Total oxidation of model volatile organic compounds over some commercial catalysts

Article abstract of DOI:10.1016/j.apcata.2012.07.018

Commercial VOC oxidation catalysts can be used as comparative materials during development of new or improved catalysts. The objective of this study was to investigate physicochemical properties of EnviCat commercial catalysts and their performance in total oxidation of three model compounds (dichloromethane, toluene and ethanol) at laboratory scale. The reactivity of model VOC was decreasing in the order ethanol > toluene > dichloromethane. The Cu-Mn/Al catalyst was found to be the most active and selective catalyst in ethanol oxidation. In oxidation of dichloromethane, the Pt-Pd/Al-Ce catalyst with 0.10 wt% Pt + Pd was the most active, while the most selective one (giving the highest HCl yield) was the Pt-Pd/Al catalyst containing 0.24 wt% Pt + Pd. In toluene oxidation, the Pt-Pd/Al catalyst with 0.24 wt% Pt + Pd possessed the highest activity; the selectivity to CO₂ was 100% for all investigated catalysts. Obtained results showed that the performance of commercial catalysts in laboratory scale tests can be different from that declared by catalyst supplier. A possible difference in catalytic performance at industrial and laboratory scale should be taken into account when industrial catalysts are used in laboratory scale tests.

Full text of DOI:10.1016/j.apcata.2012.07.018

Applied Catalysis A: General 443–444 (2012) 40–49
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Total oxidation of model volatile organic compounds over some
commercial catalysts
∗
ˇ
Lenka Mateˇjová, Pavel Topka , Kveˇtusˇe Jirátová, Olga Solcová
Institute of Chemical Process Fundamentals of the ASCR, v. v. i., Rozvojová 135, CZ-165 02 Praha 6, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Commercial VOC oxidation catalysts can be used as comparative materials during development of new
or improved catalysts. The objective of this study was to investigate physicochemical properties of
EnviCat® commercial catalysts and their performance in total oxidation of three model compounds
(dichloromethane, toluene and ethanol) at laboratory scale. The reactivity of model VOC was decreasing
in the order ethanol > toluene > dichloromethane. The Cu–Mn/Al catalyst was found to be the most active
and selective catalyst in ethanol oxidation. In oxidation of dichloromethane, the Pt–Pd/Al–Ce catalyst
with 0.10 wt% Pt + Pd was the most active, while the most selective one (giving the highest HCl yield)
was the Pt–Pd/Al catalyst containing 0.24 wt% Pt + Pd. In toluene oxidation, the Pt–Pd/Al catalyst with
0.24 wt% Pt + Pd possessed the highest activity; the selectivity to CO2 was 100% for all investigated cat-
alysts. Obtained results showed that the performance of commercial catalysts in laboratory scale tests
can be different from that declared by catalyst supplier. A possible difference in catalytic performance
at industrial and laboratory scale should be taken into account when industrial catalysts are used in
laboratory scale tests.
Received 17 February 2012
Received in revised form 13 July 2012
Accepted 16 July 2012
Available online 22 July 2012
Keywords:
Oxidation
Volatile organic compounds
Ethanol
Toluene
Dichloromethane
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
can be carried out at lower temperatures (below 500 ◦C) and is
therefore regarded as efficient, cost-effective and environmentally
Volatile organic compounds (VOC) are known as one of the
major contributors to air pollution. They may have short and long-
term adverse health effects, act as greenhouse gases and are the
major source of ground-level ozone, which is the primary con-
stituent of the photochemical smog [1,2]. Anthropogenic sources of
VOC include chemical plants, petroleum refineries, power plants,
gas stations and dry cleaners. Moreover, an increasing amount of
carcinogenic VOC like formaldehyde and acetaldehyde is produced
due to growing utilization of the ethanol-based biofuels in cars [3].
The VOC emissions are regulated by the legislation of European
Union since 1999, when the Solvent Directive was launched. In
2005, the Commission of the European Communities published the
Thematic Strategy on Air Pollution [4]. Its aim is to cut the annual
number of premature deaths caused by air pollution by 40% of the
2000 level in 2020 and to reduce the continuing damage to Europe’s
ecosystems. In order to achieve these aims, the emissions of VOC
will need to be reduced by 51% compared to their 2000 levels.
Oxidation is the most commonly used technique for VOC con-
trol. Typically, VOCs have been removed using thermal incineration
at temperatures higher than 900 ◦C. However, the thermal decom-
position is gradually being replaced by catalytic oxidation, which
friendly way to treat VOC emissions [5]. Moreover, the lower tem-
perature and the use of appropriate catalyst avoid generation of
dangerous reaction by-products like NOx [6].
The most commonly used VOC oxidation catalysts are based on
supported noble metals or transition metal oxides. Metal oxides,
mainly of Co, Cu, Ni and Mn, have the advantage of having lower
cost and greater resistance to deactivation by poisoning [7]. On the
other hand, noble metal catalysts are generally preferred because
of their high activity, excellent stability and good selectivity to CO2
[8]. Both types of catalysts have been extensively studied in the
literature [9,10].
Commercial VOC oxidation catalysts may be used as the compar-
ative catalytic materials during catalytic testing of newly developed
catalysts at laboratory scale. The aim of this study was to report
the performance of the typical commercial catalysts in total oxi-
dation of model volatile organic compounds (ethanol, toluene, and
dichloromethane) at laboratory scale and to describe their physic-
ochemical properties.
2. Experimental
2.1. Catalysts
∗
Commercial catalysts EnviCat® HHC-5557, EnviCat® VOC-
5565, and EnviCat® VOC-1544 were purchased from Südchemie,
Corresponding author. Tel.: +420 220 390 282; fax: +420 220 920 661.
E-mail address: topka@icpf.cas.cz (P. Topka).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.07.018

L. Mateˇjová et al. / Applied Catalysis A: General 443–444 (2012) 40–49
41
Germany. According to their product data sheets, EnviCat® VOC-
5565 is intended for the oxidation of VOC, EnviCat® HHC-5557
is designed for the oxidation of chlorinated VOC, and EnviCat®
VOC-1544 is recommended for the oxidation of oxygenated VOC.
