RESUMO
UV irradiation of an ethanolic sol of TiO2 nanoparticles produces a trapped Ti(III) centre and a noticeable Burstein shift. Direct evidence that the accumulated charges can drive the otherwise forbidden reduction of As(V) by conduction band electrons is presented.
RESUMO
Reduction of alizarin molecules coupled to TiO(2) nanoparticles (A@TiO(2)) occurs on UV irradiation in the presence of a sacrificial electron donor. Evidence is presented that reduction is mediated by conduction-band electrons and yields a 1,2,9,10-tetrahydroxyanthracene species which remains coupled to the TiO(2) nanoparticles. The spectrum of the reduced complex displays two overlapping broad bands centred at 480 and 650 nm which can harvest visible photons besides 900 nm, in agreement with theoretical predictions by TDDFT. The potential relevance of the dual-redox behaviour of strongly TiO(2) coupled anthraquinone dyes in the field of photocatalysis and in connection with their utilization in the development of dye-sensitized TiO(2) solar cells is briefly discussed.
RESUMO
Product selectivity and yields of cyclohexanol and cyclohexanone formation in the photocatalytic air oxidation of cyclohexane on TiO(2) particles were determined as a function of the irradiation wavelength (254 < or = lambda/nm < or = 366) and the photon flux (0.3 < or = I(0)/neinstein cm(-2) s(-1) < or = 5.0). Photonic efficiencies for total monooxygenated products (cyclohexanol + cyclohexanone) ranged from 10 to 25% depending on photon energy and fluency. The cyclohexanol-to-cyclohexanone ratio linearly increases with the incident photon flux at each wavelength and varies more than a magnitude order -- from 3 to 32%-at the same incident photon fluency -- 5 neinstein cm(-2) s(-1) -- by changing the irradiation wavelength from 366 to 303 nm. Experimental evidence indicates that both spectral and intensity effects emerge as a consequence of the change in the frequency of photon absorption per particle. A mechanism is proposed which accounts for the origin of the selectivity changes.
RESUMO
The particular features of the application of photoacoustic calorimetry to the determination of volume changes in reactions involving radical anions in aqueous solutions are addressed. Analysis of recent literature data on volume changes of redox pairs, including our previous results on the couples NO2(-)/NO2 and SO4(2-)/(SO4)*-, together with known values of molal volume changes for ionization reactions, allow us to derive an empirical correlation for these categories of reactions. Finally, the pertinence of describing the volume changes of complex processes as a simple sum of separable terms is critically examined.