Lead-Free Cs3Bi2Br9 Perovskite as Photocatalyst for Ring-Opening Reactions of Epoxides

Article abstract of DOI:10.1002/cssc.201900716

Herein, an innovative approach was developed by using stable, lead-free halide perovskite for solar-driven organic synthesis. The ring-opening reaction of epoxides was chosen as a model system for the synthesis of value-added β-alkoxy alcohols, which require energy-intensive process conditions and corrosive, strong acids for conventional synthesis. The developed concept included the in situ preparation of Cs₃Bi₂Br₉ and its simultaneous application as photocatalyst for epoxide alcoholysis under visible-light irradiation in air at 293 K, with exceptional high activity and selectivity ≥86 % for β-alkoxy alcohols and thia-compounds. The Cs₃Bi₂Br₉ photocatalyst exhibited good stability and recyclability. In contrast, the lead-based perovskite showed a conversion rate of only 1 %. The origin of the unexpected catalytic behavior was attributed to the combination of the photocatalytic process and the presence of suitable Lewis-acidic centers on the surface of the bismuth halide perovskite photocatalyst.

Full text of DOI:10.1002/cssc.201900716

Accepted Article
Title: Lead-free Cs3Bi2Br9 Perovskite as Photocatalyst for Ring-
Opening Reactions of Epoxides
Authors: Yitao Dai and Harun Tüysüz
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COMMUNICATION
Lead-free Cs Bi Br Perovskite as Photocatalyst for Ring-
3
2
9
Opening Reactions of Epoxides
[a]
[a]
Yitao Dai, and Harun Tüysüz*
[
16]
corrosive strong acids (e.g., triflic acid/silica ) or solid acids
(e.g., sulfated Zr-HTiNbO5 ), which can result in corrosion of
[
17]
Abstract: Herein, we present an innovative approach using stable,
lead-free halide perovskite for solar-driven organic synthesis. The
ring opening reaction of epoxides has been chosen as model
systems for synthesis of value-added β-alkoxy alcohols, which
require energy intensive process conditions and corrosive strong
acids for conventional synthesis. The developed concept includes
equipment or acid leaching problems. Strong acid-free catalysts
[
18]
(
e.g., Fe(III) complex/SBA-15
and metal-organic frameworks
[
19]
[20]
such as UiO-66
and Zr-MOF
) require harsh reaction
conditions (e.g., high temperatures and long reaction time) to
obtain good conversions, which can reduce the catalyst lifetime
the in-situ preparation of Cs
3
Bi
2
Br
9
and its simultaneous application
[16]
and present challenges for recyclability.
On the other hand,
as photocatalyst for epoxides alcoholysis under visible light
irradiation in air at 293 K, with exceptional high activity and
selectivity ≥ 86 % for β-alkoxy alcohols and thia-compounds.
photocatalyst-enabled organic synthesis is considered
a
promising approach, as it can be carried out through the use of
green and sustainable solar energy under mild conditions (e.g.,
3 2 9
The Cs Bi Br photocatalyst exhibits good stability and recyclability.
[
21]
room temperature and ambient pressure).
reactions of epoxides, to our knowledge, no case has been
reported. Although carbon quantum dots decorated with -SO
groups (via a H SO reflux) were shown to catalyze the reaction
For ring-opening
In contrast, the lead-based perovskite shows a conversion rate of
only 1%. The origin of the unexpected catalytic behavior is attributed
to the combination of the photocatalytic process and the presence of
proper Lewis acidic centers on the surface of the bismuth halide
perovskite photocatalyst.
3
H
2
4
[
22]
when irradiated with visible light , these materials displayed
almost the same activity in dark and light irradiation conditions
and can be considered reservoirs of strong acid, similar to the
All-inorganic halide perovskites with a general formula of ABX
A = Rb, Cs; B = Ge, Pb, Sn; and X = Cl, Br, I) have attracted
immense research interest due to their unique photovoltaic and
3
[
17,
23]
(
other reported sulfated catalytic systems.
