NH3-SCR reaction mechanisms of NbOx/Ce0.75Zr0.25O2 catalyst: DRIFTS and kinetics studies

Article abstract of DOI:10.1016/j.molcata.2016.06.023

A NbO_x/Ce_0.75Zr_0.25O₂ (NbCZ) catalyst was synthesized by a citric acid-aided sol-gel method. It shows that above 80% NO_x conversion and above 95% N₂ selectivity for the selective catalytic reduction of NO_x by ammonia over this catalyst are achieved in the temperature range 200–450?°C. Based on the DRIFTS and kinetic studies over NbCZ and Ce_0.75Zr_0.25O₂, the promotion mechanism by niobia loading was elucidated with an overall reaction pathway. Two different reaction routes, “L-H” mechanism via “NH₄NO₃?+?NO” at low temperatures (<200?°C) and “E-R” mechanism via “NH₂?+?NO” at high temperatures (>350?°C), are presented. The niobia addition increases the surface acidity and promotes the formation of nitrates species at low temperatures. In this way, the reaction between the ads-NH₃ and nitrates species is accelerated to form NH₄NO₃ intermediates, which then decompose to N₂ and H₂O. The reaction of the ads-NH₃ species with gaseous NO_x at high temperatures is also promoted due to the enhanced acidity and weakened thermal stability of nitrates after niobia loading.

Full text of DOI:10.1016/j.molcata.2016.06.023

Accepted Manuscript
Title: NH3-SCR reaction mechanisms of NbOx/Ce0.75Zr0.25O2
catalyst: DRIFTS and kinetics studies
¨
Author: Ziran Ma Xiaodong Wu Hanna Harelind Duan Weng
Baodong Wang Zhichun Si
PII:
S1381-1169(16)30245-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.06.023
MOLCAA 9929
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
4-5-2016
23-6-2016
24-6-2016
¨
Please cite this article as: Ziran Ma, Xiaodong Wu, Hanna Harelind, Duan Weng,
BaodongWang, ZhichunSi, NH3-SCRreactionmechanismsofNbOx/Ce0.75Zr0.25O2
catalyst: DRIFTS and kinetics studies, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.06.023
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NH3-SCR reaction mechanisms of NbOx/Ce0.75Zr0.25O2
catalyst: DRIFTS and kinetics studies
Ziran Maa,b,1, Xiaodong Wua,*,1, Hanna Härelindc , Duan Wenga,d, Baodong Wangb,*,
Zhichun Sid
a
The Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and
Engineering, Tsinghua University, Beijing 100084, China
b National Institute of Clean-and-Low-Carbon Energy (NICE), Beijing 102209, China
c
Competence Centre for Catalysis, Dept. of Chemistry and Chemical Engineering, Chalmers University of
Technology, SE-412 96 Göteborg, Sweden
d Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
* Corresponding author. Tel.: +86 10 62792375
E-mail address: wuxiaodong@tsinghua.edu.cn (X. Wu), wangbaodong@nicenergy.com (B. Wang)
1 These two authors contributed equally to this work.

Graphical Abstract:

Highlights:
1) NbOx/Ce0.75Zr0.25O2 catalyst shows high NH3-SCR activity in a broad temperature window.
2) Reaction mechanisms on NbCZ catalyst are verified by DRIFTS and kinetic studies.
3) “L-H” and “E-R” mechanisms are presented for NbCZ in different temperature ranges.
4) The NH4NO3+NO reaction is accelerated over NbCZ catalyst at low temperatures.
5) The NH2+NO reaction is also promoted by addition of niobia at high temperatures.

Abstract:
A NbOx/Ce0.75Zr0.25O2 (NbCZ) catalyst was synthesized by a citric acid-aided sol-gel method.
It shows that above 80% NOx conversion and above 95% N2 selectivity for the selective catalytic
reduction of NOx by ammonia over this catalyst are achieved in the temperature range 200-450 °C.
Based on the DRIFTS and kinetic studies over NbCZ and Ce0.75Zr0.25O2, the promotion
mechanism by niobia loading was elucidated with an overall reaction pathway. Two different
reaction routes, “L-H” mechanism via “NH4NO3+NO” at low temperatures (< 200 °C) and “E-R”
mechanism via “NH2+NO” at high temperatures (> 350 °C), are presented. The niobia addition
increases the surface acidity and promotes the formation of nitrates species at low temperatures. In
this way, the reaction between the ads-NH3 and nitrates species is accelerated to form NH4NO3
intermediates, which then decompose to N2 and H2O. The reaction of the ads-NH3 species with
gaseous NOx at high temperatures is also promoted due to the enhanced acidity and weakened
thermal stability of nitrates after niobia loading.
Keywords: CeO2-ZrO2 mixed oxides, Niobia modification, NH3-SCR, Mechanism, DRIFTS

