Applied Catalysis B: Environmental 30 (2001) 267–276
Catalytic NO reduction with ammonia at low temperatures on
V2 O5/AC catalysts: effect of metal oxides addition and SO2
Zhenping Zhu, Zhenyu Liu∗ , Shoujun Liu, Hongxian Niu
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
Received 27 June 2000; received in revised form 28 September 2000; accepted 28 September 2000
Abstract
The catalytic behavior of the V-M/AC (M = W, Mo, Zr, and Sn) catalysts were studied for the NO reduction with ammonia
at low temperatures, especially in the presence of SO2 . The presence of the metal oxides does not increase the V2 O5 /AC
activity but decreases it. Except V-Mo/AC, the other catalysts are promoted by SO2 at 250◦ C, especially for V-Sn/AC.
However, the promoting effect of SO2 is gradually depressed by catalyst deactivation. Changes in catalyst preparation method
can improve the catalyst stability in short-term but cannot completely prevent the catalyst from a long-term deactivation.
Mechanisms of the promoting effect and the deactivation of V-Sn/AC catalyst by SO2 were studied using Fourier transform
infrared spectroscopy (FT-IR) spectra and measurement of catalyst surface area and pore volume. The results showed that both
the SO2 promotion and deactivation are associated with the formation of sulfate species on the catalyst surface. In the initial
period of the selective catalytic reduction (SCR) reaction in the presence of SO2 , the formed sulfate species provide new acid
sites to enhance ammonia adsorption and thus the catalytic activity. However, as the SCR reaction proceeds, excess amount
of sulfate species and then ammonium-sulfate salts are formed which is stabilized by the presence of tin oxide, resulting in
gradual plugging of the pore structures and the catalyst deactivation. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Nitric oxide; Ammonia; V2 O5 /AC catalyst; Metal oxides; Sulfur dioxide
1. Introduction
Selective catalytic reduction (SCR) of NO with ammonia is a well-proven technique for the removal of
NOx from flue gases of stationary source [1]. A large
number of SCR catalysts have been studied in the literature, and V2 O5 /TiO2 or V2 O5 -WO3 /TiO2 catalyst
has been applied widely. However, most of the catalysts must be operated at above 350◦ C to avoid catalyst deactivation by SO2 [1]. It is needed to develop
low temperature SCR catalysts, which may result in
systems of low energy consumption and economic for
∗ Corresponding author. Fax: +86-351-404-1153.
E-mail address: [email protected] (Z. Liu).
retrofitting into existing boiler systems for flue gas
cleaning [2,3]. Many catalysts show high SCR activities in the absence of SO2 at low temperatures [2–7],
but they are prone to deactivation by SO2 due to the
formation of sulfate [7,8]. Therefore, the key of the
development of the low temperature SCR catalysts is
to increase catalyst resistance to the SO2 poisoning.
Recently, we reported [9–11] an activated carbon supported vanadium oxide (V2 O5 /AC) catalyst,
which is high active for the SCR reaction at low
temperatures (180–250◦ C) and shows a selectivity
to N2 of more than 95% [10]. More interestingly,
the V2 O5 /AC catalysts with low V2 O5 loadings are
not poisoned but significantly promoted by SO2 . The
promoting effect of SO2 on the V2 O5 /AC activity
0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 3 3 7 3 ( 0 0 ) 0 0 2 3 9 - 3
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Z. Zhu et al. / Applied Catalysis B: Environmental 30 (2001) 267–276
results from the formation of sulfate species on carbon surface, which provides new acid sites for NH3
adsorption [12]. Although, ammonium-sulfate salts,
such as NH4 HSO4 and (NH4 )2 S2 O7 , may be formed
on the catalyst surface at the low temperatures, the
involved ammonium ions can react easily with NO
under the reaction conditions, which avoids deposition of excess ammonium-sulfate salts on the catalyst
surface and thus the catalyst deactivation [12].
In this work, the catalytic behaviors of the V2 O5 /AC
catalysts with additional metal oxides are studied for
the SCR reaction at low temperatures, and especial
attention is focused on the effect of SO2 and its
mechanisms. The metal oxides used include tungsten
oxide (WO3 ), molybdenum oxide (MoO3 ), zirconium oxide (ZrO2 ), and tin oxide (SnO2 ). Among
them, WO3 and MoO3 were frequently studied as
additives in V2 O5 -WO3 /TiO2 or V2 O5 -MoO3 /TiO2
catalyst, which is more active, selective and stable
than V2 O5 /TiO2 alone [1,13–16].
