Gold nanoparticles supported on passivated silica: Access to an efficient aerobic epoxidation catalyst and the intrinsic oxidation activity of gold

Article abstract of DOI:10.1021/ja903730q

(Figure Presented) Well-defined and perfectly dispersed [(≡SiO)Au^I] surface species supported on silica have been obtained via surface organometallic chemistry and transformed upon mild reduction (H₂, 300°C) into small (1.8 ± 0.6 nm)

Full text of DOI:10.1021/ja903730q

Published on Web 09/25/2009
Gold Nanoparticles Supported on Passivated Silica: Access to an Efficient
Aerobic Epoxidation Catalyst and the Intrinsic Oxidation Activity of Gold
David Gajan,†,‡ Kevin Guillois,§ Pierre Deliche`re,§ Jean-Marie Basset,† Jean-Pierre Candy,†
Vale´rie Caps,*,§ Christophe Cope´ret,*,† Anne Lesage,‡ and Lyndon Emsley‡
UniVersite´ de Lyon, Institut de Chimie de Lyon, C2P2-LCOMS UMR 5265 (CNRS - CPE - UniVersite´ Lyon 1),
ESCPE Lyon, 43, Bd. du 11 NoVembre, F-69616 Villeurbanne, France, UniVersite´ de Lyon, CNRS/ENS-Lyon/
UCB-Lyon 1, Centre de RMN a` Tre`s Hauts Champs, 5 rue de la Doua, 69100 Villeurbanne, France, and Institut de
Recherches sur la Catalyse et l’EnVironnement de Lyon (IRCELYON, CNRS/UniVersite´ de Lyon), 2 AVenue Albert
Einstein, F-69626, Villeurbanne Cedex, France
Received May 7, 2009; E-mail: coperet@cpe.fr
Scheme 1
We describe a method for the direct preparation of 1.8 nm gold
nanoparticles (AuNP) on passivated silica via controlled function-
alization of the silica surface with a AuI complex, namely,
{Au[N(SiMe3)2]}4, followed by mild reduction under H2 (Scheme
1). This approach leads to a highly efficient gold catalyst for liquid-
phase aerobic epoxidation of trans-stilbene. This tailor-made
catalyst shows that AuNP are intrinsically active in aerobic
epoxidation and in preferential oxidation of CO (PROX).
Although gold had been considered to be an inert metal, Haruta
et al.1 discovered that AuNP in the 2-5 nm range exhibited unique
catalytic activity for the low-temperature oxidation of CO when
associated with a reducible oxide. Other oxidation reactions
involving supported gold catalysts and O2 were then developed,
such as the direct synthesis of H2O2 from H2,2 the epoxidation of
propene,3 and several liquid-phase aerobic oxidations (the oxidation
of alcohols,4 aldehydes,5 and hydrocarbons6). For these reactions,
the mechanism of O2 activation is an important issue because
dissociative chemisorption of O2 on gold is unlikely.7 In gas-phase
oxidation, it relies on the presence of oxygen vacancies8 and/or
surface hydroxyls on the support,9 and the gold-support interface
probably plays a key role. However, in the presence of H2, such as
in the epoxidation of propene with a H2/O2 mixture10 and in
PROX,11 alternative pathways involving the formation of highly
reactive hydroperoxo species from O2 and H2 are possible.12
Additionally, in liquid-phase aerobic epoxidation of alkenes, O2 is
probably activated by an alkyl radical produced from the hydro-
carbon solvent and initiated by ROOH over gold (Scheme 2).13,14
In all of these reactions, the size of the AuNP is also a key factor
determining the activity.15 Therefore, much effort has been devoted
to controlling the size of AuNP on a variety of supports (TiO2,
CeO2,16 Fe2O3,17 Al2O3,18 MgO,19 SiO2,20 and activated carbon21).
However, there is a still need to develop methods for controlling
the size of AuNP on supports with less reactive oxygenated surface
functionalities.
Scheme 2
at 2958 and 2902 cm-1 are characteristic of the ν(CH) of the grafted
SiMe3 ligands. Upon a subsequent treatment under H2 at 300 °C,
the disk turned deep-red, indicating the formation of AuNP and
implying the cleavage of the Au-O bond. Since the IR spectrum
displayed only vibrations characteristic of the SiMe3 groups but
no O-H or N-H vibrations, this suggested that the t SiOH groups
resulting from reduction of the gold species were further converted
into t SiOSiMe3 by reaction with HxN(SiMe3)3-x
By impregnation of a silica powder with a pentane solution of
{Au[N(SiMe3)2]}4 (1.1 equiv), the solid Au(I)/SiO2 remained white,
.
