N2O formation in the selective catalytic reduction of NO x with NH3 at low temperature on CuO-supported monolithic catalysts

Article abstract of DOI:10.1016/j.jcat.2004.10.019

The formation of nitrous oxide in the selective catalytic reduction of NO_x with ammonia was studied in nitric acid plant exhaust gas under low-temperature conditions. The important role played by the support in this process was evidenced by a comparison of CuO/γ-Al₂O₃ and CuO/TiO₂ monolithic catalysts. Textural properties and CuO crystal phases were analysed. Temperature-programmed desorption of NH ₃, NO, and NO₂ and X-ray photoelectron spectroscopy (XPS) after in situ adsorption of NO and NO₂ experiments were carried out. Even at low temperature, N₂O generation takes place at a greater rate for the catalysts supported on titania than for those on alumina. In the case of alumina, the N₂O concentration detected was independent of the CuO content. For the CuO/TiO₂ system it was associated with the active phase configuration. After in situ adsorption of NO and NO₂, a band associated with nitrate species was identified in the XPS spectra. Adsorbed NO3- species seem to be responsible for the NO₂, NO, and O₂ desorbed products detected in the NO_x-TPD. The results suggests that NO3(ads)- along with NH_x(ads) species, were involved in N ₂O formation at low temperature.

Full text of DOI:10.1016/j.jcat.2004.10.019

Journal of Catalysis 229 (2005) 227–236
www.elsevier.com/locate/jcat
N2O formation in the selective catalytic reduction of NOx with NH3 at
low temperature on CuO-supported monolithic catalysts
S. Suárez ∗, J.A. Martín, M. Yates, P. Avila, J. Blanco
Instituto de Catálisis y Petroleoquímica, C.S.I.C. C/Marie Curie 2, Campus de Cantoblanco, 28049 Madrid, Spain.
Received 12 August 2004; revised 7 October 2004; accepted 9 October 2004
Abstract
The formation of nitrous oxide in the selective catalytic reduction of NO with ammonia was studied in nitric acid plant exhaust gas under
x
low-temperature conditions. The important role played by the support in this process was evidenced by a comparison of CuO/γ -Al O and
2
3
CuO/TiO monolithic catalysts. Textural properties and CuO crystal phases were analysed. Temperature-programmed desorption of NH ,
2
3
NO, and NO and X-ray photoelectron spectroscopy (XPS) after in situ adsorption of NO and NO experiments were carried out. Even
2
2
at low temperature, N O generation takes place at a greater rate for the catalysts supported on titania than for those on alumina. In the
2
case of alumina, the N O concentration detected was independent of the CuO content. For the CuO/TiO system it was associated with the
2
2
active phase configuration. After in situ adsorption of NO and NO , a band associated with nitrate species was identified in the XPS spectra.
2
−
Adsorbed NO species seem to be responsible for the NO , NO, and O desorbed products detected in the NO -TPD. The results suggests
x
3
2
2
−
that NO
along with NH
2004 Elsevier Inc. All rights reserved.
species, were involved in N O formation at low temperature.
x(ads) 2
3(ads)
Keywords: SCR; N O formation; Low temperature; Monolithic catalyst; CuO/γ -Al O ; CuO/TiO ; XPS in situ
2
2
3
2
1. Introduction
(NO:NO2 = 1) reduction with ammonia at low tempera-
ture [4]. Wolberg and Roth identified three phases in CuO/
Al2O3 catalysts by X-ray adsorption edge spectroscopy:
isolated Cu+2, copper aluminate surface phase, and crys-
talline CuO [5]. The effect of the NiO incorporation on
the CuO/Al2O3 system was studied by Knözinger et al.
[6,7]; Ni+2 tends to occupy the octahedral sites in subsur-
face layers and in the bulk. Thus, Ni+2 leads to a Cu+2
redistribution, with an enhanced segregation of copper on
the surface. Previous studies carried out by Blanco et al.
with CuO/Al2O3 catalysts modified by the addition of small
quantities of NiO have shown superior performance in NOx
elimination from the tail gas in nitric acid plants [8,9].
The role of the support for the NO + NH3 + O2 reaction
was analysed by Centi et al. [10], who studied copper ox-
ide supported on Al2O3, TiO2, SiO2, and ZSM-5 catalysts;
at 200 ◦C CuO/TiO2 and CuO/ZSM-5 presented the highest
conversion, whereas that based on Al2O3 was the most ac-
tive at temperatures above 350 ◦C.
The selective catalytic reduction (SCR) of nitrogen ox-
ides, with ammonia as a reductant, to produce nitrogen and
water is one of the most widespread technologies for NOx
abatement [1]. Nitric acid plant flue gas releases high con-
centrations of NOx into the atmosphere. The low tempera-
ture at which the effluent must be treated (180–250◦C), and
the high concentration of NO2 present in the flue gas (250–
500 ppm) make this a singular case among NOx stationary
sources.
Copper oxide-supported catalysts have shown excel-
lent activity for NOx abatement with different reducing
agents, such as CO [2], hydrocarbons [3], or ammonia. Re-
cently, Blanco et al. have demonstrated the excellent perfor-
mance of a CuO/NiO–Al2O3 monolithic catalyst in the NOx
*
Corresponding author. Fax: +34-91 585 47 60.
E-mail address: ssuarez@icp.csic.es (S. Suárez).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.10.019
2004 Elsevier Inc. All rights reserved.

228
S. Suárez et al. / Journal of Catalysis 229 (2005) 227–236
Although many articles concerning NOx abatement can
green bodies were subsequently dried and heat treated at
500 ◦C for 4 h in air to produce monolithic supports with the
following geometric dimensions: square cell size 2.53 mm,
be found in the literature, very few consider N2O formation
as a product of the SCR reaction [11–13], even though it is
a strong greenhouse gas and contributes to the depletion of
the stratospheric ozone layer. At present there is no legisla-
tion concerning N2O concentration limits, although this is
expected to change in the near future.
