Determination of mesopore surface area in presence of micropores

A modified t-method known as $\text{t}\text{J}$ method, capable of finding mesopore surface area in absence or presence of micropores in which adsorption may be pressure dependent or pressure independent is described by the application of John's adsorption isotherm equation.     

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.     

Viscous flow in aqueous solution of quaternary ammonium salts: high concentration range

By introducing the condition that the viscosity of solutions η → η₀, the viscosity of the solvent as the concentration of the salt N → 0, we have recast Angell's equation in the following form, ln η_rel = kN/[N_o(N_o  N)]. This equation has been found to represent satisfactorily the viscosity behaviour of quaternary ammonium halides in aqueous solution up to high concentration and the values of N₀, the hypothetical concentration at which the solution becomes a glass, agree with those obtained from Angell's equation. The relationship with Vand's equation of fluidity is also discussed.     

Micropore volumes of carbons by means of a new isotherm equation

The linearized form of the isotherm equation developed by John is given by log log P = C + n log v. The log v values corresponding to relative pressure 1, on the extrapolated straight line of the experimental plot of the above equation gave micropore volume in good agreement with the values obtained by Dubinin method, especially for microporous carbons giving Type 1 and extremely low pressure isotherms. It is shown mathematically that the new equation is fundamentally the same as Dubin equation.     

A new approach to quantify the microporosity of activated carbons by analysing the N2/77 K and CO2/273 K adsorption data by the simplex flexible method

A mathematical model has been applied to N₂/77 K and CO₂/273 K adsorption isotherms for a series of activated carbons prepared by carbonising olive stones in N₂ and then activating them in CO₂ to six different levels of burn-off in the range 8–80%. Narrow and wide micropore volumes of activated carbons were calculated from the Dubinin-Radushkevich and Dubinin-Astakhov equations considering one, two and three micropore size distributions in each sample, and allowing a variation of the micropore volume and characteristic energy of each distribution with the burn-off. The flexible simplex method was applied to obtain the parameters of each distribution in the mathematical model. Generally, it was found that increasing the number of micropore size distributions above two did not significantly improve fits. Each isotherm was fitted using six parameters at most. However, various constraints were imposed, and the parameters were estimated from each isotherm using non-linear, least-squares regression analysis. The results obtained confirm the valuable use of CO₂/273 K adsorption to quantify the narrow microporosity of activated carbons. Differences between N₂/77 K and CO₂/273 K adsorption in microporous activated carbons were due to the wide microporosity. An agreement between micropore volumes obtained from CO₂/273 K adsorption and that corresponding to one of the two distributions of micropores obtained from N₂/77 K adsorption was obtained. The Dubinin-Radushkevich equation was more successful than the Dubinin-Astakhov equation in the quantification of the microporosity with N₂/77 K and CO₂/273 K. On the other hand, the exponent n of the Dubinin-Astakhov equation was better correlated with the burn-off of the carbons than with the parameter B.     

The enthalpies of immersion of active carbons,in relation to the Dubinin theory for the volume filling of micropores

It is shown that an exact expression can be derived from Dubinin's theory, for the enthalpy of immersion of active carbons. It appears that for a given liquid, the specific enthalpy of immersion is a function of the characteristic energy βE₀ of the Dubinin-Astakhov equation. The new relation is illustrated by data obtained for 10 different carbons immersed into benzene and n-heptane at 34°C. Simultaneously, results are presented for the immersion into water, not described by Dubinin's theory, but providing useful complementary information about the chemical state of the surface.     

Effect of penetrant size and shape on its transport through a thermoset adhesive: I. n-alkanes

The diffusion behaviors of a series of n-alkanes, ranging from C₆ to C₁₇, through a polyamide-type polymeric matrix have been investigated by means of mass uptake measurements. Since n-alkanes are known to display negligible interactions with the polymer matrix, this study serves to isolate the effects of penetrant size and shape on the transport process without undue interference from polymer-penetrant interactions. It is established that the diffusion of the n-alkanes through the polymer matrix studied is Fickian and proceeds via a Henry's law-type mechanism. The diffusion coefficients, D, are evaluated based on a thin-film approximation of the Fickian equation. The activation energies of diffusion, E_d, are determined from the temperature dependence of D, using the Arrhenius equation. Correlations between the Arrhenius terms, E_d and D₀, are also established which enable the prediction of diffusion coefficients for similar polymer-penetrant systems. It is also demonstrated by means of activation energy calculations and molecular simulations that the n-alkanes assume a linear geometry within the polymer matrix and diffuse along their long axes.     

