The effect of Ce on catalytic decomposition of chlorinated methane over RuOx catalysts

Article abstract of DOI:10.1016/j.apcata.2013.11.006

Ru oxide catalysts supported on Ce-Al₂O₃ with various Ce contents were prepared by wet impregnation with RuCl₃ aqueous solution and characterized by XRD, N₂ adsorption, TEM/HRTEM, H ₂-TPR, Raman and N

Full text of DOI:10.1016/j.apcata.2013.11.006

Applied Catalysis A: General 470 (2014) 442–450
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
The effect of Ce on catalytic decomposition of chlorinated methane
over RuO catalysts
x
∗
Le Ran, Zhengyi Wang, Xingyi Wang
Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2013
Received in revised form 30 October 2013
Accepted 4 November 2013
Available online 10 November 2013
Ru oxide catalysts supported on Ce-Al O with various Ce contents were prepared by wet impregnation
2
3
with RuCl3 aqueous solution and characterized by XRD, N2 adsorption, TEM/HRTEM, H2-TPR, Raman and
NH3-TPD. The catalytic combustion of dichloromethane (DCM) was investigated over the Ru/Ce-Al2O3
catalysts at first time. The Ru/Ce-Al2O3 catalysts present an outstanding catalytic activity. Ru/7%Ce-Al2O3
◦
◦
catalyst is most active, and DCM is converted completely at 270 C. At 250 C, the conversion of DCM over
the Ru/Ce-Al2O3 catalysts with 3% or higher Ce contents keeps constant within 1000 h. High stable activity
of the Ru/Ce-Al2O3 catalysts can be ascribed to the synergistic effect of acidity and high oxygen mobility.
Keywords:
Dichloromethane
CeOx–Al2O3
RuO2
©
2013 Published by Elsevier B.V.
Catalytic combustion
Chlorine
1
. Introduction
intermediates which can be oxidized to carbon dioxide [7,8]. It was
often considered that the activity for catalytic combustion of DCM
was related to the synergy between acidic role and oxidization
[9,10]. Though Pt catalysts were more preferentially acceptable
than other catalysts, the significant deactivation was observed
at an initial stage of reaction due to the interaction of DCM with
hydroxyl groups [7]. As found in our previous work, Cl species
produced during the CVOC oxidation strongly adsorbed on the
active sites and hence retarded the accesses of reactants [11]. The
removal of Cl species was often rate-controlling step in many cases
of CVOC catalytic combustion [11,12]. Raising temperature will
promote the removal of Cl species via the desorption of Cl species
or the oxidation of Cl species (Deacon reaction) [13]. However, it
is desirable to develop more active catalytic materials which can
enhance the removal of Cl species at low temperature.
Chlorinated volatile organic compounds (CVOCs) are hazardous
pollutants that are considered as the most harmful organic con-
taminants due to their acute toxicity and strong bioaccumulation
potential [1]. The safe disposal of alkyl chloride pollutants, such
as CH Cl (DCM), CHCl CCl2 (TCE) and CH Cl CH Cl (DCE), has
acquired great importance with the ever increasing concern for
environmental protections [2]. Among various available detoxifica-
tion techniques, the catalytic oxidation is an interesting one which
2
2
2
2
can be efficiently performed within the temperature range from
◦
2
50 to 550 C converting so dilute pollutants that otherwise cannot
be thermally combusted without consumption of fuel. Most stud-
ies of the catalysts used in the catalytic oxidation of CVOCs have
reported on the three types of catalysts based on noble metals,
transition metals and zeolites [3–5]. At present, the problems such
as the formation of polychlorinated compounds during treatment
process and deactivation of catalysts due to the adsorption of Cl
species on the active sites remain yet to be solved. Therefore, the
development of catalysts having high performance in converting
CVOCs is of great significance.
The catalytic oxidation of DCM, as a model of alkyl chloride pol-
lutants, has been investigated widely over Pt catalysts supported
on acidic materials [6–8], especially on ␥-alumina. DCM can react
readily with hydroxyl groups of catalysts to form the reactive
CeO presented the high activity for catalytic combustion of var-
2
ious CVOCs, which was attributed to the high ability for dissociating
C Cl bonds, high oxygen mobility and abundant oxygen defects.
Recently, it has been reported that the addition of Ce element pro-
moted for DCM decomposition over Pt/Al O3 catalyst [14]. Zhou
2
et al. found that Cr–Ce–USY zeolite catalysts possessed high activity
for catalytic decomposition of DCM, TCE and DCE [15]. In our previ-
ous studies, the introduction of Ce element can improve evidently
both the activity and stability of transition metal oxide (TMO) cat-
alysts in the catalytic combustion of chlorobenene [11,12,16]. The
high stability of ceria-doped TMO catalysts derives from the fact
that the adsorbed inorganic chlorine species or dissociated Cl can be
removed rapidly from the surface or active sites via the formation
and reoxidization of intermediate TMOyClx or TMClx.
∗ Corresponding author. Tel.: +86 21 64253372; fax: +86 21 64253372.
E-mail address: wangxy@ecust.edu.cn (X. Wang).
