Silver molybdate (Ag2MoO4) and silver tungstate (Ag2WO4) nanomaterials were prepared using two complementary methods, microwave assisted hydrothermal synthesis (MAH) (pH 7, 140 °C) and co-precipitation (pH 4, 70 °C), and were then used to prepare two core/shell composites, namely α-Ag2WO4/ β-Ag2MoO4 (MAH, pH 4, 140 °C) and β-Ag2MoO4/ β-Ag2WO4 (co-precipitation, pH 4, 70 °C). The shape and size of the microcrystals were observed by field emission scanning electron microscopy (FE-SEM), different morphologies such as balls and nanorods. These powders were characterized by X-ray powder diffraction and UV-vis (diffuse reflectance and photoluminescence). X-ray diffraction patterns showed that the Ag2MoO4 samples obtained by the two methods were single-phased and belonged to the β-Ag2MoO4 structure (spinel type). In contrast, the Ag2WO4 obtained in the two syntheses were structurally different: MAH exhibited the well-known tetrameric stable structure β-Ag2WO4, while co-precipitation afforded the metastable β-Ag2WO4 allotrope, coexisting with a weak amount of the α;-phase. The optical gap of β-Ag2WO4 (3.3 eV) was evaluated for the first time. In contrast to β-Ag2MoO4/ β-Ag2WO4, the α-Ag2WO4/ β-Ag2MoO4 exhibited strongly-enhanced photoluminescence in the low-energy band (650 nm), tentatively explained by the creation of a large density of local defects (distortions) at the core-shell interface, due to the presence of two different types of MOx polyhedra in the two structures.

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Methane storage capacities on dry and water-wetted active carbon powders are compared. Sorption isotherms of methane at temperatures of 2°C and pressures up to 8 MPa are constructed for four carbonaceous materials. Three of these materials originate from the same precursor (coconut shell), are physically activated at various burnoffs, and are mainly microporous; the fourth material is a highly mesoporous, chemically activated pinewood carbon. In the dry state, these adsorbents exhibit classical Langmuirian behavior. Wetting the materials with a constant water/carbon weight ratio of ∼1 leads to isotherms that are now characterized by a marked step. The latter occurs near the expected formation pressure of methane hydrates, thus supporting their occurrence within the porous materials. The amount of gas stored at the highest pressures investigated then ranges from 180 to 230 volumes at standard temperature and pressure (STP) per unit volume of storage vessel (V/V), depending on the material, whereas only 110-160 V/V are obtained with dry carbons (at 2°C, 8 MPa). Hence, wetting the carbonaceous adsorbents improves the methane storage capacities, thus confirming recent works. The results are discussed on the basis of the known pore texture of each adsorbent, and stoichiometries of the formed hydrates are calculated. Considerations about adsorption and desorption kinetics, and pore size distributions, are also developed.