# Pt/TiO2 Coupled with Water-Splitting Catalyst  for  Organic Pollutant Photodegradation: Insight into the Primary  Reaction Mechanism

Authors: Zizhong Zhang; Xuxu Wang; Jinlin Long; Xianliang Fu; Zhengxin Ding; Zhaohui Li; Ling Wu; Xianzhi Fu
Journal: Research Letters in Physical Chemistry
 (2008)

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## Abstract

A composited system was fabricated by coupling Pt/TiO2 with water-splitting catalyst for photooxidation of organic pollutants in aqueous solutions. The new composited system exhibits more efficient photocatalytic activity than pure Pt/TiO2 does under UV light irradiation. The promoting effect is dependent on the photo-produced H2 over the composited system. The active oxygen species, hydroxyl radical (·OH) and hydrogen peroxide (H2O2), are measured by fluorescence spectroscopy and photometric method, respectively. The results reveal that the produced H2 by photocatalytic water splitting over NiO/NaTaO3:La transfers to Pt particle of TiO2 surface, then reacts with introducing O2 to generate in situ intermediate H2O2, and finally translates into ·OH radical to accelerate the photooxidation of organic pollutants.


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## Body

## 1. Introduction

Photoinduced
charge transfer occurring on semiconductor materials can achieve direct
conversion of photo energy to chemical energy, and thus it can be used for
elimination of organic pollutants and splitting water into hydrogen. However,
the utility of semiconductor-based photocatalytic process is controlled to a
large extent by the separation efficiency of the initially formed excited
states (hvb+ and ecb−) [1]. A
variety of approaches was made to enhance electron-accepting or electron-donating ability of the material
surface to favor the interfacial charge separation and consequently increase
the photocatalytic efficiency. One approach involves addition of surface
adsorbed redox species capable of scavenging selectively either of the excited
states to the photoreaction system [2, 3]. Another promising approach concerns modification
of TiO2 with noble metals, other semiconductors, and coloring
matters to improve the separation of the excited states [4–6].Deposition of
platinum on TiO2 has been reported to enhance extremely the
photocatalytic efficiency for organic pollutant elimination due to its high electron-trapping
effect [7], although an excessive number of platinum particles per grain of TiO2 can be
detrimental to the performance of the reaction system [8]. We have
recently demonstrated that trace amount of H2 can efficiently improve
the activity of benzene photooxidation over Pt/TiO2 [9, 10]. However,
the mechanisms have not been fully understood, and a practical approach for the environmental application has not
yet to be achieved, due to the difficulties in realizing the integration of H2 gas and photocatalysis into a practical system.Herein, an alternative system was fabricated by coupling Pt/TiO2 with
water-splitting catalyst NiO/NaTaO3:La to supply the in situ H2 to enhance
photocatalytic oxidation organic pollutants in an aqueous solution, where the
obtained composited system is quite different from the classic coupled
semiconductor system. The data show that the high photocatalytic efficiency of
the composited system is attributed to the formation of more ·OH which is dependent
on the generation of in situ H2O2 from the combination between the photo-produced H2 by the NiO/NaTaO3:La
and bubbled O2 on Pt/TiO2 surface.


## 2. Experimental

### 2.1. Sample Preparation

Titanium dioxide (TiO2) particles were prepared by a sol-gel technique. Titanium
isopropoxide (0.1 mol) was first added dropwise to 100 mL of nitric acid aqueous
solution. The suspension was stirred to clear and then dialyzed to pH of ca. 4
to obtain the TiO2 sol. The sol was dried at 333 K in an oven for 3
days. The resulting solid powders were ground to fine powders and finally
calcined at 623 K for 3 hours.NaTaO3:La
was prepared by the solid state reaction according to the literature [11]. In typical,
0.02 mol Ta2O5, 0.0206 mol Na2CO3, and
0.0004 mol La2O3 were mixed and then calcined in air at
1173 K for 1 hour and 1423 K for 10 hour.Platinum supported
catalyst was prepared by the incipient wetness impregnation method. The
calcined TiO2 was impregnated with a 5.22×10−2 M aqueous solution of H2PtCl6. The
impregnated sample was dried at 393 K for 6 hours and subsequently reduced with
an NaBH4 solution (0.1 M).
After reduction, the solid sample was washed with deionized water to remove
residual ion, and finally dried in air at 333 K (denoted as Pt/TiO2).
The initial ratio of Pt to TiO2 was fixed at 1 wt%.NiO loaded catalysts were prepared by
an impregnation method from a2.36×10−2 M aqueous solution of Ni(NO3)2 and then
dried at 383 K for 2–5 hours. The sample thus obtained was subsequently calcined at 543 K for 1 hour
in air using a muffle furnace. The initial ratio of NiO to NaTaO3:La
was fixed at 0.2 wt%.


