Photocatalytic partial oxidation of methanol to methyl formate under visible light irradiation on Bi-doped TiO2: Via tuning band structure and surface hydroxyls

Article abstract of DOI:10.1039/d0ra06309f

Preparing visible light responsive catalysts for partial oxidation of methanol to methyl formate is a challenging issue. This work addresses the synthesis, characterization and theoretical calculation of Bi doped TiO2 catalysts as well as their photocatalytic performance and reaction mechanism for MF synthesis from methanol. The catalysts were prepared by a simple wet chemical method. The results of the characterization and theoretical calculation evidenced that bismuth was intercalated in the lattice of anatase by the substitution of titanium. Impurity levels were formed in the valence band, conduction band and between the two bands. The Bi 6s and 5p orbitals contributed to the formation of the impurity levels. The photo-excited electrons transited from the valence band via impurity levels, formed by Bi 6s orbitals, to the conduction band. The doping of Bi enhanced surface hydroxyls, reduced the band gaps and raised the valence band edges (VBE) of the Bi doped catalyst. The Bi doped catalysts were visible light responsive due to the reduced band gap. The surface hydroxyls were beneficial to the methanol conversion, and the rise of the VBE enhanced the redox potential of the photogenerated holes. Only moderate redox potentials and sufficient surface hydroxyls could lead to high methanol conversion and MF selectivity. This study is of great significance to the development of the photocatalytic synthesis theory and provides a green route for MF synthesis from methanol. This journal is

Full text of DOI:10.1039/d0ra06309f

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PAPER
Photocatalytic partial oxidation of methanol to
methyl formate under visible light irradiation on Bi-
doped TiO2 via tuning band structure and surface
hydroxyls†
Cite this: RSC Adv., 2020, 10, 31442
*
Yue Ma, Xuzhuang Yang,
Guanjun Gao, Zhe Yan, Haiquan Su,
Bingbing Zhang,
Yanqiu Lei and Yanbing Zhang
Preparing visible light responsive catalysts for partial oxidation of methanol to methyl formate is
a challenging issue. This work addresses the synthesis, characterization and theoretical calculation of Bi
doped TiO2 catalysts as well as their photocatalytic performance and reaction mechanism for MF
synthesis from methanol. The catalysts were prepared by a simple wet chemical method. The results of
the characterization and theoretical calculation evidenced that bismuth was intercalated in the lattice of
anatase by the substitution of titanium. Impurity levels were formed in the valence band, conduction
band and between the two bands. The Bi 6s and 5p orbitals contributed to the formation of the impurity
levels. The photo-excited electrons transited from the valence band via impurity levels, formed by Bi 6s
orbitals, to the conduction band. The doping of Bi enhanced surface hydroxyls, reduced the band gaps
and raised the valence band edges (VBE) of the Bi doped catalyst. The Bi doped catalysts were visible
light responsive due to the reduced band gap. The surface hydroxyls were beneficial to the methanol
conversion, and the rise of the VBE enhanced the redox potential of the photogenerated holes. Only
moderate redox potentials and sufficient surface hydroxyls could lead to high methanol conversion and
MF selectivity. This study is of great significance to the development of the photocatalytic synthesis
theory and provides a green route for MF synthesis from methanol.
Received 20th July 2020
Accepted 20th August 2020
DOI: 10.1039/d0ra06309f
rsc.li/rsc-advances
since 2014.7–9,12 J. C. Colmenares et al. studied the same reaction
on Pd–Au/TiO2 and Pd–Cu/TiO2.10,24 However, all studies used
Introduction
UV irradiation as light source so far. It has great potential of
application if the reaction can occur under the irradiation of
visible light.
From coal via methanol to methyl formate (MF), a key inter-
mediate to produce important chemicals such as formic acid,
formamide, acetic acid and ethylene glycol,1–5 is a promising
route in coal chemical industry.6 The photocatalytic synthesis of
MF from methanol is a new green route and has become a hot
research topic in recent years due to the ambient reaction
conditions and high MF selectivity.7–12
TiO2-based materials have been widely studied for many
reactions.13–21 The secondary alcohol photo-oxidation on TiO2
and ZnO was rst reported by J. Cunningham and B. K. Hodnett
in 1981,22 and Liu et al. reported the photocatalytic methanol
oxidation on MoO3/TiO2 and TiO2 as early as 1985.23 H. Komi-
nami et al. studied this reaction on an anatase-type TiO2 in
2010.11 In our previous studies, we have reported Au, Ag, Au–Ag
alloy and Cu nanoparticles supported on TiO2 and ZnO for
photocatalytic partial oxidation of methanol at 15 ꢀC–45 ꢀC
Titania is an effective photocatalyst for partial oxidation of
methanol because, on the one hand, there are plenty of surface
hydroxyls which can react with methanol molecules to give rise
to methoxy, a key intermediate for the reaction, at room
temperature without irradiation,2,3,25 and on the other hand, it
has suitable valence band edge that can generate photoexcited
holes being able to oxidize methoxy to formaldehyde under UV
irradiation.7–10,12,24 However, TiO2 cannot work as a photo-
catalyst for partial oxidation of methanol to MF under visible
light irradiation due to the wide band gap (3.0 eV).26 Even so,
TiO2 is still the most promising visible light responsive photo-
catalyst for this reaction if proper modication is carried out.
Doping TiO2 with metallic or non-metallic elements can form
impurity energy levels between the conduction band and
valence band of TiO2, and thus was oen employed to lower the
threshold energy for excitation in order to utilize a wide range of
solar energy.27–32 Such a modication usually works in a lot of
photocatalytic processes such as the degradation of
School of Chemistry and Chemical Engineering, Inner Mongolia Key Lab of Rare Earth
Materials Chemistry and Physics, Inner Mongolia University, Hohhot, Inner Mongolia,
010021, P. R. China. E-mail: xzyang2007@yahoo.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06309f
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organics,31,33–35 but it is a challenge in photocatalytic partial vigorously stirring, and keep stirring for 8 hours. The obtained
oxidation process, especially for partial oxidation of methanol. solid was washed with deionized water and absolute ethanol
This is because the doping process can cause not only the several times and dried under supercritical condition (245 ꢀC, 8
reduction of the band gap but also the shi of valence and/or MPa), and nally the catalyst was obtained aer calcination at
conduction band edges, leading to the variation of the redox 500 ꢀC for 2 h, and labelled as XBi-TiO2, where X ¼ n(Bi/Ti) is the
potential of the photoexcited holes and thus the partial oxida- percentage of Bi in the catalyst.
tion reaction. In addition, the doping may change the surface
hydroxyls of titania as well, remarkably inuencing the meth-
anol conversion and methyl formate selectivity.
