Effect of Methacrylic Acid in PNNPAM Microgels on the Catalytic Activity of Embedded Palladium Nanoparticles

Poly-N-n-propylacrylamide based microgels with embedded Pd-nanoparticles are synthesized with varying amounts (5-25 mol%) of methacrylic acid (MAc) comonomer. The microgels are characterized to investigate the influence of the comonomer content on catalytic activity of the hybrid particles. Since the acidity changes significantly upon copolymerization, the incorporated amount and apparent p K-a value of the MAc via potentiometric titration are determined. The thermoresponsive properties of the microgels are characterized by means of photon correlation spectroscopy at pH values of 4 and 10, which correspond to the protonated and deprotonated state of the acidic copolymer according to the determined p K(a)values. MAc shows tremendous influence on the thermoresponsivity of the microgels not only related to the amount of MAc but especially upon change in pH. The prepared microgel systems are subsequently loaded with palladium nanoparticles. Their catalytic activity is investigated by the reduction of 4-nitrophenol. The catalytic degradation of the educt is monitored via UV-vis spectroscopy and the observed absorbance decay is analyzed by a pseudo-first order kinetic. Derived reaction rate constants are normalized with respect to the surface area of the palladium nanoparticles, revealing that the charge density and hydrophilicity within the microgel template greatly affect reactivity of embedded nanoparticles.