This work employs density functional theory (DFT+U) calculations to explore initial C–H activations in C1–C3 alkanes on V2O5, MgV2O6 (meta-vanadate), Mg2V2O7 (pyro-vanadate), and Mg3V2O8 (ortho-vanadate) surfaces. These materials are selective catalysts for the oxidative dehydrogenation (ODH) of alkanes into alkenes, which offers practical and thermodynamic advantages over non-oxidative alkane dehydrogenation. The geometric and electronic properties that govern the reactivity of these materials, however, have not been explored by theory despite their importance in controlling rate determining alkane initial C–H activation during ODH catalysis. In this work, we explore fourteen low-energy surfaces of MgxV2Ox+5 (x = 0–3) exposing 64 distinct O atoms (reaction sites). C–H activation barriers are largest on Mg3V2O8, lower and similar for Mg2V2O7 and MgV2O6, and lowest for V2O5 surfaces; these predicted trends are consistent with measured ODH reactivity in earlier studies. Barriers are lowest (on average) when alkanes react with O atoms bound to a single V atom, with bridging O atoms having slightly higher barriers, and three-fold O atoms having the largest activation barriers. However, there is scattering within each subset indicating that factors beyond O-atom coordination have a significant role in the barriers. Vacancy formation energies (VFE) and the O 2p band energies were found to be weak descriptors of surface O reactivity for alkane activation barriers. Hydrogen addition energy (HAE) and methyl addition energy (MAE) values, in contrast, were found to correlate well with alkane activation barriers. MAE, however, outperforms HAE correlations because of the tendency of H* to form H-bonds with nearby surface O atoms, and those H-bonds are absent in C–H activation transition states causing scatter in the correlation of barriers with HAE. Constrained-orbital DFT methods were used to establish a theoretical thermochemical cycle that decouples surface reduction by CH3* into three components: surface distortion, orbital localization, and bond formation. These results give insights into how Mg:V ratios, surface structure (O-atom coordination), and reducibility (HAE, MAE) impact the reactivity of vanadium-based metal oxides toward alkane activation.

中文翻译：

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控制 Mg-钒酸盐和 V2O5 表面对 C1-C3 烷烃初始 C-H 活化的反应性的电子和几何特征 – DFT+U 研究


这项工作采用密度泛函理论 （DFT+U） 计算来探索 V2O5、MgV2O6（偏钒酸盐）、Mg2V2O7（焦钒酸盐）和 Mg3V2O8（邻钒酸盐）表面上 C1-C3 烷烃的初始 C-H 活化。这些材料是将烷烃氧化脱氢 （ODH） 转化为烯烃的选择性催化剂，与非氧化烷烃脱氢相比，它具有实用和热力学优势。然而，控制这些材料反应性的几何和电子特性尚未被理论探索，尽管它们在控制 ODH 催化过程中决定烷烃初始 C-H 活化的速率很重要。在这项工作中，我们探索了 MgxV2Ox+5 （x = 0–3） 的 14 个低能表面，暴露了 64 个不同的 O 原子（反应位点）。C-H 活化势垒在 Mg3V2O8 上最大，Mg2V2O7 和 MgV2O6 较低且相似，V2O5 表面最低;这些预测趋势与早期研究中测得的 ODH 反应性一致。当烷烃与结合到单个 V 原子上的 O 原子反应时，势垒最低（平均而言），桥接 O 原子具有略高的势垒，而三倍 O 原子具有最大的活化势垒。然而，每个子集内都存在散射，表明 O 原子配位以外的因素在势垒中起着重要作用。发现空位形成能 （VFE） 和 O 2p 带能是烷烃活化屏障表面 O 反应性的弱描述符。相比之下，氢加成能 （HAE） 和甲基加成能 （MAE） 值被发现与烷烃活化屏障密切相关。 然而，MAE 优于 HAE 相关性，因为 H* 倾向于与附近的表面 O 原子形成 H 键，并且这些 H 键在 C-H 活化过渡态中不存在，导致势垒与 HAE 的相关性分散。约束轨道 DFT 方法用于建立一个理论热化学循环，该循环将 CH3* 的表面还原解耦为三个部分：表面变形、轨道定位和键形成。这些结果有助于深入了解 Mg：V 比、表面结构（O 原子配位）和还原性 （HAE、MAE） 如何影响钒基金属氧化物对烷烃活化的反应性。