Inada Shiori, Hama Tetsuya, Tachibana Shogo
Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan.
Komaba Institute of Science, The University of Tokyo, 3-8-1 Komaba, Tokyo 153-8902, Japan.
J Chem Phys. 2024 Apr 21;160(15). doi: 10.1063/5.0192557.
Non-transition metal oxides, including major minerals of the early Solar System, are known to evaporate decomposing into multiple gas molecules, while maintaining their stoichiometric compositions (dissociative congruent evaporation). Here, we derived the absolute rate of this type of evaporation using the transition state theory. In our modified transition state theory, the activation energy closely corresponds to the average energy of the molecules at the transition state, reflecting the degree of decomposition at the potential energy barrier along the reaction coordinate of evaporation. By comparing the theoretical and experimental evaporation rates for the reaction MgO (s) → Mg (g) + O (g), we found that there is an activation barrier close to the product side (i.e., "late" barrier) where the decomposition is almost achieved. The present theory is advantageous to the Hertz-Knudsen equation, which is essentially formulated as the evaporation rate in equilibrium based on the detailed balance, in that it describes dissociative congruent evaporation as a non-equilibrium process and thus provides the link between the experiments and the reaction dynamics.
已知包括早期太阳系主要矿物在内的非过渡金属氧化物会蒸发分解成多个气体分子，同时保持其化学计量组成（解离一致蒸发）。在此，我们利用过渡态理论推导了这类蒸发的绝对速率。在我们改进的过渡态理论中，活化能与过渡态分子的平均能量密切相关，反映了沿蒸发反应坐标在势能垒处的分解程度。通过比较反应MgO (s) → Mg (g) + O (g)的理论蒸发速率和实验蒸发速率，我们发现靠近产物一侧存在一个活化势垒（即“晚期”势垒），在该势垒处分解几乎完成。本理论相对于赫兹 - 克努森方程具有优势，赫兹 - 克努森方程本质上是基于细致平衡将蒸发速率表述为平衡态下的速率，而本理论将解离一致蒸发描述为非平衡过程，从而建立了实验与反应动力学之间的联系。
