Adsorption Science & Technology

This paper summarises the results of four years' investigation of the thermodynamics of adsorption in activated carbon micropores. Five adsorbates, i.e. methane, methanol, ethanol, carbon dichloride and carbon tetrachloride were adsorbed at three temperatures (308, 328 and 348 K) on two polymeric microporous carbons. Adsorption isotherms, as well as differential heats of adsorption, were measured using adsorption calorimetry. From the experimental results, the entropy as well as the average heat capacity of the adsorbed molecules were calculated. The data obtained lead to the mechanism of adsorption in carbon micropores as well as to the influence of the oxidation of the carbon surface on this mechanism. By comparing the isosteric and differential heats of adsorption, it is shown that the isosteric enthalpy cannot be applied for a correct description of the adsorption energetics. Finally, some empirical relationships describing low-coverage adsorption are proposed and discussed.

Electrical conductivity measurements have been used to detect extremely small changes in the oxygen content of oxide semiconductors in contact with reducing or oxidizing gases. In continuation of previous studies, the oxygen loss of chromium oxide in H₂ and SO₂ flows, as well as the oxygen gain in an oxygen atmosphere, were studied. In SO₂, the conductivity dropped instantaneously to minimum values due to its adsorption on adsorbed oxygen sites. The treatment of Cr₂O₃ in SO₂ led to the elimination of chemisorbed oxygen and the covering of the surface with polymeric SO₂. In contrast, in an H₂ flow, the conductivity of Cr₂O₃ initially exhibited an induction period during which the value was constant. At the end of the induction period, the conductivity increased rapidly to a maximum value and then dropped sharply to a minimum. The induction period may be regarded as the time necessary to create an oxygen vacancy, the activation energy for such a process being 21.1 kJ/mol. A hydrogen molecule is then adsorbed on to the oxygen vacancy possibly as a hydride ion, and leading to the initial increase in the conductivity. The hydride ion then migrated to a chemisorbed oxygen site, where it formed a surface hydroxy group and caused a consequent decrease in the electrical conductivity. The surface then dehydroxylated due to the interaction of surface hydroxy groups with gaseous hydrogen, leaving coordinatively unsaturated surface chromium ions behind.

In an oxygen flow at 400°C and above, either SO₂ or H₂ treatments led to a sharp increase in conductivity due to oxygen adsorption. In contrast, at temperatures less than 350°C, oxygen adsorption was retarded after an SO₂ flow, possibly due to the strong adsorption of a polymeric film of SO₂. Correspondingly, after H₂ treatments, oxygen was adsorbed instantaneously at temperatures as low as 200°C, presumably because of the weak sorption of H₂ on the surface chromium ions.

After discontinuing the hydrogen flow, further oxygenation caused a subsequent decrease in the conductivity, possibly due to surface hydroxylation. Hydrogen trapped in the bulk of the Cr₂O₃ could spill over the surface and cause such a hydroxylation process.

The co-adsorption of binary mixtures of tetradecyltrimethylammonium bromide (TTAB) and nonyl phenyl ethoxylates (NP-n series) from their aqueous solutions on to quartz has been studied. None of the ethoxylates adsorbed on to the quartz when alone, but considerable adsorption took place from the mixture with cationic TTAB. The adsorption of TTAB was increased in the presence of ethoxylates below its CMC due to chain–chain interaction between the cationic/non-ionic surfactants. FT-IR spectroscopic studies indicated that micelle-like aggregate structures formed on the quartz surface even below the CMC. Zeta potential measurements were carried out in order to understand the adsorption behaviour.

The physical, chemical and catalytic properties of simple oxides and aluminosilicates of different composition, together with modifications of the same obtained by treatment with various reagents, have been studied experimentally. It has been shown that the use of simple oxides as catalysts led to the extensive oxidation of ethane while the use of aluminosilicates allowed the generation of formaldehyde, acetaldehyde and alcohols such as methanol and ethanol.

The presence of phosphorus in the modified catalysts favoured the formation of aldehydes and led to an increase in aldehyde concentration as well as the appearance of alcohols among the oxidation products of n-alkanes, e.g. ethane. Thus, the addition of phosphoric acid to γ-Al₂O₃ resulted in the formation of formaldehyde as a useful product.

The adsorption of ammonium ions, radionuclide ions (Sr²⁺ and Cs⁺) and heavy metal ions (Ni²⁺, Cr³⁺, Fe³⁺ and Hg²⁺) on natural Sokyrnytsky clinoptilolite has been investigated. Effective cation exchange of heavy metal ions and radioactive elements was observed with samples of the clinoptilolite which had been pretreated with ammonium chloride solutions. The adsorption isotherms for such cations from solutions of 0.5–1.0 mg/l ion content were studied under static conditions for adsorbent particle size fractions of 1.5–2.5 mm. Solid phases and solution aliquots were analyzed by X-ray fluorescence methods for Sr²⁺ ions while the concentrations of other ions were analyzed by spectrophotometric methods.

A series of Ni/Al₂O₃–AlPO₄ catalysts has been prepared, thereby allowing the effect of nickel content and calcination temperature on the structural and catalytic activity of such solids to be investigated. The textural parameters of all the prepared samples were determined from nitrogen adsorption studies conducted at −196°C, while thermal analysis methods (DTA, TGA and DrTGA) and X-ray diffraction techniques were used to elucidate their structures. Their surface acidities were measured by the amine titration method, while the conversion of isopropanol on the prepared catalysts was investigated using a pulse microcatalytic reactor.

Significant changes in the structure, texture, surface acidity and catalytic activity of the prepared solids were observed as a result of varying their nickel content and calcination temperatures. Thus, XRD studies demonstrated the presence of nickel metal, nickel aluminate spinel, γ-alumina and tridymite phases while both the surface areas and the porous structures were found to depend on the metal content and calcination temperatures. The catalytic selectivity for isopropanol conversion was found to depend on the number of surface acid sites present.

The ability of bentonite to remove Cu^II from aqueous and acidic solutions at different pH values has been studied for different adsorbate concentrations by varying the amount of adsorbent, temperature and shaking time. The maximum (85%) adsorption of Cu^II was achieved from aqueous solution at pH 3.8. The influence of different anions and cations in the 0–1000 μg/ml concentration range on the adsorption of copper under optimized conditions has been examined. A flame atomic absorption spectrometer was used for measuring the copper concentration before and after adsorption. Isotherm analysis of the adsorption data obtained at 25°C, 30°C, 40°C and 50°C showed that the adsorption of copper(II) on bentonite could be described by both the Langmuir and the Freundlich isotherms. Values of ΔH⁰ and ΔS⁰ were calculated from the slope and intercept of the ln K_D versus l/T plots.