Kinetics of the N2O-CO reaction on Rh(1 1 0)

Article abstract of DOI:10.1016/j.susc.2007.03.019

Our recent studies of the steady-state kinetics of the N₂O-CO reaction on Rh(1 1 0) indicate that at CO excess, the reaction rate increases with increasing temperature. At N₂O excess, the reaction rate is nearly independent of temperature at T < 520 K and rapidly decreases with increasing temperature at T > 520 K. Our present analysis of the relevant data indicates that the latter feature seems to be related to surface-oxide formation. Following this line, we propose a mean-field kinetic model making it possible to describe and clarify the experiment.

Full text of DOI:10.1016/j.susc.2007.03.019

Surface Science 601 (2007) L49–L54
www.elsevier.com/locate/susc
Surface Science Letters
Kinetics of the N O–CO reaction on Rh(110)
2
a,b,
a
a
*
Vladimir P. Zhdanov
, Osamu Nakagoe , Tatsuo Matsushima
a
Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
Boreskov Institute of Catalysis, Russian Academy of Sciences, Novosibirsk 630090, Russia
b
Received 17 January 2007; accepted for publication 14 March 2007
Available online 20 March 2007
Abstract
Our recent studies of the steady-state kinetics of the N
2
O–CO reaction on Rh(110) indicate that at CO excess, the reaction rate
increases with increasing temperature. At N O excess, the reaction rate is nearly independent of temperature at T < 520 K and rapidly
2
decreases with increasing temperature at T > 520 K. Our present analysis of the relevant data indicates that the latter feature seems to be
related to surface-oxide formation. Following this line, we propose a mean-field kinetic model making it possible to describe and clarify
the experiment.
ꢀ
2007 Elsevier B.V. All rights reserved.
Keywords: Heterogeneous catalysis; Reaction kinetics; Low index single crystal surfaces; Rh(110); N
2
2
O; CO; N ; Oxygen
One of the generic reactions occurring on the Pt-group
we have explored the reaction kinetics on Rh(110) as well
[15,16]. First, the measurements were focused on the tem-
perature dependence of the reaction rate under steady-state
metals is CO oxidation by N O [1],
2
N
2
O þ CO ! N
2
þ CO
2
:
ð1Þ
conditions in the situation when the CO and N O pressures
2
In analogy with CO or H oxidation by O , it runs via a few
steps and accordingly can be studied in detail. In addition,
this reaction is of interest from the point of view of applied
2
2
are comparable [15]. Then, the steady-state reaction
kinetics were explored as a function of CO pressure [16].
Compared to the kinetics on Pd(110), the results obtained
environmental chemistry, because N O is one of the harm-
2
on Rh(110) exhibit a qualitatively new feature at N O
2
ful ingredients of motor vehicle gas [2,3]. The early studies
excess (see below). In this Letter, we quantify our findings
by employing a mean-field kinetic model taking the prob-
able reaction steps into account.
In the experiment [16], the reaction kinetics (Fig. 1) was
measured under steady-state conditions as a function of CO
of the kinetics of CO oxidation by N O were performed on
2
supported catalysts [4,5] (see also Refs. [6,7]). In our group,
we have recently explored the corresponding kinetics on
Pd(110) under transient and steady-state conditions
ꢁ
7
[
8–12]. The measurements were executed with angular
pressure at fixed N O pressure, PN O ¼ 2:3 ꢀ 10 Torr,
2
2
resolution of desorbing N . The results obtained were
2
and surface temperatures of 470, 500, 530 and 570 K. At
each temperature, the surface was first cleaned and then
the experiment started at CO excess and, during the reac-
tion runs, the CO pressure was decreased step by step. After
each stepwise pressure change, the system was kept about
described by using a mean-field kinetic model [10] and
Monte Carlo simulations [13]. An interesting comparative
study of CO oxidation on Pd(110) by N O, O and NO
2
2
has been performed by Nakao et al. [14]. More recently,
5
5
min in order to reach a new steady state. Then, another
min was used to measure the reaction rate. Thus, the time
*
Corresponding author. Address: Boreskov Institute of Catalysis,
interval between the data points was about 10 min.
Russian Academy of Sciences, Novosibirsk 630090, Russia. Fax: +7
At CO excess, i.e., at PCO > PN O, the reaction rate is
03833 344678.
2
E-mail address: zhdanov@catalysis.ru (V.P. Zhdanov).
found (Fig. 1) to increase with increasing temperature. This
0
039-6028/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2007.03.019

