Low-temperature catalytic destruction of CCl4, CHCl3 and CH2Cl2 over basic oxides

Article abstract of DOI:10.1039/b413876g

The catalytic destruction of CCl₄, CHCl₃ and CH ₂Cl₂ in the presence of steam has been compared over a series of unsupported and alumina-supported lanthanide and alkaline earth oxides. It was found that (1) the destruction rate over basic oxides decreases with decreasing chlorine content of the CHC compound (CCl₄ > CHCl₃ > CH₂Cl₂); (2) the catalyst activity is always higher for lanthanide oxides than for alkaline earth oxides; (3) supported basic oxides are more active than their unsupported counterparts and (4) a stable destruction activity of more than 10 days can be maintained as long as an excess of steam is present in the gas stream. The reaction products are also dependent on the type of chlorinated hydrocarbon. CO₂ and HCl are the products for the destruction of CCl₄ and both compounds are formed from the reaction intermediate Cl₂CO. In the case of CHCl ₃ a mixture of CO and HCl is produced, partially formed via the hydrolysis of the reaction intermediate HClCO. Finally, the reaction products for the destruction of CH₂Cl₂ are HCl, CO and H ₂; the latter two are formed from H₂CO decomposition. In the case of supported basic oxides significant amounts of CH₃Cl are produced in the catalytic destruction of CH₂Cl₂, which is catalyzed by the partially uncovered Al₂O₃ support phase. In situ Raman and infrared spectroscopy were used to monitor the physicochemical changes taking place in the catalytic solid as well in the gas phase above the catalyst material. Based on these findings a plausible reaction mechanism is proposed. Lanthanide oxides and lanthanide oxide chlorides are both active phases in this catalytic process.

Full text of DOI:10.1039/b413876g

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R E S E A R C H P A P E R
Low-temperature catalytic destruction of CCl4, CHCl3 and
CH2Cl2 over basic oxides
Pieter Van der Averta and Bert M. Weckhuysen*ab
a Centrum voor Oppervlaktechemie en Katalyse, Departement Interfasechemie, KULeuven,
Kasteelpark Arenberg 23, 3001 Leuven, Belgium
b Departement Anorganische Chemie en Katalyse, Debye Instituut, Universiteit Utrecht,
Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands. E-mail: b.m.weckhuysen@chem.uu.nl
Received 8th September 2004, Accepted 14th September 2004
First published as an Advance Article on the web 5th October 2004
The catalytic destruction of CCl4, CHCl3 and CH2Cl2 in the presence of steam has been compared over a series
of unsupported and alumina-supported lanthanide and alkaline earth oxides. It was found that (1) the destruction
rate over basic oxides decreases with decreasing chlorine content of the CHC compound (CCl4 4 CHCl3 4
CH2Cl2); (2) the catalyst activity is always higher for lanthanide oxides than for alkaline earth oxides; (3)
supported basic oxides are more active than their unsupported counterparts and (4) a stable destruction activity
of more than 10 days can be maintained as long as an excess of steam is present in the gas stream. The reaction
products are also dependent on the type of chlorinated hydrocarbon. CO2 and HCl are the products for the
destruction of CCl4 and both compounds are formed from the reaction intermediate Cl2CO. In the case of CHCl3
a mixture of CO and HCl is produced, partially formed via the hydrolysis of the reaction intermediate HClCO.
Finally, the reaction products for the destruction of CH2Cl2 are HCl, CO and H2; the latter two are formed from
H2CO decomposition. In the case of supported basic oxides significant amounts of CH3Cl are produced in the
catalytic destruction of CH2Cl2, which is catalyzed by the partially uncovered Al2O3 support phase. In situ
Raman and infrared spectroscopy were used to monitor the physicochemical changes taking place in the catalytic
solid as well in the gas phase above the catalyst material. Based on these findings a plausible reaction mechanism
is proposed. Lanthanide oxides and lanthanide oxide chlorides are both active phases in this catalytic process.
transform dynamically from the metal oxide state (La2O3) to
the metal oxide chloride (LaOCl) or metal trichloride (LaCl3)
state due to bulk diffusion of oxygen and chlorine atoms. Both
Introduction
Recently, our group reported on a new class of catalytic
materials active in the decomposition of CCl4 in the presence
La2O3 and LaOCl are active materials in this reaction, whereas
of steam.1–4 This destruction reaction can be written as follows:
LaCl3 is catalytically inactive. It was also found that an excess
of steam in the waste stream is crucial for stable catalyst
CCl4 þ 2H2O - CO2 þ 4 HCl
(1)
activity.