1. Introduction
Selective catalytic reduction with ammonia is an effective technique for the abatement of
NOx from diesel exhaust and stationary sources. To meet stringent legislations, V2O5-WO3/TiO2
catalyst has been commercially applied in these fields for decades [1, 2]. However, some
drawbacks still remain for vanadium-based catalysts, including the narrow operation temperature
window (300-400 °C), toxicity and volatility of V2O5 species and the poor N2 selectivity in the
high temperature range due to significant N2O formation. To avoid such problems, many
researchers have been taking persistent efforts to develop novel and environmentally friendly
catalysts with high NOx conversion, high N2 selectivity and nontoxic species.
Ceria and ceria-containing materials have been widely employed as catalysts because of the
facile redox cycling between Ce3+ and Ce4+. The acid-base properties, nontoxicity and lower cost
of ceria also make them promising candidates for NH3-SCR catalysts. Recently, many solid-acid
modified Ce based catalysts have been reported, such as CeO2-WO3 [3-9], CeO2-Nb2O5 [10-15],
3-
2-
CeO2-MoO3 [16-22], CeO2-PO4 [23-26] and CeO2-SO4 [27-30]. Among them, CeO2-Nb2O5
based catalyst appears to be very promising due to its superior low-temperature activity, wide
operation temperature window and acceptable resistances to SO2 and alkali metals. The NH3-SCR
mechanisms over solid acid-ceria catalysts have been studied by some researchers. Chen et al. [3,
4] presented that the addition of tungsten oxide to CeO2 catalyst can promote the NO oxidation to
NO2, with which the adsorbed NH3 species react readily to generate N2 and H2O via an
Eley-Rideal (E-R) mechanism. The reaction of the adsorbed NH3 and nitrates species according to
the Langmuir-Hinshelwood (L-H) mechanism is another crucial reaction route on CeO2-WOx
catalyst. This mechanism is later supported by Peng et al. [8] based on in-situ DRIFTS and in-situ
Raman techniques. They proposed that CeO2-WOx catalyst follow a double-separated acid and
redox cycle, in which tungsta and ceria played critical roles, respectively. A similar “E-R”
mechanism was presented as the main reaction mechanism by Zhu et al. [16] on MxOy (M=Ni, Cu
+
and Fe)/MoO3/CeO2 catalyst. They thought that the NH3 or NH4 species adsorbed on different
acid sites are activated through deprotonation process (partial oxidation) to form NH2 (amide) or
+
NH3 species. These activated species react with the gas phase NO subsequently to form a NH2NO
nitrosamide species, from which N2 and H2O are produced. Liu et al. [20] suggested that over

CeO2-MoOx catalyst, both adsorbed NH3 and weakly adsorbed NO2, could participate in “L-H”
mechanism at low temperatures. At high temperatures, coordinated NH3 species combine with
gaseous NO, forming an intermediate which would finally decompose to H2O and N2. In the study
of Yu et al. [25], it was proposed that the SCR reaction over Zr3(PO4)2/CeO2-ZrO2 proceeds via
the combination of the adjacent surface NxOy species and ads-NH3 species via a “L-H” mechanism.
In summary, some common reaction routes were presented for solid acid-ceria catalysts.
Investigations on the mechanism of CeO2-Nb2O5 based catalyst have been rarely reported.
Casapu et al. [10] found that the slightly lower stability of the NxOy adsorbed species on the
MnNbCe surface as well as the larger amount of NO2 evolved at low temperature represents an
important effect of the niobium addition to the MnCe catalyst. Qu et al [13] suggested that the
abundance of surface adsorbed oxygen might arise from the short-range activation effect of
niobium oxide species to cerium oxide species. Recently, they pointed out that the SCR reaction
pathway over the Ce–Nb catalysts may follow both the “E-R” and “L-H” mechanisms, where the
former contributed more especially at high reaction temperatures [15]. Similarly, our previous
-
study [14] also observed the enhanced NH3 activation and NO3 formation, and the subsequent
-
promoted reaction of ads-NH3 and ads-NO3 species for the SCR reaction over NbOx/CeO2-ZrO2
catalysts according to the “L-H” mechanism. At high temperatures, the thermal stability of the
-
adsorbed NO3 species are reduced significantly, and thus the SCR reaction on this catalyst mainly
follows the “E-R” mechanism via the reaction of ads-NH2 species and gaseous NO. However, the
detailed reaction routes of CeO2-NbOx based catalysts remain to be studied. More in-situ DRIFTS
and kinetics studies would be very beneficial to help us to elucidate the aspects by niobia
modification in the surface properties of ceria based catalysts and propose the specific reaction
mechanisms of the modified catalysts at different temperatures.
2. Experimental
2.1. Catalyst preparation
Ce0.75Zr0.25O2 (CZ) catalyst was synthesized by a citric acid-aided sol-gel method using
Ce(NO3)4·6H2O and Zr(NO3)4·5H2O as the precursors according to the procedure reported in Ref.

[14]. The resultant gel was dried at 110 °C overnight and then subjected to decomposition at
300 °C for 1 h and calcination at 550 °C for 3 h under air at ambient conditions. The
NbOx/Ce0.75Zr0.25O2 (NbCZ) catalyst was subsequently synthesized by impregnating the
as-received Ce0.75Zr0.25O2 powders with C12H7NbO24 solution, with the impregnated powders
dried at 110 °C overnight and calcined at 550 °C for 3 h in static air. The loading amount of niobia
was 15 wt.%. For comparison, CZ was also treated through the above calcination procedure.
2.2. Activity measurement
The measurements of the steady-state activity were carried out in a fixed-bed quartz reactor
(inner diameter 10 mm) under the following conditions: 200 mg sample with 60-80 mesh, 500
ppm NH3, 500 ppm NO, 5% O2, 12% CO2, 5% H2O (when used), N2 as balance, 500 mL min-1
total flow rate, and GHSV= 3×105 h-1. The concentrations of the outlet gases were real-timely
monitored by a Thermo Nicolet 380 FTIR spectrometer. All the turbings in the reactor system
were heat-traced to 190 °C to prevent water condensation and ammonium nitrate deposition. The
NOx conversion (X), N2 selectivity (S) and pseudo-first order rate constant (k) of NOx reduction
were calculated according to Eqs. (1)-(3), which were based on the data collected when the
reaction substantially reached a steady state condition after about 30 min at each temperature. In
Eq. (3), F and W were the total flow rate of the flow gas and sample mass, respectively [14].
[NO]out [NO2 ]
out )100
(1)
X (%)= (1
[NO]in
[NO2 ]out  2[N2O]out
[NH3]in [NO]in [NH3]out [NO]out
(2)
(3)
S (%) = (1
)100
F
k =  ln (1 X)
W
Steady state kinetics experiments were conducted in a fixed bed quartz reactor every twenty
degrees between 180 and 260 °C. Thus stable and low conversions of NOx (< 20%) were achieved
in an approximate kinetic regime. Over this temperature range, the NH3 conversion was consistent
with NO conversion for NbCZ with no outlet N2O and NO2 detected, suggesting NH3 and NO
oxidation is negligible. A much higher gas hourly space velocity (1.2×106 h-1, with a total flow

