Novel non-hydrolytic synthesis of a V2O5-TiO 2 xerogel for the selective catalytic reduction of NOx by ammonia

Article abstract of DOI:10.1039/b408073d

A vanadia-titania mesoporous xerogel (10.5 wt% V₂O₅) was prepared from chloride precursors using a one-step non-hydrolytic sol-gel route and subsequent drying at ambient pressure; after calcination at 773 K for 5 h no crystalline V₂O₅ was detected and the resulting mixed oxide exhibited remarkable activity in the selective reduction of NO with NH₃.

Full text of DOI:10.1039/b408073d

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Novel non-hydrolytic synthesis of a V O –TiO xerogel for the selective
2
5
2
catalytic reduction of NO by ammonia{
x
a
a
a
b
Aurelian F. Popa, P. Hubert Mutin, Andr e´ Vioux,* Gerard Delahay and Bernard Coq
b
a
Chimie Mol e´ culaire et Organisation du Solide-UMR CNRS 5637, Universit e´ Montpellier 2, case 007,
Place E. Bataillon, F-34095 Montpellier Cedex 5, France. E-mail: vioux@univ-montp2.fr;
Fax: 1(0)467143852; Tel: 1(0)467143970
b
Laboratoire de Mat e´ riaux Catalytiques et Catalyse en Chimie Organique-UMR CNRS 5618, ENSCM,
rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France
8
Received (in Cambridge, UK) 28th May 2004, Accepted 16th July 2004
First published as an Advance Article on the web 20th August 2004
A vanadia–titania mesoporous xerogel (10.5 wt% V
2
O
5
) was
before measurement, the sample was degassed at 573 K to about
2 Pa. The pore size distribution was calculated from the desorption
branch of the isotherm, using the BJH method. The type IV
isotherm (Fig. 1) indicated that the material was mesoporous, with
prepared from chloride precursors using a one-step non-
hydrolytic sol–gel route and subsequent drying at ambient
pressure; after calcination at 773 K for 5 h no crystalline
V O was detected and the resulting mixed oxide exhibited
3
21
2
5
a total pore volume of 0.24 cm g and a BET surface area of
2
21
remarkable activity in the selective reduction of NO with
NH3.
78 m g ; the average pore diameter (4 V/SBET) was 12 nm.
According to t-plot analysis, microporosity was negligible. This
texture is attractive compared to that of conventional xerogel
samples of similar composition, which exhibited either much lower
Vanadia-based catalysts are known for their high efficiency in the
selective catalytic reduction (SCR) of NO by NH which is a widely
6
7
3
surface areas or significant microporosity. Catalysts derived from
two-step aerogels calcined at 573 K showed higher surface areas
used process for de-NO -ing waste gases from stationary sources
x
(eqn. (1)).
2 21
ca. 180 m g ) but they still contained a considerable amount of
(
organic species; furthermore, these solids appeared less thermally
stable than mesoporous titania or vanadia–titania prepared by
3
4
NO 1 4NH
3
1 O
2
~ 4N
2
2
1 6H O
(1)
8
other methods: upon calcination at 723 K their surface area
The best support is TiO
and resistance against poisoning by SO
preparation of V O –TiO catalysts has been intensively studied
2
-anatase, which ensures high activity
2 21
2 5
dropped to ca. 42 m g and crystallisation of rutile and V O was
. As a consequence, the
3
2
observed.
The X-ray diffraction pattern (performed on a Seifert h–h
powder diffractometer, 2h range 19–90u, using Cu K radiation) of
our sample revealed the presence of TiO -anatase only. Despite the
high calcination temperature neither TiO -rutile nor crystalline
could be detected, thus indicating a high thermal stability of
2
5
2
1
during the last two decades. Elaborated procedures have been
used to increase the activity of vanadia–titania catalysts, such as
multiple grafting, sol–gel synthesis using chemically modified
precursors and 2-step procedures to improve the dispersion of
vanadia species, or supercritical drying to increase the specific
a
2
2
2 5
V O
2,3
the anatase phase and a good dispersion of vanadia species.
