Transition from heterogeneous catalytic reaction to homogeneous one by variation of palladium particle size

Reduction of methylene blue with hydrazine was catalyzed by nearly mono-dispersed palladium particles of which diameters (2r0) ranged from 30 nm to 230 nm. The spectroscopically evaluated pseudo-first order reaction rate constants were proportional to r 0 3 α , where 3α varied from 2 to 3 with a decrease in the radius. The catalysis at large r0 proceeded heterogeneously, whereas that at small r0 occurred homogeneously. The relation between α and r0 allowed us to evaluate the thickness of the reaction layer to be 20 nm. The analysis of the diffusion–reaction model demonstrated the thickness did not vary largely with r0.