Halide
perovskites can be prepared using anti-solvent precipitation
method where the perovskite precursor salts are dissolved in
[
1]
optoelectronic properties. This class of materials includes other
[
2]
[3]
derivative perovskites such as Cs
3
Bi
2
X
9
, Cs
3
Fe
2
Br
9
and the
dimethyl sulfoxide (DMSO) and then precipitated in a low-
[
4]
[2a, 2c]
double perovskite Cs
2
AgBiX
6
. Halide perovskites have been
explored for applications in various fields such as solar cells,
polarity solvent (e.g. alcohols)
. Herein, we combine this
[
5]
perovskite synthesis approach with its application as
a
[
6]
[7]
lasers and phototransistors and have also started to gain
increasing attention in the field of heterogeneous catalysis. After
the first report in 2016 of the photocatalytic production of
hydrogen [ from HI using methylammonium lead iodide
photocatalyst for epoxide alcoholysis in an integrated step as
shown in Scheme S1. First, the in-situ synthesis of Cs
perovskite is achieved by adding the precursors (CsBr and BiBr
3
Bi
2
Br
9
3
dissolved in DMSO) directly into the target reaction solution
containing an epoxide reactant and alcohol, the latter of which
simultaneously acts as solvent, alcoholysis nucleophile, and
anti-solvent to cause the perovskite particle formation (Scheme
S1a). This one-pot approach with the integration of the
photocatalyst synthesis and reaction evaluation provides several
advantages, such as cost reduction, low environmental pollution,
and facile operation conditions. Without any purification,
separation, or post-treatment, the generated yellow suspension
8]
perovskite , several important reactions have been successfully
[
9]
catalyzed by halide perovskites under visible light irradiation.
For example, CsPbBr was reported to efficiently reduce CO to
CO , selective oxidation of benzyl alcohols to aldehydes can
be realized by formamidinium lead bromide or CsPbBr in the
meanwhile, it was also the subject of
several recent studies investigating its dynamic charge transfer
3
2
[
9b]
3
[
9c, 9d]
2
presence of TiO ;
[
10]
processes. CsPbI
3
quantum dots with a narrow bandgap (1.8
eV) have been used to promote the photocatalytic
containing the Cs
irradiated by visible light in air at room temperature (293 K) to
initiate the ring-opening reactions (Scheme S1c). This
photocatalytic system displays high activity (1333 µmol h g )
cat
and selectivity (≥ 86%) to produce 2-isopropoxy-2-phenylethanol
3
Bi
2
Br
9
(Scheme S1b) is then directly
[
9a]
polymerization of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene.
Very recently, APbBr (A = Cs or methylammonium) have been
3
-1
-1
employed for the photocatalytic α-alkylation of aldehydes with
the help of cocatalysts and base additives, which presented the
high turnover number up to 52,000 under visible light
from styrene oxide and isopropanol.
[
11]
irradiation.
could activate the C-H bonds of alkanes.
promising systems, Pb-based perovskites are unattractive for
Composite photocatalyst NiO
x
/FAPbBr
Despite these
3
/TiO
2
3 2 9
After the in-situ synthesis process, the obtained Cs Bi Br
was extracted for the following material characterizations.
According to the powder X-ray diffraction (PXRD) pattern (Figure
[
9e]
[
12]
toxicity reasons.
alternatives, but are unfortunately unstable in air.
Sn-based perovskites are possible
1a), our Cs
solvent method
(PDF#70-0493 card), presenting high crystallinity and the
3 2 9
Bi Br photocatalysts synthesized using the anti-
[
13]
[2a, 2c]
In
matched very well with the references
comparison, Bi-based all-inorganic halide perovskites can be
promising candidates for photocatalysts, as they have low-
[
24]
trigonal P3m1 symmetry.
Transmission electron microscope
[
2a, 2c, 4d, 14]
toxicity and are air-stable.
To the best of our
(TEM) and scanning electron microscope (SEM) analyses
revealed that the materials consisted of aggregated particles
with a broad size range from 50 to 500 nm (Figure 1b and Figure
S1). High resolution TEM (HRTEM) analysis (Figure 1c)
revealed lattice fringes with interplanar spacing distances of 0.40
and 0.28 nm, which match quite well with the (102) and (202)
lattice planes of crystalline Cs
ray spectroscopy (EDS) confirmed the chemical composition of
the as-prepared sample had stoichiometric ratio Cs / Bi / Br = 3:
knowledge, the photocatalytic application of Bi-based
perovskites in organic synthesis is still unknown.
