Bimetallic Supported Catalysts from Single-source Precursors - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 70

collected at adsorption edges of both metals and fitted with parameters
arising from analysis of reference samples. Very often, these fits are con-
firmed by theoretical calculations. This is much easier to perform at the
precursor stage, when the cluster is immobilized but not yet activated.
During removal of the ligands, the metal-metal bonds might become
shorter, structural changes might occur that are very interesting to follow by
EXAFS. In some cases, the replacement of some of the original ligands
by surface functions can be proven. If the clusters have been strongly re-
arranged during activation and/or agglomerated into larger nanoparticles,
it might sometimes be dicult to collect high-quality EXAFS data. The
structures can be determined more precisely when the final nanoparticles
are the most highly dispersed and the sample is uniform. For example, a
Pt-Ru/MgO catalyst was prepared by adsorption of Pt 3 Ru 6 (CO) 21 (m 3 -H)(m-H) 3
followed by ligand removal by treatment at 300 1C. 54 Infrared spectroscopy
indicated that it was not intact after adsorption on MgO. EXAFS character-
ization after activation demonstrated the presence of heterometallic
Pt-Ru bonds, which was not the case when preparing the equivalent
catalyst from a mixture of Pt(acac) 2 and Ru(acac) 3 . Similarly, the clusters
(NEt 4 ) 2 [Ir 4 (CO) 10 (SnCl 3 ) 2 ] and NEt 4 [Ir 6 (CO) 15 (SnCl 3 )] have been used for
the preparation of silica-supported Ir-Sn bimetallic catalysts. 55 Character-
ization relied heavily on EXAFS and XANES, and allowed a structural model
for the Ir-Sn species in the activated catalyst to be proposed.

d n 9 r 4 n g | 5

3.3.4.3 Further Examples of Mixed-metal Clusters as Precursors
of Nanoparticles
The group of B. C. Gates has been very active in the field. 47 A large number of
papers from their group and others such as Ichikawa report 'ship-in-a-bottle'
synthesis to form bimetallic clusters inside the super-cages of zeolites
(Faujasite, ZSM-5, ALPO-5 etc.). This occurs by ion exchange or metal salt/
complex impregnation, followed by reductive carbonylation. Once formed,
the entrapped clusters can be thermally treated by calcination and reduction
to be subsequently transformed into bimetallic very small nanoparticles
supported within zeolites. For example, Rh 6 x Ir x (CO) 16 clusters (with x
ranging from 1 to 4) were prepared by reductive carbonylation at 150-227 1C
of Rh 31 and Ir 41 introduced into zeolite NaY by ion exchange. 56 Highly
dispersed Rh-Ir nanoparticles were formed by decarbonylation of these
clusters in O 2 followed by reduction with H 2 .
A seminal study has been carried out by Nashner et al. on the preparation
of Ru-Pt carbon-supported nanoparticle catalyst from the PtRu 5 C(CO) 16
cluster. 57,58 The molecular cluster precursor was deposited on carbon
black (Vulcan XC-72) by incipient wetness from a THF solution (in quantity
corresponding to 1-2 wt% metal loading). The sample was dried in air and
activated by heating at a rate of 15 1Cmin 1 in flowing H 2 (40 mL min 1 )toa
temperature of 400 1C and keeping it at that temperature for 1 h (Figure 3.6).
The structure of the obtained nanoparticles on an atomic scale was deduced

.