Intermediate species and reaction pathways for the oxidation of ammonia on powdered catalysts

Article abstract of DOI:10.1006/jcat.2000.3154

Ammonia oxidation reaction pathways on high surface area silver powder were studied by TPD, temperature programmed reaction FT-Raman, and transient as well as steady-state ammonia oxidation experiments. NO was the main reaction intermediate yielding N

Full text of DOI:10.1006/jcat.2000.3154

Journal of Catalysis 199, 107–114 (2001)
doi:10.1006/jcat.2000.3154, available online at http://www.idealibrary.com on
Intermediate Species and Reaction Pathways for the Oxidation
of Ammonia on Powdered Catalysts
Lu Gang, B. G. Anderson, J. van Grondelle, and R. A. van Santen
Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University
of Technology, P.O. Box 513, 5600 MB, The Netherlands
Received September 12, 2000; revised December 4, 2000; accepted December 15, 2000
droxylamine (NH2OH) was the first step. This NH2OH is
Ammonia oxidation reaction pathways on high surface area converted to HNO in a next reaction step. An imide mech-
silver powder have been studied by TPD, temperature programmed anism in which the first step yields imide (NH) was pos-
reaction FT-Raman, and transient as well as steady-state ammonia
oxidation experiments. NO was found to be the main reaction in-
termediate yielding N2O as well as N2. NO could be formed even
at room temperature and when oxidized to NOx became adsorbed
tulated by Raschig (7) and Zawadzki (8). Based on the
data of secondary ion mass spectrometry (SIMS) Fogel
et al. concluded that the intermediate species HNO,
NH2OH, HNO2, and N2O were not formed during ammo-
to the silver surface, thus blocking the active sites for oxygen dis-
�
4
nia oxidation at partial pressures of 10 Torr (9). The only
intermediate specie was NO. The formation of N2 was pro-
posed to be a result of the NO reacting with NH3.
sociation. The dissociation of oxygen is thus believed to be the
rate-controlling step for ammonia oxidation. The selectivity for N2,
N2O, and NO is mainly determined by the surface oxygen coverage
and by reaction temperature. The adsorbed NOx and N2Ox species
New insights into the reaction mechanism of ammonia
are actually inhibitors for ammonia oxidation but these adsorbed oxidation were obtained from surface science experiments
species lower the surface oxygen coverage. Hence the selectivity for (10–13). Single crystals of platinum were used in these ex-
nitrogen is improved with increasing amounts of these adsorbed periments and the reaction was carried out under high vac-
species.
�c 2001 Academic Press
uum conditions. The first step of ammonia oxidation was
Key Words: ammonia; oxidation; silver; silver catalysts; reaction believed to be the formation of NH species through oxyde-
x
mechanism.
hydrogenation of ammonia. Mieher and Ho (11) concluded
that NO and N2 were formed by the combination of nitro-
gen atomswith adsorbed oxygen atomsor with two nitrogen
atoms, respectively. However Bradley et al. (13) suggested
1
. INTRODUCTION
The selective catalytic oxidation of ammonia to nitrogen that NO was directly produced by reacting NHa with Oa.
and water (SCO process) has been proposed as an effective The formation of N2 resulted from the consecutive dissoci-
ammonia abatement process (1–3). Among the catalysts ation of NO. Since only NO and N2 were produced under
published in the open literature, noble metals such as Pt and the reaction conditions of these studies, the formation of
Ir are the most active catalysts at low temperatures. How- N2O is not discussed. The recent study of Van den Broek
ever the selectivity of these metal catalysts is not satisfying, and van Santen (14, 15) showed that NH and OH were the
especially under the condition of very high O2/NH3 ratios. main surface species adsorbed on Pt and Ir sponge catalysts
Silver was also found in our laboratory to be one of the most after ammonia oxidation. The reaction of NH and OH was
active ammonia oxidation catalysts but also showed a poor thought to be the rate-determining step under their reac-
selectivity to nitrogen, also forming N2O as a main prod- tion conditions. They argued that N2O was produced by NO
uct. In order to improve the selectivity of these catalysts it reacting with adsorbed N. The STM technique was used by
is essential to develop a clear understanding of the reaction the group of M. W. Roberts (16–18) to study ammonia ox-
pathways for ammonia oxidation on these catalysts.
