Metamorphism and aqueous alteration in low petrographic type ordinary chondrites

In order to investigate the relative importance of dry metamorphism and aqueous alteration in the history of chondruies, chondruies were hand-picked from the Semarkona (petrographic type 3.0), Bishunpur (3. 1), Chainpur (3.4), Dhajala (3.8) and Allegan (5) chondrites, and matrix samples were extracted from the first three ordinary chondrites. The thermoluminescence (TL) properties of all the samples were measured, and appropriate subsets of the samples were analyzed by electron-microprobe and radiochemical neutron activation and the water and H-isotopic composition determined. The TL data for chondrules from Semarkona and Bishunpur scatter widely showing no unambiguous trends, although group B1 chondrules tend to have lower sensitivities and lower peak temperatures compared with group A5 chondrules. It is argued that these data reflect the variety of processes accompanying chondrule formation. The chondrules show remarkably uniform contents of the highly labile elements, indicating mineralogical control on abundance and volatile loss from silicates and loss and recondensation of mobile chalcophiles and siderophiles in some cases. Very high D/H values (up to approx. 8000% SMOW) are observed in certain Semarkona chondrules, a confirmation of earlier work. With increasing petrographic type, mean TL sensitivities of the chondrules increase, the spread of values within an individual meteorite decreases, and peak temperatures and peak widths show trends indicating that the TL is mainly produced by feldspar and that dry, thermal metamorphism is the dominant secondary process experienced by the chondrules. The TL sensitivities of matrix samples also increase with petrographic type. Chainpur matrix samples show the same spread of peak temperatures and peak widths as Chainpur chondruies, indicating metamorphism-related changes in the feldspar are responsible for the TL of the matrix. The TL data for the Semarkona and Bishunpur matrix samples provide, at best, only weak evidence for aqueous alteration, but the matrix contains H with approximately terrestrial D/H values, even though it contains much water. Secondary processes (probably aqueous alteration) presumably lowered the D/H of the matrix and certain chondrules. While chondrule properties appear to be governed primarily by formation processes and subsequent metamorphism, the matrix of Semarkona has a more complex history involving aqueous alteration as a meteorite-wide process

In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper.

Using a combination of complementary in situ X-ray photoelectron spectroscopy and X-ray diffraction, we study the fundamental mechanisms underlying the chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) on polycrystalline Cu. The nucleation and growth of h-BN layers is found to occur isothermally, i.e., at constant elevated temperature, on the Cu surface during exposure to borazine. A Cu lattice expansion during borazine exposure and B precipitation from Cu upon cooling highlight that B is incorporated into the Cu bulk, i.e., that growth is not just surface-mediated. On this basis we suggest that B is taken up in the Cu catalyst while N is not (by relative amounts), indicating element-specific feeding mechanisms including the bulk of the catalyst. We further show that oxygen intercalation readily occurs under as-grown h-BN during ambient air exposure, as is common in further processing, and that this negatively affects the stability of h-BN on the catalyst. For extended air exposure Cu oxidation is observed, and upon re-heating in vacuum an oxygen-mediated disintegration of the h-BN film via volatile boron oxides occurs. Importantly, this disintegration is catalyst mediated, i.e., occurs at the catalyst/h-BN interface and depends on the level of oxygen fed to this interface. In turn, however, deliberate feeding of oxygen during h-BN deposition can positively affect control over film morphology. We discuss the implications of these observations in the context of corrosion protection and relate them to challenges in process integration and heterostructure CVD.P.R.K. acknowledges funding from the Cambridge Commonwealth Trust and the Lindemann Trust Fellowship. R.S.W. acknowledges a research fellowship from St. John’s College, Cambridge. S.H. acknowledges funding from ERC grant InsituNANO (no. 279342), EPSRC under grant GRAPHTED (project reference EP/K016636/1), Grant EP/H047565/1 and EU FP7 Work Programme under grant GRAFOL (project reference 285275). The European Synchrotron Radiation Facility (ESRF) is acknowledged for provision of synchrotron radiation and assistance in using beamline BM20/ROBL. We acknowledge Helmholtz-Zentrum-Berlin Electron storage ring BESSY II for synchrotron radiation at the ISISS beamline and continuous support of our experiments.This is the final version. It was first published by ACS at http://pubs.acs.org/doi/abs/10.1021/cm502603

Acidic character of metal loaded amorphous and crystalline silico-aluminas determined by XPS and adsorption calorimetry

