Theoretical Study On The Reactions Of The Transition Metal Oxide/Nitride Clusters With Hydrocarbon

Clusters are metastable aggregates which consist of several to thousands of atoms,molecules,or ions.Clusters bridge microscopic and macroscopic world.Atomic clusters can be studied under well-controlled and reproducible conditions,and serve as ideal models that can be used to reveal the nature of active sites and the mechanisms of catalysis at a molecular level.The studies in this dissertation focus on the structures of transition metal oxide/nitride clusters and their reactivity toward hydrocarbon molecules by using experimental and theoretical calculation methods.The summary is as follows:1)Activation of alkane as Mediated by the Triatomic Anion HNbN^-Transition metal nitrides?TMNs?are reported as one of the promising replacements of costly noble metals?NMs?,such as Pt,which show high reactivity in the context of thermal methane activation.Herein,we studied the reactions of triatomic anions HNbN?with methane and ethane by mass spectrometry and photoelectron imaging spectroscopy in conjunction with density functional theory calculations.The HNbN?anions are capable of activating CH₄ and C₂H₆ through oxidative addition,exhibiting similar reactivities to free Pt atoms.The similar electronic structures of HNbN^-and Pt,especially the similar highest occupied molecular orbitals?HOMOs?,are responsible for this resemblance.It is known that the active orbital of Pt is 5d_Z² hybrid orbital,and for HNbN^?,the HOMO is mainly composed of the 4d/5s hybrid orbital.New insights into how to replace noble metals with TMNs may be derived from this study.2)Liberation of three H₂ molecules from ethene molecules as mediated by Ta₃N₂^?The reactions of Ta₃N₂^-with C₂H₄ have been investigated by mass spectrometry and quantum chemistry calculations.Ta₃N₂^-cluster can activate one ethene molecule efficiently,resulting in the formation of two H₂ molecules and Ta₃N₂C₂^-,and the newly formed Ta₃N₂C₂^-cluster is also highly reactive toward C₂H₄.Therefore,three H₂ molecules are liberated from two C₂H₄ mediated by Ta₃N₂^-at room temperature.The DFT calculations indicate that in addition to the activation of C-H bonds,the C=C double bonds in ethene are also cleaved in the two reactions.To the best of our knowledge,this is among the first to report the full dehydrogenation of C₂H₄ by gas-phase anions.This study not only extends our understanding of the properties of the rarely investigated tantalum nitride and M-N-C clusters in gas phase,but also gives new insights into the design of active sites in related condensed-phase systems at a molecular level.3)Dehydrogenation of propylene mediated by Ce–V oxide cluster CeVO₄⁺The gas-phase reaction of closed-shell cluster CeVO₄⁺towards C₃H₆ has been investigated using a reflectron time-of-flight mass spectrometer coupled with density functional theory calculations.The CeVO₄⁺cluster is reactive toward propylene,and the DFT calculations indicated that the mechanism of CeVO₄⁺+C₃H₆?CeVO₃C₃H₄⁺+H₂O involves four steps of hydrogen atom transfer,which is quite different from other reported mechanisms of H₂O formation reactions.Compared with another homonuclear closed-shell cluster V₃O₇⁺,possessing the similar mass with CeVO₄⁺,the heteronuclear oxide cluster show higher reactivity toward propylene.This work provides insights into nature of active sites and reaction mechanisms in related condensed-phase systems at a molecular level.The study on the reactions of Ta_xN_y^-clusters and nitrogen are still in progress,which is the focus of the next goal.All the above experimental data was completed by other members of the research group.