Facet-Dependent Oxygen Mobility and Reaction Pathways for Oxidative Dehydrogenation of 1-Butene over Bi₂MoO₆

The crystal facet effect is a critical factor for catalytic reactions on metal oxides due to the different atomic arrangements and physicochemical properties of diverse facets. Based on a series of combined experimental and theoretical measurements, this work investigates facet-dependent oxygen mobility and reaction pathways for the oxidation dehydrogenation (ODH) of 1-butene to 1,3-butadiene on Bi2MoO6 which exposes the {001} and {010} facets (BMO-001 and BMO-010). The results show that the oxygen mobility of BMO-001 overwhelmingly outperforms that of BMO-010, reflecting the better capacities for selective abstraction of H from 1-butene, oxygen replenishment, and bulk lattice oxygen migration. Density functional theory (DFT) calculations indicate that the rate-determining step on the {001} facet is the abstraction of the first H in 1-butene and the abstraction of the second H on the {010} facet. The existence of the [Bi2O2](2+) layer provides a favorable channel with a low-energy barrier for bulk lattice oxygen migration toward the {001} facet. Besides, complex side reactions occur on the {010} facet, including the nonselective oxidation of 1-butene, aromatization of 1-butene, and the generation of CO and subsequent formates. The total oxidation and decomposition of byproducts result in extra CO2 formation pathways. Lattice and gaseous oxygen play different roles in the above reactions. The superior oxygen mobility contributes to the high 1,3-butadiene yield for BMO-001, while the extra CO2 formation pathways lead to an abnormally high CO2 yield for BMO-010. The generated aromatic coke and formates affect the catalytic stability of BMO-010. The facet-dependent oxygen mobility and reaction pathways result in a distinct catalytic performance for 1-butene ODH.