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Stefano Fabris

Publications and source records attributed to Stefano Fabris.

5 recordsLinked to original sources
PubMed2006

Interaction of hydrogen with cerium oxide surfaces: a quantum mechanical computational study.

The interaction of the (110) and (111) surfaces of ceria (CeO(2)) with atomic hydrogen is studied with ab initio calculations based on density functional theory. A Hubbard U term added to the standard density functional allows to accurately describe the electronic structure of the two surfaces. The minimum energy configuration for the adsorbed H on each of the two surfaces is obtained. An O-H-O bridge is formed on the (110) surface, whereas an axial tricoordinated OH group results on the (111) surface. For both surfaces, the adsorption of an H atom is accompanied by the reduction of a single Ce ion (which is one of the nearest neighbors of the adsorbed atom) and by a substantial outward protrusion of the O atom(s) directly bound to H. The adsorption of atomic H on the (110) and (111) surfaces is energetically favored by -150.8 and -128.3 kJ/mol, respectively, with respect to free molecular H(2). The calculated frequencies for the OH stretching vibrational mode are 3100 cm(-1) for the (110) surface and 3627 cm(-1) for the (111) surface. The latter value is in excellent agreement with experimental data reported in the literature.

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PubMed2006

Monitoring two-dimensional coordination reactions: directed assembly of co-terephthalate nanosystems on Au(111).

We report scanning tunneling microscopy observations on the formation of 2D Co-based coordination compounds on the reconstructed Au(111) surface. Preorganized arrays of Co bilayer islands are shown to be local reaction sites, which are consumed in the formation of Co-terephthalate aggregates and regular nanoporous grids. The latter exhibit a planar geometry stabilized by the smooth substrate. The nanogrids are based on a rectangular motif, which is understood as an intrinsic feature of a 2D cobaltous terephthalate sheet and dominates over the templating influence of the quasihexagonal substrate atomic lattice. The dynamics of the Co island dissolution and metallosupramolecular self-assembly could be monitored in situ. Complementary first-principles calculations were performed to analyze the underlying driving forces and to examine general trends in 2D metal-carboxylate formation. The findings indicate the wide applicability of coordination chemistry concepts at surfaces, which moreover can be spatially confined by using templated substrates, and its potential to synthesize arrangements unavailable in bulk materials.

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PubMed2005

Electronic and atomistic structures of clean and reduced ceria surfaces.

The atomistic and electronic structures of oxygen vacancies on the (111) and (110) surfaces of ceria are studied by means of periodic density functional calculations. The removal of a neutral surface oxygen atom leaves back two excess electrons that are shown to localize on two cerium ions neighboring the defect. The resulting change of valency of these Ce ions (Ce4+ --> Ce3+) originates from populating tightly bound Ce 4f states and is modeled by adding a Hubbard U term to the traditional energy functionals. The calculated atomistic and electronic structures of the defect-free and reduced surfaces are shown to agree with spectroscopic and microscopic measurements. The preferential defect segregation and the different chemical reactivity of the (111) and (110) surfaces are discussed in terms of energetics and features in the electronic structure.

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PubMed2005

Electron localization determines defect formation on ceria substrates.

The high performance of ceria (CeO2) as an oxygen buffer and active support for noble metals in catalysis relies on an efficient supply of lattice oxygen at reaction sites governed by oxygen vacancy formation. We used high-resolution scanning tunneling microscopy and density functional calculations to unravel the local structure of surface and subsurface oxygen vacancies on the (111) surface. Electrons left behind by released oxygen localize on cerium ions. Clusters of more than two vacancies exclusively expose these reduced cerium ions, primarily by including subsurface vacancies, which therefore play a crucial role in the process of vacancy cluster formation. These results have implications for our understanding of oxidation processes on reducible rare-earth oxides.

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