Adsorption and reaction of formic acid on a (2 × 2) NiO(111)/Ni(111) surface. 3. IRAS studies on the characterization of reaction sites using CO and the behavior of surface hydroxyl species

Article abstract of DOI:10.1021/jp980320t

The decomposition of formic acid on (2 × 2) NiO(111)/Ni(111) was studied by infrared reflection absorption spectroscopy (IRAS). Formic acid molecules adsorbed dissociatively on the surface to form formate which decomposed to H₂, CO₂, H₂O, and CO at 340-415 and 520 K. The vacant sites after the decomposition of formate were probed by the adsorption of CO at 100 K. On clean NiO(111) at 100 K, IRAS peaks of two types of adsorbed CO were observed at 2146 and 2079 cm^-1, which were assigned to the CO on fully-oxidized Ni cation sites and less-oxidized Ni cation sites, respectively. The CO peaks were not observed on the formate precovered surface. The IRAS peak of adsorbed CO at 100 K on the fully-oxidized Ni cation sites appeared when the surface was heated to decompose formate around 340-415 K. After decomposition at 520 K, the intensities of both peaks of CO were recovered to the initial intensities. It was thus concluded that the sites of decompositions at 340-415 and 520 K are the fully-oxidized Ni cation sites and less-oxidized Ni cation sites, respectively. The behavior of hydroxyl species produced by dissociative adsorption of formic acid was next investigated using deuterated formic acid (DCOOD). The hydroxyl species (-OD) formed by the dissociative adsorption of formic acid (DCOOD) was observed at 2536 cm^-1 below 343 K, while the isolated OD groups appeared at 2713 cm^-1 and around 500 K. The mechanism of decomposition of formate at each temperature is discussed.

Full text of DOI:10.1021/jp980320t

J. Phys. Chem. B 1998, 102, 2979-2984
2979
Adsorption and Reaction of Formic Acid on a (2 × 2) NiO(111)/Ni(111) Surface. 3. IRAS
Studies on the Characterization of Reaction Sites Using CO and the Behavior of Surface
Hydroxyl Species
Taketoshi Matsumoto, Athula Bandara, Jun Kubota, Chiaki Hirose, and Kazunari Domen*
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226, Japan
ReceiVed: NoVember 25, 1997; In Final Form: February 11, 1998
The decomposition of formic acid on (2 × 2) NiO(111)/Ni(111) was studied by infrared reflection absorption
spectroscopy (IRAS). Formic acid molecules adsorbed dissociatively on the surface to form formate which
decomposed to H
2 2 2
, CO , H O, and CO at 340-415 and 520 K. The vacant sites after the decomposition of
formate were probed by the adsorption of CO at 100 K. On clean NiO(111) at 100 K, IRAS peaks of two
-
1
types of adsorbed CO were observed at 2146 and 2079 cm , which were assigned to the CO on fully-
oxidized Ni cation sites and less-oxidized Ni cation sites, respectively. The CO peaks were not observed on
the formate precovered surface. The IRAS peak of adsorbed CO at 100 K on the fully-oxidized Ni cation
sites appeared when the surface was heated to decompose formate around 340-415 K. After decomposition
at 520 K, the intensities of both peaks of CO were recovered to the initial intensities. It was thus concluded
that the sites of decompositions at 340-415 and 520 K are the fully-oxidized Ni cation sites and less-oxidized
Ni cation sites, respectively. The behavior of hydroxyl species produced by dissociative adsorption of formic
acid was next investigated using deuterated formic acid (DCOOD). The hydroxyl species (-OD) formed by
-
1
the dissociative adsorption of formic acid (DCOOD) was observed at 2536 cm below 343 K, while the
isolated OD groups appeared at 2713 cm and around 500 K. The mechanism of decomposition of formate
at each temperature is discussed.