Catalyst characterization
X-ray diffraction (XRD) proles were obtained using a Bruker D8
Advance diffractometer with a Cu Ka radiation operated at 40
kV. The scanning range (2q) was 10–80ꢀ. The morphology of the
samples was investigated by a FEI Tecnai S-Twin eld emission
high resolution transmission electron microscope (HR-TEM).
The optical property was determined by a UVIKON/XL UV-vis
diffuse reectance spectrometer (UV-vis) with a scanning
range of 200–800 nm. The X-ray photoelectron spectra (XPS) of
the catalysts were recorded by a Kratos Amicus spectrometer
using an Al K (1486.6 eV) radiation source. The binding energy
(BE) was adjusted by the C1s transition at 284.6 eV. The Fourier
transform infrared (FTIR) spectrum was recorded on a Bruker
Vertex 70 FTIR spectrometer. The temperature programmed
reduction with hydrogen (H2-TPR) was performed with
a Micromeritics AutoChem 2910 analyzer. The gas mixture is
10% H2 balanced with N2 and the ow rate is 50 mL minꢁ1. The
heating rate is 10ꢀC minꢁ1 10 mg sample was loaded in the
quartz tube for each run. The N2 adsorption–desorption
isotherms were measured using a Micromeritics ASAP 2020
analyzer. The specic surface areas were calculated by the
Brunauer–Emmett–Teller (BET) equation from the N2 adsorp-
tion isotherm. The pore structure parameters were obtained
from the adsorption branch using the BJH model. Prior to the
experiment of adsorption, all samples were degassed under
vacuum at 150 ꢀC for 12 h. The electrochemical properties of the
catalyst were carried out in liquid junction cells on a Shanghai
Chenhua CHI700E Chemical Station.
Bismuth has d10 electronic conguration. Bi-doped TiO2
extends the spectral range of TiO2 to visible light responsive
region and inhibits electron–hole recombination, and thus has
been extensively studied for the degradation of organic pollut-
ants,27,29,33,35,36 antibacterial application37 and photocathode for
fuel cell38 etc. However, it is seldom used in photocatalytic
partial oxidation reactions due to the more complicated reac-
tion mechanism. In this study, Bi-doped TiO2 was prepared by
the wet chemical method and the photocatalytic performance
for partial oxidation of methanol to MF was investigated. The
band structure and surface species of the catalyst were charac-
terized by a UV-visible spectrometer (UV-vis), electrochemical
workstation, X-ray photoelectron spectroscopy (XPS) and
a Fourier transform infrared spectrometer (FTIR) etc., and the
band structure and optical properties before and aer doping
was studied by the rst principal theory as well. The purpose of
this study is to investigate the effect of the Bi doping on the
band structure, especially the valence band edges and band
gaps, and the surface hydroxyls, as well as their effect on the
photocatalytic performance for partial oxidation of methanol to
MF under visible light irradiation.
Experimental
Materials
Tetrabutyl titanate (TBOT), Bi(NO3)3$5H2O, absolute ethanol
and methanol were purchased from J & K Scientic. All chem-
icals were used as received.
Photocatalytic reaction
The photocatalytic activity of the catalyst was performed in
a continuous ow magnesium–aluminium alloy reactor with
Catalyst preparation
Typically, 6.8 ml of TBOT was dissolved in 60 ml of absolute a rectangle quartz window on the top and a dividing wall-type
ethanol and keep stirring for 30 min to obtain solution A. A heat exchanger connected to the back of the reactor.7 Three
certain amount of Bi(NO3)3$5H2O was dissolved in a solution of pieces of rectangle glass which were used as the catalyst holders
30 ml of absolute ethanol and 8 ml of deionized water to obtain were installed in the bottom of the reactor, with a thermocouple
solution B. Add solution B to solution A dropwise with xed in touch with the middle catalyst holder. The catalyst of
Fig. 1 Schematic diagram of the reaction process.
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Fig. 2 XRD patterns of (a) bare TiO2, (b) 0.25Bi-TiO2, (c) 0.5Bi-TiO2, (d)
1Bi-TiO2, and (e) 2Bi-TiO2.
Fig. 3 TEM images of (a and b) bare TiO2, (c and d) 0.5Bi-TiO2.
0.02 g was loaded on the holders for each run. The dimension of
each piece of the glass holders was 26 mm ꢂ 80 mm ꢂ 1 mm.
The cooling water owed through the heat exchanger to main-
tain a constant temperature. A 500 W xenon lamp without lter
was positioned 10 cm over the quartz window of the reactor
(light intensity, 100 mW cmꢁ2). A gas mixture containing 1–
3 vol% methanol and 0.17–20 vol% O2 balanced with N2 was
supplied at the ow rate of 50 ml minꢁ1 into the reactor. Oxygen
and nitrogen in the mixture were measured by mass owmeters.
The formation rate of methyl formate was calculated by the
following eqn (3).
rM0VCS
Fr ¼
(3)
2mM
where Fr is the formation rate, mmol gꢁ1 hꢁ1; rM0 is the initial
methanol content in feed gas, mg Lꢁ1; V is the ow rate of feed
gas, L hꢁ1; C is the methanol conversion, %; S is the methyl
formate selectivity, %; M is the molecular weight of methanol, g
molꢁ1; m is the weight of catalyst, g.
