*
*
Cu20Hap
+
*
*
*
*
+
+
*
*
*
+
+
+
Cu10Hap
+
+
[19]
Hap with a Cu active phase has been achieved by Qu et al.
Five possible sites for Cu location has been proposed based on
the Hap structure and experimental results. Qu et al. claimed that
owing to the method of preparation (impregnation or co-
precipitation) copper location can be adjusted in the Hap structure.
Recently, hydroxyapatite was used for the first time as a support
for Pd and Mn species in the VOC catalytic oxidation
removal.[20,21] The Pd based catalytic formulations competed with
the best palladium systems described in literature, showing an
evidence that hydroxyapatite support could substitute
conventional ones such as zeolite or alumina. Indeed at 150 °C,
the specific rates on hydroxyapatite supports were found to be
four to six times higher than the specific rate on alumina support
in the toluene oxidation reaction.[20] As for manganese,[21] we
found that the hydroxyapatite support allowed to get well
dispersed MnOx species using Mn(II) nitrate as Mn source and
total conversion of toluene at 220 °C was obtained over MnNit-
Hap catalyst. Finally Aellach et al.[22] demonstrated the effect of
the Co species introduction in the Hap material on their catalytic
performances in the methanol total oxidation. Better Co3O4
species reducibility and activity were obtained when Co species
have been wet impregnated on hydroxyapatite.
Cu5Hap
Cu2.5Hap
Hap
10 15 20 25 30 35 40 45 50 55 60 65
2θ / º →
Figure 1. XRD patterns of pure Hap and CuxHap samples
+: CuO (PDF no. 00-048-1548) and *: Cu2(PO4)(OH) (PDF no. 01-077-0922)
The CuO monoclinic phase (PDF no. 00-048-1548) known as
tenorite with space group C2/c (five diffraction peaks marked by
(+) in Figure 1) has been detected in all the CuxHap samples.
The intensity of these peaks increased with the copper content
from 2.5 to 10 wt%, and then remained constant for the content
of 20 wt% of copper. An additional phase (fourteen diffraction
peaks marked with (*) in Figure 1) corresponding to the
orthorhombic phase Cu2(PO4)(OH) (PDF no. 01-077-0922)
known as libethenite with space group Pnnm is clearly observed
for Cu20Hap sample. The mean crystallite size of CuO and
Cu2(PO4)(OH) determined from the Rietveld refinement are
summarized in Table 1. The crystallite size of CuO increased from
12 to 42 nm with copper increase up to 10 wt% and then slightly
decreased to 33 nm for Cu20Hap sample. For the latter, the
Cu2(PO4)(OH) crystallite size is 78 nm.
The goal of the present study was to investigate how the amount
of impregnated Cu influences the performance of
Cu/hydroxyapatite when operating in the reaction of toluene
oxidation. In particular the effect of copper loading was evaluated
on the activity, selectivity and stability of Cu/Hap materials and
discussed in relation to their physicochemical properties.
Results and Discussion
Table 1. Chemical composition, crystallite size and textural properties of the
samples
1. Catalyst characterizations
Dc[a]
CuO /
nm
Dc[a]
Cu2PO4OH
/ nm
1.1. Structural and textural characterizations
[c]
[d]
Cu
wt.%
SSA
Vp
Dp
Sample
/ m2.g-1
/ cm3.g-1
/ nm
The chemical composition values (Table 1) indicate that for the
impregnated samples, copper content was close to that expected.
The X-ray diffraction patterns obtained at room temperature for
Hap and CuxHap materials calcined at 400 °C are shown in
Figure 1. The Hap solid exhibits diffraction lines corresponding to
the hydroxyapatite phase Ca5(PO4)3OH (PDF no. 00-009-0432)
confirming that the hydroxyapatite crystallizes in the hexagonal
system with space group P63/m.
The impregnation of copper on Hap does not affect the structure
of the support as its diffraction peaks position and intensity do not
significantly change with the addition of Cu and no loss of
crystallinity is observed.