idation on copper surfaces. They observed that at very low
The majority of mechanistic studies on ammonia oxida- temperatures ammonia could be oxidized to adsorbed NH2
tion have been done on platinum catalysts. Three reaction and NH species. At higher temperatures NO formation was
mechanisms have been proposed in earlier studies. The ni- also observed. On metal oxide catalysts a mechanism based
troxyl (HNO) mechanism suggests that the first step is the on the recombination of two NHx species giving rise to a
formation of a nitroxyl species that will further react to hydrazinium intermediate was also proposed (19–21).
give out N2, N2O, and NO (4). Bodenstein (5, 6) extended
Ammonia adsorption on Ag(110) has been studied
this mechanism by suggesting that the formation of hy- previously by TPD, TPRS, and EELS with and without
107
0021-9517/01 $35.00
Copyright �c 2001 by Academic Press
All rights of reproduction in any form reserved.

1
08
GANG ET AL.
coadsorption of molecular and atomic oxygen (22). It was
concluded that a diffusing nitrogen adatom was the reac-
tive intermediate in NO and N2 formation. On the basis
of a combination of XPS and VEEL spectra a dioxygen–
NH3 complex has been suggested to be a key intermediate
in the oxidation of ammonia on Ag(111) surfaces (35, 36).
No report has been published to date on the intermediate
species and reaction mechanisms on polycrystalline silver
at atmospheric pressure. In this study the ammonia reaction
pathways were studied by TPD, temperature programmed
reaction (TPR), FT-Raman, and pulsed ammonia reactions
on high surface area silver powder at high O2/NH3 ratios.
FIG. 1. Ammonia oxidation on silver powder at various temper-
atures. Reaction conditions: NH3 = 1000 ppm; O2 = 10 vol% ; total flow
rate = 50 Nml/min; catalyst weight = 0.1 g.
2
. EXPERIMENTAL
Polycrystalline silver was prepared from Ag2O
(
>99.99% ) powder. The powder was first reduced in
�
flowing H2/He overnight at 400 C and then oxidized in
flowing O2/He at 400 C for 2 h. This process was repeated
shows the results of NH3 conversion and products’ selectiv-
ity versus reaction temperatures. It can be seen that silver is
very active for ammonia oxidation and shows high conver-
�
three times to remove any carbon species and to maintain
a stable surface area. Catalytic activity measurements
were carried out in a quartz, fixed-bed reactor (4 mm in
internal diameter). The amount of silver powder catalyst
was about 0.2 g. Ammonia, oxygen, and helium flow rates
were controlled by mass flow meters. NH3, NO, and NO2
were analyzed by a ThIs Analytical Model 17C Chemilu-
minescence NH3 Analyzer, in which a high temperature
�
sion even below 160 C. At low ammonia conversion the N2
selectivity decreases with increasing temperature. At high
ammonia conversion the N2 selectivity increases with in-
�
creasing temperature. When the temperature is over 300 C
the N2 selectivity decreases again because of the formation
of NO.
converter oxidized NH3 to NOx by O2. Other substances 3.2. TPD and TPR of Ammonia on Reduced
such as N2O, N2, and H2O were analyzed by a quadruple
mass spectrometer (Baltzers OmniStar). The nitrogen
mass balance was calculated based on a combination of
these two analysis techniques. It attained a balance of
and on Oxygen-Covered Silver Powder
Figure 2 shows the NH3 TPD profile measured on com-
pletely reduced silver powder. Apparently a small amount
ofwater in the helium or in the ammonia can be adsorbed on
9
0 � 10% for all the measurements.
