CH
2I
2adsorption and reactions on TiO
2Li-Fen Liao, Chen-Fu Lien, Meng-Tso Chen, Yu-Feng Lin and Jong-Liang Lin*
Department of Chemistry, National Cheng Kung University, Tainan, Taiwan, Republic of China Received 13th December 2002, Accepted 12th March 2003
First published as an Advance Article on the web 27th March 2003
Fourier-transform infrared spectroscopy has been employed to study the adsorption, thermal transformation, and photoreactions of CH2I2on powdered TiO2. At 35C, CH2I2is adsorbed molecularly or dissociatively to
form a surface species with CHxand C–O functional groups possibly derived from C–I bond scission of CH2I2.
As the surface temperature is raised above 100C, the adsorbed CH
2I2is transformed into CH3O(a)and
HCOO(a)simultaneously, very likely via disproportionation of the reaction intermediate of dioxymethylene
(–OCH2O–). Under UV irradiation in the presence of O2, adsorbed CH2I2on TiO2decomposes to form CO(a),
HCOO(a), H2O(a), and CO2(g). But the CH2I2photoreaction is almost terminated in the absence of O2.
The possible initiation species involved in the CH2I2photodecomposition is discussed.
Introduction
Halogenated hydrocarbons are the compounds widely used in organic synthesis. Recently they are also used as precursors to generate hydrocarbon fragments on various metal or oxide-supported metal surfaces.1–3Study of adsorbed hydrocarbon fragments is important to explore the mechanisms in heteroge-neous catalytic synthesis. For instance, it has been believed that CHxis a crucial surface intermediate for the formation
of long chain hydrocarbons in Fischer–Tropsch synthesis.1 Adsorbed CH2I2 shows versatile surface reaction pathways,
depending on the nature of the substrate. On Cu(100) surface,4 CH2I2dissociates at 90 K at submonolayer coverage, whereas
at monolayer the dissociation temperature raises to 192–240 K. CH2(a)is produced on the surface after the C–I bond scission
of CH2I2. Recombination of CH2(a) generates C2H4(g) at
temperature200 K. On Al(111),5
the surface reaction of sub-monolayer CH2I2predominantly forms alkyl-aluminum
com-plex of CH3Al(H)I(g)at 470 K. Upon increasing the coverage
to one monolayer, C2H4(g)is generated at 150 K. The
appear-ance of the alkyl–aluminum complex indicates that the role of the Al(111) is not just a surface medium providing a place for the recombination of adsorbed CH2. On Rh(111),6the
disso-ciation process of CH2I2is similar to that on Cu(100),
how-ever, the CH2(a) reacts to form CH4(g) and C2H4(g). The
evolution of CH4indicates that Rh is more reactive than Cu
and leads to the dissociation of the C–H bond of CH2(a),
pro-viding the H source for the formation of CH4(g). Interestingly,
as the Rh(111) is preadsorbed with O atom, the C–I bond scis-sion of CH2I2 is retarded and the CH2(a) reacts with the
adsorbed oxygen atoms to form CH2O(g)between 170–340 K
and CO2(g) and H2O(g) between 340–460 K. The route for
the C2H4formation observed on clean Rh(111) is terminated.
On Ag(111),7 no CH
2I2 molecular desorption is detected at
submonolayer coverages, and the thermal reaction product C2H4(g) is generated at 115, 135, or 230 K, depending on
CH2I2coverage. Infrared-spectroscopic study shows the
for-mation of CH2I(a)as CH2I2(a)is heated to 155 K. 8
In the pre-sence of pre-adsorbed O on Ag(111),7 the formation of C2H4(g) decreases with increasing O coverage and CH2O(g)
becomes the main product at 225 and 270 K instead. On Mo(100),9 CH2I(a) is formed when CH2I2 adsorption layer
is heated to 135 K and CH4(g)is generated at230 K.
On TiO2, halogenated compounds have been extensively
investigated under UV illumination.10,11 One of the major focuses of these studies is to destroy these hazardous com-pounds to innocuous species from the point of the view of environmental protection and amelioration. Irradiation of TiO2using photons with energy higher than its band gap
gen-erates conduction band electrons and valence band holes which may diffuse to the surface where they react with adsor-bates and initiate photoreactions. Previously, we studied the adsorption and photoreactions of methyl iodide on TiO2.12
The present study of CH2I2is a continuation of our research
project on halogenated C1and we investigate the adsorption,
thermal reactions and photo-induced dissociation of CH2I2on
powdered TiO2 by Fourier-transform infrared spectroscopy.