### 2.2. Photocatalytic Reactions and Methods

The photocatalytic reaction was performed at room temperature in a quartz tubal
reactor surrounded with 254 nm UV lamps (Philips TUV, 4 W, Holland). The photocatalyst powders were dispersed in the salicylic acid
(SA) solution bubbled with oxygen (10 mL min-1). The concentration
of SA was analyzed by a high-performance liquid chromatograph (HPLC Waters) equipped
with a reverse phase column (Merk, LiChrospher RP-18e, 5 μm) and a UV detector with detection
wavelength of 297 nm. The mobile phase consisted of 30 mmol L-1 acetate (pH = 4.9) and the flow rate
was 1.0 mL min-1. The evolved CO2 during the reaction was collected with a Ba(OH)2 solution and then
determined by a titrate with an oxalic acid (H2C
2O4) solution (0.02 mol L-1). The evolved H2 during the reaction was
monitored by a hydrogen sensor (Dräger Pac III).Hydroxyl radical ·OH was captured by terephthalic acid to form fluorescent
2-hydroxyterephthalic acid [12] and then determined with fluorescence
spectroscopy (FS/FL920, excitation wavelength: 312 nm, and fluorescence peak:
426 nm). Hydrogen peroxide was analyzed photometrically by the POD (horseradish
peroxidase) catalyzed oxidation product of DPD (N,N-diethyl-p-phenylenediamine)
at 551 nm [13].


## 2.1. Sample Preparation

Titanium dioxide (TiO2) particles were prepared by a sol-gel technique. Titanium
isopropoxide (0.1 mol) was first added dropwise to 100 mL of nitric acid aqueous
solution. The suspension was stirred to clear and then dialyzed to pH of ca. 4
to obtain the TiO2 sol. The sol was dried at 333 K in an oven for 3
days. The resulting solid powders were ground to fine powders and finally
calcined at 623 K for 3 hours.NaTaO3:La
was prepared by the solid state reaction according to the literature [11]. In typical,
0.02 mol Ta2O5, 0.0206 mol Na2CO3, and
0.0004 mol La2O3 were mixed and then calcined in air at
1173 K for 1 hour and 1423 K for 10 hour.Platinum supported
catalyst was prepared by the incipient wetness impregnation method. The
calcined TiO2 was impregnated with a 5.22×10−2 M aqueous solution of H2PtCl6. The
impregnated sample was dried at 393 K for 6 hours and subsequently reduced with
an NaBH4 solution (0.1 M).
After reduction, the solid sample was washed with deionized water to remove
residual ion, and finally dried in air at 333 K (denoted as Pt/TiO2).
The initial ratio of Pt to TiO2 was fixed at 1 wt%.NiO loaded catalysts were prepared by
an impregnation method from a2.36×10−2 M aqueous solution of Ni(NO3)2 and then
dried at 383 K for 2–5 hours. The sample thus obtained was subsequently calcined at 543 K for 1 hour
in air using a muffle furnace. The initial ratio of NiO to NaTaO3:La
was fixed at 0.2 wt%.


## 2.2. Photocatalytic Reactions and Methods

The photocatalytic reaction was performed at room temperature in a quartz tubal
reactor surrounded with 254 nm UV lamps (Philips TUV, 4 W, Holland). The photocatalyst powders were dispersed in the salicylic acid
(SA) solution bubbled with oxygen (10 mL min-1). The concentration
of SA was analyzed by a high-performance liquid chromatograph (HPLC Waters) equipped
with a reverse phase column (Merk, LiChrospher RP-18e, 5 μm) and a UV detector with detection
wavelength of 297 nm. The mobile phase consisted of 30 mmol L-1 acetate (pH = 4.9) and the flow rate
was 1.0 mL min-1. The evolved CO2 during the reaction was collected with a Ba(OH)2 solution and then
determined by a titrate with an oxalic acid (H2C
2O4) solution (0.02 mol L-1). The evolved H2 during the reaction was
monitored by a hydrogen sensor (Dräger Pac III).Hydroxyl radical ·OH was captured by terephthalic acid to form fluorescent
2-hydroxyterephthalic acid [12] and then determined with fluorescence
spectroscopy (FS/FL920, excitation wavelength: 312 nm, and fluorescence peak:
426 nm). Hydrogen peroxide was analyzed photometrically by the POD (horseradish
peroxidase) catalyzed oxidation product of DPD (N,N-diethyl-p-phenylenediamine)
at 551 nm [13].


