Deirdre Hugi-Cleary

The comparison of experimental and calculated pore size distributions of activated carbons

Abstract Activated carbons are disorganized materials with variable pore size distributions (PSD). If one assumes that the porosity consists mainly of locally slit-shaped micropores, model isotherms can be obtained by computer simulations and used to assess the PSD on the basis of experimental isotherms. In the present study, CO2 isotherms have been measured at 273 K on seven well-characterized microporous carbons with average micropore widths between 0.65 and 1.5 nm and analysed with model isotherms obtained with standard Monte Carlo simulations. The resulting PSD are in good agreement with those obtained from a modified Dubinin equation, from liquid probes of molecular dimensions between 0.4 and 1.5 nm, from STM and from modelling based on CH4 adsorption at 308 K. The present study validates the determination of micropore distributions in active carbons based on CO2 isotherms, provided that no gate effects are present.

The characterization of microporosity in carbons with molecular sieve effects

The apparent and the real micropore size distributions (PSDs) of molecular sieve carbons can be assessed by combining the adsorption of CO2 at 273 K with immersion calorimetry into liquids of increasing molecular dimensions. On the basis of model isotherms resulting from computer simulations, the adsorption of carbon dioxide, a relatively small probe, leads to the overall PSD of the carbon (essentially the internal micropore system). Immersion calorimetry, on the other hand, reveals the distribution of the pores accessible directly from the liquid phase, that is without constrictions. Liquid CS2 probes the same volume as CO2 and can be used as a reference. The paper describes the case of an industrial molecular sieve carbon obtained by blocking partly the entrance to a relatively broad micropore system, thus limiting its accessibility to molecules with diameters below 0.5–0.6 nm. It is shown how activation by steam at 900 °C removes the constrictions and leads to a gradual overlap of the two PSDs. The distribution of the pore widths on the surface, observed directly by scanning tunnelling microscopy, is also given.

Microporosity in carbon blacks

Microporous carbon blacks can be characterized by the same techniques as activated carbons, using the classical DR equation and comparison plots based on non-porous materials. The CO2 adsorption isotherm at 273 K, combined with computer modelling, also leads to an assessment of microporosity. The results agree with independent techniques such as immersion calorimetry into liquids of variable molecular dimensions and a modified Dubinin equation. The study also confirms that the comparison plots based on N2 (77 K), CO2 (273 K) and C6H6 (293 K) do not necessarily lead to overlapping results for the total micropore volume and the external surface area of the carbons.

Pore size distributions of active carbons assessed by different techniques

(3n 21) n 3 CM (Fig. 1) the frequency of the micropore widths f(L) 5 3W L a exp[2aL ] /G(n) (4) 0 observed by STM on the surface of the solid, DN /DL, is This function, like the distribution based on the DS slightly different from the volumic distribution DW /DL, equation [1,7–9], has a single maximum and it is not since the STM analysis does not take into account the suited for the description of bimodal micropore distribuactual depth of the micropores. However, the two distions, provided that they exist. In the case of carbon CM tributions are related and STM, like TEM, confirms that the pore size distribution Eq. (4) shown in Fig. 1(b), has the micropores of active carbons are locally slit-shaped. been obtained from the adsorption isotherms of CH (253, 4 This, in turn, provides the basic model for computer 273, 308 K) and of CO (253, 273, 298 K) fitted to Eq. 2 simulations. (1). This distribution is in good agreement with the In the case of microporous carbons, it has also been histogram of Fig. 1(a), derived from liquids with molecular possible to derive PSD from adsorption data within the sizes between 0.4 and 1.5 nm. As confirmed independently framework of Dubinin’s theory for the volume filling of by STM, the experimental distribution has no secondary micropores [5,6]. A possible relation is the so-called maximum around 1.5 nm. Dubinin–Stoeckli (DS) equation [1,7,8], which applies to Computer modeling of adsorption and the determination strongly activated carbons, with relatively wide distribuof pore size distributions based on standard isotherms has tions. As reported by Daley at al. [9], good agreement is become increasingly popular and it has been discussed found between the PSD derived from the DS equation and recently [10]. At the present time, modeling is frequently the experimental distribution observed by STM on strongly based on the adsorption of N [11–13], CO [14,15] and 2 2 activated carbons. CH [16–19], but a systematic comparison with pore size 4 Recently [1], a modified Dubinin isotherm has also been distributions obtained by independent techniques is still suggested to obtain PSD in the micropore range, lacking. For example, to our knowledge, no direct com-

On the mechanisms of phenol adsorption by carbons

The removal of phenol and related compounds from dilute aqueous solutions by activated carbons corresponds to the coating of the micropore walls and of the external surface by a monolayer. This process is described by an analog of the Dubinin—Radushkevich—Kaganer equation. On the other hand, as suggested by immersion calorimetry at 293 K, in the case of concentrated solutions, the mechanism corresponds to the volume filling of the micropores, as observed for the adsorption of phenol from the vapor phase. The equilibrium is described by the Dubinin—Astakhov equation. It follows that the removal of phenol from mixtures with water depends on the relative concentrations, and the limiting factor for adsorption is either the effective surface area of the carbon, or the micropore volume.