-
1
Introduction
that formic acid adsorbs on NiO(111) to form formate, even at
1
63 K, which decomposes to desorb as H2, CO2, H2O, and CO
Considerable attention has been paid to the study about
adsorptions and reactions of molecules on single-crystal oxide
surfaces to understand the catalytic properties of oxides. The
study of oxide surfaces lags behind that of metal and semicon-
ductor surfaces since electron spectroscopies and infrared
reflection absorption spectroscopy (IRAS) have been difficult
to apply on such surfaces, especially in early times, because of
the low electron conductivity and high transparency.1 A recently
developed technique to prepare epitaxially grown films of oxides
on metal substrates enables us to apply these spectroscopies,
and there has been a large interest in these surfaces in the past
several years.2
at 340-415 and 520 K in a vacuum. Formates were found to
be in tilted bidentate configuration in a vacuum and in both
bidentate and monodentate configurations under a flow of formic
acid. Formic acid on a thick layer of NiO(111) prepared on a
Mo(110) substrate was reported to decompose at 555 K by Xu
and Goodman. The question is why the formate decomposes
at two different temperatures, 340-415 and 520 K, on our
surface. Further examination of the behavior of formate and
that of the hydroxyl species, which may play an important role
in the decomposition of formate, is required.
In this work, we investigated the characteristics of the reaction
sites for decomposition of formic acid on NiO(111) using CO
molecules as a probe. The behavior of hydroxyl species during
the decomposition of formic acid was also examined by using
deuterated formic acid.
7,8
-13
NiO single-crystal films on substrates of single-crystal metals
are typical examples of epitaxial films applied to the study of
adsorptions of molecules by several methods.4,5,7-13 The NiO-
(111) surface is known to be more active for chemical reactions
than NiO(100) because Ni cations at the surface are at a lower
Experimental Section
coordination number.8 NiO(111) is reconstructed to the (2 ×
2
) structure terminated by oxygen atoms in a vacuum; thus
The experiments were carried out in an ultrahigh vacuum
5
,9
-8
trigonal pyramids of the microfacets are disposed in order.
chamber evacuated to 10 Pa, as described in detail in the
1
1
A significant intermediate in the water-gas shift, methanol
synthesis, and selective oxidation reactions has been considered
to be the carboxylate species on the surface. One of the simplest
way to produce carboxylate is dissociative adsorption of
previous papers. The clean Ni(111) surface was prepared by
Ar ion bombardment and annealing at 1023 K and characterized
by Auger electron spectroscopy (AES) and low-energy electron
diffraction (LEED). The NiO film was grown on the clean Ni
substrate by the cycles of exposure to oxygen gas at 1000
1
4
carboxyl acid on the surface as reported on TiO2(100), NiO-
4
7,8
15
16
17
-6
(
100), NiO(111), MgO(100), ZnO(0001), and ZrO2(100).
langmuirs (1 langmuir ) 10 Torr s, 1 Torr ) 133 Pa) and
Our group has investigated the adsorption and decomposition
of formic acid on NiO(111) prepared by oxidation of Ni(111)
under vacuum and steady-state conditions.10,11 It was found
570 K and annealing at 650 K. The thickness of the NiO film
was estimated to be 5-10 monolayers from the AES peak
intensities. The hexagonal LEED pattern which indicates (2 ×
S1089-5647(98)00320-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/31/1998

2980 J. Phys. Chem. B, Vol. 102, No. 16, 1998
Matsumoto et al.
Figure 1. TPD spectra of the HCOOH-dosed NiO(111) surface. The
surface was exposed to 10 langmuirs of HCOOH at 163 K.
2) NiO(111) was observed on the NiO film. It should be noted
that the amount of oxygen on NiO(111) estimated by AES did
not change after several repetitions of thermal desorption of
formic acid or CO.
IRA spectra were acquired using a JEOL JIR-100 Fourier
transform infrared spectrometer (FTIR) with an InSb/MCT
composite detector. The resolution of the spectra was 4 cm ,
and the spectra were averaged over 512 scans.
-
1
Figure 2. IRA spectra of CO adsorbed on the NiO(111) surface at
1
00 K with various exposures.
The commercially purchased gases were purified by passing
them through a cold trap or by vacuum distillation before use.