ꢀ
The reaction temperature was from 15 to 45 C. The products
were qualied by a GC-MS and a LC-MS in batches and quan-
tied on line by a Shimadzu GC2014C equipped with a FID
detector (Fig. 1).
The methanol conversion was obtained by eqn (1), assuming
the volume ow rate was constant before and aer the reaction
due to the low reactant content in the feed gas.
Results and discussion
Catalyst characterization
rM0 ꢁ rM1
C ¼
ꢂ 100%
(1)
Fig. 2 shows the XRD patterns of TiO2 and Bi-doped TiO2. The
diffraction peaks at 25.35ꢀ, 37.60ꢀ, 48.10ꢀ, 53.89ꢀ, 55.30ꢀ, 63.20ꢀ,
69.00ꢀ, 70.18ꢀ, 75.37ꢀ are assigned to the (101), (004), (200),
(105), (211), (204), (116), (210), (215) faces of anatase, tetragonal
crystal system (reference code 00-002-0406). No diffractions
rM0
where C is the methanol conversion, %; rM0 is the initial
methanol content, mg Lꢁ1; rM1 is the methanol content in the
off gas aer reaction, mg Lꢁ1
.
2rMF
S ¼
ꢂ 100%
(2)
rM0 ꢁ rM1
where S is the methyl formate selectivity, %; rMF is the methyl
formate content in the off gas aer reaction, mg Lꢁ1
.
Table 1 The lattice parameters and average size of anatase
Lattice
parameter (A)
ꢀ
Crystal size,
nm
Cell volume,
A
ꢀ3
Samples
a ¼ b
c
Bare TiO2
0.25Bi-TiO2
0.5Bi-TiO2
1Bi-TiO2
3.7853
3.7869
3.7866
3.7880
9.5007
9.4944
9.5001
9.5036
9.5040
7.60
9.47
9.45
9.17
8.58
136.5137
136.0375
136.2365
136.2623
136.3704
Fig. 4 Raman spectra of (a) bare TiO2, (b) 0.25Bi-TiO2, (c) 0.5Bi-TiO2,
2Bi-TiO2
3.7906
(d) 1Bi-TiO2, and (e) 2Bi-TiO2.
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content, which is in line with the result calculated by the Debye–
Scherrer's equation from the XRD prole. The d(101) spacing of
anatase can be observed from the HR-TEM images, which is the
only phase that can be identied from the lattice fringes. The
d spacing enlarges with increasing the Bi content (Fig. 3d, s1h
and j†). This is consistent with the cell parameters listed in
Table 1, suggesting that Bi was intercalated into the lattice of
anatase. This is consistent with the result calculated from the
XRD.
Raman spectra can also evidence the intercalation of Bi into
the lattice of anatase. As shown in Fig. 4, the bands at 144, 197,
399, 516 and 639 cmꢁ1 are the Raman active fundamentals of
anatase.29,36,39,40 The three Raman bands at 144, 197 and
639 cmꢁ1 are denitely assigned to the three Eg modes, the
band at 516 cmꢁ1 is from the doublet of A1g and B1g modes and
that at 399 cmꢁ1 is attributed to the B1g mode. It can be
observed that the Eg bands centered at 144 cmꢁ1, attributed to
the shi of Ti–O–Ti frame of anatase, shi to higher position,
the peak width is broadened and the intensity of the peaks is
weakened with the increase of the Bi content, indicating poorer
crystallinity and evidencing the intercalation of Bi into the
lattice of anatase from another angle.
Fig. 5A shows the UV-visible diffuse reection spectra of bare
TiO2 and XBi-TiO2 samples, and Fig. 5B shows the Kubelka–
Munk plot of the samples.41 The bare TiO2 exhibits absorption
edge at 400 nm, corresponding to 3.1 eV of the band gap. The
absorption bands of the Bi doped samples exhibit obvious red
shi. With the increase of the Bi content from 0.25 wt% to
2 wt%, the absorption band edge shis from 407 nm to 427 nm,
and the band gap energy varies from 3.04 eV to 2.90 eV. The
absorption bands of the Bi doped samples extend to visible light
region. The Bi dopant in the lattice of TiO2 can form impurity
levels in the band gap of TiO2 and reduce the recombination of
the photoexcited electron and hole pair, as well as the excitation
energy.33–35,38,42 In addition, the doping process can result in
oxygen vacancies in the lattice, which play the role of photoex-
cited electron traps and is benecial to inhibiting the recom-
bination of the electron and hole as well.
The N2 adsorption–desorption results of all the ve samples
exhibit type IV isotherm, suggesting mesopores in these
samples43 (see Fig. s2†). The bare TiO2, 1Bi-doped TiO2 and 2Bi-
doped TiO2 show a H3 hysteresis loop and the samples 0.25Bi-
doped TiO2 and 0.5Bi-doped TiO2 show a H1 hysteresis loop.
The H1 loop is associated with the mesopores composed by
relatively uniform particles and the H3 loop is oen regarded as
Fig. 5 UV-visible spectra (A) and Kubelka–Munk plot (B) of the
samples. (a) Bare TiO2, (b) 0.25Bi-TiO2, (c) 0.5Bi-TiO2, (d) 1Bi-TiO2, and
(e) 2Bi-TiO2.
attributed to bismuth species could be identied in the proles.
With the increase of the Bi content from 0.25 wt% to 2.00 wt%,
the width of the (101) diffraction peak is broadened and the
intensity is weakened gradually. The average crystal sizes of
anatase becomes smaller and smaller from 9.47 to 7.60 nm,
calculated by the Debye–Scherrer's equation. The lattice
parameters of each sample were calculated and listed in Table
1. It can be observed that the cell volume gradually swells with
increasing the content of bismuth, indicating that Bi was doped
in the lattice of anatase.