�
Agand isdesorbed at about 150 C. Trace ammonia also des-
NH3 TPD and TPR experiments were performed using a
orbs at this temperature. This is mostly from water-coupled
ammonia. This result shows that a clean silver surface can-
not adsorb ammonia above room temperature. This is in
accordance with the literature (22). Figure 3 shows the NH3
TPD profile measured on oxygen precovered silver powder.
fixed-bed flow reactor system equipped with a computer-
interfaced quadruple mass spectrometer. After adsorp-
tion of NH3 at room temperature the TPD or TPR data
were recorded by mass spectrometer while the tempera-
�
ture was increased from 50 to 500 C at a heating rate of
1
�
0 C/min. FT-Raman spectra were obtained with a Bruker
FRA 106 Raman Spectrometer equipped with an in situ
reaction chamber. The standard laser was an air-cooled,
diode-pumped Nd:YAG operating at 1.064 �m with a max-
imum laser power of 500 mW. Usually a laser power of
1
50 mW was used for measures under atmospheric condi-
tions. Detection was performed by a liquid-nitrogen-cooled
InGaAs detector. One hundred scans were co-added at a
�
1
resolution of 2 cm
.
3
. RESULTS
3
.1. NH3 Oxidation on Silver Powder
at High O2/NH3 Ratios
FIG. 2. NH3 TPD on reduced silver powder. Silver powder was re-
�
�
duced at 400 C for 2 h and then in NH3/He flow for 0.5 h at 50 C. TPD was
Ammonia oxidation at high O2/NH3 ratios was carried
out on silver powder at temperatures below 400 C. Figure 1 to 400 C at a rate of 10 C/min.
performed in He = 20 Nml/min flow. Temperature was increased from 50
�
�
�

AMMONIA OXIDATION REACTION PATHWAYS
109
FIG. 4. NH3 TPR profiles on oxygen-covered silver powder. Silver
pretreatment was as in Fig. 3. TPR performed in NH3/He = 20 Nml/min
FIG. 3. NH3 TPD on oxygen-covered silver powder. The reduced sil-
ver was first oxidized at 200 C for 10 min in flowing O2 (10% )/He followed
by NH3 adsorption and TPD as described in Fig. 2.
�
�
�
flow with NH3 concentration of 1000 ppm from 50 to 400 C at 10 C/min.
Figure 4 shows the NH3 temperature programmed reac-
There are two water production peaks at 150 and 280 C, tion (TPR) profiles on an oxygen precovered Ag powder.
�
�
respectively. The peak at 150 C is similar to the water des- The NH /He was flowing constantly over the silver powder
3
�
orption peak shown in Fig. 2. The peak at 280 C can be during the increase in temperature. With increasing temper-
caused by the reaction of adsorbed OH species according ature the rates of reactions [3] and [4] are greatly increased
to the following reactions:
and the surface N/O ratio increases accordingly. Now over
the entire temperature range the only product is nitrogen.
OH + OH → H2O + O
O + O → O2.
[1] There are four N production maxima at 120, 170, 240, and
2
�
3
00 C, respectively. The first one is apparently caused by
[2]
adsorbed surface atomic oxygen. The later two appear co-
incidentally after water production has reached a minimum.
It is possible that water inhibits the migration of subsurface
oxygen onto the surface. When water desorbs, more sub-
surface oxygen may become accessible for the ammonia
oxidation reaction. It also can be seen from the water pro-
The oxygen peak appears at the same position as the water
peak. This strongly suggests that reaction step [1] is rate-
limiting. There is still a trace of NH3 desorbed at 150 C. Sig-
�
�
nificantly more N2 is produced at a temperature of 120 C.
