Hot-atom mechanism in hydrogen exchange reaction on the Ir{100} surface

Article abstract of DOI:10.1016/S0009-2614(00)00540-6

H₂ exposure was found to induce the desorption of HD and D₂ molecules from the D-precovered Ir{100} surface. This result suggests that energetic H atoms (hot H atoms) produced in the dissociation process of incident H₂ mol

Full text of DOI:10.1016/S0009-2614(00)00540-6

23 June 2000
Ž
.
Chemical Physics Letters 323 2000 586–593
www.elsevier.nlrlocatercplett
Hot-atom mechanism in hydrogen exchange reaction on
½
5
the Ir 100 surface
a
a
a
Michio Okada a,), Kousuke Moritani , Mamiko Nakamura , Toshio Kasai ,
b
Yoshitada Murata
a
Department of Chemistry, Graduate School of Science, Osaka UniÕersity, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
b
Department of Physics, DiÕision of Natural Sciences, UniÕersity of Electro-Communications, 1-5-1 Chofugaoka, Chofu,
Tokyo 182-8585, Japan
Received 11 January 2000; in final form 28 March 2000
Abstract
H2 exposure was found to induce the desorption of HD and D2 molecules from the D-precovered Ir 100 surface. This
Ä
4
Ž
.
result suggests that energetic H atoms hot H atoms produced in the dissociation process of incident H2 molecules react
with pre-adsorbed D atoms and desorb as HD molecules or produce secondary energetic D atoms via energy transfer.
Ž
.
Secondary energetic D atoms secondary hot D atoms also induce the associative reactions with pre-adsorbed D atoms and
desorb as D2 molecules. q 2000 Elsevier Science B.V. All rights reserved.
Ž .
1. Introduction
incident H D reacts directly upon making the first
collision with surface. Moreover, it has been demon-
strated in recent years that primary and secondary
Ž .
hot atoms play important roles in the H D -atom
Surface reactions of hydrogen with atomic and
molecular adsorbates on metal and semiconductor
surfaces have been paid much attention in recent
years because of the fundamental interest in the
mechanism and dynamics of these reactions. Espe-
w
x
reaction on the surface 10,13 in addition to the
Ž .
direct ER mechanism. Incident H D atoms move
around on the surface as hot atoms until the excess
Ž .
cially, abstraction of chemisorbed H D atoms by the
energy is dissipated onto the surface andror the
Ž .
gas-phase H D atoms have been extensively studied
Ž .
adsorbed H D atoms. During the collisions of hot
w
x
theoretically and experimentally 1–13 because of
its simplicity and its important roles in the catalytic
reactions. The results have been usually interpreted
Ž .
atoms with pre-adsorbed H D atoms, associative
reactions occur or secondary hot atoms are produced.
In this Letter, we will show that the hot-atom mecha-
nism plays an important role even in the reaction of
Ž
.
in terms of the Eley–Rideal ER mechanism where
Ž
.
Ž .
molecular H2 D2 with D H atoms on the surface,
i.e. hot-atom creation in the dissociative adsorption
process.
)
Corresponding author. Fax: q81-6-6850-5403; e-mail:
okada@chem.sci.osaka-u.ac.jp
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 00 00540-6

