1,720,961 research outputs found
Enhanced photocatalytic formation of hydroxyl radicals on fluorinated TiO2
No full textDirect experimental evidence of the higher concentration of hydroxyl radicals generated on fluorinated titanium dioxide (F - TiO2) under irradiation was obtained by spin-trapping EPR measurements. The faster photoinduced bleaching of the azo dye Acid Red 1 (AR1) observed in the presence of F - TiO2 was explained by the high affinity of the azo double bond towards OH radicals. Moreover, the pronounced decrease of the AR1 bleaching rate by addition of 2-propanol, as hydroxyl radicals scavenger, on F - TiO2 and not on naked TiO2 demonstrated that on fluorinated titania AR1 is mainly degraded via OH radical attack
Photocatalytic degradation of formic and benzoic acids and hydrogen peroxide evolution in TiO2 and ZnO water suspensions
No full textThe photocatalytic degradation of formic acid (FA) and benzoic acid (BA), chosen as model organic molecules with acidic properties, was investigated in TiO2 and ZnO water suspensions under different experimental conditions. Hydrogen peroxide evolution, formed through a reductive pathway started by conduction band electrons, was also simultaneously monitored during the degradation runs. The effect of different initial amounts of substrates and the dependence of the reaction rate on the initial pH of the TiO2 suspensions was interpreted under the light of a pseudo-steady state Langmuir-Hinshelwood rate form and of the electrostatic interactions occurring at the water-semiconductor interface. ZnO appeared a more effective photocatalyst than TiO2 for BA, but not for FA degradation. A much higher amount of hydrogen peroxide was detected in ZnO irradiated suspensions, both in the presence and in the absence of the substrates, mainly because of its lower photocatalytic decomposition rate on such oxide. The rate of hydrogen peroxide evolution during the photocatalytic oxidation of BA on TiO2 could be related to the rate of the oxidation process, while H2O2 could not be detected during the photocatalytic degradation of FA on this oxide, mainly because of the reduced shielding ability of this substrate
Effects of iron species in the photocatalytic degradation of an azo dye in TiO2 aqueous suspensions
No full textThe effects of Fe(III) species on the photocatalytic degradation of azo dye Acid Red 1 (AR1) have been studied in titanium dioxide aqueous suspensions under irradiation in the 315- 400 nm range. The initial increase of the photocatalytic degradation rate observed in water suspensions containing Fe(III) aquo ions (10-5 to 10-4 M) was attributed to the increased amount of dye adsorbed on the iron(III)-modified semiconductor surface. This was confirmed by the fact that iron species not adsorbed on the semiconductor, such as ferrioxalate complexes and Fe(II) species, had no kinetic effects. The mineralization kinetic profiles obtained under simultaneous sonication further confirmed the role of AR1 adsorption. The accumulation of hydrogen peroxide during the photocatalytic degradation of the dye was completely suppressed in the presence of all iron species, mainly due to the Fenton reactions consuming H2O2 in the aqueous phase, although a decrease in the rate of H2O2 formation cannot be excluded, due to the competition between adsorbed Fe(III) species and adsorbed oxygen for photo-promoted conduction band electrons
Effects of gold nanoparticles deposition and of surface fluorination on the photocatalytic activity of titanium dioxide in aqueous suspensions
No full textH2O2 evolution during the photocatalytic degradation of organic molecules on fluorinated TiO2