In this work, we extend our study to the catalytic hydrolysis
It was shown that La2O3, Pr2O3, Nd2O3 and CeO2 are very
active in this reaction and a 10 wt% La2O3/Al2O3 catalyst
exhibited a destruction capacity of 0.289 g CCl4 gꢀ1 hꢀ1 at
350 1C.1,2 Table 1 compares the obtained activity with those of
some reference materials. Although comparison is not easy
because of the different experimental conditions used, the
destruction capacity of the 10 wt% La2O3/Al2O3 catalyst is
superior over the existing oxidation catalysts.1 Based on spec-
troscopic, catalytic and theoretical investigations it was con-
cluded that the catalytic reaction consists of a destructive
adsorption step of CCl4 on basic oxygen sites followed by a
steam dechlorination step of the formed chlorinated catalytic
solid.3,4 The process starts with the splitting of this first Cl
atom during destructive adsorption, most probably catalyzed
by a Lewis acid site. The CCl3 intermediate is then stabilized by
a basic oxygen site and the formed O–CCl3 adsorbate further
decomposes by donating another Cl atom to the surface and
abstracting the bonding oxygen atom. The generated gas-phase
Cl2CO reaction intermediate has been observed experimen-
tally3 and reacts further with the surface to form CO2. The
following consecutive reactions apply for La2O3:
over basic oxides of two other chlorinated hydrocarbons
(CHCs); i.e., CHCl3 and CH2Cl2. These reactions can be
written as follows:
CHCl3 þ H2O - CO þ 3HCl
CH2Cl2 þ H2O - CO þ H2 þ 2HCl
(4)
(5)
On the basis of catalytic and spectroscopic data, a plausible
reaction mechanism is proposed explaining the different reac-
tion products formed for these two chlorinated hydrocarbons
compared to CCl4. In addition, the catalytic performances of
La2O3 materials with different surface areas are also studied,
revealing the complexity of the catalytic materials under
investigation. Finally, the role of alumina as support will be
discussed and guidelines to design more active catalytic solids
are proposed.
Experimental section
1. Catalyst preparation
3
2
32CCl4 þ La2O3 - CO2 þ 2LaCl3
(2)
(3)
The following alkaline earth and lanthanide oxides have been
studied: MgO (Aldrich, 99%, 117 m2
g
ꢀ1); CaO (Aldrich,
2LaCl3 þ 3H2O - La2O3 þ 6HCl
Depending on the reaction conditions, more specifically the
H2O to CCl4 molar ratio, the catalytic material was found to
99.9%, 7 m2
(Aldrich, 97%, 0.25 m2 gꢀ1), La2O3 (Acros, 99%, 1 m2 gꢀ1),
CeO2 (Aldrich, 99, 9%, 0.25 m2 ꢀ1), Pr2O3 (Alfa Aesar,
g g
ꢀ1); SrO (Aldrich, 99.9%, 2 m2 ꢀ1), BaO
g
5256
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 2 5 6 – 5 2 6 2
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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Table 1 Comparison of the destruction capacities of different cataly-
tic solids for the destruction of CCl4 at 350 1C, together with the
experimental conditions1
excess of steam was added to the reaction mixture. It was
shown in previous work that an excess of steam in the inlet
stream is essential for the stability of the catalyst system.3 Most
of the water in the effluent stream was trapped with an
impinger at room temperature. The remaining gases were
analyzed with a gas chromatograph (HP 4890D with an FID
detector and methanator) using a packed Hayesep Q CP
column (80–100 mesh, 3 m). Conversions have been calculated
by taking into account the amount of chlorinated hydrocarbon
before and after reaction and the corresponding sensitivity
factors of the chlorinated hydrocarbon.
Destruction
capacity/g
Loading of
CCl4 (ppm)
CCl4 hꢀ1 gꢀ1
catalyst
Catalytic system
GHSV/hꢀ1
LaMnO3
LaCoO3
500
500
6000
6000
1367
1367
15000
15000
800
0.004
0.016
0.009
0.009
0.036
0.102
0.145
0.289
Co–Y
Cr–Y
1000
1000
1000
1000
47000
47000
Cr2O3/Al2O3
Pt, Pd, or Rh/TiO2
La2O3
3. Catalyst characterization
Surface areas of the materials were determined by N2 sorption
measurements with a Micromeritics ASAP 2400 instrument.
Raman spectra were collected with a Holoprobe Kaiser Optical
RXN-532 spectrometer equipped with a holographic notch
filter, a laser Raman excitation of 532 nm and a CCD camera.
A special in situ Raman cell was used that could be heated in a
stream of reagents at elevated temperatures. Infrared spectra
were collected with a Nicolet 730 FT-IR spectrometer. A
special in situ infrared cell was designed in which a catalyst
wafer was placed in a quartz cell equipped with KBr windows.
X-ray diffraction (XRD) measurements were performed with a
Siemens D5000 diffractometer using a Ni-filtered CuKa source
with a wavelength of 0.154 nm. Scanning electron microscopy
(SEM) was performed on a Philips XL30FEG instrument. The
samples were spread out on a carbon layer. Depending on the
conductivity of the samples, a small amount of noble metal was
vaporised on the sample to prevent charging. The analysis of
the elements was carried out with an energy dispersive spectro-
meter (EDS) detector from EDAX. Coke formation has been
evaluated with thermogravimetric analysis (TGA) using a
TGA92 Setaram instrument. The materials were heated up to
800 1C (heating rate of 10 1C minꢀ1) in a He/O2 stream.
