Harry Marsh

Phenomena of catalytic graphitization

In recent years the phenomena of catalytic graphitization have developed considerably. Four types of catalytic graphitization are known to produce G-, Ts-, A- and Tn- components. The review summarizes the use of elements, alloys and compounds as catalysts. The importance of catalyst particle size is stressed as well as the method of addition of the catalyst to the carbon. Extents of graphitization induced by catalysts are markedly dependent upon the existing degree of graphitization already present in the parent carbon. The effects of graphitization at different temperatures are summarized as well as the effects caused by the ambient atmosphere, for example by oxygen and nitrogen. Mechanisms of catalytic graphitization resulting in G-, Ts-, A- and Tn- components are outlined and changes in synthetic graphites caused by catalytic graphitization are presented.

Adsorption methods to study microporosity in coals and carbons—a critique

Abstract This article discusses problems of the interpretation of adsorption data from microporous coals and carbons. Structure in coals and carbons is briefly reviewed and ultramicroporosity, microporosity and supermicroporosity are defined. Problems associated with activated diffusion, molecular sieve effects, cooperative adsorption effects, the use of polar adsorbates and the understanding of limitations and value of theoretical adsorption equations are assessed. The Dubinin-Radushkevich and Dubinin-Astakhov equations appear superior to Langmuir or BET equations in terms of differentiating and describing the microporosities of coals and carbons. Their critical use is recommended.

Formation of active carbons from cokes using potassium hydroxide

Petroleum cokes of different optical textures, a metallurgical coke and charcoals from wood were reacted with KOH in the temperature range 823–1173 K. Resultant active carbons were examined by optical and scanning electron microscopy, by assessment of effective surface areas using N2 and I2 as adsorbates and by IR spectroscopy. Green cokes rather than calcined cokes, of optical texture of mosaics were most effective in formation of active carbons, S.A. = 2700 m2g−1. A ratio of KOH:C of 4:1 was needed to obtain maximum surface areas at reaction temperatures 1.25 hr. The wood-charcoals could be effectively activated. KOH was the most effective of the alkali salts in the production of active carbons.

The characterization of microporous carbons by means of the dubinin-radushkevich equation

Abstract The Dubinin-Radushkevich (D-R) equation of adsorption predicts a Rayleigh distribution of adsorption free energy with adsorption (micropore) volume. Only when this distribution is present in microporous solids will a completely linear D-R plot result. Adsorption data of carbon dioxide at 195°K and nitrogen and argon at 77°K on various microporous polymer carbons are interpreted in terms of the D-R equation and distributions of free energy with pore volume. In no case is the complete Rayleigh distribution found to apply and corresponding deviations from linear D-R plots occur. Highly activated carbons show a bimodal distribution of free-energy values to nitrogen and argon at 77°K, but not to CO 2 at 195°K. Tentative suggestions for this behavior are presented.

Carbonization and liquid-crystal (mesophase) development: Part 1. The significance of the mesophase during carbonization of coking coals

Abstract Recent concepts of the growth processes of liquid-crystal structures, also called mesophase systems, during the carbonization of pitch substances is extended to coal carbonization. A basic model is formulated to explain the coal and coal-blend carbonization processes leading to metallurgical coke. This model explains the differences observed by optical microscopy in the size and shape of anisotropic structures seen in cokes in terms of liquid-crystal growth processes. These processes are considered to be restricted by chemical heterogeneity within the plastic phase, or to be influenced by the presence of solid surfaces (inerts) within the carbonizing system.