Fig. 3 and s1† show TEM images of bare TiO2 and XBi-TiO2
samples. The size of the nanoparticles distributed uniformly
and it becomes smaller and smaller with the increase of the Bi
Table 2 BET specific surface areas, pore volumes and Bi content of catalysts
Pore volume
Pore size
(nm)
Nominal value
n(Bi/Ti)
Actual value
n(Bi/Ti)
Samples
SBET (m2 gꢁ1
)
(m3 gꢁ1
)
Bare TiO2
0.25Bi-TiO2
0.5Bi-TiO2
1Bi-TiO2
100.60
112.30
131.50
106.80
103.50
0.16
0.35
0.41
0.28
0.23
6.65
9.43
9.64
8.62
7.80
0.00
0.25
0.50
1.00
2.00
0.00
0.23
0.44
0.93
1.96
2Bi-TiO2
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than that of the nominal one. Although the pores in the samples
were from the accumulation of the nanoparticles, it is easier to
be dispersed and thus benecial to the reaction.
Fig. 6 shows the normalized FTIR spectra of the samples. It is
considered that the amount of the total surface (Ti, Bi)–O bond
in each sample should be similar in the case that the Bi content
in the sample varied from 0 to 2 wt%. Thus, the intensity vari-
ation of the (Ti, Bi)–O vibrations in the spectrum may be from
the difference of the sample amount in the light spot due to the
inhomogeneous blending of KBr and the sample for FTIR
investigation. In order to compared the surface hydroxyls of
each sample, the stretching vibrations of (Ti, Bi)–O, the band
from 400 to 800 cmꢁ1, of each sample are considered as equal or
very close.45,46 Set the intensity of the Ti–O stretching vibration
of any sample as a standard, and those of the other samples are
normalized to the same level. So the whole spectrum of the
sample other than the standard one may enlarge or reduce the
same times as the Ti–O vibrations of the same sample. Such
a treatment is expected to eliminate the operation errors for
FTIR investigation.
Fig. 6 FTIR spectra of (a) bare TiO2, (b) 0.25Bi-TiO2, (c) 0.5Bi-TiO2, (d)
1Bi-TiO2, and (e) 2Bi-TiO2.
from the slit-like pores.43 However, it can be observed that the
onset of the hysteresis loop starts from 0.8 of the relative
pressure. Such a hysteresis loop is not from structure meso-
pores of the nanoparticles but from the pores of agglomerates
composed by nanoparticles.44 As a result, the isotherms of the
samples just indicates that the particle size of 0.25Bi-doped
TiO2 and 0.5Bi-doped TiO2 was relatively evenly distributed
while that of samples bare TiO2, 1Bi-doped TiO2 and 2Bi-doped
TiO2 was unevenly distributed. The incompact pore structure is
resulted from the supercritical drying process. Table 2 lists the
specic surface area, pore volume, pore size and Bi content of
the catalyst. The actual Bi content in each sample is slightly less
The band around 3400 cmꢁ1 is attributed to the stretching
vibration of the surface hydroxyls.47–49 In comparison of this
band in Fig. 6, it can be observed that more surface hydroxyls on
the Bi-doped samples than those on bare TiO2. The number of
hydroxyls on the Bi-doped samples rises rst and then declines
with the increase of the samples rises rst and then declines
with the increase of the Bi content, and the sample 0.5Bi-doped
TiO2 has the most surface hydroxyls.
Fig. 7 shows the XPS spectra of the catalysts. The binding
energy at 458.4 and 464.2 eV are ascribed to the 2p3/2 and 2p1/2
Fig. 7 XPS spectra of Ti 2p (A), O 1s (B), enlarged O 1s of bare TiO2 (C), and Bi 4f (D). (a) Bare TiO2, (b) 0.25Bi-TiO2, (c) 0.5Bi-TiO2, (d) 1Bi-TiO2, and
(e) 2Bi-TiO2.
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photocatalytic partial oxidation methanol at a temperature
range from 25 ꢀC to 45 ꢀC under the irradiation of visible light.
The catalysts are not active in the temperature range without
irradiation. But temperature exhibits positive effect on meth-
anol conversion, in line with the Arrhenius's rule relating to
temperature and reaction rate, under irradiation, suggesting
photon provides energy to overcome the barrier in the key step
of the reaction and the subsequent steps can occur at low
temperatures. The bare TiO2 exhibits very low activity for the
reaction. This is because the band gap of this sample is 3.1 eV,
corresponding to 400 nm of the absorption wavelength, which
just lies on the edge of the visible light spectrum. The Bi-doped
samples exhibit far better photocatalytic activity than the bare
TiO2. The methanol conversion and MF selectivity of the Bi-
Fig. 8 H2-TPR test of catalyst.
of Ti4+, respectively.50,51 A remarkable shi of the two peaks to
low binding energy can be observed in the spectrum of sample
2Bi-doped TiO2, suggesting Ti4+ in this sample obtained addi-
tional electrons.52 The binding energy of O 1s is shown in
Fig. 7B. Three oxygen species can be identied from the spec-
trum of each sample. Fig. 7C is the enlarged O 1s spectrum of
sample TiO2. The three deconvoluted peaks at 529.6, 531.6 and
533.2 eV are ascribed to oxygen in the framework of TiO2,
oxygen in surface hydroxyl and oxygen in absorbed water,
respectively.53–56 From Fig. 7B, it can be observed that the
binding energy of the framework oxygen in sample 2Bi-doped
TiO2 shis to low binding energy direction, suggesting oxygen
in the framework obtained electron as well.52 Fig. 7D shows the
binding energy of Bi 4f7/2 and 4f5/2 in the samples. From the
deconvoluted spectra, two Bi species can be identied in each
sample. The peaks near 158.7 and 164 eV are ascribed to Bi3+
species, and those near 157 and 162 eV are ascribed to Bi0,
suggesting very little amount of Bi species was reduced to Bi0
during preparation.57,58 With the increase of the Bi content, the
peak near 158.7 eV shi to high value, especially for sample 2Bi-
doped TiO2. Given that the binding energy of the framework Ti
and O in this sample shis to low value, it indicates that bond
Bi–O–Ti might be formed and electron transfer from Bi to O and
Ti might occur in these samples.52
In order to exclude that Bi3+ species was from Bi2O3, the H2-
TPR experiment was carried out for the samples (Fig. 8). For
comparison, additional H2-TPR experiment was performed
using high-purity Bi2O3 at a temperature range from 100 to
800 ꢀC. A broadened hump starting from 350 ꢀC and ending by
670 ꢀC can be observed in the prole, which can be attributed to
the reduction of Bi3+ species in sample Bi2O3.59,60 However, no
reduction peaks can be observed from the proles of the Bi-
doped TiO2 samples, indicating that no Bi2O3 species exist in
these samples. In combination of results from XRD, HRTEM,
Raman and XPS, Bi species was intercalated in the lattice of
TiO2.
