A STUDY OF SURFACE OXYGEN SPECIES FROM LA-ME-O (ME=MG, CA, SR, BA) CATALYSTS FOR METHANE OXIDATIVE COUPLING

CO2-TPD was used to study the surface basicity of La-Me-O mixed oxides and O-2-TPD, CH4-TPD were employed to study the surface active oxygen species. Comparing the CO2-TPD with O-2-TPD, we can see that the basicity of catalyst is in parallel with the catalystic activity. The stronger basicity is more profitable for the catalyst to adsorb oxygen to form active oxygen species and to activate CH4 by breaking a C-H bond, By comparing the catalytic activity, the results showed that La-Ba-O(La/Ba=7/3) catalyst had the strongest basicity, and it gave the highest CH4 conversion and C-2 selectivity, The results from the pulse reaction showed that the lattice oxygen participated in the OCM reaction without gas oxygen, and it was the selective oxygen species.

Equilibrium structure of erbium-oxygen complexes in crystalline silicon

The equilibrium structure of ErOn (nless than or equal to6) complexes in crystalline silicon has been investigated by density-functional computations. Two different geometries have been considered, corresponding to the substitutional and tetrahedral interstitial site for erbium. All atomic coordinates have been optimized by Car-Parrinello molecular dynamics. The resulting structures have low symmetry, with E-O distances of similar to2.35 Angstrom. The substitutional site is the most stable one for nless than or equal to2, while the tetrahedral interstitial is favored for n>2.

Exchange between sub-surface and surface oxygen vacancies on CeO2(111):a new surface diffusion mechanism

Using first principles calculations for O vacancy diffusion on CeO2(111), we locate a surface diffusion mechanism, the two-step O vacancy exchange one, which is more favored than the most common hopping mechanism. By analyzing the results, we identify quantitatively the physical origin of why the two-step exchange mechanism is preferred.

The influence of carbon surface oxygen groups on Dubinin–Astakhov equation parameters calculated from CO2 adsorption isotherm

We present the results of a systematic study of the influence of carbon surface oxidation on Dubinin–Astakhov isotherm parameters obtained from the fitting of CO2 adsorption data. Using GCMC simulations of adsorption on realistic VPC models differing in porosity and containing the most frequently occurring carbon surface functionalities (carboxyls, hydroxyls and carbonyls) and their mixtures, it is concluded that the maximum adsorption calculated from the DA model is not strongly affected by the presence of oxygen groups. Unfortunately, the same cannot be said of the remaining two parameters of this model i.e. the heterogeneity parameter (n) and the characteristic energy of adsorption (E0). Since from the latter the pore diameters of carbons are usually calculated, by inverse-type relationships, it is concluded that they are questionable for carbons containing surface oxides, especially carboxyls.

Identification of surface protein complexes of Streptococcus pyogenes through protein microarray technology

A systematic characterization of the composition and structure of the bacterial cell-surface proteome and its complexes can provide an invaluable tool for its comprehensive understanding. The knowledge of protein complexes composition and structure could offer new, more effective targets for a more specific and consequently effective immune response against a complex instead of a single protein. Large-scale protein-protein interaction screens are the first step towards the identification of complexes and their attribution to specific pathways. Currently, several methods exist for identifying protein interactions and protein microarrays provide the most appealing alternative to existing techniques for a high throughput screening of protein-protein interactions in vitro under reasonably straightforward conditions. In this study approximately 100 proteins of Group A Streptococcus (GAS) predicted to be secreted or surface exposed by genomic and proteomic approaches were purified in a His-tagged form and used to generate protein microarrays on nitrocellulose-coated slides. To identify protein-protein interactions each purified protein was then labeled with biotin, hybridized to the microarray and interactions were detected with Cy3-labelled streptavidin. Only reciprocal interactions, i. e. binding of the same two interactors irrespective of which of the two partners is in solid-phase or in solution, were taken as bona fide protein-protein interactions. Using this approach, we have identified 20 interactors of one of the potent toxins secreted by GAS and known as superantigens. Several of these interactors belong to the molecular chaperone or protein folding catalyst families and presumably are involved in the secretion and folding of the superantigen. In addition, a very interesting interaction was found between the superantigen and the substrate binding subunit of a well characterized ABC transporter. This finding opens a new perspective on the current understanding of how superantigens are modified by the bacterial cell in order to become major players in causing disease.

Electrochemical performance of carbon gels with variable surface chemistry and physics

This study describes the electrochemical characterization of N-doped carbon xerogels in the form of microspheres and of carbon aerogels with varied porosities and surface oxygen complexes. The interfacial capacitance of N-doped carbon xerogels decreased with increased micropore surface area as determined by N2 adsorption at −196 °C. The interfacial capacitance showed a good correlation with the areal NXPS concentration, and the best correlation with the areal concentration of pyrrolic or pyridonic nitrogen functionalities. The gravimetric capacitance decreased with greater xerogel microsphere diameter. The interfacial capacitance of carbon aerogels increased with higher percentage of porosity as determined from particle and true densities. The interfacial capacitance showed a linear relationship with the areal oxygen concentration and with the areal concentrations of CO- and CO2-evolving groups.