First, we describe the direct synthesis and characterization of
AuNP supported on hydrophobic silica. Preliminary IR studies
using a silica pellet showed that a pentane solution of {Au[N-
(SiMe3)2]}422 reacted at 25 °C with the silanols of a silica partially
dehydroxylated at 700 °C [SiO2-(700)], as evidenced by the disap-
pearance of ν(OH) silanol vibrations (3747 cm-1) and the appear-
ance of ν(NH) bands (Figure 1a,b). This is consistent with the
protonation of the N atom bonded to Au by the surface silanol and
grafting of this complex via an Au-O bond. Moreover, the peaks
Figure 1. Preparation of Aup/SiO2 monitored by FT-IR spectroscopy. (a)
Silica partially dehydroxylated at 700 °C [SiO2-(700)]. (b) After grafting of
[Au(NSiMe3)2]4 to give Au(I)/SiO2. (c) After treatment under H2 at 300
°C to give Aup/SiO2.
† C2P2.
‡ Centre de RMN a` Tre`s Hauts Champs.
§ Institut de Recherches sur la Catalyse et l’Environnement de Lyon.
9
10.1021/ja903730q CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 14667–14669 14667

C O M M U N I C A T I O N S
but elemental analysis showed that it contained 3.0 ( 0.1 wt %
Au. This corresponds to 0.15 mmol of Au/g of silica and therefore
to the grafting of ∼50% of the available surface silanols [0.26 mmol
SiOH/g for SiO2-(700)]. The IR spectrum of this solid is similar to
that obtained previously on the silica disk (e.g., the absence of
remaining t SiOH group). The N-Au bond was cleaved upon
grafting, and the liberated HN(SiMe3)2 probably reacted with
adjacent unreacted t SiOH to give t SiOSiMe3 groups and
H2NSiMe3. This is consistent with the formation of a 1:1 mixture
of t SiOSiMe3 and a surface complex (here t SiOAuI), a phenom-
enon typically observed upon grafting of similar bis(trimethylsi-
lyl)amide metal complexes.23 Moreover, the two ν(NH) bands at
3368 and 3286 cm-1 are characteristic of NH2,24 and this is
consistent with the presence of H2NSiMe3, which is probably
coordinated to AuI (Scheme 1). These hypotheses were confirmed
by solid-state NMR spectroscopy through (1) the presence of SiMe3
signals (0 ppm) and consumption of OH [a very low intensity signal
Figure 2. a) TEM image of Aup/SiO2 (2.79 wt % Au). (b) Au particle
size distribution of Aup/SiO2.
from World Gold Council (Au/TiO2-WGC, 1.5 wt % Au, 3.5 ( 0.9
nm). This is attributed to the poor dispersion (wettability) of Au/
TiO2-WGC in the liquid phase, because the powder rapidly deposits
onto the reaction vessel after only a few minutes of reaction, limiting
the accessibility to the active sites. On the other hand, Aup/SiO2,
which has a surface passivated with hydrophobic SiMe3 function-
alities, remains perfectly dispersed within the apolar medium
throughout the duration of reaction. This leads to optimized mass
transfer and hence increased initial reaction rates [from 0.023 mol
(g of Au)-1 h-1 for Au/TiO2-WGC to 0.12 mol (g of Au)-1 h-1 for
Aup/SiO2] and, when the gold dispersion on the support (as
measured by TEM) is taken into account, increased turnover
frequencies per surface metal atom, from a maximum of 18 h-1
for Au/TiO2-WGC to 40 h-1 for Aup/SiO2.
1
(∼2%) at 1.8 ppm] in the H magic-angle-spinning (MAS) solid-
state NMR spectrum (Figure S1 in the Supporting Information) and
(2) the presence of two 13C signals at 0 and 6.4 ppm associated
with t SiOSiMe3 and H2NSiMe3, respectively (Figure S2).
