LAUSR

The high-calorific value and gas infrastructure compatibility of methane (CH4) position it as a key target in artificial CO2 photoreduction. However, the challenge of manipulating the surface coverage of *CO intermediate on catalysts significantly impedes the efficiency and selectivity of CH4 production during the eight-electron reduction process. Here, a hollow nanoreactor (HoNR) photocatalyst (hS-ZnSe/CdSe) with multi-chambered branches is demonstrated, achieving efficient and selective CH4 production under visible light. In situ DRIFTS reveals that the branched mesoporous shell can effectively regulate the dispersion and concentration of *CO intermediate, thereby promoting methoxy (*CH3O) formation, which is a significantly kinetic determinant of CH4 generation. The HoNR photocatalyst demonstrates a superior CH4 production of 215.5 µmol·g-1·h-1 with an electron selectivity of 92%, which surpasses most state-of-the-art photocatalysts, especially without using any noble metal cocatalyst. Moreover, the relationship between intermediate diffusion kinetics and final product selectivity in complex geometries cavities is quantitatively established via systematic simulation. This work revolutionarily leverages chamber-branched topological architectures to drive spatiotemporally coupled cascade reactions, establishing a potential paradigm for achieving high-efficiency photocatalysis.