On the mechanism of the selective catalytic reduction of NO with higher hydrocarbons over a silver/alumina catalyst

Article abstract of DOI:10.1016/j.jcat.2004.07.026

SCR of NO with hydrocarbons has received much attention as one of the most promising and straightforward methods for reducing NO_x emissions under conditions of excess oxygen. The possibility of forming nitrogen in the gas phase by reaction of activated forms of NO_x with amines and NH₃ as well as with other organic intermediates, which could be converted to amines and/or ammonia, was studied. Nitrohexane, hexylisocyanate, heptanenitrile, hexylamine, and NH₃ were used as model compounds. Both NH₃ and hexylamine reacted in the gas phase with activated NO_x species to produce N₂. Thus, it is possible to homogeneously convert NO to N₂ by reaction with NH₃ in excess oxygen at temperatures far below those reported for selective noncatalytic reduction of NO. Nitrohexane was transformed to NH₃ in the presence of O₂ over the Ag/alumina already at 250°C and the amounts of NH₃ produced increased by the addition of H₂O. The role of hydrogen on the hydrocarbon-SCR process involved several functions. Improved oxidation of all components was obtained in the presence of H₂. In the oxidation of octane, addition of hydrogen increased the conversion from 41 to 73% at 350°C. Consequently, the improved oxidation rate of the hydrocarbon also resulted in faster formation of different oxygenates, which reacted and formed N-containing species. Hydrogen activated NO over the Ag/alumina catalyst for further reactions with the produced amines and NH₃ or with other N-containing species in the gas phase.

Full text of DOI:10.1016/j.jcat.2004.07.026

Journal of Catalysis 227 (2004) 328–343
www.elsevier.com/locate/jcat
On the mechanism of the selective catalytic reduction of NO with higher
hydrocarbons over a silver/alumina catalyst
Kari Eränen ∗, Fredrik Klingstedt, Kalle Arve, Lars-Eric Lindfors, Dmitry Yu. Murzin
Laboratory of Industrial Chemistry, Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, FIN-20500 Åbo/Turku, Finland
Received 14 April 2004; revised 15 June 2004; accepted 22 July 2004
Available online 26 August 2004
Abstract
The possibility of forming nitrogen in the gas phase by reaction of activated forms of NO with amines and ammonia as well as with
x
other organic intermediates, which can be converted to amines and/or ammonia, was investigated. The activation of NO was carried out
by passing NO together with oxygen and small amounts of hydrogen over an Ag/alumina catalyst. A special T-shaped reactor was used to
produce activated forms of NO and to feed model compounds of proposed intermediates to the gas phase after the catalyst. Nitrohexane,
x
hexylisocyanate, heptanenitrile, hexylamine, and ammonia were used as model compounds. The T-reactor tests showed that both ammonia
and hexylamine react in the gas phase with activated NO species producing N . Hydrogen was shown to have at least two main functions.
x
2
First, hydrogen contributed to improved oxidation of all involved species, resulting in faster production of key intermediates. Secondly,
hydrogen assisted in the formation of activated NO species for the gas-phase reactions. Nitrohexane was transformed to NH in the presence
x
3
◦
of O over the Ag/alumina already at 250 C and the amounts of NH produced increased in the presence of H O. Hexylisocyante was
hydrolyzed to amine and ammonia at 250 C over the catalyst in the presence of O but only to ammonia at 400 C. At 250 C the conversion
2
3
2
◦
◦
◦
2
to amine and ammonia was almost doubled by the addition of H O. Heptanenitrile was quite stable and only small amounts of NH were
2
3
◦
observed at 400 C; however, nitriles may react directly with activated forms of NO forming N . The importance of gas-phase reactions as
x
2
a part of the HC-SCR mechanism is emphasized. R-NO , R-NCO, and R-CN are intermediates for the formation of amines and ammonia,
2
which are consumed both on the surface of the catalyst and in the gas phase behind the catalyst bed by reactions with activated NO species.
x
2004 Elsevier Inc. All rights reserved.
Keywords: HC-SCR; Ag/alumina catalyst; Reaction mechanism; Gas-phase reaction; Hydrogen effect; GC-MS
1. Introduction
volved, the complete reaction mechanism is not yet fully
understood. In the review by Burch et al. [4], the main ob-
servations obtained so far have been discussed. It is gen-
erally considered that the first step of HC-SCR over oxide
catalysts involves the formation of strongly bound nitrites
and nitrates. In addition, activation of the hydrocarbon takes
place by partial oxidation to oxygenated compounds, such
as acetates. Both the nitrite and nitrate species as well as ac-
etates have been detected in FTIR studies by several research
groups. The ad-NOx species are supposed to react with the
adsorbed and partly oxidized hydrocarbon species on the
catalyst surface to yield organo-nitrogen species. Depend-
ing on the reaction conditions, the organo-nitrogen species
can be transformed to highly reactive intermediates such as
isocyanates, nitriles, amines, and oximes. These intermedi-
Selective catalytic reduction of NO with hydrocarbons
(HC-SCR) has received much attention as one of the most
promising and straightforward methods for reducing NOx
emissions under conditions of excess oxygen. Since the early
work of Held et al. [1] and Iwamoto et al. [2] a large num-
ber of different materials have been proposed and tested for
HC-SCR. Among these, Ag/alumina has shown high activity
both in laboratory and in full-scale tests [3].
Several proposals to describe the reaction mechanism
have been made. Because of the complicated reactions in-
*
Corresponding author. Fax: +358 2 215 4479.
E-mail address: Kari.Eranen@abo.fi (K. Eränen).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.07.026
2004 Elsevier Inc. All rights reserved.