Further evidence of the presence of grafted AuI species was obtained
by adsorption of PMe3. First, the 31P cross polarization MAS (CP-
MAS) NMR spectrum (Figure S3) displays two different resonances
at 8.4 and -16.6 ppm, consistent with the presence of two types of
AuI-phosphine complexes. On the basis of chemical shift values of
analogous AuI-PMe3 complexes, they have been assigned to a 1:2
mixture of [t SiOAuPMe3] and [t SiO-Au+(PMe3)2] (Scheme S1).25
Second, the signal previously observed at 6.4 ppm and attributed to
coordinated H2NSiMe3 in the 13C CP-MAS NMR spectrum of Au(I)/
SiO2 (Figure S2), is replaced by a signal at 14.2 ppm (Figure S4),
assigned to the PMe3 ligand of the two aforementioned AuI surface
species. The presence of these two species upon reaction of Au(I)/
SiO2with PMe3 shows that the well-defined t SiOAu(H2NSiMe3)
species can be in slightly different environments, as already proposed
for [(t SiO)Zr(CH2tBu)3].26
Figure 3. (a) trans-Stilbene conversion (squares) and (b) epoxide yield
(triangles) as functions of time in the presence of Aup/SiO2 (solid symbols)
and Au/TiO2-WGC (open symbols). Reaction conditions: trans-stilbene
(substrate, 0.90 ( 0.05 mmol), methylcyclohexane (solvent, 20 mL/155
mmol), catalyst (2.0 ( 0.1 µmol of Au), tert-butylhydroperoxide (0.05
mmol), air (1 atm), 80 °C, 900 rpm.
Next, heating Au(I)/SiO2 at 300 °C under H2 (500 Torr) for 20 h
caused the white solid to turn red, indicating the formation of
AuNP. Transmission electron microscopy (TEM) of this solid (Aup/
SiO2) evidenced the formation of homogeneously dispersed particles
(Figure 2a) with a size distribution centered at 1.8 ( 0.6 nm (Figure
2b). The X-ray photoelectron spectrum of Aup/SiO2 at the Au 4f
level (Figure S5) exhibits binding energies of 83.8 and 87.4 eV
for Au 4f7/2 and Au 4f5/2, respectively, consistent with Au0 having
no to little interaction with the support.27 The formation of small
AuNP probably is a consequence of the low and controlled density
of covalently bound gold complexes as well as the absence of OH
groups and halide impurities, which can induce sintering.28 Indeed,
the IR spectrum of Aup/SiO2 shows only traces of surface silanols
[<0.003 mmol/g, <0.01 OH/nm2, i.e., <1% of the initial OH density
of SiO2-(700)], indicating passivation of the silica surface. The
concomitant disappearance of the NH vibration is probably due to
the reaction of the freed t SiOH groups with Me3SiNH2 previously
coordinated to Au(I), which generates t SiOSiMe3 and NH3 (Figure
1c).
Moreover, Aup/SiO2 displays reasonable activity for low-
temperature PROX, a key process for the purification of H2 feed
used in the polymer electrolyte fuel cell (PE-FC) technology. When
this material is heated to 280 °C under a 2% CO + 2% O2 + 48%
H2 mixture balanced in He, the conversion of CO first increases
up to a maximum value and then decreases because of competition
with H2 oxidation for the limited amount of O2 present in the feed
(Figure 4). This activity profile remains similar over several cooling
and heating steps (Figure S6) and TEM analysis of the sample after
reaction (Figure S7) reveals similiar particle size as in the fresh
catalyst. At the temperature of interest for fuel cell applications
(80 °C), Aup/SiO2 is a factor of ∼2 more active in terms of rate
per surface metal atom than Au/SiO2-cd prepared by colloidal
deposition (0.9 wt % Au, 4.0 ( 1.7 nm).29 This is attributed to the
gold particle size effect (1.8 vs 4.0 nm for Aup/SiO2 and Au/SiO2-
cd, respectively).15 It should be noted, however, that Aup/SiO2 is
(∼4 times) less active than the reference catalyst from World Gold
Council (Au/TiO2-WGC, 1.5 wt % Au, 3.5 ( 0.9 nm), in agreement
with the proposed role of the titania support in PROX, namely,
promotion of O2 activation by oxygen vacancies and OH groups.
Thus, the significant activity of the fully passivated Aup/SiO2,
This material, Aup/SiO2, exhibits the best catalytic performances
observed to date in the liquid-phase epoxidation of trans-stilbene
under aerobic conditions (Scheme 2).14 After 50 h, nearly full
conversion was achieved with an epoxide selectivity of ∼80%
(Figure 3). Notably, an epoxide yield of ∼80% is greater than the
yield of 5% expected from the stoichiometric reaction of stilbene
with the peroxide initiator. By comparison, only 71% conversion
and 50% epoxide yield were reached over the reference catalyst
9
14668 J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009

C O M M U N I C A T I O N S
(3) Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566.