Photocatalytic performance
Fig. 9 shows the methanol conversion (Fig. 9a), MF selectivity
Fig.
9 Photocatalytic activity of Bi-TiO2 catalysts, (a) methanol
(Fig. 9b) and MF formation rate (Fig. 9c) of the catalysts for conversion; (b) MF selectivity; (c) MF formation rate.
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TiO2, 0.25Bi-doped TiO2, 0.5Bi-doped TiO2, 1Bi-doped TiO2 and
2Bi-doped TiO2, respectively. Combining the band gap energy of
the samples calculated from the UV-vis investigation, the band
energy diagram can be obtained as shown in Fig. 11.
Density functional theory calculations
Bismuth doped titania prepared by the wet chemical method in
this study is efficient for photocatalytic partial oxidation of
methanol to MF under visible light irradiation. Based on the
series characterizations, bismuth species were intercalated in
the lattice of anatase. However, it is not yet clear where bismuth
exists in the lattice. In order to clarify the possible state of
bismuth in the lattice of titania, the DFT method which was
implemented in the Cambridge Sequential Total Energy
Package (CASTEP) was employed to calculate possible structure,
Fig. 10 Mott–Schottky diagram of the catalyst at 500 Hz in 0.2 M
Na2SO4 solution.
doped TiO2 catalyst are all about 50%, while those of the bare
TiO2 are less than 20% and 35%, respectively, suggesting the
doping of Bi remarkably enhanced the photocatalytic perfor-
mance. However, the photocatalytic performance of the Bi-
doped TiO2 varies with the Bi content in the sample. With the
increase of the Bi content, the MF selectivity rises rst and then
goes down. The 0.5Bi-doped TiO2 exhibits the best MF forma-
tion rate. Such a phenomenon has a close relation with the ne
band structure of the catalyst, which will be further discussed
later. In order to understand the ne band structure of the
catalyst, the at band potentials of the samples were deter-
mined by the electrochemical impedance spectra and calcu-
lated by the Mott–Schottky plot. Fig. 10 presents typical Mott–
Schottky plots constructed from capacitance vs. electrode
potential measurements at 0.5 kHz in 0.2 M Na2SO4 solution. All
samples show a positive slope in Mott–Schottky plots, which
means that they exhibit n-type semiconductor properties.61 The
at band positions of bare TiO2, 0.25Bi-doped TiO2, 0.5Bi-doped
TiO2, 1Bi-doped TiO2 and 2Bi-doped TiO2 were 0.40, 0.51, 0.69,
0.62 and 0.61 V vs. RHE, respectively. In general, the conduction
band is more negative about ꢁ0.1 or ꢁ0.2 eV than the at band
for a lot of n-type semiconductor.62 If ꢁ0.2 eV is taken, the
conduction bands lie on 0.2, 0.31, 0.49, 0.42 and 0.41 V for bare
Fig. 12 Comparison of PDOS, (a) total state density; (b) fractal density
Fig. 11 Band energy diagram of the catalysts with different Bi content. of TiO2; (c) fractal density of Bi-TiO2.
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narrowing of the band gap. The calculated light absorption
spectrum is shown in Fig. S4.† An obvious red-shi can be
observed in the Bi-doped TiO2, and the absorption band extends
to visible light spectrum, which is in line with the experimental
results (Fig. 5). By comparison of the calculated and the exper-
imental results, it can be conrmed that Bi species substitute
some Ti sites to form Bi doped TiO2.
The Bi content in the catalyst has remarkable impact on the
valence band edge (VBE) and surface hydroxyls, and the VBE
and the surface hydroxyls exhibit obvious correlation with the
photocatalytic performance (Fig. 13). The surface hydroxyls
almost show linear relation with the methanol conversion and
the VBE exhibits positive correlation with methanol conver-
sion as well. The MF selectivity exhibits remarkable depen-
dence on both the VBE and the surface hydroxyls. The bare
TiO2 and 2Bi-TiO2 have similar VBE, and the surface hydroxyls
on 2Bi-TiO2 is about 2 times than those on bare TiO2. However,
the bare TiO2 exhibits the lowest methanol conversion of 18%
and MF selectivity of 36%, while 2Bi-TiO2 exhibits the highest
MF selectivity of 53% and relatively high methanol conversion
of 50%. The sample 0.25Bi-TiO2 and sample 1Bi-TiO2 have
almost equal VBE and similar surface hydroxyls, and they also
exhibit similar methanol conversion and MF selectivity. The
sample 1Bi-TiO2 has a little more surface hydroxyls than
sample 1Bi-TiO2 and the VBE of 1Bi-TiO2 is 0.03 eV higher than
that of 2Bi-TiO2, while the MF selectivity of 1Bi-TiO2 is lower
than that of 2Bi-TiO2 although the methanol conversion of 1Bi-
TiO2 is slightly higher. The sample 0.5Bi-TiO2 has the most
surface hydroxyls and VBE, but it exhibits only the highest
methanol conversion but moderate MF selectivity. From the
above facts, it is denite that the surface hydroxyls play
important roles for methanol conversion and MF selectivity.
The VBE plays a part role for MF selectivity. Actually, the
photocatalytic partial oxidation of methanol to MF must
undergo the following important steps.
