Study of the catalytic wet air oxidation of p-hydroxybenzoic acid on a fresh ruthenium catalyst supported by different oxides.

The catalytic wet air oxidation (CWAO) of p-hydroxybenzoic acid (p-HBA) was conducted in a batch reactor at 140 °C, and at a total air pressure of 50 bar over Ru-based catalysts. Four materials were selected as supports - TiO, CeO-TiO, ZrO-TiO and LaO-TiO - all of which had mesopores in their texture and pollutant adsorption capacities. The supports were prepared by the sol-gel method, and then impregnated with 3 wt% of Ru precursor. Such characterization techniques as N-sorption, XRD, XPS, H-TPR, NH-TPD, TEM, and HAADF-STEM were used to analyze the different solids. The correlation between catalytic activities and physicochemical properties was discussed. A significant specific surface area (S), a large amount of surface-active oxygen, and Lewis acidity sites were observed on cerium-containing catalysts (Ru/CeTi). Fresh Ru catalysts containing cerium showed higher activity than Ru/TiO, Ru/ZrTi, and Ru/LaTi catalysts. It is assumed that the acidic sites and surface oxygen trap the p-HBA molecule, thus increasing the catalytic properties of the Ru particles which interact with the surface oxygen through the cerium redox process (Ce/Ce). As the presence of cerium increases surface-active oxygen, it inhibits the deposition of carbon on the surface of the Ru catalyst. The pseudo-second order (PSO) model adequately described the kinetic data of the p-HBA oxidation reaction using Ru catalysts.