镍族金属团簇在催化加氢过程中的应用（英文）

Nickel Family Metal Clusters for Catalytic Hydrogenation Processes

贵金属纳米颗粒具有优异的催化活性,是异相催化反应中的重要角色。作为一种理想的研究模型,气相金属团簇被广泛应用于在原子和分子尺度探究催化反应的机理。在本专论中,我们将本课题组近年来关于氢气在镍族金属团簇上的解离吸附进行了回顾。首先,我们对比了不同金属团簇的结构演化规律和相对稳定性。随后,我们系统研究了H_2分子在金属团簇上的解离吸附行为,揭示了不同金属对H―H键的解离能力。为了表征不同金属团簇的催化活性,我们定义了两个关键参数:氢气的解离吸附能(ΔE_(CE))和H原子的连续脱附能(ΔE_(DE))。结果显示,随着H覆盖度的增大,ΔE_(CE)和ΔE_(DE)都呈现显著的下降。由于在实际的催化反应中,氢气总是维持在一定的分压下,这就意味着催化剂金属应该总是处于较高的H覆盖度下。因此,通过处于H饱和状态下的ΔE_(CE)和ΔE_(DE)来评估金属团簇的催化能力是合理可行的。我们发现,在饱和H吸附状态下,每一个Pt原子可以容纳4个H原子,而每一个Pd或Ni原子则只能吸附2个H原子。考虑到H原子在这些团簇上的脱附能力相当,Pt团簇相对较高的H吸附量将极大提高其在加氢过程中的催化活性。最后,我们系统研究了带电状态对Pt团簇催化性能的影响规律。结果显示,在H覆盖度较低时,H2分子的解离以及H原子的脱附过程受Pt团簇带电状态的影响较大。在饱和H吸附时,由于大量H原子的吸附,电荷的影响被平均化到每个Pt―H键上,导致ΔE_(CE)和ΔE_(DE)都收敛到一个非常小的区域。此外,当团簇的尺寸增大时,其所带的电荷被大量的Pt原子分摊,每个Pt原子仅携带极少的电荷,使得电荷的影响已经可以忽略。

Nanoparticles of precious metals play an important role in many heterogeneous catalytic reactions due to their excellent catalytic performance. As an idealized model, gas phase metal clusters have been extensively utilized to understand catalytic mechanisms at a molecular level. Here we provide an overview of our recent studies on H2 dissociative chemisorption on nickel family clusters. The structure evolution and the stability of the metal clusters were first compared. H2 dissociation on the clusters was then carefully addressed to understand the capability of metal clusters to break the H-H bond. Two key parameters, the dissociative chemisorption energy （△ECE） and the H sequential desorption energy （△EDE）, were employed to characterize the catalytic activity of metal clusters. Our results show that both △ECE and AEDE decline significantly as the H coverage increases. Since the catalyst is in general covered entirely by H atoms and H2 molecules in a typical hydrogenation process, and maintained at a pre-determined pressure of H2 gas, we can rationally use the calculated AEcE and AEDE values at full H saturation to address the activity of metal clusters. Our results suggest that at full H coverage, each Pt atom is essentially capable of adsorbing 4 H atoms, while each Ni or Pd atom can only accommodate 2 H atoms. Considering the similar values of H desorption energies on Pt and Pd clusters, the higher average H capacity per Pt atom could probably lead to a faster reaction rate because more active H atoms are produced on the Pt catalyst particles in the hydrogenation process. Finally, the charge sensitivity of the key catalytic properties of Pt clusters for hydrogenation was systematically evaluated. The results show that the dissociation of H2 and H desorption are strongly correlated to the charge state of the Pt clusters at low H coverage. However, at high H-capacities, both △ECE and △EDE fall into a narrow range, suggesting that the charge can be readily dispersed and that the Pt--H bonds average the interaction between clusters and H atoms. As a result, the H-capacities on charged clusters were found to be similar as the cluster size increased; in case of sufficiently large clusters, the reactivity of a fully saturated cluster was no longer sensitive to its charge state.