Aggregation-free gold nanoparticles in ordered mesoporous carbons: Toward highly active and stable heterogeneous catalysts

Article abstract of DOI:10.1021/ja403822d

A coordination-assisted synthetic approach is reported here for the synthesis of highly active and stable gold nanoparticle catalysts in ordered mesoporous carbon materials using triblock copolymer F127 as a structure-directing agent, thiol-containing sil

Full text of DOI:10.1021/ja403822d

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Article
Aggregation-free Gold Nanoparticles in Ordered Mesoporous
Carbons: Towards Highly Active and Stable Heterogeneous Catalysts
Shuai Wang, Qingfei Zhao, Huimin Wei, Jianqiang Wang,
Minhyung Cho, HaeSung Cho, Osamu Terasaki, and Ying Wan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja403822d • Publication Date (Web): 18 Jul 2013
Downloaded from http://pubs.acs.org on July 19, 2013
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Aggregation-free Gold Nanoparticles in Ordered Mesoporous Carbons:
Towards Highly Active and Stable Heterogeneous Catalysts
†
†
†
‡
§
§
Shuai Wang , Qingfei Zhao , Huimin Wei , Jianqiang Wang , Minhyung Cho , Haesung Cho ,
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§
,†
Osamu Terasaki , Ying Wan*
†
The Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of Rare Earth
Functional Materials, and Department of Chemistry, Shanghai Normal University, Shanghai 200234,
‡
P. R. China; Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied
§
Physics, Chinese Academy of Sciences, Shanghai, 201204, P. R. China; Center for Functional
Nanomaterials, Department of Chemistry, Korea Advanced Institute of Science and Technology,
Daejeon 305ꢀ701, Korea
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
*
To whom correspondence should be addressed. Tel: 86ꢀ21ꢀ6432ꢀ2516; Fax: 86ꢀ21ꢀ6432ꢀ2511;
Eꢀmail: ywan@shnu.edu.cn
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Abstract
A coordination assisted route is firstly reported here for the synthesis of highly active and stable
gold nanoparticles in ordered mesoporous carbon catalysts using triblock copolymer F127 as a
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structureꢀdirecting agent, thiolꢀcontaining silane as a coordination agent, HAuCl as a gold source
and phenolic resin as a carbon source. Upon carbonization, monoꢀdispersed gold nanoparticles with
a size of ~ 9.0 nm are reduced and simultaneously confined by the “rigid” mesoporous carbonaceous
framework. Nanoparticle aggregation is inhibited even at a high temperature of 600 °C. After
removal of the silica component, the carbonꢀbased catalysts possess the ordered mesostructure, high
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surface area (~1800 m /g), large pore volume (~1.19 cm /g) and uniform bimodal mesopore size (<
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.0 and 4.0 nm). The monoꢀdispersed gold nanoparticles are highly exposed because of the
interpenetrated bimodal pores in the carbon framework, which exhibit excellent catalytic
performance. A complete conversion of benzyl alcohol in water to benzoic acid can be achieved at
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0 °C and 1 Mpa. Benzyl alcohol can also completely convert to benzoic acid at 60 °C even under
an atmospheric pressure, showing great advantages in green chemistry. The catalysts are stable,
poison resistance and can be reused with negligible activity loss and metal leaching. The silane
coupling agent shows multifunctions: (1) coordination with gold species by thiol group to obtain
monoꢀdispersed gold species; (2) reaction with phenolic resins by silanol groups to form relatively
“
rigid” composite framework; (3) poreꢀforming agent to generate secondary pores inside carbon pore
walls that provides high surface areas, large pore volumes and accessibility to gold nanoparticles;
and (4) complete elimination that shows negligible effect on catalytic performance for Auꢀcontaining
mesoporous carbon catalysts.
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1
. Introduction
Gold nanoparticles as catalysts are of great significance in oxidations, addition to multiple CꢀC
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bonds, cyclization reactions, rearrangements, CꢀC coupling reactions. Supporting on carriers may
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disperse and fix the gold nanoparticles. The supports can range from oxides for example CeO
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, and
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TiO2,
mixed oxides including MgꢀAlꢀO,
and GaꢀAlꢀO,
to polymers such as
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poly(Nꢀvinylꢀ2ꢀpyrrolidone) (PVP), and poly(oꢀphenylenediamine) (PoPD). However, particle
growth at high temperatures, or even during reduction and reaction, accompanied by a corresponding
loss of catalytic activity, is a serious problem. This is mainly caused by the increased mobility of the
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gold particles with low melting point on the carrier. Additionally, the metal particles usually detach
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from the support in reuse, resulting in a sharp decrease in active sites.
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Recently, encapsulation of the dispersed gold nanoparticles inside porous silica, CeO , zeolite, etc.
has been proved to enhance catalytic activity and impede nanoparticles sintering under
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highꢀtemperature environments. In particular, several groups individually reported the intercalation
of monodispersed gold nanoparticles in the mesoporous silica and organosilica framework with
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assistance of thiolꢀcontaining silane or silsesquioxane. The presence of a protection shell or layer
outside active metal cores has also been reported to be feasible for enhancing the thermal stability
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against sintering. For example, Schueth and coꢀworkers developed a bottomꢀup strategy for stable
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yolkꢀshell gold nanoparticles. The gold nanoparticles are monodispersed in each zirconiaꢀbased
sphere that inhibits the aggregation among particles at higher temperatures. However, these oxide
carriers undergo change during the heat treatment, as the surface area dramatically decreases. A
facile route to stabilize monoꢀdispersed gold nanoparticles is still expected.
The advantages of carbon supports with respect to conventional oxidic supports, like silica and
zirconia, involve the stable structure which can minimize the shrinkage effect on support up to
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000 °C under inert atmosphere, chemical inertness, intrinsic hydrophobic nature, as well as easy
recovery of precious metal active phases from spent catalysts by burning away carbon. Carbonꢀbased
palladium nanocatalyst has been proved to be a highly efficient catalyst in the waterꢀmediated Heck
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coupling reaction. By comparison, immobilization of monodispersed Au nanoparticles on activated
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carbon remains a challenge, since the conventional methods such as depositionꢀprecipitation (DP)
and impregnation cannot be applied. For example, under normal conditions in the DP process,
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AuCl
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cannot be effectively deposited onto a negatively charged acidic carbon surface due to
electronic repulsion. The impregnation method is unfeasible, possibly due to the inert carbon matrix
which show weak interaction with gold nanoparticles and therefore cannot anchor tiny particles with
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high surface energy.
