Size Effect in Ethylene Oxidation over Silver Nanoparticles Supported on HOPG

A preparation procedure for a model catalyst – stable silver nanoparticles with a narrow size distribution supported on highly oriented pyrolytic graphite (Ag/HOPG) – is presented. This procedure is carried out under ultra high vacuum conditions and consists of three stages: 1 – surface defect formation by soft Ar+-treatment; 2 – silver deposition; 3 – defect annealing by heating at T = 300 °C. The analysis of oxygen forms on a silver surface was carried out by in-situ X-ray photoelectron spectroscopy (XPS). The simultaneous presence of two oxygen species (electrophilic and nucleophilic) on the silver surface was found to be necessary for high activity of the model catalyst in ethylene epoxidation. An alternative explanation of the size effect in ethylene oxidation has been suggested: size dependence of the reaction rate may result from the existence of a ring-shaped zone at the edge of silver particles in which the surface concentration of nucleophilic oxygen is not uniform. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3514

Are Au nanoparticles on oxygen free supports catalytically active?

Gold nanoparticles Au NPs on oxygen free supports were examined using near ambient pressure X ray photoelectron spectroscopy NAP XPS under CO oxidation conditions, and ex situ using scanning electron microscopy SEM and transmission electron microscopy TEM . Our observations demonstrate that Au NPs supported on carbon materials are inactive, regardless of the preparation method. Ozone O3 treatment of carbon supports leads oxygen functionalization of the supports. When subsequently exposed to a CO feed, CO is oxidized by the functionalized sites of the carbon support via a stoichiometric pathway. Microscopy reveals that the reaction with CO does not change the morphology of the Au NPs. In situ XPS reveals that the O3 treatment gives rise to additional Au 4f and O 1s peaks at binding energies of 85.25 85.6 eV and 529.4 530 eV, respectively, which are assigned to the presence of Au oxide. A surface oxide phase is formed during the activation of Au NPs supported on Au foil by O3 treatment. However, this phase decomposes in vacuum and the remaining low coordinative atoms do not have sufficient catalytic properties to oxidize CO, so the size reduction of Au NPs and or oxidation of Au NPs is not sufficient to activate A

Interactive Surface Chemistry of CO2 and NO2 on Metal Oxide Surfaces: Competition for Catalytic Adsorption Sites and Reactivity

Cataloged from PDF version of article.Interactive surface chemistry of CO2 and NO2 on BaOx/Pt(111) model catalyst surfaces were investigated via X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques with a particular emphasis on the competition between different adsorbates for the catalytic adsorption sites and adsorbate-induced morphological changes. After NO2 adsorption, nitrated BaO x/Pt(111) surfaces do not reveal available adsorption sites for CO2 at 323 K, irrespective of the presence/absence of exposed Pt sites on the surface. Although NO2 adsorption on thick BaO x(>10MLE)/Pt(111) overlayers at 323 K leads to the formation of predominantly nitrate species, NO2 adsorption on the corresponding carbonated surface leads to the formation of coexisting nitrates and nitrites. The presence of carbonates on BaOx/Pt(111) overlayers does not prevent NO2 uptake. Carbonated BaOx(1.5 MLE)/Pt(111) surfaces (with exposed Pt sites) obtained via CO2 adsorption can also further interact with NO2, forming surface nitrate/nitrite species, accompanied by the transformation of surface carbonates into bulk carbonate species. These results suggest that the nitrate formation process requires the presence of two adjacent unoccupied adsorption sites. It is apparent that in the presence of both NO2 and CO2, carbonate species formed on Lewis base (O2-) sites enable the formation of nitrites on Lewis acid (Ba2+) sites. Thermal aging, nitration, and carbonation have a direct impact on the morphology of the two-/three-dimensional (2D/3D) BaO x aggregates on Pt(111). While thermal aging in vacuum leads to the sintering of the BaOx domains, nitration and carbonation results in redispersion and spreading of the BaOx domains on the Pt(111) substrate. © 2013 American Chemical Society.

