**3. Formation and structure of the metal clusters in zeolite framework**

#### **3.1 Progress**

Already in the years 1970s, the high temperature hydrogen reduction of Ag ion to metallic silver in zeolite Y was known but it was only in 1978 when Kellerman and Texter reported the first optical measurements [32]. By measuring the Shuster-Kubelka-Munk re-emission function and fluorescence of a vacuum dehydrated sample of silver clusters they presented and characterized for the very first time the excited electronic state of monodispersed atoms as well as optical absorption of collaterally produced clusters and particles. They assigned the broad absorption peaking at 3.3 eV to "intrazeolitic silver particles". The results triggered a large interest among scientist. Three years later Gellens *et al* reported the formation of the so called "color centers" in Ag-A zeolites, as a result of the reduction of Ag+ ions upon dehydration. The authors assigned the process to the formation of linear Ag3 2+ clusters with Ag0 located inside the sodalite cage opposite the framework D4R and in between two Ag+ cations located at the D6R [33]. It was then believed that the yellow color is due to isolated cluster whereas the observed red color was associated to the formation of two or more interacting Ag3 2+ clusters in the sodalite cage to a maximum of four.

As stated above, the silicon-to-aluminum ratio is an important parameter of zeolite as this determines the number of exchangeable cations. In 1984, Johnson *and al*. detected distinct species following the reaction of sodium-exchanged zeolite Y with sodium, potassium, or rubidium vapor in dehydrated samples of zeolite Y [34]. Upon exposure to high concentrations of metal vapor he obtained stable ionic clusters (Na4 3+, K4 3+ and Rb4 3+) characteristic of small metallic particles located inside the zeolite cavities. Also interesting is that the researchers observed cluster species Ag6 q+ (q = 1, 3, 5) in which an unpaired electron is trapped at a cluster of six equivalent silver cations. In the same year ESR measurements on silver particles in zeolite A proved that during the reduction of silver-loaded zeolite A certain stages of cluster formation can be followed and detected the presence of Ag6 x+ and Ag8 y+ clusters which fit in the sodalite cage [35]. Two years later, Wang and Herron succeeded in encapsulation of CdS and PbS in zeolite Y matrix and indicated that zeolites can be thought as providing a solid solvent for this type of clusters [36]. Impressive work since, through simple experiments, they showed that their optical properties were dependent on size and state of aggregation, a property well-known already for metal particles. A fascinating conclusion was drawn: due to the fact that the transition from clusters to aggregates is not continuous but abrupt they suggested that aggregation inside the zeolite is a percolative process.

The first important review on the structure and chemistry of silver clusters in zeolites has been published by Sun and Self and the reader is encouraged to read

this excellent work which inspired many scientists [9]. The spectroscopy and light induced processes of silver clusters in zeolites have been incipiently discussed by Chen *et al*. [37] For the first time they looked at photostimulated luminescence of silver-exchanged zeolite-Y. Once irradiated with 254 nm wavelength, the photoluminescence intensity of silver atoms centered at 505 nm decreases and a new absorption band shows up around 840 nm. By photostimulation of this absorption band, the fluorescence of silver atoms is observed and the photoluminescence intensity of silver atoms increases slightly. These phenomena were considered to be caused by the charge transfer between the zeolite framework and the entrapped silver atoms. The photostimulated luminescence of silver clusters encapsulated in zeolite-Y was caused by the recombination of luminescence centers with electrons released from their traps by photo- stimulation. Charge transfer from the framework oxygens to silver cations was also reported by Seff and Kim based on a color change from brick-red to yellow [38].

Following the pioneering work presented above, small metal particles started to attract the real interest of scientists due to their peculiar optical, electronic and catalytic activity that change from bulk properties to molecular-like properties once the size decreases under a certain range. For instance, a change in the electronic properties from a band structure to molecular orbitals levels was common for isolated species of a few atoms particle. Such change was almost invariable of the nature of the constituent atoms. Of a particular interest was to control the size and coordination because these would allow fine-tuning of electronic and luminescent properties of the clusters. However, to achieve that, a deep understanding of the electronic state was much desired. Based on such understanding the revealed structure-to-function relationship could finally open perspectives for practical applications.

