Bridging the pressure and material gap in the catalytic ammonia oxidation: Structural and catalytic properties of different platinum catalysts

Article abstract of DOI:10.1016/j.jcat.2005.03.002

To understand the pressure and material gap in the platinum-catalyzed ammonia oxidation over Pt, the reaction was studied over a wide range of pressures (10^-3-105 Pa) and temperatures (293-1073 K) with different Pt catalysts: stepped Pt(533) single crystal, knitted Pt gauze, and Pt foil. Experiments were supplemented by theory applying DFT calculations. It was concluded that the primary reaction step of NH₃ oxidation, stripping of hydrogen from the NH₃ molecule, is favored by the presence of surface O and OH species. The latter species are more active for dehydrogenation of NH₂ and NH fragments. NO and N₂ are the only nitrogen-containing products detected under UHV conditions (p<10-1 Pa). N₂O was observed, however, at about 6 Pa (peak pressure) in the temporal analysis of products (TAP) reactor and under ambient pressure conditions of 100 kPa in a microstructured reactor. The pressure dependence of N₂O formation is suggested to be related to a minimum surface coverage by reaction intermediates required for N₂O formation, which is a real pressure gap phenomenon. Independently of the pressure range (10^-3-105 Pa) and the type of Pt specimen, N₂ formation prevails at low temperatures (<700 K), whereas NO production increases with temperature and becomes the dominant reaction channel at high temperature. Catalyst characterization by SEM revealed a reconstruction of the Pt surface after ammonia oxidation at ambient pressure. The degree of surface restructuring is related to the total exposure of the catalyst to the reactants. Surface roughening contributes to activation of the catalyst and changes in its selectivity.

Full text of DOI:10.1016/j.jcat.2005.03.002

Journal of Catalysis 232 (2005) 226–238
www.elsevier.com/locate/jcat
Bridging the pressure and material gap in the catalytic ammonia
oxidation: structural and catalytic properties of different platinum
catalysts
a,∗
b,∗
a
a
c
M. Baerns , R. Imbihl , V.A. Kondratenko , R. Kraehnert , W.K. Offermans ,
c,∗
b
R.A. van Santen A. Scheibe ,
a
Institute for Applied Chemistry Berlin-Adlershof, Richard-Willstätter-Str. 12, D-12489 Berlin, Germany
b
Institut für Physikalische Chemie und Elektrochemie, Universität Hannover, Callinstr. 3-3a, 30167 Hannover, Germany
Schuit Institute of Catalysis, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
c
Received 14 September 2004; revised 22 February 2005; accepted 1 March 2005
Available online 12 April 2005
Abstract
To understand the “pressure and material gap” in the platinum-catalyzed ammonia oxidation over Pt, the reaction was studied over a wide
−3
5
range of pressures (10 –10 Pa) and temperatures (293–1073 K) with different Pt catalysts: stepped Pt(533) single crystal, knitted Pt gauze,
and Pt foil. Experiments were supplemented by theory applying DFT calculations. It was concluded that the primary reaction step of NH
3
oxidation, stripping of hydrogen from the NH molecule, is favored by the presence of surface O and OH species. The latter species are
3
more active for dehydrogenation of NH and NH fragments. NO and N are the only nitrogen-containing products detected under UHV
2
2
−1
conditions (p < 10 Pa). N O was observed, however, at about 6 Pa (peak pressure) in the temporal analysis of products (TAP) reactor and
2
under ambient pressure conditions of 100 kPa in a microstructured reactor. The pressure dependence of N O formation is suggested to be
2
related to a minimum surface coverage by reaction intermediates required for N O formation, which is a real “pressure gap” phenomenon.
2
−3
5
Independently of the pressure range (10 –10 Pa) and the type of Pt specimen, N formation prevails at low temperatures (< 700 K),
2
whereas NO production increases with temperature and becomes the dominant reaction channel at high temperature. Catalyst characterization
by SEM revealed a reconstruction of the Pt surface after ammonia oxidation at ambient pressure. The degree of surface restructuring is related
to the total exposure of the catalyst to the reactants. Surface roughening contributes to activation of the catalyst and changes in its selectivity.

2005 Elsevier Inc. All rights reserved.
Keywords: Ammonia oxidation; Platinum; NH ; N O; Microstructured reactor; TAP reactor; UHV reactor; SEM; Surface restructuring of Pt catalysts
3
2
1
. Introduction
Selective oxidation of ammonia to nitric oxide over
to NO; however, in addition, other side reactions with sig-
nificant exothermicity occur, affecting the overall reaction.
It is commonly accepted that the process is mass-transfer
limited at ambient and elevated pressures [1–6]. The reac-
tion has been studied for a very broad range of temperatures
platinum-group metal (PGM) catalysts is the essential re-
action of industrial manufacture of nitric acid, which is one
of the most important bulk chemicals. The oxidation of am-
monia comprises mainly the exothermic oxidation of NH3
−8
5
(300–1500 K), pressures (10 –10 Pa), and over differ-
ent materials (Pt single crystal [7–11], polycrystalline Pt
[
12–18], supported Pt [19]). Despite the fact that oxidation
of ammonia over PGM gauzes is a highly optimized and
balanced industrial process, there is neither a generally ac-
cepted mechanism nor intrinsic kinetics of all the reaction
steps involved in the catalytic ammonia oxidation. Difficul-
*
Corresponding authors.
E-mail addresses: baerns@aca-berlin.de (M. Baerns),
ronald.imbihl@mbox.pci.uni-hannover.de (R. Imbihl), r.a.v.santen@tue.nl
R.A. van Santen).
(
0021-9517/$ – see front matter  2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2005.03.002

