The reaction mechanism of the high temperature ammonia oxidation to nitric oxide over LaCoO3

Article abstract of DOI:10.1016/j.jcat.2010.09.022

Perovskites are promising catalysts for oxidation reactions. Good NO selectivity is reported for the oxidation of ammonia into nitric oxide over LaCoO₃. More interestingly over this catalyst very little N ₂O is produced, which makes it a potential candidate for industrial ammonia oxidation. In order to further develop perovskite catalysts, an understanding of the reaction mechanism is necessary. Steady-state, TAP and oxygen exchange experiments over LaCoO₃ have been carried out. A reaction mechanism that describes the product distribution (NO, N₂O and N₂) as a function of the oxidation degree of the catalyst has been proposed. The reaction proceeds by a Mars and Van Krevelen mechanism. NO and N₂O are formed through parallel routes from ammonia via surface nitroxyl (HNO) species. Formation of nitrogen occurs through at least three routes. Decomposition of both reaction products NO and N₂O leads to N₂, the latter decomposition being the fastest. The third route to N₂ consists of the reaction of adsorbed ammonia with short-lived oxygen surface species, such as peroxide or superoxide species.

Full text of DOI:10.1016/j.jcat.2010.09.022

Journal of Catalysis 276 (2010) 306–313
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
The reaction mechanism of the high temperature ammonia oxidation to nitric
oxide over LaCoO3
⇑
Gregory Biausque, Yves Schuurman
Institut de recherches sur la catalyse et l’environnement de Lyon, IRCELYON, CNRS, Université Lyon 1, UMR 5256, 2 Avenue Albert Einstein, F-69626 Villeurbanne, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Perovskites are promising catalysts for oxidation reactions. Good NO selectivity is reported for the oxida-
tion of ammonia into nitric oxide over LaCoO3. More interestingly over this catalyst very little N2O is pro-
duced, which makes it a potential candidate for industrial ammonia oxidation. In order to further develop
perovskite catalysts, an understanding of the reaction mechanism is necessary. Steady-state, TAP and
oxygen exchange experiments over LaCoO3 have been carried out. A reaction mechanism that describes
the product distribution (NO, N2O and N2) as a function of the oxidation degree of the catalyst has been
proposed. The reaction proceeds by a Mars and Van Krevelen mechanism. NO and N2O are formed
through parallel routes from ammonia via surface nitroxyl (HNO) species. Formation of nitrogen occurs
through at least three routes. Decomposition of both reaction products NO and N2O leads to N2, the latter
decomposition being the fastest. The third route to N2 consists of the reaction of adsorbed ammonia with
short-lived oxygen surface species, such as peroxide or superoxide species.
Received 30 July 2010
Revised 21 September 2010
Accepted 22 September 2010
Keywords:
Temporal analysis of products
Nitrous oxide
Oxygen exchange
Perovskites
Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction
N2O. Research on oxides [3–8] shows that they are good catalysts
for ammonia oxidation, especially cobalt oxide (Co3O4 spinel), iron
Nitric acid is one of the most utilized nitrogen containing com-
pounds in the world. It is a commodity chemical for the production
of fertilizers, nylon, explosives and all organic compounds involv-
ing nitration.
Its industrial production proceeds through ammonia oxidation
with air into nitric oxide at high temperatures (>1073 K) and short
contact times (10ꢀ4–10ꢀ3 s) over platinum metal group (PGM) al-
loy gauzes via the Ostwald process [1]. Nitric oxide is further oxi-
dized at low temperature to a mixture of nitrogen dioxide and
dinitrogen tetroxide (N2O4) and these molecules react with water
to yield nitric acid.
The key step in this process is the ammonia oxidation reaction.
NO selectivity over PGM gauzes is in the range of 94–96%, but the
catalysts’ costs are high due to the high price of PGM’s and the plat-
inum losses through volatilization during the oxidation process. In
addition, recent regulations require the abatement of N2O, a green-
house gas with a very long life time (120 years) and a global warn-
ing potential (GWP) 310 times higher than CO2. Selectivity to N2O
over PGM gauzes is 1–2%. Currently, several different solutions
within a nitric acid plant are put in place to decompose or reduce
N2O [2,3].
oxide (a-Fe2O3) and chromium oxide.
Nitric oxide selectivity over oxides is comparable to that over
PGM but the nitrous oxide selectivity is lower than over PGM cat-
alysts. However, oxides suffer from low activity and insufficient
stability. In some ammonia plants, cobalt-based bulk oxide mono-
lith catalysts have replaced a part of the PGM gauze [7].
Crystalline mixed metal oxides have also been investigated and
they seem to offer an interesting alternative. In this group, per-
ovskites are attractive materials as they show good performance
and offer large range of materials through substitution and doping.
Appropriate formulation of these oxides leads to easy tailoring of
many desirable properties such as valence state of transition metal
ion, binding energy and diffusion of oxygen in the lattice and con-
ducting properties of the solid.
In order to improve the performance of these materials, an
understanding of the reaction mechanism and kinetics is indis-
pensable. Few studies report on these issues over oxide materials
for the high temperature ammonia oxidation [3,9,10].
Wu and co-workers [11] have compared the NO selectivity over
A substituted (Ca, Sr) LaMnO3, LaCoO3 and LaFeO3 perovskites.
They proposed a mechanism involving adsorbed oxygen surface
species (classifying the reaction of the supra facial type) and the
formation of ‘‘HNO” as an intermediate species. However, they
did not explain the formation N2O nor did they take into account
consecutive reactions of NO and N2O.