(
)
M. Okada et al.rChemical Physics Letters 323 2000 586–593
587
Ä
4
The clean Ir 100 surface is reconstructed into a
3. Results and discussion
Ž
.
w
x
1=5 close-packed arrangement 14,15 . Recently,
we have reported the H D -induced surface restruc-
turing and its related surface reactions on the
Ir 100 - 1=5 surface, using low-energy electron
Ž .
Fig. 1 shows the time dependence of the mass
signals of HD and D2 during the H2 exposure with a
pressure of 6=10y8 Torr to Ir 100 which was
Ä
4
Ž
.
Ä
4
y6
Ž
.
Ž
.
diffraction LEED and temperature-programmed
pre-exposed to 2.0 L 1Ls10
TorrPs D2 at
Ž
.
desorption TPD . In the experiments of TPD from
the H–D co-adsorbed surface, we found a decrease
of total pre-adsorbed H atoms at some initial H
coverage with increasing D2 exposure 16 . This
result suggests a possibility that pre-adsorbed H
atoms are desorbed as H2 or HD. In the present
study, we will directly identify the desorbed
molecules during the reaction and discuss a possible
model of the H–D exchange reaction based on these
results.
;160 K. The pre-adsorbed D coverage is about half
w
x
of the saturation coverage 16 . We could detect
desorbing D2 molecules during H2 exposure. The
rapid increase of both the HD and D2 mass signals
was observed just after introducing H2 into the
chamber. The number of desorbing HD and D2
molecules decreased as the exponential-like time de-
pendence after they reached the maximum. As we
reported in the previous paper 16 , we measured the
TPD spectra of D2 , HD, and H2 after the H2–D2
co-adsorption experiments and confirmed the de-
crease of the total number of pre-adsorbed species of
atoms with proper sensitivity correction of the mass
spectrometer.
w
x
w
x
2. Experimental
LEED observations and TPD measurements were
In order to confirm that this time dependence
originates in the chemical reaction induced by inci-
dent molecular H2 on the surface, we carried out the
Ž
.
performed in an ultra-high-vacuum UHV chamber
with a base pressure of 5=10y11 Torr at Osaka
University, which was equipped with LEED optics
Ž .
following experiments.
1 The quadrupole mass
Ž
.
Ž
Omicron and a quadrupole mass filter Microvi-
spectrometer in the present experiments can detect
the molecules desorbing from the sample holder and
chamber walls. Thus, we have done the same experi-
ments without the sample as those shown in Fig. 1.
The thin solid lines in Fig. 1 correspond to the time
dependence of the mass signals of HD and D2 during
the H2 exposure at 6=10y8 Torr without the sam-
ple after the preparation of D-covered surface with
D2 exposure at ;160 K. A step-like increase of HD
was observed and the HD signal kept constant during
H2 exposure. This result suggests that the HD mass
signal includes the impurity of HD molecules in the
H2 gas bottle and desorbing molecules from the
sample holder andror the chamber wall. The contri-
bution of such background to the HD mass signal is
constant and can be easily subtracted in order to get
the HD signal from the sample surface. On the other
hand, D2 signals came from only the sample surface
.
Ä
4
.
sion . A mechanically polished single-crystal Ir 100
surface 8 mm diameter, 1.7 mm thick; MaTeck
Ž
used in the present experiments was oriented within
Ä
4
0.18 of the bulk 100 plane. A sample holder was in
contact with an electron-beam heating assembly and
a liquid N2 reservoir so that the sample could be
heated to 1800 K and cooled down to ;100 K. The
sample was cleaned by successive cycles of 500 eV
Ne-ion bombardment, oxygen treatment at 1200;
1400 K in 5=10y8 Torr O2 , and final flashing at
1500 K, until no impurities could be detected by
Auger electron spectroscopy and LEED showed a
Ž
.
sharp 1=5 pattern with low background intensity.
In the present hydrogen reaction experiments, the
crystal was firstly exposed to high-purity D2
molecules at ;160 K by backfilling the chamber.
The HD and D2 molecules, which desorbed from the
Ä
4
Ž .
D-precovered Ir 100 surface during H2 exposure,
were detected by the quadrupole mass filter. The flux
of H2 molecules impinging on the surface was con-
trolled by varying the pressure of H2 backfilling the
chamber. The pressure of 1=10y6 Torr H2 corre-
without any contributions of the background. 2
Atomic H species may be produced on the filament
of the mass spectrometer. Thus, we checked the
effect of the filament of the mass spectrometer dur-
ing taking the curve shown in Fig. 1 repeating with
filament on and off, and confirmed no effect of H
sponds to 1.4=1015 H2 cmy2 sy1
.

