Total oxidation of naphthalene using copper manganese oxide catalysts

Article abstract of DOI:10.1016/j.cattod.2015.01.032

A series of copper manganese oxide catalysts have been investigated for the total oxidation of naphthalene, selected as a model compound for oxidation of polyaromatic hydrocarbons. Catalysts were prepared by coprecipitation and were calcined at 300, 400, 500 and 600 °C. The catalyst most active and selective towards CO₂ was calcined at 400 °C, and this was related to the formation of a high surface area poorly crystalline CuMn₂O₄ phase. Catalysts calcined at 500 and 600 °C were less effective in oxidising naphthalene to CO₂, and their composition of more highly crystalline CuMn₂O₄, with Mn₂O₃ for the 600 °C catalyst, formed partial oxidation products. There was also a direct relationship between the extent of reduction from temperature programmed reduction and catalyst performance.

Full text of DOI:10.1016/j.cattod.2015.01.032

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Catalysis Today
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Total oxidation of naphthalene using copper manganese
oxide catalysts
Tomos J. Clarke, Simon A. Kondrat, Stuart H. Taylor∗
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online xxx
A series of copper manganese oxide catalysts have been investigated for the total oxidation of naphtha-
lene, selected as a model compound for oxidation of polyaromatic hydrocarbons. Catalysts were prepared
by coprecipitation and were calcined at 300, 400, 500 and 600 ◦C. The catalyst most active and selective
towards CO2 was calcined at 400 ◦C, and this was related to the formation of a high surface area poorly
crystalline CuMn2O4 phase. Catalysts calcined at 500 and 600 ◦C were less effective in oxidising naph-
thalene to CO2, and their composition of more highly crystalline CuMn2O4, with Mn2O3 for the 600 ◦C
catalyst, formed partial oxidation products. There was also a direct relationship between the extent of
reduction from temperature programmed reduction and catalyst performance.
Keywords:
Copper manganese oxide
Hopcalite
Catalytic oxidation
Naphthalene
Polyaromatic hydrocarbons
VOCs
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
CeO2, prepared by precipitation, for naphthalene catalytic oxida-
tion [9]. Nanocrystalline CeO2 with a small crystallite size and high
Catalytic oxidation of volatile organic compounds (VOCs) is
recognised as being a highly effective process for controlling their
aerial emissions [1]. Whilst catalytic total oxidation of VOCs have
been studied widely, some specific types of VOCs have received
less attention. One such example is poly-aromatic hydrocarbons
(PAHs), which are produced from a wide range of combustion
processes and have been linked with serious health risks [2,3].
However, more recently, attention has started to focus on the cat-
alytic total oxidation of PAHs [4]. Naphthalene has often been
selected as a model PAH for study, as it is relatively less toxic to han-
dle and its behaviour during catalytic oxidation can be extrapolated
to more complex higher molecular weight PAHs.
In one of the earliest studies on naphthalene catalytic oxida-
tion, Zhang et al. [5] used a series of metal catalysts supported on
␥-Al2O3. The most effective were those containing platinum, and
initial studies focused on precious metals, showing that they were
generally more active than metal oxide catalysts [6,7]. The effect of
support was also important and studies have shown that platinum
supported on silica is exceptionally active for naphthalene oxida-
tion [8]. Alongside the investigation of supported metals, there
has also been a drive to develop more active metal oxide-based
catalysts, as they are cheaper and supplies are more abundant.
Previously, we investigated Co3O4, Mn2O3, ZnO, CuO, Fe2O3 and
surface area was the most active [10], and it has also been demon-
strated that activity could be improved further by the addition of
copper to CeO2 [11]. Mn2O3 also showed appreciable activity for
naphthalene oxidation, and more in depth studies using high sur-
face area Mn2O3, prepared by nanocasting, have identified that it is
potentially a catalyst worth further investigation [12]. Against this
background, it is interesting to consider the influence of adding
copper to manganese oxide for naphthalene oxidation.
