2.2.1 Gas-Phase Photocatalysis
Using the diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), the
gas-phase photocatalytic reaction can be well tracked. Szczepankiewicz et al.
revealed a new band at 3716 cm
À1 for UV-irradiated TiO 2 in the gas phase in the
presence of a hole scavenger, ascribing to OH stretching for surface Ti
3+
ÀOH
formed by electron capture at acidic Ti
4+
ÀOH centers [16]. Another band at
3683 cm
À1 in an oxygen atmosphere in the absence of hole scavengers was
attributed to a surface-bound OH radical formed by hole capture at Ti
4+ ÀOH
centers. Intensity changes and corresponding wavelength shifts for ν(TiOÀH) are
proportional to the magnitude and polarity of the electric field. Wu et al. detected a
large amount of surface peroxo species and OH groups on the TiO 2 and M (Cu, V,
and Cr)/TiO 2 catalysts after a calcination treatment at 500
C under airflow. The
photocatalytic reaction of nitric oxide (NO) on TiO 2 and transition metal-loaded
M/TiO 2 catalysts indicates nitric oxide is adsorbed on TiO 2 and M/TiO 2 in the form
of bidentate nitrites and nitrates by reacting with OH groups, peroxo, or M ¼ O
species. In addition, NO can also be adsorbed on M
n+ in the form of nitrosyls. Under
UV irradiation, bidentate nitrite was oxidized to either monodentate or bidentate
nitrate, which was induced by superoxo species oxidized from peroxo species by
photogenerated holes.
2.2.2 Aqueous-Phase Photocatalysis
The application of IR spectroscopy in aqueous or vapor photocatalysis system is
restricted by the strong IR absorption for water. To minimize the interference from
water, the attenuated internal reflection (ATR) technique has been developed for
dissecting the intermediate formed during photoirradiation, where evanescent IR
waves penetrate intro a thin layer of an aqueous solution and reach the surface of
semiconductor covered on the surface of the internal reflection element (IRE). The
spectral cell for multiple internal reflection infrared (MIRIR) experiments uses a
trapezoidal-shaped ZnSe as the internal reflection element (IRE), which was irradiated by an IR beam with 45
incident angle. Taking TiO 2 film as a demonstration, it
is applied on one face of the IRE by a dip-coating method (Fig. 2.9), which was set in
the spectral cell made by Diflon [17]. The inner volume of the cell was approximately 10 mL. The length and width of the exposed area of the IRE are 35.5 and
9.2 mm, respectively. The IR light was reflected about nine times in the IRE with
TiO 2 , as calculated from the geometry of the IRE. The spectral cell was placed in the
sample chamber of an FTIR spectrometer with a deuterated triglycine sulfate
(DTGS) detector.
The first direct in situ spectroscopic detection of primary intermediates for the
photocatalytic O 2 reduction in aqueous solutions was achieved in TiO 2 system
through IRE technology. Under the UV irradiation and in the presence of dissolved
2.2 Infrared Spectroscopy
27
Using the diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), the
gas-phase photocatalytic reaction can be well tracked. Szczepankiewicz et al.
revealed a new band at 3716 cm
À1 for UV-irradiated TiO 2 in the gas phase in the
presence of a hole scavenger, ascribing to OH stretching for surface Ti
3+
ÀOH
formed by electron capture at acidic Ti
4+
ÀOH centers [16]. Another band at
3683 cm
À1 in an oxygen atmosphere in the absence of hole scavengers was
attributed to a surface-bound OH radical formed by hole capture at Ti
4+ ÀOH
centers. Intensity changes and corresponding wavelength shifts for ν(TiOÀH) are
proportional to the magnitude and polarity of the electric field. Wu et al. detected a
large amount of surface peroxo species and OH groups on the TiO 2 and M (Cu, V,
and Cr)/TiO 2 catalysts after a calcination treatment at 500
C under airflow. The
photocatalytic reaction of nitric oxide (NO) on TiO 2 and transition metal-loaded
M/TiO 2 catalysts indicates nitric oxide is adsorbed on TiO 2 and M/TiO 2 in the form
of bidentate nitrites and nitrates by reacting with OH groups, peroxo, or M ¼ O
species. In addition, NO can also be adsorbed on M
n+ in the form of nitrosyls. Under
UV irradiation, bidentate nitrite was oxidized to either monodentate or bidentate
nitrate, which was induced by superoxo species oxidized from peroxo species by
photogenerated holes.
2.2.2 Aqueous-Phase Photocatalysis
The application of IR spectroscopy in aqueous or vapor photocatalysis system is
restricted by the strong IR absorption for water. To minimize the interference from
water, the attenuated internal reflection (ATR) technique has been developed for
dissecting the intermediate formed during photoirradiation, where evanescent IR
waves penetrate intro a thin layer of an aqueous solution and reach the surface of
semiconductor covered on the surface of the internal reflection element (IRE). The
spectral cell for multiple internal reflection infrared (MIRIR) experiments uses a
trapezoidal-shaped ZnSe as the internal reflection element (IRE), which was irradiated by an IR beam with 45
incident angle. Taking TiO 2 film as a demonstration, it
is applied on one face of the IRE by a dip-coating method (Fig. 2.9), which was set in
the spectral cell made by Diflon [17]. The inner volume of the cell was approximately 10 mL. The length and width of the exposed area of the IRE are 35.5 and
9.2 mm, respectively. The IR light was reflected about nine times in the IRE with
TiO 2 , as calculated from the geometry of the IRE. The spectral cell was placed in the
sample chamber of an FTIR spectrometer with a deuterated triglycine sulfate
(DTGS) detector.
The first direct in situ spectroscopic detection of primary intermediates for the
photocatalytic O 2 reduction in aqueous solutions was achieved in TiO 2 system
through IRE technology. Under the UV irradiation and in the presence of dissolved
2.2 Infrared Spectroscopy
27