Several attempts have been done to disperse stable gold nanoparticles on carbon by enhancing
their interaction. Rossi and Prati first showed a colloidal synthesis for finely dispersed gold
nanoparticles (AuNPs) on activated carbon surface which involved the use of preformed gold
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colloids with protecting groups. Hardacre and coꢀworkers demonstrated that gold particles
supported on activated carbon (Au/C) can be dispersed down to atomic level during the
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carbonylation in the presence of methyl iodide. But the treatment by methyl iodide may somewhat
impede the catalytic activity in dehydrogenation. The random poreꢀsize distribution of activated
carbon ranging from micropores to macropores may also inhibit their applications where diffusion
may be a rateꢀlimiting step, for example catalysis and adsorption. Vinu and coꢀworkers directly used
a nanocasting carbon nitride support with inbuilt functionalities that acts as a stabilizing, reducing,
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and poreꢀsizeꢀcontrolling agent to anchor gold nanoparticles via the traditional twoꢀstep method.
Tuel and coꢀworkers also casted gold@mesoporous carbon coating using gold nanoparticles coated
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with thiol functionalized mesoporous silica shell as a sacrificed scaffold. Gold@ordered
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mesoporous carbons with tailored pore sizes have been reported as a good candidate for given
reactions in particular for the shapeꢀselective ones. However, the stability, especially under high
temperatures, is not reported. A repeated nanocasting was adopted to cast yolkꢀshell gold
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nanoparticles supported on carbon by using the above Au@ZrO hard template. The porosity of
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the shell is imperfect, and the thick shell or micropores may influence diffusion of reactants. The
catalytic activity may be inhibited. Up to now, no results have been reported related to the oneꢀstep
synthesis of gold nanoparticles on mesoporous carbon matrix.
In this manuscript, we demonstrate a coordinationꢀassisted selfꢀassembly approach for the
intercalation of aggregationꢀfree and monodispersed gold nanoparticles inside ordered mesoporous
carbon frameworks, using triblock copolymer F127 as a structure directing agent, HAuCl as a gold
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source, preformed phenolic resins as a carbon source, 3ꢀmercaptopropyltrimethoxysilane (MPTMS)
as a coordination and coupling agent. The strong coordination between thiol group and Au species
favors the immobilization of gold in the carbonaceous matrix. Upon carbonization and removal of
silica, the carbon supported catalysts possess the highly ordered mesostructure, high surface area of
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1800 m /g, large pore volume of ~ 1.19 cm /g, bimodal mesopores with uniform primary
mesopore size of about 4.0 nm and plenty of secondary mesopores less than 2.0 nm, and
monodispersed gold nanoparticles inside the matrix with a size of ~ 9 nm. Both the mesostructure
and gold nanoparticles are sintering free at high temperatures up to 600 ºC. The heterogeneous gold
catalyst is highly active in selective aerobic oxidation of benzyl alcohol using water as a solvent,
showing a conversion close to 100 % at 90 °C under 1 MPa within 1 h; and at 60 °C under an
atmospheric pressure within 12 h. The mesoporous AuꢀC catalyst is stable and reusable. These
advantages pave a way for supported gold nanoparticles in green chemistry.
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. Experimental Section
.1 Chemicals
Poly (ethylene oxide)ꢀbꢀpoly (propylene oxide)ꢀbꢀpoly (ethylene oxide) triblock copolymer F127
= 12, 600), and 3ꢀmercaptopropyltrimethoxysilane (MPTMS, minimum 98
wt%) were purchased from Acros Chemical Inc. Chloroauric acid tetrahydrate (HAuCl ꢁ4H O, Au
(EO106PO70EO106, M
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minimum 47.8 wt%) and other chemicals were obtained from Shanghai Chemical Company. All
chemicals were used as received without any further purification. Water used in all experiments was
deionized.
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.2 Synthesis of ordered mesoporous Au-C catalysts
The synthesis involves coordinationꢀassisted selfꢀassembly of lowꢀpolymerized phenolic resins,
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MPTMS, tetraethylorthosilicate (TEOS), HAuCl , and triblock copolymer F127 template. In a
typical synthesis, 2.08 g of TEOS and 1.97 g of MPTMS were firstly mixed in the presence of 10.0 g
of ethanol for 30 min. At the same time, 3.2 g of Pluronic F127, 2.0 g of HCl (0.2 M) and 10.0 g of
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ethanol were mixed together to obtain a clear solution. To it, 6.3 mL of HAuCl solution (gold
concentration in ethanol: 48.5 mmol/L) was added. After stirring for 10 min, the mixtures of TEOS
and MPTMS, and a solution containing 2.0 g of preformed phenolic resins and 8.0 g of ethanol were
added in sequence. The lowꢀpolymerized phenolic resins were prepared according to the established
procedure as described in Supporting Information (SI). After stirring for 2 h at 40 °C, the mixture
was poured into multiple dishes. The dishes were placed in a hood to evaporate ethanol at 40 °C for
5 h, and then to thermopolymerize at 100 °C for 24 h. The asꢀmade films were scratched from the
dishes and ground into fine powders. Calcination was carried out in a tubular furnace under nitrogen
flow to obtain mesoporous goldꢀpolymerꢀsilica or goldꢀcarbonꢀsilica nanocomposites. Temperature
program was from room temperature to 350 °C with a ramp of 1 °C/min, to 600 with 5 °C/min, and
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maintenance for 3 h. The catalyst was denoted as Au(SH)ꢀSC. Then, the goldꢀcarbonꢀsilica
nanocomposite was immersed with NaOH aqueous solution (1 M) at 45 °C by four times to remove
silica component. Each time took 12 h. The sample was named as Au(SH)ꢀC. The catalysts were
dried in an oven at 80 °C overnight under vacuum, then at 150 °C for 4 h in air.
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For comparison, thiolꢀgroupꢀfree Au(0)ꢀSC and goldꢀfree (SH)ꢀSC samples were synthesized
according to the above procedure with the maintenance of the total silica and/or gold contents.
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.3 Characterization
The Xꢀray diffraction (XRD) measurements were taken on a Rigaku Dmaxꢀ3C diffractometer
using Cu Kα radiation (40 kV, 20 mA, λ = 0.15408 nm). The metallic Au sizes were estimated
according to the Scherrer formula: size = 0.89λ/βcosθ on the basis of the 111 diffraction peak in
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wideꢀangle XRD patterns. N adsorptionꢀdesorption isotherms were measured at 77 K with
Micromeritics TriStar II 3020 analyzer. The BrunauerꢀEmmettꢀTeller (BET) method was utilized to
calculate the specific surface areas (SBET). By using the BarrettꢀJoynerꢀHalenda (BJH) model, the
pore volumes and pore size distributions were derived from the adsorption branches of isotherms.