Early Spring Feeding Habits of Bearded Seals (Erignathus Barbatus) in the Central Bering Sea, 1981

The diet of bearded seals, Erignathus barbatus, near St. Matthew Island, Bering Sea, was studied during the early spring of 1981. Eighty-six percent of the 78 seals' stomachs examined contained fish. Other prey taxon groups, in decreasing order of their percentages of occurrence, were crabs (73%), clams (55%), snails (47%), amphipods (32%), shrimp (18%), mysids (13%), marine worms (13%) and cephalopods (4%). The most frequently occurring prey species were capelin, Mallotus villosus, (82%); codfishes, Gadidae (64%); narrow snow crab, Chionoecetes opilio (63%); eelpouts, Lycodes spp. (56%); longsnout prickleback, Lumpenella longirostris (49%); nutshell clams, Nuculana sp. (42%); and moon snails, Polinices sp. (27%). Seventy-seven percent of the seals examined had consumed prey from three or more different taxon groups. We identified seven food items not previously reported as prey of the bearded seal in the Bering Sea. No differences were detected between the diets of males and females and between adults and juveniles, indicating no apparent segregation of foraging by sex or age. Bearded seals in the St. Matthew Island region of the Bering Sea forage in a manner similar to their conspecifics in other areas where fish constitute a major portion of their diet. Prey selection is probably dependent on availability, and diet may be highly diversified even within a relatively small area during a short period of time. Variety in prey consumption exemplified the ability of the bearded seal to forage in the seasonally changing habitat associated with the advance and retreat of the ice front.Key words: bearded seal, Erignathus barbatus, diet, demersal and pelagic fish, benthic invertebrates, prey species diversity On a étudié le régime alimentaire du phoque barbu, Erignathus barbatus, près de l'île Saint Matthew dans la mer de Béring, tôt au printemps de 1981. Quatre-vingt six p. cent des 78 estomacs de phoques examinés contenaient du poisson. Parmi les autres groupes de taxons servant de proies, on retrouvait, par pourcentages décroissants, les crabes (73 p. cent), les myes (55 p. cent), les gastéropodes (47 p. cent), les amphipodes (32 p. cent), les crevettes (18 p. cent), les mysis (13 p. cent), les vers marins (13 p. cent) et les céphalopodes (4 p. cent). Les espèces de proies les plus courantes étaient le capelan, Mallotus villosus (82 p. cent); la morue, Gadidae (64 p. cent); le crabe des neiges, Chionoecetes opilio (63 p. cent); la lotte, sp. Lycodes (56 p. cent); Lumpenella longirostris (49 p. cent); sp. Nuculana (42 p. cent); et la natice, sp. Polinices (27 p. cent). Soixante-dix-sept p. cent des phoques étudiés avaient ingéré des proies venant d'au moins trois différents groupes de taxons. On a identifié sept produits alimentaires qui n'avaient pas encore été reportés comme constituant une proie pour le phoque barbu dans la mer de Béring. On n'a détecté aucune différence entre les régimes alimentaires des mâles et ceux des femelles, ni entre ceux des adultes et ceux des petits, ce qui indique qu'il n'existe apparemment pas de ségrégation quant au sexe ou à l'âge lors du comportement visant la quête de nourriture. Les phoques barbus de la région de l'île Saint Matthew dans la mer de Béring recherchent leur nourriture comme leurs congénères dans d'autres régions où le poisson constitue une grande partie de leur régime. La sélection des proies dépend probablement de leur disponibilité et le régime peut être hautement diversifié, même dans une zone relativement petite et durant une courte période. La variété qui se manifeste dans la consommation des proies montre bien la capacité du phoque barbu à rechercher sa nourriture dans un habitat qui varie selon les saisons et est associé à l'avancée et au retrait du front glaciaire.Mots clés: phoque barbu, Erignathus barbatus, régime alimentaire, poissons démersaux et pélagiques, invertébres benthiques, variété des espèces servant de proi

Direct Evidence for the Instability and Deactivation of Mixed-Oxide Systems: Influence of Surface Segregation and Subsurface Diffusion

Cataloged from PDF version of article.In the current contribution, we provide a direct demonstration of the thermally induced surface structural transformations of an alkaline-earth oxide/transition metal oxide interface that is detrimental to the essential catalytic functionality of such mixed-oxide systems toward particular reactants. The BaO(x)/TiO(2)/Pt(111) surface was chosen as a model interfacial system where the enrichment of the surface elemental composition with Ti atoms and the facile diffusion of Ba atoms into the underlying TiO(2) matrix within 523-873 K leads to the formation of perovskite type surface species (BaTiO(3)/Ba(2)TiO(4)/Ba(x)Ti(y)O(z)). At elevated temperatures (T > 973 K), excessive surface segregation of Ti atoms results in an exclusively TiO(2)/TiO(x)-terminated surface which is almost free of Ba species. Although the freshly prepared BaO(x)/TiO(2)/Pt(111) surface can strongly adsorb ubiquitous catalytic adsorbates such as NO(2) and CO(2), a thermally deactivated surface at T > 973 K practically loses all of its NO(2)/CO(2) adsorption capacity due to the deficiency of surface BaO(x) domains

Pd single-atom sites on the surface of PdAu nanoparticles: A DFT-based Topological search for suitable compositions

