Photocatalysts, relationships

Conventional colloid chemistry and elaitrochemistry have always been clo ly related with each other, the keywords electrophoresis, double layer theory, and specific adsorption describing typical asp ts of this relationship. In more ro nt times, new aspects have arisen which again bring colloid chemistry into contact with modem developments in electrcolloidal particles as catalysts for electron transfer reactions and as photocatalysts. In fact, the similarity between the reactions that occur on colloidal particles and on compact electrodes has often been emphasized by calling the small particles microelectrodes . 

The relationship between the depth profiles of the metal ions of the metal ion-implanted titanium oxide photocatalysts having the same number of metal... 

In spite of a great number of investigations aimed at the preparation of photocatalysts and photoelectrodes based on the semiconductors surface-modified with metal nanoparticles, many factors influencing the photoelectrochemical processes under consideration are not yet clearly understood. Among them are the role of electronic surface (interfacial) states and Schottky barriers at semiconductor / metal nanoparticle interface, the relationship between the efficiency of photoinduced processes and the size of metal particles, the mechanism of the modifying action of such nanoparticles, the influence of the concentration of electronic and other defects in a semiconductor matrix on the peculiarities of metal nanophase formation under different conditions of deposition process (in particular, under different shifts of the electrochemical surface potential from its equilibrium value), etc. 

In the present study, highly dispersed titanium oxides included within the zeolite cavities (Ti-oxide/Y-zeolite) and framework (Ti-MCM-41,-48) were prepared using an ion-exchange method and hydrothermal synthesis to be used as photocatalysts for the reduction of CO2 with H2O at 328 K. The characterization of these catalysts by means of in situ photoluminescence, diffuse reflectance absorption, XAFS (XANES and FT-EXAFS), and ESR measurements have been carried out and special attention has been focused on the relationship between the structure of the titanium oxide species and the reaction selectivity in the photocatalytic reduction of CO2 with H2O to form CH3OH. 

It has been shown that the "sol-gel method" is an effective and fascinating way to design highly active photocatalysts [4-6]. The present study deals with the preparation of Ti/Si binary oxide photocatalysts by the sol-gel method and the relationship between the local structure of the titanium oxide species and its photocatalytic properties in the photoreduction of CO2 with H2O at 328 K. 

TEM In situ TEM reveals atomic level structure activity relationships of photocatalysts under reaction conditions. 

Figure 6 shows the relationship between the coordination number of the Ti-oxides and the selectivity for N2 formation in the photocatalytic decomposition reaction of NO on the various Ti-oxide photocatalysts. 

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. 

Pooling all of the data taken at the two flow rates provides a relation between TCE conversion and UV intensity (Figure 22.4.6). Significant scatter, which exists for both flow rates, is likely due to the rapidly varying UV intensity levels during cloud events. The variable illumination makes it difficult to establish a precise correlation between an isolated UV measurement and the U V exposure received by the gas as it flows through and reacts with the photocatalyst bed. The 10 SCFM data suggests a square root relationship between UV intensity and conversion. The square root dependence was observed in a continuous flow reactor when TCE concentrations were less than 60 ppmv (Nimlos et. al., 1993). Hi er concentrations... 

Zhang, J., Hu, Y, Matsuoka, M., Yamashita, H., Minagawa, M., Hidaka, H., and Anpo, M. Relationship between the local structures of titanium oxide photocatalysts and their reactivities in the decomposition of NO. J. Phys. Chem. B 2001, 105, 8395. 

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