Efficient Solar-Driven Hydrogen Transfer by Bismuth-Based Photocatalyst with Engineered Basic Sites

Article abstract of DOI:10.1021/jacs.8b09796

Photocatalytic organic conversions involving a hydrogen transfer (HT) step have attracted much attention, but the efficiency and selectivity under visible light irradiation still needs to be significantly enhanced. Here we have developed a noble metal-free, basic-site engineered bismuth oxybromide [Bi₂₄O₃₁Br₁₀(OH)] that can accelerate the photocatalytic HT step in both reduction and oxidation reactions, i.e., nitrobenzene to azo/azoxybenzene, quinones to quinols, thiones to thiols, and alcohols to ketones under visible light irradiation and ambient conditions. Remarkably, quantum efficiencies of 42% and 32% for the nitrobenzene reduction can be reached under 410 and 450 nm irradiation, respectively. The Bi₂₄O₃₁Br₁₀(OH) photocatalyst also exhibits excellent performance in up-scaling and stability under visible light and even solar irradiation, revealing economic potential for industrial applications.

Full text of DOI:10.1021/jacs.8b09796

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Article
Efficient Solar Driven Hydrogen Transfer by Bismuth
Based Photocatalyst with Engineered Basic Sites
Yitao Dai, Chao Li, Yanbin Shen, Shujie Zhu, Mathias S. Hvid, Lai-Chin Wu, Jørgen Skibsted,
Yongwang Li, J. W. (Hans) Niemantsverdriet, Flemming Besenbacher, Nina Lock, and Ren Su
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09796 • Publication Date (Web): 05 Nov 2018
Downloaded from http://pubs.acs.org on November 5, 2018
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Efficient Solar Driven Hydrogen Transfer by Bismuth Based Photocatalyst
with Engineered Basic Sites
Yitao Dai,1,2 Chao Li,1 Yanbin Shen,3 Shujie Zhu,4 Mathias S. Hvid,2 Lai-Chin Wu,5
Jørgen Skibsted,2,6 Yongwang Li,1 J. W. (Hans) Niemantsverdriet,1,7 Flemming Besenbacher,2
Nina Lock,2,6 Ren Su1*
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1SynCat@Beijing, Synfuels China Technology Co. Ltd., Leyuan South Street II, No.1, Yanqi
Economic Development Zone C#, Huairou District, Beijing, 101407, China.
2Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, DK-
8000 Aarhus C, Denmark.
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3Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), No. 398 Ruoshui Road, Suzhou
Industrial Park, Suzhou, Jiangsu Province, 215123, China.
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4Department of Drug Design and Pharmacology, Faculty of Health and Medical
Sciences, University of Copenhagen, Universitetsparken 2, Copenhagen Ø 2100, Denmark.
5NSRRC, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu, 30076, Taiwan
6Dept. of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark.
7SynCat@DIFFER, Syngaschem BV, 6336 HH Eindhoven, The Netherlands
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ABSTRACT:
Photocatalytic organic conversions involving a hydrogen transfer step (HT) have attracted much
attention, but the efficiency and selectivity under visible light irradiation still needs to be
significantly enhanced. Here we have developed a noble metal free, basic-site engineered
bismuth oxybromide [Bi24O31Br10(OH)δ] that can accelerate the photocatalytic HT step in both
reduction and oxidation reactions, i.e., nitrobenzene to azo/azoxybenzene, quinones to quinols,
thiones to thiols, and alcohols to ketones under visible light irradiation and ambient conditions.
Remarkably, quantum efficiencies of 42% and 32% for the nitrobenzene reduction can be
reached under 410 and 450 nm irradiation, respectively. The Bi24O31Br10(OH)δ photocatalyst also
exhibits excellent performance in up-scaling and stability under visible light and even solar
irradiation, revealing economic potential for industrial applications.
