Clay Minerals (1997) 32, 665--672
Vanadium-doped titania-pillared
montmorillonite clay as a catalyst for
selective catalytic reduction of NO by
ammonia
K. BAHRANOWSKI*,
J. J A N A S , T. M A C H E J ,
AND L . A . V A R T I K I A N
E. M. S E R W I C K A
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek, 30-239 Krakdw, Poland,
and *FaculO~of Geology, Geophysics and Environmental Protection, Academy of Mining and Metallurgy,
al. Mickiewicza 30, 30-059 Krakbw, Poland
(Received 29 April 1996; revised 2 December 1996)
ABSTRACT: A series of V-doped titania-pillared clay catalysts, characterized by ICP-AES
chemical analysis, X-ray diffraction, BET surface area measurement, and ESR spectroscopy, have
been tested in the selective catalytic reduction of NO by NH3. An ESR analysis shows that V dopant
is anchored to the titania pillars. Vanadyl species with differing degrees of in-plane V - O re-covalent
bonding are produced depending on the method of sample preparation. Polymeric V species appear
as the V content is increased. Catalytic performance of these systems depends on the method of
preparation and on the V content. The best catalyst, converting 90-100% NO in the temperature
range 523-623 K, is obtained by exchange of pillared montmorillonite with vanadyl ions, at an
extent of exchange below the level where significant amounts of polymeric V species appear. The
co-pillared catalyst, containing vanadyl centres characterized by a higher degree of in-plane ncovalent bonding (according to ESR), is less selective than the exchanged samples.
Selective catalytic reduction (SCR) by ammonia is
the most common method used for the removal of
nitrogen oxides from industrial flue gases. The
process is currently performed mainly with aid of
titania-supported vanadia catalysts (Bosch &
Jansen, 1988). The catalytic performance of the
V2Oz/TiO2 system is influenced by the characteristics of the support, degree of surface coverage
with V species and structure of the V centres
involved in the catalytic step (Baiker et al., 1989;
Went et al., 1992; Duffy et aL, 1994; Lietti &
Forzatti, 1994; Topsoe et al., 1995). Best results are
obtained for catalysts with V loadings in the range
of the theoretical monolayer, deposited on a high
surface area anatase modification of titania. Topsoe
et al. (1995) have shown that the catalytic activity
is related to the presence of Bronsted acid sites
associated with the VS+-OH surface centres and that
surface V=O groups also play an important role in
the catalytic cycle. The nature and concentration of
these species depend on the V dispersion at the
support surface. Thus, preparation of the catalyst
should be directed at providing conditions for
optimization of both the Bronsted acidity and the
vanadyl concentration. In a series of recent papers
we have suggested the use of titania-pillared clay as
a support for the catalytically active V species
(Bahranowski & Serwicka, 1993; Bahranowsld et
al., 1995a,b; Bahranowski et al., 1996). Pillaring of
clays, first described in the seventies (Brindley &
Sempels, 1977; Lahav et al., 1978), is a general
preparative procedure, leading to highly porous
materials. Titania-pillared montmorillonite clay is
synthesized from the natural layered mineral by
replacement of the interlayer cations by oligomeric
Ti cationic species (e.g. Sterte, 1986). The resulting
9 1997 The Mineralogical Society
K. Bahranowski et al.
666
material, after calcination, contains titania pillars
which prop open the clay layers and thus expose the
interlayer space to gases and vapours. The
nanometer size of the individual pillars offers a
surface-to-bulk ratio unattainable with conventional
titania supports. This provides a unique opportunity
for the preparation of catalysts containing significant quantities of highly dispersed V. Additionally,
pillared clays possess substantial Bransted acidity,
which may be beneficial for the deNOx process.
For this reason it was decided to test the usefulness
of V-doped pillared montmorillonite clays for the
selective catalytic reduction of NO and compare
their performance with a classical monolayer
vanadia-titania catalyst. It is worthwhile to note
that first attempts to use the pillared type catalysts
in the deNOx process have been reported recently
by Yang et al. (1995) who claimed superior
performance of Fe 3+- and Cu2+-exchanged pillared
clays over a commercial type catalyst.
