Cu?Mn Hydroxyapatite Materials for Toluene Total Oxidation

Article abstract of DOI:10.1002/cctc.201901336

The oxidation of 800 ppmv toluene in air has been investigated over CuMnO_x ((Cu+Mn) loading: 10 wt%) dispersed on hydroxyapatite (Hap: Ca₅(PO₄)₃(OH)) with different molar Cu/Mn compositions (2; 1; 0.5) to be compared with the corresponding supported single transition metal oxide catalysts. The solids have been prepared by wet aqueous co-impregnation method followed by a calcination at 400 °C for 4 h and physicochemically characterized by different techniques. The catalytic performances of the supported Cu?Mn oxide catalysts revealed the existence of a synergetic effect in terms of activity and selectivity towards CO₂ through the formation of Cu?Mn mixed oxides. The strongest synergetic effect is observed for a Cu/Mn molar ratio of 2 as confirmed by the standardized conversion rate values determined at 190 °C (r₁₉₀) which followed the decreasing sequence (/10^?2 h^?1): Cu₂MnHap (2.57)>CuMnHap (2.04)>CuMn₂Hap (1.59)>MnHap (1.13)>CuHap (0.13). The highest activity of Cu₂MnHap can be explained by a better reducibility of the active phase in line with a high density of Cu-O?Mn interactions at the outermost layers as shown by ToF-SIMS results.

Full text of DOI:10.1002/cctc.201901336

Full Papers
DOI: 10.1002/cctc.201901336
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CuÀ Mn Hydroxyapatite Materials for Toluene Total
Oxidation
Maya Ibrahim,[a, b] Madona Labaki,[b] Nicolas Nuns,[a] Jean-Marc Giraudon,*[a] and
Jean-François Lamonier[a]
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The oxidation of 800 ppmv toluene in air has been investigated
over CuMnOx ((Cu+Mn) loading: 10 wt%) dispersed on hydrox-
yapatite (Hap: Ca5(PO4)3(OH)) with different molar Cu/Mn
compositions (2; 1; 0.5) to be compared with the corresponding
supported single transition metal oxide catalysts. The solids
have been prepared by wet aqueous co-impregnation method
selectivity towards CO2 through the formation of CuÀ Mn mixed
oxides. The strongest synergetic effect is observed for a Cu/Mn
molar ratio of 2 as confirmed by the standardized conversion
°
rate values determined at 190 C (r190) which followed the
decreasing sequence (/10À 2 hÀ 1): Cu2MnHap (2.57)>CuMnHap
(2.04)>CuMn2Hap (1.59)>MnHap (1.13)>CuHap (0.13). The
highest activity of Cu2MnHap can be explained by a better
reducibility of the active phase in line with a high density of Cu-
OÀ Mn interactions at the outermost layers as shown by ToF-
SIMS results.
°
followed by a calcination at 400 C for 4 h and physicochemi-
cally characterized by different techniques. The catalytic
performances of the supported CuÀ Mn oxide catalysts revealed
the existence of a synergetic effect in terms of activity and
Introduction
to superior resistance to poisoning as well as higher thermal
stability.[6–7] Furthermore, some metal oxides have demon-
strated similar and even in some cases, slightly higher activity
than that of noble metals. Indeed γ-MnO2 was found to be
more active of Pt/TiO2 in toluene removal in specific
conditions.[8] On the one hand, manganese oxides were found
to exhibit great activity and selectivity in total oxidation of
VOCs,[9,10] among which figures toluene[11] which is the target
VOC to abate in this paper. On the other hand, copper oxides
have also proven to be active and selective in the oxidation of
toluene,[12] as well as good promoters for the removal of coke
formed in the course of reaction.[13] Furthermore, a study
conducted by Wang[14] on the complete oxidation of toluene
revealed better catalytic performances for copper dispersed on
alumina compared with other TM (Fe, Mn, Cr, Co, Mo, and Ni).
Therefore, the use of a binary copper-manganese oxide can be
an attractive strategy to improve the performance of the
catalyst in the total oxidation of toluene. In that way, it has
already been reported in the literature that bulk[15–20] and
supported[16,21–24] CuÀ Mn binary oxides can be considered as
efficient catalysts in toluene oxidation, for which they exhibit
higher catalytic activity than the single metal counterparts.
Important parameters such as the methods of synthesis,
total TM loading, temperature of calcination, nature of the
support, play key roles in tuning the optimized Cu/Mn ratio for
the oxidation in toluene oxidation.
Volatile organic compounds (VOCs) are a large group of toxic
chemicals that are easily released into the atmosphere, resulting
in widespread environmental complications and negative
health impacts. They are, therefore, recognized as the major
contributors to air pollution. More stringent regulations have
been adopted in order to control their release and reduce their
daily emissions. Among the different VOC abatement techni-
ques, total catalytic oxidation appears as an attractive techni-
que. Indeed, this VOC treatment process is cost effective when
compared to thermal incineration method due to operating
conditions at much lower temperature and to high selectivity
towards oxidation products such as CO2 and H2O avoiding in
that way undesirable by-products such as dioxins and nitrogen
oxides.[1]
Although VOC total oxidation is generally performed over
supported noble metals known for their high catalytic perform-
ances such as palladium and platinum[2–5] increasing studies
focus nowadays on the use of cheaper transition metal oxides
(TMO). Despite being generally less active than the noble metal
based catalysts, they constitute a cost efficient alternative due
[a] M. Ibrahim, N. Nuns, Dr. J.-M. Giraudon, Prof. J.-F. Lamonier
Univ. Lille, CNRS, Centrale Lille, ENSCL
Univ. Artois, UMR 8181 – UCCS
Unité de Catalyse et Chimie du Solide
Lille 59000 (France)
E-mail: jean-marc.giraudon@univ-lille.fr
[b] M. Ibrahim, Prof. M. Labaki
Laboratory of Physical Chemistry of Materials (LCPM)/PR2N
Faculty of Sciences
The good performances of CuÀ Mn oxides in toluene
oxidation have been reported to be due to segregated CuO/γ-
Mn2O3, the presence of Cu allowing an enhanced catalytic
activity of the manganese oxide for which active sites may be
derived from the presence of Mn3+ in the near surface of the
catalyst.[20] Other teams ascribed the enhanced catalytic activity
to the formation of a mixed copper manganese oxide spinel
phase allowing easier electron transfer between copper and
manganese cations.[16,21,22,24] Behar et al. reported on bulk
Lebanese University
Fanar BP 90656 (Lebanon)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201901336
This manuscript is part of a Special Collection on the “French Conference on
Catalysis”.
