MOF包埋衍生策略缓释氧物种增强甲烷选择性氧化的活性与选择性: 基于瞬时原位红外的研究

doi:10.1016/S1872-2067(25)64803-0

摘要/Abstract

摘要：

甲烷作为氢碳比最高的化石资源, 是生产大宗化学品和燃料的重要原料. 将甲烷选择性氧化为C₁含氧化合物极具应用前景. 然而, 甲烷分子因结构对称、极化率低且C-H键能高, 其选择性氧化极具挑战. 同时, 氧化产物易被过度氧化为CO₂, 导致目标产物选择性低. 通过光催化激发的电子空穴对提供额外的反应路径, 并精准调控氧物种浓度, 在提高催化活性与选择性方面展现出独特潜力. 金属有机框架材料(MOF)凭借其结构可调控的优势, 近年来被广泛用于光催化剂改性. 因此, 本文提出通过优化MOF结构精准控制氧物种浓度, 从而同步提升甲烷氧化物生成甲醇活性与选择性的研究思路.

本文围绕甲烷选择性氧化这一关键反应过程, 采用MIL-125 (Ti)为前驱体, 系统研究金纳米颗粒(Au NPs)在不同负载方式(表面负载: Au/TiO₂与包埋: Au@TiO₂)下, 光热催化剂结构与性能的调控机制. X射线衍射、扫描电镜、透射电镜和N₂吸附-脱附表征结果显示, Au@TiO₂的Au纳米颗粒均匀包埋于TiO₂内部, 且具有更高的比表面积; Au/TiO₂则存在明显的Au颗粒聚集现象. 性能评估显示, 0.5Au@TiO₂在全光谱照射下表现出最高的CH₃OH产率(32.5 μmol/(g·h))和选择性(> 80.6%), 远优于相同Au负载量的Au/TiO₂ (7.7 μmol/(g·h), 31.6%). 电子顺磁共振和X射线光电子能谱进一步证实, Au@TiO₂会形成更多的Au³⁺和氧空位(O_V), 其中0.5Au@TiO₂的含量最高. Au³⁺和O_V有助于提升光生载流子分离效率, 因此0.5Au@TiO₂在所有样品中表现出最优的电子结构调控能力. 反应条件下的原位漫反射红外光谱(DRIFTs)结果表明, •OH不仅可用于CH₄的初步活化, 同时也是引发过氧化的主要活性物种, 而•OOH主要参与甲醇生成, 不涉及深度氧化. 在Au/TiO₂中, •OH主要以非氢键吸附的游离态存在, 虽然能快速活化CH₄生成甲醇, 但也极易诱发甲醇的过氧化(生成CO₂). 而Au@TiO₂对•OH以氢键形式吸附, 可有效的降低甲醇过氧化的几率. 切断CH₄的原位DRIFTs证明, Au@TiO₂中氢键吸附•OH可向非氢键吸附•OH进行转化, 这种缓释作用可以在不降低CH₄活化速率的前提下, 有效的阻断甲醇的过氧化路径, 从而保证了活性与选择性的同步提升.

综上, 本文通过改变Au和TiO₂的负载方式, 揭示了氢键吸附与缓释策略调控•OH迁移机制, 在保证高甲烷转化率的同时, 减少部分氧化产物的过氧化, 为构建高性能高值化甲烷光热选择氧化催化剂提供理论依据.

关键词: 甲烷选择性氧化, 金属有机框架衍生, 活性氧物种调制, 氢键吸附羟基

Abstract:

The methane selective oxidation was a "holy grail" reaction. However, peroxidation and low selectivity limited the application. Herein, we combined three Au contents with TiO₂ in both encapsulation (xAu@TiO₂) and surface-loaded (xAu/TiO₂) ways by MOF derivation strategy, reported a catalyst 0.5Au@TiO₂ exhibited a CH₃OH yield of 32.5 μmol·g^-1·h^-1 and a CH₃OH selectivity of 80.6% under catalytic conditions of only CH₄, O₂, and H₂O. Mechanically speaking, the catalytic activity was controlled by both electron-hole separation efficiency and core-shell structure. The interfacial contact between Au nanoparticles and TiO₂ in xAu@TiO₂ and xAu/TiO₂ induced the formation of oxygen vacancies, with 0.5 Au content showing the highest oxygen vacancy concentration. At the same Au content, xAu@TiO₂ generated more oxygen vacancies than xAu/TiO₂. The oxygen vacancy acted as an effective electron cold trap, which enhanced the photogenerated carrier separation efficiency and thereby improved the catalytic activity. In-situ DRIFTs revealed that the isolated OH (non-hydrogen bond adsorption) were key species for the methane selective oxidation, playing a role in the activation of CH₄ to *CH₃. However, an overabundance of isolated OH led to severe overoxidation. Fortunately, the core-shell structure over xAu@TiO₂ provided a slow-release environment for isolated OH through the intermediate state of *OH (hydrogen bond adsorption) to balance the formation rate and consumption rate of isolated OH, doubling the methanol yield and increasing the > 29% selectivity. These results showed a new strategy for the control of the overoxidation rate via a strategy of MOF encapsulation followed by pyrolytic derivation for methane selective oxidation.

