J. Chen et al. / Chemical Physics Letters 401 (2005) 104–108
105
2
À
SO –ZrO -400,
4
2
A continuous wave 244 nm laser (Coherent Innova 300
Fred) was used as the excitation source; a home-made
quartz sample cell was placed around a focal point of
an ellipsoidal mirror; the fluorescence signal from the
sample was collected with the ellipsoidal mirror and fo-
cused onto a 32 cm monochromator (Jobin-Yvon Triax
r.t.
These
fresh
catalysts:
2
4
À
SO –ZrO -700, ZrO -400 and ZrO -700 after calcina-
2
2
2
tion at 450 °C for 1 h in O flow and subsequently at
450 °C for 1 h in N flow, all exhibit a similarly broad
emission band centered at ca. 510 nm. This band is
attributed to the coordinatively unsaturated surface
sites, (Zr –O )Low Coordination, of the zirconia [21].
Similar luminescence behavior and assignment were also
reported on MgO and SrO [21,22]. After simulating
these spectra based on Gaussian curve for a fluorescence
band [23], as shown in Table 1, the emission band at ca.
510 nm consists of three sub-bands with peak positions
at ca. 470, 535 and 615 nm. Comparing the catalysts
2
2
4
+
2À
3
20) by passing through a filter with cut-off wavelength
below 280 nm; CCD (ISA Spectrum One CCD 3000)
was mounted at the focal plane in the exit of the mono-
chromator to detect the fluorescence signal. The wave-
length calibration of this setup had been carried out
with a mercury lamp, and the detected fluorescence spec-
trum of standard compound, quinine sulfate in 0.5 M
2
4
2
À
H SO , is the same as the standard spectrum [19].
2
SO –ZrO -700 and ZrO -700 with the catalysts
À
SO –ZrO -400 and ZrO -400, the full width at half
4
2
2
2
4
2
IR spectra measurements were recorded on a Thermo
Nicolet 470 FT-IR spectrometer with resolution of 4
maximum (FWHM) of these sub-bands becomes nar-
rower, and the relative areas of these sub-bands change.
Therefore, it is suggested that these three sub-bands
likely result from the different types of coordinatively
unsaturated sites on the zirconia surface for these four
catalysts.
À1
cm and scan numbers of 64. Raman spectra were col-
lected on ACTON SpectraPro 300i Raman spectrome-
ter. The excitation wavelength was 532 nm, and the
À1
resolution was 4 cm
.
Sulfated zirconia was prepared according to the liter-
ature [20]. The zirconium precursors, sulfate-doped
Zr(OH) and Zr(OH) , were from MEI. Two sulfated
After introduction of aniline to the fresh
À
SO –ZrO -400 at r.t., the fluorescence spectrum obvi-
4
2
2
4
4
zirconia catalysts were prepared by calcining sulfate-
doped Zr(OH) at 400 °C for 2 h (denoted as
ously exhibits a weak band at 350 nm and a strong band
at 422 nm, while the intensity of the band at 510 nm de-
4
2
À
2À
SO –ZrO
2
-400) and at 700 °C for 2 h (denoted as
creases dramatically. For the fresh SO –ZrO
2
-700
adsorbing aniline, the fluorescence spectrum shows the
4
4
2
À
SO –ZrO
that were investigated as comparison with sulfated zir-
2
-700), respectively. And two ZrO samples,
4
2
conia, were prepared by calcining Zr(OH) at 400 °C
4
for 2 h (denoted as ZrO -400) and at 700 °C for 2 h (de-
2
noted as ZrO -700), respectively. Aniline (AR grade)
was further purified by distillation under vacuum. These
2
2
4
À
2À
SO –ZrO -400, SO –ZrO -700, ZrO -400 and ZrO -
2
4
2
2
2
7
00 samples were individually pressed into thin disk
which was fixed in the sample cell and placed in a focal
point of the ellipsoidal mirror. Before introduction of
aniline, the sample was calcined in the sample cell at
4
for 1 h in N flow. After the sample was cooled down
50 °C for 1 h in O flow and subsequently at 450 °C
2
2
to r.t., N flow was passed through the aniline saturator
2
at r.t. to bring aniline vapour into the sample cell for 30
min. Then, the sample was purged by N flow at r.t.
2
Subsequently, the sample was increased its temperature
under N flow step by step for studying the changes of
2
adsorbates on the catalyst. At the same time, the LIF
spectra of the sample were collected in situ following
above process. On the other hand, IR characterization
was also carried out under the same treatment process
and conditions for these samples as the LIF spectra
measurements.
3
. Results and discussion
Fig. 1 shows the in situ LIF spectra of the fresh cat-
alysts and after aniline adsorption onto the catalysts at
Fig. 1. In situ LIF spectra of the fresh catalysts and after aniline
adsorption onto the catalysts at r.t.