Transmission electron microscopy (TEM) experiments were conduced on a JEM 2100 microscope
operated at 200 kV. Energy dispersive Xꢀray spectroscopy (EDX) was performed on a Philips EDAX
instrument. Scanning electron microscopy (SEM) images of Au(SH)ꢀC were taken with a
JSMꢀ7600F operated at 15 kV, 6 mm of working distance. To observe cross section of sample,
IBꢀ09010CP was used for cross section polishing at 4kV during 14 hours. SEM specimens were
prepared by fixating powder scraped off of the film to the SEM sample holder using carbon paste.
Fourier transform infrared (FTꢀIR) spectra were collected on Nicolet Fourier spectrophotometer.
Thermal gravimetric analysis (TG) curves were monitored on a Mettler Toledo 851e apparatus.
Xꢀray photoelectron spectroscopy (XPS) measurements were performed on a PerkinꢀElmer PHI
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000CESCA system with a base pressure of 10 Torr. Au contents are determined by inductively
coupled plasmaꢀatomic emission spectrometry (ICPꢀAES, Varian VISTAꢀMPX). Xꢀray absorption
spectra at the Au L edge of samples were collected in the fluorescence mode on the BL14W1 of the
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Shanghai Synchrotron Radiation Facility (SSRF). The data processing was performed using the
program ATHENA. All fits to the EXAFS data were performed using the program ARTEMIS.
UVꢀvisible diffuse reflectance spectra were recorded on a Varian Cary 300 Scan UVꢀvisible
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spectrophotometer, with an integrating sphere attachment using BaSO as the background standard.
The realꢀtime UVꢀvis spectra were determined using a Varian Cary 100 Ultravioletꢀvisible
spectrophotometer.
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.4 Activity test
The highꢀpressure oxidation reactions were carried out in a 50 mL autoclave reactor. The vessel
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was charged with catalyst (10 mg), benzyl alcohol (4.8 mmol), H O (10 mL) and KOH (14.4 mmol).
The autoclave was then purged five times with oxygen, leaving the vessel at 1 MPa gauge. Then the
reaction mixture was raised to 90 °C. In an atmospheric reaction, the same reagents were placed in a
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5 mL flask. Molecular oxygen was then introduced into the flask from an O balloon under
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atmospheric pressure and the mixture was heated to 60 °C with stirring. At different time intervals,
hot filtration was adopted to determine the Au leaching and benzyl alcohol conversion. The catalyst
was separated and submitted to continuous solid–liquid extraction with ethyl acetate (5 mL) using a
microꢀSoxhlet equipment. Then the solid catalyst was washed by a copious amount of water, and
dried at 80 °C overnight under vacuum for reuse. The extracted solution was mixed with the reaction
solution which was firstly neutralized with HCl solution and extracted with ethyl acetate (15 mL).
The mixture were weighed and analyzed. The recovered material accounted for more than 95 % of
the starting material. The benzyl alcohol oxidation was repeated three times, yielding the same initial
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rates and total reaction times within ± 10 %. For the identification and analysis of the products,
GC–MS equipped with a capillary DBꢀWax column was used by comparison with authentic samples.
The analysis was repeated at least three times for all tests, and the experimental errors were within ±
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%. Catalytic results were shown in terms of absolute yield of benzoic acid; conversion of benzyl
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alcohol; initial rate (mmol of reacted benzyl alcohol per g of catalyst and hour) calculated at the
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beginning of the reaction and turnover number (TON, initial rate/total Au concentration).
For the thiourea poisoning experiment, solution of thiourea which was prepared by dissolving the
proper amount in MilliꢀQ water, was added as a solvent. In standard experiments under an
atmospheric pressure, the reaction was performed for 6 h to ensure incomplete conversion of benzyl
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alcohol allowing for a comparison of the activity of the poisoning catalyst.
For recycling study, selective oxidation was performed with benzyl alcohol and oxygen
maintaining the same reaction conditions as described above except using the recovered catalyst.
Several parallel reactions were carried out to ensure that the catalyst amount was the same in each
run. The reused catalyst was designated as Au(SH)ꢀCꢀRy, wherein y was catalytic run. For example,
Au(SH)ꢀCꢀR2 represented the catalyst was used in the second run.
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. Results
3.1 Location and thermal stability of gold nanoparticles
Figure 1 shows the smallꢀangle (SAXRD) and wideꢀangle XRD (WAXRD) patterns for
Auꢀcontaining mesoporous carbonꢀsilica catalysts. The distinct diffraction peak can be detected in
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the SAXRD pattern for the asꢀmade Au(SH)ꢀSC, indicative of an ordered mesostructure. Only one
strong diffused diffraction peak at 2θ about 22 º is displayed in the WAXRD pattern, implying the
amorphous framework. The FTꢀIR spectrum of asꢀmade Au(SH)ꢀSC clearly shows bands at ~ 3400,
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000 – 2800, 1610, 1460, 1100 and 960 cm (SI Figure S1). The bands at ~ 3400 cm can be
assigned to the ꢀOH stretching, coming from both organic and inorganic components, i.e. a large
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amount of phenolic ꢀOH group, unꢀcrossꢀlinked benzyl alcohol and silanol groups.
The presence
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of organic bakelite component can be proved by the bands at ~ 3000 – 2800, 1610 and 1460 cm ,
which can be attributed to the CꢀH and CꢀC bond stretching of triꢀsubstituted and phenylꢀalkyl
etherꢀtype substituted aromatic ring structures or other characteristic stretching modes of phenolic
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resins. The bands at ~ 1100 and 960 cm can be assigned to the characteristic SiꢀOꢀSi and SiꢀOH
ꢀ1
vibrations. Several bands at ~ 1100 and 3000 – 2800 cm can be attributed to the CꢀO and CꢀH
stretching of copolymer F127. However, the thiol group in IR is invisible under the current
conditions, perhaps due to the weak dipoles exhibited by SꢀH groups which makes their modes
24
difficult to be detected by vibrational spectroscopy.
Carbonization of the catalyst is carried out at 600 °C. The vibrations in the FTꢀIR spectrum for
Au(SH)ꢀSC belonging to the polymer disappear, implying the conversion from polymeric to
carbonaceous framework, and the reserve of vibrations belonging to SiꢀO bonds, confirming the
presence of silica component (SI Figure S1). EDX spectrum confirms the elements of Au, C, Si, and
O in the catalyst (Figure 3A inset). Undetectable sulfur element proves the release of ꢀSH group
during calcination. The gold content, determined by the ICPꢀAES analysis, is about 2.6 wt%.
