Separation of molybdenum from tungsten solutions by solvent extraction using hydrogen peroxide

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In-Situ Vibrational Spectroscopic Studies on Model Catalyst Surfaces at Elevated Pressures

Cataloged from PDF version of article.Elucidation of complex heterogeneous catalytic mechanisms at the molecular level is a challenging task due to the complex electronic structure and the topology of catalyst surfaces. Heterogeneous catalyst surfaces are often quite dynamic and readily undergo significant alterations under working conditions. Thus, monitoring the surface chemistry of heterogeneous catalysts under industrially relevant conditions such as elevated temperatures and pressures requires dedicated in situ spectroscopy methods. Due to their photons-in, photons-out nature, vibrational spectroscopic techniques offer a very powerful and a versatile experimental tool box, allowing real-time investigation of working catalyst surfaces at elevated pressures. Infrared reflection absorption spectroscopy (IRAS or IRRAS), polarization modulation-IRAS and sum frequency generation techniques reveal valuable surface chemical information at the molecular level, particularly when they are applied to atomically well-defined planar model catalyst surfaces such as single crystals or ultrathin films. In this review article, recent state of the art applications of in situ surface vibrational spectroscopy will be presented with a particular focus on elevated pressure adsorption of probe molecules (e.g. CO, NO, O-2, H-2, CH3OH) on monometallic and bimetallic transition metal surfaces (e.g. Pt, Pd, Rh, Ru, Au, Co, PdZn, AuPd, CuPt, etc.). Furthermore, case studies involving elevated pressure carbon monoxide oxidation, CO hydrogenation, Fischer-Tropsch, methanol decomposition/partial oxidation and methanol steam reforming reactions on single crystal platinum group metal surfaces will be provided. These examples will be exploited in order to demonstrate the capabilities, opportunities and the existing challenges associated with the in situ vibrational spectroscopic analysis of heterogeneous catalytic reactions on model catalyst surfaces at elevated pressures

The effect of impregnation strategy on methane dry reforming activity of Ce promoted Pt/ZrO2

Cataloged from PDF version of article.Dry reforming of methane has been studied over Pt/ZrO2 catalysts promoted with Ce for different temperatures and feed compositions. The influence of the impregnation strategy and the cerium amount on the activity and stability of the catalysts were investigated. The results have shown that introduction of 1 wt.% Ce to the Pt/ZrO2 catalyst via coimpregnation method led to the highest catalytic activity and stability. 1 wt.%Ce-1 wt.%Pt/ZrO2 catalyst prepared by sequential impregnation displayed inferior CH4 and CO2 conversion performances with lowest H-2/CO production ratios. 1 wt.%Ce-1 wt.%Pt/ZrO2 catalyst prepared by coimpregnation showed the highest activity even for the feed with high CH4/CO2 ratio. The reason for high activity was explained by the intensive interaction between Pt and Ce phases for coimpregnated sample, which had been verified by X-ray photoelectron spectroscopy and Energy Dispersive X-Ray analyses. Strong and extensive Pt-Ce surface interaction results in an increase in the number of Ce3+ sites and enhances the dispersion of Pt. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved

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.

First-principles investigation of Nox and Sox adsorption on anatase-supported BaO and Pt overlayers

Cataloged from PDF version of article.We present a density functional theory investigation of the adsorption properties of NO and NO2 as well as SO2 and SO3 on BaO and Pt overlayers on anatase TiO2(001) surface. Mono layers, bilayers, and trilayers of BaO grow without strain-induced large scale reconstructions. While the bilayer and trilayer preserve, to a large extent, the NO2 adsorption characteristics of the clean BaO(100) surface, the effect of the support is evident in SO2 and SO3 adsorption energies, which are somewhat reduced with respect to the clean BaO(100) surface. When a Pt(100) layer is added on the TiO2 surface, four stable adsorption geometries are identified in the case of NO while NO2 is found to adsorb in only two configurations

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

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

Investigation of work function and chemical composition of thin films of borides and nitrides

Thin films of various borides, nitrides, and barium fluorides were tentatively deposited by pulsed laser deposition and by magnetron sputtering in order to develop the components of thermionic-photovoltaic devices for the high-temperature thermal to electrical conversion by solid state. To improve the device performance, the materials characterized by a low work function were selected. In the present work, the chemical composition and work function of obtained films were investigated by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy techniques. The values of work function were determined from the cut-off in the He I valence band spectra. Different films were compared and estimated on the basis of X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy results

Carbonic anhydrase inhibitors. Inhibition of red blood cell ostrich (Struthio camelus) carbonic anhydrase with a series of aromatic and heterocyclic sulfonamides.

The purification of red blood cell carbonic anhydrase (CA, EC 4.2. 1. 1) from ostrich (scCA) blood is reported, as well as an inhibition study of this enzyme with a series of aromatic and heterocylic sulfonamides. The ostrich enzyme showed a high activity, comparable to that of the human isozyme II, with k(cat) of 1. 2 center dot 10(6) s(-1) and k(cat)/K-M of 1. 8 center dot 10(7)M(-1) s(-1), and an inhibition profile quite different from that of the human red blood cell cytosolic isozymes hCA I and II. scCA has generally a lower affinity for sulfonamide inhibitors as compared to hCA I and II. The only sulfonamide which behaved as a very potent inhibitor of this enzyme was ethoxzolamide (K-1 = 3.9 nM) whereas acetazolamide and sulfanilamide behaved as weaker inhibitors (inhibition constants in the range 303-570 nM). Several other aromatic and heterocyclic sulfonamides, mostly derivatives of sulfanilamide, homosulfanilamide, 4-aminoethylbenzenesulfonamide or 5-amino-1,3,4-thiadiazole-2-sulfonamide, showed good affinities for the ostrich enzyme, with K-1 values in the range 25 - 72 nM

Fine-tuning the dispersion and the mobility of BaO domains on NO x storage materials via TiO2 anchoring sites

In an attempt to control the surface dispersion and the mobility of BaO domains on NOx storage materials, TiO2/TiOx anchoring sites were introduced on/inside the conventional γ-Al 2O3 support matrix. BaO/TiO2/Al 2O3 ternary oxide materials were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies. The synthesized NOx storage materials were studied via XRD, Raman spectroscopy, BET surface area analysis, TPD, XPS, SEM, EDX-mapping, and in situ FTIR spectroscopy of adsorbed NO2. NOx uptake properties of the BaO/TiO2/Al2O3 materials were found to be strongly influenced by the morphology and the surface structure of the TiO2/TiOx domains. An improved Ba surface dispersion was observed for the BaO/TiO2/Al2O3 materials synthesized via the coprecipitation of alkoxide precursors, which was found to originate mostly from the increased fraction of accessible TiO 2/TiOx sites on the surface. These TiO2/ TiOx sites function as strong anchoring sites for surface BaO domains and can be tailored to enhance surface dispersion of BaO. TPD experiments suggested the presence of at least two different types of NOx species adsorbed on the TiO2/TiOx sites, with distinctively different thermal stabilities. The relative stability of the NOx species adsorbed on the BaO/TiO2/Al2O3 system was found to increase in the following order: NO+/N2O 3 on alumina ≪ nitrates on alumina &lt; surface nitrates on BaO &lt; bridged/bidentate nitrates on large/isolated TiO2 clusters &lt; bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on TiO2 crystallites homogenously distributed on the surface &lt; bulk nitrates on the BaO sites located on the TiO2 domains. © 2010 American Chemical Society