EXPERIMENTAL
Materials and syntheses
Starting material. The montmorillonite used was
the <2 lam particle-size fraction of bentonite from
Milowice, Poland. The cation exchange capacity
(CEC) of the clay is 84 mEq/100 g. The clay was
exchanged with Na § ions by stirring in 1 M NaC1
solution for 24 h followed by repeated washing
with 1 u NaCI. The resulting suspension was
washed several times with distilled water until
free of CI- ions as indicated by lack of reaction of
the supernatant with silver nitrate solution. The
solid separated by centrifugation was dried in air at
353 K. This material is referred to as Na-mt.
Titania-pillared montmorillonite. Titania-pillared
clay was prepared according to the procedure
described by Sterte (1986). The pillaring agent
was obtained by adding TIC14 (Fluka) to 6 u HC1.
This mixture was then diluted to reach a final Ti
concentration of 0.82 ra. The amount of HC1
solution used corresponded to a final concentration
of 0.11 M. The solution was allowed to age for 3 h
prior to use. The pillaring agent was added
dropwise to a vigorously stirred water suspension
containing 4 g1-1 of Na-mt clay, in an amount
corresponding to 10 mmol Ti per g of montmorillonite. The resulting product was stirred for
3 h at room temperature, washed with distilled
water until the supernatant solution was free of C1-
and dried in air at 353 K. This material is referred
to as Ti-mt and after calcination at 673 K for 3 h as
Ti-PILC.
V-titania-pillared montmorillonites. Three
different procedures were used to introduce V into
titania-pillared clays. Two of them used cation
exchange of Ti-PILC with VO 2+ ions, while one,
referred to as 'co-pillaring', employed a VO 2§
containing pillaring solution. The exchanged
samples are reported as Vx-(Ti-PILC), the copillared as (Vx-Ti)-PILC, where x is the weight
percentage o f V2Os. (a) Ti-PILC was treated with a
0.001 or 0.1 M VOSO4 solution, centrifuged and
washed with distilled water until the supernatant
solution was free of sulphate ions, dried in air at
353 K and calcined 1 h at 673 K. Samples V0.8(Ti-PILC), and V3.4-(Ti-PILC) were obtained this
way. (b) In an attempt to increase the V content, the
V3.4-(Ti-PILC) sample was subjected to additional
exchange with 0.1 M VOSO4 followed by washing,
drying and calcination at 673 K. The procedure
resulted in sample V5.3-(Ti-PILC). (c) A pillaring
agent containing Ti and vanadyl ions at a ratio of
10:1 was obtained by adding TiCI4 to 6 M HC1
containing dissolved VOSO4. After dilution with
water the final concentrations were 0.82 M Ti,
0.082 M VO 2+ and 0.1 M HCI. Further treatment
followed the procedure of Ti-PILC preparation.
Sample (V0.1-Ti)-PILC was obtained this way.
Reference sample. A catalyst containing one
m o n o l a y e r o f vanadia deposited on anatase
(Tioxide, 50 m2g-l), referred to as lmV/TiO2, has
been used as a reference in the catalytic tests.
Physicochemical characterization
X-ray diffraction (XRD) analyses were performed
on oriented samples prepared on a glass slide using
a DRON-3.0 diffractometer with Ni-filtered Cu-Kat
radiation.
The BET surface area of the samples was
determined from Ar adsorption at 77 K, after
outgassing at 473 K for 2 h.
Chemical analysis was carried out on an PerkinElmer ICP-AES Plasma 40 spectrometer.
The E S R spectra were recorded at room
temperature and at 77 K with an X-band SE/X
(Technical University, Wrociaw) spectrometer.
Diphenylpicrylhydrazyl (DPPH) and NMR field
markers were used for determination o f the
resonance frequency. The ESR parameters of the
spectra were determined by best fit with the
V/Ti-pillared montmorillonite as deNOx catalyst
667
TABLE 1. Basal spacings, dool, specific surface area, S, V and Ti content of the clay samples
investigated.
Sample
dool [A]
Ti-PILC
V0.8-(Ti-PILC)
V3.4-(Ti-PILC)
V5.3-(Ti-PILC)
(V0.1-Ti)-PILC
~26.0
-26.0
~25.0
N25.0
-24.0
SBET [m2g-1]
345.6
334.2
339.7
309.5
285.9
computer simulated spectra. The ESR programs
SIMAX and COMBI, kindly made available by Jos6
C. Conesa from Instituto de Catalisis y
Petroleoquimica, CSIC, Madrid, Spain, were used
for the simulation procedure.