According to Odenbrand et al. [14], the three parameters
that influence N2O formation on V2O5/TiO2–SiO2 catalysts
are temperature, active phase content, and the presence of
H2O in the stream. An exponential relationship between
temperature and N2O formation on vanadium catalysts,
reaching a selectivity for N2O around 40% at 450 ◦C, was
shown by Turco et al. [15]. Delahay et al. [16] observed a
linear relationship between N2O formation and O2 concen-
tration in the feed.
wall thickness 0.89 mm, geometric surface 8.6 cm2 cm−3
,
and cell density 8.54 cell cm−2
.
Two series of catalysts were prepared by impregnation
of titania and γ -alumina monolithic supports with aque-
ous solutions of copper and nickel nitrates (Panreac, purity
ꢀ 99.9 wt%), while maintaining the [CuO]/[NiO] ratio at
10/1 (wt%). The role of nickel was to increase the dispersion
of copper. After impregnation, the precursors were dried at
room temperature for 24 h, and then at 100 ◦C for 12 h, and
finally treated at 500 ◦C in air for 4 h.
2.2. Catalyst characterisation
The influence of the operating conditions on copper
and vanadium oxide monolithic catalysts supported on alu-
mina and titania, was analysed in a previous article [17]:
high temperatures and [NOx]/[NO] and [NH3]/[NOx] feed
ratios promoted N2O formation; a direct relationship be-
tween the presence of NO2 and N2O production, studied
by temperature-programmed surface reaction, was also re-
ported.
This work is the third in a series of studies in which the
formation of N2O during the SCR reaction is analysed at
low temperature [4,18]. Typical nitric acid SCR unit con-
ditions were selected: [NO]/[NOx] = 0.5, [NH3]/[NOx]
around 0.8 (to minimise the ammonia slip), and temperature
T = 200 ◦C, similar to those studied in the previous arti-
cles. As the N2O formation may be affected by mass transfer
phenomena, it was considered convenient to carry out the
study with catalysts in the same monolithic configuration as
the ones used in the industrial SCR units. For this reason
monolithic supports were prepared with the use of sepiolite,
a natural magnesium silicate, as a binder [19,20].
Copper content was determined by inductively coupled
plasma (ICP) optical emission spectroscopy of acid solutions
of the ground catalysts in a Perkin–Elmer Optima 3300DV
apparatus.
Surface areas were measured by nitrogen adsorption/de-
sorption, with a Sorptomatic 1800. Samples were outgassed
overnight at 250 ◦C to a vacuum of < 1 × 10−2 Pa to ensure
a clean, dry surface. The pore volumes were determined by
mercury intrusion porosimetry (MIP) with a CE Instruments
Pascal 140/240 apparatus. By starting from vacuum and rais-
ing the pressure to 200 MPa, pore diameters from 150 µm
down to 7.5 nm can be determined. The Washburn equation
is applied to transform the applied pressure into an equiv-
alent diameter; the recommended values for the mercury
contact angle and surface tension are 141◦ and 484 m Nm−1
.
X-ray diffraction (XRD) patterns of ground samples of
the monolithic catalysts were recorded on a Seifert 3000P
powder diffractometer, with the use of Cu-Kα radiation
1
(λ = 0.15406 nm).
X-ray photoelectron spectra were acquired with a VG Es-
calab 200R spectrometer fitted with a monochromated Mg-
Kα radiation (hµ = 1253.6 eV, 1 eV = 1.6302 × 10−19 J)
120-W X-ray source and a hemispherical electron analyser.
The powdered samples were placed on a sample rod, intro-
duced into a pretreatment chamber, and degassed at 25 ◦C
and 10−3 Pa for 5 h prior to being transferred to the analy-
sis chamber. Samples were treated in situ with NO or NO2
at 200 ◦C for 30 min in the pretreatment chamber. Resid-
ual pressure during data acquisition was maintained below
3 × 10−7 Pa. The energy regions of the photoelectrons of in-
terest (Al 2p3/2, Ti 2p3/2, Si 2p, Mg 2p, Cu 2p3/2, N 1s) were
scanned a number of times in order to obtain an acceptable
signal-to-noise ratio. Accurate binding energies ( 0.2 eV)
were determined by referring to the C 1s peak at 284.9 eV.
Temperature-programmed desorption experiments of
NH3, NO, and NO2 were carried out over the selected cat-
alysts, which were placed in a stainless-steel micro-reactor,
with an internal diameter of 1 cm and a length of 15 cm,
operating at GHSV (NTP) = 20,700 h−1 and P = 114 kPa.
In order to fix the monolith of one cell in the centre of the
The aim of this work was to analyse the role of the sup-
port and the metal oxide content in the N2O formation in
the SCR reaction at low temperature over CuO/Al2O3 and
CuO/TiO2 monolithic catalysts. Temperature-programmed
desorption (TPD) experiments of NH3, NO, and NO2 and
X-ray photoelectron spectroscopy (XPS) after in situ adsorp-
tion of NO and NO2 were carried out with these catalysts in
order to relate the adsorbed species with N2O production.
