abstract
Although spectroscopic investigation of surface chemisorbed CO2 species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO2 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 CO2 species physisorbed prior to and after amine-functionalization of silica surfaces; combining C-13 NMR chemical shift anisotropy (CSA) and longitudinal relaxation times (T-1). These methods were used to quantitatively distinguish otherwise overlapping physisorbed CO2 signals, which contributed to an empirical model of CO2 speciation for the physi- and chemisorbed fractions. The quantitatively measured T-1 values confirm the presence of CO2 molecular dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO2 species with proportions of about 15%, 15%, and 70%, respectively. Our approach takes advantage from using adsorbed C-13-labeled CO2 as probe molecules and quantitative cross-polarization magic- angle spinning to study both physi- and chemisorbed CO2 species, showing that 45% of chemisorbed CO2 versus 55% of physisorbed CO2 is formed from the overall confined CO2 in amine-modified hybrid silicas. A total of six distinct CO2 environments were identified from which three physisorbed CO2 were discriminated, coined here as gas, liquid, and solid-like CO2 species. The complex nature of physisorbed CO2 in the presence and absence of chemisorbed CO, species is revealed, shedding light on what fractions of weakly interacting CO2 are affected upon pore functionalization. This work extends the current knowledge on CO2 sorption mechanisms providing new clues toward CO2 sorbent optimization.
keywords
SOLID-STATE NMR; MESOPOROUS SILICA; METHANE ADSORPTION; CARBON-DIOXIDE; C-13 NMR; CAPTURE; SPECTROSCOPY; RELAXATION
subject category
Chemistry, Physical; Nanoscience & Nanotechnology; Materials Science, Multidisciplinary
authors
Vieira, R; Marin-Montesinos, I; Pereira, J; Fonseca, R; Ilkaeva, M; Sardo, M; Mafra, L
our authors
Ildefonso Marin Montesinos
Junior Researcher
Luís Mafra
Principal Researcher
Mariana Coutinho Sardo
Assistant Researcher
Ricardo Vieira
Junior ResearcherProjects
CICECO - Aveiro Institute of Materials (UIDB/50011/2020)
CICECO - Aveiro Institute of Materials (UIDP/50011/2020)
acknowledgements
This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the Portuguese Foundation for Science and Technology/MCTES. We also acknowledge funding from project PTDC/QUI-QFI/28747/2017 (GAS2MAT-DNPSENS -POCI-01-0145FEDER-028), financed through FCT/MEC and cofinanced by FEDER under the PT2020 Partnership Agreement. The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project 022161 (cofinanced by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC). This work has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement 865974). FCT is also acknowledged by R.V. and M.I. for a Junior Researcher Position (CEECIND/02127/2017 and CEECIND/00546/2018, respectively), M.S. for an Assistant Research Position (CEECIND/00056/2020), and by J.P. for a Ph.D. Studentship (SFRH/BD/145004/2019).