Photocatalytic reactions, excited states, role

The determination of the photoluminescence parameters (excitation frequency, emission frequency, Stokes shift, fine structure parameter, and lifetime) can lead to information which, at the simplest level, indicates the presence of an electronically excited state of a species, but which can be sufficiently detailed so as to lead to a clear identification and characterization of the photoluminescent sites(J6-44). Moreover, measurements of the variations in the intensity and positions of the bands as a function of time (time-resolved photoluminescence) provide valuable kinetic data representing the reactions occurring at the surface. Although most of the photoluminescence measurements have been carried out at low temperatures for specific reasons (see Section III.C.2), there is much evidence that some of the excited states involved are present even at higher temperatures and that they play an important role in catalytic and photocatalytic reactions. Therefore, it is clear that the information obtained by photoluminescence techniques is useful and important lor the design of new catalysts and photocatalysts. 

B. Dynamics of the Excited States and Their Role in Photocatalytic Reactions... 

Thus, the results obtained with vanadium oxides not only provide useful information about the advantages of the photo-CVD Method for preparing highly dispersed vanadium oxide catalysts and far achieving a high photocatalytic activity but also directly show the significant role that the charge-transfer excited triplet state of the tetrahedrally coordinated vanadium oxide species plays in photocatalytic reactions on supported vanadium oxides (115, 116). 

Zeolite catalysts incorporated or encapsulated with transition metal cations such as Mo, or Ti into the frameworks or cavities of various microporous and mesoporous molecular sieves were synthesized by a hydrothermal synthesis method. A combination of various spectroscopic techniques and analyses of the photocatalytic reaction products has revealed that these transition metal cations constitute highly dispersed tetrahedrally coordinated oxide species which enable the zeolite catalysts to act as efficient and effective photocatalysts for the various reactions such as the decomposition of NO into N2 and O2 and the reduction of CO2 with H2O into CH3OH and CH4. Investigations on the photochemical reactivities of these oxide species with reactant molecules such as NOx, hydrocarbonds, CO2 and H2O showed that the charge transfer excited triplet state of the oxides, i.e., (Mo - O ), - O ), and (Ti - O ), plays a significant role in the photocatalytic reactions. Thus, the present results have clearly demonstrated the unique and high photocatalytic reactivities of various microporous and mesoporous zeolitic materials incorporated with Mo, V, or Ti oxide species as well as the close relationship between the local structures of these transition metal oxide species and their photocatalytic reactivities. 

M. (2009) The design of Ti-, V-, Cr-oxide single-site catalysts within zeolite frameworks and their photocatalytic reactivity for the decomposition of undesirable molecules—the role of their excited states and reaction mechanisms. Catal. Today, 142, 114-124. 

Investigations of mesoporous Mo-MCM-41 molecular sieves have shown that they include the tetrahedrally coordinated Mo-oxides, which exhibits a photoluminescence spectrum attributed to the radiative decay from the charge transfer excited triplet state. Also, the photocatalytic decomposition reaction of NO into Nj was found to proceed effectively and efficiently by the coexistence of CO. The dynamic quenching of the photoluminescence spectrum by the addition of NO or CO indicates that the eharge transfer excited triplet state of the Mo-oxides having a tetrahedral coordination plays a significant role in this reaction. 

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