La2O3/Al2O3
800
99.9%, 3 m2 gꢀ1) and Nd2O3 (Alfa Aesar, 99.9%, 3 m2 gꢀ1).
Alumina-supported (Condea, surface area of 220 m2 gꢀ1; pore
volume of 0.4 ml gꢀ1, 99% purity and zero point of charge of
8.5) alkaline earth and lanthanide oxide-based catalysts were
prepared by incipient wetness impregnation of aqueous metal
acetates. The origin and purity of the corresponding metal
acetate salts were as follows: Mg (Aldrich, 99%), Ca (Avoca-
do, 98%), Sr (Avocado, 98%), Ba (Aldrich, 99%), La (Fluka,
97%), Ce (Aldrich, 99.9%), Pr (Aldrich, 99.9%) and Nd
(Aldrich, 99.9%). After impregnation, samples were dried at
100 1C for 1 h. The impregnation was repeated until the desired
metal oxide loading was obtained. The metal loading of the
supported catalysts in moles was always equal and corresponds
to 10 wt% for lanthanum. In addition, La2O3 catalysts with
different surface areas have been prepared via the sol–gel
procedure. For this purpose, La(NO3)3 ꢁ 7H2O (Aldrich,
99.9%) was dissolved in water and subsequently NaOH
(BDH, 4 99%) was added to this solution to initiate gel
formation. The pH increased to a value around 7–8 and the
obtained gel was kept at room temperature for a fixed time,
followed by centrifugation and washing with demineralized
water. After successive washing steps to remove NaOH the gel
was dried at 100 1C, followed by calcination at 450 1C in O2 for
7 h. By varying the ripening time and the La concentration,
La2O3 materials with a different surface area could be ob-
tained. The synthesized La2O3 materials possess the following
surface area (m2 gꢀ1): 21, 26, 31, 35, 40, 53, numbers which are
significantly higher than the commercially available La2O3
Results and discussion
1. Catalytic activity of unsupported basic oxides
Fig. 1 compares the conversion of CCl4, CHCl3 and CH2Cl2
over the alkaline earth oxides and lanthanide oxides after 7 h of
operation at 350 1C. There are two important observations to
make: (1) the destruction rate decreases with decreasing chlor-
ine content of the CHC; i.e., in the order: CCl4 4 CHCl3 4
CH2Cl2. This observation is in line with the expected trend
since the Gibbs free energy of this reaction decreases with
increasing chlorine content (DG is equal to ꢀ384.5, ꢀ214.2 and
ꢀ112.3 kJ molꢀ1 for CCl4, CHCl3 and CH2Cl2, respectively).
(2) Lanthanide oxide-based catalysts are always more active
than alkaline earth oxide-based catalytic systems, regardless of
the type of CHC fed to the reactor. The maximum conversion
over La2O3 is 62% for CCl4, 37% for CHCl3 and 16% for
CH2Cl2. This comparison holds also for materials with similar
surface area; i.e., when we compare the specific conversion (%)
of e.g. BaO with that of CeO2 (both materials have a surface
area of 0.25 m2 gꢀ1) and of e.g. SrO with that of Pr2O3 (both
materials have a surface area of 2–3 m2 gꢀ1). It is, however,
important to indicate that the surface areas of the unsupported
basic oxides decrease during reaction, preventing a simple
relationship between catalyst activity and catalyst surface
area (vide infra).
material (1 m2
g
ꢀ1). All catalysts were granulated and the
0.25–0.50 mm sieve fraction was used for testing and charac-
terization.
2. Activity testing
Activity tests were performed on 1 g of a catalyst in a fixed-bed
reactor at atmospheric pressure. Details of the experimental
set-up have been published elsewhere.1,3,4 Samples were pre-
treated in 0.6 L hꢀ1 O2 flow at 450 1C overnight. During the
reaction, a He flow at 0.48 L hꢀ1 was passed through a
saturator filled with CCl4 (VEL, pro analyze), CHCl3 (BDH,
99.0–99.8%) or CH2Cl2 (BDH, 99.8%) and maintained at 0 1C
in an ice-bath in order to preserve a constant vapor pressure,
and, consequently, the same CHC concentration. The CCl4,
CHCl3 and CH2Cl2 loadings are respectively 0.00098, 48000
ppm), 0.0018 (84700 ppm) and 0.0023 (105000 ppm) mol hꢀ1
.
