Kaolins of high iron-content as photocatalysts: Challenges of acidic surface modifications and mechanistic insights

Surface modifications of natural kaolins (Felsőpetény, Hungary) with iron-content up to 9% (m/m) were carried out by varying acid concentration (5, 8 and 11 M HCl) and treatment time (1, 3 and 6 h) in order to evaluate the influence of treatment conditions on photochemical activity, amount of low coordinate Al defect sites, porosity, and acid/base character of the surface. Nitrogen adsorption and temperature-programmed desorption measurements showed that the acid treatment slightly reduced the pore volume and the surface area, while the average pore diameter and the number of acidic centers were increased. Solid-state 27Al NMR spectroscopy demonstrated the presence of mechanistically relevant Al defect sites. The distribution, concentration, and particle size of the iron oxide co-minerals were found to be influential to photochemical activity. Greater than 90% photochemical degradation efficiency of oxalic acid can be achieved by acid-treated samples upon 3 h exposure to 8 M HCl solution with good reproducibility.