Desulfurization Reactions on Ni₂P(001) and α-Mo₂C(001) Surfaces:  Complex Role of P and C Sites

Abstract

X-ray photoelectron spectroscopy and first-principles density-functional calculations were used to study the interaction of thiophene, H(2)S, and S(2) with Ni(2)P(001), alpha-Mo(2)C(001), and polycrystalline MoC. In general, the reactivity of the surfaces increases following the sequence MoC < Ni(2)P(001) < alpha-Mo(2)C(001). At 300 K, thiophene does not adsorb on MoC. In contrast, Ni(2)P(001) and alpha-Mo(2)C(001) can dissociate the molecule easily. The key to establish a catalytic cycle for desulfurization is in the removal of the decomposition products of thiophene (C(x)H(y) fragments and S) from these surfaces. Our experimental and theoretical studies indicate that the rate-determining step in a hydrodesulfurization (HDS) process is the transformation of adsorbed sulfur into gaseous H(2)S. Ni(2)P is a better catalyst for HDS than Mo(2)C or MoC. The P sites in the phosphide play a complex and important role. First, the formation of Ni-P bonds produces a weak "ligand effect" (minor stabilization of the Ni 3d levels and a small Ni --> P charge transfer) that allows a high activity for the dissociation of thiophene and molecular hydrogen. Second, the number of active Ni sites present in the surface decreases due to an "ensemble effect" of P, which prevents the system from deactivation induced by high coverages of strongly bound S. Third, the P sites are not simple spectators and provide moderate bonding to the products of the decomposition of thiophene and the H adatoms necessary for hydrogenation.

STM studies on the reconstruction of the Ni2P (101̅0) surface

“…Although Liu et al has found that P in phorsphide plays a role to activate the catalysis through both ligand and ensemble effects, [21,22] it is expected that phosphorus termination will eliminate the coordinatively unsaturated sites on the metal atoms, which are the positions for chemical bond formation with reactants. Meanwhile, the phosphorus layer will also provide protection for the bulk material in the commonly employed procedure of passivation, where the freshly synthesized material is exposed to a dilute stream of oxygen to allow handling at ambient conditions [23].…”

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].…”

“…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]. This model structure therefore explains 2 the apparent contradiction between theoretical predictions [15] and experimental STM using angle-resolved photoelectron spectroscopy (ARPES) [32,33,35] and observed a 5 Ni 3d-P 3p hybrid band (determined to be the main band) around 0-4 eV and a satellite 6 band at 8 eV in the photoelectron(PE) spectra. The satellite band was attributed to the 7 two-hole bound state resulting from the resonance behavior of the Ni 3d photoemission.…”

“…The weak ligand effect introduced by P sites helps 3 to stabilize the Ni 3d level. In addition, the presence of P dilutes the density of the active 4 Ni sites and thus reduces deactivation of the surface by S poisoning (an ensemble effect) 5 [ 15 ]. In addition Oyama et al found that the NiPS phase are active for 6 hydrodesulfurization reaction and they suggested the Ni(2) provides active site.…”

“…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.…”