Chitosan and carboxymethyl chitosan as catalyst matrix materials

Chitosan, a polysaccharide derived from crab and shrimp shells, was investigated as a matrix material for catalytic platinum, gold, and silver nanoparticles. Not only is chitosan able to stabilize nanoparticles, it is able to reduce metal ions to a zero valent state while controlling nanoparticle growth. The size, morphology, and catalytic activity of nanoparticles produced were examined in chitosan, and carboxymethyl chitosan (CMC), a chemically modified form of chitosan. Carboxymethyl chitosan is reported to have a higher metal ion binding capacity, but has not been previously tested as a support material for catalytic nanoparticles. Nanoparticles were characterized through UV-vis absorbance and transmission electron microscopy. Platinum nanoparticles have a narrow size distribution with an average size of 3.5 nm while gold and silver have much broader size ranges with average particle sizes of 23 nm and 7 nm respectively. The size and morphology of the nanoparticles did not vary significantly between the two polymers for each type of catalyst. However, a higher aggregation rate was observed in CMC. Nanoparticles supported on chitosan flakes were shown to be active, reusable catalysts. The catalytic activity of chitosan supported nanoparticles was monitored through UV-vis absorbance of the reduction of 4-nitrophenol in the presence of sodium borohydride. The characteristic absorbance peak of 4-nitrophenol is directly proportional to its concentration. By using an excess of sodium borohydride, pseudo first order rate kinetics can be assumed. Rate constants show that all three types of nanoparticles are catalytically active with silver nanoparticles having the highest rate constant followed by platinum and gold. The three catalysts did not change in activity after several runs when supported in chitosan; however, carboxymethyl chitosan supported catalysts were not appreciably retained by the matrix material.