0
926-860X/$ – see front matter © 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.apcata.2013.11.006

L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
443
As known, Ru-based catalysts have been employed for com-
mercialized chlorine production via HCl oxidation, due to its
extraordinary stability and easier Cl2 evolution [13,17]. Addition-
was focused, analyzing spot about 1 mm, on the sample under the
microscope.
ally, RuO has received considerable attention as a heterogeneous
catalyst used in oxidation of various compounds, such as carbon
2
2.3. Activity evaluation
monoxide, soot, methane and propane [18–21]. Recently, Ru/Al O3
catalyst was used in the catalytic combustion of TCE and presented
high activity [22]. Our recent work showed that the high activity
2
The activity and stability of catalysts were tested at atmospheric
pressure in a continuous flow micro-reactor made of a quartz tube
with inner diameter of 4 mm. 0.3 mL catalyst (0.185 g) as the reac-
tion bed was packed. The feeding flow rate to the reactor was set at
and stability of Ru/CeO for catalytic combustion of chlorinated aro-
2
matics were related to its ability for removal of Cl species from the
surface of catalyst via Deacon reaction [16,23]. Additionally, we also
found that Ru/TiO2 presented high activity for the combustion of
DCM, due to its high removal of Cl species [24]. In this work, we
3
−1
5
0 cm min and the gas hourly space velocity (GHSV) was main-
−
1
tained at 10,000 h . Feed stream to the reactor was prepared by
delivering liquid DCM with a syringe pump into dry gas mixture
composed of a given concentration of O2 and N2 balance, which
were metered by a mass flow controller, controlling DCM concen-
tration in the feed stream at 750 ppm or 1500 ppm. The injection
part was electrically heated to ensure complete evaporation of
DCM. The reactor temperature was measured with a thermocou-
ple located just at the exit of the micro-reactor. The temperature
used Ce-doped Al O3 as supports, and prepared Ru catalysts by
2
wet impregnation with RuCl3 aqueous solution, and investigated
in details the effect of Ce on catalytic decomposition of chlorinated
methane over Ru/Al O3 catalysts through characterization, reac-
2
tion kinetics and auxiliary experiments in the several aspects such
as the synergistic effect of high acidity, high oxygen mobility of
CeOx and the stability of RuOx exposed to Cl species in the catalytic
decomposition of DCM.
◦
was raised by steps from 150 to 450 C. The effluent gases were
analyzed on-line at a given temperature by using three gas chro-
matographs (GC), two equipped with FID and ECD, respectively, for
the quantitative analyses of organic chlorinated compounds, and
the other one with TCD for CO and CO2. The concentrations of Cl2
and HCl were analyzed by the effluent stream bubbling through
a 0.0125 N NaOH solution, and chlorine concentration was then
determined by the titration with ferrous ammonium sulphate using
N,N-diethyl-p-phenylenediamine as an indicator [25]. The concen-
tration of chloride ions in the bubbled solution was determined by
using a chloride ion selective electrode [26].
2
. Experimental
2.1. Catalyst preparation
Ce-Al O supports were prepared by a co-precipitation method.
2
3
A detailed process was as follows: the ammonia and an aqueous
solution of Al(NO ) ·9H O were added dropwise to an aqueous
3
3
2
solution of Ce(NO ) ·6H O under vigorous stirring. The pH value
3
3
2
of solution was controlled at 8–8.5. The precipitates were aged
3
. Results and discussion
◦
for 12 h, washed with distilled water, then dried at 100 C for 12 h
◦
and finally calcined in air at 550 C for 4 h. The synthesized Ce-
3
.1. Characterization of catalysts
−
1
Al O samples were impregnated with 0.198 M (0.02 gRu mL
)
2
3
◦
◦
RuCl3 aqueous solution at 25 C for 12 h, dried at 100 C for 12 h
Ru loading in RuOx catalysts, determined by XRF, is close to the
nominal value of 1 wt% (Table 1), indicating that the impregnation
◦
in a vacuum oven, and finally calcined in air at 550 C for 4 h in a
tubular furnace. Thus 1 wt% Ru/Ce-Al O catalysts with Ce content
2
3
of ruthenium with RuCl3 aqueous solution onto the Al O3 or Ce-
2
of 0% (pure Al O ), 1%, 3%, 5%, 7% and 10%, as determined by X-ray
2
3
Al O supports is effective. Fig. 1 shows the results of XRD analyses.
2
3
Fluorescence Spectrometer (XRF), were obtained.