A more cost-effective way to tackle both issues is to replace the
PGM catalysts by an oxide-based catalyst that does not produce
Pérez-Ramírez and Kondratenko [12] has shown, in the first TAP
study of ammonia oxidation over oxides, the participation of lattice
⇑
Corresponding author. Fax: +33 4 78 44 53 99.
E-mail address: yves.schuurman@ircelyon.univ-lyon1.fr (Y. Schuurman).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.09.022

G. Biausque, Y. Schuurman / Journal of Catalysis 276 (2010) 306–313
307
oxygen of pure metal oxides (Fe2O3, Cr2O3, and CeO2) in the forma-
tion of NO and put forward a Mars–Van Krevelen mechanism.
This study focuses on the oxidation of ammonia over a LaCoO3
perovskite. This is a typical mixed oxide with a rhombohedral dis-
tortion of the cubic lattice structure. It has been studied for the
combustion of hydrocarbons and organic compounds [13,14] de-
NOx [15] and hydrogenation [16,17]. Moreover, ammonia oxidation
over LaCoO3 has shown good results [3,7,11].
The temporal analysis of products or TAP [18–21] has been
recognized as an important transient experimental method for
heterogeneous catalytic reaction studies. It is especially suited
to the study of fast highly exothermic reactions, due to the small
amounts of reactants used and the operation in the diffusion re-
gime which eliminates all external mass transfer limitations [18].
TAP analysis gave insights into the reaction mechanism of high
temperature reactions, such as ammonia oxidation [12,22–24],
methane partial oxidation [25] and HCN production over plati-
num [26,27].
The time-dependent exit flow of each gas is detected by a quadru-
pole mass spectrometer.
A weight of 25 mg of catalyst (sieve fraction between 150 and
200
lm) was loaded in a quartz reactor (40 mm length and
3 mm internal diameter) between two layers of SiC (200
lm).
Two kinds of transient experiments were performed over the
sample at 973 K: multipulses of ammonia (NH3:Ar = 1:5), oxygen
(O2:CF4 = 1:1), labeled oxygen (18O2), nitrous oxide (N2O:Ar = 1:1),
or nitric oxide (NO:Ar = 1:5) and pump probe experiments, consist-
ing of pulses of one reactant followed by another one at different
time delays (depending on the experiment between 0.2 and 3 s).
Pump probe experiments have been carried out with several gas
mixtures: ammonia/oxygen, ammonia/labeled oxygen, nitrous
oxide/oxygen, NO/ammonia, 15N18O/ammonia. Before each experi-
ment, the sample was pre-treated by a series of pulses of O2 until a
stable oxygen pulse response was observed. Ammonia strongly ad-
sorbs not only on the catalysts but also on colder parts of the reactor
system. A very broad ammonia pulse is obtained, often too small for
correct quantification. Therefore, no ammonia pulse responses are
reported and the conversion is estimated from the production of
NO and N2.
2. Experimental
2.1. Catalyst synthesis and characterization
2.3. 16O2/18O2 exchange experiments
LaCoO3 has been synthesized by a modified citrate gel method.
La(NO3)3ꢁ6H2O (Sigma–Aldrich, 99.9%), Co(NO3)2ꢁ6H2O (Fluka, 98%)
and citric acid (Riedel-de Haën 99.5%) were used as precursors.
Both nitrate salts are dissolved separately into a minimum
amount of water. Citrate acid is added (molar ratio citric acid/me-
tal = 2/1) to the cobalt nitrate solution to form a citrate complex
with the cobalt while stirring at room temperature. Then the lan-
thanum solution is added and the pH is kept at 6.5–7 by using pure
ammonia to form the sol. The sol is slowly evaporated at 353 K un-
til a viscous gel is formed. The gel is dried at 433 K to evacuate NOx
from the material.
The oxygen exchange experiments were carried out in a fixed-
bed reactor containing 50 mg of sample at 1023 K. A gas mixture
of 200 mL minꢀ1 of 2% 16O2 + 4% He + 94% Ar was replaced by a
mixture of 200 mL minꢀ1 of 2% 18O2 + 98% Ar. After this experi-
ment, a back switch was performed. The changes in the gas compo-
sition were continuously monitored by a mass spectrometer (m/
e = 32, 34, 36, 40 & 4). Helium was added to the feed gas as an inert
tracer.
2.4. Steady-state experiments
Calcination is done in two steps: first at 873 K under air in a
muffle furnace in order to oxidize all the organic compounds for
6 h. To further optimize the crystal structure of the material, a sec-
ond calcination is performed in a tubular quartz cell at 1273 K un-
der flowing O2 (gas flow rate of 75 ml minꢀ1) for 10 h.
The catalyst was tested under continuous flow conditions in an
annular fixed-bed quartz reactor. The catalyst is coated on a mull-
ite tube that at the same time serves as a thermocouple well. The
mullite tube has an external diameter of 3 mm and is placed con-
centrically inside a quartz tube with an internal diameter of 4 mm.
The gas flows around the catalyst in the annular space. This reactor
configuration is well adapted for fast reactions with strong thermal
effects [29]. Due to the low flow resistance, high space velocities
can be applied resulting in very short contact times. The reactor
was placed inside a tubular furnace. The bed temperature was
measured with a thermocouple placed inside the mullite tube
measuring directly the catalyst temperature.
