Redox features in the catalytic mechanism of the "standard" and "fast" NH3-SCR of NOx over a V-based catalyst investigated by dynamic methods

Article abstract of DOI:10.1016/j.jcat.2006.09.012

The redox mechanism governing the selective catalytic reduction (SCR) of NO/NO₂ by ammonia at low temperature was investigated by transient reactive experiments over a commercial V₂O₅/WO₃/TiO₂ catalyst for diesel exhaust aftertreatment. NO + NH₃ temperature-programmed reaction runs over reduced catalyst samples pretreated with various oxidizing species showed that both NO₂ and HNO₃ were able to reoxidize the V catalyst at much lower temperature than gaseous O₂: furthermore, they significantly enhanced the NO + NH₃ reactivity below 250 °C via the buildup of adsorbed nitrates, which act as a surface pool of oxidizing agents but are decomposed above that temperature. Both such features, which were not observed in comparative experiments over a V-free WO₃/TiO₂ catalyst, point out a key catalytic role of the vanadium redox properties and can explain the greater deNO_x efficiency of the "fast" SCR (NO + NH₃ + NO₂) compared with the "standard" SCR (NO + NH₃ + O₂) reaction. A unifying redox approach is proposed to interpret the overall NO/NO₂-NH₃ SCR chemistry over V-based catalysts, in which vanadium sites are reduced by the reaction between NO and NH₃ and are reoxidized either by oxygen (standard SCR) or by nitrates (fast SCR), with the latter formed via NO₂ disproportion over other nonreducible oxide catalyst components.

Full text of DOI:10.1016/j.jcat.2006.09.012

Journal of Catalysis 245 (2007) 1–10
www.elsevier.com/locate/jcat
Redox features in the catalytic mechanism of the “standard” and “fast”
NH3-SCR of NOx over a V-based catalyst investigated by dynamic methods
Enrico Tronconi a,∗, Isabella Nova a, Cristian Ciardelli a, Daniel Chatterjee b, Michel Weibel b
a
Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
b
DaimlerChrysler AG Abteilung RBP/C, HPC: 096-E220, D-70546 Stuttgart, Germany
Received 29 June 2006; revised 20 August 2006; accepted 12 September 2006
Available online 16 October 2006
Abstract
The redox mechanism governing the selective catalytic reduction (SCR) of NO/NO by ammonia at low temperature was investigated by
2
transient reactive experiments over a commercial V O /WO /TiO catalyst for diesel exhaust aftertreatment. NO+NH temperature-programmed
2
5
3
2
3
reaction runs over reduced catalyst samples pretreated with various oxidizing species showed that both NO and HNO were able to reoxidize the
2
3
◦
V catalyst at much lower temperature than gaseous O : furthermore, they significantly enhanced the NO + NH reactivity below 250 C via the
2
3
buildup of adsorbed nitrates, which act as a surface pool of oxidizing agents but are decomposed above that temperature. Both such features, which
were not observed in comparative experiments over a V-free WO /TiO catalyst, point out a key catalytic role of the vanadium redox properties
3
2
and can explain the greater deNO efficiency of the “fast” SCR (NO + NH + NO ) compared with the “standard” SCR (NO + NH + O )
x
3
2
3
2
reaction. A unifying redox approach is proposed to interpret the overall NO/NO –NH SCR chemistry over V-based catalysts, in which vanadium
2
3
sites are reduced by the reaction between NO and NH and are reoxidized either by oxygen (standard SCR) or by nitrates (fast SCR), with the
3
latter formed via NO disproportion over other nonreducible oxide catalyst components.
2
© 2006 Elsevier Inc. All rights reserved.
Keywords: Diesel-urea SCR; Selective catalytic reduction; Standard SCR; Fast SCR; Redox properties; Redox catalytic kinetics; Dynamic methods
1. Introduction
and other stationary sources [9]. However, the specific demands
of mobile applications, associated with, for example, volume
limitations and dynamic operation, do not permit straightfor-
ward transposition of the technology. Because the working con-
ditions for mobile applications may be much colder than in
stationary installations, increasing the deNOx activity at low
temperatures represents a major development goal. A possible
solution to this issue is represented by the so-called “fast” SCR
reaction known since the early 1980s, when Kato et al. [10]
found that the reaction involving an equimolar NO and NO2
feed mixture,
Selective catalytic reduction (SCR) with NH3/urea is emerg-
ing as the most promising technology for the abatement of NOx
emissions from diesel vehicles [1–8]. This has stimulated a re-
newed interest in investigating the fundamental aspects of SCR
catalytic chemistry, also in view of the need for the transporta-
tion industry to develop design and simulation tools incorporat-
ing suitable SCR kinetic schemes.
Indeed, NH3 SCR over vanadia-type catalysts, in which 1
molecule of NO is reduced by 1 molecule of ammonia in the
presence of oxygen to give dinitrogen and water,
2NH3 + NO + NO2 → 2N2 + 3H2O,
(2)
2NH3 + 2NO + (1/2)O2 → 2N2 + 3H2O,
has represented for the last two decades the most effective com-
mercial deNOxing process for stack gases from power plants
(1)
is significantly faster than the “standard” SCR, reaction (1),
at low temperature. In practical terms, a preoxidation catalyst
located upstream of the SCR catalyst could convert a fraction
of NO in the engine exhausts to NO2 to approach the optimal
NO/NO2 equimolar feed ratio of reaction (2) [11]. Neverthe-
less, adding NO2 to the SCR reacting system introduces consid-
*
Corresponding author. Fax: +39 02 2399 3318.
E-mail address: enrico.tronconi@polimi.it (E. Tronconi).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.09.012

2
E. Tronconi et al. / Journal of Catalysis 245 (2007) 1–10
erable complexity resulting from the multiplication of primary
and secondary reaction routes and may result in the formation
of such undesired byproducts as NH4NO3 and N2O [12,13].