Hap
-
-
-
-
103
92(92)[b]
85
0.58
0.47
0.45
0.35
0.12
57
50
50
55
63
Cu2.5Hap
Cu5Hap
Cu10Hap
Cu20Hap
2.8
5.2
9.8
16.7
12
36
42
33
-
-
72(95)[b]
21
78
[a] Crystallite size. [b] After test of stability. [c] Pore volume. [d] BJH desorption
maximum pore diameter
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The nitrogen adsorption-desorption isotherms for Hap and Hap-
supported Cu samples (Figure S1) display the characteristic
hysteresis loop of a Type IV isotherm (IUPAC) lying in the P/P°
range of 0.7-1 exhibiting mesoporous character. However the
pore diameter of all the solids (not shown here) is non-uniform
and exhibited a broad distribution ranging from 5 to 160 nm, with
the maximum distribution of pore at around 57 ± 7 nm.
The specific surface area (SSA) and pore volume values of Hap
were 103 m2.g-1 and 0.58 cm3.g-1, respectively. These values
decrease when Cu content increases in the sample. For Cu20Hap
the decrease is much more pronounced, the SSA and pore
volume values being reduced by a factor of about 5 (Table 1).
This can be explained by a partial blocking of the pores of the
support by CuO and Cu2(PO4)(OH) crystallites, both phases
identified by XRD in Cu20Hap.
*
*
*
*
Cu20Hap
*
*
Cu10Hap
Cu5Hap
Cu2.5Hap
Hap
1.2. Infrared and Raman analyses
4000 3500 3000 2500 2000 1500 1000
Wavenumber / cm-1
500
→
Figure 2 shows the IR spectra of Hap and CuxHap materials
focused on 500-4000 cm−1 region. No shift in wavenumber was
observed for the different vibration bands.
Figure 2. FT-IR spectra of Hap and Hap-supported Cu materials focused on
500-4000 cm-1
The PO4 vibrational modes are observed at 565 cm−1, 602 cm−1,
962 cm−1 and 1030-1100 cm−1, as reported previously.[23,24] The
presence of adsorbed water is also detected in the range 3300-
3600 cm−1 and at about 1637 cm−1. The stretching and bending
vibration modes of OH structural groups are detected at 3572 and
For the copper impregnated solids, additional peaks at 294 cm-1,
343 cm-1 and 627 cm-1 are observed (Figure 3a). These lines are
attributed to CuO[29] and their intensities are increasing with the
copper content in the solid.
2-
633 cm−1, respectively. The presence of CO3 species with
characteristic vibration bands located at 872 cm−1, 1450 cm−1 and
1419 cm−1 is confirmed in all the materials.[25,26] Furthermore,
2-
Table 2. Raman line positions and assignment for the investigated materials
HPO4 groups might also be present, with the characteristic
vibration band at 872 cm−1. However, this latter overlaps with the
carbonate vibration band, making straightforward conclusions
Wavenumber / cm-1
-
impossible.[27] In addition, the presence of residual NO3 species
Sample
2-
-
Hap
(H)
CuO
(C)
Cu2PO4OH
(L)
CO3
(c)
NO3
(N)
is attested by the presence of a sharp nitrate band in the vicinity
of 1385 cm−1. Copper addition to the hydroxyapatite support
results in an increase in the intensity of this absorption band. This
result indicates an incomplete nitrate decomposition even after
the calcination step (4 h at 400 °C in air flow). For Cu20Hap
sample the NO3- band is very intense. This is also accompanied
by an increase in the intensity of the absorption band
characteristic of carbonates. Moreover, the infrared spectrum of
Cu20Hap shows additional bands (represented by * in Figure 2)
corresponding to (i) the stretching and bending vibrations of OH
units in the libethenite phase detected at 3405 cm-1 (with a
shoulder at 3457 cm-1) and 810 cm−1, respectively and (ii) the PO4
groups observed at 646 cm−1, 917 cm−1 and 969 cm−1.[28] This
result is in agreement with XRD one.