The catalysts in the form of spheres were crushed into a pow-
der and then sieved. The particle size fraction 0.160–0.315 mm
was employed in all experiments. The catalysts were denoted as
Pt–Pd/Al (EnviCat® HHC-5557), Pt–Pd/Al–Ce (EnviCat® VOC-5565),
and Cu–Mn/Al (EnviCat® VOC-1544).
experiments were evaluated using the OriginPro 7.5 software with
an accuracy of 5%.
2.3. Catalytic experiments
Total oxidation of toluene was carried out in a fixed-bed flow
glass reactor (inner diameter 5 mm) in the temperature range from
50 ◦C to 400 ◦C (the furnace temperature was linearly increased
with the rate of 3.5 ◦C min−1). The catalyst (0.225 g of sieved
grains with particle size of 0.160–0.315 mm) was examined at
71 m3 kg−1 h−1 space velocity. Temperature was measured inside
the reactor before and behind the catalyst bed and the average tem-
perature was taken as catalyst temperature. The inlet concentration
of toluene in the air was 1000 volume ppm. The catalysts were
used as-received without any pretreatment. Before the experiment
the catalytic bed was kept under the feed stream until the toluene
outlet concentration became constant.
Reaction products were analyzed by a gas chromatograph
Hewlett-Packard 6890 equipped with a FID detector and a capil-
lary column (HP-5 19091 J-413, 30 m × 0.32 mm × 0.25 mm with 5%
phenylmethyl silicone). Concentrations of CO and CO2 were moni-
tored using a Siemens Ultramat 23 infrared analyzer. Conversion of
toluene C and selectivity to carbon dioxide S were calculated based
on the material balance using Eqs. (1) and (2):
2.2. Characterization
The catalysts were characterized by X-ray fluorescence (XRF),
X-ray powder diffraction (XRD), field-emission scanning electron
microscopy (FE-SEM), N2 physisorption, temperature programmed
reduction by hydrogen (H2–TPR), and temperature programmed
desorption of ammonia (NH3–TPD).
Chemical analysis of catalysts was done using an ICP-AES ana-
lyzer and XP sequential WD-XRF spectrometer Thermo ARL 9400.
In order to obtain the most reliable results, the content of Pt, Pd, Cu
and Mn was determined using ICP-AES analysis, while the content
of other elements was analyzed using XRF.
The X-ray powder diffraction data were collected on a PANa-
lytical XˇıPert PRO diffractometer in Bragg-Brentano geometry with
fixed slits using Co K␣ radiation. The patterns were acquired in the
diffraction angle range 20–90◦ (2ꢀ) with a step size of 0.02◦ (2ꢀ)·
The field-emission scanning electron microscopy (FE-SEM)
images were taken using JEOL JSM-7500f scanning electron micro-
scope. The samples for FE-SEM measurements were prepared by
the powder dispersal in ethanol and subsequent dropping of the
solution onto a carbon grid or by depositing the powder sample on
a graphite tape.
N2 physisorption on catalyst powders (grain size
0.160–0.315 mm) was performed using Micromeritics ASAP
2020 instrument after drying at 105 ◦C under 1 Pa vacuum for
24 h. The adsorption–desorption isotherms of nitrogen at −196 ◦C
were treated by the standard Brunauer–Emmett–Teller (BET)
procedure to calculate the specific surface area SBET. The surface
area of mesopores Smeso and the volume of micropores Vmicro
were determined by the t-plot method. The pore-size distribution
(pore radius 100–102 nm) was calculated from the desorption
branch of the adsorption–desorption isotherm by the advanced
Barrett–Joyner–Halenda (BJH) method [11,12]. The Lecloux–Pirard
standard isotherm [13] was employed for the t-plot as well as for
the pore-size distribution evaluation.
The temperature-programmed reduction (TPR) measurements
of catalysts (0.025 g) were performed with a H2/N2 mixture
(10 mol% H2), flow rate 50 mL min−1 and linear temperature
increase 20 ◦C min−1 up to 1000 ◦C. A change in H2 concentration
was detected with a catharometer. Reduction of the grained CuO
(0.160–0.315 mm) was performed in each experiment to calculate
the absolute values of hydrogen consumed during reduction.
The temperature-programmed desorption (TPD) measurements
of NH3 was carried out to examine acid properties of the cata-
lysts surface. The measurements were accomplished with 0.05 g
of a sample in the temperature range 20–1000 ◦C, with helium as
a carrier gas and NH3 as an adsorbing gas. Prior to the measure-
ment, each sample was calcined in helium at 500 ◦C, then cooled
to 30 ◦C and an excess of NH3 (ten doses, 840 ␮l each) was applied
on the sample. Then, the sample was flushed with helium for 1 h to
remove physically adsorbed ammonia and after that heating rate
of 20 ◦C min−1 was applied. A change in NH3 concentration was
detected by the mass spectrometer Omnistar 300 (Pfeiffer Vac-
uum). During the experiments the following mass contributions
m/z were collected: 2-H2, 18-H2O, and 16-NH3. The TPR and TPD
ain − a
C =
S =
(1)
(2)
ain
ꢀ
n(ain − a/fM) −
niai/fiMi
ꢀ
n(ain − a/fM) −
(ni − 1)ai/fiMi
where ain is the GC peak area of toluene corresponding to its inlet
concentration (4.1 g m−3), a is the GC peak area of toluene, n is
the number of carbon atoms in the molecule of toluene, f is the
experimentally determined FID relative response factor of toluene
(1.05), M is the molecular weight of toluene, ni is the number of
carbon atoms in the molecule of corresponding by-product, ai is
the GC peak area of corresponding by-product, fi is the experi-
mentally determined FID relative response factor of corresponding
by-product (1.00 for ethylene), and Mi is the molecular weight
of corresponding by-product. The accuracy of the conversion and
selectivity determination was 2%.
Temperatures T50 and T90 (the temperature at which 50 and
90% conversion of the examined MC was observed, respectively)
were chosen as a measure of catalyst activity. The catalyst selec-
tivity was evaluated as the selectivity to CO2 at 95% conversion of
the examined MC (SCD,95). In order to eliminate the influence of
CO2 adsorption on the catalyst, the selectivity was computed from
GC peak areas using the material balance according to Eq. (2); at
95% conversion, the presence of CO was not detected in any of the
experiments.
