Dickbreder Tobias, Bechstein Ralf, Kühnle Angelika
Physical Chemistry I, Bielefeld University, Universitätsstraße 25, 33615 Bielefeld, Germany.
Phys Chem Chem Phys. 2021 Sep 14;23(34):18314-18321. doi: 10.1039/d1cp01924d. Epub 2021 Aug 6.
Desorption of molecules from surfaces constitutes an elementary process that is fundamental in both natural and application-oriented fields, including dewetting, weathering and catalysis. A powerful method to investigate desorption processes is temperature-programmed desorption (TPD), which offers the potential to provide mechanistic insights into the desorption kinetics. Using TPD, the desorption order, the energy barrier as well as the entropy change upon desorption can be accessed. In the past, several analysis methods have been developed for TPD data. These methods have in common that they rely on the Polanyi-Wigner equation, which requires proposing a desorption mechanism with a single (or at least dominating) desorption path. For real systems, however, several coupled desorption paths can be easily envisioned, which cannot be disentangled. Here, we analyse the influence of exchange between the first and the second adsorbate layer on the desorption process. We present a kinetic model, in which molecules can desorb directly from the first layer or change into the second layer and desorb from there. Interestingly, considering this additional desorption pathway alters the desorption spectrum considerably, even if the transient second-layer occupation remains as small as 4 × 10 monolayers. We show that the impact of this layer exchange can be described by a modified Polanyi-Wigner equation. Our study demonstrates that layer exchange can crucially impact the TPD data.
分子从表面的解吸是一个基本过程，在包括去湿、风化和催化在内的自然和应用领域中都至关重要。研究解吸过程的一种有效方法是程序升温解吸（TPD），它有潜力为解吸动力学提供机理见解。使用TPD，可以获取解吸顺序、能量屏障以及解吸时的熵变。过去，已经为TPD数据开发了几种分析方法。这些方法的共同之处在于它们依赖于波拉尼 - 维格纳方程，该方程需要提出具有单一（或至少占主导）解吸路径的解吸机制。然而，对于实际系统，可以很容易地设想出几种耦合的解吸路径，而这些路径无法解开。在这里，我们分析了第一层和第二层吸附质层之间的交换对解吸过程的影响。我们提出了一个动力学模型，其中分子可以直接从第一层解吸或转变到第二层并从那里解吸。有趣的是，考虑到这种额外的解吸途径会显著改变解吸谱，即使第二层的瞬时占据量小至4×10单层。我们表明，这种层交换的影响可以用修正的波拉尼 - 维格纳方程来描述。我们的研究表明，层交换会对TPD数据产生关键影响。