The amounts of exposure are derived from the apparent
pressures displayed by an ionization gauge without any cor-
rection and calibration.
Results and Discussion
Characterization of Reaction Sites Using CO. Tempera-
ture-programmed desorption (TPD) spectra of formic acid on
NiO(111) at 10 langmuirs are shown in Figure 1. The
desorption of H2, CO2, and CO were observed as products of
decomposition. The desorption of H2O, which is the decom-
position partner of CO, was not observed because of a high
background signal. H2 and CO2 as the products of dehydro-
genative decomposition desorbed in two temperature regions,
3
40-390 and 520 K. The presence of two regions indicates
that there are two kinds of sites or two kinds of decomposition
mechanisms. CO desorbed at 415 and 520 K. These desorp-
tions can be classified into two groups which are observed
around 340-415 and 520 K. Xu and Goodman reported that
the decomposition temperature of formic acid on a thick layer
of NiO(111) is around 555 K, where they used NiO(111)
prepared on a Mo(110) substrate by evaporation of Ni in an
oxygen environment. The desorption at 520 K in the present
study may correspond to that at 555 K in their study. The aim
of this study is to consider the reason formic acid is decomposed
at two temperature regions.
Figure 3. Temperature-dependent IRA spectra of formate adsorbed
on NiO(111). The surface was first exposed to HCOOH (10 langmuirs)
at 163 K and then heated to the stated temperatures.
sites such as oxygen vacancies, steps, or boundaries of crystal
domains. The CO desorbed at 125 K, and no CO was produced
2
7,8
during thermal desorption, indicating that CO adsorbs only
weakly without any reaction on NiO(111).
The temperature dependencies of the IRA spectra of the NiO-
(111) surface dosed by formic acid are shown in Figure 3. The
surface was exposed to the saturation coverage of formic acid
at 163 K, and the sample was subsequently heated to the stated
temperatures. Peaks were observed at 2860, 1570, 1360, and
CO molecule is often used to probe the adsorption sites as
the vibrational frequency reflects the state of adsorption sites
such as electronegativity. The IRA spectra of adsorbed CO on
NiO(111) at 100 K and various exposures are shown in Figure
-
1
778 cm and are assigned to the C-H stretching, asymmetric
O-C-O stretching, symmetric O-C-O stretching, and in-
plane deformation modes, respectively, of bidentate formate.10
The appearance of the band of the O-C-O asymmetric
stretching mode indicates that the formate molecules inclined
-
1
2
. Two peaks were observed at 2146 and 2079 cm . It is
known that the high- and low-frequency peaks are corresponding
to the adsorbed CO on the fully- and less-oxidized Ni cation
sites (denoted as HF and LF sites), respectively.18 We consider
that the HF sites are located at the (2 × 2) NiO(111) terrace
-
1
on the surface. The peak at 2947 cm was possibly due to
the C-H stretching mode of the physisorbed formic acid
molecule which disappeared around 273-323 K. The intensities
of the formate peaks decreased at higher temperature as formate
1
9
since the frequency is close to that on other typical oxides.
The frequency of the band for the CO on the LF sites is close
to that of Ni metal surfaces, and the sites are regarded as defect

Formic Acid on a (2 × 2) NiO(111)/Ni(111) Surface
J. Phys. Chem. B, Vol. 102, No. 16, 1998 2981
Figure 5. Temperature dependence of the area intensities of the
vibrational peaks of formate and CO. The areas were derived from the
spectra shown in Figure 4. The arrows a and b indicate the area
intensities of the band by the CO adsorbed on HF and LF sites,
respectively, of the initial surface without the adsorption of formic acid.
The desorption peaks of the TPD signals are indicated on the top.
Figure 4. IRA spectra of formate (HCOO) and CO coadsorbed on
NiO(111). The formate-covered NiO(111) was first heated to the stated
temperatures for a few minutes in a vacuum. Then, the CO adsorption
to the saturation coverage and the spectral measurement were carried
out at 100 K.
sites drastically gained intensity around 400 K but that on the
LF sites gained around 500 K. When the formate-covered
surface was heated to 650 K, the CO peaks recovered their initial
intensities, indicating that the surface structure was recovered.