Mixed copper manganese oxide (Hopcalite) is well known as an
active low-temperature catalyst for the oxidation of carbon monox-
ide to carbon dioxide [13,14]. Hopcalite has also been studied for the
oxidation of a limited range of VOCs including propane [15], toluene
[16] and chlorinated VOCs including chlorobenzene [17]. However,
to date, mixed copper manganese oxide catalysts have not been
studied for the total oxidation of PAHs. In this study, we have, for
the first time, investigated the performance of Hopcalite for the
total oxidation of the model PAH naphthalene, in order to estab-
lish if this inexpensive and readily available catalyst is potentially
suitable for control of PAH emissions.
2. Experimental
2.1. Catalyst preparation
Catalysts were prepared using an automated coprecipitation
technique. Copper and manganese nitrate solutions (0.25 M) were
premixed in a 1:2 ratio. Using a Metrohm titrando autotitrator, the
∗
Corresponding author. Tel.: +44 029 2087 4062.
E-mail address: taylorsh@cardiff.ac.uk (S.H. Taylor).
http://dx.doi.org/10.1016/j.cattod.2015.01.032
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: T.J. Clarke, et al., Total oxidation of naphthalene using copper manganese oxide catalysts, Catal. Today
(2015), http://dx.doi.org/10.1016/j.cattod.2015.01.032

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a total flow of 40 mL min−1 (20% O2 balance He). This was flowed
through a fixed bed of the catalyst, which was packed to a constant
volume to give a gas hourly space velocity of 60,000 h−1. The cat-
alyst temperature was controlled by a thermocouple placed in the
catalyst bed, and activity was measured over the temperature range
150–350 ◦C using incremental temperature steps. At each temper-
ature, the catalyst was allowed to stabilise and steady-state activity
was measured. The gaseous effluent was measured using an on-line
Varian 3400 gas chromatograph fitted with OV-17 and Carbosive
columns with flame ionisation and thermal conductivity detectors.
In order to analyse the stability of the catalysts on-stream, a
sample was tested at a constant temperature of 350 ◦C and products
analysed every hour for 48 h. To investigate the reusability of the
catalyst, a sample was tested for three successive cycles, raising the
catalyst temperature to a maximum of 350 ◦C for each cycle, before
cooling and restarting the cycle.
nitrate solution was mixed with a sodium carbonate solution (2 M)
at an appropriate rate to maintain a pH of 8.3 in a stirred precipi-
tation vessel maintained at 80 ◦C. The resulting precipitate was left
to age for 30 min at 80 ◦C before recovery by filtration, thoroughly
washed and then dried for 16 h at 110 ◦C. Catalysts were generated
by calcination of the precursor material in static air for 2 h at 300,
400, 500 and 600 ◦C. The heating ramp rate was 2 ◦C min−1 starting
from ambient temperature
2.2. Catalyst characterisation
Structural phase information on the materials was determined
using powder X-Ray diffraction. The data was gathered using a PAN-
alytical X’Pert Pro powder X-ray diffractometer and compared with
the ICDD database. The diffractometer was fitted with a Cu source
operating at 40 kV and 40 mA.
Thermal gravimetric analysis of the precursor was undertaken
using a Setram Labsys TGA/DTA instrument. The sample was heated
from 30 to 700 ◦C under flowing air (15 mL min−1) at a heating rate
of 10 ◦C min−1. The mass change and the heat flow of the sample
was monitored.