Total oxidation of dichloromethane and ethanol was carried out
in a fixed-bed flow quartz tubular reactor (inner diameter 8 mm)
enabling also tests with chlorinated compounds [14]. The reactor
operating under atmospheric pressure was located in a vertically
situated ceramic tubular oven. Temperature was measured outside
the reactor on its wall right before the catalyst bed.
The analysis of reaction products was performed on the Gasmet
DX-4000 N FTIR analyzer, which is able to detect almost all gaseous
compounds excluding noble gases and diatomic homonuclear com-
pounds such as O2, N2, and Cl2. The analyzer consists of a high
temperature sample cell, a temperature controller, a Peltier cooled
MCT-detector and signal processing electronics. It was calibrated to
detect following chlorinated hydrocarbons: C2Cl4, C2HCl3, CH3Cl,
CH2Cl2, CHCl3, COCl2, HCl and following oxyderivatives of hydro-
carbons: CO2, CO, CH3CHO, CH2O, CH3COOH, CH3COOC2H5, CH3OH,

42
L. Mateˇjová et al. / Applied Catalysis A: General 443–444 (2012) 40–49
Table 1
Chemical composition of catalysts in wt%.
Pt–Pd/Al
Pt–Pd/Al–Ce
Cu–Mn/Al
Cu
Mn
Pt
Pd
CeO2
MgO
CaO
Na2O
K2O
Cl
–
–
–
–
3.34
5.44
–
0.13
0.11
0.02
0.19
0.06
0.27
0.03
0.35
98.75
0.05
0.05
6.45
–
0.04
0.12
0.01
–
–
0.03
0.16
0.06
0.06
0.01
–
Al2O3
92.84
90.75
CH3CH2OH and C2H4. The measured spectra were analyzed with
Calcmet for Windows analysis software.
The initial dichloromethane (DCM) or ethanol concentration
was 1000 volume ppm in all tests (DCM ∼3.79 g m−3, ethanol
∼2.05 g m−3). The activity tests with DCM were performed in the
presence of 1.5 wt% of water to ensure sufficient amount of hydro-
gen and thus improve the selectivity towards desired HCl [15–20].
Each test was carried out with 0.870 g of fresh catalyst (grain size
0.160–0.315 mm) without any pretreatment. The flow of the reac-
tion mixture was 1.03 l min−1, which corresponded to the space
velocity of 71 m3 kg−1 h−1. The reaction temperature was linearly
increased from 100 ◦C to 500 ◦C with heating rate 3.5 ◦C min−1
(DCM oxidation) or from 50 ◦C to 400 ◦C with the same heating
rate (ethanol oxidation). The catalysts were used as-received with-
out any pretreatment. Before the experiment the catalytic bed was
kept under the feed stream until the ethanol or DCM outlet con-
centration became constant.
Fig. 1. X-ray diffraction patterns of Pt–Pd/Al, Pt–Pd/Al–Ce and Cu–Mn/Al (᭹ ␥-
Al2O3/␣-Al2O3, ꢀ cubic CeO2 (cerianite), × unidentified).
0.35 wt% of chlorine was detected, which is very likely coming from
chloroplatinic acid used during the synthesis of the catalyst.
X-ray diffraction patterns of the catalysts measured using Co
K␣ radiation are shown in Fig. 1. Due to the unknown nature
of alumina precursor(s) and the procedures and temperatures by
which the alumina precursor(s) were treated to produce the final
commercial catalysts it should be noted that the differentiation of
alumina phases presented in the examined catalysts is very com-
plex and might be extremely speculative. Based on the knowledge
of maximum temperatures to which the catalysts can be used, i.e.
680 ◦C for the Pt–Pd/Al and Pt–Pd/Al–Ce catalysts and 500 ◦C for
the Cu–Mn/Al catalyst, the ␥-, ␩- and ␣-alumina were assumed
for the identification. Because the cubic ␥-alumina and ␩-alumina
are hardly distinguishable, it cannot be surely concluded which of
these two alumina phases exhibits higher symmetry of reflections.
However, in all patterns the untypical split of (4 0 0) reflection,
(3 1 1) reflection (in Pt–Pd/Al–Ce) and (2 2 0) and (4 4 0) reflections
(in Cu–Mn/Al) can be seen. These features lead to the idea that (i)
probably some amount of another type of alumina than ␥ and ␩ is
present in the catalysts and/or (ii) identified cubic alumina might be
low-symmetric, e.g. due to strong hydroxylation. Snyder et al. [21]
studied the transformation of boehmite and bayerite to ␥-, ␩- and
ꢀ-alumina at elevated temperature. The split of all these reflections
was observed for structural transformation to ꢀ-alumina, however,
they did not observed the coexistence of ␥- and ␩- with ␪-alumina.
The split of (3 1 1) reflection can be also seen in corundum (␣-
alumina), but its other intense peaks are missing in the diffraction
patterns of the Pt–Pd/Al–Ce catalyst. In the Pt–Pd/Al–Ce catalyst,
the presence of another nanocrystalline phase – cubic CeO2 (ceri-
anite) – is confirmed by (1 1 1) reflection at 2ꢀ = 33.3◦. Other intense
CeO2 reflections (2 0 0 and 3 1 1) cannot be found because they are
either not present or are overlapped by ␥-lumina (2 2 0) and (4 2 2)
reflections. The crystallite sizes of oxidic phases were not evalu-
ated due to defects in alumina, which is far from the ideal crystal
structure. Crystalline phases of Pt and Pd oxides were not found due
to their low concentrations in both the Pt–Pd/Al and Pt–Pd/Al–Ce
catalysts. Any crystalline phases of following Cu and/or Mn oxides
were not identified: CuMn2O4, CuMnO2, Cu2O, CuO and Mn3O4. Cu
and/or Mn oxides are either amorphous or their diffraction lines
are overlapped by intense diffraction lines of aluminas.
Temperatures T50 and T90 were chosen as a measure of catalyst
activity. Selectivity to CO2 at 95% ethanol conversion (SCD,95) was
calculated according to Eq. (3):
nCD
ꢀ
SCD,95
=
(3)
nCD
+
ni
ꢀ
where nCD is the molar amount of CO2 and
ni is the sum of molar
amounts of all other detected reaction products except H2O. For
DCM oxidation, the maximum HCl yield YHCl,max and the HCl yield
at 95% DCM conversion YHCl,95 were evaluated according to Eq. (4):
cHCl
YHCl
=
(4)
in
2c
DCM
where YHCl is the HCl yield, cHCl is the output concentration of HCl,
in
DCM
and c
is the input concentration of DCM (both concentrations
are in volume ppm).