2. Experimental
2.1. Catalysts
The support, activated carbon (AC), was prepared
from a commercial coal-derived semicoke through
steam activation at about 900◦ C. Before use, the AC
was oxidized with concentrated HNO3 at 60◦ C for
1 h, followed by filtration, washing with distilled water and drying at 120◦ C for 5 h. The BET surface
areas of the AC before and after the HNO3 oxidation
are 647 and 560 m2 /g, respectively.
The 5 wt.%V2 O5 /AC catalyst was prepared by
pore volume impregnation of the oxidized AC with
an aqueous solution of ammonium metavanadate
(NH4 VO3 ) in oxalic acid. After the impregnation, the
catalyst was dried at 50◦ C overnight and then at 120◦ C
for 5 h, followed by a calcination in Ar stream at
500◦ C for 2 h. The additions of metal oxides onto the
5 wt.%V2 O5 /AC catalyst were performed separately
by an impregnation method, using aqueous solution of corresponding precursors, (NH4 )6 H2 W12 O40 ,
(NH4 )6 Mo7 O24 , ZrOCl2 , and SnCl4 . After the impregnation, the catalysts were dried and then calcined
following the same procedure under the same conditions described above. In the resulted catalysts,
V-M/AC (M = W, Mo, Zr, and Sn), the loadings of
metal oxides are 3, 5, 5, and 5 wt.% for WO3 , MoO3 ,
ZrO2 , and SnO2 , respectively.
For comparison, 5 wt.%SnO2 /AC catalyst was prepared using the same method. In addition, V-Sn/AC
catalysts were also prepared by impregnation of the
5 wt.%SnO2 /AC with aqueous solution of NH4 VO3 in
oxalic acid and by co-impregnation of the AC with a
mixed solution of SnCl4 and NH4 VO3 in oxalic acid.
All the three V-Sn/AC catalysts have the same compositions. To facilitate discussion, the three V-Sn/AC
catalysts were expressed as vp-V-Sn/AC, tp-V-Sn/AC
and co-V-Sn/AC corresponding to impregnation sequence: vanadia-tin, tin-vanadia, and co-impregnation,
respectively.
2.2. Pre-sulfation of V-Sn/AC catalysts
To better understand the effect of SO2 on activity of V-Sn/AC catalysts, the tp-V-Sn/AC catalyst
was pre-sulfated before activity measurement. The
pre-sulfation was carried out in a stream containing
1000 ppm SO2 and 3.3% O2 in Ar at 250◦ C for 5 h.
The amount of the catalyst used was 0.5 g and the
total flow rate was 300 ml/min. After the treatment,
an Ar stream (about 200 ml/min) purged the catalyst
at the same temperature for 1 h to remove the physically adsorbed SO2 . The activity measurement was
then followed.
2.3. Activity measurement
The SCR activities of the catalysts for NOx removal
were measured in a fixed bed quartz reactor of 8 mm in
diameter. Five gases, 0.5% NO in Ar, 0.5% NH3 in Ar,
15% O2 in Ar, 0.16% SO2 in Ar (when used), and pure
Ar, were used to formulate the flue gas. Before entering the reactor, the feed gases were mixed in a chamber
filled with glass wool. For experiments involving SO2 ,
NH3 in Ar was fed directly into the reactor by passing
the mixing chamber to avoid possible reaction between
SO2 and NH3 before the catalyst bed. The transient
measurements for the SO2 effect on the catalytic activity were also carried out following a procedure:
the catalyst was initially exposed to NO–NH3 –O2 –Ar
mixture for SCR reaction. At steady-state (generally
in about 1 h), 0.16% SO2 was fed into the reactor to
replace the Ar stream of the same volume and hence
Z. Zhu et al. / Applied Catalysis B: Environmental 30 (2001) 267–276
Table 1
The standard reaction conditions in activity measurements
NO (ppm)
NH3 (ppm)
O2 (%)
SO2 (ppm)
Ar
Catalyst loading (g)
Total flow rate (ml/min)
Temperature (◦ C)
500
560
3.3
500 (when used)
Balance
0.5
300
100–280
269
in an Ar stream of 300 ml/min following a temperature programmed procedure from 250 to 650◦ C at
a heating rate of 8◦ C/min, during which SO2 in the
effluent was monitored.