K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
329
ates are in the last step proposed to be directly involved in
2. Experimental
reactions with NO or NO2 and/or R-ONO leading to forma-
tion of nitrogen [4]. However, it is possible that this kind
of complex organic-inorganic reaction chain could involve
a more complex mixture of intermediates, by-products, end
products, and spectator species than noted above.
Recently, Satokawa [5] and Shibata et al. [6] showed that
hydrogen enhances the HC-SCR reaction significantly. They
found that hydrogen increased the concentration of acetate
but decreased the concentration of nitrates on the catalyst
surface during reaction conditions. Moreover, the formation
rates of nitrates and acetates also increased.
2.1. Catalyst preparation
The Ag/alumina catalyst was prepared by impregnation
of commercial alumina beads (A 201, LaRoche Chemicals
Inc.) with a silver nitrate solution according to the procedure
described in Ref. [3]. The catalyst was dried at room temper-
ature and at 100 ◦C before calcination at 550 ◦C for 3 h. The
silver content of the catalyst was approximately 2 wt%. Be-
fore testing, the catalyst was crushed and sieved to fractions
between 250 and 500 µm.
Typically, most of the studies dealing with the reaction
mechanism have been restricted to surface phenomena. In
our previous study [7], it was shown that HC-SCR over
Ag/alumina does not only take place on the surface of the
catalyst but continues in the gas phase leading to the final
products, i.e., nitrogen, carbon dioxide, and water. The evi-
dence for the gas-phase reaction was based on activity tests,
where the Ag/alumina was combined with an oxidation cata-
lyst to remove CO, which is produced over the silver catalyst
during the de-NOx process. When the Pt-oxidation catalyst
was placed immediately after the Ag/alumina, a significant
drop in the NO to N2 conversion was observed in compari-
son with the single Ag/alumina bed. As the distance between
the two catalysts was extended, the conversion of NO to N2
improved to levels close to those recorded over the single
Ag/alumina bed.
In the work of Lukyanov et al. [8] and Vassallo et al. [9]
the role of gas-phase reactions in the mechanism of HC-SCR
was studied. Lukyanov et al. proposed that the SCR reaction
over Co-ZSM-5 and H-ZSM-5 involves free radical chem-
istry and may partly be homogeneous. NO or rather NO2
was acting as an effective oxygen carrier agent for the ini-
tiation of radical formation. The catalyst was discussed to
be needed for the coupling of the N–N bond by a combi-
nation of the formed organic nitro compounds and NO or
NO2. Vasallo et al. suggested that NO acted as a homoge-
neous catalyst in the oxidation of hydrocarbons over Cu-,
Co-, and H-mordenite and that methanol selectively reduced
NO in the gas phase.
2.2. Activity tests
A quartz flow tube reactor, according to [3], was used in
the activity tests. The Ag/alumina catalyst was tested un-
der steady-state conditions in the temperature range 150–
600 ◦C. The temperature of the catalyst bed was monitored
by a K-type thermocouple connected to a temperature con-
troller (Eurotherm 900 EPC). A basic gas mixture consisting
of 500 ppm NO, 375 ppm octane, 6 vol% O2, 10 vol% CO2,
350 ppm CO, and 12 vol% H2O in He (GHSV = 60,000 h−1
and volumetric flow rate = 550 ml/min) was used in the ac-
tivity tests. The effect of hydrogen was studied by adding
1 vol% H2 to the basic gas mixture. All gases (AGA) were
of high purity and introduced into the reactor by means of
mass-flow controllers (Brooks 5850E). Water was added to
the gas mixture using a syringe pump in combination with
a controlled evaporator mixer (Bronkhorst HI-TEC). Octane
was also introduced using a syringe pump (CMA 102/Mi-
crodialysis).
A commercial Pt oxidation catalyst (Johnson Matthey)
was placed directly after the Ag/alumina catalyst in some
experiments to prove the gas-phase reactions.
The concentrations of N2, CO2, CO, and O2 were deter-
mined with the aid of a gas chromatograph (HP 6890) and
the concentration changes of NOx (NO + NO2) in the gas
mixture were recorded by a chemiluminescence NOx ana-
lyzer (API 200AH). Two condensers, kept at −5 ◦C and at
−25 ◦C, were used to trap out the water from the gas prior
to analysis.
Nitrogen formation within organic reactions usually in-
volves either hydrazine or deamination reactions [10,11].
In the latter case, nitrogen is known as one of the leaving
groups in the reaction of primary amines and the nitrosonium
ion NO+, originating from nitrous acid. The intermediate,
2.3. GC/MS studies
In order to trap unreacted species in the gas phase af-
ter the catalyst, the Ag/alumina catalyst was placed at the
entrance of a U-shaped stainless-steel tube, which was im-
mersed in liquid nitrogen. A block heater was placed around
the tube at the position of the catalyst. The isolated species
were dissolved in acetone and injected into a GC-MS (HP
6890-5973) equipped with a 15 m, 0.25 mm (diameter), and
0.50 µm (film thickness) INNOWAX column (J&W Scien-
tific). Prior to the experiments the catalyst was treated in
a gas mixture containing 6 vol% O2 at 400 ◦C for 30 min.
+
R−N2 (diazonium ion), is very unstable and undergoes
rapid fragmentation to nitrogen and an alkyl cation [11].
In this study, the possibility of forming nitrogen in the gas
phase by reaction of activated forms of NOx with amines and
ammonia as well as with other organic intermediates, which
can be converted to amines and/or ammonia, was investi-
gated. The activation of NO was carried out by passing NO
together with oxygen and small amounts of hydrogen over
an Ag/alumina catalyst.

330
K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
carried out to measure the effect of the C6 hydrocarbonchain
on the NO to N2 conversion.
2.5. FTIR analysis
In situ IR spectra were recorded on an ATI Mattson In-
finity FTIR spectrometer equipped with an IR cell made
of stainless steel. A mixture containing 1000 ppm NO and
6 vol% O2 in presence and absence of 1 vol% H2 in He
was introduced to the chamber using mass-flow controllers
(Brooks 5850E). In some tests 750 ppm of octane was intro-
duced by saturation of gas. The catalyst was pressed into a
self-supporting wafer (weight 0.021 g) and mounted in the
IR cell. In situ pretreatment of the catalyst at 400 ◦C, using
a gas mixture containing 6 vol% O2 in He, was performed
before each experiment. All experiments were conducted
at 250 ◦C. The spectra were measured by accumulating 64
scans at a resolution of 4 cm−1
.
a
Some tests were carried out in the absence of Ag/alumina in the side port.
Fig. 1. Schematic illustration of the T-shaped quartz reactor used for inves-
tigation of gas-phase reactions.
3. Results and discussion
3.1. Activity tests
The following combinations of reactants were led through
the catalyst bed, which was kept at 350 ◦C: (1) 375 ppm oc-
tane + 6 vol% O2 in He; (2) 375 ppm octane + 6 vol% O2
+ 500 ppm NO in He; and (3) 375 ppm octane + 6 vol% O2
+ 500 ppm NO + 1 vol% H2 in He.
In our previous study [7] it was shown that the 2 wt%
Ag/alumina is a very active catalyst for the reduction of
NO using octane as reducing agent. However, the activity
is limited to temperatures above 300 ◦C. As reported by
Satokawa [5], hydrogen significantly enhances the HC-SCR
reaction, especially at low temperatures. In order to investi-
gate the effect of hydrogen on the NO conversion, 1 vol% H2
was added to the basic gas mixture and the result compared
to conversions obtained without H2 is shown in Fig. 2. As
can be seen, conversion values close to 60% were recorded
already at 250 ◦C in the presence of hydrogen. On the other
hand, hydrogen alone exhibited practically no capacity to re-
duce NO to N2.
A substantial amount of carbon monoxide is formed in
parallel with the NO to N2 conversion [7]. The CO can easily
be removed by using an oxidation catalyst. However, adding
an oxidation catalyst directly after the Ag/alumina bed re-
duces the NOx conversion dramatically as shown in Fig. 3.
Interestingly, addition of 1 vol% H2 to the feed reduces al-
most completely the contribution of the gas-phase reaction
to the conversion of NO to N2, as visualized in Fig. 4. The
rather high NO conversion at 150 ◦C (over 30%) with H2
in the feed is due to the activity of the oxidation catalyst.
According to a test with only the oxidation catalyst, the ac-
tivity at temperatures higher than 150 ◦C can be related to
the Ag/alumina catalyst. It seems that the hydrogen effect is
either due to increased reaction rate of surface reactions or
reactions in the gas phase (the reactions are completed al-
ready in the pores and in the void space between particles).
The role of hydrogen will be dealt with more in detail later
in this study.
2.4. Experimental setup for gas-phase reaction studies
A special T-shaped quartz reactor, as shown in Fig. 1, was
constructed to study the formation of nitrogen from different
kinds of proposed intermediate species in the gas phase. The
basic idea was to investigate if an amine or ammonia (intro-
duced from the side of the reactor) reacts in the gas phase
with species, resulting from reactions of NO + O2 as well
as NO + O2 + H2 over Ag/alumina, to form N2. In addition,
possible intermediates, which can be converted, hydrolyzed,
or decomposed to amines/ammonia over Ag/alumina, were
tested in the same manner. In separate tests, FTIR analysis
using a heated long-path gas cell (Graseby Specac Ltd) in
combination with GC measurements were used to determine
the gas product distribution after the catalyst. Model com-
ponents for the intermediates were hexylamine, C6H13NH2
(Acros Organics), heptanenitrile, C6H13CN (Acros Organ-
ics), hexylisocyanate, C6H13NCO (Acros Organics), and
1-nitrohexane, C6H13NO2 (Aldrich).
In additional experiments, the same model intermediates
were tested for their activity to reduce NO over Ag/alumina.
These experiments were carried out by feeding 250 ppm of
the N-containing hydrocarbon together with 500 ppm NO
and 6 vol% O2 in He over the Ag/alumina bed at temper-
atures ranging from 150 to 600 ◦C. A reference run using
250 ppm n-hexane, 500 ppm NO, and 6 vol% O2 in He was