2. Experimental
2.1. Catalyst preparation
Two monolithic supports were manufactured by extru-
sion of doughs prepared by kneading boehmite (Condea) or
anatase (Millenium) powders, with sepiolite (Tolsa S.A.) as a
binder and water as described previously [4]. The γ -Al2O3:
silicate and TiO2:silicate ratios after heat treatment at 500 ◦C
were 2/3 and 1/1 by weight, respectively. The monolithic

S. Suárez et al. / Journal of Catalysis 229 (2005) 227–236
229
reactor, the space between the two bodies was filled with sili-
con carbide consisting of 0.84-mm particles. The connection
between the mass spectrometer and the reactor was a steel
capillary 1/16 inch in diameter heated to 200 ◦C to avoid
condensation. The vacuum on the system was maintained at
3 ×10−4 Pa with a total sample flow for the measurement of
0.54, [NH3]/[NOx] = 0.77, [O2] = 4 vol%, and [N2] to bal-
ance.
3. Results and discussion
10 mL min−1
.
3.1. Characteristics of the prepared catalysts
The gases at the micro-reactor inlet and the desorbed
products were detected and quantified by mass spectrome-
try (Balzers Omnistar), with the use of a Channeltron de-
tector. The following masses were evaluated: N2 (28), NO
(30), NO2 (46), O2 (16), N2O (44), NH3 (15,17). The mea-
sured ion currents were input to a solution matrix, and the
individual concentrations of the components in the gas were
determined via calibration factors. A Quantitative Analysis
Module was used with a calibration factor library. For each
component that is included in a calibration matrix, this li-
brary contains the calibration factors and the corresponding
mass numbers, allowing the estimation of the overlap from
different masses.
The catalysts were pretreated with oxygen (3 vol%) at
500 ◦C, with Ar as a gas balance. After cooling in argon
to 180 ◦C, the gas (NH3, NO, or NO2) was adsorbed until
no signal variation of the gas was detected, and was subse-
quently purged with argon until no reactant was detected in
the outlet. Finally, the temperature was increased at a rate of
5 ◦C min−1 from 180 to 500 ◦C.
We prepared two series of CuO/Al2O3 and CuO/TiO2
monolithic catalysts while varying the CuO/NiO content.
The metal oxide content and textural and mechanical proper-
ties of the alumina and titania supported catalysts are listed
in Tables 1 and 2, respectively. The number that appears in
the label of each catalyst makes reference to the CuO con-
tent. The surface area of the supports was 160 m2 g−1 for the
alumina and 110 m2 g−1 for the titania monoliths. As the ac-
tive phase content increases, a slight decrease in the surface
area and pore volume of the solids was observed, particu-
larly for high-CuO/NiO content samples.
The ratio between the CuO content and the surface
area was calculated by considering that copper was selec-
tively impregnated on the alumina particles and not on the
binder [4], and assuming a similar behaviour for the tita-
nia. The values obtained for this apparent surface density
of CuO in the catalyst are listed in Tables 1 and 2 and ex-
pressed as micromoles of CuO per square meter of alumina
(210 m2 g−1) or titania (75 m2 g−1), respectively.
3.2. Catalytic activity
2.3. Catalytic activity tests
The NOx conversion and NH3 slip as a function of the
CuO content for the CuO/Al2O3 (AlCu) and CuO/TiO2
(TiCu) catalysts and N2O produced during the SCR reaction
at 200 ◦C are represented in Figs. 1a and 1b, respectively. For
AlCu catalysts the NOx conversion curves passed through
a maximum that coincided with the minimum ammonia
Activity measurements of the monolithic catalysts were
carried out in a continuous tubular glass reactor that operated
at an integral regimen close to an isothermal axial profile.
The reactor was 75 cm long and had an internal diameter of
2.54 cm. Ammonia was fed directly into the reactor bed to
avoid the formation of ammonia salts [21]. Monoliths 20 cm
long with nine cells were used; the gap between the catalyst
and the reactor wall was filled with silicon carbide consisting
of 0.84-mm particles.
The inlet and outlet NO and NO2 concentrations were de-
termined by chemiluminescence with a Signal NO + NO2
analyser (Series 4000). Analysis of N2O and NH3 was car-
ried out by IR spectroscopy with a Signal 7000FT GFC
Analyser and with an A.D.C. Double Beam Luft Type Infra-
red Gas Analyser, respectively. The evolution of the gas con-
centration at the reactor outlet was recorder by a Multichan-
nel analogic-digital Register Yokowaga (Model 436006).
The concentration data were acquired under steady-state
conditions, after approximately 90 min of stabilization for
each point.
Table 1
Composition and textural properties of CuO/γ -Al O monolithic catalysts
2
3
Name
[CuO] [NiO]
(wt%) (wt%) (m g
S
Pore volume
(MIP)
(cm g
CuO surf. density
BET
2
−1
−2
)
(µmol m
)
3
−1
)
(alumina)
AlCu-3
AlCu-5
AlCu-6
AlCu-8
3.2
4.8
6.4
8.1
0.3
0.5
0.6
0.8
1.0
156
156
157
156
142
0.62
0.62
0.61
0.56
0.53
4.8
7.2
9.6
12.1
14.8
AlCu-10 9.9
Table 2
Composition and textural properties of CuO/TiO monolithic catalysts
2
Name
[CuO] [NiO]
(wt%) (wt%) (m g
S
Pore volume
(MIP)
(cm g
CuO surf. density
BET
2
−1
−2
)
(µmol m
(titania)
)
3
−1
)
The operating conditions for the experiments were as fol-
lows: gas hourly space velocity at normal conditions GHSV
(NTP) = 10,200 h−1, lineal velocity vL = 0.98 ms−1, pres-
sure P = 120 kPa, temperature T = 200 ◦C. The gas in-
let compositions were [NOx] = 1000 ppm, [NO]/[NOx] =
TiCu-1 1.4
TiCu-2 2.2
TiCu-4 4.0
TiCu-7 7.4
0.1
0.2
0.4
0.7
108
108
104
97
0.71
0.72
0.69
0.64
4.7
7.4
13.4
24.8

230
S. Suárez et al. / Journal of Catalysis 229 (2005) 227–236
Fig. 1. (a) NO conversion and NH slip, and (b) N O in the out-
x
3
2
let as a function of the CuO content for (Q, P) CuO/γ -Al O and
2
3
(", !) CuO/TiO monolithic catalysts. Feed compositions: [NO ] =
x
2
1000 ppm, [NO]/[NO ] = 0.54, [NH ]/[NO ] = 0.77, [O ] =
x
x
3
2
4 vol%, [N ] = balance. Operating conditions: GHSV (NTP) =
2
−1
−1
◦
10,200 h , v = 0.98 Nm s , T = 200 C, P = 120 kPa.
