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It has often been suggested that reversible hydrogen storage is hampered by the formation of higher boranes MpBnHm (M = alkaline or earth alkaline metal) acting as a boron sink in the solid state during decomposition processes of the borohydride. The thermodynamic stability of these materials poses high barriers for rehydrogenation. Though assumed to be key component in storage system optimization the ultimate proof being the crystal structure has never been determined on a higher borane in situ during thermal decomposition. We herein present a case study on a double perovskite significantly substantiating such assumptions and providing thorough experimental evidence based on synchrotron X ray diffraction. It is shown on the basis of thermogravimetric measurements (TGA) in situ mass spectroscopy and ex situ nuclear magnetic resonance (NMR) spectroscopy that the double perovskites Cs 3 Y ( \{BH\} 4 ) 6 and Cs 2 LiY ( \{BH\} 4 ) 6 release 2.9 mol \{H2\} during polymerization to the inverse perovskite Cs 3 \{BH\} 4 B 12 H 12 containing both closo and borohydride anions. Novel phases are solved ab initio and in situ Raman and mass spectroscopy are employed to follow the polymerization reaction.

Hybrid perovskites form an emerging family of exceptional light harvesting compounds. However, the mechanism underpinning their photovoltaic effect is still far from understood, which is impeded by a lack of clarity on their structures. Here we show that iodide ions in the methylammonium lead iodide migrate via interstitial sites at temperatures above 280 K. This coincides with temperature dependent static distortions resulting in pseudocubic local symmetry. Based on bond distance analysis, the migrating and distorted iodines are at lengths consistent with the formation of I2 molecules, suggesting a 2I−→I2+2e− redox couple. The actual formula of this compound is thus (CH3NH3)PbI3−2x(I2)x where x∼0.007 at room temperature. A crucial feature of the tetragonal structure is that the methylammonium ions do not sit centrally in the A-site cavity, but disordered around two off-centre orientations that facilitate the interstitial ion migration via a gate opening mechanism., The mechanism underpinning the photovoltaic effect in hybrid perovskite solar cells has remained unclear. Here, Green and co-workers suggest that iodide ions in methylammonium lead iodide perovskite migrate via interstitial sites and undergo a redox reaction to form molecular iodine and free electrons.

It has often been suggested that reversible hydrogen storage is hampered by the formation of higher boranes MpBnHm (M = alkaline or earth alkaline metal), acting as a boron sink in the solid state during decomposition processes of the borohydride. The thermodynamic stability of these materials poses high barriers for rehydrogenation. Though assumed to be key component in storage system optimization, the ultimate proof, being the crystal structure, has never been determined on a higher borane in-situ during thermal decomposition. We herein present a case study on a double-perovskite, significantly substantiating such assumptions and providing thorough experimental evidence based on synchrotron X-ray diffraction. It is shown on the basis of thermogravimetric measurements (TGA), in-situ mass spectroscopy and ex-situ nuclear magnetic resonance (NMR) spectroscopy that the double perovskites Cs3Y(BH4)6Cs3Y(BH4)6 and Cs2LiY(BH4)6Cs2LiY(BH4)6 release 2.9 mol H2 during polymerization to the inverse perovskite, Cs3BH4B12H12Cs3BH4B12H12, containing both closo and borohydride anions. Novel phases are solved ab-initio and in-situ Raman and mass spectroscopy are employed to follow the polymerization reaction.

International audience

promotiedatum: 21-6-2013 � prom-id: 10642

The stability and reactivity of epsilon, chi, and theta iron carbide phases in Fischer-Tropsch synthesis (FTS) catalysts as a function of relevant reaction conditions was investigated by a synergistic combination of experimental and theoretical methods. Combined in situ X-ray Absorption Fine Structure Spectroscopy/X-ray Diffraction/Raman Spectroscopy was applied to study Fe-based catalysts during pretreatment and, for the first time, at relevant high pressure Fischer-Tropsch synthesis conditions, while Density Functional Theory calculations formed a fundamental basis for understanding the influence of pretreatment and FTS conditions on the formation of bulk iron carbide phases. By combining theory and experiment, it was found that the formation of theta-Fe3C, chi-Fe5C2, and e-carbides can be explained by their relative thermodynamic stability as imposed by gas phase composition and temperature. Furthermore, it was shown that a significant part of the Fe phases was present as amorphous carbide phases during high pressure FTS, sometimes in an equivalent amount to the crystalline iron carbide fraction. A catalyst containing mainly crystalline chi-Fe5C2 was highly susceptible to oxidation during FTS conditions, while a catalyst containing theta-Fe3C and amorphous carbide phases showed a lower activity and selectivity, mainly due to the buildup of carbonaceous deposits on the catalyst surface, suggesting that amorphous phases and the resulting textural properties play an important role in determining final catalyst performance. The findings further uncovered the thermodynamic and kinetic factors inducing the epsilon-chi-theta carbide transformation as a function of the carbon chemical potential mu(c).