X-ray diffraction (XRD) patterns were recorded using a Bruker
D5005 diffractometer and Cu K
a radiation with 0.02° (2h) steps
over the 3–80° angular range. The crystal phase was determined
by simulating and fitting the experimental XDR patterns using
the Rietveld method [28]. The specific surface areas were deter-
mined by the BET method using nitrogen adsorption at 77 K
(Micromeritics ASAP 2020). The samples were, before analysis, de-
gassed under vacuum for 6 h.
Before coating the catalyst on the mullite tube, this specific
zone of the mullite tube was treated with diluted HF (10vol%) for
2 h at room temperature to remove silica from the mullite and thus
increase the porosity to enhance the coating adherence. Afterward
the tube is rinsed thoroughly and heated to 773 K for 2 h to remove
the epoxy resin that was used to protect the part of the tube that is
to stay free of catalyst coating. Typical suspensions were prepared
by mixing 3.96 g of water with 1 g of catalyst and 0.04 g of acetic
acid (AA) and 0.7 wt.% of TyloseÒ MH 300 P2 (methylhydroxyethyl
cellulose, MW = 95 000, Clariant) as binder [30]. The suspension is
heated at 323 K for 10 min and then cooled down to room temper-
ature and stirred for 24 h. The slurry is deposited on the mullite
tube with a syringe, while keeping the tube under horizontal rota-
tion (2000 rpm). It is well known that cobalt can diffuse into cera-
mic supports at high temperatures. Therefore, a first layer of cobalt
oxide was deposited on the mullite tube and kept at 1173 K over-
night. Then, approximately 1.5 mg of the LaCoO3 perovskite was
deposited onto the cobalt oxide layer. The length of the layer is
Chemical analysis was determined by atomic emission using an
induced plasma technique (ICP) (Activa Horiba). Scanning electron
microscopy (SEM) analysis (Jeol JSM 5800LV) was applied to deter-
mine the grain size of the samples and the thickness of the coated
layers. In the last case, the tubes were cut with a diamond saw.
2.2. Temporal analysis of products experiments
Transient experiments have been performed in the TAP-2 reac-
tor system [19,20] over LaCoO3 at 973 K. A TAP pulse response
experiment consists of injecting a very small amount of gas per
pulse into a tubular fixed-bed reactor that is kept under vacuum.
In fact, the reactor is placed on top of a vacuum chamber contain-
ing a large diffusion pump and the reactor exit is continuously
evacuated. The number of molecules that were admitted per pulse
varied between 5 and 20 nmol.
The pressure rise in the micro-reactor is small, and the gas mol-
ecules move through the reactor bed mainly by Knudsen diffusion.
1 cm and its thickness is estimated at approximately 30 lm by

308
G. Biausque, Y. Schuurman / Journal of Catalysis 276 (2010) 306–313
Table 1
The sign of the variation of the independent variable with respect to the main
observables. The range of operating conditions is given in parentheses.
5000
4000
3000
2000
1000
0
LaCoO3 rhombohedral
La(OH)3 tetragonal
L/F (220–3300 m s
T (960–
1125 K)
YO (0.1– YNH (0.01–
2
3
molꢀ1
)
0.4)
0.05)
SNO (0.82–
0.98)
–
–
–
+
–
–
0
+
+
0
x
SN (0–
–
+
2 O
2600 ppm)
XNH (0.25–
3
0.67)
YO , YNH , the mol fraction of oxygen and ammonia, respectively (mol/mol)
2
3
SEM observation of the transversal section. Two tubes have been
prepared and tested with good reproducibility. Due to the absence
of any porosity, only the outer surface of the catalyst layer partic-
ipates to the reaction. This has been verified experimentally by
varying the layer thickness and its effect on the conversion and
20
40
60
80
2θ(degree) CuKα
Fig. 1. X-ray diffraction pattern of the synthesized LaCoO3 perovskite.
selectivity. In analogy with a fixed-bed reactor (W/FNH ), the kinetic
3
data are represented by L/FNH , where L stands for the length of the
3
1.0
0.9
0.8
0.7
0.6
0.5
0.12
0.10
0.08
0.06
0.04
0.02
0.00
catalyst coating and FNH is the molar flow rate of ammonia.
3
Standard conditions consisted of a mixture of 3 vol.% NH3 and
20 vol.% O2 at a total flow rate of 400 ml minꢀ1. CF4 was added to
the mixture as an internal calibration standard. A large range of
conditions has been investigated, as indicated in Table 1. Analysis
of reactants and products was performed on-line with an IR spec-
trometer equipped with a light-pipe (Nicolet 380 with a spectral
resolution of 1 cmꢀ1). This allowed the quantitative analysis of
NH3, NO, NO2, N2O, H2O and CF4. After dissolving NH3, NOx in water
the effluent was passed through a Peltier-type condenser and fed
to a micro GC (Agilent M200). The micro GC contained a molsieve
(MS5A) and Poraplot Q column to analyze N2, O2, H2, N2O and CF4.
In this way, correct O, N and H balances were obtained (less than
5% deviation).
S
NOx
SN
2
X
NH3
S
N2O
1000
1050
1100
Temperature (K)
We report here the selectivity to NOx, the sum of NO and NO2,
although both gases have been analyzed independently by the IR
spectrometer. A gas-phase reaction occurs between NO and NO2
due to the presence of oxygen in the effluent gas. This reaction will
continue in the transfer lines from the reactor to the analyzer and
depends on the flow rate. Moreover, during TAP experiments at
very low pressure, the equilibrium is shifted to NO and no signifi-
cant amount of NO2 is detected.
