Photocatalytic discoloration of aqueous malachite green solutions by UV-illuminated TiO2 nanoparticles under air and nitrogen atmospheres: effects of counter-ions and pH.
Under air atmosphere, the photocatalytic discoloration of malachite green (MG) aqueous solutions (a triphenylmethane dye) in the presence of TiO(2) and UV light followed an oxidative pathway, involving an N-demethylation process evidenced by a blue shifting of the main absorption band with a maximum at 618 nm. This oxidative process was affected by the nature of the dye counter-ion and the pH of the solution. At pH 6.0, the oxidation was found to be faster than at pH 3.0, perhaps due to the poor interactions between MG and the semiconductor surface. Furthermore, with the presence of oxalate as counter-ion, the oxidative photocatalytic discoloration was negatively affected mainly at acidic pH. Under nitrogen atmosphere, some evidence was found about the double behaviour of MG when involved in the photocatalytic discoloration reactions pertaining to TiO(2) under these conditions. MG could be simultaneously oxidized, forming N-demethylated by-products, or reduced, thus leading to leuco-malachite green (LMG) (a colorless and toxic substance) as the main product. The LMG formation is favoured at low pH in the presence of oxalate as counter-ion.
Centro de Investigación y Desarrollo en Ciencias Aplicadas Dr J. J. Ronco (CINDECA), Departamento de Química, Facultad de Ciencias Exactas, UNLP-CCT La Plata, CONICET, 47 No. 257, 1900 La Plata, Buenos Aires, Argentina. julianregifo@quimica.unlp.edu.ar
Julián Andrés
JA
Rengifo-Herrera
Luis René
LR
Pizzio
Mirta Noemí
MN
Blanco
Christophe
C
Roussel
César
C
Pulgarin
D000388:Air; D002384:Catalysis; D003116:Color; D004396:Coloring Agents / Q000737:chemistry* / Q000528:radiation effects; D006863:Hydrogen-Ion Concentration; D007477:Ions / Q000737:chemistry; D053768:Metal Nanoparticles / Q000737:chemistry* / Q000528:radiation effects; D009584:Nitrogen / Q000737:chemistry*; D010084:Oxidation-Reduction; D012394:Rosaniline Dyes / Q000737:chemistry* / Q000528:radiation effects; D013056:Spectrophotometry, Ultraviolet; D014025:Titanium / Q000737:chemistry*; D014466:Ultraviolet Rays
D004396:Coloring Agents; D007477:Ions; D012394:Rosaniline Dyes; C005095:malachite green; C009495:titanium dioxide; D014025:Titanium; D009584:Nitrogen