Theoretical Study On The Hydrolysis Dehydrogenation Of NH₃BH₃ Catalyzed By Transition Metal Clusters

Hydrogen as an ideal energy carrier has drawn extensive attention,due to the aggravation of environmental pollution and depletion of traditional fossil energy caused by the heavy use of fossil fuels all over the world.The key factor in moving toward the"hydrogen economy"is to find safe and efficient hydrogen storage materials.Ammonia-borane?AB,NH₃BH₃?is believed to be an attractive solid hydrogen storage candidate owing to its high hydrogen content?19.6 wt%?,nontoxicity,radiation free,excellent solubility,and high dehydrogenation efficiency etc.The way of dehydrogenation from ammonia borane mainly includes thermolysis reaction in solid state,hydrolysis and methanolysis reaction in solution.Thermolysis of AB to generate hydrogen is easy to produce byproduct and it has to undergo several step reactions at very high temperature.The high cost of the methanolysis is unfavourable for the wide application of AB.While the hydrolysis dehydrogenation reaction of AB can proceed at room temperature in the presence of suitable catalysts.Although noble metal based catalysts have high activity towards the reaction of AB in liquids,the high cost of precious metals makes them less economically viable Hence,it becomes a hot topic to find high active and low-cost catalysts to make AB completely release its hydrogen under moderate conditions.In this article,the dehydrogenation styles and reaction mechanism for hydrolysis of AB catalyzed by transition metal clusters?TM?have been investigated by the first principles density functional theory method.The main contents are as follows:?1?Using the generalized gradient approximation?GGA?with Perdew-Wang's 91exchange and correlation functional?PW91?,we have examined the various possible adsorption styles of small molecules?AB and H₂O molecule?on small size alloy clusters?NiCu dimer and Ni₃Cu cluster?,and the most stable adsorption structure is found.?2?In addition,Harmonic vibrational frequencies,the frontier molecular orbitals?HOMO,LUMO?,Mulliken atomic charges,bond lengths between atoms and other related properties have been calculated for analysis and prediction of hydrolysis dehydrogenation of AB.?3?We have explored several reaction pathways for the hydrolytic AB catalyzed by Ni-Cu alloy cluster under the same calculation method.In the process,two attacking types of water molecules are considered for the hydrolytic reaction of AB:stepwise and simultaneous adsorption on the catalyst for reactions.?4?Finally,the best reaction paths were given and the possible reaction mechanism were analyzed at a molecular level.Our research find that?1?the Ni and Cu metal atoms play the distinctive roles in catalytic activity,i.e.,Ni atom takes reactions for H₂O decomposition with the formation of[OH]^-group whereas Cu atom takes reactions for hydride transfer with the formation of metal-dihydride complex.?2?The hydrolytic dehydrogenation feature of AB is that hydridic H?B?atoms from–BH₃ edge of AB would like to interact with protic H?O?atoms from H₂O,successively aggregating and forming three H₂ molecules.?3?No matter which type of water attack,we find that the interaction distancen between H?O?and H?B?on Cu atom is shorter than that on Ni atom through the bond length analysis.When H?B?and H?O?are aggregated on the Cu atom,the interaction distance is about 0.77?.?The bond length of H₂ molecule is0.747??.This indicates that hydrogen is more easily detached from Cu atom.The formation of Cu-dihydride is a prerequisite for hydrogen release with high efficiency.?4?The activation energy barrier for hydrolytic AB was most likely caused by the hydride transfer from H?B?to metal atom rather than the B-N bond broken.The formation of heterometallic NiCu bonds within distinct electronegativity might be the key factor to tune the bonding pattern of the catalyst surface to the reactant molecules?AB and H₂O?.