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INTRODUCTION
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The synthesis of important organic chemicals is essential for the production of dyes, electronics,
pigments, drugs, and foods.1 Conventional mass production of these organic compounds often
requires specialized protocols that are energy costly and environmentally unfriendly.2
Heterogeneous and homogeneous catalytic approaches have shown to be more environmentally
friendly, however, expensive catalysts and oxidants/reductants are required in most cases, which
limits their applications in industry.3-6
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The photocatalytic approach for organic synthesis has attracted much attention, as it offers the
possibility of using solar energy to drive various important chemical transformations, including
hydrogenation/dehydrogenation, N-N/C-C coupling, and Diels-Alder reactions.7-14 The
selectivity of organic photo-conversion can be also controlled, making photocatalysis an
interesting tool for synthetic applications.15-17 For example, the selective oxidation of various
primary alcohols to the corresponding aldehydes or ketones can be realized using the TiO2 based
or plasmonic metal nanoparticle photocatalyst.18,19 Meanwhile, the selective reduction of
nitroaromatics to the corresponding amines, azo-, and azoxyaromatics has been also
achieved.20,21 However, the quantum efficiency generally remains low under visible light
irradiation, ambient conditions and room temperature for most of the organic conversions, even
when using supported noble metal photocatalysts.22-25 This issue becomes more detrimental
when hydrogen transfer (HT) steps are involved in the reaction.17,26 Moreover, the unwanted
recombination of hydrogen atoms in the form of H2 may also compete with the HT reaction steps,
according to the Sabatier’s Principle.27,28 Consequently, it is still a challenge to discover a
versatile photocatalyst that can drive both photo-reduction and photo-oxidation efficiently under
mild conditions (i.e., oxidant/reductant free, room temperature, and neutral pH).29-33 Although
some noble metal loaded photocatalysts show reasonable performance in alcohol oxidation, the
cost of the material and the inconvenient operation condition (i.e., pressurized O2 and base)
render their large-scale applications difficult.12,19,34
Since base catalysts have been used frequently in hydrogenation/dehydrogenation processes
to increase the hydrogen abstraction rate,35,36 we anticipate that a photocatalyst coupled with
engineered surface basic sites could boost the HT step, thus improving the photocatalytic
performance. Although some early attempts have been reported on surface modification to
anchor basic sites on a photocatalyst,37-39 the enhancement in photocatalytic activity is usually
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limited or even negative. This may result from the strong interaction between the abstracted H
atoms with the photocatalyst,40,41 making the consecutive HT step from the catalyst to the H
acceptor thermodynamically unfavorable. Therefore, designing photocatalysts with optimized
free adsorption energies of the hydrogen atoms and adequate basicity to facilitate the HT step
under visible light irradiation is crucial for realizing efficient solar driven organic synthesis.
Herein, we demonstrate that the visible light response bismuth oxybromide with engineered
surface basic sites [Bi24O31Br10(OH)δ, (Fig. 1a)] is a promising material for performing
photocatalytic redox reactions involving HT steps under ambient conditions. The basic sites (OH
and O atoms with low coordination number) on the photocatalyst surface can promote the
oxidation of alcohols (R-OH), which will donate protons to the surface basic sites that further
accept conduction band electrons to form metastable surface adsorbed H (Hads) under irradiation
(Fig. 1b). After irradiation, these Hads atoms can rapidly be scavenged by hydrogen acceptors (A)
to complete the reaction, rather than being released as H2 gas (Fig. 1c). The efficiency and
selectivity for organic synthesis reactions involving HT steps can thus be significantly enhanced
under mild conditions by functionalizing the catalyst with engineered surface basic sites.
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Fig. 1 Design strategy & working principle of the Bi24O31Br10(OH)δ. a, Model of the two-
dimensional material that is characterized by two types of basic sites (surface OH and O atoms
with low coordination number). b, Upon visible light excitation, the basic sites can promote the
hydrogen extraction from alcohols to form Hads atoms that are attached to the under coordinated
O. c, The adsorption energy of Hads on this photocatalyst is optimized for the HT reaction rather
than for the H2 evolution.
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RESULTS AND DISCUSSION
Physical & chemical properties. The chemical composition of the as-synthesized material is
very close to Bi24O31Br10 according to X-ray photoelectron spectroscopy (XPS) and energy-
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dispersive X-ray (EDX) spectroscopy (Fig. S1-3). Temperature programmed desorption (TPD)
of CO2 reveals that the as-synthesized Bi24O31Br10(OH)δ is characterized by a broad desorption
peak, suggesting various sites with different basicity of the material (Fig. 2a). The existence of
basic sites is further confirmed by CO2-IR (Fig. S4). In comparison, the acidified
Bi24O31Br10(OH)δ (Acid-Bi24O31Br10) shows no CO2 desorption while the BiOBr only shows
CO2 desorption at elevated temperatures. The much more intense CO2 desorption peak observed
for Bi24O31Br10(OH)δ indicates that more basic sites are available for catalytic reactions
compared to e.g. BiOBr. It is also worth noting that g-C3N4 and other photocatalysts show a
negligible basicity (Fig. S5), which agrees with our previous study demonstrating that these
photocatalysts exhibit poor activity under neutral pH conditions.17 In addition, the O 1s spectra
can be deconvoluted into the lattice oxygen (529.4 eV) and the hydroxyl oxygen (530.9 eV) for
both Bi24O31Br10(OH)δ and the BiOBr reference (Fig. 2b).42 The significantly higher ratio of
hydroxyl/lattice oxygen observed for Bi24O31Br10(OH)δ compared to that of BiOBr agrees well
with the CO2 TPD, confirming that hydroxyl groups are presented at the Bi24O31Br10(OH)δ
surface. The high surface density of basic sites in the Bi24O31Br10(OH)δ photocatalyst can be
ascribed to the nature of this material. As reported previously, the structure of Bi24O31Br10 is
characterized by 2- and 3-coordinated surface oxygen as the intrinsic basic sites.43 More
importantly, our unique synthesis procedure that is performed under basic conditions (4 M KOH
aqueous) results in the partial dissolution of Br- from the catalyst surface and is eventually
replaced by OH-.
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The Bi24O31Br10(OH)δ is consists of curly nanosheets as revealed by the transmission electron
microscopic (TEM) analysis (Fig. 2c). Nitrogen adsorption analysis confirms the porous
architecture of the material with pore sizes in a wide range from ~30 nm to >100 nm (Fig. S6).
