Emission exchanged zeolite

Europium(III) exchanged zeolites have been studied by a number of research groups. Arakawa and coworkers (20, 21 ) report the luminescence properties of europium(III)-exchanged zeolite Y. Emission spectra were measured under a variety of conditions and bands for europium(II) were observed after thermal treatment of the europium(III) Y zeolites. A mechanism was proposed for the thermal splitting of water which involved the cycling of europium between the two different oxidation states. Europium MSssbauer experiments (22 ) also show that on thermal treatment of europium-(III) zeolites that europium(II) is formed. Stucky and coworkers (23, 24) studied the phosphorescence lifetime of these europium-(lll) zeolites and showed that the inverse of the lifetime (the decay constant) was linearly related to the number of water molecules surrounding the europium(III) ion in the zeolite supercages. These studies involved zeolites A, X, Y and ZSM-5. 

Lunsford and coworkers (26) have prepared a Ru(bpy)32 complex in zeolite Y and studied the quenching of oxygen and water. The emission bands of the ion-exchanged zeolite resemble those of aqueous solutions. Diffuse reflectance spectroscopy and ESCA measurements were also made in the characterization of these samples. 

Emission Spectra. The emission spectra of the uranyl acetate dihydrate in solution and in the solid state are shown in Figure 1. The fine structure in the solid state spectrum is not observed in solution. The corresponding emission spectra of uranyl-exchanged zeolites. A, Y, mordenite and ZSM-5 are shown in Figures 2-4. Excitation is carried out at 366 nm. The emission spectra have been scanned in all cases between 450 nm and at least 630 nm. 

Further observations from similar experiments are that the emission spectral lineshapes are not a function of the uranyl ion concentration in the zeolite. The lineshape does also not seem to be influenced by the relative acidity of the ion-exchanged zeolites. When the degree of hydration is changed from fully hydrated (stored in a dessicator under saturated NHi Cl aqueous soluttons) to vacuum dried at 1 x 10 Torr at room temperature, only the intensity of luminescence (and not the lineshape) changes. 

The emission spectra for uranyl-exchanged zeolites Y, mordenite and X all have differences but do show some fine structure and therefore resemble the solid state spectrum of uranyl acetate dihydrate. In fact, the spectrum of uranyl ions exchanged into sodium mordenite is very similar to that of the uranyl acetate dihydrate solid spectrum shown in Figure 1. Further support for our belief that some zeolites have a solution like environment and others have a solid like environment comes from the correlation between the crystallinity of these uranyl-exchanged zeolites and the appearance of some fine structure in the emission spectrum. We find no apparent correlation between this fine structure and the concentration of the uranyl ion in the zeolites even with a ten-fold change in the concentration of the uranyl ion. 

The most reasonable qualitative explanation of the change in the emission spectra is the above-mentioned resemblance to solution or solid state behavior which correlates well with the crystallinity of the uranyl-exchanged zeolites as determined by X-ray powder diffraction. However, there are differences between all of the zeolites which could be an indication of site symmetry and coordination in the lattice. The aluminosilicate lattice is preserved after ion-exhange except for zeolites A and ZSM-5. 

However, if transition metal exchanged zeolites are active materials in NH3-SCR, N2O emission can be also observed. N2O emission constitutes one of the main drawbacks of this system. For instance, Wilken et al. [112] reports a maximum N2O production at 200 °C over Cu-Beta zeolite. Mechanism of N2O emission is proposed to proceed through the decomposition of ammonium nitrates (reaction 19.28) ... 

Ammonia may likewise be used as reductant for selective catalytic reduction of NOx species. For this application, metal-exchanged zeolite catalysts offer new opportunities to reduce NOx emissions from lean-bum engine via the NH3-SCR process. Iron-exchanged ZSM-5 has received much attention because of its promising activity and stability in the NH3-SCR process. Correlating catalytic activities with the concentration of mononuclear and binuclear Fe species shows that both types of Fe ions and even small metal clusters are active sites for SCR,... 

A binder—free Na-Y zeolite with Si/Al ratio of 2.29 was obtained from Strem Chemical Co., La,Na—Y and Cs,Na-Y zeolites were prepared by exchanging Na-Y zeolite with LaCls and CsCl solution at room temperature. The percentage of metal ion exchanged in a zeolite has been determinated by Inductively-Coupled-Plasma Atomic Emission Spectroscopy and the number is used as prefix for the samples, e.g., Cs exchanged level of 667. is represented as 66Cs,Na-Y sample. 

As it was shown in the case of pentasil zeolites (MOR, FER, MFI), UV-VIS-NIR DR spectroscopy and UV-VIS emission spectroscopy appear to be extremely powerfiill tools for the characterization of transition metal ions in molecular sieve matrices [1,5]. The aim of this study is to employ this promissing and powerfull technique for the characterization of siting of metal cations exchanged into the (A1)MCM-41 matrix. 

Fig. 15. Low hydrocarbon emission control system utilizing a cross-flow heat exchanger TWC catalyst, A, and a zeolite-based hydrocarbon absorber system. Cold start HCs are absorbed by the hydrocarbon trap, B, until the cross-flow heat exchanger catalyst is hot enough to destroy the HCs that...

Figure 5. Relative quantum efficiencies of photoluminescence in Cu Y zeolites containing co-exchanged ions. The predicted quantum efficiencies were calculated from overlaps of the emission band of Cu with the absorption band of the coexchanged ion using the Foerster-Dexter resonance transfer theory.

Figure 2. a, emission spectrum of UOt exchanged A zeolite and b, emission spectrum of UOt exchanged ZSM-5 zeolite. Excitation was carried out at 366 nm. 

Catalytic oxidation-reduction (redox) reactions in zeolites are generally limited to reactions of molecules for which total oxidation products are desired. One important class of such reactions falls under the category of emission control catalysis. This encompasses a broad range of potential reactions and applications for zeolite catalysts. As potential catalysts one may consider the entire spectrum of zeolitic structural types combined with the broad range of base exchange cations which are known to carry out redox reactions. 

The involvement of upper excited states following irradiation of the phthalocyanine complexes Rh(Pc)(MeOH)X (where X = Cl, Br, or I) has been investigated. All compounds have the same action spectrum for photoinduced H-abstraction and the emission at 420 nm is attributed to relaxation of an upper tt, tt ) excited state. At high photonic fluxes it has been shown that biphotonic photochemistry involves the n, tt state. A study of the photoaquation of [RhCNH,), ] in fully and partially hydrated zeolite Y has been reported. Reaction within the cavities occurs with a quantum yield that is only 15—20% of that in aqueous solution this is attributed to the decrease in mobility of the water in the zeolite and to the exclusion of water from the ligand-exchange site by the zeolite lattice. ... 

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