This N2 production must come from adsorbed nitroge-
nous species on the silver surface. On Ag(111) surfaces
a dioxygen–NH3 complex has been detected by XPS and
VEEL at 80 K which is suggested to be a key intermedi-
ate in the oxidation of ammonia (35). When temperature
is increased to 210 K the dioxygens are not present and
NH species appear. The study on Ag(110) surfaces for am-
monia oxidation shows that NH groups persist on the sur-
face at temperatures well into the water desorption peak at
�
duction profile that a new peak at about 200 C appears in
addition to the two already mentioned. This peak surely
is not the result of reaction [1]. It must originate from the
following reactions:
OH + NH3 → H2O + NH2
OH + NH2 → H2O + NH
OH + NH → H2O + N.
[5]
[6]
[7]
3
10K and possiblyto significantlyhigher temperatures(22).
Thus the adsorbed nitrogen species may be NHx produced
by oxygen abstraction of hydrogen from NH3:
The possibility of reactions [5]–[7] cannot be excluded at
low temperature but at least one of these reactions, most
[3] probably reaction [7], will probably occur only at higher
NH3 + O → NH2 + OH
NH2 + O → NH + OH.
�
temperatures(>150 C). There isalso a newfeature at about
[4]
�
3
00 C in the H2O profile which is in the same position as
the fourth N2 peak in the N2 profile. This may be caused
Besides N2 there is also some N2O produced. The maxi- by the ammonia oxidation by bulk dissolved oxygen that is
mum production temperature for N2O production is higher migrated onto the silver surface at high temperature.
than that of N2. This indicates that the selectivity of am-
monia oxidation depends on the surface N/O ratio. With
3
.3. Ammonia Pulse Reaction Studies
the production of N2 the amount of NHx decreases and
the O/N increases, leading to increased N2O production to
maximum.
To further clarifythe reaction mechanism, pulse reactions
were carried out at different temperatures. Pulses of NH3

1
10
GANG ET AL.
�
�
FIG. 7. TPD profiles after NH3 oxidation reaction at 200 C on sil-
ver powder. Reaction conditions: NH3 = 1000 ppm; O2 = 10% ; flow rate =
50 Nml/min; catalyst weight = 0.1 g. TPD was performed in flowing
FIG. 5. NH3 pulse reaction profiles on silver powder at 200 C. Silver
�
first reduced at 400 C in flowing H2 (10% )/He = 40 Nml/min for 2 h and
�
then in flowing O2 (10% )/He = 20 Nml/min at 200 C. Each pulse contained
�
�
He = 20 Nml/min from 200 to 550 C at 10 C/min.
5
�l NH3.
and O2 according to the following reactions (23, 24):
(
aica 0.22 �mol) were made into a flow of O2(10 vol% )/He.
It can be seen from Fig. 5 that during the first ammonia pulse
large amounts of N2O were produced simultaneously with
N2 production. During the following ammonia pulse the
amount of N2O production was greatly reduced while N2
production increased. When the reaction temperature was
NO + NO → N2O + O
NO + NO → N2 + O2
O + O → O2.
[8]
[9]
[10]
�
raised from 200 to 400 C the situation changed completely.
To assessthe importance ofNO adsorption on Agpowder
TPD experiments were carried out. When NO was coad-
sorbed with O2 on reduced silver powder at 200 C the TPD
Large amounts of NO were produced during the NH3 pulse
but the N2 selectivity changed little (see Fig. 6). These re-
sultsindicate that some adsorbed nitrogen specieswere pro-
duced during the first NH3 pulse that greatly changed the
reaction selectivity of the following NH3 pulse reaction.
�
profiles were exactly the same as those obtained after NH3
oxidation reaction (see Fig. 8). This strongly indicates that
the adsorbed intermediate nitrogen species are mainly NO-
induced species.
It also can be seen from Fig. 7 that there is some water
produced. This may indicate the presence of some OH or
NHx species but the quantity is small. The fact that there
is a trend to desorb N2, NO, N2O, and O2 at higher tem-
perature (>500 C) suggests the existence of more than one
NO-induced species. This species may be NO since it is the
most stable species on the silver surface.