3. Results and discussion
3.1. Characterization of the catalysts
Chemical composition of catalysts determined by ICP-AES and
XRF is summarized in Table 1. Two examined catalysts are based on
Pt and Pd. The Pt–Pd/Al catalyst contains higher amount of Pt and
Pd (0.13 and 0.11 wt%, respectively) than the Pt–Pd/Al–Ce catalyst
(0.05 and 0.05 wt%, respectively). In the Pt–Pd/Al–Ce catalyst, Pt and
Pd are deposited on a mixed support composed of Al2O3 (92.84 wt%)
and CeO2 (6.45 wt%), while the Pt–Pd/Al catalyst is supported on
Al2O3 (98.75 wt%). In contrast to the catalysts containing noble met-
als, the third catalyst (Cu–Mn/Al) is based on Cu and Mn oxides
supported over Al2O3. Amount of Cu and Mn oxides is significantly
higher (8.78 wt%) than that of the noble metal catalysts. The cata-
lysts also contain traces of alkali and alkali earth metal oxides (0.55
and 0.17 wt% for Pt–Pd/Al and Pt–Pd/Al–Ce catalysts, respectively,
and 0.29 wt% for the Cu–Mn/Al catalyst). In the Pt–Pd/Al catalyst,
Three selected FE-SEM images illustrate the morphology of the
catalysts (Fig. 2). Fig. 2a (catalyst Pt–Pd/Al) shows irregular aggre-
gates of a matter composed of primary fine and relatively uniform
nanoparticles. Very similar nanoparticulate morphology can be
seen also in Fig. 2b where the Pt–Pd/Al–Ce catalyst is presented.

L. Mateˇjová et al. / Applied Catalysis A: General 443–444 (2012) 40–49
43
Fig. 2. FE-SEM images of: (a) Pt–Pd/Al; (b) Pt–Pd/Al–Ce and (c) Cu–Mn/Al.
In addition to the primary nanoparticles the larger nanoparticles
with the size around 40–45 nm are also visible in this case. The
FE-SEM image of the Cu–Mn/Al catalyst is shown in Fig. 2c. Here,
the mixture of fine nanoparticles with irregular flakes can be seen.
These flakes could be assigned either to Cu and Mn oxides or to a
type of alumina which is not present in Pt–Pd/Al and Pt–Pd/Al–Ce
catalysts.
N2 physisorption was used to determine the textural parame-
ters of the catalysts. The obtained results are summarized in Table 2.
Although the use of classic (two-parameter) BET equation for the
analysis of adsorption isotherms of microporous–mesoporous sam-
ples is not correct [22], the surface area SBET is also included
in Table 2 for comparison with literature data. The nitrogen
adsorption–desorption isotherms recorded at −196 ◦C are depicted
in Fig. 3a with the adsorbed amount, a, expressed as gas adsor-
bate volume per gram of adsorbent for all samples. Nitrogen
isotherms of all catalysts are very similar and correspond to the
combination of I and IV type isotherms according to IUPAC clas-
sification [23]. For all samples a small increase of adsorption
amount for x = p/p0 → 0 is evident, which indicates the presence of
Fig. 3. (a) N2 adsorption–desorption isotherms at −196 ◦C and (b) pore-size distributions evaluated from nitrogen physical adsorption of Pt–Pd/Al (dash-dot line), Pt–Pd/Al–Ce
(gray line) and Cu–Mn/Al (black line).

44
L. Mateˇjová et al. / Applied Catalysis A: General 443–444 (2012) 40–49
Table 2
Textural properties of catalysts and their TPR and TPD characteristics.
Catalyst
SBET (m2 g−1
)
Smeso (m2 g−1
)
Vmicro (mmliq3 g−1
)
Vp (mmliq3 g−1
a
)
H2–TPRb (mmol g−1
)
H2–TPRc (mmol g−1
)
NH3–TPDb (mmol g−1
)
Pt–Pd/Al
Pt–Pd/Al–Ce
Cu–Mn/Al
107
96
73
70
60
43
21
20
17
593
464
472
0.02
0.18
0.94
0.16
0.75
1.42
0.47
0.17
0.29
a
Net pore volume (for p/p0 = 0.985).
b
c
Quantitative data in the temperature range from 25 ◦C to 500 ◦C.
Quantitative data in the temperature range from 25 ◦C to 1000 ◦C.
micropores. A steep part of hysteresis loop (x between 0.80 and
0.985) points to the narrow mesopore-size distribution. Mesopore
surface areas, Smeso, evaluated by the three-parameter BET equation
[24] were relatively high (from 43 m2 g−1 to 70 m2 g−1); all catalysts
also include a small and comparable portion of micropores. BET sur-
376 and 488 ◦C can be recognized. As the catalyst does not comprise
other reducible components than Pt and Pd oxides, both peaks must
represent reduction of the Pt and Pd oxides present on the support
in various particle sizes. In the TPR profile of the Pt–Pd/Al–Ce cata-
lyst, three main reduction peaks can be recognized at 183, 438 and
560 ◦C together with a shoulder at 107 ◦C. The shoulder at 107 ◦C can
be ascribed to the elimination of water physisorbed on the catalyst
[30]. Reduction of both Pt oxide and surface CeO2 particles, which
are in close contact with Pt can proceed at 183 ◦C, as platinum pro-
motes the reduction of ceria at low loadings [31]. The peak at 438 ◦C
can be attributed to the reduction of surface oxygen of cerium oxide
[32]. The peak at 560 ◦C is ascribed to the formation of nonstoichio-
metric Ce oxides; this peak was identified as the main peak in the
TPR profiles of CeO2–Al2O3 supports with different CeO2 loading
and calcined at 500 ◦C [31]. Reduction curves were integrated in
the temperature range of 25–500 ◦C, potentially having relation to
catalytic oxidation reaction, and the amounts of easily reducible
components were calculated based on the calibration using CuO.
