非常规合成气一步法制低碳烯烃过程的表界面研究

Lower olefins (C2-C4) are important commodity chemicals, and C2 to C4 olefins are conventionally produced by steam cracking of naphtha. In view of lower oil reserves in China, there is a growing necessity to produce these key chemical building blocks from non-oil derived sources. Many processes have been devised to obtain lower olefins from synthesis gas, a mixture of CO and H2 that can be obtained from different carbon-containing feedstocks. A traditional route for the conversion of syngas into lower olefins is the so-called Fischer-Tropsch (F-T) process, which used metal catalysts. In this process, the selectivity of the lower olefins has been limited by the so-called ASF distribution, in which the maximum is 58%. A recently developed process is to convert syngas to C2-C4 olefins using defective oxide catalysts, and their selectivity can reach to 90%. The route is beyond the F-T process and the selectivity is not limited by the ASF distribution. Here, we make use of surface science techniques and study the reaction mechanism of the direct syngas conversion to lower olefins over oxide catalysts at atomic and molecular levels. Special attention is paid to understand the activation of C-O bonds and C-C bond coupling over the oxide surfaces, which are the most important elementary steps. The surface structures of the oxide catalysts will be examined by near ambient pressure surface science techniques under the real reaction conditions. Furthermore, modulation of the surface reactivity will be attempted via applying external field over the oxide catalysts. The mechanism of C-O activation will be illustrated, which is used to guide the design of the catalytic process with high selectivity and low consumption of water and energy.

由合成气制取低碳烯烃可减少对石油的依赖，对富煤少油的我国具有重要的战略意义。这一过程通常经由金属催化的F-T过程实现，产物分布存在ASF分布的限制（低于58%）。我们利用缺陷氧化物催化合成气一步转化，C2-C4烃类选择性超过90%，突破F-T过程中的ASF分布限制，但是CO转化率较低（<30%）。本项目重点应用表界面研究的技术和方法研究这一非F-T路线的合成气转化新过程和新机理，重点探讨缺陷氧化物表面上C-O键活化和C-C键偶联等基元过程；利用高分辨表面技术在接近真实反应条件下对反应过程进行精准表征；探索利用外场进行氧化物表面催化过程的调控；该研究将从本质上认识C-O键高效活化和C-C键可控偶联的机制，实现相关基元反应过程的有效调控；基于反应机理的认识优化催化剂设计，进一步提高CO转化率。

本项目针对合成气制低碳烯烃这一重要催化过程，基于缺陷氧化物-微孔分子筛双功能催化剂（OX-ZEO）概念，利用原位的表面技术和方法研究缺陷氧化物催化剂表面上C-O键和H-H键的活化等关键基元过程，在原子、分子水平上对催化剂活性中心及其上的反应机理进行理解。在项目执行期间主要在以下三个方面取得进展。1）构建了尺寸和缺陷状态精准可控的模型氧化物催化剂。以Au(111)为衬底，首次实现在不同氧化气氛中（O2, O3, NO2）制备单层、双层和多层的ZnO薄层结构，进一步在Au(111)表面上实现缺陷可控的MnOx薄层结构，在CeO2薄膜表面上得到尺寸和配位数可控的CeOx纳米团簇，这些模型氧化物为研究缺陷氧化物催化合成气转化机理提供理想的表面。2）氧化物表面活化CO和H2分子的机理阐述。ZnO/Au(111)模型催化剂和ZnO粉末催化剂上的原位表征研究证实H2分子在ZnO表面上的异裂过程，表面解离形成的原子氢物种在升温中以H2形式脱附；基于MnO薄层结构的表面研究显示CO在配位不饱和Mn位点处发生歧化解离，形成表面积碳，这一解离反应与MnO纳米岛的边界缺陷密度正相关；在CeOx团簇结构上观察到CO解离反应，提出Ce的配位数可以作为CO解离活性的描述符。3）原位表征方法发展。项目执行中发展和应用多种原位或准原位谱学和显微学技术，包括世界首创的近常压光发射电子显微镜和具有变温（120-800K）高压气氛的红外光谱仪，有效地研究缺陷氧化物催化剂在催化CO加氢反应中的活性结构和活性位本质。基于以上研究结果共发表论文53篇，包括Science (1篇)，Chemical Society Reviews（1篇），Nature Communications (1篇)，PNAS（1篇），JACS（1篇），ACS Catalysis（3篇），Nano Research（5篇）等，申请专利6件，培养学生12名。