obtained from Labotec Co., Ltd. Hydroquinone (reagent
grade) was purchased from Wako Pure Chemical Industries,
Ltd. These reagents have been directly used in our investiga-
tions without further puriÐcation. 2,5-Dihydroxy-p-benzo-
Results and Discussion
A visible-light irradiated silver-loaded zirconium phosphate
Ag H Zr (PO ) generated OH~ and O ~~ from aqueous
1
~x x
2
4 3
2
solution in air as shown in eqn. (1)È(6) earlier.5 Their radicals
quinone (DHQH ) as
a standard was synthesized as
2
formed DMPO-OH and DMPO-O ~ adducts with DMPO
as the spin trap reagent. However, since the reaction rate of
previously reported9 and was recrystallized from ethanol. A
phosphate bu†er solution of pH 6.0 was prepared with
KH PO and NaOH.
2
DMPO and OH~ is much larger than that of DMPO and
2
4
O ~~, only DMPO-OH was detected by the EPR measure-
The oxidation of hydroquinone (HQ) was investigated in
the dark and under irradiation. After the sample (2 mg) was
dispersed in a 2.125 cm3 phosphate bu†er solution (pH 6.0)
containing 5.9 mmol dm~3 HQ, this solution in a Pyrex tube
was irradiated with a Xe lamp (UV-37 and UV-25 cut-o†
Ðlters, Toshiba Glass Co., Ltd.) or allowed to stand in the
dark for certain times. The absorption spectra of the solutions,
which were diluted to 1/30 or undiluted (]1), in 10 mm cells
were measured using a Hitachi U-2000 spectrophotometer.
The concentration of p-benzoquinone (BQ) was calculated
2
ment. In a separate experiment, instead of DMPO, HQ was
added and its suspension was irradiated. The reaction rate of
HQ and OH~ has been reported to be 2.1 ] 1010 dm3 mol~1
s~1.8 After irradiation, the solution turned from yellow to red,
Fig. 1(a) and (b) show the absorption spectra of HQ and the
product. As shown in Fig. 1(a), in the dark, the absorption
peak of BQ at 246 nm (log e \ 4.4, in hexane)11 increased
with the amount of Ag H Zr (PO ) [0 mg (a) to 30 mg (e)]
1~x x
2
4 3
present. As shown in Fig. 2(b), during light irradiation, peaks
at 490 and 246 nm grew. The peak at 490 nm corresponded to
DHQH in aqueous solution. The R value from a thin layer
from the absorbance at 246 nm10 and its log e
\
(
j/246 nm)
4
.411 and that of DHQH was from the absorbance at 490 nm
2
f
2
chromatogram for the solutions was measured using ethyl
acetate and isopropyl alcohol as the developing solvents.
and its log e
\ 3.0; log e
was determined
(
j/490 nm)
(j/490 nm)
using A
\ 0.087 of 83.3 lmol dm~3 DHQH .
(j/490 nm)
2
Table 1 lists the R values of products in unirradiated and
Thin layer chromatography was performed using a Merk
Art. 15389 DC-Fertigplatten RP-18 F 254 S. Its 9.8 ] 5.0 cm
plate (the baseline is 0.1 cm from the bottom) was prepared
and methyl alcohol, ethyl acetate and isopropyl alcohol were
used as the developing solvents.
f
irradiated solutions. The R value (0) for the irradiated sample
f
EPR spectral measurements were performed with a JEOL
TE-300
and
ESPRIT-425
data
system.
5,5-
Dimethylpyrrolidine-N-oxide (DMPO) was used as the spin
trap reagent. The EPR spectra (X-band) of a semiquinone
radical (HBQ~) and DMPO adducts were measured with a
JEOL DATUM LC12 quartz oblique cell (ca. 0.2 cm3) for the
aqueous solution. Concentrations of HBQ~ and DMPO-OH
were determined by comparing their peak area in their EPR
spectra to that of a 3.2 lmol dm~3 TEMPO aqueous solution
as the standard.
