The study of nitric oxide adsorption and the mechanism of surface explosions in the reaction of CO + NO on Pt(100) and Pd(110) single crystal surfaces

Article abstract of DOI:10.1023/B:KICA.0000038091.88536.66

High resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) were used to study NO adsorption and the reactivity of CO_ads and NO_ads molecules on Pd(110) and Pt(100) single crystal surfaces. Compared to the Pt(100)-(1 × 1) surface, the unreconstructed Pt(100)-hex surface is chemically inert toward NO dissociation into N_ads and O_ads atoms. When a mixed adsorbed CO_ads + NO_ads layer is heated, a so-called surface explosion is observed when the reaction products (N₂, CO₂, and N₂O) synchronously desorb in the form of sharp peaks with a half-width of 7-20 K. The shape specificity of TPR spectra suggests that the vacancy mechanism consists of the autocatalytic character of the reaction initiated by the formation an initial concentration of active sites due to partial desorption of molecules from the CO_ads + NO_ads layer upon heating to high temperatures. Kinetic experiments carried out on the Pd(110) surface at a constant reaction pressure and a linear increase in the temperature confirm the explosive mechanism of the reaction NO + CO.

Full text of DOI:10.1023/B:KICA.0000038091.88536.66

Kinetics and Catalysis, Vol. 45, No. 4, 2004, pp. 598–606. Translated from Kinetika i Kataliz, Vol. 45, No. 4, 2004, pp. 632–641.
Original Russian Text Copyright © 2004 by Matveev, Sametova, Gorodetskii.
CATALYTIC REACTION MECHANISMS
The Study of Nitric Oxide Adsorption and the Mechanism
of Surface “Explosions” in the Reaction of CO + NO
on Pt(100) and Pd(110) Single Crystal Surfaces
A. V. Matveev, A. A. Sametova, and V. V. Gorodetskii
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
Received January 21, 2003
Abstract—High resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorp-
tion (TPD) and temperature-programmed reaction (TPR) were used to study NO adsorption and the reactivity
of COads and NOads molecules on Pd(110) and Pt(100) single crystal surfaces. Compared to the Pt(100)-(1 × 1)
surface, the unreconstructed Pt(100)-hex surface is chemically inert toward NO dissociation into Nads and Oads
atoms. When a mixed adsorbed COads + NOads layer is heated, a so-called surface “explosion” is observed when
the reaction products (N2, CO2, and N2O) synchronously desorb in the form of sharp peaks with a half-width
of 7−20 K. The shape specificity of TPR spectra suggests that the “vacancy” mechanism consists of the auto-
catalytic character of the reaction initiated by the formation an initial concentration of active sites due to partial
desorption of molecules from the COads + NOads layer upon heating to high temperatures. Kinetic experiments
carried out on the Pd(110) surface at a constant reaction pressure and a linear increase in the temperature con-
firm the explosive mechanism of the reaction NO + CO.
INTRODUCTION
fact forms the basis of the vacancy mechanism of oscil-
lations [4].
Mechanistic studies of the reactions CO + O2 and
NO + CO on platinum group metal (Pt, Pd) single crys-
tal surfaces are still of interest in connection with the
possibility of obtaining new experimental data that
might shed light on the mechanism of oscillations,
chemical waves, and the so-called surface “explosions”
[1–3]. It is known that oscillations of the reaction
rate of CO oxidation on platinum are associated with
the reversible phase transition of the surface Pt(100)-
Possible products of the reaction NO + CO are nitro-
gen, carbon dioxide, and nitrous oxide [7]. The route of
reaction is determined by the nature of a metal, reaction
conditions (Pi, T), and the state of adsorbed NO (molec-
ular or dissociated). At 300 K, NO adsorption in the
molecular state was found for the surfaces Pd(111),
Pd(100), Pd(112), Pt(111), Pt(210), and Pt(110)-hex;
adsorption in the molecular state with dissociation into
Nads and Oads atoms was found for the surfaces Pt(410)
hex
1 × 1. Because on the Pd single crystal surfaces
and Pt(100)-(1 × 1) [8]. In the literature, the high spec-
ificity of various platinum surfaces has been determined
in NO dissociation. The highest activity in dissociation is
seen for the surfaces (410), (210), (100); the surfaces
(111) are the least active and NO adsorbs on it only in the
molecular form [8]. It has been found that the formation
of the adsorbed oxygen layer via N–O bond dissociation
suppresses further NO molecule dissociation.
