| Skeletal nickel (Raney Ni) has been widely applied as the hydrogenation catalyst, especially in the synthesis of fine chemicals, because of their low cost and high activity. The preparation of the new Raney Ni catalysts and research on their catalytic performance are always the hot topics in catalysis area. A large amount of Raney Ni catalysts including W1-W8 have been prepared by changing both the preparation methods and alkaline leaching conditions of Ni-Al alloy. Moreover, the catalytic properties can be improved through adding other additives, known as dopants. One of the most promising tools introduced recently into heterogeneous catalysis is ultrasonic irradiation. It has been widely applied in preparation of oxide, heteropolyacid and supported catalysts.In this thesis, ultrasound was applied in the alkaline leaching process of Ni-Al mother alloy and Ni-Al-P amorphous alloy to prepare new Raney Ni and Raney Ni-P catalysts. The Mo-doped Raney Ni catalyst was prepared via impregnation of the as-prepared Raney Ni with Na2MoO4 solution followed by sonication. The hydrogenation of benzene to cyclohexane or glucose to sorbitol was used as probe reactions. The results showed that hydrogenation activity can be greatly enhanced by sonication. According to various characterizations, the promoting effect of ultrasound, amorphous structure and dopants on the catalytic performance was discussed briefly.Choosing the optimum sonication time as 15 min, the hydrogenation activity of Raney Ni increased abruptly by about 150%. While the activity of Raney Ni-P obtained through the ultrasound assisted alkaline leaching was nearly 1.5 time as high as the Raney Ni-P obtained by the regular leaching. For ultrasound-assisted Mo-doped Raney Ni catalyst, the optimum sonication time was determined as 10 min, which resulted in a nearly 0.5 time increase in the catalytic activity.According to the series characterizations including ICP, XRD, BET, SEM-EDX, XPS, DSC, EXAFS, H2-adsorption and TPD etc., together with the kinetic studies of the hydrogenation reactions, the following conclusions were drawn:1. The hydrogenation activity of the catalysts can be greatly enhanced by ultrasonic irradiation with proper time. Since ultrasound had no significant effect on the bulk structure of the as-prepared catalysts, the ultrasonic promoting effect could be mainly attributed to (1) cleaning effect, which might remove impurities on the surface and in the pore of the catalysts effectively, making more active sites exposedand in turn, enhance active surface area and enlarge catalyst pore volume and size; (2) dispersing effect, which inhibit the agglomeration of the catalyst particles effectively, resulting in the decrease of the particle size and the increase of the dispersion degree; (3) energetic effect, which supplied part of energy to keep in the catalyst, making the adsorbed hydrogen and reactant molecules activated and thus facilitate the hydrogenation. But too much energy from the ultrasound can induce the melting of the catalyst particles due to local high temperature and the collision resulted from the "bubble effect", resulting in the sintering of the small particles and even the damage of the skeletal structure, which could decrease catalytic activity.2. Under the same conditions, the catalytic activity of amorphous Raney Ni-P is higher than the crystalline Raney Ni catalyst. The promoting effect of amorphous structure could be understood by considering the following factors: (1) The uniquely short-range ordering while long-range disordering structure, together with the highly unsaturated coordination, the homogeneous distribution of active sites and the strong synergistic reaction between neighboring active sites were favorable for hydrogenation; (2) The alloying P in the Raney Ni-P could effectively protect the surface Ni active sites from oxidation; (3) Ultrasonic irradiation had no significant influence on the amorphous structure of Raney Ni-P, but sonication with suitable time may lead to the incr... |
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