A
0.2 mg/0.2 cm3 sample of an
Ag H Zr (PO ) powder suspension was mixed with 0.02
1~x x
2
4 3
cm3 DMPO spin trapping reagent (9.0 mol dm~3) and added
to the cell. The concentration of DMPO in the suspension
was 0.8 mol dm~3. The EPR spectra were measured during
irradiation with a 500 W Xe lamp (Ushio, UI-501C) with
UV-25 and UV-37 cut-o† Ðlters (Toshiba Glass Co., Ltd.),
UV-37 cut-o† and KL-40 interference Ðlters (Toshiba Glass
Co., Ltd.), or in the dark at room temperature with the follow-
ing conditions: 8 mW power, 79 lT modulation width, MnII/
MgO external standard for g values, and ^5 mT sweep width.
XPS spectra of the Ag H Zr (PO ) (Ag content \ 11
1~x x
2
4 3
wt.%) powders were measured with a Shimadzu ESCA-
000AX instrument. Mg-Ka radiation (1253.6 eV) was
1
employed as the X-ray source. Data processing12 was carried
out with an HP 340 computer (Hewlett-Packard Co., Ltd.),
which was attached to the Shimadzu ESCA-1000AX. The
ESCA-1000AX was operated at 10 kV and 30 mA under a
Fig. 1 (a) Absorption spectra of hydroquinone in supernatant after
mixing with 0 mg (a), 2 mg (b), 10 mg (c), 20 mg (d) and 30 mg (e)
Ag H Zr (PO ) for 2È3
h in the dark. Each amount of
Ag H Zr (PO ) was suspended in 2.125 cm3 of 5.9 mmol dm~3
x
pressure of 10~6È10~7 Pa. The binding energy (E ) was cali-
1~x 4 3
x
2
b
brated with C ls \ 285 eV from contaminant C. The sampling
1~x
2
4 3
hydroquinone phosphate bu†er solution (pH 6.0) and mixed for 2È3 h
in the dark. After the reaction, the suspension was centrifuged at 3000
rpm for 5 min. The obtained supernatant was diluted to 1/30 and its
absorption spectrum was measured. The concentration of p-
time was 200 ms and the number of accumulations was 20 for
the Ag 3d5@2 peak while for other elements, the number of
accumulations was 1. The Savitzky method13 was used for the
smoothing. The sample was pelletized and attached to the
sample probe with carbon tape.
benzoquinone was calculated using log e
\ 4.4 to be 4.8
(j/246 nm)
lmol dm~3, 22.9 lmol dm~3, 36.9 lmol dm~3 and 75.5 lmol dm~3,
respectively. (b) Absorption spectra of hydroquinone in supernatant
after mixing with 0 mg (a) and 2 mg (b) Ag H Zr (PO ) for 1 h
Measurements of the FT-Raman spectra were performed
with a Nihon Bunko NR-1800 instrument at room tem-
perature. The FT-Raman spectra of the Ag H Zr (PO )
1
~x
x
2
4 3
under light irradiation. 2 mg of Ag H Zr (PO ) was suspended in
~x 4 3
2.125 cm3 of 5.9 mmol dm~3 hydroquinone phosphate bu†er solution
1
x
2
1
~x x
2
4 3
(
pH 6.0) and mixed for 1 h under light irradiation using a 500 W Xe
(
Ag content \ 11 wt.%) powders were measured with an Ar
lamp (UV-25 and UV-35 cut-o† Ðlters). After the reaction, the suspen-
sion was centrifuged at 3000 rpm for 5 min. The absorption spectra of
the supernatant, which was diluted to 1/30 and undiluted ( ] 1), were
measured. The concentration of p-benzoquinone was calculated to
be 2.3 lmol dm~3 and that of 2,5-dihydroxy-p-benzoquinone using
ion laser (514.5 nm, 30 mW power) and triple mono-
chromator. The sample powders were put on the glass plate
and simply arranged on its surface with another glass plate:
entrance slit 500 lm, sensitivity 1.0 (nA/full scale) ] 100, scan
speed 600 cm~1 min~1, and accumulation number 3.
log e
\ 3.0 to be 12.5 lmol dm~3.
(j/490 nm)
284
J. Chem. Soc., Faraday T rans., 1998, V ol. 94