K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
331
Fig. 2. Activity tests over Ag/alumina in the presence and absence of hydrogen. Gas mixtures: (a) 500 ppm NO, 375 ppm C H , 1 vol% H , 6 vol% O , 10
8
18
2
2
vol% CO , 350 ppm CO, 12 vol% H O in He. (b) 500 ppm NO, 375 ppm C H , 6 vol% O , 10 vol% CO , 350 ppm CO, 12 vol% H O in He. (c) 500 ppm
2
2
8
18
2
2
2
−1
NO, 1 vol% H , 6 vol% O , 10 vol% CO , 350 ppm CO, 12 vol% H O in He. GHSV = 60,000 h and volumetric gas flow = 550 ml/min.
2
2
2
2
Fig. 3. Comparison of the conversion obtained over Ag/alumina (single bed) and over Ag/alumina in combination with a Pt oxidation catalyst (0 mm between
−1
beds). Gas mixture: 500 ppm NO, 375 ppm C H , 6 vol% O , 10 vol% CO , 350 ppm CO, 12 vol% H O in He. GHSV = 60,000 h and volumetric gas
8
18
2
2
2
flow = 550 ml/min.
3.2. GC-MS studies
species formed over the catalyst surface. The trapped species
were dissolved in acetone and injected into a GC-MS. In
the first experiment, using only octane and oxygen, some
partially oxidized hydrocarbons such as 2-propenoic acid,
crotonic acid, and acetophenone were detected. In addition,
some cyclization seemed to take place as well as dimeriza-
tion to decane. The very low amount of products in this test
In order to investigate how addition of hydrogen to the
feed influences the formed gas-phase species, GC-MS stud-
ies on trapped compounds after the catalyst were carried out.
During these experiments a U-shaped reactor was placed in
a bath filled with liquid nitrogen in order to trap (isolate)

332
K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
Fig. 4. Comparison of the conversion obtained over Ag/alumina (single bed) and over Ag/alumina in combination with a Pt oxidation catalyst (0 mm between
beds) in the presence of hydrogen. 500 ppm NO, 375 ppm C H , 1 vol% H , 6 vol% O , 10 vol% CO , 350 ppm CO, 12 vol% H O in He. GHSV =
8
18
2
2
2
2
−1
60,000 h and volumetric gas flow = 550 ml/min.
could depend on the low catalyst temperature (350 ◦C) re-
sulting in minimal activity for oxidation of octane or the lack
of a highly reactive component such as NO and/or hydrogen.
When NO was added to the gas mixture, the distribution of
species changed significantly. The majority of compounds
belonged to the nitrile group. Also some ketones, aldehy-
des, and carboxylic acids were formed. Again also some
cyclization could be observed. The nitriles may be formed
through nucleophilic addition of cyanide ions (on the sur-
face) to aldehydes and ketones (formed through partial oxi-
dation via alcohols over Ag/alumina). It is also possible that
the nitriles are by-products, which are not important dur-
ing effective HC-SCR. Another hypothesis would be that the
nitriles are hydrolyzed to amides, which are then further hy-
drolyzed to carboxylic acids and ammonia or amines [12].
In this case the reason for the high number of nitriles in the
GC-MS spectrogram would be that effective hydrolysis did
not take place. On the other hand, it has been proposed that
cyanide species on the catalyst surface are slowly converted
into isocyanate species [4]. Isocyanate species are widely
considered to be potential intermediates in the HC-SCR re-
action. Due to the slow rearranging of the CN group into the
NCO group a majority of the untransformed cyanide ions
could form the detected nitriles together with left over alde-
hydes and ketones.
gen transforms the nitriles into a more reactive form or that
hydrogen speeds up the transformation of cyanide species
to isocyanate species on the surface. On the other hand, the
effect of hydrogen might be in several parallel processes in-
cluding the activation of NO. It is possible that without H2,
the activation process of NO at this temperature is limited.
3.3. Gas-phase reaction studies involving intermediates
In all tests the formation of the desired product N2 was
determined by means of gas chromatographic analyses and
the amount of formed N2 was used to calculate the con-
version of NO. Conversion of NO in the reaction between
NO and N-containing functional groups in the gas phase
has been calculated assuming that one NO molecule reacts
with one N-containing functional group to form one mole-
cule of N2:
c
N2(out)
Conversion of NO (%) =
(1)
cNO(in)
When the reaction is taking place over the catalyst, also a
possible contribution of the hydrocarbon chain to reduction
of NO must be taken into account, especially at temperatures
above 350 ◦C. By measuring the amount of N2 produced it
was not possible to distinguish between these two possible
reactions and thus the conversions in these cases were calcu-
lated as conversion of N-containing compounds:
When hydrogen was added to the gas mixture at the
same temperature (350◦C) the GC-MS spectra changed
completely. Almost no compounds of interest, except some
partially oxidized hydrocarbons (ketones), were seen, most
probably due to the complete oxidation of the hydrocarbons
and/or very high HC-SCR activity. It could be that hydro-
Conversion of N-containing comp. (%)
2 × c
N2(out)
=
(2)
cN-containing comp. (in)