Such structure provides a high concentration of accessible surface sites and is beneficial for the
diffusion of bulky reactants (i.e., aromatic compounds or chain structured alkyl molecules). The
sample presents two types of lattice fringes with interplanar spacing distances of 0.278 and 0.299
nm according to the high resolution TEM (HRTEM) image (Fig. 2d), which match to the (117)
and (213) lattice planes of crystalline Bi24O31Br10(OH)δ reported previously.43 The X-ray
diffraction pattern (XRD, Fig. 2e) of the as-synthesized photocatalyst shows that all peaks can be
indexed to the monoclinic phase of Bi24O31Br10 (JCPDS card No. 75-0888). Notably, the (002)
peak (at ~ 9o) of our material is shifted to a lower angle in comparison with most reported
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literature values (at 10.6o),44,45 indicating an increased interplanar distance in the material along
the c-axis. This may be ascribed to intercalation of some OH- anions partially substituting the Br-
between the atomic layers as the sample is prepared under a strong basic condition.
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Fig. 2 Characterization of the Bi24O31Br10(OH)δ photocatalyst in comparison with related
materials. a, CO2-TPD of Bi24O31Br10(OH)δ, acidified Bi24O31Br10(OH)δ (Acid-Bi24O31Br10),
BiOBr and g-C3N4. b, XPS O1s spectra of Bi24O31Br10(OH)δ and BiOBr. c, TEM image (scale
bar 500 nm) and d, HRTEM image (scale bar 5 nm) of the Bi24O31Br10(OH)δ. e, XRD patterns of
the Bi24O31Br10(OH)δ and BiOBr. The diffraction pattern of monoclinic Bi24O31Br10 is presented
for comparison (JCPDS card No. 75-0888) f, PDF analysis based on synchrotron total scattering
data of the Bi24O31Br10(OH)δ in comparison with simulated PDFs of other materials with similar
compositions.
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The local structure of Bi24O31Br10(OH)δ has been further proved by the pair distribution
function (PDF) analysis of synchrotron X-ray total scattering data (Fig. 2f). The excellent match
between the PDF of Bi24O31Br10(OH)δ and the simulated Bi24O31Br10 pattern suggests that
compounds with similar compositions can be ruled out (BiOBr, Bi5O7Br, Bi4O5Br2, and
Bi3O4Br). The PDF do not provide information on OH groups due to the short O-H correlation
length and the weak scattering power of hydrogen. However, we can still conclude that the
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concentration of OH is relatively low in the material, as a small variation in Br concentration and
coordination caused by the OH substitution will significantly change the scattering pattern (see
the difference between Bi3O4Br, Bi24O31Br10, and Bi4O5Br2).
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The as-prepared Bi24O31Br10(OH)δ is characterized by an optical bandgap of 2.5 eV with an
absorption edge extended to the green light region according to the diffuse reflectance spectrum
(DRS, Fig. 3a), whereas the BiOBr reference can only absorb UV light to 375 nm. The flatband
potential (VFb) is estimated to be -0.57 V (vs. Ag|AgCl, pH = 7) according to the Mott-Schottky
analysis (Fig. 3b), which corresponds to a conduction band minimum of ~ -0.8 V and a valence
band maximum of ~ 1.7 V (vs. standard hydrogen electrode, SHE). We have further estimated
the impurity concentration in Bi24O31Br10(OH)δ compared to the BiOBr using the doping density
(Nd) derived from the slope of the Mott-Schottky plot.45 Since the two materials have a similar
dielectric constant (ε), the Nd of Bi24O31Br10(OH)δ should be two orders of magnitude smaller
than that of BiOBr according to the values of their slopes. Such low impurity concentration in
Bi24O31Br10(OH)δ is beneficial in reducing the charge carrier recombination, thus promoting the
photocatalytic performance. The band positions of the Bi24O31Br10(OH)δ derived from the DRS
and the Mott-Schottky plot are summarized in Fig. 3(c) along with some representative redox
potentials of involved reactions.
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Fig. 3 Optical and bands position characterizations of the Bi24O31Br10(OH)δ photocatalyst.
a, DRS plot of Bi24O31Br10(OH)δ in comparison with BiOBr. b, Mott-Schottky plot of the
Bi24O31Br10(OH)δ and BiOBr measured in 0.1 M Na2SO4 using a three-electrode system. c, Band
positions of the Bi24O31Br10(OH)δ photocatalyst referenced to the SHE. The redox potentials of
nitrobenzene (NB) to azoxybenzene (AOB) and aniline (AN), oxygen to the superoxide radical,
and isopropanol (IPA) to acetone (AE) are listed for demonstration.
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Fig. 4 Basic sites, hydrogen extraction and adsorption. a, DRIFT spectra of
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Bi24O31Br10(OH)δ, BiOBr, and Bi(OH)3. The samples were dried at 50 C under vacuum
conditions to remove surface adsorbed water. b, Post-mortem TPD of D2 from Bi24O31Br10(OH)δ
and BiOBr revealing the desorption of Dads from the surface. The TPD was performed using
vacuum dried samples after 450 nm irradiation in CD3OD under deaerated conditions.