R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
227
ties in the study of intrinsic kinetics of ammonia oxidation
are caused by a lack of temperature control of the exother-
mic reaction (ignition, temperature gradients) and mass-
transfer limitations. Therefore, most mechanistic studies of
ammonia oxidation were performed under ultrahigh-vacuum
nia oxidation up to a total pressure of 15 Pa, even over a long
period of catalyst operation [13,20].
It is obvious from the above background that for a de-
tailed understanding of ammonia oxidation, the reaction
should be studied under well-defined surface properties of
the catalyst and isothermal reaction conditions, emphasiz-
ing the identification of all possible reaction products and
short-lived reaction intermediates. For these purposes, a mi-
crostructured reactor made of quartz glass, the TAP reactor,
and an UHV reactor were applied. The experimental results
of this study are discussed for a broad range of total pres-
(
UHV) conditions [12,13,20–22], under which isothermic-
ity is achieved. The oxidation of ammonia reveals a strong
temperature dependence of product selectivity. N2 is the
main product at low temperatures (< 600 K); NO production
dominates above 800 K [13,20,22]. However, N2 production
prevails again when the temperature is increased further to
−3
5
sures (10 –10 Pa), temperatures (293–1073 K), and ma-
terials: stepped Pt single-crystal (Pt(533)), polycrystalline
Pt foil, and industrial knitted Pt gauze. Particular attention
was paid to reaction-induced catalyst restructuring. In ad-
dition, density functional theory (DFT) calculations were
performed to study the role of oxygen-containing surface
species in the oxidation of NH3.
1
473 K. In mechanistic and kinetic studies of ammonia oxi-
dation at UHV pressures, no information about nitrous oxide
formation is obtained, since this product was never observed
under such conditions, in contrast to continuous-flow studies
at ambient pressure [14,18]. In industrial ammonia convert-
ers, operating above 1073 K, the concentration of N2O in
the tail gas can reach 3000 ppmv [23], which is a severe
environmental issue.
To overcome ignition limitations, the kinetics of ammo-
nia oxidation was recently studied at low temperature (up to
2
. Experimental
6
50 K) over alumina supported Pt in a combined microstruc-
tured reactor and heat exchanger device at a total pressure of
bar and in a large excess of oxygen [14,24,25]. Because of
First the catalytic Pt materials used in this study are
described (Section 2.1). Since the ammonia oxidation was
1
−3
5
studied over a wide range of pressures (10 –10 Pa), suit-
able reactors were required for the various pressures, which
are presented in Section 2.2. Finally, details of the experi-
mental procedure, DFT calculations, and catalyst character-
ization are given in Sections 2.3–2.5.
improved heat transfer in this reactor, the ignition of NH3–
O2 mixtures could be avoided up to 650 K [25]. N2O was
observed as a reaction product. However, the authors did not
focus on catalyst characterization before and after the cat-
alytic reaction, which is an important aspect of the ammonia
oxidation reaction that is known to induce major changes in
catalyst morphology. Moreover, no explanation of the dif-
ferences in product distribution under UHV and ambient-
pressure conditions was given. More recently, the temporal
analysis of products (TAP) reactor was shown to be an ex-
cellent tool for studying ammonia oxidation over industrially
relevant Pt and Pt–Rh gauzes under isothermal conditions at
temperatures of industrial ammonia burners (1023–1073 K)
2
.1. Catalytic material
Three different Pt materials were used in this study.
Ambient-pressure catalytic tests were carried out with plat-
inum foil (99.95%) supplied by Alfa Aesar. The foil thick-
ness was 4 µm. TAP experiments were performed over knit-
ted Pt gauze (Multinit type 4; Degussa) with a wire diameter
of 76 µm. A Pt(533) single crystal was used for UHV studies.
The Pt(533) surface, Pt 4(111) × (100), consists of four-
atom-wide (111) terraces followed by monoatomic (100)
steps.
[
26]. N2O was identified as a reaction product [27]. The peak
pressure in the TAP reactor was ca. 160 Pa at 1073 K.
Operation of the catalysts under industrial conditions is
accompanied by strong restructuring of the surface, resulting
in development of “cauliflower” structures. These changes
occur during the first hours of operation and are assumed
to play an important role in improving catalytic perfor-
mance [28]. The phenomenon was the subject of a number of
high-temperature (> 900 K) studies [15–17,29,30]. The de-
velopment of facets depends strongly on the partial pressure
of O2 and NH3 and surface temperature, resulting in differ-
ent surface structures (flat and curved surfaces, pits) [16].
SEM studies of Lyubovsky and Barelko [15] have shown
that restructuring occurs in two stages, with initial forma-
tion of parallel facets followed by growth of micro crystals.
The initial facet formation is not uniform on all grains; this
is probably due to the structure sensitivity of reaction and
faceting [15]. No restructuring was observed during ammo-
2
2
.2. Experimental setup
.2.1. Microstructured reactor
Temperature control is an essential issue in fundamental
studies of the catalytic ammonia oxidation. Neither mecha-
nistic insights nor kinetic data could be obtained in a plug
flow reactor at ambient pressure over Pt gauze because of
the high exothermicity of the reaction and the related tem-
perature runaway. However, these obstacles were overcome
with a microstructured flow reactor, which ascertained:
– Good mass transfer due to short diffusion paths in
the microstructures (hydrodynamic diameter of reactor
channel dh = 375 µm);

228
R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
A ceramic heating element was placed in the center of
symmetry (with respect to the flow direction) of the reac-
tor, the reaction zone, and was used to control the catalyst
temperature. According to CFD simulations and tempera-
ture measurements in the gas channel, a temperature pro-
file develops along the channel axis during operation with a
flat and broad maximum in the zone of the (15-mm-wide)
central heating element. The catalyst (< 2 mm wide) was
mounted in the center of the broad maximum, providing a
measured axial temperature deviation less than 5 K in the
catalyst range. The heat released by the exothermic ammo-
nia oxidation was small (< 1%) compared with the supplied
electrical power that was required to keep the desired tem-
perature level. Thus, the assumption of isothermicity holds.
Fig. 1. Exploded view of micro-structured quartz reactor with Pt foil cata-
lyst.
2
.2.2. Temporal analysis of products reactor
The temporal analysis of products (TAP-2) reactor ap-
plies a pulse technique under vacuum conditions [31,32].
The unique features of the TAP reactor for mechanistic stud-
ies of high-temperature ammonia oxidation under isother-
mal conditions were previously reported [26]. Depending
–
Good temperature control along the catalyst; this was
due to the large ratio between the heat exchange area
and the active surface area (1:1), which were both equal
to the area of the reactor wall;
The low blank activity of the reactor walls made of
SiO2;
The ability to quickly reach high temperatures (no seal-
ing was located in the hot reaction zone; a temperature
of 850 K was reached within 2 min).
13
17
on the pulse size (10 –10 molecules), the gas trans-
port in the catalytic reactor is determined by either Knud-
sen or molecular diffusion. In the present study all exper-
iments were performed in the Knudsen diffusion regime
–
–
(
< 1015 molecules/pulse). Since under such conditions col-
lisions between molecules in the gas phase are negligible,
solely heterogeneous interactions were observed. This also
opens up the possibility for detecting short-lived reaction
intermediates, when they escape in the gas phase, and if
their concentration is not below the detection limit of QMS
In contrast to previous studies of ammonia oxidation in a
microstructured reactor made of stainless steel [14,24,25],
which were performed over a supported Pt catalyst up to
(
quadrupole mass spectrometry) analysis. Transient experi-
6
50 K, the present microstructured reactor, made from silica,
ments up to 1073 K were carried out in a homemade quartz-
glass reactor.
allowed us to study ammonia oxidation over bulk Pt catalysts
up to 850 K under isothermal conditions. An exploded view
of the microstructured reactor is shown in Fig. 1. The reactor
consisted of three plates made of quartz glass stacked on top
of each other. The total length of the reactor was 125 mm.
The plates had been microstructured by sand blasting. The
middle plate carried two reaction channels, each 500 µm
wide and 300 µm deep. They were connected to the gas inlet
and outlet by broader channels. Two holes (“gas inlet,” “gas
outlet”) at the end of the gas channels permitted fixing of
the fluid connectors. Holes at the corners of the plate were
used to connect the plates with screws. Additional clamps
2
.2.3. UHV reactor
A standard UHV system equipped with LEED, a re-
tarding field analyzer for Auger electron spectroscopy, a
scanning tunneling microscope, and a differentially pumped
quadrupole mass spectrometer (QMS) was used for rate
measurements. Under reaction conditions the UHV system
was operated as a continuous-flow reactor. During rate mea-
surements the cone (opening 2 mm), which connected the
differentially pumped QMS with the main vacuum chamber,
was brought 1 mm in front of the surface. In this way reac-
tion products from the back of the sample and from filaments
were excluded.
5
assured gas-tight sealing up to 1.3 × 10 Pa. The upper side
of the top plate contained conical holes for thermocouples
at specified locations. Material had been removed from the
quartz-glass walls (by sandblasting) to provide space for two
ceramic heating elements. We mounted a strip of the Pt foil
catalyst (10 mm deep, 4 µm thick, < 2 mm wide) on top of
the channels by fixing it between the top and channel plates.
Only two 500-µm-wide areas of Pt foil were exposed to the
gas flow. Thus, the thin Pt catalyst was located on the upper
wall of the microstructured channels and had good contact
with the channel wall.
2.3. Experimental procedure
2.3.1. Flow studies in the microstructured reactor close to
ambient pressure
Catalytic tests were carried out with feeds of different
compositions and a Pt foil “as received.” Concentrations of
NH3 (99.8%) and O2 (99.98%) were up to 3 and 4.5 vol%,

