Application to Photocatalysis

Such quenching behavior suggests that dynamic quenching, whereby quencher molecules interact with the emitting sites in their excited state to give nonradiative deactivation pathways, is the principal cause of the quenching that occurs with butenes. As a result, the quenching efficiency depends on the amount of surface quencher molecules and in turn on the equilibrium pressure of the added molecules 33, 34). 

UV irradiation of the MgO catalyst in the presence of trans-l-C Hs at 273 K led to a marked enhancement in the formation of ds-l-C Un as well as in the formation of I-C4H8. The activity of the MgO catalyst for such photoinduced reactions depends strongly on the evacuation temperature of the MgO. When MgO was degassed at higher temperatures, the yields of the photoinduced isomerization reactions increased linearly with the UV irradiation time, indicating that the reactions proceeded catalytically under UV irradiation at 273 K 96-98). 

The action spectrum of the photocatalytic isomerization reactions on MgO, defined as the plot of the reaction rate vs the wavelength of the light used, shows a good agreement with the absorption spectrum, i.e., the photoluminescence excitation spectrum of the MgO (96-98, 248). The addition of O2 or CO to MgO led to the quenching of the photoluminescence. Similarly, the rates of the photocatalytic isomerization reactions on MgO were easily inhibited by the addition of CO. its extent increasing with an increase in CO pressure. 

These results indicate that the photocatalytic isomerization reactions on MgO proceed via the same excited state for the catalysis and for the photoluminescence. Thus, the photophysical processes on MgO in the presence of a mixture of 2-C4H8 and CO are described as follows (98)  

The Stern-Volmer equation (Eq. 24) is obtained for the rate of the photocatalytic isomerization of tram-2-CJc s on MgO by using the steady-state treatment = 1 + x/cq Q], where i o and i arc the yields of the 

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J, Ragai and W, Lotfi, Effect of preparative ph and aging media on the crystallographic transformation of amorphous TiOz to anatase and ratile. Colloids Surf., 61 97-109, 1991 S, Han, S, H, Choi, S, S, Kim, M, Cho, B, Jang, D, Y, Kim, J, Yoon, and T, Hyeon, Low-temperature synthesis of highly crystalline Ti02 nanocrystals and their application to photocatalysis, Small, 1 812-816, 2005... 

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