## 3. Results and Discussion

Table1 lists
the rate constants of salicylic acid (SA) photodegradation with different
catalysts under UV light irradiation in the presence of O2. The
results show that NiO/NaTaO3:La has distinct effect on TiO2 and Pt/TiO2 for SA photodegradation. NiO/NaTaO3:La
enhances the rate of the SA photodegradation in Pt/TiO2 reaction
system, and yet has no effect on the SA photodegradation over TiO2. Two
controlled experiments are carried out respectively under UV irradiation
without catalyst and with NiO/NaTaO3:La.
The results show that NiO/NaTaO3:La
is photocatalytically inactive for SA degradation despite it was reported to be
highly active for photocatalytic splitting water into H2 even without
sacrificial agent. Therefore, it can be deduced that the NiO/NaTaO3:La
plays a promoting role for Pt/TiO2 photocatalytic degradation of SA,
and the existence of Pt is indispensable for the promoting effect of NiO/NaTaO3:La.Table 1
Rate constants for SA
photodegradation with different composited catalysts. Catalyst: 0.0500 g, the rate of NiO/NaTaO3:La
to Pt/TiO2 (or TiO2) is 25 wt%. reactant solution: 120 mL SA (5×10−4 mol L-1),
with two 254 nm UV lamps irradiation.
Photocatalystk (min-1)Pt/TiO20.00308NiO/NaTaO3:La-Pt/TiO20.00429TiO20.00289NiO/NaTaO3:La-TiO20.00286NiO/Ta2O5-Pt/TiO20.00267NiO/Sr2Ta2O7-Pt/TiO20.00415The conduction band level of the NaTaO3 and TiO2 is −1.03 eV and
−0.52 eV, respectively, while the valence band level of the NaTaO3 and
TiO2 is 2.97 eV and 2.64 eV, respectively [14, 15]. It is obvious
that both the valence and conduction band of TiO2 are sandwiched
between the corresponding bands of NaTaO3. Thus in the  NiO/NaTaO3:La-Pt/TiO2 composited
system, the promoting effect of NiO/NaTaO3:La is unexpected from the
viewpoint of coupled semiconductors [5]. It is verified by the fact that
NiO/NaTaO3:La has no effect on TiO2 for SA
photodegradation (Table 1). Furthermore, simple mechanical addition NiO/NaTaO3:La
to Pt/TiO2 suspensions cannot make them intimate contact which was
necessary to form coupled semiconductors for an acceleration in photocatalytic reaction rate [16]. Therefore, The results provide a clear
conclusion that there are other reasons attributing to NiO/NaTaO3:La promoting effect. NiO/NaTaO3:La
was well documented to be a highly efficient photocatalyst for water splitting
into H2 under UV light irradiation [11]. In our previous work, the
trace amount of H2 was found to significantly increase the activity
of Pt/TiO2 for photocatalytic oxidation of volatile organic compounds (VOC's). Therefore,
the promoting effect may be attributed to the trace amount of H2 produced from photocatalytic
water splitting by NiO/NaTaO3:La to enhance the activity of Pt/TiO2 for SA photodegradation.In order to
check the effect of NiO/NaTaO3:La, the following comparative
experiments were carried out under the same conditions. It is experimentally
verified that NiO/Ta2O5 is photocatalytically inert for
both the SA degradation and water splitting (data not shown here) [17], but has the same band
energy level as the NaTaO3 [18]. Adding NiO/Ta2O5 instead of NiO/NaTaO3:La into the Pt/TiO2 suspension, the
photodegradation rate of SA shows no change (Table 1). In contrast, replacing NiO/NaTaO3:La
with another efficient water-decomposing photocatalyst NiO (0.15 wt.%)/Sr2Ta2O7 [19], SA photodegradation can also be markedly accelerated (Table 1). The above
results confirm that the promoting effect is dependent on the water-splitting function
of NiO/NaTaO3:La.
Furthermore, we examine the photodegradation of other organic contaminations such
as phenol with the NiO/NaTaO3:La-Pt/TiO2 suspensions
under 125 W high-pressure mercury lamp irradiation for 55 minutes, showing that
both the degradation and mineralization of phenol can be enhanced significantly
from 63% to 97% and from 54% to 84%, respectively. These results demonstrate that
coupling of Pt/TiO2 with a splitting-water photocatalyst is more
efficient for the photocatalytic elimination of organic pollutants in aqueous
solution than pure what
Pt/TiO2 does.To understand
the origin of the promoting effect of NiO/NaTaO3:La, the variety of
evolved H2 in the reaction process was monitored. Figure 1 shows the change in H2 yield and SA photodegradation in the composited system with reaction time under
O2 bubbling. As H2 evolution reaches a steady state, injecting
SA into the system results in a notable decrease of H2 evolution
along with quick degradation of SA. However, as the SA is completely