Fig. 13 VBE, normalized surface hydroxyls as well as methanol
conversion and MF selectivity at 35 ꢀC of catalysts with different Bi
content.
as well as its band structure, density of states and optical
properties.63–67 The calculated results indicate that bismuth
most possibly occupied the sites of Ti substitutively but not was
in the interstitial space of the lattice (see Fig. s3 and s4†). It can
be observed in Fig. S3† that the energy levels comprising the
valence band and conduction band of Bi-doped TiO2 is more
dense than that of pure TiO2, and both the valence band and
conduction band move downward, which is consistent with the
experimental results (Fig. 11). The band gap of the Bi-doped
TiO2 is narrower than that of pure TiO2, which is resulted
from the formation of impurity levels between the valence band
and conduction of TiO2. Fig. 12 shows the total and fractal state
density of pure TiO2 and Bi-doped TiO2. Apparently, the band
positions of the Bi-doped TiO2 lowered and new impurity energy
levels are formed. The conduction and valence bands of pure
TiO2 are mainly composed by the 3d orbitals of Ti and 2p
orbitals of O. The 3d orbitals of Ti play a major role in the
formation of the conduction band, and the 2p orbitals of O
mainly contribute to the formation of the valence band. The 6s
and 6p orbitals of Bi involve in the band formation of Bi-doped
TiO2, and the 6s orbitals of Bi play the major role in the
formation of the impurity level and are responsible for the
Firstly, methanol molecules react with surface hydroxyls to
give rise to methoxy groups. This step can occur at room
temperature without irradiation. Secondly, the methoxy groups
Fig. 14 Reaction mechanism of selective oxidation of methanol to methyl formate with Bi-TiO2 catalyst under simulated sunlight.
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are oxidized to coordinated formaldehyde by the photoexcited surface of catalyst, the coordinated formaldehyde can be further
holes. If the redox potential of the hole is higher than that from oxidized to CO2 and water.
methoxy to formaldehyde the reaction can occur, or the reaction
stops. However, if the redox potential is far higher than
required, deep oxidation may occur, resulting in low MF selec-
Conclusions
tivity. Thirdly, the methoxy group reacts with coordinated A series of Bi-TiO2 catalysts were prepared by wet chemical
formaldehyde through coupling reaction to give rise to MF or is method. The catalysts exhibit visible light photoactive to partial
deeply oxidized to CO2.68 The surface hydroxyls lost during the oxidation of methanol to MF. Bismuth is intercalated in the
reaction can be recovered by oxygen in ow gas lling the lattice of anatase by substitution of titanium. Impurity levels are
oxygen vacancies and combining with hydrogen from methoxy formed in the valence band, conduction band as well as
in the second step. Apparently, surface hydroxyls are important between the two bands. The Bi 6s and 5p orbitals contribute to
intermediate reactant, the more surface hydroxyls the faster the the formation of the impurity levels. The Bi doped TiO2
reaction proceeds.49 This is why the methanol conversion is enhances the surface hydroxyls, reduces the band gaps and
proportional to the surface hydroxyls. The photocatalytic raises the VBEs. The reduced band gap makes the catalyst
oxidation of methanol can occur on bare TiO2, although it has respond to visible light. The surface hydroxyls are benecial to
the lowest VBE, indicating the VBEs of all the Bi-doped catalysts the methanol conversion, and the rise of VBE enhances the
can generate photoexcited holes that enable to oxidize methoxy. redox potential of the photogenerated holes. Only moderate
The second step and the third step determine the MF selectivity. redox potentials and sufficient surface hydroxyls can result in
In order to obtain high MF selectivity, there must be sufficient high methanol conversion and MF selectivity.
methoxy groups being able to closely contact with coordinated
formaldehyde on the surface of the catalyst. So, over strong
redox potential of the holes will result in excess formaldehyde
Conflicts of interest
and lean methoxy on the surface of catalyst, and thus low MF There are no conicts to declare.
selectivity. A moderate redox potential of the photogenerated
holes and sufficient surface hydroxyls can maintain reasonable
distribution of methoxy groups and coordinated formaldehyde
Acknowledgements
to achieve high methanol conversion and MF selectivity. This is This project was funded by the National Science Foundation of
why 2Bi-TiO2 exhibits the highest MF selectivity but the lowest China (No. 21766020, No. 21566026), the Natural Science
methanol conversion, while 0.5Bi-TiO2 exhibits the high meth- Foundation of Inner Mongolia (No. 2019MS02011), the contin-
anol conversion but moderate MF selectivity.
uously supporting project of Grassland Talent of Inner Mon-
In addition, all of the Bi-doped samples display stronger golia and the major basic research and open project of the Inner
photocurrent density (see Fig. S6†) than bare TiO2. The 0.5Bi- Mongolia Autonomous Region (30500-515330303).
TiO2 exhibits the best photocurrent, implying the best photo
response. With increasing the Bi content, the photocurrent
density increases rst and then decrease. The worse of photo
Notes and references
response for the higher Bi content samples may be resulted
from the charge trapping effect.69 The relation between the
photocurrent density and irradiation time is in line with valence
band edges of the catalysts.
1 S. P. Tonner, D. L. Trimm, M. S. Wainwright and N. W. Cant,
Ind. Eng. Chem. Prod. Res. Dev., 1984, 23, 384–388.
2 E. Tronconi, A. S. Elmi, N. Ferlazzo, P. Forzatti, G. Busca and
P. Tittarelli, Ind. Eng. Chem. Res., 1987, 26, 1269–1275.
3 A. S. Elmi, E. Tronconi, C. Cristiani, J. P. Gomez Martin,
P. Forzatti and G. Busca, Ind. Eng. Chem. Res., 1989, 28,
387–393.
Photocatalytic mechanism
The reaction mechanism of photocatalytic partial oxidation of
methanol to MF on Bi-doped TiO2 is similar to those in litera-
ture, including the steps mentioned above, except the photo-
excited process under visible light irradiation in Bi-doped TiO2.
The schematic diagram is shown in Fig. 14. The photons in the
band of visible light spectrum is absorbed by electrons in the
valence band of Bi-doped TiO2, and the electrons are excited to
the conduction band via the impurity level, mainly composed by
the 6s orbitals of Bi. The conduction band is mainly composed
by the O 2p orbitals and a small part of Ti 3d and Bi 6p orbitals.
