Meanwhile, the sintering of Pd1 into larger crystalline Pd nanoparticles (~ 5 nm) during CO treatment will deactivate the catalyst, leading to progressive activity loss in the time profile. This indicates the importance of partial sintering into redispersible nanoclusters for an optimized stability. Similar observations were reported in the 0.17% Pt@MCM-22 for NO reduction with CO and H2, Pd1/CeO2 for methane oxidation, Ni@1T-MoS2 for hydrogen evolution, Cu1/CeO2 for electrocatalytic urea synthesis , etc. In an extreme case, unprecedented chemical stability under hydrogenation conditions was shown in the Ni1Cu2/g-C3N4 catalyst without any visible decline in either activity or selectivity for at least 350 h at 160 °C. In sharp contrast, Ni1/g-C3N4 showed a slight activity increase approximately in the first six hours and then rapidly deactivated with an activity loss of ~50% in about 50 h owing to severe agglomeration of Ni atoms into Ni particles. Very recently, Matteo Cargnello et al. demonstrated an opposite deactivation pathway via the decomposition of nanoparticles into inactive single atoms at high temperatures (Figure ), which is remarkably fast and strongly dependent on the particle density and concentration of support defect sites. As shown in Figure , the thermal stability of various Pd nanoparticles on Al2O3 with dense, intermediate, and sparse loadings was probed in methane combustion, where the sparse one was expected to be most stable due to a lower probability of particle migration and coalescence or interparticle atomic exchange. Surprisingly, the dense Pd/Al2O3 catalyst showed completely stable activity after high-temperature aging, while the sparse one rapidly deactivated with the conversion decreasing from 85% to 20%. Such size dependence strongly suggests that the atomic ripening process is limited by atomic emission related to the existence of a lowest-energy Pd(OH)2 adsorbate atop tri-coordinated Al atoms in Figure rather than surface diffusion of atomic species to a nearby site. Such nanoparticle-to-single atom deactivation process is usually overlooked at low metal loadings. Since the influence of dynamic aggregation on the activity, selectivity, and stability is always correlated, a radar plot charting the key catalytic descriptors allows better evaluation of various subnanometric catalysts (Figure ). For alkyne semi-hydrogenation, the stability could be assessed by the segregation energy (Eseg), which is the differential energy between the ground state (i.e., an alloyed metallic surface) and the system after segregating one atom towards the surface. The activity depends on the adsorption of alkyne and, more importantly, the hydrogen activation energy to split molecular hydrogen, while the selectivity depends on the adsorption energy of the alkene and the ensemble area. As shown in Figures &, it is found that the preferential hydrogen adsorption on Pd induces severe segregation effects in Pd3@C3N4 or other Pd nanoparticles. Such Pd segregation or islanding is not favorable in Pd3S/C3N4 due to the polarization of the Pd-S bond, thus outperforming all state-of-the-art catalysts in alkene formation rate and durability with no sign of segregation. This highlights multiple design criteria that need to be harmonized to develop an effective SAC.