When the reaction was carried out at low O2/NH3 ratios
3
.4. Reaction Intermediates
To clarify the nature of the adsorbed species TPD exper-
iments were done on a silver powder catalyst after reaction
under different conditions. Figure 7 shows the TPD profiles
measured on a Ag powder catalyst after reaction for 1 h in
�
�
excess oxygen at 200 C. It can be seen that the production
3
profile patterns for N2O, N2, and O2 are almost same. This
may indicate that these products originate from the same
species. It is well known that NO can react to give N2O, N2,
(
e.g.,1/1) the TPD profile after reaction was totally different
�
FIG. 8. TPD profiles after NO + O2 adsorption at 200 C for 5 min.
�
FIG. 6. NH3 pulse reaction profiles on silver powder at 400 C. Reac-
tion conditions were the same as those in Fig. 5.
NO = 1000 ppm; O2 = 10% ; NO/O2/He = 20 Nml/min. TPD performed in
� �
flowing He = 20 Nml/min from 200 to 550 C at 10 C/min.

AMMONIA OXIDATION REACTION PATHWAYS
111
�
2 3
FIG. 10. Raman spectra for NO, NO , and NH adsorbed on silver
FIG. 9. TPD profiles after NH3 oxidation at 200 C on silver powder.
�
powder. NO and NO2 adsorption at 50 C on reduced silver powder; NH3
Reaction conditions: NH3 = 1% ; O2 = 1% ; flow rate = 50 Nml/min; cata-
lyst weight = 0.1 g. TPD performed in flowing He = 20 Nml/min from 200
�
adsorption at 50 C on oxygen-covered silver powder.
�
�
to 550 C at 10 C/min.
�
�
NO which will consequently be oxidized to NO2 and NO3
species. The remaining peaks in Fig. 10, such as in the posi-
(
see Fig. 9). Much more water was produced and only N2
desorbs at low temperature. As most of the OH species can
� 1
tions of 615, 670, and 985 cm , are assigned to the different
�
be desorbed as H2O at 280 C, according to reaction [1],
oxygen species on the silver surface (29).
the comparatively large amount of water must come from
NHx species. Most probably N2 and H2O result from the
following reaction:
Raman measurements were carried out on silver powder
at different temperatures following in situ ammonia oxida-
tion to observe the changes of surface species. The resulting
spectra are shown in Fig. 11. Based on previously published
Raman data on the silver–oxygen interaction (28, 29) and
on Raman and IR measurements of adsorbed N2Ox(30–
NHx + NOx → N2 + OHx .
[11]
Thus, apparently there are more NHx species on the surface
when the O2/NH3 ratio decreases.
�
1
3
3), the peak at 245 cm is assigned to surface adsorbed
�
1
atomic oxygen and the peaks at 266 and 1540 cm are as-
signed to the N–N stretching Raman band of N2Ox. The
x can be 2, 3, 4, or 5. Unambiguous determination of x is
difficult since these N2Ox species often show intensities at
nearly the same positions in Raman and IR spectra. It can
be seen from Fig. 11 that the intensity of surface atomic
oxygen decreases with increasing temperature. The inten-
sity of the N2Ox peaks increased slightly as heating began
but stayed unchanged afterward. The NO2 peak intensity
3
.5. FT-Raman Spectroscopy
FT-Raman measurements were carried out on a silver
powder with preadsorbed NO, NO2, or NH3. For NO or
NO2 adsorption a completely reduced silver powder was
used and the adsorption temperature was 50 C. For NH3
adsorption the reduced silver powder was first oxidized in
O2 at 200 C for 1 h and then NH3 was adsorbed at 100 C to
increase the Raman signal. All of the measurements were
done at room temperature to avoid the heating effects. The
resulting spectra are shown in Fig. 10. Bao et al. have studied
NO adsorption on Ag(110) by Raman and XPS (25). They
concluded that NO was not stable on silver surface. NO
was quickly oxidized to NO2 and NO3, which were strongly
adsorbed on silver surface. The SERS spectra of NO2 ad-
sorbed on the silver powder were also reported in the liter-
�
�
�
�
1
ature (26, 27). Peaks at 815, 1045, and 1285 cm after ex-
posure of silver powder to several nitrogen dioxide pulses
added into the He gas stream were observed. They assigned
the peaks at 815 and 1285 cm to adsorbed NO2 and the
peak at 1045 cm to adsorbed NO on silver surface. Com-
�
1
�
�
1
�
3
paring the Raman spectrum of NH3 adsorption with NO
and NO2 adsorption spectra in Fig. 10, it is obvious that a
spectrum similar to that of NO and NO2 adsorption appears
upon adsorption of NH3 on oxygen-covered silver powder.
