Singlet oxygen-engaged selective photo-oxidation over pt nanocrystals/porphyrinic MOF: The roles of photothermal effect and pt electronic state

Article abstract of DOI:10.1021/jacs.6b12074

The selectivity control toward aldehyde in the aromatic alcohol oxidation remains a grand challenge using molecular oxygen under mild conditions. In this work, we designed and synthesized Pt/PCN-224(M) composites by integration of Pt nanocrystals and porphyrinic metal-organic frameworks (MOFs), PCN-224(M). The composites exhibit excellent catalytic performance in the photo-oxidation of aromatic alcohols by 1 atm O₂ at ambient temperature, based on a synergetic photothermal effect and singlet oxygen production. Additionally, in opposition to the function of the Schottky junction, injection of hot electrons from plasmonic Pt into PCN-224(M) would lower the electron density of the Pt surface, which thus is tailorable for the optimized catalytic performance via the competition between the Schottky junction and the plasmonic effect by altering the light intensity. To the best of our knowledge, this is not only an unprecedented report on singlet oxygen-engaged selective oxidation of aromatic alcohols to aldehydes but also the first report on photothermal effect of MOFs.

Full text of DOI:10.1021/jacs.6b12074

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Article
Singlet Oxygen-Engaged Selective Photo-oxidation over Pt Nanocrystals/
Porphyrinic MOF: The Roles of Photothermal Effect and Pt Electronic State
Yu-Zhen Chen, Zhiyong U. Wang, Hengwei Wang, Junling Lu, Shu-Hong Yu, and Hai-Long Jiang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12074 • Publication Date (Web): 20 Jan 2017
Downloaded from http://pubs.acs.org on January 20, 2017
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Journal of the American Chemical Society
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Singlet Oxygen-Engaged Selective Photo-oxidation over Pt
Nanocrystals/Porphyrinic MOF: The Roles of Photothermal Ef-
fect and Pt Electronic State
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†
‡
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†
YuꢀZhen Chen, Zhiyong U. Wang, Hengwei Wang, Junling Lu, ShuꢀHong Yu, and HaiꢀLong
,†
Jiang*
†
Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Soft Matter Chemistry, Collaboꢀ
rative Innovation Center of Suzhou Nano Science and Technology, School of Chemistry and Materials Science, University
of Science and Technology of China, Hefei, Anhui 230026, P. R. China
‡
Department of Chemistry and Physics, Troy University, Troy, Alabama 36082, USA
ABSTRACT: The selectivity control toward aldehyde in the aromatic alcohol oxidation remains a grand challenge using molecular
oxygen under mild conditions. In this work, we have designed and synthesized Pt/PCNꢀ224(M) composites by integration of Pt
nanocrystals and porphyrinic metalꢀorganic frameworks (MOFs), PCNꢀ224(M). The composites exhibit excellent catalytic perforꢀ
mance in the photoꢀoxidation of aromatic alcohols by 1 atm O at ambient temperature, based on synergetic photothermal effect and
2
singlet oxygen production. Additionally, in opposition to the function of Schottky junction, the injection of hot electrons from
plasmonic Pt into PCNꢀ224(M) would lower the electron density of Pt surface, which thus is tailorable for the optimized catalytic
performance via the competition between Schottky junction and plasmonic effect by altering the light intensity. To the best of our
knowledge, this is not only an unprecedented report on singlet oxygenꢀengaged selective oxidation of aromatic alcohols to aldeꢀ
hydes but also the first report on photothermal effect of MOFs.
1
22ꢀ27
O2.
We reason that the integration of the photosensitizers
INTRODUCTION
into porous materials, such as metalꢀorganic frameworks
28ꢀ31
Selective oxidation of primary alcohols to aldehydes is an
important laboratory and commercial procedure for the proꢀ
duction of fine chemicals and intermediates. While the reacꢀ
tion can be traditionally effected with stoichiometric inorganic
and organic oxidants, there have been ongoing active efforts to
develop new catalytic systems using environmentally benign
reagents such as molecular oxygen.
metal nanoparticles (NPs) such as Au NPs supported on metal
oxides have been developed as effective catalysts, although
(MOFs),
could result in their synergistic and superior funcꢀ
tions. MOFs have captured widespread interests and have
found applications in many diverse fields in recent two decꢀ
ades due to their high surface area and great structural tailoraꢀ
1
ꢀ8
3
2ꢀ52
bility.
Recently, porphyrinic MOFs have been reported to
1
be an effective photosensitizer for O generation and exhibit
good catalytic activity for oxidation reactions using O as an
oxidant.
2
9
,10
1
In this regard noble
2
16ꢀ18
On the other hand, Ptꢀgroup metal nanocrystals
4ꢀ8
1
(
NCs) can not only activate O to O under light irradiation
2
2
there are still limitations including low selectivity and harsh
reaction conditions requiring high temperature or high presꢀ
sure, organic solvents, strong oxidants or corrosive environꢀ
ment. Molecular oxygen in its ground state exists in triplet
but also adsorb and activate alcohols in catalytic oxidation of
6,9,26,53,54
alcohols.
Moreover, the photothermal effect convertꢀ
ing light into heat has been recently realized on metal NCs
based on localized surface plasmon resonance (LSPR) and has
3
ꢀ
5
5ꢀ58
state ( Σ ), and upon activation it can be turned into two sinꢀ
g
been shown to promote the reactivity of metal NCs.
With
1
1
glet excited states, ꢁ and Σ , 95 and 158 kJ/mol above the
triplet state, respectively. Singlet oxygen ( O ), a mild yet
g
g
these facts, Pt NCs could be a promising catalyst for oxidation
1
1
1
2
of alcohols with O under light irradiation. Given that the opꢀ
2
1
3
efficient oxidant, has proven to be a versatile reactive oxygen
species with applications in multiple organic transformations
timized electronic state of Pt surface, similar to that of Pd, is
critical for O activation and oxidation reaction, for which the
2
1
2ꢀ21
as well as photodynamic cancer therapy,
but so far it has
formation of heterojunction structure of metal/semiconductor
22,23
5
9ꢀ63
not been applied to the oxidation of aromatic alcohols.
assumed that the O species might be effective in the oxidaꢀ
It is
is an effective approach.
Therefore, the integration of Pt
1
1
2
NCs with semiconductorꢀlike porphyrinic MOFs as O generꢀ
2
tion of alcohols and its moderate oxidation ability will merit
the selectivity to aldehydes against overꢀoxidation, although
this remains unexplored thus far. In this context, the developꢀ
ator, could achieve their synergistic effects and provide optiꢀ
mized Pt electronic state and an effective catalytic system for
alcohol oxidation.
ment of efficient photosensitizers and catalysts, which are
1
Based on the aforementioned considerations, we have raꢀ
tionally set out to stabilize Pt NCs by porphyrinic MOFs,
namely PCNꢀ224(M) (M represents the metal ion residing in
critical for both O production and the selective alcohol oxiꢀ
2
dation under mild conditions, is highly desirable.
