Quantitative Measurements of Composition, Pressure, and Density of Microvolumes of CO2–N2 Gas Mixtures by Raman Spectroscopy

Tracking carbon dioxide adsorbate intramolecular dynamics in pure silica zeolite Silicalite-1 by in situ Raman scattering

We report a series of high-quality Raman spectra of carbon dioxide (CO2) adsorbed at room temperature and at various equilibrium pressures, sampling the corresponding adsorption isotherm up to 12 bar. The observed splitting in Fermi diad resonance lines, which were additionally split into two well-resolved components, arising from at least two different CO2 species, were compared to the same quantity in high-pressure gas/solid/liquid CO2 phases. Our studies provide material specific spectral data that could be useful in the detection, identification, and dynamical characterization of CO2 deposits, inclusions, or other forms in remote locations and of various origins, e.g. geological, planetary, stellar, and deap-sea sediments.

Pressure broadening in Raman spectra of CH4-N2, CH4-CO2, and CH4-C2H6 gas mixtures

Different molecular environments change the spectrum of a given gas sample involved in a mixture compared to the spectrum of a pure gas. It is necessary to account for this effect to improve the accuracy of the analysis of the natural gas composition by Raman spectroscopy. First, the change in the main components of natural gas (methane, nitrogen, carbon dioxide, and ethane) must be considered. This work is devoted to the mutual influence of CH₄-N₂, CH₄-CO₂, and CH₄-C₂H₆ on their characteristic Raman bands in the range of 300-2500 cm^-1. The half-width and asymmetry of the Q branches of N₂, CO₂, and C₂H₆ as a function of methane concentration were obtained in the range of 1-50 bar. The averaged broadening coefficients of the rotational-vibrational lines of the ν₂ band of CH₄ perturbed by N₂, CO₂, and C₂H₆ are measured. A high-sensitivity spectrometer with a resolution of 0.5 cm^-1 based on spontaneous Raman scattering was used to obtain reliable results. The algorithm and all the necessary parameters for simulating the effect of various molecular environments on the Raman bands of the main components of natural gas are presented.

Interpretation of the pressure-induced Raman frequency shift of the ν1 stretching bands of CH4 and N2 within CH4-CO2, N2-CO2 and CH4-N2 binary mixtures

The relationships between the frequency shift of the ν₁ stretching bands of CH₄ and N₂ with pressure (or density) and composition have been previously provided in the literature as reliable parameters for accurate empirical barometers and densimeters for the direct determination of the pressure or density of gas mixtures. However, the latter results still remain a pure description of the experimental data without any interpretation of the physical mechanisms hidden behind the variation trend of the observed peak position. The present paper is devoted to interpreting the origin of the pressure-induced vibrational frequency shifts of the ν₁ stretching bands of CH₄ and N₂ within CH₄-CO₂, N₂-CO₂ and CH₄-N₂ binary mixtures at the molecular level. Two different theoretical models (i.e., the Lennard-Jones 6-12 potential approximation - LJ, and the generalized perturbed hard-sphere fluid - PHF) are used to intuitively and qualitatively assess the variation trend as well as the magnitude of the frequency shift of the CH₄ and N₂ν₁ bands for an in-depth understanding. Thereby, the contribution of the attractive and repulsive solvation-mean forces to the variation of the Raman frequency shift as a function of pressure and composition is assessed. A predictive model of the variation trend of the frequency shift of the CH₄ν₁ band as a function of pressure (up to 3000 bars), density and composition within CH₄-N₂ and CH₄-CO₂ binary mixtures is then provided.