Characterization Of The Highly Efficient Catalysts For Methane Conversion Prepared By Plasma Method

Natural gas is one of the most important flammable mineral resources. Its occurrence is widely distributed worldwide with a higher reserve than petroleum and coal. It is also cheaper. Being environmental-friendly, its role as an energy source has become increasingly important. Methane is the main component of natural gas. The effective utilization of natural gas, especially catalytic methane combustion and carbon dioxide reforming with methane, has received much attention. Although many efforts have been involved in, there still exist many problems to be solved, such as the improvement on the stability of the catalysts and catalytic activity. In this work, we attempt to use glow discharge plasma method to prepare the Pd/HZSM and Ni/Al₂O₃ catalysts for methane conversion, in order to improve the catalytic performance and also, to explore the mechanism of plasma preparation method for supported metal catalysts.Plasma preparation method includes: impregnation, glow discharge plasma treatment, calcinations. Compared with catalyst prepared by conventional method, the Pd/HZSM-5 catalysts by this method show highly dispersed and better catalytic activity, especially at low metal loading. The plasma method can reduce the usage of noble metal with the same activity and improved stability.The characterization results indicate that the plasma treatment can lead to the reduction of PdCl2, which impregnated on the support, and form highly dispersed metal particles. After calcinations, integrated cubic metal oxide is created which can keep stable structure in the methane combustion.In the process of plasma prepared Ni/Al₂O₃ catalysts, the plasma only make the metal salt, nickel nitrate, be decomposed to nickel oxides. The oxides interact with the support, which improve the dispersion and metal-support-interaction, and restrain the agglomeration in the calcinations. The plasma prepared Ni/Al₂O₃ catalysts show better low-temperature catalytic activity and stability for CO2 reforming of methane. After reaction, no un-favored carbon species formed over the catalyst.By analyzing the interaction between the catalyst and plasma, the characteristics of plasma method for catalysts preparation is summarized. The catalyst has effects on the glow of plasma during treatment. The chemical and physical states of loaded metal are greatly modified by the plasma treatment. The chemical bonds are elongated and distorted by coulomb repulsions, and then they are quickly split when collided with the energetic species in plasma. A criterion is suggested to determine whether the metal salts can be reduced or not. The ions pair (Mn+/M) with positive standard electrode potential can be reduced and vice verse. If the metal ions can be reduced, they formed metal particles in long distance with each other. If not, the metal oxide resulted from the plasma decomposition interacts with support. All these can lead to high performance of catalysts.