The overcharged surface can adsorb and dissociate oxygen
4 P. J. A. Tijm, F. J. Waller and D. M. Brown, Appl. Catal., A,
2001, 221, 275–282.
5 J. S. Lee, J. C. Kim and Y. G. Kim, Appl. Catal., 1990, 57, 1–30.
6 L. Rong, Z. Xu, J. Sun and G. Guo, J. Energy Chem., 2018, 27,
238–242.
7 C. Han, X. Yang, G. Gao, J. Wang, H. Lu, J. Liu, M. Tong and
X. Liang, Green Chem., 2014, 16, 3603–3615.
8 J. Liu, C. Han, X. Yang, G. Gao, Q. Shi, M. Tong, X. Liang and
C. Li, J. Catal., 2016, 333, 162–170.
9 X. Yang, A. Zhang, G. Gao, D. Han, C. Han, J. Wang, H. Lu,
J. Liu and M. Tong, Catal. Commun., 2014, 43, 192–196.
molecules into active oxygen atoms to ll the surface oxygen 10 J. C. Colmenares, P. Lisowski, D. Łomot, O. Chernyayeva and
vacancies resulted from the loss of surface hydroxyls during the D. Lisovytskiy, Chemsuschem, 2015, 8, 1676–1685.
reaction. Besides, H from the methanol or methoxy can react 11 H. Kominami, H. Sugahara and K. Hashimoto, Catal.
with hydroxyl to form water, in the case of lean methoxy on the
Commun., 2010, 11, 426–429.
31450 | RSC Adv., 2020, 10, 31442–31452
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
12 C. Li, X. Yang, G. Gao, Y. Li, W. Zhang, X. Chen, H. Su, 38 G. D. Bhowmick, M. T. Noori, I. Das, B. Neethu,
S. Wang and Z. Wang, Catal. Sci. Technol., 2019, 9, 6240–
6252.
M. M. Ghangrekar and A. Mitra, Int. J. Hydrogen Energy,
2018, 43, 7501–7510.
13 Y. C. Pu, G. Wang, K. D. Chang, Y. Ling, Y. K. Lin, 39 T. Ohsaka, F. Izumi and Y. Fujiki, J. Raman Spectrosc., 1978,
B. C. Fitzmorris, C. M. Liu, X. Lu, Y. Tong, J. Z. Zhang,
Y. J. Hsu and Y. Li, Nano Lett., 2013, 13, 3817–3823.
14 Y. C. Pu, Y. Ling, K. D. Chang, C. M. Liu and Y. Li, J. Phys.
Chem. C, 2014, 118, 15086–15094.
7, 321–324.
40 W. Zhao, X. Wang, H. Sang and K. Wang, Chin. J. Chem.,
2013, 31, 415–420.
´
´
41 R. Lopez and R. Gomez, J. Sol-Gel Sci. Technol., 2011, 61, 1–7.
´
´
15 Y.-H. Chiu and Y.-J. Hsu, Nano Energy, 2017, 31, 286–295.
16 Y. H. Chiu, T. H. Lai, C. Y. Chen, P. Y. Hsieh, K. Ozasa,
42 S. Murcia-Lopez, M. C. Hidalgo and J. A. Navıo, Appl. Catal.,
A, 2011, 404, 59–67.
M. Niinomi, K. Okada, T. M. Chang, N. Matsushita, 43 K. S. W. Sing, Pure Appl. Chem., 1982, 54, 2201–2218.
M. Sone and Y. J. Hsu, ACS Appl. Mater. Interfaces, 2018, 44 S. W. K. Sing, Pure Appl. Chem., 1985, 57, 603–619.
10, 22997–23008.
45 C. Y. Yen, Y. F. Lin, C. H. Hung, Y. H. Tseng, C. C. Ma,
M. C. Chang and H. Shao, Nanotechnology, 2008, 19, 045604.
46 H. Zuo, J. Sun, K. Deng, R. Su, F. Wei and D. Wang, Chem.
Eng. Technol., 2007, 30, 577–582.
47 C. P. Sibu, S. R. Kumar, P. Mukundan and K. G. K. Warrier,
Chem. Mater., 2002, 14, 2876–2881.
48 V. Augugliaro, S. Coluccia, V. Loddo, L. Marchese and
M. Schiavello, Appl. Catal., B, 1999, 20, 15–27.
49 K. L. Yeung, S. T. Yau, A. J. Maira, J. M. Coronado, J. Soria
and P. L. Yue, J. Catal., 2003, 20, 107–116.
17 J.-M. Li, C.-W. Tsao, M.-J. Fang, C.-C. Chen, C.-W. Liu and
Y.-J. Hsu, ACS Appl. Nano Mater., 2018, 1, 6843–6853.
18 Y.-H. Chiu, T.-F. M. Chang, C.-Y. Chen, M. Sone and
Y.-J. Hsu, Catalysts, 2019, 9, 430–462.
19 P. Y. Hsieh, Y. H. Chiu, T. H. Lai, M. J. Fang, Y. T. Wang and
Y. J. Hsu, ACS Appl. Mater. Interfaces, 2019, 11, 3006–3015.
20 M.-J. Fang, C.-W. Tsao and Y.-J. Hsu, J. Phys. D: Appl. Phys.,
2020, 53(14), 143001.
21 P.-Y. Hsieh, J.-Y. Wu, T.-F. M. Chang, C.-Y. Chen, M. Sone
and Y.-J. Hsu, Arabian J. Chem., 2020, DOI: 10.1016/ 50 D. O. Scanlon, C. W. Dunnill, J. Buckeridge, S. A. Shevlin,
j.arabjc.2020.05.025.
22 J. Cunninghamand and B. K. Hodnett, J. Chem. Soc., Faraday
Trans., 1981, 77, 2777–2801.
23 Y. C. Liu, G. L. Griffin, S. S. Chan and I. E. Wachs, J. Catal.,
1985, 94, 108–119.
