DFT+U Study of Properties of MoO₃ and Hydrogen Adsorption on MoO₃(010)

MoO₃ is an important catalytic material, and there exist controversial viewpoints about its surface structure, oxygen vacancy, and hydrogen adsorption, which are crucial for rationalizing the catalytic properties and reaction mechanism. To clarify these disputes, we adopted the density functional plus U (DFT+U) method to investigate properties of MoO₃ bulk and surfaces and examined atomic hydrogen adsorption. Analyses reveal that the vibration peak at 820 cm^–1 previously assigned to the vibration of asymmetrical oxygen is due to the vibration of symmetrical oxygen. On the other hand, the previously unassigned weak peaks at 899 and 723 cm^–1 are caused by the asymmetrical oxygen stretching. Single hydrogen atom adsorbs favorably at asymmetric oxygen, while the terminal oxygen becomes the favorable position for accommodating two hydrogen atoms. The H atoms occupy preferentially asymmetrical oxygen at low coverage, whereas at high coverage they favorably reside on the terminal one. Our calculations indicate that different from the previous viewpoint, water binds to the terminal oxygen defective site relatively strongly. Furthermore, the controversial viewpoints about the stability ordering of oxygen vacancy under oxidation and reduction conditions is discussed on the basis of the formation energy of oxygen vacancy and water desorption energy on defect sites.