Support Surface Chemistry Evolution During the Preparation of Metal Oxide–Activated Carbon Catalysts by Wet Impregnation: A FT-IR Spectroscopy Analysis

The present work is aimed at shedding light on the evolution of surface chemistry of a commercial activated carbon (AC) support during the preparation of supported metal oxide (MO) catalysts by the conventional wet impregnation method. Particular attention is paid to the chemical changes of oxygen-containing surface functionalities across three preparation stages of impregnation, oven-drying, and thermal treatment. AC was impregnated with aqueous solutions of several MO precursors (Al(NO3)3, Fe(NO3)3, Zn(NO3)2, SnCl2, and Na2WO4) at 80 °C for 5 h, oven-dried at 120 °C for 24 h, and heat-treated at 200 °C and 850 °C for 2 h under an inert atmosphere. The surface chemistry of the resulting catalyst samples, classified in three series by the thermal treatment, was mainly studied by FT-IR spectroscopy, complemented by elemental analysis and pH of the point of zero charge (pHpzc) measurements. During impregnation, phenolic hydroxyl and carboxylic acid groups were predominantly formed by wet oxidation of chromene, 2-pyrone, and ether-type structures found in the pristine AC. The extent of these oxidations correlated with the oxidising power of the precursor solutions. As expected, thermal treatment at 850 °C brought about markedly stronger chemical changes, with most of the above oxygen functionalities decomposing and forming less acidic structures, such as 4-pyrone groups, metal carboxylates, and C-O-M atomic groupings. All these surface chemical modifications result in a lowering of the strong basicity of the raw carbon support (pHpzc ≈ 10.5), thus leading to pHpzc values for the catalysts widely ranging from 1.6 to 9.7.