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

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

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

−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.



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).

(

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. 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

–

(

< 10¹⁵molecules/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

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

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

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

NH + OH → NH + H O

0

43

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

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

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

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

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

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

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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.

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[

(

network SPP1091, “Bridging the gap between real and ideal

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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.

(1988) 49.

[

32] J.T. Gleaves, G.S. Yablonsky, P. Phanawadee, Y. Schuurman, Appl.

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Article Doi

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

DOI: 10.1016/j.jcat.2005.03.002

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