Redox behaviour of magnetite Fe3O4nanoparticles in thermodynamic equilibrium conditions enclosed ... more Redox behaviour of magnetite Fe3O4nanoparticles in thermodynamic equilibrium conditions enclosed by non-dipolar reconstructed surfaces.
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2011
Using a combination of density functional theory calculations and statistical mechanics, we show ... more Using a combination of density functional theory calculations and statistical mechanics, we show that a wide range of intermediate compositions of ceria–zirconia solid solutions are thermodynamically metastable with respect to phase separation into Ce-rich and Zr-rich oxides. We estimate that the maximum equilibrium concentration of Zr in CeO 2 at 1373 K is approximately 2 per cent, and therefore, equilibrated samples with higher Zr content are expected to exhibit heterogeneity at the atomic scale. We also demonstrate that in the vicinity of the (111) surface, cation redistribution at high temperatures will occur with significant Ce enrichment of the surface, which we attribute to the more covalent character of Zr–O bonds compared with Ce–O bonds. Although the kinetic barriers for cation diffusion normally prevent the decomposition/segregation of ceria–zirconia solid solutions in typical catalytic applications, the separation behaviour described here can be expected to occur in mode...
In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue an... more In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue and in synthetic biomaterials, we have investigated the interaction of three constituent amino acids of the collagen matrix with two major hydroxyapatite surfaces. We have employed electronic structure techniques based on the density functional theory to study a range of different binding modes of the amino acids glycine, proline, and hydroxyproline at the hydroxyapatite (0001) and (0110) surfaces. We have performed full geometry optimizations of the hydroxyapatite surfaces with adsorbed amino acid molecules to obtain the optimum substrate/adsorbate structures and interaction energies. The calculations show that the amino acids are capable of forming multiple interactions with surface species, particularly if they can bridge between two surface calcium ions. The binding energies range from 290 kJ mol(-1) for glycine on the (0001) surface to 610 kJ mol(-1) for hydroxyproline on the (0110) surface. The large adsorption energies are due to a wide range of interactions between the adsorbate and surface, including proton transfer from the adsorbates to surface OH or PO(4) groups. Hydroxyproline binds most strongly to the surfaces, but all three amino acids should be good sites for the nucleation and growth of the hydroxyapatite (0110) surface at the collagen matrix.
The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO2(111) surface... more The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO2(111) surface have been investigated in detail by means of periodic density functional theory-based calculations, using the LDA+U and GGA+U potentials for a broad range of U values, complemented with calculations employing the HSE06 hybrid functional. In addition, the effects of the lattice parameter a0 and of the starting point for the geometry optimization have also been analyzed. From the present results we suggest that the oxidation state of single Au atoms on CeO2(111) predicted by LDA+U, GGA+U, and HSE06 density functional calculations is not conclusive and that the final picture strongly depends on the method chosen and on the construction of the surface model. In some cases we have been able to locate two well-defined states which are close in energy but with very different electronic structure and local geometries, one with Au fully oxidized and one with neutral Au. The energy difference bet...
... Neyvis Almora-Barrios*a and Nora H. de Leeuw*ab. a Department of Chemistry, University Colleg... more ... Neyvis Almora-Barrios*a and Nora H. de Leeuw*ab. a Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ. ... 27, S. Hamad, CE Hughes, CRA Catlow and KDM Harris, J. Phys. Chem. B, 2008, 112, 72807288 Article ChemPort . ...
Bulletin of the American Physical Society, Mar 19, 2010
We use density functional theory calculations with Hubbard corrections (DFT+ U) to investigate th... more We use density functional theory calculations with Hubbard corrections (DFT+ U) to investigate the electronic and redox properties of Ti-substituted zirconia (111) surfaces. It is found that titanium dopants are more likely to segregate at the surface than to migrate to the zirconia bulk. The formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. If an O vacancy is created around an isolated Ti dopant, a Ti $^{4+}\ quad\ to $ Ti $^{2+} $ reduction takes place, while if the ...
We present density functional theory calculations of the adsorption of arsenic acid (AsO(OH)3) an... more We present density functional theory calculations of the adsorption of arsenic acid (AsO(OH)3) and arsenous acid (As(OH)3) on the Al(iii)-modified natural zeolite clinoptilolite under anhydrous and hydrated conditions.
