The effect of ball milling and the addition of carbon on the physico-chemical properties as well as the OER performance of the Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) perovskite is investigated. We show that ...the ball milling process can significantly enhance the BSCF OER activity. This higher activity is associated with the formation of an amorphous surface layer produced during the ball milling process, as identified by XRD and TEM. The addition of carbon followed by ultrasonication confirms that carbon acts as a reducing agent to functionalize BSCF but is not necessarily needed during the OER, as identified by X-ray absorption spectroscopy. Our work thus demonstrates that there are at least two different physicochemical methods to enhance the OER activity of BSCF: (i) generating a surface layer with a short range order and/or (ii) modifying the BSCF electronic structure. Further study shows that these two methods can be superimposed to furthermore improve the OER activity of BSCF. The processed BSCF delivers higher mass activity than a benchmark commercial IrTiO2 catalyst, demonstrating the importance of catalyst processing parameters to functionalize highly active OER catalysts.
Display omitted
•Ru/SiO2 produces more CH4 than Ru/Al2O3 after several S poisoning–regeneration cycles.•SiO2 has a smaller S storage capacity as S+6 than Al2O3 when oxidatively regenerated.•Sads on ...Ru nanoparticles appears to stabilize them against sintering during oxidation.
The production of synthetic natural gas—SNG—from dry biomass currently involves a costly and energy-inefficient low-temperature gas cleaning step, needed to remove various sulfur-containing poisons from syngas generated from wood gasification before the sulfur-sensitive methanation step. Here we explore the use of Ru-based methanation catalysts in an alternative process, where the low-temperature gas cleaning step is omitted and the syngas from the gasifier is used directly for methanation. In this process, methanation is carried out in the presence of organic and inorganic sulfur species and is followed by periodic oxidative regeneration of the poisoned catalyst. In situ diffuse reflectance infrared Fourier transform spectroscopy and Ru K-edge X-ray absorption spectroscopy were employed to understand the mechanism of deactivation of Ru nanoparticles on two different supports: Al2O3 and SiO2. The efficiency of the regeneration is higher when Ru nanoparticles are supported on SiO2, on which only a small amount of unstable sulfate species is formed and Ru nanoparticles are more stable against sintering during oxidative regeneration due to a “passivation” effect of the adsorbed sulfur species.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
X-ray emission spectra recorded in the off-resonant regime carry information on the density of unoccupied states. It is known that by employing the Kramers-Heisenberg formalism, the high energy ...resolution off-resonant spectroscopy (HEROS) is equivalent to the x-ray absorption spectroscopy (XAS) technique and provides the same electronic state information. Moreover, in the present Letter we demonstrate that the shape of HEROS spectra is not modified by self-absorption effects. Therefore, in contrast to the fluorescence-based XAS techniques, the recorded shape of the spectra is independent of the sample concentration or thickness. The HEROS may thus be used as an experimental technique when precise information about specific absorption features and their strengths is crucial for chemical speciation or theoretical evaluation.
Full text
Available for:
CMK, CTK, FMFMET, IJS, NUK, PNG, UM
In situ X-ray absorption spectroscopy identified the role of oxidized platinum species in generating a high-activity state in the oxidation of carbon monoxide over Pt/Al
2O
3, Pt/TiO
2 and Pt/SiO
2. ...Two activity regimes were identified: low-activity at high carbon monoxide concentration and low temperature, and high activity at low carbon monoxide concentration and high temperature. The change between the two regimes occurred suddenly; there was ignition when going from low to high activity, and extinction from high to low activity. Ignition only occurred when the concentration of oxygen was higher than that of carbon monoxide. The catalyst was metallic and covered with carbon monoxide at low activity and partially oxidic at high activity. Carbon monoxide poisoned the catalyst at low activity. The partially oxidized platinum catalysts showed high activity. The partially oxidized catalyst had a lower oxygen coordination number and shorter Pt–O bond length as expected for bulk PtO
2, suggesting that a strongly defected platinum oxide was formed with a possibly square planar coordination. The catalyst structure showed a dynamic behavior and the amount of oxide depended on the concentration of reactants in the gas phase. Smaller particles of Pt/Al
2O
3 reached high activity at lower temperature than the larger particles. Pt/TiO
2 reached high activity at the lowest temperature while Pt/SiO
2 required the highest temperature. Parallel with the temperature of ignition, Pt/SiO
2 was the most oxidized while Pt/TiO
2 was the least oxidized.