Catalysis
S e l e c t i v e c a t a l y t i c r e d u c t i o n o f N O was
conducted in a pyrex glass flow microreactor at
atmospheric pressure. A mixture of 1000 ppm NO,
1000 p p m NH3, 2% Oz diluted with He was fed to
0.3 cm3 of catalyst (particle size 0.2-0.5 mm,
weight c. 0.3g) at a rate of 50 cm3 min-1,
corresponding to a space velocity equal to
10000 h -1. Effluent gases were analysed after
reaching steady state by means of gas chromatography, using a HayeSep R column for N20, a
molecular sieve 4A for N2 and 02, and TCD
detection. A KCl-alumina capillary column and a
PID detection were used for analysis of NO.
RESULTS
AND DISCUSSION
TiO2 [wt%]
0.00
0.85
3.45
5.31
0.13
44.16
43.45
41.36
40.18
38.77
clay layer, a narrow line at g = 2 is due to structural
defects, and a broad line with AHp_p ~- 1000 G,
centred on g ~ 2, is associated with some iron
oxide/oxyhydroxide impurity. The ESR spectra of
V-containing catalysts (Fig. 2) show signals attributed to V(IV), with characteristic eight-line
hyperfine patterns due to the interaction of the
unpaired electron with the 51V nucleus (I = 7/2).
The good resolution of the hyperfine structure
indicates the presence of isolated vanadyl centres.
Simulation of the spectra obtained from samples
with the highest V loading required a contribution
from the structureless signal characteristic of
reduced V ions situated in close proximity to each
other. This indicates that, on increasing the V
content, polymeric V species were formed alongside the isolated vanadyl centres.
Experimentally observed values of gll<g• and
All>A• indicate a tetragonal distortion of the
paramagnetic complex and point to the existence
of a vanadyl bond in the V species responsible for
the ESR signals. The spectra were analysed using
an axial spin hamiltonian, assuming C4v square-
Physicochemical characterization o f the
catalysts
Table 1 contains the basic data on the
physicochemical properties of the investigated
samp.les. Assuming a smectite layer thickness of
9.6 A, the observed basal spacing of c. 25-26 .~
gives the interlayer distance (i.e. the titania pillar
height) of c. 15-16 A. The surface area of all
pillared samples is high and confirms that after
pillaring the interlamellar spaces became accessible.
The natural montmorillonite used contains ~2
wt% Fe and shows a strong ESR spectrum (Fig. 1).
The line at g ~ 4.3 is due to high spin (S = 5/2)
ferric ions in the distorted octahedral sites of the
V205 [wt%]
=L,.3
2~a
_f
g=2.0
I
FIG. 1. ESR spectrum of Na-mt, recorded at 77 K.
668
K. Bahranowski et al.
g,,, 1.935
i
i
,
i
I
I
I
Ai~=169.00-10"t+cr61
I
I
1! V-(T
-PILCl
I
I
I
I
l
i
i
.
I
g~=1.980
g,. =1.980
I
I
i
I
i
i
i
I {V-Ti)- PILE
I
I
I
II
i
i
I
gu=1.938
AiI=161-09" 10-4cm -1
F~o. 2. ESR spectra of V-(Ti-PILC) and (V-Ti)-PILC catalysts, recorded at 77 K. Vertical lines are drawn to
facilitate comparison of parallel hyperfine features.
pyramidal symmetry of the ligand field surrounding
the V(IV). The ground state for such complexes is a
molecular orbital of b2 symmetry (Ballhausen &
Gray, 1962; Kivelson & Lee, 1964; De Armond et
al., 1965) and the corresponding wave function is
IB2> = ~21dxy> - 132']~pb2>
(1)
where I~pb2>are ligand orbitals of b2 symmetry, and
132 and 132' are the bonding coefficients. The squared
MO coefficient 132 which provides information
about the degree of localization of the unpaired
electron on the vanadium dxy orbital, can be
computed in terms of the empirical data according
to the following approximate relationship (Kivelsun
& Lee, 1964; De Armond et al., 1965; McGarvey,
1967):
~ = 7/6 A g l l - 5/12 Ag• - 7/6 (All - A•
(2)
Y/Ti-pillared montmorillonite as deNOx catalyst
669
TABLE 2. Parameters of the ESR spectra of V-doped titania-pillared clays.