reversible phase transitions do not occur, oscillations are
thought to be due to the penetration of oxygen atoms in the
subsurface layer of the metal. For the NO + CO reaction
on the Pt(100) surface, the vacancy-site mechanism is
proposed to describe oscillations [4]. The appearance
of extremely narrow peaks of CO2 and N2 with a half-
width of ~5 K in the reaction NOads + COads on the
Pt(100) surface during heating to T ~ 400 K was
assigned to the surface explosion [4]. The explosive
nature of the formation of CO2 molecules has also been
observed on the Pd(320) and Pd(100) single crystal sur-
The goal of this work is to study adsorption and cat-
alytic activity of the Pt(100)-(1 × 1), Pt(100)-hex, and
Pd(110) surfaces and elucidate the mechanism for the
effect of a preadsorbed oxygen layer on the reaction
NOads + COads in the mixed adsorbed layer. These stud-
ies are a necessary step toward solving the problem of
low-temperature NO conversion into N2 in the reaction
NO + CO on platinum-group metals. The first task here
is a search for conditions of nitrogen atom Nads forma-
tion during NO dissociation and the ease of their
recombination to N2 molecules at T ≈ 300 K. We used
high resolution electron energy loss spectroscopy
faces [5, 6]. The limiting step in the reaction NOads
+
COads on Pt and Pd is assumed to be the dissociative
adsorption of NO. At temperatures of ~400 K, adsorbed
oxygen atoms formed in the course of NO dissociation
readily react with the COads layer to form CO2. The
recombination of nitrogen atoms on the clean surface is
accompanied by N2 desorption. In the course of surface
explosions, autocatalytic formation of free sites that are
necessary for the NOads dissociation occurs, and this
0023-1584/04/4504-0598 © 2004 MAIK “Nauka /Interperiodica”

THE STUDY OF NITRIC OXIDE ADSORPTION
599
(HREELS), temperature-programmed desorption on the Pt(111) surface, the ν(Pt−N) vibration corre-
sponds to a frequency at 480 cm–1 [13].
(TPD) and temperature-programmed reaction (TPR).
It is known that NO adsorption on the Pt(100)-1 × 1
surface at 300 K is statistical. At the initial stage of NO
adsorption, it dissociates into Nads and Oads. As the sur-
face coverage increases, adsorption becomes molecular
[12]. The saturation coverage is θNO ≈ 0.4–0.5 ML [14].
Unlike the (1 × 1)-hex surface, on the hex surface NO
adsorption at 300 K is accompanied by the backward
reconstruction of the Pt(100)-hex surface with the for-
mation of NO1 × 1 islands [11]. Adsorption-induced
EXPERIMENTAL
Experimental studies were carried out in a vacuum
chamber of a VG ADES 400 electron spectrometer
equipped with a monochromatic EMU 50 electron gun
and a semispherical electrostatic-type energy analyzer.
The base pressure in the chamber was ~3 × 10–11 Torr.
HREEL spectra were recorded in the specular direction
at an electron energy of ~2.5 eV and an incident angle
of ~35° with a resolution of ~70–90 cm–1. TPR and
TPD spectra were obtained using a VG QXK 400 qua-
drupole mass spectrometer in the regime of linear heat-
ing at a ramp of 2−6°C/s with simultaneous recording
of ten different masses.