The static solid state V NMR spectrum of our vanadia–titania
surface area of the catalyst.
The non-hydrolytic sol–gel process offers a simple and efficient
51
4
5
way to prepare mixed oxides. This process is based on the
sample (acquired on a Bruker AM400 spectrometer) displayed two
broad resonances centred at 2225 ppm and 2570 ppm, indicating
the presence of octahedral and tetrahedral vanadium species.
condensation between chloride and alkoxide groups at moderate
temperature (around 100 uC). The alkoxide groups can also be
formed in situ, for instance by reaction of chlorides with an organic
ether, thus avoiding the use of expensive alkoxide precursors.
In this work we present the first non-hydrolytic sol–gel synthesis
of a vanadia–titania xerogel (atomic ratio Ti/V # 10), hereafter
9
More precise information on the nature of vanadium species was
obtained by micro FT-Raman spectroscopy (Labram 1B Confocal
Jobin-Yvon instrument, excitation by He–Ne laser at 632.8 nm)
(
Fig. 2).
21
Beside the peaks at 202, 390, 506, and 628 cm assigned to
anatase, three peaks were detected: two broad ones centred at about
8
peak at 1028 cm is attributed to VO stretching in monomeric
vanadyl species directly bound to the surface by three V–O–Ti
named 10 Ti/V and its performance in the SCR of NO by NH
The overall synthesis process can be represented by eqn. (2).
3
.
2
1
21
10 and 935 cm and a narrower, stronger one at 1028 cm . The
2
1
i
i
3 4 2 2 5 2
VOCl 110TiCl 121.5 Pr O ~ (V O )0.5(TiO )10 143 PrCl (2)
The preparation was carried out under argon in oven-dried
glassware. First, 0.310 g (1.77 mmol) of VOCl (Aldrich, 99%) was
introduced by syringe in a glass tube; after addition of 3.233 g
17.0 mmol) of TiCl (Acros Organics, 99.9%) and 3.926 g
3
(
4
i
(38.04 mmol) of Pr O (Acros Organics, 99%), the tube was frozen
in liquid nitrogen, sealed under vacuum and heated under
2
autogeneous pressure at 383 K for 3 days. The tube was opened
under argon and the dark-brown gel was first dried under vacuum
at room temperature and then at 393 K for 12 h. Finally, the
2
1
sample was calcined at 773 K (heating rate 10 K min , air flow
5
3 21
0 cm min ) for 5 h, turning into a beige powder.
The texture of the calcined mixed oxide was analysed by nitrogen
physisorption at 77 K on a Micromeritics ASAP 2000 instrument;
{
Electronic supplementary information (ESI) available: Temperature
Programmed Desorption of NH and isothermal catalytic tests. See http://
3
Fig. 1 Nitrogen isotherm (6-adsorption, ^-desorption) and pore size
distribution (insert) for 10 Ti/V, after 5 h calcination at 773 K.
www.rsc.org/suppdata/cc/b4/b408073d/
2
2 1 4
C h e m . C o m m u n . , 2 0 0 4 , 2 2 1 4 – 2 2 1 5
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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6
Pearson et al. for mixed sol–gel samples. It is even higher than the
TOF values reported for a catalyst prepared by multiple grafting of
vanadyl trichloride on titania aerogel or for a 2-step aerogel with
2
3
30 wt% V O .
2 5
2
However, at higher temperature, significant formation of N O
occured. This lack of selectivity, and at the same time the high
activity, could be explained by a high amount of surface poly-
vanadate species which are believed to be up to 10 times more
active than monovanadyl species but at the same time less
11
selective.
Constant temperature catalytic tests were performed at 573 ¡
K. Under steady state conditions, the conversion was only
2
marginally lower (93 ¡ 1%) than under dynamic conditions. No
deactivation was observed during 100 h,{ indicating promising
Fig. 2 RAMAN spectrum of calcined 10 Ti/V.
stability of our catalyst. In addition, the effect of SO and water on
the activity of our catalyst was investigated. Despite the high
vanadia content of the catalyst, these tests{ indicated no influence
of SO (25 vppm for 3.5 h) and only a small (ca. 4%) and reversible
2
decrease of the activity in the presence of water (1.1 vol% for 3.5 h).