Fig. 2. NH3 conversion and NOx, N2O and N2 selectivity over LaCoO3 as a function of
the catalyst temperature in an annular reactor (3% NH3, 20% O2, 77% Ar, flow rate of
400 ml minꢀ1).
tivity of 2% is observed at 973 K that decreases below the detection
limit at 1073 K. On the contrary, the nitrogen selectivity increases
with temperature. The effect of the operation conditions on the
NOx and N2O selectivity and the ammonia conversion are summa-
rized in Table 1. This table gives the sign of the variation of the
independent variable on the main observables as well as the range
of operating conditions. Thus, increasing the L/W increases the
ammonia conversion but lowers the NOx and N2O selectivity. In
fact, the NOx selectivity decreases very little with increasing
ammonia conversion from 0.25 to 0.65, less than 1% at isothermal
conditions. The N2O selectivity drops from 1500 to 1000 ppm at
1073 K and 3 mol% of ammonia. The effect of the operating vari-
ables has been conducted over a small range of ammonia conver-
sions at isothermal conditions. Increasing the oxygen partial
pressure lowers the NOx and N2O selectivity but does not influence
the ammonia conversion. Increasing the ammonia partial pressure
increases the NOx and even more the N2O selectivity. However, no
clear effect of the ammonia partial pressure on the ammonia con-
version has been observed. This might be due to the strong adsorp-
tion of ammonia on the catalyst as observed during TAP
experiments. Although no specific long-term stability test have
been performed, reproducible data under standard test conditions
were obtained even after several days of time on stream.
3. Results
3.1. Catalyst characterization
The chemical composition of the materials determined by ICP
was not fully in agreement with the nominal composition. This is
probably due to a small loss of cobalt oxide by sublimation during
calcination. Lanthanum was found in excess in the material in the
form of La(OH)3.
The XDR showed that the synthesized material consists of a
perovskite phase with a rhombohedral structure. The diffractogram
isshowninFig. 1. Thediffractogramindicatesthepresenceoflantha-
num hydroxide and Rietveld quantification attributed it to 5 wt.%.
Textural characterization by N2 adsorption has shown a very low
specific surface area of 2.4 m2/g. Crystallite sizes between 200 and
600 nm were observed by SEM for the well-crystallized sample.
3.2. Steady-state performance
Fig. 2 shows the ammonia conversion and the selectivity to the
N-containing products as a function of the temperature. The NO
selectivity is rather constant between 973 and 1073 K and then
drops significantly at higher temperatures. A rather low N2O selec-
3.3. Oxygen interaction with LaCoO3
No oxygen release was recorded when the LaCoO3 sample was
heated without further treatment to 973 K under vacuum. Then a

G. Biausque, Y. Schuurman / Journal of Catalysis 276 (2010) 306–313
309
series of 400 oxygen pulses was admitted. The integrated area un-
A
der the oxygen pulse response is plotted against the number of
oxygen pulses in Fig. 3. No oxygen has been detected at the reactor
exit during the first 40 pulses. Then a breakthrough of oxygen is
observed that levels off after 90 pulses. Even when the oxygen
pulse response is stable toward the end of the series, the pulse re-
sponse is broad indicating a strong interaction of oxygen with the
perovskite. When the pulsing of oxygen was stopped, a continuous
signal of oxygen desorbing from the sample was observed. Re-
introducing oxygen pulses resulted in a renewed oxygen uptake
depending on the time the sample was kept under vacuum.
The total amount of oxygen that has been adsorbed by the sam-
8
6
4
2
0
32O2
16O18O
36O2
ple estimated from Fig. 3 corresponds to approximately 0.5 lmol of
0.0
0.5
1.0
1.5
O atoms, which is equal to 8 l
mol/m2 by using the measured BET
Time (s)
area of 2.4 m2/g. This is in good agreement with the data of Royer
et al. [31] who reported an oxygen storage capacity for LaCoO3 of
12 l
mol/m2 at 603 K.
B
1.0
0.8
0.6
0.4
0.2
0.0
To further investigate the oxygen adsorption mechanism, 18O2
32O2
was pulsed over the sample. The first transient responses of the
oxygen isotopomers as well as those after pulsing 18O2 for
30 min are shown in Fig. 4a. A small response of 18O2 is observed
due to its large conversion and the main product is 16O2. After
30 min of pulsing 18O2, the 18O2 response has increased just like
the 16O18O response and the 16O2 response has decreased slightly
but remains the main product. Fig. 4b shows the normalized re-
sponses of the oxygen isotopomers. The normalized responses of
the products 16O18O and 16O2 are rather similar, while the reactant
18O2 normalized response is very narrow.
The total amount of exchangeable oxygen can be determined
from labeled oxygen exchange experiments under flow conditions.
The data are shown in Fig. 5. The 16O18O response increases slowly,
then goes through a maximum and slowly decreases. The 18O2 in-
creases even slower and levels off after 600 s. From these data, it is
estimated that approximately 90% of the oxygen in the perovskite
has been exchanged with gas-phase oxygen.
36O2
16O18O
0.0
0.5
1.0
1.5
2.0
Time (s)
Fig. 4. (A) Pulse responses of m/e = 36, 34 and 32 from a pulse of 36O2 over LaCoO3
at 973 K. (B) Height normalized pulse responses of m/e = 36, 34 and 32 from a pulse
of 36O2 over LaCoO3 at 973 K. For clarity reasons, only the first part of the pulse is
shown.
