甲烷及二氧化碳催化转化的第一原理及多尺度理论研究

Coupling partial oxidation with the CH4+CO2-> CH3COOH reaction helps to overcome the thermodynamic limitation to the later, and provides an atomically efficient route to utilize methane and CO2. The proposed research applies first principles based multiscale approach to the analysis of the catalytic conversion of CH4 and CO2 to value-added products over supported metal catalysts, and uses kinetic Monte Carlo (kMC) or mean-field approximation based micro-kinetic simulation to integrate the mechanistic understanding and energetic information,providing apparent kinetic parameters that can be compared directly with the experimental measurements. Both reducible (ZrO2, CeO2, ZrxCe1-xO2) and irreducible (MgO,Al2O3, In2O3) oxides will be explored as support for selected transition metals, including Fe, Co, Ni and Rh. Thermodynamics analyses of the oxide surfaces together with the supported metal clusters with varying sizes will be performed to establish the effect of reaction environment on the stability of the supporting oxides and supported metal clusters under reaction conditions. Bond breaking and making steps, including C-H and C-O bond breaking and C-C bond formation, will be followed using the density-functional theory method. Effect of oxygen, especially, lattice oxygen from the supporting oxides with different properties, on the stability of reactive intermediates as well as on bond breaking and making steps will be investigated. Reaction mechanism leading to acetic acid and other high order products will be analyzed and reactive potential profiles, including transition states and activation barrier along key reaction pathways,will be mapped out. The proposed research program is built upon the PI's broad experience in computational surface science and heterogeneous catalysis and targets issues that are important to the society. Insights into the oxidative CH4 and CO2 coupling from the proposed research will be useful in the design of highly effective catalysts that suppress methane combustion.

耦合甲烷部分氧化与CH4+CO2->CH3COOH反应有助于突破热力学对后者的限制, 是甲烷二氧化碳转化的一个原子经济反应途径。本项目利用第一原理为基础的多尺度方法研究CH4和CO2催化转化的基本物理化学过程，用动态Monte Carlo或微观反应动力学模拟综合第一原理计算得到的机理信息及能量数据，得出可与实验对比的表观动力学数据。选用可还原和不可还原两类氧化物为载体，金属元素(Fe,Co等)团簇为主催化剂，考察反应环境对负载金属团簇稳定性、C-H和C-O键断裂以及C-C偶联反应的影响；研究由CH4和CO2在氧化条件下形成乙酸及其它高阶产品的反应路径，特别是载体氧化物中晶格氧对氧化偶联的影响，搜索基元反应步骤的过渡态并计算其活化能，理清反应机理并构造反应势能面。本研究将填补在CH4和CO2氧化偶联反应机理方面的空白，对设计能抑制过度氧化的甲烷及二氧化碳转化利用高效催化剂提供指导。

自工业革命以来，化石能源的燃烧以及人类生产生活所产生的CO2使空气中CO2含量逐年稳定增加。CO2被认为是造成温室效应的主要原因。CH4作为页岩气的主要成分，储量丰富，可以为CO2转化同时提供能量和所需H源，实现CH4和CO2共转化的原子经济性反应。本项目借助第一性原理系统地研究了CH4及CO2在金属氧化物上的C−H键断裂和C−O键活化及C−C耦合反应生成乙酸及其他高阶产品的反应路径并考察了反应环境对偶联反应的影响，理清了反应机理。.d金属掺杂的可还原氧化物CeO2为CH4解离吸附提供所需还原中心。金属氧化物相较于金属可以有效提高CO2的活化效率。Zn掺杂的CeO2可以有效活化CH4的C−H键，并且将甲基稳定在表面Zn活性位上。CH4解离的活化能为0.36 eV。在Zn掺杂的CeO2表面实现了甲基碳与CO2直接C−C耦合生成乙酸根的反应。C−C耦合的活化能为0.51 eV。生成的乙酸根稳定吸附在催化剂表面。在CeO2表面引入一对Zn活性位组，可以同时稳定吸附的乙酸根与来自CH4活化的甲基，二者可以进一步C−C耦合增长C链。进一步耦合生成的二氧丙叉基通过断裂其中一个C−O键生成丙酮。此反应路径具有替代传统利用乙酸分子酮基化反应生产丙酮的潜力。在In2O3负载的ZnO催化剂表面，不仅能稳定CH4解离的甲基，也能同时将CO2吸附在表面，实现甲基碳与CO2基于Langmuir-Hinshelwood机理的C−C直接耦合，并且生成的乙酸根可以结合甲烷解离的H质子以乙酸的形式脱附，实现原子经济性100%反应。.本研究理清了CH4和CO2通过偶联反应生成乙酸及其他高阶产品（丙酮）的反应机理并绘制出相应的反应势能面，揭示了氧化物表面氧空位对C−H键断裂及C−C耦合反应的影响。本项目结果填补了CH4和CO2偶联反应机理方面的空白，对设计抑制CH4过度氧化的催化剂提供了重要的指导，具有重要的科学价值。