3. Results and discussion
3.1. Catalyst and precursor characterisation
Temperature-programmed reduction (TPR) analysis of the cat-
alysts was performed using a Quantachrome ChemBET TPD/R/O
apparatus. In total, 30 mg sample of each catalyst was heated to
600 ◦C at a rate of 15 ◦C min−1 under a flow of 10%H2/Ar at a flow
Fig. 1a shows the X-ray diffraction pattern of the catalyst
precursor (material obtained after drying the precipitate before
calcination). The only phase observed was that of the hexagonal-
rhombohedra MnCO3 phase. This was indicated by the principle
reflection at 32◦ corresponding to the (114) set of lattice planes. The
reflection associated with the (012) plane was also clearly observed
at 24.5◦, as was the (116) at 52.2◦. No evidence was observed for
reflections that corresponded to a copper-containing phase. How-
ever, when compared with a standard MnCO3 diffraction pattern,
the peaks appear shifted to higher 2Â angles by around 0.5◦. In the
standard MnCO3 sample, the (114) plane is observed at 31.4◦, the
(012) plane at 24.3◦ and the (116) plane at 51.7. This is consistent
with previous work by Porta et al., who demonstrated that when
copper is substituted into the MnCO3 lattice, the unit cell contracts
due to the smaller ionic radius of copper, site distortion due to the
d9 electron configuration as well as the higher covalency of the
copper–oxygen polyhedral [18]. Indeed, previous work on related
copper manganese oxide catalysts prepared mechanochemically
rate of 30 mL min−1
.
The surface area of the catalysts was determined using the BET
method. The catalysts were degassed by flushing with dry N2 for
0.5 h at a temperature of 120 ◦C to avoid catalysts becoming nonsto-
ichiometric by degassing under vacuum. A 5-point N2 adsorption
isotherm was measured at −196 ◦C using a Micromeritics Gemini
2360 instrument.
2.3. Catalytic activity determination
Catalyst activity was determined using a standard fixed-bed gas
flow microreactor. A 100 vppm flow of naphthalene was generated
by heating naphthalene (scintillation grade, Sigma–Aldrich) in a
tube furnace under a flow of helium. Oxygen was then added to give
Fig. 1. a. Powder X-ray diffraction patterns of precursor materials 1. Standard MnCO3, 2. Copper manganese oxide catalyst precursor. ꢀ = MnCO3. b. Thermal gravimetric
analysis profile of the catalyst precursor.
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Fig. 2. Powder X-ray diffraction patterns of calcined copper manganese oxide catalysts a. 300 ◦C, b. 400 ◦C, c. 500 ◦C, and d. 600 ◦C. ꢀ = MnCO3, ᭹ = Mn2O3, ꢁ = CuMn2O4.
showed a decrease in the lattice parameter of MnCO3, as increased
concentrations of Cu2+ were incorporated [19].
After calcination at 400 ◦C the reflections that correspond to
MnCO3 were no longer visible. There was only one observable
diffraction peak, which was a very broad feature at 36.1◦. This could
correspond with the (311) lattice plane of CuMn2O4; however, as
there are no other visible reflections, it is difficult to be certain, as
the (311) reflection is the most intense, it is unsurprising that no
others are visible, given how weak it is. The absence of other reflec-
tions and the broad nature of the peak suggest a nanocrystalline or
highly disordered material.
The TGA profile of the catalyst precursor (Fig. 1b) showed
two weight-loss events. The first broad mass loss from 100 to
350 ◦C is associated with the loss of physisorbed and chemisorbed
water from the precursor material and makes up 13% of the total
mass. The second loss at 400 ◦C was due to the decomposition
of MnCO3 with associated evolution of CO2. This second mass
loss was ca. 20% of the total mass. If the precursor material was
MnCO3 alone, the theoretical mass loss would be ca. 30%. Possi-
bly, suggesting that MnCO3 was not the only phase present. The
absence of diffraction from another phase indicates that it is either
amorphous, or at least present as very small crystallites. This inter-
pretation is somewhat speculative and the discrepancy could be
equally explained by a change of the carbonate stoichiometry,
possibly by substitution of Cu ions into the Mn lattice. Further
characterisation studies would be required to provide a definitive
explanation.
The precursor calcined at 500 ◦C showed clear evidence for for-
mation of the spinel CuMn2O4 phase. The peak at 36.3◦ can be
assigned to diffraction of the (311) planes and the reflections at
57.5 and 63.1◦ can be attributed to the diffraction from the (511)
and (440) planes, respectively.