From a more fundamental standpoint, the relationship between
the catalytic chemistry of the standard SCR and that of the less
well-investigated fast SCR, as well as the reasons why NO2 dra-
matically accelerates the reduction of NO by ammonia, have not
yet been fully elucidated.
based SCR catalyst with optimized composition. The reactivity
of NO + NH3 in the low-temperature range is investigated by
temperature-programmed reduction (TPR) methods under con-
ditions typical of real aftertreatment devices over catalyst sam-
ples subjected to various pretreatments, to examine the influ-
ence of different reducing and oxidizing agents on SCR activity.
Specifically, the relation between surface nitrates and the redox
mechanism of fast SCR is explored. Comparative experiments
over a V-free WO3/TiO2 catalyst are also performed to identify
the catalytic role of vanadium.
Because of its widespread application in the abatement of
NOx emissions from stationary sources, the mechanism of the
standard SCR reaction has been extensively investigated [9,14].
It is generally agreed that the reaction proceeds according to
a redox scheme, in which gaseous oxygen is needed for reox-
idation of the V-related catalyst sites reduced by the reaction
between NO and NH3. It is also established that at low temper-
ature, the catalyst reoxidation by gas-phase oxygen is the rate-
determining step of the overall reaction mechanism [15–20].
Concerning the fast SCR reaction (2), a much more limited
number of papers analyzing the NH3–NO/NO2 reacting system
are available in the scientific literature. This field was pioneered
by Koebel and co-workers [11,17,21,22], who extensively in-
vestigated the reactivity of NH3–NO/NO2 over powdered and
monolithic vanadia-based catalysts. Addressing possible rea-
sons for the observed higher rates of the fast SCR reaction at
low T , they proposed that gaseous NO2 would replace oxy-
gen as a more effective oxidizing agent, thus allowing faster
reoxidation of the vanadium sites. The NO2-enhanced reoxi-
dation of the catalyst was demonstrated by in situ Raman ex-
periments over V2O5/TiO2 [17], but no direct kinetic evidence
was provided to confirm that this effect could explain the order-
of-magnitude increment of the SCR. Moreover, a mechanistic
scheme in which NO2 simply replaces oxygen in reoxidiz-
ing the V sites cannot account for the significant role played
by other species in the NO/NO2–NH3 system. Recently we
showed by transient experiments over a commercial V-based
catalyst that the reduction of nitrate species (related to previ-
ously deposited ammonium nitrate formed by reaction between
NO2 and NH3) by nitric oxide proceeds at the same rate as the
fast SCR at 170 ◦C [23,24]. This suggests that such a reaction
pathway may be involved (and actually be rate-controlling) in
the low-temperature catalytic mechanism of the fast SCR reac-
tion over V-based catalysts. Such data have been rationalized
according to a reaction scheme in which NO2 forms nitrite and
nitrate species by disproportion, NO reduces nitrates to nitrites,
and NH3 reacts with nitrites to form unstable ammonium nitrite,
which readily decomposes to nitrogen and water [24]. Simi-
lar indications, obtained in this case mostly by spectroscopic
techniques, have been reported by Sachtler and co-workers for
NO/NO2–NH3 SCR over zeolite catalysts [25]. This scheme
is able to account for an extensive set of kinetic observations,
including the influence of the NO/NO2 feed ratio on the selec-
tivities to N2, NH4NO3, and N2O [13]; however, it does not
explicitly reflect the redox nature of the catalytic mechanism of
SCR.
2. Materials and methods
Unsteady SCR reactive experiments were performed at 50–
250 ◦C over a commercial extruded V2O5–WO3/TiO2 catalyst
(As ≈ 70 m2/g) with intermediate V content, originally sup-
plied as a honeycomb monolith. A significant portion of the
catalyst was crushed and ground to powder (140–200 mesh); for
each experiment, a small sample (160 mg) was collected from
the mixed powders, diluted with 80 mg of quartz, and loaded
in a flow microreactor consisting of a quartz tube (6 mm i.d.).
Using the same procedures, a commercial WO3/TiO2 catalyst
(Thann et Mulhouse S.A.) was also tested in specific runs for
comparison purposes.
The test reactor was operated at atmospheric pressure with a
total flow rate of either 120 or 280 cm3/min (STP), correspond-
ing to a gas hourly space velocity (GHSV) of about 9 × 104
or 2.1 × 105 h−1, respectively. Diluted gas streams of NO,
NO2, NH3, and O2 in He from bottled calibrated mixtures were
mixed in suitable proportions by means of mass flow controllers
(Brooks 5850 E) to achieve the desired feed composition for
each run. For H2O-containing feeds, the feed stream was passed
through a saturator maintained at a controlled temperature be-
fore entering the microreactor. For catalyst pretreatment with
HNO3, the feed stream was saturated with a suitable aqueous
solution of nitric acid.
The reactor outlet was directly connected to both a UV an-
alyzer (ABB Limas 11-HW) and a quadrupole mass spectrom-
eter (Balzers QMS 200) operating in parallel. In each experi-
ment, the UV analyzer monitored the temporal evolution of the
outlet NO, NO2, and NH3 concentrations. Helium was used as
the carrier gas, so that nitrogen (the main SCR product) could
be detected by the MS, along with the byproduct N2O, thus al-
lowing evaluation of overall N balances, which always closed
within 5% at steady state. As a consistency check, the outlet
concentration traces of NO, NO2, and NH3 also were estimated
from the MS signals after proper calibration; the steady-state
levels agreed satisfactorily with those measured by the UV an-
alyzer, whereas the transient phases were typically associated
with a slower MS response, particularly in the case of ammonia.
In view of the purposes of the present work, herein we primar-
ily report the outlet concentration traces of NO generated by the
UV analyzer.
