Highly effective photocatalytic system comprising semiconductor photocatalyst and supported bimetallic non-photocatalyst for selective reduction of nitrate to nitrogen in water

Abstract

Photocatalytic reduction of NO 3 -under UV irradiation in the presence of ethanol was carried out in the presence of semiconductor photocatalyst Pt/TiO 2 and supported bimetallic non-photocatalyst Sn-Pd/Al 2 O 3 , which were dispersed in water. This system effectively and selectively promoted the photocatalytic reduction of NO 3 -to N 2 , whereas Pt/TiO 2 or Sn-Pd/Al 2 O 3 alone showed no or little activity under the reaction conditions. The decomposition rate of NO 3 -and selectivity to gaseous nitrogen compounds (mainly N 2 ) for the present photocatalytic system were higher than those for TiO 2 photocatalyst directly-modified with Sn-Pd particles.

Combinational effect of Pt/SrTiO3:Rh photocatalyst and SnPd/Al2O3 non-photocatalyst for photocatalytic reduction of nitrate to nitrogen in water under visible light irradiation

“…As we previously reported [32], in the photocatalytic reduction of NO 3 in water under UV irradiation by the reaction system comprising Pt/TiO 2 and SnPd/Al 2 O 3 , H 2 was formed by a photocatalytic reaction over Pt/TiO 2 , and non-photocatalytic reduction of NO 3 proceeded with H 2 dissolved in water over SnPd/Al 2 O 3 (Fig. 5 (a)).…”

“…5 (a)). In fact, the amount of H 2 needed in the reaction, which was calculated from the amounts of converted NO 3 -, produced NH 4 + , and N 2 , was approximately equal to the amount of H 2 evolved by the photocatalytic reaction over Pt/TiO 2 under reaction conditions similar to those for NO 3 reduction but without NO 3 - [32].…”

“…The photocatalytic performance of precious-metal-modified semiconductor photocatalysts is determined by the kind, crystalline structure, location on the semiconductor photocatalyst, and particle size of the modifying metals. Since bare semiconductor photocatalysts like TiO 2 , ZnS, and CdS are basically inactive for the photocatalytic reduction of NO 3 -, to make them show photocatalytic activity, these photocatalysts should be modified with one or more metals, including Ru, [25] Ag [20,26], Ni [27,28], Ni-Cu [29], Pt-Cu [30], Pd-Cu [21,31], and Sn-Pd [32], which are active for thermochemical, that is, non-photocatalytic, reduction of NO 3 -. For a precious-metal-modified semiconductor photocatalyst, photogenerated electrons in the semiconductor photocatalyst should be transferred to the metal particles on the semiconductor photocatalyst, where they then reduce NO 3 with H + , which are adsorbed on the surface of the metal particles.…”

Photocatalytic reduction of nitrate using titanium dioxide for regeneration of ion exchange brine

“…Titanium dioxide (TiO 2 ) is the most frequently studied photocatalyst; such studies include applications using metal-modified TiO 2 to improve the kinetics and selectivity away from undesirable aqueous by-products ( ${\mathrm{NO}}_2^{-}$, ${\mathrm{NH}}_4^{+}$) (Kudo et al, 1987; Ranjit and Viswanathan, 1997; Kominami et al, 2001; Gao et al, 2004; Zhang et al, 2005; Sa et al, 2009; Hirayama et al, 2012; Anderson, 2012; Rengaraj and Li, 2007; Gao et al, 2006; Kominami et al, 2005; Li et al, 2010). When irradiated with incident light whose energy is larger than that of the band gap of the semiconductor, electrons are excited to the conduction band ${\mathrm{e}}_{\mathrm{cb}}^{-}$ and positive holes form in the valence band ${\mathrm{h}}_{\mathrm{vb}}^{+}$.…”

“…However, the photogenerated electron–hole pairs can recombine within a few nanoseconds (Rothenberger et al, 1985; Serpone et al, 1995); this can be overcome by adding a hole scavenger (i.e., electron donor) to trap the holes, leaving the photogenerated electrons available for nitrate reduction. Common hole scavengers such as methanol (Mori et al, 1999), ethanol (Hirayama et al, 2012), oxalic acid (Kominami et al, 2001; Gao et al, 2004), acetic acid (Sa et al, 2009), formic acid (FA), and sodium formate (Zhang et al, 2005; Sa et al, 2009) have been investigated for nitrate reduction. Among them, FA exhibits the highest activity (Zhang et al, 2005; Sa et al, 2009).…”