J.B. Parra

Sol-gel method for preparing high surface area CoAl2O4 and Al2O3-CoAl2O4 spinels

Abstract Cobalt aluminate, CoAl 2 O 4 , and Al 2 O 3 –CoAl 2 O 4 solid solutions (at Co:Al=1:4, and Co:Al=1:8) were prepared by hydrolysis of mixed metal alkoxides, followed by mild calcination of the resulting gels. Powder X-ray diffraction showed that all samples prepared were single phase materials having the spinel-type structure. The cubic lattice parameter, a 0 , was found to change gradually with aluminium content of the mixed metal oxides. Nitrogen adsorption–desorption isotherms, at 77 K, were used to determine surface area and pore texture. The BET surface area was found to be 235 m 2 g −1 for CoAl 2 O 4 , and higher values (up to 365 m 2 g −1 ) were found for the Al 2 O 3 –CoAl 2 O 4 solid solutions. Samples having the same composition were also prepared by the classical ceramic method (starting from mechanical mixtures of the parent metal oxides); for these materials the corresponding surface areas were found to be in the range 1.7–3.3 m 2 g −1 . The sol–gel method, starting from mixed metal alkoxides, was thus shown to be a convenient route to prepare cobalt aluminate spinels in a very high surface area form.

Influence of oxygen-containing functional groups on active carbon adsorption of selected organic compounds

Abstract Two commercial activated carbons (AC) were treated with HNO3 and (NH4)2S2O8. The changes in surface area and pore size distribution resulting from the oxidising treatments were studied by means of nitrogen adsorption isotherms. The modifications in the surface chemistry were evaluated by means of the point of zero charge (PZC) and the oxygen content. Selective removal of oxygen complexes by heating under N2 and CO2 flow at 723 and 1123 K was also studied. The treatment with HNO3 was found to broaden the microporosity and mesoporosity of the carbons when low concentrations of the oxidising agent were employed. Severe oxidation conditions resulted in an almost total destruction of the texture of the carbons. The point of zero charge values changed systematically with the extent of the oxidation; the more oxidised the carbon, the lower its point of zero charge. Thermal treatment at high temperatures leads to an enhancement of the carbon basicity and a slight decrease in the textural properties of the original carbons.

Influence of pyrolysis temperature on char optical texture and reactivity

In this study a set of 10 coals varying in rank and maceral composition has been devolatilised in a flat flame burner at 1000 and 1300°C in order to investigate the changes in char reactivity and structure as a function of pyrolysis temperature. The intrinsic reactivity of the chars, measured isothermally in a thermobalance at 500°C, has been related to the petrographic characteristics of the chars and their CO2 surface areas. The temperature of pyrolysis has shown to have a strong effect on char reactivity for certain coals. Overall, the increase of temperature provoked an enhanced plasticity and a more extensive consolidation during the metaplast stage of pyrolysis and reduced the amount of unfused material in the chars. This effect was more pronounced in low volatile bituminous vitrinites and low rank inertinites. The presence of inertinite derived materials in high rank coal chars enhanced their reactivities, whereas the opposite was observed for low rank coal chars. This was attributed to the fact that inertinite macerals yield both isotropic and anisotropic materials: inertinite in low rank coals increases the amount of anisotropic, less reactive char, but reduces it in high rank coals.

Kinetics of naphthalene adsorption on an activated carbon: comparison between aqueous and organic media.

The purpose of this work was to explore the kinetics of naphthalene adsorption on an activated carbon from aqueous and organic solutions. Kinetic curves were fitted to different theoretical models, and the results have been discussed in terms of the nature and properties of the solvents, the affinity of naphthalene to the solutions, and the accessibility to the porosity of the activated carbon. Data was fitted to the pseudo-second order kinetic model with good correlation coefficients for all the solution media. The faster adsorption rate was obtained for the most hydrophobic solvent (heptane). The overall adsorption rate of naphthalene seems to be controlled simultaneously by external (boundary layer) followed by intraparticle diffusion in the porosity of the activated carbon when water, ethanol and cyclohexane are used as solvents. In the case of heptane, only two stages were observed (pore diffusion and equilibrium) suggesting that the limiting stage is the intraparticle diffusion. The low value of the boundary thickness supports this observation.

High surface area nickel aluminate spinels prepared by a sol–gel method

Abstract The oxide spinel NiAl 2 O 4 and spinel-type solid solutions Al 2 O 3 –NiAl 2 O 4 (at Ni/Al=1:4, and Ni/Al=1:8) were prepared by controlled hydrolysis of mixed metal alkoxides, followed by calcination of the resulting gels. Powder X-ray diffraction showed that all samples prepared were single phase cubic materials having the spinel-type structure. The cubic lattice parameter, a o , was found to decrease gradually with increasing aluminium content of the mixed metal oxides. The specific surface area (determined by nitrogen adsorption at 77 K) was found to be in the range of 200–300 m 2 g −1 . The materials were found to be basically mesoporous, the most frequent pore radius being in the range 3.2–6.4 nm. IR spectroscopy of CO adsorbed at liquid nitrogen temperature gave a main band at 2186–2195 cm −1 , which was assigned to the CO stretching vibration of surface Al 3+ ⋯CO adducts where coordinatively unsaturated Al 3+ ions act as Lewis acid centres.