L50
V.P. Zhdanov et al. / Surface Science 601 (2007) L49–L54
500 K
470 K
100
T = 570 K
S
530 K
1
10
-
7
PCO/10 Torr
2 2
Fig. 1. AR N -desorption signal (arb. un.) as a function of CO pressure, PCO (Torr), during N O–CO reaction on Rh(110) under steady-state conditions
ꢁ
7
at PN O = 2.3 · 10 Torr and surface temperatures of 470, 500, 530 and 570 K [16]. The signal was measured at 57ꢁ off normal into the [001] direction
2
(
at this angle, the signal is nearly maximum).
feature is similar to that observed on in the N O–CO reac-
found to depend on the prehistory of the measurements. If
for example (Fig. 2) the surface is initially clean and the
reaction rate is measured at fixed reactant pressures,
PN2O ¼ 1:5 ꢀ 10 Torr and PCO = 10 Torr, first by
increasing temperature from 450 K up to 750 K and then
by decreasing it back down to 450 K, the reaction rate is
lower in the end compared to that observed in the begin-
ning. This seems to indicate that the oxide formation is
not fully reversible in this case.
To clarify the results described above, we have used a
mean-field kinetic model taking into account the proper
reaction steps. First, the reaction kinetics were analyzed
in detail assuming the surface-oxide formation to be negli-
gible (this part of our analysis is similar to that presented
earlier for Pd(110) [10]). In this case, the reaction is consid-
2
tion on Pd(110) [10]. Like in CO oxidation by O , it can
2
easily be explained taking into account that in this case
the surface is primarily covered by CO and the reaction
ꢁ
7
ꢁ7
rate is controlled by dissociative adsorption of N O. With
2
increasing temperature, the CO coverage decreases, and
accordingly the reaction rate increases.
At N O excess, i.e., at PCO < PN O, the surface is mainly
2
2
covered by oxygen. In this case, the reaction rate is nearly
independent of temperature at T < 520 K. On Pd(110), this
feature is observed as well at these temperatures and also at
higher temperatures [10]. On Rh(110), in contrast, the reac-
tion rate appreciably decreases with increasing temperature
at T > 520 K. One of the explanations of the latter feature
might be that in this case, the desorption of CO molecules
becomes faster than the reaction with adsorbed oxygen.
Our calculations presented below show however that this
is unlikely. A more probable explanation is that the oxygen
adsorbed on the Rh(110) surface forms less reactive surface
oxide resulting in reduction of the reaction rate. The latter
explanation is strongly supported by the recent STM study
of initial oxidation of a Rh(110) surface [17]. The results
obtained there explicitly indicate that on typical time scale
of UHV measurements the surface-oxide formation is kinet-
ically slow at T < 520 K and fast at T > 520 K, respectively.
ered to occur via N O dissociation, reversible CO adsorp-
2
tion, and the Langmuir–Hinshelwood (LH) reaction
between adsorbed CO and oxygen,
N
2
O
gas ! ðN
2
Þgas þ Oad
;
ð2Þ
ð3Þ
ð4Þ
COgas ꢀ COad;
CO þ O ! ðCO Þgas:
ad
ad
2
In this scheme, the N O dissociation is represented by a
2
lumped step (2). In reality, it may occur via N O adsorp-
2
tion, dissociation with the formation of adsorbed oxygen
and N , and subsequent desorption of N (see the TPD
(
(
Note that the surface-oxide formation was observed on the
111) and (100) faces of Rh as well [18,19].)
2
2
data and corresponding Monte Carlo simulations [20]).
The dissociation of adsorbed N O and N are however very
Under steady-state conditions, the kinetics of reactions
2
2
accompanied by surface-oxide formation are known to
often exhibit slow transient features. In our case, the reac-
tion kinetics shown in Fig. 1 were measured by decreasing
CO pressure. With subsequent increase of CO pressure, the
kinetics were reproducible at relatively low and high tem-
peratures, e.g., at 470 and 570 K, because in such cases
the oxide formation either does not occur or occurs rapidly
and reversibly. In general, however, the kinetics have been
fast and accordingly the whole process can be replaced by
step (2). In this approximation, the surface is covered only
by CO and O, and the equations for the corresponding cov-
erages are as follows [10]
dis
N2O
dh
O
=dt ¼ k
P
P
N O ꢁ kLH
h
O
h
CO
;
ð5Þ
ð6Þ
2
ad
CO
des
dhCO=dt ¼ k
CO ꢁ k
h
CO
CO ꢁ kLH
O CO
h h ;