The nucleophilic addition of alcohols to epoxides leads to the
production of β-alkoxy alcohols, which can be used for the
[
15]
synthesis of anti-tumoral or immunosuppressive drugs. Up to
now, most effective heterogeneous systems reported to
successfully catalyze epoxide alcoholysis reactions use
[
24-25]
3
Bi
2
Br
9
.
Energy-dispersive X-
2: 9 (Figure S2 and Table S2) with homogeneous element
[a]
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1,
5470 Mülheim an der Ruhr, (Germany)
E-mail: tueysuez@kofo.mpg.de
distribution of Cs, Bi and Br (Figure 1d-g). The nitrogen sorption
analysis confirmed that the as-prepared sample had a low
surface area of 7 m /g due to the large particle sizes (Figure S4).
4
2
The Cs
3 2 9
Bi Br sample had a direct bandgap of 2.7 eV for
Supporting information for this article is given via a link at the end of
the document.
suitable visible light absorption (Figure 1h and Figure S8).
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ChemSusChem
10.1002/cssc.201900716
COMMUNICATION
3 2 3
based photocatalysts (e.g., BiBr and Bi O ) displayed poorer
activity with conversion ≤ 13% (Table 1, Entry 4 and 6), while
CsBr showed no conversion at all under the same reaction
conditions (Table 1, Entry 5). Thus, it can be reasonably
assumed that the possible active species for epoxide alcoholysis
using the Cs
the perovskite. Moreover, when the Pb-based perovskite
prepared via in-situ anti-solvent method) was used as the
photocatalyst, a conversion rate of only 1% was achieved (Table
, Entry 7).
3 2 9
Bi Br photocatalyst are related to the Bi atoms in
(
1
Table 1. Alcoholysis of styrene oxide by isopropanol: comparison of
different photocatalysts.
a
a
Entry
Catalyst
Conversion (%)
Yield (%)
b
1
2
None
<1
1
0
c
Cs
3
Bi
2
Br
Bi
BiBr
9
(in dark)
<1
86
88
12
0
c
>99d
3
Cs
3
2
Br
9
>
99
4
5
6
7
3
13
<1
9
CsBr
Bi
CsPbBr
2
O
3
8
<1
e
3
1
o
Reaction conditions: 20 C in air, 6 h irradiation under visible light (≥ 420 nm),
2.5 mg photocatalyst, 0.1 mmol styrene oxide, 5 mL isopropanol. based on
epoxide and determined cby GC with toluene as internal standard. no
photocatalyst was added. Cs Bi Br was in-situ synthesized by adding the
precursors into the starting reaction solution as shown in Scheme S1a-b.
a
1
b
3
2
9
d
1
e
sun irradiation for 5 h by use of solar simulator. CsPbBr
3
was in-situ prepared
by adding the precursors (CsBr and PbBr
2
) into the starting reaction solution.
3 2 9
To further understand the Cs Bi Br photocatalytic system,
the ring-opening reaction of styrene oxide by isopropanol was
systematically investigated as a model reaction. According to the
recorded action spectrum obtained using monochromatic light
irradiation (Figure 2a and Figure S5), the apparent quantum
3 2 9
efficiencies (AQEs) of Cs Bi Br for epoxide alcoholysis vary in
a trend following the light adsorption profile, suggesting the
utilization of light energy for the reaction. The highest AQE of
Figure 1. a, PXRD pattern with reference (PDF#70-0493). b, TEM image. c,
HRTEM image. d-g, STEM image and element mapping. h, UV-Vis DRS
profile of Cs Bi Br .
3 2 9
0
.08% was obtained using monochromatic irradiation at 420 nm.
When the irradiation energy is lower than the optical bandgap of
Cs Bi Br (2.7 eV), the AQE drops significantly to ~0.01%, as
For the photocatalytic ring-opening of styrene oxide by
isopropanol, control tests showed that negligible reaction took
place (≤ 1% of conversion) in the absence of photocatalyst and
light irradiation (Table 1, Entry 1 and 2). In contrast, when using
3
2
9
seen in the cases of 500 and 550 nm light irradiation. It should
be mentioned that the AQE values shown here are
underestimated due to the significant light scattering in a
[
26]
Cs
and high yield (86%) for epoxide alcoholysis was observed after
h irradiation under visible light (≥ 420 nm) at room temperature
in air (Table 1, Entry 3). When the reaction was performed under
sun irradiation, the high yield up to 88% could be obtained
3
Bi
2
Br
9
as a photocatalyst, almost full conversion (≥ 99%)
solid/liquid heterogeneous medium during illumination , and
the actual number of photons absorbed by the photocatalyst
were not determined.