Quantitative data are summarized in Table 2. The highest amount of
hydrogen consumed in the temperature range 25–500 ◦C was found
with Cu–Mn/Al catalyst (0.94 mmol g−1), followed by Pt–Pd/Al–Ce
catalyst (0.18 mmol g−1). The lowest amount of reducible compo-
nents showed the Pt–Pd/Al catalyst (0.02 mmol g−1). The difference
in hydrogen consumption between Pt–Pd/Al and Pt–Pd/Al–Ce cat-
alyst, which contain similarly small amount of Pt + Pd (0.2 and
0.1 wt%, respectively), can be explained by the presence of relatively
large amount of CeO2 (6.45 wt%) in the latter catalyst.
face areas of all catalysts varied between 73 m2 g−1 and 107 m2 g−1
.
The pore-size distributions evaluated from the desorption curves of
nitrogen physisorption isotherms are presented in Fig. 3b. Pt–Pd/Al
and Pt–Pd/Al–Ce catalysts revealed the monodisperse pore struc-
ture with rmax of the main peak at 10 nm and 9 nm, respectively. The
Cu–Mn/Al catalyst showed also monodisperse pore structure, how-
ever, with larger radius of mesopores (14 nm). It is obvious that the
textural properties of the catalysts result from their composition.
The reducibility of the catalyst can be an important factor for
the catalytic oxidation [25]. Moreover, it was shown that reducing
the catalyst before testing can enhance its catalytic activity [26].
The reducibility of the investigated catalysts was examined by the
temperature-programmed reduction using hydrogen as a reduc-
tion component. Fig. 4 demonstrates that practically all reducible
compounds in the Cu–Mn/Al catalyst were completely reduced in
the temperature range 25–400 ◦C with maximum of the reduction
rate at 258 ◦C. This reduction peak comprises not only the reduction
of CuO present in the catalyst but also reduction of Mn oxides, i.e.,
reduction of Mn4+ to Mn3+ and Mn3+ to Mn2+. These findings cor-
respond to the data obtained by Buciuman et al. [27] during TPR
measurement of hopcalite (CuMn2O4) calcined at 550 ◦C, where
the main reduction peak at 320 ◦C and shoulders at 260 and 380 ◦C
were found. Reduction profiles of the catalysts containing noble
metals (Pt–Pd/Al and Pt–Pd/Al–Ce) are more complex. Following
reduction maxima can be found for the compounds of both noble
metals in the literature: PtO, PtO2 and PtAl2O4 supported on alu-
mina are reduced at 58, 110, and 227 ◦C, respectively [28]; PdO is
reduced at temperatures around 260 ◦C [29]. The TPR profile of the
Pt–Pd/Al catalyst showed the distinct peak at 115 ◦C. It could be
ascribed either to elimination of water physisorbed on the cata-
lyst or reduction of PtO2. In addition, two small reduction peaks at
TPD patterns of ammonia desorption from the examined cat-
alysts are shown in Fig. 5. Desorption of ammonia proceeded in
the temperature range of 25–450 ◦C only. It indicates that the
catalysts do not comprise very strong acidic sites. A distinct des-
orption peak can be recognized at 98 ◦C with two shoulders at
166 and 292 ◦C with the Pt–Pd/Al catalyst. It means that the
catalyst comprises acidic sites of different strength-low, medium
and strong. In contrast, the Cu–Mn/Al catalyst shows a broad peak
with maximum at 140 ◦C and a peak of medium strength, not clearly
Fig. 4. H2–TPR profiles of: (a) Pt–Pd/Al; (b) Pt–Pd/Al–Ce and (c) Cu–Mn/Al. For
clarity, the profile of Cu–Mn/Al was divided by 5.
Fig. 5. NH3–TPD profiles of: (a) Pt–Pd/Al; (b) Pt–Pd/Al–Ce and (c) Cu–Mn/Al.

L. Mateˇjová et al. / Applied Catalysis A: General 443–444 (2012) 40–49
45
distinguished. The lowest ammonia desorption peak was found
with the Pt–Pd/Al–Ce catalyst (peak maxima at 127 and 295 ◦C).
Lower acidity of the catalyst can be likely caused by the presence of
ceria (6.45 wt%) in the alumina support together with lower con-
centration of noble metals in the catalyst. The highest amount of
desorbed ammonia was observed with the Pt–Pd/Al catalyst fol-
lowed by Cu–Mn/Al and Pt–Pd/Al–Ce catalysts (Table 2).
3.2. Catalytic tests
Results of dichloromethane oxidation over investigated cata-
lysts are shown in Table 3. Comparison of T50 and T90 leads to the
conclusion that the Pt–Pd/Al–Ce catalyst is the most active one
(T50 = 409 ◦C) despite a lower content of Pt in comparison with
the Pt–Pd/Al catalyst. The Cu–Mn/Al catalyst exhibited slightly
lower catalytic activity −T50 was 416 ◦C. Surprisingly, the Pt–Pd/Al
catalyst showed the lowest activity (T50 = 452 ◦C). This ranking
in catalytic activity can be explained based on the data summa-
rized in Table 2 and depicted in Figs. 4 and 5. It is known that
in the oxidation of chlorinated methanes, the acidity of the cata-
lysts is a key factor for their sufficient catalytic activity [20,33–36].
Concerning the noble metal catalysts, which are generally more
suitable for chlorinated VOC oxidation, the Pt–Pd/Al–Ce catalyst
exhibited lower acidity than Pt–Pd/Al (0.17 and 0.47 mmol g−1
,
respectively; Table 2). On the other hand, Pt–Pd/Al–Ce showed
significantly increased reducibility (0.18 mmol g−1) in comparison
with Pt–Pd/Al (0.02 mmol g−1), although it contains lower amount
of Pt and Pd. It is evident that this increase in reducibility can be
ascribed to the addition of CeO2; the reduction of Ce4+ to Ce3+ starts
already around 180 ◦C with the maximum at 438 ◦C (Fig. 4). As the
Pt–Pd/Al–Ce catalyst exhibited significantly higher activity in com-
parison with Pt–Pd/Al and the difference in reducibility between
these two catalysts was much higher than the difference in acid-
ity, it can be concluded that the reducibility of the catalyst may be
also taken into account as an important characteristic that might
correlate with catalytic activity in the oxidation of chlorinated com-
pounds. This conclusion is supported by the superior activity of
the Cu–Mn/Al catalyst in comparison with the Pt–Pd/Al catalyst.