(4) Prati, L.; Rossi, M. J. Catal. 1998, 176, 552. Comotti, M.; Della Pina, C.;
Matarrese, R.; Rossi, M. Angew. Chem., Int. Ed. 2004, 43, 5812. Carrettin,
S.; McMorn, P.; Johnston, P.; Griffin, K.; Kiely, C. J.; Hutchings, G. J.
Phys. Chem. Chem. Phys. 2003, 5, 1329. Abad, A.; Concepcion, P.; Corma,
A.; Garcia, H. Angew. Chem., Int. Ed. 2005, 44, 4066.
(5) Corma, A.; Domine, M. E. Chem. Commun. 2005, 4042.
(6) Hughes, M. D.; Xu, Y. J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache,
D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.;
Johnston, P.; Griffin, K.; Kiely, C. J. Nature 2005, 437, 1132. Hutchings,
G. J. Top. Catal. 2008, 48, 55. Turner, M.; Golovko, V. B.; Vaughan,
O. P. H.; Abdulkin, P.; Berenguer-Murcia, A.; Tikhov, M. S.; Johnson,
B. F. G.; Lambert, R. M. Nature 2008, 454, 981. Chen, L.; Hu, J.; Richards,
R. J. Am. Chem. Soc. 2009, 131, 914. Della Pina, C.; Falletta, E.; Prati, L.;
Rossi, M. Chem. Soc. ReV. 2008, 37, 2077.
(7) Iizuka, Y.; Miyamae, T.; Miura, T.; Okumura, M.; Date, M.; Haruta, M. J.
Catal. 2009, 262, 280.
(8) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.;
Behm, R. J. J. Catal. 2001, 197, 113. Guzman, J.; Carrettin, S.; Corma, A.
J. Am. Chem. Soc. 2005, 127, 3286.
(9) Bond, G. C.; Thompson, D. T. Gold Bull. 2000, 33, 41. Costello, C. K.;
Kung, M. C.; Oh, H. S.; Wang, Y.; Kung, H. H. Appl. Catal., A 2002,
232, 159. Molina, L. M.; Hammer, B. Phys. ReV. Lett. 2003, 90, 4.
(10) Nijhuis, T. A. R.; Visser, T.; Weckhuysen, B. M. Angew. Chem., Int. Ed.
2005, 44, 1115.
(11) Caps, V.; Arrii, S.; Morfin, F.; Bergeret, G.; Rousset, J. L. Faraday Discuss.
2008, 138, 241. Quinet, E.; Morfin, F.; Diehl, F.; Avenier, P.; Caps, V.;
Rousset, J. L. Appl. Catal., B 2008, 80, 195.
(12) Sivadinarayana, C.; Choudhary, T. V.; Daemen, L. L.; Eckert, J.; Goodman,
D. W. J. Am. Chem. Soc. 2004, 126, 38.
Figure 4. Turnover frequencies (TOF, s-1) for CO oxidation over Aup/
SiO2 (diamonds), Au/SiO2-cd (squares), and Au/TiO2-WGC (triangles) as
functions of temperature under PROX conditions.
despite the absence of any such promoter, indicates that gold is
intrinsically active in the oxidation of CO under PROX conditions.
Additionally, the negligible activity observed in the absence of H2
suggests that H2 plays a critical role in the activation of O2 for
silica-supported systems, probably through the formation of OOH
species.
In conclusion, surface organometallic chemistry30 allows the
controlled formation of well-defined and dispersed [(t SiO)AuI]
surface species (0.5/nm2), which upon mild reduction (H2, 300 °C)
lead to the formation of small (1.8 ( 0.6 nm) Au particles supported
on silica passivated with SiMe3 functionalities (Aup/SiO2). This
hydrophobic Au catalyst displays improved performances in liquid-
phase aerobic epoxidation of trans-stilbene due to the smaller AuNP
and better mass transfer. The catalytic reactivity of this OH-free
material (Aup/SiO2) indicates that Au is intrinsically active in
oxidation but requires the presence of H2 or hydrocarbon, which is
consistent with the formation of HOO or ROO intermediates. Given
the accessibility of numerous bissilylamido organometallic com-
plexes,31 this synthetic strategy provides a general approach for
the preparation of tailor-made supported metallic nanoparticles with
controlled surface properties, and we are currently exploring its
scope.
(13) Lignier, P.; Morfin, F.; Mangematin, S.; Massin, L.; Rousset, J. L.; Caps,
V. Chem. Commun. 2007, 186. Lignier, P.; Mangematin, S.; Morfin, F.;
Rousset, J. L.; Caps, V. Catal. Today 2008, 138, 50.
