Abstract
We report a detailed density functional theory (DFT) study of the geometrical and electronic properties, and the growth mechanism of a Cun (n = 1-4) cluster on a stoichiometric, and especially on a defective CeO2(110) surface with one surface oxygen vacancy, without using pre-assumed gas-phase Cun cluster shapes. This gives new and valuable theoretical insight into experimental work regarding debatable active sites of promising CuOx/CeO2-nanorod catalysts in many reactions. We demonstrate that CeO2(110) is highly reducible upon Cun adsorption, with electron transfer from Cun clusters, and that a Cun cluster grows along the long bridge sites until Cu3, so that each Cu atom can interact strongly with surface oxygen ions at these sites, forming stable structures on both stoichiometric and defective CeO2(110) surface. Cu-Cu interactions are, however, limited, since Cu atoms are distant from each other, inhibiting the formation of Cu-Cu bonds. This monolayer then begins to grow into a bilayer as seen in the Cu3 to Cu4 transition, with long-bridge site Cu as anchoring sites. Our calculations on Cu4 adsorption reveal a Cu bilayer rich in Cu+ species at the Cu-O interface.
Highlights
The structure and properties of CuOx/CeO2 catalysts have been widely studied
In the optimised structure having the most negative Cu adsorption energy of À3.258 eV, the Cu atom is close to the surface and bonded with two surface O ions on top of a second-layer Ce ion, which agrees with earlier work
The fourth Cu either rises up from the surface (Fig. 12(4.3)) or moves down towards the surface, as illustrated in Fig. 12(4.4), in between two adjacent long bridge sites, to bridge Cu atoms and bond with surface/subsurface oxygen ions from two adjacent long bridge sites. In this Cu monolayer to bilayer transition, Cu–Cu interactions gradually surpass in strength Cu–O interactions and become the dominant factor, resulting in Cu atoms at the top layer occupying the space in between long bridge sites and bonding with bottomlayer Cu atoms as well as surface oxygen ions; or some Cu atoms may be incorporated into the surface, as again seen in Fig. 12(4.4), and as is observed experimentally
Summary
The structure and properties of CuOx/CeO2 catalysts have been widely studied. Chen et al used high angle annular dark field scanning transmission electron microscope (HAADF-STEM)and in situ infra-red spectroscopy, as well as density functional theory (DFT) calculations to provide experimental and theoretical evidence of a Cu bilayer on a CeO2(111) surface. A top layer of Cu0 atoms were bonded with a bottom layer of mainly Cu+ ions, which in turn were bonded with surface oxygen vacancies (in a Cu+–Ov– Ce3+ form). Kang et al recently reported experimental and theoretical evidence of an active atomic [Cu(I)O2]3À site for CO oxidation which dynamically changed to/ from [Cu(II)O4]6À via an electrophilic [Cu(II)O2(Z2-O2)]4À intermediate on the CeO2(111) surface, both of which had a lower HOMO energy compared to Cu clusters on the surface.. Kang et al recently reported experimental and theoretical evidence of an active atomic [Cu(I)O2]3À site for CO oxidation which dynamically changed to/ from [Cu(II)O4]6À via an electrophilic [Cu(II)O2(Z2-O2)]4À intermediate on the CeO2(111) surface, both of which had a lower HOMO energy compared to Cu clusters on the surface.10 Besides these combined experimental and theoretical studies, there are several computational studies focusing mainly on the atomic and electronic structures of Cu/CeO2(111) (since CeO2(111) is the most stable surface12), employing density functional theory (DFT), commonly the DFT+U approach, in which an effective Hubbard Ueff parameter is used to consider on-site Coulomb repulsions. Szabova et al reported their most stable Cu/CeO2 structure with one oxidised Cu+ and one reduced surface Ce3+ furthest away from the Cu+, with the
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