Unraveling the Critical Role of Li in Au/Li-TiO₂ Photocatalysts for Enhanced Photocatalytic Oxidative Coupling of Methane

Photocatalytic oxidative coupling of methane (OCM) represents a promising route for chemical and fuel synthesis, yet the development of photocatalysts with high activity and selectivity remains challenging. Here, we demonstrate a synergistic ensemble of Au nanoparticles and Li-doped TiO2 that significantly enhances photocatalytic OCM performance in a flow reactor. The optimized catalyst achieved a C2 hydrocarbon production rate of 4.1 mmol g–1 h–1 with selectivity up to 97% and maintained robust stability over 50 h. Comprehensive mechanistic studies, employing in situ transient absorption spectroscopy (TAS), in situ electron paramagnetic resonance (EPR), and operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) techniques, indicate that Li dopants facilitate the formation of bulk/surface defects by a unique bulk-to-surface-to-gas Li migration process. These induced defects serve a dual role: bulk defects act as effective hole traps, while surface defects provide active sites that enhance charge transfer and promote O2 adsorption/activation. Combined with Au acting as a methane activation center, this synergy promotes efficient C–H activation and C–C coupling, thus contributing to its superior catalytic performance. This work provides critical mechanistic insights into photocatalytic OCM and underscores the potential of surface and bulk engineering for efficient photocatalytic processes.