Hydrogenations of CO or CO2 are important catalytic reactions as they are interesting alternatives to produce fine chemical feedstock hence avoiding the use of fossil sources. Using monodisperse nanoparticle (NP) catalysts, we have studied the CO/H2 (i.e., Fischer-Tropsch synthesis) and CO2/H2 reactions. Exploiting synchrotron based in situ characterization techniques such as XANES and XPS, we were able to demonstrate that 10 nm Co NPs cannot be reduced at 250 °C while supported on TiO2 or SiO2 and that the complete reduction of cobalt can only be achieved at 450 °C. Interestingly, cobalt oxide performs better than fully reduced cobalt when supported on TiO2. In fact, the catalytic results indicate an enhancement of 10-fold for the CO2/H2 reaction rate and 2-fold for the CO/H2 reaction rate for the Co/TiO2 treated at 250 °C in H2 versus Co/TiO2 treated at 450 °C. Inversely, the activity of cobalt supported on SiO2 has a higher turnover frequency when cobalt is metallic. The product distributions could be tuned depending on the support and the oxidation state of cobalt. For oxidized cobalt on TiO2, we observed an increase of methane production for the CO2/H2 reaction whereas it is more selective to unsaturated products for the CO/H2 reaction. In situ investigation of the catalysts indicated wetting of the TiO2 support by CoO(x) and partial encapsulation of metallic Co by TiO(2-x).

Amorphous calcium carbonate (ACC) is a precursor to the crystalline phases of CaCO 3 , commonly found in the earliest stages of biomineral development and as one of the metastable states formed during the inorganic precipitation of calcium carbonate crystalline polymorphs. [1] Its isotropic and hydrous moldable character allows many organisms to form very complex conformations of their shells or skeletons by taking advantage of these unique properties. [2] At least two different phases of biogenic ACC have been described to date: a highly hydrated phase with one water molecule per CaCO 3 unit, and an anhydrous phase that forms as a transient phase prior to crystallization to vaterite or calcite. [3,4] Recently, the existence of polyamorphism (the existence of a substance in different amorphous modifications, akin to polymorphism in crystalline materials) in synthetic hydrated ACC has been suggested based mainly on X-ray absorption spectroscopy (XAS) and nuclear magnetic resonance (NMR) data that show different local structures of ACC precipitated from solutions at different pH values: calcite-like ACC is obtained at pH % 8.75 and vaterite-like ACC precipitates from solutions of pH % 9.8 and higher. [5,6] In addition to these two amorphous polymorphs, other studies have shown hints of aragonite local order in ACC from shells of freshwater snails, based on XAS data that show Ca-O coordination numbers of approximately 9, the theoretical value of aragonite. [7,8] These results were reproduced in synthetic samples of ACC doped with Mg 2+ , suggesting a role of this cation in the selection of the ACC amorphous polymorph. [9] Herein we show the existence of pressure-induced polyamorphism in hydrated ACC, and the formation of "aragonitic" ACC upon a decrease of the molar volume. This result suggests a possible mechanism by which Mg 2+ -a cation with smaller ionic radius than Ca 2+ -modifies the local order of ACC to an aragonite-like order by contributing to decrease the molar volume of the amorphous phase. In addition, we report the first values of the bulk modulus and the density of ACC.Experimental structure factors obtained from high-pressure X-ray diffraction experiments and fits to the experimental data using Reverse-Monte Carlo (RMC) modeling are shown in Figure 1 a. The structure factors show the typical broad oscillations of an amorphous solid, with no apparent sign of crystallization within the range of pressures studied. Pressure-induced structural changes can be identified in the diffraction data by looking at the salient feature of the S(Q) function at 11.9 GPa (see small arrow in Figure 1 a), and by plotting the position of the main diffraction peak at Q % 3.3 À1 as a function of pressure (Figure 1 b). A step-like transition is observed that can be fitted with a sigmoidal function centered at P c = 9.8 AE 0.8 GPa. Similar behavior is observed in high-pressure Raman data ( Figure 2; see raw data in Figure S7 and S8). At ambient pressure four peaks are observed: a single peak at 1081 cm À1 corresponding to th...

Chalcogenide glasses with tetrahedral networks can undergo significant densification under pressure owing to their open structures. The structural mechanisms of pressureinduced densification and the corresponding evolution of physical properties of glassy GeSe 4 alloy are studied over pressures ranging between ambient and 32.5 GPa, using X-ray scattering supplemented with 3D Monte Carlo structural modeling, Raman spectroscopy, electrical conductivity, and P− V equation of state measurements. The results demonstrate a pressure-induced, hysteretically reversible transition between low-density semiconducting and high-density metallic amorphous phases of GeSe 4 near ∼10−15 GPa. These two phases are characterized by their distinct P−V equations of state and structural mechanisms of densification. Densification in the low-density phase is dominated by large inward shifting of the second neighbors with a small amount of conversion from edge-sharing to corner-sharing GeSe 4 tetrahedra. On the other hand, densification in the high-density phase involves a gradual increase in the nearest-neighbor coordination numbers of Ge and Se atoms and the formation of Ge−Ge bonds between adjacent polyhedral units. These structural transformations are accompanied by a pressure-induced metallization that is reversible.

Recent theoretical studies of 2,6-diamino-3,5-dinitropyrazine-1-oxide (C4H4N6O5 Lawrence Livermore Molecule No. 105, LLM-105) report unreacted high pressure equations of state that include several structural phase transitions, between 8 and 50 GPa, while one published experimental study reports equation of state (EOS) data up to a pressure of 6 GPa with no observed transition. Here we report the results of a synchrotron-based X-ray diffraction study and also ambient temperature isobaric-isothermal atomistic molecular dynamics simulations of LLM-105 up to 20 GPa. We find that the ambient pressure phase remains stable up to 20 GPa; there is no indication of a pressure induced phase transition. We do find a prominent decrease in b-axis compressibility starting at approximately 13 GPa and attribute the stiffening to a critical length where inter-sheet distance becomes similar to the intermolecular distance within individual sheets. The ambient temperature isothermal equation of state was determined through refinements of measured X-ray diffraction patterns. The pressure-volume data were fit using various EOS models to yield bulk moduli with corresponding pressure derivatives. We find very good agreement between the experimental and theoretically derived EOS.