Design And Simulation Of Ammonia Borane Dehydrogenation Catalyst Based On Two-dimensional Material Surface

Today,with the continuous consumption of fossil energy,we are forced to seek some new clean,cheap and renewable energy sources to replace traditional fossil fuels.Among many new energy sources,hydrogen energy is considered as one of the best candidates to meet the growing demand for efficient and clean energy carriers due to its many advantages such as high efficiency,non-toxic,clean and renewable.However,there are still many challenges on how to store and utilize hydrogen energy efficiently and safely.In these decades,hydrogen production from ammonia-borane,a solid-state hydrogen storage material,has been regarded as a promising method for hydrogen production.At present,ammonia-borane hydrogen production mainly focuses on two aspects of hydrogen production efficiency and hydrogen production mechanism.In this thesis,we investigate the related aspects of the catalytic ammonia-borane hydrogen production on the surface of two-dimensional materials using the density functional theory（DFT）approach,mainly discussing the practical effects of the applied electric field,alkaline environment,doping and loading on the ammonia-borane hydrogen production,and focusing on the relationship between the electronic structure changes and the catalytic activity of the catalyst and molecules to further explain the catalytic mechanism.Specific studies include the following aspects:（1）Employing first-principles calculations,we have investigated the adsorption behavior of ammonia borane（NH₃BH₃,AB）molecules loaded on the surface of the two-dimensional material Mg Si P₂ in three different solvents.We found that the adsorption capacity of AB for ammonia-borane was reduced to some extent in all solventized conditions.Also,we focused on the activation of AB adsorbed on Mg Si P₂ sheets under solventized conditions with and without external electric field.The results show that AB has a better activation effect on the B-H bond of AB in alkaline aqueous solutions than in neutral aqueous solutions.The anion OH^-is more favorable for the activation of B-H bonds than the metal cations（Na⁺or K⁺）,which is more evident in the presence of an external electric field.Under an external electric field（0.003 a.u.）,the B-H bond is prolonged by about 12%compared to the free state of the AB molecule,and for the metal cation,the B-H bond is still prolonged by about 10.6%.The main reason for its enhanced activation is that the external environment enhances the transfer of electrons from the catalyst to the AB molecule,which leads to a strong activation of the B-H bond in AB.This study provides a new research method for the release of H atoms from AB molecules to promote H₂production.（2）After studying the properties related to the adsorption behavior of AB on the Mg Si P₂surface,we investigated the dehydrogenation reaction of AB on the Mg Si P₂ surface using an electrocatalytic approach.Two conventional intramolecular dehydrogenation pathways in the AB hydrolysis reaction,B-terminal dehydrogenation（rate determining step（RDS）of 0.71 e V）and B-N bond dissociation dehydrogenation（RDS of 1.00 e V）,were identified through our study.A new dehydrogenation reaction route,namely BN alternate dehydrogenation,was also explored.The reaction heat of RDS is 0.71 e V.Notably,the entire reaction process in the BN bond dissociation pathway allows the complete release of the six H protons in AB.With the help of H₂O dissociation,an H₂ output ratio of 1:6 can be achieved,which greatly improves the efficiency of hydrogen production from ammonia borane.In addition,based on the first findings,we also considered the effect of*OH^-on the decisive speed step of each dehydrogenation reaction in an alkaline environment.It was calculated that the presence of*OH^-facilitated the reduction of the reaction heat of dehydrogenation in the AB hydrolysis reaction,changing the reaction heat of RDS for B-terminal dehydrogenation from 0.71 e V to 0.15 e V,the B-N bond dissociation dehydrogenation from 1.00 e V to 0.09 e V,and the BN alternate dehydrogenation from 0.71 e V to 0.15 e V.This suggests that*OH^-greatly facilitates AB dehydrogenation to produce H₂.（3）Based on density functional theory,we also systematically investigated a series of transition metal atom-loaded P₃C（P₃C＿O）sheets to screen the most promising AB dehydrogenation catalysts.The results show that 2D Os/P₃C and Os/P₃C＿O can be an effective single-atom catalysts（SACs）.Taking the first molecule of H₂ desorption as an example,thermodynamic studies showed that the free energy barriers of AB stepwise dehydrogenation reactions were 2.07（Os/P₃C）and 1.54 e V（Os/P₃C＿O）,respectively.Interestingly,in combination with the Arrhenius formula,its calculated rate constants further quantitatively confirm the reality of the first step of AB dehydrogenation on Os/P₃C and Os/P₃C＿O substrates.We found that k_f1 at400 K is equivalent to k_f2 at 800 K,which greatly increases the temperature of the first step of AB dehydrogenation on P₃C＿O.The catalytic difference is also graphically explained by analyzing the density of states of the key intermediates of the decisive velocity step and the crystal Hamiltonian orbital overlap booster.We hope that this work will provide a promising approach for the design of catalysts for AB dehydrogenation reactions on two-dimensional material surfaces.