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Cu1.5Mn1.5O4 particles (10 nm) prepared from ionotropic alginate
Results and Discussion
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[16]
°
gel able to completely oxidize toluene at �240 C.
CuÀ Mn binary oxides supported on conventional supports
such as alumina,[23–25] titania,[16] and silica,[22] have been studied
so far.
Decomposition of the Dried Impregnated CuxMnyHapD
Samples
In the case of alumina supported CuÀ Mn oxides, Saqer
et al.[23] attributed the enhanced catalytic activity to the better
dispersion of segregated MxOy (M=Cu, Mn) species, their
increased reducibility (reactivity of surface oxygen), and their
lower tendency to form coke deposits under reaction con-
ditions. Li et al.[24] reported the best activity for a Cu/Mn atomic
ratio of 1 on catalysts pretreated with H2 in line with the
formation of the spinel phase Cu1.5Mn1.5O4. However, it was
found that the presence of water vapor had a negative effect
on catalytic activity. Wang et al.[25] associated improved catalytic
activity with the presence of highly dispersed CuÀ OÀ Mn
clusters in which part of the Cu2+ occupies the tetrahedral
vacancies (TV) of alumina support, this strong interaction
between Cu and Mn results in better redox properties.
Considering titania, the CuÀ Mn oxide supported on TiO2
obtained by incipient wetness impregnation shows an overall
activity lower than for the unsupported Cu1.5Mn1.5O4.[16]
Considering the silica based support, it was found that
MCM-41-supported CuÀ Mn oxide showed better activity than
conventional amorphous silica, β-zeolite and ZSM-5-supported
CuÀ Mn catalysts.[22] It was supposed that highly dispersed
CuÀ Mn mixed oxides on mesoporous structure possibly provide
active sites for the complete oxidation of toluene on the
mesoporous catalysts.
The present work will focus on dispersing these active metal
species on an unconventional support, which is hydroxyapatite
(Hap). Hap is a material that has attracted much attention in
numerous applications due to its non-toxicity, low cost,
abundance in the natural environment, good chemical and
thermal stability, high adsorption capacities when prepared
through solution mediated synthesis. Furthermore, its charac-
teristic properties, such as ionic exchange ability and tunable
acid-base characteristics,[26–28] have been put forward to explain
some catalytic activity differences in the VOC oxidation
process.[29–34] In fact, a review on the use of hydroxyapatite as a
multifunctional material for air, water and soil pollution control
has been recently published.[35] Previous studies have aimed to
study the effect of TM content on the performances of
copper[36] and manganese[37,38] based active phases dispersed on
Hap in the total oxidation of toluene to be correlated with the
speciation and dispersion of the TM. For the TM-based Hap
supported catalysts (TM=Cu, Mn), the maximum of activity is
achieved for a low amount of Cu (wt%: 2.5%) compared with
those of 5–10 wt% for Mn.
Thermogravimetric Analysis (TGA)
The mode of decomposition of the dried impregnated samples
(CuxMnyHapD) was investigated using TGA technique and the
resulting traces are given in Figure S2. The TGA traces (Fig-
ure S2-b) are rather similar for all samples. The first weight loss
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°
°
from 50 C to 160 C is ascribed to the departure of water. The
second stage of decomposition which takes place in the
°
temperature range of about 160–400 C is attributed to the
À [39]
decomposition of NO3 . It is found that the temperature of
°
the large DTG peak (in C) increases with Cu content owing to:
MnHapD (190)<Cu2MnHapD (300)<CuHapD (335) (Figure S2-
À
b). Furthermore, based on the theoretical NO3 content in the
Cu(II)/Mn(II) precursors and taking into account the H2O
removal contribution at low temperature, it is found that 82, 88
and 95% of the initial nitrate content decompose for CuHapD,
Cu2MnHapD and MnHapD, respectively. These results show that
À
some remaining NO3 related species can be retained by the
°
samples after calcinations performed at 400 C for 4 h.
Structural, Morphological and Textural Characterizations
The X-ray diffractograms of the fresh CuxMnyHap samples and
of the Hap support are shown in Figure 1.
The characteristic peaks of Hap agree with those corre-
sponding to the hexagonal phase Ca5(PO4)3(OH) (PDF 00-009-
0432). It is found that the Full Width at Half Maximum (FWHM)
for the (0 0 2) reflection is low as compared to the other ones in
line with the presence of anisotropic crystallites as reported
earlier in the literature.[26,40]
With the objective to improve the catalyst efficiency, this
study investigates the catalytic performances of CuÀ Mn oxides
dispersed on Hap in toluene oxidation considering a TM
content of 10 wt% to be compared with those of the single
supported TMO counterparts through the tuning of the Cu/Mn
molar ratio. The catalytic performances will be correlated with
the physicochemical properties of the catalysts.
Figure 1. XRD patterns of pure Hap and CuxMnyHap samples.
