Photocatalytic Reduction of Silver Ions on Ferric Oxides and Ferric Hydroxides

Abstract

Irradiation of an aerated aqueous suspension of silver nitrate and catalysts (ferric oxides and ferric hydroxides) with UV light (wavelength λ≥320 nm) led to the production of fine silver particles. On these catalysts, all the adsorption isotherms of silver ions fitted well to the Langmuir adsorption equation and the initial rate of Ag(I) reduction increased linearly with the increase in the initial amount of adsorbed Ag(I). The slope decreased according to the following order: α-Fe2O3 α-FeOOH γ-Fe2O3 γ-FeOOH δ-FeOOH. However, for the first three catalysts the reduction of Ag(I) only occurred when the amount of adsorbed Ag(I) reached about half the maximum coverage and also the reaction rate was nearly unaffected by N2 purging. The reduction of Ag(I) on δ-FeOOH and TiO2 upon degassing with N2 was significantly accelerated. This implies that O2 competes with silver ions for adsorption sites and reducing species on the catalyst, which is dependent on the catalyst′s properties. X-ray diffraction (XRD) analysis showed that α-Fe2O3 and δ-FeOOH were well and poorly crystallized, respectively. This indicates that the high crystallinity of the hydroxides is beneficial to the separation of photogenerated charge carriers and thus to their redox reactions with target substrates on the surface.