Palladium nanoparticles on graphite oxide and its functionalized graphene derivatives as highly active catalysts for the Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1021/ja901105a

Pd²⁺-exchanged graphite oxide and chemically derived graphenes therefrom were employed as supports for Pd nanoparticles. The influence of catalyst preparation, carbon functionalization, and catalyst morphology on the catalytic activity in the Suzuki-Miyaura coupling reactions was investigated. The catalysts were characterized by means of spectroscopy (FT-IR, solid-state ¹³C NMR, AAS, XPS), X-ray scattering (WAXS), surface area analysis (BET, methylene blue adsorption), and electron microscopy (TEM, ESEM). In contrast to the conventional Pd/C catalyst, graphite oxide and graphene-based catalysts gave much higher activities with turnover frequencies exceeding 39 000 h^-1, accompanied by very low palladium leaching (<1 ppm).

Full text of DOI:10.1021/ja901105a

Published on Web 05/25/2009
Palladium Nanoparticles on Graphite Oxide and Its
Functionalized Graphene Derivatives as Highly Active
Catalysts for the Suzuki-Miyaura Coupling Reaction
Gil M. Scheuermann,† Luigi Rumi,‡ Peter Steurer,† Willi Bannwarth,‡ and
Rolf Mu¨lhaupt†,§,
*
Freiburger Materialforschungszentrum (FMF) and Institut fu¨r Makromolekulare Chemie,
Albert-Ludwigs-UniVersita¨t Freiburg, Stefan-Meier Strasse 21-31, D-79104 Freiburg, Germany,
Institut fu¨r Organische Chemie und Biochemie, Albert-Ludwigs-UniVersita¨t Freiburg,
Albertstrasse 21, D-79104 Freiburg, Germany, and Freiburg Institute for AdVanced Studies
(FRIAS), Soft Matter Research, Albertstrasse 19, D-79104 Freiburg, Germany
Received February 20, 2009; E-mail: rolf.muelhaupt@fmf.uni-freiburg.de
Abstract: Pd2+-exchanged graphite oxide and chemically derived graphenes therefrom were employed
as supports for Pd nanoparticles. The influence of catalyst preparation, carbon functionalization, and catalyst
morphology on the catalytic activity in the Suzuki-Miyaura coupling reactions was investigated. The catalysts
were characterized by means of spectroscopy (FT-IR, solid-state 13C NMR, AAS, XPS), X-ray scattering
(WAXS), surface area analysis (BET, methylene blue adsorption), and electron microscopy (TEM, ESEM).
In contrast to the conventional Pd/C catalyst, graphite oxide and graphene-based catalysts gave much
higher activities with turnover frequencies exceeding 39 000 h-1, accompanied by very low palladium
leaching (<1 ppm).
Introduction
GO is easily available by controlled chemical or electro-
chemical oxidation of graphite via its nitrate or hydrogensulfate
salts.7-10 It is a nonstoichiometric hygroscopic compound, and
different models have been proposed concerning its structure
and composition.11-14 According to more recent studies, the
layered structure is preserved as the partial oxidation produces
sheets with graphitic domains and crumpled regions where
epoxide and hydroxyl groups are located on the basal planes
During the last few years, graphite oxide (GO)1,2 and
chemically derived graphene (CDG)3 have been rediscovered
as extremely versatile carbon materials. This renaissance is
mainly due to its importance in nanosciences (e.g., as a low-
cost alternative to carbon nanotubes in polymer nanocomposites4
and as hydrogen storage materials5 or in nanoelectronics as
insulating material or semiconductor).6
(5) Matsuo, Y.; Kume, K.; Fukutsuda, T.; Sugie, Y. Carbon 2003, 41,
2167–2169.
† FMF and Institut fu¨r Makromolekulare Chemie, Albert-Ludwigs-
Universita¨t Freiburg.
(6) (a) Eda, G.; Fanchini, G.; Chhowalla, M. Nat. Nanotechnol. 2008, 3,
270–274. (b) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud,
C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.;
First, P. N.; de Heer, W. A. Science 2006, 312, 1191–1196. (c) Freitag,
M. Nat. Nanotechnol. 2008, 3, 455–457. (d) Wang, X.; Zhi, L.; Mu¨llen,
K. Nano Lett. 2008, 8, 323–327.
‡ Institut fu¨r Organische Chemie und Biochemie, Albert-Ludwigs-
Universita¨t Freiburg.
§ FRIAS.
(1) (a) Ajayan, P. M.; Yakobson, B. I. Nature 2006, 441, 818–819. (b)
Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett,
G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Nature 2007,
448, 457–460. (c) Prud’homme, R. K.; Aksay, I. A.; Adamson, D. H.;
Abdala, A. A. U.S. Patent 092432, 2007. (d) Ruoff, R. Nat.
Nanotechnol. 2008, 3, 10–11.
(7) (a) Brodie, B. C. Ann. Chim. Phys. 1860, 59, 466–472. (b) Stauden-
maier, L. Ber. Dtsch. Chem. Ges. 1898, 31, 1481–1487. (c) Titelman,
G. I.; Gelman, V.; Bron, S.; Khalfin, R. L.; Cohen, Y.; Bianco-Peled,
H. Carbon 2005, 43, 641–649.
(8) Hummers, W. S.; Offemann, R. E. J. Am. Chem. Soc. 1958, 80, 1339.
(9) Boehm, H.-P.; Scholz, W. Liebigs Ann. Chem. 1965, 691, 1–8.
(10) (a) Boehm, H.-P.; Eckel, M.; Scholz, W. Z. Anorg. Allg. Chem. 1967,
353, 236–242. (b) Nakajima, T.; Matsuo, Y. Carbon 1994, 32, 469–
475.
(2) McAllister, M. J.; Li, J.-L.; Adamson, D. H.; Schniepp, H. C.; Abdala,
A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme,
R. K.; Aksay, I. A. Chem. Mater. 2007, 19, 4396–4404.
(3) (a) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.;
Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S.
Nature 2006, 442, 282–286. (b) Geim, A. K.; Novoselov, K. S. Nat.
Mater. 2007, 6, 183–191. (c) Li, D.; Kaner, R. B. Science 2008, 320,
1170–1171.
(11) (a) Hofmann, U.; Frenzel, A.; Wilm, D.; Csalan, E. Kolloid-Z. 1932,
61, 297–304. (b) Hofmann, U.; Frenzel, A.; Csalan, E. Liebigs Ann.
Chem. 1934, 510, 1–41.
(12) Clauss, A.; Plass, R.; Boehm, H.-P.; Hofmann, U. Z. Anorg. Allg.
Chem. 1957, 291, 205–220.