K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
333
In the cases where a hydrocarbon (no N-containing func-
tional group) was used as reducing agent, the conversion was
calculated as
in a separate test it was confirmed that introducing 1 vol% H2
to a mixture of 500 ppm NO and 6 vol% O2 flowing through
the catalyst in the bottom part and with only He from the
side did not result in any formation of N2.
2 × c
N2(out)
Conversion of NO (%) =
.
(3)
The importance of this new finding lies in the fact that
it is possible to homogeneously convert NO to N2 by reac-
tion with NH3 in excess of oxygen at temperatures far below
those reported to be valid for selective noncatalytic reduction
of NO. Depending on the gas components and their concen-
trations, NO reduction by ammonia in the gas phase should
by no means take place below 600–700◦C [13]. The crucial
point is to activate NO over the Ag/alumina catalyst for fur-
ther reaction with ammonia. Some activation seems to take
place without a reducing agent, but hydrogen is a good ac-
tivator for NO. It can be assumed that also hydrocarbons
during the HC-SCR process are involved in the activation
process of NO probably as a source of hydrogen. It is gen-
erally known that nonthermal plasma treatment induces ac-
tivation of both NO and hydrocarbon. In our case a similar
kind of activation seems to take place over the Ag/alumina
catalyst and is enhanced by addition of hydrogen.
The conversion of NO to N2 using ammonia as a reduc-
ing agent over the Ag/alumina as a function of temperature
is shown in Fig. 6. As can be seen, the conversion of NO
at 250 and 400 ◦C was 0 and 8.0%, respectively. The con-
version increased with the temperature and was about 20%
at 600 ◦C. The lower conversion in this test both at 250 ◦C
and at 400 ◦C compared to the T-reactor test may be due
to adsorbed ammonia on the catalyst, which partly inhibits
the activation of NO. A similar activity pattern as shown in
Fig. 6 was also shown by Richter et al. for the SCR of NOx
by ammonia over Ag/alumina [14]. They reported that in the
presence of hydrogen almost complete conversion of NO to
cNO(in)
In this case all the N2 formed is originating from the in-
troduced NO, which is consumed both in the formation of
N-containing species and in the reaction with these species
to form N2.
3.3.1. Ammonia
The results of the gas-phase reaction at 250 and 400 ◦C
between NO + O2 as well as NO + O2 + H2 (flowing
through the catalyst) and ammonia are shown in Fig. 5. Pass-
ing NO + O2 over the catalyst and introducing NH3 after
the catalyst bed resulted in a NO conversion of about 9.5%.
The conversion was almost exactly the same both at 250 ◦C
and at 400 ◦C. By addition of H2 to the NO + O2 mixture,
a significant improvement of the NO conversion was ob-
served. At the first point, which was analyzed 6 min after
the H2 introduction, approximately 26% of NO was con-
verted to N2 in the gas phase at both temperatures. With time
on stream the conversion slightly decreased. The NO con-
version value of 26% means that approximately half of the
available ammonia is used for the reduction of NO to N2.
According to the stoichiometry of the SCR of NO with am-
monia, the NO/NH3 ratio is usually adjusted to 1. In fact,
when 500 ppm NH3 was introduced after the catalyst bed
and 500 ppm NO + 6 vol% O2 + 1 vol% H2 was flowing
through the catalyst, the NO conversion increased to 43.7%
(not shown). Empty reactor tests showed about 3% conver-
sion and there was no effect of hydrogen addition. Moreover,
◦
◦
Fig. 5. T-reactor tests with ammonia at 250 C (2) and at 400 C (!). Gas mixture through bottom port: 500 ppm NO + 6 vol% O in presence and absence
2
of 1 vol% H in He. Gas mixture through side port: 250 ppm NH in He.
2
3

334
K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
Fig. 6. The conversion of NO over Ag/alumina as a function of temperature using NH as reducing agent. Gas mixture: 500 ppm NO, 250 ppm NH , and 6
3
3
−1
vol% O in He. GHSV = 60,000 h and volumetric gas flow = 550 ml/min.
2
◦
Fig. 7. Time on stream in situ FTIR on Ag/alumina at 250 C in a flow of 1000 ppm NO and 6 vol% O in He.
2
N2 was observed at temperatures starting from 200 ◦C. The
boosting effect of hydrogen on the NO conversion in our
T-reactor test is in good agreement with their results taking
into account that we used 500 ppm NO and 250 ppm NH3
compared to 1000 ppm NO and 1000 ppm NH3 in [14].
The effect of hydrogen on the formation of ad-NOx
species was studied by in situ FTIR at 250 ◦C. The results
of flowing NO + O2 as well as NO + O2 + H2 over the
Ag/alumina as a function of time are shown in Figs. 7 and 8.
In the case of NO + O2 flowing over the catalyst, slow for-
mation of peaks mainly at 1250, 1295, and 1550 cm−1 was
recorded. Additionally a shoulder at about 1605 cm−1 was
detected. These peaks have been attributed to monodentate
nitrate (peaks at 1250 and 1550 cm−1) and bidentate nitrate

K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
335
◦
Fig. 8. Time on stream in situ FTIR on Ag/alumina at 250 C in a flow of 1000 ppm NO, 6 vol% O , and 1 vol% H in He.
2
2
(peaks at 1295 and 1605 cm−1) [15]. When H2 was added to
the feed together with NO + O2, the formation of the nitrate
species was significantly accelerated. As a matter of fact, the
same amount of surface nitrates was achieved already in less
than 3 min in the presence of H2 compared with the amount
formed during 80 min in the absence of H2. This is in good
agreement with the work of Shibata et al. [6]. In a separate
test at 250 ◦C, simulating introduction of H2 in the T-reactor
test, the effect of introduction of H2 on adsorbed nitrates was
investigated. The adsorbed nitrates were produced in a flow
of 1000 ppm NO and 6 vol% O2 in He for 80 min. Addi-
tion of H2 to the mixture of NO and O2 resulted in rapid
growth of the bands corresponding to nitrates, especially at
1295 and 1550 cm−1, as can be seen in Fig. 9. No decrease
in peak intensity of other detected peaks or growth of new
peaks could be observed. In addition, the adsorbed nitrate
species seemed to be quite stable. In a flow of 6 vol% O2
and 1 vol% H2 (in absence of NO) no decrease in intensity
of the peaks corresponding to the nitrates could be detected,
but in a flow containing 1 vol% H2 alone, the peaks disap-
peared in about 3 min.
Addition of 1 vol% hydrogen to a flow of 750 ppm octane
and 6 vol% oxygen at 250 ◦C resulted in a similar enhancing
effect on the formation of oxygenated hydrocarbon species
on the catalyst surface. In a flow of octane and O2 slow for-
mation of very weak peaks at about 1378, 1392, 1456, and
1574 cm−1 were detected. These peaks have been assigned
to formate (1378 and 1392 cm−1) and acetate (1456 and
1574 cm−1) species on Ag/alumina [6]. Addition of H2 re-
sulted in a rapid growth of especially the acetate bands and
comparing the amount of formed acetate for 80 min with and
without H2 in the feed revealed that the amount was about
8.5 times higher in the presence of H2.
In a separate experiment the catalyst was first exposed
to a flow of 1000 ppm NO and 6 vol% O2 for 80 min at
250 ◦C to generate nitrates on the catalyst surface. As ex-
pected, addition of 750 ppm octane to the feed for 60 min
did not change the spectra due to the low temperature (no
reaction took place). However, addition of 1 vol% H2 re-
sulted in a drastic change of the peak pattern. After 3 min
the intensity of the band belonging to bidentate nitrate at
1295 cm−1 was doubled. In addition, the band at 1250 cm−1
belonging to monodentate nitrate had disappeared. Weak
bands of acetate and formate species as well as a strong
band at 1586 cm−1, which has been assigned to carboxylate
species [16], were also immediately formed and the intensi-
ties of these bands increased with time on stream. Moreover,
in the spectra recorded at 30 min after introduction of H2, a
weak band at 2228 cm−1 could be detected. This band has
been assigned to NCO species [6,16]. The intensity of the
band belonging to bidentate nitrate at 1295 cm−1 did not
change with time on stream; i.e., the intensity was the same
both at 3 min and 30 min. In addition, no increase of the
other bands of nitrates as observed in the experiment using
a flow of NO, O2, and H2 (Fig. 8) was found. Thus it seems
that in presence of octane the continuously formed nitrates
are consumed by taking part in the reaction or by desorption
into the gas phase.
In addition to the surface studies, the effect of hydrogen
addition on the gas composition after the catalyst was inves-
tigated by FTIR studies using a gas cell. At 250 ◦C no NO2
was detected when a gas mixture containing 500 ppm NO
and 6 vol% O2 was led over the catalyst. At 400 ◦C small