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In Table 1 are listed two mean crystallite sizes obtained by
the Scherrer equation from the FWHM of the peaks positioned
The X-ray diffractogram of MnHap shows a new distinct
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broad peak located at 2θ=37.1 which can be tentatively
at 10.9 and 25.9 relative to the (1 0 0) and (0 0 2) planes,
respectively. This allows us to determine a length to thickness
attributed to the akhtenskite ɛ-MnO2 (PDF no 00-030-0820)
phase (Figure S1) which was not previously detected in similar
conditions.[37]
°
°
ratio of the crystallite through the determination of the Dc(002)
/
Dc(100) ratio which is of 1.81 for the Hap support. This ratio
increases with the addition of Cu and/or Mn to get the highest
value of 2.08 for CuHap. Thus, the addition of TM leads to
slightly more elongated crystallite as compared to that of the
Hap support as already observed by Chala et al. for MnHap
samples.[38]
In addition to the characteristic peaks of Hap, we detect the
presence of new peaks for the CuHap and MnHap solids.
Indeed, the CuHap sample exhibits new peaks positioned at
Regarding the binary oxide materials, for the Cu-rich
sample, we observe an increase in intensity and broadness of
°
°
the peak located at 2θ=35.5 and of the shoulder at 2θ=38.7 .
However, the broadness of the peaks precludes discriminating a
CuO from a related CuÀ Mn spinel phase as shown in Figure 2.
By opposition, for the enriched Mn samples, i.e. CuMnHap
and CuMn2Hap, we do not observe any additional peaks to
those of the pure Hap, which could be explained either by a
high dispersion of the TMO phase(s) or/and by the formation of
supported amorphous TMO phase(s).
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2θ=35.6 , 38.8 and 48.8 associated with monoclinic CuO
tenorite (PDF No. 04-007-1375). The narrow sharp peaks are
consistent with a mean crystallite size of 45 nm (see Table 1), in
line with that of 42 nm reported by Chlala et al.,[36] indicating
that the addition of copper on Hap by wet impregnation results
in poor CuO dispersion for a 10 wt% of Cu.
The anisotropy observed by XRD was also evidenced
through SEM analysis (Figure 3). Indeed, some bundles of fiber-
like particles are observed for all our samples, indicating an
anisotropic growth of the particles with a length varying from
50 nm to almost 400 nm for all samples.
The main textural properties of the samples are summarized
in Table 2. The Hap support has a specific surface area of
102 m2.gÀ 1 and a pore volume of 0.70 cm3.gÀ 1. When adding Cu
Table 1. Mean crystallite size (DC) of Hap support and of TM related
detected phases.
Sample
Dc(002)
[nm]
Dc(100)
[nm]
Dc(002)/Dc
(100)
Dc[a] CuO
[nm]
Dc
ɛ-MnO2
[nm]
(002)
(100)
(002)/
(100)
Hap
CuHap
Cu2MnHap 37
CuMnHap 37
CuMn2Hap 38
36
39
20
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20
18
21
18
1.81
2.08
1.85
1.99
1.81
1.90
–
45
–
–
–
–
–
–
–
–
16
MnHap
35
–
°
[a] Determined from the peak at 2θ=48.8 .
Figure 3. SEM images of a) Hap, b) CuHap, c) CuMnHap and d) MnHap.
Table 2. Textural properties and H2-TPR results.
[c]
[d]
Sample
SSA[a]
Vp[b]
n(H2)th
n(H2)ex
[m2.gÀ 1
]
[cm3.gÀ 1
]
[mmol.gÀ 1
]
[mmol.gÀ 1
]
Hap
CuHap
Cu2MnHap 78
CuMnHap 79
CuMn2Hap 86
102
71
0.70
0.46
0.49
0.54
0.56
0.51
–
–
5.51 (1.58)
2.67 (1.65)
1.83 (1.69)
1.46 (1.73)
1.78 (1.82)
4.96
1.99
1.19
0.98
0.67
MnHap
78
[a] Specific surface area, [b] Pore volume, [c] Experimental amount of H2
consumed on the fresh catalysts; Theoretical amount of H2 consumed (see
text), [d] Experimental amount of H2 consumed on the used catalysts.
°
Figure 2. X-ray diffractograms of the CuxMnyHap solids in the 2θ=34–41
range. The intensity of the diffractogram of CuHap was divided by 10.
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and/or Mn, both the BET surface area and pore volume values
decrease. This decrease is higher in the case of CuHap, due
most probably to some pore blockage by CuO particles.
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Redox Characterization
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Temperature Programmed Reduction (H2-TPR) Coupled with
Mass Spectrometry (MS)
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The H2-TPR profiles of the calcined materials are illustrated in
Figure S3 and the quantitative results are reported in Table 2.
As expected the pure apatitic support does not exhibit any
reduction peak (not shown here).[41,42] . For MnHap one peak of
°
H2 consumption at 370 C was observed, followed by a tail. The
trace relative to CuHap is complex showing at high temperature
°
a strong H2 consumption due to overlapping peaks at 452 C
°
and 485 C while a low temperature H2-consumption is also
Figure 4. Evolution of the m/z=30 (NO) signal for the CuxMnyHap solids.
observed. By opposition the H2-TPR profiles of the CuÀ Mn
based materials show only two overlapping peaks in a narrow
temperature range. Interestingly, the onset of H2 consumption
decreases while the peak enlargement increases with the
increase of Cu content.
out that TNO decreases with an increase of Cu content showing
that high relative Cu content promotes the reducibility of the
materials. It should also be noted that TNO is lower for Cu/Mn=
2 and 1 as compared to those relative to the single TM based
catalyst.
Additionally, the H2-TPR profile of the fresh CuÀ Mn based
catalysts exhibits a typical profile similar to those of mixed
CuÀ Mn oxides[43–44] in line with the co-impregnation method.
