Article pubs.acs.org/Langmuir
Advanced Structural Analysis of Nanoporous Materials by Thermal Response Measurements Martin Oschatz,† Matthias Leistner,‡ Winfried Nickel,† and Stefan Kaskel*,†,‡ †
Department of Inorganic Chemistry, Dresden University of Technology, Bergstrasse 66, D-01062 Dresden, Germany Fraunhofer Institute for Material and Beam Technology (IWS), Winterbergstrasse 28, D-01277 Dresden, Germany
‡
S Supporting Information *
ABSTRACT: Thermal response measurements based on optical adsorption calorimetry are presented as a versatile tool for the time-saving and profound characterization of the pore structure of porous carbon-based materials. This technique measures the time-resolved temperature change of an adsorbent during adsorption of a test gas. Six carbide and carbon materials with well-defined nanopore architecture including micro- and/or mesopores are characterized by thermal response measurements based on n-butane and carbon dioxide as the test gases. With this tool, the pore systems of the model materials can be clearly distinguished and accurately analyzed. The obtained calorimetric data are correlated with the adsorption/desorption isotherms of the materials. The pore structures can be estimated from a single experiment due to different adsorption enthalpies/temperature increases in micro- and mesopores. Adsorption/desorption cycling of n-butane at 298 K/1 bar with increasing desorption time allows to determine the pore structure of the materials in more detail due to different equilibration times. Adsorption of the organic test gas at selected relative pressures reveals specific contributions of particular pore systems to the increase of the temperature of the samples and different adsorption mechanisms. The use of carbon dioxide as the test gas at 298 K/1 bar provides detailed insights into the ultramicropore structure of the materials because under these conditions the adsorption of this test gas is very sensitive to the presence of pores smaller than 0.7 nm.
■
characterization of nanoporous structures.21−24 One major drawback of these methods is the relatively long time that is needed for a single experiment. Hence, a more time-saving characterization tool is desirable. Recently, thermal response measurements were reported to be a useful method for the rapid screening of porous materials structures.25,26 This technique is based on optical adsorption calorimetry and measures the time-resolved temperature change (i.e., the thermal response) of a porous material during adsorption of a test gas (e.g., carbon dioxide or n-butane) in a dynamic flow cell due to the release of the heat of adsorption (Scheme 1). Since an infrared sensor is used in this method, it is designated as “InfraSORP” technique. The magnitude of the temperature change depends on the number of adsorbed molecules, the sizes of the pores where the adsorption takes place, the speed of adsorption, and the heat capacity of the material. For materials with similar chemical composition, narrow PSD, and homogeneous surface properties, the number of adsorbed molecules is proportional to the uptake of test gas.25 Moreover, the heat transfer (convection, conduction,
INTRODUCTION Nanoporous materials (i.e., materials with pore sizes below 100 nm)1 are essential in applications such as catalysis or gas adsorption.2−6 Especially carbon nanomaterials are in focus of electrochemical energy storage applications7−9 and water treatment10,11 due to their high electrical conductivity and chemical stability. Their performance in these fields is most often a function of their structural properties (e.g., the specific surface area (SSA), surface chemistry, pore size, pore volume (PV), pore geometry, and pore connectivity). Hence, a detailed characterization of these parameters is important to get a more profound understanding of fundamental mechanisms and to tailor materials for a given application. In many cases, the investigation of the pore structure by spectroscopic methods,12 X-ray scattering,13 electron microscopy,14 or calorimetry15,16 is very suitable. However, the physical adsorption (physisorption) of gases will likely remain the most frequently applied method for textural characterization and identification of accessible surface area in porous solids.17−20 Physisorption enables the evaluation of important properties (e.g., the SSA, PV, micropore volume (MPV), and pore size distribution (PSD)). Commonly, physisorption of nitrogen at 77 K, argon at 87 K, water vapor at 298 K, hydrogen at 77 K, or carbon dioxide at 273 K as well as combinations of them are used for the © XXXX American Chemical Society