The feature suggests that the decomposition of formate at 340-
decomposed. The band of the symmetric O-C-O stretching
-1
mode at 1360 cm disappeared at 373 K, but its shoulder peak
4
15 K took place mainly on the HF sites since CO started to
-
1
at 1317 cm remained until 473 K, indicating that the formate
adsorb on the HF sites at this temperature. The decomposition
of formate at 520 K, on the other hand, occurred at LF sites. It
is thus concluded that the decomposition of formate on the fully-
oxidized Ni cation sites on the terraces gave 340-415 K peaks
on the TPD signals and those on the less-oxidized Ni sites at
-
1
species giving the peaks at 1360 and 1317 cm decomposed
at 340-390 and 520 K in the TPD measurement, respectively.
It is hard to discuss the differences in structures and adsorption
sites of the two kinds of formate from frequencies of the
symmetric O-C-O stretching mode alone, and we proceeded
to use CO as the probe of the adsorption sites of formate.
Shown in Figure 4 are the IRA spectra observed after the
CO adsorption of the formate-covered NiO(111); the surface
was heated to the stated temperatures for a few minutes in a
vacuum, cooled to 100 K, and dosed by CO to the saturation
coverage. The behavior of the formate peaks at 2860, 1570,
5
20 K.
Behavior of Hydroxyl Species (-OD). It is important to
note the role of the surface hydroxyl species during the
decomposition of formate since a product of water should be
originated from the surface hydroxyl species. The behavior of
hydroxyl species during the decomposition of formate has not
been understood because of the low sensitivity of hydroxyl
species for IRAS. Moreover, the difficulty of the detection of
water in TPD also interfered with understanding the reaction
path of the water production. The use of deuterated formic acid
DCOOD enables us to detect the surface hydroxyl species on
IRAS by taking advantage of a much better signal-to-noise ratio
-
1
1
360, and 778 cm in Figure 4 is similar to that in Figure 3,
suggesting that the adsorbed CO does not affect the structure
and decomposition of the remaining formate. The changes in
the area intensities of the formate and CO bands by the heated
temperatures are displayed in Figure 5. The surface heated at
-
1
228 K was solely covered by formate to the saturation coverage,
in the O-D stretching region (∼2700 cm ) than that in the
-1
and CO molecules did not adsorb. The IRAS bands of the CO
appeared for surfaces annealed at and above 273 K. The
intensity of the band at 2170 cm- coming from CO on the HF
sites increased by increasing the annealing temperature. The
O-H stretching region (∼3700 cm ).
Hydroxyl species on NiO(111) were examined first without
adsorption of formic acid. IRA spectra of surface hydroxyl
species obtained from the dissociative adsorption of heavy water
1
-
1
band at 2090 cm corresponding to the adsorbed CO on the
LF sites gained the intensity when the surface was heated to
D O at the stated temperatures on NiO(111) are shown in Figure
2
6. At the low adsorption temperature of 163 K, two peaks were
-1
5
23 K. It must be noted that the frequency of adsorbed CO on
observed at 2709 and 2624 cm which are assigned to the O-D
the HF sites obtained from the formate covered surface around
stretching modes of the dangling and hydrogen-bonded OD
-
1
3
00 K is 17 cm higher than that on clean NiO(111) when the
groups, respectively, of the adsorbed D O molecule. In the
2
CO coverage was small. This difference in frequency is
considered to be due to interaction between CO and formate.
Formate on a surface is regarded as an anionic species, and the
shift to higher frequency of coadsorbed CO may be caused by
withdrawing of the surface electrons to lessen the back-donation
to the 2π* state of CO by formate species.19,20
The arrows a and b in Figure 5 indicate the area intensities
of the bands by CO adsorbed on the HF and LF sites,
respectively, of the initial NiO(111) surface before the adsorp-
tion of formic acid. The desorption peaks of the CO on the
HF sites. It should be noted that the peak of CO on the HF
temperature region between 228 and 550 K, a single peak was
observed at 2709-2719 cm and is assigned to the O-D
-
1
2
1,22
stretching mode of the hydroxyl species.