429,446,578,
588,606,962,
1047,1076
Hap
-
-
-
-
-
-
-
-
-
-
-
-
428,445,578,
588,606,962,
1042,1076
294,342,
627
Cu2.5Hap
Cu5Hap
Cu10Hap
428,444,578,
587,605,961,
1041,1076
295,341,
627
427,445,578,
586,609,961,
1048,1076
293,341,
627
-
-
Figure 3 shows the Raman scattering spectra of the various
materials. Hap sample exhibits the characteristic peaks of the
hydroxyapatite phase, since the line positions of the phosphate
vibration modes given in Table 2 are in agreement with those of
the literature.[22]
Cu20Hap
(1)
430,452,579,
593,606,960
299,345,
629
740
738
1056
1056
-
299,453,
554,621,
648,980,
1020
Cu20Hap
(2)
299,347,
627
450,960
Cu20Hap
(3)
299,348,
632
738,
1066
450,579,960
-
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Thus, these three Raman spectra indicate the presence of two
different copper phases for the Cu20Hap solid: CuO and
Cu2(PO4)(OH) in addition to the presence of residual NO3 and
a
-
H
CO32-, whether or not included in hydroxyapatite. Finally, the
Raman results are in agreement with the IR results which showed
the presence of the nitrate and carbonate bands in higher intensity
for this sample compared to the other solids.
H
C
H
H
C
C
Cu10Hap
1.3. XPS analyses
Cu5Hap
For Hap and CuxHap samples, the binding energy (BE) values for
Ca 2p3/2, P 2p3/2 and O 1s were found to be at 347.7, 133.5 and
531.6 eV (± 0.3 eV), respectively. These BE values are similar to
those reported for pure hydroxyapatite in the literature,[36,37]
suggesting that copper species do not influence significantly the
chemical environment of these species.
The XPS spectra of the N 1s region (Figure S2) show typically 3
photopeaks located at 400, 404 and 408 eV. The low BE
photopeak belongs likely to ammonium groups and could be
related to the use of (NH4)H2PO4 precursor for the synthesis of
the hydroxyapatite support. The two other photopeaks at 404 and
408 eV have been assigned respectively to nitrite and nitrate ions
originating most probably from the Cu(NO3)2.3H2O precursor. The
contribution of nitrates to N 1s region increases with the copper
content in agreement with the IR results.
Cu2.5Hap
Hap
200
400
600
800
1000
1200
1400
Wavenumber / cm-1
→
c
b
Due to the in-situ copper species reduction during the XPS
analysis it has not been possible to study the copper oxidation
state in CuxHap. It is interesting to notice that the greater the
copper content in the sample is, the less this in-situ reduction
phenomenon will be visible in the Cu 2p spectrum. For Cu20Hap
sample, copper species are retained as Cu2+ along the XPS
analysis, while for CuxHap (2.5 wt.% ≤ x ≤ 10 wt.%) reduced
copper (Cu+ and/or Cu0) was detected on the surface. Semi-
quantitative XPS analysis is however possible. Bulk and XPS
Cu/(Ca+P) ratios are listed in Table 3. In a general way, XPS
Cu/(Ca+P) ratios are lower than those obtained from ICP analysis,
except in the case of Cu2.5Hap for which the two ratios are almost
equal. When copper content in the material increases, the
difference between the two ratios increases (Table 3). That
means, when the copper loading increases, part of the copper
becomes not visible by XPS, and consequently can be suspected
to be present in the form of >10 nm particles (taking into account
that the analysis depth of XPS is close to this value).