The experimental H2 consumptions expressed per g of
catalyst n(H2)ex increase with Cu content from 1.46 (CuMn2Hap)
to 5.51 (CuHap). These values were compared to the theoretical
ones n(H2)th regarded as maximum values assuming Cu(+II)->
Cu(0) and Mn(+IV)->Mn(II) reductions. We noticed that n(H2)ex
exceeds n(H2)th for Cu/Mn atomic ratio �1.0. This can be
À
explained by taking into account that some reducible NO3
related entities are retained in the materials after calcination.
This assumption is supported by the FT-IR results exhibiting the
characteristic unreacted nitrate band at 1386 cmÀ 1 whose
relative intensity increases with Cu content (not shown here) as
already observed by Chlala et al.[36]
Surface Characterizations
XPS Results
À
On purpose, the degradation modes of the NO3 species
Figure 5 gives the Cu 2p core level XPS spectra for the fresh
samples and the pertinent results are given in Table 3 and S2.
have been monitored using mass spectroscopy through the
evolution of m/z signals 17, 28, 30 and 44 characteristic of NH3,
N2, NO and N2O as a function of temperature (Figure S4). For
the CuÀ Mn based catalysts the evolution of these m/z signals
over time are rather similar. A peak of high intensity at m/z=30
is observed, followed by a narrow less intense peak at m/z=28
while the contributions at m/z=17 and 44 are negligible.
Hence, during the H2-TPR experiments it turns out that the
À
NO3 entities are reduced into NO then into N2 while the
contributions of gaseous N2O and NH3 appear to be negligible.
À
Such scheme of NO3 degradation holds also for the MnHap
sample. However, in that case the relative contribution of N2 is
less pronounced.
Taking into account the ability of Cu(0) to efficiency reduce
the nitrate entities at low temperature, the TNO value has been
taken as a reference to describe the reducibility of the materials:
the easier the reducibility of the materials, the lower TNO is.
Thereby, as seen in Figure 4 the solids can be ranked according
°
to the decreasing temperature of NO appearance TNO ( C):
Cu2MnHap (174)<CuMnHap (188)<CuHap (200)<CuMn2Hap
(212)<MnHap (236). For the CuÀ Mn related catalysts it turns
Figure 5. 1st recording of the Cu 2p XPS core levels.
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À
À
the NOÀ (m/z=30), NO2 (m/z=46) and NO3 (m/z=62)
secondary ions in line with residual nitrate after calcination.
The existence of interactions between the TM and hydrox-
yapatite in TMHap (TM=Cu or Mn) materials is evidenced by
the presence of a series of cluster ions involving TM and Ca or P
and both (see Table S3, S4, 4). Based on these results, it is found
that Cu and Mn preferentially interact with Ca rather than P.
This statement has already been reported in the case of Hap
supported Mn samples by Chlala et al.[37] These observations
suggest the occurrence of an ion (CaÀ Cu/Mn) exchange at the
outermost layers oft the samples.
Table 3. XPS based data obtained for the fresh and used catalysts.
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Sample
Cu 2p3/
FWHM Cu 2p3/
Isat/
[a]
Mn
Mn/
Cu
[a]
2
[a]
2
Ipp
AOS[b]
CuHap
932.8
933.9
934.0
934.4
–
3.42
3.40
3.50
3.04
–
4.20
3.66
3.20
4.14
–
0.38
0.60
0.65
0.69
–
0.44
0.51
0.55
0.62
–
–
–
Cu2MnHap
CuMnHap
CuMn2Hap
MnHap
2.6
2.6
2.7
3.1
–
2.0
2.0
2.3
2.5
1.1
2.0
5.3
–
CuHapU
933.3
–
Cu2MnHapU 933.7
CuMnHapU 934.2
CuMn2HapU 934.4
1.1
2.3
5.1
–
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MnHapU
–
Interestingly for the CuMn based samples, the ToF-SIMS
[a] Data obtained for 1st recording, [b] Mn average oxidation state.
spectra in positive polarity exhibit
a series of mixed
+
CuxMnyOzHw (x=1; y=1; z=0,1; w=0-2) ions (see Table 4).
For example the ToF-SIMS (+) spectrum of Cu2MnHap in the
m/z 133–138 range displayed in Figure 6 evidences the
CuMnO+ and CuMnOH+ secondary ions. The presence of such
ions proves the formation of mixed CuÀ OÀ Mn oxide. However,
it is not possible to identify the mixed oxide phase just based
on the ToF-SIMS fragmentation patterns. In parallel, in order to
evaluate the relative surface density of CuÀ OÀ Mn interactions
the ToF-SIMS intensity ratio CuMnO+/(Cu+ +Mn+) for the
CuxMnyHap solids has been determined. This allows to classify
the catalysts by decreasing intensity ratio to get the following
sequence: Cu2MnHap (0.057)>CuMnHap (0.040)>CuMn2Hap
The Cu 2p3/2 binding energy (BE) for CuHap is at 932.8�0.1 eV
considering the first and second recorded XPS spectra while the
FWHM decreases from 3.4 eV to 2.7 eV. By opposition, for all the
CuxMnyHap samples, the Cu 2p3/2 BEs are much higher, located
at 934.1�0.1 eV with FWHM of 3.2�0.3 eV and Isat/Ipp ratio
about twice higher than that of CuHap, in line with a Cu(II).
However, considering the second Cu 2p XPS, it is observed for
all samples a significant Cu 2p3/2 BE decrease accompanied with
a decrease of FWHM and Isat/Ipp ratio. Based on all these
observations, it is concluded that XPS induces reduction of
copper at different extents as already observed.[36] Copper is
significantly reduced on CuHap considering the first XPS
spectra, while a partial copper reduction is observed for the
CuxMnyHap samples regarding the second recording XPS
spectra. Such different behaviors in terms of copper reduction
can be tentatively ascribed to the presence of a mixed CuMn
oxide which stabilizes the Cu towards XPS photons.