Calcination at 600 ◦C resulted in a mixture of phases, with evi-
dence of a Mn2O3 phase present with a mixed copper manganese
oxide phase. The reflection observed at 33.1◦ is characteristic of
the (222) lattice planes of Mn2O3. The reflections associated with
CuMn2O4 are now more easily observed, and they are narrower
and more intense than those after calcination at lower tempera-
tures, suggesting that the mixed copper manganese oxide phase has
longer-range crystallinity after calcination at higher temperature.
The BET surface areas for the catalysts prepared at different cal-
cination temperatures are shown in Table 1. The sample calcined at
X-ray diffraction patterns of the calcined catalysts investigated
for naphthalene oxidation are shown in Fig. 2. The catalyst calcined
at 300 ◦C still consisted of MnCO3 with no evidence for formation
of the spinel CuMn2O4 phase. This is, maybe, not unexpected, as
the TGA profile in Fig. 1b shows that significant decomposition of
MnCO3 did not begin until 400 ◦C.
Table 1
Physical properties of the tested catalysts.
Calcination
Surface
Phases present
H2 con-
T50a/◦C
T90a/◦C
T for 50%
T for 90%
CO2 selectivity
at 50%
conversion/%
temperature/◦C
area/m2 g−1
sumed/mmol g−1
naphthalene
naphthalene
coversion/◦C
coversion/◦C
Uncalcined
300
400
500
5
9
72
52
14
MnCO3
MnCO3
CuMn2O4
CuMn2O4
CuMn2O4, Mn2O3
–
–
242
229
239
269
–
250
238
270
290
–
242
228
230
248
–
250
238
258
262
–
100
99
30
20
7.4
12.4
8.7
7.2
600
Comparison catalysts
1% Pt/Al2O3
Commercial Hopcalite
–
–
–
–
250
226
280
245
–
226
–
100
245
a
Temperature for 50 and 90% conversion to CO2
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material, and hence, has the largest concentration of labile oxygen.
For calcination at 300 ◦C, the material was less reducible, presum-
ably due to the incomplete formation of the oxide phases. As calci-
nation temperatures were increased, the total amount of reducible
material decreased, this is attributed to the higher crystallinity
of these materials and the resulting decrease in lattice oxygen
mobility.
3.2. Catalytic activity
Figure 4a shows the formation of CO2 as a function of catalyst
temperature, and comparisons with a commercially available Hop-
calite and a commercial 1% Pt/Al2O3 catalyst have been made. From
data in Fig. 4a and T50 and T90 measurements for CO2 formation
(Table 1), the catalyst calcined at 400 ◦C was the most active. This
can be related to the presence of the amorphous CuMn2O4 phase
that has previously been shown to be important for CO oxidation
[13] and propane total oxidation [15]. This maximum of activity
correlates with the TPR data (Table 1), that showed that the catalyst
calcined at 400 ◦C had the highest extent of reduction and the high-
est amount of labile oxygen species. The catalyst calcined at 300 ◦C
showed lower activity, and this can be related to the presence
of MnCO3, which was not converted to the oxide during calcina-
tion. Calcination at higher temperatures increased the temperature
required for naphthalene total oxidation. This can be observed by
considering the T50 and T90 values, i.e. the temperatures required
to achieve 50 and 90% naphthalene conversion to CO2, respectively.
For example, T50 for the catalyst calcined at 500 ◦C was 10 ◦C higher
and the material calcined at 600 ◦C had a T50 40 ◦C higher than the
material calcined at 400 ◦C.