Pt(100)-hex and Pd(110) single crystals were
cleaned by Ar+ ion etching with further annealing in
oxygen and a vacuum [9]. The nonreconstructed
Pt(100)-(1 × 1) surface was prepared by the titration
reaction of the preadsorbed NOads layer with hydrogen
and further removal of the Hads layer in a vacuum
according to the procedure described in [10]. The clean
surface was characterized by the (1 × 1) diffraction pat-
tern for Pt(100)-(1 × 1) and Pd(110) single crystal sur-
faces and by the (5 × 20) diffraction pattern for the
Pt(100)-hex surface. Temperature was measured using
a chromel–alumel thermocouple welded to the edge of
a crystal. In the experiments we used nitric oxide 15NO.
phase transition hex
1 × 1 leads to the formation of
structural defects in the islands like small clusters due
to squeezing platinum atoms out of the upper layer
because of the higher atomic density of the hex phase
(>20%). The surface coverage of NO on the hex surface
is θNO ≈ 0.4–0.5 ML [14].
Vibrational Electron Spectroscopy: CO/Pt(100)
The sequence of vibrational spectra that character-
izes the molecular CO adsorption state on Pt(100)-hex
at 300 K is shown in Fig. 1c. It is known that CO
adsorption on the hex surface at coverages below criti-
cal (θ ≤ 0.1 ML) occurs with CO1 × 1 island formation
[15]. At an exposure of 0.2–3.6 L (1 L = 10–6 Torr s) CO
molecules are at the terminal state COterm characterized by
the bands ν(CO) = 2100 cm–1 and ν(Pt–CO) = 480 cm–1
[15]. The formation of structural defects due to the
RESULTS AND DISCUSSION
phase transition hex
1 × 1 occurs upon an exposure
Vibrational Electron Spectroscopy: NO/Pt(100)
of 0.2 L as follows from the appearance of bridging CO
state with a band at ~1890 cm–1 [15]. The growth of
CO1 × 1 islands with an increase in the exposure is
accompanied by a simultaneous growth of the intensity
of the CObridge and COterm bands. Upon an exposure of
To elucidate the role of reversible phase transition
Pt(100)-hex
1 × 1 in oscillations in the reaction
CO + NO, we studied the nature of adsorbed NO on the
(1 × 1) and hex surfaces by HREELS. At the initial
stage at θNO = 0.2 ML (1 ML = 1 monolayer = 1.28 ×
3.6 L, the apparent shift of the band by 6 cm–1 is due to
the dipole–dipole interaction between COads molecules
1015 cm–2) NOads islands are formed due to NO adsorp-
tion on the clean hex surface [11]. HREELS data for the
formation of saturated NO layer suggest the appearance
of two molecular NO states: NO1 × 1 on the (1 × 1) sur-
face and NOdef on defects. The stretching vibrations are
inside the adsorption island. The rate of hex
1 × 1
phase transition is characterized by the power law with
respect to the partial pressure of CO. It was expectable
that there would be a substantial difference in the posi-
tions of bands in CO adsorption on the unreconstructed
(1 × 1) and hex surfaces because the heat of CO adsorp-
tion on the (1 × 1) surface is higher [16]. However, our
spectral data [17] show that the adsorbed CO layer on
Pt(100)-(1×1) consists of terminal (ν(ëé) = 2125 cm–1)
and bridging (ν(CObridge) = 1920 cm–1) states of CO.
This fact points to the similar nature of the structural
ν(NO) = 1620 cm–1 and ν(Pt–NO) = 370 cm–1 for NO1 × 1
and ν(NO) = 1775 cm–1 for NOdef. The formation of
defects is due to the phase transition hex
1 × 1
(Fig. 1a). Figure 1b shows spectra that characterize NO
adsorption on Pt(100)-1 × 1. At saturation NO coverage
(NO = 0.5 ML), the molecular NO is characterized by
the bands ν(NO) = 1620 cm–1 and ν(Pt−NO) = 370 cm–1.
