Given the structural dynamism of SACs during the reaction, their stability should be rigorously examined in the presence of adsorbates. In general, SACs do not have static local coordinations but can switch from inactive to active structure (or vice versa) under reaction conditions. For instance, the preferred local coordination of Rh single atoms was investigated on TiO2 during calcination in O2, reduction in H2, CO adsorption, and reverse water gas shift (RWGS) reaction conditions. Theoretical and experimental studies clearly demonstrated the dynamic response of Ru single atoms in the local coordination and reactivity to various redox conditions. As shown in Figure , the surface stability diagram in H2 indicates that the preferred structure on the Rh1/TiO2 (110) surface is governed by the H and O chemical potentials. Blue, light blue, and green regions denote the preferential substitution of six-coordinated surface Ti with zero, one, or two O vacancies by a Rh atom (Rh1@TiO2, Rh1@TiO2-x, Rh1@TiO2-2x). Meanwhile, the supported Rh structure is favored (Rh1/TiO2-x and Rh1/TiO2-2×) in the orange and pink zones, respectively. It is found that hydrogen pressure stabilizes both the substitutional and the supported Rh site, but the latter is more stable in the oxygen-lean conditions. Likewise, the CO adsorption completely changes the thermodynamic site preference for the Rh, driving them to a supported site irrespective of the presence of O vacancies. This is attributed to the much stronger CO adsorption on the supported Rh (-4.89 eV for Rh1/TiO2) than the substitutional Rh (-2.74 eV for Rh1@TiO2-2x).