The impeding effect of dissolved oxygen on the photocatalytic reduction of [PdCln(H2O)4−n]2−n (n = 0–4) complexes employing different TiO2 photocatalysts in the absence of an added sacrificial reducing agent

Abstract

The photocatalytic reduction of chloro-aqua palladium(II) complexes, [PdCln(H2O)4−n]2−n (n=0–4), as a means of palladium metal recovery and waste water purification, was investigated employing four different titanium dioxide samples in a batch reactor. These suspensions were subjected to UV irradiation in the absence of an added sacrificial reducing agent (SRA) for oxygen purged, air equilibrated and nitrogen purged solutions. A commercial Degussa P25 titanium dioxide powder (DP25) was taken as standard photocatalyst and compared to three other commercial titanium dioxide powders, namely Tioxide A-HR (AHR), Sigma–Aldrich Anatase (SAA) and Sigma–Aldrich Rutile (SAR). The highest rate of photocatalytic reduction was observed for the lower chlorinated Pd(II) complexes in nitrogen, i.e. oxygen depleted solutions, at constant temperature, photocatalyst load, pH and ionic strength on all four photocatalysts. Sigma–Aldrich Rutile (SAR) was the only photocatalyst capable of reducing the highly chloride coordinated [PdCl4]2− complex, in the absence of an added organic SRA, in air equilibrated and nitrogen purged solutions. The presence of dissolved oxygen was found to be detrimental to the photocatalytic reduction of highly chloride coordinated chloro-aqua palladium(II) complexes. The effect of the structure of the complex as well as the photocatalyst surface area, crystal phase composition, aggregate particle size and band gap energy was explored in an effort to elucidate the observed differences in behaviour of the photocatalysts.