Sample
V-(Ti-PILC)
(V-Ti)-PILC
gll
g•
1.935
1.938
1.980
1.980
........
9 ..;:
..
"'
* " "
"
.
".'
-
"
''
'
t''"
.
'
"
'
""
.
'
'
'
.
.
9
.
.
'
'
.'
9
.'
"
"
.
" "i ",.-..,'....[kAY.
.
"
9
'.:
'
.
LAYER
Ti 02.
,,.
...--:
" ''''I'''''"
9
"
-
9
9
9
60.50
58.00
LAY'""L"AYER"
;
9
169.00
161.09
. -~.....:.'-..-...
.",''
'--'"
IA•
104 cm -1
9
.
,
'
.
.
'
9
."
.
9
.
' ' 2
LAYER
.
"."
.
, 9
.
9
.
9
.
.
" . .
' . .
.
Ti 02
....
.
"
Ti 02
.....LLAY--
1~2
0.80
0.76
centres present in the co-pillared sample indicates
that in this catalyst the unpaired electron is less
confmed to the V nucleus and the contribution from
the in-plane V - O n-covalent bonding is higher.
Knowing that the titania pillar height is c.
16 A, and that V is attached to the pillars, a
schematic model of the V-doped titania-pillared
m o n t m o r i l l o n i t e c a t a l y s t (Fig. 3), m a y be
proposed. The picture is, o f course, very
simplified since no information is available on
the distribution of pillars and their dimensions in
the (001) plane. It visualizes, however, the
structural analogies and differences between the
pillared sample and the classical vanadia/titania
catalyst.
where Agll = gll - ge, Ag• = g• - ge, g~ being the
free electron g value, and P is a term which takes
into account the dipole-dipole interaction of the
electron moment with the nuclear moment. In the
present work, the value of P = 146 x 10-4cm -1,
calculated by Morton & Preston (1978), has been
used. Negative values o f the All and A•
components have been taken to obtain positive
values for 132 from eqn. (2). All ESR data are
summarized in Table 2. Detailed discussion of the
spectra presented elsewhere (Bahranowski &
Serwicka, 1993; Bahranowski et al., 1996) allows
us to conclude that the ESR signals are characteristic of vanadyl species attached to the titania
pillars. Observed lowering of the 1322value for the V
9.6"A
IAII
104 era-
.
...";
FIG. 3. Schematic model of the catalyst structure.
"
-
,
K. Bahranowski et al.
670
Catalysis
1986). Addition of a small amount of V (Fig. 4b)
shifts the maximum activity to a lower temperature
but the maximum conversion remains at c. 60%.
Increase in the V content leads to a substantial
increase of the activity for SCR of NO and to the
widening of the temperature window corresponding
to high NO conversion (Fig. 4c). The maximum
conversion reaches 100% and in the range
523-623 K it is I>90%. At still higher V loading,
the maximum activity, still very high, is shifted
t o w a r d s l o w e r t e m p e r a t u r e s ( F i g . 4d).
Simultaneously, the upper limit of the temperature
window is also shifted towards lower temperature.
The maximum NO conversion obtained on the copillared sample is >60% and occurs at temperature
higher than that for samples doped with V by
Figure 4 a - e shows the results of the selective
catalytic reduction of NO over the series of pillared
catalysts investigated. For comparison, the results of
the deNOx test obtained on a classical vanadia/
titania monolayer catalyst are presented in Fig. 4f.
In addition to the NO conversion values, data on
the N20 evolution, which is indicative of the nonselective reaction pathway, are also presented.
Clearly an undoped titania-pillared sample shows
appreciable activity in the SCR process (Fig. 4a).
This is probably due to the iron oxide impurity
present in the parent montmorillonite. Supported
Fe203 catalysts are known to be among the most
active for the NO SCR reaction (Wong & Nobe,
100
100
80
-80
60
.60
40
940
20
'20
'0
0
V 5.3-(Ti-PILC )
Ti-PILC
o-e 100
l
100
i
BO z
Z 80.
