Semi‐Batch Photocatalytic Reduction of Nitrates: Role of Process Conditions and Co‐Catalysts

Abstract

Photocatalysis has been used to reduce nitrate ions from waste and drinking water. A semibatch photoreactor was developed for this process and commercial nanostructured TiO2 (P25) and TiO2 in nanosized form prepared by flame spray pyrolysis (FSP) were used as catalysts. Several noble metals were added as co‐catalyst on both titania samples and the chemical physical properties of every sample were studied by XRD, BET and UV‐Vis spectroscopy techniques. The optimum pH to reach the highest conversion of nitrates with lower selectivity toward ammonia was different for P25 and FSP samples, at basic pH and nearly neutral, respectively. The bare and doped FSP samples, with higher surface area, showed higher nitrates conversion with respect to P25‐based samples. The photocatalytic rate of nitrate reduction over undoped TiO2 was higher than that of most noble metal loaded TiO2, except Ag. The highest activity for nitrate conversion has been obtained by adding Ag on P25 and FSP, increasing the conversion up to 3.5 and 1.5 times with respect to the bare semiconductor, respectively.

Photocatalytic Selective Oxidation of Ammonia in a Semi-Batch Reactor: Unravelling the Effect of Reaction Conditions and Metal Co-Catalysts

“…The addition of some noble metals (e.g., Au) may also improve the light harvesting through the plasmonic effect. Furthermore, if metal addition is followed by thermal reduction in H 2 to reduce the metal, as reported, e.g., in [25], the presence of the metal itself can favor titania reduction during the activation in reducing environment, thus indirectly decreasing the titania band gap.…”

“…Indeed, the further increase of the rate of ammonia oxidation for pH higher than 9 indicates that electrostatically favored adsorption is not the rate-limiting step (Figure 5). Considering the negatively charged surface of both TiO 2 samples at high pH (9 and 11.5) and simultaneously the higher fraction of NH 3 with respect to NH 4 + at pH 11.5 than 9, it is suggested that for both P25 and FSP photocatalysts, the photooxidation of ammonia should be preferentially mediated by oxidizing radicals OH, as recently discussed [25]. neutral NH with respect to NH may explain the higher initial rates at pH 11.5 than at pH 9.0 (Figure 5).…”

“…The activity of the FSP titania was higher than that of the P25 sample, with generally lower selectivity to nitrite and nitrate. Some reversibility of the reaction can be also hypothesized, since the same catalysts can be active also for the photoreduction of nitrite and nitrate as recently demonstrated [25].…”

“…The catalysts were used as such or added with different noble metal co-catalysts (0.1 mol% Ag, Au, Pt or Pd). These catalysts were extensively characterized in a previous work and tested for their photoreductive properties [25]. Here, the activity and selectivity for the photooxidation of ammonia are instead being tested.…”

Photoreforming of Glucose over CuO/TiO2

“…5.6). Based on ζ-potential measurement, the point of zero charge of the P25 sample resulted 6.5, suggesting a slight positive charge of the surface under the operating conditions [41]. Furthermore, dynamic light scattering (DLS) showed that the average size of suspended particles ranged negligibly from 126 to 135 when passing from pH 3.5 to 11 [42].…”