No full textThe effect of TiO2 surface fluorination on the hydrogen peroxide evolution occurring in photocatalytic runs was investigated employing the azo dye Acid Red 1 (AR1) and two model organic molecules with acidic properties, i.e. formic acid (FA) and benzoic acid (BA), as substrates of oxidative degradation. While AR1 and BA photocatalytic degradation on fluorinated titanium dioxide (F-TiO2) was markedly faster than on unmodified TiO2, because of enhanced hydroxyl radical formation, H2O2 concentration during the photodegradation of both substrates on F-TiO2 was lower, possibly because of the reduced rate of interfacial electron transfer. By contrast, FA underwent slower photocatalytic degradation on F-TiO2, but, at the same time, hydrogen peroxide concentration was relatively high, while no H2O2 could be detected during FA photodegradation on unmodified TiO2. Photocatalytic runs in the presence of the nitrate anion, able to react with the CO2•- species produced from FA oxidation, but not with conduction band electrons, demonstrated that CO2•- plays a relevant role in H2O2 formation during FA degradation on F-TiO2. In fact, surface fluoride, having a shielding effect at the semiconductor-water interface, not only inhibits the photocatalytic decomposition of H2O2, but also favours CO2•- desorption and reaction with dissolved O2, generating H2O2. By contrast, CO2•- mainly gives electron transfer to the conduction band of naked TiO2 and surface reduction of the photocatalytically produced H2O2
Photodegradation of an azo dye and hydrogen peroxide evolution on fluorinated titanium dioxide
No full textThe photodegradation of the azo dye Acid Red 1 (AR1), but not its photomineralization, on fluorinated titanium dioxide is faster than on unmodified TiO2, while the rate of formic acid (FA) photocatalytic degradation is lower. The formation of H2O2, generated by reduction of O2 by conduction band electrons also on F-TiO2, is inhibited during AR1 photodegradation in the presence of F- because of the reduced interfacial electron transfer, while the higher concentration of H2O2 detected during FA photodegradation is due to a shielding effect of fluoride. The OH radical scavengers 2-propanol and formic acid induce a decrease in AR1 photodegradation, more pronounced on F-TiO2, evidencing the key role of hydroxyl radicals. Thus, the rate of photocatalytic degradation of substrates with a high affinity towards hydroxyl radical is enhanced on F-TiO2
Degradation of organic water pollutants through sonophotocatalysis in the presence of TiO2
No full textThe degradation of 2-chlorophenol and of the two azo dyes acid orange 8 and acid red 1 in aqueous solution was investigated kinetically under sonolysis at 20 kHz and under photocatalysis in the presence of titanium dioxide particles, as well as under simultaneous sonolysis and photocatalysis, i.e. sonophotocatalysis. The influence on the degradation and mineralisation rates of the initial substrate concentration and of the photocatalyst amount was systematically investigated to ascertain the origin of the synergistic effect observed between the two degradation techniques. The evolution of hydrogen peroxide during kinetic runs was also monitored. Small amounts of Fe(III) were found to affect both the adsorption equilibria on the semiconductor and the degradation paths. Ultrasound may modify the rate of photocatalytic degradation by promoting the deaggreagation of the photocatalyst, by inducing the desorption of organic substrates and degradation intermediates from the photocatalyst surface and, mainly, by favouring the scission of the photocatalytically and sonolytically produced H2O2, with a consequent increase of oxidising species in the aqueous phase
Effects of Au Nanoparticles on TiO2 in the Photocatalytic Degradation of an Azo Dye
No full textThe photocatalytic activity of Au modified titanium dioxide was evaluated in the photodegradation of the azo dye Acid Red 1 (AR1) under 254 nm irradiation. Noble metal nanoparticles were deposited on TiO2 either through deposition-precipitation (DP), or by immobilisation of preformed metallic sols (polyvinylalcohol (PVA)/NaBH4 or tetrakis(hydroxymethyl)phosphonium chloride (THPC)/NaOH systems). Gold nanoparticles on the photocatalyst surface had dimensions of around 3-4 nm in diameter, as determined by HRTEM analysis, and exhibited visible light plasmon absorption. THPC Au/TiO2 appears to be the most photoactive amongst the photocatalysts with a 1 wt.% Au loading, while among THPC samples with different Au loadings (0.5-20.0 wt.%) the maximum photoactivity was attained with 5 wt.% Au/TiO2. The higher AR1 photodegradation rate observed on Au/TiO2 at basic pH can be related to the higher concentration of hydroxyl anions at the interface: these are able to effectively scavenge photoproduced valence band holes, possibly in competition with Au0 oxidation to Au+
- …
