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Aiming to explain passive regeneration reaction mechanism of catalytic diesel particulate filter, the diffusion mechanism of nitrogen dioxide is studied by experiment and simulation. Due to the wall layers are coated by precious metal, a chemical reaction is genesis in 300℃ between nitric oxide and oxygen. And reaction can produce nitrogen dioxide. In cake layer, passive regeneration consumes nitrogen dioxide. Therefore, concentration gradient of nitrogen dioxide is formed. The gradient comes into a driving force of the reverse diffusion. Diffusion mechanism includes pore diffusion and Knudsen diffusion in porous media of particulate filter. Because of back diffusion of nitrogen dioxide, it can participate in passive regeneration of soot many times. Research results show that the pressure drop of filter can be controlled within 7.0% offset and NO2 concentration can be controlled within 7.3% compared the experimental values with the passive regeneration model which considering the diffusion mechanism. The nitrogen dioxide diffusion effect is more obvious with the increase of inlet temperature. When inlet temperature is increased by 100℃，the increasing rate of NO2 concentration is 1.90~1.95 times of that when temperature is increased by 50℃. The maximum concentration of NO2 is 5.69×10-4 at the position of z/L=0.9, H=0.1829mm. Analysis of regeneration process of soot retained in the filter shows that a rise in temperature of inlet gas results enhancement of NO2 diffusion, and the passive regeneration rate is accelerated. Soot mass retained in the filter is decreased by 55.94% in the first 1000s at 425℃, and soot mass in the filter layer reacts 80.53%. The total decreased soot mass is 3.68 times of the reactive mass at 325℃. The variation of concentration for NO2 and the reaction rates of soot show that diffusion of NO2 can effectively increase the passive regeneration rate and extend the active regeneration period.