The catalytic activity was also analysed in terms of naphthalene
conversion (Fig. 4b). The trend of the data is the same with 400 ◦C
being the most active for naphthalene conversion. The T50 values for
the 400 ◦C calcined catalyst were similar when CO2 yield and naph-
thalene conversion were compared. Furthermore, the T50 values for
naphthalene conversion for the 400 and 500 ◦C catalysts were sim-
ilar. However, for both the catalysts calcined at 500 and 600 ◦C,
the temperature for 50% naphthalene conversion was obtained at a
lower temperature than the T50 for CO2 formation (Table 1). These
differences between naphthalene conversion and CO2 yield indi-
cate that the catalysts calcined at 500 and 600 ◦C are less selective
towards CO2 than the catalyst calcined at 400 ◦C. These discrepan-
cies between CO2 formation and naphthalene conversion suggest
the formation of naphthalene partial oxidation products. Previous
work on naphthalene oxidation from our group has shown that par-
tial oxidation products are commonly produced, and we found that
benzoic acid and phthalic anhydride were common by-products
[9]. Based on our experience, we consider that similar products
are also produced in the study with selected catalysts, but at this
preliminary stage, we have not specifically analysed for them. It is
also possible that partial oxidation products are formed as primary
products over the most active catalyst calcined at 400 ◦C, how-
ever, they are rapidly oxidised to CO2 before exiting the catalyst
bed. The catalytic data suggests that poorly crystalline CuMn2O4 is
the most active phase for the total oxidation of naphthalene, with
more highly crystalline CuMn2O4 and less active Mn2O3 phases.
The more crystaline CuMn2O4 phase also appears to have poorer
selectivity towards CO2 than the more disordered phase, as seen
in the material calcined at 500 ◦C. Partial oxidation of naphthalene
has been observed in previous work by Garcia et al. when using
manganese oxide catalysts, this phase could be responsible for the
low-CO2 selectivity for the catalyst calcined at 600 ◦C, as Mn2O3
was a major catalyst component [9]. The formation of partial oxi-
dation products could also be directly related to the surface area
of the catalysts, as the catalyst calcined at 400 ◦C (negligible par-
tial oxidation products) has a greater surface area to re-adsorb and
Fig. 3. Hydrogen temperature-programmed reduction profiles of catalyst materials
a. 300 ◦C, b. 400 ◦C, c. 500 ◦C, and d. 600 ◦C.
300 ◦C showed a very low surface area, characteristic of manganese
carbonate. At 400 ◦C, the surface area is dramatically increased to
72 m2 g−1, and this is related to formation of a nanocrystalline
CuMn2O4 phase [14]. As the calcination temperature increased fur-
ther, the catalyst surface area decreased, as would be expected due
to the effects of sintering of crystallites of the mixed copper man-
ganese oxide phase. At the highest calcination temperature, the
formation of the low surface area Mn2O3 phase further depresses
the overall surface area of the material [12].
Temperature-programmed reduction (TPR) of the materials is
shown in Fig. 3. The results showed a complex series of over-
lapping peaks, which are often observed for copper manganese
oxides. After calcination at 300 ◦C, the catalyst showed a broad
peak centred at 202 ◦C followed by sharper peaks at 232 and 259 ◦C.
These peaks are attributed to the complex reduction of the copper-
substituted manganese carbonate phase. When calcined at 400 ◦C,
there were multiple peaks with the main peak at 250 ◦C. This is
attributed to the reduction of the nanocrystaline CuMn2O4 phase
with the other shoulders possibly arising from residual carbon-
ate reduction. At 500 ◦C, the profile is that of a typical crystalline
spinel CuMn2O4 reduction with a two stage reduction at 248 and
291 ◦C [20]. In their study of copper manganese oxide, Buciuman
et al. attribute the two-step reduction to the reduction of the two
different cations present in the spinel structure with the Cu2+ com-
ponent reducing initially to Cu0 followed by the reduction of the
Mn3+ to the Mn 2+species [20]. After 600 ◦C calcination, the high-
est studied, the two-step reduction profile remained; however,
the higher temperature peak is broadened due to the contribu-
tion from the reduction of the Mn2O3 phase to MnO. The extent of
reducibility of the catalysts was determined by measuring the total
hydrogen consumption during the reduction (Table 1). The cata-
lyst calcined at 400 ◦C contained the greatest amount of reducible
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Fig. 4. a. Light-off curve for CO2 production from catalytic naphthalene oxidation, b. Light-off curve for naphthalene conversion over the range of catalysts. ꢂ = 300 ◦C,
ꢀ = 400 ◦C, ꢁ = 500 ◦C, ꢃ = 600 ◦C, ꢀ = Commercial Hopcalite, ꢁ = 1% Pt/Al2O3. Error = 6.7%.
