RESUMEN
Two series of new photocatalysts were synthesized based on modification with Pd of the commercial P25 photocatalyst (EVONIK®). Two techniques were employed to incorporate Pd nanoparticles on the P25 surface: photodeposition (series Pd-P) and impregnation (series Pd-I). Both series were characterized in depth using a variety of instrumental techniques: BET, DRS, XRD, XPS, TEM, FTIR and FESEM. The modified series exhibited a significant change in pore size distribution, but no differences compared to the original P25 with respect to crystalline phase ratio or particle size were observed. The Pd0 oxidation state was predominant in the Pd-P series, while the presence of the Pd2+ oxidation state was additionally observed in the Pd-I series. The photoactivity tests were performed in a continuous photoreactor with the photocatalysts deposited, by dip-coating, on borosilicate glass plates. A total of 500 ppb of NO was used as input flow at a volumetric flow rate of 1.2 L·min-1, and different relative humidities from 0 to 65% were tested. The results obtained show that under UV-vis or Vis radiation, the presence of Pd nanoparticles favors NO removal independently of the Pd incorporation method employed and independently of the tested relative humidity conditions. This improvement seems to be related to the different interaction of the water with the surface of the photocatalysts in the presence or absence of Pd. It was found in the catalyst without Pd that disproportionation of NO2 is favored through its reaction with water, with faster surface saturation. In contrast, in the catalysts with Pd, disproportionation took place through nitro-chelates and adsorbed NO2 formed from the photocatalytic oxidation of the NO. This different mechanism explains the greater efficiency in NOx removal in the catalysts with Pd. Comparing the two series of catalysts with Pd, Pd-P and Pd-I, greater activity of the Pd-P series was observed under both UV-vis and Vis radiation. It was shown that the Pd0 oxidation state is responsible for this greater activity as the Pd-I series improves its activity in successive cycles due to a reduction in Pd2+ species during the photoactivity tests.
RESUMEN
TiO2 particles were prepared by sol-gel method alone and by sol-gel method combined with hydrothermal treatment. The structural and photocatalytic properties of the two series of photocatalysts were compared. XRD studies revealed that rutilization was faster in the series of photocatalysts, which had been additionally subjected to a hydrothermal process (SG-HT). The hydrothermally treated photocatalysts also displayed a higher specific surface area than those which had only been synthesized using the sol-gel process (SG) and subjected to low calcination temperatures of up to 873 K, while this tendency was inverted at higher temperatures. In accordance with the above observation, the hydrothermally treated series of photocatalysts had a lower particle size than the SG series calcined immediately after the sol-gel process up to 873 K, with this relation being inverted for the highest two temperatures which were studied (973 K and 1023 K) and which saw the commencement of rutilization. Increases in average particle size were observed for both series, with a polyhedral morphology seen as calcination temperature rose. FTIR studies highlighted the presence of the band at 2351 cm(-1) in the SG-HT photocatalysts, characteristic of surface-adsorbed CO2. This was not seen in the SG or P25 photocatalysts. In terms of photoreactivity, the best photocatalyst in the SG-HT series was that calcined at 923 K and in the SG series at 1023 K (SG-1023). Comparing these two photocatalysts and the commercial P25 photocatalyst, SG-1023 was found to be the most photoactive in both the photodegradation and the mineralization of phenol.
RESUMEN
With the goal of predicting the photocatalytic behaviour of different phenolic compounds (catechol, resorcinol, phenol, m-cresol and o-cresol), their adsorption and interaction types with the TiO(2) Degussa P-25 surface were studied. Langmuir and Freundlich isotherms were applied in the adsorption studies. The obtained results indicated that catechol adsorption is much higher than those of the other phenolics and its interaction occurs preferentially through the formation of a catecholate monodentate. Resorcinol and the cresols interact by means of hydrogen bonds through the hydroxyl group, and their adsorption is much lower than that of catechol. Finally, phenol showed an intermediate behaviour, with a Langmuir adsorption constant, K(L), much lower than that of catechol, but a similar interaction. The interaction of the selected molecules with the catalyst surface was evaluated by means of FTIR experiments, which allowed us to determine the probability of OH radical attack to the aromatic ring.