General trends in Horiuti-Polanyi mechanism vs non-Horiuti-Polanyi mechanism for water formation on transition metal surfaces

过渡金属表面上水生成经典Horiuti-Polanyi机理与非Horiuti-Polanyi机理选择的总体趋势（英文）

It is generally acknowledged in heterogeneous catalysis that hydrogenation follows the so-called Horiuti-Polanyi(HP) mechanism. In this work, a thorough investigation of the mechanism of hydrogenation of hydroxyl groups and O catalyzed by a series of transition metals was carried out through density functional theory calculations, as surface hydroxyls and O are very common species in many catalytic systems. It is found that different metal catalysts exhibit different mechanisms. On some metal catalysts, the non-HP mechanism is preferred, whereas the classic HP mechanism is favored on other catalysts. Detailed analyses of the metal-dependent mechanism shows that the activity toward the dissociation of H2 decides which mechanism is preferred. On active catalysts, such as Ni and Pt, H2 prefers to dissociate with strong H adsorption energies, which lead to the classic HP mechanism being favored. On inactive surfaces, on the other hand, the adsorption of H is weak, which results in the non-HP mechanism being preferred. The parameter η, which is a structural descriptor, was defined to understand the different mechanisms.

在非均相催化加氢反应中,氢气(H2)一直被公认为是通过两步基元步骤参加还原反应的,包括第一步的分子解离和之后的反应物与原子氢键合,即所谓的Horiuti-Polanyi(HP)机理.直到我们研究组在Ag或Au催化丙烯醛加氢还原反应理论研究中发现非HP机理加氢路径存在时,新的机理才被提出,并引起广大研究者的浓厚兴趣.考虑到表面羟基(OH)和氧(O)在非均相催化体系中广泛存在,如常见的过渡金属催化的费托合成、甲烷重整、水汽转化及氨氧化等反应,基于第一性原理的密度泛函理论计算方法,我们对OH/O在一系列过渡金属催化作用下还原生成水的微观机理进行了系统全面的探究.研究发现,不同金属对应于不同的催化氢化反应活性,以及不同的催化反应机理.在某些金属上H2以分子形式进攻反应物种的非HP机理有利,而在其它金属上经典的H2解离后参与氢化还原反应的HP机理更容易发生.详细分析显示, H2的解离活性决定了反应机理的种类:在对H2解离具有催化活性的金属(如Pt、Ni)表面,不论是(211)台阶面还是(111)平面, H2解离几乎都是无能垒过程,且伴随氢原子的强吸附,反应放热明显,导致活泼金属上HP机理更容易发生;与之相反,在不活泼的催化剂表面, H2解离很难发生,原子吸附也相当微弱,相比于断键裂解, H2更倾向于发生分子氢化的非HP机理.另外,本文还定义了一个新的结构描述符(η)来帮助理解两种机理发生的结构因素差异.η是衡量分子氢化过渡态结构(TS)中H–H键解离程度的参数,根据其定义上下限数值分别设定为H2在各催化剂表面解离过渡态的键长(Ddis)和游离分子态的键长(DH2).结果显示,易发生非HP机理的催化剂表面的TS结构对应的η参数普遍低于0.4,即H–H原子对的确是以近分子形式参与氢化反应;相反发生经典HP机理的催化剂表面,η参数普遍在0.5–0.8,即H–H即使以分子形式参与反应也是处于近解离状态,这预示了以解离吸附氢参与反应的优选性.