Pyrophosphate effect on the photocatalytic degradation of phenol over bare and Pt-deposited Bi2WO6

Article abstract of DOI:10.1039/c8nj04557g

Pyrophosphate anions (PPA) are widely present in wastewater, but their effect on semiconductor photocatalyis for organic degradation has not been established yet. In this study, we report a synergistic effect between Pt and PPA, that greatly improves photocatalytic activity of Bi₂WO₆ (BiW) for phenol degradation in aqueous solutions at pHs 5-11. BiW was synthesized by a hydrothermal method, followed by photochemical deposition of 0.5% Pt (Pt/BiW). Upon addition of PPA, rates of phenol degradation on BiW and Pt/BiW decreased and increased, respectively. The former is due to decreased O₂ adsorption, as inferred from photocatalytic and electrochemical reductions of O₂. (Photo)electrochemical measurements revealed that Pt and PPA act as electron and hole mediators, respectively. It is proposed that adsorbed PPA on the catalyst is first oxidized to a pyrophosphate anion radical, followed by phenol oxidation to regenerate PPA. However, as PPA concentration increased beyond 0.50 mM, rate of phenol degradation on Pt/BiW started to decrease, due to self-recombination of pyrophosphate anion radicals into a less reactive product.

Full text of DOI:10.1039/c8nj04557g

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Pyrophosphate effect on the photocatalytic
degradation of phenol over bare and Pt-deposited
Bi2WO6†
Cite this: New J. Chem., 2018,
42, 18873
Jie Meng, Jianjun Zhao,
Xianqiang Xiong, Xiao Zhang and Yiming Xu*
Pyrophosphate anions (PPA) are widely present in wastewater, but their effect on semiconductor
photocatalyis for organic degradation has not been established yet. In this study, we report a synergistic
effect between Pt and PPA, that greatly improves photocatalytic activity of Bi2WO6 (BiW) for phenol
degradation in aqueous solutions at pHs 5–11. BiW was synthesized by a hydrothermal method, followed
by photochemical deposition of 0.5% Pt (Pt/BiW). Upon addition of PPA, rates of phenol degradation on
BiW and Pt/BiW decreased and increased, respectively. The former is due to decreased O2 adsorption,
as inferred from photocatalytic and electrochemical reductions of O2. (Photo)electrochemical measure-
ments revealed that Pt and PPA act as electron and hole mediators, respectively. It is proposed that
adsorbed PPA on the catalyst is first oxidized to a pyrophosphate anion radical, followed by phenol
oxidation to regenerate PPA. However, as PPA concentration increased beyond 0.50 mM, rate of phenol
degradation on Pt/BiW started to decrease, due to self-recombination of pyrophosphate anion radicals
into a less reactive product.
Received 7th September 2018,
Accepted 17th October 2018
DOI: 10.1039/c8nj04557g
rsc.li/njc
absorption spectroscopy, Amano et al. observed a slow reduction of
O2 over BiW, in a time range from 200 ms to 3 ms.21 Moreover, for
Introduction
Semiconductor photocatalysts for environmental remediation observed organic degradations, a multi-electron reduction of O2
have attracted widespread attention, because various organic into H2O2 or H2O is proposed.8,9,17 Therefore, to improve photo-
pollutants can be degraded at ambient temperature only with catalytic performance of BiW, slow reduction of O2 must be
O2 as the oxidant.1–5 In principle, a semiconductor can generate enhanced. For example, deposition of Pt nanoparticles (NPs)
pairs of electrons and holes, upon excitation with band gap onto the surface of BiW (Pt/BiW) can result in remarkably
energy, in conduction and valence bands, respectively. Then, increased photocatalytic degradation of HCHO, chlorophenol,
these charge carriers may recombine to produce heat, or and rhodamine B.22–24
migrate to the surface to react with appropriate sorbents. To
date, anatase TiO2 is the most studied photocatalyst, mainly agricultural and industrial wastewater. In particular, sodium
Phosphate and polyphosphate anions are widely present in
À
ꢀ
because its electrons are capable of reducing O2 into O2 or pyrophosphate is used as a surfactant in water treatments,
HO2ꢀ. However, anatase has high band gap energy (3.2 eV), water softening reagents, electroplating liquids, and detergents
which is too large for excitation with most solar light wave- for pipes and automobile.25,26 Recently, phosphate anions have
lengths reaching the Earth’s surface.
proved to be a good co-catalyst for the hole transfer from
Recently, Bi2WO6 (BiW) has received considerable interest, Pt/BiW to phenol and 2,4-dichlorophenol.23 Thus, pyrophosphate
because its band gap is narrow (2.7 eV), and its holes are as anions may also influence semiconductor photocatalysis, which is
reactive as those of TiO2.6–17 However, organic degradation over yet to be explored.
