【作者】 李亚男；

【导师】 刘昌俊；

【作者基本信息】 天津大学 ， 化学工艺， 2012， 博士

【摘要】 本博士论文利用辉光放电等离子体和射频放电等离子体技术研究了负载Pd金属催化剂的还原与改性效应。制备和表征的催化剂包括双金属Pd-M/HZSM-5（M= Ni, Co, Cu, Ag）催化剂体系、Pd/TiO₂催化剂体系和Pd/Al₂O₃催化剂体系,考察的催化反应涵盖了甲烷催化燃烧反应及乙炔催化加氢反应。为了对催化剂及催化过程进行全面的表征,本论文使用了N2物理吸附（N2 sorption）、X射线光电子能谱（XPS）、X射线衍射（XRD）、原位X射线衍射（in situ XRD）、透射红外光谱（FT-IR）、差示扫描量热（H₂-DSC）、透射电子显微镜（TEM）、H₂化学吸附（H₂ chemisorption）、H₂程序升温脱附（H₂-TPD）、CO吸附的漫反射红外光谱（CO-DRIFT）、C₂H₄低温程序升温脱附（C₂H₄-SAT-TPD）等实验手段。为了从结构和理论角度进一步研究等离子体对负载型Pd催化剂还原及改性作用,我们利用密度泛函理论对等离子体改性后的TiO₂与金属Pd之间的作用及乙烯的吸附模型进行了系统的研究。与此同时,从水合电子的角度对负载型Pd催化剂上的金属盐前驱体的还原过程机理也进行了系统的研究。利用Ar辉光放电等离子体还原技术和传统等体积浸渍相结合的方法,成功地制备了一系列Pd基双金属Pd-M/HZSM-5（M= Ni, Co, Cu, Ag）催化剂。一系列表征证明了辉光放电等离子体过程不但不会破坏载体HZSM-5的微孔骨架结构,还可以有效地还原催化剂表面的双金属物种,改善催化剂表面的PdO颗粒分散度和稳定性,提高催化剂样品在甲烷催化燃烧反应中的低温催化活性和稳定性。辉光放电等离子体能显著增强催化剂上活性组分PdO与载体之间的相互作用,在反应过程中没有发生PdO的团聚烧结和向金属态Pd的还原。对比四种Pd基双金属催化剂,Pd-Ag/HZSM-5样品的初始反应活性、稳态反应活性和20 h的稳定性表现最好。利用H₂、Ar和O₂放电射频等离子体处理Pd/TiO₂催化剂,并对催化剂的表面性质作了较为全面的研究。H₂放电等离子体处理过程的还原效果优于Ar放电处理,并且处理后的Pd/TiO₂催化剂更容易引入金属活性组分与载体TiO₂的强相互作用（SMSI）,降低Pd对C₂H₄和H₂的吸附能力,在乙炔催化加氢反应中获得了较好的选择性。O₂放电等离子体处理可以增强Pd/TiO₂催化剂表面的钯前驱体物种与载体之间的相互作用（MPSI）,当经过200°C的H₂还原后,催化剂表面也产生了较强的金属—载体相互作用,提高了催化剂的催化反应性能,使Pd/TiO₂-OP表现出最佳的催化反应活性和乙烯选择性。利用密度泛函的计算工具,我们进一步验证了实验表征的结果。H₂、Ar和O₂放电等离子体处理使Pd/Al₂O₃催化剂表面的Pd分散度都得到了很大提高,Pd颗粒粒径变小。经过O₂放电等离子体处理后,金属前驱体和载体之间的相互作用增强。由于经过H₂放电等离子体处理的Pd/Al₂O₃上吸附的C₂H₄更容易发生脱附,因此在反应评价中的反应活性和选择性综合表现最好。利用密度泛函理论研究了水合电子团簇[（H₂O）n]-（n=1-6）,水合离子团簇[Pd（H₂O）n]²⁺（n=1-6）及水合电子还原Pd²⁺的过程。当具有高还原性的水合电子与钯离子Pd²⁺发生还原反应时,Pd²⁺首先与水分子发生水合化得到水合离子[Pd（H₂O）₄]²⁺团簇,再通过水合电子的还原作用得到[Pd（H₂O）₄] ,即:[Pd（H₂O）₄]²⁺→[Pd（H₂O）₄]+→[Pd（H₂O）₄]。通过对还原反应能量的计算,我们进一步验证了等离子体放电条件下产生具有高还原性的水合电子可以还原金属Pd²⁺,并且所放出的热量提供了进一步还原反应所需要克服的能垒,因此等离子体还原过程一旦激发后可以在室温下顺利进行直至将离子还原为金属态。更多还原

