Molecular transport in porous media plays an essential role in heterogeneous catalysis. Here, we have studied a Polymer-Membrane-Catalyst Assembly (PCMA) system consisting of PET membranes with a well-defined pore structure coated with thermo-responsive polymer (poly(n-isopropylacrylamide) or p-NIPAM) coupled to an in-situ ATR-IR cell containing palladium supported on γ-Al 2O 3. This PCMA model is designed to mimic the structure of a core-shell catalyst coated with stimulus responsive polymer. At low temperatures (<32 °C), where p-NIPAM is in its swollen state, the addition of polymer brushes caused a delay in CO chemisorption on Pd. Strikingly, when the polymer collapses (>32 °C), we observed faster CO−Pd saturation. Our multi-physics model of the CO transport and chemisorption on the PCMA suggests that the delay is caused by the strong affinity of the CO molecules with the polymer brushes. When stimulus responsive polymers are present, a complex interplay between diffusion and absorption determines the dynamic behavior of the system. Furthermore, we demonstrate that this behavior could be reversed by reducing the ratio of polymer inside the pores and the surface, in which the delay observed is now above the LCST. This new insights on the dynamics of transport, absorption, and chemisorption processes occurring on so-called “nano-reactors” provide new opportunities for developing new self-regulating catalysts with faster transitions between ON- and OFF- states.