大连理工大学李新勇教授：电催化选择性氧化甲醛同时产氢的研究

第一作者：秦美春 博士；通讯作者：范诗迎 博士
通讯单位：大连理工大学    论文DOI：10.1021/acsami.1c08363

全文速览

   大连理工大学李新勇教授课题组长期专注于绿色催化与能源催化的研究。近日，该课题组在电催化选择性氧化甲醛与同时产氢的研究中取得新进展，相关成果发表于国际顶级期刊ACS Applied Materials & Interfaces上。作者以MOF为前驱体合成了Co-N_x–C@Co催化剂，并应用于电催化析氢及选择性氧化甲醛当中，一氧化碳产率可达1993 μmol g⁻¹ h⁻¹，法拉第电流效率可达到16.2%，X射线吸收精细结构光谱结合量化计算最终确定Co-N₄ 为电催化选择性氧化甲醛反应最主要的活性位点，钴纳米颗粒则可以显著促进析氢反应的进行。

背景介绍

电催化水分解制氢被认为是最具潜力的制氢方法之一。然而，阳极上缓慢发生的析氧反应是水分解制氢的一大瓶颈。虽然涌现出许多高活性的催化剂，但析氧反应仍需要更高的过电位才能与析氢反应的速率相匹配，导致能量转换效率低。

本文亮点

在阳极反应室引入甲醛，利用甲醛的氧化反应取代析氧反应，且以MOF为前驱体合成了Co-N_x–C@Co催化剂，应用于电催化析氢和甲醛氧化；Co-N_x–C@Co可以同时实现电催化析氢和甲醛的选择性氧化产一氧化碳，结合实验和理论计算系统研究了Co-N_x–C@Co电催化析氢和选择性氧化甲醛的活性位点。

图文解析

如图1所示，以MOF为前驱体, 利用共沉淀法合成了Co-N_x–C@Co催化剂。从SEM图可得，材料呈现规则的多面体状。

Figure 1. (a) Synthesis procedure of Co-Nx-C@Co. (b) SEM image of the Co-Nx-C@Co precursor. (c) SEM image of the prepared Co-Nx-C@Co. (d) XRD curves of N-C@Co, Co-Nx-C@Co, and Co-Nx-C.

 如图2所示，结合TEM和HAADF-STEM可得，Co-N_x–C@Co既包含钴纳米颗粒又包含单原子态的钴，且钴纳米颗粒被碳层包覆，催化剂中各元素分布均匀。

Figure 2. (a) TEM image, (b) HRTEM image, (c) magnified HAADF-STEM image, and (d) HAADF-STEM image and corresponding EDS element mapping image of Co−N_x−C@Co

如图3所示，材料的XPS和XAS的表征进一步验证了催化剂中钴纳米颗粒和单原子钴的共存，且Co-N_x–C@Co中的单原子主要以Co-N₄配位的方式存在。

Figure 3. (a) N 1s spectrum and (b) Co 2p spectrum of Co-Nx-C@Co. (c) Normalized XANES spectra of Co-Nx-C@Co, Co-Nx-C, Co foil, and CoO at the Co K-edge. (d) FT-EXAFS measurements of Co-Nx-C@Co, Co-Nx-C, and Co foil. WT-EXAFS measurements of (e) Co foil and (f) Co-Nx-C@Co.

如图4所示，在反应池中加入甲醛，进行了电催化氧化实验。结果表明，相较于析氧反应，甲醛的氧化反应可以在更低电位下发生，且甲醛的氧化具有更快的动力学反应速率。

Figure 4. (a) LSV curves of Co-Nx-C@Co in 1.0 M KOH electrolyte with different concentrations of CH2O. (b) Summary of potentials to reach 10 mA cm-2 for N-C@Co, Co-Nx-C@Co, and Co-Nx-C in 1.0 M KOH electrolyte with different concentrations of CH2O. (c) Tafel slope curves of N-C@Co, Co-Nx-C@Co, and Co-Nx-C in 1.0 M KOH electrolyte containing CH2O. LSV curves of N-C@Co, Co-Nx-C@Co, and Co-Nx-C normalized by (d) ESCAs and (e) BET. (f) LSV curves of the Pt electrode in 1.0 M KOH electrolyte with and without 3.2 M CH2O

如图5所示，作者对所合成的材料进行了电催化选择性氧化甲醛活性测试，Co-N_x–C@Co催化剂的一氧化碳产率可达1993 μmol g⁻¹ h⁻¹，法拉第电流效率可达16.2%，且具有较好的析氢活性。

Figure 5. (a) CV curves of N-C@Co, Co-Nx-C@Co, and Co-Ni-C anodes in a divided cell in 1.0 M KOH electrolyte with and without CH2O addition in the anode chamber; the scan rate is 10 mV-1 s-1. The (b) CO yield, (c) CO partial current density, and (d) CO faradaic efficiency for N-C@Co, Co-Nx-C@Co, and Co-Nx-C at different potentials in 1.0 M KOH electrolyte containing CH2O in the anode chamber. (e) Cycle experiments of Co-Nx-C@Co at 1.2 V vs RHE. (f) LSV curves of N-C@Co, Co-Nx-C@Co, and Co-Nx-C for HER in a 1.0 M KOH electrolyte with and without CH2O addition in the anode chamber.

最后，利用DFT量化计算研究了甲醛选择性氧化为一氧化碳的反应路径，同时，通过吉布斯自由能图可得Co-N₄位点为甲醛选择性氧化为一氧化碳的主要活性位点。

Figure 6. (a) Reaction pathways of CH2O conversion to CO. Free energy diagrams for CH2O conversion to CO on (b) Co-N4-C and (c) Co-C.