Typical experiments were NO + NH3 TPR runs. A feed
stream consisting of NH3 (1000 ppm) + NO (1000 ppm) with
H2O (1% v/v), balance He, and no oxygen was initially ad-
mitted to the reactor at 50 ◦C. Then the catalyst temperature
Herein we address redox mechanistic features of standard
SCR and fast SCR over an industrial V2O5–WO3/TiO2 cat-
alyst for mobile applications, considered a representative V-

E. Tronconi et al. / Journal of Catalysis 245 (2007) 1–10
3
was continuously increased at 20 ◦C/min up to 250–550 ◦C,
depending on the purposes of the experiment. Such TPR exper-
iments were carried out over reference reduced and oxidized
catalyst samples, as well as over other catalyst samples sub-
jected to different pretreatments.
gen is present in the feed stream; the minor NO conversions
measured above 200 ◦C are attributed to the so-called “slow”
SCR reaction [12,14],
2NH3 + 3NO → (5/2)N2 + 3H2O.
(3)
The reference reduced catalyst samples were obtained by
exposure to NH3 (1000 ppm feed stream) at 550 ◦C for about
1.5 h. Catalyst reduction was confirmed by a peak of N2 evolu-
tion during the first few minutes, the integral amount of which
was compatible with reduction of the vanadium catalyst load.
The reference oxidized catalyst samples were obtained by ex-
posure to O2 (500 ppm) during a temperature ramp (20 ◦C/min)
up to 550 ◦C; oxidation of the catalyst was confirmed by O2
consumption and H2O evolution at temperatures above ca.
200 ◦C.
Various other pretreatments were also applied to the cata-
lyst to investigate their influence on its oxidation state and its
SCR reactivity, as discussed in the following sections. Specifi-
cally, pretreatment procedures were performed by feeding H2O
(1% v/v) in a temperature ramp up to 550 ◦C, H2O (1% v/v) at
200 ◦C, NO (1000 ppm) at 200 ◦C, NO (1000 ppm) + H2O (1%
v/v) at 200 ◦C, NH3 (1000 ppm) at 200 ◦C, NO (1000 ppm) +
NH3 (1000 ppm) at 200 ◦C, NO2 (100 or 500 ppm) at 150 ◦C,
and HNO3 (about 100 ppm) + H2O (1% v/v) at 100, 150, and
200 ◦C.
In addition to TPR runs, specific tests were carried out ac-
cording to the transient response method (TRM experiments).
This involved performing a step change of the inlet concen-
tration of one reactant (1000 ppm NH3, NO, or NO2) while
feeding other reactants in the presence of H2O (1% v/v) and bal-
ance He at 170 ◦C. In line with the SCR literature for V-based
systems [14], no significant activity in the oxidation of NO to
NO2 up to 450 ◦C was observed over the present catalysts, even
in dedicated diagnostic runs.
In fact, replicated temperature ramps performed up to higher
temperatures over the reduced catalyst confirmed conversion of
both nitric oxide and ammonia, as well as production of nitro-
gen, according to the stoichiometry of reaction (3). It is well
known that under practical conditions, that is when oxygen is
present, slow SCR is negligibly slow compared with standard
SCR.
In contrast, the catalyst sample pretreated according to the
reference oxidizing procedure appeared to be significantly more
active. Indeed, analysis of the outlet concentrations of nitric ox-
ide, ammonia, and nitrogen confirmed the occurrence of the
standard SCR reaction (1) at temperatures as low as 130 ◦C.
This is in line with literature reports [15] that standard SCR
over V-based catalysts proceeds by consuming the catalyst lat-
tice oxygen, in agreement with a redox mechanism. If oxygen
is fed to the reactor, then the lattice oxygen consumed by the
reaction can be replenished, and the reaction proceeds to steady
state. Otherwise (i.e., if gaseous oxygen is not available), the
reaction stops once all of the reactive lattice oxygen of the cat-
alyst is depleted [15]. In fact, starting from around 250 ◦C, the
evolution of NO conversion measured over the reference oxi-
dized catalyst increased only slowly, paralleling that observed
over the reference reduced catalyst, thus indicating that only the
slow SCR reaction (3) was proceeding.
3.2. Standard SCR reaction versus catalyst reoxidation
We now apply the tests assessed in the previous section to
clarify aspects of the redox mechanism associated with the stan-
dard SCR reaction. Fig. 1 illustrates results from three different
TPR runs: (A) NO concentration profile during NO+NH3 +O2
TPR over a reference reduced catalyst, (B) O2 concentration
profile during a temperature ramp for reoxidation of a refer-
ence reduced catalyst, and (C) NO concentration profile during
NO + NH3 + O2 TPR over a WO3/TiO2 catalyst.
3. Results and discussion
3.1. SCR behavior of reference reduced and reference
oxidized catalyst samples
The activity in the standard SCR reaction of reference re-
duced and oxidized catalyst samples was first analyzed by
NO + NH3 TPR. Over the reference reduced catalyst, no con-
version of NO was evident up to 200 ◦C, and only minor SCR
activity was apparent at 250 ◦C, when about 980 ppm of NO
was still present in the outlet stream.
A different picture was observed for NO + NH3 TPR over
the reference oxidized catalyst. The NO outlet concentration
began to decrease at ≈130 ◦C, and the NO conversion was
close to 8% at 250 ◦C. Occurrence of the SCR reaction was also
confirmed by stoichiometric evolution of nitrogen, the integral
amount of which was found to be in good agreement with that
measured during reference reduction of the catalyst sample.