On the XRD patterns of Al O and Ce-Al O supports, three broad
2
3
2
3
◦
◦
◦
diffraction peaks appearing at 36.9 , 45.9 and 66.6 can be ascribed
2
.2. Catalysts characterization
to spinal phase (␥-Al O , PDF #50-0741). With the increase of Ce
2 3
◦
◦
◦
content, there appear four weak peaks at 28.6 , 33.3 , 47.5 and
◦
The powder X-ray diffraction patterns (XRD) of the samples
56.5 , ascribed to CeO cerianite with a fluorite-like structure (PDF
2
◦
were recorded on a Rigaku D/Max-rC powder diffractometer using
#43-1002). A new weak and broad peak appears at 60.1 , probably
Cu K␣ radiation (40 kV and 100 mA). The diffractograms were
recorded within the 2ꢀ range of 10–80 with a 2ꢀ step size of 0.01
a reflection from CeAlO (PDF #28-0260) (the other two peaks at
3
◦
◦
◦
◦
◦
◦
33.6 and 48.3 overlap with the reflections at 33.3 and 47.5 from
◦
◦
◦
and a step time of 10 s. The nitrogen adsorption and desorption
CeO ). The diffraction peaks of RuO appear at 28.0 , 35.0 and 54.2
2
2
◦
isotherms were measured at −196 C on an ASAP 2400 system in
for all Ru/Ce-Al O catalysts [27]. According to the Scherrer equa-
2 3
◦
static measurement mode. The samples were outgassed at 160 C
tion applied to ꢀ1 1 0ꢁ reflection of RuO , the size of RuO particles
2
2
for 4 h before the measurement. The specific surface area was cal-
culated using the BET model. Ru content was determined by XRF
using a Shimadzu (XRF-1800) wavelength dispersive X-ray fluores-
cence spectrometer. Samples were prepared in the form of uniform
tablets (20 mm of diameter) by pressing (30 MPa) the powder of
catalyst. Transmission electron microscopy (TEM) analysis was car-
ried out using a JEM-2010F operated by an accelerating voltage
is estimated to be 11–20 nm, dependent on Ce content (Table 1).
The average size of RuO2 particles is estimated to be 20.9 nm for
Ru/Al O3 catalyst, while for Ru/7%Ce-Al O3 catalyst, ∼11 nm. The
2
2
decrease in lattice parameters of CeO to some extent (Table 1) can
2
be ascribed to the entrance of some Ru species into the CeO cerian-
2
◦
ite [23]. The disappearance of diffraction peak at 60.1 indicates that
the interaction of RuOx with Ce3+–O–Al. The size of CeO particles
2
of 200 kV. H -temperature programming reduction (H -TPR) was
of Ru/Ce-Al O3 catalysts increases to 6.3–9.3 nm from 5.0–7.6 nm
2
2
2
investigated by heating catalysts (100 mg) in H (5 vol.%)/Ar flow
for Ce-Al O samples. Moreover, the BET area of samples increases
2 3
gradually with the increase of Ce content in the range of 1–10%,
2
−1
◦
−1
◦
(
30 mL min ) at a heating rate of 10 C min from 50 to 750 C. The
hydrogen consumption was monitored by thermo-conductivity
confirming that the addition of Ce can promote ␥-Al O dispersion.
2
3
detector. Before the H -TPR analyses, the samples were heated
for 60 min in Ar flow at 300 C. The Raman spectra were obtained
Fig. 2(a)–(e) shows TEM images of the Ce-Al O and Ru/Ce-Al O
samples. For 1%Ce-Al O (Fig. 2(a)) and 7%Ce-Al O (Fig. 2(b)) sup-
2 3 2 3
2
2 3 2 3
◦
on a Renishaw in Viat + Reflex spectrometer equipped with a CCD
detector at ambient temperature and moisture-free conditions. The
emission line at 514.5 nm from an Ar ion laser (Spectra Physics)
ports, nanoparticles have irregular shape. HRTEM (Fig. 2(f) and (g))
confirms the existence of 5–6 nm CeO2 particles in 7%Ce-Al O3
2
+
and Ru/7%Ce-Al O samples, which is in good agreement with the
2
3

4
44
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
Table 1
Surface and structural properties of Ru/Ce-Al2O3 catalysts with various Ce content.
Ru loading a (wt%)
SBET (m /g)
2
LCeO b (nm)
NCeO c (nm)
NRuO2 d (nm)
H2 consumption (␮mol gcat)
Catalysts
2
2
Actual
Theoretical
Actual/theoretical
Al2O3
–
–
–
–
–
–
171
178
181
194
200
198
175
184
181
196
209
190
–
ND
ND
5.415
5.416
5.413
–
ND
ND
–
ND
–
–
–
–
–
–
20.9
20.8
19.7
17.6
11
ND
–
3
10
–
36
–
e
1%Ce-Al2O3
3%Ce-Al2O3
5%Ce-Al2O3
7%Ce-Al2O3
10%Ce-Al2O3
0.09
0.10
0.20
0.40
0.53
1.96
1.88
2.28
2.70
3.20
–
ND
5.0
5.8
7.6
–
ND
ND
6.3
7.1
9.3
107
179
250
357
193
229
300
372
443
–
35
101
190
376
431
679
1004
1397
–
Ru/Al2O3
0.975
0.968
0.961
0.958
0.919
0.908
Ru/1%Ce-Al2O3
Ru/3%Ce-Al2O3
Ru/5%Ce-Al2O3
Ru/7%Ce-Al2O3
Ru/10%Ce-Al2O3
5.405
5.407
5.401
a
Determined by XRF.
CeO2 lattice parameters calculated from XRD results.
CeO2 crystal size.
RuO2 crystal size, calculated by the Scherrer equation based on XRD results.
ND, not be detected.
b
c
d
e
average mean diameter estimated based on the XRD data (5–7 nm).
that the interaction between Al O3 and Ce species must deform
2
the fluorite-like structure. However, this band appears at 460 cm
−
1
CeO mainly exposes the (1 1 1) plane with the distance of 0.31 nm.