(4) (a) Ramanathan, T.; Abdala, A. A.; Stankovich, S.; Dikin, D. A.;
Herrera-Alonso, M.; Piner, R. D.; Adamson, D. H.; Schniepp, H. C.;
Chen, X.; Ruoff, R. S.; Nguyen, S. T.; Aksay, I. A.; Prud’homme,
R. K.; Brinson, L. C. Nat. Nanotechnol. 2008, 3, 327–331. (b)
Ramanathan, T.; Stankovich, S.; Dikin, D. A.; Liu, H.; Shen, H.;
Nguyen, S. T.; Brinson, L. C. J. Polym. Sci., Part B: Polym. Phys.
2007, 45, 2097–2112. (c) Liu, P. G.; Gong, K. C.; Xiao, P.; Xiao, M.
J. Mater. Chem. 2000, 10, 933–935. (d) Uhl, F. M.; Wilkie, C. A.
Polym. Degrad. Stab. 2004, 84, 215–226.
(13) (a) Ruess, G. Kolloid-Z. 1945, 110, 17–26. (b) Ruess, G.; Vogt, F.
Monatsh. Chem. 1948, 78, 222–242.
(14) (a) Beckett, R. J.; Croft, R. C. J. Phys. Chem. 1952, 56, 929–941. (b)
Cassagneau, T.; Guerin, F.; Fendler, J. H. Langmuir 2000, 16, 7318–
7324. (c) Hontoria-Lucas, C.; Lo´pez-Peinado, A. J.; Lo´pez-Gonza´lez,
J. D.; Rojas-Cervantes, M. L.; Mart´ın-Aranda, R. M. Carbon 1995,
33, 1585–1592. (d) Nakajima, T.; Mabuchi, A.; Hagiwara, R. Carbon
1988, 26, 357–361.
9
8262 J. AM. CHEM. SOC. 2009, 131, 8262–8270
10.1021/ja901105a CCC: $40.75  2009 American Chemical Society

GO and CDG as Catalysts for Suzuki-Miyaura Reaction
A R T I C L E S
and carbonyl and carboxyl groups are placed at the edges.15,16
Thermal or chemical reduction of GO leads to an expanded
graphite or even exfoliated graphitic nanoplatelets and CDG
sheets, respectively.2,17-19
by slow intercalation of PdCl2 into graphite in a chlorine
atmosphere with subsequent reduction to Pd nanoparticles.27
Several Pd nanoparticle catalysts for C-C coupling reactions
such as the Mizoroki-Heck reaction28 and the Suzuki-Miyaura
reaction29 have been described in the literature.30-32 Im-
mobilization or stabilization procedures of Pd nanoparticles in
ionic liquids,33 polymers,34 organic-inorganic fluorinated hybrid
materials,35 and glass-polymer composite materials36 as well
as on common inorganic substrates such as carbon,37-40 silica,41
alumina,42 or zeolites43 are known to yield active catalysts for
C-C coupling reactions.
In the present study, we exploit GO and its CDG derivatives
as a support for palladium clusters and nanoparticles. The
Suzuki-Miyaura reaction has been selected as model reaction
for evaluating GO- and CDG-based palladium catalysts.
Because of the functional groups present in GO, the sorption
and intercalation of ions and molecules is possible.12,20 This
feature, together with the high specific surface area of GO and
CDG of 400 m2 g-1 up to 1500 m2 g-1, makes them promising
materials for catalytic applications.2,21 To date, there are only
a few examples of GO-based materials as catalysts.21–23 More
than 50 years ago, Boehm et al. investigated the chemistry of
GO in depth and used different methods for the determination
of the specific surface of CDG inter alia the catalytic production
of HBr.21 Later, in a USSR patent, Titelman et al. described
the deposition of NiO and CuO by thermal reduction of metal
amine precursors on a GO support as a catalyst for removing
oxygen impurities from industrial gases.22 At the same time,
Kyotani et al. synthesized carbon-metal composites by thermal
treatment of Na+, Ca2+, Cu2+, and Fe2+-exchanged graphite
oxide.24 Although they did not describe any catalytic applica-
tions, they characterized the obtained materials in detail and
observed a magnetic anisotropy for the Fe sample. Kovtyukhova
and co-workers investigated the intercalation of different transi-
tion-metal complexes in GO.25 They also synthesized nanom-
eter-sized clusters of Cu and Ni in a layered carbon matrix by
thermal breakdown of their intercalated GOs but did not test
the catalytic performance of these materials.26 Mastalir et al.
were the first to determine the high catalytic activity and
selectivity of their nano-Pd/GO systems in liquid-phase hydro-
genation of alkynes.23 Ion exchange of GO with a Pd complex
and subsequent reduction to nanosized crystallites by H2 resulted
in a catalyst that surpassed the activity of commercial supported
Pd catalysts such as Pd on activated carbon (Pd/C) and Pd
graphimet. The latter is a rather uncommon catalyst synthesized
(27) (a) Croft, R. C. Aust. J. Chem. 1956, 9, 184–193. (b) Ebert, L. B. J.
Mol. Catal. 1982, 15, 275–296. (c) Lalancette, J.-M. (Ventron Corp.).
´
U.S. Patent 3847963, 1974. (d) Mastalir, A.; Notheisz, F.; Barto´k,
M.; Haraszti, T.; Kira´ly, Z.; De´ka´ny, I. Appl. Catal., A 1996, 144,
237–248.
(28) (a) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009–
3066. (b) Heck, R. F. Acc. Chem. Res. 1979, 12, 146–151. (c) Heck,
R. F.; Nolley, J. P., Jr. J. Org. Chem. 1972, 37, 2320–2322. (d)
Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971, 44,
581–583. (e) Whitcombe, N. J.; Hii, K. K.; Gibson, S. E. Tetrahedron
2001, 57, 7449–7476.
(29) (a) Suzuki, A.; Miyaura, N. J. Chem. Soc., Chem. Commun. 1979,
866–867. (b) Suzuki, A. Pure Appl. Chem. 1994, 66, 213–222. (c)
Deng, Y.; Gong, L.; Mi, A.; Liu, H.; Jiang, Y. Synthesis 2003, 337–
339. (d) Miyaura, N.; Yamada, K.; Suzuki, A. Tetrahedron Lett. 1979,
3437–3440. (e) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457–
2483. (f) Suzuki, A. J. Organomet. Chem. 1999, 576, 147–168.
(30) Djakovich, L.; Ko¨hler, K.; de Vries, J. G. The Role of Palladium
Nanoparticles as Catalysts for Carbon-Carbon Coupling Reactions
in Nanoparticles and Catalysis, 1st ed.; Astruc, D., Ed.; Wiley-VCH:
Weinheim, Germany, 2008; pp 303-348.
(31) Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W. AdV. Synth. Catal.
2006, 348, 609–679.
(32) Yin, L. X.; Liebscher, J. Chem. ReV. 2007, 107, 133–173.
(33) (a) Xu, L.; Chen, W.; Xiao, J. Organometallics 2000, 19, 1123–1127.
(b) Deshmukh, R. R.; Rajagopal, R.; Srinivasan, K. V. Chem. Commun.