BiW is very slow as the electrons of BiW are not powerful enough
Here, we report that surface modification of BiW with both
for one-electron reduction of O2.18–20 Using time-resolved infrared Pt NPs and pyrophosphate anions can result in a nearly
7.3 times increase in photocatalytic activity of BiW. A flow-
State Key Laboratory of Silicon Materials and Department of Chemistry,
like BiW was prepared in-house, followed by photochemical
deposition of Pt NPs. Solid product was characterized with
several techniques, and phenol degradation was used as a
model reaction. In an aqueous solution, phenol adsorption on
BiW was negligible, and its photolysis under UV light (l 4 320 nm)
Zhejiang University, Hangzhou 310027, P. R. China. E-mail: xuym@zju.edu.cn
† Electronic supplementary information (ESI) available: XRD patterns, absorp-
tion and XPS spectra, N2 adsorption–desorption isotherms, SEM and HRTEM
images, kinetic fitting and main intermediates of phenol degradation, the
distribution of phosphorus species calculated, anion absorption spectra and
adsorption isotherms, phenol degradation by H2O2. See DOI: 10.1039/c8nj04557g was also absent. This simplifies the evaluation of photocatalytic
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activity, without complicated effects of organic adsorption, Photocatalysis and analysis
photolysis and dye sensitization.27 Pyrophosphate anions
Reactions were carried out at 25 1C in a Pyrex-glass reactor,
attached with a water jacket. The light source was a 300 W high
pressure mercury lamp (Yamin, Shanghai), set at 10 cm from
front of the reactor. Between the lamp and reactor, a 320 nm or
420 nm cut-off filter was inserted, for UV or visible light
irradiation, respectively. Unless stated otherwise, all experi-
ments were performed at fixed conditions (0.43 mM phenol,
1.00 g LÀ1 catalyst, 0.50 mM Na4P2O7, and initial pH of 9.0).
A suspension (50 mL) containing necessary components was first
stirred in the dark for 30 min, and then irradiated with UV or
visible light. At specific time intervals, 2.0 mL of the suspension
was withdrawn, filtered through a 0.22 mm membrane, and
analyzed by HPLC (high performance liquid chromatography) on
a Dionex P680. Total organic content (TOC) was measured on a
TOC-V CPH TOC analyzer (Shimadzu). H2O2 was measured by
POD catalyzed oxidation of DPD.29 The change in solution absor-
bance at 553 nm was recorded on an Agilent 8453 UV-visible
spectrophotometer.
Adsorption isotherm of pyrophosphate on Pt/BiW in aqu-
eous solution was measured at 25 1C. A suspension (5.00 g LÀ1
Pt/BiW, 0–20 mM Na2P2O7, and pH 9.0) was stirred in the dark
overnight. Then, the concentration of pyrophosphate in the
filtrate was measured colorimetrically at 580 nm using a
phosphor molybdenum complex complex.30
Stability test of the photocatalyst was performed as follows.
Two suspensions (pH 9.0, 1.00 g LÀ1 Pt/BiW, and 0.50 mM
Na4P2O7) were prepared, and stirred in the dark for 30 min. One
suspension (50 mL) containing 0.43 mM phenol was irradiated
with UV light for 120 min, followed by analysis as above.
Then, a certain amounts of phenol stock solution and the
non-irradiated suspension were added, to restore phenol
concentration (0.43 mM) and suspension volume (50 mL). After
stirring in dark for 30 min, the suspension was irradiated for
120 min. The above procedure was repeated five times.
were observed to negatively and positively impact BiW and
Pt/BiW catalyzed reactions, respectively. To understand this
phenomenon, several experiments were carried out. These
included studying the effect of anion concentration and
solution pH, photocatalytic and electrochemical reduction of
O2, photoelectrochemical oxidation of phenol, and stability
test of Pt/BiW in the presence of anions. Furthermore, a
possible mechanism, responsible for observed differences
among the catalyst samples, is discussed.
Experimental section
Materials
N,N-Diethyl-p-phenylenediamine (DPD), horseradish peroxide
(POD), and H2PtCl6 were purchased from Sigma-Aldrich. All other
chemicals (analytic grade) were obtained from Sinopharm
Chemical Reagent Co., Ltd, including phenol, Na2WO4, Bi(NO3)3,
Na4P2O7, and NaH2PO4. Milli-Q water was used, and solution pH
were adjusted with dilute solutions of HClO4 or NaOH.
A flow-like BiW was made by a hydrothermal method.9 Briefly,
1.26 g Na2WO4Á2H2O dissolved in 40 mL H2O and 3.64 g Bi(NO3)3Á
5H2O in 30 mL 0.4 M HNO3 were mixed, and stirred for 30 min.
