Mechanism of selective catalytic oxidation of ammonia to nitrogen over Ag/Al2O3

Article abstract of DOI:10.1016/j.jcat.2009.08.011

The mechanism of selective catalytic oxidation (SCO) of NH₃ over Ag/Al₂O₃ was studied by NH₃ temperature-programed oxidation, O₂-pulse adsorption, and in situ DRIFTS of NH₃ adsorption and o

Full text of DOI:10.1016/j.jcat.2009.08.011

Journal of Catalysis 268 (2009) 18–25
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Mechanism of selective catalytic oxidation of ammonia to nitrogen over Ag/Al O
2
3
Li Zhang, Hong He *
State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
2
The mechanism of selective catalytic oxidation (SCO) of NH3 over Ag/Al O3 was studied by NH3 temper-
Received 7 January 2009
Revised 12 August 2009
Accepted 27 August 2009
Available online 1 October 2009
ature-programed oxidation, O
essence which affects the low temperature activity of Ag/Al
nism study. Different Ag species on Ag/Al significantly influence O
ent oxygen species affect the activity of NH oxidation at low temperature. The activated –NH could react
with the atomic oxygen (O) at low temperatures (<140 °C); however, the –NH could also interact with the
at temperatures above 140 °C. At low temperatures (<140 °C), NH oxidation follows the –NH mech-
anism. However, at temperatures above 140 °C, NH oxidation follows an in situ selective catalytic reduc-
tion (iSCR) mechanism (two-step formation of N via the reduction of an in situ-produced NO species by
a NH species).
2
-pulse adsorption, and in situ DRIFTS of NH
has been elucidated through the mecha-
uptake by catalysts; while differ-
3
adsorption and oxidation. The
2 3
O
2
O
3
2
3
Keywords:
Selective catalytic oxidation
O
2
3
3
NH
Ag/Al
Mechanism
uptake
In situ DRIFTS
3
2 3
O
2
x
x
O
2
Ó 2009 Elsevier Inc. All rights reserved.
1
. Introduction
the NO
Many noble metal catalysts, such as Ag (powder), Pt, Pt/Al
Rh/Al , Pd/Al , Pt-ZSM-5, Pd-ZSM-5, and Rh-ZSM-5, have been
suggested to follow this iSCR route [9–11].
Different reaction mechanisms of NH oxidation are reportedly
x 3 2
species subsequently react with NH to form N [9–11].
2 3
O ,
Selective catalytic oxidation (SCO) of ammonia (NH
nitrogen gas (N ) and H O at low temperatures is an efficient
method to abate NH pollution. This SCO reaction has two impor-
tant parameters: the selectivity and the application temperature.
To rationally develop a process for NH oxidation to N over cata-
lysts, the reaction mechanism must be clarified.
While several studies have examined the SCO process, the
mechanism of NH oxidation and N formation is still uncertain.
Three major reaction pathways have been proposed for the SCO
of NH to N over different catalysts [1–13]. Zawadzki [1] proposed
3
) with O
2
to
2
O
3
2 3
O
2
2
3
3
associated with different oxygen species [1,7,14–16]. Zawadzki [1]
proposed that O was necessary for the very rapid formation of NH,
3
2
as well as for the HNO intermediate and N
hydrazinium-type intermediate mechanism had been suggested
under conditions of little or limited O availability; the only oxygen
2
O production. The
3
2
2
available was that associated with the metal oxide catalysts, and
the catalyst testing was carried out in an essentially reducing envi-
3
2
an imide (NH) mechanism in which the first step yields NH, and
then the NH reacts with atomic oxygen (O) to form nitroxyl
ronment. At higher O
molecular O and follows the iSCR mechanism [7,14].
Recently, Gang et al. [11,13] reported that alumina-supported
Ag (Ag/Al ) catalysts were extremely active in NH oxidation
at low temperatures, and the performance of the Ag/Al catalyst
was even superior to that of noble metal catalysts. We investigated
the role of Ag species on Ag/Al in the activity and selectivity of
NH oxidation [17]. The state of the Ag species and Ag particle size
was found to significantly influence the activity and selectivity of
2 3
concentrations, NH mainly interacts with
2
(
HNO) and further conversion to N
NH could even react with molecular O
NO). This mechanism was mainly supported by the results ob-
tained on Pt or transition metal oxide catalysts [1–5]. Two other in-
sights into the reaction mechanism of NH oxidation have been
2
or nitrous oxide (N
2
O), or
2
to produce nitric oxide
2
O
3
3
(
2 3
O
3
2 3
O
proposed in recent years [6–13]. One is the SCO by a direct route
involving a hydrazinium-type intermediate [7,8]. This mechanism
has been reported mainly on the transition metal oxide catalysts
3
0
NH
3
oxidation over Ag/Al
oxidation at low temperatures (<140 °C),
whereas Ag was also active at higher temperatures (>140 °C).
Gang et al. [11,13] revealed that the NH oxidation over the Ag-
based catalysts followed the iSCR mechanism. They reported that
the NH oxidation activity at low temperatures was related to
the catalyst’s ability to promote dissociative or non-dissociative
adsorption of O [11]. However, the roles of different oxygen
2 3
O . Ag was proposed to be the main ac-
such as CuO/Al
TiO [7,8]. The other is a two-step mechanism called in situ or
‘internal” selective catalytic reduction (iSCR), which involves the
oxidation of a significant percentage of NH into NO species, and
2
O
3
, Fe
O
2 3
/TiO
2
, CrO
x
/TiO
2
, CoO
x
2
/TiO , and CuO/
tive species of NH
3
+
2
‘
3
3
x
3
*
Corresponding author. Fax: +86 10 62923563.
E-mail address: honghe@rcees.ac.cn (H. He).
