Nearly atomic precise gold nanoclusters on nickel-based layered double hydroxides for extraordinarily efficient aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c6cy00186f

A series of nickel-based layered double hydroxides supported nearly atomic precise Au₂₅ nanoclusters catalysts, and especially Au₂₅/Ni_xAl-LDH systems (x = Ni/Al, 2, 3, 4) were fabricated via modified electrostatic adsorpti

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Nearly atomic precise gold nanoclusters on nickel-based layered double
hydroxides for extraordinarily efficient aerobic oxidation of alcohols
Shuai Wang, Shuangtao Yin, Gaowen Chen, Lun Li and Hui Zhang*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A series of nickel-based layered double hydroxides supported nearly atomic precise Au25 nanoclusters
catalysts especially Au25/Ni
adsorption of captopril-capped clusters Au25Capt18 onto the predispersed positively-charged Ni
supports followed proper calcination. Detailed characterizations show that the ultrafine gold clusters of
0.9 nm were well-dispersed on the edge sites of hexagonal plate-like particles of Ni Al-LDH (x=2, 3)
originated from their ordered LDH layers with more Ni-OH sites and strong Au-LDH synergy, while
slightly aggregated to ~1.1 nm on the irregular Ni Al-LDH due to its poor layer structure along with
x
Al-LDH systems (x=Ni/Al, 2, 3, 4) were fabricated via modified electrostatic
x
Al-LDH
1
1
2
0
5
0
~
x
4
doped nickel oxide. The catalysts exhibit excellent activity for selective oxidation of 1-phenylethanol to
acetophenone with molecular oxygen under base-free, and the activity follows an increased order of
4 2 3 3
Au25/Ni Al-LDH < Au25/Ni Al-LDH < Au25/Ni Al-LDH. The Au25/Ni Al-LDH shows the highest
-1
-1
activity with TOF of 6780 h in toluene and 118500 h in solvent-free and can be applied for a wide
range of alcohols, mainly ascribed to the ultrafine gold clusters and the strongest gold-LDH interaction
associated with the highly ordered Ni
Fe-LDH systems. The Au25/Ni
activity. The least-square fit analysis yields the rate constant (k) and apparent activation energy (E
Au25/Ni Al-LDH catalysts for 1-phenylethanol oxidation, and the order of k and E values act in
3
Al-LDH layers. Similar regularity is found in the Au25/Ni
Al-LDH can be reused five times without loss of
) of
x
Mn-LDH
and Au25/Ni3-xMn
x
3
a
x
a
accordance with their reactivities.
Another important aspect is that the reactivity for alcohol
oxidation catalyzed by Au catalysts is generally increased with
Introduction
7
,10
10
reduced sizes of Au NPs.
Tsukuda et al. reported a set of
1
2
3
3
4
4
5
0
5
0
5
Since Haruta et al. reported gold nanoparticles (3.6-8 nm) 50 poly-(N-vinyl-2-pyrrolidone)-stabilised Au clusters with sizes in
dispersed on transition metal oxides exhibited high activity in the
low-temperature oxidation of CO by molecular oxygen in 1989,
supported gold catalysts have been widely explored for a variety
of organic reactions including selective hydrogenation of nitro
1.3-9.5 nm for oxidative dehydrogenation of p-hydroxybenzyl
alcohol in H O with K CO and found that the activity increased
slowly with Au clusters’ sizes reducing from 9.5 to ~4 nm and
2
2
3
then increased rapidly with further reduced size of Au clusters.
2
3
1
1
12
compounds and α, β-unsaturated carbonyl compounds, C-C
5
5
2
Corma et al. and Prati et al. prepared Au/CeO and Au/NiO,
4
5
coupling, hydrochlorination of acetylene, and oxidation of
respectively, by exploring the combinations of small-size Au (<5
nm) and nanocrystalline metal oxides (5 nm), which turned out to
obtain highly active and selective catalysts for the oxidation of
alcohols under base-free condition ascribing to the synergy
6
alcohols. Therein, alcohols oxidized to carbonyl compounds is
one of the most essential reaction in organic synthesis. The
traditional alcohol oxidation technology involves stoichiometric
quantities of inorganic oxidants, such as permanganate and 60 between Au and nanoscaled supports CeO or NiO. However, the
dichromate, which produces a large amount of wastes containing
toxic heavy metals and causes serious environmental problem.
2
required procedures for preparing fine nanoscaled support (for
Au/CeO ) and the requirement of higher O pressure (for Au/NiO)
2
2
Moreover, the additional base (e.g., NaOH, K
2
CO
3
) is needed
would limit their wide application.
while using Au nanoparticles (NPs) catalysts whether colloidal
Layered double hydroxides (LDH), a layered anionic clay
polymer-protected Au NPs or supported Au NPs in liquid- 65 consisting of a positively charged two-dimensional brucite layer
7
,8
6,9
phase aerobic alcohol oxidation, which unavoidably leads to
reactor corrosion and environmental pollutions. From the view of
green chemistry, it has great theoretical and practical significance
to develop a route for the oxidation of alcohols with molecular
oxygen upon a heterogeneous bifunctional catalyst combining Au
nanoparticles with a solid base.
with interlayer anions such as carbonate, hydroxide, and/or other
1
3,14
desired anions,
has progressively received attention as
catalysts or supports due to its unique surface acid-basic property
1
5,16
17
and potential redox nature.
Kaneda et al. firstly reported
7
0
Au/MgAl-LDH catalysts with Au NPs’ sizes within 2.7-4.6 nm
upon varied reductant (KBH , H , hydrazine) and found that their
4
2
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.
activities for 1-phenylethanol oxidation is increased with reduced
Au NPs. Wang et al. prepared Au/MgAl-LDH catalysts with
mean Au NPs’ sizes in 2.1-21 nm by deposition-precipitation
Chloroauric acid tetrahydrate (HAuCl
sodium borohydride (NaBH
Sinopharm Chemical Reagent Co., Ltd., tetraoctylammonium
S,
99%, AR) from Changzhou Pharmaceutical Factory, and
methanol (CH OH, ≥ 99.9%, HPLC) from Tianjin Shield
4
4H
2
O, > 99.9%, AR) and
1
8
4
, 99.5%, AR) were purchased from
9 3
method and studied their oxidant-free dehydrogenation properties 60 bromide (TOABr, 98%, AR) from J&K, captopril (C H15NO
5
0
5
0
5
0
5
0
5
0
of benzyl alcohol, showing that the activity is increased slightly
with Au particle reduced from 12 to ~4 nm while increased
steeply with the mean Au size further reduced to 2.1 nm. Li et
3
Specialty Chemical Ltd. Co. All of the substrate alcohols (AR)
were purchased from Aladdin. The deionized water with the
1
9
al. reported transition metals modified MgAl-LDH (M-HT, M=
3
+
2+
2+
Cr , Co , Ni ) by calcination-reconstruction steps for Au- 65 resistivity of > 18.25 MΩ was used throughout the experiments.
1
1
2
2
3
3
4
4
5
loaded catalysts (2.7-3.9 nm) and found Au/Cr-HT bearing the
-1
Preparation of Au25Capt18
highest activity (TOF=930 h ) for benzyl alcohol oxidation. Zhao
2
0
et al. reported the Au/yNiAl-LDHs catalysts with Au NPs of ~5
nm by a modified in situ reduction-deposition step, in which
AuNPs uniformly distributed on Ni–Al LDHs were shown to be
Water-soluble captopril-capped Au25 nanoclusters were prepared
using a size-focusing synthetic methodology according to the
previous report by Jin et al. Typically, 8.23 mL HAuCl 4H O
3
2
4
2
efficient catalysts for the selective oxidation of alcohols to the 70 (10 mg/mL in methanol) was added to 1.77 mL methanol in a 25
o
corresponding aldehydes or ketones under base-free conditions
with O . Clearly, further reduced size of Au active phase upon
mL single-necked flask while vigorously stirring at 25 C. Then,
TOABr (0.23 mmol) was added to the flask and the solution color
changes from yellow to deep red. After 20 min, captopril (1
mmol dissolved in 5 mL methanol) was rapidly injected into the
2
modified method for greatly improved alcohol oxidation activity
is highly desired.
Gold clusters composed of less than 100 atoms with the size < 75 reaction mixture under vigorously stirring. The solution quickly
2
nm quite different from Au nanocrystals (>2 nm) due to the
becomes white. After 30 min, an aq. solution of 5 mL of NaBH4
(2 mmol dissolved in 5 mL of ice cold water) was added rapidly
to the reaction mixture under vigorous stirring and the solution
immediately turned to brown-blackish (see photos in Fig. S1).
2
1
distinct quantum-size effect, make this type of nanomaterial
very promising in developing new generation of catalysts. Gold
clusters-supported catalysts were successful in catalyzing varied
2
2
type of oxidation reactions, such as oxidation of primary and 80 The reaction was allowed to proceed for 8 h and the reaction
2
3,24
25
secondary alcohols,
alkenes (C=C bonds), and alkanes (C-H
mixture was centrifuged (5000 r/min, 20 min) to remove
unreacted, insoluble Au(I):SR intermediate complexs. The
supernatant was collected and the solvent was removed by rotary
2
6
bonds). Among the well-defined Au clusters, thiolate-capped
Au25(SR)18 clusters, composed of an icosahedral Au13 core and a
shell of twelve Au atoms that are face-capped on the Au13 core,
have been extensively studied.
clusters (n=10, 18, 25, 39) loaded on hydroxyapatite with average
Au diameters of 1.0 ± 0.4 - 1.1 ± 0.5 nm exhibiting the excellent
activity for cyclohexane oxidation by O and TBHP. The
Au25(SR)18 clusters can also be loaded on other metal oxides,
o
evaporation (30 C, 20 min) followed by adding ethanol (20 mL)
2
1,27
26
Tsukuda et al. reported Au
n
85 and standing overnight to obtain the brown-black precipitate. The
precipitate was dried in vacuum at 30 C giving raw product.