2.5. FT-IR analyses
to maintain the initial concentrations of NO, NH3 and
O2 . For both steady-state and transient state experiments, the reaction conditions are similar to those
shown in Table 1. The concentrations of NO, NO2 ,
SO2 and O2 at the inlet and the outlet of the reactor
were simultaneously monitored by an online Flue Gas
Analyzer (KM9006 Quintox, Kane International Limited) equipped with NO, NO2 , SO2 and O2 sensors.
Fourier transform infrared spectroscopy (FT-IR)
was used to determine the nature of ammonia and
sulfur-containing species formed on catalyst surface.
A Magna-IR 550-II spectrometer (Nicolet) was used
at ambient temperature. Both the fresh and used
tp-V-Sn/AC catalysts were analyzed. Before the analyses, the sample was ground, mixed, and palletized
with potassium bromide with a sample to potassium
bromide ratio of 1:10. For the used catalysts, after the
SCR reaction, the samples were purged in Ar at the
reaction temperature for 1 h to remove physically adsorbed species and then cooled to room temperature.
2.4. Decomposition of sulfate species formed on
catalyst surface
3. Results and discussion
To understand catalyst deactivation, the decomposition of sulfate species formed on catalyst surface was carried out using V2 O5 /AC, SnO2 /AC, and
tp-V-Sn/AC catalysts. First, the catalyst was exposed
to the reaction gases at 250◦ C in the presence of
SO2 under the standard reaction conditions listed in
Table 1. After 10 h reaction, the reaction gases were
removed from the feed while maintaining Ar flow to
purge the catalyst surface to remove physically adsorbed SO2 . Finally, the decomposition of the sulfate
species formed on the catalyst surface was performed
3.1. Effect of metal oxides
Table 2 presents the steady-state NO conversions
over various catalysts at 250◦ C. In the absence of
SO2 , all of the catalysts show an increase in activity
with time on stream (see Fig. 2) and reach steady-state
NO conversion in about 1 h. In the steady-state,
V-W/AC, V-Mo/AC, V-Zr/AC, and vp-V-Sn/AC catalysts exhibit NO conversion of 85.5, 53.7, 87.2,
and 58.6%, respectively. The activity for the SCR
reaction in the absence of SO2 follows an order of
Table 2
Steady-state NO conversion over various catalysts at 250◦ Ca
Catalyst
V2 O5 /AC
V-W/AC
V-Mo/AC
V-Zr/AC
vp-V-Sn/AC
a
V2 O5 loading (wt.%)
5
5
5
5
5
Metal oxide loading (wt.%)
–
3
5
5
5
NO conversion (%)
Without SO2
With SO2
88.4
85.5
53.7
87.2
58.6
92.2
88.3
36.2b
91.7
91.2
Other experimental conditions are the same as in Table 1.
Steady-state was not reached within the experimental time due to continuous catalyst deactivation; the value given here was obtained
after 4 h SCR reaction in the presence of SO2 .
b
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Z. Zhu et al. / Applied Catalysis B: Environmental 30 (2001) 267–276
Fig. 1. NO conversion vs. reaction temperature over V2 O5 /AC,
V-Zr/AC and V-W/AC catalysts for the SCR reaction in the absence of SO2 . Conditions are the same as in Table 1.
V-Zr/AC > V-W/AC > vp-V-Sn/AC > V-Mo/AC.
However, the activities of all the catalysts are, to different extent, lower than that of V2 O5 /AC catalyst,
which shows a NO conversion of 88.4%. This result
indicates that each of the metal oxides presented in
the V2 O5 /AC catalyst has a deactivation effect for the
SCR reaction in the absence of SO2 . This is different from the observations on the V2 O5 /TiO2 catalyst
[13–16], where WO3 or MoO3 shows a promoting
effect on the activity of V2 O5 /TiO2 catalyst, which
was attributed to increase in number and strength of
Br␾nsted acid sites [13].
