Effect of Nitric Acid Pretreatment on the Properties of Activated Carbon and Supported Palladium Catalysts

The activated carbon was pretreated with different concentrations of HNO3 (from 0.5 to 67%). The amount and type of oxygen-containing groups on the activated carbon were determined with the adsorption neutralization method and temperature-programmed desorption (TPD) of CO and CO2; furthermore, the density of total acidity (DTA) has been proposed. The Pd particle size of Pd/C catalysts was measured by X-ray diffraction patterns (XRD), CO chemisorption, and TEM. The experimental results indicated that the amount of oxygen-containing groups and the total acidity on the activated carbons, the Pd particle size and catalytic activity of Pd/C catalysts are highly dependent upon the HNO3 concentration used in the pretreatment. The pretreatment of activated carbon with a low concentration of HNO3 can increase the structure parameters due to removal of the impurities, will be beneficial to create an appropriate DTA environment, and will further improve the Pd dispersion and the catalytic activity of Pd/C catalysts. The highest catalytic activity and the finest Pd particle of Pd/C catalysts can be reached at 2.5% concentration of HNO3. A too-large amount of oxygen-containing groups assembling densely on the activated carbon will influence the Pd dispersion on the activated carbon well.