Then, the raw product was extracted with minimum amounts of
methanol several times followed by adding ethanol (30 mL) and
centrifuging (3000 r/min, 10 min) to obtain brown-blackish
o
n
2
2
8
29
30
o
such as TiO
2
, CeO
2
, and MgO , showing variously enhanced 90 precipitate, which was dried at 30 C in vacuum overnight giving
activity for varied catalytic processes. In our previous work, we
first reported the LDH-loaded Au nanoclusters (NCs) catalysts
with glutathione-capped AuNCs for alcohols oxidation and found
25 18
brown-blackish product Au Capt .
Preparation of layered double hydroxides (LDH) supports
Carbonate-containing Ni Al-LDH supports were prepared by
a single-drop coprecipitation step. In detail, a 50 mL aq. solution
AuNCs/Ni
3
Al-LDH giving the highest 1-phenylethanol oxidation
x
3
1
activity. However, the pristine AuNCs and those in the
.
.
3 2 2 3 3 2
AuNCs/LDH catalysts show the average sizes of 1.5 ± 0.5 - 2.0 ± 95 containing Ni(NO ) 6H O and varied amounts of Al(NO ) 9H O
0
.7 nm, obviously larger than the critical size of Au25 clusters
upon designed Ni/Al molar ratio x (= 2, 3, or 4) with a total
cationic concentration of 1.0 M was added dropwise into a 250
mL round-bottle flask containing 50 mL mixed alkaline solution
(~0.9 nm), and the unique roles of the transition-metal-based
LDH supports are still controversial, and the nature of Au-
clusters-LDH synergy are not clear.
2
-
3+
-
3+
2+
2 3 3
of NaOH and Na CO ([CO ]/[Al ] = 2, [OH ]/[Al + Ni ] =
o
In the present work, we successfully synthesize a series of 100 1.6) under vigorous stirring. The resultant was aged at 120 C for
nearly atomic precise Ni-based LDH supported gold clusters
catalysts (~0.9 nm) by a modified electrostatic adsorption strategy
from a atomically precise captopril-capped clusters Au25Capt18 in
order to deeply study the natural effect of the LDH supports on
alcoho1 oxidation and shed new light on the synergy between 105
Au25 clusters and the supports. The reaction rate constant (k) and
6 h, then centrifuged and washed by deionized water until pH ~7
o
and dried at 60 C overnight giving the Ni
x
Al-LDH supports.
Ni(OH)
2
was also prepared by the similar step without
.
Al(NO
Ni
coprecipitation step.
3
)
3
9H
2
O employed for comparison.
x
Mn-LDH supports were obtained by a double-drop
100 mL aq. solution with
O (0.006 mol) and varied amounts of
O upon Ni/Mn ratio (x = 2, 3) and a 100 mL
A
.
apparent activation energy (E
a
) of 1-phenylethanol oxidation over
Mn(CH
3
COO)
2
4H
2
.
the Au25/Ni Al-LDH catalysts were determined and discussed.
x
Ni(NO
3
)
2
6H
2
2
-
2+
2 3 3
alkaline solution of NaOH and Na CO ([CO ]/[Mn ] = 2,
-
2+
2+
5
5
Experimental section
110 [OH ]/[Mn +Ni ]=1.6) was simultaneously added dropwise
into a 500 mL round-bottle flask with 100 mL deionized
water under vigorous stirring keeping the reaction pH of 10.
Chemicals
2
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Catalysis Science & Technology
o
The resultant was aged at 25 C for 24 h, then centrifuged
The LDH loaded Au25 clusters catalysts were prepared by a
modified electrostatic adsorption strategy. Firstly, 1.25 g Ni Al-
LDH support was ultrasonically dispersed into 50 mL deionized
water for 10 min to obtain a buffer suspension (pH ~7.98, 7.7,
and washed by deionized water until pH ~7 and dried at 60
Mn-LDH supports. Ni3-xMn
LDH (x = 0.5, 0.8, 1) supports were also prepared by the
x
o
C for 8 h giving the Ni
x
x
Fe-
5
double-drop coprecipitation step. A 100 mL aq. solution 20 7.4). Then, 10 mL of Au25Capt18 solution (3.5 mg dispersed in 10
.
with 0.006 mol Mn(CH
3
COO)
2
4H
2
O and varied amounts of
mL deionized water, pH ~6.67) was added into the above
suspension under vigorous stirring for 10 min at room
temperature. The mixture was collected by centrifugation (4000
.
.
Ni(NO
solution (NaOH and Na CO , [CO ]/[Fe ] = 2, [OH
3 2 2 3 3 2
) 6H O and Fe(NO ) 9H O and a 100 mL alkaline
2
-
3+
-
2
3
3
2
+
2+
3+
o
]
/[Mn + Ni + Fe ] = 1.6) was simultaneously added
rpm, 3 min) and then dried at 60 C overnight giving the catalyst
1
0
dropwise into a 500 mL flask containing 100 mL deionized 25 precursors Au25Capt18/Ni
x
Al-LDH. Secondly, the precursors were
calcined at 300 C for 2 h in air giving the Au25/Ni Al-LDH
catalysts. Scheme 1 depicts the design schematic of Au25/Ni Al-
LDH catalysts. The extended catalysts Au25/Ni Mn-LDH and
Fe-LDH and reference catalyst Au25/Ni(OH) were
prepared in the same way.
o
water under vigorous stirring keeping the reaction pH of 10.
x
o
The resultant was aged at 65 C for 24 h, then centrifuged
x
and washed by deionized water until the pH ~7 and dried at
x
o
6
0 C for 24 h giving the Ni3-xMn
x
Fe-LDH supports.
Au25/Ni3-xMn
x
2
3
0
1
5
Preparation of LDH supported Au25 clusters catalysts
x
Scheme 1 Design schematic of the Au25/LDH catalysts using Ni Al-LDH system as example.
Characterization
imaging system operating at 200 kV. The samples were
ultrasonically dispersed into ethanol-water solution (v/v = 1/1) for
6
7
7
8
8
9
5
0
5
0
5
0
3
4
4
5
5
6
5
0
5
0
5
0
The UV-vis spectra were obtained on a Shimadzu UV-2501PC
spectrophotometer. The thermogravimetric analysis (TG) was
performed on a Mettler-Toledo TGA/DSC 1/1100 ST thermal
analyzer. Electrospray ionization mass spectra (ESI-MS) were
recorded using a Waters Xevo G2S quadrupole time-of-flight (Q-
TOF) mass spectrometer. The sample was dispersed in methanol
and infused at a flow rate of 5 L/min. The capillary voltage was
set as 2.50 kV. The source temperature and desolvation
2
min and put two drops on a holey carbon film supported by a
copper grid. The electron probe size was approximately 0.5 nm.
The HAADF-STEM image was recorded for 8.2 s with a digital
resolution of 512 × 512 pixels.
Fourier transform infrared spectra were obtained on a Bruker
Vector-22 FT-IR spectrophotometer using KBr pellet technique
(
(
sample/KBr = 1/100). The UV-Vis diffusion reflection spectra
DRS) were recorded on a Shimadzu UV-3600 UV-Vis-NIR
o
o
temperature were 120
C and 500 C, respectively. The
spectrophotometer. The spectra were collected at 220-800 nm
with BaSO as a reference. Specific surface areas were obtained
upon BET method from N adsorption-desorption isotherms
measured on a Quantachrome Autosorb-1C-VP system. The
temperature-programmed surface reaction (TPSR) was carried
out on a Thermo Fisher TPD/R/O 1110 instrument as follows.
After the sample (55 mg) was activated at 200 C for 2 h under
helium stream (20 mL/min), it was cooled down to 40 C and
isopropanol vapor was introduced for adsorption at 40 C for 60
min. After the sample was swept with helium stream for 60 min,
the temperature was enhanced linearly with a rate of 10 C/min in
2
the argon stream and the signals of H (M/e = 2) were taken by an
online mass spectrometry (Omnistar TM) simultaneously. The
surface chemical compositions were studied on a VG ESCALAB-
2
the analysis chamber of 2 × 10 Pa using a standard Al Kα source
(1486.6 eV). The binding energy scale was referenced to the C 1s
line of aliphatic carbon contamination (285.0 eV). The studies of
CO molecules adsorbed on the catalyst were done combining
with IR on a Nicolet 380 instrument containing a controlled
desolvation gas flow was 800 L/h. The determination of
isoelectronic point (pI) of the samples was obtained from the Zeta
potential analysis on a Malvern Zetasizer Nano ZS instrument
equipped with a multipurpose autotitrator (MPT-2). Powder X-
ray diffraction (XRD) patterns were taken on a Shimadzu XRD-
4
2
6
000 diffractometer using Cu Kα radiation (1.5418 Å , 40 kV, 30
o
mA). The samples, as unoriented powders, were step-scanned in
steps of 0.02° (2θ) in the range of 3-70° using a count time of 4 s
per step. Elemental analysis for metal ions was done on a
Shimadzu ICPS-7500 inductively coupled plasma atomic
emission spectroscopy (ICP-AES) after dissolving sample in
chloroazotic acid (1 mL) followed diluted to 10 mL using
deionized water. Scanning electron microscopy (SEM) along with
elements mapping were recorded on a Hitachi S-3500N apparatus
at 20 kV. High resolution TEM was recorded on a JEM 2010
transmission electron microscope at 200 kV. The high-angle
annular dark-field scanning transmission electron microscopy
o
o
o
50 X-ray photoelectron spectrometer (XPS) at a base pressure in
-9
(
(
HAADF-STEM) along with energy dispersive X-ray spectra
EDX) were recorded on a JEOL JEM-2100F transmission
electron microscope equipped with a digitally processed STEM
environment chamber equipped with CaF
2
windows. The IR
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spectra were recorded by using wafers in the form of self-
supporting pellet of sample mounted in a homemade ceramic cell.