FIG. 11. Raman spectra measured at RT after ammonia oxidation
on silver powder at different temperatures. Reaction conditions: NH3 =
This strongly indicates that NH3 can be quickly oxidized to 1000 ppm; O2 = 10% ; NH3/O2/He = 50 Nml/min.

1
12
GANG ET AL.
�
decreased but NO3 adsorption signal stayed unchanged as ratios (13, 22). On Pt and Ag polycrystals, however, large
the temperature increased. When the temperature was in- amounts of N2O were produced at low temperatures. At
�
creased to 300 C the signals of N2Ox disappeared and the higher temperatures NO, instead of N2O, was produced.
surface atomic oxygen adsorption signal recovered again.
The possible N2O formation pathways proposed in the lit-
erature are as follows (4, 15, 23, 34):
4
. DISCUSSION
N + NO → N2O
[17]
[18]
[19]
NH3 TPD and TPR results show that NH3 does not ad-
sorb on clean silver surface above room temperature. On
an oxygen precovered silver surface NH3 dissociatively ad-
sorbs only as NHx with one or two hydrogen abstracted by
oxygen according to reactions [3] and [4]. This is considered
to be the initial step for ammonia oxidation. Reactions [3]
and [4] can be enhanced by either increasing the temper-
ature or by increasing the oxygen coverage on the silver
surface (17, 18).
NO + NO → N2O + O
HNO + HNO → N O + H O
2
2
HNO3 + NH3 → NH4NO3 → N2O + 2H2O. [20]
Reactions [19] and [20] involve the intermediate HNO
and HNO3 species that were not observed on the silver
surface. Thus, these two reactions are unlikely to occur on
silver catalysts. The difference between reactions [17] and
NHx species can consecutively react with either O or OH
according to reaction [7] to give N:
[
18] is that the dissociation of NO is required in order that
reaction [18] can proceed. This can only happen at very low
surface oxygen coverage. The chemisorption of nitric oxide
at a Ag(111) surface has been reported to form N2O at 80 K.
However at 295 K this surface is unreactive to NO(g) under
the same condition (37). Our experiments on silver pow-
[13] der also show that N2O can be produced only on reduced
silver at the initial stage of NO adsorption. When NO is in-
NHx + O → N + OHx .
[12]
The N is very reactive and gives NO or N2 according to
N + O → NO
N + N → N2.
[14]
troduced continuously to a reduced silver powder catalyst
at temperatures from 50 to 400 C no N2O is produced ex-
�
When there is preadsorbed oxygen on the silver surface
the probability of reaction [14] occurring is low due to the
high rate of reaction [13]. The Raman experimental results
clearly showed that NOx species appeared even at room
temperature upon ammonia adsorption. There was no ev-
idence for the formation of molecular nitrogen at room
temperature on silver powder. The fact that reactions [12]
and [13] can take place at room temperature indicates that
reaction [12] is not the rate-controlling step for gas phase
NO formation as suggested by many authors (11, 15, 21).