Experimental
The details of the stainless steel IR cell with two CaF2windows
for IR transmission down to 1000 cm1 used in the present study have been reported previously.13In our system, the IR cell was connected to a gas manifold which was pumped by a 60 L s1 turbomolecular pump with a base pressure of 1 107Torr. The TiO2powder was supported on a
tung-sten grid (6 cm2
). The preparation method has been described.14 In brief, TiO
2 powder (Degussa P25, 50 m2
g1, anatase 70%, rutile 30%) was dispersed in water-acetone solution to form a uniform mixture which was then sprayed onto a tungsten mesh. After that, the TiO2 sample was
mounted inside the IR cell for FTIR spectroscopy. The TiO2
sample in the cell was heated to 450C under vacuum for
24 h by resistive heating. The temperature of TiO2 sample
was measured by a K-type thermocouple spotwelded on the tungsten mesh. Before each run of the experiment, the TiO2
sample was heated to 450C in vacuum for 2 h. After the heat-ing, 10 Torr O2was introduced into the cell as the sample was
cooled to 70C. When the TiO
2temperature reached 35C, the
cell was evacuated for gas dosing. The TiO2sample after the
annealing treatment still possessed residual surface hydroxyl groups.14,15 O2 (99.998%) and CH2I2 (99%) were purchased
from Matheson and Aldrich respectively. CH2I2was purified
by several cycles of freeze–pump–thaw before introduction to the cell. Pressure was monitored with a Baratron capacitance
1912 Phys. Chem. Chem. Phys., 2003, 5, 1912–1916 DOI: 10.1039/b212387h
manometer and an ion gauge. In the photochemistry study, both the irradiation and IR beams were set 45to the normal
of the TiO2sample. The irradiation light source used was a
combination of a 350 W–Hg arc lamp (Oriel Corp), a water filter, and a band pass filter with a band width of100 nm centered at 400 nm (Oriel 51 670). The power at the position of TiO2sample was0.14 W cm2measured in the air by a
power meter (Molectron, PM10V1). Infrared spectra were obtained with a 4 cm1resolution by a Bruker FTIR spectro-meter with a MCT detector. The entire optical path was purged with CO2-free dry air. The spectra presented here have
been ratioed against a clean TiO2 spectrum providing the
background reference.
Results
Fig. 1 shows the IR spectra of TiO2after being in contact with
1.5 Torr of CH2I2vapor at 35C followed by evacuation at
35, 50, 100, 150, 200, 250, and 300C for 1 min. In the 35C spectrum, the bands at 1111, 1359, 2973, and 3058 cm1are assigned to CH2 twisting, scissoring, symmetric stretching,
and antisymmetric stretching of adsorbed CH2I2respectively,
after comparing with the IR absorptions of CH2I2 in gas
phase16 and on Rh(111).17 In addition, the band at 1226 cm1is attributed to CH2 wagging of the adsorbed CH2I2,
since the gaseous CH2Cl2 wagging mode is at 1270
cm1;18,19the bands between 2000–2500 cm1are due to the combination modes of the adsorbed CH2I2. Table 1 lists the
observed fundamental bands of CH2I2on TiO2and the
corre-sponding vibrational modes. Because the frequency detection limit of our apparatus, the bands due to the CI2 moiety of
the adsorbed CH2I2are not observed. The negative bands in
the region of 3600–3800 cm1 in the 35C spectrum signify
the loss of isolated OH groups on TiO2after CH2I2adsorption
and the enhanced broad band between 3200–3600 cm1 indi-cates the formation of hydrogen bonding. These two findings reveal the nature of the interaction between the surface OH groups and the adsorbed CH2I2molecules, i.e. the hydrogen
bonding of –O–H I–CH2I. However, the hydrogen bonding
interaction has no significant effect on the CH2absorption
fre-quencies of the adsorbed CH2I2, because they are very close to
those of free CH2I2(see Table 1). In spite of the –O–H I–
CH2I interaction, another interaction between CH2I2 and
surface Ti+4 Lewis acid site via the lone-pair electrons of I
can not be ignored. In Fig. 1, the amount of the surface CH2I2 decreases with increasing surface temperature. After
shortly annealing to 300C under vacuum, only15% residual CH2I2is present on the surface and the broad band due to the
hydrogen bonding are hardly detectable. However, the isolated OH bands are not recovered, because they are unstable at higher temperature. In the 35C spectrum in Fig. 1, there
are two bands at 1043 and 2877 cm1that do not belong to CH2I2. They are characteristic frequencies for the stretching
of C–O and CHx, implying that CH2I2 may decompose on
TiO2 at 35C and produce the surface species containing
C–O and CHx functional groups. In the previous study of
CH3I adsorption on TiO2, it has been shown that the C–I
bond of the adsorbed CH3I dissociates to form CH3O(a) on
the surface.12Scheme 1 shows three possible species responsi-ble for the 1043 and 2877 cm1because they possess C–O and CHxgroups and are derived from C–I bond scission of CH2I2.
