No abstract

No abstract

Perovskite oxides have recently been proposed as promising redox intermediates for solar thermochemical splitting of H2O and CO2, offering the benefit of significantly reduced operating temperatures. We present a systematic experimental screening of doped lanthanum manganites within the composition space La1−x(Ca,Sr)xMn1−yAlyO3 and identify several promising redox materials. In particular, La0.6Sr0.4Mn0.6Al0.4O3 and La0.6Ca0.4Mn0.6Al0.4O3 boast a five‐ to thirteen‐fold improvement in the reduction extent compared to the state‐of‐the‐art material CeO2 in the temperature range 1200–1400 °C. The materials are shown to be capable of splitting CO2 into CO fuel when isothermally cycled between low‐pO2 and high‐pCO2 environments at 1240 °C and to approach full reoxidation in CO2 with temperature swings as low as 200 °C, with mass‐specific fuel yields up to ten times that of CeO2. The underlying material thermodynamics are investigated and used to explain the favorable redox behavior.

Two-step thermochemical cycles for splitting CO 2 with Zn/ZnO and FeO/Fe 3 O 4 redox pairs using concentrated solar energy are considered. Thermogravimetric-based kinetic analyses were performed for the reduction of CO 2 to CO with Zn and FeO. Both reactions are characterized by an initial fast interface-controlled regime followed by a slow diffusion-controlled regime, which are described using a shell-core kinetic model. In the interface-controlled regime, a power rate law is applied with apparent activation energies 113.7 and 73.4 kJ mol -1 , and corresponding reaction orders 0.339 and 0.792, for the Zn/CO 2 and FeO/CO 2 systems, respectively. In the diffusion-controlled regime, limited by the ion mobility through the oxide shells, the apparent activation energies are 162.3 kJ mol -1 for Zn/CO 2 and 106.4 kJ mol -1 for FeO/CO 2 . Additional reaction mechanisms above the Zn melting point for Zn/CO 2 reactions are postulated.

International audienceCarbon is transported from Earth's surface into its interior at subduction zones. Carbonates in sediments overlying hydrothermally altered rocks (including serpentinites) within the subducted slab are the main carriers of this carbon1. Part of the carbon is recycled back to the surface by volcanism, but some is transferred to the deep Earth1, 2. Redox transformations during shallow subduction control the transfer and long-term fate of carbon, but are poorly explored1, 3. Here we use carbon stable isotopes and Raman spectroscopy to analyse the reduction of carbonate in an exhumed serpentinite-sediment contact in Alpine Corsica, France. We find that highly crystalline graphite was formed during subduction metamorphism and was concentrated in the sediment, within a reaction zone in direct contact with the serpentinite. The graphite in this reaction zone has a carbon isotopic signature (δ13C) of up to 0.8±0.1‰, similar to that of the original calcite that composed the sediments, and is texturally associated with the calcium-bearing mineral wollastonite that is also formed in the process. We use mass-balance calculations to show that about 9% of the total carbonaceous matter in the sedimentary unit results from complete calcite reduction in the reaction zone. We conclude that graphite formation, under reducing and low-temperature conditions, provides a mechanism to retain carbon in a subducting slab, aiding transport of carbon into the deeper Earth

Fully polymeric and biobased CO2 sorbents composed of oxidized nanofibrillated cellulose (NFC) and a high molar mass polyethylenimine (PEI) have been prepared via a freeze-drying process. This resulted in NFC/PEI foams displaying a sheet structure with porosity above 97% and specific surface area in the range 2.7-8.3 m(2)·g(-1). Systematic studies on the impact of both PEI content and relative humidity on the CO2 capture capacity of the amine functionalized sorbents have been conducted under atmospheric conditions (moist air with ∼400 ppm of CO2). At 80% RH and an optimum PEI content of 44 wt %, a CO2 capacity of 2.22 mmol·g(-1), a stability over five cycles, and an exceptionally low adsorption half time of 10.6 min were achieved. In the 20-80% RH range studied, the increase in relative humidity increased CO2 capacity of NFC/PEI foams at the expense of a high H2O uptake in the range 3.8-28 mmol·g(-1).

Two-step thermochemical cycles for CO 2 reduction via Zn/ZnO and FeO/Fe 3 O 4 redox reactions are considered. The first, endothermic step is the thermal dissociation of the metal oxide into the metal or a reduced valence metal oxide and O 2 using concentrated solar energy as the source of high-temperature process heat. The second, nonsolar, exothermic step is the reaction of the reduced metal/metal oxide with CO 2 , yielding CO and/or C, together with the initial form of the metal oxide that is recycled to the first step. Chemical equilibrium compositions of the pertinent reactions are computed as a function of temperature and pressure. A second-law thermodynamic analysis for the net reaction of CO 2 ) CO + 0.5O 2 indicates a maximal solar-chemical energy conversion efficiency of 39 and 29% for the Zn/ZnO and FeO/Fe 3 O 4 cycle, respectively. Efficiencies are lower for both cycles yielding C. Major sources of irreversibility are associated with the re-radiation losses of the solar reactor operating at 2000 K and the quenching of its products to avoid recombination.

Yttrium modified Ni-based double-layered hydroxides were tested as novel catalysts in dry methane reforming. The catalysts were characterized by XRF, BET analysis, XRD, TPR-H 2 , TPD-CO 2 , TGA/DSC-MS, H 2 chemisorption and TEM. Co-precipitation with yttrium (III) nitrate hexahydrate resulted in increased specific surface area, smaller Ni crystallite size, enhanced reducibility, and higher distribution of weak and medium basic sites as compared to Y-free material. The DRM catalytic tests, carried out in the temperature range of 850-600 ºC, led to a significant improvement of activity, with the 1.5 wt % Y-promoted catalyst being the most efficient in converting both CH 4 and CO 2. Moreover, the 10 h test at 700 ºC further confirmed enhanced stability for this HTNi-Y1.5 catalyst. Higher CO 2 conversion than CH 4 conversion and less CO compared to H 2 proves that side reactions are occurring simultaneously.