L
concentration at the outlet. The highest NOx conversion
(ca. 75%) was close to that expected for the [NH3]/[NOx]
ratio employed (maximum theoretical value of 77%). The
major difference between the AlCu and TiCu curves was the
copper oxide content at which this conversion was achieved:
2.2 wt% for the titania catalyst and 6.4 wt% for the alumina
catalyst. For both series the SCR activity was lower at higher
CuO contents.
Even at this low reaction temperature, N2O was detected
at the outlet as a secondary product. In spite of the low
N2O selectivity observed (lower than 10% in all cases),
the two series of catalysts displayed significant differences
with respect to their tendency towards N2O formation. For
CuO/Al2O3, the selectivity remained practically constant;
however, for CuO/TiO2 a progressive increase in the N2O
concentration with the CuO content was observed. These re-
sults indicated that the nature of the support and CuO oxide
loading were key factors in the N2O selectivity.
Fig. 2. X-ray diffraction patterns of (a) CuO/γ -Al O (b) CuO/TiO cata-
lyst with different CuO content in the range 2θ = 30 –40 .
2
3
2
◦
◦
though the N2O concentration detected in the outlet could
arise from this reaction, the ammonia balance indicates that
a different reaction path is involved, since to produce 1 mole
of N2O, 2 moles of NH3 are required.
3.3. X-ray diffraction analysis
The crystal phases formed in the AlCu and TiCu series
studied by X-ray diffraction in the 2θ = 32◦–40◦ range are
shown in Fig. 2. The variation in the reflexion intensity at
36◦ and 39◦ for CuO/Al2O3 catalysts with a CuO content
lower than or equal to 6.4 wt% were ascribed to the alu-
mina phase. At higher metal oxide concentrations two peaks
centred at 2θ = 35.5◦ and 38.7◦ appeared. According to the
ASTM 05-0661, these peaks were assigned to a CuO tenorite
crystal phase [22–24]. The effect of Ni as a dispersive agent
is obvious from a comparison of the concentration at which
the CuO crystal phases started to appear with that previously
reported by Friedman et al.
In a previous study [18], the behaviour of these two cat-
alysts for NH3 oxidation reaction in the temperature range
180–450◦C was reported; the CuO/Al2O3 catalyst (CuO =
6.4 wt%) was inactive at T = 200 ◦C, P = 120 kPa, and
GHSV (NTP) = 15,000 h−1, with a feed composition of
[NH3] = 1000 ppm and O2 = 3 vol%. Nevertheless, the
CuO/TiO2 system ([CuO] = 2.2 wt%) displayed an ammo-
nia conversion around 12% under similar conditions. Results
in Fig. 1 show how the ammonia balance was satisfied for
AlCu catalysts, whereas for TiCu the ammonia slip data cor-
roborate the superior tendency of this system towards the
NH3 oxidation reaction, especially at high CuO loading. Al-

S. Suárez et al. / Journal of Catalysis 229 (2005) 227–236
231
When the catalytic activity results (Fig. 1) and the XRD
diffractograms of the samples (Fig. 2a) were correlated,
a clear relationship between the NOx conversion decrease
with the appearance of a CuO crystal phase could be es-
tablished. The values of CuO concentration at which the
reduction in catalytic activity was observed corresponded to
8 wt% for the alumina and 4 wt% for the titania catalysts,
AlCu-8 and TiCu-4, respectively. Similar values of copper
surface density (12.1 and 13.4 µmol CuO m−2, respectively)
were obtained for the two samples (Tables 1 and 2). This re-
sult indicated how the surface area of the support affected
the active phase dispersion and the formation of the copper
oxide crystal phase.
With respect to the N2O formation (Fig. 1b), a relation
between the presence of a CuO crystal phase and N2O for-
mation could be assumed for TiCu. However, from the re-
sults obtained for AlCu systems, other parameters probably
related to the nature of the support, or copper site density
might also be involved.
3.4. Catalyst behaviour after NH3, NO, and NO2
adsorption
The species generated on the catalysts’ surfaces were
studied by TPD. The measurements were carried out with
the AlCu-6 and TiCu-2 systems, since they displayed the
highest catalytic activities (Fig. 1). The reactants NH3, NO,
and NO2 (1000 ppm) were adsorbed at 180 ◦C over an oxi-
dised catalyst, with the use of an 8-cm-long monolith of one
channel.
Fig. 3. Temperature-programmed desorption of NH after adsorption at
3
180 C for (a) AlCu-6 and (b) TiCu-2 monolithic catalysts. Desorp-
◦
tion products: (3) H O, (1) NH , (×) N , (!) NO. Feed composi-
2
3
2
tion: [NH ] = 500 ppm, [Ar] = balance. Desorption operating conditions:
3
−1
−1
GHSV (NTP) = 20,000 h , v = 0.76 m s , [Ar] = 100 vol% and
L
◦
◦
−1
.