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Basic sites, hydrogen extraction, adsorption and donation. We have first probed the basic
sites by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT, Fig. 4a). The
Bi24O31Br10(OH)δ exhibits the most densely populated surface hydroxyl groups (3600-3000 cm-1)
compared to the BiOBr and Bi(OH)3 counterparts. The isolated OH peak (~3500 cm-1) observed
for BiOBr suggests a low concentration of hydroxyls. In addition, the intense vibrational peaks in
the range of 1600-1200 cm-1 observed for both Bi24O31Br10(OH)δ and Bi(OH)3 can be assigned to
the carbonates originating from the chemically adsorbed CO2.46 The density of surface basic sites
of Bi24O31Br10(OH)δ is comparable to that of the Bi(OH)3 solid base.
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The surface basicity of Bi24O31Br10(OH)δ significantly improves the hydrogen extraction from
the hydrogen donor forming weakly anchored Hads. We speculate that the basic sites on
Bi24O31Br10(OH)δ play a role similar to that of KOH, which has been reported as a proton
extractor in many heterogeneous catalytic systems.47 This has been evidenced by the post-
mortem TPD of the photocatalyst after 450 nm irradiation under vacuum in the presence of
CD3OD (Fig. 4b, no base added). Note that fully deuterated methanol (CD3OD) is employed as
the probing molecule to qualitatively determine the bond strength of the Dads (Hads) on the
catalyst surface, as the sensitivity of the D2 signal is an order of magnitude higher than that of H2
in the mass spectrometer, and moreover it proves that the alcohol to be the proton source. The
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two D2 desorption peaks (m/z = 4, at 80 and 160 C) suggest that the D atoms have been
efficiently transferred from CD3OD to the Bi24O31Br10(OH)δ surface forming weakly bonded
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Dads, which can be further transferred to an added hydrogen acceptor. The desorption of HD has
also been observed in the P-TPD profile (m/z = 3, Fig. S5b), indicating the surface OH groups
are also involved in the hydrogen transfer reaction. In addition, neither D2 nor HD is observed in
the desorption spectra without irradiation, indicating that there is no dissociative adsorption of
CD3OD on the photocatalyst (Fig. S5b). In contrast, no detectable D2 or HD has been observed
for BiOBr and the acidified Bi24O31Br10(OH)δ after 450 nm irradiation under identical conditions,
suggesting that barely any D has been transferred to the surface of the BiOBr photocatalyst (Figs.
S5c and S5d).
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We have further investigated the formation of Hads on the Bi24O31Br10(OH)δ photocatalyst in
the presence of isopropanol by in-situ DRIFT under deaerated conditions (Fig. 5a). Clearly, the
H atoms are extracted from isopropanol and transferred to the surface of Bi24O31Br10(OH)δ upon
450 nm irradiation, as two additional OH stretching peaks appear at high wave numbers. While
the 3660 cm-1 peak corresponds to isopropanol vapors,48 the 3640 cm-1 peak can be assigned to
the hydroxyl groups consisting of Hads bound to the under-coordinated O. This hydrogen transfer
process is also accompanied by the colour change of the catalyst from yellow (fresh) to black
(after irradiation), which transforms back to a yellow powder after exposure to air. Solid state
nuclear magnetic resonance spectroscopy has further confirmed the hydrogen extraction and
adsorption process (ssNMR, Fig. 5b and Fig. S9). While the main peak at 4.1 ppm of the fresh
Bi24O31Br10(OH)δ can be assigned to the OH- groups or water molecules associated with the
surface Bi sites (i.e., Bi–OH), the minor peaks at 1.3 and 0.8 ppm of the fresh Bi24O31Br10(OH)δ
can be assigned to a small impurity of polymer contaminants. A peak with much higher intensity
is observed after blue light irradiation (450 nm) in the presence of isopropanol under vacuum
(red curve), reflecting the adsorption of hydrogen during the activation. The dominant resonance
peaks at 1.0 ppm and 0.8 ppm corresponds to the typical chemical shift region for surface
hydroxyl groups,49 suggesting that the extracted H atoms are bound to the under-coordinated O
atoms (marked as blue balls in Fig. 1). This has been further confirmed by in-situ FT-IR (Fig.
S10 and S11). It is worth noting that the intensities of the surface hydroxyl groups decrease after
the hydrogenated sample has been exposed to air for 20 h (green curve), forming surface
adsorbed water (4.6 ppm). This provides evidence that the photogenerated O-Hads can be
transferred to the acceptor under dark conditions.
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Fig. 5 In-situ spectroscopy analysis. a, DRIFT of Bi24O31Br10(OH)δ recorded at different
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irradiation times in the presence of isopropanol (IPA) under deaerated conditions. b, H MAS
NMR spectra of fresh Bi24O31Br10(OH)δ (black), after irradiation under vacuum with isopropanol
(red), and subsequent the irradiated Bi24O31Br10(OH)δ after exposed to air for 20 hours (green).