rate of 2 L min-1) and less than 200 mesh size powder catalyst were adopted to achieve kinetically
controlled conditions and absence of external mass transport limitations. The typical reactant gas
composition included 200-800 ppm NO, 200-800 ppm NH3, 5% O2, and N2 in balance.
The rate of NO reduction as a function of reactant concentrations was expressed as a
power-law rate in Eq. 4.
RNO  k[NO] [NH3] [O2 ]
(4)
where, RNO was the SCR rate, k was the apparent rate constant, and α, β, and γ were the reaction
orders for NO, NH3 and O2, respectively.
2.3. Catalyst characterization
Temperature-programmed desorption of NOx (NOx-TPD) experiments were carried out in a
fixed bed quartz reactor with the effluent gases monitored using a Thermo Nicolet 380 FTIR
spectrometer. 100 mg powder catalyst was preheated in flowing 5% O2/N2 at 500 °C for 30 min to
remove any possible impurities. After cooling down to 30 °C, the sample was exposed to a flow of
1000 ppm NO + 5% O2 in N2 for 60 min, followed by flushing with N2 for 30 min to remove the
physical-adsorbed molecules. Afterwards, the catalyst was heated up to 500 °C in 5% O2/N2 at a
rate of 10 °C min-1 for desorption of NOx.
In situ DRIFT spectra were recorded by a Nicolet 6700 FTIR spectrometer equipped with a
high-temperature environmental cell fitted with KBr window. The catalyst was finely ground and
placed in a ceramic crucible and manually flattened. Prior to each experiment, the catalyst was
pretreated at 500 °C in 10% O2/N2 with a total flow rate of 100 mL min-1 for 30 min to remove
traces of organic residues. Afterwards, background collection was performed after the sample was
flushed by 100 mL min-1 N2 for 30 min at the corresponding temperature. For NH3 adsorption, a
feeding gas containing 1000 ppm NH3 in N2 with a total flow rate of 100 mL min-1 passed through
the sample for 60 min. After purging physisorbed NH3 molecules by N2 flow for 30 min, the
DRIFT spectra of adsorbed species on catalysts were collected at the same temperature.
NO+NH3+O2 co-adsorption were carried out through a similar procedure, in which 100 ml min-1
feeding gas with 1000 ppm NO + 1000 ppm NH3 + 10% O2 in N2 was introduced to the
N2-pretreated sample.

For the NO+O2 reaction with pre-adsorbed NH3 species over the catalyst, a similar procedure
was performed for the pretreatment and the background collection steps. The catalysts were
pre-adsorbed with 1000 ppm NH3/N2 for 60 min at 180 °C, followed by N2 purging for 30 min to
remove the weakly adsorbed NH3 species. The 1000 ppm NO + 10% O2 in N2 with a total flow
rate of 100 ml min-1 was then introduced into the IR cell, and the spectra were recorded as a
function of time. Similar procedures were carried out for the NH3 and NH3+NO reaction with
pre-adsorbed NxOy species over the catalyst.
3. Results
3.1 SCR activity
Fig. 1 shows the SCR activities of CZ and NbCZ catalysts as a function of temperature.
Without Nb modification, CZ shows poor activity within the whole temperature range (100-500 °C),
with the maximum NOx conversion less than 50% and even negative values at temperatures higher
than 400 °C as a result of NH3 over-oxidation to NOx in Fig. 1a. The activity of NbCZ is greatly
improved with above 80% NOx conversion achieved at a wide temperature range from 200 to
450 °C. Furthermore, little N2O or NO2 is generated over NbCZ, accompanied by N2 selectivity
exceeding 95% within the whole temperature range. This indicates that the introduction of Nb
species to CZ could suppress the unselective catalytic oxidation of NH3 by O2 to N2O and NO2 at
high temperatures as shown in Fig. 1b. The presence of H2O in the feeding gas reduces the
catalytic activities at low temperatures (< 350 °C) but enhanced the activities at high temperatures
(> 350 °C) for both the catalysts. The inhibition effect of water on the low-temperature SCR
reaction is usually explained as an effect of a competitive between H2O and NH3/NOx on the
reaction sites [7]. The latter is usually explained as an inhibition effect of H2O on the unselective
catalytic oxidation of NH3 [31], which also leads to increased N2 selectivity at high temperatures.
Fig 1c shows the k values of CZ and NbCZ at various temperatures, which are in similar trends
with NOx conversions in Fig. 1a.
3.2 Surface acidity