As shown above, the activities of V-Zr/AC and
V-W/AC catalysts are relatively high and are close to
that of V2 O5 /AC catalyst at 250◦ C. The dependence of
their activities upon reaction temperature is shown in
Fig. 1. Activities of the catalysts increase with increasing temperature in the studied temperature range. In
the entire temperature range, the activities of V-Zr/AC
and V-W/AC catalysts is lower than that of V2 O5 /AC
catalyst. Compared to V-Zr/AC catalyst, the activity
of V-W/AC catalyst is higher at temperatures below
200◦ C, but similar at higher temperatures.
3.2. Effect of SO2
Table 2 also presents the steady-state NO conversion over the catalysts in the presence of SO2 at
250◦ C. It should be pointed out that the values given
here was obtained after 4 h SCR reaction in the presence of SO2 , at that time most of the catalysts reach
a steady-state NO conversion, except the V-Mo/AC
catalyst, which is continuously deactivated by SO2
within the experimental time. Clearly, the presence
of SO2 greatly changes the catalytic behaviors of the
catalysts. The activity of V-Mo/AC catalyst declined
with time in contrast to that in the absence of SO2 ,
suggesting deactivation by SO2 . On the contrary, NO
conversions over V2 O5 /AC, V-W/AC, V-Zr/AC, and
vp-V-Sn/AC increase from about 88, 85, 87, and 58%
in the absence of SO2 to about 92, 88, 91, and 91%
in the presence of SO2 , respectively indicating activity promotion by SO2 . This is similar to the previous
findings on V2 O5 /AC [9–11] at low temperatures
(180–250◦ C) and on V2 O5 /TiO2 [13,17–19] at higher
temperatures (>350◦ C).
Fig. 2 shows the behaviors of the catalysts in the
presence of SO2 at different temperatures. For the
V-Mo/AC catalyst, introduction of SO2 in the feed
at 250◦ C causes gradual decrease of NO conversion,
and the decrease does not reach a steady-state within
the experimental period used. When temperature is
increased to 280◦ C, NO conversion increase to a
steady-state value of about 50%, which is lower than
the value in the absence of SO2 at 250◦ C. For the
V-Zr/AC catalyst, the presence of SO2 results in an
increased and stable activity at 250◦ C, but decrease
in temperature to 220◦ C results in deactivation. For
the V-W/AC catalyst, NO conversion decreases initially upon the addition of SO2 in the feed but then
increases gradually with time on stream and reaches a
steady-state, at which the catalytic activity is slightly
higher than that before SO2 addition. NO conversion decreases with decreasing temperature with a
steady-state NO conversion at 220◦ C but continuous
deactivation at 200◦ C. Interestingly, the V-Sn/AC
catalyst is greatly promoted by SO2 and the activity
is stable at temperature as low as 200◦ C. In the presence of SO2 , its activity is close to those of V2 O5 /AC
(Table 2) and V-Zr/AC catalyst at 250◦ C, although
it exhibits a relatively low activity in the absence of
SO2 . These results indicate that promotion or deactivation of those catalysts by SO2 depends greatly
upon reaction temperature and the nature of the added
metal oxides. Note that similar to the V2 O5 /AC catalyst reported previously [9–11], V-Sn/AC catalyst
Z. Zhu et al. / Applied Catalysis B: Environmental 30 (2001) 267–276
271
Fig. 2. Behaviors of various catalysts for the SCR reaction in the presence of SO2 . Conditions are the same as in Table 1.
shows a stable activity in relatively wide temperature
range. Therefore, it is studied in detail below.
3.3. Effect of preparation method on the behavior of
V-Sn/AC
The vp-V-Sn/AC catalyst used above was prepared
by loading vanadium oxide prior to tin oxide. To understand the influence of preparation method, catalysts
of the same composition were also prepared by loading tin oxide prior to vanadium oxide (expressed as
tp-V-Sn/AC) and by co-impregnation of vanadium and
tin oxide (expressed as co-V-Sn/AC). Table 3 presents
NO conversion over the three catalysts at 250◦ C. The
activities of the catalysts are all promoted by SO2 . In
the absence of SO2 , the tp-V-Sn/AC catalyst shows
a relatively low activity compared to the other two,
but it exhibits the highest activity in the presence of
SO2 , with a NO conversion of about 97%. The activity
of co-V-Sn/AC is close to that of vp-V-Sn/AC in the
absence of SO2 but slightly higher than the latter in
the presence of SO2 . Since SnO2 /AC shows no activity for the SCR reaction independent of the absence
or presence of SO2 (Table 3), it is believed that the
SCR reaction over these catalysts occurs at vanadia
sites.