Prior to measurements, the samples were first heated to 100 C at
image of the sample with Au25Capt18 directly dispersed in water
(Fig. 1E and F) shows that the Au cluster size is 1.5 ± 0.5 nm (on
o
~150 particles), slightly larger than the critical value of Au25
o
27
heating rate 5 C/min in N
2
flow and kept for 1 h, then cooled to 60 cluster (~0.9 nm) , implying the assemblages of the ultrafine
o
5
50 C. The sample was scanned to get a background, then
exposed to a CO flow for 1 h, followed the cell purging with N
for 30 min and the IR spectra were recorded in absorbance mode
gold clusters formed by hydrogen bond between the protonated –
34
2
2
CO H groups of the ligands. To test this hypothesis and
evaluate the core diameters of individual Au25Capt18 clusters,
another sample with the Au Capt initially dissolving in pH 7.9
-1
at 4 cm resolution upon averaging 64 scans.
2
5
18
6
5
buffer solution, in which most of the –CO
2
H groups of the
2
of 3.7 for –CO H
Activity test
-
ligands are dissociated into –CO
2
given the pK
a
1
4
1
1
2
2
0
5
0
5
The liquid-phase aerobic oxidation of 1-phenylethanol was done
using a 25 mL three-necked round-bottle flask with a reflux
condenser under magnetic stirring. The substrate (5 mmol),
in captopril. Then, the obtained HRTEM clearly reveals a
dramatically reduced average Au core size of 0.9 ± 0.3 nm (on
~150 particles) (Fig. 1C and D), which is reasonably agreement
solvent (toluene, 5 mL) and catalyst (Au: 0.01 mol%) were mixed 70 with the Scherrer dimension 0.8 nm and very close to ~0.9 nm of
in the flask and then heated to the reaction temperature (60, 70，
Au25 clusters in theory, indicating the significantly improved
dispersibility and nearly atomically monodispersed Au25Capt18
o
2
80, 90 and 100 C) with O bubbling (20 mL/min) at atmospheric
3
4,35
pressure. During the reaction, 0.2 mL aliquots were pipetted
every 15 min, filtered (0.22 m) and analyzed by GC (Agilent
clusters owing to the Coulomb repulsion,
which is favorable
for the synthesis of the LDH supported Au25 clusters catalysts.
7
890A) equipped with a flame ionisation detector and an Agilent 75 The FFT images (insets in Fig. 1D and F) shows one-pair
J&W HP-5 (5% phenyl polysiloxane, 30 m × 0.25 mm × 0.25 μm)
capillary column. Biphenyl was used as an internal standard for
quantitative analysis. The solvent-free oxidation of 1-phenyl-
ethanol was done as follow: 1-phenylethanol (100 mmol),
diffraction pot in a weak diffusive ring for the sample with
Au25Capt18 dispersed in basic buffer instead of several bright
diffraction spots for the sample with Au25Capt18 directly
dispersed in water, also implying the ultrafine size of the
-4
catalyst (Au: 4.0 × 10 mol%), reaction temperature (120, 130， 80 Au25Capt18 in basic media.
o
1
40, 150 and 160 C). After reaction, the sample was centrifuged
Upon the Zeta potential analysis (Fig. S3), the isoelectric
point (pI) of Au25Capt18 in the solution is ca. 5.66. When pH > pI,
o
and washed with toluene and dried at 60 C overnight for next run.
the proton of –CO
was dissociated to form –CO
2
H group at the opposite side of the thiol group
Results and discussion
-
2
with a negative electric potential
The successful synthesis of water-soluble Au25Capt18 clusters 85 formed on the surface of Au25 clusters. Such Au25 clusters can
with high purity can be verified by FT-IR, UV-vis data and ESI-
MS. The ligation of captopril in the form of the thiolate to the Au
core was indicated by the absence of the IR band ν(S-H) at 2567
verify the high dispersion state without aggregation due to
3
3
4
4
5
5
0
5
0
5
0
5
-1
14
cm in Au25Capt18 (Fig. S2A). The UV-vis spectrum of the
original solution of Au25Capt18 (Fig. 1A(a)) shows an
exponential-like decay from the UV region into the visible region
while no surface plasmon resonance band at ~520 nm typical of
2
1
Au NPs with diameters > 2 nm, implying the ultrafine clusters
of the Au25Capt18 below 2.0 nm. Moreover, because of strong
quantum size effects, the Au25Capt18 cluster shows multiple
molecular-like transitions in the optical spectrum with at least
three well-defined bands at 670, 450, and 400 nm (Fig. 1A). The
excited state at 670 nm corresponds to a LUMO ← HOMO
transition, i.e., essentially an intraband (sp←sp) transition, which
can be viewed as a transition entirely due to the electronic and
2
1
geometric structure of the Au13 core. The peak at 450 nm arises
from mixed intraband (sp←sp) and interband (sp←d) transitions,
and the peak at 400 nm principally from an interband transition
(sp←d). Meanwhile, a broad shoulder at ~800 nm (spin-
forbidden) is observed and the 400 nm band is less pronounced,
33
confirming that native Au25Capt18 clusters are indeed anionic.
The ESI-MS result further clearly suggest the obtained gold
clusters to be Au25Capt18 (Fig. S2C).
The XRD pattern of Au25Capt18 (Fig. 1B) shows a relative
o
o
strong peak at 37.5 (0.235 nm) and a very broad feature at 64.5 .
Upon the peak width (full width at half maximum, FWHM in rad.)
o
at 37.5 , the particle dimension can be tentatively estimated as 0.8
Fig. 1 UV-vis (A) of original solution of Au25Capt18 (a) and supernatant
nm by the Debye-Scherrer formula, D=0.9λ/(FWHM · cosθ),
o
after impregnation on LDH (b), XRD (B) of Au25Capt18, HRTEM of
where FWHM is 11 (0.19 rad.) and λ is 0.1542 nm. The HRTEM
4
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Catalysis Science & Technology
2
+
Au25Capt18 dispersed in basic buffer (C, D) and directly dispersed in water
metal ions in brucite-like layers given the larger radii of Ni
3
+
(E, F) (insets: the histogram of the size distribution and FFT).
60 (0.72 Å ) than Al (0.50 Å ) as the parameter a is a function of the
mean radii of the layer metal cations. It is noted that the maximal
Scherrer dimension for both precursors and supports occurs at x =
electrostatic repulsion. When pH < pI, the Au25 clusters surface
turns to be electrostatically neutral to lose the driving force of
dispersion thus the hydrophobic interactions between Au25
clusters predominate, considering the proton of the weak acid –
3
, implying the highest crystallinity of this system. The almost
5
0
5
0
5
0
5
0
5
0
5
unaltered XRD peak positions and intensities for the precursors
compared with the supports indicates the well-kept LDH layer
structure after loading of small amount of atomically precise
Au25Capt18 clusters.
After calcination, it can be clearly seen from Fig. 2 that the
x
Al-LDH (x=2, 3) catalysts possess definite lamellar
structure though the (003) peaks shift to high 2θ angles due to the
ions and the diffuse
110) and (113) peaks owing to the reduced orderliness of the
4
layer cations. While for Au25/Ni Al-LDH, the LDH laminate
structure collapses upon the absence of the (00l) lines and the
formation of mixed oxide phase indexed to cubic NiO highly
dispersed in amorphous alumina matrix considering the broad
weak (111), (200) and (220) peaks (JCPDS no. 471049). It is
noticed that no XRD lines detected for Au phase implies the
extremely small Au clusters on the catalysts. As for reference
6
7
7
8
8
9
5
0
5
0
5
0
2
CO H group of captopril cannot be ionized. While the pI of
Ni Al-LDH supports with x = 2, 3, and 4 is determined as 9.3, 9.8
x
31
and 10.6, respectively, similar to previous report . These results
imply that the negatively charged Au25 clusters can be easily
adsorbed on the positively charged LDH supports by electrostatic
attraction at pH ~7-9 with high dispersibility. Meantime, the
hydrogen bond between the -OH groups of LDHs and partial
1
1
2
2
3
3
4
4
5
5
Au25/Ni
2-
removal of interlayer water and partial CO
3
(
2
undissociated –CO H groups of the ligands may also contribute
to the monodispersion of Au25 clusters on the LDH supports.
Then, during the synthesis process of the catalyst (Scheme 1),
the LDH supports were carefully predispersed in water forming a
basic buffer (pH~8.0), followed the desired amount of Au25Capt18
clusters were added. It was clearly seen that Au25Capt18 clusters
were fast adsorbed on the surface of LDH (photos in Scheme 1),
evidenced by the colourless supernatant, upon strong electrostatic
attraction and hydrogen bonding between the negatively charged
catalyst Au25/Ni(OH)
x = 2, 3), and Au25/Ni3-xMn
their XRD patterns show majorly collapsed LDH layers and
obvious NiO phase though Au25/Ni Mn-LDH and Au25/Ni MnFe-
2
, and extended systems Au25/Ni
x
Mn-LDH
(
x
Fe-LDH (x = 0.5, 0.8, 1) (Fig. S4),
Au25Capt18 and the positively charged Ni
forming the catalyst precursors Au25Capt18/Ni
TG plot of pure Au25Capt18 shows a constant weight loss of
x
Al-LDH nanoplates,
x
Al-LDH. As the
2
2
LDH still maintain the weak (003) peaks.