Redox behaviour of magnetite Fe3O4nanoparticles in thermodynamic equilibrium conditions enclosed ... more Redox behaviour of magnetite Fe3O4nanoparticles in thermodynamic equilibrium conditions enclosed by non-dipolar reconstructed surfaces.
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2011
Using a combination of density functional theory calculations and statistical mechanics, we show ... more Using a combination of density functional theory calculations and statistical mechanics, we show that a wide range of intermediate compositions of ceria–zirconia solid solutions are thermodynamically metastable with respect to phase separation into Ce-rich and Zr-rich oxides. We estimate that the maximum equilibrium concentration of Zr in CeO 2 at 1373 K is approximately 2 per cent, and therefore, equilibrated samples with higher Zr content are expected to exhibit heterogeneity at the atomic scale. We also demonstrate that in the vicinity of the (111) surface, cation redistribution at high temperatures will occur with significant Ce enrichment of the surface, which we attribute to the more covalent character of Zr–O bonds compared with Ce–O bonds. Although the kinetic barriers for cation diffusion normally prevent the decomposition/segregation of ceria–zirconia solid solutions in typical catalytic applications, the separation behaviour described here can be expected to occur in mode...
In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue an... more In view of the importance of the hydroxyapatite/collagen composite of both natural bone tissue and in synthetic biomaterials, we have investigated the interaction of three constituent amino acids of the collagen matrix with two major hydroxyapatite surfaces. We have employed electronic structure techniques based on the density functional theory to study a range of different binding modes of the amino acids glycine, proline, and hydroxyproline at the hydroxyapatite (0001) and (0110) surfaces. We have performed full geometry optimizations of the hydroxyapatite surfaces with adsorbed amino acid molecules to obtain the optimum substrate/adsorbate structures and interaction energies. The calculations show that the amino acids are capable of forming multiple interactions with surface species, particularly if they can bridge between two surface calcium ions. The binding energies range from 290 kJ mol(-1) for glycine on the (0001) surface to 610 kJ mol(-1) for hydroxyproline on the (0110) surface. The large adsorption energies are due to a wide range of interactions between the adsorbate and surface, including proton transfer from the adsorbates to surface OH or PO(4) groups. Hydroxyproline binds most strongly to the surfaces, but all three amino acids should be good sites for the nucleation and growth of the hydroxyapatite (0110) surface at the collagen matrix.
The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO2(111) surface... more The electronic structure and oxidation state of atomic Au adsorbed on a perfect CeO2(111) surface have been investigated in detail by means of periodic density functional theory-based calculations, using the LDA+U and GGA+U potentials for a broad range of U values, complemented with calculations employing the HSE06 hybrid functional. In addition, the effects of the lattice parameter a0 and of the starting point for the geometry optimization have also been analyzed. From the present results we suggest that the oxidation state of single Au atoms on CeO2(111) predicted by LDA+U, GGA+U, and HSE06 density functional calculations is not conclusive and that the final picture strongly depends on the method chosen and on the construction of the surface model. In some cases we have been able to locate two well-defined states which are close in energy but with very different electronic structure and local geometries, one with Au fully oxidized and one with neutral Au. The energy difference bet...
... Neyvis Almora-Barrios*a and Nora H. de Leeuw*ab. a Department of Chemistry, University Colleg... more ... Neyvis Almora-Barrios*a and Nora H. de Leeuw*ab. a Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ. ... 27, S. Hamad, CE Hughes, CRA Catlow and KDM Harris, J. Phys. Chem. B, 2008, 112, 72807288 Article ChemPort . ...
Bulletin of the American Physical Society, Mar 19, 2010
We use density functional theory calculations with Hubbard corrections (DFT+ U) to investigate th... more We use density functional theory calculations with Hubbard corrections (DFT+ U) to investigate the electronic and redox properties of Ti-substituted zirconia (111) surfaces. It is found that titanium dopants are more likely to segregate at the surface than to migrate to the zirconia bulk. The formation energy of oxygen vacancies decreases substantially in titanium-substituted surfaces with respect to undoped surfaces. If an O vacancy is created around an isolated Ti dopant, a Ti $^{4+}\ quad\ to $ Ti $^{2+} $ reduction takes place, while if the ...
We present density functional theory calculations of the adsorption of arsenic acid (AsO(OH)3) an... more We present density functional theory calculations of the adsorption of arsenic acid (AsO(OH)3) and arsenous acid (As(OH)3) on the Al(iii)-modified natural zeolite clinoptilolite under anhydrous and hydrated conditions.
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