Formation of Copper Nanoparticles in LTL Nanosized Zeolite: Kinetics Study

The kinetics of formation of copper nanoparticles in LTL type nanosized zeolite (Cu-LTL) was investigated. The water suspensions containing copper ion exchanged zeolite crystals (Cu2+-LTL) were treated with three reducing agents (hydrazine monohydrate, triethylamine, and sodium borohydride) under identical conditions. With triethylamine, copper particles (Cu-0 NPs) were not formed, while with sodium borohydride, a rapid reduction of Cu2+ leading to Cu-0 nanoparticles predominantly located on the external surface of the LTL zeolite nanocrystals was observed, whereas the reaction with hydrazine led to slow reduction and formation of Cu-0 NPs within the LTL zeolite channels. The kinetics of formation of Cu-0 NPs in the LTL zeolite suspension was investigated by in situ UV-vis spectroscopy. Additionally, the solid samples (pure LTL, Cu2+-LTL and Cu-0-LTL) were isolated before and at different stages of reduction and investigated with XRD, nitrogen sorption, and high-resolution transmission electron microscopy (HRTEM). The initial copper nanoparticles with a size of 0.2-1.6 nm were observed after 190 min of reduction in the presence of hydrazine, and under prolonged treatment (from 280 to 960 min), the copper nanoparticles tend to migrate to the external surface of the zeolite crystals and aggregate into large copper entities. The structure of the nanosized LTL zeolite crystals was preserved from the initial to the final stages of reduction.