2
Very thin particles with lattice fringes of distance about 0.32 nm
for 3%Ce-Al O3 sample and becomes strong with the increase in
2
(
Fig. 2(g)) can be observed, corresponding to the ꢀ1 1 0ꢁ lattice plane
Ce content, due to the improvement of CeO crystalline symmetry.
2
of the tetragonal RuO . Also, the particle size of RuO is estimated
As reported in the literature, Eg, A1g, and B2g modes of RuO are
2
2
2
−
1
to be 6–9 nm, similar to the results obtained by XRD analyses.
The possible catalyst structures were investigated by Raman
spectroscopy technique (Fig. 3). For pure CeOx sample, the band
located at about 523, 646, and 710 cm , respectively [30]. For
Ru/Ce-Al O3 samples, there appear two weak and broad bands at
2
−
1
520 and 630 cm , which can be attributed to Eg and A1g modes.
The band corresponding to B2g modes can not be observed, indicat-
−1
appearing at 460 cm
can be attributed to the mode (F2g) of
fluorite-like structure [28]. As known, fluorite-structure is a cubic
structure (fcc), in which cations are located on the corners and in
the face centers while oxygen atoms are located at the tetrahedral
sites. The spectra of ꢁF2g band are dominated by oxygen lattice
vibrations and are sensitive to crystalline symmetry [29]. For
ing the existence of interaction between RuO and CeOx. Moreover,
the intensity of ꢁF2g band decreases to a significant extent, which
2
further confirms the interaction between CeO and RuO . Probably,
2
2
the entrance of Ru into CeO lattice results in the decrease of CeO2
2
symmetry, especially for the samples containing low Ce content.
The results of TPR analyses for catalysts are shown in Fig. 4(a)
1
%Ce-Al O sample, the ꢁF2g band was not observed, indicating
2 3
◦
and (b). The reduction of pure CeO2 occurs at 437 and 522 C
in the range of experimental temperature (Fig. 4(a)), associated
with the reduction of surface Ce4+ ions from small and large size
of CeO2 particles. [31]. For the Ce-Al O3 samples, the peak at
2
◦
4
37 C becomes strong gradually with the increase in Ce content.
Another peak appearing at higher temperature can be assigned to
the reduction of Ce4+ species strongly interacted with Al O3 for
2
the samples with low Ce content, and the reduction of surface
4+
Ce species from large CeO2 particles for the samples with high
Ce content (Fig. 4(a)). H2 consumption (Table 1) shows that the
actual values are much lower than theoretical values (the amount
of hydrogen required for the reduction was calculated on the basis
of 2CeO + H → Ce O + H O), probably owing to the formation of
2
2
2
3
2
3+
4+
Ce –O–Al and the reduction of Ce in bulk at higher temperature
beyond the experimental range (for the samples with higher Ce
content).
0
For Ru/Al O sample, the reduction of RuO to Ru , as shown in
Fig. 4(b), occurs at 176 C. With the increase in Ce content, the initial
reduction of Ru shifts to low temperature gradually and finally
drops to 74 C for Ru/CeO2 sample. This phenomenon indicates
2
3
2
◦
4
+
◦
that the reduction of Ru species is promoted by a high degree of
4+
coordinative unsaturation and, perhaps, by neighboring Ce ions,
3+
which, in turn, undergo reduction to Ce species. Thus, the reduc-
◦
tion peak area below 300 C increases with Ce content increase,
indicating the oxygen mobility of CeO2 can be promoted by Ru
incorporation. H2 consumptions of the Ru/Ce-Al O3 catalysts are
2
much higher than the theoretical values calculated on the basis of
Fig. 1. XRD patterns of Ce-Al2O3 and Ru/Ce-Al2O3 catalysts with various Ce
contents; 1 – Al2O3, 2 – 1%Ce-Al2O3, 3 – 3%Ce-Al2O3, 4 – 5%Ce-Al2O3, 5 – 7%Ce-
Al2O3, 6 – 10%Ce-Al2O3, 7 – Ru/Al2O3, 8 – Ru/1%Ce-Al2O3, 9 – Ru/3%Ce-Al2O3, 10 –
Ru/5%Ce-Al2O3, 11 – Ru/7%Ce-Al2O3 and 12 – Ru/10%Ce-Al2O3.
0
RuO + 2H → Ru + 2H O, probably due to spilling over hydrogen
2
2
2
from Ru to the supports [32]. The ratios of actual value to theoretical
value increase significantly with Ce content increase. Additionally,

L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
445
Fig. 2. TEM images of Ce-Al2O3 and Ru/Ce-Al2O3 catalysts: (a) 1%Ce-Al2O3, (b) 7%Ce-Al2O3, (c) Ru/Al2O3, (d) Ru/1%Ce-Al2O3 and (e) Ru/7%Ce-Al2O3; HRTEM images of
%Ce-Al2O3 (f) and Ru/7%Ce-Al2O3 (g).
7
◦
the reduction peak at about 530 C disappears, implying that the
sites of supporting RuO2 is closed to these Ce species that can be
reduced readily by the dissociated H atoms on Ru species at lower
temperature.
the case of supported RuO2 catalysts, the amount of acidic sites
increases, probably due to the presence of residual Cl species in
−
the catalyst because of the usage of RuCl precursor.
