Active phase for inclined CO2 desorption on Rh(110) in steady-state CO oxidation

Article abstract of DOI:10.1016/S0009-2614(03)01169-2

The angular and velocity distributions of desorbing CO₂ and LEED spot intensities were examined in steady-state CO oxidation on Rh(110) and Pd(110). On Rh(110), in the limited CO pressure range, the angular distribution splits into bi-direction

Full text of DOI:10.1016/S0009-2614(03)01169-2

Chemical Physics Letters 377 (2003) 279–285
www.elsevier.com/locate/cplett
Active phase for inclined CO desorption on Rh(1 1 0)
2
in steady-state CO oxidation
a
Izabela Rze ꢀz nicka ,Tatsuo Matsushima
b,
*
a
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
b
Laboratory of Surface Reaction Dynamics, Catalysis Research Center, Hokkaido University, Sapporo 060-0811, Japan
Received 3 March 2003; in final form 16 June 2003
Published online: 25 July 2003
Abstract
The angular and velocity distributions of desorbing CO
CO oxidation on Rh(1 1 0) and Pd(1 1 0). On Rh(1 1 0),in the limited CO pressure range,the angular distribution splits
2
and LEED spot intensities were examined in steady-state
into bi-directional lobes collimating at ꢀ24° off normal along the [0 0 1] direction and the meta-stable (1 ꢁ 2) structure
stabilized by oxygen is maximized. On the other hand,neither inclined CO
found on Pd(1 1 0).
2
desorption nor meta-stable (1 ꢁ 2) was
Ó 2003 Elsevier B.V. All rights reserved.
1
. Introduction
experiments collimates fairly along the local nor-
mal of declining microfacets on reconstructed
A (1 1 0) plane of noble metals is frequently
Pt(1 1 0) and Ir(1 1 0),indicating the CO
tion on inclined (1 1 1) facets [4,5]. On the other
hand,no inclined CO desorption was found on
2
forma-
reconstructed into missing-row forms with or
without adsorbates [1]. The resultant surface
commonly consists of three- or four-atom-wide
microfacets with a (1 1 1) structure declining al-
ternatively about +30° or )30° in the [0 0 1] di-
rection [2]. The repulsive force exerting from its
2
Pd(1 1 0) [6] and Rh(1 1 0) [7]. In this Letter,the
presence of inclined CO reactive desorption on
2
Rh(1 1 0) during steady-state CO oxidation is re-
ported for the first time.
In steady-state CO oxidation on Pt(1 1 0),re-
formation site to the product CO in CO oxidation
on noble metals is strong enough to hold the site
orientation in the angular distributions [3]. In fact,
2
2
active CO desorption collimates fairly along the
local normal of those declining facets even when
the surface is mostly converted from the (1 ꢁ 2)
reactive CO
2
desorption in thermal desorption
into the (1 ꢁ 1) form [8]. Furthermore,the CO
2
desorption shifts to the surface normal at low
CO pressures where the (1 ꢁ 2) surface is highly
covered by oxygen [9]. The observation of
*
Corresponding author. Fax: +81-11-706-4748.
E-mail address: tatmatsu@cat.hokudai.ac.jp (T. Matsu-
shima).
inclined CO
2
desorption requires low-oxygen
0
009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0009-2614(03)01169-2