6
As shown in Figure 2b, the time-dependent photocatalytic
activity profile of the Cs Bi Br catalyst indicates that the initial 2
3 2 9
1
after 5 h. This promotion is mainly due to the partial UV light in
the sunlight, which could generate the more energic photo-
excited charges to accelerate the reactions. Even in the scaled
test with the epoxide amount increased up to 1 mmol, the
satisfactory activity (80% yield) could be reached under 1 sun
irradiation at room temperature after 48 h. Notably, this facile
photocatalytic system shows high regioselectivity to the β-alkoxy
alcohol without the formation of any other alcohol regioisomers.
The main by-products come from the oxidation of styrene oxide
to benzaldehyde and isopropyl benzoate, meanwhile the
oxidation of isopropanol to acetone was also observed.
h of irradiation serves as an activation period, as the conversion
increased with longer irradiation time. Similar catalytic behavior
was also observed when isopropanol was replaced by 1-butanol
as the nucleophile under 14 h irradiation (Figure S6). The longer
illumination time required to get higher conversion for the 1-
butanol case is mainly attributed to the stronger steric effect, as
3 2 9
the in-situ formed Cs Bi Br prepared in 1-butanol has similar
crystallinity and light adsorption ability as the photocatalyst
prepared in isopropanol (Figure S8 and S9). The observed
induction period can be ascribed to the initial adsorption of the
alcohols on the catalyst surface and their reaction with the
photogenerated charges to accumulate sufficient radicals that
Furthermore, the Cs
3
Bi
2
Br
9
perovskite photocatalyst used
[
27]
milder reaction conditions but showed a comparable reaction
serve as active nucleophiles in the ring-opening reaction.
-1
-1
rate (1333 µmol h
g
cat ) to reported thermal heterogeneous
Additionally, the excess alcohol (as solvent) may also lead to the
slower adsorption of the epoxide and its activation at catalyst
surface, which could also cause this induction period behavior. It
should be noted that due to the presence of DMSO (used as
catalysts (Table S1) that also avoided the use of strong acids
but required harsh reaction conditions (e.g., elevated reaction
temperature and long reaction time). In addition, other bismuth
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ChemSusChem
10.1002/cssc.201900716
COMMUNICATION
solvent to dissolve the perovskite precursors) in the reaction
solution, some of the photocatalyst could have been re-
dissolved (Table S3). However, these soluble species in the
reaction solution had no significant effect on the ultimate
photocatalytic performance. As shown in Figure 2b, removing
the photocatalyst using hot filtration from the reaction mixture
applications of bismuth halide perovskite for more challenging
coupling reactions.
Table 2. Photosynthesis of β-alkoxy alcohols via ring-opening
reactions of epoxides over Cs
3
Bi
2
Br
9
under visible light irradiation.
Entr
y
Time
(h)
Con.
(%)
Sel.
(%)
Epoxide
Alcohol
Product
a
a
after
4
h
of reaction did not generate significant further
conversion (from 40% at 4 h to 43% at 6 h).
1
4
4
28
23
95
94
2
3
4
14
95
4
5
4
6
40
90
86
≥99
b
6
4
4
32
33
94
93
b
7
8
4
10
86
94
94
9
14
Figure 2. a, Action spectrum of Cs
irradiation at wavelengths of 420 nm, 450 nm, 475 nm, 500 nm and 550 nm. b,
Time-dependent photocatalytic reaction over the Cs Bi Br (≥420 nm) in air at
93 K for 6 h (-■- for conversion and -●- for selectivity). The dashed line
shows the conversion in the absence of solid catalyst after hot filtration.
The general application scope of our in-situ synthesized
Cs Bi Br photocatalytic system is demonstrated through the
3 2 9
3 2 9
Bi Br under monochromatic light
1
0
4
4
8
2
94
95
3
2
9
2
11
ring-opening reactions of various epoxides by different alcohols.
