Liu Ping, Rodriguez José A, Asakura Takeshi, Gomes João, Nakamura Kenichi
Department of Chemistry, Brookhaven National Laboratory, Building 555, Upton, New York 11973, USA.
J Phys Chem B. 2005 Mar 17;109(10):4575-83. doi: 10.1021/jp044301x.
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.
采用X射线光电子能谱和第一性原理密度泛函计算方法，研究了噻吩、H₂S和S₂与Ni₂P(001)、α-Mo₂C(001)及多晶MoC的相互作用。一般来说，表面的反应活性按照MoC < Ni₂P(001) < α-Mo₂C(001)的顺序增加。在300 K时，噻吩不吸附在MoC上。相比之下，Ni₂P(001)和α-Mo₂C(001)能够轻易地使噻吩分子解离。建立脱硫催化循环的关键在于从这些表面去除噻吩的分解产物（CₓHᵧ碎片和S）。我们的实验和理论研究表明，加氢脱硫（HDS）过程中的速率决定步骤是吸附的硫转化为气态H₂S。Ni₂P比Mo₂C或MoC更适合作为HDS的催化剂。磷化物中的P位点起着复杂而重要的作用。首先，Ni-P键的形成产生了微弱的“配体效应”（使Ni 3d能级略有稳定，并存在少量的Ni→P电荷转移），这使得噻吩和分子氢的解离具有较高活性。其次，由于P的“集合效应”，表面存在的活性Ni位点数量减少，这防止了系统因强吸附S的高覆盖度而失活。第三，P位点并非简单的旁观者，它对噻吩分解产物和氢化所需的H原子提供适度的键合。