Besides the diffused peaks at 22 º, the peaks at 2θ of 38, 44, 65 and 78 º are also observed in the
WAXRD pattern for Au(SH)ꢀSC, attributed to the 111, 200, 220 and 311 diffractions of
faceꢀcentered cubic (fcc) metallic gold (JCPDS PDFꢀ04ꢀ0784). The {111} lattice space is estimated
to be 0.24 nm. The diffractions are wide, implying tiny nanoparticles. Estimation with the Scherrer
formula gives rise to the gold particle size of about 9.2 nm (the size of perpendicular to the 111 plane,
the same in the latter). If 200, 220 and 311 diffractions are applied for estimation, the particle size
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even reduces to about 7 nm. No diffraction belonging to gold oxides is found. The well resolved
SAXRD pattern for Au(SH)ꢀSC can be retained, with a slight shift to higher 2θ values compared to
that for the asꢀmade sample, suggesting the stable mesostructure framework with a small shrinkage
0 2
(about 28 %). The cell parameter (a ) for the mess structure is calculated to be 9.1 nm (Table 1). N
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sorption isotherms of Figure 2 display typical typeꢀIV curves with distinct capillary condensation
steps for Au(SH)ꢀSC, characteristic of mesoporous solids with uniform pore size. This phenomenon
is coincident with the XRD pattern. Narrow poreꢀsize distribution curve with the mean value of 4.2
nm is calculated from the adsorption branch. The hysteresis loop for the catalysts belongs to the H2
25
type. Desorption occurs at relative pressures close to the fluid cavitation pressure. This
phenomenon has also been observed in mesoporous carbon FDUꢀ15 and carbonꢀsilica composites
with the highly ordered 2D hexagonal mesostructure, which may be originated from the nonꢀuniform
2
2,23b,26
pore surface of carbon materials, as well as partially blocked larger mesopores.
The BET
2
3
surface area is 418 m /g and the pore volume is 0.29 cm /g for the Auꢀcontaining mesoporous
silicaꢀcarbon catalyst. The estimated pore wall thickness is 4.9 nm, on the basis of the hexagonal
pore structure. It should be noted that the gold nanoparticle size is apparently larger than either the
pore size or wall thickness. The result implies that gold particles occupy both pore space and wall.
TEM images for Au(SH)ꢀSC show typically stripeꢀlike and hexagonally arranged pores, indicative
of the stable ordered 2D hexagonal mesostructure (Figure 3). Notably, monodispersed gold
nanoparticles which locate inside mesochannels with expansion to the pore walls are found within
the whole catalyst particles. Distortion in ordered porosity caused by deposition of Au nanoparticles
is unobvious. The ordinate in Figure 3C presents a normalized number (0 – 150) of particles,
revealing nanoparticles with a mean size of 8.7 nm. No large particles aggregate outside ordered
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mesopores, clearly indicating that the monodispersed gold nanoparticles are intercalated in the
mesoporous carbonaceous matrix.
The XPS analysis is a surface sensitive technique with a sampling volume that extends from the
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7
surface to a depth of 1 – 5 nm. The diffused peaks with binding energy of about 84 and 88 eV can
be assigned to Au4f5/2 and Au4f7/2 in the metallic state (Figure 4). However, these two XPS signals
are extremely weak. The surface and nearꢀsurface concentration of metallic gold is far lower than the
value from the ICPꢀAES analysis. This observation can be attributed to the immobilization of gold
particles in the pore system, confirming the previous conclusion from electron microscopy analysis.
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Similar phenomena have been reported by Lu et al. and our group. The binding energy signals for
nickel or iron are extremely weak for metalꢀcontaining mesoporous carbons in which metal or oxide
nanoparticles are involved inside the mesopore walls. Only Au, C, O and Si are detected, confirming
the absence of any contamination from chlorine and sulfur for the catalyst.
3
.2 Gold nanoparticles in mesoporous carbon with a high surface area
A further step to dissolve silica component from the integrated silicaꢀcarbon frameworks was
carried out. The Si signals disappear and the O signals pronouncedly reduce in the XPS spectrum for
the Au(SH)ꢀC catalyst (Figure 4), proving the elimination of SiO . The EDX pattern further reveals
2
the absence of element Si (Figure 3D inset). The ICPꢀAES analysis displays that Au(SH)ꢀC has a
gold content of 5.9 wt%. Gold ions are undetectable in the collected basic solution after immersing
with the goldꢀcontaining catalyst, suggesting the stability of gold nanoparticles and negligible
leaching during etching of silica.
The mesoporous pure carbonꢀbased gold catalyst Au(SH)ꢀC shows wellꢀresolved SAXRD and
WAXRD patterns (Figure 1), proving the maintenance of the 2D hexagonal mesostructure, and fcc
Au nanoparticles with a size about 9.4 nm. TEM images confirm the above results (Figure 3DꢀF).
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Both the ordered pore arrays and the sinteringꢀfree and well dispersed nanoparticles with a mean size
of about 9.0 nm are presented. Figure 3G shows a representative particle, revealing the characteristic
Au {111} lattice fringe at 0.24 nm. By comparison with the images for the mother Au(SH)ꢀSC
catalyst, we can observe neither a distinct migration nor growth of gold nanoparticles. The particles,
which locate inside mesochannels with expansion to the pore walls, stay intact, although the pore
walls of the carbon mesostructure seems looser than the mother catalyst. These observations further
suggest that the removal of silica component has a minor effect on both the mesostructure and gold
nanoparticles.
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To exclude the location of Au nanoparticles on the external surface of mesostructured solids, SEM
images for both surface and cross section of Au(SH)ꢀC were taken (Figure 5). Typical rough surface
of mesoporous carbon solids with opened mesoporous is observed. Large gold nanoparticles cannot
be obviously found. After cross section polishing, Au nanoparticles within the mesoporous carbon
pore network appear (which are confirmed by EDX). Brighter contrast of Au nanoparticle at the
leftꢀtop of Figure 5B corresponds to the particle exposed to the surface, while the other two particles
are embedded in carbon network. Sizes of carbon pores and Au nanoparticles are measured, as
shown in the image (gold particle size, primary pore and wall thickness of ~ 9.1, 4.0 and 4.9 nm,
2
respectively), which are analogous to the values calculated from the N sorption and WAXRD results.
Though the size of Au nanoparticles is obviously larger than that of pores, carbon network was not
severely damaged by gold deposition.