2
0
021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.08.011

L. Zhang, H. He / Journal of Catalysis 268 (2009) 18–25
19
species in the activity and the reaction mechanism over the Ag/
Al have not been studied in detail. The role of different oxygen
species in the activity and reaction mechanism of NH oxidation
over Ag/Al needs to be determined. Since the active species
on Ag/Al for NH oxidation are different in different tempera-
2.3. O
2
-pulse adsorption
2 3
O
3
O
2
uptake was determined by O
2
-pulse adsorption at various
-pretreated Ag/Al O catalysts
2 2 3
2
O
3
temperatures over the fresh and H
using a Quantasorb-18 automatic instrument (Quanta Chrome
Instrument Co.). Prior to the O -pulse adsorption, 300 mg of sam-
ple was pretreated in situ with flow of 5 vol.% /Ar
(40 cm min , 10 °C min ) at 400 °C for 2 h. Then, He gas
(40 cm min ) was passed over the sample for 1.5 h. After cooling
to the desired temperature in He, O pulses (4.46 mol) were then
injected in a He carrier over the sample, with a time interval be-
tween O pulses of almost 100 s. The O signal was analyzed online
with an Autosorb-1-C TCD controller. A similar O -pulse adsorp-
tion experiment was performed over the fresh Ag/Al , without
the H pretreatment prior to the O -pulse adsorption.
2
O
3
3
ture regions [17], investigating the role of different Ag species in
the reaction process is important. Such information may be useful
2
a
H
2
3
ꢀ1
ꢀ1
ꢀ1
in the development of an improved process of NH
Ag/Al
In this study, the role of different oxygen species in the activity
and reaction mechanism of SCO of NH over Ag/Al was studied
-pulse adsorption was used to measure the O uptake
. Temperature-programed oxidation (TPO) and diffuse
3
oxidation over
3
2
O
3
.
2
l
3
2 3
O
in detail. O
on Ag/Al
2
2
2
2
2
O
3
2
reflectance infrared Fourier transform spectroscopy (DRIFTS) were
used to clarify the possible reaction mechanisms.
2 3
O
2
2
In this article, we have elucidated the essence which affects the
low temperature activity of Ag/Al
study. Different Ag species significantly influence O
alysts, and different oxygen species affect low temperature activity
and reaction routes. Adsorbed NH mainly interacts with chemi-
sorbed oxygen atom on Ag/Al at low temperatures (<140 °C).
In contrast, at temperatures above 140 °C, adsorbed NH can also
react with gas phase O to produce N . Accordingly, the reaction
route of NH oxidation over Ag/Al is different in different tem-
perature regions. At low temperatures (<140 °C), NH oxidation
follows the –NH mechanism, while at temperatures above
2
O
3
through the mechanism
2.4. Fourier transform infrared (FTIR) spectroscopy studies
2
uptake by cat-
In situ DRIFTS spectra were recorded in a NEXUS 670-FTIR
3
equipped with a smart collector and a liquid N -cooled MCT detec-
2
2
O
3
tor. The sample (about 30 mg) for study was finely ground and
3
placed in a ceramic crucible. A feed gas mixture, controlled by mass
3
ꢀ1
2
2
flow meters, was supplied at a flow rate of 100 cm min . The wa-
3
2
O
3
fers were first treated at 500 °C in a flow of high purity 10 vol.% O2/
3
N for 0.5 h and then cooled to room temperature. At each temper-
2
ature, the background spectrum was recorded in flowing N2 and
1
40 °C, it follows the iSCR mechanism.
was subtracted from the sample spectrum obtained at the same
temperature. All spectra were recorded at a resolution of 4 cm
with 100 accumulated scans.
ꢀ
1
The isotope experiment was performed in an IR cell, which was
connected to a vacuum system, and it could be kept below
2
. Materials and methods
ꢀ4
1
0
Torr by the pump. In this paper, the spectra are displayed in
2.1. Catalyst preparation
absorbance.
2 3
The catalyst used in this study was 10 wt.% Ag/Al O prepared
2
ꢀ1
3. Results
by an impregnation method using
and an appropriate amount of AgNO
c
-Al
2 3
O powder (250 m g )
ꢀ3
3
(7.874 mg cm ) aqueous
3
3.1. NH -TPO
solution. After impregnation, the excess water was removed in a
rotary evaporator at 80 °C. The sample was first dried at 120 °C
overnight followed by calcination at 600 °C in air for 3 h. The
NH
and H
NH oxidation activity over the two catalysts and the variety of
products produced in different temperature regions (Fig. 1). NH
3
-TPO experiments were performed separately on the fresh
2
-pretreated 10 wt.% Ag/Al catalysts to investigate the
2 3
O
2 3
weight ratio of Ag is measured with respect to the support c-Al O .
3
3
3 3
2.2. NH temperature-programed oxidation (NH -TPO)
NH
3
temperature-programed oxidation (NH
3
-TPO) experiments
catalysts
were carried out over fresh and H
2
-pretreated Ag/Al O
2 3
using a fixed-bed continuous flow micro-reactor system equipped
with a computer-interfaced quadruple mass spectrometer (Hiden
WR13012 UK). Typically, 100 mg of sample was pretreated by
3
ꢀ1
2
heating in a flowing stream (40 cm min ) of O /He (10 vol.%)
ꢀ1
from room temperature to 500 °C (25 °C min ). This was done to
remove the water and desorb unwanted impurities. The tempera-
ture was held at 500 °C for 0.5 h and then cooled to room temper-
2 3
ature under flowing gas (O /He). Prior to the NH oxidation, the
catalyst was reduced in situ by heating from room temperature
ꢀ1
3
ꢀ1
2
to 400 °C (10 °C min ) in a H /Ar (5 vol.%) flow (40 cm min ).
