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Stable and catalytically active nano particles with small sizes are of paramount importance in catalysis, but the preparation of such particles has been a great challenge due to their propensity to aggregate into large and less active particles. Herein, we would like to report the formation of ultra-small and uncommon RhO 2 particles within an organic cage in the homogeneous solution, and the soluble assembly showed both better durability and high activity towards ammonia borane methanolysis to generate hydrogen gas. A volcanic plot of the turnover frequencies (TOFs) over the molar amount of Rh per cage was demonstrated, and RhO 2 -30@COP1-T showed a TOF of around 286 min−1 at room temperature. Our assembly also showed better durability during the catalysis in contrast to NPs made from pure rhodium acetate, and there was no aggregation formed after five runs. We further demonstrated the Michaelis-Menten model through the kinetic study, an indication that the catalytic reaction happened within the organic cage. The actual active sites of the catalyst was zero-valent metal Rh. Interestingly the installation of azobenzene groups at the opening of the cage led to gated activity through the trans-cis conformation interchange, due to the different steric hindrances affecting the diffusion rates of reactants. • Ultra-small RhO 2 particles are formed through mild conditions. • RhO 2 particles are formed within an organic cage in the homogeneous solution. • Light-gated catalysis phenomenon through the azo conformation. [ABSTRACT FROM AUTHOR]