2001, 1544–1545. (c) Yang, X.; Fei, Z.; Zhao, D.; Ang, W. H.; Li,
Y.; Dyson, P. J. Inorg. Chem. 2008, 47, 3292–3297.
(15) Lerf, A.; He, H. Y.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998,
102, 4477–4482.
(16) (a) Boukhvalov, D. W.; Katsnelson, M. I. J. Am. Chem. Soc. 2008,
130, 10697–10701. (b) Kudin, K. N.; Ozbas, B.; Schniepp, H. C.;
Prud’homme, R. K.; Aksay, I. A.; Car, R. Nano Lett. 2008, 8, 36–41.
(17) Schniepp, H. C.; Li, J.-L.; McAllister, M. J.; Sai, H.; Herrera-Alonso,
M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.;
Aksay, I. A. J. Phys. Chem. B 2006, 110, 8535–8539.
(34) (a) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett.
2000, 2, 2385–2388. (b) Hu, J.; Liu, Y. B. Langmuir 2005, 21, 2121–
2123. (c) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003,
125, 8340–8347. (d) Narayanan, R.; El-Sayed, M. A. Langmuir 2005,
21, 2027–2033. (e) Gallon, B. J.; Kojima, R. W.; Kaner, R. B.;
Diaconescu, P. L. Angew. Chem., Int. Ed. 2007, 46, 7251–7254.
(35) Niembro, S.; Shafir, A.; Vallribera, A.; Alibe´s, R. Org. Lett. 2008,
10, 3215–3218.
(18) (a) Boehm, H.-P.; Clauss, A.; Hofmann, U.; Fischer, G. O. Z.
Naturforsch. 1962, 17, 150. (b) Bourlinos, A. B.; Gournis, D.; Petridis,
D.; Szabo´, T.; Szeri, A.; De´ka´ny, I. Langmuir 2003, 19, 6050–6055.
(c) Li, D.; Mueller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G.
Nat. Nanotechnol. 2008, 3, 101–105.
(36) Mennecke, K.; Cecilia, R.; Glasnov, T. N.; Gruhl, S.; Vogt, C.;
Feldhoff, A.; Vargas, M. A. L.; Kappe, C. O.; Kunz, U.; Kirschning,
A. AdV. Synth. Catal. 2008, 350, 717–730.
(19) (a) Stankovich, S.; Piner, R. D.; Chen, X. Q.; Wu, N. Q.; Nguyen,
S. T.; Ruoff, R. S. J. Mater. Chem. 2006, 16, 155–158. (b) Stankovich,
S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.;
Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–
1565.
(37) (a) Marck, G.; Villiger, A.; Buchecker, R. Tetrahedron Lett. 1994,
35, 3277–3280. (b) Tagata, T.; Nishida, M. J. Org. Chem. 2003, 68,
9412–9415. (c) Arvela, R. K.; Leadbeater, N. E. Org. Lett. 2005, 7,
2101–2104. (d) Sajiki, H.; Kurita, T.; Kozaki, A.; Zhang, G.; Kitamura,
Y.; Maegawa, T.; Hirota, K. Synthesis 2005, 537–542. (e) Maegawa,
T.; Kitamura, Y.; Sako, S.; Udzu, T.; Sakurai, A.; Tanaka, A.;
Kobayashi, Y.; Endo, K.; Bora, U.; Kurita, T.; Kozaki, A.; Monguchi,
Y.; Sajiki, H. Chem.-Eur. J. 2007, 13, 5937–5943.
(20) (a) Thiele, H. Kolloid-Z. 1937, 80, 1–20. (b) Slabaugh, W. H.; Seiler,
B. C. J. Phys. Chem. 1962, 66, 396–401. (c) De´ka´ny, I.; Kru¨ger-
Grasser, R.; Weiss, A. Colloid Polym. Sci. 1998, 276, 570–576.
(21) Boehm, H.-P.; Clauss, A.; Fischer, G. O.; Hofmann, U. Z. Anorg. Allg.
Chem. 1962, 316, 119–127.
(38) (a) Heidenreich, R. G.; Ko¨hler, K.; Krauter, J. G. E.; Pietsch, J. Synlett
2002, 1118–1122. (b) Huang, J.; Wang, D.; Hou, H.; You, T. AdV.
Funct. Mater. 2008, 18, 441–448.
(22) Titelman, G. I.; Karamanenko, S. V.; Novikov, Y. N.; Gorozhankin,
E. V.; Golosman, E. Z. (Novomoskovskij Gni I Pi Azotno SU, State
Scientific Research and Planning Institute of the Nitrogen Industry
and of Organic Synthesis USSR.). USSR Patent SU 1806005, 1993.
(39) Ko¨hler, K.; Heidenreich, R. G.; Krauter, J. G. E.; Pietsch, J. Chem.-
Eur. J. 2002, 8, 622–631.
´
´
(23) (a) Mastalir, A.; Kira´ly, Z.; Patzko´, A.; De´ka´ny, I.; L’Argentiere, P.
(40) Sakurai, H.; Tsukuda, T.; Hirao, T. J. Org. Chem. 2002, 67, 2721–
2722.
´
Carbon 2008, 46, 1631–1637. (b) Mastalir, A.; Kira´ly, Z.; Benko,
M.; De´ka´ny, I. Catal. Lett. 2008, 124, 34–38.
(24) Kyotani, T.; Suzuki, K.; Yamashita, H.; Tomita, A. Tanso 1993, 160,
255–265.
(41) (a) Bigi, F.; Coluccia, S.; Maggi, S.; Martra, G.; Mazzacani, A.; Sartori,
G. Res. Chem. Intermed. 2003, 39, 285–291. (b) Bedford, R. B.; Singh,
U. G.; Walton, R. I.; Williams, R. T.; Davis, S. A. Chem. Mater. 2005,
17, 701–707. (c) Jana, S.; Dutta, B.; Bera, R.; Koner, S. Inorg. Chem.
2008, 47, 5512–5520. (d) Erathodiyil, N.; Ooi, S.; Seayad, A. M.;
Han, Y.; Lee, S. S.; Ying, J. Y. Chem.-Eur. J. 2008, 14, 3118–3125.
(25) Kovtyukhova, N. I.; Karpenko, G. A.; Chuiko, A. A. Russ. J. Inorg.
Chem. 1992, 37, 566–569.
(26) Kovtyukhova, N. I. Theor. Exp. Chem. 1995, 31, 77–91.
9
J. AM. CHEM. SOC. VOL. 131, NO. 23, 2009 8263

A R T I C L E S
Scheuermann et al.
