The research team, led by Tim-Patrick Fellinger with contributions from Simon W. J. Dietzmann, Asad Mehmood, Jian Liang Low and colleagues, set out to understand a puzzling observation: commercial activated carbons and advanced nitrogen-doped carbons frequently show extremely sharp peaks in their calculated ultramicropore distributions, with full widths at half maximum far below what physical considerations should allow. For pores differing in size by even a single carbon layer, theory suggests a peak width of at least 0.125 nanometres, yet some measurements report peaks as narrow as 0.04 nanometres. Such precision would imply pores defined to a fraction of an atom’s diameter, which is physically implausible in the disordered, amorphous world of non-graphitic carbon.

To resolve the mystery, the researchers prepared a pair of elegantly matched model materials. Starting from a zinc-containing zeolitic imidazolate framework, they synthesised a zinc-nitrogen-carbon material containing tetrapyrrolic ZnN4 sites, atomically dispersed metal centres coordinated by four nitrogen atoms embedded in the carbon plane. Acid extraction of the zinc then produced a second material in which the metal ions were removed but the carbon framework remained essentially unchanged, leaving behind tetrapyrrolic H2N4 cavities with nitrogen-nitrogen distances of roughly 0.4 nanometres. Spectroscopic analysis using extended X-ray absorption fine structure confirmed the tetrapyrrolic coordination in both materials, while X-ray photoelectron spectroscopy tracked the transformation from ZnN4 to H2N4 sites. Powder X-ray diffraction confirmed that no change in graphitisation occurred, meaning only the zinc was removed and the carbon skeleton was preserved.

With these precisely defined materials in hand, the team measured gas sorption using three different probe gases: nitrogen at 77.4 kelvin, carbon dioxide at 273 kelvin, and argon at 87.3 kelvin. Both materials displayed type I isotherms characteristic of microporous solids, and zinc extraction increased the apparent specific surface area by about 23 percent. But then came a surprise. At the lowest accessible relative pressures, around ten to the minus seven, the zinc-containing material actually adsorbed more nitrogen and more carbon dioxide than the supposedly more porous metal-free version. This trend reversal was counterintuitive: if the removal of zinc created new ultramicropores, the leached material should have taken up more gas, not less.

The key clue came from argon. Unlike nitrogen and carbon dioxide, argon is essentially non-polar and possesses no quadrupole moment, the asymmetry in its charge distribution that allows it to interact with electrically polarised surfaces. Argon adsorption followed the expected trend, with the metal-free material consistently taking up more gas across the entire pressure range, and no reversal appeared at low pressures. Because the anomalous behaviour depended on the quadrupole moment of the gas, with carbon dioxide showing the strongest effect, followed by nitrogen and effectively none for argon, the researchers concluded that the origin was microscopic and chemical rather than macroscopic and structural. The specific interactions between polar gas molecules and the embedded nitrogen-metal sites were distorting the measurements.

The pore size distributions told the same story. Analysis of nitrogen and carbon dioxide isotherms produced razor-thin apparent ultramicropore peaks, with full widths at half maximum of roughly 0.09 and 0.04 nanometres respectively, at apparent pore diameters between 0.53 and 0.82 nanometres depending on the gas used. Argon, in contrast, revealed only broader supermicropores centred around 1.3 nanometres with a full width of about 0.6 nanometres, features the team assigns to genuine physical porosity. The apparent surface areas themselves also diverged depending on the gas: for the zinc-containing material, nitrogen analysis suggested 750 square metres per gram while argon gave 566, with nitrogen consistently overestimating the accessible surface.

To explain these observations at the molecular level, the team turned to electronic density functional theory calculations of adsorption energies for each gas on three sites: plain graphitic carbon, the tetrapyrrolic H2N4 cavity, and the ZnN4 centre. For all three gases, adsorption at ZnN4 was stronger than on graphitic carbon, and the deviation grew with increasing quadrupole moment of the adsorbate. The zinc atom carries a substantial partial charge of about plus 1.6 electrons due to its ionic bonding character, creating a strongly polarised site that attracts quadrupolar molecules like carbon dioxide and nitrogen through quadrupole-ion interactions. Binding energies at these sites fall squarely in the range normally associated with ultramicropore filling, so the porosity analysis software interprets them as pores smaller than 0.7 nanometres, even though no pore physically exists there. Carbon dioxide also binds preferentially to the H2N4 cavity through quadrupole-dipole interactions aligned with the nitrogen-hydrogen bonds, though more weakly than at ZnN4.