In order to determine the types of surface acid sites and their stability, DRIFT spectra of
adsorbed ammonia species at different temperatures on CZ and NbCZ were collected in the range
of 4000-2500 and 1800-900 cm-1, and the results are shown in Fig. 2. The bands at 1150 and 1194
cm-1 are assigned to the symmetric bending vibration of NH3 coordinated to Lewis acid sites [7,
13, 20], while those at 1418 and 1667 cm-1 are attributed to the asymmetric and symmetric
+
bending vibrations of NH4 bonded to Brønsted acid sites, respectively [7, 12, 13]. In the N-H
stretching region, bands at 3390 and 3258 cm-1 are indicative of NH3 adsorbed on Lewis acid sites
[3, 12, 13]. The negative bands at 3768 and 3674 cm-1 are assigned to the O-H stretching in the
surface hydroxyl groups [3, 7, 32]. It is worth noting that there are two types of Lewis acid sites
on CZ, probably corresponding to the unsaturated Cen+ and Zrn+ ions [8, 29], as indicated by two
bands corresponding to symmetric bending vibrations of NH3 at 1150 and 1194 cm-1, respectively.
Amide (NH2) species (1550, 1300 and 1095 cm-1) [3, 20, 33] deriving from the deprotonation of
ammonia via partial oxidation are not stable at temperatures above 150 °C.
Fig. 2b shows the DRIFT spectra of NH3 adsorption on NbCZ. New bands at 3158, 3000 and
2750 cm-1 are attributed to NH4+ species bonded to Brønsted acid sites [12, 34, 35], while those at
1590 cm-1 and in the range of 1050-1250 cm-1 are assigned to NH3 species coordinated to Lewis
acid sites [3, 7, 20]. Clearly, the numbers of both Lewis and Brønsted acid sites increases on
NbCZ as indicated by the stronger bands compared with those on CZ. Considering the decrease in
the surface coverage of Cen+/Zrn+ after niobia loading, these additional Lewis acid sites should
derive from the formation of unsaturated Nbn+ cations. According to our previous study [14], the
additional Brønsted acid sites results mainly from Nb-OH bonds, which is formed by combination
of surface H2O and Nb=O bond as indicated by the bands at 973-991 cm-1 [13, 36]. Different from
+
CZ, the bands of NH3/NH4 and NH2 (at 1560 and 1295-1330 cm-1) species on NbCZ can be
observed at the temperature as high as 350 °C, indicating more NH3 or NH2 like-species available
at high temperatures. NH2 is important for the high-temperature SCR reaction as an intermediate
in the “E-R” route [3, 20, 31, 35].
Fig. 3 shows the Lewis and Brønsted acidities, which were obtained by calculating the
integrated areas of bands at 1095-1212 and 1418-1430 cm-1 in the DRIFT spectra in Fig. 2,
respectively. The data of IR intensity were normalized by the catalyst surface areas (102 and 74

m2 g-1 for CZ and NbCZ, respectively). Both the amounts of Lewis and Brønsted acid sites follow
the order of NbCZ > CZ, demonstrating the contribution of niobia to the total surface acidity. It is
noted that Lewis acid sites on NbCZ appear to be more strongly coordinated to NH3 at the
temperatures higher than 350 °C. Due to its stronger interaction with ammonia than Cen+ or Zrn+
cations, the unsaturated Nbn+ cations in the surface NbOx species could increase the thermal
stability of adsorbed ammonia on NbCZ.
Fig. 4 shows the TPD profiles of NOx (NO2 and NO) for CZ and NbCZ catalysts in the
temperature range 30-500 °C. Both the catalysts show several obvious NOx desorption peaks in
low (50-110 °C) and high (250-450 °C) temperature ranges, corresponding to the desorption of
-
-
weakly adsorbed NxOy (NO- or NO2 ) species and decomposition of NO3 (nitrates), respectively
[32, 37, 38]. The total amounts of desorbed NOx were estimated from these profiles and the values
are 1.92 and 1.00 μmol m-2 for CZ and NbCZ, respectively, indicating the inhibition effect of Nb
species on the NOx adsorption. The NO desorption peak centered at 420 °C on CZ is mainly
produced by the thermodynamic balance of NO2-NO cycle from the NO2 derived from
decomposition of nitrates [37], and hereby should be ascribed to NO2 when calculating the
NO2/NOx (NO2+NO) ratio. In this way, the calculated NO2/NOx ratios are 99% and 93% for
NbCZ and CZ, respectively, indicating that more NO2 than NO like-species desorbs from the
former catalyst. This illustrates that almost all the NOx adsorbed species over NbCZ are nitrates.
Furthermore, the NO2 desorption peak shifts to lower temperature for NbCZ, suggesting the
reduced thermal stability of nitrates after niobia loading. These probably result from the abundant
active oxygen species at the Nbn+-O2--Cen+ interface and the acidic nature of surface NbOx species,
as discussed in our previous study [14]. It has been reported the unstable nitrates would readily
participate in a “L-H” reaction route as the reactants, while the stable nitrates could potentially
deactivate the ceria based catalysts by blocking the interaction of Cen+ active sites with NH3 [3, 20,
25, 39]. In this sense, the less stable nitrates on NbCZ are more favorable to NH3-SCR reaction
than those stable ones on CZ.
3.3 DRIFTS studies
3.3.1. Reaction between NOx and ad-NH3 species

