Photoelectrocatalytic oxidation of methane over three-dimensional ZnO/CdS/NiFe layered double hydroxide
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摘要: 将甲烷以低能耗的方式直接转化为甲醇等高附加值的化学品一直是可持续化工产业的重要目标和重大挑战。本文制备了三维（3D）ZnO/CdS/NiFe层状双金属氢氧化物（LDH）核/壳/分层纳米线阵列（NWAs）结构材料并将其用于室温、模拟阳光照射下甲烷的光电催化氧化。结果表明3D ZnO/CdS/NiFe-LDH具有优异的光电化学性能及催化活性，甲烷气氛下的光电流密度达到了6.57 mA·cm−2（0.9 V vs RHE），其催化甲烷生成甲醇及甲酸产量分别是纯ZnO的5.0和6.3倍，两种主要产物的总法拉第效率达到54.87%。CdS 纳米颗粒（NPs）的沉积显著提升了复合物对可见光的吸收，促进了光生载流子的分离。而具有三维多孔结构的NiFe-LDH纳米片的引入改善了甲烷氧化表面反应动力学，起到了优异的助催化作用；并且有效抑制了O2•-的产生，防止O2•-进一步将甲醇及甲酸氧化为CO2，提高了甲醇及甲酸的选择性。最后，提出了三维ZnO/CdS/NiFe-LDH复合材料光电催化甲烷转化为甲醇及甲酸的机理，为甲烷低能耗转化为高价值化学品提供了新思路。Abstract: The direct conversion of methane into methanol and other high value-added chemicals with low-energy consumption has always been an important goal and a major challenge for the sustainable chemical industry. In this paper, a three-dimensional (3D) ZnO/CdS/NiFe layered double hydroxide (LDH) shell/core/hierarchical nanowire arrays (NWAs) structure material was fabricated and utilized for photoelectrocatalytic oxidation of methane at room temperature under simulated sunlight. Results show that the ZnO/CdS/NiFe-LDH photoanode exhibites excellent photoelectrochemical performance and catalytic activity. The photocurrent density under the methane atmosphere reached 6.57 mA·cm−2 at 0.9 V (vs RHE). Yields of methane oxidation products, which mainly are methanol (CH3OH) and formic acid (HCOOH), catalyzed by the synthesized ZnO/CdS/NiFe-LDH composite are 5.0 and 6.3 times those of pure ZnO, respectively. The total Faraday efficiency of the two main products reach 54.87%. The deposition of CdS nanoparticles (NPs) significantly facilitates the absorption of visible light and promotes the separation of photo-generated carriers. The introduction of NiFe-LDH nanosheets with a three-dimensional porous structure improves the surface reaction kinetics of methane oxidation, acting as an excellent co-catalyst. It also effectively inhibites the production of O2•-, preventing O2•- from further oxidizing methanol and formic acid into CO2, which improves the selectivity of methanol and formic acid. Finally, this paper proposed a mechanism of the photoelectrocatalytic oxidation of methane to methanol and formic acid over 3D ZnO/CdS/NiFe-LDH composite material, which provides a new idea for the conversion of methane into high-value chemicals with low-energy consumption.
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Key words:
- photoelectrocatalysis /
- methane /
- methanol /
- ZnO /
- NiFe-LDH
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图 2 形貌及元素组成。（a）ZnO的扫描电镜图；（b）ZnO/CdS的扫描电镜图；ZnO/CdS/NiFe-LDH的扫描电镜图（c）、X射线能谱图（d）、透射电镜图（e）及高分辨透射电镜图（f）
Figure 2. Morphology and element composition: (a) SEM images of ZnO; (b) SEM images of ZnO/CdS; SEM images (c), EDS spectra (d), TEM image (e)，and HR-TEM image (f) of the ZnO/CdS/NiFe-LDH sample
图 3 ZnO、ZnO/CdS、ZnO/CdS/NiFe-LDH的X射线衍射图
Figure 3. XRD patterns of ZnO, ZnO/CdS, and ZnO/CdS/NiFe-LDH photoanodes
图 4 ZnO、ZnO/CdS、ZnO/CdS/NiFe-LDH的紫外‒可见光吸收谱
Figure 4. UV–vis absorption spectra of ZnO, ZnO/CdS, and ZnO/CdS/NiFe-LDH
图 5 光阳极的线性伏安扫描曲线。（a）ZnO；（b）ZnO/CdS；（c）ZnO/CdS/NiFe-LDH；（d）在持续通入甲烷且光照条件下对比图
Figure 5. LSV: (a) ZnO; (b) ZnO/CdS; (c)ZnO/CdS/NiFe-LDH photoanodes; (d) comparison in the presence of CH4 under illumination
图 6 ZnO、ZnO/CdS、ZnO/CdS/NiFe-LDH光阳极在持续通入甲烷下的（a）1.1 V（vs RHE）下的计时电流（J‒t）曲线；（b）光照条件下与黑暗条件下的电化学阻抗谱；（c）1.1 V（vs RHE）下的电流‒时间稳定性曲线
Figure 6. (a) Chronoamperometric J‒t curves collected at 1.1 V (vs RHE) under chopped illumination conditions; (b) EIS plots under dark and light illumination; (c) current–time curves for stability measurement collected at 1.1 V (vs RHE) of ZnO, ZnO/CdS, ZnO/CdS/NiFe-LDH photoanodes saturated with methane
图 7 外加电压为1.1 V(vs RHE)，光照强度为AM 1.5G、100 mA·cm−2，电解液为0.5 mol·L−1 Na2SO4水溶液条件下ZnO、ZnO/CdS、ZnO/CdS/NiFe-LDH光电催化氧化甲烷转化为甲醇（a）甲酸（b）产量随时间变化曲线图以及2.5 h内生成甲醇及甲酸的法拉第效率（c）
Figure 7. Yields of CH3OH (a) and HCOOH (b) in the photoelectrocatalytic oxidation of CH4 with ZnO, ZnO/CdS, and ZnO/CdS/NiFe-LDH catalysts, with a potential of 1.1 V (vs RHE) under simulated sunlight illumination (AM 1.5G, 100 mA·cm−2), and the electrolyte is 0.5 mol·L-1 Na2SO4 aqueous solution; (c) Faradaic efficiencies of CH3OH and HCOOH for ZnO, ZnO/CdS, and ZnO/CdS/NiFe-LDH for a 2.5-h operation
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