The Mn 3s core levels for the Mn containing samples have
been investigated. From the splitting between the two
multiplet spin components a Mn average oxidation state (AOS)
has been estimated which gives a value of 3.1 for MnHap while
those for CuxMnyHap decrease to 2.7–2.6.
The Mn/Cu atomic ratio compared to the nominal Mn/Cu
shows a Mn surface enrichment which becomes more pro-
nounced with increasing Mn content.
Table 4. List of ToF-SIMS positive secondary ions detected in CuÀ Mn
based solids.
+
+
+
+
CuxMnyCavOzHw
n.d.[a]
CuxMnyPtOzHw
CuxMnyCavPtOzHw
n.d.[a]
CuxMnyOzHw
CuMnPH
CuMn
CuMnPH2
65CuMn
65CuMnPH2
CuMnO
65CuMnO
CuMnOH
65CuMnOH
[a] Not detected.
Time of Flight Secondary Ions Mass Spectrometry (ToF-SIMS)
Regarding the Hap support, the peaks displayed in the positive
ToF-SIMS spectra include Ca+ (m/z=40), CaH+ (m/z=41), CaO+
(m/z=56), CaOH+ (m/z=57) and Ca2O+ (m/z=96), Ca2PO3
+
+
(m/z=159), Ca2PO4
(m/z=175), CaPO3H+ (m/z=120),
CaPO4H2 (m/z=137), Ca2PO3H+ (m/z=160), Ca3P2O3H+ (m/z=
+
231), Ca4P2O4H+ (m/z=287), Ca5P2O5H+ (m/z=343) and
+
Ca5(PO4)3 (m/z=485) which are the characteristic secondary
ions observed for Ca5(PO4)3(OH).[45–49] In the negative polarity,
OÀ (m/z=16), HOÀ (m/z=17), PÀ (m/z=31), POÀ (m/z=47),
À
À
PO2 (m/z=63), and PO3 (m/z=79) are found. It should be
mentioned that some mineral impurities such as Na+ (m/z=
23), Mg+ (m/z=24), K+ (m/z=39) and some hydrocarbons
Figure 6. ToF-SIMS (+) spectrum in the m/z 133–138 range collected from
the Cu2MnHap sample.
+
CxHyOz were detected as well. Additionally all samples exhibit
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(0.025). This result highlights the greater contribution of
CuÀ OÀ Mn interactions for Cu2MnHap.
(210)@CuHap (255). 90% of toluene conversion was achieved
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°
°
for CuHap and MnHap at temperatures of 302 C and 224 C,
°
respectively, while a temperature below 220 C was required for
CuÀ Mn based catalysts. Indeed, the same order of decreasing
Catalytic Oxidation of Toluene
activity was maintained for all catalysts for both T90 (CO2) and
T
10 (CO2) (see Table S5).
The conversion of toluene into CO2 over Hap and CuxMnyHap
catalysts as a function of temperature is given in Figure 7. It
should be noted that the toluene conversion is significantly
enhanced by the addition of single TM with good redox
properties over the hydroxyapatite support. Additionally, the
catalysts perform even better when adding Cu and Mn on the
support by co-impregnation. Toluene conversion into CO2
based on T50 (CO2) (temperature at which 50% of toluene is
Considering a standardized conversion rate determined at
°
190 C (r190), the catalysts can be ranked by decreasing activity
(/10À 2 hÀ 1) following the same previous sequence: Cu2MnHap
(2.57)>CuMnHap (2.04)>CuMn2Hap (1.59)>MnHap (1.13)@
CuHap (0.13). It should be noted that r190 for Cu2MnHap catalyst
is 2 times higher and 20 times higher compared to those
obtained with MnHap and CuHap, respectively.
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T50 (CO2) as a function of the Mn/(Cu+Mn) molar ratio
(Figure 8) shows a parabolic resembling curve indicating a
better activity in terms of specific activity for the supported
mixed oxides. It should be noted that the specific activity is the
highest for a Cu/Mn ratio of 2.0. Moreover, the production of
CO is lower over CuÀ Mn based catalysts as compared to those
of the single TMHap samples, to become non-detectable over
the Cu2MnHap sample (Figure S5) in line with previous studies
showing a better activity for CO oxidation into CO2 for such Cu/
Mn stoichiometry.[50–51] Figure 8 clearly shows a nice correlation
between the catalytic activity in terms of T50(CO2) and the
reducibility of the catalysts in terms of the onset reduction
temperature (Tor). This correlation highlights the importance of
the redox behavior of the catalysts for such reaction.
Interestingly, as shown in Figure 9, the standardized rate r190
increases linearly with the CuMnO+/(Cu+ +Mn+) ToF-SIMS
intensity ratio. It is recognized that the formation of CuÀ OÀ Mn
entities, in which the polar covalent MnÀ O bond is weakened
by the presence of Cu, promotes oxygen lability.[52–53] Further-
more, the electron transfer between copper and manganese
cations is made easier.
°
converted into CO2 in C) decreases in the following order:
Cu2MnHap (196)>CuMnHap (202)�CuMn2Hap (205)>MnHap
Figure 7. CO2 yield for CuxMnyHap catalysts (GHSV=14,500 hÀ 1; 800 ppmv
toluene in air).
Figure 8. T50 (CO2) (left axis) and Tonset reduction (right axis) vs. Mn/(Cu+Mn)
molar ratio.
Figure 9. r190 as a function of CuMnO+/Cu+ +Mn+ intensity ratio for the
mixed CuxMnyHap catalysts.