The IR spectra (Fig. S5A) of the Au25/Ni
o
4
3.6% at 300 C (Fig. S2B), in good agreement with the theoretic
o
x
Al-LDH catalysts
compared with the precursors and the supports mainly show the
typical IR bands of the CO -LDH phase. The bands at 3489
3
cm of the precursor and support with Ni/Al ratio of 3 present as
content of ligands, the catalyst precursors were calcined at 300 C
to remove the ligands, giving the Au25/Ni Al-LDH catalysts.
Fig. 2 presents the XRD patterns of the Au25/Ni Al-LDH
catalysts compared with the precursors and the supports. Both the
Ni Al-LDH supports and the Au25Capt18/Ni Al-LDH precursors
show typical hexagonal crystal structure of LDH materials
without other crystal phase. The characteristic reflections of the
supports (Fig. 2(a ,b ,c )) and catalyst precursors (Fig. 2(a’,b’,c’))
x
2-
13
x
-1
a narrower peak than the others, implying a more ordered cation
x
x
13,36
distribution in the former,
in line with its XRD data.
1
3,31
Differently, beside the similar IR bands assigned to the LDH
supports, the IR spectra of the Au25Capt18/Ni Al-LDH precursors
show clear C-H mode of CH and CH groups of the captopril
ligands, indicating the soft-land of Au25Capt18 clusters on the
x
0
0
0
3
2
show a series of (00l) peaks as sharp, symmetric, and strong lines
at low 2θ angles, implying good crystallinity of the LDH moieties
x x
in Ni Al-LDH and Au25Capt18/Ni Al-LDH samples. The FWHM
of (003) peak is increased with increasing Ni/Al ratio x, implying
the reduced crystal size of LDH phase with increasing Ni content,
just reflected by the Scherrer dimensions (Table S1). The
intensities of (110) and (113) peaks are enhanced with increasing
Ni/Al ratios for the samples with x of 2 and 3, suggesting that the
metal cations in the brucite-like layers become highly ordered
with increasing Ni content, in accordance with their crystal sizes
along (110) plane (Table S1). While those for the sample with
Ni/Al ratio of 4 are inferior to those of x = 2 and 3, which is
probably due to the formation of Ni(OH)
species weakening the crystallinity of the LDH structure though
the content of Ni(OH) is too less to be detected. Furthermore, it
can be seen from Table S1 that the lattice parameter c (= 3d003
2
phase from excess Ni
2
)
for all the precursors and supports is increased with Ni/Al ratios
corresponding to gradual downshift of 2θ angles. One reason is
the reduced layer charge density with increasing Ni content
weakening the electrostatic attraction between the layer cations
and the interlayer anions resulting in larger interlamellar spacing,
another is the strong redox ability of Ni species makes laminates
95
Fig. 2 XRD patterns of the catalysts Au25/Ni
LDH (b), Au25/Ni Al-LDH (c) and the corresponding precursors
Au25Capt18/Ni Al-LDH (a’, b’, c’) and supports Ni Al-LDH (a , b , c ).
2 3
Al-LDH (a), Au25/Ni Al-
3
6
distorted. The parameter a (= 2d110) is also enhanced in the
4
same order, implying the decrease of the arrangement density of
x
x
0
0
0
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Page 6 of 15
LDH supports. However, the catalysts Au25/Ni
only show the typical IR bands similar to corresponding 60 while Au25/Ni
precursors but greatly reduced C-H and clearly weak as(COO )
x
Al-LDH (x=2, 3)
regularity and smooth surface in spite of a little aggregation,
Al-LDH shows serious aggregation, irregular plate
shape and rough surface probably maybe due to its collapsed
LDH layer along with NiO phase doped.
The SEM-mapping of Au25/Ni Al-LDH (Fig. S8) indicates
x
that the elements Ni, Al and Au are uniformly dispersed in the
4
-
-
1
-
-1
(
1553 cm ) and 
LDH layer structure and the weak remains of the ligands. As for
Au25/Ni Al-LDH, absent C-H and very weak absorptions below
s
(COO ) (1332 cm ), implying the well-kept
5
0
5
0
5
0
5
0
5
0
5
4
-
1
2-
1
700 cm associated with CO
3
anions indicate the complete 65 catalyst samples. Moreover, the edge sites of LDH nanoplates
removal of the ligand and the partial preservation of the LDH
layers, though XRD fails to detect the (003) line, leading to a
may predominantly draw Au25 clusters due to the exposure of
more unsaturated dangling M-OH groups with high potential for
smaller hexagonal LDH nanoplates (20-40 nm) than the larger
ones (~50-80 nm) reported previously.
The HRTEM images of the Au25/Ni Al-LDH catalysts (Fig. 3)
x
clearly show that the ultrafine Au25 clusters are highly dispersed
on the edge sites of the hexagonal LDH platy particles, probably
due to the more pending M-OH functional groups near the edge
3
6,37
1
1
2
2
3
3
4
4
5
5
mixed oxides containing well-defined nickel oxide phase.
Considering that the thiolate ligands may maintain the structural
3
1
2
9
integrity of the Au25Capt18
clusters in Au25/Ni Al-LDH (x = 2, 3) are superior to that of
Au25/Ni Al-LDH. Partial remains of the ligands may also repress
,
thus the structural integrity of Au25
70
x
4
the mobility of the Au25 clusters beneficial to the monodisperse of
the atomically precise Au25 clusters on the LDH supports.
2 3
sites of the supports. The Au25/Ni Al-LDH and Au25/Ni Al-LDH
UV-vis/DR spectra show that all the samples (Fig. S5B) have 75 present approximately atomic precise Au clusters of 0.9 ± 0.3 nm
similar optical absorption probably due to the major common and 0.9 ± 0.2 nm, almost same as the pure Au25Capt18
LDH phases though Au25/Ni Al-LDH shows very weak bands respectively, implying the existence of strong metal-support
interaction between Au25 clusters and Ni Al-LDH (x = 2, 3)
supports which have the better layer structure even after 300 C
80 calcination. While Au25/Ni Al-LDH has Au cluster size of 1.1 ±
,
4
due to its poor layer structure. Two main absorptions around 380
and 650 nm with a respective shoulder at high wavelength are
x
o
2
+
ascribed to the d-d spin-allowed electronic transitions of a Ni
4
3
3
ion in an octahedral field, due to the A2g(F) → T1g(P) transition
0.4 nm, slightly larger than the pure Au25Capt18, indicating the
less strong Au25-LDH interaction for this sample with largely
collapsed LDH layers structure along with partial NiO doped thus
Au25 clusters are easy to migrate during the calcination leading to
3
3
36,38
and
A2g(F) →
T
1g(F) transition,
respectively. However,
there is no band at ~520 nm, typical for the localized surface
2
1
plasmon resonance of Au NPs >2 nm, observed for both
precursors and catalysts, implying the ultrafine Au clusters <2 nm 85 slight increase of Au cluster size. Furthermore, the high-
existed in the Au25/Ni
studied, Au25/Ni Al-LDH shows the highest Ni octahedral
coordination symmetry, implying its best layer cations’ order.
The N adsorption-desorption analyses for all the catalysts
and the supports (Fig. S6) show similar isotherms characteristics 90 nearly parallel to each other, implying the possible epitaxial
x
Al-LDH cataysts. Among all the catalysts
magnified HRTEM images (insets in Fig. 3) show that all the
catalysts exhibit both the nanocrystalline Au with (111) lattice
fringes (0.235 nm) and the LDH with (015) lattice fringes (0.225
2
+
3
2
x
nm). These two lattice fringes for Au25/Ni Al-LDH (x = 2, 3) are
to each other. All the samples presents a Type IIb N
2
adsorption
growth of Au25 clusters on the LDH supports favoring the
ultrafine Au25 cluster sizes and the strong gold-LDH synergy
upon the epitaxial diffusion of Au25 clusters onto the LDH
adsorption isotherm upon IUPAC classification, though their full
adsorption-desorption isotherms show Type H3 hysteresis closing
0
0
at ~0.4 (p/p ) but Au25/Ni
4
Al-LDH at ~0.25 (p/p ) associated with
4
surface. While these two lattice fringes for Au25/Ni Al-LDH are
the aggregates of the plate-like particles resulting in slit-shaped 95 not parallel to each other but have certain angle of the cross,
3
6,39
mesopores.
uniform mesopores from 2 to 50 nm. The BET areas (SBET) of the
catalysts Au25/Ni Al-LDH with x=2, 3, and 4 as 127, 108, and
81 m /g, respectively, are larger than corresponding supports
The pore size distributions reveal a series of non-
probably due to the lower match between the Au25 clusters and
the support owing to the collapsed LDH layer with the
incorporated NiO phase leading to the slightly aggregated Au25
x
2
1
clusters. The FFT images of Au25/Ni
x
Al-LDH (x = 2, 3) further
2
(
113, 105, 126 m /g) because the interlayer anions and water 100 depict the brighter diffraction dots for LDH phase than for Au25
molecules are deprived partially upon calcination leading to the
formation of mesoporous. The increment of SBET of the catalysts
4
nanoclusters, just contrary to those of Au25/Ni Al-LDH, also
demonstrating the better kept LDH layer of the former and even
smaller Au25 sizes than the latter, which may strengthen the
synergestic effect between the Au25 clusters and the LDH thus in
compared to supports reduce in an order of Au25/Ni
4
Al-LDH
Al-LDH
2
2
(
54.9 m /g) < Au25/Ni
2
Al-LDH (14.3 m /g) < Au25/Ni
3
2
(2.2 m /g), in line with the destruction degree of LDH layers, i.e., 105 favor of the alcohol oxditaion reaction.
