Zeolite-supported Molecular Metal Complex Catalysts - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 32

Table 2.1 Characterization techniques to investigate zeolite-supported molecular
metal complex catalysts.

Technique Information Limitations

EXAFS
spectroscopy

Metal-ligands and metal-
support interactions
(inter-atomic distances
between an absorber metal
atom and backscatter atoms
and number of backscatter
atoms)

Average information; not
suitable for high
temperature experiments;
cannot distinguish light
backscatter atoms (such as
C, O, and N etc.)

d n 9 r 4 n g | 7

XANES
spectroscopy

Oxidation state of supported
metal complexes

Average information

TEM Location of supported metal

complexes and their nuclearity

Local information; electron
damages to samples

IR spectroscopy Ligands on a metal; species

present on the surface;
uniformity of supported metal
complexes by CO as a probe
molecule 18

Assignments of bands may not
be straightforward

NMR
spectroscopy

Ligands on a metal; dynamical
uniformity of supported metal
complex (e.g., supported
rhodium diethene complex 24 )

Assignments of bands may not
be straightforward

UV-visible
spectroscopy

Ligand field environment of
supported metal complexes

Not specific to identify
supported species

Density
functional
theory

Estimated spectral assignments;
structural models of supported
metal complexes; prediction of
intermediate species in a
catalytic cycle

Simplified model for a support

.

multiple species gives the average inter-atomic distance as described in
Section 2.4.1 (the limited number of structural parameters are allowed to be
optimized in analysis 45 and thus it is dicult to model several structures in
one model). Determinations of the number of atoms surrounding a par-
ticular atom are accompanied by relatively large errors ( 20%). Rejection of
one structural model over other models is not straightforward and requires
careful error analysis 46 as well as information from other techniques. Un-
fortunately, detailed processes of EXAFS analysis are not provided in many
papers. Characterization of metal-low-Z backscatter contributions (e.g.,
metal-oxygen and metal-carbon) by EXAFS is more challenging than that of
metal-high-Z backscatter contributions (e.g., metal-metal). This diculty is
relieved when supported metal complexes have terminal CO ligands because
multiple scattering effects of EXAFS provide a powerful analysis tool. 47
Structural uniformity facilitates characterization by EXAFS. Bare et al.
used EXAFS and XRD to elucidate structures of active sites of highly