模型催化剂上甲烷电催化活化转化的反应机理与构效关系

Aiming at the main research direction of the current Major Research Plan  the exploration of electro-catalyzing carbon-based small molecules, we plan to carry out the study of electrocatalytic activation and conversion of CH4. Methane is a primary starting chemical for industrial synthesis. Direct conversion CH4 into other valuable chemicals by electrochemistry is a promising approach for effectively utilizing CH4. However, the poor activity and selectivity of catalysts severely limit this approach in application. To address this problem, it needs to understand the reaction mechanism and structure-activity relationship of electro-catalyzing CH4. Taking our advantages in long-term electrocatalytic mechanism studies, especially in single-crystal electrodes, in situ FTIR spectroscopy, and shape-controlled synthesis of nanocrystals, as well as our finding that Pt(100) possesses an exceptional high catalytic activity for CH4 activation, in this project, we want to reveal the reaction mechanism and structure-reactivity of CH4 activation by combining model electrocatalysts with spectroscopic studies. We want to build three kind of well-defined model catalysts, that is, metal single crystal electrodes, single crystal electrodes modified with oxide, and shaped nanocrystals, to study the structure-activity relationship in methane electrocatalysis. And we will further use electrochemical in situ FTIR spectroscopy, Raman, and SFG to detect the reaction intermediates. Based on the knowledge obtained, we try to reveal the reaction mechanism at the molecular level. Our project will do contribute to the one of main objects of Major Research Plan about catalytic activation of chemical bonds concerning carbon, and also contribute to core science tasks about the selective activation and directional transformation of carbon-based molecules on interface.

针对指南重点研究方向“碳基小分子电催化转化的新探索”，本项目以甲烷电催化活化转化为研究重点。甲烷来源广泛，是一个非常基础的化工原料。将甲烷一步电化学转化成化学品前景诱人，但受限于催化剂活性低、选择性差的瓶颈，这迫切需要从分子水平认识甲烷电催化活化反应机理和构效关系，实现催化剂理性设计。基于我们在电催化机理研究的长期积累，特别是单晶模型催化剂、原位谱学和纳米晶可控制备的优势，以及近期发现Pt(100)单晶电极对甲烷异常高的活化能力，本项目拟通过构筑结构明确的“金属单晶”、“单晶面-金属氧化物界面”、“形貌可控纳米晶”三类模型催化剂，研究甲烷电催化的构效关系；综合运用红外、拉曼和SFG三种优势互补的原位振动光谱，检测反应中间体，结合理论计算，揭示甲烷电催化活化的反应机理。本项目有望为重大研究计划总体目标“涉碳化学键催化活化”和核心科学问题“碳基载能分子在表界面的选择活化和定向转化”贡献力量。

甲烷等低碳烷烃来源非常丰富，目前主要是作为燃料，通过燃烧而利用。将甲烷作为化工合成原料，往往要先将其转化为合成气（CO + H2），再通过费托合成来制备目标产物，合成路线长，能耗高，效率低。若能将甲烷等低碳烷烃一步法转化成高价值产品，具有重要意义。本项目针对重大研究计划总体目标“涉碳化学键催化活化”和核心科学问题“碳基载能分子在表界面的选择活化和定向转化”，以甲烷电催化活化转化为研究重点，拟通过运用结构明确的铂单晶模型催化剂，运用电化学原位红外光谱检测表面吸附态中间体，并结合DFT理论计算，从分子水平研究甲烷在常温下电催化转化的表面过程和反应机理。取得如下研究成果：1）以Pt(100)，Pt(111)、Pt(110)三个低指数晶面，以及Pt(S)[n(100)×(110)]和Pt(S)[n(100)×(111)]两个系列Pt高指数晶面为模型催化剂，揭示了甲烷在不同表面结构铂单晶电极上电催化转化的构效规律，发现该反应对长程有序(100)晶面非常敏感，只有当平台原子宽度大于5个原子的(100)表面位才能活化甲烷，产物为多重吸附态CO。2）通过DFT理论计算，结合13C同位素原子示踪的原位红外光谱，揭示仅在Pt(100)晶面上，甲烷的吸附态中间体CHx才能够与表面含氧物种结合，进一步发生转化，而在Pt(111)和Pt(110)晶面上该步骤能垒非常高而难以进行，从而合理解释了所观察到的甲烷电催化转化构效规律。基于所获得的构效规律，指导通过纳米晶形状控制合成制备具有（100）晶面择优的立方体铂纳米晶催化剂，获得了对甲烷转化具有一定活性的纳米催化剂。3）拓展研究了不同表面结构铂单晶面对乙烷和丙烷等低碳烷烃的电催化转化的构效规律，同样观察到这些反应对Pt(100)具有高度的敏感性。特别是首次发现丙烷能够在Pt(100)晶面上电催化转化为乳酸，并且作为主要的氧化产物，这有望为基于石油路线制备乳酸提供一个新思路。本项目研究成果在ACS Catal、Energy Environ Sci.、ACS Energy Lett等期刊上发表标注论文14篇论文，培养毕业4名博士生。