Au25/Ni
3
Al-LDH maintains the highest layer structure orderliness.
3
Fig. 4 shows typical HAADF-STEM image of Au25/Ni Al-
The SEM images of all the samples (Fig. S7) indicate that the
x
Al-LDH supports have clear platelet-like morphology with an
LDH and size distribution for Au25 clusters upon ca. 100 clusters.
The comparison of Au content in EDX spectra (Fig. S9) confirms
that the bright spots in Fig. 4A are indeed Au25 clusters. The
Ni
even surface and the crystallite size in the ab-direction is ca. 20-
4
3
0 nm. As shown in Fig. S7(a',b',c'), the morphology of the 110 average diameter of the Au25 clusters in Au25/Ni Al-LDH
catalyst precursors is analogous to those of the supports,
suggesting the adsorption of Au25 clusters onto the supports has
little effects on the features of the supports, in line with the XRD
appeares to be 0.9 ± 0.2 nm, comparable to that of the pristine
Au25Capt18 cluster, indicating the well-preserved sizes of the Au25
clusters during the calcination. Close inspection of Fig. 4A
reveals that the monodispersed Au25 clusters are distributed
and IR data. Then, the Au25/Ni
show the similar platelet-like morphology for Au25/Ni
and Au25/Ni Al-LDH especially the latter with well-kept
x
Al-LDH catalysts (Fig. S7(a,b,c))
2
Al-LDH 115 mostly at the edge of the LDH nanoplates, in line with the
HRTEM result.
3
6
| Journal Name, [year], [vol], 00–00
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Catalysis Science & Technology
5
Fig. 4 (A) Representative HAADF-STEM image of Au25/Ni
3
Al-LDH and
(B) size distribution of the Au clusters. The bar in panel (A) is 20 nm.
Table 1 shows the catalytic activities of the catalysts in
1
1
2
2
3
3
4
4
0
5
0
5
0
5
0
5
oxidation of 1-phenylethanol to acetophenone with molecular
oxygen as a sole oxidant at atmospheric pressure under base-free
conditions. Initially, the catalysts were evaluated at 60, 70, 80, 90
o
and 100 C in toluene. Pristine Ni
x
Al-LDH supports display
verylow conversion of 1-phenylethanol (<4%), indicating that
Au25 clusters are the active species for the aerobic alcohol
1
9,40,41
oxidation reaction as previously reported.
The pure
Au25Capt18 clusters also fail to show any activity could be
attributed to the existence of the ligands which significantly
inhibit the access of reactants to the Au25 clusters’ surfaces. It can
be clearly seen that in terms of the same catalyst, the conversion
and TOF of 1-phenylethanol oxidation increase with the reaction
o
temperatures. At 80 C, the TOF values increase in an order of
-
1
-1
Au25/Ni
Au25/Ni
4
3
Al-LDH (4960 h ) < Au25/Ni
2
Al-LDH (5310 h ) <
-
1
Al-LDH (5870 h ). The Au25/Ni Al-LDH exhibits the
3
highest TOF value. Specially using a little higher catalyst loading,
3
1
exactly same conditions as our previous work, Au25/Ni
3
Al-LDH
-1
gives the TOF of 6607 h , which is ca. 16.2% better than in the
-1
formerly published 5687
h
on AuNCS/Ni
3
Al-LDH-0.22
3
0
-1
(
AuNCs: ~1.5 nm) and dramatically higher than 1101 h over
Au/2Ni-Al (AuNP: 4.5 nm) which employed about five times
lower amount of substrate, five times higher catalyst loading and
o
20
four times higher amount of toluene at 100 C in Zhao’s report.
Also, the Au25/Ni Al-LDH catalyst gives the conversion of 10.2%
of the oxidation of 1-phenylethanol in the absence of O
3
2
.
Moreover, besides similar dependence in activity-temperature
o
(
(
120, 130, 140, 150 and 160 C) and activity-catalyst composition
x
x = 2, 3, 4) to those in toluene, the Au25/Ni Al-LDH catalysts
clearly exhibit extraordinarily high activity in the solvent-free
o
oxidation of 1-phenylethanol (Table 1). The TOF at 160 C of
-1
118500 h with 23.7% yield of acetophenone is achieved for
Au25/Ni
3
Al-LDH, which is much better than those of the
-1 8
-
previously reported Au/PI (20000 h ) , Au/MgAl-HT (37000 h
1
42
-1 31
)
, AuNCs/Ni
3
Al-LDH-0.22 (46500 h ) , and Au/Mg
3
Cr-HT
-1 19
(81000 h ) under similar catalytic reaction conditions. These
results unequivocally demonstrate the significant increase in
aerobic oxidation activity for alcohols by nearly atomic precise
monodispersed Au25/Ni
Au25/Ni Al-LDH catalyst with ultrafine Au25 clusters (~0.9 nm)
and well-kept LDH layer structure presents the highest activity.
50 Similar regularity is also found in the Au25/Ni Mn-LDH and
Au25/Ni3-xMn Fe-LDH systems for aerobic oxidation of 1-
x
Al-LDH catalysts. Importantly, the
3
Fig. 3 HRTEM images of Au25/Ni
2 3
Al-LDH (a), Au25/Ni Al-LDH (b) and
Au25/Ni Al-LDH (c).
4
x
x
-1
2
phenylethanol. The TOF of 4070 h for Au25/Ni Mn-LDH with
better LDH layer structures is obviously higher than that of
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Catalysis Science & Technology
Page 8 of 15
[
a]
31
Table 1 Aerobic oxidation of 1-phenylethanol over varied Au catalysts.
the Au clusters’ solution. 2) The deposition of atomically
precise Au25 clusters may provide more favourable Au25 surface
30 electronic state thus a highly active Au clusters catalyst probably
through a beneficial modification for the electronic structure of
the Au core. 3) The proper layer charge density of the supports
Catalyst
Au/wt
%
T
/ C
60
Conv.
/%
Sel.
/%
TOF
-1 [c]
[
b]
o
/h
Au25/Ni
2
Al-LDH
0
.197
45.3
50.1
53.1
57.2
61.9
51.8
55.4
58.7
99.1
63.8
67.8
10.2
32.0
40.1
49.6
52.5
55.5
19.5
23.7
14.2
21.8
40.7
37.1
34.1
31.6
27.8
99
99
99
99
99
99
99
99
4530
5010
5310
5720
6190
5180
5540
5870
7
8
9
0
0
0
3
such as 0.25 for Ni Al-LDH may be beneficial to the mono-
dispersion of Au25 clusters onto the edge sites of LDH support
thus strengthen the Au -LDH synergy, compared with the LDH
supported non-atomically precise Au nanoclusters catalysts.
After reaction, the Au25/Ni Al-LDH catalyst was filtered out,
washed with solvent for another five recycles. The activities of 1-
phenylethanol oxidation were well-kept in all the recycles. The
40 ICP data for reaction mixture show no Au species detected,
implying the strong interaction between Au25 clusters and LDH
support. In addition, after filtering out the catalyst, no further
conversion of 1-phenylethanol was observed, indicating that the
catalyst was heterogeneous in nature. The recovered catalyst after
five runs were also characterized by HRTEM (Fig. S10), which
shows almost same morphology as the fresh one, indicating the
high structural stability of the present catalyst.
3
5
2
5
1
00
60
31
Au25/Ni
3
Al-LDH
0
.191
3
7
0
0
0
0
8
[
d]
8
9
99 6607
99
6380
1
00
99
6780
[
e]
1
00
60
99 1020
Au25/Ni
4
Al-LDH
0
.195
99
99
99
99
99
99
99
99
99
99
99
99
99
99
3200
4010
4
5
7
8
9
0
0
0
4960
5250
Furthermore, Au/Ni
variety of alcohols to corresponding carbonyl compounds with
50 high activity, indicating a high versatility of the nearly atomic
precise Au25/Ni Al-LDH catalyst. The Au25/Ni Al-LDH exhibits
3
Al-LDH could also selectively oxidise a
1
00
5550
[
f]
Au25/Ni
Au25/Ni
Au25/Ni
2
3
4
Al-LDH
Al-LDH
Al-LDH
0
0
0
0
0
0
0
0
0
.197
.191
.195
.203
.201
.198
.191
.185
.179
160
160
160
80
97500
118500
71000
2180
[
f]
f]
x
3
[
comparable or higher activities not only for benzylic secondary
alcohols including 1-phenylethanol and benzhydrol (Table 2,
entries 1 and 5), but also for aliphatic secondary alcohols such as
cyclohexanol (Table 2, entry 9), compared with Au/HT catalysts
previously reported (Table 2, entries 2-4, 6-8 and 10-12) at high
3
Meanwhile, Au25/Ni Al-LDH presents higher
activity of benzylic primary alcohols oxidation (Table 2, entry
Au25/Ni(OH)
2
Au25/Ni
Au25/Ni
Au25/Ni
2
3
2
Mn-LDH
Mn-LDH
80
4070
5
5
80
3710
MnFe-LDH
20,31,42
80
3410
conversion.
Au25/Ni2.2Mn0.8Fe-LDH
Au25/Ni2.5Mn0.5Fe-LDH
80
3160
80
2780
13), compared with literature values (Table 2, entries 14, 15 and
[
a]
19,20,31
1
-phenylethanol (5 mmol), catalyst (Au: 0.01 mol%), toluene (5 mL), O
2
60 16).