R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
229
respectively. The concentration of Ne (99.99%) as an in-
ternal standard was always 10 vol%; the balance was Ar
averaged to improve the signal-to-noise ratio. Ne (99.998%),
Xe (99.99%), O2 (99.995%), 14NH3 (99.98%), and NH3
(99.9%) were used without further purification. Isotopically
labeled ammonia was purchased from ISOTEC (USA).
Blank tests with NH3–O2 mixtures in the TAP reactor
filled with SiO2 particles showed only very low activity at
temperatures higher than 1023 K. The main product of the
blank runs was nitrogen; its molar fraction was lower than
0.01 even at 1073 K.
15
(
99.99%). All gases were dosed by electronic mass flow
controllers. For temperature-programmed ammonia oxida-
tion, a flow of fixed composition was fed to the reactor at
room temperature, then the temperature in the catalyst zone
of the reactor was changed stepwise (25, 50, and 100 K).
Temperature was held constant at each level for at least 20
min. Blank activity in the whole temperature range (300–
8
33 K) was found to be too low to be measured. Effluent gas
composition was continuously monitored by QMS (Balzer
Omnistar GSD 300). For quantitative evaluation, all of the
feed components and reaction products were calibrated with
respect to Ne. Back pressure was adjusted by a needle valve
to 102 kPa. Feed gas composition was measured in each ex-
periment during blank runs at room temperature.
We tested mass transfer limitation of the fast reaction by
replacing the diluting gas Ar with He in the catalytic exper-
iments at various temperatures. Diffusion coefficients in He
are significantly higher (approximately twofold) than in Ar
and, hence, result in higher mass transfer coefficients. Thus,
deviations between the measurements with Ar and He indi-
cate that mass transfer influences catalytic results [33].
2.3.3. Flow studies in the UHV reactor
+
Samples were prepared by repeated cycles of Ar bom-
−
3
barding (E = 1 keV, pAr = 2 × 10 Pa, t = 15 min), heat-
−4
ing in oxygen (pO = 1 × 10 Pa), followed by annealing
2
to 1300 K. The heating and cooling rate in the tempera-
−1
ture cycling experiments was 100 K min . Gases of purity
99.99% for oxygen and 99.5% for ammonia were used. All
pressures given for the UHV studies are uncorrected. NO
formation was followed in a QMS via an AMU of 30 and
N2 via an AMU of 28. The CO contribution to the signal of
AMU 28 was determined from the signal of an AMU of 12
originating from CO fragmentation and then subtracted. The
background was taken from rate measurements at 300 K,
with the assumption that the catalytic activity at this temper-
ature is negligible. To determine the absolute reaction rates,
the QMS was calibrated against the ionization gauge in the
main chamber.
2
.3.2. Pulse studies in the TAP reactor under vacuum
conditions
The Pt gauze (0.119 g) was packed (packing density
.2 × 10 kgm ) between two layers of quartz particles
3
−3
4
(
dp = 250–355 µm) in the isothermal zone of the quartz TAP
2.4. DFT calculations
microreactor (i.d. = 0.006 m, L = 0.04 m). Before the ex-
periments the Pt gauze was pretreated at ambient pressure
Electronic structure calculations within the framework of
density functional theory (DFT) were performed with the
VASP code [34,35]. Ultrasoft pseudo-potentials, introduced
by Vanderbilt [36] and generated by Kresse and Hafner
[37], were used to describe the ion-electron interactions. The
one-electron Kohn–Sham eigenfunctions were expanded in
plane waves with an energy cutoff of 400 eV. The PW-91
functional accounting for the exchange-correlation of the
electrons was included in the code and exploited for the
calculations [38]. A platinum slab, formed by a periodical
expansion of a unit cell (supercell approach), consisting of
five layers of Pt atoms and separated by five metal layers
replaced by vacuum (∼ 13.8 Å), served as an adsorbent.
Adsorbates were adsorbed on both sides of the slab, main-
taining S2 space group symmetry. In such way we prevented
long-range dipole–dipole interactions between mirror unit
cells. We used a DFT-optimized Pt–Pt bond length of 2.82 Å
instead of the experimental bulk distance of 2.77 Å to avoid
unrealistic forces on the metal atoms. A uniform 5 × 5 k-
point sampling mesh within the first Brillouin zone, obtained
via the Monckhorst package, was applied. The k-point sum-
mation was done with an electronic temperature of 0.15 eV,
and the total energy was extrapolated to zero temperature.
Since in previous work on the O/Pt(111) and NO/Pt(111)
adsorbate system [39] and comparable adsorbate systems
like O/Ru(0001) [40] and O/Pd(211) and NO/Pd(211) [41]
−1
in a flow of 30 ml min of pure oxygen at 1073 K for 1 h
to remove carbon deposits. Before pulse experiments with
NH3 the gauze was additionally pretreated with O2 pulses
before each experiment to recover the loss of oxygen species
in the previous experiment. Interaction of ammonia with
oxygen-free Pt was performed after pretreatment in a flow
−1
of 10 ml min of pure H2 at 1073 K for 4 h. After any pre-
treatment the Pt gauze was exposed to vacuum conditions
−6
(
10 Pa). Then, pulses containing a small amount of re-
14
actant (< 5 × 10 molecules per pulse, resulting in a peak
pressure of ca. 6 Pa) were injected into the microreactor.
Transient experiments were carried out in the tempera-
ture range between 323 and 1073 K with steps of 100 K,
14
15
with mixtures of O2/ NH3/Ne = 1:1:1, O2/ NH3/Ne =
14
1
:1:1, O2/ NH3/Ne = 2:1:1, or NH3/Ne = 1:1. Transient
responses of reactants and products were analyzed at the
reactor outlet by mass spectroscopy at atomic mass units
(
AMUs) related to N2O (44.0, 30.0, 28.0), O2 (32.0), NO
(
(
1
(
30.0), N2 (28.0), H2O (18.0), NH3 (15.0), Ne (20.0), and Xe
15
132.0). In experiments with a mixture of NH3/O2/Ne =
15
:1:1, transient responses were recorded at AMUs N2O
46.0, 31.0, 30.0), O2 (32.0), 15NO (31.0), N2 (30.0), and
15
Ne (20.0). When a signal was not unique for one component,
this is indicated in the text and the figures. In all experi-
ments, pulses were repeated 10 times for each AMU and

