The onset of mass transport limitations triggers the stimulus responsiveness of polymer coated catalysts

A series of Pd/Al₂O₃ catalysts coated with N-isopropylacrylamide polymer (p-NIPAM) brushes with increasing particle size of the support were prepared to study the interplay between mass transport limitations and the ability of the thermo-responsive catalysts to modify reactivity during reaction. Nitrite hydrogenation in water was chosen as probe reaction, which is a key step in the reduction of nitrites and nitrates from drinking water. The results show that the apparent activation energy decreases on increasing temperature above the LCST, i.e. the temperature where the polymer undergoes a phase change, for catalyst with particle sizes between 38 and 100 μm, alluding to a significant increase in mass transfer limitations. In sharp contrast, both p-NIPAM modified catalysts with smaller particle size and uncoated catalysts do not show this change in activation energy with temperature. Detailed multi-physics mass transport and reaction modelling indicated that the transport of the limiting reagent, hydrogen in this case, is severely reduced when the p-NIPAM collapses at temperatures above its LCST. It is concluded that effective reaction control using stimulus responsive polymers requires the system to be close to the mass transport limitation regime, to maximize the effect of the conformation change of the polymer on the catalyst performance and achieve sharp and reversible transitions from active to inactive.