Direct Experimental Evidence of the Effects of Clay Particles’ Basal-to-Lateral Surface Ratio on Methane and Carbon Dioxide Adsorption

International audienceThe amount and size of the charge-balancing cations on the exposed faces of clay particles are supposed to be one of the key factors affecting the specificity of adsorption of gases at clay surfaces. However, the trends characterizing the thermodynamics of gas adsorption, reported for different members of 2:1 phyllosilicate phases, do not systematically follow neither the difference in their layer charge, determining the number of charge-balancing cations, nor the nature of the latter. To better understand the specificity of CH4 and CO2 molecular interactions with different clay phases, the adsorption isotherms were measured for isoionic pure-phase illite and montmorillonite up to pressures of 9 MPa at ambient temperature. For both gases, higher adsorption capacities per unit of specific surface area for montmorillonites in comparison to illites could not be explained by the existing theory relying on the properties of exposed charge-balancing cations on the surface. Instead, the slopes of the isotherms perfectly correlate with the shape of clay particles, characterized by their basal-to-lateral faces’ aspect ratio, which is assessed by derivative isotherm summation (DIS) using argon adsorption at 77 K. Montmorillonite clays, characterized by higher fractions of high-energy hydroxylated particles’ edges (∼45%) in the total specific surface area, featured stronger adsorption interactions with CO2 and CH4 in comparison to illites, for which the contribution of edges to the surface area is only 20%. This introduces a new important factor controlling the mechanism of CH4 and CO2 adsorption by clay minerals

Thermodynamic data of adsorption reveal the entry of CH4_4 and CO2_2 in a smectite clay interlayer

International audienceThe ability of smectite clays to incorporate gases in their interlayers is shown to be conditioned by interlayer spacing, depending, in turn, on phyllosilicate layer composition and effective size of the charge-balancing cations. As illustrated by earlier in situ X-ray diffraction and spectroscopic characterization of the gas/clay interface, most smectites with small-size charge-balancing cations, such as Na+ or Ca2+, accommodate CO2 and CH4 in their interlayers only in a partially hydrated state resulting in the opening of the basal spacing, above a certain critical value. In the present study CH4 and CO2 adsorption isotherms were measured for Naand Mg-exchanged montmorillonite up to 9 MPa using a manometric technique. The process of dehydration of these clays was thoroughly characterized by thermogravimetric analysis and powder X-ray diffraction. A dramatic decrease in specific surface area and methane and carbon dioxide adsorption capacities for fully dehydrated samples in comparison to partially dehydrated ones is assigned to the shrinkage of interlayer spacing resulting in its inaccessibility for the entry of CH4 and CO2 molecules. This observation is direct evidence of CH4 and CO2 adsorption capacity variation depending on the opening of smectite clay interlayer spacing.</p

Microporosity and nanostructure of activated carbons: characterization by X-ray diffraction and scattering, Raman spectroscopy and transmission electron microscopy

International audienceMicroporosity and structure of a set of activated carbons was studied by combination of N 2 and CO 2 adsorption, Transmission Electron Microscopy (TEM), X-ray diffraction and scattering and multiwavelength Raman spectroscopy. It is shown that correlations between measured parameteres may be established for a given set of activated carbons, most often obtained from a same precursor. Comparison of results of TEM images processing and of Small-angle scattering with adsorption data suggests that super-micropores (0.7-2 nm) are highly variable in shape and strongly deviate from the ideal slit pore model. These pores are likely located in between disordered continuous graphene stacks. It is shown that Small-angle scattering is mostly caused by supermicropores; contribution of other types of porosity is of secondary importance. For a set of carbons with similar structure, a reasonable correlation between Guinier radii and pore width obtained from N 2 adsorption can be found; however, the reason for the observed offset between the data sets remain uncertain. Sensitivity of the Raman scattering to atomic scale processes leads to poor or unclear correlations between the spectroscopic and structural data, although some notable exceptions are noted

Nanostructure characterization of carbide-derived carbons by morphological analysis of transmission electron microscopy images combined with physisorption and Raman spectroscopy

A novel image processing method based on mathematical morphology is applied in order to characterize the nanostructure of carbide-derived carbons (CDCs) observed using high resolution transmission electron microscopy (HRTEM). The analysis provides information about the shape and disordered arrangement of the defective polyaromatic units forming the CDC nanostructure. Individual fringes, basic structural units, and continuous domains are analysed. Hierarchical polycarbosilane-based CDCs obtained at different pyrolysis/chlorination temperatures are investigated. The information collected is interpreted with respect to the different synthesis conditions. This analysis is supported by Raman spectroscopy measurements and porosity evaluation with nitrogen (−196 °C) and carbon dioxide (0 °C) physisorption. The CDCs show only minor differences in the carbon nanostructures. The HRTEM image analysis is sensitive enough to illuminate the slight variations. An increase in carbon ordering at higher synthesis temperature and a rather folded structure with higher tortuosity of the fringes with increasing micropore volume is observed. These findings are complementary to the data obtained from nitrogen and carbon dioxide physisorption experiments as well as from Raman spectroscopy, showing a less defective microstructure of the CDCs prepared at higher pyrolysis/chlorination temperatures. The results also prove the precise control over the nanostructure of CDCs provided by the synthesis temperature