In this review, the synthesis methods of single-atom catalysts (SACs), and the mechanisms of SACs for CO₂electrochemical reduction, photocatalytic reduction, and thermo-catalytic reduction were thoroughly summarized.

A novel Hilbert fractal photo microreactor (PMR) was proposed to investigate the photochemical synthesis of 2,5-hexanedione (2,5-HDN) through direct C–C coupling of acetone in the continuous-flow mode. The conventional fluorinated ethylene propylene (FEP) capillary PMR was used for comparison. The flow in the Hilbert fractal PMR was numerically analyzed by computational fluid dynamics (CFD). It was found that the original laminar flow was disturbed by the continuous bending in the Hilbert fractal configuration, leading to irregular distribution of velocity. The secondary flow was intensified by the increasing inlet velocity. Experimental results revealed that the reaction time was shortened by both PMRs from several hours of batch reaction to a couple minutes. Overall, the selectivity to 2,5-HDN decreased with the increasing residence time and H₂O₂ concentration, while the conversion of acetone followed an opposite trend. Compared with the capillary PMR, the Hilbert fractal PMR exhibited the most advantages at 2.5 min and permitted the increase of the 2,5-HDN selectivity by 7% for 5% H₂O₂. These observations were attributed to the introduced flow disturbance, leading to the enhanced mass-transfer rate. Furthermore, the Hilbert fractal PMR offered a higher apparent quantum yield (AQY), but its maximum generation rate of 2,5-HDN (10.5 mmol h⁻¹) was lower than that in the capillary PMR (20.2 mmol h⁻¹) mainly due to a much smaller reactor volume.