1 Ligand-Free Sub-Nanometer Metal Clusters in Catalysis
11
Fig. 1.11 Left: Aberration-corrected HR–TEM micrograph of Au@EVOH; the circles in yellow
indicate Au clusters, the circle in red indicates a 2 nm Au NP, and the inset shows the interplanar
crystallographic distance for Au. Top right: Photograph of neat EVOH (left, colorless), Au@EVOH
(middle, yellow) and the material synthesized with carvacrol, which leads to plasmonic Au nanoparticles (right, red). Bottom right: A representative EDX spectrum of the area indicated for clusters,
showing the presence of Au (Fig. from Ref. [50]. Copyright © by The Royal Society of Chemistry
2017)
1.2.3.5 Single Crystal X-Ray Diffraction
Perhaps, the more powerful technique for the structural determination of metal clusters is single crystal X-ray diffraction (SC–XRD). For ligand-stabilized clusters, this
technique has been routinely employed, and for instance, the SC–XRD structure of
a quasi-linear Pd 4 cluster stabilized and protected by polyarene ligands has been
reported (Fig. 1.12) [47, 48]. However, this compound does not find application
in catalysis since ligand exchange with the reactants triggers decomposition of the
cluster even below 0 °C. In principle, decomposition may not occur in a ligandfree cluster suitably accommodated within a solid support, thus enabling heterogeneous catalysis by metal clusters. Indeed, the same quasi-linear Pd 4 cluster could
be obtained in a robust and crystalline MOF structure with ability to incorporate
and reduce metal cations [20]. The use of this type of MOFs has recently opened
not only the possibility to prepare ligand-free few-atom sub-nanometer clusters but
also to characterize them by SC–XRD and do catalysis with them. The MOF acts as
a convenient crystalline matrix to host the ligand-free metal clusters and to obtain
suitable monocrystals for diffraction, as it has been achieved not only for Pd but also
for Pt clusters [45, 60].
1.2.3.6 Other Techniques
Dynamic light scattering (DLS) and zeta potential measurements
These techniques have been long employed to determine the size (DLS) and charge
(zeta potential) of colloidal Au nanoparticles with the same instrumentation. Recent
11
Fig. 1.11 Left: Aberration-corrected HR–TEM micrograph of Au@EVOH; the circles in yellow
indicate Au clusters, the circle in red indicates a 2 nm Au NP, and the inset shows the interplanar
crystallographic distance for Au. Top right: Photograph of neat EVOH (left, colorless), Au@EVOH
(middle, yellow) and the material synthesized with carvacrol, which leads to plasmonic Au nanoparticles (right, red). Bottom right: A representative EDX spectrum of the area indicated for clusters,
showing the presence of Au (Fig. from Ref. [50]. Copyright © by The Royal Society of Chemistry
2017)
1.2.3.5 Single Crystal X-Ray Diffraction
Perhaps, the more powerful technique for the structural determination of metal clusters is single crystal X-ray diffraction (SC–XRD). For ligand-stabilized clusters, this
technique has been routinely employed, and for instance, the SC–XRD structure of
a quasi-linear Pd 4 cluster stabilized and protected by polyarene ligands has been
reported (Fig. 1.12) [47, 48]. However, this compound does not find application
in catalysis since ligand exchange with the reactants triggers decomposition of the
cluster even below 0 °C. In principle, decomposition may not occur in a ligandfree cluster suitably accommodated within a solid support, thus enabling heterogeneous catalysis by metal clusters. Indeed, the same quasi-linear Pd 4 cluster could
be obtained in a robust and crystalline MOF structure with ability to incorporate
and reduce metal cations [20]. The use of this type of MOFs has recently opened
not only the possibility to prepare ligand-free few-atom sub-nanometer clusters but
also to characterize them by SC–XRD and do catalysis with them. The MOF acts as
a convenient crystalline matrix to host the ligand-free metal clusters and to obtain
suitable monocrystals for diffraction, as it has been achieved not only for Pd but also
for Pt clusters [45, 60].
1.2.3.6 Other Techniques
Dynamic light scattering (DLS) and zeta potential measurements
These techniques have been long employed to determine the size (DLS) and charge
(zeta potential) of colloidal Au nanoparticles with the same instrumentation. Recent