Study of the equilibrium surface composition of Co1-yFe2+yO4--I : Cation composition with stoichiometric oxygen content

The equilibrium composition at the surface of the mixed spinel Co1-yFe2+yO4 is calculated as a function of the composition parameter y and of the degree of inversion. The ionic standard chemical potentials include the Born-Mayer repulsive energy and the vibrational contribution. The calculated results turn out to be weakly dependent on the bulk inversion, and indicate a pronounced tendency of iron to segregate at the surface. These results are compared and found to be in agreement with some experimental data obtained by AES. In a subsequent paper we will take into account the gas-phase and the charge-transfer effects.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22764/1/0000319.pd

Study of the equilibrium surface composition of Co1-yFe2+yO4--II : The role of vacant and chemisorbed oxygen and charge transfer effect

In Part I of this work we have calculated the equilibrium cation composition at the surface of Co1-yFe2+yO4 considering a stoichiometric oxygen content. In this paper we extend our model to include the effect on the cation surface composition of the equilibrium with the gas-phase. We take into account both the formation of surface oxygen vacancies and chemisorption. This model allows for an interpretation of AES experimental data obtained after different thermal treatments under gas fluxes or in UHV. While all previous calculations were based on the assumption that Co3+ and Fe2+ ions are never present simultaneously, here we discuss finally the effects of the charge transfer from Co2+ to Fe3+.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22765/1/0000320.pd

Generating multi-chain configurations of an inhomogeneous melt from the knowledge of single-chain properties

Mean-field techniques provide a rather accurate description of single-chain conformations in spatially inhomogeneous polymer systems containing interfaces or surfaces. Intermolecular correlations, however, are not described by the mean-field approach and information about the distribution of distance between different molecules is lost. Based on the knowledge of the exact equilibrium single-chain properties in contact with solid substrates, we generate multi-chain configurations that serve as nearly equilibrated starting configurations for molecular dynamics simulations by utilizing the packing algorithm of Auhl and co-workers [J. Chem. Phys. 119, 12718 (2003)] for spatially inhomogeneous systems, i.e., a thin polymer film confined between two solid substrates. The single-chain conformations are packed into the thin film conserving the single-chain properties and simultaneously minimizing local fluctuations of the density. The extent to which enforcing the near-incompressibility of a dense polymer liquid during the packing process is able to re-establish intermolecular correlations is investigated by monitoring intermolecular correlation functions and the structure function of density fluctuations as a function of the distance from the confining solid substrates.Comment: 10 pages, 8 figure

Magnesium oxide as a catalyst support: The influence of chlorine

The properties of MgO when used as a support material for Ru-Au catalysts have been investigated by thermal analysis, chemical analysis, surface area measurements and X-ray diffraction. During impregnation the support undergoes a bulk hydration but heating to 673 K restores the oxide. In a sample impregnated by water only, the dehydration occurs at 651 K and the surface area increases from 15 to> 300 m2 g-1, due to the formation of small pores. Great changes in the DTA peak temperature and in the surface area after dehydration are found in the catalysts and in MgO impregnated with HCl solutions. It is shown that these changes are accurately correlated to the amount of chlorine which remains in the solid (deriving from the metal precursor compounds or HCl). The formation of basic magnesium chlorides, shown by X-ray diffraction, seems to favour an easier release of water (i.e., lower DTA peak temperatures and lower apparent activation energies) and to reduce the formation of small pores.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23933/1/0000179.pd

Bimetallic Ru-Au catalysts: Effect of the support

Ru-Au catalysts supported on SiO2 were characterized by using H2 and O2 chemisorption, wide-angle X-ray scattering, diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. Catalytic activity was measured for the hydrogenolysis of propane and ethane. The hydrogenolysis activity of ruthenium decreased by two orders of magnitude with addition of gold. This suggested that Ru and Au did not exist as separate particles but formed bimetallic aggregates. Chemisorption and XPS experiments showed a surface composition similar to the bulk. A comparison was made with a previously studied Ru-Au-on-MgO system, on which an enrichment of Ru on the surface of bimetallic Ru-Au clusters was discovered. It is suggested that the strength of the metal-support interaction can affect the surface composition of multimetallic supported systems.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/24360/1/0000629.pd

Bimetallic ruthenium-gold-on-magnesia catalysts: Chemicophysical properties and catalytic activity

Bimetallic Ru-Au catalysts supported on MgO were prepared, their atomic composition varying from 100% Ru to 100% Au. Samples were characterized after impregnation and drying at 110 [deg]C and after further reduction by hydrogen at 400 [deg]C. X-Ray line broadening analysis, small-angle X-ray scattering, transmission electron microscopy, gas chemisorption, diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) analysis were used. Catalytic activity was measured for the oxygen transfer between CO and CO2. The Ru and Au precursor compounds appear to be modified, and have probably interacted in the bimetallic samples, already after impregnation and drying. Relevant amounts of metal oxides are found in some "reduced" catalysts; a metal-support interaction is proposed for Au/MgO. The amounts of the Ru and Au oxidized species found by EXAFS in the examined bimetallic sample are significantly different from those found in Ru/MgO and Au/MgO, respectively. The DR spectra of the bimetallic samples are not a simple combination of those of the monometallic catalysts. The Ru/Au atomic ratios measured by XPS are higher than the analytical ones. The fraction of surface Ru atoms, measured by hydrogen chemisorption, increases on decreasing the bulk Ru/Au atomic ratio. The Au/MgO sample has an activity which is two orders of magnitude higher than that of the remaining samples. All of this suggests the existence of a Ru surface enrichment and also an Ru-Au chemical interaction. These conclusions are also supported by an ir study of CO chemisorbed on the same samples.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23184/1/0000111.pd

Surface modification of starch based biomaterials by oxygen plasma or UV-irradiation

Radiation is widely used in biomaterials science for surface modification and sterilization. Herein, we describe the use of plasma and UV-irradiation to improve the biocompatibility of different starch-based blends in terms of cell adhesion and proliferation. Physical and chemical changes, introduced by the used methods, were evaluated by complementary techniques for surface analysis such as scanning electron microscopy, atomic force microscopy, contact angle analysis and X-ray photoelectron spectroscopy. The effect of the changed surface properties on the adhesion of osteoblast-like cells was studied by a direct contact assay. Generally, both treatments resulted in higher number of cells adhered to the modified surfaces. The importance of the improved biocompatibility resulting from the irradiation methods is further supported by the knowledge that both UV and plasma treatments can be used as cost-effective methods for sterilization of biomedical materials and devices.I. P. thanks the FCT for providing her a postdoctoral scholarship (SFRH/BPD/8491/2002). This work was partially supported by FCT, through funds from the POCTI and/or FEDER programs, The European Union funded STREP Project HIPPOCRATES (NNM-3-CT-2003-505758) and the European NoE EXPERTISSUES (NMP3-CT-2004-500283)