Assuming the same nature of the alumina support this difference
in catalytic activity can be correlated with a relatively very high
reducibility of the Cu–Mn/Al catalyst (0.94 mmol g−1) due to the
high amount of reducible Mn and Cu species.
In the oxidation of chlorinated VOC, the selectivity of the cat-
alysts is at least as important as their activity due to possible
formation of harmful chlorinated by-products (e.g. perchloroethy-
lene, chloroform, trichloroethylene) that can be produced apart
from chlorine and desired HCl [37,38]. From comparison of HCl
yields at the temperature of 95% DCM conversion (Table 3) it is
seen that the HCl yields achieved with Pt–Pd/Al–Ce and Cu–Mn/Al
catalysts are comparable (72% and 73%, respectively). On the other
hand, with the Pt–Pd/Al catalyst the 95% conversion was not
achieved up to 500 ◦C and the value of YHCl,95 is therefore not
available. The maximum HCl yield was decreasing in the order
Pt–Pd/Al > Pt–Pd/Al–Ce > Cu–Mn/Al (Table 3).
Fig. 6. Concentration profiles of dichloromethane and all detected products of
dichloromethane oxidation: (a) Pt–Pd/Al; (b) Pt–Pd/Al–Ce and (c) Cu–Mn/Al.
Dichloromethane concentration in air 1000 vpm, 1.5 wt% of water, space
velocity 71 m3 kg−1 h−1
0.160–0.315 mm.
, ; catalyst particle size
temperature ramp 3.5 ◦C min−1
the presence of ceria in the Pt–Pd/Al–Ce catalyst enhanced its selec-
tivity to CO2. The Cu–Mn/Al catalyst produced traces of CO (5 ppm)
and CHCl3 (20 ppm) (Fig. 6c). Other possible harmful by-products
such as perchloroethylene, trichloroethylene, methylchloride or
phosgene were not detected with any of the investigated catalysts.
The laboratory tests with grained catalyst have shown that
the Pt–Pd/Al–Ce catalyst is more active and also more selec-
tive to CO2 than the Pt–Pd/Al catalyst. In the case of the
Cu–Mn/Al catalyst, the selectivity to CO2 was higher than that
of the Pt–Pd/Al catalyst. However, undesirable harmful chlori-
nated by-product – chloroform – was formed. The selectivity
of the Cu–Mn/Al catalyst corresponds to the observations of
Miranda et al. [39] and Döbber et al. [40] who studied supported
manganese oxides. Miranda et al. [39] investigated oxidation of
trichloroethylene (TCE) over grained commercial Mn/Al2O3 cata-
lyst. They observed the formation of undesired higher chlorinated
by-products as tetrachlorethylene (PCE), tetrachloromethane
(TTCM) or trichlormethane (TCM) mainly at lower temperatures.
The amount of TTCM and TCM formed was decreased by the
Except HCl and CO2 as main products, following by-products
were found in the output stream when the 1.5 wt% of water was
injected into the input stream: CO, CH2O, and CHCl3. The concen-
tration profiles of desired HCl and CO2 together with all detected
by-products of DCM oxidation are shown in Fig. 6a–c. Over Pt–Pd
based Pt–Pd/Al and Pt–Pd/Al–Ce catalysts, traces of CO (20 ppm
and 10 ppm, respectively) and CH2O (15 ppm and 5 ppm, respec-
tively) were detected besides CO2. No chlorinated intermediates
were found. The Pt–Pd/Al–Ce catalyst exhibited better results than
Pt–Pd/Al: a reaction temperature of 480 ◦C was sufficient for total
DCM oxidation yielding CO2, HCl and chlorine. When the concen-
tration profiles shown in Fig. 6a and b are compared, it is visible that

46
L. Mateˇjová et al. / Applied Catalysis A: General 443–444 (2012) 40–49
Table 3
Activity and selectivity of catalysts in total oxidation of ethanol, toluene and dichloromethane.
Catalyst
Ethanola
Toluenea
Dichloromethaneb
T50 (◦C)
T90 (◦C)
SCD,95 (%)
T50 (◦C)
T90 (◦C)
SCD,95 (%)
T50 (◦C)
T90 (◦C)
YHCl,max (%)
YHCl,95 (%)
Pt–Pd/Al
Pt–Pd/Al–Ce
Cu–Mn/Al
166
180
142
178
212
184
82
78
99
214
270
282
223
285
301
100
100
100
452
409
416
500
452
477
80
77
74
–
72
73
a
Concentration in air 1000 vpm, space velocity 71 m3 kg−1 h−1, temperature ramp 3.5 ◦C min−1; catalyst particle size 0.160–0.315 mm.
b
Concentration in air 1000 vpm, 1.5 wt% water, space velocity 71 m3 kg−1 h−1, temperature ramp 3.5 ◦C min−1; catalyst particle size 0.160–0.315 mm.
addition of water to the reaction mixture. On the other side,
the presence of water decreased the stability of the Mn catalyst.
Similarly, the higher chlorinated methanes were also detected as
by-products of chloromethane oxidation over zirconia-supported
manganese oxide by Döbber et al. [40]. Gu et al. [41] examined
CuMn/ZrO2–TiO2–Al2O3 catalyst in DCM oxidation. They proved its
high efficiency since a total DCM conversion was reached at 470 ◦C.
CHCl3 was the only intermediate with maximum concentration
at 380 ◦C. This performance was ascribed to the highly dispersed
CuMnOx phase. Moreover, the authors also found out that the addi-
tion of ZrO2 improved the reducibility of the active phase. Vu et al.
[42] investigated the 5 wt% CuMnOx/TiO2 catalyst in the oxidation
of chlorobenzene (CB). They reported high catalyst activity, selec-
tivity and long-term stability of this system, which was attributed
to the formation of the Mn1.6Cu1.4O4 spinel phase. However, the
deactivation of the catalyst was observed and the formation of oxy-
chlorinated copper and manganese species were presumed to be
responsible to the catalyst partial deactivation.