Then, the suspension was transferred into a 150 mL Teflon-lined
autoclave, and heated at 160 1C for 20 h. Next, the solid product
was collected, washed thoroughly with water, dried at 80 1C, and
calcined at 350 1C for 3 h. Pt/BiW was prepared by a photochemical
method.23,28 A suspension containing 4 mL CH3OH, 120 mL
H2PtCl6, and 0.90 g BiW was irradiated with a 300 W mercury
lamp for 3 h. After filtration, the solid product was thoroughly
washed with water, and dried at 80 1C. The amount of H2PtCl6
in the filtrate was analyzed by inductively coupled plasma mass
spectroscopy, from which, the content of Pt in Pt/BiW was
calculated to be 0.50 wt%.
(Photo)electrochemical measurement
Working electrodes of BiW or Pt/BiW films were made by the
doctor blade method.23 A gel containing photocatalyst and PVA
(poly vinyl alcohol) was coated onto fluorinated tin oxide (FTO)
glass, followed by calcination at 500 1C for 1 h. Then, the glass
was cut into several pieces (each had an exposed area of 1 cm2).
(Photo)electrochemical measurements were carried out in
0.5 M NaClO4 at pH 9.0, on a CHI660E Electrochemical Station
(Chenhua, Shanghai) using Ag/AgCl reference electrode, and
Characterization
X-ray diffraction (XRD) patterns were recorded on a D/max-2550/PC
diffractometer (Rigaku). From the (113) peak width at
2y = 28.31, average diameter of BiW crystallites (dXRD) was
calculated using Scherrer equation. Adsorption–desorption
isotherms of N2 on solid products were carried out at 77 K
on a Micromeritics ASAP2020 apparatus. Then, the Brunauer–
Emmett–Teller specific surface area (Asp), the Barrett–Joyner–
Halenda average pore size (dp), the t-plot micropore (Vm), and
total pore volume (Vt) were calculated. Diffuse reflectance
spectrum was recorded on a Shimadzu UV-2550 with BaSO4
as the reference. Reflectance (R) was transferred into Kubelka–
Munk absorbance. X-ray photoelectron spectroscopy (XPS) was
performed on a Kratos AXIS UItra DLD spectrometer. Binding
energies were calibrated with C 1s at 284.8 eV. Scanning
platinum gauze counter electrode. Scan rate was set at 10 mV sÀ1
.
The light source was a 500 W Xe lamp, equipped with a 320 nm
cut-off filter.
Results and discussion
Solid characterization
electron microscopy (SEM) and high-resolution transmission Solid product was characterized with several techniques, and
electron microscopy (HRTEM) were performed on a Hitachi results are shown in Fig. S1–S3 of ESI.† Briefly, XRD patterns of
SU-8010 and JEM-2100F electron microscope, respectively.
BiW and Pt/BiW were similar and in accordance with that for
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orthorhombic Bi2WO6 (PDF no. 39-023). Average diameters of
Bi2WO6 in BiW and Pt/BiW were calculated to be 13.6 and
13.9 nm, respectively. No diffraction of metallic Pt was observed
for Pt/BiW, mostly due to low content or amorphous nature of
Pt. In the absorption spectrum of BiW, there was a wide band,
assigned to the electron transition from O 2p or Bi 6s orbitals to
the empty orbital of W 5d.31 The spectral onset was located at
approximately 450 nm, equal to a band gap energy of 2.75 eV.
After Pt loading, the solid changed color from pale-yellow to
dark-gray. In the absorption spectrum of Pt/BiW, there was an
additional tailing band at wavelengths longer than 450 nm, due
to localized plasma resonance of Pt NPs.32 Chemical states of
Pt in Pt/BiW were examined by XPS. There were three peaks at
78.3, 73.8, and 70.5 eV. The first was assigned to PtO2,33
whereas the others were assigned to Pt 4f5/2 and Pt 4f7/2
,
respectively, of metallic Pt.23,32,33 In the adsorption–desorption
isotherms of N2, there was a hysteresis loop that indicated
presence of mesopores in either BiW or Pt/BiW. However, on
comparison with BiW, Pt/BiW had a smaller average pore
size, micropore volume, and total pore volume, but a larger
surface area (Table 1). These observations indicate that micro-
pores and mesopores of BiW are occupied or blocked by Pt
NPs. In SEM images of BiW and Pt/BiW, there were flower-
shaped spheres. In TEM and HRTEM images of Pt/BiW, the
large particles were seen to be coated with tiny particles
(10–20 nm), which had a lattice distance of 0.223 nm
matching that of Pt(111) facet. Furthermore, all elements
(Pt, W, Bi, and O) in Pt/BiW were detectable by energy disperse
X-ray spectroscopy.
Fig. 1 (A) Time profiles of phenol degradation, and (B) corresponding
changes in TOC, measured in an aqueous solution at initial pH 9.0, under
conditions of (a) BiW, (b) BiW + Na4P2O7, (c) Pt/BiW, (d) Pt/BiW + Na4P2O7,
and (e) no additives.
