Understanding the mechanism of molecular precursor condensation by in-situ TEM: towards the rational design of porous carbon materials for CO₂ absorption

Abstract text (incl. figure legends and references)

Carbon-based materials are sustainable, metal-free functional materials used for various applications; for instance—owning to their porosity—they are often exploited for CO₂ capture. Their CO₂ uptake is usually enhanced by introducing heteroatoms (e.g., nitrogen) and adjusting the porous structure. While nitrogen doping can be achieved through one-step condensation of cheap nitrogen-containing molecular precursors, the porous structure—if no porogen or template is applied—can be adjusted only by synthesis conditions. The choice of the latter remains rather empirical due to the lack of a fundamental understanding of the condensation mechanism.

Our study aims to broaden the fundamental understanding of the condensation process and propose guidelines for designing new porous carbon materials for CO₂ absorption. To realize this goal, we performed in-situ heating experiments inside a TEM and followed the condensation ex-situ by analyzing the reaction products formed at different temperatures and pressures and evaluating their adsorption properties. As a model molecular precursor, we use uric acid, which forms carbonaceous materials with high nitrogen content. To distinguish the porosity in bulk and the surface of the studied particles, we combined bright-field STEM (internal structural information) with secondary electron imaging in STEM (morphological information).

The in-situ condensation in TEM starts at lower temperatures (250 °C) compared to synthesis in nitrogen atmosphere (425 °C from thermogravimetrical analysis). Figure 1 shows the secondary electron (SE) and bright-field (BF) signals from the same particle during heating. The surface of the particle is smooth at 25 ºC (Figure 1 a, SE) and becomes porous at 250 ºC (Figure 1b, SE, mesopores of ~21 nm), indicating the first temperature-induced changes – the beginning of the condensation process. Upon further isothermal heating at 250 ºC, the subsequent mass loss occurs from within the bulk of the particle through the porous surface resulting in the loss of mass first from the pre-surface area (Figure 1 c, BF) and after that from the bulk of the particle (Figure 1 d, BF).

Figure 1. Secondary electron (SE) and bright-field (BF) STEM images of the particle a) at 25 °C before the in-situ heating, b) immediately after reaching 250 °C, c) after 4 minutes at 250 °C, d) after 64 minutes at 250 °C.

We found that only samples in vacuum (1.5·10^-4 bar) have pores of about 14 nm in diameter at the surface, while in nitrogen (1 bar), the surface of the samples appears smooth. The pressure in the reaction chamber influences the mass transfer from the surface of particles leading to more porous surfaces when synthesizing materials at low pressures, as observed in ex-situ samples in vacuum and during in-situ TEM heating (3·10^-10 bar). As a result, vacuum samples demonstrate 2 times higher CO₂ uptake at 500 °C than nitrogen samples.

Thus, we reported the condensation of a molecular precursor in-situ for the first time. We showed that varying pressures and reaction rates result in particles with different porosity; the porosity of the surface of the particles during the early stages of condensation governs the subsequent release of volatiles and the development of a hierarchical pore structure.

We gratefully acknowledge financial support by the Max Planck Society and Dr. Janina Kossmann and Dr. Nieves Lopes for the synthesis and CO₂ sorption characterization