As discussed above, a C-A mechanism can possibly be Ga-hydride-centered (region c, Fig. ), and these sites could be produced through a hydrogen gas treatment. To explore this possibility, we have analyzed the free energy landscapes of the Ga-hydride-centered C-A mechanism on both 3CN and 4CN sites (Figs. ). High activation barriers are involved in the β-hydride elimination steps (4CN: 2.5 eV, 3CN: 2.3 eV), and the alternative β-hydride transfer, producing Ga-ethyl, exhibits much lower activation barriers. Further, high activation barriers of ethylene insertion are consistently observed for the 4CN sites (2.9 eV). Therefore, we predict that the Ga-ethyl-centered C-A mechanism is the most favorable pathway on the Ga/SiO2 system, and the 3CN Ga sites exhibit a much higher turnover frequency than the 4CN ones. Hence, the 3CN sites likely have a major effect on the overall reactivity of the catalyst.