34
Various photosensitizers, such as metal NPs, porphyrins and
their derivatives, have been reported to effectively generate
the center of porphyrin ligand), to afford composite catalysts,
Pt/PCNꢀ224(M). The optimized Pt/PCNꢀ224(Zn) composite
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combines the advantages of both Pt NCs and PCNꢀ224(Zn),
nologies 7890B GC System equipped with an Agilent Techꢀ
nologies 5977B MSD Mass Spectrometer. The metal disperꢀ
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lying in their photothermal effect and O production ability,
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and exhibits excellent catalytic activity and selectivity in the
oxidation of primary alcohols to aldehydes, via a singlet oxyꢀ
sion was measured by CO chemisorption using a fully autoꢀ
mated chemisorption analyzer (Micromeritics, Autochem II
2920) at 323 K. Prior to CO titration, the catalysts were treated
at 423 K for 60 min under a hydrogen gas flow and then
cooled to 323 K under a helium gas atmosphere. The metal
dispersion was calculated assuming a stoichiometry of one CO
molecule per surface metal atom.
genꢀengaged oxidation process by using O under very mild
2
conditions involving visible light irradiation (Scheme 1). To
the best of our knowledge, this is the first report on the singlet
oxygenꢀengaged oxidation of aromatic alcohols and also the
first finding of photothermal effect by MOFs.
Scheme 1. Schematic illustration showing the singlet oxy-
gen-engaged selective oxidation of alcohols over Pt/PCN-
Preparation
of
ligand.
The
tetrakis(4ꢀ
carboxyphenyl)porphyꢀrin (TCPP) and other metalꢀinvolved
TCPP (MꢀTCPP) ligands were synthesized based on previous
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24(M) using molecular oxygen under visible light irradia-
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tion
reports with minor modifications.
Preparation of PCN-224(M) (M: Zn, Ni, Co, Mn and 2H).
PCNꢀ224(M) were prepared based on the reported procedure
34
with minor modifications. For PCNꢀ224(M) (M: Zn, Co, and
H): typically, ZrCl (120 mg), ZnꢀTCPP or CoꢀTCPP or
2
4
H TCPP (40 mg), benzoic acid (1.2 g) and 0.5 mL of acetic
2
acid in 7.5 mL of DMF were ultrasonically dissolved in a 20
mL Teflonꢀlined autoclave and heated in 120 ºC oven for 24 h.
For PCNꢀ224(Ni): ZrCl (40 mg), NiꢀTCPP (40 mg), benzoic
4
acid (1.0 g) and 0.5 mL of acetic acid in 7.5 mL of DMF were
ultrasonically dissolved in a 20 mL Teflonꢀlined autoclave and
heated in 120 ºC oven for 24 h. For PCNꢀ224(Mn): ZrCl (40
4
mg), MnꢀTCPP (40 mg), benzoic acid (1.8 g) and 0.5 mL of
acetic acid in 7.5 mL of DMF were ultrasonically dissolved in
a 20 mL Teflonꢀlined autoclave and heated in 120 ºC oven for
24 h. For all samples, dark brown crystals were harvested by
filtration and washing.
MATERIALS AND METHODS
Preparation of ZIF-8. The synthesis of ZIFꢀ8 powder was
Materials and equipment. All chemicals were from comꢀ
mercial suppliers without further purification unless otherwise
mentioned. Powder Xꢀray diffraction (XRD) patterns were
64
based on a previous procedure with some modifications.
Typically, Zn(NO ) ·6H O (1.68 g) was dissolved in 80 mL of
3
2
2
methanol, followed by a mixture of 2ꢀmethylimidazole (3.70 g)
with 80 mL of methanol was added into above solution with
vigorous stirring for 24 h. The white product was separated by
centrifugation and washed thoroughly with methanol, and
TM
measured on a Japan Rigaku SmartLab rotation anode Xꢀray
diffractometer equipped or Holland X’Pert PRO fixed anode
Xꢀray diffractometer equipped with graphite monochromaꢀ
tized Cu Kα radiation (λ = 1.54178 Å). The UVꢀVis absorpꢀ
tion spectra were recorded on a UVꢀVis spectrophotometer
o
finally dried for 12 h at 50 C. The ZIFꢀ8 powder was further
o
activated at 200 C under vacuum prior to use.
(UVꢀ2700, Shimadzu) in the wavelength range of 200ꢀ800 nm.
Preparation of branched Pt nanocrystals (~10 nm). The
The content (6.3 wt%) of Pt in Pt/PCNꢀ224(Zn) was quantified
by an Optima 7300 DV inductively coupled plasma atomic
emission spectrometer (ICPꢀAES). The size, morphology and
microstructure of Pt/PCNꢀ224(Zn) were investigated by
transmission electron microscopy (TEM) on JEOLꢀ2010 with
an electron acceleration energy of 200 kV and a scanning elecꢀ
tron microscopy (SEM, Zeiss Supra 40 scanning electron miꢀ
croscope at an acceleration voltage of 5 kV). The Xꢀray phoꢀ
toelectron spectroscopy (XPS) measurements were conducted
by using an ESCALAB 250 highꢀperformance electron specꢀ
trometer using monochromated Al Kα radiation (hν = 1486.7
eV) as the excitation source. Nitrogen sorption isotherms were
measured by an automatic volumetric adsorption equipment
branched Pt nanocrystals (NCs) in good dispersion were preꢀ
6
5
pared according to reported procedure with modifications.
Typically, a volume of 2.5 mL of ethylene glycol (EG) was
o
preheated at 120 C for 10 min. Then, 94 µL of 0.375 M polyꢀ
vinylpyrrolidone (PVP, M.W. = 55,000) and 47 µL of 0.0625
M H PtCl ·6H O aqueous solution was added to the EG every
2
6
2
3
0 s interval over a 16 min period with constant vigorous stirꢀ
ring (total volumes for PVP and H PtCl ·6H O solutions are 3
2
6
2
mL and 1.5 mL, respectively). The resulting mixture was kept
o
at 120 C for an additional 10 min and then cooled down to
room temperature. The product was centrifuged at 14,000 rpm
for 5 min and washed thoroughly with water for several times
to remove excess PVP. The precipitate was collected and reꢀ
dispersed in water to obtain an aqueous solution of Pt NCs (2
mg/mL) with size of ~10 nm.
(Micromeritics, ASAP 2020). Prior to nitrogen adsorpꢀ
tion/desorption measurements, the asꢀsynthesized samples
were activated in DMF and acetone, and then dried in vacuum
o
at 60 C. Following that, the product was dried again by using
Preparation of Pt/PCN-224(M) and Pt/ZIF-8 composites.