The spectra also show the band of a stretching vibration
ν(Pt–O) = 540 cm–1 from Oads atoms formed via the dis- defects formed in the course of the preparation of the
clean (1 × 1) surface according to the procedure
described in [10]. CO adsorption on the Pt(100)-(1 × 1)
surface at 300 K occurs in a statistical manner with sur-
face coverage up to θCO ≈ 0.75 ML [15].
sociation of NOads molecules [12]. The corresponding
vibration of Nads expected at 300–600 cm–1 is not
resolved in the spectrum because of the overlap with
the frequencies ν(Pt–O) and ν(Pt−NO). For instance,
KINETICS AND CATALYSIS Vol. 45 No. 4 2004

600
MATVEEV et al.
(c)
2106
NO/Pt(100)-hex
1620
CO/Pt(100)-hex
(‡)
350
480
O
N
×50
×100
ν(Pt–NO)
1890
1775
370
3.6 L
1.4 L
NO/Pt(100)-1 × 1
(b)
ν(Pt–NO)
1620
×300
370
O
N
2100
ν(Pt–O)
540
×100
O
0.4 L
0.2 L
×250
0
1000
2000
0
1000
2000
Energy loss, cm–1
Fig. 1. HREEL spectra obtained in the adsorption of (a, b) NO and (c) CO at 300 K: (a) Pt(100)-hex, exposure 5.0 L (NO);
(b) Pt(100)-(1×1), exposure 0.3 L (NO); (c) Pt(100)-hex, exposure 0.2–3.6 L (CO).
Temperature-Programmed Reaction on Pt(100)
occurs at lower temperatures and leads to the formation of
N2 and CO2 in the form of individual peaks at Tdes ~ 350 K.
Figure 2 shows the TPR spectra of NO + CO on
Pt(100)-hex. The nature of the spectra suggest the
mechanism of the surface explosion, which expresses
itself as anomalously narrow desorption peaks of the
products N2 and CO2 [4]. According to spectral data,
adsorption of NO and CO molecules on Pt(100)-hex at
T < 400 K occurs without dissociation into Nads and Oads
atoms. Adsorption is accompanied by the phase transi-
Preliminary activation of the NOads layer at 270 K is
accompanied by the formation of additional peaks in
the reaction products: the low-temperature peak of CO2
at Tdes ~ 270 K and the high-temperature peak of N2 at
Tdes ~ 435 K (Fig. 4). According to spectral data
(Fig. 1b), the dissociative adsorption of NO on Pt(100)-
(1 × 1) occurs with the formation of the Oads that readily
reacts with COads at T ~ 250 K [17]. This fact explains
the appearance of the low-temperature peak of CO2 at
270 K. The formation of the high-temperature peak of
N2 at 435 K is probably due to the recombination of
strongly bound Nads atoms adsorbed on the (1 × 1) sur-
face defects. It is known that on the clean Pt(100) sur-
face the layer of atomic Nads is stable at 300 K. Nitrogen
atoms recombine at high temperatures and form N2 pre-
sented by two peaks at 390 and 450 K [18]. Earlier, we
observed the high-temperature formation of N2 at
490 K as the dissociation of NO molecules on the
defects of the Pt(111) surface [19].
tion hex
1 × 1 resulting in the formation of the
islands of unreconstructed surface covered by the
mixed NOads + COads layer. The reaction in the
NO1 × 1 + CO1 × 1 islands is initiated by the partial des-
orption of the NOads layer upon heating to T ~ 350 K,
which is accompanied by the appearance of vacancies
on the metal surface (free adsorption sites) that are nec-
essary for the dissociation of nitric oxide into Nads and
Oads atoms. Then, the reaction occurs in the autocata-
lytic regime of surface explosion characterized by nar-
row peaks of desorption of N2 and CO2 molecules at
395 K.
On the unreconstructed surface of the Pt(100)-(1 × 1)
surface, reaction in the NOads + COads layer occurs in a
different manner (Fig. 3). In the successive filling of the
Temperature-Programmed Desorption on Pd(110)
TPD spectra at different exposures to NO at Tads
(1 × 1) surface, the reaction between COads and NOads 100 K are shown in Fig. 5. At low exposures (0.2–1.0 L),
=
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THE STUDY OF NITRIC OXIDE ADSORPTION
601
395 K
CO2
N2
200
400
600
800
T, K
Fig. 2. Synchronous formation of CO and N on Pt(100)-hex as a result of surface explosions of the mixed layer formed by the
2
2
consecutive adsorption of NO (0.3 L) and CO (0.3 L) at 120 K.