3
The results of NH -TPD analyses for catalysts are shown in
3
3.2. Catalytic activity
Table 2. For pure Al O3 sample, the amount of acidity is esti-
2
mated to be 294 ␮mol/g (Table 2). For the Ce-Al O3 catalysts, the
2
Under the reaction condition of 750 ppm DCM, 20% O , N
2
2
amount of acidic sites increases from 340 ␮mol/g for 3%Ce-Al O3
−1
2
balance and GHSV = 10,000 h , DCM conversion over the fresh cat-
alysts as the function of temperature is shown in Fig. 5(a), while T50%
and T90% (the temperature needed for 50% and 90% conversion) as a
function of Ce content, shown in Fig. 5(b). It can be seen in Fig. 5(a)
that pure Al O support seems active for DCM decomposition with
to 375 ␮mol/g for 7%Ce-Al O . The increased new acidic sites could
2
3
result from Ce species highly dispersed on Al O surface. When Ce
2
3
content was too high, such as 10%, the acidity decreases to a sig-
nificant extent, due to the coverage of CeO over Al O surface. In
2
2
3
2
3
◦
T50% and T90% of 304 and 341 C. With the incorporation of Ce,
the conversion curves shifts significantly to low temperature. T
decreases from 329 C for 1%Ce-Al O catalyst to 310 C for 7%Ce-
90%
◦
◦
2
3
Al O . Ru/Ce-Al O catalysts with the Ce contents of 1–7% possess
2
3
2
3
◦
higher activity and T90% drops in the range of 260–297 C (Fig. 5(b)).
DCM conversion reached the complete up to 340 C over Pt/Al O
6] and to 400 C over Pd supported catalysts [33]. Obviously, Ru
◦
2
3
◦
[
based catalysts are more active than other noble metal oxide cata-
lysts for DCM decomposition. It is interesting to find that the change
trend of T50% and T90% with the increase of Ce is consistent with that
4+
of the temperatures at which Ru can be reduced at initial time,
indicating that the reducibility of catalysts is responsible for the
activity (Fig. 5(b)). On the other hand, T50% and T90% is also related to
the acidic amount of catalysts. As reported previously, the acidity
is favorable for activating C Cl bond [6–8]. Although Ru/10%Ce-
Al O possesses the best oxygen mobility, its amount of acidic sites
2
3
is lest. As a result, the activity of Ru/10%Ce-Al O catalyst decreases
2
3
significantly.
Fig. 6 presents the stability of catalysts in DCM catalytic decom-
position under reaction condition as the above mentioned. Based
on the analyses of all component including reactants and products
in the effluent, chlorine uptake on the surface of catalysts treated in
◦
reaction feed at 250 and 270 C for 1000 min is quantitatively deter-
mined by means of Cl species balance and shown in Table 2 (TPSR on
the above treated catalysts indicated that no adsorption of organic
◦
chlorinated compound deposits). At 250 and 270 C, DCM conver-
Fig. 3. Raman spectra of catalysts; 1 – 1%Ce-Al2O3, 2 – 3%Ce-Al2O3, 3 – 5%Ce-Al2O3, 4
7%Ce-Al2O3, 5 – 10%Ce-Al2O3, 6 – Ru/1%Ce-Al2O3, 7 – Ru/3%Ce-Al2O3, 8 – Ru/5%Ce-
Al2O3, 9 – Ru/7%Ce-Al2O3, 10 – Ru/10%Ce-Al2O3 and 11 – CeO2.
sion on Ru/Al O catalyst decreases within the first 400 min rapidly
from 45% and 61% to zero, indicating that the stability of Ru/Al2O3
–
2
3

4
46
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
Table 2
The catalytic performance of Ce-Al2O3 and Ru/Ce-Al2O3 catalysts with various Ce content.
Cl uptakea (mmol/g−1
)
Clb (at%)
T50/T90c ( C)
◦
Samples
NH3 uptake (␮mol/g)
cat
◦
◦
C
250
C
270
CHCl3
CH3Cl
Al2O3
294
318
340
375
350
381
396
470
1.77
–
–
1.65
–
–
–
–
–
–
0.58
0.52
0.38
0.35
262/301
262/297
265/295
262/301
258/295
250/281
226/267
205/245
305/362
397/348
285/325
272/313
260/292
259/281
246/278
235/275
1
3
7
%Ce-Al2O3
%Ce-Al2O3
%Ce-Al2O3
–
–
d
Ru/Al2O3
1.46
1.16
0.81
0.68
0.87
0.83
0.64
0.51
Ru/1%Ce-Al2O3
Ru/3%Ce-Al2O3
Ru/7%Ce-Al2O3
a
Cl uptake, determined by subtraction of the amount of HCl and Cl2 measured at the reactor outlet from the total chlorine amount.
b
c
◦
Determined by XPS for the used catalysts for 1000 min at 270 C.
