Studies On The Preparation And Catalytic Performance Of Pd Catalysts Using Glow Discharge And Radio Frequency Plasmas

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.