Study On Carbon Surface Micro-Enviroment And Pd Precursor Adsorption Behavior Of Pd/C Catalyst

In order to prepare size-assembled Pd/C catalyst, investigations of Pd precursor adsorption behavior and the growth process of Pd particle size during Pd/C catalyst preparation were investigated. HNO₃, NaOH and NH₃·H₂O solutions were used for the pretreatment of activated carbon supports, and also high temperature treatment was performed for some of the carbon samples. A series of Pd/C catalysts were prepared by impregnation method using H₂PdCl₄ as precursor and the pretreated carbon as supports.During the impregnation process of Pd/C preparation, a new method Na₂S titration was proposed for the determination of Pd precursor equilibrium uptake (EPU) on the carbon surface. Adsorption experimental results indicated that Pd precursor equilibrium uptake is mainly influenced by PZC (point-of-zero-charge) value of the activated carbon. Adsorption state of Pd precursor on the surface of activated carbon was determined by H₂-TPR and XPS analysis. The results obtained suggested that Pd adsorption state is influenced by the content and distribution of surface oxygen-containing groups on the surface of activated carbon.Pd particle size of Pd/C catalyst is characterized by XRD and TEM analysis. Pd particle size of Pd/C catalyst is not highly dependent upon the PZC or EPU value, while the DTA value used to describe the content of surface acidic oxygen-containing groups and their distribution is found to be the most important factor that influences Pd particle size formed during the impregnation process of Pd/C preparation. Conclusively, Pd particle size is mainly influenced by the DTA value of the activated carbon via the altering of Pd⁰ nucleation and H₂PdCl₄ precursor diffusion rates. Although pH altering step and the reduction of Pd/C can also influence the Pd particle size, the most important factor is the Pd⁰ particles formed in the impregnation step. Furthermore, a series of particle size-assembled Pd/C catalysts were prepared for the hydrogenation of nitrobenzene and 3,4-dichloronitrobenzene, and high conversions and selectivities were obtained.