Photoluminescence bulk catalysts

Turning to non-metallic catalysts, photoluminescence studies of alkaline-earth oxides in dre near-ultra-violet region show excitation of electrons corresponding to duee types of surface sites for the oxide ions which dominate the surface sUmcture. These sites can be described as having different cation co-ordination, which is normally six in the bulk, depending on the surface location. Ions on a flat surface have a co-ordination number of 5 (denoted 5c), those on the edges 4 (4c), and dre kiirk sites have co-ordination number 3 (3c). The latter can be expected to have higher chemical reactivity than 4c and 5c sites, as was postulated for dre evaporation mechanism. 

Photoluminescence (PL) is widely applied to investigate surfaces and surface chemical phenomena with a high degree of sensitivity. The technique provides extremely rich information when applied to the study of photoluminescence sites on bulk oxides with a large surface to volume ratio on sites located on the surface of a support, for example oxide-supported catalysts on sites that can be modified by thermal treatments (calcination, reduction, etc.) and when the local environment of the emitting sites is altered by the adsorption of molecular probes. By way of introduction, basic photophysical aspects essential for the rationalization of PL data will be summarized. 

Figure 35 shows the photolumincsccnce spectrum of CdS supported on PVG with a relatively high loading 196). Peaks are observed near 520, 560, and 680 nm. The 680-nm peak is associated with the sulfur vacancy since the presence of excess sulfide ions quenches the photoluminescence however, the presence of excess cadmium has no effect on the emission. The 520- and 560-nm photoluminescence are associated with the major bulk emission 197-199). The 520-nm emission is attributed to the band-to-band transition, and the 560-nm emission is attributed to a typical radiative clcctron-hole recombination at the particle surface. As shown in Fig. 35 (b), the addition of H2O to the catalyst has a significant effect on the spectrum. The 560-nm photoluminescence is completely quenched, as expected if the radiative recombination of electrons and holes occurs at the surfaces where H2O molecules easily interact with these electrons and holes, thereby reducing the energy and intensity of the photoluminescence. On the other hand, the 520-nm emission from the bulk emitting sites is not affected by the addition of H2O. The photoluminescence... 

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