The exact structure of metal clusters stabilized be the zeolite matrix remained under debate for a long time. Gellens et al. investigated the electronic properties of silver clusters using extended Hűckel framework [39]. They calculated isolated Ag3 molecules in linear or nearly linear geometries and a strong similarity with the electronic spectrum of yellow Ag clusters were found. The proposed model for Ag3 encaged in the zeolite framework appeared to be weakly interacting with the zeolite lattice. Very interesting is that the scientists suggest the occurrence of an electron transfer of approximately two electrons from the cluster toward the zeolite framework increasing the charge density on the Si/Al model.

Until 1990s, simple structures as dimers and trimers of alkali-metal elements or noble metals were only experimentally investigated with UV–VIS and IR spectroscopy, Raman, ESR and EXAFS techniques [40–42]. These experiments indicated various shapes such as linear, bent or triangular structure and even interconversions between such structures were proposed. Precise studies of the effect of cluster shape, structure and interactions with the zeolite environment have been rather limited mainly due to a combination of instrumentation and techniques used. Other difficulties were found due to a distribution of particle size or problems with high mobility of precursors. Zeolite X and Y have been utilized as scaffolds to restrict and stabilize metal agglomeration. In such matrices, for instance, geometric models based on EXAFS experimental results revealed the formation of Pd2, Pd3 and Pd4 clusters occupying adequate positions of the sodalite cages [43]. This showed again a "molecular" cluster elegantly stabilized in an open-framework zeolite. Texter *et al* encountered similar difficulties when investigated the formation of charged silver clusters of activated dehydration of zeolite A [44]. The nuclearity of these clusters formed in the sodalite unit was uncertain, believed to be between 6 and 14 while the dominant cluster had an absorption band centered at 2.72 eV and a higher band at 3.8 eV. Closely, more details about the cluster nuclearity and interactions

#### *Zeolites as Scaffolds for Metal Nanoclusters DOI: http://dx.doi.org/10.5772/intechopen.96876*

with the framework have been brought via far-infrared experiments [45]. The cage vibrational mode characteristic for Ag0 atoms isolated in zeolite Y has been identified at 89 cm−1 while silver-silver stretching modes were identified for zeolites A and Y encapsulated Agn q+ clusters with n = 2, 3 and 6. Via IR spectroscopy the presence of silver microcrystals located on the external surface of zeolite has been also demonstrated. Although important steps have been made, the structure of the metal cluster remains largely uncertain and very few have succeeded in providing strong evidences. Spectroscopic experiments, although widely qualitative, remain the basis in identifying a cluster species. Generally, larger clusters give rise to lower optical bandgap and inconsistencies in assignment of these bands were sometimes observed in literature.

A first attempt to establish the structure of the model Ag clusters in zeolite sample was made in 2004 by employing a combination of steady-state spectroscopy and K-edge XANES/EXAFS analyses [46]. The absorption spectra showed bands at 255 and 305 nm and from Ag K-edge EXAFS results, the structure of the cluster was presumably identified as Ag4 2+. The coordination number 3.3 and the Ag-Ag distance of 2.7 Å suggest that the cluster consisted of 3 or 4 Ag atoms. The amount of the clusters increases with the Ag/Al ratio of Ag zeolites. Further spectroscopic investigations on very small particles Ag2S and PbS in zeolite A showed photoluminescence in the visible range and lifetimes as long as 300 μs and these properties were shown to be strongly dependent on co-cations [47]. A broad investigation has been carried out for Li<sup>+</sup> , Na+ , K+ , Rb+ , Cs+ , Mg2+, Ca2+, and Sr2+. Interestingly, the unusual long luminescence lifetime has been interpreted as excitation energy transfer between Ag2S and Ag4S2, a concept that comes often forward in literature.
