Multifunctional chitosan hydrogel entrapped carbon nitride composite supported catalysts for efficient water pollutant reduction and hydrogen generation

A multifunctional composite hydrogel (CH/CN) was prepared by encapsulation of carbon nitride (CN) into the chitosan (CH) matrix crosslinked with formaldehyde. The 3D hydrogel network was also very effective for the electrostatic coordination of metal ions, in that it contained abundant hydroxyl (-OH) and amino (-NH2) functional groups, and physically trapped the in situ reduced zero valent nanoparticles (M0 = Cu0, Ag0, Ni0, Co0, Fe0) in its porous structure to prevent agglomeration and leaching effects. These formed composites (CH/CN@M0) were carefully examined for catalytic reduction of water contaminants and H2 production through NaBH4 methanolysis. CH/CN@Ag0 showed the highest activity for the reduction of pollutants, with 4-nitrophenol (4-NP) being reduced to 4-aminophenol completely within 8.0 min, and with a pseudo-first order rate constant (kapp) of 0.593 min–1. This high efficiency is believed to be due to a synergistic Langmuir-Hinshelwood mechanism in which the highly porous CH/CN matrix functions as an adsorbent hub to trap the reactants around the Ag0 active sites and promote the quick electron transfer from BH4- to 4-NP. The catalyst also exhibited excellent performances in real water matrixes, such as sea, well and irrigation water, with efficiencies of 4-NP reduction of 97–99%. Considering the production of H2, the CH/CN hydrogel also shows the best catalytic performance with a maximum hydrogen generation rate (HGR) of 3600 mL g–1 min–1 and the minimal activation energy of 15.81 kJ mol–1, due to the synergistic effect of the nitrogen rich CN framework and the high surface area of the porous CH hydrogel matrix. The recyclability of both CH/CN and CH/CN@Ag0 was observed for a number of successive cycles without significant loss of activity. These results establish CH/CN based hydrogels as sustainable, cost effective, and high performance catalytic platforms for simultaneous environmental remediation and clean energy generation.