Besides a capillary condensation at middle relative pressures in the N isotherm curves, a
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0
remarkable sorption at relative pressure of P/P of 0.01 ꢀ 0.3 is observed for Au(SH)ꢀC (Figure 2),
compared with its mother composite Au(SH)ꢀSC. Accordingly, Au(SH)ꢀC possesses an extremely
2
3
high BET surface area of 1826 m /g and large pore volume of 1.19 cm /g which is almost 4 times
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higher than the mother catalyst, and a bimodalꢀlike poreꢀsize distribution with the most probable
pore sizes of 4.0 and less than 2.0 nm. The primary mesopore size is similar to Au(SH)ꢀSC, while the
secondary one is much smaller. Since the micropore volume of Au(SH)ꢀC is low (SI Figure S2), the
secondary mesopores contribute the majority of surface area and pore volume. These phenomena
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have also been observed for mesoporous silicaꢀcarbon materials.
Silica and carbon can form an
interpenetrating framework. Once silica nanoparticles are etched, plenty of voids (< 2 nm secondary
mesoproes) are formed inside carbon pore walls. Therefore, the original mesoporous Au(SH)ꢀSC
composites also possibly possess interpenetrating inorganicꢀinorganic framework instead of a carbon
coating on mesoporous silica. Upon removal of silica component, secondary voids are generated
inside the carbon walls. Gold nanoparticles are free of immigration and aggregation because of the
confinement by carbon walls and mild washing by NaOH solution. The penetrated voids in walls can
therefore provide high surface area, and more importantly, high accessibility to gold nanoparticles
which partially locate inside pore walls. In addition, the generation of edges in the interface between
carbon and gold can therefore be expected.
3
.3 The role of thiol group
A reference sample Au(0)ꢀSC was synthesized with the same procedure as Au(SH)ꢀSC but in the
absence of MPTMS. Au(0)ꢀSC shows a well resolved SAXRD pattern (SI Figure S3A), indicating
the highly ordered mesostructure with the domain size calculated to be 11.6 nm. The pore size
distribution for this catalyst is also narrow, centered at about 6.9 nm (SI Figure S4). But the WAXRD
pattern of Au(0)ꢀSC gives very sharp diffractions for fcc Au (SI Figure S3B), implying the presence
of large Au particles with a diameter above 100 nm. The phenomenon implies the macrophase
2
8b
separation of the ordered mesostructure and large metallic particles. TEM images indeed show
both ordered mesostructure domains without distinct nanoparticles and large particle aggregation
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outside the carbonaceous support even heating at 600 ºC (SI Figure S5). EDX analysis reveals that
no gold nanoparticles can be detected in the ordered domains (SI Figure S5B inset).
We thereafter traced the synthesis in the absence/presence of MPTMS with the realꢀtime UVꢀvis
4
spectra (Figure 6A). The pristine ethanolic HAuCl solution with yellowish color exhibits an
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absorption band centered at 320 nm, which is attributed to the ligand metal π→σ* orbital transition
ꢀ
in (AuCl
4
) species. Negligible changes in the solution color and UVꢀvis spectra can be found after
successive addition of triblock copolymer F127 and TEOS. Pronounced difference takes place once
the pristine solution contains functional monomer MPTMS. The solution varies to colorless. The
absorption intensity due to the π→σ* orbital transition decreases dramatically, suggesting the
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disappearance of AuꢀCl coordination. A further addition of lowꢀpolymerized phenolic resin can
only generate several strong absorption below 300 nm, which are originated from the benzeneꢀring
frameworks, but shows a minor effect on the absorption by Au species. Individual interactions
4
between ethanolic HAuCl solution with reactants were also investigated (SI Figure S6). The
remarkable change in absorbance by Au species occurs only with the presence of MPTMS, that is, a
distinct reduce in intensity of the band at 320 nm assigned to the AuꢀCl coordination. These
30
observations demonstrate the change in the coordination atoms with Au from Cl to S.
Figure 7 shows the XANES data at the Au L edge for the synthetic solutions of Au(SH)ꢀSC and
3
3
+
thiolꢀfree Au(0)ꢀSC, a Au reference of HAuCl
4
solution and gold foil. The thiolꢀfree synthetic
3+
solution for Au(0)ꢀSC displays a spectrum of similar shapes and edge energies with the Au
reference, which exhibits a large absorption feature at a lower binding energy (labeled a) than the
bulk gold metal. This shift to a lower binding energy is due to the 2p to 5d transition that precedes
3
1
the excitation to the bulk. The results indicate that gold in thiolꢀfree synthesis solutions is in the
3+
Au state and the coordination with Cl, the same as the pristine HAuCl
4
solution. The spectrum for
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Au(SH)ꢀSC synthesis solution has a leftꢀshift binding energy, a much smaller aꢀfeature, and
weakened b and cꢀfeatures, probably due to a small concentration of 5d holes, implying the distinct
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reduce in valence and change in coordination state. The reduction of Au(III) species to Au(I)
3
3
thiolate has also been demonstrated by the thiolꢀcontaining PTMPꢀPMAA ligand,
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pꢀHSCH (C H )C(CH ) , etc.
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3 3
The Fourier transforms of the EXAFS data for the thiolꢀfree synthesis solution of Au(0)ꢀSC is
much similar with the HAuCl solution reference. Both show a strong AuꢀCl coordination on the
4
first shell with the bond length of about 2.29 Å and a coordination number (CN) about 3 (Figure 7B,
Table 2), indicating the maintenance of the gold state and coordination in the pristine solution. By
comparison, the synthesis solution of Au(SH)ꢀSC displays a strong coordination between Au and S,
with a CN of 2 and the bond length of 2.30 Å. The replacement of the AuꢀCl by AuꢀS bond is in
coincidence with the UVꢀvis spectra. Neither Au(SH)ꢀSC nor thiolꢀfree Au(0)ꢀSC solution exhibits a
similar shape, edge energy and AuꢀAu coordination with Au foil, excluding the reduction of metallic
35
gold nanoparticles in solution.
The asꢀmade thiolꢀfree Au(0)ꢀSC displays a wide absorption band centered at about 500 nm due to
the metallic Au surface plasma resonance, indicating the reduction of gold ions to metal at 100 °C.
The WAXRD pattern for the asꢀmade Au(0)ꢀSC indeed show the diffraction peaks belonging to
metallic gold nanoparticles (SI Figure S7). On contrast, a featureless UVꢀvis spectrum is observed
for the asꢀmade Au(SH)ꢀSC material, indicating the absence of gold nanoparticles and the reserve of
Auꢀthiol complex (Figure 6B). It should be commented, however, that gold clusters having less than
36
2
nm in diameter do not exhibit generally intense surface plasmon band. Thus, the gold clusters, if
formed, are very small. No characteristic UVꢀvis absorption can be observed for all carbonꢀbased
catalysts after carbonization due to strong absorption of light by carbon.