T = 180–500 C at a rate of 5 C min
3.5. NH3: Temperature-programmed desorption
between the N2 desorption profiles of the two catalysts is
the initial temperature at which N2 started to desorb. Even at
a temperature as low as 180 ◦C, TiCu-2 produced nitrogen,
indicating that NHx(ads) species generated on the titania cat-
alyst are more active than those produced over the alumina
system.
The total amount of desorbed N2, H2O, NH3, and NO was
calculated by integration of the areas under the curves in the
temperature range between 180 ◦C and 500 ◦C (Table 3). N2
and NO are selectively produced by oxidised copper species
(dispersed or aggregated) according to the equations
Temperature-programmed desorption curves recorded
after NH3 adsorption over alumina and titania copper-
supported catalysts are shown in Fig. 3a and 3b, respectively.
At the starting temperature of 180 ◦C and after the gas purge,
only chemisorbed NH3 would remain on the catalyst sur-
face. Broad ammonia desorption bands between 180 ◦C and
420 ◦C, with a maximum at 250 ◦C characterised the des-
orption profiles of both catalysts. At higher temperatures
N2, NO, and H2O were desorbed.
These results indicated the presence of at least two types
of acid sites where NH3 can be adsorbed. The first type, char-
acterised by the NH3 desorption band centred at 250 ◦C, was
ascribed to Brønsted acid sites of the support. This band was
also observed in the NH3 TPD of alumina, titania, or mag-
nesium silicate without active phased incorporation [25].
The second type, responsible for the N2, H2O, and NO
desorption curves, was assigned to dissociatively adsorbed
ammonia species such as NHx (x = 0–2) that could react
with lattice oxygen to produce N2, H2O, or even small quan-
tities of NO (Fig. 3a). The NH3 TPD pattern of the supports
(figure not included) showed no N2 desorption, which im-
plied that the oxygen necessary to produce either N2 or H2O
was provided by the active phase [26]. The major difference
2NH3 + 6CuO → N2 + 3H2O + 3Cu2O,
2NH3 + 10CuO → 2NO + 3H2O + 5Cu2O.
(1)
(2)
From the TPD experiments and considering the reaction
stoichiometry of Equations (1) and (2), we evaluated the
Cu+2 species accessible to react with ammonia [26]. The re-
sults obtained (Table 3) point out that although metal oxide
content on the AlCu-6 catalyst (6.4 wt%) was three times
larger than that over TiCu-2 (2.2 wt%), the copper species
accessible in the latter was higher (93 vs 73%), thus ex-
plaining the similar performance towards NOx conversion
displayed by the two systems (Fig. 1a).

232
S. Suárez et al. / Journal of Catalysis 229 (2005) 227–236
3.6. NO and NO2: Temperature-programmed desorption
the NO TPD curves (not shown) obtained for raw alu-
mina, titania, and magnesium silicate materials treated at
500 ◦C. The results obtained in these previous experiments
showed that the magnesium silicate presents a NO des-
orption band centred at 250 ◦C, and the alumina or tita-
nia desorbed the NO together with N2O, NO2, and O2
around 420 ◦C and 380 ◦C, respectively. Thus, in Fig. 4a
the first band centred at 245 ◦C, observed for both Al/Sep
and AlCu-6, was assigned to the silicate NO adsorption
sites. This band remained unaltered when the active phase
was added to the support, indicating that the NO ad-
sorption sites over the magnesium silicate remained un-
changed.
TPD experiments were carried out in order to determine
the adsorption capacity of the catalysts towards NO and
NO2. Experiments were conducted with the procedure as
used for NH3 TPD. The product desorption curves after NO
or NO2 adsorption for AlCu-6 and TiCu-2 and those for the
corresponding supports are shown in Figs. 4 and 5, respec-
tively.
The supports and catalysts displayed a NO desorp-
tion curve with two bands centred at different temper-
atures. Bands were assigned according to the shape of
Fig. 4. Temperature-programmed desorption of NO after adsorption
at 180 C for supports (filled) and catalysts (empty). (a) AlCu-6 and
Fig. 5. Temperature-programmed desorption of NO after adsorption
2
at 180 C for supports (filled) and catalysts (empty). (a) AlCu-6 and
◦
◦
alumina support, (b) TiCu-2 and titania support. Desorption products:
alumina support (b) TiCu-2 and titania support. Desorption prod-
(2, 1) O , (Q, P) NO , (", !) NO, (F, E) N O. Feed composi-
ucts: (Q, P) NO , (2, 1) O , and (", !) NO. Feed composition:
2
2
2
2 2
tion: [NO] = 1000 ppm, [Ar] = balance. Desorption operating conditions:
[NO ] = 1000 ppm, [Ar] = balance. Desorption operating conditions:
2
−1
−1
−1
−1
GHSV (NTP) = 20,000 h , v = 0.76 m s , [Ar] = 100 vol% and
GHSV (NTP) = 20,000 h , v = 0.76 m s , [Ar] = 100 vol% and
L
L
◦
◦
−1
◦
◦
−1
T = 180–500 C at a rate of 5 C min
.
T = 180–500 C at a rate of 5 C min
.
