Take the CO2 reduction reaction (CO2RR) as an example; most SAC catalysts tend to convert CO2 into C1 products such as HCOOH, CH3OH, and CH4 but fail to obtain the more profitable C2 + hydrocarbon products (ethanol, ethene, etc.) due to the lack of nearby active sites for the C-C coupling steps between two * CO intermediates. (FE) of 91% at -0.7 V and an onset potential of -0.4 V. This is further validated by Karabiner et al. using operando X-ray absorption (XAS) under the working conditions of CO2RR. For the gas-phase hydrogenation of CO2, the single-atom Cu-Zr catalyst with Cu1-O3 units is the sole contributor to the desirable methanol. Meanwhile, the presence of small copper clusters or nanoparticles leads to the formation of CO by-product. Similarly, such dynamic aggregation of Pt SACs to nanoparticles will induce the switching of the two-electron pathway to the fourelectron pathway in the oxygen reduction reaction (ORR), producing H2O instead of H2O2. The reason lies in the side-on adsorption configuration of O2 on Pt nanoparticles, in comparison with the end-on configuration on Pt SACs with a shorter O-O bond length. The change in catalytic selectivity via dynamic aggregation is universal to many reactions, whose (irreversible)