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•Metakaolins reactivity impacts the behavior in the presence of alkaline solution.•Highly reactive metakaolin accelerates the consolidation of the material.•The metakaolin reactivity ...can generate the formation of one or several networks.•Nanostructure variations after consolidation influence the mechanical properties.•A descriptive model of the mechanism of geopolymer formation was proposed.
Geopolymer materials are obtained by the alkaline activation of aluminosilicate sources, the best of which is metakaolin. However, every raw material is different, and very few comparative studies have been done on different metakaolin sources. The aim of this work is to develop methods for the prediction of the working properties of geopolymer materials based on the reactivity of the metakaolin employed. Infrared spectroscopy showed direct relationships between the wettability, the Si/Al ratio and the kinetics of conversion of Si–O–Si bonds to Si–O–Al bonds. Moreover, it was demonstrated that the presence of impurities and the reactivity of the metakaolin can generate the formation of one or several networks. Finally, a descriptive model of the mechanism of geopolymer formation was proposed that takes into account the quality of metakaolin used.
Gradually reduced graphene oxide materials were successfully synthesized by performing successive chemical treatments on graphite oxide synthesized using Hummers' method. These materials were deeply ...characterized using complementary spectroscopic techniques (X-ray induced photoelectron spectroscopy, UV- spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy). It was shown that reduction processes allow removing oxygenated functional groups. Nevertheless, defects in the carbon structure materials formed subsequently to the oxidation step remain after such reducing treatments. CO stripping coupled with in-situ Fourier transform infrared spectroscopy measurements revealed that the reduction extent of the carbon-based materials affects the electron density on Pt thus engendering modifications in the adsorption strength of CO molecule.
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•xMo/Al2O3 catalysts (x=2–12wt%) are tested in Biginelli reaction for DHPM.•XRD and FFT analysis revealed the presence of Al2(MoO4)3 phase on Mo/γ-Al2O3.•Al2(MoO4)3 phase could be responsible to ...generated Brønsted acid sites.•DHPM formation is related to Brønsted acid sites and Al2(MoO4)3 phase.•Mo/Al2O3 catalysts are highly stable and reusable materials for Biginelli reaction.
Alumina supported Mo catalysts with varying metal loading (2–12wt%) were tested in the Biginelli reaction under solvent free conditions for synthesis of 3,4-dihydropyrimidin-2-(1H)-one (DHPM). The DHPM yield increased with Mo loading, going through a maximum for 7wt% Mo. Alumina supported Mo catalysts appeared to be highly stable and reusable materials for this reaction.
A suite of characterization techniques that included X-ray diffraction, Fast Fourier Transform analysis and FTIR of adsorbed pyridine revealed that the DHPM formation is related to the existence of Brønsted acid sites that would be linked to the presence of Al2(MoO4)3 phase.
Precursors are critical parameters in geopolymerization mechanisms because they govern the reaction kinetics as well as the working properties of the final materials. This study focuses on the effect ...of alkaline solutions on geopolymer formation. Toward this end, several geopolymer samples were synthesized from the same metakaolin and various alkaline solutions. First, the solutions were characterized by thermogravimetric analysis as well as DTA–TGA, infrared spectroscopy, and MAS-NMR spectrometry. The structural evolution of the formed geopolymers was investigated using infrared spectroscopy. The measurement of mechanical strength was tested by compression. The results provide evidence of relationships between the chemical composition, the extent of depolymerization of the alkaline solutions, the kinetics of Si–O–Si bond substitution by Si–O–Al and the compressive strength. For a given aluminosilicate source, the nature and the quantity of siliceous species in the activation solution appear to lead to variation in the reactivity and, consequently, to the formation of various networks that control the kinetics of formation of geopolymers and their mechanical properties.
Different unsupported platinum nanoparticles are synthesized via water-in-oil microemulsion and colloidal methods (PAA). TEM, adatom adsorption and cyclic voltammetry measurements give very coherent ...results concerning the surface structure of the crystallites. The combination of these results with CO stripping experiments leads us to assign the oxidation peak multiplicity to surface structure rather than to pure size effect.
During the synthesis of geopolymer materials, all of the phenomena and reactions that occur should be included in the analysis of the synthesis. This work aims to study the role that siliceous ...species play in an activation solution in the presence of pure metakaolin. The formation of a gel phase during the synthesis of a K-geopolymer was shown. Different mixtures were analyzed by infrared spectroscopy. During material consolidation, there is always competition between the geopolymer network and the gel that is governed by a change in the siliceous species. The heat treatment of various gels and solids provided evidence of various networks in geopolymer materials. Finally, the change in the crystallinity of the silica in the activation solution led to a change in the kinetics of the polycondensation reactions, in agreement with previous work.
