Bimetallic Supported Catalysts from Single-source Precursors - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 76

times without a decrease in selectivity or activity. Alumina-supported
catalysts were also prepared from mixed-metal Ru-Pt clusters such as
[Ru 5 PtC(CO) 15 ] 2 , and compared to analogous materials prepared from in-
organic salts. 72 Using cluster precursors allowed excellent control of
bimetallic composition and smaller particles in the final catalyst, which was
more active and selective than commercial or salt-derived counterparts in the
production of hydrogen via the steam reforming of ethanol. Other connected
successful stories concerned the use of Ru-Sn mixed-metal clusters of varying
stoichiometries immobilized on Davison 923 mesoporous silica, 73 atrime-
tallic Ru-Pt-Sn cluster supported on silica, 74 and bimetallic Ir-Bi nano-
particles obtained from clusters deposited onto MCM-41. 75 Gold-iron
carbonyl clusters were also used as precursors of bimetallic nanoparticles
supported on ceria 76 or the siliceous mesoporous SBA-15 77 , and shown to be
superior for the total oxidation of methanol and toluene, chosen as
representative VOCs.
We have applied the same concepts to immobilize mixed-metal clusters on
carbon supports. However, it was found that the less polar carbonaceous
surface was not ideal to interact with clusters: sintering and agglomeration
could not be avoided on activated carbon used as such. 78 On pristine carbon
nanotubes also, agglomerates were found on the external walls together with
individual clusters aligned in the tip region. 79 This was ascribed to a sta-
bilizing effect of oxygenated groups known to be present in higher quantities
at the tips together with a 'pincer' effect of carbon layers extremities.
Therefore, we started functionalizing carbon materials with chelating
phosphine groups, in order to anchor mixed-metal clusters on their surface
by ligand exchange. 80 In addition, by modifying the last step of the synthesis
methodology, charged ammonium groups could also be fixed on carbon-
aceous surfaces (Figure 3.10). Mixed-metal clusters were thus immobilized
on functionalized carbon surfaces before being transformed into supported
bimetallic nanoparticles by thermal activation. This led successfully to
highly dispersed Ru-Pt nanoparticles derived from mixed-metal clusters
supported on activated carbon. 81 However, in the case of Ru-Au clusters, the
ligand exchange reaction led to ejection of Au atoms from the bimetallic
precursors to lead to very small Ru-rich nanoparticles together with much
bigger Au-only sintered particles (Figure 3.11). The same phosphine-
functionalized activated carbon support was used to anchor Pd clusters that
were activated thermally into Pd nanoparticles active in nitrobenzene hy-
drogenation. 82 The functionalization methodology could be successfully
transposed to carbon nanofibers, 83 ordered mesoporous carbons 84 and
carbon nanotubes. 85 In this last case, Fe-Co nanoparticles were obtained on
multi-walled carbon nanotubes and carbon nanofibers (Figure 3.12) using
the cluster precursors [HFeCo 3 (CO) 12 ] and (NEt 4 )[FeCo 3 (CO) 12 ], that were
shown by magnetic characterization to be blocked super-paramagnetic Fe-
Co nanoparticles together with paramagnetic ions. TEM indicated that the
nanoparticles were better dispersed and of smaller sizes on functionalized
than on pristine carbon supports. Very recently, we have developed a new

d n 9 r 4 n g | 5

.