Experimental Section
MB technique was applied according to McAllister et al.2 with some
modifications. Four milligrams of the material was sonicated for
Catalyst Synthesis. GO was prepared by oxidation of graphite
flakes (KFL 99.5, min 20% > 100 µm, Kropfmu¨hl AG) according
to the method described by Hummers and Offemann.8 Graphite
(15 g) was stirred in concentrated sulfuric acid (350 mL) at room
temperature (rt), and sodium nitrate (7.5 g) was added. After 16 h,
the mixture was cooled to 0 °C and potassium permanganate (45
g) was added. After 2 h, the green slurry was allowed to come to
rt, and after being stirred for 3 h the whole batch was carefully
poured into a 2-L beaker filled with ice-cold water. Subsequently,
hydrogen peroxide (3%) was added in excess and the mixture was
stirred overnight and then filtered. Workup was accomplished by
several washings with a mixture of HCl/H2O2 (1:1, 5%) and
filtration followed by several washings with water and centrifugation
until the supernatant did not show anymore precipitation with
AgNO3 solution. Freeze-drying yielded a dark brown solid that was
carefully powdered in a ball mill (60-µm mesh) with cooling. The
obtained GO still had a sulfur content of 2% corresponding to 6%
sulfate. This value was sufficiently low, and after Pd exchange and
workup (see below) the sulfur content was reduced to 0. The ion
exchange with Pd (i.e., the adsorption of Pd2+ ions by the
functionalized GO) was achieved by dispersing GO (2.5 g) in water
(200 mL). Subsequently, palladium acetate (250 mg, 1.1 mmol)
was added and the mixture was vigorously sonicated for 5 min.
Despite the low solubility of palladium acetate in water, cation
exchange took place overnight. The suspension exhibited a slight
vinegar smell, and after several washings with water and acetone
the Pd2+-GO was dried in vacuo at 40 °C and gently powdered
through a 150-µm mesh regaining a water content of 4.5%. The
powder density was roughly estimated to be 0.3 g cm-3, and the
Pd content amounted to 3.4%. The generation of Pd0 nanoparticles
within the GO support (sample Pd0-CDG-H2) was done by bubbling
hydrogen through a suspension of Pd2+-GO (0.5 g) in ethanol (100
mL) for 45 min with a flow of 40 mL min-1. After evaporation of
the solvent, the obtained material was again powdered through a
60 min in 100 mL of an aqueous solution of MB (0.04 mg L-1
)
with sodium hydroxide (1.000 mL, 0.1 N). Following this, the
remaining MB concentration was determined via UV-vis spec-
troscopy (absorption measured at 246 nm) after centrifugation. The
reported value of 2.54 m2 of surface area per milligram of MB
adsorbed was used to calculate the surface area of the samples.
Nanoparticles were characterized by transmission electron micros-
copy (TEM) in suspension on a 200-µm mesh Cu grid and as a
microtome cut of the samples embedded in epoxy resin with a LEO
CEM 912 operating at 120 kV. Particle size was determined
counting at least 200 particles from different images by the aid of
iTEM computer software by OlympusSIS. The morphology of the
materials was observed via environmental scanning electron
microscopy (ESEM) in an Electroscan ESEM 2020. An acceleration
voltage of 23 kV was applied, and the samples were coated with
gold/palladium before analyses. The wide-angle X-ray scattering
(WAXS) measurements were carried out on a Siemens D 5000
diffractometer at 40 kV and 30 mA with Cu KR radiation in the 2θ
range from 1° to 100° (step size 0.0156°, step time 1 s, transmission
rotation and detection by a Braun PSD ASA S). Interplanar
distances were calculated using Bragg equation. X-ray photoelectron
spectra (XPS) were recorded on a Perkin-Elmer PHI ESCA System.
X-ray source was a Mg standard anode (1253.6 eV) at 12 kV and
300 W. Samples were dried at rt in a high vacuum overnight,
compressed with 75 MPa, and again dried in a high vacuum
overnight, and the surface was sputtered with argon for 20 s before
measurements.
General Procedure for the Kinetic Measurements. The vessels
of the ASW 2000 were charged with palladium catalyst (x µmol,
y mol %), sodium carbonate (106 mg, 1 mmol, 2 equiv), boronic
acid (0.55 mmol, 1.1 equiv), and aryl bromide (0.50 mmol, 1 equiv).
After automatic addition to each vessel of ethanol (2 mL) and water
(2 mL), the mixtures were shaken (600 min-1) at rt. After 10, 30,
60, 120, 180, 240, 300, 360, 420, and 1440 min, small probes (20
µL) were taken and given in HPLC vessels with acetonitril (950
µL + 30 µL acetic acid). From these probes, the conversion was
calculated via HPLC.
General Procedure for the Suzuki Reaction. A pressure tube
was charged with boronic acid (0.55 mmol, 1.1 equiv), sodium
carbonate (106 mg, 1 mmol, 2 equiv), aryl bromide (0.5 mmol, 1
equiv), and palladium catalyst 2 (4.0 mg, 1.25 µmol, 0.25 mol %).
Ethanol (2 mL) and water (2 mL) were added, and the flask was
sealed with a Teflon screw cap and stirred in a preheated oil bath
(80 °C) for the time indicated, followed by taking 10 µL of the
mixture for HPLC analysis. After cooling to rt, the mixture was
filtered over a small column of silica gel/kieselgur, and the column
was washed with DCM (10 mL). The product was extracted into
the organic phase (2 × 10 mL DCM), the united organic phases
were dried over MgSO4 and filtered, and the solvent was removed
under reduced pressure. In some cases, the pure product was
obtained after column chromatography (d ) 2.5 cm, h ) 10 cm,
CH/EE 4:1).
150-µm mesh, showing the same elemental composition as the Pd2+
-
GO precursor. Partial reduction of Pd2+-GO with hydrazine hydrate
(sample Pd0-CDG-N2H4) was performed as follows. A volume of
0.16 mL (165 mg, 3.3 mmol, 48.5 equiv of wrt Pd) of hydrazine
hydrate (99.9% N2H5OH) was added to a suspension of Pd2+-GO
(0.2 g) in water (50 mL). After being stirred overnight, the grayish
slurry was centrifuged and washed with water several times. The
residue was then dried in vacuo at 40 °C and gently powdered
through a 150-µm mesh, regaining a water content of 0.5%. The
powder density was roughly estimated to be 0.3 g cm-3, and the
Pd content amounted to 3.4%. Sample Pd0-CDG-EXP was obtained
by placing Pd2+-GO (0.5 g) in a Schlenck tube under an argon
atmosphere and heating the material with a Bunsen burner to
500-600 °C for 2 min. Expansion of the GO support took place
and yielded a sootlike material with a powder density of ap-
proximately 0.01 g cm-3 and a Pd content of 6.0%. All catalysts
were not dried and were stored at rt in air to allow for easy handling.
