Dft Study On Mo-based Catalysts On Ammonia Synthesis And Decomposition

Ammonia(NH₃)plays a significant role in the global economy and society.It can not only be used as raw material of nitrogen fertilizer in agricultural production,but also provide CO_x-free hydrogen through ammonia decomposition.Haber-Bosch am-monia synthesis is the main way to develop nitrogen(N₂)resources in the atmosphere and store the hydrogen.Current iron-based process requires expensive equipment to operate at high temperatures and high pressure.Ruthenium-based catalysts have high activity,but ruthenium is too expensive and the catalyst life is short.With the increasing demand for human production and energy,the development of new catalysts to realize ammonia synthesis at low temperature and with high efficiency,as well as the reverse process of ammonia decomposition to produce hydrogen,has become a hot research topic.In the present study,we investigated ammonia decomposition on the Mo₂N(100)and(111)surfaces using density functional theory calculations.The step-wise dissociation of ammonia over the surface Mo atom on Mo₂N(100)has activation barriers>1.0 e V for all three N-H bonds.In contrast,the activation barriers for dissociating the first and second N-H bond in three-fold Mo sites on the(111)surface are only 0.64 and 0.22 e V,respectively,whereas the activation barrier for breaking the last N-H bond is 1.12 e V.Coupling of NH_x species with intrinsic surface N atoms does not always promote N-H bond activation but the formation of N_latNH_x species needs to overcome a high activation barrier.Consequently,N-N coupling is not expected to have significant contributions to overall ammonia dissociation.Our results also showed that recombinative desorption of N atoms involves formation of highly activated meta-stable di-nitrogen species and its subsequent desorption as N₂ molecule.The overall ammonia decomposition is limited by this recombinative desorption process,with an overall energy cost of?2.61 e V on Mo₂N(100)and?1.29 e V on Mo₂N(111).On both surfaces,the intrinsic surface N atoms of the nitride actively participate in the formation of desorbed nitrogen,making the process follow a Mars van Krevelen mechanism.Further study were conducted to elucidate the mechanism of ammonia synthesis on the nitridated Mo₂N(111)surfaces.An ensemble consisting of four Mo atoms arranged in roughly a rhombic structure(Mo₄)was found highly active for N₂ adsorption and activation,with an activation barrier of 0.58 e V to break the N-N bond.However,subsequent hydrogenation of the adsorbed NH_x(x=0,1 and 2)species on the Mo₄ site becomes rate-limiting as the activation barriers increase up to 1.47 e V.Nitridation in close proximity to the Mo₄ site can significantly improve the activity for NH_xhydrogenation,and the activity is more sensitive to the location rather than the overall coverage of N.The Mo₄ site modified anisotropically by surface N adatoms next to the site maintains its high reactivity towards dissociative N₂ adsorption while reduces the activation barriers for NH_x hydrogenation.Bonding with the N adatoms distorts geometrically the Mo₄ ensembles and modifies electronically the active sites.Microkinetic analysis based on the energetic results indicate that the Mo₄ ensembles modified with N adatom at the corner site formed in intermediate N coverages are highly active towards ammonia formation.The present study demonstrates the importance of the local structure of the active site for catalytic ammonia synthesis and is helpful to the design of novel active ammonia synthesis catalysts and the selection of optimal operating conditions.We evaluate the defined atomic ensemble effect(Mo monomer,dimer,trimer and tetramer)on ammonia synthesis on Ni(111)surfaces alloys in which Mo is responsible for the dissociation of N₂ whereas Ni is for hydrogenation.Mo aggregation induces the facile shift from non-active adsorption to activated adsorption of N₂,which accelerates the dissociation(0.5-0.8 e V)of N-N triple bonds.However,the adsorbed NH₂ on Mo dimer is too stable and its hydrogenation becomes the rate-determining step(>1.5 e V).The microkinetic analysis based on bifunctional model suggests that single Mo atoms embedded are most active for the ammonia synthesis in the temperature range of 600-700 K and a wide range of pressure.When operation temperature is decreased to 450-550 K,Mo dimers becomes more active than Mo single atom.Our work provides a rational design of Mo-based catatlysts for efficient ammonia synthesis even under low-temperature and pressures.