Subsequently, the sample was kept at this temperature for 2 h;
the temperature was then lowered to 50 °C, followed by a He flush-
ing period for 1 h. At 50 °C, the He flow was switched to a flow of
NH
3 2
500 ppm, O 10 vol.%, He as carrier for 2 h, and the total flow
3
ꢀ1
rate was 100 cm min . Then the sample temperature was in-
creased from 50 °C to 500 °C at a rate of 10 °C min and the TPO
ꢀ1
Fig. 1. NH
-pretreated (b) 10 wt.% Ag/Al
, 10 vol.%; He as carrier; flow rate, 100 cm min ; catalyst weight, 0.1 g (W/
3
temperature-programed oxidation (NH
3
-TPO) profiles over fresh (a) and
data were recorded with mass spectrometer. A similar experiment,
H
2
2
O
3
catalysts. Reaction conditions: NH
3
, 500 ppm;
without the H
2
pretreatment prior to the NH
3
-TPO, was also per-
3
ꢀ1
O
2
ꢀ
1
formed over fresh Ag/Al
2
O
3
.
F = 0.06 g s cm ).

2
0
L. Zhang, H. He / Journal of Catalysis 268 (2009) 18–25
desorbed over the fresh Ag/Al
2
O
3
(Fig. 1a) at temperatures from
5
0 °C to 300 °C. No product was formed below 140 °C. Upon heat-
ing, N
2
was formed/desorbed as the main product from 160 °C to
5
00 °C; N O was the second main product detected in the temper-
2
ature range (160–350 °C), while at higher temperatures (>350 °C),
significant amounts of NO were formed/desorbed. However, over
the H
temperatures (<160 °C). The selectivity of products of NH
strongly depended on the reaction temperature. N O and N
2 2 3 3
-pretreated Ag/Al O (Fig. 1b), NH desorbed mainly at low
3
–TPO
2
were
2
the main products detected in the low temperature range (80–
1
60 °C); N was found to be the main reaction product at temper-
2
atures above 160 °C, while at higher temperatures (>350 °C), sig-
nificant amounts of NO were formed.
Compared to the fresh Ag/Al
lower temperature (<140 °C) over the H
Most products of NH oxidation at low temperatures were N
and N . However, at higher temperatures, the main products of
NH oxidization over the fresh and H -pretreated Ag/Al cata-
lysts were similar. This demonstrates that H pretreatment en-
hances the low temperature activity of NH oxidation over Ag/
Al and that the low temperature (<140 °C) products are differ-
ent from those produced at higher temperatures.
2
O
3
, NH
3
could be oxidized at a
2
-pretreated Ag/Al
2 3
O .
3
2
O
2
3
2
2 3
O
Fig. 3. NH
conditions: NH
0.1 g (W/F = 0.06 g s cm ).
3 2 3
-TPO profiles over the fresh 10 wt.% Ag/Al O catalyst, reaction
3
ꢀ1
2
3
, 500 ppm; He as carrier; flow rate, 100 cm min ; catalyst weight,
ꢀ
1
3
2 3
O
to form O species, while molecular O
2
cannot be adsorbed dissocia-
tively on the surface of fresh Ag/Al O . In conjunction with our TPO
2 3
3
.2. O
2
-pulse adsorption
results mentioned above (Fig. 1), we found that the NH3 could be
oxidized at a lower temperature (<140 °C) over the H -pretreated
Ag/Al O in the presence of the chemisorbed oxygen atom. How-
2
The effect of the O
treated Ag/Al catalysts was investigated to elucidate the O
take of the catalysts at 100 °C and 160 °C. As seen in Fig. 2,
molecular O was readily chemisorbed on the H -pretreated Ag/
Al ; in contrast, little molecular O was adsorbed on the fresh
Ag/Al
Gang et al. [11,13] reported that adsorbed oxygen atom ap-
peared on the reduced Ag catalysts, and the dissociation of O
was believed to be the rate-controlling step for NH oxidation
13]. Lefferts et al. [18] also detected oxygen atom on a H -pre-
was only present for a
2
-pulse adsorption on the fresh and H
2
-pre-
2
3
2
O
3
2
up-
2 3 3
ever, over the fresh Ag/Al O , NH could also be oxidized at higher
temperatures in the absence of O. Because the molecular O could
2
2
2
not be adsorbed on the fresh catalyst, the adsorbed NH3 mainly
2
O
3
2
interacted with the gas phase O or the lattice oxygen under these
2
2
O
3
.
conditions. According to our other TPO experiment over fresh Ag/
Al O (Fig. 3) in the absence of gaseous O (NH –He feed), we found
2
3
2
3
2
that little product of NH oxidation was formed below 300 °C. The
3
3
adsorbed NH3 mainly reacts with the gas phase O2 over fresh Ag/
[
2
Al O in the presence of gaseous O . This demonstrates that the
2
3
2
treated Ag surface, and the molecular O
2
oxygen species have a significant influence on the low temperature
short time after removing air from the sample. Based on our O
pulse adsorption results, we conclude that molecular O
sociatively chemisorbed on the surface of H -pretreated Ag/Al
2
-
(<140 °C) activity of NH oxidation over Ag/Al O . This means that
3
2 3
2
can be dis-
2 3
the dissociation of O is the rate-controlling step for NH oxidation.
0
+
2
2
O
3
Ag is the main Ag species on H -pretreated Ag/Al O , while Ag
2
2 3
2 3
is the main Ag species on fresh Ag/Al O [17]. The presence of dif-
ferent Ag species on the catalyst clearly affects the chemisorption
of O
. The presence of Ag0 caused the adsorption of O species
and must have been a reason for the enhanced low temperature
activity of NH oxidation over H -pretreated Ag/Al
uptake of H -pretreated Ag/Al
was higher than that at 100 °C (45.73
that the chemisorption of O
ture, in agreement with other reports [11,18,19].