3.0
3.4. Ammonia interaction with LaCoO3
32O2
36O2
2.5
2.0
1.5
1.0
0.5
0.0
After a series of 100 oxygen pulses, a series of ammonia pulses
has been carried out and the products have been recorded (not
shown). The main product was NO, followed by N2. The NO selec-
tivity amounted to 92%. A small signal at m/e = 44 has been ob-
served, which is attributed to N2O. The formation of NO
decreased slowly while the N2 formation increased with the num-
ber of NH3 pulses. After a very long series of NH3 pulses, the forma-
tion of hydrogen was observed. Probably at this stage, the catalyst
is in a too reduced state, which is no longer representative for the
selective ammonia oxidation. The data reveal the participation of
oxygen from the perovskite into the oxidation of ammonia, as no
16O18
O
0
200
400
600
800
Time (s)
Fig. 5. Exit flows as a function of time for an isotopic oxygen exchange experiment
over LaCoO3 perovskite at 1023 K.
0.16
0.12
0.08
0.04
0.00
gas-phase oxygen was supplied during this experiment. Afterward
a re-oxidation of the catalyst resulted in an oxygen uptake and re-
stored the initial NO selectivity when pulsing NH3. A broad NO sig-
nal indicates that NO strongly adsorbs on LaCoO3.
3.5. Pump–probe experiments
The effect of gas-phase oxygen on the ammonia oxidation was
further investigated by pump–probe experiments. Fig. 6 gives an
example for the main product responses on an ammonia/oxygen
and an oxygen/ammonia pump–probe experiments with a time
delay between the two pulses of 3 s. This delay has been varied
0
50
100
150
200
Pulse number
Fig. 3. Integrated O2 pulse response (ꢂ10 nmol O2 & 10 nmol Ar per pulse) as a
function of the pulse number.

310
G. Biausque, Y. Schuurman / Journal of Catalysis 276 (2010) 306–313
pulse. In this case, its attribution to N2O is not unambiguous as it
A
0.6
0.4
0.2
0.0
can also be attributed to CO2 formed by oxidation of carbon that
might be present on the catalyst. For this reason and the fact that
the signal is very small and noisy, the transient response is not
shown in Fig. 6. The m/e = 28 response could be attributed to nitro-
gen as the signal intensity was much larger than the m/e = 44 sig-
nal and the formation of CO could be discarded by comparing the
responses at m/e = 14 and 12.
The NO selectivity has been calculated assuming that NO and N2
are the only nitrogen containing products formed and taking into
account the amounts on both the NH3 and O2 pulse. The NO selec-
tivity in these pump–probe experiments varied between 90% and
95% while the ammonia conversion was estimated at approxi-
mately 75%. When increasing the time delay between the ammonia
pulse and the oxygen pulse, the ammonia conversion and NO and
N2 yields are hardly affected.
Pulses of N2O over LaCoO3 are readily converted into nitrogen,
with a conversion of more than 95%. No significant amount of oxy-
gen is detected indicating that the catalyst retains the extracted
oxygen atom. To investigate the oxidation state of LaCoO3 on the
N2O decomposition, O2/N2O pump–probe experiments were con-
ducted. Fig. 8 shows that the N2O concentration decreases (thus
the N2O conversion increases) while the N2 formation increases
with the increasing time delay between the oxygen and N2O pulse.
Changing the time delay from 0.5 to 2.0 increases the conversion
from 85% to 93%. Thus, the lower the oxidation degree of the cata-
lyst is, the faster is the N2O decomposition rate.
Ar
CF
4
O2
N2
NO
2
0
4
6
8
10
Time (s)
B
0.6
Ar
0.4
0.2
0.0
CF4
O2
NO
N2
The decomposition of NO is much less than N2O. Conversions
less than 2% were observed for NO pulses over LaCoO3. Again no
oxygen release has been observed. On the other hand, pulses of la-
beled 15N18O are almost completely converted into mainly 15N16O.
Thus, LaCoO3 is capable of exchanging its oxygen atoms with those
of nitric oxide. Again the influence of the oxidation state of the cat-
alyst on the NO decomposition has been studied by pump–probe
experiments. In this case, the time delay in the pump–probe exper-
iment had hardly any effect on the rate of the NO decomposition.
The interaction between ammonia and nitric oxide was studied
by NH3/15N18O pump–probe experiments. On the NH3 pulse, the
formation of NO was observed and on the 15N18O pulse the forma-
tion of 15N16O (m/e = 31), 15N2 + NO (m/e = 30) and 15N14N (m/
e = 29). Due to the overlap of NO and 15N2 of the m/e = 30 response,
the quantification of the product distribution during this experi-
0
2
4
6
8
10
Time (s)
Fig. 6. (A) NO, N2 and O2 pulse responses of a NH3–O2 pump–probe with a 3 s delay
at 973 K. (B) NO, N2 and O2 pulse responses of a O2–NH3 pump–probe with a 3 s
delay at 973 K.
and the effect on the product distribution is shown in Fig. 7. Fig. 6
shows that NO is the main product formed on the ammonia pulse
and N2 the main product formed on the oxygen pulse. Note that
ammonia adsorbs strongly and NHx species will be present longer
than the time-scale of the pump–probe experiment. In fact upon
introduction of the oxygen pulse after the ammonia pulse, the
NO response drops quickly and after a delay a renewed NO forma-
tion can be observed. On both pump–probe experiments, the nitro-
gen response consists of a first narrow peak followed by a broader
signal. A very small signal at m/e = 44 was observed on the oxygen
ment is not too accurate. An approximate ratio of 15N2 to 15N14
N
of 3 is estimated, which is not significantly different than the ratio
of 15NO to 14NO on the 15N18O pulse.