Characterization of the Catalysts. The palladium content of
the samples was determined by digestion of the samples in hot aqua
regia followed by analysis on an atom absorption spectrometer
(AAS) Vario 6 by Jena Analytics. Before elemental analysis on a
VarioEL by Elementaranalysensysteme GmbH, the catalysts were
dried in vacuo for 16 h and WO3 was added. Thermogravimetric
analyses (TGA) were performed on an STA 409 thermobalance
by Netzsch under a nitrogen flow (75 mL min-1) with a heating
rate of 1 °C min-1. The surface area was measured using the
Brunauer, Emmet, and Teller (BET) method57 on a Porotec
Sorptomatic 1990 and by methylene blue (MB) adsorption. The
Results and Discussion
Synthesis and Characterization of the CDG Catalysts. The
GO obtained by the Hummers-Offemann method8 had an
oxygen content of 41 wt %. This composition can be described
in good accordance with the empirical formula C6H2O3 proposed
by Boehm et al.9 This highly oxidized carbonaceous material
was dispersed in water, and after addition of palladium acetate
ion exchange took place overnight. After catalyst recovery, the
palladium content of the obtained Pd2+-GO 2 amounted to 3.4
wt % (0.32 mmol g-1) as verified by AAS. Palladium(II)
supported on GO was reduced to produce a variety of catalysts
comprising palladium(0) nanoparticles and CDG. The obtained
catalysts are listed in Table 1.
(42) (a) Augustine, R. L.; O’Leary, S. T. J. Mol. Catal. 1992, 72, 229–
242. (b) Biffis, A.; Zecca, M.; Basato, M. Eur. J. Inorg. Chem. 2001,
1131–1133.
(43) (a) Djakovitch, L.; Koehler, K. J. Am. Chem. Soc. 2001, 123, 5990–
5999. (b) Dams, M.; Drijkoningen, L.; Pauwels, B.; Van Tendeloo,
G.; De Vos, D. E.; Jacobs, P. A. J. Catal. 2002, 209, 225–236.
9
8264 J. AM. CHEM. SOC. VOL. 131, NO. 23, 2009

GO and CDG as Catalysts for Suzuki-Miyaura Reaction
A R T I C L E S
Table 1. Catalysts Investigated in the Suzuki-Miyaura Coupling Reaction Including Conventional Pd/C 1 as Benchmark
oxygen contentb
(wt %)
Pd loadingc
(wt %)
particle diameterd
(nm)
surface area MBe
(m2 g-1
surface area BETf
sample
compositiona
)
(m2 g-1
)
Pd/C 1g
n.d.
41
28
25
19
10
n.d.
n.d.
820
n.d.
n.d.
220
650
Pd2+-GO 2
C6H1.9O3.2
C6H0.9O1.7
C6H1.2O1.5
C6H0.5O1.1
3.4
3.4
3.5
6.0
4 ( 1h
7 ( 2
54 ( 28
3 ( 1
1050 (430)
350 (250)
410 (130)
890 (410)
Pd0-CDG-H2 3
Pd0-CDG-N2H4 4
Pd0-CDG-EXP 5
a Calculated from elemental analysis. b Calculated from elemental analysis wrt the support material (CH) only. c Measured by AAS after digestion of
the wet samples in aqua regia. d Average size and standard deviation as measured by TEM analysis. e Modified methylene blue adsorption technique.
Values in parentheses were obtained from measurements at pH e 7. f BET adsorption isotherm. g 10% Pd on activated charcoal (Sigma-Aldrich, product
No. 75990). h Particles generated in situ (i.e., TEM analysis was performed with the recollected catalyst).
Figure 1. Thermogravimetric analysis of the catalysts under nitrogen.
Figure 2. Interlayer distance (bottom) and oxygen content (top) (wrt to
Reduction of a Pd2+-GO 2 suspension in ethanol with H2
CH only) monitored by WAXS and elemental analysis, respectively, during
reduction of Pd2+-GO 2 with H2.
generated Pd0 nanoparticles with a diameter of 5-9 nm (sample
Pd0-CDG-H2 3). The GO support was also reduced to a CDG
functional groups were decomposed in the first degradation step.
It was found that the oxygen content (i.e., the degree of reduction
of Pd0-CDG-H2 3) could easily be adjusted by simply varying
the reaction time. During the reduction of Pd2+-GO 2 with H2,
aliquots were withdrawn from the suspension, spray-dried, and
subsequently analyzed by elemental analysis and WAXS. The
results are shown in Figure 2.
The layered structure of the GO support was destroyed during
reduction as the pendant functional groups were decomposed
and Pd nanoparticles with diameters larger than the GO
interlayer distance were formed. Obviously, the GO support was
reduced to a CDG material with lower oxygen content and lower
degree of functionalization. Morphology of the catalysts was
examined by ESEM and WAXS. Pd2+-GO 2 and the partially
reduced samples Pd0-CDG-H2 3 and Pd0-CDG-N2H4 4 consisted
of platelets, the latter showing a slightly exfoliated structure of
agglomerated GO and CDG layers, respectively (Figure 3).
However, in the WAXS diffractogram the characteristic [001]
reflex of a layered material was absent in all reduced samples
as nonuniform expansion in the c-axis of the support took place
(Figure 4). Reflexes in the WAXS pattern of sample Pd0-CDG-
N2H4 4 can be attributed to cubic palladium.
material containing 28 wt % oxygen. When reducing an aqueous
suspension of Pd2+-GO 2 with hydrazine hydrate, Pd0 clusters
of 26-82 nm in diameter were produced. This was accompanied
by partial reduction of the GO support to a CDG with an oxygen
content of 25 wt % (sample Pd0-CDG-N2H4 4). “Regraphitiza-
tion” was observed accompanied by precipitation of a shiny
layer of less hydrophilic CDG. In contrast to the reports of
Stankovich et al., no remaining nitrogen was detected in this
material.19 Thermal expansion of Pd2+-GO accompanied by
thermal reduction accounted for the formation of a sootlike
carbon material consisting of agglomerates of CDG with an
oxygen content of 19 wt % (sample Pd0-CDG-EXP 5). Thermal
degradation of the different catalysts by means of TGA exhibits
very different degradation profiles as expected for the degree
of functionalization of each sample (Figure 1). The reduction
of the nonreduced Pd2+-GO 2 took place in two distinct steps
with an onset temperature of 165 and 350 °C accounting for 22
and 60% mass loss, respectively (exclusive of water). With
decreasing oxygen content of the samples the first degradation
step diminished and onset temperatures were shifted to higher
values.
The thermal “regraphitization” of GO is accompanied by the
release of CO, CO2, and steam.2,15 Hence, mass loss in TGA
cannot be correlated with the carbon content of the samples.
Presumably, the decomposition of different types of oxygen-
containing functional groups is responsible for the different
degradation steps. This is in accordance with IR data of sample
Pd0-CDG-EXP 5 where only a weak band at 1725 cm-1 could
be assigned to CdO groups. Signals indicating epoxy and
carboxyl groups were absent, thus suggesting that these
In GO remaining crystalline features from graphite such as
strong hk0 reflexes can be observed. Notably, in the CDG
samples these reflexes disappeared completely, attesting to an
unordered structure of the CDG. A weak reflex at 21.5° (d )
4.13 Å) appears in the diffractogram of the thermally reduced
Pd0-CDG-EXP 5. Although expansion and delamination of the
layers during synthesis lead to a rather amorphous material, this
can be ascribed to the formation of “regraphitized” carbon
9
J. AM. CHEM. SOC. VOL. 131, NO. 23, 2009 8265

A R T I C L E S
Scheuermann et al.