The spectra are shown on the same intensity scale, acquired at 14.09 T (599.6 MHz) using a
spinning speed of 13.0 kHz. c-g, Photographs of the stepwise HT process: fresh
Bi24O31Br10(OH)δ-isopropanol suspension without irradiation, under irrdiation (450 nm),
hydrogenated catalyst (black suspension) with the generation of acetone, nitrobenzene addition,
nitrobenzene reduction by Habs under dark conditions and recovery of the photocatalyst.
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We have further confirmed the stepwise HT path described in Fig. 1 by a control experiment
(Fig. 5c-g). Prior to irradiation, a suspension of the photocatalyst dispersed in isopropanol shows
a pale-yellow color under ambient N2 atmosphere (Fig. 3d). Blue light irradiation (450 nm, 1 h)
has been then applied to excite the photocatalyst in suspension (Fig. 3e), which gradually turns
black (Fig. 3f). Gas chromatography-mass spectrometry (GC-MS) analysis of the suspension
reveals that acetone (135 µmol) has been generated, suggesting that the Bi24O31Br10(OH)δ
photocatalyst is hydrogenated by the Hads (270 µmol). By dosing nitrobenzene (100 µmol) into
the hydrogenated photocatalyst suspension under dark conditions (Fig. 3g), the black color of the
suspension has been gradually recovered to the pale-yellow color (Fig. 3h). GC analysis of the
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suspension suggests that azoxybenzene (7.5 µmol) has been generated, indicating that 45 µmol
of Hads has been consumed. Although not all Hads has been used, it proves that irradiation is not
necessary for the consecutive HT step once Hads atoms are formed. Moreover, neither H2 (m/z =
2) nor O2 (m/z = 32) has been produced according to in-situ mass spectrometry (MS, Fig. S12
and S13), suggesting that additional Hads react with oxygen atoms in nitrobenzene to form water.
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Table 1. Photocatalytic conversion of various compounds using the Bi24O31Br10(OH)δ
photocatalyst.
Functional
group
Con. Sel.
/% /%
Entry
1
Model substrate
Nitrobenzene
Product
Azoxybenzenea
≥ 99 ≥ 98
N=O
Azobenzeneb
2
3
4
≥ 99 ≥ 99
≥ 99 ≥ 99
Reduction
Benzoquinone
Hydroquinonec
C=O
C=S
Fluorene-9-thione Fluorene-9-thiold
≥ 99 ≥ 99
≥ 99 ≥ 99
-OH
(Acyclic)
1-Hexanol
Hexanale
5
6
Cyclohexanonee
Cyclohexanol
-OH (Cyclic)
95
97
≥ 99
≥ 99
Oxidation
Benzaldehydef
Benzyl alcohol
-OH
(Aromatic)
7
Reaction conditions: a: 8 mM reactant, 50 mg catalyst in 10 mL isopropanol under N2
atmosphere, 450 nm LED (440-460 nm, 4 h, 0.03 W∙cm-2); b: reaction condition as a, but 410 nm
LED (400-420 nm, 8 h, 0.01 W∙cm-2); c: reaction condition as a, 450 nm LED for 1 h; d: reaction
condition as a, 450 nm LED for 2 h; e: 8 mM reactant, 50 mg catalyst in 10 mL acetonitrile
under aerated conditions, 450 nm LED for 24 h; f: reaction condition as e, 450 nm LED for 4 h.
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Applications in organic synthesis. The Bi24O31Br10(OH)δ photocatalyst can be used to drive a
series of photocatalytic reduction and oxidation reactions involving a HT process under base-free
conditions (Table 1). Nitrobenzene can be fully reduced into azoxybenzene or azobenzene under
blue light (450 nm) or purple light (410 nm) irradiation (Table 1, entries 1 and 2) in the presence
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of isopropanol, which is a similar result as obtained using g-C3N4 as we reported recently.17
While a full conversion of 8 mM nitrobenzene to azoxybenzene (24 mM isopropanol needed)
only needs 4 hours irradiation, the oxidation of 8 mM isopropanol to acetone in acetonitrile
requires 24 hours of identical irradiation under aerated conditions. Therefore we speculate that
the photo-induced isopropanol dehydrogenation is the rate determining step. Since there is no
gas-phase byproduct according to our online mass spectrometry analysis, all products remain in
the liquid phase. The mass balance has been further confirmed by the quantitative GC and UV-
vis analysis of the liquid phase containing the generated acetone and the aromatic compounds,
respectively (Fig. S17-S19 in the SI), revealing the high yield of azoxybenzene and azobenzene.
Since the redox potentials of nitrobenzene/azoxybenzene, nitrobenzene/azobenzene and
nitrobenzene/aniline are very close (~ -0.8 V vs. SHE),50,51 we consider that the selectivity might
not be explained by the different in redox potentials but rather the reaction intermediates
(nitrosobenzene [NBS] and N-phenylhydroxylamine [NPH]) during nitrobenzene reduction.
