Theoretical Study Of Nitrogen Reduction Reaction On Fe₂B Surface

The Haber-Bosch process is an industrial process for producing NH₃ from hydrogen and nitrogen with the assistance of catalyst and high temperature and pressure.It is considered as one of the most important discovery in the 20th century and has made tremendous contributions to the growth of population in the world.The ammonia synthesis through Haber-Bosch industry is a high energy-consumption process,consuming 1%～2%of the global energy supply.The development of ammonia synthesis processes under mild conditions has long been a goal pursued by researchers for a long time,and the realization of this goal is highly dependent on the design of new catalysts which have high activity under low temperature.An ideal catalyst should have a high activity to the breaking of N≡N bonds,meanwhile it should not interact strongly with the intermediate species（NH_x,x=0-3）.Single transition metal catalyst can hardly meet the expectation because nitrogen reduction reaction on the transition metal surface follows the Br（?）nsted–Evans–Polanyi（BEP）principle.That is,both the dissociation barrier of N₂ molecules and the desorption energy of NH_x have a linear relationship with the adsorption energy of N atoms on the catalyst surface.Limited by this linear relationship,only transition metals（Fe and Ru）located at the top of the volcano curve are considered as suitable catalysts for ammonia synthesis.Among them,iron-based catalyst has been developed as the main catalyst for industrial ammonia synthesis due to its excellent catalytic activity and low cost.Very recently,non-metallic B atoms are also considered as active species for nitrogen reduction reaction due to their special electronic properties.In this paper,based on density functional theory calculations,we studied the N₂reduction reaction on the（001）surface of Fe₂B catalyst.We found that N₂decomposition on Fe₂B（001）surface has a low barrier,and the dissociated N atoms has low diffusion barrier and the most energetically favorable adsorption site is on the top of B atom.Under low coverage,we calculated four different reaction paths of nitrogen reduction reaction.It is found that N₂ dissociation barrier is 1.08 eV for the dissociative mechanism,while the energy barrier for the associative mechanism（^*N-N+^*H→^*N-NH）is 1.66 eV,demonstrating the NH₃ synthesis process on Fe₂B（001）surface follows the dissociative mechanism.Moreover,the adsorption and dissociation behavior of N₂and H₂ molecules under different coverages are also investigated.It is found that the adsorptions of N₂ and H₂ molecules on the Fe₂B surface are gradually weakened as the N coverage increases.Under a N coverage of 50%,the most stable structure of N₂molecule adsorbed on the catalyst surface changes from the lateral adsorption to the top adsorption,which makes the desorption of N₂ from the catalyst to be easier than its dissociative adsorption.Based on this calculation result,we propose that the catalyst surface with 50%coverage of N atoms is a more suitable surface which should be considered for ammonia synthesis.We then studied the NH₃ synthesis process on Fe₂B（001）surface with 50%coverage of N atoms and found that the synthesis of NH₃on this surface has more energetically favorable reaction path than most of previously reported catalysts.Our research revealed the N₂ reduction mechanism on the surface of Fe₂B catalysts and examined the effect of N coverage on the reaction path and energy profile,which provided insights for the development of efficient catalysts for nitrogen reduction reaction.