Recently, it was reported that CO2 can serve as a carboxylating agent for the selective conversion of CH4 into CH3COOH using Co3O4 as a bifunctional catalyst. However, despite the sequential activation of both reactants, the practical CH3COOH productivity remained insufficient for industrial applications. Herein, a series of Co3O4-encapsulated mesoporous silica catalysts (denoted as Co3O4@mSiO2) were synthesized. These bifunctional core@shell architectures featured precisely tuned core diameters and shell thicknesses, achieved by systematic control of the precursor ratios. TEM and XRD analyses confirmed the successful encapsulation of nanocrystalline Co3O4 within the mesoporous silica framework, with dimensions ranging from 4.7 to 18.8 nm (core) and 16.9 to 60.1 nm (shell). Catalytic evaluations revealed that the CH3COOH yield is governed by a synergistic interplay between the core and shell dimensions. Specifically, the optimized architecture (9.9 nm core and 24.4 nm shell) delivered a peak yield of 194.2 μmol gCo3O4−1, significantly outperforming counterparts with smaller cores or thinner shells. Ex situ FT−IR spectroscopy elucidated that this optimal configuration maximized CO2 activation as surface carbonate species, facilitating the subsequent reaction with CH4. Furthermore, the mesoporous silica shell effectively suppressed sintering, ensuring structural integrity and stable performance over 13 consecutive reaction cycles.