Table 3
Steady-state NO conversion over various V-Sn/AC catalysts at
250◦ Ca
Catalyst
vp-V-Sn/Acb
tp-V-Sn/Acc
co-V-Sn/Acd
SnO2 /AC
a
NO conversion (%)
Without SO2
With SO2
58.6
46.1
56.9
3.7
91.2
97.3
95.1
1.8
Other experimental conditions are the same as in Table 1.
Prepared by loading vanadium oxide prior to tin oxide.
c Prepared by loading tin oxide prior to vanadium oxide.
d Prepared by co-impregnation of vanadium and tin oxides.
b
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Z. Zhu et al. / Applied Catalysis B: Environmental 30 (2001) 267–276
Fig. 3. NO conversion vs. reaction time over vp-V-Sn/AC and
tp-V-Sn/AC catalysts at 250◦ C. Other conditions are the same in
Table 1.
Fig. 3 shows the relationship between NO conversion and time on stream over the vp-V-Sn/AC and
tp-V-Sn/AC catalysts. After the SCR reaction reaching
a steady-state in the presence of SO2 , the vp-V-Sn/AC
shows a stable activity for 10 h but then is deactivated
gradually to 60% in 48 h. The tp-V-Sn/AC catalyst
is more stable than the vp-V-Sn/AC, showing a stable activity for 50 h in the presence of SO2 . However,
it also is deactivated gradually with time afterwards,
with decrease in NO conversion to 80% at 78 h. This
result suggests that variation in preparation can improve the stability of the V-Sn/AC catalyst but cannot
depress the deactivation completely.
The different behaviors of the tp-V-Sn/AC and
vp-V-Sn/AC catalysts may possibly result from dif-
ferent interaction among vanadium and tin species
and the surface of the support carbon and/or different
distribution of vanadium and tin species. In the case
of the tp-V-Sn/AC catalyst, prepared by loading tin
oxide prior to vanadium oxide, the deactivation of
the catalyst in the presence of SO2 may be mainly
due to the pore plugging derived from the formation
of ammonium-sulfate salts as described below. In the
case of the vp-V-Sn/AC catalyst, prepared by loading vanadium oxide prior to tin oxide, some of tin
species may possibly locate over vanadia particles
and cover the surface of the latter partially. During
the SCR reaction in the presence of SO2 , tin species
may be converted into tin sulfate salts, which is in
company with a growth in molecular size. Such a
change may result in a further cover of vanadium sites
(active sites for the SCR reaction) and quicken the
rate of catalyst deactivation in contrast to that from
ammonium-sulfate salts only.
3.4. Deactivation of V-Sn/AC catalyst
The reason of the deactivation of V-Sn/AC in the
presence of SO2 is studied through characterization.
Table 4 presents physical characteristics of the fresh
and used tp-V-Sn/AC catalysts. The fresh tp-V-Sn/AC
catalyst shows a BET surface area of 480 m2 /g and a
pore volume of 0.258 ml/g. Additionally, it exhibits
a microporous structure, with a micropore area of
362 m2 /g and a micropore volume of 0.17 ml/g. The
SCR reaction in the absence of SO2 does not cause
obvious loss in BET surface area and pore volume
of the catalyst. However, a reaction in the presence
of SO2 significantly decrease the BET surface area
and pore volume of the catalyst, from 480 m2 /g and
Table 4
The physical characteristics of the fresh and used tp-V-Sn/AC catalysta
tp-V-Sn/AC
BET surface
area (m2 /g)
Micropore
area (m2 /g)
Total pore
volume (ml/g)
Micropore
volume (ml/g)
Fresh
Used-12ASb
Used-10PSc
Used-78PSd
480
476
126
42
362
354
52
4
0.258
0.250
0.078
0.036
0.170
0.165
0.024
0.001
Measured by N2 adsorption at −196◦ C.
After 12 h reaction at 250◦ C in the absence of SO2 .
c After 10 h reaction at 250◦ C in the presence of SO .
2
d After 78 h reaction at 250◦ C in the presence of SO (partially deactivated).
