Effects of the state of the active palladium component and the additives on alumina supported Pd catalyst for the total oxidation of n-hexane were investigated.
The catalysts were characterized with BET, XRD, XPS and CO chemisorption. TPO (temperature-programmed oxidation) experiments were also carried out.
The reaction rates of n-hexane oxidation over the supported palladium catalysts increased significantly when the catalysts were pre-reduced at temperatures higher than 400℃. Metallic palladium ($Pd^0$) seemed to be more active than oxidized palladium (PdO) for the oxidation of n-hexane at lower temperatures even if the palladium oxide was known to be active phase for high-temperature combustion of methane. Oxidation state and particle size of palladium were the main parameters for determining the catalytic activities. Larger particle size and reduced surface were more desirable for VOC oxidation.
The oxidation activity of reduced palladium catalysts, however, decreased gradually during oxidation reaction at lower temperatures and became equivalent to the activity of oxidized palladium catalysts. This low-temperature deactivation was attributed to the deposition of partially oxidized organics on the active sites.
M-modification (M = La, Ce and W) showed positive effects on the catalyst performance after high-temperature reduction and there was an optimum M-loading. Among these promoters, tungsten was the most effective, being regardless of impregnation sequence. The W added Pd catalysts exhibited the lower value of Pd 3d binding energy and the higher amount of CO chemisorbed than Pd only catalyst.