The DRIFT spectra of CZ and NbCZ catalysts with the pre-adsorbed NH3 upon exposing in
NO+O2 at 180 °C as a function of time are shown in Fig. 5. Generally, the amount of the NH3
species adsorbed on the catalysts decreases upon exposing in NO+O2, indicating their
consumption by reacting with gaseous NOx/surface NxOy species. After pre-adsorption of
ammonia, NH3 species coordinated to Lewis acid sites (1148, 1198, 3275 and 3380 cm-1) and a
+
few NH4 species bonded to Brønsted acid sites (1433 cm-1) are present on the surface of CZ.
Bands assigned to NxOy species such as bridging nitrates (1606 and 1157) [3, 40], bidentate
nitrates (1012, 1550 and 1579 cm-1) [20, 40], monodentate nitrates (1527 and 1246 cm-1) [20, 32,
-
40] nitrates, NO- (1184 cm-1) [10, 40], NO2 (1494 cm-1) [40] and trans-N2O22- (1403 cm-1) species
[4, 40] are readily evident after introduction of NO+O2 for 1 min. These bands increase in
intensity gradually with time, potentially due to the basic nature of ceria. It is worth noting that
NH3 species coordinated to Lewis acid sites (3275 and 3380 cm-1) resides even after 30 min,
suggesting a slow consumption of NH3 species by NxOy species or gaseous NOx over CZ.
The bands of NH3-derived species, i.e. coordinated NH3 (1196, 1597, 3242 and 3358 cm-1),
NH4+ (1420 and 1665 cm-1) and amide species (1326 cm-1), are much more pronounced on NbCZ
catalyst. When the catalyst is purged with NO+O2 for just 3 min, all the bands ascribed to
ammonia ad-species diminish, accompanied by the strengthened bands corresponding to NxOy
species. This means that niobia loading can accelerate the reaction between surface NxOy/gaseous
NOx and ad-NH3 species. Moreover, the nitrates related bands are obviously lower in intensity on
this catalyst after exposing in NO+O2 for 30 min, confirming again that surface sites are less
saturated by NxOy species after niobia loading.
3.3.2. Reaction between NH3 and ad-NxOy species
The DRIFT spectra of catalysts with the pre-adsorbed NxOy species upon exposing in NH3 at
180 °C as a function of time are shown in Fig. 6. After pre-adsorption in NO+O2, the surface of
the catalysts are covered by bridging nitrates (1606-1613 and 1157-1160 cm-1), bidentate nitrates
(1009-1012, 1543-1556 and 1572 cm-1) and monodentate nitrates (1524-1538 and 1236-1252
cm-1). Additional bands at 1033 cm-1 on CZ can be assigned to cis-N2O22- species [4, 40]. CZ and
NbCZ show similar variations of the DRIFT spectra after introduction of NH3. Unlike the case of

pre-adsorbed NH3 in Fig. 5, most of the pre-adsorbed NxOy species are stable on the catalysts even
after the introduction of NH3 for 30 min, and the bands of NH3-associated species are not
observed within the first 5 min. These indicate that it is quite difficult for NH3 to be adsorbed on
the NOx-pre-adsorbed catalyst, and the reaction between ad-NH3 and ad-NxOy species is probably
+
limited by strong NxOy coverage. Nevertheless, it is seen that the bands ascribed to NH4 on
Brønsted acid sites (1430 and 1665 cm-1) and those to NH3 coordinated to Lewis acid sites (3379
and 3247 cm-1) are significantly stronger on NbCZ than those on CZ (1430, 3363 and 3264 cm-1)
after 30 min. This fact verifies again that the introduction of niobia could increase the number of
acid sites and induce NH3 and NOx to be adsorbed on Nbn+/Nb-OH and Cen+ sites separately,
which can compensate for the side effect of competitive adsorption of NH3 and NxOy on Cen+ sites.
3.3.3. Reaction between NO+NH3 and ad-NxOy species
It is worth studying for co-introducing NO and NH3 on the NxOy pre-adsorbed catalysts since
there are always these two gaseous reactants in SCR reaction. As shown in Fig. 7, the nitrate
related bands including bridging nitrates (1606-1613 and 1157-1160 cm-1), bidentate nitrates
(1012-1014, 1543-1556 and 1572 cm-1) and monodentate nitrates (1524-1538 and 1235-1252
+
cm-1), as well as NH4 on Brønsted acid sites (1430 and 1665 cm-1) and NH3 coordinated to Lewis
acid sites (3363-3379 and 3247-3264 cm-1), are observed in the spectra. NH4NO3 species with
bands at 1360 and 1380 cm-1 [4, 41, 42] is formed on CZ after 3 min in Fig. 7a, which is not
observed in the case of passing NH3 over CZ with pre-adsorbed NxOy (Fig. 6a). These bands, as
well as those nitrate related ones, always exist with time. It indicates that the formation of
NH4NO3 requires co-feeding gaseous NO and NH3, and it as an intermediate cannot react further
and accumulate on CZ.
Interestingly, the intensities of NH4NO3 related bands reach the maximum immediately on
NCZ after co-introduction of NO and NH3 and then decrease with time. Meanwhile, the intensities
of the nitrate related bands decline much more quickly on this catalyst. These indicate that in the
presence of gaseous NH3 and NO, niobia loading facilitates the reaction between surface NxOy
species and ads-NH3 species and accelerates the decomposition of NH4NO3. Therefore, it is
plausible to suggest that the SCR reaction over NbCZ catalyst at 180 °C may occur via the