In Fig. 1, as the surface temperature is increased higher than 100C, new peaks appear at 1027, 1127, 1364, 1552, 2837, and 2937 cm1. Among them, the 1364 and 1552 cm1bands are assigned to the COO symmetric and antisymmetric stretching of HCOO(a), which have been detected in formic acid
disso-ciation on TiO2.20 The other four bands are attributed to
CH3O(a). The 1027 and 1127 cm1are due to the C–O
stretch-ing and the 2837 and 2937 cm1are due to the CH3symmetric
and antisymmetric stretching based on the previous IR study of CH3OH dissociative adsorption on TiO2.
21,22
Note that the previous study has demonstrated that thermal decomposi-tion of CH3O(a) on TiO2 in a vacuum does not generate
HCOO(a). 23
Fig. 2 compares the reaction rates of adsorbed CH2I2and
CH2Br2on TiO2by holding the surface temperature at 55C
for 180 min in a closed cell to demonstrate the effect of the nature of carbon-halogen bond. In the 0 min spectrum of
Fig. 1 Infrared spectra of TiO2taken after being in contact with1.5
Torr of CH2I2at 35C followed by evacuation at 35, 50, 100, 150, 200,
250, 300C for 1 min. All of the spectra were measured at 35C with
50 scans. Scheme 1
Table 1 Comparison of vibrational frequencies/cm1of CH2I2
CH2I2(g)
(ref.16)
CH2I2/Rh(111)
at 100 K (ref.17)
CH2I2/TiO2
at 310 K (this work) Assignment
3047 3030 3058 nas(CH2)
2968 2940 2973 ns(CH2)
1353 1350 1359 d(CH2)
— — 1226 o(CH2)
CH2Br2, the bands located at 1192, 2987, and 3071 cm1are
assigned to CH2 wagging and stretching of adsorbed
CH2Br2.24Assuming the same CH2stretching extinction
coef-ficients for both adsorbed CH2Br2 and CH2I2, the initial
adsorbed amount of CH2I2at 35C is estimated to be about
twice that of adsorbed CH2Br2. In the case of CH2I2,
HCOO(a)(1358 and 1551 cm1) and CH3O(a)(2831 and 2929
cm1) are produced after 180 min annealing at 55C. If both
the adsorbed CH2Br2and CH2I2have the same rate of thermal
decomposition forming CH3O(a)and HCOO(a), the amounts
of these two products derived from CH2Br2 are expected to
be a half of those from CH2I2 after taking the difference in
the initial amounts of CH2Br2and CH2I2into account. Since
no CH3O(a)and HCOO(a)bands are observed in the case of
CH2Br2 after the same thermal treatment as that of for
adsorbed CH2I2, it suggests that the thermal decomposition
rate of CH2Br2is much lower than that of CH2I2. This
differ-ence in the thermal decomposition rate for CH2Br2and CH2I2
may be explained by the stronger bond strength of C–Br bond (70 kcal mol1) than that of C–I (55 kcal mol1).