Table 2 Rate constants of phenol degradation under UV or visible lighta
Phenol degradation and intermediates formation
kobs (l 4 320 nm)
kobs (l 4 420 nm)
Fig. 1A shows time profiles of phenol degradation under UV
irradiation in an aqueous solution at initial pH 9.0, measured
with BiW or Pt/BiW and in the absence or presence of 0.5 mM
Solid
BiW
No PP
PP
P
No PP
PP
P
3.0
22.6
0.60
51.2
0.26
61.1
0.77
1.90
0.26
2.01
0.15
2.40
Na4P2O7. In each case, the concentration of phenol in aqueous Pt/BiW
phase decreased exponentially with time. All kinetic curves
a
PP, pyrophosphate; P, phosphate; kobs, apparent rate constant (10À3 minÀ1),
satisfactorily fit the first-order rate equation, and the resulting
apparent rate constants (kobs) are given in Table 2. First, the rate
of phenol degradation over Pt/BiW was 7.3 times larger than
that over BiW, a trend that is in agreement with previously
reported literature.20–22 Control experiments in the dark or
without catalysts under UV light showed a negligible decrease
in phenol concentration with time. Therefore, the observed
time-dependent decrease in phenol concentration is surely due
to a reaction initiated by BiW photocatalysis. Second, on
addition of 0.5 mM Na4P2O7, the rate of phenol degradation
measured with or without 0.50 mM Na2P2O7 or NaH2PO4 at initial pH 9.0.
over BiW decreased by a factor of 5.0. However, the rate of
phenol degradation over Pt/BiW increased by a factor of 2.27.
A control experiment with Pt/BiW showed that 2 mM NaClO4
had a negligible effect on phenol degradation under UV light.
Thus, the observed effect of Na4P2O7 is due to the anions of
Na4P2O7. These observations indicate that pyrophosphate anions
had a negative and positive impact during BiW and Pt/BiW
photocatalyzed degradation of phenol, respectively. Alternatively,
4À
Pt and P2O7 may undergo an interaction that improves the
Table 1 Physical parameters of solid producta
photocatalytic performance of BiW. Moreover, under visible
light (l 4 420 nm), additive effects and co-effect of Pt and
dXRD
Asp
Vm
Vt
4À
Solid
BiW
(nm) Eg (eV) (m2 gÀ1
)
dp (nm) (cm3 gÀ1
)
(cm3 gÀ1
)
P2O7 on phenol degradation were also observed (Fig. S4,
4À
ESI†). But in this case, the co-effect of Pt and P2O7 was not
13.6
Pt/BiW 13.9
2.76
2.76
16.9
17.8
18.7
14.2
0.0016
n.d.
0.0887
0.0834
as pronounced as that under UV light (Table 2). This might be
due to increased absorption of incident light by the black Pt
NPs of Pt/BiW, which reduces the numbers of photons reaching
BiW, and hence slows down the BiW-photocatalyzed reaction.
a
dXRD, crystal diameter; Eg, band gap energy estimated from absorption
spectra; Asp, surface area; dp, average pore diameter; Vm, micropore
volume; Vt, total pore volume; and n.d, not detectable.
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Consequently, all further photocatalytic experiments described
below were carried out under UV light (l 4 320 nm).
During phenol degradation, catechol (CA), 1,4-benzoquinone
(BQ), and hydroquinone (HQ) were detected as the main inter-
mediates, and their formation is shown in Fig. S5 (ESI†). First, BQ
was observed in all samples, and its formation rate was in the
4À
4À
order of Pt/BiW 4 Pt/BiW + P2O7 4 BiW 4 BiW + P2O7
.
Second, HQ and CA were observed with Pt/BiW photocatalytic
reactions, but they were negligible with BiW. On addition of
P2O74À, the rates of HQ and CA formation on Pt/BiW increased
and decreased, respectively. Third, the total concentrations of
intermediates (BQ, HQ, and CA) were analyzed, and resultant
formation rate followed a trend similar to that of BQ formation.
That is, for the production of total intermediates, Pt/BiW was more
active than BiW, and P2O74À was disadvantageous to both BiW and
Pt/BiW. Such trend in activity for intermediates formation was
slightly different from that observed for phenol degradation, the
4À
latter showed a trend of Pt/BiW + P2O7 4 Pt/BiW 4 BiW 4
BiW + P2O74À. However, the amount of total intermediates
produced at any given time was much lower than the amount
of phenol degraded. For example, with Pt/BiW at 60 min, the
amount of phenol degraded was 0.42 mM, but the amount of
total intermediates produced was only 10 mM. These observa-
tions indicate that the intermediates also degrade in situ. Thus,
Fig. 2 Photocatalytic degradation of phenol over Pt/BiW as a function of
(A) Na4P2O7 concentration at initial pH 9.0, and (B) solution pH, in (a)
absence, and (b) presence of 0.5 mM Na4P2O7. The open circles in (A)
represent calculated amount of pyrophosphate adsorption (see text), while
red circles represent corresponding rate constants of phenol degradation.