Besides the catalyst activity it is also important to discuss the
differences in product formation. Fig. 1B compares the product
distribution in the outlet gas stream of La2O3 after 7 h of
operation at 350 1C. Similar observations were made for the
other unsupported alkaline earth metal and lanthanide oxide
catalysts. It was found that catalytic destruction of CCl4
streams results in the formation of CO2 and only traces of
CO. In the catalytic destruction of CHCl3 the main product is,
The space velocity (GHSV) was 800 hꢀ1 and as Table 1
indicates the waste streams under investigation are heavily
loaded with CHCs as compared to literature studies. Water
was added to the reactor at the rate of 0.0012 L hꢀ1 via a
Metrohm dosimeter and evaporated when in contact with the
reactor walls and bed. The H2O to CHC molar ratio was 61, 28
and 22 for CCl4, CHCl3 and CH2Cl2, respectively, and thus an
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 2 5 6 – 5 2 6 2
5257

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Table 2 CH2Cl2 conversion (%) in the presence of steam at 350 1C
over La2O3 catalysts with different surface areas
Surface area before
reaction/m2 gꢀ1
Surface area after
reaction/m2 gꢀ1
Conversion (%) after
7 h on stream
1
21
26
31
35
40
53
1
14
12
13
18
30
22
16
16
13
15
19
18
27
material. Thus, there is no simple correlation between catalyst
activity and surface area; i.e., the number of exposed La-ions at
the catalyst surface. Most probably, the synthesis conditions
are responsible for the creation of more active sites at the
La2O3 surface. Currently, we are studying the surface acid–
base properties of these different La2O3 materials with infrared
spectroscopy and probe molecules (CO, CO2 and pyridine) in
order to obtain a better insight into the number of potential
active sites and if possible to correlate these active sites with
catalytic performances.
2. Catalytic activity of supported basic oxides
The idea of increasing the surface area of the active La2O3
phase can be further explored by spreading La2O3 at the
surface of a support oxide. It was indeed found that higher
conversions were obtained by supporting basic oxides onto
Al2O3. Other supports such as TiO2 and SiO2 also gave good
results, but the conversion levels were about 10–15% lower
than in the case of alumina-supported basic oxides. We also
found that lanthanide acetates were the best salts for impreg-
nating lanthanide ions onto the alumina support and less active
catalyst materials were made if nitrate or sulfate was the
counteranion. Another important aspect is the stability of the
Al2O3 support under reaction conditions. First of all, we
observed that the surface area of alumina does not significantly
Fig. 1 (A) Conversion of CCl4 (white bars), CHCl3 (black bars) and
CH2Cl2 (traced bars) over unsupported alkaline earth metal and
lanthanide oxides after 7 h of operation at 350 1C. (B) Selectivity for
the conversion of CCl4, CHCl3 and CH2Cl2 over La2O3 after 7 h of
operation at 350 1C: CO (white bars), CO2 (black bars) and CH3Cl
(traced bars).
however, CO, which is the expected reaction product based on
reaction 4. The amount of CO2 is always negligible and floats
around 1–2%. CH2Cl2 destruction over La2O3 also results in
the formation of CO and small amounts of CO2 (2–4%). This
is in line with reaction 5. A plausible explanation for the
formation of CO is the direct decomposition of the expected
reaction intermediates CH2O and CHClO, according to:
decrease during reaction and remains around 200 m2 gꢀ1
.
Secondly, XRD indicates that no AlCl3 and/or a-Al2O3 phases
are formed during catalytic destruction of the different CHCs
for prolonged reaction times. As a consequence, alumina seems
to be a good support to keep the active phase dispersed at its
surface.
CH2O - CO þ H2
CHClO - CO þ HCl
(6)
(7)
The results of the conversion of the different CHCs over
alumina-supported alkaline earth metal and lanthanide oxides
are shown in Fig. 2A. To make relevant comparisons, the metal
loadings of the supported catalysts in moles are equal to that of
the best performing catalyst, 10 wt% La on Al2O3. It can be
concluded that the catalyst activity drastically increased by
spreading the active basic oxide compound on a carrier sup-
port. In addition, the two main observations described above
for unsupported basic oxides are still valid for the supported
basic oxide catalysts as well. Our best performing catalyst,
10 wt% La on Al2O3, converts at 350 1C 100% of CCl4, 68%
of CHCl3 and 53% of CH2Cl2.
In contrast to unsupported basic oxides, alumina-supported
basic oxides form besides CO significant amounts of CH3Cl in
the destruction of CH2Cl2 (Fig. 2B). The ratio CO : CH3Cl is
around 2 : 1 for a La on Al2O3 catalyst, and similar ratios have
been observed for other catalytic systems. The formation of
CH3Cl could be explained by the mechanism proposed by Van
den Brink et al.5 These authors have studied the oxidation of
CH2Cl2 over bare Al2O3 in the presence of H2O and observed
the formation of CH3Cl. Their reaction mechanism involves
the reaction of CH2Cl2 with a surface hydroxyl group of
alumina and the formed chloromethoxy species is further
converted into a chemisorbed formaldehyde analogue. In a
It can be postulated that the small amounts of CO2 can be
formed by the combination of CO and Cl2 to Cl2CO, which is
easily hydrolyzed with H2O at 350 1C to CO2 and HCl:
CO þ Cl2 - COCl2
Cl2CO þ H2O - CO2 þ HCl
(8)
(9)
It should, however, be stated that we have never observed any
Cl2 in the exit gas stream during the catalytic destruction of the
three CHCs under investigation.
