Timothy J. Mays

Structured carbon adsorbents from clay, zeolite and mesoporous aluminosilicate templates

Abstract Porous carbons templated from inorganic materials such as zeolites and mesoporous molecular sieves received considerable attention in the last decade. In this context, we discuss the effects of different templating structures on the carbons yielded. We describe templating using a commercial cationic clay (Wyoming bentonite), a commercial zeolite (Tosoh H-Beta) and a synthetic mesoporous aluminosilicate (Al-MCM-48), as exemplars of the categories of inorganic templates we have studied. The main carbon precursor used was furfuryl alcohol, complemented in some materials by an additional treatment using propylene carbon vapour deposition. The structures of the templates and carbons were compared using electron microscopy and powder X-ray diffraction. Carbons were further characterised with elemental analysis and nitrogen adsorption. The templating process is more complex than previously reported; the combination of these different techniques enhances the understanding of its mechanisms. A careful study of the synthesised carbons’ pore size distributions using DFT with various pore geometries was carried out and comparison with two commercial carbon adsorbents made, in order to assess the potential of such templated carbons for gas separation and gas storage.

Theoretical and experimental studies of methane adsorption on microporous carbons

Abstract GCEMC molecular simulations of methane adsorption in model slit-shaped carbon pores show that variations in the density of carbon atoms in the pore wall have a significant influence on adsorbed methane density, although variations in inter-layer spacing and the number of layer planes in the pore wall, n, (n > 2) have little effect. A model is proposed for calculating the influence of pore wall thickness on the stored volumetric methane capacities of a void-free microporous carbon monolith and model monoliths formed from close-packed powders and fibers. Volumetric methane capacities for the models were in reasonable agreement with measured values for carbon monoliths made from KOH-activated meso-carbon microbeads and for compacted activated carbon fibers.

Preparation and characterisation of chemisorbents based on heteropolyacids supported on synthetic mesoporous carbons and silica

Abstract The preparation of chemisorbents based on tungsto- and molybdophosphoric acids supported on two types of synthetic mesoporous carbons and two types of mesoporous silica is described. Strong solid acids with good accessibility to acid sites may potentially be effective adsorbents for the removal of basic molecular impurities, such as amines, from ultrapure manufacturing environments. Prepared materials were characterised by scanning electron microscopy, nitrogen adsorption, Fourier-transform infrared spectroscopy, powder X-ray diffraction, and equilibrium ammonia uptake. Composites of SBA-15 with heteropolyacids were synthesised. It was shown that the inclusion of HPAs into SBA-15 results in the loss of long range order. Adsorbents based on the HPAs impregnated into the supports with the open-pore morphology (Novacarb and SBA-15) were found to be promising materials. A composite of tungstophosphoric acid with sol–gel SiO2 was found to have the highest ammonia uptake.

Comparison of adsorption methods for characterizing the microporosity of activated carbons

Adsorption methods for characterizing the microporosity of activated carbons are discussed critically. Three methods—the αs-method and those based on the Dubinin-Radushkevich and Jaroniec-Choma isotherm equations—are compared with respect to the parameters that characterize the microporous structure of a solid. It is shown that the isotherm equations that account for the structural heterogeneity of activated carbons give values of the micropore volume similar to that obtained by the αs-method.

Computer simulations of adsorption processes in carbonaceous adsorbents

The increasing availability of powerful computers has resulted in a rapid increase in computer simulations of adsorption processes in carbonaceous adsorbents. These simulations, which are based on assumed intermolecular potential functions applied to an adsorbate in a model carbon pore, provide insights into adsorption in carbon micropores that are not available from other methods. In this paper a model carbon pore is described that takes account of the disordered microstructure of carbonaceous adsorbents. The capabilities of the computer simulations are illustrated using two examples: the determination of pore size distributions and the estimation of methane storage capacity in carbonaceous adsorbents.

Direct Evidence for Solid-like Hydrogen in a Nanoporous Carbon Hydrogen Storage Material at Supercritical Temperatures.

Here we report direct physical evidence that confinement of molecular hydrogen (H2) in an optimized nanoporous carbon results in accumulation of hydrogen with characteristics commensurate with solid H2 at temperatures up to 67 K above the liquid-vapor critical temperature of bulk H2. This extreme densification is attributed to confinement of H2 molecules in the optimally sized micropores, and occurs at pressures as low as 0.02 MPa. The quantities of contained, solid-like H2 increased with pressure and were directly evaluated using in situ inelastic neutron scattering and confirmed by analysis of gas sorption isotherms. The demonstration of the existence of solid-like H2 challenges the existing assumption that supercritical hydrogen confined in nanopores has an upper limit of liquid H2 density. Thus, this insight offers opportunities for the development of more accurate models for the evaluation and design of nanoporous materials for high capacity adsorptive hydrogen storage.