280
I. Rze ꢀz nicka, T. Matsushima / Chemical Physics Letters 377 (2003) 279–285
coverage conditions. In the steady-state CO ox-
idation on Pd(1 1 0),however,no splitting CO
3. Results and discussion
2
desorption is found in the temperature range
from 420 to 500 K below the kinetic transition
3.1. Rh(1 1 0)
(
missing-row structure is observed [10–12]. The
KT),although the c(2 ꢁ 4)-O lattice due to the
3.1.1. Kinetics
The steady-state CO
formation rate on
Rh(1 1 0) became observable above the surface
2
lack of inclined CO desorption on this surface
2
may be due to the instability of the (1 ꢁ 2) re-
construction without oxygen above 355 K [13].
On the other hand,the (1 ꢁ 2) reconstruction on
Rh(1 1 0) has been reported to be stable up to
480 K [14]. STM work indicates that oxygen on
Pd(1 1 0) and Rh(1 1 0) forms O–Rh–O–Rh zig-
temperature (T ) of 400 K and was maximized at
S
around 480 K. Its CO pressure (PCO) dependence
changed sharply below the KT. The reaction
showed a first order in CO at low PCO,and with
increasing PCO in the middle of the active region
(below the KT),the CO
2
formation per PCO in-
ꢁ
zag chains extending along the [110] direction
12,15]. Such oxygen is not reactive enough to
creased about twice and showed a rapid decrease
above the KT (Fig. 1). Such kinetic behavior was
[
À7
induce inclined CO2 desorption. Thus,the in-
clined desorption on Rh(1 1 0) (1 ꢁ 2) might be
observed at low-oxygen coverage conditions with
observed in the pressure range from 1 ꢁ 10 to
À5
1 ꢁ 10 Torr.
high CO
2
formation up to 480 K. These condi-
3.1.2. Angular distributions
tions can be obtained just below the KT in
steady-state CO oxidation [16]. Angular and ve-
locity distribution measurements under such
limited conditions can be performed by means of
steady-state angle-resolved measurements [17,18].
2
The above enhancement of CO desorption was
remarkable at h ¼ 24°. The AR signal at h ¼ 24°
runs below that at h ¼ 0° at lower PCO values
(Fig. 2a),and,around the middle of the active
region,it overcomes the other. The former is
2
. Experimental
The apparatus consists of a reaction chamber,
a chopper house,and an analyzer [17]. All the
chambers were evacuated by individual pumping
systems. The reaction chamber has XPS and
þ
LEED optics,an Ar
gun,and a quadrupole
mass spectrometer (QMS) for the angle-inte-
grated (AI) signal. In the chopper house,a cross-
correlation chopper was rotated at 98.04 Hz,
yielding a time resolution of 20 ls. Another
QMS in the analyzer detected the angle-resolved
(AR) signal. A Rh(1 1 0) or Pd(1 1 0) single
crystal was rotated to change the desorption
angle (h; polar angle) in a plane including the
[
0 0 1] direction [3–5]. The partial pressures of
13
CO (PCO) and O
dosing gases continuously. Hereafter,the isotope
2
O
(P 2 ) were kept constant by
Fig. 1. Variation of the (AI) steady-state CO
served with CO pressures at T
Torr. The rate was determined by QMS in the reaction chamber
2
desorption ob-
À6
1
3
S
¼ 480 K and PO2 ¼ 1:0 ꢁ 10
C is simply designated as C in the text. The
LEED patterns were taken over a long integra-
tion because of the reduced beam current to the
level of non-visible spots.
as the difference in the CO signal between the desired surface
2
temperature and room temperature. Typical experimental
errors are indicated by bars.

I. Rze ꢀz nicka, T. Matsushima / Chemical Physics Letters 377 (2003) 279–285
281
Fig. 2. (a) Variation of the (AR) steady-state CO
À6
2
desorption observed at h ¼ 0° (ꢂ) and 24° (ꢃ) with CO pressures at T
S
¼ 480 K and
O2 ¼ 1:0 ꢁ 10 Torr. The CO
2
P formation rate was determined by QMS in the analyzer as the difference between the signal at desired
angle and that when the crystal was away from the line-of-sight position. (b–d) Angular distributions of CO
2
in the plane along the
[
0 0 1] direction and typical deconvolutions are shown in polar coordinates. The ordinate was normalized to the signal in the normal
À7
À7
À6
direction at the inset (d). PCO is: (b) 1:7 ꢁ 10 Torr,(c) 4 :0 ꢁ 10 Torr,and (d) 1 :50 ꢁ 10 Torr. The components drawn by broken
6
8
4
curves show: (a) cos h,(b) cos h,(c) cos ðh ꢀ 24°Þ,and (d) cos h. The solid curve is the summation of the components. Typical
experimental errors are indicated by bars.
named Ôactive A,Õ and the latter, Ôactive B.Õ The
boundary at which the CO
signal at h ¼ 24° be-
comes equal to that at h ¼ 0° is called Ôsite
switching (SS)Õ [8,9]. At PCO below this point,CO
form. It actually split into a bi-directional form
(Figs. 2b,c). In the inhibited region,the signal
2
1:4
closely followed a cosine distribution,as cos
(Fig. 2d).
h
2
desorption sharply collimated along the surface
4
normal in a cos h form by considering a single
component. The observed distribution above the
3.1.3. Velocity distribution
In the three regions with different angular dis-
tributions,we examined the velocity distributions
SS in Ôactive BÕ became broader than a cosine

