paramount importance to determine the atomic positions of the silver atoms in the
zeolitic framework. Such knowledge could allow the understanding of the relationship between the structure and the physicochemical properties of these
sub-nanometer structures. Unfortunately, silver-exchanged zeolites are extremely
sensitive to damage caused by the electron beam, especially in the case of topologies
having a relatively low Si/Al ratio such as LTA and FAU zeolites. During one of the
first high-resolution TEM (HR-TEM) studies on silver-exchanged zeolites (FAUX,
LTA, LTL, MOR, RHO), a particular effect was observed [104]. The formation of
silver nanowires as a result of the reduction of silver cations incorporated in the
zeolite frameworks by the incident electron beam was reported. This new approach
to synthesize silver nanowires was later applied for the fabrication of well-defined
silver nanoparticles in LTA and FAU zeolites [105]. In this report, a twofold
mechanism, in which the electron beam irradiation performed the breaking of
chemical bonds in the zeolite framework and simultaneously a rapid diffusion of
Ag cations in the amorphous materials occurred, resulting in the aggregation of
larger silver nanoparticles from silver cations and clusters. Recently, high-angle
annular dark-field scanning transmission electron microscopy (HAADF-STEM) was
used to image individual Ag atoms in LTA zeolites [106]. An octahedral Ag 6 cluster
was clearly visualized in the sodalite cages of fully exchanged heat-treated Ag-LTA
zeolites. HAADF-STEM images yielded an intensity which scales with the atomic
number Z and the thickness of the sample [107]. Therefore, the silver atoms
appeared with higher intensity in comparison to the elements of the zeolite framework (Si, Al, and O). The use of aberration-corrected TEM at relatively low electron
doses opened new avenues to study silver-exchanged zeolites at atomic scales, as
illustrated by the work of Altantzis and collaborators, where the structure of luminescent silver clusters confined in FAU zeolites was unraveled (Fig. 5) [80].
3.5 Electron Spin Resonance (ESR) Spectroscopy
and Thermogravimetric Analysis (TGA)
ESR experiments on silver-exchanged zeolites have provided valuable information
related to the structure and magnetic properties of silver clusters, as well as their
location, stability, and reactivity with different ligands. For instance, the stabilization
of Ag 2
+
, Ag 3
2+ , and Ag 6
n+ species have been reported on Ag-LTA zeolites under γ
irradiation, depending on the silver loading and dehydration conditions
[108, 109]. Whereas, Ag 6
n+ clusters have been exclusively observed in hydrogenreduced samples [110]. The formation of these paramagnetic species was related to
the electron capture from diamagnetic precursors such as Ag 3
+ or Ag 6
2+ . ESR
analysis has also contributed to the elucidation of the formation mechanisms of
silver clusters in zeolites. For instance, Michalik and Kevan described the silver
clustering in the sodalite cage of Ag-LTA zeolites based on ESR analysis [109]. The
92
E. Coutino-Gonzalez et al.
zeolitic framework. Such knowledge could allow the understanding of the relationship between the structure and the physicochemical properties of these
sub-nanometer structures. Unfortunately, silver-exchanged zeolites are extremely
sensitive to damage caused by the electron beam, especially in the case of topologies
having a relatively low Si/Al ratio such as LTA and FAU zeolites. During one of the
first high-resolution TEM (HR-TEM) studies on silver-exchanged zeolites (FAUX,
LTA, LTL, MOR, RHO), a particular effect was observed [104]. The formation of
silver nanowires as a result of the reduction of silver cations incorporated in the
zeolite frameworks by the incident electron beam was reported. This new approach
to synthesize silver nanowires was later applied for the fabrication of well-defined
silver nanoparticles in LTA and FAU zeolites [105]. In this report, a twofold
mechanism, in which the electron beam irradiation performed the breaking of
chemical bonds in the zeolite framework and simultaneously a rapid diffusion of
Ag cations in the amorphous materials occurred, resulting in the aggregation of
larger silver nanoparticles from silver cations and clusters. Recently, high-angle
annular dark-field scanning transmission electron microscopy (HAADF-STEM) was
used to image individual Ag atoms in LTA zeolites [106]. An octahedral Ag 6 cluster
was clearly visualized in the sodalite cages of fully exchanged heat-treated Ag-LTA
zeolites. HAADF-STEM images yielded an intensity which scales with the atomic
number Z and the thickness of the sample [107]. Therefore, the silver atoms
appeared with higher intensity in comparison to the elements of the zeolite framework (Si, Al, and O). The use of aberration-corrected TEM at relatively low electron
doses opened new avenues to study silver-exchanged zeolites at atomic scales, as
illustrated by the work of Altantzis and collaborators, where the structure of luminescent silver clusters confined in FAU zeolites was unraveled (Fig. 5) [80].
3.5 Electron Spin Resonance (ESR) Spectroscopy
and Thermogravimetric Analysis (TGA)
ESR experiments on silver-exchanged zeolites have provided valuable information
related to the structure and magnetic properties of silver clusters, as well as their
location, stability, and reactivity with different ligands. For instance, the stabilization
of Ag 2
+
, Ag 3
2+ , and Ag 6
n+ species have been reported on Ag-LTA zeolites under γ
irradiation, depending on the silver loading and dehydration conditions
[108, 109]. Whereas, Ag 6
n+ clusters have been exclusively observed in hydrogenreduced samples [110]. The formation of these paramagnetic species was related to
the electron capture from diamagnetic precursors such as Ag 3
+ or Ag 6
2+ . ESR
analysis has also contributed to the elucidation of the formation mechanisms of
silver clusters in zeolites. For instance, Michalik and Kevan described the silver
clustering in the sodalite cage of Ag-LTA zeolites based on ESR analysis [109]. The
92
E. Coutino-Gonzalez et al.