Sedimentation of supsensions: implications of theories of hindered settling

Expressions are derived for the parameters A in Steinour's theory and n Richardson and Zaki's expression dealing with the hindered settling of powdered materials.It is shown that n is simply related to ?₁, the initial liquid volume fraction of a uniformly mixed suspension for which [Q(1 — ?)] was maximum value, Q being the linear settling rate of the suspension/supernatant interface and ? the initial liquid volume fraction of the suspension. For systems obeying the Richardson-Zaki expression, the settling rate at ?₁ approaches the limit V_s exp^?1, where V_s is the estimated Stokes' law limiting velocity for the system, when ?₁ approaches unity (i.e. infinite dilution).Highly hindered systems have large values for n, and ?₁ values approaching unity; for such systems Q_?1 ? V_s exp^?1. It is suggested that such behaviour implies the existence of relatively long-range forces within suspensions, hindering settling, and that particle-liquid (including particle-liquid-particle) forces are of importance in addition to particle-particle interactions.Evidence is presented that hindrance to settling is directly proportional to the polarity of the solid/liquid system and that the density of charge on the superficial particle surface is the dominant factor in determining hindrance. Maximum reduction of surface polarity without causing flocculation is suggested as the most efficient condition for separation by settling under gravity and pumping off supernatant liquor.n and ?₁ can be considered as useful indices of hindrance to sedimentation. ?₁ has particular significance as the initial liquid volume fraction at which solids flux has maximum value (for any suspension which obeys the Richardson-Zaki equation).     

Dependence of carbon dioxide sorption on the petrographic composition of bituminous coals from the Czech part of the Upper Silesian Basin, Czech Republic

The effect of the rank and of the maceral composition of bituminous coal on carbon dioxide sorption capacity was studied on the basis of samples from two coal mines (Darkov, ?SM) from the Czech part of the Upper Silesian Basin. The samples from the two mines cover a small but very significant section of coalification within the transition zone between high-volatile bituminous A coal and medium-volatile bituminous coal, where porosity and sorption properties pass through their minima. The coal porous system was characterized by the micropore volume evaluated using the sorption isotherm of carbon dioxide and the volumes of meso-, macro- and coarse pores were determined by high-pressure mercury porosimetry. The micropore fraction in the coal porous system ranged between 53% and 75%. It was particularly high in coals with high vitrinite content, namely collotelinite, and also in coals with high inertinite content. The carbon dioxide sorption capacity was determined from the carbon dioxide sorption isotherms measured using a gravimetric sorption analyzer at 298 K until a relative pressure of 0.015 p/p_s, and was interpreted by characteristic parameters of the Dubinin and Langmuir equations. It was found that the adsorbed amount of CO₂ in the ?SM coal increases with the content of vitrinite and collotelinite, whereas no increase or only a slight increase was observed for the Darkov coal. The tendency of adsorption capacity to depend on maceral composition, and also to some extent on coalification, observed for the ?SM coal, may be related to higher microporosity due to the coalification process or oxidative processes leading to the formation of pseudovitrinite.     

Reaction intermediates as a controlling factor in the kinetics and mechanism of oxygen reduction at platinum electrodes

A rotating platinum disk electrode was used to study oxygen reduction over the entire pH range and to compare the kinetics in acid and alkaline solutions. In both solutions two Tafel regions are confirmed. At low cd's ?V/?ln i = ?RT/F and at high cd's it is ?2RT/F. In the low cd region, the reaction order with respect to H₃O⁺ is $\text{3}\text{2}$ in acid, and $\text{1}\text{2}$ in alkaline solutions. For the high cd region the reaction orders are 1 and 0 in acid and alkaline solutions, respectively. Transition of the kinetics from the low to the high cd region, as well as the fractional reaction orders at low cd's, are interpreted in terms of the first charge transfer step as rate determining in both cd regions but under Temkin adsorption conditions in the low, and Lagmuirian conditions in the high cd region. Coverages, θ, with oxygen intermediates are also determined as a function of potential and pH. At all pH's, substantial coverages are observed only at potentials in the low cd region.The usual Tafel slope of ?120 mV for the first charge transfer step as rate determining is not observed at low cd's since it is modified by the dependence of activation energy on θ, and hence on V. At high cd, θ is low and ineffective and the usual slope for the first charge transfer step is observed. At all pH's the transition from low to high cd's occurs at potentials, V_T, that decrease 60 mV as pH increases one unit. Dependence of V_T and i_T on pH is discussed. Fractional reaction orders at low cd's are also related to θ. Real reaction orders with respect to H₃O⁺ are modified by the dependence of activation energy on θ, and hence on pH. The change of the kinetics from the acid to the alkaline region is discussed.The range of cd's accessible to measurements is limited at both high and low cd's. At high cd's it is limited by the supply of oxygen and/or H₃O⁺ from the solutions. At low cd's, V—log i relationship extends only to a rest potential which decreases 60 mV as pH increases one unit. The limits of measurements and the nature of the rest potential are discussed.     