Effect of texture and surface chemistry on adsorptive capacities of activated carbons for phenolic compounds removal

Three microporous commercial activated carbons (AC) were used for the adsorption of phenol and salicylic acid from aqueous solution. Equilibrium adsorption data were obtained by the bottle-point technique. Long periods of time were needed to reach the equilibrium between the active carbon and the aqueous solution. Adsorptive capacities obtained from the isotherms were calculated following the ASTM 3860 procedure. Carbons with a high oxygen content were found to present lower adsorptive capacities for both phenol and salicylic acid. Column tests were also used for the uptake of phenol and salicylic acid, and adsorptive capacities were calculated from the breakthrough curves. Adsorptive capacities calculated from the breakthrough curves were found to be significantly lower than those evaluated from the equilibrium isotherms.

Effect of gasification on the porous characteristics of activated carbons from a semianthracite

Abstract Activated carbons with degrees of burn-off between 20% and 80% have been prepared from a semianthracite char by gasification in steam at 1123 K. The adsorption isotherms of nitrogen at 77 K and CO2 at 273 K have been determined. The pore size distribution has been calculated from t plots, and helium and mercury densities. The ungasified char has a narrow pore size distribution and mainly consists of very fine microporosity. The pores have constrictions that do not allow adsorption of nitrogen, but the microporosity can be measured by the adsorption of CO2 at 273 K. Smic and CO2 (D-R) surface areas of the ungasified char are about the same. The gasification of the char broadens the micropore distribution. The small microporosity calculated from t plots increases with burn-off between 20%–35%. Gasification to higher degrees of burn-off causes burning of the walls between the adjacent pores and eliminates pore constrictions. This results in widening the micropores, creating meso and macroporosity and decreasing microporosity. When the degree of gasification is 80%, the small microporosity disappears almost completely, although the total porosity has been increased almost three times. While the nitrogen surface area increases with burn-off, the CO2 surface area decreases at higher degrees of burn-off.

Effect of coal preoxidation on the development of microporosity in activated carbons

Abstract Activated carbons have been prepared from a semianthracite preoxidised in air to different degrees. The activation has been carried out in steam at 850 °C to 50 ± 1% burnoff. The adsorption isotherms of nitrogen at 77 K and CO2 at 273 K have been determined. The adsorption isotherms indicate that the activated carbons obtained from non-oxidised coal has a poor porosity. The porosity increases with increase in the degree of coal preoxidation. The BET surface area increases from 160 m2g−1 in the case of activated carbon obtained from nonoxidised coal char to 847 m2g−1 for the activated carbon from oxidised coal char, t-plots obtained show that the degree of oxidation enhances both small and medium microporosity, the increase in small microporosity being much larger. The activated carbons obtained from preoxidised coal chars are largely microporous in character.

Characterization of activated carbons by the BET equation: an alternative approach

The adsorption isotherms of nitrogen at 77 K have been determined on two series of activated carbons and two samples of activated carbon fibres. The carbons were highly microporous and contained varying amounts of different forms of microporosity. Characterization of the carbons was undertaken using an alternative linear form of the BET equation and the results of the BET parameters obtained have been compared with those obtained from the usual classical linear form of the equation. The surface areas calculated from the two linear forms were similar while the C values were quite different. The C values obtained from the alternative linear form appeared to be more reasonable, being consistent with and related to the microporous character of these carbons. The alternative linear form gave BET plots which were more sensitive to deviations from linearity allowing the relative pressure range for the application of the BET equation to be determined without any ambiguity and more precisely.

Preparation of active carbons from coal: Part III: Activation of char

Active carbons with a burn-off of 52% have been prepared from four coals of different rank and origin after preoxidation to different degrees at 543 and 473 K, and further carbonization at 1123 K. The activation has been carried out with CO2 at 1123 K at two flow rates viz. 7 cm3 min−1 and 500 cm3 min−1. Active carbons have also been prepared from a preoxidized coal by activation to different degrees of burn-off between 10 and 80%. The effect of the degree of oxidation, the flow rate of the activating gas and the extent of burn-off on the porous structure development of active carbons has been examined. The active carbons prepared from unoxidized coal have poor textural characteristics (porosity, N2 and CO2 surface area). Nevertheless, the textural characteristics are enhanced as the degree of preoxidation of the coal is increased. The low flow rate of CO2 (activating gas) produces active carbons with a better microporous character. The degree of activation (the extent of burn-off) of the carbon determines the porous structure of the active carbon. At low degrees of burn-off (less than 50%) the product is largely microporous. At higher degrees of burn-off between 35–65% the product has a mixed porous structure and contains all types of pores. Active carbons with a given textural character can be obtained by controlling the degree of oxidation of coal and the degree of activation of the carbonized material.