The peak position
-
1
shifted from 2719 to 2709 cm on increasing the substrate
temperature from 228 to 550 K and might be due to the
anharmonic coupling with low-frequency modes such as the
frustrated translation of the hydroxyl groups as suggested for
the CO on metal surfaces. The frequency of this peak suggests
that the hydroxyl species do not interact with each other through
hydrogen bonding. The peak of the hydroxyl species disap-
peared at 650 K. Spectrum a was obtained after the surface
23

2982 J. Phys. Chem. B, Vol. 102, No. 16, 1998
Matsumoto et al.
Figure 8. TPD spectra of the DCOOH dosed on the NiO(111) surface.
The surface was exposed to 10 langmuirs of DCOOH at 163 K.
ing, and in-plane deformation modes of bidentate formate
-
1
(
DCOO-), respectively. A peak was observed at 2536 cm
between 228 and 323 K and is assigned to the O-D stretching
mode of surface hydroxyl species produced by the dissociation
of formic acid to formate. The frequency of 2536 cm is by
80 cm lower than that of the isolated hydroxyl species from
-
1
-
1
1
Figure 6. IRA spectra of NiO(111) exposed to 10 langmuirs of D
at the stated temperatures. Spectrum a was obtained after the surface
was dosed by D O at 500 K and cooled to 163 K.
2
O
D2O. This indicates that the present hydroxyl species was
perturbed by the coadsorbtion of formate through either the
hydrogen bonding or electronic change of the adsorption site
by the adsorption of anionic formate on the neighboring sites.
The hydroxyl peak disappeared above 323 K, and the hydroxyl
species possibly desorbed at this temperature as D2O. In
conclusion, we considered that a part of water is produced and
desorbs below 343 K by the decomposition of formic acid.
2
When the surface temperature was raised to 473 K, a new
-1
peak appeared at 2713 cm and was also assigned to the O-D
stretching mode of the hydroxyl species. The frequency of the
band is the same as that of the hydroxyl species formed from
D2O, indicating that these hydroxyl species are isolated. This
type of hydroxyl species is considerably produced by the
decomposition of the formate remaining at higher temperature
-
1
to give the symmetric O-C-O stretching band at 1317 cm .
Although the TPD signal of water could not be detected, the
IRA spectra clearly proves that there are two origins for the
desorption of water by the decomposition of formic acid: one
from the formate-perturbed hydroxyl species around 343 K and
the other from isolated hydroxyl species around 650 K.
TPD spectra of the DCOOH-dosed surface were measured
to reveal which hydrogen in the formic acid is the source for
the hydrogen and water production, as shown in Figure 8. The
locations of TPD peaks of CO2 and CO are almost the same as
those observed for the HCOOH-dosed surface. It is seen that
D2 was produced from DCOOH but the signals of H2 or HD
were hardly observable. It clearly indicates that the hydroxyl
species produced at the oxygen sites on dissociative adsorption
of formic acid desorbed primarily as water but not as hydrogen.
It is thus reconfirmed that the disappearance of the IRAS peaks
of the hydroxyl species is due to the desorption as water.
Desorption at 343 K of water produced from hydroxyl groups
should leave behind an oxygen vacancy. However, the AES
ratio of the O/Ni signals remained unchanged after several
repetitions of TPD, and the thus created vacancies were
apparently recovered by the oxygen atoms supplied by the
further decomposition of formate. As a matter of fact, the
change by the desorption of the hydroxyl species was not
observed in the CO-probe measurements, suggesting that the
CO does not adsorb on the oxygen-vacancy sites generated from
Figure 7. Temperature-dependent IRA spectra of formate (DCOO)
adsorbed on NiO(111). The surface was exposed to DCOOD (10
langmuirs) at 163 K and then heated to the stated temperatures.
was dosed by D2O at 500 K and was cooled to 163 K. A single
-1
peak was observed at 2721 cm . The frequency shift depend-
ent on the substrate temperature indicates that this peak is
assigned to the band of isolated hydroxyl species.