1.0
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
O2
SNO
0.8
N2
0.6
0.4
0.2
SN
N2O
1.5
2
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.5
1.0
2.0
NH3/O2 time delay (s)
O2/N2O time delay (s)
Fig. 7. Effect of the time delay between NH3 and O2 during
a
pump–probe
Fig. 8. Effect of the time delay between O2 and N2O during a pump–probe
experiment on the NO and N2 selectivity at 973 K.
experiment on the concentrations of N2O, O2 and N2 at 973 K.

G. Biausque, Y. Schuurman / Journal of Catalysis 276 (2010) 306–313
311
species that favors N2 formation. This illustrates the importance of
the catalyst oxidation state and the presence of reactive oxygen
surface species on the NO selectivity. Thus we examine the oxygen
activation over LaCoO3 in more detail using the labeled oxygen ex-
change experiments.
The theory of isotopic oxygen exchange is well developed and
three classes of exchange mechanisms are known:
The first mechanism is the equilibration reaction, which re-
quires no participation of oxygen atoms from the solid, referred
to as R0:
4. Discussion
The high temperature ammonia oxidation has been mainly
studied over platinum experimentally [22–24,32–37] and theoret-
ically [38–40] but in a lesser extent over metal oxides [3,9,10,12]
Few studies in the literature [3,11] report on the reaction mecha-
nism of ammonia oxidation at high temperatures over perovskites.
In the recent literature, it is generally admitted that the main
features for the ammonia oxidation over platinum consist of an
oxidative stepwise dehydrogenation of adsorbed ammonia assisted
by oxygen adatoms or adsorbed hydroxyl groups [38,40,41]. The
proposed mechanisms include steps for the recombination of
nitrogen adatoms with oxygen adatoms to NO and recombination
of two nitrogen adatoms to nitrogen. Nitrous oxide is formed by
the reaction between NO and nitrogen adatoms. Novell-Leruth
et al. [40] showed by DFT calculations over Pt(1 0 0) that the oxida-
tive dehydrogenation route is far more favorable than a route
through nitroxyl (HNO) or hydroxylamine (NH2OH) intermediates.
Although the ammonia oxidation over platinum and LaCoO3
perovskite has many features in common (e.g. the product distri-
bution as a function of temperature), there exist many differences
as well (e.g. the side reactions involving NO and N2O; the NO selec-
16O2ðgÞ þ 18O2ðgÞ ꢀ 218O16OðgÞ
ð1Þ
The second mechanism constitutes a simple heteroexchange
involving only one solid oxygen atom (R1):
16
18
18O2ðgÞ þ
O
ꢀ
18O16OðgÞ
þ
O
O
ð2Þ
ð3Þ
ðsÞ
16
ðsÞ
ðsÞ
18
18O16OðgÞ
þ
O
ðsÞ
ꢀ
16O2ðgÞ þ
The third reaction mechanism involves the exchange of two
oxygen atoms of the solid simultaneously (complex heteroex-
change, R2):
18O2ðgÞ þ 216OðsÞ
ꢀ
16O2ðgÞ þ 218OðsÞ
16
ð4Þ
ð5Þ
tivity dependence on PO ). This is likely due to the difference in the
18
16O2ðgÞ þ
O
þ
O
ꢀ
ðsÞ
18O16OðgÞ þ 216OðsÞ
2
ðsÞ
surface between platinum and LaCoO3, the latter exposing mainly
oxygen atoms with a small amount of oxygen vacancies [42]. Thus,
a different interaction with the reactants and products can be ex-
pected and the occurrence of different reaction intermediates. In
the case of ammonia oxidation over oxides, nitroxyl or hydroxyl-
amine intermediates are most often proposed [11,37,43]. This is
in line with observations of these intermediates by infrared spec-
The first mechanism can be excluded under TAP conditions as
collisions between gas-phase molecules can be neglected in the
Knudsen diffusion regime. TAP data allow easy discrimination be-
tween R1 and R2 as the primary and secondary products of these
two reaction mechanisms are different. For the case of oxygen ex-
change over LaCoO3, shown in Fig. 3, 32O2 is the main product.
Moreover, the normalized response of 16O18O is broad, indicating
it is a secondary product and not a primary product (in which case
it would resemble more the shape of the 36O2 normalized re-
sponse). Because the 18O fraction on the surface is very small, the
consecutive reaction step (5) is rather slow thus hardly narrowing
the response of 32O2. Thus, the complex heteroexchange R2 mech-
anism is the dominant mechanism. However, the slight delay in
the normalized m/e = 32 signal compared to the m/e = 34 implies
a small contribution of the simple exchange mechanism (R1).
The oxygen exchange flow experiments can be analyzed along
the lines detailed in Sadovskaya et al. [48]. Applying this analysis
to the oxygen flow data over LaCoO3, no distinction can be made
between the mechanisms R1 and R2 as the surface reaction is much
faster than the bulk diffusion. However, the TAP data show that the
R2 mechanism dominates. Royer et al. estimated an activation en-
ergy of approximately 76 kJ/mol for the initial 18O exchange over
LaCoO3 [49].
}
troscopy at low temperatures [44,45]. Moreover, Bruggemann
and Keil [46] have established a reaction mechanism based on ni-
troxyl or hydroxylamine intermediates for the oxidation of ammo-
nia over HZSM-5 by DFT. Accordingly, the reaction mechanism
proposed here involves nitroxyl or hydroxylamine intermediates,
although no direct evidence exists for these species at high tem-
peratures on perovskites.
