Charge-compensating exchangeable activity

In this chapter we describe the use of polyelectrolytes carrying redox-active centers on electrode surfaces with particular emphasis on organized layer-by-layer redox polyelectrolyte multilayers (RPEM). In redox-active polyelectrolyte multilayers the polyion-polyion intrinsic charge compensation can be broken by ion exchange driven by the electrochemical oxidation and reduction forming extrinsic polyion-counterion pairing. In this chapter we describe the structure, dynamics and applications of these systems. 

Self-Activation. Although pure substances do not normally luminesce, zinc sulfide that has been fired in the presence of a halogen luminesces bright blue [5.311], [5.312], The luminescence center is assumed to be a cation vacancy. The charge compensation occurs through exchange of S2- by Cl-. 

It seems that practical implementation of this type of selective catalysts will require a medium in which (very) polar products can be removed from the zeolite phase. Unfortunately, no attention has been paid in literature to such issues. On the contrary, some attention has been devoted to host modification after exchange of NaY with other alkali metal cations [37]. The cyclohexene epoxidation activity increases with decreasing size of the charge compensating cation pointing to the influence of steric effects or of electrostatic effects on the activity. In competitive experiments using cyclohexene and 1-octene as feed, the reactivity of the smaller substrate is suppressed, indicating that competitive sorption is involved as well [37],... 

A Fe(97)-BEA catalyst, prepared by conventional ion-exchange procedure and calcined at 773 K, almost contains iron-binuclear-oxo-species in charge compensation of the BEA structure. The re-oxidation of iron(II) species by N2O leads to new oxo-species reducible at lower temperature than the dimer species. Among H2, CO, propene and NH3 in the reduction of N2O, CO, propene and NH3 are selective reductants. For these three reductants a similar light-off temperature of ca 638 K is obtained in the selective catalytic reduction of N2O. It should be pointed out that CO is efficient from 473 K but its activity is limited by the reoxidation of Fe to Fe species by N2O. 

The Fe species formed during the preparation of Fe/ZSM-5 catalysts by ion exchange in aqueous medium or in the solid state were studied. XRD, EPR, Mdssbauer spectrocopy (MOSS) and chemical analysis (AAS) were used to sample characterization. The catalysts were evaluated through the propane oxidation in the range from 373 to 773 K. The MOSS data evidenced the presence of Fe" species in charge-compensation sites and a more content of hematite (Fe203) in the catalysts prepared in aqueous medium. In the propane oxidation, the activity of the Fe/ZSM-5 can be correlated with the amount of Fe-cationic species, confirming that they are the responsible for the catalytic activity. 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

Hydrotalcite was also used as support. A hydrotalcite is regarded as a very basic support. It consists of platelets in a brucite structure, with Mg2+ ions octahedrally coordinated by O atoms. Part of the Mg2+ ions are isomorphically exchanged for Al3+ ions, creating a positive charge on the framework. Between the platelets, this positive charge is compensated with CO3 or OH- ions. The OH- ions located at the edge of the platelets are active in basic catalysis. 

In the interior of these channels, which are characteristic of zeolites, are water molecules and mobile alkali metal ions, which can be exchanged with other cations. These compensate for the excess negative charge in the anionic framework resulting from the aluminum content. The interior of the pore system, with its atomic-scale dimensions, is the catalytically active surface of the zeolites. The inner pore structure depends on the composition, the zeolite type, and the cations. The general formula of zeolites is... 

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