The XPS technique in combination with microcalorimetry was used to picture the acid character of metal ion loaded zeolite and silica-alumina samples. Co, Cu, and Ni ions were loaded on ZSM-5 and SiO2-Al2O3 (about 1 mmol/g) by three different procedures: ion exchange, impregnation, and solid-state ion-exchange. The samples prepared by ion exchange of the two matrixes presented high values of the 2 p3/2 XPS bands of the metal ions, indicating the occurrence of charge transfer from the ions to the support matrix. This permitted the stabilization of the metal phases as isolated ions rather than as oxide clusters. Microcalorimetric experiments of ammonia adsorption were performed in order to determine the number, strength (i.e., adsorption enthalpy), and strength distribution of the acid sites on the samples and on the relevant matrixes. The N 1s XPS lines of ammonia adsorbed on the surfaces were decomposed into two component peaks, assigned to Br\uf6nsted (average value of BE, 402.2 eV) and Lewis (average value of BE, 400.4 eV) acid sites. The relative intensities of the two peak components were measured for the quantitative determination of Br\uf6nsted and Lewis acid site concentrations. Coupling the information from adsorption calorimetry and XPS spectroscopy of N 1s adsorbed lines, the absolute numbers of Br\uf6nsted and Lewis acid sites on each sample were determined. The two support matrixes were protonic acids (65 and 60% of Br\uf6nsted acid sites for ZSM-5 and SiO2-Al2O3, respectively), and the acidity of ZSM-5 was greater than that of SiO2-Al2O3 considering both the number of total acid sites and the acid strength. The presence of metal ions deposited on the two matrixes deeply changed the respective proportions of Br\uf6nsted and Lewis sites. A huge increase in the Lewis acid site population of the ZSM-5-based samples (70, 85, and 90% of Lewis sites for the samples containing Co, Cu, and Ni, respectively) and of the SiO2-Al2O3-based samples prepared by ion exchange (55, 60, and 70% of Lewis sites for the samples containing Co, Cu, and Ni, respectively) was observed

XPS study of the adsorption of SO2 and NH3 over supported tin dioxide catalysts used in de-NOx catalytic reaction

Alumina and titania supported tin dioxide catalysts presenting various Sn loadings were prepared by impregnation. The acidity and basicity of the samples were determined by adsorption of ammonia and sulfur dioxide, respectively, using adsorption microcalorimetry and X-ray photoelectron spectroscopy. The surface reactivity of the samples and the chemical composition of the adsorbed species were determined as well as the heats of adsorption of the probe molecules. The catalysts were tested in the selective catalytic reduction of NO by C2H4. Both calorimetry and XPS experiments have shown that the SnO2/TiO2 series of samples was markedly more acidic than the SnO2/Al2O3 series, as the N/(Ti+Sn) molar ratios were noticeably higher than the corresponding N(Al+Sn) ratios. The number of acidic sites seemed to increase with the tin content when tin dioxide was well dispersed on the support. They are of Lewis type. It was shown that sulfur dioxide adsorption led to the formation of three types of species: SO2, sulfites, and sulfates. The basicity of the SnO2/Al2O3 series of samples was weaker than that of the alumina support and passed through a minimum around 12 wt % Sn. On the contrary, as the acidity, the basicity of the SnTi series did not seem dependent on the Sn concentration. This can be correlated to the bad dispersion of SnO2 on TiO2. In the NO reduction by C2H4 reaction, the turnover frequency mainly depends on the Sn dispersion, and the Sn centers are very active even at low amounts

Acid properties of iron oxide catalysts dispersed on silica-zirconia supports with different Zr content

The acid properties of surfaces comprising dispersed iron oxide supported on pure silica and silica\u2013zirconia supports with variable zirconia to silica ratio (ZrO2/SiO2 molar ratio from 0.03 to 0.4) have been studied. The surface acidity of the bare supports and relevant iron oxide covered supports were studied by thermal programmed desorption (TPD) of 2-phenylethylamine (PEA) probe, performed in a thermogravimetric analyzer (TGA). Results from N 1s XPS study of the surfaces saturated with PEA probe complemented the study, permitting the determination of the nature of the acid sites on the different samples. A regularly increasing amount of the number of acid sites and acid strength of the supports was observed increasing the zirconia content on the silica matrix. After the iron oxide support coverage, the number of acid sites of the Fe-samples changed, it increased or decreased compared with that determined on the relevant supports depending on the acidity of the bare support. The average acid strength of the Fe-samples was higher than that of the supports and turned towards a prominent Lewis acidity, in any case. The predominant Lewis nature of all the Fe-catalysts was also confirmed by the results of the reaction of \u3b1-pinene oxide (POX) isomerization that gave the corresponding aldehyde with good selectivity. Graphical abstrac

Surface acidic properties of supported binary oxides containing CuO coupled with Ga2O3 and SnO2 studied by complementary techniques