The data were reproducible, as confirmed by several repli-
cated runs over both prereduced and preoxidized catalyst sam-
ples. The data indicate that the reduced commercial V catalyst
is virtually inactive in the standard SCR reaction when no oxy-
It is apparent that curve (B), although very noisy due to ex-
perimental limitations with the signal acquisition from the MS,
essentially overlaps curve (A). Accordingly, the same activa-
tion threshold prevailed for both reoxidation of a reference re-
duced catalyst (curve (B)) and standard SCR in the presence of
gaseous oxygen over a reference reduced catalyst (curve (A)).
In contrast, the NO+NH3 +O2 TPR experiment over the V-free
catalyst (curve (C)) showed the onset of some NO conversion
only above roughly 300 ◦C, a threshold temperature well above
that measured for the V-based catalyst, thus confirming that the
low-temperature activity in the standard SCR reaction can be
ascribed to the well-known redox properties of the vanadium
catalyst component [9,12,14].
Accordingly, the data in Fig. 1 support that V reoxidation is
the rate-limiting step in the standard SCR reaction. Of course,
such a result has significant implications for SCR kinetic mod-
eling and is in agreement with indications reported previously.

4
E. Tronconi et al. / Journal of Catalysis 245 (2007) 1–10
Fig. 2. NO concentration profiles during NO + NH TPR (1000/1000 ppm +
3
Fig. 1. Curve (A) NO concentration profile during NO + NH + O (1000/
3
2
◦
He) over catalysts after different pretreatments: (A) H O (1% v/v) at 200 C;
2
1000/500 ppm + He) TPR over a reference reduced catalyst. Curve (B) O
2
◦
◦
(B) H O (1% v/v) in T -ramp up to 550 C; (C) NO (1000 ppm) at 200 C;
2
concentration profile during reoxidation (O feed = 500 ppm + He) of a ref-
2
◦
(D) NH (1000 ppm) at 200 C; (E) NO (1000 ppm) + H O (1% v/v) at
3
2
erence reduced catalyst. Curve (C) NO concentration profile during NO +
◦
◦
200 C; (F) NO (1000 ppm) + NH (1000 ppm) at 200 C; (G) reference oxi-
3
NH +O (1000/1000/20,000 ppm+He) TPR over a WO /TiO catalyst; flow
3
2
3
2
3
◦
dized catalyst; (H) reference reduced catalyst. Other conditions as in Fig. 1.
rate = 280 cm /min (STP). Heating rate = 20 C/min.
lyst (curve (H)). Thus, it appears that exposure of the oxidized
catalyst to atmospheres containing either H2O or NO or NH3
alone or H2O + NO together at 200 ◦C did not reduce the cat-
alyst. H2O did not reduce the oxidized catalyst even at 550 ◦C.
Further supporting this conclusion is the fact that no nitrogen
evolution was detected during all such pretreatments.
In contrast, curve (F) in Fig. 2 shows the results of a simi-
lar experiment over an oxidized catalyst sample that had been
exposed to a mixture of NO and NH3 (1000 + 1000 ppm) at
200 ◦C before the TPR run. In this case, deNOx activity was
quite similar to that observed over the reference reduced cat-
alyst (curve (H)); moreover, N2 evolution was observed dur-
ing the pretreatment. Although additional spectroscopic evi-
dence would be needed in this respect, these data suggest that
NO+NH3 jointly were able to reduce the present V catalyst al-
ready at 200 ◦C. This indeed seems consistent with the results
in Fig. 1. As discussed in the previous section, at 200 ◦C, SCR
activity is limited by catalyst reoxidation, whereas the reduc-
tion step of its redox mechanism was found to proceed already
rapidly at the same temperature.
Using different approaches, Lietti and Forzatti [15] and Marsh-
neva et al. [20] concluded that the slow step in the SCR reaction
over V2O5–WO3/TiO2 catalysts is associated with V catalyst
oxidation by gaseous oxygen.
Having established the redox nature of the standard SCR re-
action, we next assess the roles of different species in the related
reduction and oxidation steps, respectively.
3.3. Activities of different species in catalyst reduction
To identify the species responsible for catalyst reduction dur-
ing the standard and fast SCR reactions, we studied the activity
of different potential reducing agents participating in the SCR
reacting system (NO, NH3, H2O, and combinations thereof),
according to the following procedure. A fresh catalyst sample
was first oxidized, then pretreated with one of the candidate re-
ducing agents. After the pretreatment, NO+NH3 TPR was run,
and the resulting NO reduction activity was compared with the
corresponding data obtained over both the reference oxidized
and reduced catalysts.
The results are summarized in Fig. 2, which compares
the NO reduction activity in the TPR runs performed over
the catalyst samples after various pretreatments. Specifically,
such pretreatments involved feeding H2O (1% v/v) at 200 ◦C
(curve (A)), H2O (1% v/v) at a temperature ramp up to 550 ◦C
(curve (B)), NO (1000 ppm) at 200 ◦C (curve (C)), NH3
(1000 ppm) at 200 ◦C (curve (D)), NO (1000 ppm) + H2O (1%
v/v) at 200 ◦C (curve (E)), and NO + NH3 (1000 + 1000 ppm)
at 200 ◦C (curve (F)). In addition, curves obtained for the
reference oxidized (curve (G)) and of the reference reduced
(curve (H)) catalyst samples are reported for comparison.
Fig. 2 shows that in curves (A)–(E), the observed deNOx ac-
tivity was essentially identical to that typical of the reference
oxidized catalyst (curve (G)) and remained evidently higher
than the activity measured over the reference reduced cata-
3.4. Mechanistic implications: catalyst reduction
In this section we briefly discuss some mechanistic impli-
cations of the present results concerning the catalyst reduction
phase in the redox mechanism of the standard and fast SCR re-
actions. Our data do not support a direct reducing action of NO
alone. This is in agreement with the view, supported by many
authors, that NH3 rather than NO is activated (i.e., oxidized) by
V-based catalysts in the first step of the standard SCR catalytic
mechanism [14]. Our data also do not support that at 200 ◦C,
ammonia alone is directly activated by deeply reducing the V
catalyst. In fact, as suggested by Fig. 2 and by the data on N2
evolution during catalyst pretreatment, under the present exper-
imental conditions, the catalyst reduction step in the redox SCR

E. Tronconi et al. / Journal of Catalysis 245 (2007) 1–10
5
mechanism seems to require a cooperative action (co-presence)
of both NO and NH3. In this respect it should be also considered
that the reducing action of NH3 is enabled by the presence of
V atoms in adjacent sites [26,27]; however, exploring the affect
of V surface density is beyond the scope of the present study.