4À
it is highly possible that P2O7
degradation of BQ and CA.
is also beneficial to the
To further verify phenol mineralization, the change in TOC
with time was monitored using Pt/BiW as photocatalyst, and
the result is shown in Fig. 1B. As irradiation time increased,
the TOC in aqueous phase decreased. Interestingly, the rate of
4À
TOC decrease obtained in the presence of P2O7 was about concentration are shown in Fig. S6 (ESI†). In a solution of
4À
1.4 times larger than that measured in the absence of P2O7
.
0.50 mM Na4P2O7, the major form of pyrophosphate is greatlÀy
4À
This observation confirms that P2O7 indeed has a positive pH-dependent. For examples, the species are (a) 15% H3P2O7
3À
effect on the intermediate degradation on Pt/BiW.
and 84% H2P2O72À at pH 3, (b) 97% H2P2O72À and 3% HP2O7
at pH 5, (c) 65% HP2O73À and 35% P2O74À at pH 9, and (d) 2%
Effect of variables
3À
4À
HP2O7 and 98% P2O7 at pH 11. On comparing the corres-
To optimize photocatalysis conditions, phenol degradation on ponding rates of phenol degradation (Fig. 2A), it follows that
Pt/BiW was performed at different pHs and [Na2P2O7] while HP2O73À is probably the most active species that participates in
other conditions remained constant, where [Na2P2O7] represents the photocatalytic reaction on Pt/BiW.
Na2P2O7 concentration. As [Na2P2O7] increased, the rate of phenol
In an aqueous solution at pH 9.0, however, the distribution
oxidation at initial pH 9.0 increased initially, and then decreased of phosphorus species remains unchanged with [Na2P2O7].
(Fig. 2A). A maximum reaction rate was observed at approximately Thus, an observed change in rate of phenol degradation with
0.50 mM Na4P2O7. In the range of [Na2P2O7] examined, however, change in [Na2P2O7], due to changing phosphorous structure, is
the rates of phenol degradations were consistently larger than less likely. To understand the effect of [Na2P2O7], an adsorption
those without Na2P2O7. As the initial solution pH increased, the isotherm of pyrophosphate on Pt/BiW (5.0 g LÀ1) in an aqueous
rate of phenol degradation increased initially, and then decreased solution at pH 9.0 was measured. As the anion concentration in
after reaching a maximum at an initial pH 9.0 (Fig. 2B). This aqueous solution at equilibrium increased, the amount of
observation was obtained either in the absence or presence of anions adsorbed on Pt/BiW increased till it reached a certain
0.5 mM Na4P2O7. Moreover, in the wide range of tested initial limit (Fig. S7A, ESI†). The isotherm fit Langmuir adsorption
pH (5–11), Na2P2O7 always had a positive effect on phenol degra- equation well (Fig. S7B, ESI†), and resulting maximum amount
dation, except at pH 3.0. Therefore, the optimum condition was of adsorption and the adsorption constant were 0.24 mmol gÀ1
0.5 mM Na2P2O7, and initial pH 9.
and 1.07 mMÀ1, respectively. Using these parameters, the amount
,
In an aqueous solution, H4P2O7 has four dissociation of anion adsorption corresponding to anion concentration in
constants.34 The distribution of phosphorus species in aqueous photocatalysis solution was calculated, and the data was compiled
solution was calculated while considering these constants, and in Fig. 2A. At [Na2P2O7] lower and higher than 0.50 mM, there are
plots with a function of solution pH and pyrophosphate positive and negative correlations, respectively, between the rate
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constant of phenol degradation and the amount of anion adsorp- and the result is shown in Fig. 3B. In the dark, H2O2 in aqueous
tion. The positive effect of Na2P2O7 is due to surface phosphorus solution is highly adsorbed onto BiW. On addition of Na4P2O7,
species on Pt/BiW. The basis of negative impact is not known at the the amount of H2O2 adsorbed on BiW was reduced. This is due
moment, and will be discussed in the section on Possible mecha- to inhibition of H2O2 adsorption by surface adsorbed pyro-
nism. Moreover, Na2P2O7 can absorb light in aqueous solutions at phosphate anions. Under UV light, the concentration of H2O2
wavelengths shorter than 350 nm (Fig. S7C, ESI†). This would in aqueous phase decreased with time, which indicates H2O2
reduce the number of photons (l 4 320 nm) reaching Pt/BiW, decomposition occurring over the irradiated BiW. Upon addi-
and hence, decrease the rate of phenol degradation. But, the molar tion of Na4P2O7, the rate of H2O2 decomposition decreased, due to
absorptivity of Na2P2O7 at 320 nm is only 0.8. Thus, the solution a decrease in the amount of H2O2 adsorption. With Pt/BiW,
filter effect of Na2P2O7, responsible for the observed large decrease however, the concentration of H2O2 in solution decreased quickly
in the rate of phenol degradation at high [Na2P2O7], is less likely.
