Ag⁰ and Au⁰ nanoparticles encapsulated in sol-gel matrices as catalysts in reductive de-halogenation reactions

Halogenated pollutants have low solubility, are toxic, tend to accumulate in food chains and are the contaminates most often found in the subsurface environment. Various approaches to achieve their degradation have been applied, including bioremediation, chemical methods based on zero valent iron and bimetallic particles, and electro-catalysis. In this review we discuss our recent results concerning Ag⁰ and Au⁰ nanoparticles incorporated within SiO₂ matrices via the sol-gel synthesis route as heterogeneous catalysts for the reductive de-halogenation of halogenated organic compounds by NaBH₄. Matrices with different compositions were prepared in order to optimize the preparation procedure. The reduction percentage and the product distribution are affected by the matrix composition and the reduction conditions; pH, substrate concentration and NaBH₄ rate of addition. The role of the encapsulated metal nanoparticles on the reductive de-halogenation of halo-acetic acids is discussed.