论文部分内容阅读
研究发现,Pd和Co_3O_4催化剂均可有效地催化甲烷燃烧反应,且Pd掺杂的Co_3O_4催化剂上甲烷反应活性优于单纯的Pd和Co_3O_4催化剂,可见两者存在明显的协同效应.然而由于Co_3O_4本身复杂的表面配位环境,相关理论模拟研究依然较少.同时,由于甲烷分子中C–H键有着非常高的键能,且该分子具有很高的对称性,导致C–H键活化往往是甲烷选择转化和完全燃烧反应中最困难的一步.由于Co_3O_4表面电子结构比较复杂,因此本文基于Co_3O_4(001)晶面的两种不同暴露面来构建和模拟Pd掺杂Co_3O_4表面Pd.O位点的甲烷反应活性.对于Co_3O_4(001)–A晶面,暴露面金属离子只有未饱和的八面体Co~o,而(001)–B晶面,还有四面体Cot.由于Pd取代Cot后所形成的Pd/(001)–B面更不稳定,因而选择了较稳定的Pd替换Co~o结构模型.基于第一性原理PBE+U计算的Pd/(001)表面甲烷活化能垒来探讨Pd掺杂对Co_3O_4表面催化活性的影响.计算表明,甲烷在Pd掺杂的(001)面上最低解离能垒为0.68 eV,明显低于在Co_3O_4(001)和(011)面的(分别为0.98和0.89 eV),表明Pd掺杂的(001)表面催化活性要远高于纯的Co_3O_4(001)和(011)表面.为了进一步理解Pd掺杂影响Co_3O_4表面甲烷反应活性的原因,我们计算了反应位点相关原子的Bader电荷.结果表明,当CH3δ–吸附于Pd/(001)–A面Pd位点时,Pd较(001)面上Co位点能从CH3~(δ–)获得更多电子,这与Pd较Co有更强的氧化性一致.我们也对比了(001)–A,(001)–B,Pd/(001)–A和Pd/(001)–B在氧气分压为常压及不同温度下表面能的大小,并发现在与反应相关的温度区间(001)–A表面较(001)–B表面更为稳定,同样地Pd/(001)–A表面也较Pd/(001)–B表面更为稳定,且Pd/(001)–A表面与(001)–A表面稳定性差别不大,因此Pd单原子掺杂的(001)表面模型在热力学上较为稳定,且根据计算的能垒,(001)–A和Pd/(001)–A表面对甲烷活化的贡献最大.为了更好与实验结果对比,我们构建了简单的动力学模型,并计算了甲烷在Co_3O_4(001),(011)和1%,2%,3%Pd掺杂的Co_3O_4(001)表面的甲烷燃烧速率.计算表明即使较低量的Pd也可明显提高甲烷燃烧速率,与实验数据吻合较好,表明掺杂Pd显著增加Co_3O_4催化甲烷燃烧.
The results show that both Pd and Co_3O_4 catalysts can effectively catalyze the methane combustion reaction, and the catalytic activity of methane on Pd-doped Co_3O_4 catalyst is better than that of pure Pd and Co_3O_4 catalysts. However, due to the complex nature of Co_3O_4 itself However, due to the high bond energy of C-H bonds in methane molecules and the high symmetry of the molecules, the activation of C-H bonds is often caused by methane Choose the most difficult step in the conversion and complete combustion reaction.Because the electronic structure of Co_3O_4 surface is complex, two different exposed surfaces of Co_3O_4 (001) crystal plane were used to construct and simulate Pd.O sites on Pd-doped Co_3O_4 surface Methane reactivity. For the Co_3O_4 (001) -A crystal plane, the exposed metal ions have only the unsaturated octahedron Co ~ o, while the (001) -B crystal plane and the tetrahedron Cot. Since Pd is substituted for Cot The Pd / (001) -B surface is more unstable, so the more stable Pd is chosen to replace the Co ~ o structure model.According to the Pd / (001) surface methane activation energy barrier calculated by the first principle PBE + U, Pd Doping on the catalytic activity of Co_3O_4 surface The calculated results show that the lowest dissociation barrier of methane on Pd-doped (001) surface is 0.68 eV, which is obviously lower than that on Co_3O_4 (001) and (011) surfaces (0.98 and 0.89 eV, respectively) The catalytic activity of doped (001) surface is much higher than that of pure Co_3O_4 (001) and (011) surface.To further understand the reason that Pd doping affects the methane reactivity of Co_3O_4 surface, we calculated the Bader Charge.The results show that when CH3δ-adsorbed on the Pd / (001) -A Pd site, Pd can get more electrons from CH3 ~ (δ-) than Co site on the (001) (001) -A, (001) -B, Pd / (001) -A and Pd / (001) -B at partial pressures of oxygen at different pressures and temperatures (001) -A surface is more stable than the (001) -B surface in the temperature range associated with the reaction, and the Pd / (001) -A surface is also more stable than the Pd / (001) -B The surface is more stable and the surface stability of Pd / (001) -A is not much different from that of (001) -A. Therefore, the Pd monoatomic (001) doped surface model is thermodynamically stable and calculated according to the energy Base, (001) -A and Pd / (001) -A surface to methane In order to compare with the experimental results, we constructed a simple kinetic model and calculated the effect of methane on Co_3O_4 (001), (011) and 1%, 2% and 3% Pd doped Co_3O_4 001), the methane combustion rate was calculated.The calculated results show that even low Pd can improve the methane combustion rate, which is in good agreement with the experimental data, indicating that Pd doping significantly increases the catalytic combustion of methane over Co_3O_4.