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A goldꢀfree sample (SH)ꢀC was also synthesized. The reaction occurring between preformed
phenolic resols which have plenty of phenyl or benzyl hydroxyl groups and silane coupling agent
containing reactive thiol groups facilitates the coꢀassembly of mesostructure. The phenolic
resinꢀbased composites have the ordered mesostructure and highꢀcontent, chemically accessible thiol
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groups which can adsorb heavy metal ions and be further oxidized to sulfonic acid.
After
37
carbonization, the rigid silicaꢀcarbon framework was obtained. A further etching step in basic
solution leads to the formation of carbon solid with an ordered mesostructure, a cell parameter of 8.6
2
nm, a pore size of 4.1 nm and a high surface area of 1621 m /g (SI Figure S8 and S9, Table 1). These
features are analogous to those for the Au(SH)ꢀC catalyst. The results confirm the formation of rigid
framework in the goldꢀcontaining mesoporous catalyst from the coꢀassembly of MPTMS, TEOS and
phenolic resin, as well as the minor effect on mesostructure by gold deposition.
3.4 Selective aerobic oxidation of benzyl alcohol in water
Organic reactions in water have been extensively investigated for the fine and pharmaceutical
chemical industries, due to the fact that they offer the possibility of providing environmentally
38
benign reaction conditions by reducing the burden of the organic solvent disposal. The oxidation of
alcohols to carbonyl compounds in water under mild conditions is highly desired.
2
The selective oxidation of benzyl alcohol with molecular O in water was chosen as a probe
reaction to evaluate the activities of gold nanoparticles inside ordered mesoporous carbon
frameworks. Using potassium hydroxide as a base, the reaction over Au(SH)ꢀC is rather fast under
90 ºC and 1 MPa (Figure 8). An almost complete conversion of benzyl alcohol to benzoic acid is
achieved within 60 min. To the best of our knowledge, this is one of the highest activities for a
6
ꢀ7,39
selective oxidation of benzyl alcohol over a heterogeneous catalyst in water (SI Table S1).
A
mild condition of 60 ºC and atmospheric pressure was then adopted. The initial reaction rate reduces
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ꢀ
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to 115.2 mmol g h with a TON value of 384 h . But benzyl alcohol can still almost complete
selectively oxidized to benzoic acid within 12 h (Figure 9). The atomic economic reaction with a
1
00 % yield in the presence green solvent of water under atmospheric pressure shows great
advantages in green chemistry.
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It should be noted that both benzaldehyde and benzyl benzoate are formed together with benzoic
acid during reaction regardless of pressure and temperature. The yields for byproducts are low, with
a maximum of 22 % for benzaldehyde at 10 min and 3 % for benzyl benzoate at 20 min under 1 MPa.
With the prolongation of reaction, the selectivity for these substances reduces. At a reaction time of
5
0 min under high pressure or 12 h under atmospheric pressure, benzoic acid is detected as the
exclusive product. This observation may suggest the intermediate of benzaldehyde during the
oxidation from benzyl alcohol to benzoic acid. Esterification is possibly responsible for the
formation benzyl benzoate that was also found by Tsukuda and coꢀworkers using the PVPꢀstabilized
6
gold nanoclusters as a catalyst. Accompanied with the conversion of most reactant, benzyl benzoate
hydrolyzes and disappears.
Figure 10 shows the normalized conversions of the reaction as a function of the concentrations of
thiocyanate. Note that the concentrations of thiocyanate are shown relative to the total number of Au
atoms. The gradual addition of the poison to the catalyst gives rise to a slow decay of the catalytic
−
activity. The conversion reduces only by 50% on adding equal molar amount of NCS and Autotal
.
−
Even with a molar ratio of NCS /Au = 33, about 37% activity can be remained. It is much different
with the heterogeneous Au catalysts showing a dramatic decrease in activity with addition of
20
Sꢀcontaining compounds. For example, addition of 6 mol% thiourea/Autotal leads to an almost
2
0
complete loss of selective oxidation activity for glucose over Au/C. Baiker and coꢀworkers had
found that the competitive adsorption of substrate over poison could improve the poisoning
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resistance.
We would also like to contribute the interesting poisonꢀresistance behavior over
mesoporous Auꢀcontaining carbon catalyst to the strong adsorption of benzyl alcohol. The majority
of thiocyanate is residue in water solution that may be inaccessible to gold nanoparticles. The high
activity can therefore be retained at a high poison concentration. This result may further confirm the
intercalation of gold nanoparticles inside carbon matrix instead of anchoring on external surface of
particle. The poisoning mechanism is deserved to be separately reported.
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One of the key factors that must be considered for heterogeneous catalysts operating in three
phase systems is the possibility that active components can leach into the reaction mixture, leading
to catalyst deactivation or formation of an active homogeneous catalyst. The liquid phase of the
reaction mixture is collected by hot filtration after reactions. This operation can avoid the
reꢀdeposition of any soluble species on the support during cold centrifugation. ICPꢀAES analysis
confirms an undetectable amount of Au in the reaction mixtures. Even if a long time reaction (24 h)
was carried out or a large amount of the catalyst (0.1 g) was added, negligible gold leaching amount
is detected after reactions. In a separated test, the oxidation mixture is hot filtered to collect the
liquid phase after 2 h stirring under atmospheric pressure. The conversion of benzyl alcohol is 47 %.
Insignificant change for the conversion of benzyl alcohol after a further 2 h reaction is observed with
the same conditions as the initial reaction except in the absence of the solid catalyst (< 3 %, less than
the experimental error). These results clearly suggest that gold leaching could be approximately
excluded in the present oxidation reactions. Therefore, the reactions approximately undergo
4
0
heterogeneous conditions, where the reactant selectively adsorbs on active centers and then
converts to the product.
For recycling study, aerobic oxidation of benzyl alcohol was performed maintaining the same
atmospheric reaction conditions except using the recovered catalyst. The time plots for the catalytic
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conversion of benzyl alcohol and yield of benzoic acid over the recovered Au(SH)ꢀC catalyst are
similar to those over the fresh one (SI Figure S10). We also tested the initial reaction rate (within 0.5
h) and conversion of benzyl alcohol to benzoic acid at 12 h in successive runs and complied in
Figure 11. It can be clearly found that obvious changes are undetected for catalytic performance after
five runs. The reaction mother liquors after hot separation with the catalyst in each run were
collected and mixed. The gold content, in the final mixture, is below the detected line by the
ICPꢀAES analysis, implying the negligible metal leaching during the repeated reactions. These
results indicate that the goldꢀcontaining mesoporous carbon catalyst is stable and can be reused.