In an attempt to enhance the catalytic performances of
unsupported basic oxides we have synthesized a series of
La2O3 materials ranging in surface area between 1 and 52 m2
g
ꢀ1. The results of these materials for the destruction of
CH2Cl2 are summarized in Table 2. Comparable results have
been obtained for the two other CHCs as well. It is evident that
there is no clear trend between the initial surface area of La2O3
and the conversion activity, although the material with the
highest surface area (52 m2 gꢀ1) is the most active catalyst. On
the other hand, the catalyst surface area of La2O3 materials is
significantly reduced under reaction conditions and in the case
of the most active La2O3 the surface area is reduced to 22 m2
g
ꢀ1. Although this material is not the material with the highest
surface any more, it is still the best performing catalyst
5258
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T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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Fig. 3 Scanning electron micrograph of a La2O3/Al2O3 catalyst
before reaction (A) and after 7 h on stream in the destruction of
CH2Cl2 at 350 1C (B), together with the relative amount of La and Cl
as determined with EDAX along the indicated analysis line. The
bottom right line has a length of 10 mm.
always low (0–3 wt% coke) and its amount increases with
decreasing chlorine content of the CHC and is higher for
alumina-supported lanthanide and alkaline earth metal oxide
than for their unsupported counterparts. For example, coke
formation equals 0.0004 mol hꢀ1 for La2O3/Al2O3 and 0.00004
mol hꢀ1 for La2O3 for the destruction of CH2Cl2 at 350 1C,
implying that the acidity of the alumina support takes part in
the coke formation. Another check for coke formation is the
gaseous carbon balance over supported catalysts. This balance
is 100% for CCl4, and floats between 95 and 100% for the two
other CHCs, fairly in line with the TGA results.
Fig. 2 (A) Conversion of CCl4 (white bars), CHCl3 (black bars) and
CH2Cl2 (traced bars) over alumina-supported alkaline earth metal and
lanthanide oxides after 7 h of operation at 350 1C. (B) Selectivity for
the conversion of CCl4, CHCl3 and CH2Cl2 over La2O3/Al2O3 after 7 h
of operation at 350 1C: CO (white bars), CO2 (black bars) and CH3Cl
(traced bars).
Finally, it is important to recall that the results presented in
Figs. 1 and 2 only hold for a reaction temperature of 350 1C. It
is obvious that the conversion increases with increasing reac-
tion temperature and reaches 100% over a La2O3/Al2O3
catalyst for CCl4, CHCl3 and CH2Cl2 at reaction temperatures
of 350, 400 and 450 1C, respectively. This is in line with the
C–H and C–Cl bond strengths (Eb ¼ 411 vs. 327 kJ molꢀ1).
Another important reaction parameter is the presence of
steam. In line with our previous experiments for the catalytic
destruction of CCl4,3 we observed for the destruction of CHCl3
and CH2Cl2 a positive influence of the presence of steam both
on activity and catalyst stability. Indeed, switching off the
steam results in an instant drop in catalyst activity, while
deactivated catalysts can regain their activity by switching on
the steam (Fig. 4). Thus, steam is essential for the performance
of the catalytic solids. If steam is present in sufficient amounts;
i.e., [H2O] : [CHC] Z 20, a stable activity could be maintained
for more than 10 days. This is illustrated in Fig. 5.
second step, two chemisorbed formaldehyde molecules will
react with HCl and H2O to form CH3Cl and CO. This
disproportionation reaction, involving a hydride shift, is re-
quired to obtain the end products, including the starting
material Al2O3. Steam converts the formed AlCl3 again to
Al2O3 and takes part directly in the disproportionation reac-
tion. This hypothesis has been further tested by using our bare
Al2O3 support as catalyst material and as expected CH3Cl and
CO have been formed out of CH2Cl2 under identical catalytic
conditions.
This reaction, however, suggests that alumina-supported
alkaline earth metal and lanthanide oxide catalysts are not
fully dispersed and that free Al2O3 patches are present at the
catalyst surface. This is indeed the case as illustrated in Fig. 3
by a SEM picture of a La2O3/Al2O3 catalyst before and after
catalyzing the decomposition of CH2Cl2 for 7 h at 350 1C. The
grey material is the alumina phase covered with only very small
amounts of La, whereas the white spots are La-enriched
phases. This is evident from the EDAX analysis along the
indicated scanning lines. Before reaction the La-rich phases
have a size of about 10 mm and are composed of most probably
La2O3. After reaction La-rich phases of equal or smaller size
are observed. In contrast, these phases contain comparable
amounts of La and Cl, pointing towards the formation of
LaOCl. Here again, alumina is not fully covered by a La-rich
phase, although clearly some Cl is covering Al2O3. Thus,
although lanthanide acetate salts have been used for the
preparation of the best performing supported catalysts,
patches of lanthanide oxide or lanthanide oxide chloride are
still inhomogeneously distributed over the alumina surface in
the catalytic solids.