345 K
CO2
350 K
N2
200
400
600
800
T, K
Fig. 3. Synchronous formation of CO and N on Pt(100)-(1×1) for the mixed layer formed by the consecutive adsorption of NO
2
2
(0.3 L) and CO (0.3 L) at 120 K.
the desorption spectrum is characterized by the NO exposure to 2.0 L leads to the appearance of additional
peak at 490 K and the peaks of dissociation products:
N2 (495 K), N2O (495 K), and O2 (815 K), which are in
low-temperature states of NO adsorption which are
characterized by the desorption peaks at 345, 265, and
agreement with data reported in [20]. An increase in the ~200 K. The studies of NO adsorption on palladium
KINETICS AND CATALYSIS Vol. 45 No. 4 2004

602
MATVEEV et al.
345 K
270 K
CO2
350 K
435 K
N2
200
400
600
800
T, K
Fig. 4. Synchronous formation of CO and N on Pt(100)-(1 × 1) for the mixed layer formed by the consecutive adsorption of NO
2
2
(0.3 L, 270 K) and CO (0.3 L, 120 K).
265 K
495 K
345 K
490 K
NO(m/e = 31)
4
4
3
3
2
2
1
1
N2(m/e = 30)
495 K
815 K
O2(m/e = 32)
4
3
4
2
3
2
1
1
N2O(m/e = 46)
200
400
600
800
1000
T, K
200
400
600
800
1000
T, K
15
Fig. 5. TPD spectra for NO, N , N O, and O upon NO adsorption on Pd(110) at 100 K at exposures equal to (1) 0.2, (2) 0.4,
2
2
2
(3) 1.0, and (4) 2.0 L.
KINETICS AND CATALYSIS Vol. 45 No. 4 2004

THE STUDY OF NITRIC OXIDE ADSORPTION
603
485 K
530 K
N2
NO
460 K
CO
CO2
455 K
820 K
O2
485 K
N2O
200
400
600
800
1000
T, K
200
400
600
800
1000
T, K
Fig. 6. Formation of reaction products N , CO , N O, and O on Pd(110) for the mixed layer formed by adsorption of CO molecules
2
2
2
2
15
at 100 K (0.1 L) and additional adsorption of NO at exposures equal to 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 L (bottom-up increase
in the exposure). Heating ramp is 6°C/s.
single crystal surfaces (100), (111), (110), (112), and surface in the same range of temperatures as from the
(320) [5, 24] showed that the NOads layer is molecular
clean surface (Fig. 5). The results obtained are close to
at T < 300 K and these molecules dissociate at T ≥ 300 K the characteristics of the surface explosion on the
[22]. It has also been found that Nads and Oads atoms
readily diffuse into the subsurface layer of palladium at orption peaks of N2 (483 K) and CO2 (471 K) at
Pd(100) surface observed at 470–485 K [6]. The des-
T ~ 300 K and reverse if the temperature is increased. It
has been shown that the dissolution of Oads atoms in the
subsurface layer of Pd is due to the low activation bar-
Pd(100) and Pd(110) practically coincide, which sug-
gests that this reaction is structure-insensitive. How-
ever, compared to Pt(100), the reactivities of NOads and
COads molecules on Pd(110) were much lower, as
becomes clear from comparing the spectra of Figs. 6
and 2–4. On the Pd(110) surface, the reaction products
N2 (485 K) and CO2 (460 K) are formed at ~100 K
higher temperatures compared to Pt(100).
rier for the process Oads
Odissolv [2]. The surfaces of
palladium can be arranged in a series according to their
reactivity toward N–O bond cleavage: Pd(100) >
Pdpolycryst > Pd(111) > Pd(110) [22].
Temperature-Programmed Reaction on Pd(110)
The effect of the layer of atomic oxygen Oads on the
reaction (COads + NOads)/Pd(110) is illustrated in Fig. 7.