Based on all these findings it is possible to summarize that sup-
ported Cu–Mn catalysts could be promising for the oxidation of
chlorinated VOC due to relatively high catalytic activity and low
production costs. However, the efficient form of active species has
to be tailored depending on the type of oxidized compounds. The
negative features of these types of catalysts seem to be a formation
of by-products [39–41] and lower resistance against deactivation
by chlorine [42]. On the other hand, the investigated Pt–Pd cata-
lysts did not produce any harmful chlorinated intermediates even
if their activity and selectivity to CO2 and HCl was relatively low
(the Pt–Pd/Al catalyst). For this reason the noble metal catalysts
can be reasonably considered as more appropriate for Cl-VOC oxi-
dation than the catalysts based on metal oxides, which was also
concluded by Ojala et al. [43].
It should be noted that the results of catalytic test at laboratory
scale can be affected by the form of catalysts (powders contrary
to monoliths/pellets/spheres) and their preparation procedure
[20,37]. For example, in ref. [20], where the original monolithic
form of Pt–Pd catalysts (1:4 Pt–Pd ratio) supported on alumina
and alumina-ceria was investigated, the catalytic activity exhibited
inverse trend than in this study. Moreover, significantly different
results of DCM oxidation were observed with two Pt–Pd/alumina
catalysts of the same composition but provided by two different
manufacturers [37].
Results of ethanol oxidation are summarized in Table 3 and
Fig. 7a–f. The catalytic activity of the catalysts decreased in the
order Cu–Mn/Al > Pt–Pd/Al > Pt–Pd/Al–Ce. This trend is also clearly
visible from the temperature dependence of ethanol concentra-
tion for each catalyst (Fig. 7a, c and e). Comparison of selectivity
to CO2 at 95% ethanol conversion (Table 3) leads to the conclu-
sion that the Cu–Mn/Al catalyst, in contrary to Pt–Pd catalysts,
is the most selective one (SCD,95 = 99%), while the Pt–Pd/Al and
Pt–Pd/Al–Ce catalysts possess lower selectivity to CO2 (SCD,95 = 82%
and 78%, respectively). CH3CHO, CO and CH2O were found with
the Pt–Pd/Al–Ce (Fig. 7c and d) and Cu–Mn/Al catalysts (Fig. 7e
and f), which corresponds to the well-accepted mechanism of
ethanol oxidation [44]. The by-products detected during ethanol
oxidation over the Pt–Pd/Al catalyst are shown in Fig. 7a and b.
Apart from CH3CHO (up to ∼400 ppm), a significant amount of
CH3COOH (∼100 ppm) and traces of CO (∼5 ppm), CH2O (∼6 ppm)
and ethylacetate (∼3 ppm) were found. These by-products agree
well with the suggested mechanism of ethanol oxidation over
Pt-based catalysts [44]. Different performance of the investigated
Pt–Pd catalysts is likely connected with higher Pt loading of the
Pt–Pd/Al catalyst in comparison with the Pt–Pd/Al–Ce catalyst (0.24
versus 0.10 wt% Pt + Pd). A higher activity of the Pt–Pd/Al catalyst
may be also linked to its different surface acid properties (a higher
amount of acid centers) as illustrated by NH3–TPD profiles (Fig. 5,
Table 2).
Results of toluene total oxidation over examined catalysts are
shown in Table 3 and the light-off curves are depicted in Fig. 8.
The courses of the light-off curves indicate relatively complex
behavior of the examined catalysts in toluene oxidation. The low-
temperature step in toluene conversion observed between ∼140 ◦C
and 260 ◦C in the case of Cu-Mn/Al catalyst could be connected
with chemisorption of toluene on the catalyst. Correctness of this
assumption is supported by the comparison of toluene conversion
with CO2 evolution during the experiment. From Fig. 8 it is seen that
increase in toluene conversion over Cu–Mn/Al catalyst between
∼140 ◦C and ∼260 ◦C is not accompanied by corresponding increase
in CO2 concentration. On the other hand, a very intense peak in CO2
concentration is observed when the reaction temperature reaches
∼275 ◦C. This peak likely arises from the oxidation of a large amount
of toluene previously adsorbed on the catalyst surface. Therefore,
the low-temperature step in toluene conversion can be ascribed to
toluene chemisorption on the catalyst.
The activity of the catalysts decreased in the order
Pt–Pd/Al > Pt–Pd/Al-Ce > Cu–Mn/Al (T90 was 223, 285, and 301 ◦C,
respectively). Observed trend in catalytic activity is in agreement
with the generally known fact that noble metal catalysts are
more active in the total oxidation of aromatic VOC than the
catalysts based on transition metal oxides [45]. On the other hand,
the evident positive effect of the presence of ceria in alumina
support on toluene oxidation was recently reported [46,47]. Del
Angel et al. [46] and Abbasi et al. [47] found that the combustion
temperature of toluene diminished as the content of the ceria in
␥-alumina support increased. It was also shown that the ability
of ceria to exchange oxygen due to the redox cycle Ce4+ ↔ Ce3+
can be related to the improvement in toluene oxidation. The XPS
study revealed that the population of the Ce4+ species increases
with increasing Ce content in the catalyst. Then, the higher is the
Ce4+/Ce3+ ratio in the catalyst, the higher is the activity of the
ceria–alumina catalyst in toluene oxidation [46]. In addition, it
was confirmed that ceria–alumina can enhance the reduction of
Pt-oxide species and their mutual interaction can be responsible
for better performance of the catalyst in toluene oxidation as well
[47]. According to the H2–TPR study in this work, the positive
effect of ceria on the reducibility of the catalyst was observed when
Pt–Pd/Al–Ce and Pt–Pd/Al catalysts were compared. However, due
to different Pt and especially Pd content in the catalysts, no clear
correlation between performance of these catalysts in toluene
oxidation and their reducibility could be made. The positive

L. Mateˇjová et al. / Applied Catalysis A: General 443–444 (2012) 40–49
47
Fig. 7. Concentration profiles of ethanol and all detected products of ethanol oxidation: (a), (b) Pt–Pd/Al; (c), (d) Pt–Pd/Al–Ce and (e), (f) Cu–Mn/Al. Ethanol concentration in
air 1000 vpm, space velocity 71 m3 kg−1 h−1, temperature ramp 3.5 ◦C min−1; catalyst particle size 0.160–0.315 mm.
effect of the increased Lewis acidity of ␥-alumina-based catalyst in
comparison with ceria-␥-alumina-based catalysts on the efficiency
of toluene combustion was not confirmed [46]. However, in the
noble metal catalysis the activity depends also on the noble metal
loading [9,48]. Thus, remarkably lower noble metal loading in
the Pt–Pd/Al–Ce catalyst (0.1 wt%) in comparison with Pt–Pd/Al
catalyst (0.24 wt%) could be the main reason for higher T90 of
the former catalyst and for the relatively large difference in T90
between them (62 ◦C).