−1
Gas composition of feed: 750 ppm chlorinated methane, 20% O2 and N2 balance; GHSV = 10,000 h
.
d
◦
Obtained at 285 C.
catalyst cannot be improved essentially compared with pure Al O .
for 1000 min are estimated to be 1.16, 0.81 and 0.68 mmol Cl/gcat,
respectively. Raising reaction temperature to 270 C, the chlorine
2
3
◦
Moreover, no inorganic chlorine compound can be detected in
the effluent during the deactivation process, implying that the Cl
species deposited on the active sites on Ru/Al O catalyst and prob-
uptakes of Ru/1%Ce-Al O , Ru/3%Ce-Al O and Ru/7%Ce-Al O cat-
2
3
2
3
2
3
alysts decreased by 20–30% and their corresponding conversions
keep at 52%, 81% and 96%, respectively. Ru/7%Ce-Al O catalyst in
2
3
ably resulted in the deactivation of Ru/Al O catalyst. The chlorine
2
3
2
3
◦
◦
uptake of Ru/Al O catalyst at 250 C is estimated to be 1.46 mmol
particular was tested at 270 C for another 1000 h with the conver-
sion as high as 96%. Therefore, the removal of these Cl species is a
key step to improve the stability of catalysts.
2
3
◦
Cl/gcat. By raising temperature to 285 C, substantial decrease in
DCM conversion is not observed, and the conversion can be main-
tained at about 65% for 1000 min. The corresponding chlorine
uptake decreases to 0.68 mmol Cl/gcat, indicating the removal of Cl
The analyses of containing-carbon product are shown in
Fig. 7(a)–(c). CO2 is main containing-carbon product over the
species is promoted by raising temperature. The stability of Al O in
Ru/Ce-Al O3 catalysts, which is in coincident with the results
2
3
2
DCM decomposition was studied widely, and its deactivation was
obtained during DCM decomposition over noble metal oxides
supported on acidic materials [6,7]. The selectivity for CO2
increases with the increase in temperature and reaches almost
ascribed mainly to the consumption of hydroxyl group [6]. For the
◦
Ru/Ce-Al O catalysts, the stability at 250 C increases with Ce con-
2
3
◦
tent. The activity of Ru/1%Ce-Al O catalyst cannot be maintained
100% at 325 C at which no other containing-carbon product
2
3
◦
at 250 C, similar to the case of Ru/Al O catalyst. However, raising
can be detected. CH Cl can be detected at different level for all
2
3
3
Ce content to 3%, the conversion keeps at 49–52% within 1000 min.
This phenomenon is consistent with previously reported results
that Ce catalysts supported on acidic zeolites presented good stable
activity for the CVOCs catalytic combustion [34]. On Ru/7%Ce-
Al O catalyst, a higher stable activity with DCM conversion of
catalysts (Fig. 7(b)). The amount order of CH Cl is produced:
3
Al O > Ce-Al O > Ru/Al O > Ru/Ce-Al O , lined with that of Cl
2
3
2
3
2
3
2
3
uptake (Table 2). The formation of CH Cl should be promoted by
3
chlorination. Over Ru/7%Ce-Al O catalyst with low Cl uptake, the
2
3
lowest amount (<8 ppm) of CH Cl is formed, while up to 45 ppm
2
3
3
◦
8
0% can be reached at 250 C. The chlorine uptakes of Ru/1%Ce-
CH Cl is obtained over Ru/Al O catalyst with high Cl uptake. Al O
3 2 3 2 3
◦
−1
having highest Cl uptake (1.77 mmol g ) presents 50% selectivity
Al O , Ru/3%Ce-Al O and Ru/7%Ce-Al O catalysts used at 250 C
2
3
2
3
2
3
Fig. 4. H2-TPR profiles of Ce-Al2O3 (a) and Ru/Ce-Al2O3 (b) with various Ce contents; (a): 1 – 1%Ce-Al2O3, 2 – 3%Ce-Al2O3, 3 – 5%Ce-Al2O3, 4 – 7%Ce-Al2O3, 5 – 10%Ce-Al2O3,
and 6 – CeO2; (b): 1 – Ru/Al2O3, 2 – Ru/1%Ce-Al2O3, 3 – Ru/3%Ce-Al2O3, 4 – Ru/5%Ce-Al2O3, 5 – Ru/7%Ce-Al2O3, and 6 – Ru/CeO2.

L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
447
Fig. 6. DCM conversion with time on stream over Ru/Ce-Al2O3 catalysts with various
Ce contents at different temperatures; gas composition: 750 ppm DCM, 20% O2 and
−
1
N2 balance; GHSV = 10,000 h
.
The activity order for the catalytic combustion of chlorinated
methane over the Ru/Ce-Al O catalysts is CHCl > CH Cl > CH Cl .
2
3
3
3
2
2
3.3. TPSR study
In order to investigate the process involving Cl species, the
feed containing DCM of 2000 ppm and oxygen of 20% pass through
Ru/Al O and Ru/7%Ce-Al O catalysts and the effluent was mon-
2
3
2
3
Fig. 5. The conversion curves of DCM over catalysts with various Ce contents (a)
and T50%, T90%, the initial reduction temperature (TR) and the acidic amount of Ru/Ce-
Al2O3 catalysts as functions of Ce contents (b); gas composition: 750 ppm DCM, 20%
itored by mass spectroscopy within a given range of temperature
(Fig. 8(a) and (b)). It can be seen in Fig. 8(a) that for Ru/Al O cat-
2
3
◦
alyst, the evolvement of CO2 occurs at 223 C, and increases with
−1
O2 and N2 balance; GHSV = 10,000 h
.