Preparation and characterization of aluminum pillared K10 and KSF for adsorption of trimethoprim

Montmorillonite KSF and K10 were used as precursor materials for synthesis of aluminum pillared K10 and KSF (Al-K10 and Al-KSF) which characterized by TGA, XRD, SEM and FT-IR spectroscopic analysis. The sorption of trimethoprim (TMP) which is commonly employed as an antibiotic onto Al-K10 and Al-KSF was also investigated as a function of adsorbent dosage, solution pH, contact time and temperature. The adsorption kinetics was interpreted using pseudo-first-order, pseudo-second-order kinetic models and intraparticle diffusion model. The pseudo-second-order model provided the best correlation. Adsorption isotherm parameters were obtained from Freundlich, Langmuir and Dubinin–Radushkevich (DR) isotherm models. Adsorption of TMP onto Al-K10 and Al-KSF was physical in nature and ion-exchange mechanism for DR equation, respectively. Al-K10 exhibits higher removal capacity at lower adsorbent dosages in comparison with Al-KSF. The removal capacity was increased by increasing pH. ΔH⁰, ΔS⁰ and ΔG⁰ showed that adsorption of trimethoprim was endothermic, increasing randomness and not spontaneous in nature.     

What kind of pore size distribution is assumed in the Dubinin–Astakhov adsorption isotherm equation?

Two most sophisticated methods of carbon porosity characterization (high resolution α_s-plot and the procedure proposed by Nguyen and Do, (ND)) were utilized for the assessment of porosity from the series of numerically generated adsorption isotherms. Basing on the Dubinin–Astakhov (DA) adsorption isotherm equation, two series of adsorption isotherms of nitrogen (T=77.5 K) were generated for constant E₀ and different n values, and for constant n and different E₀. They were described by the both above-mentioned methods. The types of obtained α_s-plots as well as the pore size distribution curves (PSD) lead to suggestions about the basic features of the DA and the meaning of the parameters of this adsorption isotherm equation.     

Physico-chemical studies on the interaction of transition metal ions with Schiff bases: polarographic study of copper(II) complex of salicylaldehyde-serine

Copper(II) complex of Schiff base derived from salicylaldehyde-serine has been studied by polarographic method of analysis at 30° and at constant ionic strength in aqueous medium. The plots of i_dvs √h and i_dvsC were linear and passed through the origin, showing that the reduction process was diffusion controlled. The plots of log (i/i_d?ivsE_de were linear in all concentrations of Schiff base but the slope showed that the reduction process was irreversible. The kinetic parameters were determined by Koutecky's method. The composition and stability constant were determined by DeFord and Hume's method.     

Elucidating the mechanism of carbocationic polymerization of α-methylstyrene

Summary The polymerization of -methylstyrene (MeSt) using the H₂O/SnCl₄ initiating system and ethyl chloride solvent has been investigated over the temperature range from –40° to –122°C in the presence and absence of the proton trap 2,6-di-tert-butylpyridine (DtBP). Arrhenius (In ¯M_n versus 1/T) plots obtained with poly (methylstyrene) (PMeSt) samples prepared in the absence of DtBP reveal the existence of two sharply defined temperature regimes (see Figure 1): at higher temperatures from –40 to –86°C, the slope of the Arrhenius plot yields H_{M n} = –4.90±0.25 kcal/mole whereas at lower temperatures, from –86° to –122°C,H_{M n} = –0.3 kcal/mole. With PMeSt samples prepared in the presence of DtBP the H_{M n} obtained for the higher temperature regime increases to –1.67 ± 0.20 kcal/mole whereas the H_{M n} reflecting the lower temperatures remains the same as that obtained in the absence of DtBP. These observations are readily explained by postulating a change in mechanism at –86°C: Evidently the ¯M_n of PMeSt is determined over the higher temperature regime by chain transfer to monomer which is frozen out at lower temperatures where termination becomes ¯M_n determinant. In the absence of DtBP chain transfer to monomer is operative which leads to the higher Arrhenius slope over the higher temperature regime; however, over the lower temperature regime where chain transfer is absent and termination is ¯M_n controlling, the Arrhenius slope remains unchanged. Evidence obtained from a Mayo plot (negligible intercept in the 1/¯M_n versus 1/[MeSt]_o plot, Figure 2) with samples prepared at –92°C, corroborate this postulate. Molecular weight dispersities (¯M_W/¯M_n) as a function of temperature have been determined in the presence and absence of DtBP (Figure 3). The proton trap affects ¯M_W/¯M_n only over the higher temperature regime which also suggests that chain transfer to monomer is frozen out at –86°C and that the polymerization becomes termination dominated at low temperatures.     

