Hierachical Control Of Supported Pd-Based Trimetallic Catalysts For Selective Hydrogention Of Acetylene

Ethene, as an important chemical raw materials, is mainly used for the production of polyethylene and other organic chemicals. Limited by the preparation conditions, ethene feed gas produced by the thermal cracking of naphtha inevitably contains on order of 1% of acetylene. Trace acetylene could cause catalyst employed for ethene polymerization poisoning and antioxidant capacity of polyethylene products decreasing, thus it is important to effectively remove trace acetylene in ethene feed gas. Due to the simple, green and environmental-friendly process as well as low cost, catalytic hydrogenation method by Pd-based catalysts has been commonly adopted in industry. With the higher requirement for the residual content of acetylene in ethene feed gas, design and synthesis of high-performed supported Pd-based catalyst become a hot topic in ethene industry.Diversification of active components, as an effective method for synthesis of high-performed catalysts, means that introduction of other metal to single active metal facilitates the performances of catalysts because the synergetic effect between different components modifies the geometric and electronic environments of active component. In this paper, co-reduction method followed by sol-immobilization was adopted to synthesize supported ocboctahedral PdAuAg catalysts, and then the catalyst was applied to discuss the impact of diversification of active components on the performances of catalyst. Experimental results showed that PdAuAg catalyst with the atomic ratio of 1:1:2 exhibited optimal ethene selectivity in the selective hydrogenation of acetylene. In situ diffuse infrared spectroscopy of CO (in situ CO-DRIFTS) and X-ray photoelectron spectroscopy (XPS) revealed that the introduction of Au and Ag caused the quantity of non-active metal components exposed in the surface increasing, and then separated Pd active sites in catalyst increasing, meanwhile the electron density of Pd raising, these factors could facilitate ethene selectivity of trimetallic catalyst. The reduced activity of trimetallic catalyst could be ascribed to the weakened activation/dissociation abilities of H due to the reduction of continuous Pd active sites in the surface.In recent years, super-structured nanaoparticles are widely used in hydrogenation, oxidation, coupling reactions owing to their rough surfaces, rich defective sites and cooperative interactions between building blocks. In this paper, super-structured PdAuAg2 mesocrystals were fabricated via altering surfactant and reducing agent. PdAuAg2 mesocrystals with an average size of ca.41.1 nm exhibited a uniform alloy structure and typically sea urchin-like morphology, and the formation process of these mesocrystals was regarded as a result of homogeneous nucleation and oriented attachment. Defective sites in the mesocrystals, like stacking fault and grain boundary were characterized by HRTEM. Compared with ocboctahedral PdAuAg2 catalyst, mesocrystal PdAuAg2 catalyst showed better catalytic activity and ethene selectivity. Based to the results of gas sensing test, H2-temperature programmed desorption(TPD) and apparent activation energy test, the enhanced activity could be contributed to the low activation barrier and the high concentration of defective sites in the surface which facilitated the adsorption/activation of reactant molecules and the dissociation of resultant. The results of Br- capping (100) facets experiments and X-ray diffraction indicated that the improved ethene selectivity could be ascribed to the increase in the ratio of (111) to (100) facets and the formation of Pd-C phase in mesocrystal catalyst. In addition, in situ CO-DRIFTS and XPS revealed that the cooperative interactions between building blocks increased the quantity of separated Pd active sites, and then further improved the ethene selectivity.