Fig. 3 shows the spectra taken during photoirradiation of adsorbed CH2I2 on TiO2in a closed cell after the indicated
times. After 180 min photoirradiation, the CH2I2 bands, at
1111, 2973, and 3058 cm1, are only slightly reduced, accom-panying with the formation of HCOO(a) at 1361 and 1552
cm1and CH3O(a)at2830 and 2930 cm1which are hardly
discernible in the scale used. Because the TiO2temperature is
increased up to 55C during the photoirradiation, the
forma-tion of HCOO(a)and CH3O(a)is not attributed to photoeffect,
but instead the thermal effect as demonstrated in Fig. 2. Fig. 4 shows the infrared spectra taken before and after the indicated
times during UV exposure of CH2I2adsorbed on TiO2surface
in 10 Torr of O2in a closed cell. In constrast to the case of
without O2 in Fig. 3, the bands at 1111, 2973, and 3058
cm1 due to adsorbed CH2I2 are considerably reduced after
180 min irradiation, and thus leading to the enhanced bands at 1359, 1552, 1616, 2119, 2212, 2349, and 2874 cm1. The 1359 and 1552 cm1 are attributed to HCOO(a), 1616 to
adsorbed H2O, 2119 and 2212 cm1 to adsorbed CO, and
2349 to gaseous CO2. 25
It has been known that the fundamen-tal frequencies of gaseous CO and HI are 2143 and 2230 cm1 respectively.26 Therefore the strong band at 2119 cm1 in Fig. 4 is attributed to adsorbed CO(a), instead of adsorbed
HI. However, the possibility for the formation of HI in the CH2I2 photoreactions can not be completely ruled out.
Furthermore, it is impossible in the present study to address the issues regarding the presence and the adsorption state of iodide produced from CH2I2 photodecomposition on TiO2,
because the vibrational frequencies of surface iodine is far below our detection limit of 1000 cm1. But the study of CH3I decomposition on TiO2 (110) surface has shown that
I(a) is stable up to 300C as evidenced by X-ray
photo-electron spectroscopy, however it is desorbed from the surface above this temperature.27 The reaction pathway of CH2I2(a)! HCOO(a)+ H2O(a)+ CO(a)+ CO2(g) summarizes
the products detected by our infrared spectrometer in the photodecomposition of CH2I2 on TiO2. Recently, we have
also studied the photoreactions of adsorbed CH2Cl228 and
CH2Br229 on TiO2 in the presence of 16O2. Strong bands
at 2127 cm1 in the CH2Cl2 case and at 2123 cm1 in the
CH2Br2 case are observed and attributed to adsorbed CO.
Furthermore, the photoreactions of CH2Cl2(a)in the presence
of 18O2 produces two bands at 2077 and 2127 cm1 which
Fig. 3 Infrared spectra taken before and after the indicated times
during photoirradation of a TiO2 surface covered with CH2I2. The
CH2I2-adsorbed surface was prepared by exposing a clean TiO2
sur-face to1.1 Torr of CH2I2 followed evacuation at 35C. All of the
spectra were measured with five scans.
Fig. 2 Infrared spectra taken before and after holding the TiO2
surface covered with CH2I2(a) or CH2Br2(b) at 55C for 180 min.
The four traces in the 2700–3200 cm1region have been multiplied
by a factor of two. The CH2I2-adsorbed and CH2Br2-adsorbed
sur-faces were prepared by exposing a clean TiO2surface to1.5 Torr
of CH2I2and CH2Br2, followed evacuation at 35C. All of the spectra
are attributed to C18O(a)and C16O(a)respectively.30The
rela-tive amounts of CH2I2(a) and HCOO(a), CO(a), and CO2(g)
as a function of photoirradiation time are shown in Fig. 5. The amount of CH2I2(a)decreases with photoirradiation time
and the decreasing rate is higher in the initial stage. The amounts of CO(a), HCOO(a) and CO2(g) increase
monotoni-cally. Since the CO(a) is produced as an intermediate, its
increase with photoirradiation time shows the rate for its for-mation from CH2I2decomposition is higher than the rate for
its consumption to form CO2(g). Since the surface temperature
was increased to 55C during the photoillumination of TiO2, a
separate thermal control experiment was carried out by hold-ing the CH2I2-adsorbed TiO2at 55C in 10 Torr O2for 180
min without photon exposure. Fig. 6 compares the spectra of the TiO2with the same initial amount of adsorbed CH2I2after
photoirradiation or surface heating at 55C for 180 min. It is
found that HCOO(a)is produced by the surface heating,
how-ever its amount is only50% of that produced by photoirra-diation. Furthermore, no CO(a)and CO2(g)are detected after
the surface annealing at 55C. Photoreaction of HCOO(a)on
TiO2 in the presence of O2has been investigated previously
and CO2(g)is the only product observed from HCOO(a)
photo-decomposition.31 Therefore the CO(a) formation observed in
Fig. 4 is not from HCOO(a)but instead directly from CH2I2
photodecomposition in the presence of O2.