c
H
C
C
3
2
N
L
L
L L
C/L
L
H/L
H
H
1
200
400
600
800
1000
1200
1400
Wavenumber / cm-1
→
Figure 3. Raman spectra of (a) Hap, Cu2.5Hap, Cu5Hap and Cu10Hap and (b)
Cu20Hap. (H: Hap; C: CuO; L: Cu2(PO4)(OH); N: NO3- and c: CO3
2-
)
Laser beam focused on different locations led to different Raman
spectra for Cu20Hap, suggesting an heterogeneity in the
composition of this sample (Figure 3b). New lines have been
detected in addition to the characteristic lines of the Hap and CuO
phases. Spectrum 1 shows an intense peak at 1056 cm-1
characteristic of nitrates[30,31] and another peak at about 740 cm-1
which could be attributed to carbonates inserted in
hydroxyapatite.[32] Spectrum 2 reveals seven other lines at 299,
453, 554, 621, 648, 980 and 1020 cm-1 likely related to the
libethenite phase.[33] In spectrum 3 two lines are clearly observed
Table 3. Atomic bulk and surface ratios for CuxHap samples; T10, T50, T90 and EA
obtained in toluene oxidation for Hap and CuxHap catalysts
Cu/(Ca+P)
ICP
Cu/(Ca+P)
XPS
T10
/ °C
T50
/ °C
T90
/ °C
EA /
kJ.mol-1
Solid
Hap
-
-
293
196
200
208
390
215
220
232
>400
231
238
246
-
Cu2.5Hap
Cu5Hap
Cu10Hap
0.030
0.057
0.122
0.027
0.024
0.038
108
114
124
at 738 and 1066 cm-1 which can be derived from two characteristic
2-
vibrational modes of the CO3
hydroxyapatite.[32,34,35]
group inserted in
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CaNO2+
Cu20Hap
0.267
0.091
223
248
277
158
This result is in line with the evolution of mean crystal domain size
evaluated from X-ray line broadening of XRD patterns. With
average crystallite size of 12 nm for CuO in Cu2.5Hap sample
(Table 1), whole copper is expected to be visible by XPS. Besides
XPS and bulk Cu/(Ca+P) ratio values are very close. With mean
crystal domain sizes for CuO higher than 33 nm in CuxHap (x  5
wt.%) samples (Table 1), copper is not expected to be fully
observable by XPS, that explains why XPS Cu/(Ca+P) ratio
becomes significantly lower than those obtained from ICP
analysis (Table 3).
Cu20Hap
Cu10Hap
Cu5Hap
Hap
Cu2.5Hap
1.4. ToF-SIMS analyses
ToF-SIMS has been performed in order to get understandings into
the elemental composition in the outer-most atomic layers of
samples. The high sensitivity of the ToF-SIMS analysis (study of
trace levels up to 1 ppm) allows to provide molecular information
about surface and interfaces of the materials through the
detection of molecular ions.
85.85
85.9
85.95
m/z →
86
86.05
CuNO3-
Cu20Hap
It is found the characteristic secondary ions of hydroxyapatite in
the negative and positive spectra of Hap (Figure S3), as
previously reported.[21] The ToF-SIMS spectra of the CuxHap
samples identify the presence of residual NO3- in interaction with
+
copper and calcium demonstrated by the presence of the CaNO2
and CuNO3- ions with m/z = 86 and 125.5 respectively. Figure 4
clearly shows that the intensity of these ions increases when
copper is added to Hap. This increase is moderate and in the
same extent for Cu2.5Hap, Cu5Hap and Cu10Hap. However for
Cu20Hap a huge increase of CaNO2 and CuNO3 normalized
intensities is observed, which is in agreement with IR results.
Cu5Hap
+
-
Cu2.5Hap
Hap
Cu10Hap
-
Moreover, Cu2PO4 ion (m/z = 222.8) was detected only in the
ToF-SIMS (-) spectrum of Cu20Hap solid, which can be correlated
with the presence of the libethenite phase detected by XRD, IR
and Raman for this sample.
124.8
124.9
125
125.1
m/z →
Figure 4. Normalized counts for CaNO2+ and CuNO3- peaks for CuxHap
Lishil et al. showed using LEIS technique that Ca ions and O ions
are more exposed to the first atomic layer of hydroxyapatites,
irrespective of the Ca/P ratio.[15] Therefore the calcium enrichment
in the first top layer well explains the formation of Ca2CO3H+ ions
at the surface of the hydroxyapatites (Figure 5a). The normalized
intensity of Ca2CO3H+ ions is much bigger for Cu20Hap.
Additionally, a semi-quantitative study based on ToF-SIMS
showed that the Ca+/P+ ratio is 27.4 for Hap support (Figure 5b)
and it remains constant after copper addition up to 10 wt% in Cu.
However, for the highest copper content (20 wt%), this ratio
increases to reach 123.3. This result could be directly linked to
the libethenite phase formation. Some phosphates are no more
used for hydroxyapatite formation, thus calcium species are able
to react with CO2 to form calcium carbonates (Figure 5a).