LaCoO3 contains oxygen vacancies (VꢁꢁOs) as well as electronic
point effects (hꢁ, e0) that allow for a finite compositional variation,
LaCoO3ꢀd, governed by the oxygen partial pressure [47]. In this
study, theinteractionof ammoniawith a LaCoO3 perovskitehasbeen
investigated at different degrees of its oxidation state by varying the
oxygen gas phase partial pressure. Before presenting a reaction
mechanism, the main reaction paths are summarized first.
Ammonia readily reacts over a pre-oxidized sample without
gas-phase oxygen into mainly ꢂ90% NO, ꢂ5% N2 and a small
amount of N2O. Thus, the reaction between adsorbed ammonia
with surface oxygen from the sample seems to be the principal
reaction route at steady-state conditions. The surface oxygen at
the surface can be replenished by the diffusion of oxygen from
the bulk or by supplying gas-phase oxygen. The reaction proceeds
by a Mars and Van Krevelen mechanism. This is confirmed by
changing the time delay between the oxygen and ammonia in
the pump–probe experiments. The conversion expressed as the
sum of NO and N2 formed remains constant with the time delay.
If no gas-phase oxygen is supplied, the catalysts will be more
and more reduced by the ammonia conversion and the selectivity
to nitrogen increases. A fast gas-phase oxygen activation on the
catalyst to minimize the number of reduced sites is thus crucial
for a good NO selectivity. Pérez-Ramírez and Kondratenko [12] also
attributed a Mars and Van Krevelen mechanism for the ammonia
oxidation over Cr2O3, CeO2 and Fe2O3, although only the latter
oxide showed participation of bulk oxygen.
In conclusion, oxygen chemisorbs on the surface and exchanges
rapidly both atoms with those of the surface, after dissociation and
oxygen diffusion through the bulk is the rate-controlling step for
further exchange.
For perovskites having the general formula ABO3, the following
global redox reaction takes place [11,50]
4Bnþ þ 2VO þ O2ðgÞ ¼ 4Bðnþ1Þþ þ 2O2ꢀ
ð6Þ
This redox reaction is a four electron process and has been
extensively studied over a lot of different oxide materials [47,51].
It consists of the following elementary steps, here given in Krö-
ger–Vink notation [51,52]:
adsorption : O2 þ VÅÅ þ e0 ¼ ðO2ÞÅ
ð7Þ
ð8Þ
ð9Þ
Os
Os
dissociation : ðO2ÞÅ þ VOÅÅ þ e0 ¼ 2OOÅ
Os
s
s
incorporation : OÅO þ VOÅÅ þ e0 ¼ OOX þ VÅOÅ
However, pump–probe TAP experiments show an increased N2
production at the moment of the introduction of gas-phase oxygen.
One might thus postulate the presence of a reactive oxygen surface
s
s
b
b
redox couple : Co0Co ¼ CoCXo þ e0
ð10Þ

312
G. Biausque, Y. Schuurman / Journal of Catalysis 276 (2010) 306–313
Eqs. (7)–(9) show that several surface oxygen species can co-ex-
tally (Fig. 8), as the higher the oxidation state the lower the surface
oxygen vacancy concentration.
To account for the additional nitrogen production on the oxygen
pulse during a O2/NH3 pump–probe experiment, the following
steps are added to the reaction mechanism:
ist on the surface: physisorbed O2, superoxide (Oꢀ2 ), peroxide (O-)
and surface lattice oxygen (O2ꢀ) [51]. Furthermore, the adsorption
of oxygen on LaCoO3 requires two vacancies that are each associ-
ated with two cobalt cations. Cobalt cations can present two redox
couples: Co4+/Co3+ and Co3+/Co2+. The first redox couple is pro-
posed in A0 substituted perovskites [50] and is typically associated
NH3;ads þ OÅ ! NH2;ads þ OHÅ
ð20Þ
ð21Þ
ð22Þ
Os
Os
NH2;ads þ OÅ ! NHads þ OHÅ
with the
a-desorption peak in the oxygen TPD [53]. The second
Os
Os
NHads þ HNOX ! N2ðgÞ þ H2OðgÞ
couple is proposed for LaCoO3 and associated with b-desorption
peak [50,53]. This desorption peak is observed above 1000 K and
thus corresponds well with the temperature range used for the
ammonia oxidation.
Based on the active site for oxygen activation and the nitroxyl
intermediate described above, the following reaction route to NO,
the main product observed experimentally, is proposed:
Os
In step (20), adsorbed ammonia reacts with an oxygen surface
species (peroxide), formed in step (8), rather than with a lattice
oxygen species. The surface lifetime of the surface oxygen species
is very short. The NHads species then are further reduced by NHOads
species to form nitrogen (step (22)). Rather than a peroxide spe-
cies, the reactive oxygen species could be a superoxide.
This reaction mechanism addresses the product distribution
(NO, N2O, N2) as a function of the degree of oxidation of the cata-
lyst surface. Over an oxidized surface, the selectivity to NO will be
very high as the nitroxyl HNO intermediate species will react with
lattice oxygen to yield NO, rather than a second-order reaction be-
tween two nitroxyl HNO species to N2O. The decomposition of N2O
requires an oxygen vacancy, thus a partly reduced surface. Simi-
larly, the formation of N2 by step (22) is a second-order process,
thus favored at high ammonia partial pressures. The catalyst sur-
face must enable fast oxygen incorporation into its vacancies to
minimize the formation of superoxide or peroxide species. The
oxygen exchange experiments (Fig. 5) show that this is indeed
the case.
