J. Am. Chem. Soc. 1999, 121, 7933-7934
7933
of the PtO2/1b colloid afforded a solid with a platinum content
of 15.2%, which is completely redispersable in water. Alternative
approaches for the workup of the crude colloidal solution are a
dialysis process or ion exchange.8 Platinum contents of more than
30% can be attained by a combination of these workup procedures
if so desired.
Water-Soluble Colloidal Adams Catalyst:
Preparation and Use in Catalysis
Manfred T. Reetz* and Michael G. Koch
Max-Planck-Institut fu¨r Kohlenforschung,
1 Kaiser-Wilhelm-Platz, D-45470 Mu¨lheim/Ruhr, Germany
To leave no doubt that the colloids are indeed composed of
PtO2 particles, additional methods of characterization were
applied. For example, X-ray photoelectron spectroscopy (XPS)9
of the prepared colloids and of commercial Adams catalyst clearly
showed the presence of Pt(IV) species, as demonstrated by the
similarities in the Pt 4f regions. The binding energy, EB, amounts
to 74.5 eV both in colloidal PtO2 and in Adams catalyst. Analysis
of colloidal PtO2 and commercial Adams catalyst by extended
X-ray absorption fine structure (EXAFS) spectroscopy10 also gives
evidence for a very similar composition and structure of the
particles. For example, measurements of the PtO2 colloid at the
Pt LIII edge revealed an oxygen backscatterer at 2.0, 3.6, and 3.8
Å and a platinum backscatterer at 3.1 Å, with a coordination
number of 6 for platinum. These values are in full agreement
with those obtained for commercial Adams catalyst in the present
study and those reported in the literature for pure R-PtO2.11
To test whether the preformed water-soluble PtO2 colloids can
be immobilized on solid supports quantitatively while maintaining
their original size, aqueous solutions of a sulfobetaine-stabilized
PtO2 colloid (1.9 nm; 14.4% Pt) were stirred with various forms
of Al2O3, namely Alox N (neutral),12a Alox S (acidic),12a and
Puralox (neutral),12b the ratio of colloid to solid support being
chosen so as to obtain materials having about 5% platinum (by
ReceiVed March 1, 1999
In 1922 Adams reported that a certain form of bulk PtO2,
prepared by the reaction of H2PtCl6 with NaNO3 at 450 °C, is an
excellent hydrogenation catalyst.1 Indeed, Adams catalyst has
continued to play a significant role in heterogeneous catalysis.2
Platinum dispersed on solid supports such as Al2O3 or charcoal
also constitute commercially available catalytically active
systems.2c,2d,3 Here we report on the preparation and application
of water-soluble nanosized colloidal PtO2 stabilized by carbo- or
sulfobetaines 1 or 2, respectively.4
-
-
C12H25(CH3)2N+(CH2)nCO2 C12H25(CH3)2N+(CH2)3SO3
1a n ) 3
2
1b n ) 1
In comparison to the vast literature on zerovalent transition
metal colloids stabilized by polymers, surfactants, or special
ligands,5 much less is known concerning the corresponding
nanosized transition metal oxides.6 A particular challenge is the
preparation of concentrated solutions of water-soluble nanopar-
ticles of metal oxides under conditions which resist agglomeration
to insoluble bulk material. Such aqueous colloidal solutions would
allow the practical immobilization of preformed nanoparticles in
membranes or on solid supports. We have previously demon-
strated that carbo- and sulfobetaines of the types 1 and 2,
respectively, are excellent water-soluble stabilizers for a number
of transition metal colloids, the latter being prepared by electro-
chemical reduction of the corresponding metal salts in organic
or aqueous medium.7 We therefore speculated that the corre-
sponding metal oxide colloids could be accessible by simple
hydrolysis/condensation of metal salts under basic aqueous
conditions in the presence of the above surfactants.
(1) (a) Voorhees, V.; Adams, R. J. Am. Chem. Soc. 1922, 44, 1397-1405.
(b) Adams, R.; Shriner, R. L. J. Am. Chem. Soc. 1923, 45, 2171-2179.
(2) (a) Adams, R.; Voorhees, V.; Shriner, R. L. In Org. Synth. Coll., Vol.
I., 2nd ed.; Blatt, A. H., Ed.; Wiley: New York, 1941; pp 463-470. (b)
Frampton, V. L.; Edwards, J. D., Jr.; Henze, H. R. J. Am. Chem. Soc. 1951,
73, 4432-4434. (c) Paquette, L. A., Ed. Encyclopedia of Reagents for Organic
Synthesis, Vol. 6; Wiley: Chichester, 1995. (d) Augustine, R. L. Catalytic
Hydrogenation; Dekker: New York, 1965. Previous work on platinum oxide
catalysts: (e) Willsta¨tter, R.; Waldschmidt-Leitz, E. Chem. Ber. 1921, 54, 4,
B113-B138, and literature cited therein.
(3) Ertl, G.; Kno¨zinger, H.; Weitkamp, J., Eds. Handbook of Heterogeneous
Catalysis, Vol. 1-5; VCH: Weinheim, Germany, 1997.
