Improvement in the methylene blue adsorption capacity and photocatalytic activity of H2-reduced rutile-TiO2 caused by Ni(II)porphyrin preadsorption

H2-reduced rutile-TiO2 xerogel (Ti-700), obtained via the sol–gel process, was found to strongly adsorb the Ni(II)-5,10,15,20-tetrakis(4-carboxyphenyl)-porphyrin (NiTCPP) from a methanolic solution, despite its
very low specific surface area (SBET ≈ 2 m2 g−1). UV/vis spectroscopy analysis showed that after calcination
at 700 ◦C and reduction under H2 flow at 400 ◦C, the TiO2-xerogel increased its NiTCPP-adsorption capacity
by surface area unit by up to 120 times. The effect of the porphyrin presence in the catalytic performances
of TiO2-xerogels was studied through three kinetics models: (i) the pseudo-first-order kinetic model;
(ii) the pseudo-second-order kinetic model, which are used to describe the adsorption rate based on
the adsorption capacity of the catalysts; and (iii) the Langmuir–Hinshelwood kinetic model which is
used to describe the photocatalytic degradation rate of methylene blue (MB). A significant improvement
in the efficiency of Ti-700 was observed after the porphyrin-adsorption process (NiTCPP/Ti-700): MB-adsorption
capacity at equilibrium and the apparent MB-photoconversion constant, kapp, of NiTCPP/Ti-700 were both up to 2 times higher than those observed for the Ti-700.