230
R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
no net magnetization was found, we assumed that the mag-
netic moment of the adsorbates was totally quenched by
adsorption to the slab. Therefore, all of our calculations
were done in a non-spin-polarized way, except for open shell
atoms and molecules in the “gas phase.” The minimum en-
ergy path (MEP) for the reactions was determined by the
nudged elastic band method (NEB) [42,43] with improved
tangent estimate [44]. The goal of the NEB search was to
find a configuration close to the transition state (TS) of the
reaction. This configuration was further optimized with a
quasi-Newton algorithm to minimize the forces and hence
to determine the equilibrium position of the TS. In some
subtle cases, it turned out to be necessary to refine this TS
search with “smarter” NEB methods, like the climbing im-
age nudged elastic band (CI-NEB) and variable nudged elas-
tic band (VNEB) methods [45].
Fig. 2. Molar fractions of N , H O and H vs. temperature when
2
2
2
14NH /Ne = 1/1 mixture was pulsed in the TAP reactor over Pt gauze
3
In all the calculations the total energy converged to ap-
having been pre-treated in O2.
−1
proximately 5 kJ mol with respect to the k-point sampling,
energy cutoff, and cell size. If not otherwise stated, no re-
strictions on the atoms were implied for geometry optimiza-
tion.
on oxygen-covered single-crystal surfaces of Pt(443) and
Pt(533) with a high reactive sticking coefficient up to 0.2.
Since ammonia decomposition on the bare Pt surfaces is
negligible below 600 K, adsorbed oxygen clearly activates
ammonia decomposition on Pt. Therefore it can be con-
cluded from experimental results that stripping of hydrogen
2
.5. Catalyst characterization
To follow surface-morphology changes of the catalysts,
atoms from the NH molecule by oxygen species is the dom-
3
they were characterized by scanning electron microscopy
SEM) before and after exposure of the catalyst to ammo-
nia oxidation. Images were recorded on a Hitachi-S4000
equipped with a field emission gun (FEG) (cold) in SE mode.
The electron accelerating voltage was 15 kV.
inating primary reaction step in the activation of ammonia.
(
This conclusion is consistent with results obtained in UHV
studies on Pt(111) [21] and Pt(100) [8], as well as for studies
on Pt sponge up to 573 K [46]. As outlined below, this is also
in agreement with theoretical considerations. N , H , and
2
2
H2O were the only reaction products observed at the reactor
outlet with NH3 pulsing over O2-treated Pt gauze (Fig. 2).
3
. Results
Nitric oxide and nitrous oxide were not observed. N for-
2
mation started above 573 K and increased continuously with
First, results from theoretical and experimental studies
temperature, in contrast to the temperature dependence of
N formation found in experiments with different NH –O
on the role of oxygen-containing surface species in NH3
decomposition/activation are presented. Then, data for cat-
alytic ammonia oxidation on polycrystalline Pt foil, knitted
Pt gauze, and a single Pt(533) crystal under ambient pres-
sure, low vacuum (ca. 6 Pa), and UHV conditions, respec-
tively, are shown. The catalytic data are supplemented by
SEM images of the respective catalyst surfaces. Thereafter,
the information will be compared and discussed, with the in-
tention of understanding those factors that influence surface
restructuring and product distribution as a function of total
pressure.
2
3
2
mixtures, as described in Section 3.3 (Figs. 5 and 6). H and
2
H O were detected as hydrogen-containing products in ex-
2
periments on NH pulsing in the absence of gas-phase O
2
3
(Fig. 2), where hydrogen was the main product. However,
H O was the only hydrogen-containing product observed
2
when NH –O mixtures were pulsed over Pt gauze (Figs. 5
3
2
and 6). For discussion of these differences see Section 4.2.
To further evaluate the reaction pathways of ammonia
activation by adsorbed oxygen-containing species, NH de-
3
composition over a flat Pt(111) surface was studied by DTF
calculations. The (111) surface is usually assumed to be
the main face of polycrystalline Pt. In Table 1 the en-
ergy profiles are presented for the reaction of ammonia
on a Pt(111) surface without co-adsorbed species, in the
presence of adsorbed oxygen atoms, and in the presence
of hydroxyl groups. We performed the respective calcu-
lations while considering computed binding energies and
adsorption-site preferences of possible intermediates, reac-
tants, and products of ammonia oxidation evaluated by Of-
fermans et al. [47]. The activation energy for the abstrac-
3
.1. Ammonia activation on Pt surfaces
In a broad temperature range (323–1073 K), no gas-phase
reaction products were observed with NH3 pulsing over Pt
gauze pretreated with hydrogen. Hence, a hydrogen-treated
Pt surface is inactive for NH3 decomposition at short resi-
dence times (10 s) in the TAP reactor. In contrast, Pt gauze
pretreated with O2 shows high activity for NH3 conversion
−3
(see Fig. 2). As shown previously [49], ammonia adsorbs