Figure 3. ESEM images of the catalysts in 800-fold magnification (the
scale bar is 50 µm). The inset in the image of sample Pd0-CDG-EXP 5
shows a particle at 175-fold magnification (scale bar is 250 µm). The typical
accordion structure of this particle due to expansion of the layers can be
observed.
Figure 5. TEM images of the catalysts. Nanoparticles formed simulta-
neously during the reduction of the GO. Palladium nanoparticles in sample
Pd2+-GO 2 were generated in situ during the Suzuki-Miyaura coupling
reaction. The SAED in the inset shows a hexagonal pattern that can be
ascribed to “regraphitized” regions.
shows particles with wormlike structures but with stacks of
unchanged graphite crystallites having sharp X-ray diffraction
peaks. Moreover, the typical surface area of these materials is
rather low (<100 m2 g-1), whereas sample Pd0-CDG-EXP 5
inhibits a much higher specific surface area of 650 m2 g-1
according to BET measurements. The results of the modified
MB adsorption technique were approximately double the surface
area values obtained by the BET method. This is possibly due
to agglomeration of the dry powder and its exfoliation in alkaline
aqueous suspension, respectively. The Pd dispersion (i.e., the
average Pd crystallite size of the catalysts) could not be
determined by these adsorption experiments. However, disper-
sion was determined by measuring the particle diameter via
TEM. Excitingly, thermal reduction of the GO support was
accompanied by the formation of highly dispersed nanoparticles
with an extremely narrow size distribution of 3.3 ( 0.7 nm
(sample Pd0-CDG-EXP 5 in Figure 5).
Moreover, the palladium nanoparticles had a spherical shape
as confirmed by microtome cuts of the embedded samples. Pd0
nanoparticles generated in situ and by hydrogen were also quasi-
spherical and dispersed all over the carbon support. Those
generated by hydrogen exhibited a wider size distribution of 7
( 2 nm. However, nanocrystallites of cubic Pd generated by
the hydrazine reduction accumulated in several spots that were
randomly distributed in the CDG support. XPS measurements
confirmed the findings of the WAXS analysis of this sample;
nanocrystallites consisted of Pd0. The peaks occurring in the
XP spectra of nonreduced sample Pd2+-GO also matched
literature values for Pd2+ ions.45 Nevertheless, the spectrum of
the thermally reduced sample Pd0-CDG-EXP 5 showed a broad
signal. This revealed that the nanoparticles observed by TEM
consisted of both Pd0 and Pd oxides.
Figure 4. WAXS patterns of GO- and CDG-Pd composites. The broad
peak between 20 and 30° can be attributed to remaining reflexes of the
capillary tube material. The d-spacings of >1.5 nm could not be detected
because of the limited resolution (due to weak reflexes of the materials and
the insufficient low-angle resolution of the instrument).
regions. This is well-known for different types of soot where a
weak [002] interference stems from randomly arranged parallel
graphene layer stacks exhibiting larger d-spacings than pristine
graphite.44 Stacking is due to van der Waals attractive interac-
tions. As a result, selected area electron diffraction (SAED)
eventually shows sharp reflexes of ordered regions (see inset
of Figure 5). In addition, “regraphitization” can be enhanced
by compression of the material. WAXS patterns of tablets
formed by compression of the reduced samples with 75 MPa
show a broad peak between 17 and 30° owing its highest
maximum to the [002] reflex of hexagonal carbon with a
d-spacing of 3.37 Å. It has to be noted that thermally reduced
GO is different from commercially available expanded graphite
produced from graphite intercalation compounds. The latter also
Catalyst Screening in C-C Coupling Reactions. The Suzuki-
Miyaura reaction is one of the most widely used synthetic
protocols in modern chemistry and in industrial application.46
(44) Hofmann, U.; Wilm, D. Z. Elektrochem. 1936, 42, 504–522.
9
8266 J. AM. CHEM. SOC. VOL. 131, NO. 23, 2009

GO and CDG as Catalysts for Suzuki-Miyaura Reaction
A R T I C L E S
Figure 6. Catalyst screening in the Suzuki-Miyaura reaction. Comparison of Pd-GO/CDG catalysts 2-5 and Pd/C 1. (a) Coupling of 4-methoxyphenylboronic
acid and 4-bromo-1,2-difluorobenzene (1 mol %). (b) Coupling of 4-methoxyphenylboronic acid and 4-bromo-1,2-(methylene dioxy)benzene (0.25 mol %).
(c) Coupling of 4-methoxyphenylboronic acid and 3-bromobenzaldehyde with different Pd amounts (catalyst Pd2+-GO 2).
The organo boronic acids are largely unaffected by the presence
of water, tolerate a broad range of functional groups, and yield
nontoxic byproduct. In recent years, the focus of the research
has been directed on the synthesis of heterogeneous catalytic
systems.37,40,47,48 These systems show the same or even higher
activities than homogeneous catalysts, do not require working
under an inert atmosphere, and are very easy to recover, though
the activities often drop in recycling experiments.40,48
The supported Pd-GO/CDG catalysts described above were
applied to the Suzuki-Miyaura coupling reaction. Commercially
available Pd on charcoal (Pd/C) 1 was used as reference. All
catalysts were easily suspended in the solvent with stirring or
shaking but allowed for facile separation from the reaction
mixture when at rest. To establish an approximate activity
ranking among the catalysts, a Suzuki-Miyaura coupling
reaction with reactive substrates and 1 mol % Pd was monitored
at rt (Figure 6a).
8% of product after 24 h. Subsequently, the maximum enhance-
ment in activity was investigated by lowering the Pd loadings.
Six different amounts of Pd were applied to the coupling reaction
of 3-bromobenzaldehyde with 4-methoxyphenylboronic acid and
Pd2+-GO 2 at rt (Figure 6c). With the highest loading of 0.5
mol % the reaction was completed after 1 h with a yield of
93% of product. Decreasing the Pd concentration to 0.03 mol
% still gave a conversion of 80% after 4 h. By further decreasing
the amount of Pd to 0.016 mol %, the conversion dropped
substantially to 65%. However, the activity of the catalyst is
quite remarkable at concentrations that low. When the same
Pd concentration was applied at 80 °C, the reaction was
surprisingly fast, converting 93% of the halo aryl after 10 min.
When 0.004% Pd was applied at 80 °C, the system remained
highly active, converting 27% of the halo aryl after 10 min.
This results in a turnover number (TON) of 6700 and a turnover
frequency (TOF) of 39 000 h-1. To the best of our knowledge,
this is the highest TOF observed in the Suzuki-Miyaura reaction
with a heterogeneous catalyst. Heterogeneity of the catalytic
systems was confirmed by determining the Pd leaching (mecha-
nistic aspects are discussed below). After complete reaction,
samples from reaction mixtures with 1 mol % Pd were analyzed
by ICP-AAS, showing very low leaching of 1.1 ppm in the case
of the highly active catalyst Pd2+-GO 2 and values of <1 ppm
for the Pd0-CDG catalysts 3-5.
