Efficiently Light-Driven Nonoxidative Coupling of Methane on Ag/NaTaO₃: A Case for Molecular-Level Understanding of the Coupling Mechanism

Abstract

Metal-decorated oxide semiconductors are overwhelming photocatalysts for nonoxidative coupling of methane (NOCM). However, the overall NOCM mechanism remains an unopened black box, which hinders the design of high-performance catalysts. Herein, we systematically studied a series of noble metal (Ag, Au, Pt, Pd, Cu, and Ni)-decorated oxides (NaTaO3, CaTiO3, LiNbO3, and TiO2) for NOCM. We proposed that the active sites for H abstraction and C–C coupling of CH4 are spatially separated. Specifically, NaTaO3 only completes the initial H abstraction of CH4 activation, while metal nanoparticles are responsible for the final C–C coupling. Noble metals dominate NOCM by significantly decreasing the energy barrier of CH4 dissociation and promoting C–C coupling. Among various metals, Ag is preferential for the weak adsorption of ·CH3 intermediates and subsequent metal-induced C–C coupling. This contributes to Ag/NaTaO3 the highest C2H6 yield of 194 μmol g–1 h–1 and stoichiometric H2 with 11.2% quantum efficiency. This work provides a molecular-level insight into the CH4 coupling mechanism on metal-decorated photocatalysts.

Photocatalytic Oxidative Coupling of Methane to Ethane Using Water and Oxygen on Ag3PO4-ZnO

“…These results clearly demonstrated that the decrease of the activity on 1.5 Ag 3 PO 4 -ZnO may be caused by the reduction of Ag + , and then remained stable when the Ag NPs and Ag 3 PO 4 -ZnO form a stable heterojunction. According to a previous study, the Ag NPs favor the desorption of *CH 3 , so the formation of Ag NPs prevents the excessive oxidation of *CH 3 and improves the selectivity of C 2 H 6 and C 2 –C 3 hydrocarbon production …”

“…According to a previous study, the Ag NPs favor the desorption of *CH 3 , so the formation of Ag NPs prevents the excessive oxidation of *CH 3 and improves the selectivity of C 2 H 6 and C 2 −C 3 hydrocarbon production. 34 Study of the Process Mechanism. To investigate the charge separation efficiency of the photocatalysts, TPC, EIS, and steady-state PL tests were carried out.…”

Co0−Coδ+ Interface Double‐Site‐Mediated C−C Coupling for the Photothermal Conversion of CO2 into Light Olefins

“…To further confirm the reaction mechanism, we collected in situ DRIFTS spectra to investigate the reaction pathways occurring on the different samples during the hydrogenation of carbon-13 labelled carbon dioxide ( 13 CO 2 ) (Figure 4c). The relative intensity of the peaks observed at 3012 cm À 1 (Figure 4a) and 3002 cm À 1 (Figure 4c) corresponding to 12 CH 4 and 13 CH 4 , respectively suggest that over the Co/ MAO catalyst after the introduction of light, CH 4 was predominantly formed from CO 2 . In addition, in the spectra collected immediately on the CoÀ CoO x /MAO after light irradiation, the positions of the * 13 CO peaks observed at 2146 and 2034 cm À 1 were significantly different from that of the * 12 CO peak (Figure 4b).…”

“…More importantly, the different electron distribution and valence state of two different metal active sites at the interface may cause the charge distribution of the C 1 intermediate to be significantly different, thus reducing the C 1 electrostatic repulsion, promoting the C−C coupling reaction. In addition, the active sites on the interface can be proposed to overcome the long‐distance migration of C 1 intermediates on the asymmetric active sites [13] . Therefore, the construction of charge asymmetric active sites at the interface of two‐phase catalysts has great potential in the CO 2 reduction reaction used to form C 2+ hydrocarbons.…”

“…In addition, the active sites on the interface can be proposed to overcome the long-distance migration of C 1 intermediates on the asymmetric active sites. [13] Therefore, the construction of charge asymmetric active sites at the interface of twophase catalysts has great potential in the CO 2 reduction reaction used to form C 2 + hydrocarbons.…”