(
)
588
M. Okada et al.rChemical Physics Letters 323 2000 586–593
Ä
4
atoms produced in the mass spectrometer, although
incident atomic H can easily produce HD and D2 on
tively, for Ir 100 pre-exposed to 3.3 L D2. The inset
shows the H2 flux dependence of the integrated
amount of desorbing molecules. The flux of H2
molecules was controlled by the pressure of H2.
Below a pressure of 3=10y8 Torr, the amount of
w
x
the D-precovered surface 10,13 .
The reaction rate curve for HD and D2 is shown
as a function of the H2 flux in Fig. 2a,b, respec-
desorbing HD and D2 increases linearly with the
y8
Ž
.
flux region I . Above a flux of 3=10
Torr
Ž
.
region II , on the other hand, the amount of desorb-
ing HD and D2 molecules increases much more
slowly with flux than that in region I and approaches
asymptotically to a constant value. Integration of the
reaction rate curves after subtracting the background
is proportional to the amounts of desorbing HD and
D2 as reaction products. Fig. 3 shows the initial-D-
Ž
.
coverage uD dependence of amounts of desorbing
HD and D2 molecules at a H2 pressure of 6=10y8
Torr. The total yields of HD and D2 are mainly
determined by the concentration of the pre-adsorbed
D atoms in region II.
What mechanism produces HD and D2 molecules?
At the sample temperature of ;160 K, it is impossi-
ble that pre-adsorbed D atoms react directly and
desorb as D2 molecules. We could not detect the
desorbing D2 molecules without H2 exposures for
the D-pre-adsorbed surface at ;160 K. Thus, addi-
tional energy will be needed for the associative
reaction and the desorption processes of pre-ad-
sorbed D atoms. We will propose the following me-
chanism as shown in a potential diagram of Fig. 4.
An incident H2 molecule dissociates on a vacant site
and is accelerated to the surface in the strong attrac-
tive well. The gained energy corresponding to the
dissociative adsorption energy will be transferred to
the lateral motion of H atoms via the corrugation of
the surface. Energetic H atoms, i.e. hot H atoms
Ž .
Ž .
Fig. 1. Time dependence of the mass signals of a HD and b D2
during the H2 exposure under a pressure of 6=10y8 Torr from
Ä
4
the Ir 100 surface pre-exposed to 2.0 L D2 at ;160 K. Introduc-
tion of H2 starts at the zero minute. Dots and thin solid lines
Ä
4
indicate the mass signals with and without the Ir 100 sample,
respectively. The mass signals are given in arbitrary units that are
Ž .
Ž .
and b . Thick solid curves in
Ž .
Ž .
and b corre-
common to
a
a
Ž .
Ž
.
spond to the fitting function of RHD t s Aa exp ykt q
Ž
.
ÄŽ .. .4
Ž
Ž
Ba exp ykt = 0.55q a y0.55 exp ytr4.4 y a exp ykt
2
Ž .
Ž
.
and RD t sCa exp y2kt , respectively, where A, B, C, k,
2
Ž .
a correspond to the
and a are constants. The dashed curves in
Ž
.
different HD desorption mechanisms see text .

(
)
M. Okada et al.rChemical Physics Letters 323 2000 586–593
589
Ž .
Ž .
Ä
4
Fig. 2. The H2 pressure dependence of the reaction rate versus time curves for a HD and b D2 from the Ir 100 surface pre-exposed to
3.3 L D2 at ;160 K. The insets show the H2 pressure dependence of the integrated amount of desorbing HD and D2. Below a pressure of
y8
3=10y8 Torr, the integrated amount of desorbing HD and D2 increases linearly with H2 pressure region I , while above 3=10
Torr,
Ž
.
Ž
.
it increases more slowly with H2 pressure than that in region I region II .

(
)
590
M. Okada et al.rChemical Physics Letters 323 2000 586–593
y3
Ž
.
system of HrIr, the mass ratio of H to Ir 5.2=10
is so small that the energy dissipation into the sub-
strate is small during one collision. As a result, HU
atoms are considered to survive for a long time.
During moving around the surface, a primary HU
atom collides with a pre-adsorbed D atom and des-
U
Ž
.
orbs as HD or creates a secondary hot D atom D .
DU also creates desorbing HD or D2 molecule due to
the reaction with adsorbed H or D, respectively.
Furthermore, we have experienced from the TPD
measurements that the pre-adsorbed atoms are moved
from one site to another by the collision of HU
andror DU 16 . This primary and secondary hot-
w
x
atom mechanism has been proposed for reaction
Ž .
between the incident H D atomic beam and pre-ad-
w
x
sorbed hydrogen atoms 10,13 . Here, we can con-
clude that the hot-atom process is important for the
Ž
.
reaction between thermal H2 D2 molecule and
pre-adsorbed hydrogen atoms on the metal surface.
We have analyzed the reaction rate curves shown
w
x
in Fig. 1a,b based on a simple model 13 . In this
experimental condition of region II, a sufficient
amount of H2 molecules impinges. That is, an
amount of adsorbed H atoms comparable to that of D
Ž .
a HD
Fig. 3. Initial-D coverage dependence of the amount of
Ž . Ä 4
and
b D2 molecules desorbing from the Ir 100 surface during
H2 exposure under a pressure of 6=10y8 Torr. The amount of
the desorbing molecules is determined by the integration of the
reaction rate versus time curves after subtracting the background
Ž
.
Ž .
Ž
Ž .
.
see Fig. 1 . Solid lines in
a
and b correspond to the fitting
function of g1a and g2 a 2 see text .
U
U
Ž
.
H , are generated through this process. H sur-
Fig. 4. Schematic potential diagram of H2 and H interacting with
vives until the excess energy is dissipated into the
Ä
4
Ž .
the Ir 100 surface. U z is the interaction potential, where z is
surface andror pre-adsorbed atoms. In the present
the distance between the particles and the surface.