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125
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Ca2CO3H+
Cu20Hap
b
a
123.3
100
75
50
25
0
Cu10Hap
Hap
Cu5Hap
140.7
140.9
141.1
141.3
m/z →
25.5
30.2
27.5
27.4
2.5
7.5
12.5
17.5
Cu content / wt% 
Figure 5. (a) Normalized counts for Ca2CO3H+ peak for Hap and Cu5Hap,
Cu10Hap and Cu20Hap and (b) Ca+/P+ ratio for Hap and CuxHap samples
Figure 6. Rate of toluene conversion in function of copper content in CuxHap.
Feed composition: 800 ppmv toluene in air, temperature: 215 °C
The apparent energy activation EA values for toluene oxidation
over CuxHap catalysts have been calculated from the Arrhenius
plots (Figure S5). EA value depends on the copper content in the
sample (Table 3). The lower the EA value is, the easier the toluene
can be oxidized. The values of EA over CuxHap were close to
those (~100 kJ.mol-1) over CuO/Al2O3.[38] The lowest EA value for
Cu2.5Hap well explained the highest activity of this catalyst in the
toluene oxidation.
2. Catalytic performances
2.1. Catalytic oxidation of toluene
The evolution of the toluene conversion into CO2 over Hap and
CuxHap catalysts in function of temperature is illustrated in
Figure S4. T10, T50 and T90, the temperatures at which 10 %, 50 %
and 90 % of toluene are converted into CO2 respectively, have
been extracted from these light-off curves and are listed in Table
3. It is worth to note traces of benzene and benzaldehyde
products in the temperature range [130–220 °C] when toluene is
not fully converted.
Cu addition remarkably enhances the toluene conversion of the
Hap support. 90 % of toluene conversion is achieved for Cu based
catalysts at temperature below 277 °C while for pure Hap the
required temperature is higher than 400 °C. Toluene conversion
into CO2 based on T50 (°C) decreases as follows: Cu2.5Hap (215)
> Cu5Hap (220) > Cu10Hap (232) > Cu20Hap (248) >> Hap (390).
Thus the improvement in toluene conversion observed over
CuxHap seems to be related to the decrease in the copper
content in the sample. Then the catalytic activity was compared
by expressing the rate of toluene conversion per mole of copper
and evaluated at 215 °C (r215) (Figure 6). Similar order of
decreasing activity (mol.h-1.mol-1Cu) was obtained: Cu2.5Hap
(1.116) > Cu5Hap (0.385) > Cu10Hap (0.096) >> Cu20Hap
(0.019). It is clear that the catalyst with the lower copper loading
has by far the highest rate: r decreased by ten and sixty times
when the reaction was performed on Cu10Hap and Cu20Hap
respectively instead of Cu2.5Hap.
Table 4 lists the catalytic properties (T50) of various Cu based
materials obtained in toluene (various concentrations) total
oxidation. Clearly this comprehensive overview highlights the
remarkable catalytic activity of Cu2.5Hap materials. In particular
Wang[10] and Kim[39] have both investigated the catalytic oxidation
of toluene over Cu-based γ-Al2O3 catalysts, Cu loading varying
from 1 to 15 wt%. They reported 5 wt% Cu/γ-Al2O3 as the most
active catalyst owing to the well-dispersed copper (+II) species at
the alumina surface. Above 5 wt% Cu content in the sample, large
CuO crystals formation led to the catalytic activity decrease.
Therefore the better activity achieved over Cu2.5Hap for toluene
oxidation can be related to the presence of well dispersed Cu
species on the hydroxyapatite. Increasing the copper content in
the sample led to the formation of CuO with higher crystallite size
(Table 1) which are less active in toluene oxidation. The huge
promotion of the activity per mol of copper (Figure 6) puts forward
that the copper dispersion is not the sole parameter influencing
the catalytic activity. Besides comparing the catalytic performance
of Cu2.5Hap to that of Cu5%/Al2O3,[39] the T50 is lowered by 60 °C
suggesting that the Hap support could contribute to activity
increase. Soylu et al.[40] and Carabineiro et al.[13] showed that the
acidity of the catalyst plays a dominant role in toluene oxidation
reaction. The toluene adsorption takes place with methyl and
phenyl groups interaction with the catalyst surface, followed by H
and C subtraction.[41-43] In particular Carabineiro et al.[13] proposed
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10.1002/cctc.201601714
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that toluene must be not too strongly adsorbed (weak acid sites)
in order to favor afterward the oxygen attack from the
surface/lattice of the catalyst. It is well known that hydroxyapatites
are not inert supports and can develop both acid and basic
reduction may contribute to the H2 consumption and makes the
interpretation of the reducibility of copper by H2-TPR impossible.
