Metal-doped TiO$_2$ nanoparticles were prepared by the sol-gel and hydrothermal synthesis methods. The estimated quantum yields (QYs) of Pt/Cr-doped TiO$_2$ nanoparticles was ca. 0.3 \%. Cr-, Co- and N-doped TiO$_2$ showed the photocatalytic activity for IPA degradation to CO$_2$, but only Cr-doped TiO$_2$ produced H$_2$ photocatalytically in the presence of methanol-water aqueous solution under visible light ($lambda$ $>$ 420 nm). Thus, in the case of Pt/Co-doped TiO$_2$, the electron excited to the conduction band has a sufficient reduction potential to reduce H$^+$ ion, but hole in the valence band has lower oxidation potential than required for CH$_3$OH degradation to CO$_2$. Therefore, Cr- , Co- and N-doped TiO$_2$ show the different activity for the photocactalytic reaction of gases and solution phases. The results of the calculated electronic structure and experimental optical properties are correlated to schematically to describe the possible mechanism of the photocatalytic behavior of the system under study.