Table 3
◦
◦
TPD results from 180–500 C after ammonia adsorption at 180 C
Name
NH
(µmol g
N
H O
2
(µmol g
NO
−1
(µmol g )
NH
dissociatively
(µmol g
CuO
accessible
(%)
3
2
3(ads)
−1
−1
−1
)
(µmol g
)
)
−1
)
Al/Sep
AlCu-6
Ti/Sep
TiCu-2
135
57
91
–
78
–
–
274
–
–
21
–
–
177
–
–
73
–
51
43
121
–
86
93

S. Suárez et al. / Journal of Catalysis 229 (2005) 227–236
233
Table 4
Desorbed products after NO or NO adsorption at 180 C
thors assigned the high temperature band to the transfor-
◦
mation of Cu+–ONO− into Cu+–ONO2 species. At high
−
2
temperatures the nitrate complex is decomposed into NO
and O2, reducing the Cu+ to Cu0. Centi et al. [32,33] also
identified nitrates species adsorbed to copper catalysts.
Considering the results obtained in this work for cata-
lysts and supports, it could be suggested that nitrate species
Mn+–(O–NO2)−, where M is Cu, Ni, Al, or even Ti, could
be responsible for the NO and O2 desorption bands observed
in the NO TPD, according to Eq. (3)
Gas
adsorbed
Catalyst NO
(µmol g
NO
(µmol g
O
N O
2
2
2
−1
−1
−1
−1
(µmol g )
)
)
(µmol g
)
NO
AlCu-6 50
Al/Sep 41
TiCu-2 17
5
9
1
7
17
10
10
11
2
10
2
Ti/Sep
38
6
NO
AlCu-6 76
Al/Sep 61
TiCu-2 47
239
231
102
92
165
130
83
5
11
13
18
2
Ti/Sep
23
61
NO3 ↔ NO(g) + O2(g)
.
(3)
Nevertheless, in order to explain the formation of N2O
and that of NO2 in the supports, other reactions should be
considered. One possibility is the NO autooxido-reduction
reaction yielding NO2 + N2O, as shown in Eq. (4). Accord-
ing to this reaction the [NO2]/[N2O] ratio is 1, which is
consistent with the data reported in Table 4.
The second NO desorption band observed at higher tem-
perature (425 ◦C) for the Al/Sep system and associated
with different nitrogen oxides and oxygen was character-
istic of Al2O3. Copper and nickel oxide incorporation on
alumina/silicate system causes a decrease in the desorption
temperature to 360 ◦C and produced a relative increase in
the oxygen desorbed fraction. The bond strength of the NO
in the catalyst was modified because the sites of NO ad-
sorption on the alumina were altered by the metal oxides
or because aluminium sites were blocked by copper/nickel
sites. The desorption profiles at high temperature show that
the support presented a higher tendency to produce N2O,
which was drastically reduced by incorporation of the metal
oxides.
3NO ↔ NO2(g) + N2O(g)
.
(4)
The product desorption curves for AlCu-6 and TiCu-2
catalysts after NO2 adsorption are displayed in Fig. 5, to-
gether with those for the supports. The systems show a
higher adsorption capacity for NO2 than for NO, particularly
the alumina supported catalysts. Along with the NO2 band,
N2O, NO, and O2 were also observed. Although the N2O
band seems to be negligible (because of the graph scale), the
data quoted in Table 4 clearly indicate that N2O was also
produced, and it is especially promoted over TiO2 systems.
The reaction described in Eq. (4) could explain this nitrous
oxide formation.
Copper/nickel oxide incorporation led to a greater quan-
tity of desorbed NO2 and a decrease in the desorption b−and
temperature. As discussed above for NO TPD, NO3(ads)
species could justify the NO and O2 desorption in an
equimolecular ratio. However, an excess oxygen concen-
tration was observed in all systems, indicating that other
reactions that involve oxygen production are taking place.
Apostolescu et al. [28] have recently studied the adsorp-
tion of NO2 over alumina by thermal decomposition of the
species formed. They suggested that this process takes place
via formation of nitrite species−, which are subsequently ox-
idized by NO2 to yield NO3 and gaseous NO. Thermal
treatment leads to decomposition of nitrate species in the
high temperature region. The latter reaction results in the
formation of NO2(g) and O2(gas) in the molar ratio of 4 and
the reformation of the Al2O3 surface, according to the fol-
lowing reaction:
In Fig. 4b the results obtained for the TiCu-2 system and
the Ti/Sep support are shown. The curves display a behav-
iour similar to that found for the alumina system. The band
corresponding to the magnesium silicate was also observed
at low temperature, and the band centred around 400 ◦C was
attributed to the NO adsorption of the titania surface. In this
case, when the active phase was incorporated over the Ti/Sep
there was a large decrease in the band intensities, along with
a shift to lower temperatures, indicating a reduction in the
number of NO adsorption sites and a weakening of their
strength. That the reactivity of the NO adsorbed species on
the catalyst based on TiO2 was higher than the reactivity of
the species on the catalyst based on Al2O3 was clear from
a consideration of the shift of the high temperature band.
In the former the peak appeared at 345 ◦C, whereas in the
latter the band was centred at 365 ◦C. Moreover, the results
show that the magnesium silicate NO adsorption sites were
also modified by the addition of CuO/NiO, suggesting that
the active phase impregnation process was not selective for
TiO2, and there was a partial deposition of the metal oxide
in the silicate.
The quantities of desorbed NO, NO2, O2, and N2O for
supports and CuO/NiO catalysts are summarised in Table 4.
The capacity of these systems to adsorb NO (Figs. 4a and
4b) was significantly lower than their capacity to adsorb am-
monia. The NO adsorption studies carried out on previously
oxidised CuO/Al2O3 catalyst by Knözinger et al. [27] dis-
played two desorption bands centred at 220 ◦C and 400 ◦C,
respectively. Taking into account the FTIR spectra, the au-
−
2NO3(ads) ↔ 2NO2(g) + 0.5O2 + O2−
.