V.P. Zhdanov et al. / Surface Science 601 (2007) L49–L54
L51
1000
450
500
550
600
650
700
750
TS (K)
-desorption signal (arb. un.) as a function of temperature at PN O ¼ 1:5 ꢀ 10ꢁ Torr and PCO = 10 Torr. The temperature was first
7
ꢁ7
Fig. 2. AR N
increased and then decreased as indicated by arrows.
2
2
Table 1
where
Kinetic parameters for the reaction steps occurring on the metal phase
dis
N2O
n
k
¼ ð1 ꢁ 2h
O
Þð1 ꢁ hCOÞ p jN O
ð7Þ
dis
2
Parameter
Value
Dimension
Ref.
dis
N2O
k
Eq. (7)
Fitting
Fitting
KT
Eq. (8)
AM [22]
AM [22]
KT
Eq. (9)
TST [21]
TPD [22]
TPD [22]
is the N O-dissociation rate constants (j
is the impinge-
N O
2
2
p
n
dis
0.5
3
3.2 · 10
–
–
s
ment rate constant, pdis is the dissociation probability at
low coverages, and n P 2 is the exponent used below as a
fitting parameter),
5
5
ꢁ1
ꢁ1
j
N O
2
ad
Torr
Torr
k
CO
g
pad
0.3
0.7
4.0 · 10
–
–
ad
CO
k
k
¼ ½ð1 ꢁ hCOÞ=ð1 ꢁ hCO þ ghCOÞꢂp j
¼ mdes exp½ꢁðEdes ꢁ AhCOÞ=k Tꢂ
are the CO adsorption and desorption rate constants (jCO
is the impingement rate constant, pad is the sticking coeffi-
cient at low coverages, g < 1 is the parameter related to the
precursor states, mdes is the pre-exponential factor, Edes is
the activation energy at low coverages, and A > 0 is the
parameter taking repulsive lateral CO–CO interactions into
account), and
ð8Þ
ð9Þ
ad CO
ꢁ1
ꢁ1
des
CO
j
CO
des
CO
s
B
k
m
1
6
ꢁ1
des
10
37
4
s
E
des
kcal/mol
kcal/mol
A
a
k
LH
Eq. (10)
TST [21]
TPD [3]
13
ꢁ1
mLH
ELH
a
10
27
s
kcal/mol
CO oxidation on Rh is structure insensitive, and for this step we use
the parameters corresponding Rh(111) (note also that the value of ELH
employed is close to that predicted by the DFT calculations [23]).
k
LH ¼ mLH expðꢁELH=k
B
TÞ
ð10Þ
is the rate constant of the LH step (mLH and ELH are the
coverage-independent Arrhenius parameters). Note that
expression (7) implies that one of the coverages is low
and that the oxygen saturation coverage is 0.5 ML (this
corresponds to the formation of the (1 · 2) missing-row
With the specification above, the model reasonably
reproduces the experiment at CO excess in the whole tem-
perature range (cf. Figs. 1 and 3). At N O excess, the exper-
2
iment is reproduced as well but only for T < 520 K. For
T > 520 K, the model does not predict a drop of the reac-
tion rate with increasing temperature. As already noted,
the latter is indicative that the LH step is sufficiently fast
in order to maintain the high-reactive regime at T > 520 K.
To describe the drop of the reaction rate at T > 520 K,
we have complimented steps (2)–(4) by surface-oxide
formation,
islands). Expression (8) does not contain h , because the
O
effect of adsorbed oxygen on the CO-adsorption rate
is weak.
The bulk of the values of the kinetic parameters used in
Eqs. (7)–(10) were obtained (Table 1) by employing the
results of independent adsorption measurements (AM),
TPD experiments, elementary kinetic theory (KT), and
transition-state theory (TST). The only two fitting param-
eters were pdis and n. The value of the former parameter
was chosen to reproduce the position of the kinetic transi-
tion from one kinetic regime to another. The value of the
latter parameter was selected to fit the reaction order in
O ! O :
ð11Þ
ad
ox
The full-scale description of the latter step is hardly possi-
ble at present, because the data indicating what happens in
this case on the atomic scale under chemically reactive con-
ditions are lacking. Nevertheless, using our kinetic data
CO at T = 470 K and PCO > PN O
.
2