Carbons of high surface area. A study by adsorption and high resolution electron microscopy

Five carbons of high surface area, ~ 2000–3000 m2g−1 are studied by adsorption of carbon dioxide at 195 and 273 K. Effective surface areas are calculated using Langmuir and Dubinin- Radushkevich equations. Structure in these carbons is assessed by phase contrast high resolution transmission electron microscopy. The lamellae or constituent layers of these carbons are resolved as fringe images. Careful examination of thin sections of these carbons shows significant differences in separation distances of lamellae which indicate differences in the size and shape of the supermicroporosity which exists as the space between the lamellae. These differences correlate closely with the effective surface areas. The supermicroporosity consists of cage-like voids 1–5 nm dia., the cages being separated by walls of 1–3 carbonaceous layers in thickness. The filling of such supermicroporosity by a mechanism of increasing adsorption potential or cooperative adsorption adequately accounts for high internal volumes of up to 1.7cm3g−1 and of effective surface areas of about 3000 m2g−1. The size and shape of supermicroporosity can be deduced from micrographs.

Reactivity of coal macerals and lithotypes

Abstract Variations in the rank of coal used for power generation can cause problems with the efficiency of combustion. This paper describes an examination of the effects of variations in the maceral composition of coals, in association with rank, upon reactivity in combustion. Using a simple reactivity test, the reactivity ranges for two rank series of single seam coals were determined along with the reactivity ranges obtained from studying lithotypes of various maceral concentrations of a single coal. Reactivity tests on pure maceral concentrates show significant reactivity differences between macerals so that coal type (maceral/lithotype content) must be considered as important a parameter as rank. Changes in maceral concentration alone could account for 45% of the rank reactivity variation without changing rank.

Carbonization and liquid-crystal (mesophase) development. 15. A common stage in mechanisms of coal liquefaction and of coal blends for coke making

Abstract This paper discusses the processes of coal liquefaction and co-carbonization of coal/pitch blends in terms of physical and chemical properties of the fluid phases found in both pyrolysis systems. Mechanisms of development of thermal plasticity in coals are outlined. In coal liqudfaction the importance is stressed of hydrogen-donor vehicles interacting with the products of thermal depolymerization of coal. The concept of variations in the facility of solvation and solvolysis of additives in co-carbonizations can explain the variations observed in degrees of interaction of a single coal with several additives. Possibly, the hydrogen-donor facility of an additive may be as important in assessments of modifying ability as an average molecular structure. The possibility exists of using an analysis of optical texture of cokes resulting from the fluid coal/solvent pyrolysis systems to characterize the effectiveness of solvents in coal liquefaction systems as distinct from coal blending co-carbonization studies.

Inhibiting effect of incorporated nitrogen on the oxidation of microcrystalline carbons

Abstract Nitrogen-containing carbons were prepared by carbonization of sucrose with additions of urea, glucosamine hydrochloride, uracil, hydroxymethyl pyridine, or picolylamine. The development of their nitrogen content during charring, carbonization at 1100°C, and subsequent activation with CO2 at 850°C was followed by chemical analysis as a function of the quantity of nitrogen additive. The time needed to reach a constant burn-off during activation with CO2 was considerably longer than with a nitrogen-free sucrose carbon. A similar inhibition of oxidation was also observed in the oxidation with 5% O2-95% Ar. Kinetic data were obtained from thermogravimetric experiments. The nitrogen contained in the carbons was liberated during combustion largely as NOx.

The process of activation of carbons by gasification with CO2-II. The role of catalytic impurities

Abstract The effect of iron and nickel impurities on the development of microporosity during the gasification of 850°C polyfurfuryl alcohol carbon with carbon dioxide has been investigated. The impurities, initially added as an atomic dispersion, agglomerate during carbonisation and gasification. Gasification by carbon dioxide is catalysed only in the vicinity of the metal agglomerate, resulting in the development of macro- and transitional pores with little change in micropore structure. The metal becomes inactive, catalytically, by conversion to the oxide when subsequent uncatalysed gasification changes the micropore structure. In the nickel impregnated carbon the microporosity, developed after the catalytic reaction, is similar to that developed in the pure carbon. However, a larger amount of transitional and macroporosity is present in the iron impregnated carbon. Thus, the subsequent development of microporosity, when the iron is inactive, is affected by these larger pores and does not follow the same course as that of the pure carbon.