The Pt/PCNꢀ224(M) or Pt/ZIFꢀ8 composites were prepared by
introducing asꢀsynthesized MOF into the synthetic system of
Pt NCs (0.0256 mmol). Typically, the mixture of PCNꢀ224(M)
or ZIFꢀ8 (50 mg) in ethylene glycol (EG, 2.5 mL) was ultraꢀ
o
the “outgas” function for 12 h at 80 C. Electron spin resoꢀ
nance spectra (ESR) were recorded on JESꢀFA200 electron
paramagnetic resonance spectrometer under visibleꢀlight irraꢀ
diation (λ > 400 nm). Catalytic oxidation reaction products
were analyzed and identified by gas chromatography (GC,
Shimadzu 2010 Plus with a 0.25 mm × 30 m Rtx®ꢀ5 capillary
column). GCꢀMS analysis was performed on an Agilent Techꢀ
o
sonically dispersed, and then preheated at 120 C for 10 min.
Following that, 94 µL of 0.375 M polyvinylpyrrolidone (PVP,
M.W. = 55,000) and 47 µL of 17 mM H PtCl ·6H O aqueous
2 6 2
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(4) superoxide dismutase (SOD, 1350 unit/mL, 500 ꢂL), reꢀ
solution was added to the EG every 30 s interval over a 16ꢀ
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min period with constant vigorous stirring (total volumes for
PVP and H PtCl ·6H O solutions are 3 mL and 1.5 mL, reꢀ
spectively.
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The 2,2,6,6-tetramethyl-4-piperidone hydrochloride (4-
oxo-TMP) oxidation and ESR analysis. A 10 µL aqueous
suspension of PCNꢀ224(Zn) (3.2 mg/mL) or 1 µL aqueous
suspension of Pt nanocrystals (2 mg/mL) or 10 µL aqueous
suspension of Pt/PCNꢀ224(Zn) (3.2 mg/mL) was mixed with
500 µL of 4ꢀoxoꢀTMP solution (50 mM). The measurements
were conducted at room temperature under incident light irraꢀ
diation with USHIO Optical Modulex SXꢀU1501XQ (500 W,
λ > 400 nm) or in dark.
o
spectively). The resulting mixture was kept at 120 C for an
additional 10 min and then cooled down to room temperature.
The obtained product was collected by centrifugation, and
washed thoroughly several times with water and ethanol. The
synthesized sample was further dried overnight at 323 K under
dynamic vacuum for further use. The PVP concentration was
changed to 1.125 M for synthesis of ~2.5 nm Pt NCs and no
PVP was added for ~14 nm Pt NCs, while other parameters
were fixed.
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Confirmation experiments for singlet oxygen production.
Some classical probe molecules, such as 1,3ꢀ
Preparation of Pd octahedron nanocrystals (~15 nm).
1
6
The Pd octahedron nanocrystals were prepared according to
diphenylisobenzofuran (DPBF),
9,10ꢀdimethylanthracene
6
6
6
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reported procedure. Typically, 0.105 g of PVP, 0.060 g of
citric acid and 0.060 g of Lꢀascorbic acid were dissolved in 8
mL of deionized water in a 3ꢀneck flask. The mixture was
heated in air at 120 ºC for 5 min. The Pd stock solution (0.065
g K PdCl in 3 mL of deionized water) was then added dropꢀ
(
DMA), singlet oxygen sensor green (SOSG, Molecular
6
8
69
Probe) and terpene hydrocarbons (αꢀterpinene), have been
used to detect singlet oxygen. The test based on DPBF was
carried out using a visible light source at room temperature.
Typically, 10 mg of DPBF was dissolved in 4 mL of DMF
with 10 µL aqueous suspension of PCNꢀ224(Zn) (3.2 mg/mL).
The stock solutions of fluorescent DMA or SOSG were preꢀ
pared by dissolving 6 mg of DMA in 6 mL of dimethyl
formamide or 100 ꢂg of SOSG in 1 mL of methanol. Then 2
2
4
wise into the flask within 30 min. The reaction was kept at 120
ºC in air for 3 h. The precipitate was washed with acetone and
then with ethanol thoroughly to remove the PVP and other
molecules by centrifugation.
Photocurrent measurement. Photocurrent measurements
were performed on a work station (Chenhua Instrument,
Shanghai, China) in a standard threeꢀelectrode system with the
photocatalystꢀcoated FTO as the working electrode, Pt plate as
the counter electrode, and Ag/AgCl as the reference electrode.
A 300 W Xenon lamp with a visible light cutꢀoff filter (> 400
nm) was used as the light source. A 0.2 M Na SO electrolyte
ꢂL of the sensor solution was mixed with 3 mL of D O with or
without 10 µL of aqueous suspension of PCNꢀ224(Zn) (3.2
mg/mL).
2
The 5,5-dimethyl-1-pyrroline N-oxide (DMPO) ring
opening reaction and corresponding ESR analysis. A 10 µL
aqueous suspension of Pt/PCNꢀ224(Zn) (3.2 mg/mL) was
mixed with 0.5 mL of water. Then 20 µL of DMPO was added
into the solution. The measurements were conducted at room
temperature under incident light irradiation with USHIO Optiꢀ
cal Modulex SXꢀU1501XQ (500 W, λ > 400 nm).
2
4
was bubbled with nitrogen for 20 min before the measureꢀ
ments. The asꢀsynthesized Pt NCs, PCNꢀ224(Zn), and
Pt/PCNꢀ224(Zn) composite (5 mg) were added into 2 µL
Nafion and 1 mL ethanol mixed solution, and the working
electrodes were prepared by dropping the suspension (100 µL)
onto the surface of a FTO plate. The working electrodes were
dried at room temperature, and the photoresponsive signals of
the samples were measured under chopped light at 0.6 V.
Alcohol oxidation reactions conducted at ambient tem-
perature. In general, a mixture of 1.25 mg of Pt NCs or 20 mg
of 6.3 wt% Pt/PCNꢀ224(Zn) (determined by ICP analysis) or
PCNꢀ224(M), 20 µmol of alcohol compounds (or toluene) and
1
0 mL water was placed in a threeꢀneck flask (25 mL). The
Mott-Schottky plot measurement. MottꢀSchottky plots of
PCNꢀ224(Zn) were measured on a electrochemical workꢀ
station (Zahner Zennium) in a standard threeꢀelectrode system
with the photocatalystꢀcoated electrode as the working elecꢀ
trode, Pt plate as the counter electrode, and Ag/AgCl as the
reference electrode at frequencies of 1000, 1500, and 2000 Hz,
respectively. A 0.5 M Na SO solution deoxygenated using N
catalytic oxidation reaction was initiated at ambient temperaꢀ
o
ture (~16 C) with 1 atm O under visible light irradiation with
2
a 300W Xe lamp (λ > 400 nm, PLSꢀSXE300, Beijing Philae
Technology Co., LTD, China). The solution temperature of the
reaction system was monitored throughout the reaction. After
the reaction, the catalyst was separated by centrifugation,
thoroughly washed with water and ethanol, and then reused in
subsequent runs. The yield of the product was analyzed by GC
with nꢀdodecane as the internal standard. For comparison, 20
mg of Pt/ZIFꢀ8 as a catalyst was examined for the reaction
while all other reaction parameters and processes were kept
the same. For the testing of oxidation ability of H O for benꢀ
2
4
2
stream was used as the electrolyte. The preparation procedure
of working electrode is similar to that for the photocurrent
measurement described above.