Indeed, upon stirring an aqueous solution of PtCl4 and
carbobetaine 1a (molar ratio, 1:4) in the presence of excess NaOH
at 50 °C for 7 d, complete consumption of the yellow platinum
salt was observed with formation of a deep red-brown colloidal
solution of PtO2/1a (eq 1). The reaction was monitored by UV-
(4) Reetz, M. T.; Koch, M. G. Patent applied for DE-A 19852547.8, 1998.
(5) (a) Schmid, G., Ed. Clusters and Colloids: From Theory to Applica-
tions; VCH: Weinheim, Germany, 1994. (b) Fendler, J. H., Ed. Nanoparticles
and Nanostructured Films; Wiley-VCH: Weinheim, Germany, 1998.
(6) Examples of metal oxide colloids: (a) Lume-Pereira, C.; Baral, S.;
Henglein, A.; Janata, E. J. Phys. Chem. 1985, 89, 5772-5778. (b) Harriman,
A.; Thomas, J. M.; Millward, G. R. New J. Chem. 1987, 11, 757-762. (c)
Kalyanasundaram, K.; Gra¨tzel, M. Angew. Chem. 1979, 91, 759-760; Angew.
Chem., Int. Ed. Engl. 1979, 18, 701. (d) Christensen, P. A.; Harriman, A.;
Porter, G.; Neta, P. J. Chem. Soc., Faraday Trans. 2 1984, 80, 1451-1464.
(e) Claerbout, A.; Nagy, J. B. Preparation of Catalysts V. In Stud. Surf. Sci.
Catal., Vol. 63; Poncelet, G., Ed.; Elsevier: Amsterdam, 1991; pp 705-716.
(f) Pileni, M. P. Langmuir 1997, 13, 3266-3276. (g) Reetz, M. T.; Quaiser,
S. A.; Winter, M.; Becker, J. A.; Scha¨fer, R.; Stimming, U.; Marmann, A.;
Vogel, R.; Konno, T. Angew. Chem. 1996, 108, 2228-2230; Angew. Chem.,
Int. Ed. Engl. 1996, 35, 2092. Examples of metal sulfide colloids: (h) Weller,
H.; Eychmu¨ller, A. Semiconductor Nanoclusters: Physical, Chemical, and
Catalytic Aspects. In Stud. Surf. Sci. Catal., Vol. 103; Kamat, P. V.; Meisel,
D., Eds.; Elsevier: Amsterdam, 1996; pp 5-22.
PtCl4 H O/base8 PtO2 colloid
(1)
2
stabilizer
vis spectroscopy (disappearance of the PtCl4 absorption at 250
nm and appearance of a plasmon absorption in the range 200-
800 nm) and by transmission electron microscopy (TEM). TEM
analysis of the final colloid showed the presence of 1.8 ( 0.3-
nm-sized particles (Figure 1). The existence of nanoparticles in
solution was demonstrated by small-angle X-ray scattering
(SAXS). The use of stabilizer 1b also gave 1.8-nm-sized PtO2
particles. The condensation process can also be performed at
reflux temperature for 2.5 h with Li2CO3 as a base, resulting in
1.9 ( 0.4 nm PtO2/1b colloids. High-resolution TEM analysis of
the samples revealed the existence of lattice planes, demonstrating
the nanocrystalline character of the particles, as in the case of
PtO2/1b (Figure 2).
The reaction could also be performed using sulfobetaine 2 and
Li2CO3 as the base, leading to 1.7 ( 0.3-nm-sized PtO2/2 colloids.
Other Pt(IV) salts such as H2PtCl6 can also be employed for the
preparation of PtO2 colloids.4 The addition of excess acetonitrile
to the colloidal solution resulted in the essentially complete pre-
cipitation of the PtO2 colloid as a brown solid. The precipitation
(7) (a) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401-
7402. (b) Reetz, M. T.; Helbig, W.; Quaiser, S. A. In ActiVe Metals:
Preparation, Characterization, Applications; Fu¨rstner, A., Ed.; VCH: Wein-
heim, Germany, 1996; pp 279-297. (c) Reetz, M. T.; Helbig, W.; Quaiser,
S. A., EP 0672 765 A1, March 4, 1995.
(8) Dialysis was performed on 0.01 M solutions of the colloids using a
dialysis hose. Whereas the PtO2/2 colloid isolated by precipitation contains
small amounts of Li (0.6%) and Cl (4.5%) in addition to Pt (13%), samples
obtained by dialysis are essentially Li- and Cl-free: Li (0.02%), Cl (<0.1%).
(9) We thank W. Gru¨nert and M. Heber (Ruhr-Universita¨t Bochum) for
the XPS measurements.
(10) We thank U. Kolb for the EXAFS studies.
(11) (a) Hoekstra, H. R.; Siegel, S.; Gallagher, F. X. In Platinum Group
Metals and Compounds; Advances in Chemistry Series, Vol. 98; American
Chemical Society: Washington, DC, 1971; pp 39-53. (b) Muller, O.; Roy,
R. J. Less-Common Met. 1968, 16, 129-146.
10.1021/ja9906498 CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/25/1999