Although spectroscopic investigation of surface chemisorbed CO 2 species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO 2 molecules in confined spaces is less trivial as they are often indistinguishable resorting to (isotropic) NMR chemical shift or vibrational band analyses. Herein, we undertake for the first time a thorough solid-state NMR analysis of CO 2 species physisorbed prior to and after amine-functionalization of silica surfaces; combining 13 C NMR chemical shift anisotropy (CSA) and longitudinal relaxation times ( T 1 ). These methods were used to quantitatively distinguish otherwise overlapping physisorbed CO 2 signals, which contributed to an empirical model of CO 2 speciation for the physi- and chemisorbed fractions. The quantitatively measured T 1 values confirm the presence of CO 2 molecular dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO 2 species with proportions of about 15%, 15%, and 70%, respectively. Our approach takes advantage from using adsorbed 13 C-labeled CO 2 as probe molecules and quantitative cross-polarization magic-angle spinning to study both physi- and chemisorbed CO 2 species, showing that 45% of chemisorbed CO 2 versus 55% of physisorbed CO 2 is formed from the overall confined CO 2 in amine-modified hybrid silicas. A total of six distinct CO 2 environments were identified from which three physisorbed CO 2 were discriminated, coined here as “gas, liquid, and solid-like” CO 2 species. The complex nature of physisorbed CO 2 in the presence and absence of chemisorbed CO 2 species is revealed, shedding light on what fractions of weakly interacting CO 2 are affected upon pore functionalization. This work extends the current knowledge on CO 2 sorption mechanisms providing new clues toward CO 2 sorbent optimization.

Adsorption isotherms obtained through volumetric measurements are widely used to estimate the gas adsorption performance of porous materials. Nonetheless, there is always ambiguity regarding the contributions of chemi- and physisorption processes to the overall retained gas volume. In this work, we propose, for the first time, the use of solid-state NMR (ssNMR) to generate isotherms of CO2 adsorbed onto an amine-modified silica sorbent. This method enables the separation of six individual isotherms for chemi- and physisorbed CO2 components, a feat only possible using the discrimination power of NMR spectroscopy. The adsorption mechanism for each adsorbed species was ascertained by tracking their adsorption profiles at various pressures. The proposed method was validated against conventional volumetric adsorption measurements. The isotherm curves obtained by the proposed ssNMR-assisted approach enable advanced analysis of the sorbents, revealing the potential of variable-pressure NMR experiments in gas adsorption applications.

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