It should be noted that in most cases, the reactions displayed in
Table 2 were performed under consistent reaction time (4h) to
enable comparison, and that higher conversions could be
possible with further optimization of reaction conditions.
o
Reaction conditions: 20 C in air, under visible light (≥ 420 nm) illumination,
a
1
2.5 mg Cs
using toluene as internal standard. using 31.25 mg in-situ formed
Cs Bi Br
3 2 9
Bi Br , 0.1 mmol epoxide, 5 mL alcohol. Determined by GC
b
3
2
9
.
[
19, 22]
As shown in Table 2, through 4 h visible light (≥ 420 nm)
irradiation at room temperature in air, the in-situ formed
Based on related literature
, we hypothesized that the
surface of our Cs Bi Br photocatalyst contained the acid sites
3
2
9
Cs
3
Bi
2
Br
9
photocatalyst can be used to ring-open other
needed for epoxide activation. To investigate this, we performed
epoxides, such as 1,2-epoxybutane, 2-methyl-1,2-epoxypropane, UV-Vis diffuse reflectance spectroscopy (DRS) analysis with
and cyclohexene oxide to produce the corresponding β-alkoxy
alcohols (Table 2, Entries 1-3). These epoxide substrates were
found to react more slowly than styrene oxide, likely due to their
lower electron density and increased steric hindrance. For the
cases of different alcohols (Table 2, Entries 4-11), higher
catalyst precursor amounts were required for successful in-situ
synthesis of the photocatalyst when using methanol and ethanol
as solvents due to their strong polarity (Table 2, Entries 6-7); in
these cases the lower conversions in comparison with the
isopropanol example is mainly due to the dissolution of
alizarin as the adsorbate, which has been reported to accurately
[29]
assess the surface acidity of metal oxides and sulfides. Pure
uncoordinated alizarin displays absorbance at 2.856 eV, which
is due to the intramolecular charge-transfer (IMCT) from the
catechol moiety to the entire ring system as the lowest energy
[
30]
transition. After the adsorption of alizarin onto the Cs Bi Br
9
3
2
perovskite surface, the IMCT band is clearly shifted to 2.42 eV
(Figure 3a). This shift originates upon adsorption of the molecule
onto the perovskite surface since the two original protons in
alizarin are replaced by the metal cation (e.g. Bi in our case),
[
28]
[29]
Cs
3
Bi
2
Br
9
.
When 1-butanol was used a solvent (Table 2,
which has lower degrees of electron withdrawal. Based on the
Entry 8), much lower conversions (10%) were observed because
of the steric effect from the long carbon chain. This hindrance is
more obvious in the cases of 2-butanol and tert-butanol (Table 2, at 2.38 eV) and weaker than that for Al O (IMCT band at 2.48
Entries 10-11). However, the longer light irradiation time can
lead to high conversion even in these cases (Table 2, Entry 5
and 9).
recorded acidity scale in the reported literature, the acidity of
Cs Bi Br is stronger than that for amphoteric ZnO (IMCT band
3
2
9
2
3
[
29]
eV). Therefore, it can be concluded that weak Lewis acid sites
exist on the Cs Bi Br surface. In addition, we also observed a
3
2
9
similar IMCT band shift when performing the same alizarin
In addition, this photocatalytic protocol can be extended to
other nucleophiles such as thiol to produce thia-compound with
a yield of up to 84% (Table S4), which confirms widespread
adsorption measurement on Bi O3 (IMCT band at 2.44 eV,
2
Figure S12a), which agrees with its acid-base properties as an
[
31]
amphoteric oxide.
This property may contribute to the
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ChemSusChem
10.1002/cssc.201900716
COMMUNICATION
photocatalytic activity of Bi
Entry 6), which is unfortunately limited by its wider bandgap (2.9
eV, Figure S12b) in contrast to Cs Bi Br . Similarly, the activity
of BiBr (Table 1, Entry 4) may also come from its mild Lewis
acid nature.
2
O
3
for epoxide alcoholysis (Table 1,
Cs
3
Bi
2
Br
9
is proposed as shown in Figure 3c. First, under visible
light irradiation, the photogenerated electrons and holes appear
in the conduction band (CB) and valence band (VB) positions,
respectively, of the Cs
photogenerated holes and the superoxide anions (O
produced from O reduction by the photogenerated electrons
react with the alcohol (e.g., isopropanol) to form alcohol radicals
or anions, which serve as the active nucleophiles for the ring-
opening reaction. Meanwhile, the Bi-based Lewis acid sites on
the photocatalyst activate the epoxide (e.g., styrene oxide) by
coordinating the basic oxygen atom in the three-membered
3
2
9
3
3 2 9
Bi Br perovskite. Subsequently, the
[
32]
●¯
In contrast, the CsPbBr
3
showed much weaker
2
)
Lewis acidity with an IMCT band shift of 2.19 eV (Figure S13a),
even though it has enhanced light absorption efficiency due to
smaller bandgap (2.32 eV, Figure S13b). This also agrees well
2
[
9c, 29]
with the reported literatures.