2
3
2
2 3
O .
mol g 1
mol g ). This demonstrates
is affected by tempera-
ꢀ
)
O
2
2
2
O
l
3
at 160 °C (76.58
l
ꢀ1
2
on Ag/Al
2
O
3
3
3.3. FTIR study of NH adsorption
FTIR spectra of the adsorbed species due to contact with the
fresh and H -pretreated 10 wt.% Ag/Al catalysts at different
temperatures with NH are presented in Figs. 4 and 5. As shown
in Fig. 4, after the sample was treated with NH at room tempera-
ture, the bands were observed at 1695, 1614, 1483, 1389, and
2
2 3
O
3
3
ꢀ
1
ꢀ1
1
232 cm . The bands at 1695 and 1483 cm were due to the
asymmetric and symmetric deformation modes of NH
3
coordi-
þ
nated on Brønsted acid sites (NH ) [6,20–23], respectively. The
4
ꢀ1
band at 1389 cm was observed under similar conditions on Ni/
Al by Amblard et al. [6] and on Au/MO -Al by Lin et al.
2
O
3
x
/c
2 3
O
[
24]. According to the literature [6,24] and the peak behavior when
Fig. 2. O
at various temperatures. Fresh 10 wt.% Ag/Al
pretreated 10 wt.% Ag/Al
.46 mol/pulse; He as carrier; flow rate, 40 cm min ; catalyst weight, 0.3 g.
2
uptakes on fresh 10 wt.% Ag/Al
2
O
3
and on H
2
-pretreated 10 wt.% Ag/Al
2
O
3
ꢀ1
the sample was heated, the band at 1389 cm could be assigned to
NH ad-species on the
1232 cm could be assigned to the asymmetric and symmetric
2
O
3
:
100 °C (a), 160 °C (b);
H
2
-
,
þ
2 3
c-Al O support. The bands at 1614 and
2
O
3
:
100 °C (c); 160 °C (d). Reaction conditions:
O
2
4
3
ꢀ1
ꢀ1
4
l

L. Zhang, H. He / Journal of Catalysis 268 (2009) 18–25
21
ꢀ1
respectively. The band at 1456 cm was ascribed to –NH deforma-
tion modes [8,14,24,25]. These results indicate that upon heating,
the adsorbed NH
3
can be activated to form –NH
2
and –NH interme-
diates through abstraction of hydrogen.
NH
NH
3
! NH
2
þ H
ð1Þ
ð2Þ
2
! NH þ H
2
The same experiment was performed on the H -pretreated Ag/
Al
244 cm were observed after NH
ing, the bands of NH coordinated on Brønsted acid sites disap-
peared, followed by the bands ascribed to NH coordinated on
Lewis acid sites, and finally the band of –NH. A new weak band
2 3
O catalyst. Similar bands at 1689, 1606, 1477, 1394, and
ꢀ1
1
3
adsorption (Fig. 5). Upon heat-
3
3
ꢀ1
at 2154 cm was observed from 100 °C to 160 °C, which disap-
peared at temperatures above 160 °C.
ꢀ1
The bands at 2222 cm are reportedly due to N–N stretching
modes of gas phase N O over the reduced Ag catalyst [26]. A sim-
2
ꢀ
1
ilar band at 2200 cm upon NH
signed to adsorbed N O [27]. The appearance of more than one
form of adsorbed N O is often explained by the simultaneous pres-
ence of N- and O-bonded molecules and/or by adsorption on differ-
ent sites. However, the N–O modes of N O were not observable in
many cases because of the strong bands associated with the sub-
strate. When the N–O modes of adsorbed N O were detected, they
were registered in the 1262–1220 cm region [28]. As shown in
3
oxidation on ZnO has been as-
Fig. 4. FTIR spectra of the adsorbed species arising from contact of NH
with the fresh 10 wt.% Ag/Al at room temperature and successive purging with
at various temperatures.
3
(500 ppm)
2
2 3
O
N
2
2
2
2
ꢀ
1
ꢀ1
Figs. 4 and 5, a band was observable at 1260 cm , but it did not
disappear synchronously with the band at 2160 cm ; hence these
two bands should not be assigned to one ad-species. The band at
260 cm might be due to the shift in the band of NH coordi-
3
nated at Lewis acid sites (1244, 1232 cm ).
To assure the correct assignment of the band (2154,
ꢀ1
ꢀ
1
1
ꢀ
1
ꢀ1
2
160 cm ), we investigated the interaction of N
2 2 3
O and Ag/Al O
(
the results are shown in Supplementary material). We found that
ꢀ1
the bands in the 2150–2300 cm region were observable due to
the asymmetric N–N–O stretching modes of gas phase N O, but
they disappeared quickly after purging with N for several minutes.
Therefore, the band at 2154 (2160) cm should not be assigned to
the gas phase N O over the Ag/Al . Our results suggest that this
band (2154, 2160 cm ) should be assigned to adsorbed N O.
Compared to the results on the fresh Ag/Al (Fig. 4), the bands
of NH adsorbed on Brønsted and Lewis acid sites of the H -pre-
treated Ag/Al disappeared at lower temperatures. While the
intensity of these bands decreased with a reduction in tempera-
ture, the bands of –NH
appeared, indicating that NH
Al can be activated to form –NH
lower temperatures. Together with our O
(Fig. 2), this indicates that both adsorbed NH
vated at lower temperatures (<140 °C) on H
Al , which may explain the enhanced low temperature activity
of NH oxidation.
2
2
ꢀ1
2
2 3
O
ꢀ1
2
2 3
O
Fig. 5. FTIR spectra of the adsorbed species arising from contact of NH
with the H -pretreated 10 wt.% Ag/Al at room temperature and successive
purging with N at various temperatures.
