Metal-doped carbon aerogels as catalysts during ozonation processes in aqueous solutions

The efficiency of Co(II)-, Mn(II)-, and Ti(IV)-doped carbon aerogels for the transformation of ozone into OH radicals was investigated. The carbon aerogels had a markedly acid surface character (pHPZC≅3–4) with very high surface oxygen concentrations (O≅20%). X-ray photoelectron spectroscopy (XPS) analyses of the samples showed the oxidation state of the metals was +2 for Co and Mn and +4 for Ti. The presence of Mn(II)-doped carbon aerogel enhanced ozone transformation into OH radicals, whereas the presence of Co(II) and Ti(IV) carbon aerogels presented no activity in this process. Moreover, it was observed that an increase in the concentration of Mn in the surface of the aerogel increases its efficiency to transform ozone into OH radicals, with an Rct value ([OH]/[O3]) of 5.36×10−8 for the aerogel doped with 16% of surface Mn(II) compared to an Rct of 2.68×10−9 for conventional ozonation. Regardless of the aerogel used, XPS analysis of the ozonated aerogel samples showed an increase in the concentration of surface oxygen when the exposure to ozone was longer. However, presence of oxidized metal species after ozone treatment was only detected in the case of the Mn-doped aerogel, (Mn(III) and Mn(IV)). CO2 activation of carbon aerogel produced a marked increase in its efficiency to transform ozone into OH radicals compared with non-activated sample. The efficiency of Mn activated carbon aerogel to transform ozone into OH radicals was greater than that of Witco commercial activated carbon or H2O2 in the ozonation of water from Lake Zurich (Zurich, Switzerland).