Key words: Methane selective oxidation, Metal-organic framework derived, Reactive oxygen species modulation, Hydrogen bonded adsoprotion, hydroxyl groups

李可欣, 袁浩, 杨祖保, 胡准. MOF包埋衍生策略缓释氧物种增强甲烷选择性氧化的活性与选择性: 基于瞬时原位红外的研究[J]. 催化学报, 2025, 78: 202-214.

Ke-Xin Li, Hao Yuan, Ralph T. Yang, Zhun Hu. MOF encapsulation derived slow-release oxygen species to enhance the activity and selectivity of methane selective oxidation: A transient DRIFTs Study[J]. Chinese Journal of Catalysis, 2025, 78: 202-214.

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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(25)64803-0

https://www.cjcatal.com/CN/Y2025/V78/I11/202

图/表 11

Table 1 The amount of reactants in the preparation of xAu@MIL-125.

Sample	DMF (mL)	Au NPs
0.25Au@MIL-125	8.750	0.250
0.5Au@MIL-125	8.500	0.500
1.5Au@MIL-125	7.500	1.500

Table 2 The amount of reactants in the preparation of xAu/MIL-125.

Sample	MIL-125 (g)	Methanol (mL)	Au NPs (mL)
0.25Au@MIL-125	0.25	9.750	0.250
0.5Au@MIL-125	0.25	9.500	0.500
1.5Au@MIL-125	0.25	8.500	1.500

Fig. 1. (a) Synthesis strategy for xAu@TiO2 and xAu/TiO2. (b) The XRD patterns of xAu@TiO2 and xAu/TiO2. The SEM images for 0.25Au@TiO2 (c), 0.25Au/TiO2 (d), 0.5Au@TiO2 (e), 0.5Au/TiO2 (f), 1.5Au@TiO2 (g), and 1.5Au/TiO2 (h). The HRTEM images for 0.5Au@TiO2 (i) and 0.5Au/TiO2 (j).

Fig. 2. The product yield and oxygenate selectivity for CH4 conversion over xAu@TiO2 (a) and xAu/TiO2 (b) with different Au NPs contents. Reaction conditions: 50 mg photocatalyst, 20 mL D.I. Water, 5 bar O2, 10 bar CH4, 2 h, 300 W Xe lamp. EPR experiments with DMPO as the spin-electron trapping agent for monitoring the generation of ?OH (c) and ?OOH (d) over xAu@TiO2 and xAu/TiO2.

Fig. 3. The XPS spectra of Au 4f (a), Ti 2p (b), O 1s (c), and VB (d) for xAu@TiO2 and xAu/TiO2 catalysts.

Fig. 4. Mott-Schottky plots for 0.25Au@TiO2 (a), 0.25/TiO2 (b), 0.5Au@TiO2 (c), 0.5Au/TiO2 (d), 1.5Au@TiO2 (e), and 1.5Au/TiO2 (f). Steady-state PL (g), time-resolved photoluminescence decay profiles (h), EIS Nyquist plots (i), and photocurrent density (j) at 0.5 V vs. Ag/AgCl under 300 W Xe lamp irradiation.

Fig. 5. In-situ DRIFT of 0.5Au/TiO2 and 0.5Au@TiO2 under adsorption and Xe lamp irradiation at 4000-3200 cm-1 (a), 2850-2810 cm-1 (b), and 1800-1400 cm-1 (c).

Table 3 The attribution of the characteristic peaks of in situ DRIFT.

Peak position (cm^-1)	Attribution	Ref.
3684	Isolated OH	[44]
3545	*OH (H₂O)	[45]
3430	*OH	[46]
3246	*OOH	[50]
850-960	proxy	[51]
970	*Ti-OOH	[47]
934	*Ti-(OO)	[47]
1690	*OH	[44]
1646	*OH₂	[44]
1636	H₂O (bending vibration)	[45]
3015	*CH₄	[46]
2824	Chemisorbed CH₄	[46]
1539	*CH₄	[46]
2347	CO₂	[4]
2127	CO	[4]
1720	*HCOOH (C=O)	[49]
1717	*CH₂O	[49]
1581	Bidentate *HCOO	[49]
1568	*OCO	[49]
1556	*OCH₃(CH₃OH)	[47]
1499	*HCHO	[48]
1456	*CH₃	[46]
1450	*CH₂	[46]

Fig. 6. The in-situ DRIFTs of 0.5Au/TiO2 (a-c,g) and 0.5Au@TiO2 (d-f,i) in the presence of H2O, O2 and CH4 under light irradiation at 50 °C. The in-situ DRIFTs intensity of the CH3OH and CO2 over 0.5Au/TiO2 (h) and 0.5Au@TiO2 (j).

Fig. 7. The in-situ DRIFTs of 0.5Au/TiO2 (a,b) and 0.5Au@TiO2 (d,e) under light after switched H2O+O2 atmosphere. The in-situ DRIFTs intensity of the Iso OH, *OH, Ti-OOH and CO2 over 0.5Au/TiO2 (c) and 0.5Au@TiO2 (f).

Fig. 8. The methane selective catalytic mechanism over Au/TiO2 and Au@TiO2.

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