The behavior of surface hydroxyl species produced by the
decomposition of formic acid was different from that produced
by the adsorption of water. Figure 7 shows IRA spectra of the
NiO(111) surface dosed by deuterated formic acid DCOOD in
the same way as that of Figure 2. The peaks at 2160, 1589,
-
1
1
331, and 775 cm were assigned to the C-D stretching,
antisymmetric O-C-O stretching, symmetric O-C-O stretch-

Formic Acid on a (2 × 2) NiO(111)/Ni(111) Surface
J. Phys. Chem. B, Vol. 102, No. 16, 1998 2983
Figure 10. Temperature dependence of IRA spectra of formate
adsorbed on hydroxyl (-OD)-precovered NiO(111). The surface was
exposed to 10 langmuirs of D O at 500 K to produce isolated hydroxyl
2
Figure 9. Schematic model for the decomposition of formic acid on
NiO(111).
species. The hydroxyl-covered NiO(111) was then exposed to the 10
langmuirs of DCOOD at 163 K and then heated to the stated
temperatures. All spectra were normalized against the spectrum of the
initial hydroxyl-covered NiO(111) at 500 K.
the desorption of water due possibly to the morphology of the
sites or blocking by the nearest formate species. We propose
the scheme for the decomposition reaction as displayed in Figure
9.
Adsorption of Formic Acid on Hydroxyl-Covered NiO-
111). Next investigated was the adsorption of formic acid on
Conclusion
(
the NiO(111) surface which was precovered by the D2O-
originated OD groups. The surface was prepared by exposure
to the 10 langmuirs of D2O vapor at 500 K. This hydroxylation
condition is known to produce the largest amount of the isolated
hydroxyl species on NiO(111) and disrupt the (2 × 2)
The findings are summarized as follows.
(1) There are two decomposition temperatures, 340-415 and
520 K, for the formate on NiO(111). The frequency of the CO
adsorbed on the partially decomposed surface indicated that the
decompositions at 340-415 and 520 K took place on the fully-
oxidized Ni cation sites and the less-oxidized Ni cation sites,
respectively.
5
microfacets. Figure 10 shows IRA spectra of formic acid
adsorbed on hydroxylated NiO(111) as a function of tempera-
ture, where the shown spectra are the ratio spectra divided by
those of hydroxylated NiO(111). When the surface was dosed
at 163 K, a large reduction of the peak intensity, that is the
(2) The IRAS peaks of surface hydroxyl species (OD)
examined during formate decomposition on clean and hydroxyl-
-1
ated NiO(111) revealed that the 2530 cm peak of the hydroxyl
species interacting with formate disappeared around 323 K,
indicating that the hydroxyl species desorbs as water. On
heating the substrate to 473 K, the isolated hydroxyl species
was formed from the decomposition of formate, giving the peak
-
1
negative peak on the ratio spectra, was observed at 2711 cm ,
-
1
and a peak appeared at 2534 cm which was assigned to the
hydroxyl species perturbed by formate. The intensity of the
-
1
2
534 cm peak was almost twice that observed on clean NiO-
-
1
(111). The feature suggests that the isolated hydroxyl species
at 2711 cm . It desorbed at 650 K with further heating.
initially produced on the preparation transformed its structure
to the hydroxyl species perturbed by formate. This peak
disappeared around 323 K as was the case on clean NiO(111),
confirming that the formate-affected hydroxyl species desorbs
References and Notes
(1) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides,
1
st ed.; Cambridge University Press: New York, 1994; Chapter 1.
around 323 K irrespective of their origin. At the temperature
(2) Kuhlenbeck, H.; Od o¨ rfer, G.; Jaeger, R.; Illing, G.; Menges, M.;
higher than 473 K, the negative peak at 2711 cm-1 disappeared,
Mull, Th.; Freund, H.-J.; Pohlchen, M.; Staemmler, V.; Witzel, S.;
Scharfshwerdt, C.; Wennemann, K.; Liedtke, T.; Neumann, M. Phys, ReV.
