Anchored vanadium oxides

C. Photoluminescence and Local Structure of Anchored Vanadium Oxide... 

Figure 19 shows the typical photoluminescencc spectrum of the anchored vanadium oxide catalyst prepared by photo-CVD methods (a), its corresponding excitation spectrum (b), and the UV absorption spectrum of the catalyst (c) (56,115,116). These absorption and photoluminescence spectra (phosphorescence) are attributed to the following charge-transfer processes on the surface vanadyl group (V=0) of the tetrahedrally coordinated VO4 species involving an electron transfer from to V and a reverse radia-... 

Figure 20 shows the second-derivative spectrum of the photolumincs-cence shown in Fig. 19. The well-resolved fine structure indicates that the energy gap between the (0 -> 0) and (0 - 1) vibrational transition bands is 1040 cm in good agreement with the vibrational energy of the surface V=0 bond obtained by IR or Raman measurements. These findings indicate that the photon energy absorbed by the vanadium oxide/SiOa catatyst is mainly localized on the V=0 surface bonds.The Raman spectrum of the anchored vanadium oxide/Si02 catalyst observed at 78 K exhibits a peak at 1050 cm in good agreement with the results obtained by Went...

UV irradiation of anchored vanadium oxide/Si02 catalysts at 280 K in the presence of CO leads to the photoformation of CO2 (i.e., the photoreduction of the catalyst by CO) accompanied by small amounts of a photoin-duced adsorption (photouptake) of CO (56). After photoreduction of the anchored catalysts and further evacuation of excess CO, O2 was introduced onto the catalysts at pressures less than 530 Pa and at 280 K for a few... 

The Franck-Condon analysis of the vibrational fine structure of the photoluminescence spectrum of the anchored vanadium oxide observed at 77 K indicates that the equilibrium V-0 bond distance of the vanadyl group is elongated in the charge-transfer excited state by 0.013 nm compared with the ground state value (725). UV irradiation of the anchored vanadium oxides at 280 K in the presence of CO led to the photoformation of CO2. Since the photoformation of CO2 from CO is accompanied by the removal of oxygen from the oxide (i.e.. the photoreduction of the oxide), such an elongation of the equilibrium nuclear distance of the V-0 bond in the excited state is closely associated with the facile photoformation of CO2 on the anchored vanadium oxides. In other words, the O hole trapped centers in the electron-hole pair state of the (V -0 ) complex exhibit a high reactivity similar to 0 anion radicals 66). 

Adsorption of lUO on Highly Dispersed Anchored Vanadium Oxide Catalysts... 

B. Photoc-atalysis on SL PPOR I KD C-ATALYSTS 1. Photocatalysis on Anchored Vanadium Oxides... 

The photoluminescence of the anchored vanadium oxide prepared by the photo-CVD method was quenched in both its intensity and its lifetime by the addition of trans-2-C4H8, suggesting that the molecules interact with... 

The results obtained with vanadium oxide catalysts prepared by the impregnation method show a remarkable contrast with those obtained with anchored vanadium oxide catalysts (63,116). As shown in Fig. 64. the yields of the photocatalytic isomerization as well as the yields of the phosphorescence of the oxide increase with the content of the vanadium ions and then decrease, even when the catalyst contains 0.1 wt% V. When the vanadium content is high, an increase in the efficiency of the radiationless deactivation due to the aggregation of the vanadium oxide species is observed. 

The samples were repeatedly washed with benzene, dried and then hydrolyzed with HjO and dried again. Finally, these samples were calcined in O2 at 773 K. The concentrations of the anchored vanadium oxide samples were determined to be 1.30 (for V/Si02-sample 1),2.23 (for V/Si02-sample 2), and 3.07 (for V/Si02-sample 3) V wt %, respectively, by plasma emission spectrometry. 

Fig. 1. Luminescence spectra of anchored V/SiOj catalysts at 77 K with excitation wavelength 280 nm, 5 nm slit width and concentrations of anchored vanadium oxide 1 1.3 2 2.23 3 3.07 V wt%, [5]...

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