282
I. Rze ꢀz nicka, T. Matsushima / Chemical Physics Letters 377 (2003) 279–285
Fig. 3. Velocity distributions of desorbing CO
À6
2
in three reaction zones: (a) active A,(b) active B,and (c) inhibited regions,at
T
S
¼ 480
K and P 2
O
¼ 1:0 ꢁ 10 Torr. Typical deconvolutions are shown by broken curves. The temperature in h i was estimated from the
mean translational energy.
of desorbing CO . The velocity distribution always
showed a single peak although the sharpness
changed significantly (Figs. 3a–c).
Above the KT,the velocity distribution was
mostly described by a Maxwellian distribution at
2
perature of the fast component was estimated to be
1600 ꢀ 70 K at h ¼ 0°. Thus,the angular distri-
6
bution was deconvoluted into 0:05 cos h þ
0:01 cos h.
The velocity distribution in Ôactive BÕ is inter-
esting because the average translational tempera-
ture reached 1380 K at h ¼ 24° and decreased to
1260 K at h ¼ 0°. These velocity distributions in-
dicate the presence of inclined desorption. In the
same way as above,the fast component was esti-
mated to have 1610 ꢀ 50 K at h ¼ 24° and 1510 ꢀ
40 K at h ¼ 0° (Fig. 3b). In this deconvolution,the
thermalized component was maximized. However,
as long as only these two components are consid-
ered,their fractions can be determined uniquely
because of the significant difference in the peak
position between them. Furthermore,the resultant
fraction must fit the deconvolution of the angular
distribution. Thus,the angular distribution in
T
S
regardless of the h value. The distribution at
h ¼ 0° involved a fast component of about 1050 ꢀ
50 K. Its amount was about 15% or less. Here,
1
the slow component described by a Maxwell dis-
tribution at T was fitted to the observed signal and
S
the remaining signal after subtraction of the slow
one was then fitted to a modified Maxwellian
form. A typical deconvolution is shown by broken
curves in the figure. The resultant energy of the
fast component is indicated (in the temperature
units) as ThEi ¼ hEi=2k,where hEi is the mean
translational energy and k is the Boltzman con-
stant. The thermalized component yields a cosine
distribution and the angular distribution was then
deconvoluted into a cosine form and a sharp
component consisting of the remaining signal.
Thus,the angular distribution in the inhibited re-
8
Ôactive BÕ was approximated as 0:42 cos ðh þ 24Þ þ
8
0:42 cos ðh À 24Þþ 0:12 cos h.
In the other deconvolution,the fast component
was divided into the normally directed and in-
clined components by assuming the same sharp-
ness as the fast component in the active A for the
normally directed one and the collimated angle of
24° for the inclined components consisting of the
remaining signal. The distribution was appr-
4
gion was approximated as 0:15 cos h þ 0:85 cos h.
The fast component at 24° was reduced to about
7
0% and showed a temperature of 860 K.
In Ôactive A,Õ the velocity distribution involved a
larger fast component. The average translational
temperature reached 1450 K at h ¼ 0° and 1330 K
at h ¼ 24°,being consistent with the normally di-
rected and sharp angular distribution. The tem-
6
11
oximated as 0:15 cos h þ 0:26 cos ðh þ 24Þþ
0:26 cos ðh À 24Þ þ 0:2 cos h. The slow component
11

I. Rze ꢀz nicka, T. Matsushima / Chemical Physics Letters 377 (2003) 279–285
283
had to be included to fit with the velocity data below
.7 km/s. The contribution from the fast and nor-
mally directed component could not be completely
excluded because of the similar velocity distribu-
tions. However,the normally directed component
was still minor.
appeared above the KT (Fig. 4e). Their spot in-
tensities were monitored by a CCD-video at the
acceleration voltage of 70 eV. The intensity of the
half-order spot at (À1=2; 1) was maximized in
Ôactive B.Õ Concomitantly,the integral order spots
at (0; 1) and (À1; 1) decreased at the SS and in-
creased steeply just below the KT. On the other
hand,the intensity of the (2 ꢁ 2)p2mg at
(À1; À1=2) was fairly constant below the SS and
was suppressed above it. Above the KT,a clear
(1 ꢁ 1) pattern (Fig. 4e) was observed. Such in-
tensity changes were observed in the range of
0
On this surface,the observation of desorption
from declining facets was obscured by the reactive
2
CO desorption from other places,the high con-
tribution of the cosine component,and the rela-
tively slow velocity of the fast component.
3
.1.4. LEED study
Sharp changes in the LEED spot intensity were
T
S
¼ 430–500 K (Figs. 4a,b). The surface structure
sharply changed as ð2 ꢁ 2Þp2mg-O ! ð1 ꢁ 2Þ !
ð1 ꢁ 1Þ. Both transformations were accelerated at
found in Ôactive B.Õ The well-known (2 ꢁ 2)p2mg-
O structure [19] (Fig. 4c) appeared at low P
.
higher T because both the increase and decrease
CO
S
With increasing the PCO,the half-order spots due
of the (1 ꢁ 2) spot systematically shifted to lower
to the (1 ꢁ 2) structure (Fig. 4d) intensified at
P
S
CO from the SS or KT points with increasing T ,
around the middle of the active region and dis-
as typically shown at 480 K in Fig. 4. The decrease
À7
Fig. 4. Rh(1 1 0); spot intensities of (2 ꢁ 2)p2mg,(1 ꢁ 2),and (1 ꢁ 1) structures with CO pressures at fixed O
at (a) T
¼ 430 K and (b) T
at 430 K are shown for: (c) active A,(d) active B,and (e) inhibited regions. The accelerating voltage was 70 eV.
2
pressure of 1 ꢁ 10 Torr
S
S
¼ 480 K. The vertical lines indicate the site-switching and kinetic transition points. Typical LEED patterns