Multilayer single-solute adsorption from dilute solutions on energetically heterogeneous solids

The isotherm equations for multilayer single-solute adsorption from dilute solutions on energetically heterogeneous solids are derived by solving the integral equation for different energy distributions and local adsorption isotherm of BET-type. This procedure is analogous to that used in multilayer gas adsorption on heterogeneous solid surfaces. The equations, obtained for quasi-gaussian energy distribution, are examined using the adsorption data of n-valeric acid, n-amyl alcohol, aniline and cyclohexanol from dilute aqueous solutions on different types of carbon adsorbents of specific surface areas from 18.4m₂/g to 124m₂/g. The best equation for describing the above data is the modified Langmuir-Freundlich one, denoted in the paper by k-LF equation.     

General scheme of adsorption,electroreduction and catalytic hydrogenation of nitro-compounds of platinum—I. Kinetics and mechanism of adsorption

The kinetics and mechanism of adsorption of nitro-compounds (nitro-methane, nitro-ethane and nitro-benzene) on platinum and platinum group metals have been investigated by complex potentiodynamic pulses' and charging curves' techniques. The adsorption of nitro-compounds on platinum in the potential range from 0 to 1 V has been shown to proceed mainly with the formation of two types of chemisorbed particles. For potentials more positive than 0.35 V nitro-compounds are adsorbed due to a nitro-group with the formation of NPt and OPt bonds. In this region of potentials the adsorption of nitro-compounds is not accompanied with the flow of considerable amounts of electricity through the electrode—solution interface. These adsorbed particles react very rapidly with adsorbed hydrogen with the formation of semi-reduced chemisorbed particles, whose formation can be observed at 0 < E_r < 0.35 V, that is in the range of hydrogen desorption on platinum. Two hydrogen atoms are spent on the formation of a semi-reduced chemisorbed particle from a nitro-compound molecule, and the particle formed occupies 4 adsorption centres on the surface.The dependence of steady-state coverage of the platinum electrode surface with chemisorbed particles in nitro-compounds solutions is described by the Temkin isotherm and the adsorption kinetics by the Roginsky—Zel'dovich equation.     

Sub-micron dispersed-phase particle size in polymer blends: overcoming the Taylor limit via solid-state shear pulverization

A comparison was made of the fineness of dispersion in immiscible polymer blends achieved by a continuous mechanical alloying technique, solid-state shear pulverization, relative to that achieved by melt mixing. Two polymer blend systems were investigated. A polystyrene (PS)/polyethylene (PE) wax blend was studied because, based on a classic analysis by G.I. Taylor, melt mixing was expected to yield a number-average dispersed-phase domain size, D_n, well above 1 μm. A PS/high density polyethylene (HDPE) blend was also studied because it was known to produce a sub-micron number-average dispersed-phase particle size when mixed by twin-screw extrusion. In the case of the PS/PE wax blend at compositions ranging from 1 to 15 wt% polyethylene wax, pulverization resulted in nearly identical D_n values (typical value of 0.7 μm) independent of minor-phase content; these D_n values were an order of magnitude smaller than the anticipated Taylor limit for melt-mixed blends. In contrast, PS/PE wax blends made by batch, intensive melt mixing yielded D_n values between ∼3 μm at both 1 and 5 wt% minor-phase content and 17.5 μm at 15 wt% minor-phase content. The increase in D_n with increasing dispersed-phase content in the melt-mixed blend is a consequence of coalescence present during melt processing; such effects are disallowed in the pulverization process occurring in the solid state. Scanning electron microscopy of a 95/5 wt% PS/HDPE blend provided D_n values of 500 and 270 nm in the twin-screw extruded and pulverized samples, respectively. Fractionated crystallization studies further corroborated the ability of pulverization to result in a finer, nanoscopic dispersion of the minor phase as compared to extrusion.     