◦
temperature. At 300 C, this signal keeps constant. The signals of HCl
◦
and Cl appear at 300 C at the same time. This phenomenon can be
2
for CH Cl·CHCl , as a product of chlorination, is detected at higher
explained by the fact that HCl produced from acidic sites transfers
3
3
◦
than 260 C. Although the CHCl amount is trace, it is much higher
toward the active sties for Deacon reaction (HCl + O → Cl + H O)
3
2
2
2
over Ru/Al O catalyst than those over the Ru/Ce-Al O3 catalysts
on RuOx and is oxidized into Cl (thus, H O is formed). The differ-
2 2
2
3
2
(
Fig. 7(c)), of which Ru/7%Ce-Al O3 catalyst presents the lowest
ence between the temperatures of appearing CO and inorganic Cl
2
2
amount of CHCl , <1 ppm. In order to understand these phenomena,
species indicates the strong adsorption of these Cl species. Thus,
3
the activity of catalysts for CH Cl and CHCl3 catalytic combustion
the evolvement of HCl and Cl can still be observed when DCM in
3
2
was investigated and the corresponding T50% and T90% are listed in
Table 2. It can be seen that all Ru catalysts under study are highly
active in CH Cl and CHCl decomposition, especially for the Ru/Ce-
the feed is cut off. For Ru/7%Ce-Al O3 catalyst, the signals of HCl
2
and Cl can be detected at lower temperature (Fig. 8(b)). Moreover,
2
the above mentioned temperature difference is smaller, compared
with the case of Ru/Al O catalyst. Obviously, the incorporation of
3
3
Al O catalysts. CH Cl and CHCl will be oxidized quickly if formed.
2
3
3
3
2
3
Fig. 7. The product distributions during DCM decomposition over Ru/Ce-Al2O3 catalysts with various Ce contents; (a) CO2, (b) CH3Cl and (c) CHCl3; gas composition: 750 ppm
−1
DCM, 20% O2 and N2 balance; GHSV = 10,000 h
.

4
48
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
Fig. 9. The conversion curves of DCM decomposition over Ru/7%Ce-Al2O3 and
Ru/Al2O3 catalysts under various oxygen concentrations at GHSV = 10,000 h ; (a)
750 ppm DCM; (b) 1500 ppm DCM.
Fig. 8. The mass spectroscopy of reactants and products during TPSR; (a) Ru/Al2O3,
−
1
(
b) Ru/7%Ce-Al2O3; gas composition: 2000 ppm DCM, 20% O2 and N2 balance;
−
1
GHSV = 10,000 h
.
7
0
50 or 1500 ppm DCM, the rate increases within 0.2–20% [O] by
.6 ␮mol/min gcat. The rate at 3% [O] or higher is much lower than
Ce into Al O decreases the strength of Cl adsorption on active sites
so that the escape of HCl species can occur at lower temperature.
2
3
those obtained over Ru/7%Ce-Al O3 catalyst. These phenomena
2
indicate that the activity of Ru/Al O catalyst is not quite sensitive
toward the change of active oxygen species on RuOx. In other words,
2
3
3.4. Kinetics consideration
In order to understand the kinetics of DCM decomposition, the
effect of oxygen concentration ([O]) on conversion was investi-
gated on Ru/7%Ce-Al O3 and Ru/Al O3 catalysts under 750 and
2
2
1
500 ppm DCM, and the results are shown in Fig. 9(a) and (b). For
Ru/7%Ce-Al O catalyst, significant shifts of conversion curves to
2
3
low temperature were observed as [O] increases. This shift occurs
to a much smaller extent for Ru/Al O3 catalyst. Moreover, at a
2
given temperature, with the increase in [O] the conversion under
at 1500 ppm DCM increases more than that at 750 ppm DCM. Here,
◦
we calculate the reaction rates at 250 C (Fig. 10) and obtain a
considerable linear relation of rate with [O] within 0.2–3% [O] at
7
1
50 ppm DCM. For Ru/7%Ce-Al O catalyst, the rate increases from
.4 ␮mol/min gcat at 0.2% [O] up to 4.77 ␮mol/min gcat at 3% [O],
2 3
indicating that the process involving oxygen may be slow step.
Moreover, raising DCM concentration up to 1500 ppm, a more quick
increase in rate within 3–20% [O] than that at 750 ppm DCM can be
observed. This larger dependence of rate on [O] can be explained
by the fact that Ru/7%Ce-Al O3 catalyst possesses high oxygen
2
mobility and the active oxygen effectively increases with gas [O]
so as to promote the removal of Cl species from the surface of
Ru/7%Ce-Al O catalyst [11,12]. In the case of Ru/Al O3 catalyst,
◦
Fig. 10. The effect of oxygen concentration on DCM decomposition at 250 C over
2
3
2
−1
Ru/7%Ce-Al2O3 and Ru/Al2O3 catalysts at GHSV = 10,000 h ; the data within bracket
the dependence of rate or conversion on [O] is not strong. At either
are the conversion.