A third route for the formation of N2 is through the decomposi-
tion of NO, as observed by pulsing NO over the LaCoO3. Teraoka
et al. [50] studied the NO decomposition of NO over La0.8Sr0.2CoO3
in a fixed-bed flow reactor. They observed an inhibition by oxygen
on the NO decomposition rate. They proposed a reaction mecha-
nism where NO chemisorbs on two adjacent surface oxygen vacan-
cies followed by the formation of dinitrogen. The vacancies are
regenerated by desorption of molecular oxygen from the surface.
This reaction mechanism corresponds well with the results pre-
sented here on the ammonia oxidation. Oxygen desorption is prob-
NH3;ads þ OX ! H2NOHX
ð11Þ
ð12Þ
ð13Þ
ð14Þ
ð15Þ
ð16Þ
Os
Os
H2NOHX þ OOX ! HNOXO þ H2O þ VÅÅ þ 2e0
Os
Os
s
s
HNOOX þ OOX ! NOOÅ þ OHÅO þ e0
s
s
s
s
NOOÅ ! NOðgÞ þ VÅÅ þ e0
Os
s
2OHOÅ ! H2OðgÞ þ OX þ VOÅÅ þ e0
Os
s
s
5CoXCo þ 5e0 ! 5CoC0 o
NH3;ads þ 2:5OX þ 5CoCXo ! NOðgÞ þ 1:5H2OðgÞ þ 2:5VÅÅ þ 5CoC0 o
Os
Os
ð17Þ
The subscript ads in steps (11)–(17) refers to physisorbed or
chemisorbed species. Ammonia probably adsorbs through interac-
tion of its lone electron pair on the nitrogen atom with a cation on
the surface. This will weaken the N–H bond and oxygen insertion
will become possible followed by a dehydrogenation steps to give
a nitroxyl HNO species. The alternative route would be first a dehy-
drogenation step to give NHads species followed by oxygen insertion
yielding the same nitroxyl species. The last dehydrogenation step
then leads to NO- that desorbs as NO. The electroneutrality is estab-
lished by the change in oxidation state of the cobalt cations (step
(16). Thus steps (11)–(15) are accompanied by five Co3+/Co2+ redox
changes. The surface oxygen vacancies (VꢁꢁOs) are annihilated by
either bulk diffusion or by gas-phase oxygen (steps (7)–(9)). Steps
(11)–(15) are not elementary steps, as the oxidizing species cannot
be the surface anions O2ꢀ (due to its ꢀ2 oxidation state), but rather
singly or doubly ionized anionic vacancies [54]. This process occurs
through electron transfer that is expected to occur rapidly due to the
good electrical conductivity of LaCoO3 [50,51]. The net reaction is an
incorporation of surface lattice oxygen into the NO molecule,
according to the Mars and Van Krevelen mechanism.
ably too slow to be observed on the time-scale of
experiment. Thus, the following step can be added:
a
TAP
NOðgÞ þ VÅÅ þ e0 ! NOÅO
ð23Þ
ð24Þ
2NOÅO !O2sOÅO þ N2ðgÞ
s
s
s
Moreover, by using labeled N18O oxygen scrambling with the
perovskite has been observed, which can be represented as:
N18OðgÞ
þ
16OOX þ VOÅÅ þ e0 ! ½16O—N—18OꢃÅO
The formation of N2O takes place by the recombination of two
nitroxyl species:
s
s
s
! N16OðgÞ
þ
18OXO þ VÅOÅ þ e0
ð25Þ
s
s
2HNOXO ! N2OðgÞ þ H2O
ð18Þ
This process resembles the simple oxygen exchange (step (2)). A
possible mechanism is the adsorption of N18O with the oxygen
atom into an oxygen vacancy site adjacent to a surface oxygen an-
ion. Next, a nitrite 16ON18O- intermediate species is formed that
now can desorb as N16O (or N18O).
s
Over platinum N2O formation has beenproposed by thecombina-
tionoftwoadsorbed NOspecies [55,56]. On thecontrarytoplatinum,
over LaCoO3 no N2O formation has been observed when pulsing NO.
Reaction (18) is proposed over different oxides [37,43] and is in anal-
ogy with hyponitrous acid decomposition [57]. Cheskis et al. [58]
experimentally studied the decay rates of HNO in the gas phase. They
mainly found N2O as a product and concluded that the reaction of
two HNO molecules leads directly to this product by hydrolysis.
N2O can then be decomposed into N2, involving an oxidation of
an oxygen vacancy site:
The formation of 15N14N in small amounts (15N2:15N14N = 3:1)
during the 14NH3/15NO experiments might indicate another, minor,
formation route of nitrogen. Pérez-Ramírez et al. [23] proposed for
the 15N14N formation over platinum a recombination of NHx frag-
ments with adsorbed NO. Because 14NO is formed on the ammonia
pulse, the 15N14
N formation occurs probably through the
15NO/14NO decomposition reaction, rather than through the reac-
tion with a NHx fragment.
N2OðgÞ þ VÅÅ þ e0 ! OÅO þ N2ðgÞ
ð19Þ
Os
s
The steady-state results summarized in Table 1 support the
reaction mechanism proposed here. The decrease of the NOx and
N2O selectivity with increasing contact time indicates that these
Step (19) explains the dependence of the rate of N2O decompo-
sition on the oxidation state of the surface, as observed experimen-

G. Biausque, Y. Schuurman / Journal of Catalysis 276 (2010) 306–313
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