Activated carbons chemically modified by concentrated H2SO4 for the adsorption of the pollutants from wastewater and the dibenzothiophene from fuel oils

The surface properties, porosities, and adsorption capacities of activated carbons (AC) are modified by the oxidation treatment using concentrated H2SO4 at temperatures 150-270 degreesC. The modified AC was characterized by N-2 adsorption, base titration, FTIR, and the adsorption of iodine, chlorophenol, methylene blue, and dibenzothiophene. The treatment of AC with concentrated H2SO4 at 250 degreesC greatly increases the mesoporous volume from 0.243 mL/g to 0.452 mL/g, specific surface areas from 393 m(2)/g to 745 m(2)/g, and acidic surface oxygen complexes from 0.071 meq/g to 1.986 meq/g as compared with the unmodified AC. The base titration results indicate that the amount of acidic surface oxygen groups on the modified AC increases with increasing the treatment temperatures and carboxyls and phenols are the most abundant carbon-oxygen functional groups. The carboxyl groups, COO- species, and hydroxyl groups are detected mainly for the sample treated at 250 degreesC. The mesoporous properties of the AC modified by concentrated H2SO4 were further tested by the adsorption of methylene blue and dibenzothiophene. The AC modified by concentrated H2SO4 at 250 degreesC has much higher adsorption capacities for large molecules (e.g., methylene blue and dibenzothiophene) than the unmodified AC but less adsorption capacities for small molecules (e.g., iodine). The adsorption results from aqueous solutions have been interpreted using Freundlich adsorption models.

Preparation and characterization of multi-walled carbon nanotubes supported PtRu catalysts for proton exchange membrane fuel cells

A series of PtRu nanocomposites supported on H2O2-oxidized multi-walled carbon nanotubes (MWCNTs) were synthesized via two chemical reduction methods - one used aqueous formaldehyde (HCHO method) and the other used ethylene glycol (EG method) as the reducing agents. The effects of the solvents (water and ethylene glycol) and the surface composition of the MWCNTs on the deposition and the dispersion of the metal particles were investigated using N-2 adsorption. TEM. ICP-AES. FTIR and TPD. The wetting heats of the MWCNTs in corresponding solvents were also measured. The characterizations suggest that combination of the surface chemistry of the MWCNTs with the solvents decides the deposition and the dispersion of the metal nanoparticles. These nanocomposites were evaluated as proton exchange membrane fuel cell anode catalyts for oxidation of 50 ppm CO contaminated hydrogen and compared with a commercial PtRu/C catalyst. The data reveal superior performances for the nanocomposites prepared by the EG method to those by the HCHO method and even to that for tile Commercial analogue. Structure performance relationship of the nanocomposites was also studied. (C) 2005 Elsevier Ltd. All rights reserved.

Lithium and oxygen adsorption at the b-MnO2 (110) surface

The adsorption and co-adsorption of lithium and oxygen at the surface of rutile-like manganese dioxide(b-MnO2), which are important in the context of Li–air batteries, are investigated using density functional theory. In the absence of lithium, the most stable surface of b-MnO2, the (110), adsorbs oxygen in the form of peroxo groups bridging between two manganese cations. Conversely, in the absence of excess oxygen, lithium atoms adsorb on the (110) surface at two different sites, which are both tricoordinated to surface oxygen anions, and the adsorption always involves the transfer of one electron from the adatom to one of the five-coordinated manganese cations at the surface, creating (formally) Li+ and Mn3+ species. The co-adsorption of lithium and oxygen leads to the formation of a surface oxide, involving the dissociation of the O2 molecule, where the O adatoms saturate the coordination of surface Mn cations and also bind to the Li adatoms. This process is energetically more favourable than the formation of gas-phase lithium peroxide (Li2O2) monomers, but less favourable than the formation of Li2O2 bulk. These results suggest that the presence of b-MnO2 in the cathode of a nonaqueous Li–O2 battery lowers the energy for the initial reduction of oxygen during cell discharge.

Theoretical analysis of the energy levels induced by oxygen vacancies and the doping process (Co, Cu and Zn) on SnO2 (110) surface models

Density functional calculation at B3LYP level was employed to study the surface oxygen vacancies and the doping process of Co, Cu and Zn on SnO2 (110) surface models. Large clusters, based on (SnO2)(15) models, were selected to simulate the oxidized (Sn15O30), half-reduced (Sn15O29) and the reduced (Sn15O28) surfaces. The doping process was considered on the reduced surfaces: Sn13Co2O28, Sn13Cu2O28 and Sn13Zn2O28. The results are analyzed and discussed based on a calculation of the energy levels along the bulk band gap region, determined by a projection of the monoelectron level structure on to the atomic basis set and by the density of states. This procedure enables one to distinguish the states coming from the bulk, the oxygen vacancies and the doping process, on passing from an oxidized to a reduced surface, missing bridge oxygen atoms generate electronic levels along the band gap region, associated with 5s/5p of four-/five-fold Sn and 2p of in-plane O centers located on the exposed surface, which is in agreement with previous theoretical and experimental investigations. The formation energy of one and two oxygen vacancies is 3.0 and 3.9 eV, respectively. (C) 2001 Elsevier B.V. B.V. All rights reserved.