Temperature dependence of adlayers on Pt(1 1 1) and Au(1 1 1) in a sulfuric acid solution studied by in situ IRAS

Article abstract of DOI:10.1016/S0009-2614(01)00374-8

Temperature dependence studies of adsorption of sulfuric acid species on Pt(111) and Au(111) electrodes were carried out using in situ infrared reflection absorption spectroscopy. A temperature-dependent shift of the interconversion potential between HSO₄^-/H₃O⁺ and H₂SO₄ on a Pt(111) electrode was observed. A temperature-dependent frequency shift of the absorption bands of HSO₄^- was also observed on both Pt(111) and Au(111) electrodes in the potential region where a √3×√7 structure evolved. Modelling experiments in ultrahigh vacuum revealed that ordering of the overlayer water molecules played an important role in the frequency of the absorption bands of HSO₄^-.

Full text of DOI:10.1016/S0009-2614(01)00374-8

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8 June 2001
Chemical Physics Letters 340 ꢀ2001) 425±430
www.elsevier.nl/locate/cplett
Temperature dependence of adlayers on Ptꢀ1 1 1) and Auꢀ1 1 1)
in a sulfuric acid solution studied by in situ IRAS
Y. Shingaya, M. Ito *
Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223, Japan
Received 14 September 2000; in ®nal form 13 March 2001
Abstract
Temperature dependence studies of adsorption of sulfuric acid species on Ptꢀ1 1 1) and Auꢀ1 1 1) electrodes were
carried out using in situ infrared re¯ection absorption spectroscopy. A temperature-dependent shift of the intercon-
‡
version potential between HSOÀ=H3O and H2SO4 on a Ptꢀ111) electrode was observed. A temperature-dependent
4
frequency shift of the absorption bands of HSOÀ4 was also observed on both Ptꢀ1 1 1) and Auꢀ1 1 1) electrodes in the
p
p
potential region where a 3 Â 7 structure evolved. Modelling experiments in ultrahigh vacuum revealed that ordering
of the overlayer water molecules played an important role in the frequency of the absorption bands of HSOÀ4 . Ó 2001
Elsevier Science B.V. All rights reserved.
1. Introduction
ꢀAES) [22], thermal desorption spectroscopy
ꢀTDS) [17±19] measurements. The results show
that bisulfate species are adsorbed at potentials
more positive than 400 mV and form an ordered
A Ptꢀ1 1 1) electrode in an aqueous H2SO4 so-
lution is one of the fundamental systems in elec-
trochemistry and has been studied widely [1±16]
since Clavilier published a characteristic cyclic
voltammogram ꢀCV) in the system [1,2]. It is well
known that the CV pro®le is ascribed to adsorp-
tion and desorption of both hydrogen and anions
[14±16]. The adsorption of sulfuric acid species on
Ptꢀ1 1 1) has been studied by in situ measurements
such as IR [3±9], scanning tunneling microscopy
ꢀSTM) [10,11], radiochemistry [12] and chrono-
coulometry [13]. Sulfuric acid species on Ptꢀ1 1 1)
have also been observed under UHV conditions
using IR [17±21], low-energy electron di€raction
ꢀLEED) [20±22], Auger electron spectroscopy
p
p
3 Â 7 structure in the potential range between
500 and 700 mV [19]. Despite the thorough studies
of the system, almost nothing was known about
the temperature dependence of the CV. Recently,
Zolfaghari et al. reported a quite interesting tem-
perature dependence of the CV pro®le for a
Ptꢀ1 1 1) electrode in a 0:5 M H2SO4 solution [23].
In this Letter, it is shown that current peaks cor-
responding to hydrogen adsorption or desorption
shift to a negative direction and that sharp spikes
at around 450 mV shift to a positive direction with
a rise in temperature. It is also reported that
asymmetric peaks at around 700 mV shift to a
negative direction with a rise in temperature.
In this Letter, we discuss the in¯uence of tem-
perature changes on the adlayer of sulfuric acid
species on Ptꢀ1 1 1) and Auꢀ1 1 1) electrodes using
* Corresponding author. Fax: +81-45-5661701.
E-mail address: masatoki@chem.keio.ac.jp ꢀM. Ito).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ꢀ 0 1 ) 0 0 3 7 4 - 8