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°
°
Stability Tests
200 C to 390 C allows to quadruple the toluene conversion
which amounts now to 18% as observed in Figure 12.
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9
°
Stability tests of CuxMnyHap catalysts were conducted at 200 C
in similar conditions to those of the catalytic tests. As can be
seen in Figure 10, a decrease in toluene conversion is noted for
all CuxMnyHap catalysts. A quantification of the deactivation
resistance of our catalysts was performed through the use of an
activity coefficient a200 determined by calculating the ratio of
toluene conversion after 24 h of test to that obtained at the
beginning of the test. The evolution of the values of a200 as a
function of Mn/(Cu+Mn) molar ratio is given in Figure 11.
Therefore, the used catalysts can be ranked by decreasing
Characterizations after Stability Tests
In order to get insight about the deactivation, additional
characterizations have been performed on the used catalysts.
The XRD patterns of the CuxMnyHapU samples reveal no
modification in the positions and relative intensities of the
peaks previously observed in fresh samples, nor does it reveal
any new crystalline phase.
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resistance
towards
deactivation:
CuHap@MnHap>
By opposition the reducibility of the used catalysts is
significantly altered compared to that of the fresh catalysts in
terms of H2-TPR profiles as shown in Figure 13 and in terms of
H2 consumption amount (Table 2). For the CuHapU and
Cu2MnHapU samples it is found that n(H2)ex is higher than n
(H2)th indicating that NO3À related moieties are still incorporated
in the catalysts. For the MnHapU sample is found a very
complex H2-TPR trace showing at least five overlapping
contributions. Furthermore, the H2 consumption amount is low
and results in a Mn AOS of 2.7 highlighting the reduction of the
MnOx species. For the used CuMn based catalysts, it is found
that the onset reduction temperature is all the lower when the
Cu content is higher, as previously observed for the fresh
samples. All the traces are initially composed of an intense
narrow peak followed by a complex tail constituted of over-
lapping components in a wide temperature window (up to
Cu2MnHap�CuMnHap�CuMn2Hap.
The effect of the temperature of activation was investigated
in the case of CuMnHap. Increasing the temperature from
°
700 C). Furthermore, a global H2-consumption decrease is
noted compared to that observed on the fresh samples, which
becomes all the more pronounced the higher the Mn content
is.
Interestingly, it should be noticed that the amount of H2-
TPR consumed for the low temperature peak corresponds to
the total conversion of Cu(II) into Cu(0) within the margin of
Figure 10. Time course for toluene oxidation over CuxMnyHap catalysts
activated at 200 C. Feed composition: 800 ppmv toluene in air, Ttest =200 C.
°
°
Figure 12. Time course for toluene oxidation over CuMnHap catalyst
°
°
Figure 11. Evolution of activity coefficient a200 as a function of Mn/(Cu+Mn)
molar ratio.
activated either at 200 C or 390 C. Feed composition: 800 ppmv toluene in
°
air, Ttest =200 C.
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Figure 13. (a) H2-TPR reduction profiles for CuxMnyHapU solids and b) zoom of the H2-TPR reduction profiles for CuMnHapU, CuMn2HapU and MnHapU.
uncertainty (see Table 5). As a consequence, the H2-TPR profiles
of these two CuMn based catalysts consist in the successive
reductions of Cu(II) into Cu(0) and of Mnx+ into Mn(II).Taking
into account the remaining amount of H2 consumed in the TPR
experiments, a Mn AOS of 2.8 and 2.6 has been estimated for
the CuMnHapU and CuMn2HapU samples, respectively. There-
fore, the Mn entities appear more reduced after the stability
test as previously noticed.[38] The total oxidation of toluene over
CuÀ Mn mixed oxide can be described by a Mars-van Krevelen
model in which the toluene is oxidized by the catalyst and the
gaseous dioxygen restore the oxidized state of the catalyst.
Taking this model into account, the Mn reoxidation can be
herein the rate limiting step of the process. A possible
In this study the performances of Cu–Mn spinel oxide
materials were investigated in steam reforming of methanol. A
decomposition of the spinel was noted following the catalytic
reaction. It was claimed that the difficulty in oxidizing Mn(II)
leads to the spinel decomposition. To conclude, such a similar
behavior may have likely occurred in our experiments.
Cu 2p core level XPS spectra for the used samples that are
illustrated in Figure S6 show that the copper state is still (+II)
for the supported CuÀ Mn oxides (Table 3). By opposition the
Mn AOS decreases after durability test (Table 3) while the Mn/
Cu atomic ratios remain practically unchanged.
decomposition of the mixed CuxMnyOz species present in fresh Conclusion
CuÀ Mn samples into CuO and MnOx after catalytic reaction
could account for such a behavior.
This assumption is in line with the work of Papavasiliou
et al.[54]
Hydroxyapatite supported binary CuÀ Mn (TM: 10 wt%) oxides
with different atomic Cu/Mn composition (2; 1; 0.5), were
successfully synthesized by co-impregnation and calcined at
°
400 C for 4 h. The performances of these catalysts were
compared to those of the single TM in the catalytic oxidation of
toluene in dry air. Adding Cu to Mn promotes in any case the
total oxidation of toluene into CO2 with the highest activity and
CO2 selectivity obtained for the catalyst having a Cu/Mn molar
ratio of 2. These improved catalytic performances could be
correlated with improved reducibility induced by the presence
of a supported mixed copper-manganese phase. However, the
catalysts deactivate with time. Such a deactivation of Hap
supported CuÀ Mn based oxides with time may be explained by
a decomposition of the mixed entities.
Table 5. H2 consumption amounts and Mn AOS.
