Co/Pd催化剂上CH_4/CO_2等温两步反应直接合成乙酸的热力学

Thermodynamics of Direct Synthesis of Acetic Acid from CH_4 and CO_2 by a Two-Step Isothermal Reaction Sequence on Co/Pd Catalysts

采用量子化学密度泛函理论对CH4／CO2两步法合成乙酸反应中表面碳化物CH4（x=0～3）在Co和Pd模型表面上不同吸附活性位上的吸附能、空间构型和反应吉布斯自由能进行了系统性的比较研究．计算结果表明，CH4／CO2两步反应在单一金属Co或Pd催化剂上在常压等温条件下不能有效进行，但在CO和Pd组成的双金属催化剂上，两步反应在常压等温下可以进行．在Co和Pd双金属催化剂上，金属Co活化CH4生成金属碳化物CH，Co（x=0，1）为热力学允许反应，其后CHx溢流到金属Pd上形成CH、Pd（y=1～3）碳化物，最后CO2插入CHyPd生成乙酸，后两者在常压等温情况下也为热力学允许反应，并且在435K以上可以与前者构成等温循环．计算结果与实验结果吻合．

The adsorption energy of surface carbon species CHx（x = 0-3）, space configurations of CH4 on different active sites of Co and Pd surfaces, and the Gibbs free energy change have been investigated using the density functional theory method for the direct synthesis of acetic acid from CH4 and CO2 by a two-step reaction se- quence. The calculation results show that the two-step reaction cannot be effectively carried out on the single Co or Pd catalyst under atmospheric pressure and isothermal conditions. However on binary Co/Pd catalysts, the reaction that Co activates CH4 leading to CHxCo （x = 0, 1） is favorable in thermodynamics. Subsequently, the reactions that CHx overfall from Co to Pd producing CHyPd （y = 1-3） and CO2 inserts into CHyPd forming acetic acid are also advantageous in thermodynamics under atmospheric pressure and isothermal conditions. These two reactions combine with former reaction to make an isothermal cycle above the temperature 435 K. The calculated results are in agreement with the experimental results.