Ab initio description of non-adiabatic effects in dissociative adsorption at surfaces

基于微观理解的预测材料科学建模需要对原子尺度上所有潜在的基本过程有透彻的了解。单个气相分子在金属表面的(解离)吸附是这样一个基本过程，对于任何涉及暴露在现实气体环境中的表面的应用来说，都是至关重要的，多相催化只是一个突出的例子。不幸的是，我们目前对这一基本过程的理解甚至对于像02这样最简单但普遍存在的双原子分子也是非常肤浅的。由于缺乏关于撞击分子与表面电子结构相互作用的可靠和可分辨的信息(以所谓的势能面(PES)的形式总结)，经验理论引用了有争议的情景来解释实验上可达的、积分的动量，如表面的粘着概率。在这些场景中，经常被讨论的是非绝热效应在解离中的作用，即可能违反Born-Oppenheimer近似和不能即时跟踪核运动的电子分布。澄清这一问题的见解只能来自密度泛函理论(DFT)等第一性原理理论，这些理论涉及对体系电子结构的可靠而近似的描述。这种理论完全基于绝热DFT-PES，过去主要是针对氢在金属表面的解离而发展起来的，而非绝热效应显然不起作用。最近的方法学发展现在也使得在有约束的DFT方法中计算非绝热PESS成为可能。相应地，我们提出了第一个系统的研究，解决了非绝热效应在02离解中的真正重要性。为了明确地确定这些效应所起的作用，这需要同时解决一些其他尚未完全了解的问题，如描述电子关联所需的准确性或表面迁移率的影响。聚焦于O2在一组具有代表性的表面上的解离动力学，元素周期表上出现的趋势图有望有助于首次全面理解这一最重要的分子的(解离)吸附，这也可能对其他广泛存在的硅原子如N2、NO或CO的解离提供一些指导。

A predictive materials science modeling based on microscopic understanding requires a thorough knowledge of all underlying elementary processes at the atomic scale. The (dissociative) adsorption of individual gas phase molecules at metal surfaces is such an elementary process that is of crucial relevance for any application involving surfaces exposed to realistic gas environments, with heterogeneous catalysis forming just one prominent example. Unfortunately, our present understanding of this fundamental process is even for a most simple, but ubiquitous diatomic molecule like 02 very shallow. Lacking reliable and resolved information about the interaction of an impinging molecule with the surface electronic structure (summarized in form of a socalled potential-energy surface (PES)), empirical theories have invoked controversial scenarios to account for experimentally accessible, integral kinetic quantities like the sticking probability at the surface. Among these scenarios is the frequently discussed role of non-adiabatic effects in the dissociation, i.e. a possible violation of the Born-Oppenheimer approximation and electron distributions that are not able to follow the nuclear motion instantaneously. Clarifying insight into this matter can only come from firstprinciples theories like density-functional theory (DFT), which involve a reliable, though approximate description of the electronic structure of the system. Based exclusively on the adiabatic DFT-PES, such theories have in the past been predominantly developed for H2 dissociation at metal surfaces, where non-adiabatic effects apparently play no role. Recent methodological developments have now also enabled the calculation of diabatic PESs within constrained DFT approaches. Correspondingly, we propose a first systematic investigation addressing the real importance of non-adiabatic effects in 02 dissociation. To unambiguously pin down the role played by these effects, this requires to concomitantly tackle a number of other, not yet fully understood issues like the required accuracy in the description of electron correlation or the influence of surface mobility. Focusing at the dynamics of 02 dissociation at a representative set of surfaces, the emerging trend picture over the periodic table is expected to contribute to a first comprehensive understanding of the (dissociative) adsorption of this most important molecule, that may also give some guidance on the dissociation of other widespread diatomics like N2, NO or CO.