Sample
n(H2)th[a] [mmol.gÀ 1
]
n(H2)ex[b] [mmol.gÀ 1
]
Mn AOS[c]
CuMnHapU
CuMn2HapU
MnHapU
0.84
0.58
–
0.84
0.65
–
2.8
2.6
2.7
[a] Theoretical amount of H2 consumed corresponding to the reduction of
CuO to Cu0, [b] Experimental amount of H2 consumed relative to the low
temperature reduction peak considered as symmetric, [c] Manganese
average oxidation state.
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Halenda) method. Prior to data collection, each sample (�500 mg)
Experimental Section
Material Preparation
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°
was outgassed under vacuum at 200 C for 2 h.
Scanning Electron Microscopy
Synthesis of the Stoichiometric Hap Support
Scanning electron micrographs were recorded on a JEOL JSM
7500F operating at an accelerating voltage of 5 kV. Prior to analysis,
a thin chromium coating was performed on the samples in order to
reduce potential charging effect.
Stoichiometric hydroxyapatite, with a Ca/P molar ratio of 1.67, was
prepared by a co-precipitation technique.[26] 150 mL of an aqueous
solution of Ca(NO3)2 ·4H2O (0.0835 mole, Riedel-de Haën, purity
�98%) was added dropwise to 500 mL of a NH4H2PO4 solution
(0.05 mole, Fluka; purity �99%) under stirring at pH 10 (by properly
adding ammonia (25%; Scharlau) to the reaction media) under
reflux to give a white precipitate. The maturation duration was 1 h
under constant stirring. After filtration, washing with hot deionized
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Thermogravimetric Analysis(TGA)
Thermogravimetric analysis (TGA) was carried out using a TA
Balance instrument, model: SDT 2960 DSC-TGA X, under a flow of
20% O2 in helium (100 mL.minÀ 1), goÀin1g from room temperature to
°
water and drying (80 C for 18 h) a calcination step was performed
in dry air (400 C for 4 h, 2 C.minÀ 1) to get the Hap support.
°
°
°
°
600 C with a heating rate of 5 C.min
.
Synthesis of the Catalysts (10wt% of TM)
Infrared Spectroscopy (IR)
CuHap: In a typical experiment 1.76 g of Cu(NO3)2 ·3H2O (Sigma
Aldrich, purity�99%), was dissolved in 50 mL of deionized water
and the apatitic support (4.0 g) was then added. The pH of the Cu
(II) nitrate aqueous solution (50 mL) was 4.0. This pH turned out to
be 4.6 when the Hap support (solid) was added to the Cu(II)
FTIR (Fourier transform infrared) spectra were recorded at room
temperature, under vacuum, in a spectral range spanning from 200
to 4000 cmÀ 1, while using a Nicolet 460 spectrometer. These spectra
were an average of 256 scans with a spectral resolution of 4 cmÀ 1
.
Thin, disk-shaped pellets of the catalysts were prepared by
compressing 30 mg of an intimate mixture, composed of 1 mg of
powdered sample combined with 100 mg of dried KBr. A spectrum
of an empty cell was first recorded. This spectrum served as a
background spectrum, which would be subsequently subtracted
from that of the samples analyzed.
°
solution (50 mL). Rotary evaporation (Büchi Rotavapor R-114; 60 C;
20 rpm; 70 mbar) was used to remove water. After 30 min of
evaporation (volume of the solution: 35 mL) the pH decreased to
4.3 to keep stable after further evaporation of the solution. The
recovered powder was dried and calcined in similar conditions to
those relative to the Hap support. MnHap: the synthesis was carried
in a similar way as for CuHap using Mn(NO3)2 ·4H2O (Sigma Aldrich
purity �97%) as Mn(II) precursor. The CuMn2Hap, CuMnHap and
Cu2MnHap samples (Cu/Mn molar ratio: 0.5; 1; 2) were synthesized
by dissolving the suitable Cu(II) and Mn(II) precursors in 50 mL of
deionized water and subsequently treated in a similar manner to
before. The weight percentages of Cu and Mn were 3.66% and
6.34%, 5.36% and 4.64%, 6.98% and 3.02% for CuMn2Hap,
CuMnHap, and Cu2MnHap, respectively.
X-Ray Photoelectron Spectroscopy (XPS)
XPS measurements were carried out on an AXIS Ultra DLD Kratos
spectrometer, that employs a monochromatic AlÀ Kα source (hν=
1486.6 eV), operating at 120 W. The X-ray photoelectron spectra
were recorded at ambient temperature, while using an ultra-high
vacuum system (base pressure: 10À 9 mbar). All binding energies
were calibrated with respect to adventitious carbon C 1s peak BE
located at 284.8 eV. Data processing was performed using CasaXPS
software. The average oxidation state (AOS) of Mn was calculated,
based on the magnitude of the Mn 3 s multiplet splitting. XPS
spectra of Cu 2p core level were recorded twice: at the beginning
and at the end of XPS analysis. XPS quantification of Cu was
performed according to the method described in the work of Ye
et al.[15]
Characterization
X-Ray Diffraction (XRD)
XRD studies were carried out using a Bruker AXS D8 Advance
powder diffractometer equipped with a LynxEye Super Speed
detector and a CuÀ Kα radiation (λ=1.5418 Å), operating in the
°
°
°
10 �2θ�55 range with a step of 0.02 and a count time of 5 s.
The identification of the crystalline phases present in a sample was
accomplished by comparing the registered patterns with the ICDD
PDF-4 database cards, processed on EVA software. Furthermore, the
mean crystallite size of the Hap supported Cu and/or Mn oxide
catalysts, was determined by the Scherrer equation corrected from
the line broadening of the apparatus using LaB6 as an internal
reference.