L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
449
oxygen mobility is low for Ru/Al O catalyst without Ce species. As
2
3
known from the stability and TPSR tests, the evolvement of HCl
◦
from Ru/Al O at 250 C is difficult, which can be related to low
2
3
oxygen mobility of Ru/Al O catalyst.
2
3
To obtain more insight into the promotion function by CeO2,
the physico-chemical properties of different catalysts were char-
acterized. The results show that after being used for 1000 min at
◦
2
70 C for the combustion of DCM (750 ppm DCM, 20% O2 and N2
balance), the specific surface area of all catalysts under study rarely
change, no phase transformations are observed in their XRD pat-
terns and the obvious deposit of coke on the catalyst surface was
not detected by TG techniques. The change of XPS spectra of species
(
Ce 3d, Ru 3d, Al 2p and O 1s) on the used catalyst surface was not
observed. However, Cl 2p peak appearing at around 198 eV on XPS
spectra and EDS results (not shown) confirm the presence of chlo-
rine species on all of the used catalysts. The amount of deposited
Cl decreases from 0.58 at.% for Ru/Al O3 catalyst to 0.35 at.% for
2
Ru/7%Ce-Al O catalyst, estimated by XPS. This result is consistent
2
3
with the order of Cl uptake over various catalysts in Table 2. Consid-
ering no changes of binding energy for various species of the used
catalysts (see Support Information), therefore, the deactivation of
Fig. 11. The effect of water on the activity of Ru/Al2O3 and Ru/7%Ce-Al2O3 catalysts;
insert: the stability at 250 ◦C in wet feed; wet: 3% H O, dry: absence of water; gas
2
−
1
composition: 750 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h
.
catalysts is related to the strong adsorption of HCl or Cl produced
2
from the decomposition of DCM.
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.
species from active sites is critical to promote the stability of Ru
catalysts.
2
013.11.006.
4. Conclusions
From the above results, it can be concluded that oxygen species
can efficiently react with the Cl species that adsorbed with moder-
ate strength on the surface. For Ru/Al O3 catalyst or Ru/Ce-Al O3
catalysts with low amount of Ce, surface oxygen migrates to a
smaller extent and the adsorption of Cl species is strong. So, these
catalysts need high temperature to remove the adsorbed Cl species,
and present a low stable activity. There are more active oxy-
Ru oxide catalysts supported on Ce-Al O and Al O were pre-
2
3
2
3
2
2
pared by wet impregnation with RuCl aqueous solution and tested
3
in catalytic combustion of DCM. The characterization of XRD, N2
adsorption, TEM/HRTEM, H -TPR, Raman and NH -TPD shows the
2
3
existence of interaction between Ce species and Al O . As a result,
2
3
the acidity increases to some extent. The reducibility of Ru/Ce-
Al O is greatly increased by the addition of Ce. During the catalytic
gen species for Ru/7%Ce-Al O3 catalyst to remove the Cl species
2
2
3
adsorbed on the surface of catalysts during the reaction, so that
more active sites can be accessible. Therefore, the stability of
decomposition of DCM, Ru/Ce-Al O catalysts present an outstand-
2
3
ing stable activity. The measurement of chlorine uptake shows the
Cl deposition is responsible for the deactivation of Ru/Al2O3 cata-
Ru/7%Ce-Al O catalyst in the catalytic combustion of DCM is pro-
2
3
◦
moted at 250 C, due to a larger decrease in chlorine species deposit.
lyst. High stable activity of Ru/Ce-Al O catalysts (1000 h, higher
2
3
◦
than 96% conversion at 270 C for Ru/7%Ce-Al O catalyst) can be
2
3
3.5. The effect of water on DCM decomposition
ascribed to the synergistic effect of high acidity, high oxygen mobil-
ity of CeOx and the stability of RuOx exposed to Cl species. Therefore,
Because of water detected in the exhaust gas, it is one part
Ru/Ce-Al O catalysts will become useful in the practice of removal
2
3
of the work to investigate the effect of water on the activity of
Ru/7%Ce-Al O and Ru/Al O3 catalysts for DCM decomposition
of alkyl chlorides.
2
3
2
(Fig. 11). In the presence of 3% water, the low conversion section
on Ru/7%Ce-Al O catalyst shifts significantly to high temperature.
Acknowledgements
2
3
◦
The temperature needed for 15% conversion increases from 162 C
◦
◦
to 202 C. At higher than 250 C, the conversion curve becomes
approaching to that without water. At low temperature, water is
We would like to acknowledge the financial support from
National Basic Research Program of China (No. 2010CB732300,
2011AA03A406) and National Natural Science Foundation of China
(No. 21277047) and Commission of Science and Technology of
Shanghai Municipality (11JC1402900).
+
−
dissociated probably on CeO species into H and OH species so as
2
to retard the transfer of active oxygen. This inhibition was observed
during the combustion of trichloroethylene over cerium oxide in
wet air [35]. For Ru/Al O catalyst, where the inhibition of 3% water
2
3
on DCM decomposition is observed at low temperature only to a
smaller extent. With raising temperature, the adsorption of water
becomes difficult, and the effect of water becomes weak. Product
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