While photocatalytic reduction of a NBS/NPH mixture (1:1 in molar ratio) using
Bi24O31Br10(OH)δ results in mainly azo- and azoxybenzene (Con. >99%, Sel. > 97%, Table S1),
the photoreduction of NBS/NPH mixture using TiO2 leads to the formation of amine (Sel. of
82%). Therefore the Bi24O31Br10(OH)δ photocatalyst promotes the N=N coupling of NBS and
NPH and inhibits the full reduction of the intermediates, thus resulting in a high selectively to
azo- and azoxy-aromatic products.
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Under blue light irradiation (450 nm), compounds having -quinone (C=O) and -thione (C=S)
functional groups can be also reduced to their corresponding quinols (C-O) and thiols (C-S)
under deaerated conditions, as representatively demonstrated by the photo-reduction of
benzoquinone and fluorene-9-thione into hydroquinone and fluorene-9-thiol, respectively (Table
1, entries 3 and 4). Moreover, selective oxidation of alcohols to their corresponding aldehydes or
ketones can be also achieved in the presence of oxygen (Fig. S14). The 1-hexanal, cyclohexanol,
and benzyl alcohol can be selectively photo-oxidized to hexanal, cyclohexanone, and
benzaldehyde, respectively, suggesting that such an approach can be generalized for the
oxidation of other aliphatic- (acyclic and cyclic) and aromatic- alcohols. In comparison, neither
the reported high-performance photocatalysts (i.e., Au/TiO2, Pt/Ta2O5, Cu/Graphene, and g-
C3N4) nor the acidified Bi24O31Br10(OH)δ photocatalyst show activity at neutral pH (Table S2),
again suggesting the vital role of the surface basic sites in promoting the HT process for the
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photocatalytic reduction and oxidation reactions.29 In addition, by tuning the irradiation
wavelength of the light sources, the Bi24O31Br10(OH)δ photocatalyst can also realize controlled
photo-conversion of various nitroaromatic compounds into their corresponding azoxy- and azo-
compounds under base free conditions (Tables S3 and S4). Nevertheless, the Bi24O31Br10(OH)δ
photocatalyst exhibits superior photocatalytic performance compared to most other reported
photocatalyst materials under ambient conditions (Table S5).
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Quantum efficiency evaluation. The photocatalytic performance of the Bi24O31Br10(OH)δ is
significantly enhanced under basic conditions. Figure 6a compares the quantum efficiency (QE)
of Bi24O31Br10(OH)δ and g-C3N4 for nitrobenzene conversion as a function of irradiation
wavelength under basic conditions (10 mM KOH, pH = 12). Here g-C3N4 is used as the reference
material as it presents a reasonable performance in selective photocatalytic nitrobenzene
reduction under visible light irradiation,17 whereas the BiOBr barely absorbs visible light, and
thus shows no activity. The Bi24O31Br10(OH)δ shows remarkably high QEs of 42% and 32%
under 410 nm and 450 nm irradiation, which is approximately 30 and 80 times better than that of
the g-C3N4 reported previously.17 Noticeably, a QE of 5% can be achieved even under green
light irradiation (500 nm) for the Bi24O31Br10(OH)δ photocatalyst. In addition, the QE of
Bi24O31Br10(OH)δ exhibits a coherent trend with its light absorption spectra compared to that of
g-C3N4, suggesting an efficient utilization of the visible light to drive the photocatalytic reactions
(Fig. S15). Furthermore, we estimate that the basicity of our Bi24O31Br10(OH)δ photocatalyst is
not far from optimum (pKb ≈ 2.7), as the performance is 1/6 to that of at pH = 12 (pKb = 2),
which is the optimum reaction condition based on a previous report.17 The efficient and selective
synthesis of azobenzene and azoxybenzene by photocatalysis under eco-friendly conditions is a
promising replacement for the risky diazo-coupling process for dye industry, despite the need of
basic conditions to maximize the efficiency.
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Scaling-up and solar driven photocatalytic performance. The photocatalytic hydrogen
transfer process employing the Bi24O31Br10(OH)δ photocatalyst shows favorable potential for
scaling-up in terms of the starting concentration of the reactants, the reaction volume, and
stability, as representatively demonstrated by the photocatalytic nitrobenzene conversion (Fig.
6b-d). Full conversion of 40 mM nitrobenzene to azoxybenzene is achieved within 2.5 h, which
is 67 times faster in comparison with the reaction catalyzed by g-C3N4 (Fig. 6b). The
Bi24O31Br10(OH)δ photocatalyst works at even higher nitrobenzene concentration (80 mM, ~ 1
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vol%) where the g-C3N4 failed. This is a further confirmation of its potential for industrial
application. The full conversion time of nitrobenzene to azoxybenzene increases logarithmically
upon an increase in the nitrobenzene concentration due to the increased light absorption of the
generated orange-colored azoxybenzene. However, the full conversion time can be significantly
reduced for other hydrogen transfer reactions with transparent products, as exemplified by the
benzyl alcohol oxidation to benzaldehyde showing a linear dependence between the full
conversion time and the reactant concentration (Fig. 6b).
