INTRODUCTION The development of efficient processes in photocatalysis is a challenging task. In particular, N-containing pollutants, such as inorganic ammonia, nitrites and nitrates, and some organic N-containing compounds (dyes, pesticides, drugs, etc.), are harmful contaminants for drinking water, inducing acute and/or chronic diseases, especially affecting infants and children. Furthermore, when released in waste waters, they contribute to eutrophication, or possibly contaminate ground water. We developed innovative photocatalytic processes for the abatement of N-compounds, focusing on selectivity towards innocuous N2, to be applied for the treatment of waste waters to meet legislative specifications. The photocatalytic performance of the samples, to be correlated with nanomaterials properties, has been checked for the photoreduction of nitrate ions and the photooxidation of ammonia. EXPERIMENTAL TiO2 has been prepared in nanosized form by using an innovative flame pyrolysis (FP) approach, able to synthesise single or mixed oxide nanoparticles, characterized by homogeneous particle size and good phase purity. The TiO2 samples was prepared from a solution of Titanium(IV)-isopropylate (Aldrich, pur. 97 %) in xylene and propionic acid. Several metals such as Pd and Ag were added as co-catalysts with a 0.1-0.5 wt% concentration by wet impregnation from a solution of nitrate precursors. RESULTS AND DISCUSSION Photocatalytic reduction of NO3− and oxidation of NH3/NH4+ in water were carried out in a specifically designed Pyrex reactor, with a top quartz window The catalysts were suspended in an aqueous solution of NH4Cl or NaNO3. The reactor was operated in semi-batch mode: the solution containing the pollutant to be photoconverted was added at the beginning of the reaction in batch mode, whereas a gas stream continuously flowed through the reactor. The gas was composed by He during the conditioning-outgassing phase preliminary to every measurement. He was fed in continuous mode also during the nitrate photoreduction tests, whereas it was substituted by synthetic air (80 vol% He + 20 vol% O2) during the ammonia photooxidation experiments. A trap for ammonia, possibly stripped from the reactor, was placed downstream. Comparison with commercial nanostructured P25 TiO2 revealed several differences. The TiO2 sample prepared by FP exhibited higher activity with respect to the commercial sample, achieving ca. 20 % conversion of NH3 after 5 h, without indication of decay or deactivation. By contrast, the commercial sample was less active and most of all its activity was rapidly ruled out. Some induction period was observed, due to the need of sample conditioning upon irradiation. The most interesting point is that no trace of nitrites or nitrates was observed, confirming a full selectivity to the desired product, i.e. molecular N2. The addition of Pd further improved the conversion up to 32%. The photoreduction of nitrates was also tested over the same samples. Very low conversion (<5%) was achieved with pure TiO2 catalysts, whereas Pd doping improved conversion by ca. one order of magnitude under the same experimental conditions. The key problem remained process selectivity: 100% to the undesired NH3 for the undoped photocatalyst, max 30% for the Pd-doped sample prepared by FP. CONCLUSION These results on both reactions let us conclude that a two step process can be designed to cope with the insufficient selectivity to N2 during the nitrate photoreduction step. The ammonia produced is removed during a second, fully selective step of photooxidation. The flame pyrolysis procedure is a viable technique for the preparation of either bare or metal-doped semiconductors in nanosized form, to be used for the photocatalytic abatement of inorganic N-containing pollutants in waste or drinking waters. ACKNOWLEDGMENTS The financial support of Fondazione Cariplo (Italy) through the measure “Ricerca sull’inquinamento dell’acqua e per una corretta gestione della risorsa idrica”, project “DEN - Innovative technologies for the abatement of N-containing pollutants in water”, grant no. 2015-0186, is gratefully acknowledged, as well as the valuable help of Veronica Praglia and Alberto Riva.

Nanostructured photocatalysts for the photooxidation of ammonia and photoreduction of nitrates from waste waters / I.G. Rossetti, M. Compagnoni, E. Bahadori, A. Tripodi, G. Ramis, F. Freyria, M. Armandi, B. Bonelli. ((Intervento presentato al convegno ANM tenutosi a Aveiro nel 2017.