The foregoing results may be consistent with the following
interpretations:
1. Ammonia, already adsorbed onto nonreducible acidic
sites, is initially activated (oxidized) by adjacent vanadium re-
dox sites, but the reaction proceeds to a significant extent only
after interaction with gaseous NO according to an Eley–Rideal
scheme,
NH*3 + V5+=O ↔ V=O[NH3],
(4a)
(4b)
V=O[NH3] + NO → N2 + H2O + V4+–OH,
where NH*3 represents adsorbed ammonia.
Fig. 3. NO concentration profiles during NO + NH TPR over catalysts after
A similar proposal was formulated by Topsøe et al. on the
basis of in situ FTIR evidence [19]. However, the chemical na-
ture of the initial NH3 activation, which does not reduce the cat-
alyst deeply (with nitrogen evolution), as documented herein,
remains to be clarified. Moreover, this interpretation is some-
what at variance with recent kinetic data showing considerable
inhibition of ammonia on the low-temperature SCR of NO and
pointing to the existence of an optimal NH3 surface coverage
[18,28–31]. It was also found that such an inhibiting action of
ammonia, already reported by other authors [32–34], cannot
be accommodated by a simple Eley–Rideal kinetic approach
assuming a reaction between adsorbed ammonia and gaseous
nitric oxide [18,30].
2. Nitric oxide is oxidized by the V catalyst to nitrite species,
but the equilibrium is highly unfavorable and shifts to the right
only in the presence of NH3 (adsorbed onto nearby acidic sites),
which reacts with nitrites to give N2 and H2O via decomposi-
tion of unstable ammonium nitrite intermediates, for example,
according to
3
◦
different pretreatments: (A) O (500 ppm) at T = 150 C; (B) NO (100 ppm)
2
2
◦
◦
at 150 C; (C) HNO (∼100 ppm) at 150 C; (D) reference oxidized catalyst;
3
(E) reference reduced catalyst. Other conditions as in Fig. 1.
3.5. Activity of different species in catalyst reoxidation
Similar to the investigation of reducing agents in Section 3,
in this section we test the reactivity of different oxidizing
species, namely O2, NO2, and HNO3, in the catalyst reoxida-
tion step of the redox SCR mechanism. Fig. 3 compares the
deNOx activity observed in NO + NH3 TPR runs over three
catalyst samples. Each sample was first reduced according to
the reference procedure, then pretreated at 150 ◦C with a dif-
ferent oxidizing agent—O2 (curve (A)), NO2 (curve (B)), or
HNO3 (curve (C))—and finally tested in a NO + NH3 TPR run.
Data collected in the same TPR experiment over the reference
oxidized catalyst sample (curve (D)) and the reference reduced
catalyst sample (curve (E)) also are displayed for comparison.
It is apparent from Fig. 3 that the NO evolution recorded
when the catalyst sample was pretreated in oxygen flow at
150 ◦C (curve (A)) essentially overlaps with the curve obtained
over the reference reduced catalyst (curve (E)): hence, oxygen
was apparently unable to reoxidize the prereduced V catalyst at
150 ◦C. This is consistent with the data in curve (B) of Fig. 1,
which suggest that the onset of catalyst reoxidation by oxygen
occurred above 200 ◦C under our experimental conditions.
In contrast, NO evolution traces recorded after exposing
the reduced catalyst both to NO2 (curve (B)) and to HNO3
(curve (C)) show that NO conversion was initiated already at
about 130 ◦C, a threshold temperature very similar to that typ-
ical of the reference oxidized catalyst (curve (D)). These data
then suggest that both NO2 and HNO3 were able to effectively
reoxidize the V catalyst at 150 ◦C.
V
5+=O + NO ↔ V4+–ONO,
(5a)
(5b)
V4+–ONO + NH3* → N2 + H2O + V4+–OH.
In this case, the inhibiting action of ammonia could be more
easily explained by either a competitive adsorption of NH3 onto
the V sites involved in NO activation or an adverse electronic
interaction of adsorbed NH3 with the vanadium oxidizing cen-
ters [18].
In any case, both (4a)+(4b) and (5a)+(5b) yield the overall
catalyst reduction step,
NO + NH*3 + V5+=O → N2 + H2O + V4+–OH.
(6)
For the standard SCR reaction, reoxidation of the reduced V
sites closing the redox cycle is then carried out by gaseous oxy-
gen according to
Notably, two similar TPR experiments were repeated af-
ter exposing prereduced catalyst samples to HNO3 at 200 and
100 ◦C. The results (not reported here) showed behavior quite
similar to that of curve (C) for the sample pretreated at 200 ◦C,
whereas with pretreatment at 100 ◦C, the catalyst behaved like
the reference reduced sample and thus likely was not oxidized.
Accordingly, the temperature threshold for catalyst reoxidation
V4+–OH + (1/4)O2 → V5+=O + (1/2)H2O.
(7)
For fast SCR, however, other strong oxidizing agents—namely,
NO2 and HNO3—are present in the reacting environment in
addition to O2. Thus, it is of interest to compare their oxidizing
activity, as we discuss in the following sections.