3
,3´,5,5´-tetramethylbenzidine (TMB) oxidation. TMB (5
mg) was mixed with 3 mL of HAc/NaAc buffer solution (0.2
2
2
M : 0.2 M) and 1 mL of H O. Then, an aqueous suspension of
zyl alcohol, 1.25 mg of Pt NCs, 1 mL of H O , 9 mL of H O
and 20 µmol of benzyl alcohol was mixed in the absence of
light irradiation.
2
2 2 2
100 µL of PCNꢀ224(Zn) (3.2 mg/mL), or 10 µL of Pt nanoꢀ
crystals (2 mg/mL), or 100 µL of Pt/PCNꢀ224(Zn) (3.2
mg/mL), was added into the reaction mixture with O bubbling
2
Alcohol oxidation reactions conducted at a fixed tem-
perature (T = 16 C). In general, a mixture of 20 mg of 6.3
wt% Pt/PCNꢀ224(M) or PCNꢀ224(M), 20 µmol alcohol and 10
mL water was placed in a photoꢀcatalytic reactor. The temperꢀ
ature of mixture solution was fixed at 16 C by flowing conꢀ
densate water. The catalytic oxidation reaction was carried out
in the presence of 1 atm O under visible light irradiation with
a 300W Xe lamp (λ > 400 nm). The yield of the product was
analyzed by GC with nꢀdodecane as the internal standard.
o
at 20 °C under visible light irradiation. The concentration of Pt
NCs was kept the same as that in Pt/PCNꢀ224(Zn). The samꢀ
ples were taken at different time intervals for UVꢀVis measꢀ
urements. The measurements were also performed under other
o
gas environments (N and air). To verify the type of active
2
oxygen species, different scavenger molecules were added into
the solution prior to the UVꢀVis measurements: (1) carotene
2
(10 mg); (2) mannite (2 mg); (3) catalase (200 unit/mL, 2 mg);
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Alcohol oxidation reaction over Pd octahedron nano-
crystals. In general, a mixture of 2.5 mg of Pd octahedra, 20
µmol of benzyl alcohol and 10 mL of water was placed in a
ning electron microscopy (SEM) image shows that PCNꢀ
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5
6
224(Zn) has uniform size (~800 nm) and is mostly in octaheꢀ
dral shape (Figure S5b), and Pt NCs are uniformly dispersed
on the surface of PCNꢀ224(Zn) in the composite (Figure 1d
and S5c,d).
threeꢀneck flask (25 mL). The catalytic oxidation reaction was
o
initiated at 80 C with 1 atm O in the absence of visible light
2
irradiation.
The O activation is a key step in the O ꢀinvolved oxidation
2
2
reactions. To examine the ability of Pt/PCNꢀ224(M) compoꢀ
RESULTS AND DISCUSSION
sites for O activation, TMB was employed as the initial model
2
A series of chemically stable PCNꢀ224(M) (M = Zn, Ni, Co,
compound to undergo oxidation over Pt/PCNꢀ224(Zn) as a
representative. Upon oxidation by different active oxygen
species, the colorless TMB was changed to two colored prodꢀ
ucts with characteristic UVꢀVis peaks at 370 and 652 nm (parꢀ
tially oxidized product I, blue), as well as 450 nm (fully oxiꢀ
o
Mn, 2H) were obtained via solvothermal reactions at 120 C
3
4
according to previous procedure (Figure S1). Their structures
contain both channels and cages with sizes as large as ~19 Å
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
Figure S2). N2 sorption experiment on the representative
7
0
PCNꢀ224(Zn) indicates high porosity and a large BET surface
dized product II, yellow), respectively (Figure S6). Interestꢀ
ingly, TMB molecules were mainly oxidized into partially
oxidized intermediate by Pt NCs, PCNꢀ224(Zn), and Pt/PCNꢀ
2
area of 2018 m /g (Figure S3). UVꢀVis spectrum of PCNꢀ
2
(
24(Zn) shows strong adsorption in the range of 300ꢀ800 nm
Figure 1a), revealing the photon absorption and electronꢀhole
2
24(Zn) composite (Figure S6), revealing their moderate oxiꢀ
separation ability upon visibleꢀlight irradiation. The positive
slopes of MottꢀSchottky plots for PCNꢀ224(Zn) at frequencies
of 1000, 1500 and 2000 Hz are consistent with the behavior of
a typical nꢀtype semiconductor (Figure 1b). The flat band posiꢀ
tion (LUMO) determined from the intersection is ~ꢀ0.80 V vs
Ag/AgCl or ꢀ0.58 V vs normal hydrogen electrode (NHE). The
valence band (HOMO) is then calculated to be 1.25 V vs
NHE, considering that the band gap energy of PCNꢀ224(Zn) is
estimated to be 1.83 V from Tauc plot (Figure S4).
dation power, which is essential for selective oxidation of priꢀ
mary alcohols into aldehydes but not carboxylic acids. This
result prompted us to compare the oxidation of benzyl alcohol
with O by Pt/PCNꢀ224(Zn) versus KMnO . While excellent
2
4
selectivity for benzaldehyde was achieved with Pt/PCNꢀ
24(Zn), the stronger oxidant KMnO gave benzoic acid as a
2
4
main product (Figure S7).
