Photooxidation of naphthalenesulphonic acids in presence of transition metal-doped carbon aerogels
The aim of the present study was to analyse the activity of carbon aerogels doped with transition metals (Co(II), Mn(II) and Ti(IV)) in the photooxidation process of naphthalenesulphonic acids (1-naphthalenesulphonic (NS), 1,5-naphthalenedisulphonic (NDS) and 1,3,6-naphthalenetrisulphonic (NTS)) from aqueous medium. Results derived from the textural characterization of the aerogels showed that there is no significant influence of the metal added during preparation process (Co(II), Mn(II) or Ti(IV)) on the textural properties of the aerogels. Determination of pH of point of zero charge (pHPZC) and X-ray photoelectron emission spectroscopy (XPS) analyses of the different samples showed them to have highly acidic surfaces (pHPZC ≈ 3–4) with considerably high surface oxygen concentrations (Os ≈ 20%) regardless of the metal present in the aerogel. XPS analysis showed an oxidation state of +2 for Co and Mn, and +4 for Ti. Results obtained demonstrated that photodegradation of naphthalenesulphonic acids with UV radiation (254 nm) is not effective. Quantum yield (Φ) values at a wavelength of 254 nm were close to zero, showing the low reactivity of these compounds in the photodegradation process. In experiments using a medium-pressure lamp, it was detected that the photodegradation rate of these acids decreased as the number of sulphonic groups present in the aromatic rings increased. Thus, quantum yield values (Φ) for NS, NDS and NTS were 0.019, 0.008 and 0.002, respectively. Presence of Ti(IV) or Co(II) aerogels or of the aerogel prepared without transition metal addition did not affect the photooxidation rate of naphthalenesulphonic acids. However, the addition of Mn(II)-doped carbon aerogel accelerated the elimination rate of these contaminants. These results could indicate that Mn(II)-doped carbon aerogel may act as photocatalyst in NTS photooxidation process. A rise in the surface concentration of Mn increased the NTS photooxidation rate. Activation of Mn aerogel with CO2 markedly increased its microporosity whereas its photocatalytic activity was not enhanced. Presence of tert-butanol (OH radical scavenger) decreased the NTS photodegradation rate, in presence of the Mn aerogel, confirming that oxidation by OH radicals is the main photooxidation pathway. Moreover, the addition of hydrogen peroxide accelerated the NTS photodegradation rate; however, in order to maintain the efficiency of the added hydrogen peroxide, the appropriate dose must be used. Comparison of the results obtained from the photocatalytic activity of the Mn(II) aerogel with TiO2 during photodegradation of naphthalenesulphonic acids showed that TiO2 was more efficient.
Journal: Applied Catalysis B: Environmental - Volume 69, Issues 1–2, 1 December 2006, Pages 93–100