CdS in zeolite

Figure 4. Absorption (dotted), excitation (solid) and emission (dashed) spectra of 6.5wt% CdS in zeolite Y. The absorption spectrum was taken at room temp and the others at 77K.

Low concentrations of Cd(l) species can be obtained by allowing Cd vapor to react with Cd and trapping the resultant Cd(I) species in a zeolite . When Cd -exchanged zeolite A is exposed to Cd metal vapor at 350°C for 2.5 d, both Cd and Cd2 " are formed, the latter having a Cd-Cd internuclear distance of 235 pm. 

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

Zeolites can be ion-exchanged with cations or impregnated with various metals to modify their performance for use in applications such as separations, adsorption and catalysis. For example, faujasite zeolites exchanged with Na, Li, K, Ca, Rb, Cs, Mg, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd, Ag, Cd, In, Pt, H, Pb, La, Ce, Nd, Gd, Dy and Yb have been made and studied due to their use in separation and catalysis [135]. The ability to determine the distributions of these cations in the zeolitic structure is one of the key parameters needed in understanding adsorption mechanisms and molecular selectivities. Little has compiled an excellent reference... 

Zeolite A, sodalite, zeolite X, and chabazite Size-quantized CdS formed in zeolite channels... 

Unfortunately however, the cations most often found in natural and synthetic zeolites are not ideal for NMR studies. The 23Na nucleus has a large quadrupolar moment, while cations of most spin j nuclei (such as Y, Rh, Ag, Cd, Pb, Hg, Os, and Pt) are of relatively little interest to the zeolite chemist. For these reasons much attention has been paid to 7Li, which has a small quadrupolar moment, and to 20ST1, an attractive spin 5 nucleus. On the other hand, the 23Na resonance can be used as a sensitive probe for the electric field gradient in zeolites and the effects brought about by sorbed molecules. 

We cannot explain why we find that Na-A exhibits a higher selectivity for Cd + in the Cl" system than Gal (1) found. Perhaps this difference relates to the difference in our batches of zeolite. 

One can see that with decreasing average thickness of the silica grain boundaries below about 1 run the intensity of the PL strongly decreases. The investigations being currently done in our laboratory should decide if this decrease is due to a delocalization of the electrons and holes. Such an effect has been described for CdS clusters trapped in zeolites [41]. To optimize the electroluminescence from nc-Si films will require to find the optimum compromise. 

Quantization effects have not only been observed with colloidal solutions, but also with nano crystals deposited on Pt or SnOa [38,39] and with semiconductor clusters (CdS,PbS) produced in zeolites [40,41]. The same effect has been found with thin single crystalline semiconductor layers (super lattices) (see e.g. Refs. [42,43]. In this case the quantization occurs only in one dimension, and the shift of energy states is much less, i.e, in the order of 0.1 eV, whereas the shift in a colloidal particle (quantization in 3 dimensions) amounts to several eV. 

A recent, well-defined example of such clusters is the existence of CdS superclusters in zeolites (86,87). The authors prepared very small CdS clusters in zeolites. In zeolite Y, for example, there are sodalite cages (5... 

We note that zeolites have also been used as hosts for a number of other intriguing "nanocomposites", for example in the field of encapsulated quantum-size semiconductor particles such as Se, CdS, CdSe, PbS, and GaP.3h32,33,34,35 phe encapsulation of metals such as Bi, Hg, Sn and Ga in zeolites has been described by Bogolomov.36 These studies demonstrate the enormous versatility of zeolite host systems for studies and control of structural/electronic relationships. 

The effect of exchanging Cd " into zeolite LTA has been studied by Cd NMR, enabling the identification of 2 different cadmium coordination states in the hydrated form of the zeolite. The effect of cadmium exchange on the silicon and aluminium environments of this zeolite was also investigated using Si and Al NMR spectroscopy (Eldewick et al. 1999). 

Raman spectra gave evidence for quantum size effects in CdS clusters confined in zeolite pores.433 The Raman spectra of nanocrystals of CdSi xSex... 

M.R. Steele, PM. Macdonald, and G.A. Ozin, Topotactic Metal Organic Chemical Vapor Deposition in Zeolite Y Structure and Properties of CH3MY from MOCVD Reactions of (CH3)2MHY, where M = Zn, Cd. J. Am. Chem. Soc., 1993, 115, 7285-7292. 

Within the bivalent cation-exchanged Y series, the order Ca, Mg, Cd > Sr > Ba has been observed 125,127) in benzene propylation at 250-300 . Activity of the CaY for the propylation reaction reached a sharp peak in activity when 50% of the Na+ ions had been replaced by Ca++ 125). This increase in activity corresponds with the increasing occupation by Ca++ of the Su sites in zeolite Y at higher degrees of Ca++ exchange, as predicted by Pickert et ai. 40). 

Since all of the CdS clusters reside in the sodalite cages of the zeolite Y framework, the larger supercages of the structure are still available for absorption of substrate molecules - in this case olefins for photo- oxidation via electron transfer. Colloidal CdS in free solution has been used for such oxidations previously(19) and in a competitive oxidation of styrene and 1,1-diphenylethylene we find that unconfined bulk CdS will effect oxidation in a ratio of 1 2 for these two olefins (irradiation at 365nm). In the zeolite confined system we find however that the ratio becomes 1 1 ie a slight shift in selectivity toward the smaller substrate as may be expected on the basis of size/diffusion effects. From the viewpoint of the enzyme mimic, we have here a system... 

CdS, PbS, CdSe, and ZnS clusters arc the semiconductor compounds studied most extensively in zeolites.[39,40] In addition, ternary systems such as ZnCdS have attracted the interest of many researchers.[41] An ordered array of cadmium clusters assembled in zeolite A was prepared in an attempt to produce heavy metal clusters with different distributions and gcometries.[42] (Fig. 8)... 

The various tunable properties of zeolites have inspired a great variety of concepts in electrochemistry with zeolite-modified electrodes. For example, silver ions inside the zeolite pore system arc not electrochemically active in amperometric detection. Flowever, indirect analyte detection can occur when the analyte causes the removal of silver ions into the solution where they are electrochemically detected.[94] This indirect approach was extended to different copper-exchanged zeolites and demonstrated for the detection of several non-elcctroactive ions including alkali metal, ammonium and calcium.[95] A zeolite-modified electrode (ZME) with high selectivity towards Pb over Cd in cyclic voltammetry was prepared via electrophoretic deposition of zeolite Y, coated with Nafion.[96]... 

---