0.25
0.25
0.20
0.15
0.10
0.05
0.00
a
b
Without quencher
Adding OH quencher
Adding H O quencher
O2
⋅
air
N2
0.20
2
2
-
a
b6
Adding O2 quencher
Pt NCs
PCN-224(Zn)
Pt/PCN-224(Zn)
LUMO
-
0.58 V
1.83 eV
.25 V
0.15
5
0.10
0.05
0.00
1
1
4
HOMO
Adding O quencher
2
3
2
0
10
20
30
40
0
10
20
Time (min)
Pt/PCN-224(Zn)
30
40
2
000 Hz
1500 Hz
000 Hz
Time (min)
-0.8 V
1
0.25
cPt/PCN-224(Zn)
d
1
Pt and PCN-224(Zn) mixture
0
0.20
0.15
0.10
Pt NCs
PCN-224(Zn)
blank
3
00
400
500
600
700
800
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Potential (V) vs Ag/AgCl
Pt/PCN-224(Zn) + dark
Wavelength (nm)
Pt/PCN-224(Zn) + carotene
Pt NCs
c
d
0.05
0.00
PCN-224(Zn)
blank
Pt (0.195 nm)
0
10
20
30
40
Time (min)
Figure 2. UVꢀVis peak intensity for the TMB oxidation product
vs reaction time (a) over Pt/PCNꢀ224(Zn) in various gas environꢀ
ments, (b) over Pt/PCNꢀ224(Zn) in the presence of different scavꢀ
engers, and (d) over different catalysts in bubbled O . (c) ESR
Figure 1. (a) UVꢀVis spectra of PCNꢀ224(Zn), Pt NCs and
Pt/PCNꢀ224(Zn). (b) MottꢀSchottky plots for PCNꢀ224(Zn) in 0.2
M Na SO aqueous solution (pH 6.8); inset: the energy diagram of
2
spectra of the samples after mixing 4ꢀoxoꢀTMP solution with
PCNꢀ224(Zn), Pt NCs, Pt/PCNꢀ224(Zn) in the absence or presꢀ
ence of carotene under visible light irradiation or in dark. A curve
for the 4ꢀoxoꢀTMP solution was collected as the reference.
2
4
the HOMO and LUMO levels of PCNꢀ224(Zn). (c) TEM and
HRTEM (inset) images for Pt NCs. (d) TEM and enlarged TEM
images for Pt/PCNꢀ224(Zn).
The dramatically different oxidation rates for TMB in variꢀ
ous gas environments (N , O and air) suggest that active oxiꢀ
Pt NCs were obtained by reacting H PtCl ·6H O in ethylene
2
2
2
6
2
dative species is indeed evolved from O (Figure 2a and S8).
glycol (EG)/H O in the presence of polyvinylpyrrolidone
2
2
65
To clarify which active oxygen species is actually generated in
our system, various scavengers including carotene, mannite,
catalase, and superoxide dismutase (SOD) were introduced
into reaction system to specifically inhibit the generation of
(
PVP) at 120 °C for 10 min. Transmission electron microsꢀ
copy (TEM) and highꢀresolution TEM (HRTEM) images
clearly show the branched Pt NCs in tripod or tetrapod shape
with good dispersion, uniform size (~10 nm), and great crysꢀ
tallinity (Figure 1c and S5a). The broad adsorption of Pt NCs
in UVꢀVis spectrum indicates the SPR property (Figure 1a).
The Pt/PCNꢀ224(M) composite was assembled by preꢀaddition
of PCNꢀ224(M) into the synthetic system of Pt NCs. The crysꢀ
tallinity and structure of PCNꢀ224(M) are well maintained
upon combination with Pt NCs (Figure S1). The slight deꢀ
crease of surface area in reference to parent PCNꢀ224(Zn) is
ascribed to the mass occupation of Pt NCs (Figure S3). Scanꢀ
1
•−
71
O , •OH, H O , and O , respectively. As shown in Figure
2
2
2
2
2
b, only carotene significantly suppresses the oxidation of
TMB, indicating that the main active species generated by
1
Pt/PCNꢀ224(Zn) composite is O . The fact that oxidation is
2
not completely inhibited by carotene suggests that some minor
1
oxidation pathways other than O oxidation might be also in
2
effect. Furthermore, 4ꢀoxoꢀTMP, a wellꢀknown probe moleꢀ
1
1
cule for O due to its ability to trap O to afford the stable
2
2
ACS Paragon Plus Environment

Page 5 of 10
Journal of the American Chemical Society
1
o
nitroxide radical 4ꢀoxoꢀTEMPO, further confirms the O genꢀ
both Pt NCs and PCNꢀ224(M), and 36 C for Pt/PCNꢀ224(Zn),
2
o
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
eration through electron spin resonance (ESR) measurement.
evidently higher than the resulting temperature (18 C) of the
ESR spectra display 1:1:1 triplet with a gꢀvalue of 2.030 charꢀ
acteristic signals of 4ꢀoxoꢀTEMPO for all the samples (Figure
reaction system in the absence of catalyst or the presence of
ZIFꢀ8 (Table S1). Both Pt NCs and PCNꢀ224(M) obviously
have photothemal effect, which is synergistically enhanced in
Pt/PCNꢀ224(M). We propose that visible lightꢀresponsive
porphyrin unit in PCNꢀ224(M) is responsible for converting
the light to heat, as other nonꢀporphyrin based MOFs like ZIFꢀ
8 cannot. To the best of our knowledge, neither the photoꢀ
thermal effect of MOFs nor the synergistic photothermy beꢀ
tween Pt NCs and MOFs has been reported previously.
7
2
1
2c), demonstrating the formation of O or at least a species
2
1
that displays chemical properties of O . Amongst Pt/PCNꢀ
2
2
24(Zn), its two components, and the simple physical mixture
of two components, Pt/PCNꢀ224(Zn) exhibits the strongest
UV absorbance for TMB oxidation product (Figure 2d) and
the strongest signals of 4ꢀoxoꢀTEMPO (Figure 2c), confirming
1
the synergy between Pt NCs and the MOF for O producꢀ
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
16,26
tion.
The signals are almost completely quenched by caroꢀ
Table 1. Oxidation of benzyl alcohol by O over different
2
tene or in the dark, again manifesting that they are originated
catalysts under visible light irradiation started from ambi-
1
1
from O indeed and light is necessary for O generation. To
o
a
2
2
ent temperature (16 C)
1
further confirm the generation of O based on Pt/PCNꢀ224(Zn)
by light irradiation, a variety of classical O probe molecules
2
b
1
Time
(min)
30
Yield
(%)
84
o
2
Entry
Catalyst
T ( C)
including DPBF, DMA, and SOSG, have been examined. All
34
36
36
26
28
26
28
28
28
16
16
27
27
30
28
28
28
30
30
26
obtained results corroborate each other and reveal that active
1
O species is indeed formed in our reaction system (Figure
2
1
Pt/PCNꢀ224(Zn)
50
>99
>99
39
S9ꢀ11).