284
I. Rze ꢀz nicka, T. Matsushima / Chemical Physics Letters 377 (2003) 279–285
is controlled by the conversion of (1 ꢁ 2) after
removal of oxygen and not by the accumulation of
CO(a) [9]. The steady (0; 1) spot intensity above
layer. In fact,XPS measurements showed that the
oxygen coverage in Ôactive BÕ was about half of the
signal intensity in Ôactive A,Õ with (2 ꢁ 2)p2mg-O
having a half monolayer of O(a) [14]. The inten-
sity–voltage character of LEED showed remark-
able differences in the accelerating voltage of
50–200 eV between both (1 ꢁ 2) structures.
the KT increased with T
S
,suggesting faster sur-
face-atom orderings at higher T
S
.
This (1 ꢁ 2) at the steady state did not return to
the (1 ꢁ 1) by being heated to 600 K after removal
of the reactant gasses. It yielded a (2 ꢁ 2)p2mg
pattern,indicating the back diffusion of oxygen
from the subsurface layer. On the other hand,the
3.2. Results on Pd(1 1 0)
(
1 ꢁ 2) structure,which was prepared by reduction
On Pd(1 1 0),the rate increased linearly with
increasing PCO to the KT,where it dropped sud-
denly and showed negative orders in CO [9]. CO
with hydrogen after a short exposure to oxygen,
was quickly converted into the (1 ꢁ 1) above 500
K. This indicates that the (1 ꢁ 2) at the steady state
is not the same as the meta-stable missing-row form
described previously [14]. The (1 ꢁ 2) at the steady
state may be stabilized by oxygen in the subsurface
2
desorption collimated along the surface normal
6
below the KT,showing a cos h form along the
[0 0 1] direction. Above the KT,it showed a cosine
form. The velocity distribution curve involved two
À7
Fig. 5. Pd(1 1 0); (a) spot intensities of c(2 ꢁ 4)-O,(1 ꢁ 2)-CO,and (1 ꢁ 1) structures with CO pressures at fixed O
Torr at T
at 450 K are shown for (b) and (c) the active region and (d) the inhibited region.
2
pressure of 1ꢁ10
S
¼ 450 K and the accelerating voltage of 70 eV. The vertical lines indicate the kinetic transition point. Typical LEED patterns

I. Rze ꢀz nicka, T. Matsushima / Chemical Physics Letters 377 (2003) 279–285
285
desorption components,a fast one and a slow one.
The former showed high translational energy and
was suppressed above the KT. This component
for her 2000–2004 scholarship. This work was
supported in part by Grant-in-Aid No. 13640493
for General Scientific Research from the Japan
Society for the Promotion of Science.
10
2
showed a cos h form. CO desorption collimated
sharply along the surface normal,similarly to that
in Ôactive AÕ on Rh(1 1 0). No Ôactive BÕ was ob-
served in the steady-state conditions on Pd(1 1 0).
LEED observations were consistent with the
absence of bi-directional desorption. We monitored
the LEED intensity on Pd(1 1 0) around the KT,as
shown in Fig. 5. The LEED pattern changed from
c(2 ꢁ 4)-O [12] into the (1 ꢁ 1) without passing the
meta-stable (1 ꢁ 2) below the KT and showed the
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Acknowledgements
[
Rze ꢀz nicka Izabela is indebted to the Ministry of
Education,Science,Sports and Culture of Japan