3
We also note that Au25/Ni Al-LDH displays higher
[
b]
[c]
bubbling (20 mL/min), 1 h. Based on ICP. Moles of alcohol converted per
activity for the selective oxidation of allylic alcohols, for example,
cinnamyl alcohol can be selectively oxidized to cinnamyl
aldehyde with 93.3% yield in 4 h (Table 2, entry 17). Moreover,
it should be noted that for all these alcohols oxidation activity
[
d]
mole of Au per hour.
mol%), toluene (10 mL), O
ethanol (5 mmol), catalyst (Au: 0.01 mol%), toluene (5 mL), preevacution by
1-phenylethanol (10 mmol), catalyst (Au: 0.015
[
e]
5
0
5
0
5
2
(20 mL/min), 1 h, same as in ref 31. 1-phenyl-
[
f]
Ar for 1 h before heating to reaction Temp., all in Ar (8 mL/min). 1-phenyl- 65 data, the amount of the Au25/Ni
3
Al-LDH catalyst employed in the
-4
2
ethanol (100 mmol), catalyst (Au: 4.0×10 mol%), O (20 mL/min), 30 min.
reaction is almost 2 times less than that of Au/2Ni-Al beside 4
2
0
times lower amount of toluene used, implying the excellent
catalytic activity of the present nearly atomic precise Au25/Ni Al-
LDH catalyst upon the modified electrostatic adsorption method.
catalysts. These observations imply that the layer structure of the 70 All these results indicate that the Au25/Ni Al-LDH catalysts are
highly effective for the aerobic oxidation of alcohols.
To reveal the natural reason for different alcohols oxidation
performance of the series of Au25/Ni Al-LDH catalysts, we study
-
1
1
1
2
2
Au25/Ni
3
Mn-LDH (3710 h ) with collapsed LDH layers though
Al-LDH
3
both show lower TOF values than those of Au25/Ni
x
x
LDH supports may play important role on the alcohol oxidation,
regardless of their varied specific surface areas just as that the
Au25/Ni
4
Al-LDH with collapsed LDH layers possesses poor
x
2
alcohol oxidation activity despite of its larger SBET (181 m /g)
the macroscopic kinetic process of the catalysts, containing the
75 effect of the Ni/Al molar ratios and reaction temperatures on the
reaction rate for the aerobic oxidation of 1-phenylethanol. Time-
evolution of conversion, C, was monitored from the measured
yield of product as show in Fig. S11. It can be seen that the time-
conversion plots are linear up to approximately 60% conversion
2
than Au25/Ni
The unprecedentedly high aerobic oxidation activity of
Au25/Ni Al-LDH for 1-phenylethanol may be not only ascribed to
3
Al-LDH (108 m /g).
3
the nearly atomic precise Au25 clusters and higher LDH layer
regularity, but also to the strongest Au25-LDH interaction. The
reason comes from three aspects. 1) The modified electrostatic 80 of 1-phenylethanol in all the reactions in toluene (Fig. S11(A)),
adsorption strategy upon LDH-predispersed buffer for highly
efficient electrostatic adsorption of the negatively charged
Au25Capt18 clusters as near monodispersion over the edge sites of
the hexagonal LDH nanoplates, compared to the simple
implying that the product inhibition is very small. As shown in
Fig. 5 (in toluene) and Fig. S12 (solvent-free), the term -ln(1-C)
is increased linearly with the reaction time (h) for all Au25/Ni
x
Al-
LDH catalysts, indicating that the reaction is first order with
impregnation-adsorption by directly adding LDH powders into 85 respect to 1-phenylethanol. The rate constant, k, was therefore
8
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Catalysis Science & Technology
obtained from the slope of the -ln(1-C) plot as a function of
Table 2 Aerobic oxidation of various alcohols to corresponding carbonyl
[
a]
reaction time. The rate constant k values for 1-phenylethanol 60 compounds over Au25/Ni
3
Al-LDH.
o
oxidation at 60-100 C in toluene (Table S2) are larger than those
Entry Substrate
Au/substrate
Conv. Sel.
Time
[h]
o
at 120-160 C without solvent (Table S3). The k values of 1-
[
mol %]
[%]
[%]
5
0
5
0
5
phenylethanol oxidation with and without solvent at the same
temperature are increased in an order of Au25/Ni Al-LDH <
Au25/Ni Al-LDH < Au25/Ni Al-LDH.
A good linear correlation is obtained by plotting of lnk
versus the inverse of the temperature, and apparent
1
2
3
4
5
6
7
8
9
0.115
0.115
0.45
99.9
99
0.25
[
b]
4
99.9
99
99
0.5
0.33
4
[c]
[e]
2
3
99
0.25
>99
99.9
99.9
99
>99
99
0.115
0.115
0.45
3
1
1
2
2
activation energy, E
least-square fit analysis yields E
the optimal catalyst Au25/Ni Al-LDH and 17.91 kJ/mol for
Au25/Ni Al-LDH, respectively, both smaller than the 21.62
kJ/mol for Au25/Ni Al-LDH in toluene (Fig. 5). Meanwhile,
the E values for 1-phenylethanol oxidation under solvent-
free conditions are 19.01, 16.9 and 22.56 kJ/mol,
respectively, for Au25/Ni Al-LDH samples with x = 2, 3, and
(Fig. S12). The E values of all the Au25/Ni Al-LDH
a
, could be estimated with the slope. The
[
[
[
b]
c]
e]
99
4
a
values of 15.31 kJ/mol for
96
4
3
0.25
99
>99
99
4
2
0.115
0.115
0.45
99.2
98.3
93
4
4
[
b]
c]
a
10
11
99
4
[
97.8
>99
99
4
[e]
x
1
1
1
1
1
1
2
3
4
5
6
7
0.25
11
4
4
a
x
0.115
0.115
0.2
99.5
95.3
96
0.5
2
catalysts for 1-phenylethanol oxidation reaction in toluene
are lower than corresponding ones obtained in solvent-free
[
[
[
b]
d]
e]
99
>99
>99
95
1
reactions, and the E
with and without solvent are decreased in an order of
Au25/Ni Al-LDH > Au25/Ni Al-LDH > Au25/Ni Al-LDH.
The highest k and the lowest E values of Au25/Ni Al-LDH
a
values of 1-phenylethanol oxidation
0.25
57
2
0.115
0.115
0.25
93.3
77.0
54
4
4
2
3
[
[
b]
e]
18
96
4
a
3
imply its optimal electronic structure and strong metal-
support interaction between the ultrafine Au clusters with
19
99
2
[a]
Alcohols (1 mmol), Au25/Ni
3
Al-LDH (Au: 0.115 mol%), toluene (5
nearly atomic precise and the Ni
highest layer regularity.
3
Al-LDH support with the
o
[b]
mL), 80 C, O
2
bubbling (20 mL/min).
Alcohols (1 mmol), AuNCs/
o
Mg
3
Al-LDH-0.23 (Au: 0.115 mol%), toluene (5 mL), 80 C, O
2
bubbling
[c]
(
20 mL/min) in ref 30. Alcohols (1 mmol), Au/HT (0.1 g, 0.45 mol%),
The XPS analysis was used to explore the effect of the
electronic structure of the Au25/Ni Al-LDH catalysts on the
catalytic activity (Fig. 6). The Au 4f7/2 binding energy (BE)
of Au25Capt18 located at 84.7 eV exhibits a ~0.7 eV shift to
the higher BE compared to the bulk gold (84.0 eV) owing to
o
[d]
6
5
toluene (5 mL), 80 C, air (1 atm) in ref 42.
Alcohols (1 mmol),
3
3
4
4
5
5
0
5
0
5
0
5
x
o
Au/MgCr-HT (Au: 0.2 mol%), toluene (10 mL), 100 C, O
2
bubbling (20
[e]
mL/min) in ref 19. Alcohols (1 mmol), Au/2Ni-Al (0.05 g, Au 1 wt%),
o
toluene (20 mL), mole ratio of alcohol/Au=400, 80 C, O
2
bubbling (15
2
1
mL/min) in ref 41.
the electron donation from Au25 to the surface thiolates,
approving the formation of Au-S bond for the Au25Capt18
cluster. The Au25/Ni Al-LDH (x = 2, 3, 4) catalysts exhibit
7
7
8
8
9
0
5
0
5
0
2 3
Au25/Ni Al-LDH < Au25/Ni Al-LDH. This trend is in
7
x
accordance with the activity results, just as Tsukuda et al.
reported that the catalytic activity of Au NPs for aerobic
oxidations is enhanced by an increase in the amount of
negative charge on the Au core, therefore further suggesting
that the interaction between the Au25 clusters and the LDH
the Au 4f7/2 BE at 83.42, 83.34 and 83.52 eV, respectively,
clearly smaller than 84.0 eV of the bulk gold, ascribed to the
lower coordination number of surface Au atoms of ultrafine
Au25 cluster (< 2 nm), indicating that the Au25 clusters on the
7
,41,43
catalysts are negatively charged
and there exists an
supports increases in an order of Au25/Ni
Au25/Ni Al-LDH < Au25/Ni Al-LDH.
4
Al-LDH <
electron transfer from Ni Al-LDH to the surface of the Au25
x
2
3
clusters. The reason for electron transfer from LDH supports
to Au25 clusters can be explained as follows. Given that
electron affinity of neutral gold particle is size-dependent,
In order to get more insight into the surface characteristics of
the catalysts, Ni 2p and O 1s spectra were carefully analysed for
the catalysts and pristine supports (Fig. S13, Fig. S14, and Table
S4). The Ni 2p3/2 BE values of Ni
with increasing Ni/Al ratios because the amount of positively
charged AlO octahedra partially substituted neutral NiO
octahedra in the layer become less and the electron cloud density
surrounding the nickel nuclei increase.
LDH catalysts, the peaks at ~856 and ~862 eV are assigned to the
spin-orbit spilt lines and shake-up satellite of Ni 2p3/2
than those of corresponding Ni Al-LDH supports, indicating that
4
2
as the size is remarkably reduced to less than ~5 nm, surface
Au atoms on the nanoparticles behave more like a single Au
atom, are highly active and easily anionically ionized.
