Reduction of Exchange Ions

Cations of noble metals in zeolites are easily reduced by H2, and this is the method most commonly applied. Proper activation and reduction treatments are essential to obtain the highest metal dispersions. Empirically determined treatment conditions are usually employed to obtain well dispersed metals, and generalizations about them are difficult. For example, prior to the final reduction in H2, it is usually necessary to eliminate any NH3 produced during the thermal decomposition of an amine complex or a NH4 ion in the zeolite. The reduction of metal ions in the presence of evolving NH3 can easily lead to the formation of agglomerated metal. Furthermore, Dalla Betta and Boudart [129] pointed out 

Thermal activation of the complexes in flowing O2 prior to the reduction by H2 is essential if highly dispersed Pt clusters in zeolites are to be obtained. 

however, more oxophilic metals than Pt are used (e.g. Ru), then pretreatment in O2 before reduction should be avoided because this leads to the formation of metal oxides such as Ru02. In these cases, pretreatment under vacuum or in an inert gas is recommended, with the reduction to be carried out in the presence of H2. 

---

There are numerous reports of attempts to prepare metal clusters (as distinguished from metal carbonyl clusters) in zeolite cages, [105, 108, 123-125] most often by reduction of exchange ions in the cages or by decarbonylation of metal carbonyl clusters. One of the challenges has been to confine the resultant clusters within the cages, and often the literature reports have failed to provide suffident... 

In this work, the state of silver, the formation and the final size and dispersion of the particles are investigated. The combination of TPR/MS and UV-Vis. spectroscopy allows a better comprehension of the transformations occurring during thermal/gas treatment In addition, TEM shows that the exchanged zeolite exhibits the smaller particles with best dispersion that can be attributed to the stabilization, and next, to the reduction of exchanged ions... 

---