【Abstract】 This dissertation focuses on using glow discharge plasma and radio frequency plasma technology to study preparation and modification of supported Pd metal catalysts. The prepared and characterized metal catalysts cover bimetallic Pd-based catalysts including Pd-M/HZSM-5 （M= Ni, Co, Cu, Ag）, Pd/TiO₂ and Pd/Al₂O₃. The investigated catalytic reactions contain methane combustion and selective hydrogenation of acetylene. In order to fully characterize the catalysts and catalytic process, a series of techniques were employed, including N2 sorption, X-ray photoelectron spectroscopy （XPS）, X-ray diffraction （XRD）, in situ XRD, Fourier transform infrared spectroscopy （FT-IR）, differential scanning calorimetry （DSC）, transmission electron microscope （TEM）, H₂ temperature-programmed desorption （H₂-TPD）, H₂ chemisorption, CO diffuse reflectance Fourier transform infrared spectroscopy （CO-DRIFT）, C₂H₄ subambient temperature-programmed desorption （C₂H₄-SAT-TPD） and so on.From the viewpoint of theoretical chemistry, the plasma catalysis technique can be classi?ed into two main functions of reduction and modification. First, with using density functional theory method （DFT）, we detailedly studied the interaction of C₂H₄ adsorption on the Pd4 cluster which is loaded on perfect and defective anatase TiO₂（101） surface. And then, representative models of hydrated election and hydrated Pd ions regarding the reduction process were studied to clarify the advantages of molecular simulation. It is essential to understand the interactions between plasma and catalysis. Moreover, finding out the main reasons leading to the beneficial effect could provide useful information for further enhancement of performance as well.Highly dispersed Pd-M/HZSM-5 （M= Ni, Co, Cu, Ag） catalysts have been synthesized via conventional incipient wetness impregnation followed by novel glow discharge plasma reduction. In the present work, plasma treatment could not only maintain the microporous framework of HZSM-5, but achieve better dispersion and stabilization of PdO particles on the HZSM-5 as well, which lead to a higher initial activity and stability in reaction tests. Also the stronger interaction between PdO and support favored the stability performance, avoiding the sintering or reduction of PdO. Among the investigated catalysts, the Pd-Ag/HZSM-5 sample exhibits the best activity for methane combustion.Non-thermal RF plasma modification has been applied to Pd/TiO₂ catalysts. It is confirmed that supported Pd precursors could be effectively reduced to the metallic state during the room temperature plasma treatment in H₂ and Ar. Plasma treatments also enhanced the surface active sites of Pd/TiO₂ catalysts and improved the dispersion of Pd metal particles. In addition, plasma treatments could induce strong metal-support interaction with lower reduction temperature （200°C）, accompanied with the reduced adsorption capacity of H₂ and C2H4, which lead to an enhanced catalytic performance on selective hydrogenation of acetylene. On the basis of the experimental results of inducing SMSI and minimizing the detrimental effects of high temperature calcinations by O₂ plasma at room temperature, theoretical study was applied to confirm the enhanced effect on selectivity performance.The present theoretical calculations on hydrated electron and hydrated Pd ions were designed to clarify explicitly the exothermic reduction process from Pd²⁺ to Pd0 or [Pd（H₂O）₄]²⁺ to [Pd（H₂O）₄] via metastable state. Since the non-thermal plasma is a mixture of electrons, highly excited atoms and molecules, ions, radicals, etc., the hydration configurations, electron density and HOMO of the given clusters were also considered. We extrapolated two specific processes that the hydrated electron reduced the Pd²⁺ from precursor to the target Pd0. It is likely that once the plasma was excited, the reduction could occur gradually at room temperature, which is still lack of a theoretical proof and detailed description by any other groups as yet.更多还原