decomposed, the production of H2 progressively comes after its former steady state
(Figure 1). This indicates that the produced H2 is partly consumed to
accelerate the SA photodegradation over Pt/TiO2. This is supported
by the result that introducing H2 from an outer bottle instead of NiO/NaTaO3:La
into the Pt/TiO2 reaction system, the rate of SA photodegradation
was enhanced and comparable. In combination with the results of the SA
degradation (Table 1), it is deduced that the promoting effect of NiO/NaTaO3:La
to accelerate Pt/TiO2 for SA photodegradation is correlated to the
produced H2 consumed by Pt particle on TiO2 in the presence
of O2.Figure 1
Hydrogen evolution and SA degradation
over NiO/NaTaO3:La-Pt/TiO2 with oxygen bubbling under UV
light illumination.Photocatalytic
degradation of SA and phenol is initial from the attack of ·OH radical [20].
Figure 2 shows the plots of increase in fluorescence intensity at 426 nm against
illumination time for the reaction system. The linear increase in fluorescence
intensity for NiO/NaTaO3:La-Pt/TiO2 system is higher than
that for pure Pt/TiO2 system, suggesting that a larger amount of ·OH
radical was produced in NiO/NaTaO3:La-Pt/TiO2 composited
system. Thus we conclude that the consumed H2 is converted to a larger amount of active
oxidative species ·OH to induce quicker degradation of SA and phenol.Figure 2
Fluorescence spectra (insert) and
induced fluorescence intensity (426 nm) against illumination time for terephthalic
acid solution on (a) Pt/TiO2 and (b) NiO/NaTaO3:La-Pt/TiO2 samples under UV irradiation.In the
presence of H2 and O2, Au supported Ti-based catalysts
were reported to selective vapor-phase epoxidation of propylene. The reaction
is likely due to the in situ preparation of H2O2 from H2 and O2 at perimeter interface of the catalyst [21]. Thus it is possible that in the NiO/NaTaO3:La-Pt/TiO2 system, the produced H2 by photocatalytic water splitting and
bubbled O2 are primarily combined to form H2O2 on Pt/TiO2. Figure 3 shows the absorbance (at 551 nm) of the produced
H2O2 against illumination time for the reaction system. It
is obvious that a larger
amount of H2O2 was produced on NiO/NaTaO3:La-Pt/TiO2 reaction system than that on Pt/TiO2 reaction system. The produced H2 and bubbled O2 can be responsive to the generation of more amount of
H2O2 for the composited system. It is confirmed by the
result that as the Pt/TiO2 solutions were bubbled both with H2 and O2 in the dark, some amount of H2O2 was detected. The effect of H2O2 on the photocatalytic
activity was investigated earlier. Shiraishi and
Kawanishi [22] have declared
that the photocatalytic activity is closely related to the formation of H2O2.
Additional dosage of H2O2 into the TiO2 suspension was often used and found to efficiently enhance the degradation of organic
compounds due to the generation of ·OH radical by the direct photolysis or the
photoinduced electron reduction of H2O2 [23]. Thereby, in
the composited reaction system, the produced H2 and introducing O2 directly combine to form in situ H2O2 on Pt particle of TiO2 firstly, and then the H2O2 traps the photoinduced electron on Pt particle surface or is photocleaved to
form ·OH radical. Moreover, the larger amount of H2O2 is produced by the
composited reaction system not only simply, but also practicably, and it may be
useful for photocatalytic selective oxidation reaction by in situ H2O2 via the NiO/NaTaO3:La
and Pt/TiO2 co-deposition on suitable support.Figure 3
Absorption intensity (551 nm) of DPD/POD reagent after reaction with H2O2 against illumination time in the aqueous solution of (a) NiO/NaTaO3:La-Pt/TiO2 and (b) Pt/TiO2.


## 4. Conclusions

This work opens up a new efficient composited system for improving the efficiency of the photocatalytic
process. Coupled with the water-splitting catalyst, NiO/NaTaO3:La can efficiently promote the photocatalytic
performance of Pt/TiO2 for organic pollutant elimination in aqueous
solutions. It is shown that the in situ H2O2 is not only simply, but also practicably formed in the composited system by
directly combining H2 produced by photocatalytic water splitting
with introducing O2 on Pt particle of TiO2, and then the in situ H2O2 is
photocleaved or reduced by photogenerated electron to produce ·OH radical to
accelerate photooxidation reaction. This work is clearly very useful to explore
a new efficient and practical route for photocatalytic elimination of organic contaminations.



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*Source: 810457-2008-03-24.xml*