(5)
(s)
However, the experimental data given in Table 4 show a
NO2/O2 ratio of only about 1.5. The complex mechanism
that takes place in these systems seems to indicate that the
nitrate decomposition to NO and O2 (Eq. (3)) and its decom-
position to NO2 and O2 (Eq. (5)) may occur simultaneously.

234
S. Suárez et al. / Journal of Catalysis 229 (2005) 227–236
Thus, considering the oxygen desorbed as a sum of the two
contributions, a good fit may be obtained.
After NO or NO2 treatment, a chemical shift of 0.8 eV
towards higher binding energy was observed, suggesting that
NxOy species, strongly adsorbed to cupric ions, modified the
density of the electronic cloud, producing a variation in the
charge distribution of the complex. In the case of TiCu-2 the
shift was observed only after NO2 adsorption.
3.7. X-ray photoelectron spectroscopy results
In order to determine the nature of the NxOy adsorbed
species, XPS experiments were carried out over the selected
catalysts. Fresh samples and samples subjected to in situ ad-
sorption of NO and NO2 were studied. The N 1s, in addition
to Cu 2p3/2, Si 2p, Mg 2p, Al 2p3/2, or Ti 2p3/2 core levels,
were measured for TiCu-2 and AlCu-6 samples. The XPS
binding energy (BE) values for copper and nitrogen and the
N/Si, N/Cu surface atomic ratio are compiled in Table 5. All
values are related to the C 1s binding energy of 284.1 eV.
The main XPS feature of the CuAl-6 sample was the ap-
pearance of spin–orbit split Cu 2p1/2, and 2p3/2, along with
their shake-up satellite, indicating the characteristic peaks of
Cu+2. The spectra reveal that copper species are in an oxi-
dation state of Cu2+, attributed to the large peak with a max-
imum around 934.5 eV and the satellite peak at a binding
energy around 10 eV higher [7,23]. The ratio between these
two peaks was about 2. The presence of reduced species was
ruled out, because of the oxidant conditions used in the cata-
lyst treatment. Furthermore, the intensity of the satellite peak
did not show any modification with the gas pretreatment.
The Cu+2 main peak presented a shift to a higher bind-
ing energy (934.5 eV) with respect to the theoretical peak
corresponding to CuO (933.5 eV). According to Friedman
and Freeman [23], this is due to the formation of a CuAl2O4
spinel phase. Moreover, the deconvolution of this peak in-
dicates two contributions, one centred at 933.6 eV and the
other at 934.5 eV. Thus, the presence of both CuO and
CuAl2O4 spinel phases defines the catalyst structure.
In all cases, the intensity of the Cu+2 main peak for
TiCu was higher than the intensity of the peaks for Cu–Al.
This may be indicative of the higher dispersion of copper
species in the former (corroborating the data obtained by
NH3 TPD) or symptomatic of the higher porosity of the alu-
mina support, where copper species may be located inside
the mesopores, where they would be inaccessible to the XPS
technique.
The N 1s spectra are represented in Fig. 6. The spectra are
characterized at least by two bands, centred around 400 and
407 eV. The former peak may be assigned to N2O or NO ad-
sorbed species, whereas the latter coi−ncides with that of the
NaNO3 spectra, suggesting that NO3 species may be gen-
erated in the adsorption process [30]. The existence of these
bands even in the fresh samples has been attributed to NxOy
remainders from the copper and nickel nitrate salts used for
catalyst preparation. With increasing oxidative power of the
adsorbed molecule, the bands at lower binding energy dis-
appear, becoming more important those centred at higher
BE. These observations are reflected in the contribution of
each band to the total area under the curves (Table 5). The
N/Si atomic ratio increased according to the series: no pre-
treatment < N−O < NO2. The results suggest that strongly
adsorbed NO3 species are generated on the catalyst sur-
face after exposure to nitrogen oxide, especially NO2. The
NO and NO2 pretreatment produces a shift of the band at
407 eV with respect to the fresh sample. This effect is in ac-
cordance with the shift detected in Cu+2 spectra (Table 5)
and corroborates the idea that soft electronic effects pro-
duced by nitrate species adsorbed on Cu+2 sites may shift
the e−lectronic cloud towards the nitrate species, generating a
NO3 (δ−) and Cu+2 (δ+).
For CuTi the same general trend was observed. Two Cu+2
phases were detected in the XPS analysis. The deconvolu-
tion of the Cu 2p3/2 peaks centred at 934.5 eV indicated the
presence of a CuO phase (933.0 eV), and that at 935 eV cor-
responded to a Cu–O–Ti phase, as previously described for
CuO/TiO2 catalysts by Cordoba et al. [29].
The influence of the support is reflected when the N/Cu
atomic ratios of the two catalysts are compared. The N/Cu
Table 5
◦
Binding energies of core electrons, and N/Si surface atomic ratio, for the fresh sample, and after adsorption of NO, NO , at 200 C
2
Pre-treatment
Cu 2p
Cu 2p
N 1s
N/Si
N/Cu
3/2
3/2
satellite
(a) AlCu-6 fresh catalyst
NO
934.5
935.3
935.5
943.5
399.3 (60)
407.9 (38)
399.2 (33)
407.1 (67)
400.4 (14)
407.2 (86)
–
–
943.4
943.4
0.031
0.051
0.32
0.43
NO
2
(b) TiCu-2 fresh catalyst
NO
934.5
934.5
935.3
943.1
943.0
943.6
399.8 (61)
407.7 (39)
400.1 (26)
406.8 (74)
399.9 (25)
406.9 (75)
0.041
0.051
0.098
0.42
0.52
0.99
NO
2

S. Suárez et al. / Journal of Catalysis 229 (2005) 227–236
235
appear at relatively higher temperatures than those corre-
sponding to the AlCu-6 catalyst.
