Photoluminescence surface sites

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. 

This review deals with the applications of photolurainescence techniques to the study of solid surfaces in relation to their properties in adsorption, catalysis, and photocatalysis, After a short introduction, the review presents the basic principles of photolumines-cence spectrosajpy in relation to the definitions of fluorescence and phosphorescence. Next, we discuss the practical aspects of static and dynamic photoluminescence with emphasis on the spectral parameters used to identify the photoluminescent sites. In Section IV, which is the core of the review, we discuss the identification of the surface sites and the following coordination chemistry of ions at the surface of alkaline-earth and zirconium oxides, energy and electron transfer processes, photoluminesccncc and local structure of grafted vanadium oxide, and photoluniinescence of various oxide-... 

Originally, photoluminescence spectroscopy was applied to characterize the local coordination of metal ions as well as to probe structural perturbations that occur due to alkaline earth and rare earth metal ions in oxides such as silica and alumina. Emphasis has turned to elucidating the mechanisms of catalytic and photocataljTic reactivity, i.e., the characterization, at the molecular level, of the active surface sites as well as the significant role of these sites in catalysis and photocatalysis. 

It has also been shown that the photoluminescence spectrum of MgO observed at approximately 330-420 nm appears upon surface dehydroxyla-tion 98). In other words, the intrinsic surface sites which appear upon removal of the surface OH groups [Eq. (14) are linked to the photoluminescence observed with the well-degassed powdered MgO catalysts 96-98) ... 

Degassing ZrOa at high temperatures leads to the appearance of an abnormal absorption and photoluminescence spectra which could be attributed to the formation of surface sites in low coordination or coordinatively unsaturated surface sites (see Section lV.A.2.b) (101-104). Moreover, Zr02... 

Figure 58 shows the photoluminescence spectrum of the undoped MgO degassed at the same temperature as the methane oxidative coupling reaction together with the photoluminescence speclrmn of the 3 mol% Li-doped MgO (Fig. 58, 2) and its deconvoluted curves (Fig. 58, 2-a and 2-b). In addition to a characteristic photolumincscence spectrum at around 370 nm, attributed to the surface sites in low coordination on MgO, the Li-doped MgO exhibits a new photoluminescence band at about 350-550 nm with a at about 450 nm (Fig. 58, 2-b). The intensity of this new emission depends on the amount of Li doped. The excitation spectr um corresponding to this new emission is evident at about 260-290 nm 100, 240), which suggests that surface sites with a coordination number of four may be associated with this new photoluminescence. 

Figure 60 also show s the Stcrn-Volmer plots for the quenching of the photoluminescence intensity. There is good agreement between the Stern-Volmer plots for the photoluminescence intensity and the yields of the photocatalytic isomerization reactions, indicating that both the photocatalytic reaction and the photoluminescence proceed through the same excited state of MgO, i.e., the charge-transfer excited state on the four-coordinated surface sites on MgO (96-98, 240). 

Anpo and Che write about applications of photoluminescence techniques, which are powerful but only seldom used methods for the identification of surface sites and their local environments, particularly on oxide surfaces. The dynamics of energy and electron transfer processes are discussed in light of catalytic and photocatalytic phenomena. 

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. 

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