The most striking validation came from a new analytical approach. Because standard pore size distribution fitting cannot directly yield adsorption energies, the researchers applied a multi-Langmuir analysis to the carbon dioxide isotherms, deconvoluting the data into contributions from three classes of binding sites: the carbon support, real micropores, and the strongest specific sites. For the strongest site class, the fitted adsorption free energies differed by 0.036 electron volts between the zinc-containing and metal-free materials, a value that matches almost exactly the 0.035 electron volt difference predicted by the density functional theory calculations for carbon dioxide binding at ZnN4 versus H2N4 sites. The internal consistency between experiment and simulation is compelling evidence that the phantom pores are indeed the chemical fingerprints of atomically dispersed metal-nitrogen sites. Encouragingly, the extracted maximum capacities at these sites, between 0.08 and 0.13 millimoles per gram, are comparable to active site densities independently reported for iron-nitrogen-carbon catalysts, hinting that simple gas sorption could one day serve as a quantitative probe of catalytically active sites, a quantity that has remained notoriously difficult to measure.

The practical implications for the field are substantial. The authors recommend that narrow ultramicropore peaks arising from in-plane functionalities be flagged as artefacts, for example with an asterisk, and that corresponding pore volumes be reported as apparent rather than true micropore volumes. For atomically dispersed metal-nitrogen-carbon materials and related nitrogen-doped carbons, surface areas and total pore volumes are best determined from argon sorption, which avoids the overestimation caused by specific adsorption. Beyond correcting current practice, the findings open a genuine opportunity: gas sorption, one of the most widely accessible characterisation techniques in materials chemistry, could be transformed into a tool for detecting and quantifying specific surface sites and in-plane functionalities, provided new analysis kernels are developed. That would help researchers disentangle morphological properties such as porosity and surface area from chemical properties such as active site geometry and concentration, both crucial for optimising carbon-based materials in energy storage, conversion and catalysis. What looked like a flaw in the measurement may turn out to be one of its most useful features.

Subject of Research: Pore size distribution analysis artefacts in non-graphitic carbons and atomically dispersed M-N-C materials caused by specific gas adsorption

Article Title: A Pore or not a Pore? Understanding Pore Size Distributions of Non‐Graphitic Carbon and Atomically‐Dispersed M‐N‐C Materials

Article References: Dietzmann, S. W. J., Mehmood, A., Low, J. L., Wu, S.-H., Prinz, C., Buzanich, A. G., Radnik, J., Appel, P. A., Emmerling, F., & Fellinger, T.-P. (2026). A Pore or not a Pore? Understanding Pore Size Distributions of Non‐Graphitic Carbon and Atomically‐Dispersed M‐N‐C Materials. Advanced Science, 13(50), Article e76048. https://doi.org/10.1002/advs.76048

Image Credits: AI Generated

DOI: 10.1002/advs.76048

Keywords: pore size distribution, gas sorption, M-N-C materials, nitrogen-doped carbon, ultramicropores, ZnN4 sites, density functional theory, activated carbon, BET surface area, argon sorption, Langmuir analysis, electrocatalysis

Carbonate rocks, notorious for their heterogeneity and complex pore networks, have long presented a challenge for geoscientists attempting precise measurements of their pore structures. Traditional techniques such as MIP and SEM, while highly effective in capturing detailed pore characteristics, are often labor-intensive and costly, limiting their widespread application, especially for large-scale studies. The integration of SIP, a geophysical method that measures the electrical polarization of porous media subjected to alternating electrical currents, emerges as a promising alternative capable of non-invasively probing pore architecture.

The team, comprising Panwar, Sharma, Kalita, and colleagues, meticulously combined SIP measurements with MIP and SEM imaging to derive more constrained and reliable pore size distributions. The core of their work underscores a pivotal advancement: the application of spectral induced polarization to effectively serve as a bridge between microscale imaging techniques and larger-scale petrophysical evaluations. By doing so, the researchers provide an accessible, scalable pathway to decode the intricate microstructure of carbonate rocks.

Spectral induced polarization offers significant advantages by capturing the frequency-dependent electrical response of rock samples. This response is intrinsically linked to the geometrical and chemical properties of the pores filled with conductive fluids, such as brine. By analyzing the SIP spectra, the researchers were able to infer detailed pore size distributions, revealing subtle variations within carbonate matrices that were previously challenging to quantify non-destructively. This non-invasive nature of SIP positions it as a powerful tool in geophysical investigations, offering critical insights without altering or destroying samples.

To validate their novel SIP-derived pore size estimations, the researchers conducted extensive comparisons with mercury intrusion porosimetry and high-resolution scanning electron microscopy analyses. MIP is a classical approach in which mercury is forced into the pores under pressure, providing precise measurements of pore throat sizes. SEM, on the other hand, furnishes detailed qualitative and quantitative imaging at nanometer scales, revealing the morphology and spatial distribution of pores. The concordance between the SIP results and these established methods illustrated the robustness and accuracy of the new approach.