Discussion
In the present study, when a clean TiO2 surface at 35C is
exposed to CH2I2(g), CH2I2 is adsorbed molecularly and can
dissociate to form a surface species containing CHxand C–O
functional groups. On TiO2, the adsorbed CH2I2and isolated
surface OH groups forms hydrogen bonding. However the possible acid-base interaction between the lone pairs on the
iodine atom of the adsorbed CH2I2and the Ti+4surface Lewis
acid sites can not be ruled out. This type of interaction between the oxygen lone pairs of C=O groups and Ti4+ions on TiO
2
has been observed by the red-shift of the C=O stretching frequency.23Due to the detection limit of our infrared spectro-meter, it is impossible for us to detect the CI2frequencies of
the adsorbed CH2I2, but, the observed CH2 frequencies are
similar to those of gaseous CH2I2. In our previous adsorption
study of CH3I on TiO2, the C–I bond can dissociate at 35C
to form CH3O(a). 12
In the present case of CH2I2on TiO2, the
C–I bond dissociates as well, forming the surface species likely as shown in Scheme 1. The amount of adsorbed CH2I2
decreases with increasing surface temperature under vacuum due to desorption and dissociation. CH2I2 decomposes
ther-mally on TiO2 into CH3O(a) and HCOO(a) simultaneously.
Based on the product distribution and bond energy difference between C–H and C–I, it is proposed that the CH2I2reaction is
via dioxymethylene. Previously, Busca et al. investigated the thermal reactions of formaldehyde on TiO2 by FTIR, they
Fig. 4 Infrared spectra taken before and after the indicated times
during photoirradation of a TiO2 surface covered with CH2I2 in 10
Torr of O2. The CH2I2-adsorbed surface was prepared by exposing
a clean TiO2 surface to1.5 Torr of CH2I2followed evacuation at
35C. All of the spectra were measured with five scans.
Fig. 5 Relative concentrations of CH2I2(a), CO(a), CO2(g), and
HCOO(a)as a function of photoirradiation time for the photoreactions
of adsorbed CH2I2on TiO2in the presence of O2. The CH2I2-adsorbed
TiO2surface was prepared by exposing a clean TiO2surface to1.5
Torr of CH2I2followed by evacuation at 35C. The maximum amount
of each species is scaled to 1.
Fig. 6 Infrared spectra taken after 180 min photoirradation (a) and
surface heating at 55C (b) for a TiO
2surface covered with CH2I2
in 10 Torr of O2. The surface was prepared by exposing a clean
TiO2surface to1.5 Torr of CH2I2followed evacuation at 35C. Both
found the formation of dioxymethylene (–OCH2O–) below
30C, which decomposed via oxidation to form HCOO
(a)
and via Cannizzaro-type disproportionation to form HCOO(a)
and CH3O(a)concomitantly prior to 150C.20Furthermore the
difference in bond energies of C–H (100 kcal mol1) and C–I (55 kcal mol1) implies that the C–I bonds of the adsorbed CH2I2 on TiO2 break preferentially thermodynamically, and
are likely to form –OCH2O–. This argument is also supported
by the comparison of the decomposition of CH2I2and CH2Br2
to form HCOO(a) and CH3O(a) on TiO2 in Fig. 2. The
adsorbed CH2Br2has a smaller thermal reaction rate than that
of CH2I2, since the C–Br bond energy is70 kcal mol1,15
kcal mol1higher than that of C–I bond.
