Electronic and geometric features controlling the reactivity of Mg-vanadate and V2O5 surfaces toward the initial C–H activation of C1–C3 alkanes – A DFT+U study

Abstract

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 Mg_xV₂O_x+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.