with time, even in the dark. Thus, the two curves obtained in
absence and presence of Na4P2O7 nearly overlapped. These
observations indicate that Pt NPs of Pt/BiW have a catalytic
effect on the thermal decomposition of H2O2. This would be
responsible for the observed low production of H2O2 from
Pt/BiW. Moreover, formation, adsorption, and decomposition
of H2O2 occur simultaneously during the photocatalysis with
either BiW or Pt/BiW. Thus, the real amount of H2O2 formed
from each photocatalyst should be much larger than that
detected in aqueous phase. Therefore, it is hard to assess the
catalytic activity for photocatalytic reduction of O2 using only
H2O2 as a probe.
Photocatalytic reduction of O2 to H2O2
During phenol degradation, H2O2 was also monitored, and the
results are shown in Fig. 3A. As the irradiation time increased, the
concentration of H2O2 formed in the aqueous phase of BiW also
increased. On addition of Na4P2O7, the concentration of H2O2
formed at any given time was greatly reduced. Such negative effect
of Na4P2O7 on O2 reduction is in good agreement with that on
phenol oxidation. However, the concentration of H2O2 formed on
Pt/BiW was lower than that on BiW. This trend in O2 reduction is
not in agreement with that in phenol oxidation. Moreover, on
addition of Na4P2O7, the concentration of H2O2 produced from
Pt/BiW was increased, but remained lower than that from BiW
alone. In this case, the concentration of H2O2 increased with time
up to 20 min, and then, decreased eventually.
(Photo)electrochemical measurement
Fig. 4 shows the electrochemical reduction of O2 on a BiW or
Pt/BiW film electrode in 0.5 M NaClO4 at initial pH 9. As the
applied bias shifted cathodically, the electrode current
increased negatively. At a given potential, the electrode current
obtained under air was much larger than that measured under
N2, which indicated that O2 reduction was the dominant process.
However, the onset potential for O2 reduction on a Pt/BiW
electrode (À0.12 V vs. Ag/AgCl) was more positive than that on
BiW electrode (À0.22 V vs. Ag/AgCl). Moreover, the current with
Pt/BiW electrode at any given potential was approximately one
order of magnitude larger than that with BiW electrode. These
observations indicate that the electrochemical reduction of O2 on
Pt/BiW is easier than that on BiW. This is in agreement with the
literature that suggests Pt to be a good catalyst for O2 reduction.35
On addition of 0.50 mM Na2P2O7 into the electrolyte, the
current density of each electrode was reduced, without a
change in the onset potential for O2 reduction. Since at a given
potential, the electron concentration at the electrode is con-
stant, it follows that the observed decrease in current on
addition of Na4P2O7 is mostly due to decreased O2 adsorption
at the electrode. Moreover, such inhibition effect of Na4P2O7 on
Pt/BiW electrode was weaker than that on BiW electrode. This
might be due to the catalytic effect of Pt NPs on O2 reduction,
which offsets the negative effect of anions on O2 adsorption.
But in the photocatalytic degradation of phenol, Pt/BiW in the
presence of Na4P2O7 was more active than in the absence of
Na4P2O7 (Fig. 1A and Table 2). Such discrepancy between two
experiments will be discussed later.
It is known that Pt can catalyze thermal decomposition of
H2O2.35 Thus, a separate experiment with H2O2 was carried out,
Fig. 5 shows the photoelectrocatalytic oxidation of phenol
on a Pt/BiW film electrode in 0.5 M NaClO4 at initial pH 9.0.
In the potential range from À0.2 V to 0.5 V (vs. Ag/AgCl), no
oxidation of phenol was observed in the dark, in the absence or
Fig. 3 (A) Formation of H2O2 in presence of 0.43 mM phenol. (B) Adsorption
in dark and photodecomposition of 0.10 mM H2O2 without phenol. Condi-
tions: (a) BiW, (b) BiW + Na4P2O7, (c) Pt/BiW, (d) Pt/BiW + Na4P2O7.
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Fig. 6 Stability test of Pt/BiW for phenol degradation in the presence
of 0.5 mM Na2P2O7. Each run was performed for 120 min, and then a
certain amount of phenol was supplied to restore phenol concentration
at 0.43 mM.