As TiO2absorbs photons with energy higher than its band
gap, conduction band electrons and valence band holes are generated at the same time. They are original sources for the reduction and oxidation of adsorbates on TiO2. In the present
study of CH2I2on TiO2, CH2I2photoreaction proceeds only
in the presence of O2, forming CO(a), HCOO(a), H2O(a)and
CO2(g). The fact of no photoreaction for CH2I2 on TiO2in
the absence of O2 indicates that the photodecomposition of
adsorbed CH2I2in O2is not due to direct photodissociation
of adsorbed CH2I2, i.e. the photoreaction of CH2I2on TiO2
is mediated by TiO2. In the previous studies of
electrochemis-try and electron dissociative attachment, CH2Br2and CH2Cl2
were found to form transient molecular anions followed by dissociation into Br and CH2Br
for CH2Br2 and Cl and
CH2Cl
for CH2Cl2. 32,33
Therefore it is possible that the photoreaction of CH2I2on TiO2is due to the photogenerated
electrons in the conduction band. However, if this is the case, one would expect that the photoreaction rate of the adsorbed CH2I2in the absence of O2is higher than that in the presence
of O2, because of the electron-scavanging property of O2.
Besides, based on the spectra in Fig. 3 taken during photoillu-mination of the adsorbed CH2I2in the absence of O2, except
the slightly decreased CH2I2(a) bands and the enhanced
HCOO(a) and CH3O(a) bands due to the thermal effect, no
new bands are observed. Therefore the conduction band electrons may not play important role in the photoinduced reaction of CH2I2on TiO2.
The photodecomposition of the adsorbed CH2I2 to form
CO(a), HCOO(a), H2O(a)and CO2(g) is an oxidative process.
The possible species involved in the TiO2-mediated
photooxi-dation of CH2I2 are hole, OH
, and oxygen anions, such as O, O2, O3, etc.10,11 The efficiency for hole capture by
the CH2I2(a)depends on the match of the energy levels between
CH2I2 occupied orbitals and TiO2 valence band edge. OH radicals are possibly generated because the TiO2surface
pos-sesses isolated OH groups. On annealed TiO2 surfaces upon
UV irradiation at 77 K, ESR has shown the existence of O3, O23, O2species.
34
However only O2is shown to be
stable at room temperature. In the band gap irradiation of TiO2colloids suspended in aqueous solutions, ESR has also
shown the trapped hole species to be oxygen surface anion radical covalently bound to titanium atoms.35 It has been reported that CH2Br2reacts with Oor O2in gas phase to
produce Br.36The neutral species of Br, CH2O, or OBr are
suggested to be formed in the reactions.36In addition, theore-tical calculations suggest that Oinvolves in the photodecom-position of CH3O(a) and CH4 on semiconductors.37,38 The
exact species for the initiation of photooxidation of CH2I2
on TiO2is still under discussion.
Conclusion
As TiO2is exposed to CH2I2at 35C, CH2I2is adsorbed
mole-cularly or dissociatively to form a surface species with the structure as shown in Scheme 1 from C–I bond scission of CH2I2. The adsorbed CH2I2 is thermally transformed into
CH3O(a)and HCOO(a), probably via –OCH2O–, and
photo-chemically transformed into CO(a), HCOO(a), H2O(a), and
CO2(g)in the presence of O2.
Acknowledgements
We gratefully acknowledge the financial support of the National Science Council of the Republic of China (Grant NSC 91-2113-M-006-012) for this research.
References
1 B. E. Bent, Chem. Rev., 1996, 96, 1361.
2 F. Solymosi and J. Rasko, J. Catal., 1995, 155, 74.
3 M. D. Driessen and V. H. Grassian, J. Am. Chem. Soc., 1997, 119,
1697.
4 I. Kova´cs and F. Solymosi, J. Phys. Chem. B, 1997, 101, 5397.
5 M. Hara, K. Domen, M. Kato, T. Onishi and H. Nozoye, J. Phys.
Chem., 1992, 96, 2637.
6 C. W. Bol and C. M. Friend, J. Am. Chem. Soc., 1995, 117, 11 572.
7 K. C. Scheer, A. Kis, J. Kiss and J. M. White, Top. Catal., 2002,
20, 43.
8 G. Wu, D. Stacchiola, M. Collins and W. T. Tysoe, Surf. Rev.
Lett., 2001, 8, 303.