6
E. Tronconi et al. / Journal of Catalysis 245 (2007) 1–10
◦
Fig. 5. NO concentration profiles during T -ramps at 20 C/min. Curve (A)
feed = NO + NH + NO (1000/1000/500 ppm). Curve (B) feed = NO + NH
3
2
3
◦
Fig. 4. NO and O concentration profiles during T -ramp at 20 C/min af-
(1000/1000 ppm) over reduced catalyst pretreated with NO (500 ppm) at
2
2
2
◦
◦
ter catalyst pretreatment with HNO (∼100 ppm) at 150 C. Feed flow rate =
150 C. Curve (C) feed = NO + NH + O (1000/1000/500 ppm).
3
3
2
3
280 cm /min (STP).
Because catalyst reoxidation is the rate-limiting step of the
SCR reaction, as documented in Section 2, an important impli-
cation is that the overall SCR deNOx kinetics will be strongly
enhanced in the presence of NO2 and/or HNO3. In fact, the pro-
moting action of NO2 when added to the NO + NH3 reacting
system, leading to the so-called fast SCR, is well known in the
SCR literature.
Notably, the behavior observed after oxidizing the catalyst
with either NO2 or HNO3 was similar. In fact, as presented and
discussed in other works from our group [13,23,24], over V-
based SCR systems at low temperatures NO2 readily undergoes
the disproportion reaction
by HNO3 lies between 100 and 150 ◦C under our experimental
conditions.
Another important feature becomes apparent on inspection
of Fig. 3. For the catalyst samples pretreated with NO2 and
HNO3 (curves (B) and (C), respectively), the behavior dur-
ing the NO + NH3 temperature ramp was identical to that of
the TPR run over the reference oxidized catalyst (curve D) up
to about 170 ◦C; above this temperature, however, curves (B)
and (C) exhibited a marked increment of deNOx activity, fol-
lowed by a sudden drop in NO conversion at around 250 ◦C.
Interestingly, such a temperature threshold corresponds to the
onset of nitrate decomposition, as indicated by the main de-
sorption peaks of O2 and NO2 detected during TPD runs af-
ter buildup of nitrates onto the catalyst by exposure to either
HNO3 (see Fig. 4) or NO2 (with similar results). The forma-
tion/storage of nitrate species onto pure TiO2, WO3/TiO2, and
V2O5–WO3/TiO2 catalysts from NO2 has been reported by sev-
eral authors [35–38]. Thus, Figs. 3 and 4 provide evidence in
favor of a strong promoting action of adsorbed nitrates on the
NO + NH3 SCR reactivity at low temperature. The surface ni-
trates would then be decomposed and depleted above ≈250 ◦C,
explaining the subsequent drop in NO conversion.
2NO2 + H2O ↔ HONO + HNO3,
(8)
so that the reactivities of NO2 and HNO3 can hardly be distin-
guished.
Fig. 3 further suggests that the buildup of nitrates onto the
V-based catalyst (possibly onto sites associated with the W and
Ti components) may play an important role in enhancing the
deNOx efficiency compared with the NO + NH3 + O2 stan-
dard SCR below 250 ◦C. In this respect we have previously
reported evidence pointing to a significant participation of ni-
trates in the mechanism of the fast SCR reaction; specifically,
data from dedicated transient reactivity experiments at 170 ◦C
showed that the rate of fast SCR (2) equals the rate of reduction
of nitrate species (related to NH4NO3) by NO [23,24].
Additional transient experiments illustrated in Fig. 5 provide
more insight into the role of nitrates in the catalyst reoxidation
step, and also into its relationship with the rate of fast SCR.
In Fig. 5, curve (A) represents the evolution of NO conversion
observed during a TPR run at 150–300 ◦C with a feed includ-
ing NO (1000 ppm) + NO2 (500 ppm) + NH3 (1000 ppm). At
150 ◦C, the NO conversion was >20% due to the fast SCR re-
action. At this low temperature, however, NO2 and NH3 also
reacted to form NH4NO3, which partially built up onto the
3.6. Mechanistic implications: catalyst reoxidation and role of
nitrates
As for the catalyst reduction step in Section 4, herein we
discuss the results of Figs. 3 and 4 in relation to their rele-
vant mechanistic implications. So far, only Raman evidence
has been reported in the literature to support that NO2 affects
the reoxidation of V-based SCR catalysts more efficiently than
gaseous oxygen [17]. In this respect, the data in Fig. 3 provide
a direct confirmation that NO2 can reoxidize the reference re-
duced catalyst at a much lower temperature than O2 under SCR
reactive conditions. They further prove that HNO3 is a much
better oxidizing agent as well.

E. Tronconi et al. / Journal of Catalysis 245 (2007) 1–10
7
catalyst [13,24], as pointed out by the following analysis of am-
monia, NO2, and nitrogen traces (not shown in the figure):
catalyst discussed earlier, as well as over a commercial V-free
WO3/TiO2 catalyst. The experiments were designed to analyze
separately the two consecutive reaction steps—nitrate buildup
and their reduction by NO—that previous works identified as
the two major stages in the mechanism of the overall fast SCR
reaction at low temperatures [23,24].
Fig. 6A shows the experiment performed over the V-based
catalyst at 170 ◦C [23]. In the first stage, NH3 and NO2 alone
were fed to the reactor, thereby forming and depositing ammo-
nium nitrate onto the catalyst as confirmed by the consumption
of both reactants and the production of nitrogen, in agree-
ment with reaction (9). Formation of ammonium nitrate, as
estimated from the lack in nitrogen balance, was also in line
with (9). DTA-TG measurements confirmed that the endother-
mic decomposition of ammonium nitrate did not start until
173–174 ◦C, slightly above the temperature of the present ex-
periment. Thus, the formed NH4NO3 remained on the catalyst
sample.
