MOF encapsulation derived slow-release oxygen species to enhance the activity and selectivity of methane selective oxidation: A transient DRIFTs Study

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

Abstract

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

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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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64803-0

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

Figures/Tables 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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