Pd supported on mesoporous activated carbons with high oxidation resistance as catalysts for toluene oxidation

Mesoporous activated carbons were obtained by chemical activation of kraft lignin with H3PO4 and used as supports for the preparation of carbon-based Pd catalysts with low palladium content (0.5%). The catalytic properties of the carbon-based Pd samples were evaluated in the catalytic oxidation of toluene. The effect that a thermal treatment at 900 °C in inert atmosphere carried out before and after the Pd-deposition produced on the structure and activity of the catalysts was analyzed. The catalysts obtained show high external surface areas and mesopore volumes. The chemical activation with H3PO4 yielded carbon supports with a significant amount of surface phosphorus, in form of COPO3, CPO3 and C3PO groups. These phosphorus groups act like physical barrier, increasing the oxidation resistance of the catalysts and avoiding the burn-off of the carbon substrate during the oxidation of the VOCs. TEM analysis confirmed the presence of well-dispersed Pd particles, with sizes between 5 and 12 nm. A kinetic study of the catalytic oxidation of toluene was performed. The reaction seems to proceed through a Langmuir–Hinshelwood mechanism, whose rate-limiting step is the surface reaction between adsorbed toluene and oxygen adsorbed dissociatively, with an activation energy value of 83 kJ mol−1. Toluene and xylene were oxidized to CO2 and H2O in the temperature range of 150–400 °C at a space velocity of 19,000 h−1.