Actually the NO formed cannot desorb at low temperatures
and quickly reacts with other adsorbed oxygen to produce
NOx which will strongly adsorb on the silver surface and
block the active sites for oxygen dissociation. Since reac-
tion [14] is not significant at high O2/NH3 ratio conditions,
the formation of nitrogen may occur as follows:
cept for at the initial time. Hence reaction [17] is the most
likely reaction candidate for the formation of N2O. To give
further evidence for this theory, the SCR reaction was also
carried out on reduced silver powder. The results are shown
in Figs. 12 and 13. It can be seen from Fig. 12 that the pro-
duction of N2O is not significant when NO reacts with NH3
alone on the silver catalyst. In the presence of oxygen NO
can react quickly with NH3 to give rise to N2O and N2 (see
Fig. 13). These experimentsclearlyindicate that N2 and N2O
are not formed mainly from NO alone. NO cannot react di-
rectly with NH3 on silver either. The role of O2 is actually
NHx + NO → N2 + OHx
NO + NO → N2 + O2.
[15]
[16]
Reaction [16] actually involves the same NO dissociation
and atomic nitrogen combination as in reaction [14]. This
occurs only at low temperatures on reduced silver and oxy-
gen is a strong inhibitor of this reaction.
The mechanism of N2O formation from ammonia is not
very clear. In low pressure ammonia oxidation studies on
Pt and Ag single crystals, no N2O formation was found over
FIG. 12. NO + NH3 SCR reaction on reduced silver powder. Re-
action conditions: NH3 = 194 ppm; NO = 74 ppm; catalyst weight =
a large temperature interval and a large range of NH3/O2 0.1 g; NH3/NO/He = 100 Nml/min.

AMMONIA OXIDATION REACTION PATHWAYS
113
thus the oxygen dissociation rate which is determined by
the rate of NO removal. The selectivities to N2 and to N2O
are also controlled by the relative rates of reactions [15]
and [17]. Actually, the relative rates of these two reactions
are controlled by the surface oxygen coverage. At high sur-
face oxygen coverage the reaction from NHx to NO and
N is very fast so that the surface NHx concentration de-
creases and the rate of reaction [15] decreases accordingly.
The selectivity is thus mainly toward N2O. On the contrary
the reaction becomes more selective to nitrogen when the
surface oxygen coverage decreases.
As the surface NO2, NO3, and N2Ox species block some
of the active sites for oxygen dissociation, they become in-
hibitors for ammonia oxidation. However the adsorption
of these species on the silver surface decreases the oxy-
gen coverage. So the selectivity to nitrogen increases with
increasing amounts of these adsorbed species. The Raman
spectra for ammonia oxidation clearly showed that atomic
surface oxygen intensity decreased with increasing temper-
FIG. 13. NH3 + NO + O2 SCR reaction on reduced silver powder.
Reaction conditions: NH3 = 168 ppm; NO = 74 ppm; O2 = 10% ; catalyst
weight = 0.1 g; NH3/NO/O2/He = 100 Nml/min.
to produce adsorbed NHx and N species that can then react
�
�
ature from 50 C up to 300 C. This is due to the fact that the
readily with NO to produce N2 and N2O.
�
amount of adsorbed N2Ox species increases with increasing
Apparently NO2, NO , and N2Ox species are produced
3
�
temperature. At a temperature higher than 350 C N2Ox de-
by NO reacted with O or O2:
composes and is removed from the surface so the oxygen
NO + O → NO2
NO + 2O → NO3
NO + NOx → N2Ox .
[21] coverage again increases. The steady-state tests for ammo-
�
nia oxidation on silver powder at different temperatures
[22]
(
Fig. 1) also show that the nitrogen selectivity increases with
�
[23] increasing temperature up to 300 C. With further increase
of temperature, the selectivity to nitrogen decreases due to
Thus the overall ammonia reaction pathway on silver
powder in an oxygen-rich atmosphere can be described
schematically as in Fig. 14. It can be seen clearly from this
scheme that NO is the key intermediate. Even at room tem-
perature the NO can be quickly produced. Since NO can-
not desorb at low temperature, it blocks the active sites
for oxygen dissociation. So the reaction can continue only
when the adsorbed NO is removed. At moderate tempera-
the increasing rate of NO desorption. Ammonia pulse reac-
tions again indicated the effect of adsorbed NOx and N2Ox
species on the product selectivity. After the first ammonia
�
pulse at 200 C some adsorbed NOx and N2Ox species were
produced that partially blocked the surface and decreased
the surface oxygen coverage. In subsequent ammonia pulse
reactionsthe selectivityto nitrogen wasimproved due to the
�
lower surface O/N ratio. At higher temperature (400 C) the
�
tures (below 300 C) the NO can only be removed either as
desorption rate of NO became rapid so that the NO was
formed instead of N2O. Since little NOx can be adsorbed
on the silver surface at this temperature there is almost
no selectivity difference among three consecutive ammo-
nia pulses.