336
K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
Fig. 9. The effect of addition of 1 vol% H to the flow of 1000 ppm NO and 6 vol% O in He on nitrates adsorbed on Ag/alumina. Catalyst temperature:
2
2
◦
250 C.
amounts (below 10 ppm) of NO2 were observed. Addition
of 1 vol% H2 led to a significant conversion of NO to NO2,
being 68% at 250 ◦C and 57% at 400 ◦C under steady-state
conditions. Also strong peaks related to water were present.
In addition, a small peak at 1260 cm−1 was observed. This
peak could possibly be attributed to trans-HONO [17]. No
peaks corresponding to formation of N2O were observed.
It is clear that the rate of formation of nitrates on the cat-
alyst is drastically changed in the presence of small amounts
of hydrogen. In the T-reactor test with NH3 (and also in the
corresponding tests with other intermediates described in the
following sections) addition of H2 to the flow of NO + O2
resulted in such a strong formation of ad-NOx species that a
decrease in the NOx concentration at the reactor outlet could
be detected for at least 10 min as observed from NOx ana-
lyzer values. Moreover, in a recent study it was shown that
hydrogen dissociated and exchanged with deuterium (D2)
on oxidized Ag/alumina to form HD [18]. This could mean
that the catalyst has an ability to form HONO and HONO2
from the nitrates on the catalyst surface in the presence of
dissociated hydrogen. It is possible that electron-deficient
oxides of nitrogen such as NO+ and NO2+ are formed from
the decomposition of HNO2 and HNO3. In the case of ze-
olites, NOx activation by interaction with acidic hydroxyls
was considered to take place and generate nitrosonium ions
NO+ in a process involving H+ [19,20]. Iglesias-Juez et al.
[21] have detected these kinds of species on Ag/alumina dur-
ing reaction conditions and showed that acidic hydroxyls are
recovered by interaction with the hydrocarbon or derived
products, thus closing the catalytic cycle and regenerating
the active surface species. It can be speculated that H2 in our
case is acting in the same manner. Furthermore, Hadjiivanov
et al. [20] discussed that NO+ and nitrate precursor mole-
cules can be simultaneously formed by disproportionation
of N2O3 and N2O4.
Another possible explanation of the effect of hydrogen
is that it boosts the formation of highly reactive radicals,
such as NO·, NO2·, O·, H·, and OH·. Richter et al. [14] have
proposed that hydrogen generates on short-term scale zero-
valent silver. Reactive O (and/or OH) species are formed via
dissociative interactions of O2 (and H2) with these metal-
lic silver species. These reactive oxygen atomic species
should then accelerate the necessary oxidative transforma-
tion of gaseous NO to adsorbed nitrite/nitrate species. The
increased concentrations of NO2 in the gas phase in the pres-
ence of H2 in our case might be due to the reaction between
O radicals and NO. It is possible that also some of the formed
NO2 species are of radical character. Whether these kinds of
radicals directly react with NH3 or N-containing hydrocar-
bons forming N2 is unclear. It is worth noting that in an on-
going parallel work dealing with EPR combined with matrix
isolation technique, there were indications of low molecular
weight radicals trapped in the matrix behind the Ag/alumina
catalyst.
It is commonly known that NO+ reacts with alkylamines
to form an alkyl diazonium ion [11]. In case of a primary
amine this diazonium ion is very unstable and decomposes
to nitrogen and to a highly reactive alkyl cation. This alkyl
−
cation reacts with nucleophiles, for example, NO2 to give
nitroalkanes. The nitrosation of a primary amine with NO+
proceeds according to the following reaction [11]:
R–NH2 + NO+ → R–+≡N: → N ↑ + R+.
(4)
N
2

K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
337
As shown in the following section, a similar type of deam-
ination reaction is indeed taking place after the Ag/alumina
catalyst bed and is thus suggested to be part of the HC-SCR
reaction. As shown above, it seems that some kind of acti-
vated NOx species also reacts with ammonia. Whether these
are the same kind of species that react with the amine is not
clear.
Anyway, one important role of the Ag/alumina catalyst
during the HC-SCR is to feed the gas phase with acti-
vated oxides of nitrogen as well as ammonia and/or amines
(formed by surface reactions between hydrocarbons and
NOx). Reaction between these activated nitrogen oxides and
ammonia is proposed to be the final step in the HC-SCR
mechanism over Ag/alumina.
In a separate test the temperature dependence of the gas-
phase reaction between hexylamine and activated forms of
NOx was examined and the result is shown in Fig. 11. In
this experiment 250 ppm hexylamine was continuously in-
troduced after the catalyst bed and a mixture of 500 ppm
NO, 6 vol% O2 and 1 vol% H2 was flowing through the cat-
alyst. As can be seen, the temperature strongly influences on
the conversion and the highest conversion (about 26%) of
NO was obtained between 200 and 250 ◦C. This means that
half of the available amine was consumed at these tempera-
tures in the gas-phase reaction.
3.3.3. Formation of amines and/or ammonia from
hexylisocyanate, heptanenitrile, and nitrohexane over
Ag/alumina
3.3.2. Hexylamine
Isocyanate species and nitro compounds are widely pro-
posed to be important intermediates in the HC-SCR reaction.
In addition, the GC-MS analysis of the compounds trapped
behind the Ag/alumina catalyst revealed a large formation
of nitriles during the HC-SCR reaction. Also in the in situ
FTIR experiments bands corresponding to the isocyanate
group were detected. Isocyanates are easily hydrolyzed to
amines in the presence of water. Also nitriles are known to
undergo hydrolysis in the presence of H2O and as noted ear-
lier, the first hydrolysis product is the corresponding amide,
which is then further hydrolyzed to carboxylic acid and
ammonia [12]. Nitrocompounds such as R-NO2 have been
proposed as intermediates originating from the reaction of
adsorbed NOx species (mainly nitrates) and partly oxidized
hydrocarbons (mainly acetates) [4]. As noted earlier, the ni-
trosation of primary amines results in the formation of a
highly reactive alkyl cation. This alkyl cation reacts with
The results of the gas-phase reaction of species formed
by passing NO + O2 as well as NO + O2 + H2 through
the catalyst with hexylamine at 400 ◦C are shown in Fig. 10.
Passing NO + O2 over the catalyst and introducing hexy-
lamine after the catalyst bed resulted in a NO conversion of
about 4.3%. When H2 was added to the feed together with
NO + O2 a significant increase of the NO conversion was
obtained. The conversion increased by time and the max-
imum conversion obtained was approximately 18% in the
gas phase. The corresponding empty reactor test showed a
conversion of about 3.5% with no effect of H2 addition. It
is obvious that the role of hydrogen is to activate NO over
the Ag/alumina catalyst as explained in the previous section
and hereby generate enough reactive NO+ species, which
react with the amine to form diazonium ions according to
reaction (4). These diazonium ions are then subsequently de-
composed forming N2.
◦
Fig. 10. T-reactor tests with hexylamine at 400 C with catalyst (1) and without catalyst (2) in the bottom part. Gas mixture through bottom port: 500 ppm
NO and 6 vol% O in the presence and absence of 1 vol% H in He. Gas mixture through side port: 250 ppm hexylamine in He.
2
2

