An Evaluation of the Reliability of the Results Obtained by the LBET, QSDFT, BET, and DR Methods for the Analysis of the Porous Structure of Activated Carbons

Mirosław Kwiatkowski,Elżbieta Broniek

doi:10.3390/ma13183929

Mirosław Kwiatkowski, Elżbieta Broniek

Open Access

https://doi.org/10.3390/ma13183929

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Abstract

This paper presents the results of an analysis of the impact of the activator to the product of carbonized materials mass ratio on the porous structure of activated carbons prepared from mahogany, ebony, and hornbeam wood by carbonization and chemical activation with potassium hydroxide. The analyses were carried out on nitrogen adsorption isotherms using the Brunauer–Emmett–Teller (BET), Dubinin-Radushkevitch (DR), and Quenched Solid Density Functional Theory (QSDFT) methods, as well as the numerical clustering-based adsorption analysis (LBET) method. The activated carbons with the best adsorption properties and homogeneous surfaces were prepared at a mass ratio of R = 3. The analyses suggest the significant potential of producing adsorbents characterized by a large surface area and adsorptive capacity from raw materials such as mahogany, ebony, and hornbeam wood. The analyses in question also included an evaluation of the usability and reliability of the results obtained with the employed methods of structural analysis. Particular focus was placed on the limitations of adsorption models and on critically analyzing the output data. Our study shows the unique advantages of the LBET method compared to the other methods used. The LBET method allowed us, for example, to determine the degree of heterogeneity of the surface of the studied activated carbons and the shape of the clusters of adsorbate molecules formed in the pores of the studied material, as well as obtain information about the distribution of adsorption energy on the first adsorbed layer. This study also demonstrates the limitations of the methods used and the necessity to use LBET and QSDFT methods simultaneously for porous structural analysis. The simultaneous analysis of the adsorption isotherms via the LBET and the QSDFT methods makes it possible to choose the optimal preparation conditions while considering the properties of the original raw material. The analyses also suggest the complementary character of the employed methods and the scope of the useful and reliable information that can be obtained with these methods.

Highlights

New tasks related to protecting air and surface water, as well as implementing sustainable development principles, have resulted in an increased demand for environmentally friendly, cheap, and widely available adsorbents [1,2,3]
The results of the research presented in this article highlight the significant potential of producing activated carbons with a very high adsorption capacity and large specific surface area from mahogany, ebony, and hornbeam wood through activation with potassium hydroxide
The activated carbons with the best adsorption properties were prepared at a mass ratio of the activator to the products of carbonized wood equal to R = 3

Summary

Introduction

New tasks related to protecting air and surface water, as well as implementing sustainable development principles, have resulted in an increased demand for environmentally friendly, cheap, and widely available adsorbents [1,2,3]. The most common adsorbents are activated carbons, which are composed of a carbon structure that contains small amounts of heteroatoms, such as oxygen and hydrogen, along with, depending on the type of raw material used, various mineral substances. The skeletal structure of activated carbons can be considered a mixture of graphite-like crystallites separated by disordered carbon consisting of complex aromatic-aliphatic forms and inorganic matter derived from the raw material. These graphite-like crystallites consist of several parallel flat graphite layers that are randomly oriented and interconnected. Activated carbons have a very large specific surface area resulting from a significant proportion of micropores that constitute about 90–95% of the carbon’s total porosity. Most of the adsorption process takes place in the micropores, but larger pores, such as mesopores and macropores, play a very important role in any adsorption process because they act as transport pores through which the adsorbate molecules reach the micropores [1,2,3]

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