No abstract

No abstract

No abstract

Nickel has been considered a promising nonprecious metal for propane dehydrogenation (PDH) because of its appealing ability to activate alkane molecules; however, synthesis of selective and stable Ni sites remains a challenge. Herein, we describe an aluminasupported isolated Ni(II) site by settling Ni 2+ cations into Al 3+ vacancy on γ-Al 2 O 3 as a selective and stable Ni-based catalyst for PDH. Based on the results from combined characterizations, including in situ X-ray adsorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and in situ diffuse reflectance infrared Fouriertransform spectroscopy (DRIFTS), atomically dispersed Ni(II) sites with bonding to oxygen ions from the alumina support are demonstrated. PDH catalyst tests shows that the Ni(II) single-site catalyst delivers superior performance compared to a supported metallic Ni NP catalyst, possessing >93% propylene selectivity at considerable propane conversions (15−45%), which surpasses Ni nanoparticle (NP) catalysts. The correlation of selectivity to propylene on different Ni structures with the structural characterizations suggest that the coordinatively unsaturated tetrahedral Ni(II) sites facilitate the desorption of propylene, which inhibits the side reactions for coking. Moreover, the atomically dispersed Ni(II) sites remain in its local structure in the reaction-regeneration cycles, as evidenced by in situ XAS. This study on alumina-supported nickel catalysts affords insights into the nature of selective and stable nickel sites involved in the PDH reaction.

Molecular dynamics calculations were performed in an attempt to characterize the mechanism by which attractive forces affect the velocity correlation function in simple Lennard-Jones liquids. The relationships between the presence of attractive forces, the rigidity of molecular cages, and the decay of velocity correlations were studied by molecular dynamics runs in which a single mobile particle moved in a rigid cage environment. Our results suggest that the principal role of the attractive forces is to enhance the cohesiveness of the cages and their ability to confine particles within them.