Considering the electron affinity of a single Au atom of 2.3
x
Al-LDH shift to lower values
6
6
4
4
eV, larger than the threshold value of electron transfer, the
driving force for electron transfer from the LDH surface to
Au atoms originates from surface conjugation and the
differential Fermi energy level, and it takes some charge to
13,20
x
As for the Au25/Ni Al-
46
,
higher
x
4
4,45
equalize the Fermi levels.
values of Au25/Ni Al-LDH (x = 2, 3, 4) catalysts show 0.58,
.66 and 0.48 eV downshift from the bulk Au, respectively,
indicating that the amount of electrons transferred from LDH
to Au25 clusters increases in an order of Au25/Ni Al-LDH <
In detail, the Au 4f7/2 BE
there are a strong interaction between supports and Au25 clusters
and probably a electron transfer from Ni-OH octahedra to Au25
clusters through the interaction of Au-O(H)-Ni, in conformity
with the Au 4f XPS data.
x
0
4
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Page 10 of 15
improvement of reactivity. Moreover, the charge density of the
LDH layer, regarding the reactivity, is affected by the orderliness
of the layer cations. The well ordered arrangement of cations on
the LDH layer surfaces, corresponding to the uniform distribution
3
3
4
0
5
0
4
7
of the charge density , directly causes the maximal synergistic
effect between the Au25 clusters and the Ni Al-LDH.
It should be mentioned that the recovered Au25/Ni
3
3
Al-LDH
catalyst after five runs were also characterized by XPS (Fig. S15),
which shows almost same surafce Au oxide state as the fresh one,
supporting the high recyclability of the present nearly atomic
precise Au25/Ni
The electronic state of Au25 clusters and the surface acidity of
the Au25/Ni Al-LDH catalysts were further studied by CO
3
Al-LDH nanocatalyst.
x
adsorption in combination with IR spectra (Fig. 7). In order to
discriminate between the adsorption properties of the surface Au
atoms and the supports, CO adsorption was first studied on pure
Fig. 5 -ln(1-C) against time and Arrhenius plots for the aerobic oxidation
of 1-phenylethanol catalyzed by Au25/Ni
reaction temperatures. Reaction conditions: 1-phenylethanol (5 mmol),
catalyst (Au: 0.01 mol%), toluene (5 mL), O (20 mL/min).
x
Al-LDH (x = 2, 3, 4) at different 45 supports. The Ni Al-LDH supports did not show any pronounced
x
-1
IR bands in 2000-2100 cm when exposed to a CO flow. Only
-1
5
2
prominent broad band occurred at ca. 2170 cm can be assigned
to the stretching mode of CO adsorbed on Brønsted acid sites
δ+
18,48
associated to AlO-H centres.
The fact that the LDH and
5
5
6
6
7
7
8
8
0
5
0
5
0
5
0
5
LDH-derived mixed oxides hold both strong acidity and basicity
1
8
48
have ever been reported by Wang et al. and Corma’s group. It
is believed that CO molecules are adsorbed atop of Au atoms of
clusters and surfaces, thus, the stretching frequency (νCO) of
adsorbed CO mainly reflects the electron density on the
adsorption sites, and the νCO is red- and blue-shifted compared
with that of free CO as CO is adsorbed on anionic and cationic
4
9-52
Au sites, respectively.
x
In our cases of Au25/Ni Al-LDH
-1
catalysts, besides the bands at ~2170 cm due to the CO adsorbed
-1
on supports, prominent IR bands at 2102, 2097 and 2108 cm for
Au25/Ni Al-LDH, Au25/Ni Al-LDH and Au25/Ni Al-LDH,
respectively, exhibit prominent red-shift, compared to literature
2
3
4
-1
0
7,53-55
values in 2110-2125 cm upon Au carbonyls of Au NPs
,
attributing to the greater π back-donation from the negatively
charged Au25 clusters. Also, all the bands marked by arrows in
Fig. 7 with an asymmetric broadening from the low-frequency
side are clearly observed, which can be assigned to CO adsorbed
on Au25 clusters and the low frequency broadening may be
associated with the CO adsorbed on sites at the perimeter of Au
clusters of the catalysts considering their absence in the supports
Fig. 6 Au 4f XPS spectra of Au25/Ni
2 3
Al-LDH (a), Au25/Ni Al-LDH (b),
Au25/Ni Al-LDH (c) and Au25Capt18
4
.
The curve fitting of O 1s spectra of Ni
Au25/Ni Al-LDH reveals that there are two oxygen species
including surface OH groups and lattice O species. The amount
of surface OH groups of Ni Al-LDH increases with increasing
Ni/Al ratios. The curve fitting of O 1s spectrum of Au25/Ni Al-
LDH reveals that surface OH (52.40%) is 4.99% lower than those
of Ni Al-LDH (57.39%) and the surface OH of Au25/Ni Al-LDH
60.52%) is 4.16% lower than that of Ni Al-LDH (64.68%),
while for Au25/Ni Al-LDH, the surface OH (46.56%) is 22.77%
lower than that of Ni
interaction between the Au clusters and a Mg(OH)
x
Al-LDH and
1
1
2
2
0
5
0
5
x
2
-
4
8,56,57
and the similarity to previous reports.
It is worth mentioning that the obviously observed IR bands
x
2
o
of Au25/Ni
within 30 min, rather analogous to the observations of the CO
adsorbed on the Au cores of Au:PVP (~1.3 nm) in CH Cl at 295
x 2
Al-LDH at 50 C slowly decay during N evacuation
2
3
(
3
2
2
7
K in purging argon previously reported by Tsukuda et al. ,
4
4
3
probably owing to the similar ultrafine Au core in the present
4
Al-LDH (69.33%). Dai et al. studied the
’s basal plane
catalysts to Au:PVP system. The νCO value for both Au25/Ni
LDH and Au25/Ni Al-LDH is greatly red-shifted from that of
Au25/Ni Al-LDH, implying more negatively charged Au clusters
2
Al-
2
by using a DFT theory, and found strong interaction of the Au
clusters with the surface –OH groups via a short bond between
edge Au atoms and O atoms of the hydroxyls helping the Au
clusters against sintering and stabilizing Au clusters via Au-OH
linkage thus contributing to the CO-oxidation activity. The
3
4
on the former two samples. As compared with the red-shifted
values, we can know that the amount of negative charge of Au25
clusters on the Au25/Ni
Au25/Ni Al-LDH
x
Al-LDH catalysts increases in an order of
Au25/Ni Al-LDH Au25/Ni Al-LDH,
<
2
<
3
amount of surface –OH groups of Au25/Ni
with the order of alcohol oxidation activities, indicating that the –
OH function groups on Au25/Ni Al-LDH is advantageous to the
activation of substrate alcohols and further results in the
x
Al-LDH is consistent
4
consistent with the XPS analysis and alcohol oxidation activities,
suggesting the more negatively charged Au25 clusters on the
catalysts is more favourable for the oxidation of alcohols.
x
1
0
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dehydrogenation process of the adsorbed isopropanol, that means,
the Au25/Ni Al-LDH catalysts have excellent dehydrogenation
abilities especially Au25/Ni Al-LDH showing the highest rate for
x
3
3
4
4
5
5
6
6
5
0
5
0
5
0
5
oxidation of alcohols due to its strongest Au-LDH synergetic
effect associated with both the ultrafine nearly atomic precise
Au25 clusters and the well-kept Ni Al-LDH layer structure.
3
The present results implies that the strong Au25-LDH
interactions featured as the critical effect on the electronic
structure of the Au25 clusters play key roles on the various
aerobic alcohol oxidation activity of the Au25/Ni
catalysts. Meanwhile, unprecedentedly higher oxidation activity
of Au25/Ni Al-LDH than most previously reported catalysts may
x
Al-LDH
3
also be benefited from the existence of Brønsted basic sites (M-
δ-
OH ) on the catalyst for the initial O-H bond cleavage, as most
1
9,40-42
reports previously pointed,
as well as the existence of
Fig. 7 CO adsorption FT-IR spectra of Au25/Ni
LDH (b), Au25/Ni Al-LDH (c), Ni Al-LDH (a
Ni Al-LDH (c ).
2
Al-LDH (a), Au25/Ni
3
Al-
surface nickel sites and Au25 clusters for improving the cleavage
of C-H bond to create the carbonyl compound. The existence of
Brønsted acid sites on the surface of the catalysts may also
4
2
0
), Ni Al-LDH (b ), and
3
0
4
0
1
8
facilitate the alcohol oxidation process as Wang et al. reported.
Based on the above analysis, the nature of the active sites of the
Au25/Ni Al-LDH catalysts are illustrated in Scheme 2.
x
From the above results, the following correlations between
the electronic structures and the oxidation activity of the
Au25/Ni
clusters (Au25/Ni
electronic charge to CO molecules exhibit higher activity toward
aerobic oxidation than the larger Au clusters (Au25/Ni Al-LDH).
2) When the Ni/Al molar ratio increases from 2 to 4, the areas of
x
Al-LDH catalysts can be summarized. (1) Smaller Au
x
Al-LDH (x = 2, 3) that can donate more
4
(
coordination sites for CO molecules are obviously reduced as
shown in Fig. 7. Contrary to expectations from decrease in the
area available for catalysis, the activity of Au25/Ni Al-LDH
3
reaches the highest. This result can be explained in that the
decrease of the cluster surface area available for catalysis is
compensated by the enhancement of activity per unit surface area
by the donation of more electronic charge from the LDH support.
