Oxygen photostimulated adsorption

Clearly, the quantum yield is a constant only if n = 0 and m = 1. Work by Emeline et al. (1998a, 2000c) has established that the reaction orders m and n can be interdependent. That is, the reaction order m varies with reagent concentration [M] and the order n depends on photon flow p. Eor example, for the photodegradation of phenol over TiOi and the photostimulated adsorption of oxygen on ZrOi, m 1 if n O, whereas n 1 if m 0. Results of kinetic studies (see Fig. 5.17) of both photostimulated processes have shown that the interdependence of the reaction rate can be generalised by eq. 5.91, which is identical with eq. 5.65. 

Figures 5.42 and 5.43 give two more examples that demonstrate, respectively, the band-like and step-hke dependencies of the quantum yields for the photostimulated adsorption of oxygen, hydrogen and methane on dispersed MgO and Zr02 solids. The case of strong light absorption nsnally corresponds experimentally to the fnndamental absorption of light by the solid photocatalyst. Accordingly we have analysed the spectral variations of the qnantum yields for the case of intrinsic fnndamental absorption, and in particular at the fundamental absorption edge of the solids.

Figure 5.42 Absorption spectrum of overlapping absorptions of F and F colour centres in powdered ZrOi (curve 2), and quantum yields of photostimulated adsorption of molecular oxygen on photoexcitation of ZrOi in these bands. Reprinted with permission from Emeline et al. (2000a). Copyright (2000) American Chemical Society.

The experimental manifestations of the spectral variations in Fig. 5.44 are illustrated in Fig. 5.45 for the photostimulated adsorption of molecnlar oxygen on reduced and oxidised surfaces of solid ZrOa. Reduction or oxidation of the surface evidently changes the surface defectiveness, and in some cases even the surface structure. The latter also affects the surface potentials and probably also the direction of the subsurface electric field. 

Figure 5.45 Experimental spectral dependencies of quantum yields (/> of the photostimulated adsorption of oxygen on (1) oxidised and (2) reduced surfaces of Zr02.

Figure 5.46 Experimental spectral dependencies of the quantum yields (f) of photostimulated adsorption of (1) oxygen and (2) hydrogen on the surface of Degussa P25 Ti02. Reprinted with permission from Emeline et al. (2002a). Copyright (2002) Elsevier Science S.A.

---