Methane Activation by Different Early Actinide (Ac, Th, Pa, U, and Np) Oxides and Their Ions: A Density Functional Theory Investigation

Oxyphile actinide oxides and their ions are potential candidates for methane activation due to their unique electronic configurations. The comprehensive activation mechanisms of methane catalyzed by a series of early actinoid ions and their oxide ions (Ac3+ and AcO+; Th4+, ThO2+ and ThO+; 3Pa3+, 3PaO+, PaO+/PaO3+ and PaO2+; 4UO+, 3UO2+/UO2+, 3UO2, and UO22+; 3NpO+/5NpO+, 2NpO2+/4NpO2+, 2NpO2/4NpO2, NpO2+/3NpO2+, 2NpO22+, 2NpO3, and NpO3+) were systematically investigated in this study. Theoretical calculations in the present work suggest that Ac3+, Th4+, and PaO3+ can easily tear methane apart and extract hydrogen from it, but the reaction pathway of Pa3+···CH4 → HPa3+-CH3 needs a barrier of 47.6 kcal·mol–1. Judging from the activation barriers of the actinide oxide ions, AcO+, 2ThO+, 3UO2, and 2NpO22+ are ideal catalysts for methane activation with the lowest barriers of 32.8, 40.1, 31.1, and 33.1 kcal·mol–1, respectively.