The highest activity was reached with Pd2+-GO 2 and the
H2-reduced derivative Pd0-CDG-H2 3 followed by Pd0-CDG-
EXP 5 and Pd/C 1. After 30 min, the reaction shows complete
conversion with catalyst 2, in comparison to 93% with 3, 38%
with 5, and only 17% with Pd/C 1. All catalysts lead to complete
conversion except for the hydrazine-reduced sample Pd0-CDG-
N2H4 4. This catalyst displayed the lowest activity and 77%
conversion after 24 h. On the basis of these findings, a
Suzuki-Miyaura coupling with a less electrophilic and thus less
reactive aryl bromide and with lower Pd concentration of 0.25
mol % was performed (Figure 6b). Here, the difference in
activity between the catalysts is much more conspicuous. While
catalysts 2, 3, and 5 yielded the coupling product with 83, 90,
and 72% conversion, only 21% was achieved with Pd/C 1.
Again, Pd0-CDG-N2H4 4 showed the lowest activity, affording
Furthermore, the scope of the Suzuki-Miyaura reaction with
Pd2+-GO 2 was thoroughly investigated (Table 2). The reactions
proceeded extraordinarily well with a wide range of aryl
bromides containing electron-donating groups (7-11), electron-
withdrawing groups (12-18), or aromatic substituents such as
that in 2-bromonaphthalene (19-21). However, coupling reac-
tion of 2-bromopyridine with both boronic acid leads only to
modest conversion (20a). In most cases, the formation of
homocoupling products was not observed and never exceeded
4%.
(45) (a) Briggs, D.; Seah, M. P. Practical Surface Analysis: By Auger and
X-ray Photoelectron Spectroscopy, 2nd ed.; Wiley: Chichester, U.K.,
1990; Vol. 1. (b) Kumar, G.; Blackburn, J. R.; Albridge, R. G.;
Moddeman, W. E.; Jones, M. M. Inorg. Chem. 1972, 11, 296–300.
(46) Tsuji, J. Palladium Reagents and Catalysts: New PerspectiVes for the
21st Century, 1st ed.; Wiley: Chichester, U.K., 2004.
In addition, the recyclability of the catalysts was studied in
the coupling reaction forming 3-(4′-methoxyphenyl)benzalde-
hyde 12a at 80 °C and with 0.25 mol % Pd. With Pd2+-GO 2,
completion of the reaction was accomplished after 20 min in
the first run. The activity slightly dropped in the next two runs,
dramatically dropping in the fourth, showing just 19% conver-
sion (Table 3).
(47) (a) LeBlond, C. R.; Andrews, A. T.; Sun, Y.; Sowa, J. R. Org. Lett.
2001, 3, 1555–1557. (b) Kitamura, Y.; Sako, S.; Udzu, T.; Tsutsui,
A.; Maegawa, T.; Monguchi, Y.; Sajiki, H. Chem. Commun. 2007,
5069–5071.
(48) (a) Arcadi, A.; Cerichelli, G.; Chiarini, M.; Correa, M.; Zorzan, D.
Eur. J. Org. Chem. 2003, 2003, 4080–4086. (b) Lyse´n, M.; Ko¨hler,
K. Synthesis 2006, 692–698.
9
J. AM. CHEM. SOC. VOL. 131, NO. 23, 2009 8267

A R T I C L E S
Scheuermann et al.
Table 2. Suzuki-Miyaura Reactions of Different Aryl Bromides with 0.25 mol % Pd2+-GO 2
The Pd0 nanoparticles that formed in situ on the Pd2+-GO
catalyst 2 (cf. Figure 5) agglomerated and accumulated in several
spots on the support in the course of recycling. However,
formation of Pd black did not take place. Reuse of the Pd0-
CDG catalysts 3-5 was accompanied by a substantial loss in
activity in the second run already (10-27% conversion). This
behavior cannot be explained by leaching during the reaction
as the Pd amount in the reaction mixture after the first run
exhibited very low values of <1 ppm. The activity in recycling
experiments strongly depended on the recycling procedure.
9
8268 J. AM. CHEM. SOC. VOL. 131, NO. 23, 2009

GO and CDG as Catalysts for Suzuki-Miyaura Reaction
Table 3. Recycling Experiment with Pd2+-GO 2a
A R T I C L E S
Scheme 1. Three-Phase Testa
run
conversion (%)b
1
2
3
4
100
83
74
19
a Formation of 3-(4′-methoxyphenyl)benzaldehyde was catalyzed with
a total TON of 1100. b This is in line with the results found in TEM
analyses (Figure 7).
a Suzuki reaction with a solid-phase-bound aryl iodide. The conversion
was determined by HPLC after cleavage from the solid support.
oxygen-promoted ligand-free Suzuki-Miyaura coupling.49 The
findings described above are in line with the poor performance
of commercially available Pd/C 1. Of course, the impregnation
method (i.e., the fabrication procedure) has a strong impact on
the activity of Pd/C.39 However, the high Pd dispersion in Pd/C
is in sharp contrast to the low activities, which are probably
due to the inert behavior of the support. It is worth noting that
the Pd-GO/CDG catalysts did not show a decrease in activity
after six months, despite their high degree of functionalization.
Moreover, they were stored in air at rt to allow for easy handling.
To shed more light on the mechanism of the Suzuki-Miyaura
coupling with the heterogeneous Pd-GO/CDG catalysts,31 three-
phase tests50,51 and filtration experiments52 were carried out.
In the three-phase test, a solid-phase-bound aryl iodide is
subjected to the reaction conditions (Scheme 1).
If the catalytic species is truly immobilized, the spatial
separation from the iodide will prevent any conversion, while
a soluble catalyst can reach the substrate by diffusion and thus
effect conversion. The experiment was carried out with Pd/C
1, Pd2+-GO 2, and Pd0-CDG-EXP 5 in the presence and in the
absence of 4-bromophenol as a soluble substrate. In both
experiments for all three catalysts, more than 90% conversion
of the solid-phase-bound iodide was observed. These results
suggest that the catalysts are operating by a soluble catalytic
species and do not depend on a soluble aryl halide for generation
of the active species from the precatalyst. Further experimental
support was provided by the observation that the reaction
continues after hot filtration of the reaction mixture and leads
to nearly the same conversion (Figure 8).
Upon completion of the reaction and cooling to rt, the Pd
species were probably redeposited on the support, which would
be in accordance with the low Pd leaching. Nevertheless, it is
also conceivable that the support acts as a reservoir and only a
small fraction of active catalytic species is released into the
solution. These results are in accordance with the widespread
hypothesis that Pd nanoparticles function as a reservoir. Only
a small amount of metal goes into solution as Pd0 atoms and
catalyzes the reaction, returning to the solid support after
completion of the transformation (dissolution-redeposition
mechanism).30,39,51,53 This is not contradictory to the role of
the support discussed above since the reaction can still be
Figure 7. Pd particles formed on catalyst Pd2+-GO 2 after the first run
(left) and fourth run (right). Agglomeration of the particles can be observed.
