Microscopic View of Carbon Dioxide Adsorption in Amine-Decorated Porous Silicates for Direct Air Capture

Organic-inorganic hybrid sorbents, based on mesoporous silicates and polymers, are a leading candidate for CO₂ capture under ambient pressure, which takes advantage of the high surface area of the silicates. [1]. An impregnation process allows active functional groups, such as polymer-based alkyl amines, to decorate the internal surface of mesoporous silicates and improve the adsorption-desorption kinetics at a reduced energy expense [2]. The mesopore surface was shown to interact with aqueous polyethyleneimine (PEI) to create the amine-modified sites for CO₂ adsorption [3]. However, the beam-sensitive specimen has limited electron microscopy in measuring these adsorption sites' structure and chemistry within the nanometer-sized mesopores [4]. In this work, we adopted a new approach that combines in situ cryogenic scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) in an environmental transmission electron microscope (ETEM) to probe the interactions among CO₂, PEI, and mesoporous silicates.

The ETEM is equipped with a monochromated electron source to achieve an EELS energy resolution of up to 80 meV at 80 kV and a custom-built optical spectroscopy system to measure Raman shifts from a ten-micrometer-in-diameter area centered around the electron-beam location [5]. The amine-modified silicates were prepared by impregnating about 60 wt.% of PEI with a molecular weight of 800 g/mol into pristine silicates (e.g., MCM-41). A liquid-nitrogen-cooled holder was used to freeze the amine-modified sample in the ETEM and, therefore, mitigated electron beam damages as monitored with the spatially correlated Raman spectra. We used in situ EELS hyperspectral imaging to benchmark the mesopore surface’s interaction with CO_2, either with or without PEI impregnation. For the pristine MCM-41 mesopores, we obtained carbon K-edge maps based on the distinct π* peak at ≈ 290 eV to show physisorption in a CO₂ environment and subsequent desorption in the ETEM baseline vacuum [6]. On the other hand, the amine-modified MCM-41 was activated at 60 °C in the ETEM baseline pressure and then exposed to CO₂. After evacuating CO₂, we froze the sample in situ by adding liquid nitrogen to the cooling holder. The approach allowed us to analyze EELS data obtained from specimens in the frozen state, whether they had been exposed to CO₂ or not. We identified the energy loss near edge structure (ELNES) associated with the carbonyl core to π*_C=O transition (≈ 289 eV) in ammonium carbamate [7], which indicates the formation of chemisorbed species when exposed to CO₂ under a water-free condition. Another ELNES of the amide core to π* transition (≈ 401 eV) was also detected to support the above observation. Compared to the ETEM result, the same type of PEI film on a metal substrate was examined using polarization modulation-infrared reflection adsorption spectroscopy (PM-IRRAS). The carbamate peak intensity changes in response to the adsorption and desorption processes confirmed carbamate formation in a dry CO₂ environment.

We show here that in situ cryogenic STEM-EELS enables the direct observation of CO₂ chemisorption and the ability to benchmark the properties and performance of PEI-decorated MCM-41 at the nanoscale. We continue pushing the frontier of understanding the dynamic structure and chemistry in a broad range of beam-sensitive materials with our new approach to elucidate the fundamental principle that leverages adsorption sites for new direct air capture technologies.

References:
[1] S Choi, JH Drese, and CW Jones, ChemSusChem 2 (2009), p. 785.
[2] WJ Son, JS Choi, and WS Ahn, Microporous Mesoporous Mater. 113 (2008) p. 31.
[3] X Xu et al., Energy Fuels 16, (2002) p. 1463.
[4] J Young et al., Energy Environ. Sci. 14 (2021), p. 5377.
[5] WCD Yang et al., ACS Appl. Mater. Interfaces 11 (2019), p. 47037.
[6] WCD Yang et al., Nat. Mater. 18 (2019), p. 614.
[7] WG Urquhart and H. Ade, J. Phys. Chem. B 106 (2002), p. 8531.