5
Fig. 8 TPSR diagram of isopropanol on Au25/Ni
LDH (b), Au25/Ni Al-LDH (c), Ni Al-LDH (a
Ni Al-LDH (c ).
2
Al-LDH (a), Au25/Ni
3
Al-
4
2
0
), Ni Al-LDH (b ), and
3
0
4
0
To obtain more insights into the gold-support interactions, we
compared the TPSR of pre-adsorbed isopropanol on Au25/Ni Al-
LDH catalysts and Ni Al-LDH supports. Generally, isopropanol
will undergo dehydrogenation over basic sites to form acetone 70 their comparable core sizes (Fig. 3). This implies that the higher
and H and dehydration over acid sites to yield ethylene and
reactivity of the former is due to the higher electron-donating
ability of Ni Al-LDH, although the possibility that the surface
areas available for catalysis are smaller for Au25/Ni Al-LDH than
for Au25/Ni Al-LDH is not totally excluded. All of these
indicating the quite weak dehydrogenation abilities of the pure 75 correlations indicate that the catalytic activity of small (<1.5 nm)
(3) The charge state of Au25 clusters supported on Ni
more negative than that by Au25/Ni Al-LDH, and the activity for
oxidation is much higher than that of Au25/Ni Al-LDH, despite
3
Al-LDH is
1
1
2
2
3
0
5
0
5
0
x
2
x
2
2
8
5
H
2
O. The evolution of H
shown in Fig. 8. Clearly, quite weak H
for all the three LDH supports, quite similar to Liu’s report,
2
for Au25/Ni
x
Al-LDH catalysts is
3
2
signals could be detected
3
1
9
2
LDH supports, in conformity with their undetected alcohol
oxidation activities. However, exceptionally strong H signals for
Au25/Ni Al-LDH catalysts can be achieved at much lower
temperatures compared to corresponding LDH supports. The gaps
Au25 clusters for aerobic oxidation is enhanced by an increase of
negative charge on the Au core. This may provide an empirical
rule for the fabrication of supported active Au catalysts; more
electronic charge could be deposited into high-lying orbits of
2
x
of the desorption temperatures between the catalysts and the 80 ultrafine Au25 clusters by doping with electropositive elements
o
24
supports are in an increasing order of Au25/Ni
4
Al-LDH (13 C) <
such as Pd or by interaction with nucleophilic sites of stable
o
o
Au25/Ni
that all the Au25/Ni
dehydrogenation activity compared with pristine supports. This
trend is just in accordance with the order of alcohol oxidation 85 that cleavage of the C-H bond at the benzylic position is the rate-
2
Al-LDH (27 C) < Au25/Ni
3
Al-LDH (33 C), suggesting
supports.
7
,26,59
x
Al-LDH catalysts have much higher
Tsukuda et al.
studied the effect of electronic structures
of Au clusters on aerobic alcohol oxidation, and demonstrated
activity, indicating the gradually increasing Au25-LDH synergetic
interactions in Au25/Ni Al-LDH catalysts, as the above XPS
determining step upon kinetic isotope effect. Particularly,
7
Tsukuda et al. have ever reported the anionic Au cores of
x
shows. These results indicate that the loading of Au25 onto the
surface of the LDH support significantly facilitate the
Au:PVP on the electron donotation from PVP in aerobic
oxidation of p-hydroxybenzyl alcohol with K CO additive, and
2 3
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Page 12 of 15
x
Scheme 2 The nature of the active sites of the Au25/Ni Al-LDH catalysts.
proposed that an extra electron from the gold readily transfers to
the LUMO (π*) of adsorbed O , based on similar observations of
Au cluster anions via gas-phase studies,
O-O bond and activates oxygen molecule to form superoxo- or
peroxo-like species for further catalytic reaction, which may play
an essential role in the alcohol oxidation. In the present work, the
5
0
5
0
5
0
5
0
5
0
2
4
9,60
which weakens the
1
1
2
2
3
3
4
4
5
lower activation energy E
a
for aerobic oxidation of 1-phenyl-
ethanol over Au25/Ni Al-LDH compared to other samples clearly
3
Scheme 3 The possible reaction pathway of the aerobic oxidation of
indicates the greatly promoted synergy between Au25 and Ni-OH
groups from the higher LDH layer regularity. It is shown that the
catalytic activity is enhanced with increasing electron density on
the Au core.
alcohols over Au25/Ni
x
Al-LDH (red balls stand for electron-rich Au).
δ-
Final step, the Au-H hydride is oxidized rapidly by adsorbed
active oxygen to form a water molecule facilitated by the AlO-
5
5
δ+
18
H
acid sites of the support. Thus, the original metallic sites are
Generally, the aerobic oxidation of alcohols catalyzed by
LDH-loaded Au catalysts may involve the assumption that
the rate-determining step, the β-H elimination of a metal
alkoxide intermediate on the heterogeneous metal surface, is
recovered to realize the catalytic oxidation cycle. Upon this
reaction mechanism, the LDH with best ordered layer structure
such as Ni Al-LDH providing uniform distribution of Ni Al-OH
3 3
60 groups, is the key factor for the β-H elimination of metal alkoxide
by the strongest synergetic effect between the nearly atomic
precise gold cluster and the LDH support, besides the promotion
effect of the weak Brønsted-base sites on the LDH supports.
9
,61
the same on metal oxides loaded noble metal catalysts,
and uniquely, the basic sites on the LDH supports activate
the O-H bond of the alcohols to promote the formation of the
1
7,19,41
metal alkoxide.
The activation of oxygen by supported
6
2
61
Moreover, in the absence of oxidant, we surmise that the
metals or surface oxygen vacancies of the supports is
another critical step for formation of water. Thus upon all
the above experimental and characteristic results and the
previous findings
aerobic oxidation mechanism for alcohols on Au25/Ni
LDH (Scheme 3). Step 1, hydroxyl hydrogen of alcohol
δ+
6
5
Brønsted acid sites (i.e., AlO–H ) on the Ni Al-LDH support
3
might participate in the reaction with the Au hydride to produce
2
0,42,62
molecular H . We will carry out further studies to elucidate this
2
, we tentatively propose a possible
mechanism in our future work.
x
Al-
δ-
Conclusions
x
molecule attacks a weak basic Ni-OH site on the Ni Al-
LDH, resulted from an abstraction of proton by the hydroxyl
group on the support, to promote the formation of Ni-
alkoxide intermediate at the interface and the simultaneous
7
7
8
8
0
5
0
5
We have successfully synthesized a series of Ni-based LDH
supported nearly atomic precise Au25 clusters catalysts via a
modified electrostatic adsorption method of Au Capt onto the
2
5
18
formation of a water molecule. Step 2, molecular oxygen O
2
pre-dispersed Ni-based LDH followed proper calcinations.
Especially, the Au25/Ni Al-LDH catalysts exhibit excellent
adsorbed on the Au sites near the interface between Au25
x
clusters and LDH support is directly activated via the double
activity for aerobic oxidation of 1-phenylethanol under no
adventitious base with molecular oxygen as a sole oxidant and the
6
3,64
linear O
s
-Au-O
a
model,
and the H atom on the β-carbon
of the Ni-alkoxide is simultaneously coordinated with Au25
clusters. The adsorption-desorption equilibrium could be
obtained quickly in step 1 or 2 upon the promotion of
activities increase in an order of Au25/Ni
LDH < Au25/Ni Al-LDH. The kinetic studies show E
7.91, 15.31, and 21.62 kJ/mol for Au25/Ni Al-LDH, Au25/Ni
LDH and Au25/Ni Al-LDH in toluene and 19.01, 16.9, and 22.56
kJ/mol under solvent-free conditions, respectively. The catalyst
Au25/Ni Al-LDH behaves the highest activity owing to the
4
Al-LDH < Au25/Ni
values of
Al-
2
Al-
3
a
1
2
3
6
2
electron transfer to O
determining step,
2
.
Step 3, also believed as the rate-
the Ni-alkoxide intermediate
4
9
,61,62
undergoes a β-hydride elimination facilitated by the peroxo-
3
δ-
like species to afford metal-hydride (Au-H )species at the
strongest Au25-LDH synergistic effect between the ultrafine Au25
clusters (~0.9 nm) with most negative charge on the clusters’
surface and the LDH support with most Ni-OH sites from the best
layer orderliness. We believe that the layer orderliness of the
LDH supports affect directly the synergistic effect of Au25 and
6
5
catalyst interface accompanied by the formation of the
corresponding carbonyl product. The coordinatively
unsaturated metal atoms are more active for the cleavage of
this C–H bond with the assistance of Au-O-O-Au species.
Therefore, the nearly atomic precise Au25 clusters catalyst
with ultrafine Au particles exhibited much higher activity.
x
Ni Al-LDH, that is, the higher orderliness of the LDH supports,
the stronger interaction between Au25 and supports and the higher
1
2
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6
5
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activity of 1-phenylethanol oxidation besides the ultrafine
clusters facilitate the aerobic oxidation of alcohols. The similar
regularity is also found in Au25/Ni Mn-LDH and Au25/Ni3-
Mn Fe-LDH systems though these two systems give lower 1-
phenylethanol oxidation activity than Au25/Ni Al-LDH. These
findings may provide new insights into atomic-scale design of
environmentally benign and reusable LDH-based Au clusters
catalysts for variety of highly efficient heterogeneous catalysis
processes.
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Table of Contents (TOC)
5
Nearly atomic precise Au25/Ni
x
Al-LDH catalysts obtained via modified electrostatic adsorption of Au25Capt18 onto predispersed Ni
x
Al-LDH followed
proper calcination shows extraordinary alcohol oxidation property with O
2
due to ultrafine Au cluster, ordered LDH layer and strong Au25-LDH synergy.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 15