<p>Hydrogen-assisted dehydrochlorination process (HaDHC) is an attractive route for the production of fluorine-containing olefins under relatively mild conditions, so far lacking the highly efficient metal-based catalysts and their design strategy. Here, we report a nano-MgF₂ supported Pd-Ag catalyst with a tunable Pd dispersion, which is optimized by the Pd-Ag alloy degree in the fresh catalyst and subsequently in situ chlorination during the induction period of reaction. After the in situ restructuring process, the resulting Pd-Ag/nano-MgF₂ catalysts with atomically dispersed Pd sites exhibited an excellent catalytic performance for HaDHC of 1,1,1,2-tetrafluoro-2-chloropropane (HCFC-244bb) to 2,3,3,3-tetrafluoropropene (HFO-1234yf), the new-generation refrigerant, with a conversion of ca. 60% and HFO-1234yf selectivity of ca. 82% at 270 °C. Characterization results reveal that the Pd-Ag alloy degree in the fresh catalyst can be facilely tuned by changing the impregnation sequences for bimetallic precursors during catalyst preparation due to different metal-support interactions. Constructing a high Pd-Ag alloy degree offers a high in situ chlorination degree of the catalyst surface to finally get highly isolated Pd sites. Adsorption and computational results demonstrate that the chemisorbed hydrogen species on the single atom Pd sites (Pd-F and Pd-F₃ sites) boost the HFO-1234yf formation, while the spillover hydrogen species derived from the large Pd ensembles (Pd₆, Pd₇, and Pd₈ clusters) contribute to the formation of deep hydrogeneration product, 1,1,1,2-tetrafluoropropane (HFC-254eb).</p><p></p>