Also CuMnOx/␥-Al2O3 catalysts have been studied and consid-
ered as very active and promising in toluene oxidation [49]. Li
et al. [49] found out that bimetallic CuMnOx/␥-Al2O3 catalysts with
various molar ratios of Cu/Mn exhibit higher catalytic activity in
toluene oxidation than monometallic CuOx/␥-Al2O3 and MnOx/␥-
Al2O3 catalysts. It is mainly attributed to the formation of the
Cu1.5Mn1.5O4 spinel phase in the bimetallic catalysts. However, in
the Cu–Mn/Al catalyst investigated in this work the presence of
such structure, which could be responsible for the catalytic activity
comparable to Pt–Pd/Al–Ce catalyst, can not be confirmed due to
overlapping of alumina diffraction lines with that of Cu–Mn oxides
in the X-ray diffraction pattern of the catalyst.
In VOC oxidation reactions, selectivity of catalysts to CO2 is
often even more important than their activity as some by-products
formed during the oxidation can be more detrimental to the envi-
ronment than the initial compounds. For example, benzene can be
formed during toluene oxidation [50]. Generally, noble metal cat-
alysts exhibit higher selectivity to CO2 than metal oxide catalysts
[43]. Investigated commercial catalysts revealed high selectivity in
Fig. 8. Light-off curves of toluene total oxidation for Pt–Pd/Al (ꢁ), Pt–Pd/Al–Ce
(ꢂ) and Cu–Mn/Al (᭹) together with dependence of CO2 concentration on reac-
tion temperature for Cu–Mn/Al (ꢀ). Toluene concentration in air 1000 vpm,
space velocity 71 m3 kg−1 h−1, temperature ramp 3.5 ◦C min−1; catalyst particle size
0.160–0.315 mm.

48
L. Mateˇjová et al. / Applied Catalysis A: General 443–444 (2012) 40–49
the total oxidation of toluene (SCD,95 = 100% for all catalysts) and no
by-products were detected.
superior activity in comparison with Cu–Mn catalyst, while selec-
tivity to CO2 was 100% in all cases.
It may be interesting to compare the activity of commercial
catalysts in the oxidation of model compounds investigated at labo-
ratory scale with the information provided to industrial customers
in catalyst data sheets. In our case, the Cu–Mn/Al catalyst was
reported as “high performance catalyst for the oxidation of oxy-
genated VOC like alcohols, aldehydes, and ketones”. Indeed, our
results confirmed that this catalyst was the most active and selec-
tive one in the oxidation of ethanol. The results of dichloromethane
oxidation were more complicated. The Pt–Pd/Al catalyst was
described as “high performance catalyst for the total oxidation of
halogenated (chlorinated) VOC”. Nevertheless, the highest activity
in dichloromethane oxidation was accomplished with Pt–Pd/Al–Ce,
which was labeled as “high performance catalyst for the total oxida-
tion of VOC”. On the other hand, the highest HCl yield was reached
with the Pt–Pd/Al catalyst (80%), followed by Pt–Pd/Al–Ce (77%).
When we take into account that in oxidation of chlorinated VOC the
selectivity (i.e. the HCl yield) is considered to be more important
than activity [43], it can be concluded that these results are in agree-
ment with the information provided by catalyst supplier. However,
this is not the case of toluene oxidation. Here, the Pt–Pd/Al catalyst
was the most active one, although it was recommended for the oxi-
dation of chlorinated VOC. On the other hand, Pt–Pd/Al–Ce reported
as “high performance catalyst for the total oxidation of VOC” was
much less active (T50 was by 56 ◦C higher). The selectivity to CO2
was 100% for all tested catalysts.
The most active in the oxidation of dichloromethane was
the Pt–Pd/Al–Ce catalyst containing Pt and Pd (∼1:1) supported
on alumina–ceria. The superior activity of this catalyst may be
attributed to the presence of CeO2, which increased its ability to
completely oxidize reaction by-products to CO2 in the tempera-
ture range 400–500 ◦C. On the other hand, the highest HCl yield
was achieved with Pt–Pd/Al, probably due to higher acidity of this
catalyst in comparison with Pt–Pd/Al–Ce. No harmful chlorinated
intermediates were detected with both Pt–Pd catalysts, while chlo-
roform was produced over the Cu–Mn/Al catalyst.
It is necessary to state that the short-lasting laboratory tests
could not cover other important features of commercial cata-
lysts – their stability under reaction conditions and durability. The
obtained results showed that the activity of commercial catalysts in
laboratory scale tests can differ from that declared by catalyst sup-
plier. A possible difference in catalytic performance at industrial
and laboratory scale should be taken into account when industrial
catalysts crushed into a powder form are used in laboratory scale
tests. Nevertheless, comparison of the results obtained on new or
improved oxidation catalysts with those found on commercial cat-
alysts, though acquired only at laboratory scale, is always valuable.
Acknowledgements
The financial support of the Ministry of Industry and Trade of
the Czech Republic (project No. FR-TI1/059) is gratefully acknowl-
edged. The authors highly appreciate the kind support, fruitful
discussions and help with experiments provided by Satu Pitkäaho,
Dr. Satu Ojala and Prof. Riitta L. Keiski from the University of Oulu,
Finland. The authors thank Filip Novotny´ from the Czech Technical
University in Prague for FE-SEM images.
These results illustrate that the performance of commercial cat-
alysts in laboratory scale tests may differ from that declared by
catalyst supplier. This discrepancy may be connected with the fact
that the catalysts, which were originally in the form of spheres,
were crushed into a powder before the tests and therefore the
physicochemical properties of the catalyst layer were changed.
Thus, when industrial catalysts are crushed and employed in a
powder form in laboratory scale tests (as e.g. in ref. [51]), such
possible differences in catalytic performance should be taken into
account.
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