Therefore the activation of the catalysts during the first six hours
and the increase of SSA value of Cu10HapU in comparison with
SSA value of fresh Cu10Hap which contains higher nitrates
content could be explained by the reduction of theses species.
2-
sites.[15] In Hap, Ca2+ and HPO4 ions can act as Lewis and
Brønsted acid sites, respectively. Therefore the possible
presence of such ions at the surface of low copper content
material could also contribute to the increased catalytic
performance of Cu2.5Hap. In this regard further characterization
of CuxHap acid-base properties are required in order to assess
the importance of such properties of Hap in the toluene oxidation.
100
80
Cu2.5Hap
Table 4. Toluene content in air, Volume Hourly Space Velocity VHSV or Gas
Hourly Space Velocity GHSV and temperature at 50 % toluene conversion into
CO2, T50, on our best catalyst and different copper systems from the literature
60
Cu10Hap
40
VHSV /
Toluene /
ppm
Catalyst
m3.kg-1.h-1
T50 / °C
215
Reference
(GHSV) / h-1
30
(15000)
present
work
Cu2.5Hap
800
20
0
5% Cu/γ-Al2O3
4000
4400
3.6
~ 275
207[a]
[10]
[12]
8% CuO/Ce0.8Zr0.2O2
(33000)
15% Cu/γ-Al2O3
5% Cu/γ-Al2O3
9.5% CuO/HCLT
9% Cu/SBA-16
1000
900
200
6
305[a]
275[a]
330
[38]
[39]
[40]
[44]
0
5
10 15 20 25 30 35 40 45
Time / h →
1000
880
(15000)
(1.2)
Figure 7. Time-on-stream plot of toluene conversion into CO2 over Cu2.5Hap
and Cu10Hap catalysts. Feed composition: 800 ppm toluene in air, temperature:
215 °C
~ 337[a]
10% CuO/SiO2
10% CuO/SBA-15
5% Cu/ZrO2
~ 317
~ 325
~ 245[a]
880
(1.2)
60
[45]
[46]
1000
[a] Temperature at 50 % toluene conversion
Conclusions
2.2. Stability tests
In this study we have shown that hydroxyapatite-supported
copper materials prepared by the wet impregnation method have
a great potential as noble metal-free based catalysts for the
toluene removal. Copper exchange with hydroxyapatite led to the
libethenite phase when high copper content (20 wt%) is deposited
on the support. Such copper based phase (Cu2(PO4)OH) is not
very active in the toluene oxidation. However the lowest copper
content (2.5 wt%), impregnated over hydroxyapatite support led
to the best catalyst for toluene total oxidation owing to well
dispersed CuO species and possibly to the participation of the
hydroxyapatite in the toluene activation. Despite of the presence
The stability of the Cu2.5Hap and Cu10Hap catalysts was
evaluated by a long-run testing at 215 °C for 45 h (Figure 7).
Cu2.5Hap and Cu10Hap exhibit an increase of toluene
conversion from 45 % to 67 % and 18 % to 32 % respectively
during the first six hours under the stream, and remained stable
till the end of the stability test. This result has demonstrated a
remarkable behavior of these materials since they do not
deactivate under our experimental conditions but instead activate
in the first hours to remain stable thereafter. The XRD analyses
(not shown) of the samples after stability test did not show any
structural modification. An increase in SSA value is observed for
used Cu10Hap (Table 1), while it remains the same for used
Cu2.5Hap. Interestingly, the ToF-SIMS analyses allow to
conclude that nitrates species are consumed during the toluene
catalytic oxidation at 215 °C (Figure S6). Redox mechanism is
often proposed for the VOC oxidation over transition metal
oxides.[47] Following the reduction of Cu2+ to Cu0 by toluene, the
nitrates could be reduced into gaseous N2 on metallic copper.