On the use of standard DRK isotherms in Dubinin's t /F method

Abstract It is shown that the adsorption of benzene, carbon tetrachloride, dichloromethane and nitrogen by a typical non-porous carbon black follows the Dubinin–Radushkevich–Kaganer equation. The requirement for temperature invariance is fulfilled, with an average characteristic energy Eo=10.8 kJ mol−1. This expression is compared with the standard isotherms for benzene and carbon tetrachloride at 293 K proposed by Dubinin and used as a reference in the so-called t/F method, which leads to the non-porous surface area of active carbons. It appears that Dubinin’s isotherm contains inconsistencies, which are compensated for internally. Alternative DRK expressions, applicable to different vapours, are therefore proposed. The present study also shows the limits of Dubinin’s method with respect to comparison plots at higher relative pressures.

Adducts of zirconium and hafnium tetrachlorides with neutral Lewis bases. Part I. Structure and stability: a vibrational and NMR study

Abstract Raman and NMR spectroscopy were used to establish the structure and stability of several MCl 4 · 2Ladducts (M=Zr, Hf; L=neutral Lewis base) in CHCl 3 and CH 2 Cl 2 . Only trans adducts were found for MCl 4 ·2(Me 2 N) 3 PO, cis-trans equilibria were observed for HfCl 4 ·2Cl 2 (Me 2 N)PO, MCl 4 ·2Cl(Me 2 N) 2 - PO and MCl 4 ·2(MeO) 3 PO and only the cis configuration was present for ZrCl 4 ·2Cl 2 (Me 2 N)PO, MCl 4 · 2Cl 2 (MeO)PO and MCl 4 ·2L (L=Me 2 O, Me 2 S, Me 2 Se, Cl 3 PO). 1 H NMR was used to determine the relative stabilities of some of the adducts and the sequence found was: (Me 2 N) 3 PO⪢Cl(Me 2 N) 2 PO > (MeO) 3 PO⪢Cl(MeO) 2 PO>Cl 2 (Me 2 N)PO). The observed structures and stabilities are explained using steric and electronic arguments. A variable pressure study of the cis-trans equilibrium on ZrCl 4 ·2(MeO) 3 - PO yielded the reaction volume, δ V °=+3.8±0.4 cm 3 mol −1

Phenol Adsorption from Dilute Aqueous Solutions by Carbons

The adsorption of phenol from dilute aqueous solutions by seven activated carbons and one non-porous carbon black is reported. It is confirmed that the equilibrium can be described by a modified Dubinin-Radushkevich-Kaganer equation, with exponent n = 4 and Eg = (1.03 ′ 0.18)E o . At low equilibrium concentrations, phenol and its derivatives are adsorbed as monolayers by both non-porous and porous carbons. However, water is preferentially adsorbed on the oxygen-containing surface complexes, which reduces the area available to phenol and its derivatives by 71 m 2 per mmol of surface oxygen.

The characterization of non-porous surfaces by a combination of the BET and the Dubinin-Radushkevich-Kaganer (DRK) theories

The characterization of non-porous surfaces by the BET and the DRK techniques should be complementary. However, the monolayer capacities N a m obtained by these techniques may be different, with trends for the adsorption of simple molecules on a given surface. Assuming that the BET monolayer is correct, its use in the DRK equation leads to exponents n different from the standard value n = 2. This exponent is directly related to the width of Χ(e), the distribution of the gas-solid adsorption energy e for a given system. Therefore, the analysis based on an extended DRK equation leads to information on the heterogeneity of the surface. For example, the data for adsorption of simple molecules physisorbed on a non-porous silica, on the (111) face of rhombic sulphur and on graphitised carbon blacks lead to average exponents of 1.2, 2.0 and 2.2 respectively, with specific scaling factors β. In the case of nitrogen adsorbed on SiO 2 , the value n = 1.8 suggests the presence of specific interactions.

Stereoselectivity in reactions of metal complexes Part XVIII. Kinetics and stereoselectivity in electron-transfer reactions between [Co(L)H2O]+ (L=N,N′-[(pyridine-2,6-diyl)bis(methylene)]bis[proline] (promp) and N, N′-[(4-methoxypyridine-2,6-diyl)bis(methylene)]bis[proline] (MeO-promp) and optically active iron(II) complexes

The new optically active pentadentate ligands N,N′-[(4-methoxypyridine-2,6-diyl)bis(methylene)]bis[(R)- or (S)- proline], (R)- or (S)-MeO-promp, and their CoIII complexes have been synthesised. Electron-transfer kinetics between [Co(L)H2O]+ (L=N,N′-[(pyridine-2,6-diyl)bis(methylene)]bis[proline] (promp) or MeO-promp) and the optically active [Fe(S,S)-L′] (L′=promp, MeO-promp or N,N′-[(pyridine-2,6-diyl)bis(methylene)bis[alanine] (alamp) have been measured by circular dichroism. The observed stereoselectivity is always in favour of the heterochiral diasteroisomeric pair. Mean kδδ/kδδ ratios of 2.0, 2.1, 1.9 and 2.4, 2.0, 1.9 were observed for [Co(promp)H2O]+ and [Co(MeO-promp)H2O]+, respectively, with the three optically active FeII complexes. Substitution in the pyridine moiety of the ligands has no influence on the stereoselectivity of the reaction. An outer sphere mechanism involving the pyridine moiety of one or both of the reacting complexes can therefore be excluded.