Photocatalytic reduction of metals in presence of combustion synthesized nano-TiO2

The photocatalytic reduction of the metal ions, Cu(II) and Cr(VI), was investigated in presence of solution combustion synthesized (CS) nano-$TiO_2$. The efficiency of this catalyst was compared with that of commercial titania, Degussa P-25. For the reduction of $Cu^{2+}$, the first-order rate constants were 0.003 and 0.018 $min^{-1}$ in presence of Degussa P-25 and CS $TiO_2$, respectively. For the reduction of $Cr^{6+}$, the initial rate constants were 0.007 and 0.247 $min^{-1}$ in presence of Degussa P-25 and CS $TiO_2$, respectively. Thus, the rate of reduction of Cu II) and Cr(VI) is higher in CS $TiO_2$ compared to Degussa P-25 $TiO_2$. The effect of pH and the effect of initial concentration on the photocatalytic reduction of Cu(II) and Cr(VI) in presence of solution combustion synthesized $TiO_2$ and Degussa P-25 catalyst were investigated. The variation of the photoreduction rate of Cu(II) and Cr(VI) with respect to pH correlates with the variation of the driving force, which is the difference between the standard reduction potential of the metal ion and the energy level of the electrons in the conduction band. The other metal ions like Zn(II), Cd(II), Pb(II) and Mn(II) could not be reduced in both the catalytic systems and this is attributed to the thermodynamic infeasibility of their reduction by photo generated electrons.