(
)
M. Okada et al.rChemical Physics Letters 323 2000 586–593
591
atoms exists on the surface at the maximum position
of the desorbing curve. We assume that the follow-
cal reaction within the initial coverage of uD. We
have to introduce this ‘effective uD’ denoted as a,
because some D atoms still remain on the surface
without desorbing at the end of the exchange reac-
ing three types of reactions with hot atoms of HU and
U
Ž .
Ä
4
D occur on the D H -covered Ir 100 surface during
the H2 exposure.
w
x
tion 16 . Moreover, the ‘effective uD’ may have the
physical concept that D atoms located far from the
periphery in the D-adsorbed island cannot contribute
to the hot-atom formation and the chemical reactions
discussed here. The D atoms far from the periphery
in the island still remain unchanged during the reac-
H) primary qD ad
HD g ,
Ž .
D2 g ,
Ž .
HD g ,
Ž .
HD g .
Ž .
1
Ž
.
Ž
.
Ž .
D) secondary qD ad
2
Ž .
3
Ž .
Ž
Ž
Ž
.
.
.
Ž
Ž
Ž
.
.
.
D) secondary qH ad
H) secondary qD ad
tion, since it may be difficult for HU primary to
Ž
.
In reaction 1, primary HU atoms produced in the
dissociation process of H2 react with pre-adsorbed D
atoms and desorb as HD molecules. In reaction 2,
secondary DU atoms produced in the collision with
primary HU react with pre-adsorbed D atoms and
penetrate into the inside of an island due to larger
U
Ž
.
diffusion barrier. The coverage of D secondary
U
Ž
.
Ž .
produced in the reaction of H primary qD ad
U
Ž
.
Ž
.
H ad qD secondary , is also proportional to the
U
Ž
.
‘effective uD’, since H primary are sufficiently
desorb as D2 molecules. In reaction 3, secondary DU
supplied.
U
Ž
H
.
Ž .
atoms produced from adsorbed D H atoms
The total H and D coverage during the reaction
Ž .
react with adsorbed H D atoms and desorb as HD
Ž
.
can be written as us1y0.55 exp ytr4.4 which
molecules.
Ž .
was estimated from the uptake curve of H D adsorp-
tion at PH s6=10y8 Torr 16 . Here, t min
.
Next, we will derive the rate equations of HD and
D2 production from the three types of chemical
w
x
Ž
is the reac2tion time and the initial coverage of
pre-adsorbed D atoms is 0.45. As a result, the de-
Ž .
reactions 1–3. The production rate RHD
via reactions of 1 and 3 can be written in a
t of HD
Ž .
Ž .
sorption rate of HD and D2 , RHD
shown in Fig. 1 can be represented by the form of
Ž .
t and RD t ,
2
U
Ž .
w
Ž
w
. x w
Ž
. x
form of RHD t sk1
H
primary
D ad
.xw Ž .x4
k3 D secondary H ad q H secondary D ad ,
q
U
U
Ä
w
Ž
.xw Ž .x
Ž
Ž
.
Ä
RHD t s reaction 1 A effective uD q reaction 3
where k1 and k3 are rate constants of reactions 1 and
Ž
. 4 Ž .
A
effective uD PuH sA a exp ykt qB a
3, respectively. On the other hand, the production
Ž
Ž
.
Ä
Ž
Ž
..
exp ykt P 0.55 q a y 0.55 exp ytr4.4 y a
Ž .
rate RD t of D2 via reaction 2 can be written in the
.
4
Ž .
Ä
Ž
exp ykt and R t s reaction 2 A effective
2
D2
Ž
) Ž
.xw Ž .x
2
.
24
.
Ž .
w
form RD t sk2 D secondary D ad , where k2
uD sC a exp y2kt , where A, B, C, k, and a
are constants. Here, we approximate the time depen-
dence of the ‘effective uD’ and ‘effective total cover-
2
Ž
.
sk3 is a rate constant of reaction 2. Here, we
U
Ž
.
assume that the sufficient amounts of H primary
U
1
w
Ž
are supplied in region II and, as a result, D sec-
Ž
.
Ž
.
Ž
age’ suH qeffective uD as a exp ykt and 0.55
U
.x
w
Ž
.x
ondary and H secondary are proportional to
.
Ž
.
qa y0.55 exp ytr4.4 , respectively. The mea-
sured curves are reasonably well fitted with these
functions, as shown by the thick solid curves in
Fig. 1. The best-fitted value of 1rk is 5.9 min. The
more rapid decay of the D2 desorption rate than that
of the HD desorption rate can be clearly explained
by the D2 desorption rate which is proportional to
a 2. Furthermore, the peak shift of the D2 desorption
rate relative to that of HD can be also explained by
the existence of reaction 3. Here, in this fitting
process, we introduce a fitting parameter a of the
‘effective uD’, and a in the best fitting of the
experimental curve in Fig. 1 gives 0.14, which agrees
quite well with the value of 0.15 determined from
w Ž .xŽ
.
w Ž .xŽ
.
D ad seffective uD and H ad suH , respec-
tively. Thus, the desorption rate of HD by reaction 3
Ž
.
is proportional to effective uD PuH , while that by
reaction 1 is proportional to the effective uD. On the
other hand, the desorption rate of D2 is proportional
to effective uD 2. Here, we introduce the ‘effective
uD’ in place of uD as D ad . The ‘effective uD’
means the coverage of D participating in the chemi-
Ž
.
w Ž .x
1
Ž
.
M. Okada, K. Moritani: On the HrIr 111 system, it is
confirmed that HU primary are sufficiently supplied even for the
Ž
.
w
x
Ž
.
TPD experiments 16 . The total D coverage deter-
higher pre-adsorbed D coverage of uD unpublished .

