Artifacts and misinterpretations in gas physisorption measurements and characterization of porous solids.

This contribution provides a critical review of gas physisorption in the textural characterization of porous solids, with the focus on the artifacts in experimental data that lead to serious misinterpretation of the results derived from the analysis of adsorption isotherms. Apart from the problems related to the determination and interpretation of the BET area, we paid particular attention to the issues associated with the determination of pore size distribution; for example, the choice of the correct branch of the hysteresis loop and the network effects. Pitfalls in the analyses using either the classical macroscopic or the advanced microscopic (DFT, GCMC) methodology are addressed. The ultimate aim is to provide guidance for proper calculations and correct interpretation of physisorption data.

Characterization of non-graphitized carbon blacks: a model with surface crevices.

Experimental isotherms for argon and nitrogen adsorption on two non-graphitized carbon substrates, Carbopack B and Cabot BP280, do not obey Henry's Law in the range of pressures accessible to the most sensitive MKS pressure transducers. At high pressures, close to the bulk coexistence pressure (P0), the isotherms at temperatures below the bulk triple point temperature cross the P0 axis at a finite loading, a behaviour which is interpreted as incomplete wetting. It was found that the adsorbed density at P0 for Cabot BP280 is lower than that for Carbopack B which is, in turn, only slightly lower than that for the highly graphitized Carbopack F, suggesting that there is a long-range effect of the surface structure in non-graphitized carbon blacks, in the accumulation of higher layers, especially for Cabot BP280. We have carried out extensive Monte Carlo simulations to compare experimental observations with a molecular model for substrate surfaces decorated with crevices of molecular dimensions. From the analysis of the experimental data, it was found that the typical width of crevices is of the order of 0.65-0.9 nm. In the high pressure region, the crossing of the P0 axis by isotherms at temperatures below the bulk triple point temperature can be explained by an adsorbate structure which is less dense and more disordered than the fcc structure of the bulk crystal, with a consequent raising of the coexistence pressure between the adsorbate and the gas phase above P0. Adsorbate loading at the point where the isotherm crosses the P0 axis for Cabot BP280 is lower than for Carbopack B which can be attributed to a higher concentration of crevices leading to a lower adsorbate density and an irregular arrangement of atoms at the interface separating the adsorbed phase and the gas phase. This results in weaker gas-adsorbate interactions which supresses the build-up of higher layers. We suggest that the use of the adsorbed density at the bulk coexistence pressure, at temperatures below the bulk triple point temperature, can be a useful tool for assessing the presence and concentration of surface crevices on non-graphitized carbon black.

On the transition from partial wetting to complete wetting of methanol on graphite.

The transition from partial wetting to complete wetting for methanol adsorbed on a highly graphitized thermal carbon black, Carbopack F, over a range of temperature from the triple point at 185 K to 298 K, was investigated using Monte Carlo simulation and high-resolution experiments. At 190 K, (above the triple point) both the experimental and simulated adsorption isotherms cut the P/P0 axis at a finite loading; a feature of partial wetting that has not been recognized previously in the literature. This occurs because most O- and H-atoms in the second layer of the adsorbate point towards the adsorbent surface to form hydrogen bonds with molecules in the first layer and therefore the interface between the bilayer adsorbed film and the gas phase consists mainly of methyl groups, preventing the system from forming higher layers. At temperatures above 263 K, methanol adsorption increases with pressure and wets the surface as the pressure approaches the bulk coexistence pressure P0. This is because the O-H and O-CH3 bonds of methanol in the region above the second layer have random orientation, and adsorption in higher layers takes place via hydrogen bonding. From extensive simulations of methanol adsorption on adsorbents of different strength over a wide temperature range, a parametric map has been constructed which identifies the regions of non-wetting, partial wetting and complete wetting. For a given surface strength, wetting is favoured at higher temperatures, and at a given temperature there is a transition from non-wetting on weakly adsorbing substrates to either partial wetting or to complete wetting on strong adsorbents at temperatures below or above the roughening temperature Tr of 260 K.

Water adsorption on carbon - A review.

Water adsorption on carbonaceous materials has been studied increasingly in the recent years, not only because of its impact on many industrial processes, but also motivated by a desire to understand, at a fundamental level, the distinctive character of directional interactions between water molecules, and between water molecules and other polar groups, such as the functional groups (FGs) at the surfaces of graphene layers. This paper presents an extensive review of recent experimental and theoretical work on water adsorption on various carbonaceous materials, with the aim of gaining a better understanding of how water adsorption in carbonaceous materials relates to the concentration of FGs, their topology (arrangement of the groups) and the structure of the confined space in porous carbons. Arising from this review we are able to propose mechanisms for water adsorption in carbonaceous materials as the adsorbate density increases. The intricate interplay between the roles of FGs and confinement makes adsorption of water on carbon materials very different from that of other simple molecules.