CD
t./3 60
t'~
Lt_l
Z
CD
L.J
C9
Z
b,o
60
-40
o
rrl
r--
20
20 t~
0
IO0
0
(V 0.1 -Ti) -PI Lf_
VO.8- ( l i - P I LC )
100
i
80-
~
'
'
o~
f
C
80
60-
60
40-
40
20"
20
0
0-)
V3.4-/Ti-PIk C.)
473
573
,
Im Vl,Ti0 2
673
473
TEMPERATURE
,
573
,
,
673
[K]
FIc. 4. NO conversion and N20 yield in SCR of NO with NH3: (a)-(e) on pillared clay catalysts, (f) on a
conventional one monolayer vanadia-titania catalyst. Conditions as described in experimental section.
V/Ti-pillared montmorillonite as deNOx catalyst
exchange procedures. The maximum conversion on
the reference lmV/TiO2 catalyst is 85% which is
lower than that observed for the V3.4-(Ti-PILC)
catalyst.
The profiles of N20 evolution show that for
different catalysts, the onset of the non-selective
reaction occurs at different temperatures and the
position of its maximum vs. the maximum of NO
conversion depends on the catalyst. Thus, in the
pure Ti-PILC sample (Fig. 4a), the maximum N20
evolution occurs before the maximum of NO
conversion is attained, while in V-doped samples
prepared by the exchange technique (Fig. 4b-d),
the maximum NO conversion occurs before the
N20 concentration reaches its peak. In the V3.4(Ti-PILC) and V5.3-(Ti-PILC) catalysts, virtually
no nitrous oxide evolves in the high NO conversion
range.
On the other hand, in the case of the co-pillared
catalyst (V0.1-Ti)-PILC, the maximum N20 yield
coincides with the maximum NO conversion.
The observed phenomena may be explained in
terms of different catalytic properties of V centres
present at the catalyst surface. Use of ESR has
shown that depending on the preparative procedure
and the V content, different vanadyl species exist at
the surface of the catalysts studied. Thus, for
catalysts prepared by co-pillaring the in-plane n
V - O bonding is more covalent than in the catalysts
obtained by the exchange procedure. Moreover, in
the latter, the increase in V content leads to the
appearance of interacting V centres, which points to
the formation of polymeric vanadate species. It has
been reported (Went et al., 1992) that polyvanadate
species are more active but less selective than
dispersed V monomers. The behaviour of the V-rich
V5.3-(Ti-PILC) sample is consistent with the above
finding and the observed shift of maximum activity
towards the lower temperature may be attributed to
the higher activity of the polymeric V species
present in this catalyst. Simultaneously, because of
the lower selectivity of polyvanadate species, the
upper limit of the temperature window also shifted
downwards. Vanadium species generated during copillaring are less selective than those present in the
exchanged catalyst. The fact that the maximum
N20 evolution parallels the maximum NO conversion supports this and is a highly undesirable
phenomenon from a practical point of view. This
is mainly due to the shift in the maximum of NO
conversion to higher temperatures. It is possible that
the poorer performance of the co-pillared catalysts
671
is associated with the more covalent nature of the
V - O in-plane ~-bonding characteristic of the V
species present in these samples. A change in the
degree of covalency of the bonds between V and O
may in turn affect the strength of surface V - O H
Br~nsted acid centres which, as demonstrated by
Tops~e et al. (1995), play an important role in the
deNOx catalytic cycle.
CONCLUSIONS
It has been shown that V-doped pillared clays
represent an interesting possibility as potential
catalysts for SCR of NO with ammonia. Catalytic
performance of these systems depends on the
method of preparation and on the V content. The
best catalyst, converting 90-100% NO in the
temperature range 523-623 K, is obtained by
exchange of pillared montmorillonite with vanadyl
ions, at an extent of exchange below the level
where a significant amount of polymeric V species
appear. The ESR analysis shows that co-pillaring
generates vanadyl centres characterized by a higher
degree o f in-plane ~-covalent bonding. The
resulting catalyst is less effective than the
exchanged samples.
ACKNOWLEDGMENTS
The authors wish to thank Professor B. GrzybowskaSwierkosz for providing the reference vanadia/titania
sample. The work was partially financed by the State
Committee for Scientific Research, KBN, Warsaw,
within the research project 6 P201 034 07.
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