TPR spectra were recorded after consecutive adsorp-
tion of molecules 18O2 (0.2 L) + 15NO (1.0 L) at 100 K
on the clean metal surface and further adsorption of CO
(0.1–0.5 L). The oxygen layer was initially heated to
Spectra shown in Fig. 6 point to the explosive char-
acter of the reaction of 15NOads (0.05–0.30 L) and COads
(0.10 L) adsorbed on the Pd(110) surface at 100 K. The
formation of CO2 occurs at 460 K. The formation of N2
and N2O occurs synchronously at T ~ 485 K. Oxygen
and unreacted NO and CO molecules desorb from the 200 K to remove molecular O2ads. Figure 7 shows CO2
KINETICS AND CATALYSIS Vol. 45 No. 4 2004

604
MATVEEV et al.
495 K
495 K
NO(m/e = 31)
260 K
190 K
N2(m/e = 30)
4
3
2
1
495 K
CO2 (m/e = 46)
N2O, CO2 (m/e = 46)
495 K
270 K
NO (m/e = 33)
4
3
2
4
3
2
1
1
200
400
600
800
1000
T, K
200
400
600
800
1000
T, K
Fig. 7. Effect of atomic oxygen layer O on the formation of reaction products N , CO , and N O on Pd(110) for the mixed layer
ads
2
2
2
15
formed by the consecutive adsorption of O (0.1 L, 200 K) and NO (100 K) with additional CO adsorption at 100 K and exposures
2
(1) 0.1, (2) 0.2, (3) 0.3, (4) 0.5. Heating ramp is 6°C/s.
peaks at 270 and 495 K in contrast to TPR spectra in
Fig. 6. The low-temperature peak of CO2 probably
results from the existence of two parallel reaction
routes:
Hysteresis: Pd(110)
Figure 8 illustrates the hysteretic character of a
change in the rate of formation of reaction products in
the course of slow heating and cooling of the Pd(110)
sample in the reaction mixture NO + CO at a constant
partial pressure of reactants. It is seen that with an
increase in temperature the rate of N2 formation is char-
acterized by two temperature maximums at 495 and
545 K, whereas when the temperature is decreased
there is only one temperature maximum (at 530 K). Let
us analyze this hysteresis using the vacancy-site mech-
anism proposed to explain the surface explosions
observed in the reactions NO + CO and NO + H2 on
Pt(100) [1–4, 6]. The reaction mechanism is described
as a sequence of the following steps:
COads + 18Oads
C16O18Ogas (m/e = 46) + 2* (1)
(where * is the active site of the surface);
2) COads + 15N18Oads C16O18Ogas
(m/e = 46) + 15Nads + *.
(2)
The possibility of the second reaction on palladium was
suggested in [23]. The formation of nitric oxide with
the isotopic composition 15N18Oads is due to the isotope
1) CO + *
2) NO + *
COads,
NOads,
18
exchange reaction 15N16Oads
+
O
ads
15N18Oads
+
16
O
ads
and is characterized by the desorption peak
3) NOads + *
4) 2Nads
Nads + Oads,
N2 + 2*,
CO2 + 2*,
N2O + 2*.
15N18Oads with T = 495 K (Fig. 7). In the presence of the
oxygen layer, the high-temperature formation of mole-
cules CO2, N2, and N2O occurs in a synchronous man-
ner in the form of narrow peaks at Tdes ~ 495 K pointing
to the explosive mechanism of the reaction.
5) COads + Oads
6) NOads + Nads
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THE STUDY OF NITRIC OXIDE ADSORPTION
605
495 K
545 K
N2
CO2(×3)
N2O(×3)
200
400
600
800
T, K
–8
Fig. 8. Hysteresis of the rate of formation of molecules N , CO , and N O in the reaction NO + CO on Pd(110) (P(CO) = 7.6 × 10 Torr,
P(NO) = 3.8 × 10 Torr. Heating and cooling ramp is 2°C/s.