International audienceSilica–alumina (SA) was used as support for dispersing CuO, Ga2O3, SnO2 oxide phases and the corresponding binary couplings (CuGa/SA and CuSn/SA samples). The acidity of uncovered SA and of SA-supported oxides was studied from a qualitative (nature of acid sites) and quantitative (number, acid strength, and strength distribution of acid sites) points of view by an integrated approach. The adsorption and desorption of basic probes (ammonia and pyridine) onto the acidic surfaces were monitored by the coupled volumetric–calorimetric technique, and by XPS and FT-IR spectroscopy. The results show that the Brönsted acidity of the SA support was converted into predominantly Lewis acidity upon metal oxide deposition. The high dispersion of the copper oxide phase was responsible for this phenomenon. Differences of acid strength distribution were noticed among the samples. Moderate acidity was associated with the Cu sites (100 kJ mol-1 150 kJ mol-1) increased with the presence of Ga and Sn. The experimental results are discussed in the light of the intrinsic acidity of the Cu, Ga, and Sn metal ions derived from electronic parameters

XPS analyses of polystyrene thin films treated under DC pulsed discharges conditions in nitrogen, oxygen and oxygen-argon mixtures

This paper is devoted to polystyrene thin film treatment under DC pulsed discharges in nitrogen, oxygen and oxygen-argon mixtures. XPS analysis is used in order to investigate the new chemical bonds generated by the reactive particles colliding the surface from the plasma. The experimental conditions are close to the best running conditions deduced from [16,75,86]. Correlations between microscopic analyses obtained by XPS through the knowledge of the surface chemical composition and macroscopic ones realized by studies of water drop deposited on the films (contact angle measurements) are presented. It is pointed out that the wettability is highly improved when the percentage of oxygen atoms incorporated in polystyrene surface reaches a high value. C − O, C = O and O − C = O are the main chemical bonds and it is found that the oxygen uptake happens during a very short time (about one second for the treatment duration time, i.e. about 10 ms on account of the duty factor value). A high oxygen rate is observed, correlated to the formation of a maximum $\Delta \theta/\theta_i$ plateau value (wettability). Reaction processes in the plasma bulk and on the polystyrene surface are proposed, taking into account the bond energies of polystyrene. As a consequence, the polystyrene treatment is interpreted assuming two steps for the reaction processes: in a first step, chemical bonds are broken by energy transfer from reactive particles to the surface and in a second step, new chemical bonds are generated. In this spirit, the main reactive plasma particles are described and surface reactions are proposed on the polystyrene surface

Supported Binary Oxide Catalysts Containing CuO Coupled with Ga2O3 and SnO2

Two series of binary oxide catalysts (CuGa/SA and CuSn/SA) containing CuO coupled with Ga2O3 or SnO2 were prepared by dispersing the metal phases onto a high surface area acidic silica-alumina ( SA) support by an adsorption method. Similar total amounts of metals ( about 1.6 atom(Met) center dot nm(-2)) were deposited onto the support, in different proportions, to obtain samples with weight percent of copper varying from 3.3 to 6.5% The SA support was first covered by the copper precursor, Cu(C2H3O2)(2), and then gallium (Ga(NO3)(3) center dot H2O) or tin (SnCl4 center dot 5H(2)O) precursors were deposited on the dried Cu-containing sample. The calcined materials were characterized by surface techniques (N-2 adsorption and XPS), to detect the surface morphology and chemical state of metal species, and by FT-IR and adsorption calorimetry after CO adsorption. Nanosized metal phases were observed in every case. Besides Cu(II), the surfaces contained Cu(I) as well as Cu(delta(+)) ions ( with 1 < delta < 2) stabilized by the interaction with the acid centers of the support. Gallium oxide preferentially covered the copper oxide phase, while tin dioxide mainly deposited on the bare surface of the support. The reducibility properties of the different supported metal oxides were investigated by TPR from both the qualitative and quantitative points of view. The catalytic performances of the simple and binary surfaces were comparatively studied in different reactions ( decomposition and reduction of N2O and NO, and CH4 combustion) considered as test reactions to investigate the reducing or oxidizing properties of the materials. The observed differences in catalytic activity among the catalysts are discussed in relation with the surface composition and properties

Étude de la structure électronique du benzothiazole et de ses homologues azoté, oxygéné et sélénié par spectroscopie photoélectronique (HeI, HeII)

Les structures électroniques comparées du benzothiazole, du benzimidazole, du benzoxazole et du benzosélénazole sont analysées à l'aide de la spectroscopie photoélectronique et des méthodes de calculs de chimie quantique STO-3 G et EHT. Les conclusions tirées de ces dernières méthodes sont confirmées par une étude comparative des intensités de bandes dans les spectres enregistrés avec les radiations Hel et Hell