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Y. Shingaya, M. Ito / Chemical Physics Letters 340 ,2001) 425±430
‡
in situ infrared re¯ection absorption spectroscopy
ꢀIRAS). A UHV modelling technique is also exe-
cuted in order to discuss structure of water layers
conversion between the HSOÀ=H3O ion pair and
4
the H2SO4 adlayer on a Ptꢀ1 1 1) electrode [17].
Therefore, it is expected that the potential at which
the interconversion occurs shifts upon a variation
in temperature. In situ IRAS measurements were
carried out at various temperatures in order to
investigate the temperature dependence of the in-
terconversion. Fig. 1 shows in situ IRA spectra
from a Ptꢀ1 1 1) electrode in a 0:5 M H2SO4 solu-
tion obtained at 297 K ꢀsolid line), and 350 K
ꢀdotted line), respectively. The spectra were ob-
tained with 4 cmÀ1 resolution and 6 scans co-ad-
dition during an anodic potential sweep of
200±800 mV. The spectrum at 200 mV was re-
ferred to the background and the sweep rate was
5 mV/s. Absorption bands in the spectra are
‡
on the adsorbed HSOÀ=H3O species.
4
2. Experimental
In situ IRAS measurements were carried out
using a FTS-45RD Fourier transform infrared
spectrometer ꢀBIO-RAD) with a liquid nitrogen
cooled MCT detector. CaF2 was used as a prism
window and an incident angle of IR beam was 65°.
IRAS measurements were carried out with a ki-
netic sampling method with 4 cmÀ1 resolution
during a potential sweep of 5 mV/s. Six consecu-
tive spectra were co-added, and the interval of the
electrode potentials between co-added spectra was
10 mV. The temperature of the system was con-
trolled with an electric heater which was wrapped
around the IR cell. The reference electrode was
Hg=Hg2SO4=0:5 M H2SO4, but values of elec-
trode potential are quoted versus the standard
hydrogen electrode ꢀSHE) in this Letter. The ref-
erence electrode was maintained at room temper-
ature.
The experimental details of the UHV modelling
technique were described elsewhere [19]. A home-
made UHV apparatus was used. The Ptꢀ1 1 1)
surface was prepared by a conventional cleaning
method. The adsorbed sulfuric acid species were
formed from H2O and SO3 in the UHV chamber.
IRAS measurements under UHV conditions were
carried out using a system 2000 Fourier transform
infrared spectrometer ꢀPerkin-Elmer).
3. Results and discussion
3.1. Temperature dependence of in situ IRA spectra
from a Pt,1 1 1) electrode in a sulfuric acid solution
A large temperature dependence of asymmetric
current peaks was reported at around 700 mV in a
CV of a Ptꢀ1 1 1) electrode in a 0:5 M H2SO4 so-
lution [23]. We paid attention to this point. We
reported previously that the current peak was as-
cribed to the current associated with the inter-
Fig. 1. In situ IRA spectra from a Ptꢀ1 1 1) electrode in a
0:5 M H2SO4 solution. The spectra were obtained at 297 K
ꢀsolid line) and 350 K ꢀdotted line).

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Y. Shingaya, M. Ito / Chemical Physics Letters 340 ,2001) 425±430
427
assignable to the SO3 symmetric stretching of
HSOÀ4 and the SO2 symmetric stretching of H2SO4
[19]. The spectral features of both temperatures
resemble each other in both frequency and inten-
sity on the whole.
We reported the frequency shift of S±O
stretching band of HSOÀ4 adsorbed on electrode
surface is related to an electronic state of the metal
and an orientation of hydration water molecules
on the surface [24]. It is interesting to note that the
deviation of the peaks from lower ꢀsolid line) and
higher ꢀdotted line) temperature phases is largest
p
p
at potential ranges where a 3 Â 7 structure is
formed and smallest at negative potential ranges
where the coverage of sulfuric acid species on the
surface is not enough to form ordered adlayer.
Since a discontinuity in the band center frequency
vs. potential plot appeared at the potential where
‡
the interconversion between HSOÀ=H3O and
4
H2SO4 occurs at room temperature [17], the band
center frequency was plotted as a function of po-
tential as shown in Fig. 2. The upper and lower
plots represent the anodic and cathodic sweeps,
respectively. In the graphs, it is shown that the
discontinuous points of the frequency shift toward
negative potentials as the temperature increases.
This means that the potential at which the inter-
‡
conversion between HSOÀ=H3O and H2SO4 ad-
4
layer takes place depends on temperature. This
®nding coincides with the result of the CV.
This temperature dependence of the intercon-
version potential can be explained as follows. We
reported previously that the HSOÀ=H3O adlayer
forms a hydrogen bonding network as shown in
Fig. 3. The model is based on UHV modelling of
‡
Fig. 2. The band center frequencies of adsorbed sulfuric acid
species as a function of the electrode potential: ꢀA) anodic
sweep, ꢀB) cathodic sweep. The spectra were obtained at 297,
317, 331 and 350 K.
4
‡
the HSOÀ=H3O adlayer on Ptꢀ1 1 1) [19]. The fact
4
‡
that the asymmetric bending mode of H3O ap-
peared in the IRA spectrum while the symmetric
mode was missing indicates that the molecular C3
‡
axis of the adsorbed H3O is parallel to the surface
[19]. TDS measurements of water molecules
showed the existence of one additional hydration
water in the adlayer. However, the bands of hy-
dration water were absent in the IRA spectrum.
The results indicate that the hydration water is
adsorbed in a molecular plane parallel to the sur-
face as shown in the ®gure. It is clearly shown that
‡
Fig. 3. A schematic diagram of HSOÀ; H3O and H2O on
Ptꢀ1 1 1).
4
p
p
the 3 Â 7 structure was stabilized through the

