R.Sh. Mikhail

Adsorption in relation to pore structures of silicas: I. Organic vapor adsorption on microporous silica gel

Abstract Adsorption-desorption isotherms of benzene, cyclohexane, isopropanol, and carbon tetrachloride were measured on a microporous silica gel which had been precharacterized by nitrogen adsorption. The surface areas were obtained from the BET equation (SBET) and from the de Boer Vt − t plot (St). The pore-size distribution was obtained from the application of the MP-method. A prerequisite to the use of the de Boer Vt − t plot and to the application of the MP method is to have t-curves for all the adsorbates used, and these are offered in this investigation. Certain problems connected with the adsorption in micropores are discussed. For the nonspherical molecules such as benzene and cyclohexane, agreement between SBET and St was obtained only when the molecules were considered to be vertically oriented to the pore wall. The thickness t, as well as the area occupied by a single molecule, depends thus on the orientation on the surface. There is evidence to believe that the “opposite wall effect” may induce this orientation to a limit of one to three molecular layers on the surface. For the nearly spherical molecules, their packing in narrow pores might be denser two-dimensionally than three-dimensionally, and in such cases the packing factor is particularly small. A delay in the onset of the downward deviation in the Vt − t plot is noticed in some cases, and a possible explanation is given.

Thermal decomposition of ammonium metavanadate

Abstract Ammonium metavanadate is subjected to thermal treatment in the range 180–550°C and structural and surface area changes are studied by X-ray diffraction and N 2 adsorption. DTA and TGA show the existence of a large endotherm (260°C) possibly composed of several stages, and a much smaller one (340°C) which is responsible for evolution of the molecule of water associated with the V 2 O 5 initially formed. Three exotherms also appear and explanations for their presence are given. The phases formed, as well as the specific surface areas, are determined for the products obtained in a vacuum and in the presence of water vapour, and changes in surface areas are related to the phase transformations and dehydration of the products formed during thermal treatment.

Adsorption in relation to pore structures of silicas II. Water vapor adsorption on wide-pore and microporous silica gels

Adsorption-desorption isotherms of water vapor at 35°C and of nitrogen at −196°C were measured on two silica gels which were precharacterized to contain only wide pores, and on one silica gel which contained only narrow pores. The silica gels were heat treated at 120°, 400°, and 1000°C. Specific surface areas were calculated by various methods, and pore-structure analysis could be performed with water vapor as adsorbate. The results were checked against those obtained from nitrogen. Wide-pore silica gels contained composite surfaces, i.e., partly hydrophilic, partly hydrophobic; but after the exposure to water vapor, particularly at about saturation pressure, they became hydrophilic. In such cases the total surface areas could be obtained from the desorption and not the adsorption isotherms. Reduction in the pore size was shown to increase the hydrophilicity of the surface, and the microporous silica gel proved to be totally hydrophilic. Tentative explanations are offered for these phenomena, and it has been shown that besides the hydroxyl content of the surface, the porous structure influences to a considerable degree the extent of adsorption interaction. In narrow pores it seems that dispersion forces are involved besides the hydrogen bonding.

Surface properties and catalytic activity of pure and silica-coated aluminas

Abstract The surface properties of predominantly microporous, mesoporous, and nonporous alumina samples were studied and compared with samples modified by coating with a fixed amount of silica on their surfaces. The samples were characterized in terms of their specific surface areas, pore structure, and the chemistry of the surface, namely, surface acidity and surface OHs. An attempt was made to relate the activity toward cumene cracking with the chemistry as well as the predominant pore size. This study will hopefully reveal the role of the pore structure of alumina in determining its adsorption and catalytic activity and also the effect of doping with silica in modifying the surface properties of alumina.

