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amorphous titania coating which selectively oxidizes benzyl alco-
hol to benzaldehyde [27], but all require complex and multistage
synthetic methods for their preparation.
Monoclinic bismuth vanadate, BiVO4, has a band gap of 2.4 eV
and has been used extensively for visible light photoelectrochem-
ical water oxidation under bias [28–32]. Of relevance to this work
bismuth vanadate has been used for the selective photocatalytic
oxidation of benzyl amines to their corresponding imines (up to
97% conversion, 91% selectivity in 7 h) using a broad band Xe lamp
and a 420 nm filter [33].
Our motivation for this work was to investigate photocatalytic
synthesis using inexpensive LED visible light sources and earth
abundant metal oxides for selective oxidation. In related visible
light photocatalysis with metal oxides, low conversions are often
observed and attributed to rapid charge carrier recombination
which is common for metal oxides. Therefore, we also targeted
nanostructured materials to increase the probability of charge car-
rier diffusion to the surface. The key features which control the
selectivity of heterogeneous photocatalytic oxidation are also
poorly understood.
Herein we report the synthesis of BiVO4 nanoparticles (nan-
BiVO4) using a simple hydrothermal method and their application
to the aerobic photooxidation of benzyl alcohols to benzaldehydes.
Illumination with a blue LED gave benzaldehydes in high yield and
selectivity. The activity of nan-BiVO4 under these conditions was
shown to be 10 times greater than titanium dioxide and 30 times
greater than bulk BiVO4. Importantly, it was also observed that
conversion is limited by even small quantities of benzoic acid
and that the photoreaction is significantly inhibited by over-
oxidation.
evaporate. Brunauer, Emmett and Teller (BET) N2 surface area anal-
ysis was performed on samples dried under nitrogen for 6 h at
80 °C, and nitrogen adsorption isotherms were measured at 78 K
on a Micromeritics Tristar 3000. 1H Nuclear magnetic resonance
(NMR 400 MHz) spectra were recorded on a JEOL ECX400 and
ECS400 spectrometers at room temperature. Chemical shifts are
referenced to the deuterium lock of residual solvent. Gas Chro-
matography (GC) was performed on an Agilent/HP 6890, with an
injection volume of
1 lL, using helium as a carrier gas at
1 mL minÀ1, a flame ionisation detector at 250 °C, and Chrompack
DB-5 ms column between 90 and 300 °C with ramp rate
20 °C minÀ1. Time resolved photoluminescence was performed on
an Edinburgh Photonics FLS 980 spectrometer, irradiating
a
1 mg mLÀ1 acetonitrile dispersion with an Edinburgh instruments
picosecond pulse light emitting diode, k = 380 nm. Photocatalysis
was carried out using an Et Lumiere 30 W blue LED array,
kmax
= 470 nm, krange = 400–560 nm, with an irradiance of
245 mW cmÀ2 measured using
Photometer at a distance of 2 cm.
a
ITL 1400-A Radiometer
2.3. Catalyst preparation
Synthesis of BiVO4 nanoparticles (nan-BiVO4): nan-BiVO4 was
prepared by a modified literature method [34]. Bismuth nitrate
pentahydrate (4.85 g, 10 mmol) and EDTA (2.93 g, 10 mmol) were
added to 2 M nitric acid (100 mL) and stirred for 30 min until clear.
Ammonium metavanadate (1.17 g, 10 mmol) was added to this
solution and stirred for 2 h giving a green-yellow solution. The
solution was heated at 90 °C for 6 h in a Teflon-lined autoclave
and after cooling to room temperature the resulting mixture was
centrifuged at 4000 rpm for 30 min to yield a yellow powder,
which was washed alternately with distilled water and ethanol
and then dried overnight at 60 °C. Yield = 0.420 mg (13.0%).
