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