One of the core challenges addressed in the study was refining SIP spectral models to accurately capture the electrochemical polarization mechanisms governing electrical responses in complex carbonate structures. Unlike sandstones or clastic reservoirs, carbonates exhibit wide variations in pore connectivity, sizes, and mineral compositions that influence SIP signals. The research team developed refined computational models to disentangle these effects, enabling improved extraction of pore size-related information from measured SIP data.

Furthermore, the combined method proved instrumental in differentiating between microporous and macroporous domains within carbonate samples. This differentiation is crucial because fluid flow dynamics and storage capacity are strongly governed by the distribution of pore sizes. Through detailed SIP spectral analyses, the team revealed previously inaccessible details about the dual-porosity nature prevalent in many carbonate systems, a feature that traditional single-technique methods often overlook or underestimate.

The implications of this research extend beyond sedimentary geology alone. Accurate pore size characterization is vital for enhancing oil recovery techniques, optimizing carbon sequestration strategies, and predicting contaminant transport in aquifers. By reliably estimating pore size distributions through a synergistic SIP-MIP-SEM framework, the study lays the groundwork for improved subsurface models, which ultimately lead to better resource management and environmental stewardship.

An exciting aspect highlighted by this research is the potential for non-destructive, rapid field applications. Given that spectral induced polarization can be performed on core samples or directly in boreholes, this approach opens up possibilities for real-time subsurface monitoring. Compared to traditional MIP or SEM analysis, which require time-consuming sample preparations, SIP measurements may streamline workflows and reduce operational costs on exploration and extraction sites.

Moreover, the integration methodology proposed by Panwar and colleagues can be expanded and adapted to other rock types and fluid systems. While the study focused on carbonate samples saturated with brine solutions to mimic natural conditions, the underlying principles of SIP as a pore size proxy are broadly applicable. This versatility presents avenues for future research exploring parameter calibration across diverse lithologies, fluid chemistries, and geophysical settings.

The image accompanying the study offers a compelling visualization of the spectral induced polarization and corresponding pore attributes derived through complementary techniques. Such graphical representations not only elucidate the scientific concepts but also facilitate the communication of complex subsurface properties to multidisciplinary audiences, including industry professionals and policy makers.

As environmental challenges increasingly necessitate efficient subsurface characterization, innovations like these play a pivotal role in advancing sustainable geoscience practices. Enhanced pore network characterizations contribute to more accurate predictions of fluid flow behavior under changing climatic and operational conditions, informing risk assessments and mitigation strategies.

In summary, this pioneering research bridges the gap between microscale analyses and bulk geophysical measurements, delivering a sophisticated yet practical approach for understanding the pore size distributions of carbonate reservoirs. By meticulously validating SIP-derived parameters with established MIP and SEM datasets, the authors underscore the reliability and applicability of their method, setting a new standard for carbonate rock characterization.

This work exemplifies how interdisciplinary techniques can converge to solve longstanding geological questions. The fusion of physics-based spectral analyses with microscopic imaging unlocks a comprehensive view of pore structures that neither approach could fully achieve in isolation. Such advances not only push the frontiers of academic research but also hold transformative potential for the energy sector and environmental management.

The future trajectory inspired by this study envisions the broader deployment of spectral induced polarization as a routine diagnostic tool in geosciences. With ongoing improvements in instrumentation and computational modeling, SIP could become integral to real-time reservoir characterization and monitoring. This promises to accelerate both exploration efforts and the responsible stewardship of subsurface resources.

Considering the pressing global need to understand complex geological formations in a cost-effective and environmentally conscious manner, the integration of SIP with MIP and SEM data represents a critical step forward. Innovations in measurement methodologies will be vital as the demands on carbonates and other reservoirs continue to escalate in the coming decades.

Ultimately, the study by Panwar, Sharma, Kalita, and colleagues sets a new benchmark in the quest to unravel the intricacies of carbonate pore systems. Their work eloquently demonstrates the power of combining spectral-induced polarization insights with meticulous laboratory techniques to yield pore size distributions of unprecedented accuracy and detail—an achievement with far-reaching scientific and practical consequences.

Subject of Research: Pore size distribution characterization in carbonate rocks using spectral induced polarization combined with mercury intrusion porosimetry and scanning electron microscopy.

Article Title: Constraining spectral induced polarization-derived pore size distributions in carbonates using MIP and SEM.

Article References:
Panwar, N., Sharma, R., Kalita, H. et al. Constraining spectral induced polarization-derived pore size distributions in carbonates using MIP and SEM. Environ Earth Sci 84, 683 (2025). https://doi.org/10.1007/s12665-025-12641-2

Image Credits: AI Generated

DOI: https://doi.org/10.1007/s12665-025-12641-2