Finally, in order to facilitate comparison, we have performed a
2
2
1
bonds; the broad peaks at 935 and 810 cm are ascribed to VO
stretching and V–O–V stretching in polymeric metavanadates,
respectively. The absence of a sharp and strong line at about
10
21
995 cm proved once more that the presence of crystalline vanadia
can be ruled out.
Accordingly, the tetrahedral species detected by NMR are most
likely monomeric vanadyls while the octahedral species would
correspond to vanadium atoms either in polymeric metavanadate
species or to vanadium atoms participating in the mixed oxide
network.
test under the same experimental conditions on the V
TiO EUROCAT catalyst, which was chosen as a reference catalyst
in a joint European project on de-NO
complex system than 10 Ti/V, containing about 3 wt% V
2 5 3
O –WO /
2
1
2
x
.
EUROCAT is a more
, 9 wt%
2 5
O
1
3
WO , 6.5 wt% SiO and 78 wt% TiO .
3 2 2
Temperature programmed desorption (TPD) of NH (Micro-
3
The EUROCAT catalyst, which contains 3.5 times less vanadia,
appeared significantly less active but more selective than 10 Ti/V
meritics AutoChem 2910) showed that the maximum amount of
2
1
(
Fig. 3). However, at equal conversion, selectivity of EUROCAT
was only marginally better; for instance, at 75% conversion we
found 20 ppm N O for EUROCAT and 25 ppm for our 10 Ti/V
catalyst.
adsorbed NH { was about 1.3 mmol g of catalyst.
3
The SCR catalytic activity was evaluated in a continuous flow
quartz glass reactor; an aliquot of 20 mg (about 0.04 cm ) of
3
2
catalyst was used. The feed mixture consisted of 0.2% NO, 0.2%
NH and 3% O (in volume), with He as balance gas; the total gas
flow was 138 cm min , thus resulting in a very high gas hourly
The non-hydrolytic sol–gel method presented here is a simple
and attractive method to prepare vanadia–titania catalyst starting
from low-cost (albeit moisture sensitive) chloride precursors and
using ambient pressure drying. After calcination at 773 K, the
catalyst presented excellent textural properties as well as high
dispersion of the active vanadia phase. As a consequence, this
material was very efficient in the SCR of NO by ammonia at
moderate temperature.
3
2
3
21
2
1
space velocity (GHSV) of 207 000 h . The temperature was raised
2
1
up to ca. 650 K at a constant rate of 6 K min ; the effluent
concentration in NO, NH , O , N , H O and N O was con-
3
2
2
2
2
tinuously monitored by on-line sampling to a quadrupole mass
spectrometer (Balzers QMS 421) equipped with Faraday and
channeltron detectors. Prior to the tests, in situ activation was done
We gratefully acknowledge financial support of this work by the
Centre National de la Recherche Scientifique (France), markedly
for funding an associate researcher position for A.F.P.
by heating the aliquot up to 770 K for 30 min under O –He flow.
2
The catalytic data indicated that our V
was highly active: NO conversion reached 50% at 500 K, with
negligible N O formation (about 8 ppm). At 423 K (7% NO
2 5 2
O –TiO catalyst (Fig. 3)
2
conversion) the turnover frequency (TOF), expressed as mol NO
converted per mol V per hour was 2.2 (on the basis of the nominal
vanadium content). This value is about 8 times higher than the
TOF values reported by Schneider et al. for a sample with a
similar vanadium content prepared from a 2-step aerogel or by
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3
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8
9
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2 Special issue on EUROCAT deNOx SCR project, Catal. Today, 2000,
2
1
1
1
Fig. 3 NO conversion and N
the EUROCAT catalyst ($).
2
O production for: calcined 10 Ti/V (%) and
56(4).
13 J. C. V e´ drine, Catal. Today, 2000, 56, 333.
C h e m . C o m m u n . , 2 0 0 4 , 2 2 1 4 – 2 2 1 5
2 2 1 5