Study On The Performance And Mechanism Of CoNPs@ZIF-8 Nanocatalysts For Hydrolysis Of NaBH₄ To Hydrogen Production

Non-renewable fossil fuels are the main composition of the energy we can utilize,and the by-products of their combustion are environmentally harmful.As the energy is remaining decreased day by day,Human beings are looking for a renewable green new energy to replace fossil fuels.Hydrogen is considered to be the most promising green energy in the 21st century.Hydrogen generation from hydrogen-rich molecules is the key to the present energy and environmental problems.Among all hydrogen-rich molecules,NaBH₄ has attracted a great attention since its discovery by Schlesinger in the 1950's due to i)high purity of hydrogen;ii)controllable hydrogen production rate;iii)satisfactory hydrogen storage capacity;iv)high stability in alkaline solution;v)safe and pollution-free.Sodium borohydride hydrolysis catalyzed by noble-metal nanocatalysts has long been proposed as a viable possibility,but it requires expensive costs.Herein,the NaBH₄ hydrolysis is catalyzed by single earth-abundant Fe,Co,Ni,and Cu nanoparticles supported by a metal organic framework?MOF?,ZIF-8.These nanostuctures were characterized by Transmission electron microscopy?TEM?,X-ray photoelectron spectroscopy?XPS?,Brunauer-Emmett-Teller?BET?and Inductively coupled plasma optical emission spectroscopy?ICP-OES?.It appears that CoNPs@ZIF-8 is the most efficient and selective catalyst in this series.The turnover frequency?TOF?related to the total atoms number within the CoNP reaches 14023 mL_H2 min^-1 gCo^-1 at 30 ? and 25717 mL_H2 min^-1 gCo^-1 at 40 ?,and the TOF?s?related to the number of surface atoms of the CoNP reaches 19476 mL_H2 min^-1 g_surface Co^-1 at 30? and 35718 mL_H2 min^-1 gSurface Co^-1 at 40?.This compares with and overtakes most of the previous composite non-noble metal nanoparticles systems,showing an activity even superior to that of some noble metal nanoparticle catalysts.Kinetic studies including a high kinetic isotope effect disclosed using D₂O?k_H/k_D=6.85?and the favorable effect of NaOH lead to the suggestion of an oxidative addition of a O-H bond of water in the rate-determining step assisted by hydrogen bonding with a hydridic hydrogen of sodium borohydride.