Temperature Programmed Reduction (H2-TPR) Coupled with
Mass Spectrometry (MS)
The H2-TPR experiments were carried out on a Micromeritics
Autochem II (2920) instrument, equipped with a U-shaped quartz
reactor. Temperature-programmed reduction experiments were
performed on the fresh (200 mg) and used catalysts (100 mg)
°
previously cleaned in Ar at 150 C for 1 h. After cooling down the
samples were flushed with 5 vol% H2/Ar gaseous mixture
°
°
°
N2 Physisorption Analysis
(50 mL.minÀ 1) from 25 C to 800 C (10 C.minÀ 1). The monitoring of
hydrogen consumption was performed by a thermal conductivity
detector (TCD). A cold trap, composed of isopropanol and liquid
nitrogen, was used to remove water from the gas before the
detector. The quantification of the amount of H2 consumed was
accomplished by integration of the H2-TPR profiles. The H2-TPR
analysis was coupled with mass spectrometry (MS), able to identify,
through the study of m/z signals, the various produced gaseous
molecules at the exit of the reactor.
Nitrogen absorption-desorption measurements were conducted
with a Micromeritics Tristar II Surface Areas and Porosity apparatus.
The specific surface areas were calculated according to the BET
(Brunauer, Emmet and Teller) method, based on the evaluation of
the quantity of physisorbed N2, whereas, pore volume and size
distribution were determined by the BJH (Barrett, Joyner and
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°
performed at 200 C for 24 h following the same experimental
procedure exposed before. The used catalysts were labelled with
the suffix U.
Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
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Positive and negative Time-of-Flight Secondary Ion Mass Spectrom-
etry (ToF-SIMS) measurements were performed with a TOF·SIMS5
spectrometer (ION-TOF GmbH Germany) equipped with a bismuth
liquid metal ion gun (LMIG). The compacted samples were
bombarded with pulsed Bi3+ primary ion beam (25 keV, 0.25 pA)
rastered over a 500×500 μm2 surface area. With a data acquisition
of 100 s, the total fluence does not amount up to 1012 ions/cm2
ensuring static conditions. Charge effects were compensated by
means of a 20 eV pulsed electron flood gun. In these experiments,
the mass resolution (m/Δm) was about 3200 at m/z=175 for
Ca2PO4+. The secondary ions were identified by their exact mass,
coupled with the appropriate intensities for the expected isotope
pattern.
Acknowledgements
The “DepollutAir” project (grant number 1.1.18) of the European
Program INTERREG V France-Wallonie-Flanders (FEDER), Chevreul
institute (FR 2638), Ministère de l’Enseignement Supérieur et de la
Recherche and Région Hauts-de-France are acknowledged for the
funding and their support for this work. This research has also
been partially supported by a French–Lebanese research project
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°
(PHC CEDRE N 32933QE), a grant from the Lebanese CNRS 2016–
°
2017 project N 01-08-15, a PhD grant from the Lebanese
Catalytic Tests
University and ARCUS E2D2 project.
The catalytic tests were carried out in a continuous-flow fixed-bed
Pyrex reactor, with 200 mg of catalyst, under atmospheric pressure.
The reactor was set in an electric furnace, and the reaction
temperature was continuously monitored by two thermocouples, Conflict of Interest
one attached to the outside of the reactor and one inserted into a
thermowell, placed in the middle of the catalyst bed. The reactive
The authors declare no conflict of interest.
gas mixture consisted of 800 ppmv of toluene diluted in dry air
(100 mL.minÀ 1) corresponding to a gas hourly space velocity (GHSV)
of 14,500 hÀ 1
.
Keywords: Catalytic oxidation · Toluene · Copper · Manganese ·
Hydroxyapatite
À 1
°
The catalysts were activated in air at 390 C for 2 h (75 mL.min
then submitted to the reactive mixture for 1 h before to decrease
)
the temperature from 390 C to 25 C at a rate of 0.5 C.minÀ 1. The
gaseous products were analyzed quantitatively by gas chromato-
graph (7860 A Agilent Gas Chromatograph) equipped with two
detectors and two columns: a Thermal Conductivity Detector (TCD)
that identifies permanent gases (CO, CO2, etc.), separated on a
Restek Shin Carbon ST/Silco HP NOC 80/100 micro packed column;
and a Flame Ionization Detector (FID) that recognizes hydrocarbons
and aromatic compounds (toluene, benzene, etc.), separated on a
capillary column CP-Wax 52 CB: 25 m, Ø 0.25 mm×1.2 μm.
°
°
°
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Toluene conversion (Ct) was assessed according to the Equation (1):
½toluene�i À ½toluene�t
Ct ð%Þ ¼
� 100
(1)
½toluene�i
where [toluene]i and [toluene]t were respectively toluene inlet and
outlet concentrations.
The yield in CO2 was determined by the Equation (2):
½CO2�t � 100
CCO2 ð%Þ ¼
(2)
7 � ½toluene�i
where [CO2]t was the CO2 outlet concentration.
The standardized rate of toluene conversion r190 expressed in mole
of reacted toluene per hour and per mol of TM, was evaluated as
follows [Equation (3)]:
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ppmTol � FT � Ct
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Manuscript received: July 24, 2019
Revised manuscript received: October 8, 2019
Version of record online: ■■■, ■■■■
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Appetite for oxidation: Binary
copper-manganese oxides supported
on hydroxyapatite were shown to be
very efficient catalysts for toluene
oxidation. Higher activity and CO2 se-
lectivity were achieved over these bi-
metallic materials compared with
their single metal counterparts due
to an intimate interaction between
Cu and Mn leading to better redox
properties.
M. Ibrahim, Prof. M. Labaki, N. Nuns,
Dr. J.-M. Giraudon*, Prof. J.-F.
Lamonier
1 – 12
CuÀ Mn Hydroxyapatite Materials
for Toluene Total Oxidation
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