combination of surface NxOy and ads-NH3 species to NH4NO3 and its further decomposition to
H2O and N2.
3.3.4 NH3+NO+O2 co-adsorption
To identify surface species presented on catalyst under reaction conditions, the DRIFT
spectra of CZ and NbCZ following contact with NH3+NO+O2 at various temperatures are shown
in Fig. 8. NO- species (1180 cm-1), bridging nitrates (1604 and 1213 cm-1), bidentate nitrates (1554,
2-
1576 and 1008 cm-1), monodentate nitrates (1529 and 1237 cm-1) and cis-N2O2 (1026 cm-1) are
present on CZ. The bands attributed to coordinated NH3 to Lewis acid sites (3380 and 3275 cm-1)
are also detected. The oxidation of weakly adsorbed NO- species (1180 cm-1) to more stable
nitrates is observed at 150-250 °C. The NO related band (1690 cm-1) appears at temperatures no
less than 250 °C. With the temperature increasing, the NH3 related bands diminishes and the
nitrate related bands increase in intensity, indicating that the surface of CZ is dominantly covered
with nitrates especially at high temperatures. It is difficult for ammonia to be adsorbed on the
surface strongly covered with nitrates. The formation of NH4NO3 (1380 and 1360 cm-1) can be
observed in the temperature range 200-350 °C. This species disappear at high temperatures, which
may be caused by the competitive oxidation of ammonia in the presence of oxygen.
For NbCZ catalyst, bridging nitrates (1606 and 1210 cm-1), bidentate nitrates (1565 and 1015
cm-1) and monodentate nitrates (1531 and 1237 cm-1) are observed in Fig. 8b. Compared with CZ,
NO- species is absent on NbCZ. Instead, nitrates turn to be the main NxOy species even at a
temperature as low as 150 °C, which should be due to facile oxidation of adsorbed NO- to nitrates.
Again, the bands associated with nitrates decrease in intensity more rapidly with temperature,
indicating that the thermal stability of nitrates is greatly weakened by niobia addition. These
correlate well with the NOx-TPD results in Fig. 4. NH4NO3 species (1380 and 1360 cm-1), an
intermediate from the reaction between surface ads-NH3 and nitrates species, appears in the
temperature range 150-300 °C. This confirms again that the “L-H” mechanism is the main
reaction route over NbCZ at low temperatures (< 300 °C). Compared with CZ, the NH3 derived
bands disappear on NbCZ. Considering the stronger acidity of NbCZ and less inhibition effect of
nitrates on NH3 adsorption (as shown in Figs. 6 and 7), this abnormal phenomenon is probably

caused by the accelerated SCR reaction which consumes most of the surface adsorbed NH3 on
NbCZ.
3.4 Kinetic studies
To determine the reaction orders with respect to NO and NH3 over NbCZ at various
temperatures, the concentration of NO/NH3 was varied from 200 to 800 ppm with another reactant
(NH3/NO) and O2 kept at 500 ppm and 5%, respectively. Regression analysis of the experimental
data was performed to calculate the reaction orders with confidence intervals of 99%, and the
results are listed in Table 1. The rates of NO reduction as a function of the inlet NO and NH3
concentrations at 180-260 °C are shown in Fig. 9.
As shown in Fig. 9a, the rates of NO reduction at different temperatures are found to increase
linearly with NO concentration. The reaction order for NO (α) is between 0 and 1, which is
explained by the fact that NO can participate in SCR reaction in the form of both gaseous and
surface adsorbed NxOy species [43]. Therefore, SCR reaction may follow both “E-R” and “L-H”
mechanisms over NbCZ catalyst. It is interesting to note that the reaction rate at 180 °C almost
keeps steady when NO concentration increases from 200 to 800 ppm. The reaction order for NO
concentration (α=0.365) is close to zeroth-order, indicating that NO involves in SCR reaction
mainly as adsorbed species at low temperatures according to “L-H” mechanism. At the
temperatures higher than 180 °C, the value of α is more close to 1, suggesting that gaseous NO
plays a more critical role in the reaction, which is possibly associated with the more unstable
nature of nitrates species. In this case, “E-R” mechanism accordingly becomes the dominating
reaction route on NbCZ at high temperatures.
It is noted in Fig. 9b that NO reduction rate decreases slightly with increasing NH3
concentration in the presence of 5% O2 at 180 and 200 °C. The reaction orders (β) with respect to
NH3 are found to be -0.177 and -0.096. These values are close to zeroth-order for NH3, suggesting
that NH3 needs to be initially adsorbed on the catalyst in order to react with surface NxOy/gaseous
NOx. Interestingly, the correlations between NO reduction rate and NH3 concentration at
220-260 °C show diverse features in the low and high NH3 concentration ranges. The absolute
value of β1 increases from 0.172 to 0.366, whereas that of β2 decreases from -0.140 to -0.057. A
similar case has been reported by Orlink on a V2O5/TiO2 catalyst [44]. It is usually explained by

that a side NH3-oxidation reaction besides the NH3-SCR reaction accounts for the decreased
apparent NO reduction rate at high NH3 concentrations [45-47]. However, in this study, NH3
oxidation is negligible at 180-260 °C (not shown) [14]. Therefore, the inhibition effect of
excessive NH3 on NO reduction rate results probably from the competitive adsorption of NH3 and
NO, in which the adsorptive sites for NxOy species may be blocked by surface NH3 species. This
speculation is supported by the fact that the absolute value of β (180-200 °C) or β2 (220-260 °C)
decreases with temperature, as the “E-R” reaction route is more dominant at high temperatures
and surface NxOy species plays a less important role.
4. Discussion
As shown in Fig. 1, the SCR activity of CZ is quite poor at 180 °C, while about 80 % NOx
conversion is achieved over NbCZ. At this temperature, more NH3 is adsorbed on NbCZ as shown
in Fig. 3, indicating a stronger NH3 adsorption capacity derived from niobia loading. Based on the
DRIFTS studies, Cen+ sites are the main sites for both NH3 and NO adsorption on CZ (Eqs. 5 and
6), while NO and NH3 are preferentially adsorbed on Cen+ and Nb-OH/Nbn+ sites of NbCZ (Eqs.
6-8), respectively. All the NH3 related bands diminish quickly when passing NO+O2 over NbCZ
with pre-adsorbed NH3 (Fig. 5b). Some nitrates species are also removed by purging NH3+NO
over this modified catalyst (Fig. 7b). In the NH3+NO+O2 co-adsorption experiment (Fig. 8), both
the NH3 and NO derived species are observed and evolve with temperature, indicating the
co-adsorption and reaction of the two reactants. These results suggest that both adsorbed NH3 and
NxOy species could participate as important intermediates in the SCR reaction over NbCZ, which
is further verified by the approximate zeroth-orders for NH3 and NO at 180 °C in the kinetic study
(Fig. 9). Thus, both adsorptions of NH3 and NOx are important intermediate processes in SCR
reaction and the reaction over NbCZ at low temperatures is believed mainly to follow an “L-H”
route.
As shown in Figs. 7 and 8, NH4NO3 formed by the reaction between ads-NH3 and NxOy
species is a critical intermediate species in this “L-H” reaction route (Eq. 9). Fig. 10 shows the
evolutions of the band annotated as bidentate nitrate estimated from Figs. 6 and 7 as a function of