In situ XAS identified surface-oxidized platinum as the active species at high activity during carbon monoxide oxidation over supported platinum catalysts. At low activity, the platinum catalysts were metallic and poisoned by carbon monoxide.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Methane steam reforming on Rh/Al2O3 catalysts promoted by Sm2O3 and/or CeO2 was studied to clarify the role of the promoters on catalytic and structural properties. Promoted catalysts showed higher ...reaction rates and stability than Rh/Al2O3 due to the stronger interactions of Rh and the Sm2O3–CeO2 containing support, avoiding sintering of active phase. Display omitted
► Promoters led to higher reaction rates of Rh/Al2O3 catalysts in methane steam reforming. ► EXAFS showed instability of the Rh particles when supported on unpromoted Al2O3. ► Sm2O3 and CeO2 containing supports have stronger interaction with the active phase. ► The presence of promoters avoided sintering and oxidation of the metal particles.
The role of Sm2O3 and CeO2 promoters on the structural properties and catalytic behavior of Rh/xSm2O3–yCeO2–Al2O3 catalysts during methane steam reforming (MSR) was investigated. Promoted catalysts showed higher reaction rates per surface Rh atom and improved stability compared to Rh/Al2O3. In situ X-ray absorption revealed that the structure of Rh particles in Rh/Al2O3 changes drastically during MSR, while it was stable in the presence of Sm2O3–CeO2. Sintering of the active metal phase was the main cause of deactivation. STEM images showed stronger Rh agglomeration of the unpromoted catalyst with time on stream.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We report results from an extensive study on the speciation of zinc (Zn) and its relation to the mobility and bioavailablity of this element in a smelter contaminated soil and an
in situ remediated ...area of this soil 12 yr after the application of cyclonic ash and compost. Emphasis was placed on the role of neoformed precipitates in controlling Zn speciation, mobility and bioavailability under different environmental conditions. Twelve years after remediation, the pH of the treated and non-treated soil differed by only 0.5 pH unit. Using state-of-the-art electron and X-ray microscopies in combination with micro-focused extended X-ray absorption fine structure (μ-EXAFS) spectroscopy, no major differences in Zn speciation were found between samples of the treated and non-treated soil. In both soils, 30% to 50% of Zn was present in smelter related minerals (willemite, hemimorphite or gahnite), while 50% to 70% of Zn was incorporated into newly formed Zn precipitates. Contrary to the non-treated soil, the treated soil did not contain gahnite or sphalerite; it is possible that these minerals were dissolved under the higher pH conditions at the time of treatment. Desorption experiments, using a stirred flow technique with a 0.1 mol/L CaCl
2 (pH 6.5) and a HNO
3 (pH 4.0) solution were employed to determine the exchangeable Zn fraction and the Zn fraction which will be mobilized under more extreme weathering conditions, respectively. No significant differences were found in desorption behavior between the treated vs. non-treated soil. Bioavailability tests, using the
R. metallidurans AE1433 biosensor showed that ∼8% of total Zn was bioavailable in both the treated and non-treated soils. It was concluded that the incorporation of Zn into newly formed precipitates in both the treated and non treated soils leads to a significant natural attenuation of the exchangeable/bioavailable Zn fraction at near neutral pH conditions. At lower pHs, conditions not favorable to the formation of Zn precipitates, the pool of Zn associated with the secondary Zn precipitates is potentially more bioavailable.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Organisms have developed metal regulatory mechanisms in response to changes in the bioavailability of trace metals. Just as metal bioavailability dictates cellular uptake, intracellular metal ...speciation determines the availability of metals to exert biological effects. However, the missing link in understanding the relationship between metal uptake and biological responses is the ability to accurately measure intracellular metal speciation. We conducted Pb exposure studies on the well-characterized model green alga Chlamydomonas reinhardtii and identified temporal changes in intracellular Pb speciation under conditions relevant for fresh water ecosystems using resonant X-ray emission spectroscopy (RXES), which possesses enhanced sensitivity to functional group chemistry relative to X-ray absorption spectroscopy (XAS). Analysis of RXES maps show that only a small fraction of total intracellular Pb was complexed by thiol groups. Initial sequestration of Pb in oxides and inorganic phosphate was followed by binding of Pb to organic phosphate, suggesting potential interference in vital cellular functions. These results contrast proposed detoxification responses involving complexation by thiol groups from peptides.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