A second disadvantage of the alumina-supported basic oxide
catalysts is the formation of some coke at the catalyst surface
during reaction as observed with TGA of the catalytic solids
after 7 h on stream at 350 1C. The amount of coke is, however,
Fig. 4 Effect of steam on the catalytic activity of a La2O3/Al2O3
catalyst at 350 1C for the destruction of CCl4 (K), CHCl3 (m) and
CH2Cl2 (’): (A) switching off the steam and (B) switching on the
steam again.
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 2 5 6 – 5 2 6 2
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Table 3 Raman shifts of Ln2O3, LnOCl and LnCl3 reference com-
pounds, together with those of the catalytic solids treated with CCl4,
CHCl3 and CH2Cl2 at 350 1C for 7 and 36 h
Element
La
Solid
Raman shift/cmꢀ1
Ref.
La2O3
LaOCl
107, 195, 410
125, 188, 215, 335,
440
7, 8
7, 9, 10
LaCl3
La2O3 treated with
CCl4 for 7 h
108, 180, 211
106, 191, 409
7, 11, 12
This work
La2O3 treated with
CCl4 for 36 h
La2O3 treated with
CHCl3 for 7 h
La2O3 treated with
CHCl3 for 36 h
La2O3 treated with
CH2Cl2 for 7 h
La2O3 treated with
CH2Cl2 for 36 h
CeO2
125, 188, 215, 335,
440; 108, 180, 211
122; 185, 209, 333,
438
This work
This work
This work
This work
This work
Fig. 5 CCl4 conversion in the presence of steam at 300 1C as a
function of time on steam over La2O3/Al2O3 (A) and La2O3 (B)
catalysts over an extended time period.
125, 188, 215, 335,
440; 108, 180, 211
121, 185, 209, 332,
439
3. In situ spectroscopic characterization
125, 188, 215, 335,
440; 108, 180, 211
102, 250, 454
125, 188, 217, 321,
469
In situ Raman spectra collected during CCl4, CHCl3 and
CH2Cl2 decomposition in the presence of steam (reactions 1,
4 and 5) over La2O3 after 7 h of reaction are shown in Fig. 6
and compared with those of La2O3, LaOCl and LaCl3 refer-
ence compounds. The spectra indicate that La2O3 is the main
catalytic phase present in the reactor in the presence of CCl4,
whereas LaOCl is formed in the presence of either CHCl3 or
CH2Cl2. In any case, no significant amounts of LaCl3 could be
detected after 7 h on stream. The presence of LaOCl in the case
of CHCl3 and CH2Cl2—containing less chlorine atoms per
molecule than CCl4—can be explained by the higher loadings
of both CHCs in the feed. On the other hand, longer reaction
times (e.g. 36 h) lead to the formation of both LaOCl and
LaCl3 (Table 3). Finally, it is important to notice that the
375 cmꢀ1 Raman peak has been assigned previously to a
luminescence effect.3
Similar in situ Raman spectra have been obtained for CeO2,
Pr2O3 and Nd2O3 after 7 and 36 h on stream. Table 3
summarizes the Raman shifts of some reference materials as
well as the Raman shifts of the catalytic solids after 7 and 36 h
on stream. As discussed above, in the case of CCl4 after 7 h on
stream CeO2, Pr2O3 and Nd2O3 are the main phases, whereas
CeOCl, PrOCl and NdOCl are the dominant phases in a
catalyst destroying CHCl3 or CH2Cl2 for 7 h. Thus, the family
Ce
7
7
CeOCl
CeCl3
CeO2 treated with
CCl4 for 7 h
108, 183, 217
100, 246, 450
7
This work
CeO2 treated with
CCl4 for 36 h
CeO2 treated with
CHCl3 for 7 h
CeO2 treated with
CHCl3 for 36 h
CeO2 treated with
CH2Cl2 for 7 h
CeO2 treated with
CH2Cl2 for 36 h
Pr2O3
125, 188, 217, 321,
469; 108, 183, 217
122, 186, 213, 319,
467
This work
This work
This work
This work
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125, 188, 217, 321,
469; 108, 183, 217
122, 185, 214, 318,
466
125, 188, 217, 321,
469; 108, 183, 217
105, 192, 415
121, 185, 216, 346,
458
Pr
13
9
PrOCl
PrCl3
Pr2O3 treated with
CCl4 for 7 h
108, 186, 216
102, 190, 412
14
This work
Pr2O3 treated with
CCl4 for 36 h
Pr2O3 treated with
CHCl3 for 7 h
Pr2O3 treated with
CHCl3 for 36 h
Pr2O3 treated with
CH2Cl2 for 7 h
Pr2O3 treated with
CH2Cl2 for 36 h
Nd2O3
120, 186, 216, 345,
457; 108, 187, 216
119, 182, 213, 344,
456
This work
This work
This work
This work
This work
121, 186, 217, 346,
458; 109, 187, 217
118, 181, 212, 343,
455
120, 185, 217, 346,
457; 108, 186, 215
102, 190, 434
121, 184, 220, 353,
465
Nd
9
9
NdOCl
NdCl3
Nd2O3 treated with
CCl4 for 7 h
104, 184, 219
100, 186, 431
15
This work
Nd2O3 treated with
CCl4 for 36 h
121, 185, 221, 353,
467; 105, 185, 219
119, 180, 218, 350,
462
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Nd2O3 treated with
CHCl3 for 7 h
Nd2O3 treated with
CHCl3 for 36 h
Nd2O3 treated with
CH2Cl2 for 7 h
Nd2O3 treated with
CH2Cl2 for 36 h
121, 184, 220, 354,
466; 105, 184, 219
119, 180, 218, 350,
462
Fig. 6 In situ Raman spectra collected during CCl4, CHCl3 and
CH2Cl2 decomposition in the presence of steam over La2O3 after 7 h
of reaction at 350 1C: (1) La2O3 dehydrated in N2 at 550 1C; (2) La2O3
after reaction with CCl4; (3) La2O3 after reaction with CHCl3; (4)
La2O3 after reaction with CH2Cl2; (5) La2O3 after reaction with
CH2Cl2; this material is the one collected from the reactor in point B
of Fig. 5; (6) LaOCl dehydrated in N2 at 800 1C; and (7) LaCl3
dehydrated in N2 at 550 1C.