Catalyst stability
Fig. 6 shows the five repeated results of phenol degradation
in an aqueous suspension containing 1.0 g LÀ1 Pt/BiW and
0.5 mM Na2P2O7 at initial pH 9. For each run, the irradiation
time was fixed at 120 min, and the starting conditions were
adjusted to be approximately the same (see Experimental
section for details). As the irradiation time progressed, the
concentration of phenol in aqueous phase decreased. However,
the rate of phenol degradation decreased from one run to the
next. The relevant apparent rate constants of phenol degradation
(10À2 minÀ1), obtained from the first run to the last one, were 2.60,
1.59, 1.35, 1.05, and 1.00, respectively. Such rate decrease of phenol
decomposition is mostly due to the formation and accumulation
of various intermediates in the suspension, which would then
compete with phenol for the active species. After the first run,
however, there was a large decrease in the rate of phenol oxidation,
as compared with others. One possible reason for that is formation
of a colored intermediate, such as BQ, shielding off the light
absorbed by Pt/BiW, and hence, slowing down phenol oxidation.
In fact, the suspension color changed from white to brown
between 1st and 2nd runs. Since BQ is degradable (Fig. S5A, ESI†),
it follows that the catalyst could be used repeatedly.
Fig. 4 Dark current–voltage curves of (A) BiW, and (B) Pt/BiW film
electrodes, measured under N2 (dotted lines) and under air (solid lines),
in the (a) absence and (b) presence of 0.5 mM Na4P2O7.
Fig. 5 Current–voltage curves of Pt/BiW film electrode, measured in the
presence of 0.43 mM phenol, under N2 in the dark (dotted lines) and under
UV light (solid lines), (a) without and (b) with addition of 0.5 mM Na4P2O7.
Possible mechanism
First, Pt NPs in Pt/BiW can catalyze the electrochemical
reduction of O2 (Fig. 4). This positive effect of Pt should be
operative in the photocatalytic reduction of O2. As a result,
presence of Na4P2O7. Under UV light, anodic current was Pt/BiW is much more active than BiW for the photocatalytic
observed at approximately 0.01 V (vs. Ag/AgCl). As the bias oxidation of phenol, either under UV or visible light (Table 2).
shifted anodically, the electrode current increased positively. Second, pyrophosphate anions in aqueous solution can adsorb
This is indicative of phenol oxidation by the photogenerated onto the catalyst (Fig. S7, ESI†). The surface anions would inhibit
holes of Pt/BiW film electrode. On addition of 0.50 mM the adsorption of O2, leading to a decrease in the concentration of
Na4P2O7, the onset potential of phenol oxidation shifted H2O2 formed from the photocatalytic reduction of O2 on BiW
negatively. Meanwhile, the electrode photocurrent increased (Fig. 3A), and in currents of O2 reduction in the dark on BiW and
significantly. These observations indicate that the photo- Pt/BiW film electrodes (Fig. 4). Third, the anions can catalyze
electrochemical oxidation of phenol in the presence of Na4P2O7 photoelectrochemical oxidation of phenol over Pt/BiW (Fig. 5).
is easier than in the absence of Na4P2O7. In other words, This positive effect of anions should be also operative in the
pyrophosphate anions have a catalytic effect on phenol oxida- photocatalytic oxidation of phenol over Pt/BiW, as observed, under
tion on Pt/BiW.
UV or visible light (Table 2). Forth, the electrons and holes of BiW
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3À
ꢀ
are photogenerated and consumed in pairs. An acceleration of (1.44 V vs. NHE).39 Since P2O7
is reactive to phenol, the rate
electron transfer would promote the hole transfer, and vice versa. of phenol degradation is proportional to [Na4P2O7] up to
3À
ꢀ
In the photocatalytic process, electron reduction of O2 is catalyzed 0.50 mM (Fig. 2A). However, P2O7
radicals can recombine
by Pt, and hence the hole oxidation of phenol is speeded up. to form an unknown product,25 the rate of formation of which
Similarly, the hole oxidation of phenol is catalyzed by pyrophos- would increase with increased radical concentration. This may
phate, and subsequently, electron reduction of O2 should be explain why the rate of phenol degradation, after reaching a
accelerated. This has been observed from the photocatalytic maximum at 0.50 mM Na4P2O7, decreases with further increase
reduction of O2 on Pt/BiW, but not on BiW (Fig. 3A), the latter is in [Na4P2O7] (Fig. 2A). Interestingly, at [Na4P2O7] up to 8 mM,
due to decreased O2 adsorption. However, in an electrochemical the rate of phenol degradation was still larger than that in the
process, O2 is reduced by external electrons without the involve- absence of Na4P2O7. It means that the recombination product
3À
3À
ꢀ
ꢀ
ment of hole transfer. Therefore, the anion-caused decrease in O2 of P2O7
is less reactive than P2O7
itself, but it is still able
adsorption leads to current decrease in O2 reduction on both BiW to oxidize phenol.
2À
ꢀ
and Pt/Pt/BiW film electrodes (Fig. 4).