338
K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
Fig. 11. The temperature dependence on the conversion of NO in the gas-phase reaction using hexylamine as reducing agent. Gas mixture through bottom
port: 500 ppm NO, 6 vol% O , and 1 vol% H in He. Gas mixture through side port: 250 ppm hexylamine in He.
2
2
Table 1
◦
The formation of R-NH and/or NH from hexylisocyante, heptanenitrile, and 1-nitrohexane, under dry and wet conditions at 250 and 400 C
2
3
◦
◦
400 C
Intermediate
Condition
250 C
Conversion to R-NH (%)
2
Conversion to NH (%)
3
Conversion to R-NH (%)
2
Conversion to NH (%)
3
a
Hexylisocyanate
Heptanenitrile
1-Nitrohexane
Dry
23.0
43.0
0.0
0.0
0.0
8.0
13.1
0.0
5.6
8.0
0.0
0.0
0.0
0.0
0.0
0.0
36.0
46.0
17.2
23.6
36.2
55.5
b
Wet
Dry
Wet
Dry
Wet
0.0
13.3
Gas mixture: 250 ppm of N-containing hydrocarbon and 6 vol% O , (1 vol% H O) in He.
2
2
a
No H O in the feed.
2
b
With 1 vol% H O in the feed.
2
NO2− to give nitroalkanes and this offers an alternative route
for producing these species.
corresponding value in the presence of water was 46%. At
this temperature no amine was detected. It is evident that if
isocyanate species are formed over the catalyst, they are eas-
ily transformed into amines and ammonia, which, as shown
above, can react with activated nitrogen oxides in the gas
phase to form N2.
In the case of heptanenitrile, no conversion to amine
or ammonia was observed at 250 ◦C and under dry condi-
tions. Adding water to the feed resulted in a conversion of
5.6% of the nitrile to ammonia, the rest being unreacted.
At 400 ◦C, the corresponding conversions to ammonia were
17.2% (dry) and 23.6% (wet). No amine or amide formation
was detected in any of these tests. It is quite clear that hep-
tanenitrile is much more stable in terms of hydrolysis and
decomposition than the corresponding hexylisocyanate.
At 250 ◦C and under dry conditions, 8% of nitrohexane
was converted to NH3 and at 400 ◦C the corresponding value
In order to test if amines and/or ammonia are formed from
isocyanates, nitriles, or nitroalkanes over the Ag/alumina,
experiments were carried out where mixtures of 250 ppm
hexylisocyanate, heptanenitrile, or nitrohexane and 6 vol%
O2 in presence and absence of 1 vol% H2O were fed over
the Ag/alumina catalyst. The components at the reactor out-
let were analyzed by FTIR using a gas cell and the results
are shown in Table 1.
Under dry conditions at 250 ◦C, about 23 and 8% of the
introduced hexylisocyanate was converted into amine and
ammonia, respectively. When 1 vol% of water was added
to the feed, the conversion of hexylisocyanate to amine and
ammonia increased to approximately 43 and 13.1%, respec-
tively. Interestingly, at 400 ◦C about 36% of hexylisocyanate
was converted into ammonia under dry conditions and the

K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
339
Table 2
◦
T-reactor tests with hexylisocyanate, heptanenitrile, and 1-nitrohexane at 250 and 400 C
Time
Feed from
Feed from side
port
Hexylisocyanate
Heptanenitrile
1-Nitrohexane
(min.) bottom port
◦
◦
◦
◦
◦
◦
400 C
250 C
400 C
250 C
400 C
250 C
X
Max.
X
Max.
X
Max.
X
Max.
X
Max.
X
NO
(%)
Max.
NO
NO
NO
NO
NO
a
a
a
a
a
a
(%)
X
NO
(%)
X
NO
(%)
X
NO
(%)
X
NO
(%)
X
NO
X
NO
(%)
(%)
(%)
(%)
(%)
(%)
6
30
36
66
90
NO + O
NO + O
NO + O
NO + O
NO + O
NO + O
NO + O
O
2
0.0
0.0
0.5
0.0
0.0
0.0
0.5
0.0
0.0
4.0
0.5
18.1
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
+ R–X
+ R–X
+ R–X
2
2
2
2
2
2
2
2
2
2
0.9
5.2
15.5
15.5
6.7
8.9
18.0
18.0
1.1
1.5
0.0
0.0
6.3
7.0
8.6
8.6
0.8
3.2
4.0
4.0
6.7
9.8
18.1
18.1
+ R–X + H O
2
96
+ R–X + H O
6.0
4.5
28.1
28.1
9.7
10.5
23.0
23.0
0.9
0.9
2.8
2.8
7.9
6.7
11.8
11.8
4.5
4.5
6.7
6.7
14.5
13.0
27.8
27.8
2
126
150
156
186
216
+ R–X + H O
2
NO + O + H
+ R–X + H O
2
2
2
2
2
2
NO + O + H
+ R–X + H O 30.4
28.1
28.1
28.1
23.7
12.8
11.0
23.0
23.0
23.0
8.5
0.5
0.5
2.8
2.8
2.8
23.3
8.1
6.0
11.8 14.3
11.8
11.8
6.7
6.7
27.0
21.3
27.8
27.8
2
2
NO + O + H
+ R–X + H O 16.5
3.6
2
2
NO + O + H
+ R–X + H O 14.4
2
2
a
Maximum conversion of NO obtained if all the R-NH and/or NH formed react(s) with NO to form N . R–X = hexylisocyanate, heptanenitrile or
2
3
2
1-nitrohexane. Gas concentrations: 500 ppm NO, 6 vol% O , 1 vol% H , 250 ppm hexylisocyanate, heptanenitrile, or 1-nitrohexane, 1 vol% H O, balance: He.
2
2
2
was 36.2%. In the presence of 1 vol% H2O, the conversion to
NH3 increased to 13.3% at 250 ◦C and to 55.5% at 400 ◦C.
Thus, it seems that ammonia is easily formed from nitro-
hexane especially at higher temperatures. No conversion of
nitrohexane to amines was detected in any of these tests.
ter addition of H2, an excess of activated forms of NOx is
released, resulting in high N2 formation. The excess of these
species depends on whether the catalyst has become satu-
rated by nitrates due to the flow of NO + O2 through the
bed in the first part of the test. At the moment of hydro-
gen addition, a lot of activated NOx species desorbs into the
empty space after the bed and reacts further with the avail-
able amines and ammonia. Under steady-state conditions,
the amount of the reactive species desorbed into the gas
phase is probably much lower. It is worth noting that in this
case there is also a possibility for the activated NOx species
to react not only with formed amine and ammonia (from iso-
cyanate), but also with unreacted hydrocarbon species and
water. This might explain the more pronounced drop in con-
version of NO compared to the test with ammonia, where a
similar trend, but weaker, was visible. Also at 400 ◦C, the
same type of increase in activity was observed when H2 was
added to the bottom feed. In this case as well, the conver-
sion observed at the first measurement point (about 23%)
is exactly what one could expect if the activated forms of
NOx react with all the ammonia available. No nitrogen was
formed directly by reaction of hexylisocyanate and oxygen
over the Ag/alumina catalyst as evidenced by a separate test.
Maximum NO conversions of about 1.5 and 7% in the
gas phase at 250 and 400 ◦C, respectively, were achieved
by passing NO + O2 from the bottom and heptanenitrile
together with O2 from the side. Adding water to the feed
from side did not change the activity pattern. The low con-
version at 250 ◦C was expected, as there was simply no NH3
in the gas phase. When hydrogen was introduced to the bot-
tom part of the feed, an increase in the conversion of NO
was again observed. The conversion at the first experimen-
tal point reached values over 8.5% at 250 ◦C and 23.3% at
400 ◦C. The amount of N2 formed at this first point exceeds
the conversion values correspondingto the amount of ammo-
nia available and apparently the activated NOx species react
3.3.4. T-reactor tests with hexylisocyanate, heptanenitrile,
and nitrohexane
Similar kinds of T-reactor tests were performed with
hexylisocyanate, heptanenitrile, and nitrohexane as with am-
monia and hexylamine. However, in order to form ammonia
and amines from these reactants, the T-reactor was equipped
with two similar Ag/alumina beds, one was placed in the side
tube and the second one was placed in the bottom part. Mix-
tures of NO and O2 as well as NO, O2, and H2 were fed from
the bottom of the reactor and hexylisocyanate, heptaneni-
trile, or nitrohexane and O2 with and without the addition of
water was fed from the side. The results of these tests are
shown in Table 2.
Passing NO + O2 from the bottom and hexylisocyanate
together with O2 from the side resulted in a NO conversion
of about 5.2 and 8.9% at 250 and 400 ◦C, respectively. The-
oretically, according to the amounts of amine and ammonia
formed at 250 ◦C, the conversion of NO could in the best
case be 15.5% (dry conditions) and 28.1% (wet conditions).
At 400 ◦C the corresponding values are 18 and 23%, respec-
tively. Adding water together with hexylisocyanate did not
affect the conversion probably due to the limited amount of
activated NOx species present. As expected, the conversion
of NO was significantly improved in the gas phase immedi-
ately when hydrogen was added to the bottom feed contain-
ing NO and O2. At the first measurement point (wet condi-
tions) at 250 ◦C the conversion was about 30%, which means
that all the available amine and ammonia is consumed. How-
ever, a drastic drop in the activity took place as a function of
time. It might be that during the first experimental point af-