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Y. Shingaya, M. Ito / Chemical Physics Letters 340 ,2001) 425±430
hydrogen bonding network between the ion pairs
and the water molecule. The network structure is
unfavorable as far as entropy is concerned. Den-
sity functional molecular orbital calculations also
indicate that formation of a hydrogen bonding
‡
network in ionic HSOÀ=H3O ꢀH2O† clusters is
4
n
accompanied by destabilization due to entropy
factors [25]. On the contrary, the H2SO4 adlayer
does not form a hydrogen bonding network and
an adsorbed H2SO4 may translate or rotate on the
surface, so that an H2SO4 adlayer can bene®t from
the entropy factor due to a temperature rise.
Therefore, the HSOÀ=H3O adlayer becomes un-
‡
4
stable as temperature increases and the equilib-
‡
rium potential between HSOÀ=H3O and H2SO4
adlayer shifts toward the negative.
4
Another ®nding in Figs. 1 and 2 is that while the
frequency of the SO2 symmetric stretching of
H2SO4 is almost independent of temperature, the
frequency of the SO3 symmetric stretching of
HSOÀ4 increases with rising temperature. In the
UHV measurements, the frequency of the SO2
symmetric stretching of H2SO4 adsorbed on
Ptꢀ1 1 1) is independent of temperature, which is in
good agreement with the result of in situ IRAS for
H2SO4 and is quite reasonable. The frequency shift
of the HSOÀ4 band upon variation in temperature
is about 0:085 cmÀ1=K. Since the integrated in-
tensity of the band is almost the same, a dipole±
dipole coupling e€ect is excluded from the reason
of the frequency shift. One of the candidates is the
e€ect of the hydronium ꢀoxonium) cation as a
counter cation. The oxonium perchlorate crystal
has a layer structure similar to that of the
Fig. 4. IRA spectra of adsorbed HSOÀ4 and 2 ML H2O on
Ptꢀ1 1 1) in UHV at 110 K ꢀa). After annealing at ꢀb) 138 K, ꢀc)
151 K, ꢀd) 162 K, ꢀe) 186 K.
‡
water layer on top of the HSOÀ=H3O adlayer
4
would become important. Fig. 4 shows IRA
spectra of adsorbed HSOÀ4 and H2O on the
Ptꢀ1 1 1) surface obtained under UHV conditions.
Spectrum ꢀa) was obtained by the formation of
‡
‡
HSOÀ=H3O adlayer. And it has been reported
about a 1 ML HSOÀ=H3O adlayer followed by
4
4
‡
that the H3O orientations in the hydrogen
the addition of 2 ML H2O at 110 K. The ad-
sorption bands at 1232 and 3395 cmÀ1 can be as-
signed to the SO3 stretching mode of the adsorbed
HSOÀ4 and to the OH stretching mode of the H2O
molecules in the hydration layer on top of the
bonding network of oxonium perchlorate became
disordered at high temperatures [26]. Therefore,
one possible reason for the frequency shift might
be
HSOÀ=H3O adlayer.
a change in the conformation of the
‡
‡
HSOÀ=H3O adlayer, respectively. Spectra ꢀb) to
4
4
ꢀe) were obtained by heating the sample stepwise
to 138, 151, 162 and 186 K. The absorption bands
of the HSOÀ4 and H2O adlayer change together by
annealing. While the intensity of the H2O band
decreases at annealing temperatures higher than
151 K because of thermal desorption, the fre-
quency of the HSOÀ4 band shifts continuously to
3.2. UHV modelling of HSOÀ4 and H2O on Pt,1 1 1)
A more reasonable explanation for the tem-
perature-dependent frequency shift of the ad-
sorbed HSOÀ4 can be given from the results of
UHV modelling experiments. The structure of the