镍基粘结型催化剂上甲烷解离和CO歧化反应动力学研究

用脉冲微反色谱技术研究了在镍基粘结型催化剂上CH_4解离和CO歧化反应动力学,以及稀土氧化物的催化作用.因碳在表面沉积对活性有影响,引入了积碳因子,得到了满意结果.稀土氧化物的合适加入量均可降低反应活化能.对CH_4解离反应,La_2O_3和CeO_2的适宜值均为3%(mass),而CO歧化反应La_2O_3为4.5%(mass),CeO_2为3%(mass).在稀土氧化物加入量为0-6%(mass)内,活化能降低范围:CO歧化为33kJ/mol,CH_4解离为44kJ/mol.认为吸附态的甲烷脱除第一氢原子是反应控制步骤,稀土氧化物通过对吸附态的甲烷分子和邻近Ni原子的正电吸引,改变了甲烷解离的机理,使活化能降低.CO借助稀土氧化物中晶格氧转移使其更易实现歧化反应,使活化能降低.发现CO歧化反应中生成的碳对活性影响比较小,认为这是由于沉积在镍表面上的碳迁移到载体上的缘故.

Kinetics of CH_4 Dissociation and CO Disproportionation on Cohered Nickel Catalyst

The kinetics of CH4 dissociation and CO disproportionation on cohered nickel catalysts and the contribution of La2O3 and CeO2 to the catalytic reactions were studied by the pulse-reaction-chromatograph technique. The activity of the catalysts was decreased by the carbon deposited on the surface of nickel. Satisfying results were obtained by introducing the carbon-deposition factor into the kinetic equations.Appropriate quantities of the oxide of rare earth metal added to the catalyst can reduce the activation energy of the reactions. The values are: 3% (mass)of both La2O3 and CeO2 for CO disproportionation;4.5%(mass)La2O3 and3%CeO2 for CH4 dissociation. The reduction of activation energy is within 44kJ/mol for CH4 and 33kJ/mol for CO when the oxide of rare earth metal is added to the catalysts in an amount of 0-6%(mass). According to the results of experiments, it is assumed that the dissociation of the first hydrogen atom from adsorbed CH4 is the rate controlling-step. The electric charge attraction between the rare earth metal oxide and adsorbed CH4 and nickel atoms causes the reduction of activation energy of the CH4 dissociation, while for the CO disproportionation, the reduction of activation energy results from the transfer of the lattice oxygen in the oxide of rare earth metal.The carbon formed in CO disproportionation shows relatively little, influence on the activity of the catalyst, because carbon is considered to be able to migrate from the nickel catalyst to the surface of catalyst support.