Thus, weak Lewis acid sites on
Pb-based perovskite could be the key point for its poor activity in
ring-opening reactions.
[
34]
heterocyclic ring of epoxide.
Finally, the desired alcoholysis
product is generated by the backside attack of the activated
epoxide by the alcohol radicals or anions via the traditional
[
19]
nucleophilic substitution reaction pathway.
In summary, through the integration of Cs
3
2 9
Bi Br halide
perovskite synthesis and its photocatalytic evaluation in a one
pot, an efficient approach has been established for the ring-
opening reaction of various epoxides in different alcohols and
thiols to enable synthesis of β-alkoxy alcohols and thia-
compounds at room temperature and under visible light
irradiation. This Cs Bi Br photocatalyst system not only avoids
3 2 9
the utilization of strong acids, but also displays high activity and
good selectivity for epoxide alcoholysis. In comparison, the lead
3
counterpart CsPbBr shows the very poor activity with a conversion
rate of 1%. According to the surface acidity characterization and
control experiments, the high photocatalytic activity can be
ascribed to the coupling effect between photocatalysis and
proper Lewis acid sites on surface of bismuth-based material,
which can help to activate the epoxides.
Acknowledgements
The authors acknowledge Silvia Palm and Norbert Pfänder for
recording the electron microscopy images and Jutta Rosentreter
for conducting the GC/GC-MS analysis. The authors thank Prof.
Candace K. Chan for assistance with the revision of this article,
Dr. Cristina Ochoa Hernández for scientific discussions. This
work is supported by the MAXNET Energy consortium of the
Max Planck Society.
Figure 3. a, UV-Vis spectra of the sample Cs
3 2 9
Bi Br adsorbed with alizarin
(
red line), clean Cs Bi Br (blue line) and pure alizarin (black line). b,
3
2
9
Performance results of the control experiments with the addition of scavengers
such as oxalic acid (OA), triethylamine (TEA), benzoquinone (BA) and KI
under visible light irradiation for 5 hours. c, Proposed photocatalytic cycle for
epoxide alcoholysis reaction over Cs Bi Br with proper Lewis acids sites on
3 2 9
surface (The yellow atoms are Bi, pink atoms are Br, and blue atoms are Cs).
Keywords: halide perovskite • epoxides • ring-opening reactions
To further investigate the effect of surface acid sites, we
conducted additional experiments where various additives were
added to the photoreactor (Figure 3b). After addition of an
organic acid (oxalic acid, 0.3 mmol), the photocatalytic activity
was obviously promoted with the conversion increased from
•
visible light photocatalysis • Lewis acid
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3 2 9
75% (in the case where only Cs Bi Br photocatalyst was used)
to 94% after 5 h of the reaction time. The presence of this
organic acid could help the activation of the epoxide, which
would in turn benefit the final ring-opening reaction. In contrast,
when the same molar amount of an organic base, triethylamine,
was added during the reaction, the photocatalytic system was
totally deactivated. The poisoning from base addition can be
ascribed to the occupation of surface acid sites of Cs Bi Br by
3 2 9
basic molecules. The significant influence of the acid-base
reaction environment on the conversion strongly supports the
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[
[
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[
●
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Photocatalytic
epoxides over halide perovskite:
The lead-free Cs Bi Br is prepared
ring-opening
of
Yitao Dai and Harun Tüysüz*
Page No. – Page No.
3
2
9
in-situ and simultaneously used for the
alcoholysis of various epoxides under
visible light irradiation. This novel
catalyst indicated excellent catalytic
performances, good stability and
Lead-free Cs
Photocatalyst
Reactions of Epoxides
3
Bi
2
Br
for
9
Perovskite as
Ring-Opening
recyclability.
unexpected
The
origin
of
this
is
catalytic
behavior
ascribed to presence of proper acidic
centers at the halide perovskite.
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