R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
231
Table 1
a
a
Reaction barrier Eact and energy change ꢀE for the dehydrogenation of
NHx on Pt{111}
−
1
−1
ꢀE (kJ mol )
Reaction
Eact (kJ mol
)
NH → NH + H
112
131
134
41
122
145
30
51
−4
3
2
NH → NH + H
2
NH → N + H
39
NH + O → NH + OH
41
3
2
NH + O → NH + OH
−14
2
NH + O → N + OH
29
NH + OH → NH + H O
−15
−67
−25
3
2
2
NH + OH → NH + H O
0
43
2
2
NH + OH → N + H O
2
a
Eact and ꢀE are determined from the energy difference between the
most stable initial and the lowest transition state and between the initial and
final states respectively, neglecting possible lateral interactions.
Fig. 3. Product distribution vs. temperature for temperature-programmed
ammonia oxidation over Pt foil in micro-structured reactor (feed
2
50 ml/min, 3% NH , 4.5% O , balance Ar + Ne; filled symbols indicate
3
2
tion of hydrogen atoms from NH3 molecules is lowered by
both chemisorbed oxygen and hydroxyl species. Most im-
portantly, adsorbed oxygen atoms only decrease the activa-
tion barrier for the abstraction of the first hydrogen atom
from NH3 molecules significantly, whereas OH species al-
low easier abstraction of all hydrogen atoms down to the
formation of adsorbed nitrogen species. Based on both ex-
perimental and theoretical studies, it is concluded that the
ammonia decomposition is initiated by stripping of hydro-
gen from ammonia molecules via adsorbed oxygen atoms,
whereas hydroxyl groups dominate in the further dehydro-
genation of ammonia fragments.
the heating branch, open symbols indicate the cooling branch).
7
23 and 823 K is observed in the second temperature cycle,
but the hysteresis is less pronounced. Increasing the time on
stream at each temperature level to 24 h during heating and
to ∼ 12 h during cooling results in a more significant hys-
teresis of product formation versus temperature. At 823 K
no constant activity could be reached within 24 h, indicating
an ongoing change on the catalyst surface.
The increase in catalyst activity in the first tempera-
ture cycle appears to be more significant at lower temper-
atures. This indicates either that Pt is activated only for low-
temperature ammonia oxidation or the activation is masked
by mass transfer limitations at higher temperatures. As men-
tioned above, the catalyst activity does not differ signifi-
cantly between the cooling branch of the first temperature
cycle and the heating branch of a second temperature cycle.
Thus, we carried out the test for mass-transfer limitations by
activating Pt foil in a first temperature cycle with Ar (3%
NH3, 4.5 vol% O2, 250 ml/min) and comparing the results
with a following second temperature cycle with He instead
of Ar. The conversion curves start to deviate slightly at 473 K
and significantly at 523 K. This indicates an influence of
mass transfer on reaction rates starting at about 523 K on
activated Pt foil or about 623 K over fresh Pt foil. Activation
of Pt catalyst during repeated temperature cycles in ammo-
nia oxidation would therefore be masked by mass-transfer
limitations. Both the onset of a continuing activation and the
test for mass-transfer limitation indicate a maximum temper-
ature of roughly 523–623 K for steady-state tests of intrinsic
kinetics.
To investigate the influence of feed composition on prod-
uct formation in the ammonia oxidation, the reaction was
also studied over a fresh Pt foil catalyst with an NH /O
ratio of 1. The product gas composition at different temper-
atures is shown in Fig. 4. Less NO and N O are produced
compared with the previously discussed oxygen-rich feed.
N becomes the main product throughout the whole temper-
ature range. NO production increases with temperature. For-
mation of N2 and N2O passes through a maximum around
3
.2. Product distribution in ammonia oxidation under
ambient pressure conditions
Distribution of reaction products vs. temperature is shown
in Fig. 3 for the first heating and cooling cycles of a fresh
Pt foil catalyst, with a feed containing an excess of O2 over
NH3 (4.5 vol% O2, 3 vol% NH3). The catalytic reaction
was measured at each temperature for 1 h. The only reaction
products other than H2O were N2, N2O, and NO. Nitrogen
was the main product at low temperatures and was first ob-
served at 423 K. At this temperature small amounts of N2O
are produced as well. An increase in temperature resulted
in increasing formation of nitrogen and nitrous oxide. NO
was detected at 623 K and became the dominating product
at higher temperatures, and N2 and N2O formation passed
through a maximum at 623 and 723 K, respectively.
Whereas steady-state catalyst operation is achieved at
4
23–623 K after an initial activation period, an ongoing
time-dependent activation of the catalyst was observed at
23 and 823 K. This results in a different product distrib-
7
ution during cooling in the first temperature cycle (Fig. 3),
with enhanced formation of nitrogen and nitrous oxide in the
temperature range from 400 to 723 K. The activation of the
catalyst is permanent: when the temperature program is re-
peated over the same Pt-foil catalyst, product formation dur-
ing cooling in the first temperature cycle and during heating
in the second cycle are almost the same. Further activation at
3
2
2
2

232
R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
Fig. 4. Product distribution vs. temperature for temperature-programmed
ammonia oxidation over Pt foil in microstructured reactor (feed
Fig. 5. Molar fractions of N O, NO and N when a mixture of
2
2
NH /O /Ne = 1/1/1 was pulsed in the TAP reactor over Pt gauze at dif-
4
00 ml/min, 3% NH , 3% O , balance Ar + Ne; filled symbols indicate
3
2
3
2
ferent temperatures.
the heating branch).
7
73 K. Both maxima are shifted toward higher temperatures
as compared with those previous experiments using an ex-
cess of oxygen.
3
.3. Product distribution in ammonia oxidation under
transient vacuum conditions
In contrast to pulsing of NH3 over Pt gauze without
gas-phase oxygen (Fig. 2, Section 3.1), N2, NO, and N2O
were formed when mixtures NH3/O2/Ne = 1/1/1 and
NH3/O2/Ne = 1:2:1 were pulsed over Pt gauze (p < 6 Pa)
(Figs. 5 and 6). H2O was the only hydrogen-containing re-
action product. The formation of N2O in these experiments
was proved by pulsing of a NH3/O2 mixture containing iso-
Fig. 6. Molar fractions of N O, NO and N on pulsing a mixture
2
2
15
NH /O /Ne = 1/2/1 in the TAP reactor over Pt gauze at different tem-
topically labeled ammonia ( NH3). No signal at AMU 44.0,
which is related to CO2 formation, was observed in these
experiments at any temperature (Fig. 7). Short-lived inter-
mediates such as NH2, NH, and HNO were not detected in
the gas phase when the respective AMUs were monitored.
All of the products observed in the catalytic tests using Pt
gauze are formed catalytically, since the transient experi-
ments were carried out in the Knudsen diffusion regime,
where interactions in the gas phase are unlikely.
3
2
peratures.
N2 and N2O formation are shifted to a lower temperature
(473 K) in the excess of oxygen. NO appears in the gas phase
at lower temperature, and its content increases with rising
temperature more strongly as compared with a mixture with
NH3/O2/Ne = 1/1/1.
Similar to results of ambient-pressure experiments (Sec-
tion 3.2), the product distribution is a function of tempera-
ture (Figs. 5 and 6). Upon pulsing of a NH3/O2/Ne = 1/1/1
mixture, the formation of N2O starts above 473 K (Fig. 5).
After passing through a maximum at ca. 600 K, its amount
decreases continuously with a further increase in tempera-
ture. N2 was the only nitrogen-containing product at 373 K;
its concentration passes through a maximum at ca. 600 K.
NO appears in the gas phase above 500 K and becomes the
main product at higher temperatures (Fig. 5). When the ra-
tio of O2–NH3 is increased from 1 to 2, the selectivity for
N2O and NO (Fig. 6) increases. This is in good agreement
with results obtained at ambient pressure (Section 3.2) and
literature data on steady-state ambient pressure ammonia ox-
idation [48]. From Fig. 6 it is also clear that the maxima of
3.4. Product distribution in ammonia oxidation under UHV
conditions
Results of temperature-programmed ammonia oxidation
over cleaned Pt(533) are shown in Fig. 8 for two differ-
ent ratios of NH /O (1/1 and 1/3). Reaction starts around
3
2
500 K with the formation of N , which is the only nitrogen-
2
containing product at low temperatures (< 600 K). NO was
observed above 600 K. The formation of NO increases with
temperature and decreases slightly above 900 K. AMU 44,
which belongs to N O and background CO , was monitored
2
2
throughout the experiment. Since no change in the intensity
of the signal at AMU 44 was observed, no formation of N O
2
was identified in detectable amounts. A slightly higher cata-
lyst activity was observed in the cooling branch (not shown

