Study on the Application of Non-thermal Plasma Catalysis in the Conversion of CO2and Removal of NOx
|Course||Physical and chemical|
|Keywords||non-thermal plasma plasma catalytic reaction decomposition of CO2 reforming reaction selective catalytic reduction of NO with methane decompositionof NO synergistic function|
The conversion of CO2and removal of NO is important for eco-environment protection. However, due to the stability of CO2molecules and the low concentration of NO in oxygen-rich exhaust, the conversion of CO2and removal of NO is difficult. Non-thermal plasma (NTP) could activate and convert gaseous molecules through inelastic electron impact reactions at ambient temperature. This characteristic is very important for reactions need very high activation energy or reactions limited seriously by thermodynamic equilibrium, such as the decomposition of CO2, CO2reforming of hydrocarbons, and the decomposition of NO. The non-equilibrium characteristics of NTP can promote or realize reactions which are difficult or impossible to proceed through conventional chemical methods. On the other hand, the advantage of catalytic reactions is their ability for the directional conversion of reactants to target products. The combination of NTP with catalyst is expected to enhance the efficiency of conventional chemical reactions. At present, study on plasma catalytic reactions has been a new domain of chemical disciplines.The present work is focused on the study of the conversion of CO2and the low-temperature removal of NO under plasma catalytic conditions. The main results are as follows:1. Characteristics of the decomposition of CO2in a dielectric packed-bed plasma reactorThe characteristics of the decomposition of CO2in a dielectric packed-bed plasma reactor were investigated. The influence factors including the physical-chemical properties of the packing dielectrics, plasma discharge power and the length of plasma discharge zone were investigated. It was found that the dielectric properties and morphology of packing dielectric pellets play important roles in the decomposition of CO2due to their influence on the electron energy distribution in the plasma; the acid-base properties of the packing materials also affect the reaction through the chemisorption of CO2on the basic sites of the materials; heterogeneous reactions on the solid surfaces of the dielectric materials also play a role in the reaction, which was confirmed through the investigation of the influence of the length of plasma discharge zone on the reaction. The conversion of CO2is initiated by the electron impact dissociation reaction, and thus the distribution of electron energy in NTP is one of the most important factors influencing the decomposition efficiency of CO2. At the same time, the reversion reaction of CO2decomposition, the oxidation of CO, is an important side reaction, inhibiting the conversion of CO2. To promote the conversion of CO2, it is important to inhibit the oxidation of CO.2. NTP assisted CO2reforming of propane over Ni/y-Al2O3catalystTo promote the conversion of CO2in NTP, adding reducing agent, such as hydrocarbons, to the reactant gases is a superior measure since hydrocarbon molecules could act as scavengers of oxygen species generated from the dissociation of CO2. NTP assisted CO2reforming of propane was studied in combination with a Ni/γ-Al2O3catalyst, which is the first example of NTP assisted dry reforming of propane using a catalyst. The CO2reforming of propane was investigated in three reaction modes:NTP reaction, thermal catalytic reaction, and plasma assisted catalytic reaction modes. A synergistic function between NTP and the catalyst has been obtained:the activation temperature of the catalyst is reduced; the conversion of the reactants is higher than the summed results in NTP reaction and thermal catalytic reaction; the selectivity of syngas is much higher than in the other two reaction modes. The cause of the synergistic function between the plasma and the catalyst was analyzed based on the comparison of the reaction results in the three reaction modes and the analyzation of primary reaction processes in the plasma assisted catalytic reforming reaction.3. NTP assisted selective catalytic reduction of NO over B2O3/γ-Al2O3catalystsAs for the low-temperature removal of NOx under plasma catalytic conditions, the selective catalytic reduction of NO with methane was first investigated in a plasma-enhanced catalytic system. The work was focused on the investigation of the reaction processes of active chemicals generated by NTP on the post-plasma catalysts, and the optimization of the catalytic system. There is a synergistic effect between the plasma and the catalyst, which is due to the further reactions of the active chemicals generated by NTP on the post-plasma catalyst. NO2, HCHO, CH3NO and CH3NO2were detected as the active intermediate products. B2O3/γ-AI2O3was a superior catalyst for the reaction. NH3-TPD and NOx-TPD characterizations showed that the Brφnsted acidity of the catalysts is of prior importance in the reduction of NOx by the chemicals generated by NTP and adsorbed-NOx species on the catalysts also play a role in the conversion of NOx. The results of plasma assisted selective catalytic reduction of NO indicated that with the assistance of NTP, the temperature window of the reaction is obviously lowered, and the low-temperature removal efficiency of NOx is greatly enhanced.4. High-efficiency removal of NOx using a combined adsorption-discharge plasma catalytic processIn order to further increase the economy and the applicability of NOx removal processes, from the aspect of fuel economy and the improvement of energy efficiency, a combined adsorption-discharge plasma catalytic process was used for the removal of NOx in oxygen-rich environment. The work was focused on the investigation of the decomposition characteristics of adsorbed NOx on the catalysts in Ar and N2plasmas. It was found that the cause for the much lower conversion of adsorbed NOx in N2plasma than in Ar plasma is that in N2plasma, the reverse reaction of NOx decomposition, NOx formation reaction, is an important side reaction. The momentary increase of oxygen species derived from the decomposition of adsorbed NOx is considered to be the main cause as their collisions with nitrogen species can generate NOx again. Thus, solid carbon was added to the catalyst to act as a scavenger for oxygen species generated from the decomposition of adsorbed NOx in N2plasma. A highly improved NOx removal rate of97.8%was obtained, and the energy efficiency is0.758mmol NOx/W-h, which is comparative to that in almost all NOx removal processes. During the conversion of adsorbed NOx, the carbon was also progressively burned off. The process provides an interesting prospect for the simultaneous and centralized removal of NOx and carbon particulates in combustion gases, such as diesel engine exhaust, at a rather low energy cost.