Results described above were obtained when centrifugation was
applied to separate the catalysts from the reaction mixture and
after washings with water and diethylether. However, simple
filtration followed by washings in the same manner led to a
massive decrease in activity in the second run (4-6%). This is
in contrast to the new addition of substrates to the reaction
mixture after the completed first run, which gave again 100%
conversion.
Many correlated aspects have to be taken into account when
commenting on the reasons for the different activities among
the catalysts. The shape and size of the nanoparticles and their
size distribution (i.e., the Pd dispersion) are the most easily
understandable aspect. Regardless of the exact mechanism of
the reaction, a high surface area is expected to increase the
activity of a heterogeneous catalyst. Good accessibility to the
catalytic particles by the reagents is an additional premise, and
thus the surface area of the support also plays an important role.
From this it follows that the catalysts having the smallest Pd
nanoparticles and highest specific surface areas, namely Pd2+
-
GO 2 and Pd0-CDG-EXP 5, should display the highest activities.
This is not true for the latter, which can be ascribed to the nature
of the particles. They consist of a mixture of Pd oxides and
Pd0, and thus not all the Pd is available for the coupling reaction,
which is catalyzed by Pd0 species. Moreover, Pd oxides behave
rather inertly at the applied reaction conditions and leaching of
Pd0 is negligible. In contrast, Pd0 formation on the catalyst Pd2+-
GO 2 via in situ reduction by the solvent under basic conditions
leads to the highest Pd dispersion. The dispersion is supposed
to be even higher during the reaction than suggested by the
final particle size measured after complete reaction. It is obvious
from the foregoing that Pd0-CDG-N2H4 4 accounts for the lowest
activity. Furthermore, the role of the support has to be
considered. In general, one of the main influences of the support
is attributed to its acidity or basicity.32 The extent of this effect
in turn varies with the characteristics of the reagents. In
conclusion, the different kinetic profiles of catalyst Pd2+-GO 2
and Pd0-CDG-H2 3 are possibly due to the dissimilar degree of
derivatization (41 vs 28% oxygen content). This is corroborated
by recent studies by Han et al., who proposed a model for the
(49) Han, W.; Liu, C.; Jin, Z. L. Org. Lett. 2007, 9, 4005–4007.
(50) (a) Collman, J. P.; Kosydar, K. M.; Bressan, M.; Lamanna, W.; Garrett,
T. J. Am. Chem. Soc. 1984, 106, 2569–2579. (b) Smith, M. D.; Stepan,
A. F.; Ramarao, C.; Brennan, P. E.; Ley, S. V. Chem. Commun. 2003,
2652–2653. (c) Tzschucke, C. C.; Bannwarth, W. HelV. Chim. Acta
2004, 87, 2882–2889.
(51) Davies, I. W.; Matty, L.; Hughes, D. L.; Reider, P. J. J. Am. Chem.
Soc. 2001, 123, 10139–10140.
(52) Sheldon, R. A.; Wallau, M.; Arends, I.; Schuchardt, U. Acc. Chem.
Res. 1998, 31, 485–493.
9
J. AM. CHEM. SOC. VOL. 131, NO. 23, 2009 8269

A R T I C L E S
Scheuermann et al.
Figure 8. Filtration experiment. Formation of 4-methoxy-3′,4′-methylenedioxybiphenyl. (a) Without filtration. (b) Filtration of the catalyst from the reaction
mixture after 60 min.
influenced positively by homogeneous active species interacting
with the surface of GO/CDG. Filtration experiments give further
evidence. After filtration of the catalysts from the reaction
mixture, a distinct decrease in activity can be observed.
Therefore, it can be speculated that the reaction proceeds via a
support-aided homogeneous mechanism.
profound understanding of the structure-activity relationship
of Pd-GO/CDG catalysts further work has to be done. Currently,
the rediscovery of GO and its derivatives has led to a much
better understanding of its nature and processabilty. Recent
progress such as the selective ion passage through functionalized
graphene pores54 or the Langmuir-Blodgett assembly of GO55
offers attractive new opportunities for designing highly active
catalysts with hierarchic architectures and tailored nanocata-
lysts.56
Conclusions
In summary, immobilization of Pd2+ on graphite oxide (GO)
via cation exchange and subsequent chemical reduction afforded
Pd0 nanoparticles and chemically derived graphenes (CDG). The
(GO)-Pd and the (CDG)-Pd catalysts were successfully applied
to the Suzuki-Miyaura coupling reaction. These novel hetero-
geneous catalysts were readily available and easy to handle as
they are stable in air. Extraordinary high activities with turnover
frequencies (TOF) of up to 39 000 h-1 and very low leaching
make them an attractive alternative to commercially available
Pd catalysts such as Pd on charcoal. Reuse of the catalysts can
be achieved, albeit with loss in activity depending on the
recycling procedure. However, recovery of the noble metal is
possible because of very low leaching.
Acknowledgment. We gratefully acknowledge financial support
by the DFG Priority Research Program SFB428. TEM images were
measured by Dr. Ralf Thomann.
Note Added after ASAP Publication. After ASAP publication
on May 25, 2009, Table 2 was reprocessed to better display the
data. The corrected version was published May 27, 2009.
Supporting Information Available: Characterization of GO
and the Pd-GO/CDG catalysts and experimental procedures and
spectroscopic data for all Suzuki coupling products. This
material is available free of charge via the Internet at http://
pubs.acs.org.
The present work shows the outstanding versatility of GO as
a starting material for transition-metal nanoparticle supports.
By simple variation of the reduction methods and procedure
parameters size and shape of the nanoparticles and the func-
tionalization of the support can be varied. Nevertheless, for a
JA901105A
(54) Sint, K.; Wang, B.; Kra´l, P. J. Am. Chem. Soc. 2008, 130, 16448–
16449.
(55) Cote, L. J.; Kim, F.; Huang, J. J. Am. Chem. Soc. 2009, 131, 1043–
(53) (a) De La Rosa, M. A.; Velarde, E.; Guzma´n, A. Synth. Commun.
1990, 20, 2059–2064. (b) Reetz, M. T.; de Vries, J. G. Chem. Commun.
2004, 1559–1563. (c) Gaikwad, A. V.; Holuigue, A.; Thathagar, M. B.;
ten Elshof, J. E.; Rothenberg, G. Chem.-Eur. J. 2007, 13, 6908–
6913.
1049.
(56) Schlo¨gl, R.; Abd Hamid, S. B. Angew. Chem., Int. Ed. 2004, 43, 1628–
1637.
(57) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60,
309–319.
9
8270 J. AM. CHEM. SOC. VOL. 131, NO. 23, 2009