B 1991, 43, 1969.
indicating that the isolated hydroxyl species was recovered by
decomposition of formic acid.
(
3) Wulser, K.; Langell, M. A. Catal. Lett. 1992, 15, 39.
(4) Troung, C. M.; Wu, M. C.; Goodman, D. W. J. Phys. Chem. 1992,
7, 9447.
5) Rohr, F.; Wirth, K.; Libuda, J.; Cappus, D.; B a¨ umer, M.; Freund,
H.-J. Surf. Sci. 1994, 315, L977.
6) Ventrice, C. A.; Bertrams, Th.; Hannemann, H.; Brodde, A.;
Neddermeyer, H. Phys. ReV. 1994, B49, 5773.
7) Xu, C.; Goodman, D. W. J. Chem. Soc., Faraday Trans. 1995, 91,
709.
The temperature-dependent change of the formate peaks on
hydroxylated NiO(111) was not different from that on the clean
NiO(111). The intensity ratio of the antisymmetric and sym-
metric O-C-O stretching bands varies by the tilt angle of
formate from the surface normal, and the invariance suggests
that the tilt angle on hydroxylated NiO(111) was the same as
that on clean NiO(111). The formate stood vertical to the
surface in the condition of the steady-state reaction under a
continuous flow of formic acid.13
9
(
(
(
3
(
(
8) Xu, C.; Goodman, D. W. Catal. Today 1996, 28, 297.
9) Sch o¨ nnenbeck, M.; Cappus, D.; Klinkmann, J.; Freund H.-J.;
Petterson L. G. M.; Bagus, P. S. Surf. Sci. 1996, 347, 337.

2984 J. Phys. Chem. B, Vol. 102, No. 16, 1998
Matsumoto et al.
(
10) Bandara, A.; Kubota, J.; Wada, A.; Domen, K.; Hirose, C. Surf.
Sci. 1996, 364, L580.
11) Bandara, A.; Kubota, J.; Wada, A.; Domen, K.; Hirose, C. J. Phy.
Chem. 1996, 100, 14962.
12) Kubota, J.; Bandara, A.; Wada, A.; Domen, K.; Hirose, C. Surf.
Sci. 1996, 368, 361.
13) Bandara, A.; Kubota, J.; Wada, A.; Domen, K.; Hirose, C. J. Phys.
Chem. B 1997, 101, 361.
(18) Yoshinobu, J.; Ballinger, T. H.; Xu, Z.; J a¨ nsch, H. J.; Zaki, M. I.;
Xu, J.; Yates, J. T., Jr. Surf. Sci. 1991, 225, 295.
(
(19) Zaki, M. A.; Kn o¨ zinger, H. J. Catal. 1989, 119, 311.
(20) Vesecky, S. M.; Xu, X.; Goodman, D. W. J. Vac. Sci Technol.
(
1
994, A12, 2114.
21) Sanders, H. E.; Gardner, P.; King, D. A.; Morris, M. A. Surf. Sci.
994, 304, 159.
22) Kondo, J. N.; Yuzawa, T.; Kubota, J.; Domen, K.; Hirose, C. Surf.
(
(
1
(
(
(
(
(
14) Henderson, M. A. J. Phys. Chem. 1995, 99, 12919.
15) Wu, M.-C.; Goodman, D. W. Catal. Lett. 1992, 15, 1.
16) Petrie. W. T.; Vohs. J. M. Surf. Sci. 1991, 245, 315.
17) Dilara, P. A.; Vohs. J. M. J. Phys. Chem. 1993, 97, 12919.
Sci. 1995, 343, 71.
(23) Clark, R. J. H.; Hester, R. E. Spectroscopy of Surfaces; John Wiley
& Sons, Ltd: New York, 1988; Chapter 8.