Based on the results obtained by XPS after SCR reac-
tion in the presence of NO2 catalyst, Blanco et al. proposed
a Langmuir–Hinshelwood mechanism for CuO/Al2O3 [31].
In the studied system, copper remains as the Cu+ species
under steady-state conditions, and these species are more
easily reoxidised by NO2 than they are by O2 or NO. The
proposed mechanism involved dissociatively adsorbed am-
monia on Cu+2-forming NH2 species and a reduced site.
Nitrogen dioxide can be transformed to NO according to
the process Cu2O + NO2 → 2CuO + NO(ads), leading to a
high concentration of NO adsorbed to the surface. Centi et al.
[32,33], who analysed the behaviour of CuO catalysts sup-
ported on Al2O3 and ZSM-5 by ESR, UV–vis, etc., proposed
the formation of ammonium nitrates species, generated by
the transformation of nitrites to nitrates in CuO and in the
presence of O2 and NHx(ads), as intermediates to N2O for-
mation. For CuO/ZSM-5 samples, Mizumoto et al. [34,35]
suggested that N2O formation is related to the SCR reac-
tion and to reduced copper sites. For Pt/Al2O3 Yamaguchi
et al. [36] proposed that NH(ads) species, along with NO or
reduced sites, are responsible for N2O formation. Otto et al.
[37] pointed out that reduced sites generated by the disso-
ciative NH3 adsorption reacted with two NO molecules to
produce N2O. Meier and Gut [38] put forward the reaction
between NO and NH3 as responsible for the N2O forma-
tion. In the NO/NO2 TPD experiment carried out in this
work, the N2O desorption was negligible and occurred at
a higher temperature than the SCR reaction. Thus it may
be assumed that the N2O does not come from the NO or
NO2 alone. But the oxidation state of the catalyst was dif-
ferent under a NOx or SCR atmosphere, where ammonia
was also present. In fact, Mizumoto and Otto et al. sug-
gested that reduced sites may be involved in the N2O for-
mation.
The results obtained in this work point out the important
role of the support in the formation of N2O as a secondary
product during the SCR reaction. For alumina samples, the
N2O concentration detected in the reactor outlet was inde-
pendent of the CuO/NiO loading. The ammonia oxidation
reaction or even the reaction between NOx and NH3 were
excluded as a possible way to produce N2O. In fact, when
the NOx conversion started to decrease, no appreciable in-
fluence of the N2O amount was observed. Over catalysts
supported on titania, a progressive increase in the N2O for-
mation with the metal loading was observed, suggesting
that properties related to active phase configu−ration are in-
volved in the N2O formation process. The NO3(ads) adsorbed
species generated in the presence of a NO/NO2 gas mix-
ture observed by XPS and responsible for the NO2, NO, and
O2 desorption of the NO/NO2 TPD could react with NHx
species (and was more reactive in the case of CuTi catalysts)
to produce N2O, according to the mechanism proposed by
Centi et al.
Fig. 6. N 1s core level spectra for fresh samples and after adsorption of NO,
and NO at 200 C.
2
◦
atomic ratios are higher over the titania-based catalysts than
for the alumina-based catalysts. These observations are in
good agreement with the NO2 TPD results, where the NO2,
NO, and O2 desorbed bands of TiCu-2 were less intense and

236
S. Suárez et al. / Journal of Catalysis 229 (2005) 227–236
4. Conclusions
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The influence of the support and CuO/NiO content in the
N2O formation during the SCR reaction at low temperature
was studied. For this purpose CuO–NiO/TiO2 and CuO–
NiO/γ -Al2O3 monolithic catalysts were analysed, with the
use of magnesium silicate as a binder. N2O formation is a
complex process in which different intermediates may be
involved, especially when supports of several components
are used. The studied supports had a tendency towards N2O
formation in the presence of NO/NO2, and this ability is
strongly affected by deposition of the active phase. The cat-
alytic activity results indicate that the ammonia oxidation re-
action was not the main path for the N2O formation, even at
high metal oxide loading under the selected operating condi-
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J. Mater. Sci. 28 (1993) 4113.
−
tions. The existence of NO3(ads) species in the catalyst after
adsorption of NO or NO2 was demonstrated by NO or NO2
TPD and XPS. The presence of dissociatively adsorbed am-
monia responsible for the nitrogen formation observed by
NH3 TPD is related to NHx adsorbed species. Thus, the reac-
[20] J. Blanco, M. Yates, P. Avila, A. Bahamonde, J. Mater. Sci. 29 (1994)
5927.
−
tion between these NO3(ads) and NHx(ads) species adsorbed
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Garcia de la Banda, Appl. Catal. 55 (1989) 151.
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on Cu2+, and especially reactive on the CuTi system, is sug-
gested as a feasible path for producing N2O.
Acknowledgments
[25] S. Suárez, Desarrollo de catalizadores SCR altamente selectivos a ni-
trógeno para plantas de ácido nítrico Ph.D. Thesis, Madrid (2002).
[26] J.M. Rodriguez Blas, Desarrollo de catalizadores monolíticos basa-
The authors gratefully acknowledge the CICYT (projects
MAT 2001-1597 y MAT 2000-0080-P4-02) and the Min-
istry for Science and Technology (Spain) (I3P program) for
financial support and Prof. J.L.G. Fierro for the XPS mea-
surements and his valuable contribution to the XPS spectra
interpretation.
dos en CuO/NiO para la eliminación de NO , Ph.D. Thesis, Madrid
x
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