FuÀrthermore, HP2O7
is probably less reactive than
In the photocatalysis of Bi2WO6, hole oxidation of H2O to HPO4 ꢀ, as inferred from Table 2. In an aqueous suspension of
ꢀ
form OH radicals is often proposed, which is thermodynami- Pt/BiW at initial pH 9.0, the rate of phenol degradation in the
cally possible. In an aqueous solution at pH 0, the valence band presence of 0.5 mM NaH2PO4 was larger than that in the presence
edge potential of BiW is 3.0 V versus normal hydrogen electrode of 0.5 mM Na4P2O7, under UV or visible light. On one hand, this is
3À
(NHE),9 which is more positive than the oxidation potential of in agreement with the reduction potential of P2O7 ꢀ, which is less
4À
2À
ꢀOH radicals (2.8 vs. NHE).36 However, it is known that P2O7
in aqueous solution can react with OH (9 Â 10 M
positive than that of PO4 ꢀ (2.25–2.65 V vs. NHE at pH 0).40 On the
s
À1).37 If other hand, this is not in agreement with the adsorption of
5
À1
ꢀ
ꢀOH radicals are involved, then the rate of phenol degradation P2O74À, which would be higher than that of PO43À. In an
should decrease on addition of Na2P2O7, which is not the case aqueous suspension of Pt/BiW, the maximum amount of
adsorption measured at pH 9.0 (0.24 mmol gÀ1
,
4À
in our observations. To verity this doubt, a separate experiment P2O7
3À
with 0.1 M H2O2 and 0.22 mM phenol was carried out under UV Fig. S7, ESI†) was 4 times larger than that of PO4 adsorption
light irradiation at wavelengths longer than 240 nm. As the measured at pH 7.0 (0.058 mmol gÀ1).23 In both cases, the rate
irradiation time increased, phenol concentration decreased of phenol degradation on Pt/BiW in aqueous suspension
ꢀ
(Fig. S8, ESI†), which indicates phenol oxidation by OH radicals increases with the amount of anion adsorption. Consequently,
formed from H2O2 photolysis. In the presence of 10 and 30 mM the slower degradation of phenol with Na4P2O7 than that with
2À
ꢀ
Na4P2O7, rates of phenol degradation were decreased by 57.7 and Na3PO4 supports the above proposal that HP2O7
is less reactive
À
70.8%, respectively. These observations indicate that pyrophos- than HPO4 ꢀ. Moreover, the optimum pH at which maximum rate
phate can scavenge ꢀOH radicals in aqueous solutions at pH 9. of phenol degradation was reached was 9.0 for pyrophosphate, but
Therefore, the previous mechanism of ꢀOH production from hole 7.0 for phosphate.23 Therefore, although pyrophosphate and
oxidation of H2O on BiW is less likely.
phosphate behave similarly, their structural difference affect the
We propose that pyrophosphate anions are first oxidized by photocatalytic reactions of BiW and Pt/BiW differently.
the valence holes of BiW, and the resulting pyrophosphate
anion radicals then oxidize phenol, as outlined in Scheme 1,
a proposal that is thermodynamically possible. Reducing poten-
tials for pyrophosphate anions are not known, but they can be
Conclusions
oxidized to P2O7
dynamic calculation, Daniel et al. estimated a reduction photocatalysis by BiW and Pt/BiW, respectively. However, in
by ꢀOH or SO4
À 25,37,38 Through a thermo- We have shown that Na2P2O7 is detrimental and beneficial to
ꢀ
.
3À
ꢀ
3À
potential of 1.9 V vs. NHE for P2O7
at pH 10.25,37 Such both systems, pyrophosphate anions play similar roles. First,
ꢀ
potential is more negative than that of BiW valence band edge, electrochemical reduction of O2 on BiW or on Pt/BiW is
and is also more positive than the reduction potential of phenol inhibited by pyrophosphate, due to decreased O2 adsorption.
But with Pt/BiW, this negative effect of pyrophosphate is offset
by Pt-catalyzed reduction of O2. Second, the adsorbed pyropho-
sphate species on catalysts can mediate the hole transfer from
BiW to phenol. But with BiW, this positive effect of pyrophos-
phate is offset by the anion-caused decrease in O2 adsorption.
Based on (photo)electrochemical measurement, it follows that
3À
Pt and HP2O7 act as electron and hole mediators, respec-
tively. Due to mutual promotion between the electron and hole
transfers, photocatalytic degradation of phenol over Pt/BiW is
greatly enhanced on addition of Na2P2O7. Moreover, an optimum
condition for photocatalysis (initial pH 9, and 0.50 mM Na2P2O7)
has been obtained to maximize the rate of phenol degradation.
However, Pt noble metal is expensive, and it should be replaced by
Scheme 1 Possible mechanism for the synergistic effect of Pt and
pyrophosphate on Bi2WO6-photocatalytic degradation of phenol.
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18 S. Sun, W. Wang and L. Zhang, J. Phys. Chem. C, 2012, 116, 19413.
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18880 | New J. Chem., 2018, 42, 18873--18880 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018