L52
V.P. Zhdanov et al. / Surface Science 601 (2007) L49–L54
-
-
-
-
-
-
-
1.4
1.6
1.8
2.0
2.2
2.4
2.6
530 K
00 K
5
T =570 K
S
4
70 K
0
0
0
.6
.4
.2
T =470 K
S
5
00 K
5
30 K
0
.0
.4
0
0
0
0
0
.3
.2
.1
.0
T =570 K
S
4
70 K
-
7.5
-7.0
-6.5
-6.0
Log ( P )
CO
1
0
ꢁ7
Fig. 3. Reaction rate, W (ML/s), and adsorbate coverages calculated under steady-state conditions for PN O = 2.3 · 10 Torr and surface temperatures
2
of 470, 500, 530 and 570 K. The surface-oxide formation is neglected.
and recent results of other relevant studies, we may suggest
one of the likely scenarios.
The available STM data [17] show that on Rh(110) the
surface oxide forms nm-sized islands and accordingly the
corresponding kinetics should be described in terms of
the theory of first-order phase transitions. Phenomenolog-
We are interested in the drop of the reaction rate with
increasing temperature. Taking into account that the
dependence of the rate constants on temperature is usually
stronger than on coverage, we use for kox the simplest
representation
k
ox ¼ mox expðꢁEox=k
B
T Þ;
ð13Þ
ically, it can be done by introducing the critical oxygen cov-
cr
erage, h , and considering that the oxide islands grow or
O
cr
cr
O
where mox and Eox are the coverage-independent Arrhenius
parameters. With these approximations, Eq. (12) yields
shrink respectively at h < hO and h > h , where h is
O
O
O
the local O coverage of the metal phase. In our case
Fig. 1), the reaction runs start in the situation when CO
(
h ¼ 1 ꢁ exp½ꢁk ðt ꢁ t Þꢂ;
ð14Þ
ox
ox
0
is in excess, the O coverage is negligible, and accordingly
there is no oxide. With decreasing CO pressure, the equa-
tion for the surface-oxide coversage can be represented as
where t is the time corresponding to the moment when h
0
O
cr
is equal to h .
O
ꢀ
cr
In addition, we take into account that the catalytic cycle
on the metal phase is much faster than the oxide formation
and also faster than the reaction steps on the oxide. In this
case, the local reaction rate, Wmet, and adsorbate coverages
on the metal phase are given by the steady-state solutions
dhox
dt
0
for
h
O
< hO
¼
ð12Þ
cr
O
k ð1 ꢁ h Þ for h > h ;
ox
ox
O
where kox is the oxide-formation rate constant dependent
in general on coverage and temperature.