12 h
30
1
Since oxidative species other than O could be responsible
2
2
3
4
Pt NCs
for our observed reactions, we checked whether any of them is
formed in significant amount. The radical trapping agent
60
56
3
6
0
0
5
•−
PCNꢀ224(Zn)
DMPO is particularly useful for identifying O2 and •OH. The
5
ESR spectra display a 1:1:1 triplet signal consistent with
1
30
37
DMPO/ O . Neither 1:1:1:1 quartet signal from DMPO/•O H
Pt + PCNꢀ224(Zn)
Pt/PCNꢀ224(Zn)
Pt/PCNꢀ224(Ni)
Pt/PCNꢀ224(Co)
Pt/PCNꢀ224(Mn)
Pt/PCNꢀ224
2
2
6
3
6
0
0
0
62
nor 1:2:2:1 quartet signal from DMPO/•OH was detected
Figure S12), disqualifying radicals as responsible oxidants in
52
(
c
5
7
3
86
our reaction system. αꢀTerpinene is an excellent probe comꢀ
pound for our system because it will be converted to ascaꢀ
30
60
59
1
•−
6
7
8
9
ridole with O but to other aromatic products with O and
•
97
2
2
69
OOH. Ascaridole was detected as the nearly exclusive
3
6
0
0
52
product after 4 h in our reaction conditions, again corroboratꢀ
83
1
ing O engagement (Figure S13). Our proposed mechanism is
2
1
30
60
61
consistent with the generation of O but inhibited conversion
2
•−
81
of O to O based on PCNꢀ224(Zn) upon light irradiation
2
2
30
41
(Scheme S1).
60
62
With the above information in hands, we are in a position to
investigate the photoꢀoxidation of aromatic alcohols by O2
over Pt/PCNꢀ224(M) or related catalysts under visible light
10
11
Pt/ZIFꢀ8
Pt/PCNꢀ224(Zn)
Pd NCs
60
52
d
30
16ꢀ35
35
e
irradiation. The reaction has been conducted with benzyl alcoꢀ
12
60
80
25
34
36
38
>99
23
o
f
hol in water with 1 atm O at ambient temperature (16 C). The
13
Pt NCs
120
30
2
data for catalytic oxidation of benzyl alcohol in Table 1 indiꢀ
cates that Pt/PCNꢀ224(Zn) exhibits the highest catalytic activiꢀ
ty among all catalysts with 100% selectivity for aldehyde proꢀ
duction and the reaction is complete in 50 min (entry 1). The
reaction does not produce benzoic acid even it is extended to
longer reaction time, manifesting the superb selectivity of
catalyst (entry 1). Based on the 3.4% of accessible Pt sites
from a CO titration experiment (Figure S14), the turnover
Pt/PCNꢀ224(Zn)
15
1
4
+
carotene
60
31
g
1
5
Pt/PCNꢀ224(Zn)
12 h
>99
a
Reaction conditions: catalyst (20 mg, 6.3 µmol Pt), alcohol (20
2
µmol), 10 mL H O, visible light (λ > 400 nm, 100 mW/cm ), O
(1 atm). Yield of aldehyde was analyzed by GC, and nꢀdodecane
was used as the internal standard. At a fixed reaction temperature
of 16 C. The reaction was from 16 to 35 C within 30 min by
external heating in the absence of light irradiation, with the heatꢀ
ing speed similar to that by photothermal effect. Pd NCs (2.5 mg),
80 C, in the absence of light. H O (1 mL), H O (9 mL), alcohol
(20 µmol), without O bubbling and visible light irradiation. alꢀ
2
2
b
c
o
d
o
ꢀ1
frequency (TOF) is calculated to be as high as 269 h . Under
e
identical conditions, only 56% of benzyl alcohol is oxidized
over Pt NCs after 60 min, and negligible activity can be deꢀ
tected with PCNꢀ224(Zn) (entries 2, 3), suggesting that alcoꢀ
hols can be activated on the Pt surface while the MOF has no
o
f
2
2
2
g
2
cohol (1 mmol), catalyst (31.5 µmol Pt).
9
such ability. The mere 62% yield for the physical mixture of
The synergistic photothemal conversion of Pt/PCNꢀ224(Zn)
is an important finding as photoꢀinduced heat is highly desiraꢀ
ble for endothermal reactions. When the reaction temperature
Pt NCs and PCNꢀ224(Zn) (entry 4), significantly lower than
that of the composite, should be ascribed to the absence of
interaction between separated Pt NCs and the MOF.
o
was fixed at 16 C to eliminate photoꢀinduced heat, only 52%
An unusual photothermal effect has been observed for the
reaction. Upon visible light irradiation for 30 min, the reaction
and 86% yields can be obtained for Pt/PCNꢀ224(Zn) after 30
min and 60 min, respectively (Table 1, entry 5 and Figure 3).
Pt/ZIFꢀ8 gave a similar activity to Pt NCs (entries 2, 10),
o
o
temperature spontaneously increases from 16 C to 26 C for
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 10
mainly because of that ZIFꢀ8 cannot convert light into heat. It entry 14), it showed no response to varying amounts of other
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
is noteworthy that the catalytic activities of Pt/PCNꢀ224(M)
composites increase with more d electrons of central metals
(no metal, Mn, Co, Ni, Zn) in the porphyrin (entries 1, 6ꢀ9).
To exclude the influence of photothermal effect, reactions
with different catalysts were conducted under identical condiꢀ
quenchers even at over two orders of magnitude of theoreticalꢀ
ly consumed amount (Figure S17). These results unambiguꢀ
ously rule out the other three types of ROS and demonstrate
1
the involvement of O from O activation by Pt/PCNꢀ224(M)
2
2
in the reaction of benzyl alcohol. The above multiple evidence
o
1
tions at a fixed temperature (16 C), and the same activity orꢀ
collectively supports that O is indeed generated (see SI, Secꢀ
2
der was observed (Table S2). It is assumed that the d electron
number of the metal centered in metalloporphyrin in MOFs
influences the triplet states of porphyrin, which play important
tion 4) and is truly engaged and responsible in our benzyl alꢀ
cohol oxidation system. We have proposed a tentative reaction
pathway (Scheme S3). However, the exact mechanism, includꢀ
1
roles in both the activation ability of triplet O (Scheme S2)
ing the specific roles played by O to boost the reaction, reꢀ
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
19,22
and possibly the energy transfer between MOF and Pt,
mains to be clarified by more experiments and/or theoretical
1
reasonably explaining the distinct activity in Pt/PCNꢀ224(M).
calculations. Since O alone is inefficient at oxidizing benzyl
2
1
alcohol, we propose that visible light, photothermal effect, O ,
2
100
and the diamagnetism of metal ion residing in porphyrin cenꢀ
ter synergistically contribute to the excellent activity and seꢀ
lectivity in the oxidation of benzyl alcohol to benzaldehyde
e
c
t
n
u
a
b
o
istur
it
h
80
60
40
t, w
h
nal d
li
g
xter
e
using O over the Pt/PCNꢀ224(M) composite.