3
(500 ppm)
3
2
2
2
O
3
2 3
O
2
ꢀ
1
ꢀ1
2
(1581, 1377 cm ) and –NH (1456 cm
adsorbed on H -pretreated Ag/
and –NH intermediates at
-pulse adsorption results
and O can be acti-
-pretreated Ag/
)
3
2
2
O
3
2
deformation modes of NH
3
coordinated on Lewis acid sites [6,20-
2
2
3], respectively. Correspondingly, the bands in the NH stretching
ꢀ1
3
2
region were also observed at 3401, 3356, 3270, and 3149 cm
8,14,25].
The intensities of these bands decreased gradually with the in-
crease in the sample temperature (Fig. 4). Upon heating, first, the
bands of NH coordinated on Brønsted acid sites disappeared, fol-
lowed by the bands ascribed to NH coordinated on Lewis acid
sites. This desorption sequence seemed to coincide with the ex-
pected adsorption strength of these ad-species. The NH coordi-
nated on Brønsted acid sites was weaker than that coordinated
on Lewis acid sites. Desorption of the unreacted NH and the acti-
vation of NH were responsible for these decreases. A new weak
2
[
2 3
O
3
3
3
3 2
3.4. FTIR study of the interaction of NH with O
3
The NH oxidation mechanism was studied with respect to the
3
behavior of adsorbed NH3 species interacting with O2 on the sur-
3
faces of fresh and H -pretreated 10 wt.% Ag/Al O catalysts using
2
2 3
3
in situ DRIFTS. Fig. 6 shows the in situ DRIFT spectra of the fresh
Ag/Al O in a flow of NH + O at various temperatures. The bands
ꢀ1
band was visible at 2160 cm from 160 °C to 220 °C, but vanished
at temperatures above 220 °C. The assignment of this new peak
will be discussed in the following section.
2
3
3
2
ꢀ1
of NH coordinated on Brønsted acid sites (1691, 1487, 1389 cm
3
)
disappeared at temperatures above 140 °C. However, the bands
ꢀ1
With increasing temperatures, intensities of three bands, 1581,
coordinated on Lewis acid sites (1614, 1232 cm ) decreased grad-
ually and vanished at higher temperatures (>220 °C). While these
band intensities decreased, the band intensity of –NH
ꢀ
1
1
456, and 1377 cm , increased following the decrease of the
ꢀ
1
bands assignable to NH
assigned to amide (–NH
3
. The bands at 1581 and 1377 cm were
) scissorings and (–NH ) waggings,
ꢀ1
2
2
(1456 cm ) increased from 100 °C. Three new bands appeared at

2
2
L. Zhang, H. He / Journal of Catalysis 268 (2009) 18–25
and N
<140 °C; Fig. 7). In contrast, no adsorbed NH
the fresh Ag/Al under this condition, although it could have
2
O on the H
2
-pretreated Ag/Al
2
O
3
at low temperatures
(
3
was oxidized on
2 3
O
been activated to form some –NH intermediates (Fig. 6).
The presence of surface –NH indicates the dissociative adsorp-
tion of NH
Our DRIFT results (Fig. 7) showed that at lower temperatures, the
bands of adsorbed NH were smaller in the presence of O than
without O (Fig. 5). Considering that gas phase O can be chemi-
sorbed as oxygen atom on the surface of H -pretreated Ag/Al
this indicates that the adsorbed NH is more readily activated to
form –NH in the presence of oxygen atom, confirming the previous
reports [1]. The first elementary process in NH oxidation on H
3
and/or the presence of H-abstraction on the surface.
3
2
2
2
2
2 3
O ,
3
3
2
-
pretreated Ag/Al
2
O
3
is likely the reaction of NH
3
with O as follows:
NH
3
þ O ! NH þ H
2
O
ð3Þ
As shown in Fig. 7, –NH and the adsorbed N
2
O bands were vis-
ible at low temperatures (<140 °C); some nitrate ad-species were
observed at temperatures above 140 °C, as seen on the fresh Ag/
Fig. 6. FTIR spectra of the fresh 10 wt.% Ag/Al
2 3
O catalyst treated with a flow of
5
00 ppm NH + 10 vol.% O /N at various temperatures.
3
2
2
2
Al O
2
(Fig. 6). This clearly shows that over the H -pretreated Ag/
3
2
Al O
3
, –NH is the key intermediate of NH
3
oxidation at low temper-
atures (<140 °C), and the SCO pathway at low temperatures is dif-
ferent from the reaction route at temperatures above 140 °C. In
contrast, at temperatures above 140 °C, the SCO of NH
3
over fresh
2 2 3
and H -pretreated Ag/Al O catalysts both follow a similar reaction
route.
3
A key role of –NH and –HNO species in NH oxidation was pro-
posed by Zawadzki [1] and supported by Germain and Perez [30].
Zawadzki [1] suggested that on Pt or metal oxide catalysts, the
–
NH could interact with oxygen atom to form –HNO species; then
the –NH and –HNO could react with each other giving rise to N
while two –HNO species would be intermediates in N O formation.
2
,
2
However, we found no band assignable to –HNO (Fig. 7), perhaps
because the –HNO reacted quickly.
To discover the –HNO species, the concentration of NH
duced to lower the reaction rate. First, we evacuated the gaseous
NH after exposure of the H -pretreated Ag/Al to NH for 1 h
to leave the adsorbed NH on the surface of the catalyst. A flow
of O was then supplied at various temperatures, and the subse-
quent spectra were collected (Fig. 8). Upon heating, –NH
3
was re-
3
2
2
O
3
3
3
2
Fig. 7. FTIR spectra of the H
2
-pretreated 10 wt.% Ag/Al
2 3
O catalyst treated with a
flow of 500 ppm NH + 10 vol.% O
3
2
/N at various temperatures.