This study proposes a thermodynamic model that is able to predict the pore water chemical composition of clayey rocks at temperatures up to 80°C. The model is based on equilibrium with a ...quartz/kaolinite/calcite/dolomite/Mg-chlorite mineralogical buffer and accounts for the sorption properties of both the montmorillonite and illite phases.
Hydrothermal alteration experiments were also performed at 80°C on centimeter-sized samples from the Callovo-Oxfordian clay-rich formation (Paris Basin, France) to assess the validity of the proposed model at this temperature. The experiments were performed in both open and closed systems, and the initial CO2(g) partial pressure (pCO2(g)) used in each system was fixed to log pCO2(g)=−0.4. After 15months of alteration, the aqueous and gas phases were extracted at 80°C and analyzed. The evolution of the clay mineralogy was also characterized using X-ray diffraction profile modeling of experimental patterns and transmission electron microscopy (TEM).
The composition of the Callovo-Oxfordian pore water predicted by our model at 25°C is in good agreement with that reported in the literature and measured in the geological formation. At 80°C, experiments in open systems were used to assess the kinetics of alteration process and the time needed to reach equilibrium for several dissolution–precipitation mineral reactions. For the closed system, it is shown that the proposed model well predicts the chemical composition of the fluid extracted at 80°C, considering the existing variability in the solubility constants for both Mg-chlorite and kaolinite minerals. Despite the need for additional alteration experiments to better constrain the temperature-dependence of the predicted pCO2(g) values, the proposed model provides reliable predictions of pore fluid composition at 80°C, a temperature expected in the framework of radioactive waste storage.
► We propose a pore water model at 25 and 80°C for sedimentary basins. ► Model is based on a mineralogical buffer and ion-exchange reactions. ► Model at 25°C is validated by published experimental values measured previously in situ. ► Hydrothermal alteration experiments performed with argillite from the COx validate the model at 80°C.
► Carbon-supported Pt-based electrocatalysts synthesized by Pechini method. ► Electrocatalytic ethanol oxidation reaction (EOR) at 90
°C. ► A single 5
cm
2 DEFC delivers a power density of 37
mW
cm
...−2. ► Acetaldehyde is the main oxidation product.
Carbon-supported Pt-based electrocatalysts were synthesized by Pechini method for the ethanol oxidation (EOR). Physicochemical characterizations were helpful to estimate the diameters of the obtained materials ranging from 2
nm to 5
nm. Main electrochemical experiments were carried out at 90
°C
i.e. under the working conditions of performing the single 5
cm
2 direct ethanol fuel cell (DEFC). Pt
80Sn
20/C was the anode catalyst which has given the highest power density of 37
mW
cm
−2. Importantly, the IR spectroscopy measurements associated with the qualitative analysis done at the output of the anodic compartment of the fuel cell have shown that ethanol oxidation on Pt
80Sn
20/C was mainly a two-electron sustainable process.
The electrodeposition of platinum and ruthenium was carried out on carbon electrodes to prepare methanol anodes with different Pt/Ru atomic ratios using a galvanostatic pulse technique. ...Characterizations by XRD, TEM, EDX and atomic absorption spectroscopy indicated that most of the electrocatalytic anodes consisted of 2 mg cm-2 of Pt-Ru alloy particles with the desired composition and with particle sizes ranging from 5 to 8 nm. Electrochemical tests in a single DMFC show that these electrodes are very active for methanol oxidation and that the best Pt/Ru atomic ratio in the temperature range used (50-110 deg C) is 80:20. The influence of the relaxation time tuff was also studied and it appeared that a low toff led to smaller particle sizes and higher performances in terms of current density and power density.
Syntheses of supported Pt/C nanoparticles via a water-in-oil microemulsion method are described, including some variations in the surfactant removal step (chemical reagents, thermal treatment, ...washing procedure, surfactant batch, etc.). Their influence on the microstructure and on the electrochemical activity is evaluated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) allow domain size distributions and corresponding domain sizes (
L
v and
D
m) to be evaluated. Thermal treatment under reductive atmosphere at only 200
°C leads to the sintering of the platinum particles. Under H
2 3% in He atmosphere conditions (close to fuel cell working conditions) it is shown that
L
v increased with temperature following two kinetics according to the temperature range (25–200
°C and 200–500
°C). Change of the surfactant causes large differences in
L
v. For catalysts prepared with the same precursor batch, very close
L
v are obtained, whereas electrochemical measurements (hydrogen adsorption–desorption and CO stripping) display marked differences.