2
2
2
–8
On the Pd(110) surface at T < 430 K, the rate of nous peaks; (2) the reaction occurs via the Langmuir–
reaction product formation in the saturated adsorption Hinshelwood mechanism with the formation of CO2
layer of reactants is insignificant. At T > 430 K desorp- and N2 only under conditions for formation of vacant
tion of some NOads and COads molecules is accompa- adsorption sites in a high concentration; these are nec-
nied by the formation of vacant sites that are necessary essary for the dissociative adsorption of NO; (3) the
for the dissociation of nitric oxide. The rate-limiting vacancy model is proposed to describe hysteresis on the
step is the reaction of NOads dissociation (step 3) with Pd(110) surface; (4) in the reaction NOads + COads, the
the coverages of highly reactive Oads and Nads atoms activity of the Pt(100) surface is much higher than those
leading to the formation of N2, CO2, and N2O in subse- of the Pd(110) and Pd(100) surfaces.
quent steps (4–6). This process initiates a drastic
One might expect that the formation of NO1 × 1 and
increase in the rate of formation of N2 and CO2 (the for-
CO1 × 1 islands with a high local density (θ ~ 0.5) in the
ward branch of hysteresis), which is typical of the reac-
mixed adsorption layer on the Pt(100)-hex surface
tion NOads + COads in the regime of surface explosion
would make it possible to initiate the bimolecular dis-
(see Fig. 6). The second maximum in the formation of
proportionation reaction NOads + COads
Nads +
nitrogen molecules is probably due to the recombina-
tion of strongly bound Nads (Tdes ~ 550 K), which is
accumulated on the surface due to the low contribution
of nitrogen atoms to the reaction that produces N2O mol-
ecules (step 6). The backward branch of hysteresis in the
range T ~ 380–430 K is characterized by the high cata-
CO2gas + * that frees active sites for further dissociation
of NOads molecules. However, the results obtained for
the Pt(100)-hex surface did not confirm this hypothesis.
On the Pd(110) surface, the mechanism of CO2 mole-
cule formation at low temperatures in the reaction
lytic activity in the formation of CO2. This is associated COads + 15N18Oads
C16O18Ogas (m/e = 46) + 15Nads + *
can be interpreted in the framework of the “molecular
catalysis” mechanism with the participation of molecu-
lar forms of reactant adsorption, but this should be fur-
ther confirmed by other studies.
with the high concentration of Oads atoms formed by the
dissociation of NOads molecules at high temperatures.
The results of this work led us to determine the fol-
lowing main features of the reaction NO + CO on
Pt(100) and Pd(110) surfaces: (1) the interaction
Kinetic experiments carried out at a constant pres-
NOads + COads is autocatalytic with explosion-like for- sure of the reaction mixture and with a linear increase
mation of products CO2 and N2 in the form of synchro- in the temperature confirm the explosive character of
KINETICS AND CATALYSIS Vol. 45 No. 4 2004

606
MATVEEV et al.
the reaction NO + CO. Pd(110) shows the maximum 06246) and the Dutch Science Society (grant no. 047-
catalytic activity in the temperature range 450–650 K.
015-002).
REFERENCES
CONCLUSIONS
1. Ertl, G., Adv. Catal., 1990, vol. 37, p. 213.
The study of critical phenomena, including phase
transitions, hysteresis, oscillations, surface explosions,
etc., is very interesting for understanding the factors
affecting the kinetics of heterogeneous catalytic reac-
tions. Despite tremendous complexity in the experi-
mental studies of critical phenomena due to their high
sensitivity to external influence, it has become possible
to study the mechanism of these phenomena in detail
for the reaction NO + CO on platinum-group metals.
On the Pt(100)-hex surface, the rate-limiting step is the
step of activated dissociation of NO molecules at
T > 400 K. HREELS studies show that the unrecon-
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ACKNOWLEDGMENTS
Kawasaki, K., Surf. Sci., 1991, vol. 259, nos. 1/2, p. 109.
This work was supported by the Russian Foundation
for Basic Research (grant nos. 02-03-32568 and 02-03-
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