2
2
ꢀ1
ꢀ1
(
1371 cm ) and –NH (1456 cm ) appeared at temperatures
ꢀ
1
above 70 °C; a new band was observed at 1529 cm from 70 °C
to 120 °C (Fig. 8). Similar behavior was observed for adsorbed
1
576, 1545, and 1302 cmꢀ1 at temperatures above 140 °C and in-
creased significantly with rising temperature. These bands could
be assigned to nitrate ad-species [28,29], and the assignments of
different nitrates are discussed in a more detailed manner in the
following section.
Fig. 7 shows the results of the similar experiment performed on
the H
Brønsted acid sites and Lewis acid sites decreased at a lower tem-
perature in the presence of O than without O (Fig. 5). The –NH
band (1456 cm ) appeared at 80 °C and increased significantly
at temperatures above 140 °C. The band of adsorbed
2 2 3 3
-pretreated Ag/Al O . The bands of NH coordinated on
2
2
ꢀ1
2
N O
ꢀ1
(
2154 cm ) was visible from 80 °C to 140 °C. In addition, it could
be noted that the band assigned to nitrate ad-species (1302,
ꢀ
1
1
579 cm ) appeared at 140 °C. The bands at 1579, 1545,1456,
ꢀ
1
and 1302 cm increased significantly when the temperature rose
to 160 °C, demonstrating that at higher temperatures (>140 °C)
some nitrate ad-species are formed on the H
Al , similar to the results found on the fresh Ag/Al
However, on the fresh Ag/Al (Fig. 6), no N O (2154 cm ) ad-
2
-pretreated Ag/
2
O
3
2 3
O
(Fig. 6).
ꢀ
1
2
O
3
2
ꢀ
1
species appeared with the –NH band (1456 cm ) at low tempera-
tures (<140 °C). Based on these findings, together with the TPO re-
Fig. 8. FTIR spectra of the H
NH (500 ppm) at room temperature, followed by purging with N
successive heating in 10 vol.% O /N2 at various temperatures.
2
-pretreated 10 wt.% Ag/Al
2
O
3
taken after adsorption of
sults (Fig. 1), we conclude that the adsorbed NH
3
could be
3
2
for 30 min and
activated to form –NH and further be converted to produce N
2
2

L. Zhang, H. He / Journal of Catalysis 268 (2009) 18–25
23
Fig. 9. FTIR spectra of the adsorbed NH
pretreated 10 wt.% Ag/Al at 100 °C (The sample was exposed to 20 Torr NH
0 min firstly, and then it was out gassed and exposed to 30 Torr O ).
3
interacting with the 16O
2
and 18
O
2
over H
2
-
2
O
3
3
for
3
2
O (2154 cm 1) with a band at 1529 cmꢀ1; this band may have
O, and the peak at
around 1529 cm was near the reported NO stretching band of
HNO species [7,8,24]. Alternatively, –HNO could be responsible
ꢀ
N
2
been an intermediate in the formation of N
2
ꢀ1
–
for two bands. The adsorbed –HNO species have been proposed
to be responsible for a N@O stretching band in the 1550–
ꢀ
ꢀ
1
1
1
1
400 cm
400 cm
range, and for a –NH bending band in the 1450–
ꢀ1
region [7]. Since the two bands (1529, 1456 cm
)
appear at the same time, this explanation is likely.
To help identify the –HNO species, the experiments were also
1
8
16
conducted with
FTIR spectra obtained after either
the adsorbed NH on the H -pretreated Ag/Al
00 °C. The band near 1529 cm after exposing to
O
2
and
2
O using FTIR. Fig. 9 illustrates the
18
16
O
2
or
O
2
interacting with
catalyst at
3
2
2 3
O
ꢀ1
16
1
O
2
(Fig. 9)
after replacing with the
2
(Fig. 9), which implies that this band is due to a species con-
Fig. 10. FTIR spectra of the adsorbed species arising from NO + O2 (500 ppm NO, O2
ꢀ1
was red-shifted to around 1490 cm
10 vol.%) adsorption over the H
2 2 3
-pretreated 10 wt.% Ag/Al O at 160 °C (a) and after
1
8
successive purging with N2 at 160 °C (b).
O
taining oxygen.
This implies that –NH may be converted to –HNO ad-species in
the presence of O and that these ad-species must be the interme-
would decrease. According to our results (Fig. 10b), the band at
diates in the formation of N
2 2
and N O at low temperatures. At high-
ꢀ1
ꢀ1
1456 cm
2
decreased significantly after purging with N , while
er temperatures (>140 °C), the bands of –HNO (1529 cm ) and
ꢀ1
ꢀ1
the band at 1302 cm did not vary in intensity. Therefore, the
band at 1456 cm
vated surface nitrate ad-species on Ag/Al
adsorbed N
bands of nitrates increased. This result indicates that the reaction
route of NH oxidation over Ag/Al is different in different tem-
perature regions.
2
O (2154 cm ) vanished synchronously, while the
ꢀ1
should be assigned to adsorbed water-sol-
. In addition, it could
2 3
O
3
2 3
O
ꢀ1
be noted that this band at 1456 cm was in the same region as
the –NH band. As shown in Fig. 7, –NH appeared at a lower tem-
perature (80 °C) after NH
surface nitrate ad-species were visible at a higher temperature
160 °C) going with other nitrate ad-species; therefore, they
should be assigned to different ad-species at different tempera-
3
activation, while the water-solvated
3
2
.5. FTIR study of NO + O adsorption
(
FTIR spectra of the adsorption of NO in the presence of O
2
on
ꢀ1
the H -pretreated 10 wt.% Ag/Al at 160 °C were observed to
investigate the nitrate ad-species on the catalyst, and the results
are shown in Fig. 10a. The bands at 1614, 1579, 1545, 1456, and
2
2 3
O
tures. The band at 1302 cm was also close to that of bidentate
nitrate on Ag/Al [32,33]; thus we are inclined to assign this
2 3
O
band to bidentate nitrate.