reaction time. It keep almost unchanged on CZ in the presence and absence of gaseous NO, while
different trends are observed on the modified catalyst. Combined with high SCR activity of NbCZ,
it implies that the presence of NO is a necessary condition for reactions between ads-NH3 and
NxOy species. It is clear that Nb loading accelerates the reaction between ads-NH3 and NxOy
species as indicated by the fast formation of NH4NO3 within the first 1 min, while it takes about 5
min for the NH4NO3 related band to reach the maximum intensity on CZ. As the reaction
continues, NH4NO3 species accumulates on CZ, while it is consumed quickly on NbCZ, indicating
the promoted decomposition of ammonium nitrates by the addition of niobia to the catalyst. It has
been reported that the direct decomposition of NH4NO3 would produce N2O [48], which is not
detected at the temperatures below 300 °C over NbCZ in this study (Fig. 1b). According to the
literatures [25, 32, 39, 41, 48-50], NH4NO3 does not behave as a terminal product in a stream
containing NO and is suggested to be efficiently reduced to NH4NO2 (Eq. 10), from which N2 and
H2O are swiftly produced (Eq. 11). Hence, the reduction of NH4NO3 to NH4NO2 by NO and the
simultaneous feasible decomposition of NH4NO2 are considered as the important steps of the
“L-H” reaction route for NbCZ catalyst at low temperatures. Similar mechanisms have been also
reported for some Ce [4, 25] and Mn catalysts [32, 51].
On the other hand, it is noticeable in Figs. 4 and 8b that the adsorbed nitrates species are
unstable and their thermal stability is significantly weakened by niobia loading. In this case, the
SCR reaction on NbCZ at high temperatures (> 350 °C) is suggested to follow an “E-R”
mechanism. The increased reaction order (α) for NO (Table 1) also indicates that gaseous NO
behaves a more crucial role in the NH3-SCR reaction route at high temperatures. It is concluded
by a lot of researchers [3, 4, 8, 16, 20] that this “E-R” mechanism involves the formation of amide
(NH2) species via deprotonation of NH3, and the subsequent reaction between NH2 and gaseous
NO to generate N2 and H2O. In our study, it is found that both the coordinated NH3 to Lewis acid
+
sites and NH4 to Brønsted acid sites can participate the SCR reaction, in agreement with the
literatures [7, 8, 20]. However, it should also be addressed that the stable nitrates on CZ could
occupy the surface active sites and prohibit the competitive adsorption of NH3 as shown in Fig. 8a.
A similar inhibition effect by NO adsorption has been proposed by researches on catalysts such as
WOx-CeO2/TiO2 [3], MoOx-CeO2 [20] and ZP/CZ [25].

According to the above discussion, the NH3-SCR reaction mechanism over NbCZ catalyst
can be proposed as “L-H” and “E-R” mechanisms at low and high temperatures, respectively, and
the tentative mechanisms are presented in Fig. 11. The introduction of niobia to CZ can increase
the NH3-SCR activity significantly at both low and high temperatures. The promotion effects of
niobia on the low-temperature activity include the following aspects. Firstly, it is proposed in Figs.
2-
4 and 8b that the NxOy species on NbCZ is mainly nitrates, while large amount of NO- or N2O2
species are present on CZ. This indicates that the introduction of niobia facilitates the oxidation of
NO-like (NO- and N2O2) species on Cen+ sites to nitrates, which can be ascribed to the better
reducibility derived from the Nbn+-Cen+ interaction that induces the generation of more Ce3+
cations and weakly bonded oxygen species[12,13]. According to the XPS measurements in our
previous work [14], the surface atomic ratios of Ce3+/Ce are 18% and 39% of CZ and NbCZ, and
the relative ratios of weakly bonded oxygen Oα/O are 9% and 17%, respectively. Meanwhile, the
weak bonding of NO2-like species on NbCZ at relatively low temperatures may also enhance the
activity by facilitating the “fast SCR” reaction. Secondly, the introduced abundant Nbn+ and
Nb-OH sites as acid sites increase the surface acidity of the catalyst, making ammonia and NOx
adsorbed on Nbn+/Nb-OH and Cen+ sites separately rather than their competitive adsorption on
Cen+ sites. This weakens the inhibition effect on NH3 adsorption by stable nitrates and promotes
the occurrence of the SCR reaction. Finally, it has been reported that solid acids can catalyze both
the reaction of NH4NO3 to NH4NO2 [48, 52] and the further decomposition process of NH4NO2
[39, 50] at significantly low temperatures (< 100 °C). However, if the accumulated NH4NO3 could
not be efficiently reduced by NO at 180 °C, it may deactivate the catalyst by blocking the “L-H”
reaction route [48]. Accordingly, the reaction of ads-NH3 with nitrates species, the reduction of
NH4NO3 to NH4NO2 species and the further decomposition of NH4NO2 species are all accelerated
on NbCZ catalyst as indicated in Fig. 10, resulting in a superior low-temperature activity.
At high temperatures, the stable nitrates on CZ occupy Cen+ active sites and inhibit the
adsorption of NH3. This, as well as the competitive oxidation of NH3, leading to negative
conversion of NOx. Contrarily, the unstable nitrates on NbCZ could provide more Cen+ sites
available for NH3 adsorption. More importantly, the abundant Nb=O bonds in surface NbOx
species could also promote the activation of NH3 to NH2 as shown in Fig. 2b (Eq. 12). In this way,

