"Hidden" CO 2 in Amine-Modified Porous Silicas Enables Full Quantitative NMR Identification of Physi- and Chemisorbed CO 2 Species

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