R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
233
Surface morphology of the Pt-foil catalyst changes dra-
matically after its use in ammonia oxidation under ambient
pressure conditions during 12 h of the temperature ramp
from 293 to 823 K and back to 293 K. The surface is vis-
ibly roughened (Figs. 9c, d), which is accompanied by an
increase in catalyst activity. Small crystals start to grow from
the surface in some domains (Fig. 9d), and other areas are
covered by flat triangular structures growing on top of each
other. All new surface structures exhibit sharp edges. The
crystal planes appear to be flat and some of the planes con-
tain holes. The further development of these structures was
observed when the Pt foil was used under reaction conditions
for a longer period of time (10 days). The respective SEM
images are shown in Fig. 9e and f. Crystal structures on the
surface became larger. Crystals seem to be arranged in layers
on top of each other. They do not appear to protrude so much
from the surface, but rather preferably arrange parallel to the
surface and parallel to each other. Structures no longer fea-
ture sharp edges, but look rather smooth and rounded. The
occasional appearance of rounded holes in crystal surfaces
might indicate local melting. As observed, the changes in
Pt morphology under reaction conditions led to roughening
of the platinum surface. Because of the small sample size
the respective surface areas could not be measured with any
satisfactory accuracy. Hence, a proportionality could not be
established between the increase in catalyst activity and the
increase in surface area, and no statement can be made about
whether the activation is due to the formation of differently
active sites.
Fig. 7. Transient responses recorded during pulsing a mixture of
15
NH /O /Ne = 1/1/1 in the TAP reactor over Pt gauze at 573 K.
3
2
In contrast to ambient-pressure ammonia oxidation, no
surface restructuring is visible in SEM after repeated expo-
sure of Pt gauze catalyst to oxygen pretreatment at 1073 K
and ammonia oxidation under transient conditions (323–
1073 K). Similar results were obtained for a single crystal
Pt(533), which was used for ammonia oxidation under UHV
conditions. LEED and STM demonstrated that the micro-
scopic structure of Pt(533) changes reversibly under reac-
tion conditions [11]. Under certain conditions a doubling of
the step height of the initially monoatomic steps on Pt(533)
occurs that is associated with a change in selectivity from
preferential N2 formation to NO as the main product. Under
the relatively mild reaction conditions of an UHV experi-
ment, however, the microscopic structural changes are not
associated with variations in the macroscopic surface topog-
raphy (Fig. 11a). Only by tilting the crystal can some kind
of structure be made visible (Fig. 11b), which is ascribed to
the initial procedure for polishing the crystal. Faceting and
growth of smaller crystals from the surface are not observed
by SEM.
Fig. 8. Temperature dependence of the N and NO production on Pt(533)
2
under UHV conditions using reaction mixtures with NH /O ratios of 1
3
2
−
2
(
closed symbols) and 0.3 (open symbols) at fixed p
= 1 × 10 Pa.
NH3
here), as previously reported and discussed for large oxy-
gen excess [11]. It was also shown before in detail [11]
that the break in reaction rates around 700 K (Fig. 8) can
be explained by a reversible change in catalyst morphology.
LEED and STM indicated the formation of double-atomic
steps, which disappeared with further heating [49].
Product formation over Pt(533) strongly depends on the
ratio of NH3 to O2 (Fig. 8). A higher partial pressure of oxy-
gen results in increased N2 formation at all temperatures, but
this effect is more pronounced at low temperatures. The on-
set of nitrogen production shifts slightly toward lower tem-
peratures. A higher oxygen/ammonia ratio also improves
NO formation in the whole temperature range where nitric
oxide is formed. Selectivity for NO is increased (Fig. 8).
3
.5. Reaction-induced changes of surface morphology
4
. Discussion
SEM images of fresh Pt-foil and Pt-gauze catalysts are
given in Figs. 9a, b and Fig. 10a, respectively. Some textural
damage caused by the manufacturing process is seen. In gen-
eral, the Pt surface is smooth and flat with occasional holes.
Section 4.1 gives a possible explanation for morphologi-
cal changes induced on the surface of different Pt materials
during ammonia oxidation over a broad range of total pres-

234
R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
Fig. 9. SEM of Pt foil before and after use in ammonia oxidation in micro-structured reactor: fresh Pt foil (a, b), Pt foil after temperature programmed ammonia
oxidation at 293–823 K for 12 h (c, d) and Pt foil after temperature programmed ammonia oxidation at 293–823 K for 10 d (e, f).
Fig. 10. SEM of Pt gauze before (left) and after (right) the studies of ammonia oxidation under transient conditions in the TAP reactor.

R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
235
Fig. 11. SEM of Pt(533) after repeated ammonia oxidation under UHV conditions.
sures (10 3–10 Pa). A relationship between these changes
and catalytic properties is suggested. The influence of total
pressure and Pt material on product distribution in the am-
monia oxidation reaction is discussed in Section 4.2.
−
5
with pure oxygen or ammonia. Lyubovski and Barelko [15]
observed in a time-resolved SEM study that restructuring
of Pt wire used for ammonia oxidation at ca. 1373 K can
be divided into two phases. They defined an initial phase
of formation of parallel facets. The facets are about 5 µm
apart. These facets somehow resemble the pattern shown
in Fig. 9c. Another similarity between the present results
and their work is the fact that not all grains are etched to
the same extent, perhaps because of different initial crys-
tal orientations. The initial phase of reaction-induced re-
structuring of the surface [15] is followed by formation of
individual and randomly located microcrystals with well-
formed crystal faces, where the new crystals extend into the
surrounding volume. This state of progressed etching de-
scribes a high-resolution image as shown in Fig. 9d quite
well. Finally, those authors observed a slow growth of larger
crystals (∼ 30 µm long, 5 µm thick) that seems to be the
beginning of cauliflower growth. No such features were ob-
served in the present study. According to Nilsen et al. [17],
these cauliflowers are associated with ammonia oxidation
for a long time and at temperatures above 1073 K. Ammo-
nia oxidation at lower temperatures produces what they call
“nodules;” this might resemble an enlarged version of the
structures developed in the microstructured reactor foil after
extended periods of ammonia oxidation (Figs. 9e, f).
4
.1. Influence of reaction conditions on the surface
morphology of different Pt materials
No macroscopic roughening or restructuring of the Pt sur-
face was observed by SEM up to a 5000-fold magnification,
when Pt gauze or a Pt(533) single crystal was repeatedly
exposed to ammonia oxidation under vacuum in the TAP
reactor and UHV conditions, respectively. This is in agree-
ment with previous data in the literature. Using optical mi-
croscopy, Gland and Korchak [20] did not see faceting of Pt
single crystals used in UHV ammonia oxidation. Pignet and
Schmidt [13] studied restructuring phenomena of Pt wires
that had been used for many hours in ammonia oxidation at
a total pressure of 10 Pa. They concluded that the surface
stays “quite smooth.”
In contrast to low-pressure ammonia oxidation, under in-
dustrial ammonia oxidation conditions, growth of so-called
cauliflowers significantly changes the surface of Pt wire. Al-
though this final cauliflower state of catalyst etching was not
found in the present work, results from Figs. 9e and f seem
to be similar to initial stages of such cauliflower growth. The
phenomenon has been systematically studied by SEM on Pt
wires [15–17] and spheres (d ∼ 0.6 mm) [50] that were pre-
pared under flow conditions of ammonia oxidation with a
total pressure of 1 bar and various feed compositions. Expo-
sure times were in the range of 15 min [15] to 386 h [17].
All of the investigations of catalytic etching have been car-
ried out in the regime of ignited ammonia oxidation (above
Taking into account the above discussion, the differences
in surface restructuring that occur after the ammonia oxi-
dation reaction under low and ambient pressure conditions
can be explained as follows. Since significant surface re-
structuring of Pt requires the presence of NH and O at the
3
2
same time [16], the reaction intermediates are suggested to
be responsible for the structural changes. This assumption
is supported by results of Nilsen et al. [17], who found no
9
00 K). The results of our experiments make it possible
significant etching of Pt surfaces when passing NO, N O, or
2
for the first time to examine surface morphology after reac-
tion at 650–850 K with reactant partial pressures in the kPa
range. McCabe and co-workers [16] obtained a “phase dia-
gram” illustrating the effect of temperature (973–1773 K)
and NH3/O2 ratio on surface morphology. Depending on
the gas-phase composition and temperature, different types
of facets were observed. Surface restructuring was more ex-
tensive and rapid in an ammonia/air mixture as compared
H O over the catalyst. The concentration of reaction inter-
mediates on the surface may play a key role in the restruc-
turing process. A similar idea was previously put forward by
McCabe and co-workers [16]. These authors concluded that
2
Pt restructuring depends on the total amount of NH and
3
O passed over the catalyst. Thus it can be suggested that
2
the failure to observe any macroscopic changes in surface
morphology by SEM after ammonia oxidation at low reac-