2
In consideration of the potential for practical applications,
the reaction with increased amount of substrate was also conꢀ
ducted (entry 15). With 1 mmol (~0.10 g) of alcohol the reacꢀ
tion took longer time (12 h) to finish, but it is remarkable that
exclusive and quantitative formation of benzaldehyde is still
achieved without external heat input. Noteworthily, in addition
to the representative Pt/PCNꢀ224(Zn) with ~9 nm Pt NCs deꢀ
scribed above, the Pt/PCNꢀ224(Zn) with Pt NCs of ~3 and ~14
nm have been synthesized (Figure S16) and they also present
o
20
0
0
10
20
30
40
50
60
Time (min)
Figure 3. The oxidation yield of benzyl alcohol vs reaction time
over Pt/PCNꢀ224(Zn) in the presence or absence of light irradiaꢀ
tion or external heating, while other reaction parameters being
kept unchanged.
1
00% selectivity toward benzaldehyde, though lower activity
compared to that with ~9 nm Pt NCs, in the oxidation of benꢀ
zyl alcohol under identical reaction conditions (Table S3).
To assess the importance of light, the catalytic oxidation
over Pt/PCNꢀ224(Zn) was carried out with all parameters unꢀ
altered except changing light irradiation to heating, in which
the yield was only 35% after 30 min, far below that under
visible light irradiation (entry 11 and Figure 3).
a
100
b100
90
90
80
70
80
70
60
50
40
30
20
1
0 min
20 min
30 min
60 min
6
0
0
5
1
0 min
While all experimental evidences point to the conclusion
40
1
20 min
30 min
50 min
30
that O or some other species from it is responsible for our
2
1
3,22,23
20
observed alcohol oxidation, which was rarely reported.
60
80
100
120
mW/cm2
)
140
60
80
100
120
140
1
Pd octahedron nanocrystals was reported to generate O soleꢀ
ly and oxidize glucose. We synthesized Pd NCs according to
literature procedures and applied it to benzyl alcohol oxidation
2
2
Light intensity
(
Light intensity (mW/cm )
1
3
Figure 4. The oxidation yield of benzyl alcohol over Pt/PCNꢀ
224(Zn) at different light intensities under visible light irradiation
at (a) spontaneous reaction temperatures, and (b) a fixed reaction
(
Figure S15). The reaction achieved complete conversion to
o
benzaldehyde after 1 h at 80 C in water (entry 12). More imꢀ
portantly, when the reaction was performed at ambient temꢀ
o
temperature (16 C).
perature, 35% conversion to benzaldehyde was achieved after
1
To further understand the role of visible light and the interꢀ
action between Pt and PCNꢀ224(M), the catalytic performance
of Pt/PCNꢀ224(Zn) as a representative has been investigated
for benzyl alcohol oxidation under varying light intensities
1
2 h (data not shown). This is clear that O is engaging the
2
selective oxidation of alcohol to aldehyde, albeit in low effiꢀ
ciency with Pd NCs. Moreover, ESR analysis of the radical
trapping agent DMPO excludes the formation of the radicals
2
•
−
(60~140 mW/cm , λ > 400 nm). The reaction yields increase
(e.g., O2 and •OH), as indicated above (Figure S12). Another
possible oxidant that could be formed in our reaction system is
hydrogen peroxide. However, the direct introduction of H O
2
with enhanced light intensity from 60 to 100 mW/cm , and
then decrease when the light intensity is further increased
(Figure 4a). The positive relationship between reaction yields
2
2
1
into the benzyl oxidation reaction to replace O results in
2
2
and light intensity at the range of 60ꢀ100 mW/cm can be exꢀ
much lower activity, indicating that H O is not the responsiꢀ
2
2
plained by photothermal effect, as stronger light intensity
leads to a higher reaction temperature which is favorable for
the endothermic process (Figure S18 and Table S4). The drop
of reaction yield after the light intensity further increases beꢀ
ble oxidant in our reaction system (entry 13). To further exꢀ
clude the formation of other reactive oxygen species (ROS)
1
and/or the conversion of O into other ROS in the reaction
2
system, four specific scavengers that exclusively inhibit the
2
1
•−
yond 100 mW/cm , however, indicate that other factors also
generation of O , •OH, H O , and O species, respectively,
2
2
2
2
control the reaction. Reactions were thus conducted at a conꢀ
were introduced into the reaction system in varying amounts.
o
stant temperature (16 C) to exclude photothermal effect, and
While the reaction yield quickly decreased with increasing
1
similar correlation between reaction yield and light intensity
was observed (Figure 4b). The results imply the presence of
electron transfer between Pt NCs and PCNꢀ224(Zn) under
amount of the O quencher carotene (only 31% yield was
2
1
obtained even after 60 min by adding 20 mg of O quencher;
2
ACS Paragon Plus Environment

Page 7 of 10
Journal of the American Chemical Society
from the plasmonic Pt NCs to reach the MꢀS interface and
light irradiation and thus the electron density of Pt surface
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
changes along with light intensity. It has been reported that O2
activation and oxidation reaction are positively correlated with
the electron density on the surface of Pdꢀgroup metals.
enter the LUMO of PCNꢀ224(Zn) at high light intensity (>100
mW/cm ). The flow of electrons in Pt/PCNꢀ224(Zn) compoꢀ
site at different light intensities is well consistent with the
catalytic results and the mechanism is illustrated in Scheme 2.
2
13
Meanwhile, the modulation of the charge state of active metal
surface has been well documented in the noble metꢀ
al/semiconductor (MꢀS) hybrid configuration based on the
electron transfer via Schottky junction and/or plasmonic efꢀ
The recycling stability of a heterogeneous catalyst is of
great importance for its practical application. Delightedly, the
catalytic activity of Pt/PCNꢀ224(Zn) is well retained even after
six runs of benzyl alcohol oxidation without any treatment or
activation (Figure 5a). No significant loss of crystallinity for
PCNꢀ224(Zn) was observed from the PXRD patterns (Figure
S21) and aggregation of Pt NCs does not occur from TEM
image after reaction (Figure S5b), suggesting excellent recyꢀ
clability and stability of Pt/PCNꢀ224(Zn). XPS measurements
show that the binding energies of Pt 4f5/2 and 4f7/2 remain well
before and after reaction, indicating the unchanged oxidation
state of Pt during the reaction and excellent stability of
Pt/PCNꢀ224(Zn) (Figure S22).
59ꢀ63
fect.
In our system, both Pt NCs and PCNꢀ224(Zn) are able
to accept photons of visible light and can be excited to proꢀ
duce photoelectrons. Upon visible light irradiation with relaꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
tively low intensity (<100 mW/cm ), PCNꢀ224(Zn) is readily
photoexcited and electronꢀhole pairs are separated. The elecꢀ
trons on the conductive band (LUMO) of PCNꢀ224(Zn), with
the behavior of an nꢀtype semiconductor, flow to the Fermi
level of Pt, promoted by the presence of Schottky junction.