Fig. 5. a. Conversion of naphthalene to CO2 in successive reaction cycles using the catalyst calcined at 400 ◦C, ꢃ = Cycle 1, ꢀ = Cycle 2, ꢂ = Cycle 3, ◦ = Cycle 4. Error = 6.7%;
b. Time-on-stream catalyst stability test of material calcined at 400 ◦C. Error = 6.7%.
further oxidise partially oxidised products when compared with
the catalysts calcined at higher temperatures.
time-on-stream. The results of temperature cycling experiments
are shown in Fig. 5a. After the first light-off curve, there was a
slight deactivation of the catalyst. Subsequent light-off curves were
superimposable with each other, indicating that after the initial
use, there was no further catalyst deactivation. The catalyst was
also subjected to an accelerated ageing time-on-stream experiment
for 48 h at 350 ◦C. As shown in Fig. 5b, the activity of the catalyst
decreased from 100 to around 90% within the first 4 h of testing. The
activity then stabilised at this level for the remainder of the exper-
iment and there was no further evidence of catalyst deactivation.
The deactivation observed could be due to several factors, such as
loss of surface area or increased surface concentration of species
like CO2 or partially oxidised products. Further studies would be
required to confirm the definitive reason.
It is interesting to benchmark the performance of the coprecip-
itated copper manganese oxide catalysts, as there are no previous
data available for naphthalene oxidation (Table 1). When compared
with a commercially available copper manganese oxide catalyst,
the coprecipitated catalyst calcined at 400 ◦C performed similarly
with comparable T50 and T90 values. However, the composition
of the commercial catalyst is more complex than copper man-
ganese oxide alone and also contains promoter materials that are
not present in the coprecipitated catalysts prepared in this study.
Commonly, supported noble metal catalysts have been successfully
used for naphthalene total oxidation. Comparison with a com-
mercial 1% Pt/Al2O3 catalyst demonstrated that the coprecipitated
copper manganese oxide catalysts were more active when com-
pared in terms of T50 and T90
.
4. Conclusions
The stability of the most active catalyst, prepared by calci-
nation at 400 ◦C, was evaluated by cycling through a series of
light-off experiments and by measuring activity as a function of
Precipitated copper manganese oxide catalysts are active for
naphthalene total oxidation. A poorly crystalline CuMn2O4 phase
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was more active than the crystalline spinel phase. The optimum
calcination temperature for these catalysts was 400 ◦C. Lower cal-
cination temperatures were insufficient to generate the active
CuMn2O4 phase. At higher calcination temperatures, the increased
crystallinity of the catalysts and lower surface areas resulted in
lower total oxidation activity, also demonstrated by the production
of more undesirable partially oxidised products. A direct relation-
ship between the performance for naphthalene oxidation and the
extent of reduction of the catalyst was observed, indicating that
the lability of oxygen species of the catalyst are important. The
most active catalyst, prepared after calcination at 400 ◦C, demon-
strated higher activity for naphthalene oxidation than a supported
platinum catalyst, and was at least equivalent to a commercial
Hopcalite catalyst. Furthermore, the catalyst was stable on reuse,
showing a slight deactivation after the first use, but no further
deactivation thereafter. Slight deactivation was also observed with
prolonged time-on-stream, but this was only in the first 4 h and then
stable high activity was maintained.
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Please cite this article in press as: T.J. Clarke, et al., Total oxidation of naphthalene using copper manganese oxide catalysts, Catal. Today
(2015), http://dx.doi.org/10.1016/j.cattod.2015.01.032