V.P. Zhdanov et al. / Surface Science 601 (2007) L49–L54
L53
of Eqs. (5) and (6) (Fig. 3). The total reaction rate can
accordingly be represented as
The values of the other two free parameters, Eox = 35
kcal/mol and pER = 0.15, were chosen to fit the experiment.
The agreement between the experiment and calculations is
found to be good (cf. Figs. 1 and 4). As expected from the
discussions above, our model predicts that the surface-
oxide formation is negligible at T < 520 K and very fast
at T = 570 K. For T = 530 K, the oxide formation occurs
on the time scale comparable with the time scale of
measurements.
W ¼ W metð1 ꢁ hoxÞ þ W ox
h
ox
;
ð15Þ
where Wox is the local reaction rate on the oxide.
On the oxide patches, the reaction may occur via the LH
or Eley–Rideal (ER) mechanism (in the context of the
oxide chemistry, the former one is often called a Mars
van Kleveren mechanism). Alternatively, the oxygen form-
ing oxide may migrate to the metal patches and then react.
For Pt(110), for example, the DFT calculations [24] indi-
cate that the ER pathway,
In summary, the N O–CO reaction on Rh(110) exhibits
2
unusual behaviour at N O excess and T > 520 K. Specifi-
2
cally, the reaction rate rapidly decreases with increasing
temperature. Our analysis of the corresponding kinetics
and the other relevant recent reports indicate that this fea-
ture is likely related to surface-oxide formation. For this
reaction scenario, we have constructed a mean-field kinetic
model making it possible to describe and clarify the mea-
sured kinetics.
COgas þ Oox ! ðCO
2
Þgas
;
ð16Þ
seems to be preferable. To be specific, we take into account
only this pathway (for the LH scheme the results were
proved to be similar) and use the simplest first-order
expression for the corresponding rate,
Finally, it is appropriate to note that the concept that
under the lean conditions the rate of the CO conversion
W ox ¼ kERPCO
;
ð17Þ
where kER = pERjCO is the reaction rate constant (pER is
the reaction probability).
to CO on the Pt-group metals may be reduced due to
2
the formation of surface oxide is not new. Its wide circula-
tion started after the famous publication by Sales, Turner
and Maple [25], where they complemented their experimen-
tal studies of kinetic oscillations in CO oxidation on poly-
crystalline Pt, Pd and Ir at atmospheric pressure by a MF
kinetic model including surface-oxide formation as a feed-
back between the low- and high-reactive regimes. Although
at present the reports on oxide formation during CO oxida-
tion are numerous (see e.g. the data for Pt(110) [26], sup-
ported Pt [27], Rh [28] and Pd [29], Monte Carlo
simulations [30] and references therein), the understanding
of the mechanistic details of this process under chemically
reactive conditions is still far from complete. Thus, this
important aspect of catalytic reactions occurring with par-
ticipation of adsorbed oxygen remains open for further
experimental and theoretical studies.
According to our measurements (Fig. 1), the drop of the
reaction rate at T > 520 K starts just after reaching the con-
dition PCO ’ P
. Our calculations (Fig. 3) show that h
O
N2O
cr
is low in this case. This indicates that h seems to be low
O
cr
O
cr
O
as well. To be specific, we use h ¼ 0:1. (Note that h
may depend on temperature. For our present calculations,
cr
O
this effect is not important as long as h is low.)
According to TST [21], the pre-exponential factors for
elementary rate processes occurring in adsorbed overlayers
1
3
ꢁ1
are typically comparable or higher than 10
s . The oxide
formation occurs however on the island boundaries, and
accordingly the corresponding apparent pre-exponential
factor in Eq. (13) is expected to be a few orders of magni-
1
3
ꢁ1
tude lower than 10
s . In our calculations, we employ
1
1
ꢁ1
mox = 10
s .
-1.4
-1.6
-1.8
-2.0
-2.2
-2.4
-2.6
500 K
5
30 K
470 K
T =570 K
S
-7.5
-7.0
-6.5
Log ( P )
CO
-6.0
10
Fig. 4. Steady-state reaction rate, W (ML/s), calculated taking into account surface-oxide formation. The reaction conditions are the same as in Figs. 1
and 3.

L54
V.P. Zhdanov et al. / Surface Science 601 (2007) L49–L54
[15] T. Matsushima, O. Nakagoe, K. Shobatake, A. Kokalj, J. Chem.
Acknowledgement
Phys. 125 (2006) 133402.
[
[
16] T. Matsushima, Phys. Chem. Chem. Phys., in press.
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One of the authors (V.P. Zhdanov) is grateful for the
Guest Professorship at the Catalysis Research Center,
Hokkaido University.
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Kresse, M. Schmid, P. Varga, J. Yuhara, X. Torrelles, C. Quiros, J.N.
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