Zeolite redox behaviour

Baba T, Akinaka N, Nomura M and Ono Y (1993), Reversible redox behaviour between silver cations and silver metal particles in hydrated Ag-Y zeolites , J Chem Soc, Faraday Trans, 89, 595. 

Besides the use as a model system, copper exchanged zeolites themselves are of technical interest, e.g. as hydrocracking catalyst components [1]. The copper ion redox behaviour is important for cocatalytic effects during catalyst activation (reduction of Fe-zeolites ) and catalyst regeneration. 

Catalytic N2O decomposition was investigated over FeZSM-5 prepared by a novel exframework method. This method comprises the introduction of Fe in the MFI framework, followed by calcination and steam treatment to extract the iron to extra-framework positions. The ex-framework method induces superior activity for direct N2O decomposition, compared to FeZSM-5 catalysts prepared by solid and aqueous ion-exchange methods. The extraordinary catalytic performance of ex-FeZSM-5 is attributed to the highly dispersed state of the Fe in the zeolite matrix and the Fe(III)/Fe(II) redox behaviour of a significant fraction of the iron centers in the catalyst. 

Finally the Redox behaviour, in general was shown to depend on the zeolite structure, on the Si/Al ratio and on the nature of the major charge balancing cation. For example divalent copper can accept a Jt electron from an appropriate aromatic molecule with adequate ionization potential to form a radical cation... 

The importance of XAS spectroscopy in oxidation catalysis research results from its abiUty to provide information on the local structure of active redox sites under reaction-Uke conditions, which can often hardly be obtained by other methods, such as XRD which is restricted to crystaUine material, EPR spectroscopy which only detects species with unpaired electrons, or UV-vis diffuse reflectance and Raman spectroscopy which are less sensitive to strongly absorbing TMI in reduced valence states. Thus, in situ EXAFS has been widely used to determine the local environment and redox behaviour of TMI in nanoporous oxides. A typical example is discussed in 19.3.2.2a for Fe-ZSM-5 zeolites. Another major application is the determination of size and shape of supported metal clusters, especially for particles smaller than... 

Wamken, M., Lazar, K., and Wark, M. Redox behaviour of Sn02 nanoparticles encapsulated in the pores of zeolites towards reductive gas atmospheres smdied by in situ diffuse reflectance UV/Vis and Mossbauer spectroscopy. Phys. Chem. Chem. Phys. 2001,3, 1870-1876. 

FTIR and DRIFTS studies were used to characterise the structures of silica-supported VOx species. vVO modes were used to monitor redox behaviour of a P-VOPO4 catalyst. FTIR of a vanadium oxide/ZSM-5 zeolite suggests the presence of the species (Si-0)3V=0, of C3V symmetry. TR measurements were used to probe the lowest electronically-excited state of (TPP)V=0. ... 

The CpFe(C0)2 2 hy adduct exhibits no catalytic activity in the temperature range studied (200-300°C). However cyclo-pentene and cyclopentane are detected through GC monitoring of the gas-phase. A redox reaction Fe(0)/H+ is still occurring as for the Fe3(C0)12"HY adduct the evolved hydrogen allows the reduction of the cyclopentadienyl ligands. This behaviour already provides evidence for the location of the Fe(l) complexes within the large cavities of the zeolite. 

This different behaviour with respect to the direct decomposition could be still related to a redox mechanism if one considers the comparable fraction of Cu reduced by the treatment with CO instead of the fraction reduced by He in Cu-HZSM5 and Cu-NaY. (Table 2). Tlie alternative explanation is that a redox cation is not essential for the reduction of NOx with hydrocarbons [6, 14]. The marked dependence of the CO conversion on the type of zeolite and on the... 

Multifunctional catalysis, in which reactions consisting of several reaction steps are carried out by a shorter synthesis route, is becoming increasingly important in organic synthesis. Molecular sieve catalysts, too, help to combine several catalytic steps and tailor them optimally to one another [15, 18, 24], In this respect, molecular sieves like zeolites can be used as carriers for catalytically active components such as transition metals, noble metals. In addition the catalytic behaviour of these components the intrinsic acidic or basic or redox properties of the zeolites combined with shape selective feature are still present. 

Using this technique, Kwak et al. have studied the possible involvement of anion transport in Prussian blue in acetonitrile and propylene carbonate solutions. It is known that cation transport is dominant during the redox reaction of Prussian blue films and that the electroactivity and ion transport behaviour of Prussian blue films are highly limited by the pore size because Prussian blue films have a rigid zeolitic structure. The electroactivity in aqueous solutions... 

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