236
R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
tant pressures is due to considerably lower amounts of NH3
and O2 to which Pt(533) single crystal or Pt gauze were ex-
posed under UHV and vacuum conditions. For comparison,
increasing temperature (Fig. 2). A similar tendency was pre-
viously reported for ammonia oxidation over Pt(100) with
−8
molecular beams at about 10 Pa [22] and over stepped
Pt 12(111) × (111) [20]. The shift of the maximum in N2
formation toward lower temperature with increasing oxy-
gen partial pressure may be explained by the fact that an
increase in oxygen partial pressure results in a higher surface
coverage by oxygen species, which favors oxidation of sur-
face nitrogen species to NO over their recombination to N2.
Therefore N2 formation decreases at higher temperatures,
when NO is formed. On the other hand, the theoretical and
experimental results show that adsorbed O and OH species
contribute to the hydrogen stripping from NH3 molecules
(Section 3.1), whereby ammonia conversion at low temper-
atures is increased, and thus to N2 formation. In addition,
from our previous work on high-temperature (1023–1073 K)
ammonia oxidation, it was concluded that the oxygen cover-
age and the nature of oxygen species are important in NO
formation [26].
−7
2
ca. 5×10 mol/cm of ammonia was pulsed over Pt gauze
during ammonia oxidation in the TAP reactor, Pt(533) was
−
4
2
exposed to about 10 mol/cm of ammonia during temper-
2
ature ramping, and ca. 10 mol/cm of NH3 was passed over
the Pt foil during ammonia oxidation (3 vol% NH3 and O2,
respectively) in the microstructured reactor at ambient pres-
sure for 10 h.
The above changes in catalyst morphology must also be
reflected on an atomic scale. Previous experiments with sin-
gle crystals of Pt(533) and Pt(443) did demonstrate that the
step structure changes easily, even under mild reaction con-
ditions, and, furthermore, that the activation barrier for this
microscopic restructuring is much lower under reaction con-
ditions than in the presence of one reactant only [49]. De-
tailed, atomically resolving STM studies have revealed that
the mobility of surface Pt atoms is dramatically increased
in the presence of adsorbates such as atomic hydrogen [51].
Quantum chemical calculations of ammonia adsorption on
stepped Pt(111) surfaces indicate a similar trend that can
be attributed mechanistically to a lowering of the binding
strength of a Pt atom bound to the substrate if it is attached
to an adparticle [52]. Nevertheless, under vacuum conditions
the buildup of such changes on an atomic level was not suf-
ficient for detection by SEM.
Qualitatively the observations made with the single-
crystal planes are in agreement with the results obtained with
polycrystalline Pt under much higher pressure. An additional
factor, which has to be taken into account, is the penetration
of oxygen into the Pt bulk under reaction conditions, which
was established recently [53]. This phenomenon is probably
connected with restructuring of the surface, whereby the sur-
face will open up channels for oxygen penetrating into the
Pt bulk. The extent to which the formation of new oxygen
species just accompanies the restructuring of the surface or
whether it is the actual driving force for restructuring need to
be shown in future investigations. Mechanistically, a bridge
needs to be established between the microscopic structural
changes observed with LEED and STM and the macroscopic
topographical changes that are seen with SEM on Pt cata-
lysts exposed to high-pressure reaction conditions.
For all types of Pt catalyst, NO formation during NH3
oxidation in the presence of gas-phase O2 starts at ca. 550 K
and dominates above 600–700 K. These findings are consis-
tent with UHV studies reported in the literature. Gland and
Korchak [20] studied the ammonia oxidation over stepped Pt
−8
−6
12(111) × (111) in the pressure range from 10 –10 Pa,
which is an order of magnitude below the pressure in the
present UHV experiments. Nitrogen was the main product
at low temperatures. Significant formation of NO started at
about 570 K; NO was the main product at high temperatures.
However, its production decreased at the highest tempera-
tures under vacuum conditions. The decline in NO formation
above 900 K is due to a decrease in the sticking coeffi-
cients of the reactants with increasing temperature. It has
to be emphasized that N2O was not observed in any of the
above-described UHV studies or in the present study over a
Pt(533) single crystal under UHV conditions. In contrast to
these UHV results, N2O was observed in the present study
when a mixture of ammonia and oxygen was pulsed over
knitted Pt gauze at about 6 Pa in the TAP reactor at tem-
peratures from 400 to 800 K. The formation of N2O was
already demonstrated at temperatures above 1073 K under
transient conditions [26]. However, the cited study was per-
formed at a peak pressure of 160 Pa, which is significantly
higher than in the present study. In contrast to the lower pres-
sures (Knudsen diffusion regime), at which all homogeneous
reaction steps are suppressed, at higher pressures, that is, in
the regime of molecular diffusion, N2O formation might be
also influenced by gas-phase interactions. The proof of N2O
in the Knudsen diffusion regime (ca. 6 Pa) seems therefore
noteworthy, since this product was not observed at similar
pressures of ca. 10 Pa in earlier studies [12,13]. Aside from
this, the distribution of N2 and NO formation versus tem-
perature is similar, and raising the oxygen content has the
same effects of increasing NO selectivity and shifting the
maxima of NO and N2 formation toward lower temperatures.
N2O was also observed, when ammonia oxidation was per-
4
.2. Effect of total pressure on catalytic performance of
different Pt materials
Independently of the total pressure, at temperatures below
7
00 K nitrogen is the main product of ammonia oxidation
over Pt(533) single crystal, knitted Pt (gauze), and Pt foil.
The temperature dependence of N2 formation passes through
a maximum. The position of the maximum for N2 production
is a function of the ratio of NH3 to O2. The higher the ratio,
the higher is the temperature of the maximum (Figs. 3–6).
In an extreme case, such as ammonia pulsing without gas-
phase oxygen, N2 formation increases continuously with

