Tuning radical generation rate for efficient CH4 photooxidation to CH3OH over AgPd alloy and Co3O4 cascade active sites

doi:10.1016/S1872-2067(25)64854-6

Abstract

Abstract:

The direct conversion of methane into methanol under mild conditions represents a highly appealing pathway. Regulating the generation of hydroxyl radicals (•OH) is a representative method, but excessive release of •OH will inevitably lead to the over-oxidation of CH₃OH. Here, we design AgPd alloy and Co₃O₄ cascade active sites on the TiO₂ surface (AgPd-Co/TiO₂) to control the release rate of •OH to improve the selectivity of CH₃OH. By incorporating Co₃O₄ as a hole buffer and storage center, the kinetics of •OH generation at the TiO₂ interface can be effectively modulated. This confines the spatial distribution of •OH to the active sites of the AgPd alloy, thus facilitating the directional combination of •CH₃ and •OH. The optimal AgPd-Co/TiO₂ photocatalyst demonstrates outstanding catalytic performance with the selectivity of CH₃OH reaching up to 93% in the liquid phase. AgPd-Co/TiO₂ exhibited significantly enhanced selectivity relative to reported TiO₂-based photocatalytic systems, while simultaneously achieving comparable methanol yields. This research offers valuable insights for the precise design of composite photocatalysts to achieve highly selective methane oxidation.

Key words: Methane conversion, AgPd alloy, Co₃O₄, TiO₂, Photocatalysis

Shuqi Liang, Zhen Xiao, Jinni Shen, Wenxin Dai, Zizhong Zhang. Tuning radical generation rate for efficient CH₄ photooxidation to CH₃OH over AgPd alloy and Co₃O₄ cascade active sites[J]. Chinese Journal of Catalysis, 2026, 80: 189-199.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(25)64854-6

https://www.cjcatal.com/EN/Y2026/V80/I1/189

Figures/Tables 6

Fig. 1. (a,b) XRD patterns of different photocatalysts. (c,d) TEM images of Ag1Pd0.5-1Co/TiO2. (e) High-resolution TEM of Ag1Pd0.5-1Co/TiO2. (f) Element mapping of Ag (pink), Pd (cyan), Co (yellow) of Ag1Pd0.5-1Co/TiO2. XPS spectra for Ag 3d (g), Pd 3d (h), and Co 2p (i) of different photocatalysts.

Fig. 2. (a) Performance test of samples featuring different Ag loading ratios. (b) Photocatalytic CH4 oxidation performance over TiO2, Ag1Pd0.5/TiO2, 1 wt% Co3O4/TiO2, Ag1Pd0.5-1Co/TiO2. (c) Comparisons with the representative photocatalysts on the yield and selectivity of CH3OH in liquid phase. Effect of CH4 pressure (d) and O2 pressure (e) on photocatalytic CH4 oxidation into CH3OH over Ag1Pd0.5-1Co/TiO2. (f) Effect of H2O volume on photocatalytic CH4 oxidation into CH3OH over Ag1Pd0.5-1Co/TiO2.

Fig. 3. In situ XPS spectra of Ti 2p (a), O 1s (b), Co 2p (c), Ag 3d (d), and Pd 3d (e) in dark and under light irradiation for 10 min. (f) Time-resolved PL spectra of TiO2, 1 wt% Co3O4/TiO2, and Ag1Pd0.5-1Co/TiO2.

Fig. 4. (a) In-situ EPR spectra of DMPO-·OH over different photocatalysts. (b) Radical trapping agent experiment over Ag1Pd0.5-1Co/TiO2. (c). GC-MS spectra of CH3OH produced in the presence of 18O2 and H216O or 16O2 and H218O over Ag1Pd0.5-1Co/TiO2. (d) PL spectra of 7-hydroxycoumarin by ?OH radicals generated over Ag1Pd0.5-1Co/TiO2 with or without O2 incorporation. (e) DMPO-·OH over Ag1Pd0.5-1Co/TiO2 with the addition of scavengers at different concentrations. (f) The relationship between the relative intensity of ?OH in metal-loaded catalysts and the selectivity of CH?OH in liquid products.

Fig. 5. (a) Gibbs free energy profiles for O2 hydrogenation on AgPd/TiO2 and AgPd-Co/TiO2. The numbers in parentheses denote the electron transfer to oxygen. Color code: light blue (Ti), red (lattice O), cyan (Pd), gray (Ag), blue (Co), orange (oxygen), and green (hydrogen) spheres. In-situ IR spectra of Ag1Pd0.5/TiO2 (b) and Ag1Pd0.5-1Co/TiO2 (c) with different light irradiation times under the atmosphere of coexistence of CH4, O2. (d) The IR absorption peaks of the relevant intermediates emerged within the range of 2250?2500 cm?1. Color code: red (Ag1Pd0.5/TiO2), blue (Ag1Pd0.5-1Co/TiO2).

Fig. 6. The reaction mechanism of CH4 photocatalytic conversion to CH3OH over Ag1Pd0.5-1Co/TiO2.

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Tuning radical generation rate for efficient CH₄ photooxidation to CH₃OH over AgPd alloy and Co₃O₄ cascade active sites

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