ꢀ
1
ꢀ1
1
302 cm were detected. In addition, a weak band at 1750 cm
3
According to our FTIR spectra results on NH oxidation (Figs. 6
was also seen. Based on the previous studies of the NO + O
2
reac-
and 7), the bands assignable to nitrate ad-species (1614, 1579,
ꢀ1
tion on Al
2
O
3
or Ag/Al
2
O
3
catalysts, these peaks were tentatively
1545, 1456, and 1302 cm ) appeared in the same regions as
NO + O ad-species. This result indicates that adsorbed NH could
be oxidized to form NO and adsorbed as nitrate ad-species on
the catalysts in the presence of O
ꢀ1
assigned to adsorbed
N
2
O
4
(1750 cm
)
and monodentate
2
3
ꢀ
1
ꢀ1
ꢀ1
(
1545 cm ), bidentate (1579 cm ), and bridging (1614 cm ) ni-
ꢀ1
trates [26,28,31–34]. The bands at 1456 and 1302 cm
were
2
.
ꢀ1
ꢀ1
close to the IR bands at 1411 and 1302 cm
water-solvated surface nitrates on Al [31]. Since the adsorbed
water could be removed from the surface by purging with dry N
at 160 °C, band intensity of the water-solvated surface nitrates
for adsorbed
It could be noted that a band (1614 cm ) of bridging nitrates
was located at the same position as the asymmetric deformation
2 3
O
3
modes of NH coordinated on the Lewis acid sites, which may ex-
2
plain the appearance of this peak at higher temperatures while the

2
4
L. Zhang, H. He / Journal of Catalysis 268 (2009) 18–25
the bands assignable to adsorbed N
served in the absence of O . Other studies have found that in the
absence of O , activated –NH could also interact with lattice oxy-
gen (O ) to produce a small quantity of N O [7,8,14]. Our results
Figs. 4 and 5) demonstrate that in the absence of O , NH can
and –NH intermediates and produce
O (2160, 2154 cmꢀ1) were ob-
2
2
2
2ꢀ
2
(
2
3
dehydrogenate to form –NH
2
some products; however, this kind of reaction would not be sus-
tainable under O
in detail.
2
-free conditions and will not be discussed here
Based on the in situ DRIFTS results of NH
3
oxidation (Fig. 6) on
coordinated on
the surface of fresh Ag/Al , the bands of NH
2
O
3
3
Brønsted acid sites disappeared at 140 °C, while the bands coordi-
nated on Lewis acid sites decreased gradually at higher tempera-
tures. However, our NH
results showed that over the fresh catalyst, the adsorbed NH
hardly be oxidized at low temperatures (<140 °C). Therefore, the
NH coordinated on Brønsted acid sites had no influence on the
activity; only the NH coordinated on Lewis acid sites contributed
to this reaction activity. A similar result was found on the H -pre-
treated Ag/Al (Fig. 7). These results indicate that Brønsted acid
sites are not necessary for NH oxidation, in agreement with the
previous findings in the literature [7,8,14].
3
-TPO (Fig. 1) and in situ DRIFTS (Fig. 6)
3
could
3
3
2
Fig. 11. FTIR spectra of the surface species arising from the interaction of NH
500 ppm) with the in situ-formed NO on the H -pretreated 10 wt.% Ag/Al at
60 °C.
3
2 3
O
(
1
x
2
2 3
O
3
other bands (1232, 1244 cm 1) of NH
ꢀ
coordinated on the Lewis
3 2
4.2. Interaction of NH with O
3
4
.2.1. Interaction of NH
It has been (Figs. 7 and 8) demonstrated that the reaction path-
way at low temperatures (<140 °C) over H -pretreated Ag/Al is
totally different from the route at temperatures above 140 °C. Oxy-
gen atom has been found to be chemisorbed on the surface of H
pretreated Ag/Al , and it is more active than the gaseous O or
3 2
with O at low temperatures (<140 °C)
acid site disappeared (Figs. 6 and 7).
2
2 3
O
3
3 x
.6. Interaction of NH with the in situ-formed NO
2
-
To elucidate the reaction pathway, the interaction of NH
the in situ-formed NO species was investigated as follows: first,
NO ad-species were formed in situ on the H -pretreated Ag/
Al by exposing the catalyst to NH and O at 160 °C for 1 h. Then
the sample was purged with N at this temperature for 0.5 h. Final-
ly, 500 ppm of NH was introduced when the in situ DRIFTS spectra
were recorded. As shown in Fig. 11, the bands attributable to ni-
trate ad-species (1614, 1579, 1545, 1456, 1302 cm ) were ob-
served after exposing the Ag/Al to NH and O at 160 °C for a
brief period. Following NH inflow for 1 min, the monodentate ni-
trate band (1545 cm ) vanished, indicating that NH reacted with
the NO ad-species quickly. However, the intensity of the bands
1579, 1302 cm ) attributable to bidentate nitrates only de-
creased slightly during this period. Therefore, we conclude that
3
with
2
O
3
2
x
2
ꢀ
lattice oxygen (O ). At low temperatures (<140 °C), O reacts more
favorably with–NH to form –HNO intermediates (Fig. 8). The –NH
and –HNO are the key intermediates of the –NH mechanism. Za-
wadzki [1] previously proposed the –NH mechanism, but provided
x
2
2
O
3
3
2
2
3
no evidence of the –NH and –HNO intermediates or O
tion. Since both the O chemisorption and the intermediates of –
NH and –HNO were detected in our experiments, we are inclined
2
chemisorp-
ꢀ1
2
2
O
3
3
2
to attribute the low temperature reaction pathway of NH
3
oxida-
3
ꢀ1
tion over Ag/Al O to the –NH mechanism. The main reaction
2 3
3
routes at low temperatures (<140 °C) are as follows:
x
ꢀ1
(
O ! 2O
ð4Þ
ð5Þ
ð6Þ
ð7Þ
2
NH þ O ! HNO
NO
NH
x
adsorbed as monodentate nitrate reacted more favorably with
compared to the other nitrate ad-species. This result clearly
NH þ HNO ! N
2
þ H
2
O
x
HNO þ HNO ! N
2
O þ H
2
O
indicates that the SCO of NH
temperature follows an iSCR mechanism, which involves the oxi-
dation of a significant percentage of NH into NO species, along
with adsorbed NO species interacting with NH and being reduced
to N with N O as a by-product. The in situ-formed NO is the inter-
mediate of this iSCR mechanism.