121, 184, 221, 353,
466; 105, 184, 220
of the lanthanide oxides (Ln2O3) gradually transforms during
reaction into the corresponding lanthanide oxide chloride
5260
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 2 5 6 – 5 2 6 2
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4

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Table 4 Catalytic activity (conversion in %) of bulk La2O3, LaOCl
and LaCl3 for CCl4, CHCl3 and CH2Cl2 destruction after 7 h on stream
in the presence of steam at 350 1C. The reported activity of LaCl3
materials should be considered with care since hydrated LaCl3 easily
transforms during heating in the reactor into LaOCl so the activity of
LaCl3 could be partially due to the presence of LaOCl instead of LaCl3
Compound
CCl4
CHCl3
CH2Cl2
La2O3
LaOCl
LaCl3
62
60
5
34
26
24
15
14
11
(LnOCl) and after prolonged reaction times (i.e., 36 h) partially
to lanthanide chloride (LnCl3). This change is also associated
with a decrease in surface area. This observation confirms the
earlier reported dynamics of the catalyst systems under inves-
tigation.3 The almost complete transformation of Ln2O3 into
LnOCl takes place according to:
Fig. 7 Reaction scheme for the destruction of CCl2XY (with X ¼ H
and Y ¼ Cl for CHCl3 and X ¼ Y ¼ H for CH2Cl2) over La2O3 in the
presence of steam.
partially transformed into LaOCl during heating in the reactor.
Further studies should clarify these observations. Finally, we
recall that these bulk transformation phenomena as observed
with in situ Raman spectroscopy could not be detected for the
alumina-supported lanthanide oxide catalysts because of
strong laser fluorescence.3
1
2
1
2
Ln2O3 þ CCl4 - 2LnOCl þ CO2
(10)
This implies that LnOCl, in addition to Ln2O3, is an active
phase. This hypothesis has been evaluated by testing the
activity of bulk LaOCl in the destruction of CH2Cl2 and
CHCl3. As expected, similar activities were obtained as in the
case of La2O3 (Table 4). Continuing the reaction for extended
time periods or shutting down the steam supply results in the
(partial) transformation of the catalyst material to LaCl3
(Table 3). These processes can be written as follows:
In situ infrared spectra of the gas phase have been collected
to investigate the formation of possible gaseous reaction inter-
mediates. Table 5 summarizes the FT-IR absorption bands of
the gas phase molecules detected during reaction over La2O3
and La2O3/Al2O3 at 350 1C in the presence of steam and CCl4,
CHCl3 or CH2Cl2, together with their assignments. Unlike in
the case of CCl4 where COCl2 could clearly be identified as a
reaction intermediate,3 no other gaseous molecules other than
those found by gas chromatography in the gas outlet (e.g. CO
and CO2) were detected. Since we anticipate that the same
reaction mechanism is applicable as previously proposed for
the destruction of CCl4 over La2O3, CH2O and CHClO are the
expected reaction intermediates for the destruction of CH2Cl2
and CHCl3, respectively. Their absence in the infrared gas
phase spectra could be explained by their low stability under
the applied reaction conditions. Both reaction intermediates
will be readily hydrolysed in the presence of steam resulting in
the formation of CO and H2/HCl according to reactions 6 and
7. We indeed observed high amounts of CO in the in situ
FTIR cell for the catalytic destruction of CH2Cl2 and CHCl3
(Table 5). Other evidence for the catalytic decomposition of
HClCO comes from the work of Jasien.6 This author per-
formed theoretical calculations on the AlCl3-catalyzed decom-
position of HClCO. It was found that the presence of AlCl3
resulted in a large decrease of the activation energy for the
decomposition reaction of HClCO.
