金属-氧化物界面对甲烷干重整反应催化作用机制的理论研究

The dry reforming of methane is of great significance in efficient utilization of natural gas resources and alleviation of atmospheric pollution. Because of the extreme complexity of methane dry reforming, the activity and lifetime of current available catalysts are insufficient. As the activity and stability of a dry reforming catalyst is strongly related to the structure of heterogeneous metal-oxide interface, it is possible to enhance the catalytic performance through regulating the catalyst composition and structure, if the reaction mechanism was well understood. In this project, therefore, quantum chemical methods and experimental techniques including the in-situ infrared spectra were employed to investigate the properties of metal-oxide interface and their relation to the catalytic performance in methane dry reforming. The structures of different metal-oxides, including traditional metal-on-oxide and the reverse oxide-on-metal catalysts such as ZrO2-Pd, ZrO2-Ni, Al2O3-Ni, will be built; the interfacial structure of these catalysts, the reaction energy of adsorption, decomposition, and reforming of CO2 and methane, and the reaction rate-limiting step are found, to reveal the reaction mechanism of methane dry reforming and the effect of interfacial structure of metal-oxides on the intrinsic causes of catalytic activity. The results obtained may provide a theoretical foundation for a deep understanding on the reaction mechanism of methane dry reforming and relation between the metal-oxide interface structure and catalytic performance, which is of great benefit to the development of new efficient catalyst for methane dry reforming through adjusting the local structure of catalyst.

甲烷与二氧化碳干重整制合成气对于天然气资源高效利用和大气环境改善具有重大意义。然而由于干重整反应过程极其复杂，目前催化剂的活性和寿命都难令人满意。金属-氧化物的异质界面结构是影响催化性能的重要因素，对其进行深入研究，根据催化反应特性对催化剂结构进行合理调变，是提高其催化性能的关键。为此，本项目拟采用量子化学方法，结合原位红外表征等实验手段，构建不同金属-氧化物（包括传统的metal-on-oxide和oxide-on-metal反转催化剂，如ZrO2-Pd、ZrO2-Ni、Al2O3-Ni等）结构模型，研究其界面结构和性质；对二氧化碳和甲烷的吸附、分解、重整等过程进行能量计算，寻找反应速控步，揭示甲烷干重整反应机理，阐明金属-氧化物的界面结构对甲烷干重整的催化作用机制；揭示影响催化剂活性和稳定性的内在因素，为优化催化剂局域结构、探索新型高效甲烷干重整催化剂、提高其活性和稳定性提供理论基础。

甲烷与二氧化碳干重整制合成气对于高效利用天然气资源和降低环境污染具有重大意义。然而由于该反应过程极其复杂，目前催化剂的活性和选择性都难令人满意。金属-氧化物界面结构是影响催化性能的重要因素，深入研究界面结构对甲烷干重整反应的影响有助于根据催化反应特性对催化剂结构进行合理调变，从而提高其催化性能。为此，本项目采用密度泛函理论，结合微观动力学分析及原位红外表征等实验手段研究金属-氧化物界面结构对甲烷干重整反应活性与选择性的影响。首先研究了不同金属-氧化物（包括传统的氧化物担载金属和金属担载氧化物反转催化剂，如ZrO2-Pd、NiIn-ZrO2、Ni-Al2O3、NiCo-Al2O3、Ir-Al2O3等）界面结构和性质。研究发现在γ-Al2O3(110)表面，Irn团簇倾向于吸附在非羟基覆盖区域，即由Al2O3的表面羟基、终端氧和铝原子形成的凹谷中；Co-Ni及Ni纳米棒优先落位于配位不饱和表面Al和O形成的凹谷上方。随后对二氧化碳和甲烷的吸附重整等过程进行了能量计算和微观动力学分析，揭示了各体系甲烷干重整反应机理。研究发现Co-Ni表面的甲烷干重整反应机理为甲烷逐步脱氢至CH随后被CO2分解得到的O氧化生成CHO，CHO进一步脱氢生成产物CO。反应速控步为CH被氧化生成CHO。而在金属-氧化物界面，如存在暴露界面位点的ZrO2-x@Pd(111)反转催化剂或Al2O3担载的Ni或Co-Ni纳米棒Ni@Al2O3、Co-Ni@Al2O3传统催化剂，反应分子优先吸附在界面位点处；若不存在配位不饱和的暴露界面位点即异质结结构如不存在O空位的ZrO2@Pd(111)或Ni-In@ZrO2，强金属-载体电子相互作用亦会影响催化剂表面的反应物种吸附能力从而影响反应活性与选择性。适当产生界面O空位（如Pd担载还原价态的ZrO2-x@Pd(111)反转催化剂或ZrO2担载Ni-In金属间化合物Ni-In@ZrO2-x传统催化剂）有助于提高甲烷干重整中CO选择性。ZrO2体相结构（单斜相、四方相、无定型相）也将影响反应活性与选择性。最后，该课题发现Al2O3/Al担载Ni-In金属间化合物InNi3C0.5展现出优异的CO2还原制CO性能（接近于热力学反应平衡值的CO2转化率、高达99%的CO选择性、高稳定性）。该课题结果为优化催化剂局域结构、探索新型高效甲烷干重整催化剂提供了理论基础。