Zeolites metal redispersion

Work with the objective of comparing oxo-ions with oxide particles in order to test the validity of this reasoning has been reported by Chen et al. who used a catalyst that initially contains Fe oxo-ions, [HO-Fe-0-Fe-OH] +. These sites were first converted to Fc203 particies by a simpie chemical treatment. This was followed by another treatment, which redispersed these Fc203 particies back to oxo-ions. The change in particle size was monitored by a spectroscopic method based on the observation that in zeolites metal ions and oxo-ions, that are attached to the wall of a cage, give rise to a typical IR band caused by the perturbation of the vibrations of the zeolite lattice. 

The chemical stability tests were intended to quantify structure loss under conditions encountered in either catalyst manufacturing or post-regeneration metal redispersion processes ("rejuvenations") Zeolites were first calcined in dry air for two hours at 4 2dC, after which slurries of 20 g zeolite in 200 mL of treatment solution were prepared and stirred for two hours. The treated zeolites were collected on Buchner funnels, washed twice by reslurrying in 200 mL portions of water, dried at 110DC, and (optionally) calcined in dry air for one hour at 482°C. 

Oxidative redispersion of Pd and Cu aggregates in zeolite NaHY has been reported (76,143,205). First the metal (M = Pd or Cu) is oxidized to oxide particles ... 

The CO molecule is a well-known coordinating ligand for many transition metals. Although CO has been reported to be a reducing agent for metal ions such as Cu " and Ag" in zeolites (214,215), it also induces oxidative redispersion of Rh particles in zeolites at room temperature (704). With FTIR, it has been established that the presence of H2O is crucial for the reduction of Ag" in zeolite according to the following reaction ... 

The first two steps require corrosive oxidizing agents forming mobile metal complexes. Chlorine fulfills these two criteria and can be used effectively to redisperse large particles on the external zeolite surface. Foger et al. [ 138,139] have studied the redispersion of platimun by chlorine in various zeoUtes. However, temperatures in excess of 350°C are required in these processes, and part of the metal may be lost. 

Isolated Pd-atoms in sodalite cages obtained by reducing Pd + ions on SI sites were re-oxidized into Pd + cations located on ST sites merely by heating the zeolite in oxygen above 450 K [21]. In the same way, Pd-clusters filling adjacent supercages were re-oxidized into Pd + cations by calcination in oxygen above 470 K [21]. The mechanism of oxidative redispersion of metal clusters into cations involved the participation of zeolite protons ... 

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