Following carbon condensation by in situ TEM: towards a rational understanding of the processes in the synthesis of nitrogen-doped carbonaceous materials

Abstract

Porous carbonaceous materials obtained from biomass have been an important class of CO₂ sorbents since ancient times. Recent progress in carbon-based adsorbent technology is based on the implication of the concept of heteroatom doping. In this respect, the synthesis of carbonaceous materials through one-step condensation of cheap nitrogen-containing molecular precursors is an attractive strategy for obtaining such N-doped carbons. The design of the adsorbents obtained by this route relies on the careful adjustment of synthesis parameters, such as the temperature, the heating rate, and the atmosphere. However, in most cases, the latter choice remains rather empirical due to the lack of a fundamental understanding of the condensation mechanism of molecular precursors. In this work, we followed the structural, morphological, and chemical evolution of a molecular precursor (uric acid) at the nanoscale using a combination of in situ condensation inside a scanning transmission electron microscope with ex situ analysis of the products of condensation at different temperatures, atmospheres, and heating rates, and correlate our findings with the CO₂ sorption properties of the obtained materials. 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 found that synthesis in vacuum enables effective condensation at considerably low temperatures (500 °C). Using a higher heating rate (10 °C min⁻¹) suppresses structural ripening and preserves the optimal size of micropores, thus giving a CO₂ uptake twice as high compared to samples synthesized in a nitrogen atmosphere, which is commonly used, preserving the same selectivity.