Effects of CO2 activation of carbon aerogels leading to ultrahigh micro-meso porosity
Graphical abstract
Introduction
The synthesis of porous materials with tailor-made high surface areas and pore volumes in the micro- and mesoporous range has become a subject of great interest in the last decades, driven by the need to obtain highly featured materials with controlled properties (uniformity in pore sizes, volumes and shapes) in multidisciplinary fields [1], [2]. Within this context, the characterization of the porous structure of materials has become a valuable tool and it is commonly obtained from the analysis of experimental adsorption isotherms of different gases (N2, CO2, Ar, H2) at various temperatures [3]. Both classical (Dubinin–Radushkevich, BJH or t-plot) and molecular simulation methods have been extensively used for this purpose, allowing the determination of the apparent surface areas, pore volumes and pore size distributions (PSD).
While most of these methods are well established for microporous materials (zeolites, activated carbons, silica) and thus provide reasonable (and comparable) results, the analysis of the mesoporosity is not yet so well accomplished, particularly for materials displaying extremely high pore volumes and where pore blocking and/or cavitation phenomena can occur [4], [5], [6], [7], [8]. The evaluation of mesoporosity relies on the analysis of the isotherm in the regions of capillary condensation and evaporation and is associated to hysteresis phenomena and the existence of metastable states. The origin and interpretation of hysteresis in adsorption is a long standing and widely discussed issue, and yet discrepancies remain [3], [6], [7], [8], [9]. A general consensus in this discussion is that the shape of the loop provides useful qualitative information about the mesoporous structure of the materials, as adsorption hysteresis depends on the size and shape of the pores and the connectivity of the pore system, among other factors.
A simple one dimensional carbon slit pore model and the related adsorption theoretical models based on the density functional theory (DFT) in its local [10] and nonlocal (NLDFT) [11] versions have been used for more than two decades to describe and characterize the pore structure of activated carbons. However, over time with enhanced accuracy of the adsorption measurements the visible artifacts in the PSD calculation results have been reported and discussed [12], [13]. It is now well understood that the flat and energetically uniform surface of simple slit pores is a source of the problem due to the fact that the adsorption process in such pores exhibit the so called layering transitions which do not occur on real heterogeneous surfaces.
In this work we use a new 2D-NLDFT-HS model for carbons with energetically heterogeneous and geometrically corrugated pore walls [14], [15]. This improved carbon slit pore model gives an excellent fit to the experimental data and is free of common artifacts usually obtained in the PSD analysis when the standard NLDFT model was used.
The objective of this work was to analyze the effects of CO2 activation on the porosity of the ultrahigh pore volume carbon aerogels. The changes in both the micro and mesopore range were evaluated using 2D-NLDFT-HS model [15] and compared with the results of conventional methods (BJH, t-plot) in terms of pore volumes, surface areas and pore size distributions.
Section snippets
Materials
Details of the preparation of the carbon aerogels by the polymerization of resorcinol (R), formaldehyde (F) and water (W) mixtures using carbonate as catalyst (C) have been reported elsewhere [16], [17]. Briefly, the precursors (molar ratio R/C 200, R/W 0.06) were mixed in sealed glass moulds and kept at 40 °C for 24 h and 70 °C for 120 h. After a controlled water–acetone exchange, the materials were dried under CO2 supercritical conditions (sample G4) and carbonized at 800 °C (sample G4c). The
Results and discussion
In this work we present the analysis of the porous features of carbon aerogels with ultrahigh micro- and mesopore volumes by comparing different methods based on the analysis of the nitrogen adsorption isotherms at its boiling point. The carbon aerogels have been synthesized by the polycondensation of resorcinol-formaldehyde mixtures in alkaline medium as described elsewhere [16], [17]. The effect of the various synthesis and processing conditions on the characteristics (morphology, texture,
Conclusions
We have analyzed the effects of CO2 activation on the porosity of ultrahigh pore volume carbon aerogels, by comparing classical (BJH, t-plot, DR) and molecular simulation methods commonly used for the textural characterization of porous solids. The new 2D-NLDFT-HS model for carbons with energetically heterogeneous and geometrically corrugated pore walls was used to obtain the pore size distribution of a series of carbon aerogels with ultrahigh micro-/mesoporosity and obtained by physical
Acknowledgments
The authors thank the financial support of the Spanish MINECO (grant CTM2011/023378).
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