Surface Structure Of Cu/ZnO Catalyst And Its Activation To Hydrogen Molecule

Cu/ZnO is widely used as a catalyst for methanol synthesis from syngas.It is also one of candidate catalysts for higher alcohol synthesis.However,the atomic nature and catalytic mechanism of Cu/ZnO system remains obscure,which hinder further impromentation.In this thesis,by means of ab initio method,we investigated its possible surface landscape and the activation mechanism of H₂ molecule.Firstly,we investigated the ZnO substrate,which itself is utilized as catalyst for some hydrogenation process.Because ZnO sits in the borderline between ionic and covalent substances,and because both Zn and O are hybridized as sp³,many phenomenons are local,such as surface relaxation,surface reconstruction,and electron localization of n-type surface defects.For H₂ adsorption and subsequent dissociation,experimentally there are two types,one is fast and reversible,called typeâ… ,while another is slow and irreversible,called typeâ…¡.After many calculations,we designated H₂ adsorption at perfect(10(?)0)surface as the typeâ… . Reason for low barrier is that H-H bond is torn apart by strong electronic field near polar Zn-O dimers.Thus stronger the electronic field,lower the activation energy. For H₂ adsorption at n-type defect sites,because the lack of neighboring polar Zn-O dimers,oxidative addition of H₂ molecule onto oxygen vacancy(V_o) or zinc interstitial(Zn_i) is needed during the dissociation.Concerning energy of dissociation,we found that it is moderate exothermic for H₂ adsorption over two perfect nonpolar surfaces.An abnormal phenomenon is that this value increases as coverage,probabely due to the interaction between adjacent H^δ+ and H^δ-.For defect sites,dissociation of H₂ at V_o is endothermic,while at Zn_i is much exothermic. Considering the above mentioned value of reaction barrier,the dissociation at Zn_i might be designated as typeâ…¡.Our following work is copper dissociation above ZnO.It is indicated in our work that deposited copper frameworks are dictated by its ZnO substrate.And, because the limited bonding ability of copper atom,whose electronic configuration is 3d¹⁰4s¹,interaction between Cu and ZnO and interaction among deposited coppers are generally competitive.Thus,above relatively small and highly symmetric(10(?)0)surfaces,copper depositions follow the order of regular zigzag structures,then undertake a rearrangement to form new zigzag structures,and finally encaged to 3D clusters.While,above relatively larger and lower symmetric (1120) surface,copper depositions follow the order of irregular islands or zigzag structures,2D streaks,2D surface layers and finally flat,disk like cages.Migration barriers of deposited coppers are generally low,indicating some metallic nature between Cu and ZnO,and,this could be the major reason for the easy deactivation of Cu/ZnO catalyst.