| Pd/C catalysts are widely accepted as industrial catalysts for the liquid phase reactions, such as hydrogenation, oxidation, coupling reaction, et al., which play an irreplaceable role in the production of medicine, dyes, pesticides and many other fine chemicals. The controllable preparation of Pd/C catalysts and the improvement of their sulfur resistance, are key techniques for the successful development of many green and efficient chemical processes. Although the Pd particles could be tailored by the surface modification of carbon surface, it is still difficult to prepare Pd particles with controllable properties due to the complexity of surface oxygen-containing groups. On the other hand, Pd/C catalysts can be easily poisoned by sulfur impurities remaining in industrial raw materials, largely limiting their practical applications. In essence, these problems are closely related with the morphology of Pd nanoparticles. Therefore, this paper mainly focused on the construction and morphology change study of Pd particles, correspondingly, the researches were carried out from the following two aspects:(1) The controllable synthesis of Pd particles through the regulation of types of oxygen-containing groups on carbon surface;(2) The study of chemical morphology changes of sulfur deactivated, and regenerated Pd particles. The main research contents of this paper and the innovation are mainly manifested in the following aspects:(1) Phenolic groups were mainly introduced on carbon surface by a novel hydrothermal treatment method(T=180-240℃). The number of phenolic groups(0.106~0.290 mmol/g) increased as the increasement of hydrothermal temperatures, meanwhile, while, the average diameter of Pd particles decreased gradually(6.15~3.65 nm). Based on the characterizations of XRD, TEM, H2-TPR and the theoretical calculation, the enhanced binding between palladium clusters and oxygen groups modified carbon nanotubes(CNTs) was studied. The results suggested that phenolic group greatly enhanced the interaction between Pd particles and carbon support, which was the main reason for the improvement of Pd dispersion.(2) Subnano Pd clusters(< 2 nm) were successfully prepared on activated carbons using the “strong electrostatic interaction” between metal precursor and carbon support. The size effect of subnamometer Pd particles was investigated by using cinnamaldehyde, o-chloronitrobenzene hydrogenation as probe reactions. The results indicated that the electronic effect was dominant rather the geometric effect on hydrogenation for Pd particles smaller than 2 nm.(3) Type of oxygen containing-groups on carbon nanotubes(CNTs) was regulated by nitric acid, hydrothermal, and thermal oxidation treatment. The selectivity of cyclohexanone could be tailored between 0 and 87.3% at ~100% conversion for phenol aqueous phase selective hydrogenation. Further studies showed that the selectivity of cyclohexanone decreased as the increasement of the amount of C-O groups. The main reason was that C-O had a significant change in the adsorption and desorption behavior of cyclohexanone on catalyst surface.(4) The reasons for the deactivation of Pd/C catalyst used for the hydrogenation of industrial nitrobenzene sulfonic acid(80℃, 0.8 MPa) were studied. The results indicated that the hydrogenolysis of 3-nitrophenyl sulfone over Pd/C catalyst would generate sulfur fragments, which caused the serious deactivation of Pd/C catalyst. It was found that the initial activity of Pd/C catalyst could be substantially recovered after treating it in air at temperatures as low as 100℃. And the catalyst could be reused for at least 20 times without the significant loss of activity. The change of the valence of sulfur species on Pd surface before and after thermal oxidation were characterized by XRD, XPS, and FT-IR. The recovery of catalytic activity was attributed to the formation of high valence sulfur species with the occupation of oxygen under heating, which largely weakened the strength of Pd–S bond.(5) The reasons for the deactivation of Pd-based catalysts by thiophene using hydrogenation of naphthalene(240℃, 5.0 MPa) as a model reaction were studied. The results showed that sulfur species not only adsorbed on metal surface, but also penetrated in to the bulk of Pd. It was found that the initial activity of Pd based catalysts could be partially or completely recovered by thermal oxiadation for carbon(200℃) or γ-Al2O3(300℃), respectively. The formation and oxidation processes of palladium sulfide over two supports were characterized by XRD, XPS, and TEM. The results showed that the supports played an important role in recovering activity for Pd-based catalysts with sulfur atoms penetrating into the bulk of Pd. |
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