Gassorption analysis of in-plane functionalized carbons reveals small, uniform ultramicropore signatures as potential artefacts of specific adsorption of the probe gases. Low-pressure Langmuir fitting links these features to distinct functionalities via adsorption energies, enabling straightforward quantification of otherwise inaccessible surface-site densities, demonstrated here for tetrapyrrolic ZnN 4 and H 2 N 4 sites.
ABSTRACT
Pore size analysis is essential for understanding and optimizing structure-performance relations of functional carbon-based materials including activated carbons, supercapacitor electrodes and atomically dispersed metal-nitrogen-doped carbon (M-N-C) catalysts. Pore size distribution (PSD) plots based on gas sorption porosimetry often show narrow micropores that are related to the adsorptive properties of named materials, which must be considered as artefacts arising from approximations in classical density functional theory (cDFT) models. By selectively preparing specific in-plane functionalities using pyrolytic template-ion (salt templating) reactions, we herein show that those apparent pores can be explained by preferential adsorption of the adsorbate molecules to specific in-plane functionalities. Tetrapyrrolic Zn-N 4 sites are present in ZIF-8 derived carbons, which are converted by Zn-extraction into nitrogen-doped carbons (NDC) comprising tetrapyrrolic H 2 N 4 sites. DFT-based calculation of adsorption energies allows the conclusive assignment of corresponding adsorption phenomena in comparative N 2 vs. CO 2 vs. Ar adsorption measurements additionally using Langmuir analysis. While the assignment of artefacts may improve the discussion of porosity, the determination of specific adsorption sites may be utilized as a valuable tool in materials science. Advanced models for the important material classes may allow accelerated progress in important energy-related research fields.
