利用离子液体界面工程实现非均相分子催化剂高效生产双氧水

doi:10.1016/S1872-2067(21)63946-3

摘要/Abstract

摘要：

双氧水是一种广泛应用的必要化学品. 目前, 双氧水大规模制备主要通过高能耗的蒽醌氧化还原-蒸馏浓缩过程实现. 近年来, 利用可再生电能经氧还原反应实现分散式双氧水制备引起了广泛的兴趣. 在此过程中, 高效电催化剂的作用非常重要. 除已报道的基于贵金属及其合金、金属氧化物和碳基催化剂外, 氮原子稳定的单原子金属钴催化剂(Co-N/C)因其优异的性能受到越来越多的关注. 尽管通过调节单原子钴活性位点的周围环境可以提升其催化活性, 但常用的高温热解方法仍很难制得原子精度结构, 因此, 仍需探索其他简单有效的催化活性提升方法.

本文通过简单的界面工程方法成功地制备了具有单原子金属钴活性位点的非均相分子催化剂, 可高效生产双氧水. 首先将具有典型Co-N/C结构的酞菁钴分子(CoPc)分散于多壁碳纳米管(CNT)表面, 得到一类具有位点结构并充分暴露的非均相分子催化剂(CoPc/CNT), 避免了由于活性位点结构不一和扩散传质可能带来的影响. 随后利用溶剂互溶法将疏水离子液体均匀地覆盖在CoPc/CNT表面, 实现界面工程改性, 并通过调控体系中离子液体与CoPc/CNT催化剂的质量比(x), 制备了一系列精确控制离子液体覆盖层厚度的催化剂(CoPc/CNT-x). 采用高角球差扫描透射显微镜、X射线光电子能谱和同步辐射X光吸收光谱技术表征了单原子Co活性位点的结构, 结果表明, 离子液体包裹不影响钴原子的局部环境和电子结构.

离子液体包裹可有效提升催化剂活性, 且提升程度与离子液体层厚度密切相关. 进一步实验结果证实, 性能提升主要来自两方面. 首先, 与水溶液相比, 离子液体可以通过更大的氧溶解度(2.12 × 10^-3 mol/L)在活性位点周边建立富氧环境, 从热力学角度提升氧还原反应活性. 其次, 其疏水性可以使生成的双氧水分子快速从催化剂表面脱离, 避免其富集并进一步还原降解. 实验发现最佳离子液体层厚度为8 nm, 该厚度下催化剂生成双氧水的动力学电流密度是参比催化剂(CoPc/CNT)的近4倍, 且生成双氧水选择性提升至92%, 在两电极电解槽测试中可以稳定地实现3.71 mol_H2O2 g_cat^-1 h^-1的产率. 本工作证明界面工程可以作为一类简单易行的方法, 有效提升电化学制备双氧水的效率.

关键词: 双氧水, 离子液体, 氧还原反应, 单原子催化剂, 非均相分子催化剂

Abstract:

Efficient and selective oxygen reduction reaction (ORR) electrocatalysts are critical to realizing decentralized H₂O₂ production and utilization. Here we demonstrate a facile interfacial engineering strategy using a hydrophobic ionic liquid (IL, i.e., [BMIM][NTF₂]) to boost the performance of a nitrogen coordinated single atom cobalt catalyst (i.e., cobalt phthalocyanine (CoPc) supported on carbon nanotubes (CNTs). We find a strong correlation between the ORR performance of CoPc/CNT and the thickness of its IL coatings. Detailed characterization revealed that a higher O₂ solubility (2.12 × 10^-3 mol/L) in the IL compared to aqueous electrolytes provides a local O₂ enriched surface layer near active catalytic sites, enhancing the ORR thermodynamics. Further, the hydrophobic IL can efficiently repel the as-synthesized H₂O₂ molecules from the catalyst surface, preventing their fast decomposition to H₂O, resulting in improved H₂O₂ selectivity. Compared to CoPc/CNT without IL coatings, the catalyst with an optimal ~8 nm IL coating can deliver a nearly 4 times higher mass specific kinetic current density and 12.5% higher H₂O₂ selectivity up to 92%. In a two-electrode electrolyzer test, the optimal catalyst exhibits an enhanced productivity of 3.71 mol_H2O2 g_cat^-1 h^-1, and robust stability. This IL-based interfacial engineering strategy may also be extended to many other electrochemical reactions by carefully tailoring the thickness and hydrophobicity of IL coatings.

Key words: Hydrogen peroxide, Ionic liquid, Oxygen reduction reaction, Single-atom catalyst, Heterogeneous molecular catalyst

于子勋, 刘畅, 邓业煜, 李默涵, 厍芳馨, Leo Lai, 陈元, 魏力. 利用离子液体界面工程实现非均相分子催化剂高效生产双氧水[J]. 催化学报, 2022, 43(5): 1238-1246.

Zixun Yu, Chang Liu, Yeyu Deng, Mohan Li, Fangxin She, Leo Lai, Yuan Chen, Li Wei. Interfacial engineering of heterogeneous molecular electrocatalysts using ionic liquids towards efficient hydrogen peroxide production[J]. Chinese Journal of Catalysis, 2022, 43(5): 1238-1246.

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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(21)63946-3

https://www.cjcatal.com/CN/Y2022/V43/I5/1238

图/表 5

Fig. 1. Catalyst synthesis and characterization. (a) Schematic illustration of the synthesis of IL coated CoPc/CNT catalysts; (b) HAADF-STEM image of the CoPc/CNT catalyst; BF-STEM (c), STEM-EDX mapping result (d), and HAADF-STEM image (e) of the IL coated CoPc/CNT-1 catalyst; XPS survey scans (f), and N 1s (g) and Co 2p (h) high-resolution XPS spectra of different catalysts.

Fig. 2. Electrochemical performance of CoPc/CNT catalysts. LSV curves of the CoPc/CNT and CoPc/CNT-x catalysts collected in O2 saturated electrolyte (a), and the calculated H2O2 selectivity (λH2O2) and electron transfer number (n) (b); (c) The mass kinetic current density (jK-mass) of different catalysts toward H2O2 production. Inset shows the jK-mass at 0.2 V vs. RHE.

Table 1 ORR performance of CoPc/CNT catalysts. Performance parameters were collected at 0.2 V.

Catalyst	U_disk/ V	U_ring/ V	n	H₂O₂ (%)	FE_H2O2 (%)	j_K/ mA cm^-2
CoPc/CNT	0.55	0.54	2.39	80	67	16.45
CoPc/CNT-0.2	0.59	0.58	2.33	83	71	18.46
CoPc/CNT-1	0.64	0.62	2.15	92	86	22.51
CoPc/CNT-1.5	0.66	0.64	2.18	91	84	31.41
CoPc/CNT-2	0.65	0.62	2.30	85	74	22.18

Fig. 3. (a) Relationship between the half-wave potential increment (ΔE1/2) and the H2O2 selectivity differences to the x value in CoPc/CNT-x catalysts; (b) O2 solubility measurement.

Fig. 4. H2O2 production performance of two-electrode electrolyzers assembled using CoPc/CNT-1.5 and CoPc/CNT. Electrolyzer voltage responses (a), and H2O2 selectivity (b) at different current densities; (c) Chronopotentiometry test results and the amount of H2O2 produced.

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