Theoretical Study On The Geometry And Electronic Structure Of Tungsten Oxide Clusters Supported On The MgO Films

Supported tungsten oxide and bimetallic tungsten oxide have attracted great attention in catalysis yield due to their wide applications in hydrocarbons dehydrogenation,hydrogenation,oxidation reduction reactions et al.In order to further study the structure-activity relationship between structure and catalytic performance,the configurations and electronic structures of CrW₂O₉ clusters deposited on the MgO?001?surface and MgO/Ag?001?films,W₃O₉ clusters deposited on the defective MgO/Ag?001?films have been investigated using first-principles molecular dynamic simulations combined with quantum mechanical calculations.Our results show that,the CrW₂O₉ clusters exhibit completely different structures,when deposited on MgO?001?surface and MgO/Ag?001?films.On the MgO?001?surface,CrW₂O₉ clusters are vertically adsorbed on the surface via two W-O bonds and four Mg-O bonds.Whereas on the MgO/Ag?001?films,CrW₂O₉ clusters are parallelly adsorbed on the films via four Mg-O bonds.The electronic structures show that just a little charge transfer?0.73 e?occurs from the MgO?001?surface to the CrW₂O₉ clusters,which originates from the formation of adsorption dative bonds at interface.While on the MgO/Ag?001?films,the CrW₂O₉ clusters obtain more electrons?1.72 e?and the electrons are mainly originated from the metal Ag by direct electron tunneling.Furthermore,our work reveals a progressive Lewis acid site where spin density preferentially localizes around the Cr atom for CrW₂O₉/MgO/Ag?001?system,indicating that bimetallic tungsten oxide CrW₂O₉ clusters potentially have better catalytic activities with respect to that of pure W₃O₉ clusters.When W₃O₉ clusters deposited on the defective MgO/Ag?001?films,the W₃O₉ clusters interact with the defective MgO/Ag?001?films via eight Mg-O bonds and the oxygen vacancy on the MgO/Ag?001?films is healed by one terminal oxygen?Ot?atom.Accordingly,the adsorption energy of the dispersed W₃O₉ clusters on the defective MgO/Ag?001?films is increased significantly compared with that on the single crystal MgO?001?defective surface.The further investigations of electronic structures show that after deposition,the W₃O₉ clusters obtain 2.74 e,which mainly from the metal Ag substrate.Further analysis show that a net transfer of two electrons from the metal Ag to the W₃O₉ clusters through the thin MgO dielectric barrier by direct electron tunneling,only a little charge transfer occurs from the oxygen vacancy to the W₃O₉ clusters via covalent bonds.Furthermore,the scanning tunneling microscope?STM?and high resolution electron energy loss spectroscopy?HREELS?demonstrate that the STM and HREELS of the W₃O₉ clusters deposited on the defective MgO/Ag?001?films are completely different from that of the W₃O₉ clusters deposited on MgO single crystal surface.