A. J. Logsdail, S. M. Woodley, C. R. Catlow, M. J. Powell,
R. G. Palgrave, I. P. Parkin, G. W. Watson, T. W. Keal,
P. Sherwood, A. Walsh and A. A. Sokol, Nat. Mater., 2013,
12, 798–801.
51 A. Sanchez-Martinez, O. Ceballos-Sanchez, C. Koop-Santa,
´
˜
´
24 P. Lisowski, J. C. Colmenares, D. Łomot, O. Chernyayeva and
D. Lisovytskiy, J. Mol. Catal. A: Chem., 2016, 411, 247–256.
E. R. Lopez-Mena, E. Orozco-Guareno and M. Garcıa-
Guaderrama, Ceram. Int., 2018, 44, 5273–5283.
25 G. Ramis, G. Busca and V. Lorenzelli, J. Chem. Soc. , Faraday 52 H. Li, D. Wang, P. Wang, H. Fan and T. Xie, Chem.–Eur. J.,
Trans., 1987, 183, 1591–1599.
2009, 15, 12521–12527.
26 A. Fujishima and K. Honda, Nature, 1972, 238, 37–38.
27 U. Alam, M. Fleisch, I. Kretschmer, D. Bahnemann and
M. Muneer, Appl. Catal., B, 2017, 218, 758–769.
28 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga,
Science, 2001, 293, 269–271.
53 D. Chen, D. Yang, Q. Wang and Z. Jiang, Ind. Eng. Chem. Res.,
2006, 45, 4110–4116.
54 Z. Zhang, J. Long, X. Xie, H. Zhuang, Y. Zhou, H. Lin,
R. Yuan, W. Dai, Z. Ding, X. Wang and X. Fu, Appl. Catal.,
A, 2012, 425–426, 117–124.
29 Y. Hu, Y. Cao, P. Wang, D. Li, W. Chen, Y. He, X. Fu, Y. Shao 55 A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri,
and Y. Zheng, Appl. Catal., B, 2012, 125, 294–303.
S. Cappelli, C. L. Bianchi, R. Psaro and V. Dal Santo, J. Am.
Chem. Soc., 2012, 134, 7600–7603.
56 T. K. Sham and M. S. Lazarus, Chem. Phys. Lett., 1979, 68,
426–432.
30 Y. Huang, J.-J. Cao, F. Kang, S.-J. You, C.-W. Chang and
Y.-F. Wang, Aerosol Air Qual. Res., 2017, 17, 2555–2565.
31 L. Kumaresan, A. Prabhu, M. Palanichamy, E. Arumugam
and V. Murugesan, J. Hazard. Mater., 2011, 186, 1183–1192. 57 J. Ma, J. Chu, L. Qiang and J. Xue, RSC Adv., 2012, 2, 3753–
32 M. Schraml-Marth, A. Wokaun and A. Baiker, J. Catal., 1990,
124, 86–96.
33 A. Khaleel, M. Meetani, N. S. Al Kaabi, K. Abdul al, S. Salem,
3758.
58 W. Yuqi, L. Gongxuan and L. Shuben, J. Phys. Chem. C, 2009,
2, 9950–9955.
S. Mansour, S. H. Ahmed, M. Bakiro, T. Ramachandran, 59 M. Kang, Y.-R. Ko, M.-K. Jeon, S.-C. Lee, S.-J. Choung,
F. Hamed and A. Alzamly, J. Water Reuse Desalin., 2019, 9,
31–46.
J.-Y. Park, S. Kim and S.-H. Choi, J. Photochem. Photobiol.,
A, 2005, 173, 128–136.
34 P. A. K. Reddy, B. Srinivas, P. Kala, V. D. Kumari and 60 Y. Xie, C. Zhang, D. Wang, J. Lu, Y. Wang, J. Wang, L. Zhang
M. Subrahmanyam, Mater. Res. Bull., 2011, 46, 1766–1771. and R. Zhang, New J. Chem., 2019, 43, 15368–15374.
35 S. Sood, S. K. Mehta, A. Umar and S. K. Kansal, New J. Chem., 61 S. B. Taieb, I. B. Assaker, A. Bardaoui, M. Gannouni,
2014, 38, 3127–3136.
36 S. Sajjad, S. A. Leghari, F. Chen and J. Zhang, Chem.–Eur. J.,
2010, 16, 13795–13804.
37 P. Magesan, P. Ganesan and M. J. Umapathy, Optik, 2016, 16,
5171–5180.
A. Souissi, S. Nowak, L. Mouton, S. Ammar and
R. Chtourou, Int. J. Hydrogen Energy, 2016, 41, 6230–6239.
62 L. Li, J. Yan, T. Wang, Z. J. Zhao, J. Zhang, J. Gong and
N. Guan, Nat. Commun., 2015, 6, 5881.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 31442–31452 | 31451

View Article Online
RSC Advances
Paper
63 V. Milman, K. Refson, S. J. Clark, C. J. Pickard, J. R. Yates, 67 C. Di Valentin, E. Finazzi, G. Pacchioni, A. Selloni and
S. P. Gao, P. J. Hasnip, M. I. J. Probert, A. Perlov and
M. D. Segall, J. Mol. Struct.: THEOCHEM, 2010, 954, 22–35.
64 S. J. Clark, M. D. Segall, C. J. Pickardii, P. J. Hasnipiii and
M. I. J. Probertiv, Z. Kristallogr., 2005, 220, 567–570.
65 J. E. Jaffe, Z. Lin and A. C. Hess, Phys. Rev. B, 1998, 57, 11834–
11837.
E. Giamello, Chem. Mater., 2008, 20, 3706–3714.
68 K. R. Phillips, S. C. Jensen, M. Baron, S. C. Li and
C. M. Friend, J. Am. Chem. Soc., 2013, 135, 574–577.
69 Y. H. Lu, W. H. Lin, C. Y. Yang, Y. H. Chiu, Y. C. Pu,
M. H. Lee, Y. C. Tseng and Y. J. Hsu, Nanoscale, 2014, 6,
8796–8803.
66 K. Choudhary and F. Tavazza, Comput. Mater. Sci., 2019, 161,
300–308.
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