340
K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
directly with a part of the –CN group. On, the other hand,
the effect of hydrogen was only short termed and the activ-
ity decreased by time to values obtained without hydrogen
in the feed.
increase in conversion was obtained at 250 ◦C. When octane
was used as the reducing agent in the standard activity test,
there was almost no conversion of NO at 250 ◦C (see Fig. 2,
test without H2). This explains why no increase in conver-
sion is observed in the T-reactor test at 250 ◦C. The reaction
to generate intermediates, which form amines, ammonia, or
nitriles, is simply not taking place at such low temperatures
due to poor hydrocarbon oxidation activity.
Due to the same type of conversion increase as in the
T-reactor tests above, it is reasonable to assume that am-
monia and/or amines are also formed in the reaction of NO
+ O2 + octane over Ag/alumina. On the other hand, in
the T-reactor test with heptanenitrile, higher conversion of
NO than expected (from the amount of NH3 available) was
observed. Because of this, it seemed that the nitrile group re-
acted directly with activated NOx and it is possible that also
the nitriles are responsible for the increase in conversion in
the test with octane. The detected nitriles in the GC-MS stud-
ies support this theory.
Passing NO + O2 from the bottom and nitrohexane to-
gether with O2 from the side resulted in a maximum NO
conversion in the gas phase of about 3 and 9.8% at 250
and 400 ◦C, respectively. Adding water to the feed of ni-
trohexane and O2 increased the conversion slightly. When
hydrogen was added to the bottom part of the feed, the ex-
pected increase in the conversion of NO was observed. The
conversion of NO measured 6 min after H2 introduction was
14.3% at 250 ◦C and 27.0% at 400 ◦C. The conversion of NO
at 250 ◦C exceeds the expected value of 6.7% but at 400 ◦C
the conversion corresponds exactly to the amount of ammo-
nia available, i.e., about 27%.
3.3.5. T-reactor test with octane
To test if the reaction of octane with NO and O2 over the
Ag/alumina catalyst generates ammonia and/or amines af-
ter the catalyst bed, similar types of T-reactor tests as above
were performed at 250 and 350 ◦C. In this case, mixtures
of O2, NO + O2 or NO + O2 + H2 in He were fed through
the catalyst in the bottom part and NO + O2 + octane was
flowing through the catalyst placed in the side port. The re-
sults of these tests are shown in Fig. 12. When O2 (6 vol%
in He) was fed from the bottom and NO + O2 + octane from
the side, the conversion at 250 and at 350 ◦C was about 5 and
26%, respectively. Adding NO to the bottom feed did not
change the conversion. However, when H2 was added to the
feed together with NO and O2, a similar kind of increase in
conversion at 350 ◦C was observed as in the cases above. No
3.3.6. Activity of hexylamine, hexylisocyanate,
heptanenitrile, and 1-nitrohexane as reducing agents
Hexylamine, hexylisocyanate, heptanenitrile, and 1-nitro-
hexane were also tested as reducing agents for NO over the
Ag/alumina catalyst and their activities were compared to
the activity of hexane. The results of these tests are shown in
Fig. 13. Using hexylamine as reducing agent, two maxima
of the conversion curve at 250 and 550 ◦C were recorded.
A comparison of this activity pattern with the corresponding
curve obtained using hexane reveals that the first maximum
is probably due to the reaction involving the amine group.
With hexane as reducing agent this maximum is missing be-
◦
◦
Fig. 12. T-reactor tests with octane at 250 C (2) and at 400 C (!). Gas mixture through bottom port: 500 ppm NO and 6 vol% O in the presence and
2
absence of 1 vol% H in He. Gas mixture through side port: 500 ppm NO, 250 ppm octane, and 6 vol% O in He.
2
2

K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
341
Fig. 13. Conversion of N-containing compounds to N as a function of temperature using hexane, hexylamine, hexylisocyante, heptanenitrile, and 1-nitro-
2
−1
hexane as reducing agents. Gas mixture: 500 ppm NO, 6 vol% O , and 250 ppm of reducing agent in He. GHSV = 60,000 h and volumetric gas flow =
2
550 ml/min.
cause the formation of amines or other reactive intermediates
is restricted to higher temperatures. The second maximum is
probably obtained as a combined effect of the hydrocarbon
chain together with the remaining amine or its decomposi-
tion product ammonia.
A similar trend was observed using hexylisocyanate and
nitrohexane as reducing agents. In the case of hexylisocyante
the first maximum is probably due to the reaction involv-
ing the amine group and ammonia formed by the hydrolysis
and decomposition of the isocyanate. The second maximum
seems to originate from both the hydrocarbon chain and
the ammonia formed from the isocyanate group. With ni-
trohexane the first maximum was not as pronounced as for
hexylisocyanate.
Opposite to the cases of hexylamine, hexylisocyanate,
and nitrohexane there was only one clear maximum at
500 ◦C in the activity pattern when using heptanenitrile as
reducing agent. The activity related to this intermediate is re-
stricted to higher temperatures because of its stability. If the
temperature is high enough (over 400 ◦C) the heptanenitrile
is a very active reducing agent for NO. The activity seems
to be a result of formed ammonia in combination with the
hydrocarbon chain or possibly of the –CN group itself.
activated forms of NOx to form N2. Upon oxidation of these
N-containing compounds over an oxidation catalyst, NO is
reproduced. It is well known that ammonia and amines, for
instance, are oxidized to NO. The presence of ammonia and
amines in the gas phase could explain the observed behav-
ior when adding the oxidation catalyst after the Ag/alumina
bed. The T-reactor tests show that activated forms of NOx
react with these species in the gas phase producing N2. In
addition, it seems that also nitriles react in the same way.
The role of hydrogen on the HC-SCR process is inter-
esting, as it has at least two main functions. First of all,
improved oxidation of all components involved is obtained.
In the presence of hydrogen the oxidation of octane is im-
proved as evidenced by a test in which 1 vol% H2 was added
to a mixture of 375 ppm octane and 6 vol% O2 at 350 ◦C.
Without H2 in the feed the conversion of octane was 41.2%.
By introduction of 1 vol% H2, the conversion increased to
73%. In addition, as shown earlier, the rate of formation of
adsorbed NOx species is much higher in the presence of hy-
drogen than in the absence of H2. The improved oxidation
of the hydrocarbon also results in faster formation of dif-
ferent oxygenates which react with these ad-NOx species
to form N-containing compounds. From these N-containing
compounds amines and ammonia are easily formed as shown
by our FTIR gas cell experiments. Secondly, in this study hy-
drogen has been shown to activate NO over the Ag/alumina
catalyst to react with the produced amines and ammonia
or with other N-containing species in gas phase, but the
same reactions certainly also take place on the surface of
the Ag/alumina catalyst. Hydrocarbons could be a source of
3.4. Mechanistic aspects
It is clear that gas-phase reactions are involved in the
HC-SCR process over Ag/alumina. It is also evident that
the components participating in these gas-phase reactions in-
clude a nitrogen-containingfunctional group that reacts with

