We report the fabrication of ultrathin Co-Fe spinel oxide nanosheets with adjustable stoichiometry confined in mesoporous SiO2 shells (CoxFe3-xO4@mSiO(2)) by encapsulating Fe-Co layered double hydroxides (FeCo-LDHs) with mesoporous SiO2 shells followed with a calcination process. In this way, the unique 2D structured CoxFe3-xO4@mSiO(2) nanosheets offers high specific surface area and intimate contact with NH3. The tunable stoichiometry of CoxFe3..04 nanosheets regulate the electron structure and thus optimize the nitrogen desorption ability. The encapsulation of mSiO(2) shells not only effectively facilitates the generation of ultrathin Co-Fe spinel oxides nanosheets with abundant active sites, but also protects CoxFe3-xO4 nanosheets from detachment and agglomeration during the NH3 decomposition reaction. Benefiting from these advantages, the optimal Co0.89Fe2.11O4@mSiO(2) nanosheet catalyst possesses 88% conversion of ammonia at 600 degrees C with a space velocity of 60 000 cm(3) g h(-1) and maintains even 48 h without attenuation.