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W. Wu et al. / Catalysis Communications 40 (2013) 1–4
the catalytic test, 80 mg of In2S3 powders (99.98%, Alfa Aesar Co.)
was suspended in 80 mL of 4-NA (98%, Alfa Aesar Co.) aqueous solu-
3.2. Reusability of In2S3 photocatalyst
tion (20 mg/L) in
a
reactor (100 mL). After adding 50 uL of
Generally, the stability of a catalyst is a very important factor for its
practical applications. As shown in Fig. 2, the photoreduction activity of
the In2S3 photocatalyst does not obviously decrease in the recycling ex-
periments (Experimental details see Supplementary data). Its catalytic
activity can keep at ~ 100% in the 5th cycle of testing. XRD patterns
(see Fig. 3) indicate that the crystal structure of the In2S3 photocatalyst
is intact after the reaction. The XRD patterns of the catalyst can be well
indexed to tetragonal In2S3 (JCPDS card no. 051-1160). Moreover, the
In3+ concentration changes in aqueous solution have been measured
by ICP-AES. The In3+ concentrations before and after the reaction are
0.33 and 0.36 ppm, respectively. The slight change in the In3+ concen-
trations before and after the reaction can be ignored due to the instru-
mental detection limit (0.04 ppm for the In element). These results
confirm that the In2S3 photocatalyst has high stability for the photo-
catalytic hydrogenation of 4-NA under visible light irradiation.
triethanolamine (TEOA, A.R., Sinopharm Chemical Reagent Co.), the
suspension was stirred in the dark for 30 min to ensure eliminate
oxygen in the system by purging with nitrogen (>99.95%). As the
reaction proceeded, 4 mL of the suspension was taken at a certain in-
terval and was filtrated. The change of the 4-NA concentration during
the reaction was analyzed by measuring the absorbance at 380 nm
with a Cary 50 UV-vis spectrophotometer (Varian Co.). The whole
photocatalytic process was carried out under N2 bubbling with a
flow rate of 60 mL/min.
2.2. Characterization
Experimental details for the electron spin resonance (ESR), X-Ray
diffraction (XRD), inductively coupled plasma-atomic emission spec-
trometry (ICP-AES) and GC-MS analysis were described in detail in
Supplementary data.
3.3. Effect of hole scavengers
As mentioned in our previous work, the addition of a hole scaven-
ger is proved to be an efficient way to enhance the photocatalytic activ-
ity of the In2S3 photocatalyst for the photocatalytic hydrogenation of
4-NA [5–7]. Therefore, other hole scavengers (CH3OH, (NH4)2C2O4,
HCO2NH4 and Na2SO3) have been used to investigated the photocata-
lytic hydrogenation of 4-NA over the In2S3 photocatalyst upon purging
with N2. As shown in Table S1, only ~ 30% of 4-NA is converted to PPD
over the In2S3 photocatalyst in the presence of CH3OH, (NH4)2C2O4,
HCO2NH4 or Na2SO3 as the hole scavenger after 90 min of visible
light irradiation. However, the photoreduction activity of the In2S3
photocatalyst is dramatically increased when TEOA is used as the hole
scavenger. These results reveal that TEOA is a efficient hole scavenger
for the photoreduction reactions over the In2S3 photocatalyst.
3. Results and discussion
3.1. Photocatalytic hydrogenation of 4-NA
Fig. 1 shows UV-vis spectral changes of the 4-NA aqueous solution
over the In2S3 photocatalyst as a function of irradiation time in the
presence of TEOA under visible light irradiation (λ ≥ 420 nm). A de-
crease in the absorption of 4-NA at 380 nm along with simultaneous
appearance of two peaks at 238 and 305 nm has been observed in this
work. The peaks observed at 238 and 305 nm can attribute to the char-
acteristic peaks of PPD. After 90 min of visible light irradiation, 100% of
4-NA can be converted to PPD over the In2S3 photocatalyst. GC-MS
analysis results (see Fig. S1 and S2) indicate that 4-NA is completely re-
duced to PPD and no other products is detected in the present system.
Furthermore, the control experiment (4-NA and TEOA without the
In2S3 photocatalyst) exhibits negligible photocatalytic hydrogenation
of 4-NA (see Fig. S3), indicating the photolysis of 4-NA can be ignored
under visible light irradiation in the presence of TEOA. These results re-
veal that the In2S3 photocatalyst shows the catalytic activity for the
photocatalytic hydrogenation of 4-NA under visible light irradiation
in the presence of TEOA as the hole scavenger upon purging with N2.
3.4. ESR analysis
5, 5-Dimethyl-1-pyrroline N-oxide (DMPO) spin-trapping ESR
technique has been introduced to investigate the photocatalytic
hydrogenation of 4-NA over the In2S3 photocatalyst. Fig. 4 shows
DMPO spin-trapping ESR spectra of the In2S3 photocatalyst in dark
and under visible light irradiation (λ ≥ 420 nm). Sextet characteristic
peaks of the DMPO- · O−2 adduct are observed in the In2S3 photo-
catalyst methanol suspension under visible light irradiation. This
confirms that the In2S3 photocatalyst can produce photoinduced elec-
trons under visible light irradiation. It has been reported that the
redox potential of the conduction band (CB) of the In2S3 photocatalyst
Fig. 1. UV-vis spectral changes of the 4-NA aqueous solution over the In2S3
photocatalyst as a function of irradiation time in the presence of TEOA under visible
light irradiation (λ ≥ 420 nm).
Fig. 2. Reusability of the In2S3 photocatalyst for the photocatalytic hydrogenation of
4-NA.