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The XRD patterns, TEM images and N sorption isotherms clearly show that the reused catalysts
retain the ordered mesostructure, high surface area, and uniform mesopores (SI Figure S11, S12, S13
and Table 1). Obviously, gold nanoparticles are aggregationꢀfree after use. Both the TEM images
and WAXRD patterns reveal the particle size of ~ 9.5 nm, almost the same as the fresh catalyst. The
Au(SH)ꢀCꢀR6 catalyst, after five catalytic run, shows a gold content of 5.9 wt%, further confirming
the free of metal leaching.
4. Discussion
The synthesis for ordered mesoporous carbon supported gold nanoparticles involves oneꢀstep
coordinationꢀassisted selfꢀassembly approach.
A goldꢀthiol coordination occurs between Au species and thiol group in MPTMS in the initial
solution, as evidenced by the EXAFS and UVꢀvis absorption analysis. The coordination can stabilize
11
gold species. Simultaneously, the silane coupling agent containing reactive thiol groups can react
41
both with preformed phenolic resins and TEOS. In this case, the interactions among gold, silane,
and resins are strengthened in solution. Triblock copolymer assembly at the interface is originated by
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the hydrogenꢀbonding interaction between hydrophilic EO segments and hydroxyl groups of resins
and silicates and then induced by solvent evaporation.
The subsequent thermopolymerization step at 100 ºC involves further solidification of the integral
26
organicꢀinorganic frameworks. The soꢀderived asꢀmade films are flaxen and transparent without
obvious macrophase separation. Au in low chemical state and strong coordination with ꢀSH is very
stable. The mobility and reactive ability may therefore be inhibited. The thiol groups coordination
with gold ions possibly locate either inside the carbonaceousꢀsilicate walls or on the pore surface. By
comparison, gold ions lack of coordination with thiol moieties are easily reduced to metallic
nanoparticles at this stage possibly by unreacted formaldehyde in the phenolic resin.
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During heating treatment, triblock copolymer decomposes to CO, CH et al. The mesopores are
4
open. Phenolic resins carbonize and a large amount of small molecules is released. Carbon and
silicate form an integrated framework with the ordered mesostructure. The relatively “rigid”
37
interpenetrated framework is thereafter fixed. Gold species can decompose or reduce to metallic
state. The neighbored gold species gather together and grow once the thiol groups are released. The
growth of gold nanoparticles inside mesopore channels is feasible with the possibility of penetration
into pore walls. But the particle growth is confined by the rigid silicaꢀcarbon framework, and the
crystal growth energy in the solidified pore walls is high. As a result, the particles homogenously
disperse in the matrix and are confined by the pore walls with adjacent to six cylindrical pores. The
growth is ended with the particle size about 9.0 nm. Gold seems “get stuck”. The simultaneous
formation of gold nanoparticles and relatively rigid silicaꢀcarbon framework favors the intercalation
of Au in pore walls. Unobvious distortion of ordered porosity occurs after gold deposition, although
the gold diameter is apparently larger than either pore size or wall thickness. At the same time,
carbon species may deposit over the metallic gold catalysts, in particular at the corner of the
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interface between gold and carbonaceous pore wall. These carbon species which can be regarded
as part of carbonaceous pore walls play a further immobilization role on gold nanoparticles.
Aggregation of nanoparticles outside nanopores is inhibited. The avoidance of postꢀreduction is
4
3
interesting because treatment at higher temperatures always generates bigger gold nanoparticles.
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On contrast, gold species lack of thiol group coordination will grow to large nanoparticles above 100
nm, move outside pores and separate with carbon ordered mesostructure at an elevated temperature,
due to the improved immobility of metallic nanoparticles in a relatively “soft” framework.
Therefore, a preformed rigid framework should be favorable for inhibiting growth of small
metallic gold clusters during reduction. A preliminary experiment shows that 5.0 nmꢀgold
nanoparticles can be confined in the mesoporous carbonaceous framework with an additional
lowꢀtemperature thermopolymerize at 150 ºC, as evidenced by the TEM image and XRD pattern
(
Figure S14).
The present catalyst is totally different with the traditional metal catalyst supported by porous
carriers. The sintering of gold nanoparticles on the surface of ordered nanopores cannot be avoided
due to both the weak interaction between surface and metal and the open nanopores. Metal easily
4
4
migrate outside pores to form large particles. The presence of carbon and silica can modify the
45
gold surface energy, and this may result in the expression of “active” surfaces. The spaceꢀrelated
4
5a
growth may also lead to the development of specific facets. Therefore, the “active” {111} surface
is highly exposed.
After removal of silica component (Au(SH)ꢀC), carbon wall can support the mesostructure, and
retain unchanged, only with the generation of plenty of secondary mesopores inside pore walls
which are originated from the silica nanoparticles in the interpenetrated framework. The silane
coupling agent including thiol group and silica can be completely eliminated, that would show
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negligible effect on the catalytic result of Auꢀcontaining mesoporous carbon catalysts. The gold
nanoparticles are stable in basic solution. Dissolving and etching can be avoided in this step. The
particles are reserved, occupying both pore space and wall, and being exposed. Special corners and
edge sites of gold surface atoms may be generated by the secondary pores, that are expected by the
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surprising activity.
Besides primary pores, bulky organic molecules can easily enter the
47
secondary mesopores of carbon with the relatively hydrophobic surface. Accordingly, gold
nanoparticles are highly accessible. This kind of carbon support differs from the most reported
mesoporous carbon for example CMKꢀ3 or FDUꢀ15 whose surface areas are mainly contributed by
23a,48
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micropores.
Diffusion limitation is much more serious in these cases.
The benzyl alcohol molecules can be enriched inside both primary and secondary mesopores, and
therefore, are accessible to gold nanoparticles with highly exposed active surface atoms and special
corner or edges. The intermediate alkoxide are firstly formed, and then converted to aldehyde and
49
water by selective CꢀH bond (α to the oxygen) activation. The AuꢀH covered catalyst surface might
2
be reactivated by O and OOH. The formation of strong oxidization agent may facilitate the further
oxidation of aldehyde to acid. Simultaneously, the presence of water can reduce the barrier of O2
50
hydrogenation. Although benzyl benzoate is also produced, it can be reꢀadsorbed on gold
nanoparticles and hydrolyze in the presence of aqueous base solution to generate benzoic acid. As a
result, benzoic acid is the exclusive oxidation product. Gold nanoparticles, which are confined by the
carbon walls, are highly stable. Due to the strong adsorption of benzyl alcohol from water in the
mesopores of carbon with the relatively hydrophobic surface, the transportation of soluble
thiocyanate is inhibited. As a result, the attack to gold nanoparticles by poison molecules is inhibited.
