The confinement of refined catalytically active substances in special nanospaces is beneficial for promoting reaction processes by taking advantage of the quantum confinement effect and enlarged atomic efficiency. Herein, we present a redox nanoporous molecular sieve nanocatalyst containing size-confined active MnO_x in the siliceous nanospace of mesopores via a functional micelle-assisted in situ embedding strategy. We also demonstrate its excellent environmental catalytic efficiency toward the oxidative degradation of methyl orange in the liquid phase (8 min) and volatile toluene in the gas phase (T₉₀/T₁₀₀ = 225/240 °C, under the mass space velocity of 60 000 ml g⁻¹ h⁻¹ and concentration of 1000 ppm, which surpass noble metal catalysts and most composite catalysts). This was achieved by virtue of eliminating diffusion limitations, and great active atom-efficiency. In this synthetic strategy, the well-made resultant MnO_x had a refined size and greater interaction with diffusing reactant molecules without impeding their diffusion and mass transfer. Excellent and durable catalytic efficiency was demonstrated, due to the superior characteristics concerning the great dispersion of MnO_x and favorable structure, thereby confirming that high catalytic removal efficiency depends on the amount of accessible MnO_x rather than the loading of active species. Our findings provide the first example of a heterogeneous environmental catalyst that affords highly efficient catalytic elimination of organic contaminants in both liquid and gaseous systems.