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Figure 6c depicts the photocatalytic nitrobenzene conversion under solar irradiation (800 mL,
80-fold scaling-up in volume). Full conversion of nitrobenzene to azoxybenzene with high
selectivity (>95%) is achieved within one full day of irradiation (9 h), whereas the g-C3N4 only
converts ~20% of nitrobenzene under similar reaction conditions. Further irradiation of the
Bi24O31Br10(OH)δ–reactant suspension leads to full conversion of azoxybenzene to azobenzene,
revealing that the selectivity of this process can simply be tuned through the irradiation time for
application. In comparison, the g-C3N4 photocatalyst fails to convert nitrobenzene to azobenzene
under solar irradiation. The sequential full conversion of nitrobenzene to azoxybenzene and then
to azobenzene suggests that the nitrobenzene molecules adsorb much more strongly on the
catalyst surface compared to the azoxybenzene intermediate, thus resulting in a consecutive
conversion fashion. It should be noted that the performance of the Bi24O31Br10(OH)δ
photocatalyst has not been optimized, as the solar radiation intensity already decreased
significantly in the autumn when the experiment was performed (Fig. S16). We estimate the QE
for the 80-fold scaled-up test to be ~6%, which is close to that of the QE under 500 nm LED
irradiation. Nevertheless, the Bi24O31Br10(OH)δ photocatalyst shows excellent stability in the
recycle tests (Fig. 6d). A full conversion of nitrobenzene with high selectivity to azoxybenzene
can be realized under blue light irradiation (450 nm) for five consecutive runs. Our one-step
synthesis of azoxy- and azoaromatics from photo-reduction of nitrobenzene under ambient
pressure, room temperature conditions show high selectivity and efficiency compared to other
heterogeneous-, homogeneous-, and electrocatalytic approach that require harsh reaction
conditions and expensive catalysts.52-59 Therefore we consider that the robust, stable
Bi24O31Br10(OH)δ photocatalyst has the potential to be utilized for industrial synthesis of azoxy-
and azoaromatics, which are generally produced inefficiently by reducing nitrobenzene using Bi
containing metal as the reducing agent.60
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Fig. 6. Photocatalytic performance. a, Quantum efficiencies of the Bi24O31Br10(OH)δ (red
circles) and g-C3N4 (black circles) for nitrobenzene (NB) conversion as a function of the
irradiation wavelength (left scale). Absorption spectra of Bi24O31Br10(OH)δ and g-C3N4 are
plotted (right scale). b, Photo-conversion of NB to azoxybenzene under deaerated conditions and
benzyl alcohol (BA) to benzaldehyde under aerated conditions using Bi24O31Br10(OH)δ (red) and
g-C3N4 (black) at different starting concentrations (450 nm irradiation, 10 mM KOH and 50 mg
catalyst). c, 800 mL up-scaled selective NB conversion under solar irradiation (conversion:
black, selectivity: blue and red). The test was performed in Beijing (30/10/2017-01/11/2017, 10
oC). d, Stability test of the Bi24O31Br10(OH)δ under 450 nm irradiation. Reaction conditions: 50
mg catalyst in 10 mL isopropanol with 8 mM NB under N2 atmosphere. The catalyst powders
were rinsed by acetone prior to each run.
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CONCLUSIONS
Herein, we report the design of a noble metal free, visible light response Bi24O31Br10(OH)δ
photocatalyst with surface basic sites to boost both photocatalytic reduction and oxidation
reactions involving a hydrogen transfer process under ambient conditions. The surface basic sites
promote the hydrogen transfer from alcohols to form metastable Hads under irradiation, thus
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giving rise to high quantum efficiencies for photocatalytic organic synthesis. We anticipate that
its high stability and favorable performance in up-scaling makes the inexpensive
Bi24O31Br10(OH)δ photocatalyst a promising candidate for industrial application.
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EXPERIMENTAL SECTION
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Synthesis of Bi24O31Br10(OH)δ. The photocatalyst Bi24O31Br10(OH)δ was synthesized by a
microwave based precipitation method in aqueous solution. Bi(NO3)3∙5H2O (0.5 g) and NaBr
with a molar ratio of 1:15 were dissolved in 2 M HNO3 aqueous solution at room temperature.
The mixture was stirred for 10 minutes, and the pH of the solution was adjusted to 13 using
aqueous KOH (4 M). The resulting white suspension was stirred vigorously for 30 min and then
transferred to a microwave reactor heated for 4 min at a power of 240 W. Subsequently, the
yellow precipitate was filtrated and washed three times with ethanol and deionized (DI) water to
eliminate any possible residuals. Finally, the product was dried in an oven at 60 oC for 12 h.
TEM. The detailed nanostructure of the as-synthesized Bi24O31Br10(OH)δ was investigated using
a TALOS F200A transmission electron microscope with a TWIN lens system, an X-FEG
electron source and a Ceta 16M Camera.