The catalyst exhibits an interestingly poisoningꢀresistance behavior.
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. Conclusion
The oneꢀstep construction of the sinteringꢀstable goldꢀcontaining mesoporous carbonaceous
catalysts is on the basis of coordinationꢀassisted triblock copolymerꢀtemplating approach. The silane
coupling agent containing thiol group is essential for the aggregationꢀfree active gold nanoparticles
in the carbonaceous matrix, which can coordinate with gold species by thiol group to obtain
monoꢀdispersed gold species; react with phenolic resins by silanol groups to form relatively “rigid”
composite framework; and generate secondary pores inside carbon pore walls to provide high
surface areas. The silane coupling agent can be completely eliminated with negligible effect on the
catalytic performance of Auꢀcontaining mesoporous carbon catalysts. During heating treatment,
metallic gold is in situ reduced, accompanied with the opening of mesopores, and rigidifying of
framework. This process facilitates the immobilization and stabilization of monodispersed gold
nanoparticles in the carbonaceous framework. Aggregation of nanoparticles is inhibited even with a
high carbonization temperature of 600 °C. Once inorganic component of silica is removed,
mesoporous pure carbonꢀbased catalysts can be obtained with maintenance of monodispersed gold
0
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nanoparticles (~ 9.0 nm), high specific surface area (~ 1800 m /g), large pore volume (1.19 cm /g),
uniform primary pore size distribution (4.0 nm) with plenty of secondary mesopores (< 2.0 nm)
inside carbon pore walls. The mesoporous carbonꢀbased gold catalyst exhibits an almost complete
conversion of benzyl alcohol to benzoic acid within 60 min at 90 °C and 1 Mpa, or 12 h at 60 °C and
atmospheric pressure. Negligible activity loss, metal leaching and mesostructure destruction are
observed for the reꢀused catalyst. The catalyst is highly poisoningꢀresistance. Even with a molar ratio
for thiocyanate:Au of 33, about 37% activity can be remained. The high activity and stability can be
assigned to aggregationꢀfree metallic gold nanoparticles, highly exposed surface atoms by bimodal
pores, confinement of nanoparticles by stable carbon mesostructure, and large carbon mesopores
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with the relatively hydrophobic surface facilitating the selective adsorption of substrate in water and
mass transfer.
Supporting Information
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Preparation of lowꢀpolymerized phenolic resins; Oxidation of benzyl alcohol in water using Au
based or bimetallic AuꢀPd based supported catalysts; FTꢀIR spectra and tꢀplots for asꢀmade
goldꢀcontaining mesoporous catalysts, the catalyst calcined at 600 °C (Au(SH)ꢀSC); and after
2
dissolution of silica (Au(SH)ꢀC); XRD patterns, N sorption isotherms, poreꢀsize distribution curves
and TEM images for the thiolꢀfree sample Au(0)ꢀSC, goldꢀfree sample (SH)ꢀC, supported 5.0
nmꢀgold catalyst, fresh Au(SH)ꢀC catalyst, and the catalysts after the first and third runs
(Au(SH)ꢀCꢀR2 and Au(SH)ꢀCꢀR4); UVꢀvis spectra for monoꢀconstituent and biꢀconstituent synthesis
solutions in ethanol; and time conversion plots over the reused catalyst Au(SH)ꢀCꢀR2; TEM image,
and WAXRD pattern for the supported 5.0 nmꢀgold catalyst.This information is available free of
charge via the internet at http://pubs.acs.org.
Acknowledgements
This work was supported by State Key Basic Research Program of China (2013CB934102), NSF
of China (21073122 and 21173149), Ministry of Education of China (PCSIRTꢀIRT1269 and
2
0123127110004), Shanghai Sci. & Tech. and Edu. Committee (11JC1409200, DZL123 and
S30406).
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2278.
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Table 1. Structural and textural parameters for goldꢀcontaining mesoporous catalysts.
a
b
c
d
Au content a0
wt%) (nm) (m /g) (cm /g) (nm) (nm) (nm)
As made Au(SH)ꢀSC n.d.
S
Vt
Dp
dAu
dAu
BET
Sample
2
3
(
12.7 ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
Au(SH)ꢀSC
Au(SH)ꢀSC
Au(SH)ꢀC
Au(0)ꢀCS
Au(0)ꢀC
(SH)ꢀC
2.6
2.8
5.9
n.d.
n.d.
ꢀꢀꢀ
9.1
8.6
8.9
418
383
0.29
0.29
4.2
4.0
4.0
6.9
6.0
4.1
3.9
3.8
9.2
9.1
9.4
50
50
ꢀꢀꢀ
8.7
8.4
8.9
n.d.
150
ꢀꢀꢀ
1826 1.19
0.50
11.0 1331 1.23
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
11.6 413
8.6
8.6
8.5
1621 1.06
1693 1.11
1624 1.06
Au(SH)ꢀ CꢀR2
Au(SH)ꢀ CꢀR4
5.9
5.9
9.5
9.6
9.5
n.d.
a
b
c
Estimated by ICPꢀAES and TG analysis;
cell parameter of the mess structure;
Calculated from the Scherrer formula according to the 111 diffraction;
d
Estimated from TEM analysis of average diameter.
Table 2. Fit parameters of Au L
thiolꢀfree Au(0)ꢀSC catalysts, and the reference aqueous HAuCl
3
edge EXAFS spectra for the synthetic solutions of Au(SH)ꢀSC and
solution and Au foil.
4
a
b
2
ꢀ3
2
c
d
Sample
Solution for Au(SH)ꢀSC
Solution for Au(0)ꢀSC
Shell
AuꢀS
AuꢀCl
AuꢀCl
AuꢀAu
N
R
σ (10 Å )
2.2±0.9
2.5±1.6
1.9±1.0
8.3±0.5
∆ E (eV)
7.5±1.1
11.2±2.8
9.1±0.9
5.2±0.5
0
2.0±0.2
3.0±0.6
3.5±0.4
10.6±0.8
2.30±0.01
2.29±0.01
2.27±0.01
2.86±0.01
HAuCl
Au foil
4
solution
a
b
c
Coordination number;
Distance between absorber and backscatterer atoms;
Debye–Waller factor;
d
Inner potential correction.
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