The rapid increase of the concentration of atmospheric CO2 and the imperative for solar-fuel generation highlight the urgent need for advanced catalytic materials. Single-atom catalysts (SACs) anchored on two-dimensional (2D) materials present a powerful approach by combining atomically isolated active sites with tunable coordination environments, defect chemistry, and enhanced charge-transport pathways intrinsic to 2D supports. This review explores SAC/2D systems for photocatalytic CO2 reduction, beginning with the fundamental thermodynamics and kinetics of CO2 activation, multielectron proton-coupled transfer, hydrogen-evolution competition and C1 vs. C2+ product selectivity. Then, it covers synthesis strategies for anchoring atomically dispersed metal sites on 2D supports, and discusses advanced characterization techniques‒including atomic-scale imaging, operando coordination spectroscopy and time-resolved carrier-dynamics measurements; that link structure and photocatalytic function. Photocatalytic performance trends across carbon-based, metal-oxide and emerging 2D supports are analysed to illustrate how planar confinement, support polarity, interfacial coupling, and charge-carrier behaviour regulate activity, selectivity, and stability. Further, a unified design framework is established, and three framework-guided design strategies i.e., heteronuclear dual-atom sites, heterostructure interfaces and internal-field/polarization engineering, are highlighted as promising strategy to overcome several intrinsic and scale-up challenges i.e., single atom instability, low multicarbon (C2+) selectivity and rapid recombination. Finally, by aligning mechanistic insight with material design and pointing toward reproducible high-loading synthesis and device-oriented configurations (e.g., thin-films, flow-reactors), the review outlines a pathway toward selective, stable and scalable SAC/2D photocatalyst systems for solar-driven CO2 reduction.
