Abstract: A series of activated carbon used as supports were pretreated with different concentrations of HNO3. Pd/C catalysts were then prepared by impregnation using H2PdCl4 solution as the Pd precursor. The results of adsorption experiments in the impregnation process indicated that the equilibrium uptake of the Pd precursor on the surface of activated carbon decreased with increasing HNO3 concentration for the pretreatment. When undiluted HNO3 was used, only 62.54% of Pd precursor was adsorbed on the surface of activated carbon, and about 37.46% of Pd precursor was still reserved in the slurry. The Pd precursor equilibrium uptake was highly dependent upon the point-of-zero charge of the activated carbon. Compared with the surface oxygen anchoring mechanism, the surface charge model gives a better explanation of the adsorption experimental results. Pd particle size was initially decreased with increasing HNO3 concentration (≤5%) and then increased with further increasing HNO3 concentration (>10%). No obvious correlation between the Pd precursor equilibrium uptake and the Pd particle size was found because the particle size of Pd was also affected by Pd precursor adsorption species and their amounts. Formation of PdCly-x adsorption species was inhibited by the increase of oxygen-containing groups on the activated carbon surface. When a low HNO3 concentration (≤5%) was used for the pretreatment of activated carbon, PdCly-x and Pd0 were the main adsorption states of Pd precursor, whereas when the HNO3 concentration was high (>5%), PdCly-x was not detected.

Key words: palladium, activated carbon, supported catalyst, equilibrium uptake, particle size, point-of-zero charge, oxygen-containing group