Metal Nanoparticles Prepared In Spherical Polyelectrolyte Brush Nanoreactor And Their Catalytic Activity

In recent years, metal nanoparticles have received increasing attention due to their fascinating chemical and physical characteristics and potential technological applications. Because of their high surface-to-volume ratio, metal nanoparticles exhibited excellent catalytic performance in hydrogenation, oxidation, and reduction reactions compared to their bulk materials. Spherical polyelectrolyte brush (SPB) is a core-shell structured nanoparticle with unique properties introduced by the polymer chains, such as thermo-sensitivity and pH-response. Due to the Donnan effect of the polyelectrolyte chains, the inside environment such as pH value and ions concentration can be totally different from the one outside. The thickness of the SPB (L) can be tunable by the temperature, pH value and salt concentration. This provides us a new method to synthesize the metal composite nanoparticles and nano-scale enzyme carrier.The SPB studied here consists of a polystyrene (PS) core and poly(acrylic acid) (PAA) chains. At first, we use the SPB as nanoreactor for the generation of Ni-NPs with controlled size and size distribution. Using the reduction of 4-nitrophenol as the model reaction to test the catalytic activity of the Ni-NPs composite nanoparticles. We also synthesized the silver nanoparticles in SPB nanoreactor and using the reduction of 2-nitrophenol and 4-nitrophenol to test the catalytic activity of the Ag-NPs composite nanoparticles. Finally, we synthesized the large size SPB which has a 300 nm'PS core. Then we used these particles to prepare the silver nanoparticles and test their catalytic activity aiming at the large size silver catalyst in chemical industry.This study focused on the synthesis and catalytic activity characterization of Ni-NPs and Ag-NPs in SPB and main work includes:1. Using SPB as nanoreactors, the Ni2+ ions were concentrated as counterions within the SPB layer and reduced by NaBH4 to narrowly dispersed Ni-NPs. When the reaction temperature of the reduction decreased from 303 to 273 K, the average diameter of the obtained Ni-NPs reduced from 7.5 to 3.4 nm. The size of the prepared Ni-NPs can be well controlled by the reaction temperature. The Ni-NPs immobilized on SPB showed high catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol by NaBH4 as monitored by the UV-vis spectrophotometer. From the kinetics data we obtained the activation energy of 41,7 kJ/mol, the pre-exponential factor of 1.56×104 s-1, and the entropy of activation of 80.3 J/(mol×K).2. The size-controlled Ag-NPs were synthesized in SPB nanoreactor. The average size of Ag-NPs synthesized under the temperatures 273 K,303 K and 333 K are 2.5 nm,2.8 nm and 3.1 nm, respectively. The Ag-NPs immobilized on SPB showed high catalytic activity for the reduction of 4-nitrophenol and 2-nitrophenol as monitored by the UV-vis spectrophotometer. From the kinetics data we obtained the activation energy Ea, pre-exponential factor A, and the entropy of activation△S for the reduction of 2-nitrophenol were 54.4 KJ/mol,5.66×106 s-1, and 129.3 J/(molxK), while for the reduction of 4-nitrophenol they were 59.4 KJ/mol,2.85×107 s-1, and 142.7 J/(molxK), respectively. From the data we can concluded that the Ag-NPs can show higher catalytic activity when as catalyst for the reduction of 2-nitrophenol than 4-nitrophenol. When compared to the Ni-NPs, we found that the Ni-NPs had the higher catalytic activity than Ag-NPs for the reduction of 4-nitrophenol.3. We synthesized the PS core with the size of 300 nm and SPB with the size of 700 nm. Using TEM we observed the microstructure of the SPB. The large size SPB has the same salt effect and pH-response behaviors as small size one. The Ag-NPs were successful generated in the large SPB particles and also showed good catalytic activity to the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride.