María Domingo-García

Modifications produced by O2 plasma treatments on a mesoporous glassy carbon

A glassy carbon, P3, obtained by carbonization of polyfurfuryl alcohol was used to study the changes produced by oxygen plasma treatments which were compared to those obtained by molecular oxygen. The sample was selected for its well defined mesoporosity. The aim was to study the influence of porosity on the reactivity of the sample in oxygen plasma. The results were compared to previously reported data obtained using a glassy carbon with no macro or mesoporosity. It is concluded that the influence of mesoporosity is almost negligible in such a way that only the external surface was affected by plasma treatment.

Modifications produced by O2 and CO2 plasma treatments on a glassy carbon : comparison with molecular gases

Abstract A carbon material obtained by pyrolysis of polyfurfuryl alcohol was submitted to four different treatments. Two of these were carried out using molecular gases (oxygen and carbon dioxide) and two using plasma of the same molecules. The textural and chemical modifications produced by these treatments were studied by N2 and CO2 adsorption at 77 and 273 K, respectively, mercury porosimetry and Fourier Transformed Infrared Spectroscopy (FTIR). In addition, the changes in the external surface produced by the treatments were observed by Scanning Electron Microscopy (SEM). Molecular carbon dioxide treatment opens and creates microporosity. However, plasma treatments mainly produce constrictions at the entrance of the micropores. The effect of the molecular treatments on the external surface is not easily observed, while plasmas treatments produce pits on the surface. The statistical analysis of these have been obtained in order to establish their evolution.

Gas chromatographic determination of adsorption isotherms, spreading pressures, london force interactions and equations of state for n-alkanes on graphite and carbon blacks

Adsorption isotherms were obtained for C6C9n-alkanes on a graphite and two carbon blacks at 363°K by gas chromatography using the peak maximum elution method. The surface areas of the adsorbents were obtained by applying the Brunauer—Emmett—Teller equation, and found to depend on the hydrocarbon used. Increasing differences from that found with nitrogen at 77°K were observed with increasing chain length of the hydrocarbon. This can be attributed to adsorbate—adsorbate lateral interactions. The London component of the surface free energy of the solid, vL2, was calculated from the spreading pressure. The values obtained show that the homogeneity of the solids increases in the order V3 < V3G < Acheson graphite. On the other hand, for the same adsorbent, vL2 depends on the hydrocarbon used. Equations of state were also determined. It was found that for a spreading pressure below 2 mN m−1 the n-alkanes obey the ideal equation, but spreading pressures above 2 mN m−1 the behaviour of the hydrocarbons increasingly deviates from that of ideality as the spreading pressure and chain length of the n-alkane increase. This is attributed to the increase in adsorbate—adsorbate lateral interactions.

Dynamic adsorption of ammonia on activated carbons measured by flow microcalorimetry

Several activated carbons of different textural and chemical surface characteristics have been used to study the ammonia adsorption. The textural characteristics were determined by N2 and CO2 adsorption at 77 and 273 K. The chemical surface groups were estimated by thermal programmed desorption followed by mass spectrometry (TPD-MS) and by selective titrations in aqueous solutions. The ammonia adsorption was studied under dynamic conditions from N2 diluted flow using a flow adsorption microcalorimeter. The ammonia adsorption consists of reversible and irreversible components. The former is assigned to physisorption process while the latter is adsorption on chemical groups. The plots of the differential heats of adsorption versus the cumulative adsorption point to the existence of a wide distribution of acid sites some of which are very strong. However, they are not always easily accessible to NH3 because constrictions in the pores-network hinder the access which forces to re-arrangement.

The dynamic adsorption of several hydrocarbons on active carbons

Abstract The adsorption of several hydrocarbons on different active carbons prepared from agricultural byproducts has been studied at high temperatures using a gas-solid chromatographic technique. The thermodynamic functions of adsorption were determined and the differences in the heats of adsorption have been used to evaluate porosity of the samples. An attempt to determine the London component of surface free energy of the active carbons has been made.

A gas-solid chromatographic study of the adsorption of hydrocarbons and alcohols on graphite

SummaryThe Henrys law region of the adsorption isotherms for several hydrocarbons and alcohols on a graphite was studied by gas-solid chromatography. The thermodynamic functions at zero surface coverage were determined. The low heats of adsorption indicate a highly homogeneous surface. A constant increment of the standard free energy with the number of carbon atoms in the lineal adsorbates was found. Attemps were made to estimate the London contribution to the surface free energy of the adsorbents. A non-specific interaction between the graphite and the adsorbates was shown.

Adsorption of hydrocarbons on graphites and graphitized carbon black at zero surface coverage

Abstract The Henrys law region of the adsorption isotherms for a homologous series of n -alkanes on graphites and a graphitized carbon black was studied by gas chromatography. The three thermodynamic functions at zero surface coverage were determined. A linear relationship between the standard free energy, the enthalpy and the entropy and the number of carbon atoms was found. The low heats of adsorption at zero coverage indicated a virtual absence of high-energy sites. Attempts were made to estimate the London contribution to the surface free energy of the adsorbents; it was estimated to be 100 mN m −1 for the graphitized carbon black and 80 mN m −1 for the graphites.

Activated carbons as supports for nickel catalysts

Abstract Several activated carbons, prepared using Ni(NO 3 ) 2 ·6H 2 O and Ni(CO) 4 as precursors, are used as supports for nickel catalysts. The influence of the textural characteristics of the supports (surface area, porosity and chemical surface groups) on metal dispersion and the influence of the preparation method, nickel content and reduction conditions (heating rate, temperature and period of reduction) on the characteristics of the catalysts are studied. Finally, the activity of these catalysts in n-butane hydrogenolysis is tested.

Evaluation of the dispersive component of the surface energy of active carbons as determined by inverse gas chromatography at zero surface coverage

The methods to obtain the dispersive component of the surface energy (gamma(s)(d)) of active carbons (AC) from inverse gas chromatography (IGC) measurements usually render values much higher than those obtained by other techniques. In this paper this is ascribed to two factors: (i) the high temperatures at that IGC measurements are carried out and (ii) the microporosity of the AC. It is shown that the temperature dependence of the area of the methylene group is an important factor in the high gamma(s)(d) values. Thus, corrections for this dependence should be considered in the calculations. In relation to microporosity, the cooperative effect of the pore walls is also an important factor to be considered in the evaluation of gamma(s)(d). The values gamma(s)(d) obtained after these corrections have their own physical meaning related to ideal flat carbon surfaces. Critical comments are made about some reported relationships between gamma(s)(d), obtained from IGC, and the BET surface area or pore volume of AC as determined from nitrogen adsorption at 77K. These are based on the very different experimental conditions at which nitrogen and IGC measurements are carried out.

Carbon molecular sieves produced by the fixation of sulphur surface complexes

SummaryA Saran carbon has been treated with carbon disulphide and sulphur in order to introduce sulphur surface complexes, a commercial activated carbon was also used. Samples obtained were characterized by their surface areas and porosities. The adsorption of n-hexane, benzene and cyclohexane has been measured out at high temperatures and under dynamic conditions using a gas chromatographic technique. Results were compared with those obtained when oxygen-surface complexes were introduced and it can be concluded that when CS2 is used with a highly microporous carbon, such as Saran, a molecular sieve for cyclohexane (mean size 0.58 nm) is produced at determined amounts of sulphur fixed.