Received: February 6, 2015 Revised: March 13, 2015
A
DOI: 10.1021/acs.langmuir.5b00490 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
acid. Polymerization of the hydrocarbon was achieved by heating the mixture to 373 K for 6 h followed by subsequent heating to 433 K for another 6 h. The infiltration was repeated with a 10 mL of aqueous solution of 1.6 g of sucrose, to which was added 0.18 g of 96% sulfuric acid, again followed by heating to 373 and 433 K. Carbonization was carried out under flowing argon atmosphere in a horizontal tubular furnace. The material was heated to 1173 K with a rate of 150 K/h and annealed for 2 h. Silica dissolution was achieved by treating the composite material in a hydrofluoric acid (35% aqueous solution)/ water/ethanol mixture (1:1:1 by volume) for 3 h followed by washing with large amounts of ethanol and drying at room temperature (RT). Synthesis of Ordered Mesoporous Silicon Carbide (OM-SiC). A 2 g sample of SBA-15 template was infiltrated in a mortar with a mixture of 2.1 mL of liquid allylhydridopolycarbosilane SMP-10 (purchased from Starfire Systems) and 0.5 mL of p-divinylbenzene by the incipient wetness technique. Pyrolysis of the SMP-10/SBA-15 composite was carried out under constant argon flow at 1073 K for 2 h with a heating rate of 60 K/h. Silica dissolution, washing, and drying of the OM-SiC were performed similar to CMK-3. Synthesis of Ordered Mesoporous Silicon Carbide-Derived Carbon (OM-SiC-CDC). OM-SiC-CDC was prepared by silicon removal from OM-SiC using a high-temperature chlorine treatment. An ∼1 g sample of the mesoporous carbide was transferred to a quartz boat and placed within a quartz tube in the isothermal zone of a horizontal tube furnace. After flushing with 150 mL/min argon, the material was heated to 1073 K with a heating rate of 450 K/h. When this value was reached, the gas flow was changed to a mixture of 80 mL/min chlorine and 70 mL/min argon for 3 h. The furnace was then cooled to 873 K under an argon flow of 150 mL/min, and the gas flow was changed to 80 mL/min hydrogen for 1 h in order to remove chlorine and metal chloride species adsorbed in the pores of the CDC after high-temperature chlorination.31 The pristine OM-SiC-CDCs was cooled to RT under flowing argon. Synthesis of Microporous TiC-CDCs. An ∼3 g sample of crystalline TiC powder (Sigma-Aldrich, 95%; particle size ∼4 μm) underwent high-temperature chlorine treatments under the same conditions as for the above-described OM-SiC-CDC. The maximum chlorination temperatures were 873 K/600 °C (TiC-CDC-600), 1073 K/800 °C (TiC-CDC-800), or 1273 K/1000 °C (TiC-CDC-1000). Volumetric Adsorption Measurements. Nitrogen physisorption measurements were performed at 77 K with 50−100 mg sample on a Quadrasorb apparatus (Quantachrome Instruments, USA). All numerical values are calculated from these measurements. SSAs were calculated using the multipoint BET equation (p/p0 = 0.01−0.1 for the TiC-CDCs and p/p0 = 0.05−0.2 for the templated samples). Total micromesopore volumes (PVMicro+Meso) are calculated at p/p0 = 0.95. Low-pressure nitrogen physisorption measurements were performed with 10−20 mg sample on an Autosorb 1C (Quantachrome Instruments). Pore size distributions (PSDs) were calculated from these measurements using the quenched solid density functional theory (QSDFT) method32 for nitrogen at 77 K on carbon with slit/ cylindrical pores (with only cylindrical pores for OM-SiC). Micropore volumes (PVMicro) are the cumulative pore volumes at a diameter of 2 nm. Carbon dioxide physisorption measurements were performed at 273 K on an Autosorb 1C (Quantachrome Instruments). Cumulative pore volumes