Higher light intensity would induce the delivery of more elecꢀ
trons to the Pt surface, making it more electronꢀrich, which is
1
beneficial to the activation of O to O and thus improves the
2
2
a 100
b
catalytic activity. When the light intensity further increases
2
80
(>100 mW/cm ), the hot electrons of Pt NCs can be generated
by the excitation of surface plasmons (plasmonic effect),
which is more competitive than Schottky junction for electron
transfer between Pt NCs and PCNꢀ224(Zn). As a result, the
electrons from Pt NCs are injected into the conductive band
60
40
20
0
(
LUMO) of PCNꢀ224(Zn), producing the electronꢀdeficient
1
surface of Pt sites, which is disadvantageous to O generation
2
1
2
3
4
5
6
and lowers catalytic activity. The above results indicate that
the electronic density of Pt surface can be flexibly regulated
Cycles
Figure 5. (a) Recyclability of Pt/PCNꢀ224(Zn) in the oxidation of
benzyl alcohol. (b) TEM image of Pt/PCNꢀ224(Zn) after catalytic
reaction.
by adjusting light intensity, which holds the key to modulation
1
of O generation and the catalytic activity of alcohol oxidaꢀ
2
tion (Scheme S4).
1
Encouraged by the excellent catalytic performance of
Pt/PCNꢀ224(Zn) in the oxidation of benzyl alcohol, we proꢀ
ceeded to investigate the scope of this reaction. Diverse substiꢀ
tuted benzyl alcohols and heteroaromatic alcohols were comꢀ
pletely oxidized with varying reaction time, but quantitative
conversion as well as complete selectivity for aldehyde over
carboxylic acid or other oxidizing products was obtained in
each case (Table 2). The reactions were completed within 60
min with benzyl alcohol (entry 1) and its substitutions with
Scheme 2. Proposed mechanism for the O generation and
2
electron transfer between Pt and PCN-224(M) in the oxidation
of benzyl alcohol over Pt/PCN-224(M) under visible light ir-
radiation. VB, valence band; CB, conduction band; E repre-
f
sents the Fermi level of PCN-224(M) or Pt NCs; E ´represents
f
the Fermi level of Pt/PCN-224(M) composite
electronꢀwithdrawing groups (ꢀNO , ꢀCl, ꢀBr) (entries 2ꢀ4),
2
while it requires at least 120 min for those bearing electronꢀ
donating groups (ꢀCH , ꢀNH ) or furfuryl alcohol (entries 5ꢀ7).
3
2
The 4ꢀmethoxybenzyl alcohol is an abnormality here (entry 8).
Despite the presence of the electronꢀdonating methoxy group,
the reaction rate is not slowed down as compared to benzyl
alcohol (entry 1). In addition to electronic effect, steric hinꢀ
drance also significantly slows down the reaction (entries 4, 9,
and 10). In the absence of steric hindrance, the 1,4ꢀ
benzenedimethanol can be quickly converted (entry 11), with
a similar catalytic efficiency to that of benzyl alcohol. Howevꢀ
er, under identical conditions, Pt/PCNꢀ224(Zn) does not preꢀ
sent any activity in the oxidation of toluene (entry 12), which
generally requires harsh conditions like stronger oxidant or
high temperature/pressure.
This electron transfer between Pt NCs and PCNꢀ224(Zn)
was further studied by transient photocurrent measurement for
PCNꢀ224(Zn), Pt NCs and Pt/PCNꢀ224(Zn) (Figure S19).
While the current density of separate PCNꢀ224(Zn) or Pt NCs
stays positively correlated with light intensity from 60 to 120
2
mW/cm , the current density of Pt/PCNꢀ224(Zn) composite
initially increases with increasing light intensity up to 100
Table 2. Photo-oxidation of various aromatic alcohols over
2
a
mW/cm then subsequently decreases when light intensity is
Pt/PCN-224(Zn)
further increased. The results corroborate with the trend obꢀ
served for reaction yield vs light intensity and indicate imꢀ
proved charge separation in Pt/PCNꢀ224(Zn) driven by the
Schottky barrier at relatively low light intensity (<100
2
mW/cm ) and the reverse injection of energetic hot electrons
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light irradiation, Pt/PCNꢀ224(Zn), synergizing the ability of its
Time Conversion Selectivity
Entry
Substrate
b
b
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
individual component for the O generation, effectively actiꢀ
(min)
(%)
(%)
2
1
vates O into O , a mild and crucial oxidant for the highly
2
2
selective oxidation of primary alcohols; 3) Among all PCNꢀ
1
2
3
4
5
6
50
>99
100
224(M) explored, PCNꢀ224(Zn) exhibits the greatest ability of
1
O activation and O production due to the strongest diamagꢀ
2
2
1
0
2+
netism with the d configuration of Zn among different metꢀ
60
50
40
>99
>99
>99
>99
>99
>99
100
100
100
100
100
100
als residing in the porphyrin center in the MOF; 4) The elecꢀ
1
tronic state of surface Pt, closely associated with O generaꢀ
2
tion, can be adjusted flexibly by the competition between
Schottky junction and plasmonic effect with varying light inꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
tensities, to achieve optimized O activation; 5) The moderate
2
1
oxidation ability of O effectively prevents the overꢀoxidation
2
and thus gives 100% selectivity to aldehyde products. This
work represents the first report on the acceleration of catalytic
organic oxidations by regulating electronic states of metal
surface in metal/MOF composites and their photothermal efꢀ
fect under visible light irradiation, which will open up a new
door to the development of metal/MOF composites, or even
metal/semiconductors, for photocatalytic organic reactions.
120
120
120
7
8
ASSOCIATED CONTENT
Supporting Information.
Additional figures mentioned in the text. This material is available
free of charge via the ACS Publications website at
http://pubs.acs.org.
50
>99
>99
100
100
9
300
AUTHOR INFORMATION
Corresponding Author
jianglab@ustc.edu.cn
1
1
0
1
180
50
>99
100
Notes
The authors declare no competing financial interest.
c
>99
ꢀ
100
ꢀ
ACKNOWLEDGMENT
We are grateful to the reviewers for their insightful comments and
valuable suggestions. This work was supported by the NSFC
12
120
(
(
21673213, 21371162, and 21521001), the 973 program
2014CB931803), the Recruitment Program of Global Youth
a
Reaction conditions: catalyst (20 mg, 6.3 µmol Pt), alcohol (20
Experts, and the Fundamental Research Funds for the Central
Universities (WK20160190075).
2
µmol), 10 mL H O, visible light (λ > 400 nm, 100 mW/cm ), O
2
2
o
b
(
1 atm), starting temperature: 16 C. Yield of aldehyde was anaꢀ
lyzed by GC, and nꢀdodecane was used as the internal standard.
c
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