The second part of the run involved removing NO2 from the
feed stream. Thus NH3 readily recovered its feed concentration
level, and the reaction was no longer observed, even though
residual ammonium nitrate was still present on the catalyst. Af-
terwards, NO was admitted to the reactor while ammonia was
still continuously fed (t = 8500 s); NO and NH3 immediately
reacted in equimolar amounts, with production of N2. But the
reaction also involved consumption of NH4NO3 and in fact pro-
ceeded only until depletion of the ammonium nitrate deposited
onto the catalyst.
2NH3 + 2NO2 → NH4NO3 + N2 + H2O.
(9)
With increasing temperature up to about 250 ◦C, NO conversion
was significantly enhanced, eventually exceeding 80%. No-
tably, this is higher than the stoichiometric limit of 50% associ-
ated with the feed concentration of NO2, a clear indication that
the reaction proceeded at the expense of nitrates initially stored
on the catalyst. In fact, NO conversion again dropped suddenly
when the temperature exceeded 250 ◦C (i.e., the temperature
threshold corresponding to the onset of nitrate decomposition
[see Fig. 4]), and eventually approached the steady-state stoi-
chiometric 50% limit.
Curve (B) in Fig. 5 displays the evolution of NO conver-
sion during a temperature ramp in which only NO and NH3
(1000 ppm each) were fed over a reduced catalyst sample that
had been previously pretreated with NO2 (500 ppm). The simi-
larity between curves (A) and (B) is quite evident. In particular,
in the range 150–225 ◦C the temperature dependence of the fast
SCR reaction in the presence of gaseous NO2 (as inferred from
curve (A)) is virtually the same of the NO + NH3 reaction in
the presence of adsorbed nitrates (curve (B)). This strongly sug-
gests that, as in standard SCR, the rate-determining step in the
fast SCR reaction is still associated with reoxidation of the V
sites, which is, however, accomplished more effectively by ad-
sorbed nitrates generated from NO2 than by gaseous oxygen, as
in the case of standard SCR.
For comparison purposes, Fig. 5 displays also curve (C)
(same as curve (A) in Fig. 1) associated with NO conversion
during a NO + NH3 + O2 temperature ramp. The drastically
lower reactivity of NO in standard SCR, in the absence of ei-
ther gaseous NO2 (as in curve (A)) or adsorbed nitrates (as in
curve (B)), is clearly apparent.
We also note that replacing O2 with gaseous NO2 in the
role of V-oxidizing agent, as proposed by Koebel et al. [17] to
account for the higher fast SCR rates, cannot explain the tran-
sient data of the temperature ramps in Figs. 3 and 5. Instead,
V reoxidation must be effected by adspecies stored on the cata-
lyst, either nitrates or possibly molecularly adsorbed NO2. Such
adsorbed species act as reservoirs of oxidizing agents, thus per-
mitting the significant increment of NO conversion observed as
the temperature is raised.
Data from this and other similar experiments [24] have been
interpreted assuming that the conversion of ammonium nitrate
occurred via the reaction between NO and nitric acid (in equi-
librium with ammonium nitrate), which was thus reduced to
nitrous acid with formation of NO2. Then in the presence of ad-
sorbed NH3, nitrous acid would produce N2 via decomposition
of NH4NO2, whereas NO2 would react readily with ammonia
to form more NH4NO3 and N2 [24]. Such a scheme is in fact in
agreement with the observed overall stoichiometry,
NO + (1/2)NH4NO3 → (3/2)N2 + (5/2)H2O.
(10)
To analyze the role of vanadium in this global reactivity, an
identical experiment was run over the V-free WO3/TiO2 cat-
alyst (Fig. 6B). The first part of this experiment shows that
feeding NH3 and NO2 to the reactor resulted in nitrogen pro-
duction, similar to what was seen over the V-based catalysts and
in agreement with reaction (9), indicating that formation and
deposition of ammonium nitrate occurred over the WO3/TiO2
catalyst, just as for the V-based catalyst. Accordingly, vanadium
seems unnecessary for the formation of ammonium nitrate onto
the catalyst surface.
A very different picture appears in the second and final
part of the experiment, when NO was admitted to the reac-
tor (roughly at t = 8250 s). In contrast to the V-based catalyst
(Fig. 6A), on feeding NO to the WO3/TiO2 catalyst, where ni-
trates had been stored previously, here we found no conversion
of either nitric oxide or ammonia, or any nitrogen production.
This indicates that the V-free sample did not allow reduction
of nitrate species by NO, at least under the experimental condi-
3.7. Role of vanadium redox properties in the fast SCR
reaction
Data reported in the previous sections showed that buildup
of nitrates onto the V-based catalyst plays a crucial role in en-
hancing SCR deNOx efficiency. It was suggested that this re-
sults from the fact that nitrates reoxidize the vanadium sites
more efficiently than gaseous oxygen. However, nitrates also
are formed onto sites related to W and Ti [35], the most abun-
dant components of the SCR catalyst. Accordingly, to assess
the role of vanadium (in particular, its redox properties) in the
mechanism of the fast SCR reaction, more dedicated transient
runs were carried out on a comparative basis over the V-based

8
E. Tronconi et al. / Journal of Catalysis 245 (2007) 1–10
◦
Fig. 6. Transient experiments at T = 170 C: formation of NH NO (feed: 1000 ppm NH , 1000 ppm NO , 1% H O); reduction of NH NO by NO (feed:
4
3
3
2
2
4
3
1000 ppm NH , 1000 ppm NO, 1% H O in He). (A) V O –WO /TiO catalyst; (B) WO /TiO catalyst. Inset: TPD run performed at the end of experiment (B).
3
2
2
5
3
2
3
2
tions used here. Notably, the presence of unreacted NH4NO3 on
the V-free catalyst sample was confirmed by an additional TPD
experiment performed at the end of this run (see Fig. 6B, inset).
