Heterogeneous photocatalytic reduction of metal ions with very negative reduction potentials

The last couple of decades have seen the widespread usage of photocatalytic reactions with semiconductors such as TiO₂ as photocatalyst. This method finds application in energy conversion/storage and also in environmental remediation such as wastewater treatment and cleaning of drinking water. That most of these studies are concentrated on the photooxidation of hazardous organic substances and for improving the efficiency of photocatalysts is a fact worth mentioning. Studies on the behavior of metal ions in heterogeneous photocatalytic systems, especially on the photoreduction of metal ions with very negative reduction potentials, are rare. In the present study, Cd 2+ and Mn2+ were used as model ions to study the photocatalytic reduction behavior of such species. Novel findings include observations that: (a) certain organic additives facilitate the photoreduction of Cd 2+ and Mn2+ on the UV-irradiated TiO₂ surfaces; (b) the pH of the suspensions in these cases plays a key role; and (c) the (dark) adsorption of Cd2+ and Mn2+ on the TiO ₂ surface is facilitated by formation of surface complexes with organic additives; and (d) the oxidation state of Mn2+ is Mn 0 after the photocatalytic reaction.; It is known that coupled semiconductors can greatly increase the efficiency of photocatalytic reactions. A new method to prepare coupled semiconductors (e.g., TiO₂/CdSe, TiO₂/PbSe), namely underpotential photocatalytic deposition, has been developed. In this method, Se is first deposited photocatalytically on the surface of TiO₂, which in turn is used as a catalyst to deposit Cd2+ or Pb2+. The formation of CdSe or PbSe on the surface of TiO₂ (composite semiconductors) is a thermodynamically favorable process. The efficacy of TiO₂ and TiO₂/CdSe for the photocatalytic reduction of dichromate species and photooxidation of methylene blue has been compared. Dramatic performance improvements were observed using the coupled semiconductor configuration.