(
)
592
M. Okada et al.rChemical Physics Letters 323 2000 586–593
mined by TPD spectra decreased from 0.45 to 0.30
by 0.15 after the completion of the chemical reac-
tion. Thus, this reaction model may be a proper view
of the exchange reaction, although it is simplified.
According to the above discussion on the reaction
mechanism, the total yield of desorbing HD and D2
molecules is expected to be proportional to a and
a 2, respectively. Assuming that a contributing to
tional to the pressure of H2 molecules. Thus, a
sufficient amount of HU atoms participating in the
exchange reaction is considered not to be supplied,
because they dissipate and adsorb before colliding
with other pre-adsorbed atoms.
4. Conclusions
2
the reaction is proportional to uD, a can be de-
scribed as b1uD qb2 , where b1 and b2 are con-
stants. Thus, the total desorption yields of HD and
D2 can be described as g1a and g2 a 2, respectively,
where g1 and g2 are constants. In this experiment,
b2 sy0.45b1 q0.15, where as0.15 at uD s0.45,
since the total D coverage determined by TPD spec-
tra decreased from uD s0.45 to 0.30 by 0.15 after
the completion of the chemical reaction. The solid
curve of total yield in Fig. 3 indicates the curve fitted
to these relations of g1a and g2 a 2 for HD and D2 ,
respectively, where g1rg2 is determined from the
experimental results for the total yields of HD and
D2 shown in Fig. 3. The experimental data points
indicated by solid circles are in good agreement with
the calculated curve. This result gives additional
support to the above-discussed simplified-reaction
mechanism mediated by hot hydrogen atoms. The
hot-atom mechanism plays an important role even in
the hydrogen-exchange reaction due to thermal
In summary, we found the desorption of D2
Ä
4
molecules from the D-precovered Ir 100 surface
under H2 exposure. This kind of reaction requires
additional energies, which are supplied by the ener-
getic HU atoms produced in the dissociation process
of H2 molecules. This hot-atom mechanism proves
to play an important role in the hydrogen exchange
reaction on the surface. The surface structure depen-
dence of the exchange reaction, which was previ-
w
x
ously reported by Ali et al. 17 , is one of the future
studies in order to understand further the hot-atom
mechanism.
Acknowledgements
This work is supported by Nissan Science Foun-
dation and also partially supported by the REIMEI
Research Resources of Japan Atomic Energy Re-
search Institute.
Ž
.
Ž .
molecular H2 D2 with D H atoms adsorbed on the
surface.
For the region of uD shown in Fig. 3, correspond-
ing to region II, a sufficient amount of HU atoms are
supplied for the hydrogen-exchange reaction via the
dissociation process on many available vacant sites
at a pressure of 6=10y8 Torr. The coverage of
available vacant sites is about 0.4 and 0.1 at a
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.
.
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