Heating the WO3/TiO2 catalyst in inert atmosphere provoked
the decomposition of the residual ammonium nitrate, leading
primarily to the evolution of N2O [11,22,24].
cussed thus far and, by identifying the related role of the vana-
dium redox properties, provides a unifying approach to the
mechanisms of the standard SCR and fast SCR reactions.
Standard SCR:
NO + NH*3 + V5+=O → N2 + H2O + V4+–OH,
V4+–OH + (1/4)O2 → V5+=O + (1/2)H2O.
Fast SCR:
(6)
(7)
3.8. A unifying redox scheme for standard SCR and fast SCR
NO + NH*3 + V5+=O → N2 + H2O + V4+–OH,
V4+–OH + NO3− + H+ → V5+=O + NO2 + H2O,
(6)
Herein we propose a set of reactions that formally summa-
rizes the catalytic chemistry underlying the experiments dis-
(11)

E. Tronconi et al. / Journal of Catalysis 245 (2007) 1–10
9
(i) Explains the higher rate of the fast SCR as due to the faster
reoxidation of V sites by nitrates (see Fig. 3).
(ii) Agrees with the behavior observed during transient runs,
when the fast SCR occurred also in the absence of gaseous
NO2 due to the involvement of surface nitrates (see Fig. 5).
(iii) Is consistent with the reducing action of NO on adsorbed
nitrates and ammonium nitrate, observed and reported
herein (Fig. 6A) and in previous publications [13,23,24],
and explains why this reaction does not proceed effectively
on a V-free catalyst (Fig. 6B).
(iv) Can account not only for the deNOx reactions, but also for
the formation of observed byproducts, such as ammonium
nitrate and N2O, both originating from reactions of sur-
face nitrates with ammonia at low and high temperatures,
respectively [12,13,23,24].
4. Conclusion
NO + NH3 TPR runs in the absence of oxygen were used to
examine mechanistic features of the SCR reaction over an in-
dustrial V-based catalyst for diesel exhaust aftertreatment under
representative low-temperature conditions. Through such meth-
ods, we have confirmed that reoxidation of the vanadium sites
by gaseous oxygen is the rate-limiting step in the redox catalytic
mechanism of the standard SCR (NO + NH3 + O2) reaction.
However, the main contribution of the present work con-
cerns the mechanism of the less well-investigated fast SCR
(NO + NH3 + NO2) reaction. TPR data offer direct experimen-
tal evidence that reoxidation of V sites is significantly acceler-
ated in the presence of NO2 and/or HNO3; this explains why
fast SCR is so much faster than standard SCR at low temper-
atures. Our data further suggest that this is not a direct effect,
as was previously proposed in the literature, but rather results
from the crucial participation of nitrates adspecies, formed from
NO2, in reoxidizing the reduced V sites and thus in promot-
ing the rate of the SCR reaction over V-based catalysts. This
aspect is critical to rationalize the transient behavior of the
fast SCR reacting system and for SCR dynamic kinetic mod-
eling.
The present results open the way to a new interpretation of
the complete NO/NO2–NH3 low-temperature catalytic chem-
istry involved in both the standard and fast SCR reactions over
V-based catalysts. It relies on a unifying redox approach in
which the rate-controlling reoxidation of the vanadium catalyst
sites involves gaseous oxygen in the NH3 SCR of NO only, but
is carried out more effectively by surface nitrates when NO2 is
added to the reacting system. Nitrates are formed via dispropor-
tion of NO2 in a secondary catalytic cycle that does not require
a redox function.
Scheme 1. Redox catalytic cycles of the standard and fast SCR reactions over
V O –WO /TiO catalysts. V=O and V–OH are oxidized and reduced vana-
2
5
3
2
dium sites, respectively. S=O is a non-reducible oxidic site.
(12)
(13)
2NO2 + O2− ↔ NO2− + NO−3 ,
NO−2 + NH3* → N2 + H2O + O2− + H+.
The steps associated with the NO + NH3 + O2 standard SCR,
reactions (6) and (7), were discussed in Section 4. Reaction (6)
accounts for reduction of the V5+-sites; reaction (7) is the rate-
limiting reoxidation step involving gaseous oxygen.
In the case of the NO + NH3 + NO2 fast SCR, the reduc-
tion of the V sites still occurs according to the same global
reaction (6), but the rate-determining step in the redox process
(i.e., the reoxidation of V sites) is radically changed, in this
case being carried out by surface nitrates according to reac-
tion (11), which replaces the much slower reaction (7). The
introduction of nitrates brings about additional complexity to
the SCR chemistry, however, so reaction (12) represents forma-
tion of nitrite and nitrate ad-species via disproportion of NO2,
whereas reaction (13) accounts for the decomposition of nitrites
to N2 via reaction with NH3. In agreement with the experimen-
tal observations shown in Fig. 6, no redox catalyst function is
involved in steps (12) and (13), which are assumed to occur
over nonreducible oxidic sites, represented here schematically
as O2−, possibly associated with the W- or Ti-catalyst compo-
nents. A similar sequence has been invoked in the literature to
explain the formation of ammonium nitrate from NO2 + NH3
observed over TiO2 [35] and over zeolites [25], as well as the
formation of nitrates from NO2 over Al2O3 [39]. The redox cy-
cles of the vanadium sites for the standard SCR and fast SCR
reactions are schematically depicted in Scheme 1.
To the best of our knowledge, this redox scheme is consis-
tent with most experimental observations concerning NH3-SCR
chemistry over V-based catalysts available in the literature. It
may prove useful as well to interpret mechanistic data for NH3-
SCR over zeolite-based catalysts. A transient Mars–Van Krev-
elen kinetic model in close agreement with the SCR chemistry
discussed herein is currently under development.
In summary, the proposed redox mechanism:

10
E. Tronconi et al. / Journal of Catalysis 245 (2007) 1–10
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