2
a
b
Z. Zhu et al. / Applied Catalysis B: Environmental 30 (2001) 267–276
0.258 ml/g to 126 m2 /g and 0.078 ml/g after 10 h reaction and further to 42 m2 /g and 0.078 ml/g after 78 h
reaction. Especially, the micropore area and micropore
volume lose more significantly and, only 4 m2 /g micropore area and 0.001 ml/g micropore volume remain
after 78 h reaction in the presence of SO2 . This dramatic change is in company with partial deactivation
of the catalyst (refer Fig. 3). This result clearly indicates that the deactivation of the tp-V-Sn/AC catalyst
in the presence of SO2 is due to catalyst pore plugging
and surface area loss. Note that after 10 h reaction, no
catalyst deactivation was observed as shown in Fig. 3,
although the surface area and pore volume of the catalyst decrease significantly. The reason may be that
the remained surface area (126 m2 /g) is still enough
to allow the reaction occurring in a high NO conversion under the conditions used. It is worth to note that
the BET surface area of V2 O5 /TiO2 , the catalyst frequently used in practice, is generally about 50 m2 /g
[17], which is much lower than the value of the present
catalyst, even after 10 h reaction in the presence of
SO2 .
Fig. 4 illustrates the FT-IR spectra of fresh and
used tp-V-Sn/AC catalysts, along with the spectrum
of the AC. No obvious absorption is found for the AC
Fig. 4. FT-IR spectra of the AC (a) and the tp-V-Sn/AC catalysts
(b–e). (b) Fresh; (c) after 12 h SCR reaction in the absence of
SO2 ; (d) after 10 h SCR reaction in the presence of SO2 ; (e) after
78 h SCR reaction in the presence of SO2 .
273
under the used analysis conditions. The spectrum of
the fresh tp-V-Sn/AC catalyst exhibits a weak band
at 794 cm−1 and a very broad band in the range of
1000–1300 cm−1 , which peaks at 1072 cm−1 . The
broad band is very similar to the spectra of the
V2 O5 /AC catalyst [12] and can be assigned to the
stretching frequency of V5+ =O species. The band at
794 cm−1 can be assigned to the stretching vibration
of V–O–V. These assignments are similar to but with
noticeable difference from the reports in the literatures [20–22]. Frederickson and Hausen [20] showed
that bulk V2 O5 exhibits two IR bands at 1020 and
825 cm−1 , they were assigned to the stretching vibration of V5+ =O and V–O–V, respectively. The broad
band of the present catalyst may possibly result from
the lowered symmetry of vanadium species or existing
of some different species.
After the SCR reaction in the absence of SO2 , the
tp-V-Sn/AC shows a spectrum roughly similar to that
of the fresh one. But a new weak band appears at
1406 cm−1 , which can be assigned to NH4 + species
chemisorbed on the Brønsted acid sites [23,24]. After
10 h SCR reaction in the presence of SO2 , the broad
band gets narrow and the peak shifts to 1109 cm−1 . Simultaneously, a new band appears at 608 cm−1 , which
may be assigned to the characteristic frequencies of
SO4 2− ion. Free SO4 2− ion shows two infrared-active
bands at 1104 (ν 1 ) and 613 cm−1 (ν 2 ) [25]. Therefore,
this result suggests that sulfate species be formed during the SCR reaction in the presence of SO2 . In addition, the increased intensity of the band at 1406 cm−1
suggests an increased NH3 adsorption, which agrees
with the previous observation [12] that the formation
of sulfate species on the V2 O5 /AC catalyst significantly enhances the NH3 adsorption. After 78 h SCR
reaction in the presence of SO2 , the intensity of the
bands at 1406 and 608 cm−1 further increase, which
suggests that the amount of the NH4 + and SO4 2−
ions on the catalyst surface increase with increase in
time on stream and the catalyst deactivation (Fig. 3).
It should be pointed out that there is obvious change
in absorption in the range of 900–1300 cm−1 , the reason is not clear because the signals in this range is
very complicated, which may include absorption of
both sulfate and vanadium species. One possible reason may be that the ν 1 band of SO4 2− is splited into
two or three peaks resulting from a lowered symmetry of SO4 2− ion from Td point group to C3v or C2v
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Z. Zhu et al. / Applied Catalysis B: Environmental 30 (2001) 267–276
point groups [25]. Such splitting of SO4 2− ion was
observed by Chen and Yang [19] on titania surface.