R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
237
formed over Pt foil in the microstructured reactor at ambient
pressure (Section 3.2). N2O formation peaks around 750 K
mechanism of N2O formation is valid, a possible explana-
tion for the effect of total pressure on N O formation might
2
(Figs. 3 and 4). It should be mentioned in particular that un-
be the requirement of high surface coverage of reactants or
der ambient pressure catalytic tests were carried out under
conditions eliminating ignition. This confirms the advantage
of microstructured reactors for the study of highly exother-
mic ammonia oxidation, as was established before by Re-
brov et al. [24] at lower temperatures (< 650 K). A study by
Pignet and Schmidt [12] exemplifies the difficulties usually
associated with studies of ammonia oxidation close to am-
bient pressure. They investigated the reaction over Pt wires
and were unable to obtain any temperature-controlled cat-
alytic data with a feed of 7.5% NH3 and 20% O2 in Ar due
to ignition of the reaction feed.
intermediates [Eqs. (1)–(3)] to form nitrous oxide. Usually,
a high surface coverage is promoted by lower temperature
and high pressure, and only under such conditions was a
significant N O selectivity observed experimentally. It was
2
shown that for ambient pressure conditions (Section 3.2),
N O formation undergoes a maximum at 750 K. For lower
2
pressures of the TAP reactor, the highest N O concentra-
2
tion was observed at 573 K, that is, ca. 180 K lower than
at ambient pressure. For reduced pressure, the temperature
range where a significant coverage of the surface with ad-
sorbates is expected also shifts to lower temperatures. The
maximum of N O formation (with N O originating from a
The present study on ammonia oxidation within a broad
total-pressure range and under isothermal conditions reveals
that different Pt materials, single Pt(533) crystal, knitted Pt
gauze, and a Pt foil, show very similar catalytic performance
with respect to NO and N2 formation. The main difference
between ammonia oxidation over Pt(533) single crystal un-
der UHV conditions and in tests over polycrystalline ma-
terials at higher pressures (> 1 Pa) is the absence of N2O
under UHV. With the present knowledge, we cannot exclude
the possibility that N2O is preferentially formed over poly-
crystalline materials only. However, we assume that the total
pressure plays the most important role, as discussed below.
The temperature of maximum N2O formation under vac-
uum conditions in the TAP reactor is lower by about 180 K
than that under ambient pressure conditions for a similar
ratio of NH3/O2 (Figs. 3–6). N2O formation is shifted to-
ward lower temperatures with decreasing absolute pressure.
If this trend is linearly extrapolated to lower pressures (UHV
conditions), the maximum of N2O selectivity would be in a
temperature range where hardly any ammonia is converted.
Since N2O formation is only heterogeneously catalyzed
2
2
surface reaction of second order) should also shift to a lower
temperature. This was indeed observed experimentally in
TAP experiments. At even lower pressures of UHV the sur-
face coverage is probably too low to form N O in amounts
2
that can be analytically detected. Thus, a possible explana-
tion for the pressure-related differences in N O production is
2
the described kinetic effect, although mechanistic effects or
the influence of surface defects on nitrous oxide production
cannot be excluded.
5. Conclusions
Different total pressures (10 3–10 Pa) and catalytic ma-
terials (Pt single crystal, knitted Pt gauze, and Pt foil) were
applied in the oxidation of ammonia. These variables did not
influence the general temperature dependence of N2 and NO
formation. That is to say, the “pressure gap” can be bridged
for the catalysis of the main reaction paths. N2 is prefer-
entially formed at low temperatures and passes through a
maximum. NO formation increases continuously with tem-
perature and decreases slightly only at the highest tempera-
tures applied and at low pressure, because of a decrease in
the sticking coefficients of reactants.
−
5
(
Section 3.3), surface coverage by intermediates, from which
N2O is formed, obviously plays an important role. Three dif-
ferent mechanistic models for N2O formation are discussed
in the literature:
−3
The main difference between UHV (10 Pa) and TAP
6 Pa) and ambient pressure conditions (100 kPa) is the ab-
–
–
–
N2O is formed via recombination of two assumed HNO
intermediates [Eq. (1)] [54,55];
N2O originates from recombination of two surface NO
molecules [Eq. (2)] [56];
(
sence of the reaction product N2O under UHV conditions
when a stepped Pt(533) single crystal is used. This can be
explained by the fact that N2O formation strongly depends
on surface coverage by reaction intermediates, which is a
function of partial pressures of feed and product molecules.
No significant restructuring of the surfaces of the differ-
ent Pt materials was observed by SEM when Pt gauze and
Pt(533) single crystal were repeatedly used for ammonia
oxidation under vacuum in the TAP reactor and UHV con-
ditions, respectively. In contrast to low-pressure ammonia
oxidation, significant reconstruction of the Pt surface occurs
after ammonia oxidation at ambient pressure. The pressure
dependence of surface-morphology changes is related to the
extent of catalyst exposure to reactants. The surface rough-
ening is accompanied by an increase in activity of the foil
N2O is formed via interaction between NHx and NO
[Eq. (3)] [27].
HNO–s + HNO–s → N2O + H2O + 2s;
NO–s + NO–s → N2O + O–s + s;
NO–s + NHx–s + O–s → N2O + HxO + 3s.
(1)
(2)
(3)
It should be emphasized that the above equations do not
represent elementary reaction steps, but are typically lumps
of elementary step. All proposed reaction mechanisms of
N2O formation [Eqs. (1)–(3)] include the interaction of two
adsorbed species on the surface. So without stating which

238
R. Kraehnert et al. / Journal of Catalysis 232 (2005) 226–238
catalyst. This effect will be studied more closely in future
work.
[26] J. Pérez-Ramírez, E.V. Kondratenko, V.A. Kondratenko, M. Baerns,
J. Catal. 227 (2004) 90.
[
27] J. Pérez-Ramírez, E.V. Kondratenko, Chem. Commun. (2004) 376.
28] R.J. Farrauto, C.H. Bartholomew, in: Fundamentals of Industrial Cat-
alytic Processes, Chapman & Hall, London, 1997, p. 481.
[
Acknowledgments
[29] E. Bergene, PhD thesis, Surface characterisation of Pt and Pt/Rh gauze
The authors thank Deutsche Forschungsgemeinschaft
DFG) for financial support in the frame of the competence
catalysts, Trondheim, 1990, p. 119.
30] E. Bergene, O. Tronstadt, A. Holmen, J. Catal. 160 (1996) 141.
31] J.T. Gleaves, J.R. Ebner, T.C. Kuechler, Catal. Rev.-Sci. Eng. 30 (1)
[
[
(
network SPP1091, “Bridging the gap between real and ideal
systems in heterogeneous catalysis.” V. Kondratenko thanks
the Institute for Applied Chemistry Berlin-Adlershof e.V. for
a Ph.D. fellowship. The provision of SEM results by Gisela
Weinberg and Robert Schlögl was highly appreciated.
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