A GCMC simulation and experimental study of krypton adsorption/desorption hysteresis on a graphite surface.

Adsorption isotherms and isosteric heats of krypton on a highly graphitized carbon black, Carbopack F, have been studied with a combination of Monte Carlo simulation and high-resolution experiments at 77K and 87K. Our investigation sheds light on the microscopic origin of the experimentally observed, horizontal hysteresis loop in the first layer, and the vertical hysteresis-loop in the second layer, and is found to be in agreement with our recent Monte Carlo simulation study (Diao et al., 2015). From detailed analysis of the adsorption isotherm, the latter is attributed to the compression of an imperfect solid-like state in the first layer, to form a hexagonally packed, solid-like state, immediately following the first order condensation of the second layer. To ensure that capillary condensation in the confined spaces between microcrystallites of Carbopack F does not interfere with these hysteresis loops, we carried out simulations of krypton adsorption in the confined space of a wedge-shaped pore that mimics the interstices between particles. These simulations show that, up to the third layer, any such interference is negligible.

On the explanation of hysteresis in the adsorption of ammonia on graphitized thermal carbon black.

We present a Monte Carlo simulation and experimental study of ammonia adsorption on graphitized thermal carbon black. Our new molecular model for the adsorbent is composed of basal plane graphene surfaces with ultrafine pores grafted with hydroxyl groups at the junctions between graphene layers. The simulated adsorption isotherms and isosteric heats are in good agreement with the experimental data of Holmes and Beebe, and the simulations reproduce the unusual experimental hysteresis of ammonia adsorption on an open graphite surface for the first time in the literature. The detailed mechanisms of adsorption and desorption, and the origin of hysteresis, are investigated by the microscopic analysis of the adsorbate structures to show that restructuring occurs during adsorption. The main results from this work are: (i) at the triple point, ammonia adsorbs preferentially around the functional groups to form clusters in the ultrafine pores and spills-over onto the basal plane as the loading is increased; followed by a 2D condensation on the graphite surface to form a bilayer adsorbate; (ii) at the boiling point, adsorption occurs on the basal plane due to the increasing importance of thermal fluctuations (an entropic effect); (iii) the isosteric heat is very high at zero loading due to the strong interaction between ammonia and the functional groups, decreases steeply when the functional group is saturated, and eventually reaches the heat of condensation as the fluid-fluid interaction increases.

Modification of commercial NaY zeolite to give high water diffusivity and adsorb a large amount of water.

By using NaY zeolites as desiccant materials, commercial NaY zeolite was alkali treated with 1 M NaOH aqueous solution and then Mg(2+) ion-exchanged by 0.5 M Mg(NO3)2 aqueous solution. Alkali treatment (AT) of NaY zeolite removed silicon atoms selectivity from the framework of Y-type zeolite and enhanced water diffusivity of Y-type zeolite. On the other hand, Mg(2+) ion-exchange of NaY zeolite increased the amount of water adsorbed. Prepared Y-AT-Mg zeolite had both water adsorption velocity and a large difference of water adsorbed amount between adsorption at 30 °C and desorption at 100 °C.

The interplay between molecular layering and clustering in adsorption of gases on graphitized thermal carbon black--spill-over phenomenon and the important role of strong sites.

We analyse in detail our experimental data, our simulation results and data from the literature, for the adsorption of argon, nitrogen, carbon dioxide, methanol, ammonia and water on graphitized carbon black (GTCB), and show that there are two mechanisms of adsorption at play, and that their interplay governs how different gases adsorb on the surface by either: (1) molecular layering on the basal plane or (2) clustering around very strong sites on the adsorbent whose affinity is much greater than that of the basal plane or the functional groups. Depending on the concentration of the very strong sites or the functional groups, the temperature and the relative strength of the three interactions, (a) fluid-strong sites (fine crevices and functional group) (F-SS), (b) fluid-basal plane (FB) and (c) fluid-fluid (FF), the uptake of adsorbate tends to be dominated by one mechanism. However, there are conditions (temperature and adsorbate) where two mechanisms can both govern the uptake. For simple gases, like argon, nitrogen and carbon dioxide, adsorption proceeds by molecular layering on the basal plane of graphene, but for water which represents an extreme case of a polar molecule, clustering around the strong sites or the functional groups at the edges of the graphene layers is the major mechanism of adsorption and there is little or no adsorption on the basal planes because the F-SS and FF interactions are far stronger than the FB interaction. For adsorptives with lower polarity, exemplified by methanol or ammonia, the adsorption mechanism switches from clustering to layering in the order: ammonia, methanol; and we suggest that the bridging between these two mechanisms is a molecular spill-over phenomenon, which has not been previously proposed in the literature in the context of physical adsorption.

On the isosteric heat of adsorption of non-polar and polar fluids on highly graphitized carbon black.