Studies on water and nitrogen adsorption on hardened cement pastes I development of surface in low porosity pastes

Abstract Adsorption studies of water vapor at 35°C and nitrogen at −196°C were performed on low porosity portland cement pastes of ages between 1.5 hours and 3 months. Nitrogen areas and total pore volumes were found not only to be much smaller than water areas as previously published, but their changes with paste age are basically different. Calculations of hydraulic radii refute the hypothesis that the areas inaccessible to nitrogen are interlayer areas. From both water and nitrogen results, it was possible to speculate qualitatively on the mechanism of early hydration in low porosity pastes.

Kinetics of the thermal decomposition of calcium hydroxide

Abstract The kinetics of the thermal decomposition in vacuo of calcium hydroxide was investigated at 260°, 300°, 350°, and 385°C. The rate of the reaction increases during the first 10% to 25% decomposition; then it decreases. The decomposition is assumed to take place at the calcium hydroxide-calcium oxide interface; during the first stage this interface increases, and during the second stage it decreases. The second stage, or roughly the final 75% to 90% of the decomposition, is interpreted by a mechanism based on the assumption that the interface moves inward with a slowly diminishing velocity. This mechanism leads to an energy of activation of 14.3 kcal./mole. The energy of activation calculated without consideration of any mechanism is 14.9 ± kcal./mole. Because the decomposition of calcium hydroxide is endothermic to the extent of 25.2 kcal./mole, the low energy of activation of the decomposition indicates that the reaction takes place in more than one step. A two-step mechanism is proposed for the second stage of decomposition.

The interaction of methanol vapor with Taurus-Littrow orange soil☆

Abstract Adsorption of methanol vapor at 22°C, was measured on the Taurus-Littrow Orange soil sample 74220,29 part of the Apollo 17 return. Two complete adsorption-desorption cycles were carried out after out-gassing at 150°C, and a third cycle after outgassing at 300°C. It is demonstrated that specific interactions dominate the low-pressure region of the adsorption isotherm, while “fracturing” occurs at high relative pressures. A new method of data analysis is proposed, namely, the n s - n R plot, where n s is the number of the adsorbed layers on the sample, and n R the number on a reference nonporous solid. Advantages of this plot over the conventional de Boer t -plot are discussed, and it is shown that both fracturing and specific interactions can be detected through its use.

Studies on premix water-soluble polymer cement pastes - I

Abstract Studies were carried out on the effect of adding some water-soluble polymers to the cement mix on some important physico-chemical and mechanical properties of the hardened pastes. The polymer was found to act as a retarder for hydration at early ages, then it acts as an accelerator. The compressive strength results were found to be affected by the nature of the hydration products, their degree of compaction, their average grain size, and by induced orientation of the crystallites. All these factors are affected by the presence of the polymer.

Surface changes induced in aluminas by coating with silica

Abstract Adsorption isotherms of nitrogen at -196 °C, of argon at -183 °C and of benzene and water at 35 °C were measured on alumina samples of various porosities before and after coating with a fixed amount of silica. The effects of the coating on the structure and surface properties of the aluminas were determined by differential thermal analysis, X-ray diffraction and pore structure analysis of the various samples calcined at different temperatures. The most obvious effects were as follows: coating microporous alumina with silica reduces the number of pores but does not significantly reduce their sizes; coating mesoporous alumina reduces the size but not the number of pores; coating non-porous alumina produces a porous layer on top of the non-porous substrate.

Effects of doping on the catalytic activity of nickel oxide

Abstract A study was made of the effect of doping with Li+, Na+ and Fe3+ on the catalytic activity of NiO prepared in air by heating the hydroxide at 300–500 °C. Investigations of the kinetics and mechanisms of the catalytic oxidation of CO on the various samples have shown that retardation of reaction by Li+ and Na+ doping is due to the slow adsorption of oxygen anions on Ni2+ ions, which are transformed to Ni3+ on doping. Fe3+ doping, on the other hand, increases the surface concentration of the catalytically active Ni2+ sites on which both O2 and CO are adsorbed, and consequently promotes the catalytic reaction. These results agree with the electronic theory of catalysis.