Besides the amounts of experimental H2 consumed during H2-
TPR experiments for the Cu-Hap samples were much higher than
the ones obtained theoretically for the reduction of Cu2+ species
-
of residual NO3 species after calcination step at 400 °C, the
catalytic performances of Cu/Hap are very stable for 45 h time-
on-stream, the nitrates being eliminated by reduction through a
redox mechanism implying metallic copper.
Experimental Section
1. Synthesis of the Hap support and supported Cu
catalysts
[15]
The hydroxyapatite support was prepared as follows
:
into Cu0. As residual NO3 coming from the Cu precursor and
-
0.0835 mole of Ca(NO3)2·4H2O (Sigma-Aldrich, purity ≥ 99.9%)
2-
CO3 incorporated in Hap are potential reducible species, their
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10.1002/cctc.201601714
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placed in 150 mL of distilled water was added dropwise, under
stirring and at 80 °C to 500 mL of an aqueous solution containing
0.05 mole of (NH4)H2PO4 (Sigma-Aldrich, purity ≥ 98.0%). The pH
of the solution was kept at 10 during synthesis by adding a 25%
NH3.H2O solution. The obtained precipitate was filtered, washed
with hot water, and dried overnight at 80 °C. The solid was finally
calcined at 400 °C for 4 h under air flow. The Ca/P atomic ratio
was found to be 1.79.
Several copper-loaded hydroxyapatites were prepared by the
conventional impregnation method carried out in a rotary
evaporator with 50 mL of copper nitrate Cu(NO3)2.3H2O (Sigma
Aldrich, purity: 99.1 %) solution containing the appropriate
amount of Cu and 2 g of Hap. The resulting mixture was heated
at 60 °C. The residue was then dried overnight at 80 °C and
calcined 4 h at 400 °C under air flow. The solids are noted
CuxHap, where x represents the copper weight percentage of Cu
(x= 2.5, 5, 10 and 20).
2.4. Infrared spectroscopy experiments
Fourier-transform transmission infrared (FT-IR) spectra were
recorded on a Nicolet 460 spectrometer at room temperature. A
KBr pellet containing the compound was prepared by intimately
mixing about 1 mg of powdered sample with 100 mg of dry
powdered KBr. The spectral range covered was 500 to 4000 cm-
1 with 2 cm-1 spectral resolution.
2.5. Raman spectroscopy
The Raman spectra were recorded in air at room temperature
using
a Raman microprobe (Labram HR). This system
incorporated a Peltier-cooled detector and an Ar+-ion laser
(488 nm) with ± 1.5 cm-1 spectral resolution, 0.4 mW laser power
and 120 seconds acquisition time for each scan. The presented
spectra corresponded to the average of 3 scans. The Raman
spectrometer was standardized by means of Si line at 520.6 cm-
2. Characterization of the materials
2.1. Elemental analyses
1
.
2.6. X-ray photoelectron spectroscopy
Quantification of Cu, Ca and P content was done by inductively
coupled plasma-optical emission spectroscopy (ICP-OES).
Analyses were carried out on an Agilent Technologies 700 Series
spectrometer (REALCAT platform - Lille University). For analysis
five milligrams of solid were dissolved in 50 mL aqua regia
solution.
XPS analyses were done on an AXIS Ultra DLD Kratos
spectrometer. This instrument operated with a monochromatised
source (Al Kα = 1486.7 eV) and a charge compensation gun. C
1s binding energy at 285 eV was used as reference for
determination of different core level binding energies. A mixed
Gaussian/Lorentzian peak fit procedure was used to simulate the
experimental photo-peaks using CasaXPS processing software.
2.2. X-ray diffraction analyses
For quantitative analysis
a nonlinear Shirley background
Powder X-ray diffraction data were recorded on a Bruker AXS D8
Advance diffractometer functioning at room temperature in a
Bragg–Brentano geometry. This instrument is equipped with a
LynxEye Super Speed detector. Data were collected using the Cu
Kα line in a 2θ range from 10 to 65° with a step size of 0.02° and
5 s counting time per step. LaB6 standard sample was used to
determine the instrument resolution. The mean crystallite size of
the copper oxides was evaluated by Rietveld refinement. For peak
profiles description the Thompson–Cox–Hastings pseudo-Voigt
function was selected. Here, an anisotropic size broadening
model based on linear combination of spherical harmonics was
used to simulate the size broadening, and 6 additional parameters
were refined. From this refinement, the individual size of Hap
crystallite was derived for each crystallographic plane. From the
refinement, the integral breadth of each peak was deduced and
the corresponding crystallite size was calculated using the
Scherrer formula (D=λ/βcosθ where λ is the wave length, β the
integral breadth and θ the peak position).