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CO2-TPD. The basicity of the samples was investigated using a chemisorption analyser
(AutoChem II) coupled with a mass spectrometer (MS, OmniStar GSD, Pfeiffer) according to
the following procedure: First the dried photocatalyst powders were loaded into a U-shaped
o
quartz tube in the chemisorption analyser, and purged with He (50 mL∙min-1) at 50 C for 0.5
hour to clean the sample; Then the sample was saturated with CO2 gas via pulsed injection of
o
CO2 in a He stream (50 mL∙min-1) at 50 C; Subsequently the sample was purged using a He
o
flow (50 mL∙min-1) for 1 h to remove physically adsorbed CO2 at 50 C; The TPD spectra were
eventually recorded by heating the sample to 450 oC at a ramp rate of 10 oC∙min-1 with a He flow
(50 mL∙min-1).
Synchrotron X-ray total scattering. The total scattering measurements were performed at the
P02.1 beamline at PETRA III (DESY Hamburg, Germany). The sample was packed in a Kapton
capillary (1.0 mm ID and 1.05 mm OD), and a two-dimensional scattering image was collected
on a Perkin Elmer detector (4040 cm2, pixel size: 200200 m2) using a photon energy of 60
keV for at least 1 min (for CeO2 calibration standard) or at least 30 s (for prepared catalysts and
references) at ambient temperature and pressure.
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Mott-Schottky analysis. Mott-Schottky plots were recorded by a potentiostat (Autolab
PGSTAT302N) in a 0.1 M Na2SO4 electrolyte solution by performing impedance measurements
at a fixed frequency of 100 Hz between the applied voltage range of -0.6–0.6 V. A three-
electrode configuration was used for the analysis, where catalyst powder coated on highly
oriented pyrolytic graphite (HOPG) was used as working electrode (WE), Ag|AgCl/Sat. KCl was
used as reference electrode (RE), while the counter electrode (CE) was a Pt mesh.
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DRIFT. The DRIFT characterization was carried out using a Bruker VERTEX 70 FTIR
spectrometer. Before data collection, the loaded powder samples were dried in situ under
o
vacuum conditions at 50 C for 1 h. In addition, the basicity of the sample was studied by
introducing a CO2 flow (10 vol% in Ar, 50 mL∙min-1) to the cell until the sample was saturated.
1
Solid State NMR. The H NMR spectra were acquired on a Varian Direct-Drive VNMRS-600
spectrometer (14.09 T) using a custom-made CP/MAS NMR probe for 4 mm out diameter rotors.
The spectra were taken at a spinning speed of νR = 13.0 kHz, a 2.0 μs (~30o) excitation pulse, a
4-s relaxation delay, and 256 scans. The experiments were performed at ambient temperature
using air-tight end-capped zirconia rotors packed with the samples in an argon-filled glovebox.
The 1H chemical shifts are relative to tetramethylsilane (TMS).
Post-mortem TPD. TPD analysis of the used photocatalysts was conducted following the
procedures described below: Firstly 50 mg of photocatalyst dispersed in 10 mL of deuterated
methanol (CD3OD) was irradiated for 6 h (450 nm LED) under 1 bar N2 atmosphere at RT; Then
the suspension was kept stirring at room temperature under vacuum conditions until fully dried;
Finally, the dried photocatalyst was loaded into a U-shaped quartz tube in a chemisorption
o
o
analyser and heated to 500 C at a ramp rate of 10 C∙min-1 with an Ar gas flow rate of 50
mL∙min-1. Meanwhile the desorbed gases were monitored by the coupled MS in real time.
Stepwise HT process test. The experiment was performed as follows: firstly a 250 mg of
Bi24O31Br10(OH)δ was dispersed in 20 mL isopropanol under 1 bar N2 atmosphere; then a 450
nm LED light was used to irradiate the yellow suspension for 1 hour; after the light was turned
off, a 2 mL suspension was collected, centrifuged, and analyzed by GC-MS; subsequently, a 100
µmol of nitrobenzene was added to the suspension and kept in dark for 12 hours under 1 bar of
N2 atmosphere under continuous magnetic stirring; finally, a 2 mL suspension was collected for
GC analysis.
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ASSOCIATED CONTENT
Supporting Information
Detailed photocatalyst preparation, physical-chemical characterization and photocatalytic
performance evaluation (Figs. S1-19). The performance of reference photocatalysts,
photocatalytic reduction of various nitro-aromatics to azo/azoxy-compounds, synthesis of
asymmetric azo-aromatic compounds and summary of reported photocatalytic performances are
shown in Tables S1-5. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
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*suren@synfuelschina.com.cn
Funding
RS would like to thank the NSFC for financial support (projects number: 21503257 and
21601198). YD greatfully acknowledges the Sino-Danish Centre (SDC) for a PhD scholarship.
NL acknowledges the VILLUM Foundation Young Investigator Programme (VKR023449) and
the Danish National Research Foundation (Carbon Dioxide Activation Center, DNRF 118).
Syngaschem BV acknowledges funding from Synfuels China Technology Co. Ltd.
Notes
Authors declare no competing interests.
ACKNOWLEDGMENT
We thank Ms. Xiaoping Wang, Ms. Caixia Hu, and Ms. Yiling Bai from Synfuels China
Technology Co. Ltd. for TEM, NMR, and TPD analysis. We also thank Dr. Dorthe B. Ravnsbæk
for experimental assistance at PETRA III.
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