342
K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
Fig. 14. Schematic illustration of the proposed reaction mechanism during HC-SCR over Ag/alumina. Reactions 1–4 are accelerated by addition of hydrogen.
◦
◦
Higher concentrations of ammonia and amines are formed in presence of water in steps 5–8. Steps 5 and 6 proceed at 250 C, but at 400 C step 5 is not taking
place. The transformation of nitrile to NH in step 7 is restricted to high temperatures. Step 8 is pronounced at high temperatures. (a) From the literature [4].
3
hydrogen and may at temperatures where partial oxidation
takes place play an important role in the activation proce-
dure of NOx.
4. Conclusions
The possibility of forming nitrogen in the gas phase by re-
action of activated forms of NOx with amines and ammonia
or with other organic intermediates, which can be converted
to amines and/or ammonia, was investigated. Both ammonia
and hexylamine were shown to react in the gas phase with
activated NOx species to produce N2. This new finding re-
veals that it is possible to homogeneously convert NO to N2
by reaction with NH3 in excess oxygen at temperatures far
below those reported for selective noncatalytic reduction of
NO. Components such as R-NO2, R-NCO, and R-CN were
proved to be intermediate species from which amines and
ammonia are formed for the last step of the HC-SCR reac-
tion mechanism. Nitrohexane was transformed to NH3 in the
presence of O2 over the Ag/alumina already at 250 ◦C and
the amount of NH3 produced increased by the addition of
H2O. Hexylisocyante was hydrolyzed to amine and ammo-
nia at 250 ◦C over the catalyst under conditions of excess
oxygen but only to ammonia at 400 ◦C. At 250 ◦C the con-
version to amine and ammonia was almost doubled by the
addition of H2O. Heptanenitrile was very stable and only
small amounts of NH3 were observed at 400 ◦C; however,
nitriles can react directly with activated NOx species form-
ing N2.
The observations made within the present study fit well
into the mechanism proposed by Meunier et al. [16], where a
similar kind of Ag/alumina catalyst was used. We would like
to stress the importance of gas-phase reactions as a part of
their proposed mechanism. A schematic illustration, of the
proposed mechanism of HC-SCR over Ag/alumina includ-
ing the new observations made within this study is shown
in Fig. 14. In this mechanism, R-NO2, R-NCO, and R-CN
are intermediates for the formation of amines and ammo-
nia, which are consumed both on the surface of the catalyst
as well as in the gas phase behind the catalyst bed by reac-
tion with activated NOx species. The nitriles are quite stable
and transform to ammonia only at high temperatures. There-
fore, it is unlikely that these species are the key intermediates
at temperatures below 400 ◦C. Isocyanate species, on the
other hand, can be responsible for the activity also at low
temperatures, as they easily hydrolyze to form amines and
ammonia at temperatures below 250 ◦C. It was shown that
hydrogen boost the formation rate of isocyanate and thereby
enhances the low-temperature activity during the HC-SCR
process.

K. Eränen et al. / Journal of Catalysis 227 (2004) 328–343
343
The role of hydrogen on the HC-SCR process involves
References
several functions. Improved oxidation of all components was
obtained in the presence of H2. In the oxidation of octane,
addition of hydrogen increased the conversion from 41 to
73% at 350 ◦C. As a consequence, the improved oxidation
rate of the hydrocarbon also results in faster formation of
different oxygenates, which react and form N-containing
species. The rate of formation of adsorbed NOx species was
also found to be much higher in the presence of hydrogen.
In addition, hydrogen was shown to activate NO over the
Ag/alumina catalyst for further reactions with the produced
amines and ammonia or with other N-containing species in
the gas phase. The activated NOx species may be of radical
or ionic character.
[1] W. Held, A. König, T. Richter, L. Puppe, SAE Tech. Paper Ser.
No. 900496 (1990).
[2] M. Iwamoto, H. Yahiro, S. Shundo, Y. Yu-u, N. Mizuno, Appl.
Catal. 69 (1991) L15.
[3] K. Eränen, L.-E. Lindfors, A. Niemi, P. Elfving, L. Cider, SAE Paper
2000-01-2813 (2000).
[4] R. Burch, J.P. Breen, F.C. Meunier, Appl. Catal. B 39 (2002) 283.
[5] S. Satokawa, Chem. Lett. (2000) 294.
[6] J. Shibata, K. Shimizu, S. Satokawa, A. Satsuma, T. Hattori, Phys.
Chem. Chem. Phys. 5 (2003) 2154.
[7] K. Eränen, L.-E. Lindfors, F. Klingstedt, D.Yu. Murzin, J. Catal. 219
(2003) 25.
[8] D.B. Lukyanov, G. Sill, J.L. d’Itri, W.K. Hall, J. Catal. 153 (1995) 265.
[9] J. Vassallo, E. Miró, J. Petunchi, Appl. Catal. B 7 (1995) 65.
[10] C.C. Clark, Hydrazine, Mathieson Chemical Corporation, Baltimore,
MD, 1953.
[11] S.N. Ege, Organic Chemistry, Houghton Mifflin, Boston, 1999.
[12] J. March, Advanced Organic Chemistry, Wiley, New York, 1992.
[13] B. Leckner, M. Karlsson, K. Dam-Johanssen, C. Weinell, P. Kilpinen,
M. Hupa, Ind. Eng. Chem. Res. 31 (1992) 1477.
Acknowledgments
[14] M. Richter, R. Fricke, R. Eckelt, Catal. Lett. 94 (2004) 115.
[15] K.I. Hadjiivanov, Catal. Rev.-Sci. Eng. 42 (2000) 71.
[16] F.C. Meunier, J.P. Breen, V. Zuzaniuk, M. Olsson, J.R.H. Ross,
J. Catal. 187 (1999) 493.
[17] Application of Fourier Transform Spectroscopy to Air Pollution Prob-
lems, Ohio State Univ. Research Foundation, Columbus, EPA-600/3-
78-057 (1978) 83.
Markku Reunanen at the Laboratory of Wood Chemistry,
Åbo Akademi University, is gratefully acknowledged for
the GC-MS analysis. The financial support by the Commis-
sion of the European Communities in the framework of the
GROWTH program (G5RD-CT-2001-00595) is also grate-
fully acknowledged. This work is part of the activities at the
Åbo Akademi Process Chemistry Centre within the Finnish
Centre of Excellence Programme (2000-2005) by the Acad-
emy of Finland.
[18] J. Jensen, Ms. thesis, Åbo Akademi (2003).
[19] T. Gerlach, F.-W. Schütze, M. Baerns, J. Catal. 185 (1999) 131.
[20] K. Hadjiivanov, J. Saussey, J.L. Freysz, J.C. Lavalley, Catal. Lett. 52
(1998) 103.
[21] A. Iglesias-Juez, A.B. Hungría, A. Martínez-Arias, A. Fuerte, M.
Fernández-García, J.A. Anderson, J.C. Conesa, J. Soria, J. Catal. 217
(2003) 310.