Isosteric heat of adsorption is indispensable in probing the energetic behavior of interaction between adsorbate and solid, and it can shed insight into how molecules interact with a solid by studying the dependence of isosteric heat on loading. In this study, we illustrated how this can be used to explain the difference between adsorption of non-polar (and weakly polar) fluids and strong polar fluids on a highly graphitized carbon black, Carbopack F. This carbon black has a very small quantity of functional group, and interestingly we showed that no matter how small it is the analysis of the isosteric heat versus loading can identify its presence and how it affects the way polar molecules adsorb. We used argon and nitrogen as representatives of non-polar fluid and weakly polar fluid, and methanol and water for strong polar fluid. The pattern of the isosteric heat versus loading can be regarded as a fingerprint to determine the mechanism of adsorption for strong polar fluids, which is very distinct from that for non-polar fluids. This also allows us to estimate the interplay between the various interactions: fluid-fluid, fluid-basal plane and fluid-functional group.

Packing effects on argon and methanol adsorption inside graphitic cylindrical and slit pores: a GCMC simulation study.

Using Grand Canonical Monte Carlo simulation, we have studied the effects of confinement on argon and methanol adsorption in graphitic cylindrical and slit pores. Linear chain, zigzag and incomplete helical packing are observed for argon adsorption in cylindrical pores. However, for methanol adsorption different features appear because the electrostatic interactions favour configurations that maximize the hydrogen bonding among methanol molecules. We have found zigzag chains with hydrogen-bonded structures for methanol adsorption in cylindrical and slit pores. To investigate how dense the adsorbed phase is and how many molecules could be packed per unit physical volume of the solid, we consider two different definitions of pore density; one based on the physical volume and the other on the accessible volume. That based on accessible volume gives a measure of the fluid density, while that based on the physical volume gives a measure of how much adsorbate can be stored per unit volume of the adsorbent. It is found that the adsorbate is denser in cylindrical pores, but that slit pores can pack more molecules per unit solid volume. We also discuss the effects on the isosteric heat of argon and methanol of pore size, pore geometry and loading.

Capillary condensation of adsorbates in porous materials.

Hysteresis in capillary condensation is important for the fundamental study and application of porous materials, and yet experiments on porous materials are sometimes difficult to interpret because of the many interactions and complex solid structures involved in the condensation and evaporation processes. Here we make an overview of the significant progress in understanding capillary condensation and hysteresis phenomena in mesopores that have followed from experiment and simulation applied to highly ordered mesoporous materials such as MCM-41 and SBA-15 over the last few decades.

Characteristics and humidity control capacity of activated carbon from bamboo.

Activated carbons were prepared from bamboo by chemical activation with K2CO3 or physical activation with CO2. The structural and surface chemical characteristics of the activated carbons were determined by N2 adsorption-desorption and Boehm titration, respectively. The water vapor adsorption properties of the activated carbons with various pore structures (preparation conditions) were examined. The relationship between water vapor adsorption capacity and pore properties, and the humidity control capacity of the prepared activated carbons are also discussed. The water adsorption isotherms show a region of rapidly increasing uptake of water vapor, and the relative humidity corresponding to those regions was different according to the preparation conditions, especially activation temperature. Water vapor adsorption capacity was improved with larger pore volume and surface area, but the humidity control capacity in a certain specific humidity region differed greatly according to the relative humidity corresponding to the steeply rising regions of the isotherms. In the typical operating conditions of an adsorption heat pump, RH 10-35%, the bamboo-sourced activated carbon that was prepared at 873K by potassium carbonate activation with impregnation ratio 1.0 had the highest humidity control capacity.

Spectroscopic study for photocatalytic decomposition of organic compounds on titanium dioxide containing sulfur under visible light irradiation.

Yellowish S-containing TiO2 (S-TiO2) powders were prepared by calcination of a mixture of titanium(III) chloride and ammonium thiocyanate solutions. Three kinds of S-TiO2 were prepared by varying the concentration of ammonium thiocyanate (0.5, 1 or 13 M). X-ray photoelectron spectroscopy spectra of the S-TiO2 showed that sulfur atoms existed on the surface of TiO2 powders. But the peaks assigned to S disappeared after Ar+ etching, which means that these atoms were not doped in the bulk of the TiO2 powders. While UV-visible absorption spectra of S-TiO2 showed that the absorption edges of these photocatalysts were seen to shift to a longer wavelength (lower band gap energy) than those of undoped rutile TiO2 prepared and commercial anatase type TiO2 (ST-01). The S-TiO2 (1 M) exhibited higher photocatalytic activity than ST-01 for degradation of methylene blue in aqueous solution under visible light irradiation (lambda > 400 nm). It was also confirmed by IR spectroscopy that acetaldehyde in oxygen under visible light irradiation (lambda > 400 nm) was decomposed to acetic acid by the S-TiO2 and ST-01 at the first decomposition step.