and the zeolite framework by FT-IR spectroscopy, this analytical tool provides
indirect information that could be helpful, for instance, in the analysis of the role
of water on the optical properties of silver loaded zeolites.
3.2 X-Ray-Based Techniques and Theoretical Modeling
X-ray diffraction (XRD) is a useful tool used for determining the atomic and
molecular structure of a crystal, in which the crystalline atoms cause the diffraction
of an X-ray beam into many specific directions. By measuring these angles and
intensities of the diffracted beams, the density of electrons within the crystal
structure can be estimated. From this electron density, the mean positions of the
atoms in the crystal structure can be determined, as well as their chemical bonds and
degree of disorder. XRD has been one of the preferred characterization techniques to
study the crystalline and well-ordered structures of natural and synthetic zeolites.
This type of analysis has been extended to metal exchanged zeolites, for instance, a
detailed review on the extraframework metal cation distributions in faujasite zeolites
through XRD was recently published [89]. In the case of silver-containing LTA
zeolites, the dynamical change of colors that resulted from the heat treatment of
silver-exchanged LTA zeolites was systematically studied via XRD by two different
research groups. Kim and Seff [63] attributed this color change to the presence of
partially reduced octahedral Ag 6 clusters within the sodalite cages of the zeolite
framework, whereas Jacobs and collaborators [90] suggested that the coloration was
due to the presence of linear Ag 3 clusters. Further investigation on Ag-LTA samples
pointed out that the formation of silver clusters with different sizes could be achieved
by varying the initial silver loading [19]. Next to silver-exchanged LTA zeolites,
other zeolite topologies containing different silver species, mostly utilized for
catalytic applications, have been analyzed by XRD. For instance, Lee and collaborators [91] reported the formation of Ag 2
2+ , Ag 3
+
, Ag 3
2+ silver clusters in faujasite
zeolites dehydrated under oxygen flow. In a similar study, Kim and co-workers
determined the crystal structure of fully Ag-exchanged faujasite zeolites under
reducing environments [92], they found the presence of larger silver clusters (Ag 4
n
+
, Ag 8
m+ ) when the samples were exposed to a H 2 gas flow.
X-ray absorption fine structure (XAFS) has also been widely applied in the
characterization of silver-exchanged zeolites (Fig. 4). This technique provides information at atomic scale in structures lacking long-range order (such as sub-nanometer
clusters), including metal-to-metal and metal-to-ligand bonding, as well as for
estimating the cluster nuclearity. By using this technique metallic and cationic silver
clusters composed of 2–8 Ag atoms such as dimers (Ag 2 ), trimers (Ag 3
+
) and
hexamers (Ag 6
+4 ) have been reported for silver-exchanged zeolites [93–95]. Additionally, XAFS studies have revealed the great influence that the confinement
scaffold (zeolite topology) possesses on the physicochemical properties of silver
clusters [96, 97]. However, the data interpretation of the results obtained by XAFS
on silver-exchanged zeolites should be performed with caution, a recent study
Highly Luminescent Metal Clusters Confined in Zeolites
89
indirect information that could be helpful, for instance, in the analysis of the role
of water on the optical properties of silver loaded zeolites.
3.2 X-Ray-Based Techniques and Theoretical Modeling
X-ray diffraction (XRD) is a useful tool used for determining the atomic and
molecular structure of a crystal, in which the crystalline atoms cause the diffraction
of an X-ray beam into many specific directions. By measuring these angles and
intensities of the diffracted beams, the density of electrons within the crystal
structure can be estimated. From this electron density, the mean positions of the
atoms in the crystal structure can be determined, as well as their chemical bonds and
degree of disorder. XRD has been one of the preferred characterization techniques to
study the crystalline and well-ordered structures of natural and synthetic zeolites.
This type of analysis has been extended to metal exchanged zeolites, for instance, a
detailed review on the extraframework metal cation distributions in faujasite zeolites
through XRD was recently published [89]. In the case of silver-containing LTA
zeolites, the dynamical change of colors that resulted from the heat treatment of
silver-exchanged LTA zeolites was systematically studied via XRD by two different
research groups. Kim and Seff [63] attributed this color change to the presence of
partially reduced octahedral Ag 6 clusters within the sodalite cages of the zeolite
framework, whereas Jacobs and collaborators [90] suggested that the coloration was
due to the presence of linear Ag 3 clusters. Further investigation on Ag-LTA samples
pointed out that the formation of silver clusters with different sizes could be achieved
by varying the initial silver loading [19]. Next to silver-exchanged LTA zeolites,
other zeolite topologies containing different silver species, mostly utilized for
catalytic applications, have been analyzed by XRD. For instance, Lee and collaborators [91] reported the formation of Ag 2
2+ , Ag 3
+
, Ag 3
2+ silver clusters in faujasite
zeolites dehydrated under oxygen flow. In a similar study, Kim and co-workers
determined the crystal structure of fully Ag-exchanged faujasite zeolites under
reducing environments [92], they found the presence of larger silver clusters (Ag 4
n
+
, Ag 8
m+ ) when the samples were exposed to a H 2 gas flow.
X-ray absorption fine structure (XAFS) has also been widely applied in the
characterization of silver-exchanged zeolites (Fig. 4). This technique provides information at atomic scale in structures lacking long-range order (such as sub-nanometer
clusters), including metal-to-metal and metal-to-ligand bonding, as well as for
estimating the cluster nuclearity. By using this technique metallic and cationic silver
clusters composed of 2–8 Ag atoms such as dimers (Ag 2 ), trimers (Ag 3
+
) and
hexamers (Ag 6
+4 ) have been reported for silver-exchanged zeolites [93–95]. Additionally, XAFS studies have revealed the great influence that the confinement
scaffold (zeolite topology) possesses on the physicochemical properties of silver
clusters [96, 97]. However, the data interpretation of the results obtained by XAFS
on silver-exchanged zeolites should be performed with caution, a recent study
Highly Luminescent Metal Clusters Confined in Zeolites
89