As described above, the deactivation of the
V-Sn/AC catalysts in the presence of SO2 may be
mainly due to the formation of ammonium-sulfate
salts on the catalyst surface, which plugs the pore
structure and thus deactivates the catalyst. This behavior is obviously different from that of V2 O5 /AC
catalyst [9], which is rather stable under similar conditions and shows no sign of deactivation in 260 h.
The reason of the stabilization of the V2 O5 /AC catalyst was studied recently [12], which showed that
the improved decomposition and high reactivity with
NO of the formed ammonium-sulfate salts effectively
avoids the deposition of excess ammonium-sulfate
salts on the catalyst surface.
To further understand the deactivation of the
V-Sn/AC catalyst, decomposition of the sulfate
species (or ammonium-sulfate salts) formed on the
tp-V-Sn/AC catalyst was performed and compared
with those on the V2 O5 /AC and SnO2 /AC catalysts.
As shown in Fig. 5, SO2 in the effluent was monitored
and used to indicate the decomposition progress. Obviously, the decomposition temperature of the sulfate
species on the surface of SnO2 /AC and tp-V-Sn/AC
is higher than on the V2 O5 /AC surface. This suggests
that tin oxide stabilize the formed sulfate species,
Fig. 5. Decomposition of sulfate species formed on various catalysts during the SCR reaction in the presence of SO2 . Initial SCR
reaction conditions: 250◦ C, time on stream of 10 h, others are the
same in Table 1. Decomposition conditions: Ar of 300 ml/min,
from 250 to 650◦ C at a heating rate of 8◦ C/min, 0.5 g catalysts.
resulting in accumulation of ammonium-sulfate salts
and then the deactivation of the V-Sn/AC catalyst.
It is worth to point out that the deactivation of the
V-Sn/AC catalyst may also be associated with formation of tin sulfate through a reaction between tin
oxide and the sulfate species. Similar phenomenon
has been observed on some other catalysts, such as
CuO/AC [7] and MnOx /Al2 O3 [8]. At low temperatures, these catalysts are severely deactivated by SO2
due to formation of metal sulfates. However, for the
present catalyst, there has been no direct evidence yet
to indicate this mechanism, although it is possible.
As a conclusion, the V-Sn/AC catalyst is severely
deactivated by SO2 mainly due to the formation of
ammonium-sulfate salts on the surface, the salts are
stabilized by the presence of tin oxide. However, it
seems contradict with the findings that SO2 significantly promotes the activity of the V-Sn/AC catalysts
in the initial reaction period (Figs. 2 and 3, Table 3).
This phenomenon will be elucidated in the following
study on the mechanism of SO2 promotion.
3.5. Mechanism of SO2 promotion for V-Sn/AC
To better understand the promoting role of SO2 in
the SCR reaction over V-Sn/AC catalyst, an experiment consisting of six consecutive parts, as shown in
Fig. 6, was carried out over tp-V-Sn/AC catalyst at
250◦ C. In the experiment the outlet SO2 concentration was monitored to indicate the behavior of SO2 .
As shown in part I, the SCR reaction in the absence
of SO2 shows a steady-state NO conversion of 46%.
Upon introduction of SO2 into the feed (part II), NO
conversion increases gradually from 46% to a new
steady-state value of 97%. Interestingly, the outlet SO2
concentration simultaneously increases and follows a
pattern much similar to that of NO conversion. The removal of SO2 from the feed, in part III, does not cause
any decline in NO conversion. A heating treatment of
the catalyst in Ar from 250 to 650◦ C (part IV) was performed, during which a considerable amount of SO2
was detected, and the SO2 release profile was similar
to that for tp-V-Sn/AC in Fig. 5. After the reactor was
cooled to the original temperature of 250◦ C and the
reaction gases were re-fed under the same conditions
in part I (part V, without SO2 ), NO conversion reaches
a steady-state value of 66%, which is much lower than
that in parts II and III but is higher than that in part
Z. Zhu et al. / Applied Catalysis B: Environmental 30 (2001) 267–276
275
Fig. 6. Effect of SO2 on the activity of tp-V-Sn/AC catalyst at 250◦ C. Reaction conditions are the same as in Table 1. (I) NO–NH3 –O2
reaction; (II) adding SO2 into the feed; (III) removing SO2 from the feed; (IV) treating the catalyst by TPD of SO2 in Ar from 250 to
650◦ C and then cooling down to 250◦ C; (V) and (VI) following processes as the same as in (I) and (II), respectively.