Knudsen diffusion of various gases through a barium sulphate packed bed

The diffusion coefficients, D_k, of helium, krypton, carbon dioxide and air through a packed bed of barium sulphate calculated from the Knudsen diffusion flow equation were found to be a cumulative parameter obtained from the contribution of the diffusion coefficient through the voids of the packed bed, D_v, and the diffusion coefficient through the pores, D_p, within barium sulphate.The surface area of compacted and uncompacted material obtained from low temperature gas adsorption and the Brunauer, Emmett and Teller equation (BET), together with the void sizes and pore sizes in compacted material obtained by Barrett, Joyner and Halenda (B.J.H.) isotherm analysis, when compared with the surface area values obtained by Knudsen diffusion and permeametry, indicated that the coefficients D_v and D_p evaluate the void and pore areas in compacted barium sulphate, respectively.     

Dependence of particle fluctuation velocity on gas flow, and particle diameter in gas fluidized beds for monodispersed spheres in the Geldart B and A fluidization regimes

In this paper we present new experimental data on the steady-state, mean squared, fluctuation velocity, or granular temperature, of Geldart B polymer, glass, nickel, and stainless steel monodispersed spheres averaged over the wall of a gas fluidized bed, as a function of gas flow and sphere diameter. The granular temperature is obtained by Acoustic Shot Noise technology—namely power spectral analysis of the steady state vibrational energy of the wall excited by random sphere impact, and calibrated by hammer excitation over the wall. The new data extends to polymer and metallic spheres the experimental discovery of a 1996 paper of Cody et al. that the fluctuation velocity of Geldart B glass spheres when scaled to the gas superficial velocity, U_s, is inversely proportional to sphere diameter, directly proportional to a fundamental length scale, D_o^B, and is a universal function of U = (U_s / U_mf). We also demonstrate that the new data is consistent with the diameter dependence of the fluctuation velocity that can be derived from both the 1997 paper of Menon and Durian, who measured random sphere motion near the wall through the spectroscopy of scattered laser light, and the 1992 paper of Rahman and Campbell, who measured the average granular pressure of random sphere impact on a porous steel membrane. While the inverse scaling of the fluctuation velocity with sphere diameter, and the existence of a fundamental length scale for gas fluidization, D_o^B, had not been a feature of any published fundamental model, or computer simulation, of the steady state granular temperature of spheres in gas fluidized beds, we show that it is a feature of two recent dense kinetic fluidization models published in 1999, by Buyevich and Kapbasov, and Koch and Sangani. Both theories implicitly define a fundamental length scale for the fluctuation velocity, D^? = (μ_f² / ρ_p²g)^1 / 3, where ρ_p is the sphere density, μ_f is the gas viscosity, and g is the laboratory gravitational field. The new data for polymer, glass, nickel and stainless steel spheres presented in this paper, defines D_o^B = (56 ± 2)D^?. We use the Anderson-Jackson stability model to show that the length scale D_o^B, also defines a stability length scale, such that for D < D_o^B(D > D_o^B), the uniform dense phase of the fluidized bed is stable (unstable), against one dimensional, first order fluctuations in sphere concentration. The length scale, D_o^B is thus the theoretical equivalent to the empirical scaling length introduced by Geldart, D_B/A, to distinguish spheres (D > D_B/A) that bubble at fluidization, from spheres (D < D_B/A) that fluidize before bubbling. Finally, we present new experimental data, on the remarkable changes in the granular temperature, bed expansion, and bed collapse time, between Geldart B and Geldart A monodispersed glass spheres, and compare that data to granular temperature, and bed expansion, for Geldart A rough, non-spherical, log-normal dispersed diameter catalytic particles.     

EFFECT OF CHEMICAL COMPOSITION ON RELATIONSHIP BETWEEN REFRACTIVE INDEX AND ABBE VALUE FOR BINARY SYSTEMS*

A procedure is outlined for deriving the relationship between n_D and v for binary glass systems, using constants previously published by Huggins and Sun.¹ As an example, the Na₂O-SiO₂ system is discussed and the results are plotted in two ways. An equation is deduced for the slope of the n_D versus v curve at any composition, and initial values of this slope, at the SiO₂ end, are tabulated for various M_mO_n-SiO₂ systems.