as confirmed by the kinetic study (Fig. 9), activated NH2 species readily reacts with gaseous NOx,
resulting in promoted high-temperature activity of NbCZ catalyst (Eq. 13).
NH3 + Cen+  Cen+ -NH3 (ads)
(5)
NO + Cen -O2 O*  Cen -NO3 (ads)
(6)
(7)
+
NH3 + Nb-OH  Nb-O-NH4 (ads)
NH3 + Nbn+  Nbn+ -NH3 (ads)
(8)
NH3(ads) + NO3 (ads) + H  NH4NO3 (ads)
NH4NO3(ads) + NO  NH4NO2(ads) + NO2
NH4NO2 (ads)  H2O + N2
(9)
(10)
(11)
(12)
(13)
Nb=O+ Nbn+ (Cen+ ) + 2NH3  Nb-O-NH4 + Nbn+ (Cen+ )-NH2 + e
Nbn+ (Cen+ )-NH2 (ads) + NO  N2 + H2O
5. Conclusions
NbOx/Ce0.75Zr0.25O2 catalyst exhibits above 80% NOx conversion and above 95% N2
selectivity in the temperature range from 200 to 450 °C. Based on the in situ DRIFTS and kinetic
+
-
studies, it is proposed that both a “NH4 +NO3 ” reaction route at low temperatures and a
“NH2+NO” reaction route at high temperatures can be accelerated by niobia loading. The
enhanced acidity by niobia addition can accelerate the reaction between ads-NH3 and nitrates
species to form NH4NO3, the reduction of NH4NO3 by NO and the further catalytic decomposition
of NH4NO2 to N2 and H2O at low temperatures. At higher temperatures, more stable acid sites
arising from niobia enhance the ammonia adsorption capacity and decrease the stability of nitrates
species, contributing to the reaction of activated NH2 species with gaseous NO.
Acknowledgements
The authors would like to acknowledge the financial support from projects of the National
Natural Science Foundation of China (Nos. 51202126 and 51372137) and the Ministry of Science

and Technology, PR China for financial support from Project 2015AA034603.
This work has been partly performed within the Competence Centre for Catalysis (KCK) at
Chalmers University of Technology. KCK is financially supported by Chalmers University of
Technology and financially supported by the Swedish Energy Agency and the member companies:
AB Volvo, ECAPS AB, Haldor Topsøe A/S, Scania CV AB, Volvo Car Corporation AB and
Wärtsilä Finland Oy.

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Table captions
Table 1 Reaction orders for NO and NH3 concentrations over NbCZ at various temperatures.
Figure captions
Fig. 1. (a) NOx conversions, (b) N2 selectivities and (c) k values of the catalysts for NH3-SCR reaction.
Catalysts: (1) CZ, (2) NbCZ, (3) CZ with 5% H2O, and (4) NbCZ with 5% H2O. Reaction conditions:
500 ppm NH3, 500 ppm NO, 5% O2, 12% CO2 and N2 in balanced.
Fig. 2. DRIFT spectra of adsorbed species on (a) CZ and (b) NbCZ arising from contact with NH3+N2
at different temperatures.
Fig. 3. The integrated area for bands of NH3 adsorbed on (a) Lewis and (b) Brønsted acid sites over the
CZ and NbCZ per catalyst surface area as a function of temperature.
Fig. 4. TPD profiles of NO and NO2 over (1) CZ and (2) NbCZ catalysts.
Fig. 5. DRIFT spectra at 180 °C upon passing NO+O2 over the (a) CZ and (b) NbCZ catalysts with
pre-adsorbed NH3 species.
Fig. 6. DRIFT spectra at 180 °C upon passing NH3 over the (a) CZ and (b) NbCZ catalysts with
pre-adsorbed NxOy species.
Fig. 7. DRIFT spectra at 180 °C upon passing NH3+NO over the (a) CZ and (b) NbCZ catalysts with
pre-adsorbed NxOy species.
Fig. 8. DRIFT spectra of adsorbed species on the (a) CZ and (b) NbCZ arising from contact with
NO+NH3+O2 at different temperatures.
Fig. 9. Dependence of NO reduction rate on (a) NO and (b) NH3 concentrations over NbCZ catalyst at
180-260 °C.
Fig. 10. Variations of intensity of bidentate nitrate related band (1543-1556 cm-1) obtained from Figs.
6 and 7 and those of NH3NO4 related band (1360 cm-1) from Fig. 7 as a function of reaction time over
(a) CZ and (b) NbCZ at 180 °C.
Fig. 11. The tentative NH3-SCR mechanisms over CZ and NbCZ catalysts in different temperature
ranges.

Table 1 Reaction orders for NO and NH3 concentrations over NbCZ at various temperatures.
β
Temperature (°C)
α
β1
β2
180
200
220
240
260
0.365
0.738
0.844
0.831
0.826
-0.177
-0.096
0.172
0.357
0.366
-0.140
-0.080
-0.057
Note: β1 and β2 is the reaction order for NH3 concentration at low and high concentrations, respectively.