Structure of model bimetallic PdAu nanoparticles is analyzed aiming to find Pd:Au ratios optimal for existence of Pd1 single-atom surface sites inside outer Au atomic shell. The analysis is performed using density-functional theory (DFT) calculations and topological approach based on DFT-parameterized topological energy expression. The number of the surface Pd1 sites in the absence of adsorbates is calculated as a function of Pd concentration inside the particles. At low Pd contents none of the Pd atoms emerge on the surface in the lowest-energy chemical orderings. However, surface Pd1 sites become stable, when Pd content inside a Pd-Au particle reaches ca. 60%. Further Pd content increase up to almost pure Pd core is accompanied by increased concentration of surface Pd atoms, mostly as Pd1 sites, although larger Pd ensembles as dimers and linear trimers are formed as well. Analysis of the chemical orderings inside PdAu nanoparticles at different Pd contents revealed that enrichment of the subsurface shell by Pd with predominant occupation of its edge positions precedes emergence of Pd surface species

Role of the Exposed Pt Active Sites and BaO2 Formation in Nox Storage Reduction Systems: A Model Catalyst Study on BaOx/Pt(111)

Cataloged from PDF version of article.BaOx(0.5 MLE - 10 MLE)/Pt(111) (MLE: monolayer equivalent) surfaces were synthesized as model NOx storage reduction (NSR) catalysts. Chemical structure, surface morphology, and the nature of the adsorbed species on BaOx/Pt(111) surfaces were studied via X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). For theta(BaOx) = 2.5 MLE) were found to be amorphous. Extensive NO2 adsorption on BaOx(10 MLE)/Pt(111) yields predominantly nitrate species that decompose at higher temperatures through the formation of nitrites. Nitrate decomposition occurs on BaOx(10 MLE)/Pt(111) in two successive steps: (1) NO(g) evolution and BaO2 formation at 650 K and (2) NO(g) + O-2(g) evolution at 700 K. O-2(g) treatment of the BaOx(10 MLE)/Pt(111) surface at 873 K facilitates the BaO2 formation and results in the agglomeration of BaOx domains leading to the generation of exposed Pt(111) surface sites. BaO2 formed on BaOx(10 MLE)/Pt(111) is stable even after annealing at 1073 K, whereas on thinner films (theta(BaOx) = 2.5 MLE), BaO2 partially decomposes into BaOx indicating that small BaO2 clusters in close proximity of the exposed Pt(111) sites are prone to decomposition. Nitrate decomposition temperature decreases monotonically from 550 to 375 K with decreasing BaOx coverage within theta(BaOx) = 0.5 to 1.0 MLE. Nitrate decomposition occurs at a rather constant temperature range of 650-700 K for thicker BaOx overlayers (2.5 MLE < theta(BaOx) < 10 MLE). These two distinctly characteristic BaOx-coverage-dependent nitrate decomposition regimes are in very good agreement with the observation of the so-called "surface" and "bulk" barium nitrates previously reported for realistic NSR catalysts, clearly demonstrating the strong dependence of the nitrate thermal stability on the NOx storage domain size

Purification and characterization of recombinant human renin for X-ray crystallization studies

<p>Abstract</p> <p>Background</p> <p>The renin-angiotensin-aldosterone system (RAS) cascade is a major target for the clinical management of hypertension. Although inhibitors of various components of this cascade have been developed successfully, development of renin inhibitors has proven to be problematic. The development of these inhibitors has been hindered by poor bioavailability and complex synthesis. However, despite the challenges of designing renin inhibitors, the enzyme remains a promising target for the development of novel treatments for hypertension. X-ray crystallographic data could greatly assist the design and development of these inhibitors. Here we describe the purification and characterization of recombinant human renin for x-ray crystallization studies.</p> <p>Results</p> <p>A cDNA encoding the full length of native human preprorenin (406 amino acid residues) was introduced into the HEK-293 cell line. A clonal cell line expressing prorenin was generated and grown under serum free conditions in a hollow fiber bioreactor. Prorenin was constitutively secreted and purified directly from the conditioned medium. Concanavalin A chromatography effectively enriched and purified prorenin to 90% homogeneity in a single step. Prorenin was converted to active renin by trypsin digestion to remove the propeptide. Active renin was further purified using a cation exchange column followed by a gel filtration column. Biochemical characterization of the recombinant enzyme showed both binding and catalytic properties were essentially identical to previously reported activities for purified renin. Crystals were grown using this material in our X-ray structure studies, and high resolution diffraction was obtained.</p> <p>Conclusion</p> <p>This present work describes a simple and efficient method for the generation and purification of active human renin. The protein is highly pure and is suitable for supporting structural biology efforts.</p