N2O or N2 through surface reaction [17] and reaction [15],
respectively. At even higher temperatures NO can directly
desorb as one of the products. The rate-controlling step is
5
. CONCLUSIONS
Silver is a very active catalyst for ammonia oxidation. At
�
low temperatures (below 300 C) mainly N2 and N2O are
produced. At higher temperatures NO instead of N2O be-
comes one of the products. NO is an important reaction
intermediate for this reaction. Even at room temperature
the NO can be produced and adsorbed on the silver sur-
face in the form of NOx. Since NO cannot desorb at low
temperature it blocks the active sites for oxygen dissocia-
tion. The dissociation of oxygen is thus believed to be the
rate-controlling step for ammonia oxidation. The selectivity
FIG. 14. Scheme illustrating the pathways of NH3–O2 reactions.

1
14
GANG ET AL.
to N2, N2O, and NO is determined by surface oxygen 13. Bradley, J. M., Hopkinson, A., and King, D. A., J. Phys. Chem. 99,
1
7,032 (1995).
coverage and temperature. Low surface oxygen coverage
favorsnitrogen formation. Adsorbed NOx and N2Ox species
are actually inhibitors for ammonia oxidation but they also
lower the surface oxygen coverage. Hence, the selectivity
1
1
4. Van den Broek, A. C. M., Van Grondelle J., and Van Santen, R. A.,
J. Catal. 185(2), 297 (1999).
5. Van den Broek, A. C. M., Ph.D., thesis, Eindhoven University of Tech-
nology, (1998).
to nitrogen is improved by increasing the amount of these 16. Carley, A. F., Davies, P. R., and Roberts, M. W., Chem. Commun. 17,
1
793 (1998).
adsorbed species on silver surface.
1
1
7. Carley, A. F., Davies, P. R., Kulkarni, G. U., and Roberts, M. W., Catal.
Lett. 58, 97 (1999).
8. Afsin, B., Davies, P. R., Pashusky, A., Roberts, M. W., and Vincent,
D., Surf. Sci. 284, 109 (1993).
Generally speaking, silver alone, like Pt, is not a good
catalyst for selective ammonia oxidation to nitrogen be-
cause too much N2O is produced. It follows from the above
conclusions that blocking of the sites for oxygen dissocia- 19. Amores, J. M. G., Escribano, V. S., Ramis, G., and Busca, G., Appl.
Catal. B 13, 45 (1997).
0. Trombetta, M., Ramis, G., Busca, G., Montanari, B., and Vaccari, A.,
Langmuir 13, 4628 (1997).
tion is an effective way to improve the nitrogen selectivity,
but also would result in a loss of catalyst activity.
2
2
2
2
2
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4. Otto, K., Shelef, M., and Kummer, J. T., J. Phys. Chem. 74(13), 2690
ACKNOWLEDGMENTS
The authors gratefully acknowledge the support of the Netherlands
Technology Foundation (STW) under the auspices of the Netherlands
Organization for Scientific Research (NWO).
(
1970).
2
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5. Bao, X., Wild, U., Muhler, M., Pettinger, B., Schl o¨ gl, R., and Ertl, G.,
Surf. Sci. 425, 224 (1999).
6. Von Raben, K. U., Dorain, P. B., Chen, T. T., and Chang, R. K., Chem.
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