9 G. Wu, M. Kaltchev and W. T. Tysoe, Surf. Rev. Lett., 1999, 6, 13.
10 M. A. Fox and M. T. Dulay, Chem. Rev., 1993, 93, 341.
11 M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann,
Chem. Rev., 1995, 95, 69.
12 C. Su, J.-C. Yeh, C.-C. Chen, J.-C. Lin and J.-L. Lin, J. Catal.,
2000, 194, 45.
13 P. Basu, T. H. Ballinger and J. T. Yates, Jr., Rev. Sci. Instrum.,
1988, 59, 1321.
14 J. C. S. Wong, A. Linsebigler, G. Lu, J. Fan and J. T. Yates, Jr.,
J. Phys. Chem., 1995, 99, 335.
15 Y. Suda, T. Morimoto and M. Nagao, Langmuir, 1987, 3, 99.
16 H. Sponer, Molekulspektren und Ihre Anwendung auf Chemische
Probleme I, Springer, Berlin, 1935; Merck FT-IR Atlas, ed. K. G. R. Pachler, F. Matlok and H.-U. Gremlich, VCH, Darm-stadt, 1988.
17 G. Kliv’enyi and F. Solymosi, Surf. Sci., 1995, 342, 168.
18 T. Shimanouchi and I. Suzuki, J. Mol. Spectrosc., 1962, 8, 222.
19 J. L. Duncan, G. D. Nivellini and F. Tullini, J. Mol. Spectrosc.,
1986, 118, 145.
20 G. Busca, J. Lamotte, J.-C. Lavalley and V. Lorenzelli, J. Am.
Chem. Soc., 1987, 109, 5197.
21 C.-C. Chuang, C.-C. Chen and J.-L. Lin, J. Phys. Chem. B, 1999,
103, 2439.
22 W.-C. Wu, C.-C. Chuang and J.-L. Lin, J. Phys. Chem. B, 2000,
104, 8719.
23 C.-C. Chuang, W.-C. Wu, M.-C. Huang, I.-C. Huang and J.-L.
Lin, J. Catal., 1999, 185, 423.
24 R. S. Dennen, E. A. Piotrowski and F. F. Cleveland, J. Chem.
Phys., 1968, 49, 4385.
25 L.-F. Liao, C.-F. Lien, D.-L. Shieh, M.-T. Chen and J.-L. Lin,
J. Phys. Chem. B, 2002, 106, 11 240.
26 N. B. Colthup, , L. H. Daly and S. E. Wiberley, Introduction to
Infrared and Raman Spectroscopy, Academic Press, New York, 3rd edn., 1990.
27 C. Su, J.-C. Yeh, C.-C. Chen, J.-C. Lin and J.-L. Lin, J. Catal.,
2000, 194, 45.
28 M.-T. Chen, C.-F. Lien, L.-F. Liao and J.-L. Lin, J. Phys. Chem.
B., in press.
29 M.-T. Chen, Y.-F. Lin, L.-F. Liao, C.-F. Lien and J.-L. Lin,
Int. J. Photoenergy, submitted.
30 J. Fan and J. T. Yates, Jr., J. Am. Chem. Soc., 1996, 118, 4686.
31 L.-F. Liao, W.-C. Wu, C.-Y. Chen and J.-L. Lin, J. Phys. Chem.
B, 2001, 105, 7678.
32 L. A. Pinnaduwage, C. Tav, D. L. McCorkle and W. X. Ding,
J. Chem. Phys., 1999, 110, 9011.
33 M. Fedurco, C. J. Sartoretti and J. Augustynski, Langmuir, 2001,
17, 2380.
34 P. Meriaudeau and J. C. Vedrine, J. Chem. Soc., Faraday Trans. 2,
1976, 72, 472.
35 O. I. Micic, Y. Zhang, K. R. Cromack, A. D. Trifunac and M. C.
Thurnauer, J. Phys. Chem., 1993, 97, 7277.
36 R. Thomas, Y. Liu, C. A. Mayhew and R. Peverall, Int. J. Mass
Spectrom. Ion Processes, 1996, 155, 163.
37 A. B. Anderson and N. K. Ray, J. Am. Chem. Soc., 1985,
107, 253.
38 M. D. Ward, J. F. Brazdil, S. P. Mehandru and A. B. Anderson,
