N-isopropylacrylamide polymer brushes alter the micro-solvation environment during aqueous nitrite hydrogenation on Pd/Al₂O₃ catalyst

Nitrite contaminants in freshwater streams, resulting from run-off of fertilizers and livestock farming, are a major ecological challenge. Here, we have developed a new family of catalysts based on Pd/Al2O3 coated with N-isopropylacrylamide polymer (p-NIPAM) brushes that increase N-N bond formation over N-H bond formation, promoting nitrogen selectivity by 3-fold, reaching >99% for the Pd/Al2O3 containing 20 wt% carbon in the form of p-NIPAM, without significant drops in catalytic activity (TOF of c.a. 6.8 ± 1.1 min−1). Strikingly, rigorous mass transport studies revealed that the presence of p-NIPAM does not limit the transfer of molecules during the hydrogenation of nitrites in aqueous phase. These observations were corroborated by detailed reaction kinetics in which similar activation barriers for nitrites disappearance of 30–34 kJ mol−1 were obtained regardless the polymer content. The observed reaction orders for nitrites were similar on the coated and un-coated catalysts, indicating that the rate determining step, most likely NOX-H bond formation, remained unaltered. The apparent barriers for ammonia formation, however, drastically increased from 41 ± 3 kJ mol−1 on Pd/Al2O3 to 63 ± 4 kJ mol−1 and 76 ± 5 kJ mol−1 on the 7 and 20 wt% C counterparts, respectively. Contrary to the widely accepted operation mode of thermo-responsive catalysts containing p-NIPAM brushes, we demonstrated that these polymers modify the chemical environment near the active site as shown by in-situ ATR data, tuning the catalyst selectivity without altering the molecular transport. These results will facilitate the development of more selective catalysts for liquid phase reactions relevant for drinking water purification.