2LaOCl þ CCl4 - 2LaCl3 þ CO2
(11)
At this stage the catalyst material becomes less active and
separate catalytic experiments on LaCl3 for the destruction of
CH2Cl2 and CHCl3 show that the initial activity of LaCl3 is
indeed smaller than those of LaOCl and La2O3 (Table 4).
However, whereas in the case of CCl4, LaCl3 seems to be
almost completely inactive, this is certainly not the case for
CHCl3 and CH2Cl2. One explanation could be that LaCl3 is
Table 5 FT-IR absorption bands of the gas phase molecules detected
in the presence of a La2O3 catalyst at 350 1C
Chlorinated
hydrocarbon
Absorption
bands/cmꢀ1
Assignments
based on ref. 16
CCl4
2363, 2340 (most
intense)
2175, 2112
C
O stretch of CO
Q
2
C
Q
O stretch of CO
3550–3200, 1628
OꢀH stretch and
HꢀOꢀH bending of H2O
O stretch of COCl
1820, 1831
3033, 2967
C
Q
2
A plausible reaction scheme summarizing the current experi-
mental observations is given in Fig. 7. The catalytic destruction
occurs via an initial adsorption step of the chlorinated hydro-
carbon on the basic oxide surface, in which the chlorine atoms
are attached to the catalyst surface, followed by a dechlorina-
tion step of the formed chlorinated solid by steam and the
formation of hydrochloric acid. CO is proposed to be formed
from the reaction intermediate HClCO (for CHCl3) or H2CO
(for CH2Cl2), whereas CO2 is proposed to be formed from the
reaction intermediate Cl2CO (for CCl4).
CHCl3
CꢀH stretch
2408, 2398
2363, 2340
CꢀCl overtone stretch
C O stretch of CO
Q
2
2175, 2112 (most
intense)
3550–3200, 1628
C
O stretch of CO
Q
OꢀH stretch and
HꢀOꢀH bending of H2O
CꢀCl overtone stretch
CꢀCl stretch
1542, 1442
1218, 1201
3001, 2995
2695, 2686
2363, 2340
2175, 2112 (most
intense)
CH2Cl2
CꢀH stretch
CꢀCl overtone stretch
C
O stretch of CO
2
Q
Conclusions
C
Q
O stretch of CO
The following conclusions can be drawn from this work:
(1) Basic oxides are very active materials for the destruction
of CCl4, CHCl3 and CH2Cl2 in the presence of steam between
200 and 450 1C. The ease of catalytic destruction decreases
with decreasing chlorine content of the chlorinated
1464, 1455
3550–3200, 1628
CꢀCl overtone stretch
OꢀH stretch and
HꢀOꢀH bending of H2O
CꢀCl stretch
1280, 1261
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 5 2 5 6 – 5 2 6 2
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hydrocarbon and is the highest for alumina-supported lantha-
nide oxides. Stable destruction activities are achieved as long as
excess steam is added to the gas stream.
(Utrecht University). The authors also thank H. De Winne
(KULeuven) for catalytic testing, O. Manoilova (Utrecht
University) for the Raman experiments and M. Versluijs-
Helder (Utrecht University) for SEM measurements. B.M.W.
acknowledges the Dutch Science Foundation (NWO-CW)
for a Van der Leeuw and VICI grant.
(2) The catalytic hydrolysis of CCl4 leads to the formation of
CO2 and HCl, whereas the catalytic destruction of CHCl3 and
CH2Cl2 results in respectively a mixture of CO and HCl and a
mixture of H2, CO and HCl. CO and HCl are proposed to be
formed directly from HClCO (for CHCl3), H2CO decomposi-
tion results in the formation of CO, HCl and H2 (for CH2Cl2),
whereas CO2 and HCl are formed from Cl2CO (for CCl4).
Alumina-supported lanthanide oxides also produce significant
amounts of CH3Cl, which is associated with the catalytic
performance of the partially uncovered alumina support phase.
Another disadvantage of the alumina support is the enhanced
formation of coke. Future work clearly should be directed
towards the use of other and most probably non-oxidic catalyst
supports, such as carbon nanofibers.
(3) Catalytic destruction of CCl4, CHCl3 and CH2Cl2 with
lanthanide oxides is assumed to proceed over a basic surface
oxygen site. The active phases La2O3 and LaOCl contain these
basic surface oxygen sites and their relative concentration is
related to the preparation method of the La2O3 material.
Future infrared work with probe molecules should unravel
structural differences between catalytically different LaOCl and
La2O3 materials with increasing surface areas.
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Acknowledgements
This work was supported by the Instituut voor Aanmoediging
van Innovatie door Wetenschap en Technologie in Vlaanderen
(IWT-Vlaanderen; project ADV/990177). The authors ac-
knowledge fruitful discussions with R. Schoonheydt (KULeu-
ven), G. Mestl (Nanoscape), I. Langhans (CQ), K. Ooms (CQ),
D. van Deynse (Tessenderlo Chemie), Y. Sergeant (Tessender-
lo Chemie), M. Belmans (Tessenderlo Chemie) and T. Visser
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