CuX@SWCNT (X=Cl, Br, I) nanostructures were prepared by capillary filling of 1.4–1.6nm single-walled carbon nanotubes (SWCNT) with copper halides. The structure of CuX@SWCNT (X=Cl, Br, I) represents ...a distorted two-layer hcp of halogen atoms arranged along the SWCNT. The EXAFS and the high angle angular dark field (HAADF) HRTEM data indicate that Cu is partially coordinated by C. According to the optical absorption, valence band photoemission spectroscopy and work function measurements, a Fermi level (FL) downshift as compared with the initial value for the nanotubes and a corresponding charge transfer from the nanotubes to the 1D crystals is observed for CuX@SWCNT nanostructures. The FL shift increases in the sequence CuI<CuBr<CuCl due to an increase of the electron affinity for the halogen atoms. The XPS data confirm the acceptor effect of copper halides and indicate that metallic and semiconducting nanotubes behave differently. Raman spectroscopy performed under electrochemical charging allowed estimation of the value of charge transfer between the nanotube walls and the intercalated 1D crystal. The X-ray absorption and emission spectra for carbon and copper thresholds revealed a new energy level composed of the carbon 2рz and copper 3d-orbitals. This indicates the Cu–C bonding, which in line with the structural HAADF HRTEM and EXAFS data.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Multi-metallic aerogels have recently emerged as a novel and promising class of unsupported electrocatalyst materials due to their high catalytic activity and improved durability for various ...electrochemical reactions. Aerogels can be prepared by a spontaneous one-step gelation process, where the chemical co-reduction of metal precursors and the prompt formation of nanochain-containing hydrogels, as a preliminary stage for the preparation of aerogels, take place. However, detailed knowledge about the homogeneity and chemical distribution of these three-dimensional Pd-Pt aerogels at the nano-scale as well as at the macro-scale is still unclear. Therefore, we used a combination of spectroscopic and microscopic techniques to obtain a better insight into the structure and elemental distribution of the various Pd-rich Pd-Pt aerogels prepared by the spontaneous one-step gelation process. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDX) were employed in this work to uncover the structural architecture and chemical composition of the various Pd-rich Pd-Pt aerogels over a broad length range. The Pd
80
Pt
20
, Pd
60
Pt
40
and Pd
50
Pt
50
aerogels showed heterogeneity in the chemical distribution of the Pt and Pd atoms inside the macroscopic nanochain-network. The features of mono-metallic clusters were not detected by EXAFS or STEM-EDX, indicating alloyed nanoparticles. However, the local chemical composition of the Pd-Pt alloys strongly varied along the nanochains and thus within a single aerogel. To determine the electrochemically active surface area (ECSA) of the Pd-Pt aerogels for application in electrocatalysis, we used the electrochemical CO stripping method. Due to their high porosity and extended network structure, the resulting values of the ECSA for the Pd-Pt aerogels were higher than that for a commercially available unsupported Pt black catalyst. We show that the Pd-Pt aerogels possess a high utilization of catalytically active centers for electrocatalytic applications based on the nanostructured bimetallic framework. Knowledge about the homogeneity and chemical distribution of the bimetallic aerogels can help to further optimize their preparation by the spontaneous one-step gelation process and to tune their electrocatalytic reactivity.
Pd-Pt aerogels prepared by a one-step gelation process exhibit alloy features with different elemental distributions over a broad length scale.
PDF