Nanostructured photocatalysts for the photooxidation of ammonia and photoreduction of nitrates from waste waters

I.G. Rossetti;M. Compagnoni;E. Bahadori;A. Tripodi;
2017

Abstract

INTRODUCTION The development of efficient processes in photocatalysis is a challenging task. In particular, N-containing pollutants, such as inorganic ammonia, nitrites and nitrates, and some organic N-containing compounds (dyes, pesticides, drugs, etc.), are harmful contaminants for drinking water, inducing acute and/or chronic diseases, especially affecting infants and children. Furthermore, when released in waste waters, they contribute to eutrophication, or possibly contaminate ground water. We developed innovative photocatalytic processes for the abatement of N-compounds, focusing on selectivity towards innocuous N2, to be applied for the treatment of waste waters to meet legislative specifications. The photocatalytic performance of the samples, to be correlated with nanomaterials properties, has been checked for the photoreduction of nitrate ions and the photooxidation of ammonia. EXPERIMENTAL TiO2 has been prepared in nanosized form by using an innovative flame pyrolysis (FP) approach, able to synthesise single or mixed oxide nanoparticles, characterized by homogeneous particle size and good phase purity. The TiO2 samples was prepared from a solution of Titanium(IV)-isopropylate (Aldrich, pur. 97 %) in xylene and propionic acid. Several metals such as Pd and Ag were added as co-catalysts with a 0.1-0.5 wt% concentration by wet impregnation from a solution of nitrate precursors. RESULTS AND DISCUSSION Photocatalytic reduction of NO3− and oxidation of NH3/NH4+ in water were carried out in a specifically designed Pyrex reactor, with a top quartz window The catalysts were suspended in an aqueous solution of NH4Cl or NaNO3. The reactor was operated in semi-batch mode: the solution containing the pollutant to be photoconverted was added at the beginning of the reaction in batch mode, whereas a gas stream continuously flowed through the reactor. The gas was composed by He during the conditioning-outgassing phase preliminary to every measurement. He was fed in continuous mode also during the nitrate photoreduction tests, whereas it was substituted by synthetic air (80 vol% He + 20 vol% O2) during the ammonia photooxidation experiments. A trap for ammonia, possibly stripped from the reactor, was placed downstream. Comparison with commercial nanostructured P25 TiO2 revealed several differences. The TiO2 sample prepared by FP exhibited higher activity with respect to the commercial sample, achieving ca. 20 % conversion of NH3 after 5 h, without indication of decay or deactivation. By contrast, the commercial sample was less active and most of all its activity was rapidly ruled out. Some induction period was observed, due to the need of sample conditioning upon irradiation. The most interesting point is that no trace of nitrites or nitrates was observed, confirming a full selectivity to the desired product, i.e. molecular N2. The addition of Pd further improved the conversion up to 32%. The photoreduction of nitrates was also tested over the same samples. Very low conversion (<5%) was achieved with pure TiO2 catalysts, whereas Pd doping improved conversion by ca. one order of magnitude under the same experimental conditions. The key problem remained process selectivity: 100% to the undesired NH3 for the undoped photocatalyst, max 30% for the Pd-doped sample prepared by FP. CONCLUSION These results on both reactions let us conclude that a two step process can be designed to cope with the insufficient selectivity to N2 during the nitrate photoreduction step. The ammonia produced is removed during a second, fully selective step of photooxidation. The flame pyrolysis procedure is a viable technique for the preparation of either bare or metal-doped semiconductors in nanosized form, to be used for the photocatalytic abatement of inorganic N-containing pollutants in waste or drinking waters. ACKNOWLEDGMENTS The financial support of Fondazione Cariplo (Italy) through the measure “Ricerca sull’inquinamento dell’acqua e per una corretta gestione della risorsa idrica”, project “DEN - Innovative technologies for the abatement of N-containing pollutants in water”, grant no. 2015-0186, is gratefully acknowledged, as well as the valuable help of Veronica Praglia and Alberto Riva.
Settore ING-IND/25 - Impianti Chimici
Nanostructured photocatalysts for the photooxidation of ammonia and photoreduction of nitrates from waste waters / I.G. Rossetti, M. Compagnoni, E. Bahadori, A. Tripodi, G. Ramis, F. Freyria, M. Armandi, B. Bonelli. ((Intervento presentato al convegno ANM tenutosi a Aveiro nel 2017.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/2434/618534
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