The bands assignable to NH ad-species (1697, 1614, 1478, and
following NH inflow, as shown
in Fig. 11. Their intensity increased with the exposure time. How-
3 2 2 3
on the H -pretreated Ag/Al O at this
3
x
4
.2.2. Interaction of NH
At higher temperatures (>140 °C), NH
treated Ag/Al does not follow the –NH mechanism (Figs. 7
and 8) mentioned above. Under this condition (>140 °C), the inter-
action of NH with O caused the formation of NO ad-species on
the Ag/Al , and the in situ-formed NO species could react
quickly with NH following NH inflow (Fig. 11). Ramis et al. [14]
proposed an SCR route; i.e., in the presence of NO, the –NH would
react with NO to produce N , and –NH would react with NO to pro-
duce N O as a by-product. Zawadzki [1] also suggested that –NH
would react with molecular O to produce NO at higher tempera-
tures, but the author did not propose the in situ reaction between
the NO and –NH . Since the key intermediate (NO ) of the SCR
routes proposed by Ramis et al. [8,14,35–38] was detected in our
experiment (Figs. 6 and 7), and the NH could rapidly react with
the in situ-formed NO ad-species (Fig. 11), we are inclined to ac-
cept that the NH oxidation over Ag/Al at higher temperatures
3
with O
2
at high temperatures (>140 °C)
x
x
3
oxidation over H -pre-
2
2
2
2 3
O
3
ꢀ1
3
2
x
1
398 cm ) appeared on Ag/Al
2
O
3
3
2
O
3
x
ꢀ1
3
3
ever, the bands at 1529 and 1456 cm assignable to –HNO species
increased at the same time. Together with our results mentioned
above, this indicates that the adsorbed –NH might have interacted
with the previously adsorbed surface O to form –HNO species un-
der this condition.
2
2
2
2
x
x
4
. Discussion
3
4
.1. NH adsorption
3
x
3
2 3
O
As shown in Figs. 4 and 5, with the adsorption of NH
3
and an in-
(>140 °C) follows the iSCR mechanism. The reaction pathways are
crease in temperature,–NH and –NH intermediates appeared and
2
as follows:

L. Zhang, H. He / Journal of Catalysis 268 (2009) 18–25
25
NH þ O
NH
þ NO ! N
NH þ NO ! N
H þ OH ! H
A possible role of molecular O
has also been proposed in the literature [1,8] but may require high-
er temperatures compared to the interaction of NH with oxygen
atom, which is in agreement with our above-mentioned results.
We found that O could also be adsorbed dissociatively to form
surface oxygen atom on H -pretreated Ag/Al at higher temper-
2
! NO þ OH
þ H
O þ H
ð8Þ
product. At temperatures above 140 °C, the SCO of NH3 follows
an iSCR mechanism. That is, the –NH mainly reacts with molecular
O to form NO. Then, the in situ-formed NO interacts with the NH
2 x
2 2
and is reduced to N , with N O as a by-product.
2
2
2
O
ð9Þ
2
ð10Þ
ð11Þ
2
O
Acknowledgments
2
3 x
in NH oxidation to produce NO
This work was financially supported by the Chinese Academy of
Sciences (KZCX1-YW-06-04) and the National Natural Science
Foundation of China (10735090).
3
2
2
2 3
O
atures (160 °C; Fig. 2), and the coverage of active oxygen species
increases with an increase of temperature up to160 °C. Fig. 8 shows
Appendix A. Supplementary material
that the adsorbed NH
3
could be quickly activated to form –NH
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2009.08.011.
intermediates via reaction (3) in the presence of oxygen atom.
However, at higher temperatures (>140 °C), the –NH mainly inter-
acted with the oxygen species to form NO intermediates (Fig. 8).
This demonstrates that the increase of the ratio (O/NH) favors
the formation of NO at higher temperatures. We could infer that
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. Conclusions
Chemisorbed oxygen atom enhances the low temperature
(
<140 °C) activity of NH
3
oxidation over Ag/Al
mainly interacts with adsorbed NH
2
O
3
; in contrast, gas
at temperatures
phase O
2
3
above 140 °C. The pathway of NH
low temperatures (<140 °C) differs from that at temperatures
above 140 °C. At low temperatures (<140 °C), NH oxidation fol-
lows the –NH mechanism. That is, the adsorbed NH first reacts
with oxygen atom to form a –NH intermediate. Then the –NH
interacts with the chemisorbed oxygen atom to form a –HNO inter-
3 2 3
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mediate. The interaction of –NH and –HNO would produce N
H
2
and
2
O directly, while two –HNO would interact to form a N O by-
2