I. Re-introduction of SO2 in the feed (part VI), as expected, increased the NO conversion to a steady-state
value close to that in part II, which is also in company
with a similar change in SO2 concentration.
These results clearly indicate that the promoting effect of SO2 is not from the gas phase SO2 but from
the formation of some sulfur-containing species on
the catalyst surface. This is also supported by another
experimental observation. Fresh tp-V-Sn/AC catalyst
was pre-sulfated by SO2 + O2 at 250◦ C to allow it being modified by the sulfur-containing species before
the SCR reaction. After this treatment, the catalyst activity was measured under the standard reaction conditions in the absence of SO2 and the result showed
that the treated catalyst shows a steady-state NO conversion of 97.5%, which is much higher than the value
(46.1%) and is almost the same as the value (97.3%)
for the untreated catalyst in the absence and presence
of SO2 , respectively (Table 3 and Fig. 3). In addition,
introduction of SO2 into feed gases did not further increase the activity of the treated catalyst. This result
is also useful for possible use of V-Sn/AC catalyst in
the SO2 -free situations.
Furthermore, the sulfur-containing species formed
on the catalyst surface are identified as sulfate species
by FT-IR spectrum of tp-V-Sn/AC catalyst after 10 h
SCR reaction in the presence of SO2 (Fig. 4d). In the
FT-IR spectrum, the intensity of NH4 + ions simultaneously increases with sulfate species, compared to
the situation after the SCR reaction in the absence of
SO2 (Fig. 4c). These observations can be used to indicate the mechanism of the promoting effect of SO2 on
the catalytic activity, since the catalyst has not been
deactivated at this stage and the SO2 promotion is still
significant (Fig. 3).
Conclusively, the promoting effect of SO2 on the
V-Sn/AC activity is attributed to the formation of
sulfate species on the catalyst surface, which provides new Brønsted acid sites and thus increases the
ammonia adsorption and catalytic activity. Similar results have also been obtained for V2 O5 /AC [10–12],
V2 O5 /TiO2 [17,18], and TiO2 [19]. Chen et al. observed that the sulfated TiO2 shows a solid superacid
property which increases the catalyst surface acidity
[19]. Recently, we also showed [12] that the promoting effect of SO2 on the V2 O5 /AC activity is from
the formation of sulfate species on carbon surface,
which significantly increases the surface acidity and
ammonia adsorption.
As described above, both the promotion and the
deactivation of V-Sn/AC catalyst in the presence of
SO2 are due to the formation of sulfate species on
catalyst surface. In other words, the formed sulfate
species, on one hand, provide acid sites for ammonia
276
Z. Zhu et al. / Applied Catalysis B: Environmental 30 (2001) 267–276
adsorption and thus promote the catalytic activity, on
the other hand, react with ammonia to transform into
ammonium-sulfate salts, such as NH4 HSO4 and/or
(NH4 )2 S2 O7 [1], which subsequently plug the catalyst pore structure and thus deactivate the catalyst.
In the initial period of the SCR reaction in the presence of SO2 , instanced in the case of 10 h reaction
for tp-V-Sn/AC shown in Fig. 3, NO conversion is
greatly enhanced by the formation of sulfate species
on the catalyst surface and the increased ammonia
adsorption (Fig. 4). In this period, the catalyst is not
deactivated because the amount of the formed sulfate
species or the ammonium-sulfate salts is relatively
low (Fig. 4) and because the catalyst still has a large
surface area for the reaction, although the catalyst
pore structure was partially plugged (Table 4).
However, as the SCR reaction proceeds in the presence of SO2 , more and more ammonium-sulfate salts
are formed (Fig. 4) and more and more pore structures, especially for micropore, are plugged (Table 4).
This then gradually deactivates the catalyst, similar to
the situation of tp-V-Sn/AC after 78 h reaction in the
presence of SO2 (Fig. 3).
Acknowledgements
The authors gratefully acknowledge the financial
supports from the Natural Science Foundation China
(29633030, 29876046), Chinese Academy of Sciences, and the Shanxi Natural Science Foundation.
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