First-principles Study On The Mechanism Of Ammonia Borane Hydrolysis On Single/Dual Atom Catalysts

The large-scale using of fossil energy has aggravated the deterioration of the environment and climate.Meanwhile,with the massive exploitation of fossil energy,mankind will also face a severe energy crisis.Hydrogen energy is one of the clean energies,which is considered one feasible solution to replace fossil energy.While,the storage and transportation of hydrogen limits its large-scale application in current stage.Ammonia borane(AB)exhibits excellent hydrogen storage performance and three mole hydrogens can be released via hydrolysis processes.Stable and efficient catalysts are prerequisite to achieve AB full hydrolysis at room temperature.In this paper,based on density functional theory calculations,the mechanisms of AB hydrolysis on Pt-based single-atom,homonuclear diatomic,and heteronuclear diatomic catalysts were intensively studied,which helps to understand the catalytic mechanisms and provide some clues for the design of high-efficiency catalysts for the full hydrolysis of ammonia borane at room temperature.The main research contents and results include:(1)Based on density functional theory calculations,the mechanism of ammonia borane hydrolysis on nitrogen-doped graphene supported Pt single-atom system(Pt1C2N1)was investigated.It was found that the dissociation of the first H atom at the B end of the AB molecule was the rate-limiting step of hydrolysis,with an energy barrier of 0.68 eV,which indicated that the hydrolysis of AB can proceed at room temperature.Furthermore,27 transition metal single-atom catalysts supported by C2N1-graphene(M1-C2N1)and Pt-based single-atom catalysts with different coordination environments(Pt1-CxNy)were screened for potential single atom catalysts for AB hydrolysis.By considering the adsorption of AB,hydrogen desorption,dissociation of AB and further consideration of hydrolysis mechanisms,it was found that Pt1-C1N2 has a lower energy barrier for AB hydrolysis than Pt1-C2N1,and the rate-limiting step is the attacking of the second H2O molecule to the B atom with an energy barrier of 0.60 eV.The microkinetic analysis shows that the theoretical hydrogen production rate on Pt1-C1N2 and Pt1-C2N1 can be compared with the existing experimental results under the experimental conditions,indicating that nitrogen-doped graphene supported Pt single-atom system is expected to be potential catalysts for AB hydrolysis.(2)On the basis of AB hydrolysis on single-atom catalysts,the AB hydrolysis of AB on Pt single atom and Pt homonuclear diatomic(Pt2/graphene and Pt1/graphene)supported by reduced graphene oxide supported were comparatively studied to understand the effect of dual atoms.On Pt1/graphene,the rate-limiting step of the AB hydrolysis is that the formation of the third hydrogen molecule via the combination of the H atom dissociated from the third H2O molecule and the H atom adsorbed on the Pt atom,with an energy barrier of 0.81 eV.Different from Pt1/graphene,the bridging oxygen atoms between two Pt atoms in the Pt2/graphene structure is easily hydrogenated to hydroxyl groups under reaction conditions,which suggests that the real configuration should be 2HPt2-20H/graphene.The rate-limiting step of the hydrolysis of ammonia borane is the dissociation of H2O molecules adsorbed on the bottom Pt atoms with an energy barrier of 0.71 eV.The bridging hydroxyl in 2HPt220H/graphene effectively reduces the adsorption energy of H atom and H2 molecule,making the migration of H atom on the bridging hydroxyl group and the desorption of H2 on the Pt atom easier,thereby promoting the hydrolysis of AB to produce hydrogen.(3)Based on previous work on single atom and homonuclear diaomic catalysts,single atom(Pt1/C3N4,Ni1/C3N4),mixed single atom(Pt1+Ni1/C3N4)and heteronuclear diatoms(Pt1Ni1/C3N4)supported on g-C3N4 were prepared and investigated to understand the effect of the disctance of supported metal atoms.Theoretical studies showed that the rate-limiting step of ammonia borane hydrolysis on Pt single-atom(Pt1/C3N4)is the dissociation of the third H2O molecule with an energy barrier of 0.85 eV;while the rate-limiting step of ammonia borane hydrolysis on Pt1Ni1/C3N4 is the dissociation of the second H2O molecule,with an energy barrier of 0.72 eV.Therefore,Pt1Ni1/C3N4 has higher catalytic activity than Pt1/C3N4.Similar to that of Pt2/graphene system,the bridging hydroxyl group between Pt and Ni atoms has a great promotion effect on the hydrolysis reaction of ammonia borane by reducing the energy barrier of proton transfer.Furthermore,the rate-limiting steps involved the dissociation of water.Therefore,this work investigated the dissociation energy barriers of H2O molecules on Pt1/C3N4,Pt1Ni1/C3N4,Ni1/C3N4),Pt1+Ni1/C3N4.The results showed that the distance between Pt and Ni atoms has a regulating effect on the hydrolysis efficiency of ammonia borane for hydrogen production,providing a genearal picture for understanding ammonia borane hydrolysis activity.