(14) Lignier, P.; Morfin, F.; Piccolo, L.; Rousset, J.-L.; Caps, V. Catal. Today
2007, 122, 284.
(15) Haruta, M. Gold Bull. 2004, 37, 27.
(16) Tsubota, S.; Cunningham, D. A. H.; Bando, Y.; Haruta, M. Stud. Surf. Sci.
Catal. 1995, 91, 227.
(17) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115,
301.
(18) Ivanova, S.; Petit, C.; Pitchon, V. Gold Bull. 2006, 39, 3.
(19) Guzman, J.; Gates, B. C. Nano Lett. 2001, 1, 689.
(20) Okumura, M.; Tsubota, S.; Haruta, M. J. Mol. Catal. A: Chem. 2003, 199,
73. Zanella, R.; Sandoval, A.; Santiago, P.; Basiuk, V. A.; Saniger, J. M.
J. Phys. Chem. B 2006, 110, 8559. Zhu, H. G.; Ma, Z.; Clark, J. C.; Pan,
Z. W.; Overbury, S. H.; Dai, S. Appl. Catal., A 2007, 326, 89. Yin, H. F.;
Ma, Z.; Overbury, S. H.; Dai, S. J. Phys. Chem. C 2008, 112, 8349.
(21) Prati, L.; Martra, G. Gold Bull. 1999, 32, 96.
(22) Bunge, S. D.; Just, O.; Rees, W. S. Angew. Chem., Int. Ed. 2000, 39,
3082.
(23) Anwander, R.; Roesky, R. J. Chem. Soc., Dalton Trans. 1997, 137. Gauvin,
R. M.; Delevoye, L.; Hassan, R. A.; Keldenich, J.; Mortreux, A. Inorg.
Chem. 2007, 46, 1062.
(24) Avenier, P.; Lesage, A.; Taoufik, M.; Baudouin, A.; De Mallmann, A.;
Fiddy, S.; Vautier, M.; Veyre, L.; Basset, J.-M.; Emsley, L.; Quadrelli,
E. A. J. Am. Chem. Soc. 2007, 129, 176.
(25) Bauer, A.; Schneider, W.; Angermaier, K.; Schier, A.; Schmidbaur, H. Inorg.
Chim. Acta 1996, 251, 249. de Silva, E. N.; Bowmaker, G. A.; Healy,
P. C. J. Mol. Struct. 2000, 516, 263.
(26) Rataboul, F.; Baudouin, A.; Thieuleux, C.; Veyre, L.; Cope´ret, C.; Thivolle-
Cazat, J.; Basset, J. M.; Lesage, A.; Emsley, L. J. Am. Chem. Soc. 2004,
126, 12541.
Acknowledgment. D.G. and K.G. thank the “Cluster de
recherche Chimie de la Re´gion Rhoˆne-Alpes” and Project ANR-
08-JCJC-0090-01 ACTOGREEN from the French National Re-
search Agency, respectively, for graduate fellowships. We are also
grateful to M. Boualleg and L. Veyre for TEM images.
(27) Radnik, R.; Mohr, C.; Claus, P. Phys. Chem. Chem. Phys. 2003, 5, 172.
Arrii, S.; Morfin, F.; Renouprez, A. J.; Rousset, J. L. J. Am. Chem. Soc.
2004, 126, 1199.
(28) Veith, G. M.; Lupini, A. R.; Rashkeev, S.; Pennycook, S. J.; Mullins, D. R.;
Schwartz, V.; Bridges, C. A.; Dudney, N. J. J. Catal. 2009, 262, 92.
(29) Quinet, E. Ph.D. Dissertation, University of Lyon, 2008.
(30) Cope´ret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, J.-M. Angew.
Chem., Int. Ed. 2003, 42, 156.
(31) Alyea, E. C.; Bradley, D. C.; Copperthwaite, R. G. J. Chem. Soc., Dalton
Trans. 1972, 1580.
Supporting Information Available: Full experimental details,
Figures S1-S7, and Scheme S1. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16,
405.
(2) Edwards, J. K.; Solsona, B.; Ntainjua, N. E.; Carley, A. F.; Herzing, A. A.;
Kiely, C. J.; Hutchings, G. J. Science 2009, 323, 1037.
JA903730Q
9
J. AM. CHEM. SOC. VOL. 131, NO. 41, 2009 14669