Bimetallic Supported Catalysts from Single-source Precursors - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 80

layers, such as an oxide, or modified by dopants. Bimetallic colloids are
prepared by co-reduction of different metal salts. The immobilization is
carried out by dipping the supports into organic or aqueous media con-
taining the suspended precursor colloidal particles at ambient temperature.
The surfactant shell is removed by annealing in a tube furnace in a mixture
of 10% O 2 in N 2 at 300 1C followed by a reductive step at 300 1C
in pure hydrogen. One key advantage of this method is the ease of
manipulation of the alloy composition by varying the salts ratio in the co-
reduction step.
H. Bonnemann et al. used the 'precursor' concept for manufacturing
multimetallic fuel cell catalysts with very small particle sizes ( o 2 nm) and
high metal loading (420 wt% metal). Bimetallic nanoparticles are expected
to provide improvements in catalytic activity, resistance to poisoning and
long-term stability in fuel cells. Methods of preparation for monometallic or
heterometallic colloidal nanoparticles soluble in organic (termed 'organo-
sols') or aqueous (termed 'hydrosols') media have been reported. 90 Most
transition metals are amenable to their synthetic methods. The reducing
agent is usually a hydrotriorganoborate. The origin of bimetallic effects was
sought at the precursor stage by characterizing in detail bimetallic colloidal
precursors by EXAFS: it was found that synergistic effects could be ascribed
to a geometric lattice structure effect rather than electronic band structure
effects. 91 The catalytic performance of 'sols' immobilized on a support be-
fore removing the stabilizing ligand layer has also been investigated. It was
found that these systems could already be active for a variety of reactions
(depending on the nature of metal(s)) without the activation step. In add-
ition, the resistance to poisoning and deactivation was increased by the
protective layer and enantioselective transformations could be carried out.
In this latter case, colloidal platinum (particle size 2 nm) was stabilized by a
chiral ammonium group derived from dihydrocinchonidine and tested in
the enantioselective hydrogenation of ethyl pyruvate: adsorbed on SiO 2 or
Al 2 O 3 supports, it gave the (R)-enantiomer in 81% and 85% ee respectively.
Finally, highly active PEM-FC and DMFC fuel cell anode catalysts with a
narrow particle size distribution were obtained for instance by preparing a
colloid from Pt(acac) 2 and Ru(acac) 3 in the presence of Al(CH 3 ) 3 as both
reducing agent and stabilizer in toluene. 92 The organic stabilizer was then
modified by reacting with polyethylenglycol-dodecylether to obtain more
hydrophilic characteristics. The organoaluminium colloids were supported
on high-surface-area carbon by dropping it from a dispersion in toluene to a
suspension of Vulcan carbon in the same solvent. The dried powder was
then heated in an air/Ar mixture followed by pure hydrogen to remove the
surfactant stabilizer. XPS characterization after activation indicated typical
parameters for zero-valent noble metals, but Al was still present in partially
oxidized form. HRTEM demonstrated that adsorption on the Vulcan support
leaves the particle sizes and size distributions practically unaffected
(10% changes), while the activation procedure causes some temperature-
induced sintering of the particles (

d n 9 r 4 n g | 5

.

20%) to give final metallic PtRu par-
ticles of 1.5-2 nm average size. The obtained PtRu(AlR 3 )/Vulcan catalysts

B