Density Function Theoretical Investigation on the Ni3PP Structure and the Hydrogen Adsorption Property of the Ni2P(0001) Surface

Abstract

Electronic and structural properties of phosphorus terminated structure of Ni 2 P(0001) surface(Ni 3 PP) are investigated by density functional theoretical (DFT) calculation. Phosphorus adsorption largely stabilizes the Ni 2 P(0001) surface by creating Ni-P bonds on the Ni trimer. Atomic hydrogen can adsorb on the topmost P site but its adsorption energy is much lower than its adsorption energy on the Ni trimer site of Ni 3 P 2 surface. Our results suggest that the Ni trimer is the key factor for high catalytic activity.Nickel phosphide (Ni 2 P) shows a high catalytic activity for the hydrodesulfurization[1] and other hydrotreatment catalyses towards hydrodenitrogenation and hydrodeoxidation and so on.[2] The key issue to understand the hydrogenation reactions is interaction between hydrogen with Ni 2 P surface in an atomic level. The bulk structure of Ni 2 P is composed of two different layers with different composition, Ni 3 P 2 and Ni 3 P 1 , aligning alternately along the [0001] direction as shown in Figure 1(a). Rodriguez et al. showed that Ni 3 P 2 surface was more stable surface than Ni 3 P 1 surface and they discussed the hydrogen adsorption properties on the Ni 3 P 2 surface.[3] However, scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) studies on the Ni 2 P(0001) have revealed that the Ni 2 P(0001) surface is mainly (about 80 %) covered with phosphorus on the Ni three fold site of Ni 3 P 2 surface after 953 K annealing under UHV conditions with minor amount of uncovered Ni 3 P 2 surface. [4][5][6] This new phosphorus covered surface is called as Ni 3 PP surface. Similar phosphorus terminated surface was reported on the reconstructed (10-10) surface by STM observation [7]. The phosphorus termination of the surfaces were supported by photoemission spectroscope (PES) study. [8][9][10] Moreover, recent in situ XAFS studies show that the adsorption of sulfur on the Ni 2 P creates the active phase. [11][12][13] Therefore the chemically modified Ni 2 P surface is interesting not only the fundamental problem but also the catalysis applications.In the present study we have explored the density functional theoretical (DFT) work on the Ni 3 PP surface to reveal the mechanism of P-covered surface formation and its hydrogen adsorption properties in comparison with the Ni 3 P 2 .DFT calculations were performed using the Vienna abinitio Simulation Package code (VASP 5.2.12).[14-16] The Perdew-Burke-Ernzerh exchange-correlation functional with a generalized gradient approximation [17,18] was used. Projector-augmented wave approach (PAW) [19,20] have been used together with plane wave basis sets. The optimized bulk Ni 2 P lattice constants were a = b = 0.5876 nm, c = 0.3365 nm, α = β = 90°, γ = 120°. The Ni 2 P(0001) surface was modeled with 12 atomic-layers periodic slab with a ca. 3.5 nm vacuum layer. The chemical formula of Ni 3 P 2 and Ni 3 PP are Ni 36 P 18 and Ni 36 P 19 , respectively. The kinetic energy cutoff of 300 eV was used and the Brillouin zone was sampled by a 4 × 4 × 1 Monk...

An Investigation of Ni2P Single Crystal Surfaces: Structure, Electronic State and Reactivity

“…In addition, since the P 2 atoms possess a slight negative charge (-0.07e), they provide additional sites for 3 hydrogen adsorption [15,36]. This theoretical proposal has also been supported by 4 studies of the electronic structure of Ni2P [35].…”

“…Many efforts have been made to date to elucidate the surface properties of Ni2P 10 single crystal surfaces, using X-ray photoelectron spectroscopy (XPS) [22,23], scanning 11 tunneling microscopy (STM) [24][25][26][27][28], low-energy electron diffraction (LEED) [24][25][26][27][28][29][30], 12 photoemission electron microscopy (PEEM) [25], photoelectron diffraction [31] and 13 photoemission spectroscopy (PES) [30,[32][33][34][35]. Theoretical calculations using density 14 functional theory (DFT) [11,14,15,[36][37][38] have also played an important role in 15 understanding the catalytic properties of the surface. These studies have indicated that 16 the Ni2P single crystal surface is stabilized by reordering of the P atoms.…”

“…3. In this structure, P adatoms stabilize the surface by bonding with the 35 dangling bonds of three-fold Ni atoms [36]. Further analysis showed that the Ni3P-P structure accounts for 80±10% of the total surface, while the remainder (about 20%) is 1 composed of an uncovered Ni3P2 structure [29].…”

“…The P atoms are preferably segregated on the surface in order to stabilize the surface Although the P atoms can serve as stable adsorption sites for atomic hydrogen 4 [15,36], surface P may also reduce the catalytic activity since P sites do not have the 5 same ability to promote the dissociative adsorption of molecules as the Ni sites [36]. 6 3-fold Ni hollow sites are active in adsorbing S, H and OH species for different 7 reactions [15].…”

“…6 3-fold Ni hollow sites are active in adsorbing S, H and OH species for different 7 reactions [15]. Atomic hydrogen adsorbs stronger on 3-fold Ni hollow sites than the 8 topmost P sites of the Ni3P-P surface [36]. The removal of the P might be necessary 9 from the 3 fold Ni site to activate the Ni2P surface but too many removal of P would 10 reduce the stability of Ni2P structure.…”