Synthesis of bulk BiVO4 [35]: Bismuth nitrate pentahydrate
(5.53 g, 12 mmol) and ammonium vanadate (1.41 g, 12 mmol)
were dissolved in conc. nitric acid (20 mL) and 5 M NaOH
(20 mL) separately. After stirring for 30 min, these solutions were
mixed forming a yellow precipitate. The resulting mixture was
placed in 3 Â 23 mL Teflon-lined autoclaves and heated to 240 °C
for 16 h. After cooling to room temperature the resulting mixtures
were combined and centrifuged at 4000 rpm for 30 min to yield a
yellow powder, which was washed with distilled water and then
dried overnight at 60 °C. Yield = 1.34 g (34.5%).
2. Experimental
2.1. Materials
Bismuth nitrate pentahydrate (98%), ethylenediaminete-
traacetic acid (EDTA, 98.5%), 4-methoxybenzyl alcohol (98%),
4-methylbenzyl alcohol (98%), 4-ethylbenzyl alcohol (99%),
4-isopropylbenzyl alcohol (97%), biphenyl-4-methanol (98%),
4-chlorobenzyl alcohol (99%), 4-bromobenzyl alcohol (99%),
4-iodobenzyl alcohol (97%), 4-trifluoromethylbenzyl alcohol
(98%), 4-nitrobenzyl alcohol (99%), 4-hydroxybenzyl alcohol
(99%), benzyl alcohol (99%), 4-nitrobenzaldehyde (99%),
4-methoxybenzaldehyde (99%), methyl 4-formylbenzoate (99%),
4-(trifluoromethyl)benzaldehyde (98%) and cuminaldehyde (98%)
(all Sigma-Aldrich), acetonitrile (Fisher Scientific), methyl
(4-hydroxymethyl) benzoate (99%) and 4-acetamido benzyl alco-
hol (97%) (Alfa Aesar), ammonium metavanadate (98%) (Riedel-
de Haen) and P25 titanium dioxide (Degussa), were used as
received.
2.4. Photocatalytic reactions
Bismuth vanadate (32.3 mg, 100 mmol) was added to a Schlenk
flask containing benzyl alcohol stock solution (1 mL, 0.1 mmol in
acetonitrile) and acetonitrile (9 mL). The mixture was left to stir
for 30 min to disperse the catalyst under a dioxygen atmosphere
via a balloon. The mixture was then irradiated with a 30 W blue
LED array at a distance of 2 cm with an irradiance of 245 mW cmÀ2
.
2.2. Characterisation
The mixture reached ca. 40 °C by the end of the reaction and after
irradiation, the catalyst was removed using centrifugation at
4000 rpm for 30 min. For GC analysis, 1 mL of supernatant was
Powder X-ray Diffraction (PXRD) data was acquired using a
Bruker-AXS D8 Advance instrument fitted with a Lynxeye detector
and acquired with Cu Ka radiation between 10 and 70° 2h with a
taken and 1 lL injected. For NMR analysis, the supernatant was
reduced in volume using a rotary evaporator at 65 mbar at 20 °C,
and the residue dissolved in d6-DMSO containing maleic acid as
an internal standard.
0.02° step size. UV–vis Diffuse Reflectance Spectra (DRS) were
recorded on an Ocean Optic Inc. HR2000+ High Resolution Spec-
trometer. Scanning Electron Microscopy (SEM) was performed on
a FEI Sirion scanning electron microscope and a JEOL Schottky field
emission scanning electron microscope, at an accelerating voltage
of 15 kV. Bulk BiVO4 samples were prepared by spreading a small
amount of powder onto carbon tape mounted on an aluminium
stub. Bismuth vanadate nanoparticles samples were dispersed in
ethanol (1 mg mLÀ1), this dispersion was dropped onto a copper
TEM grid and left at room temperature to allow the solvent to
3. Results and discussion
3.1. Synthesis and characterisation of nan-BiVO4
Photocatalytic efficiency is reduced by electron-hole recombi-
nation processes which can be partly characterised by the bulk