CHAPTER 3 – Active Carbon Fibers

This chapter focuses on the pore and surface structures of carbon fibers, especially those fibers that have been treated, or activated, so that they contain a large number of pores narrower than 50 nm (micropores and mesopores). These active carbon fibers (ACF) are of increasing interest as adsorbents and catalyst supports, in competition with the more traditional, particulate forms of active carbons (PAC). Compared with PAC, there is improved access of adsorptive or reactive fluids to pores and active surface sites in ACF, together with generally higher pore volumes and surface areas. ACF can also be consolidated into a wide range of textiles, felts, and composites, allowing greater flexibility in the forms of materials based on ACF, and the ease (and hence low cost) with which they may be contained and handled compared with PAC. Other advantages are that materials based on ACF do not suffer from channeling, settling or attrition to the same extent as PAC packed in beds or columns.

Supercritical hydrogen adsorption in nanostructured solids with hydrogen density variation in pores

Experimental excess isotherms for the adsorption of gases in porous solids may be represented by mathematical models that incorporate the total amount of gas within a pore, a quantity which cannot easily be found experimentally but which is important for calculations for many applications, including adsorptive storage. A model that is currently used for hydrogen adsorption in porous solids has been improved to include a more realistic density profile of the gas within the pore, and allows calculation of the total amount of adsorbent. A comparison has been made between different Type I isotherm equations embedded in the model, by examining the quality of the fits to hydrogen isotherms for six different nanoporous materials. A new Type I isotherm equation which has not previously been reported in the literature, the Unilan-b equation, has been derived and has also been included in this comparison study. These results indicate that while some Type I isotherm equations fit certain types of materials better than others, the Tόth equation produces the best overall quality of fit and also provides realistic parameter values when used to analyse hydrogen sorption data for a model carbon adsorbent.

A new methodology for designing hydrogen energy storage in wind power systems to balance generation and demand

Wind power is one of the cleanest and safest of all the renewable commercial methods of generating electricity. However, wind energy is difficult to use due to its stochastic variability. Energy storage can overcome the main drawback, allowing energy production to match energy demand. In this paper, new hydrogen storage systems are investigated as a way to solve this problem. A new methodology, based on the differences between wind power generation and load variability, was developed in this study to optimize the technology, energy capacity and power transfer of the hydrogen energy storage method for specified applications. The dynamics of the complete hydrogen cycle energy storage and recovery mechanism was investigated, specifically for potential applications such as power smoothing and peak lopping. A time dependant model of the efficiency of various hydrogen storage technologies, including high pressure compression, low temperature liquefaction, metal hydrides and complex hydrides, has been developed. Based on this study, a practical hydrogen energy storage system for a 5MW micro-grid application was designed. The micro-grids electrical load requirements and the available wind energy in the local area are used as an example to demonstrate the new methodology. Using these results, the paper demonstrates that a practical system to provide hydrogen energy storage within this micro-grid using high temperature water electrolysis and high pressure compression tank storage is the best way forward.

Isosteric enthalpies for hydrogen adsorbed on nanoporous materials at high pressures

A sound understanding of any sorption system requires an accurate determination of the enthalpy of adsorption. This is a fundamental thermodynamic quantity that can be determined from experimental sorption data and its correct calculation is extremely important for heat management in adsorptive gas storage applications. It is especially relevant for hydrogen storage, where porous adsorptive storage is regarded as a competing alternative to more mature storage methods such as liquid hydrogen and compressed gas. Among the most common methods to calculate isosteric enthalpies in the literature are the virial equation and the Clausius–Clapeyron equation. Both methods have drawbacks, for example, the arbitrary number of terms in the virial equation and the assumption of ideal gas behaviour in the Clausius–Clapeyron equation. Although some researchers have calculated isosteric enthalpies of adsorption using excess amounts adsorbed, it is arguably more relevant to applications and may also be more thermodynamically consistent to use absolute amounts adsorbed, since the Gibbs excess is a partition, not a thermodynamic phase. In this paper the isosteric enthalpies of adsorption are calculated using the virial, Clausius–Clapeyron and Clapeyron equations from hydrogen sorption data for two materials—activated carbon AX-21 and metal-organic framework MIL-101. It is shown for these two example materials that the Clausius–Clapeyron equation can only be used at low coverage, since hydrogen’s behaviour deviates from ideal at high pressures. The use of the virial equation for isosteric enthalpies is shown to require care, since it is highly dependent on selecting an appropriate number of parameters. A systematic study on the use of different parameters for the virial was performed and it was shown that, for the AX-21 case, the Clausius–Clapeyron seems to give better approximations to the exact isosteric enthalpies calculated using the Clapeyron equation than the virial equation with 10 variable parameters.