subtraction was applied. XPS quantification included singlet
peaks in the form of C 1s, O 1s and N 1s as well as doublet pairs
in the form of Ca 2p, Cu 2p and P 2p. The Cu/(Ca+P) surface
atomic ratio is calculated in order to access to an indirect copper
oxide dispersion. An XPS Cu/(Ca+P) close to the bulk ratio means
that all Cu atoms are visible by XPS, and consequently that size
of the copper oxide particles remains below 10 nm (analysis depth
of XPS close to this value). If XPS Cu/(Ca+P) ratio falls
significantly below the bulk ratio, copper oxide forms in large
particles (>> 10 nm).
2.7. Time of flight - secondary ion mass spectrometry
ToF-SIMS analyses were achieved on
a
TOF.SIMS 5
spectrometer (ION-TOF GmbH Germany) implemented with a
liquid bismuth ion source. The compacted samples were
bombarded with a pulsed 25 keV Bi3+ primary ion beam rastered
over an area of 500 × 500 μm2. Static conditions were ensured
with a data acquisition of 100 s (the total fluence does not amount
up to 1012 ions/cm2). A 20 eV pulsed electron flood gun was
employed to compensate the charge effects. In these
experiments the mass resolution (m/∆m) was higher than 4000 at
2.3. N2 physisorption analyses
Textural properties (specific surface area, pore volume and
diameter) of materials were investigated by the physisorption
technique which have been carried out by the Micromeritics
Tristar II analyser. Before nitrogen adsorption, the solids were
outgassed under vacuum at 200 °C for 4 h.
+
m/z = 159 for Ca2PO3 .
3. Catalytic performances measurement
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10.1002/cctc.201601714
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Toluene catalytic oxidation experiments were done at
atmospheric pressure in a continuous flow fixed bed reactor
located in an electrical furnace. For each catalytic run, 200 mg of
fresh catalyst were put in the micro reactor. The reactive gas
mixture comprised 800 ppmv of toluene diluted in air (100 mL.min-
1) corresponding to a Volume Hourly Space Velocity (VHSV) of
500 mL.g-1.min-1 (30 m3.kg-1.h-1). Before each run, the samples
were pretreated 2 h at 390 °C under air flow of 75 mL.min-1.
Catalysts performances were evaluated in the 390-30°C
temperature range, with a decreasing rate of 0.5 °C.min-1. The
concentrations of the gaseous reactants and products were
analyzed on line by a 7860A Agilent Gas Chromatography
equipped with two detectors (TCD and FID) connected
respectively to two columns (Restek Shin Carbon ST/Silco HP
NOC 80/100 micro packed and capillary column Cp-Wax52CB25
m, Ø = 0.25 mm).
fellowships by the Agence Universitaire de la Francophonie (AUF)
– Région du Moyen-Orient. The authors are grateful in particular
to Prof. Rose-Noëlle Vannier for Rietveld refinements, Martine
Trentesaux and Laurence Burylo for XPS and XRD
measurements, respectively.
Keywords: Volatile Organic Compound • Catalytic Oxidation •
Copper • Toluene • Hydroxyapatite
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The authors thank the Chevreul Institute (FR 2638) for its help in
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the « Ministère de l’Enseignement Supérieur et de la Recherche »,
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Hydroxyapatite supported copper
D. Chlala, J.-M. Giraudon, N. Nuns, M.
Labaki, J.-F. Lamonier*
Toluene
in air
oxides were showed as efficient free
noble catalysts for the toluene
oxidation. High activity, CO2 selectivity
and stability are achieved over low
content Cu materials. Copper in
association with hydroxyapatite can be
considered as a realistic thinkable
material to replace noble metal based
solids in VOC oxidation catalytic
formulations.
Page No. – Page No.
Cu/Hydroxyapatite: highly active free-
noble catalysts for the total oxidation
of toluene
CO2
H2O
Copper content
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