Jehane Ragai

Ion-exchange and surface properties of titania gels from Ti(III) solutions

Abstract The present investigation deals with the surface properties, thermal behavior, and ion-exchange characteristics of titania gels prepared from titanous chloride under different conditions of pH and temperature. The surface properties, exchange characteristics, and structures of these gels were studied with the aid of thermogravimetric analysis, nitrogen adsorption, visible spectrophotometry, and X-ray diffraction. Studies of the exchange behavior of four different cations, Cu2+, Ni2+, Co2+, and Cr3+, do not suggest a clearcut relationship between the extent of surface area and the ion-exchange properties. It is suggested that the “bridged OHs” on the titania surface are responsible for the cation-exchange properties of the gels. A high pH of preparation would lead to a large number of available exchangeable OH groups, whereas a low pH of preparation would lead to the formation of gels with low exchange capacities because of minimal exchangeable sites. In the case of the trivalent Cr3+ ion and its large hydrated radius the size of the pores seems to play a crucial role in terms of selectivity. Thermogravimetric studies reveal the presence of two types of water (interstitial and rigidly bound water in the form of hydroxyl ions) in samples prepared at a high pH. An additional weight loss appears in the case of samples prepared at a low pH and seems to arise from the loss of coordinately bound water ligands.

Pore structures and bulk properties of titania gels prepared from titanous chloride

Abstract A series of titanium dioxide gels has been prepared by the oxidation of precipitated hydrous Ti(III) oxide, obtained by the addition of aqueous ammonia or ammonium carbonate to aqueous solutions of titanous chloride. The gels were dried in air at 110°C and their structures and surface properties studied with the aid of DTA, X-ray diffraction, nitrogen adsorption, and spectrophotometry. Analysis of the nitrogen isotherms has revealed that the gels exhibited a wide range of different pore structures, extending from narrow micropores to wide mesopores. The development of the gel porosity appears to have been largely controlled by the mode of removal of water ligands from the hydration sphere of the cations, enlargement of the micropore volume being obtained by the replacement of water by carbonato ligands and pore widening by increase in the pH of precipitation (over the pH range 2.3–7.1). All the gels were X-ray amorphous in their original state: some were converted directly to rutile at 500°C while others underwent the more usual transformation into the intermediate anatase form. It is postulated that the rapid removal of the water ligands can result in the formation of a defect structure which controls the development of the pore structure and also the behavior of the gels on heat treatment.

Preparation of reactive nickel oxide by the controlled thermolysis of hexahydrated nickel nitrate

Abstract Sample Controlled Thermal Analysis is used to highlight the role of the experimental pressure on the thermolysis of hexahydrated nickel nitrate to nickel oxide. At 5 × 10 −2 mbar, dehydration occurs before denitration begins. Formation of an amorphous material occurs, corresponding to the composition of the monohydrate. At 5 mbar, a basic salt is formed. Both these thermal decompositions occur in the solid state whereas at 100 mbar melting occurs. The nature of the nickel oxide formed depends strongly on the experimental pressure. Increasing this pressure increases the temperature of nickel oxide formation. At 5 × 10 −2 mbar an oxide of 80 m 2 g −1 is obtained whereas at 5 mbar and 100 mbar no significant surface area accessible to nitrogen is observed.

Effect of preparative pH and ageing media on the crystallographic transformation of amorphous TiO2 to anatase and rutile

A study of the various parameters affecting the crystallographic transformation of amorphous titania gels (prepared from TiCl3) to anatase and rutile has been conducted. Analysis of the effect of the preparative pH (3.0, 7.5 and 11.0) and of the ageing media (HCl, H2O, air and NH4OH) suggests that coordinated water as well as the presence of chloride ions (bulk or surface) play an important role in the transformation of amorphous titania gels to the anatase and rutile modifications. The formation or depletion of anion vacancies as well as the presence of interstitial ions seem to be the crucial parameters controlling the rate of transformation of anatase to rutile. A higher heating temperature as well as a higher preparative pH generally lead to an accelerated conversion. In most cases first-order kinetics are followed. A nucleation process with high kn predominates with NH4OH and H2O as ageing media, whereas with air a propagation-growth process in general controls the kinetics of the transformation. Ageing in HCl gives ill-defined kinetic curves. A notably low activation energy for nucleation (≈52 kcal mol−1) was observed in the case of the low pH gels aged in ammonia.

Surface and bulk properties of ancient Egyptian mortars, part II: Adsorption and infrared studies

Abstract Adsorption studies carried out on Ancient Egyptian mortars suggest that these materials are essentially nonporous with a certain degree of mesoporosity. Analysis of the adsorption data was achieved by means of two reference nonporous materials. Surface activity is observed as a result of heat-treatment. Infrared studies confirm that the samples contain gypsum, calcite and quartz sand with a clearcut predominance of gypsum in the mortars extracted from the Sphinx statue. A correlation is perceived between these results and the X-Ray Diffraction results obtained in Part I of this series.

Hydrous oxides of titanium : cation exchange properties and kinetics of exchange

The ion exchange properties of hydrous titania gels of different particle sizes, precipitated from titanous chloride through the agency of ammonium carbonate and hydroxide have been studied. Such studies were carried out under acidic and alkaline conditions with respect to Cu2+, Ni2+, Co2+ and Cr3+ ions.In the case of gels precipitated by ammonium carbonate, oxygen gas was used as the oxidizing agent whereas with ammonium hydroxide as precipitant, oxidation was performed with hydrogen peroxide.Ion exchange capacities were determined by visible spectrophotometry. Increasing the pH of preparation lead to an increase in exchange capacities of the hydroxide precipitated gels that are characterized to be mesoporous. Such an increase is not observed in the case of carbonate precipitated microporous gels. It is shown that in the latter case the NH4+ions generated by the initial interaction of (NH4)2CO3 with the acidic titanous chloride lead to the formation of titania exchangers that are predominantly in the ammonium form. The textural characteristics of the exchanger resulting from different conditions of preparation is a significant contributing parameter to the resulting data.Ageing of the microporous titania samples markedly reduces the exchanger capacity of the smaller Ni2+ ions but increases that of the bulkier Cr3+ as a result of the presence of some wide pores that appear upon agglomeration. The presence of Cr3+ ions in the hydroxo form in solution seems to inhibit its exchange with the appropriate surface species.Studies on the kinetics of exchange with respect to the Ni2+ ions seem to indicate that a particle diffusion mechanism is partly or completely responsible for the rate of exchange.

Surface and related studies in sulphated oxides of zirconium

Zirconium oxide superacids with different sulphate ion contents were prepared from Zr(SO4)2 using urea as precipitating agent. These oxides were subjected to different heating temperatures. Characterization was carried out by means of infrared spectroscopy, X-ray diffraction and nitrogen adsorption studies. The surface acidity was determined by the method described by Boehm [1]. Infrared results confirmed the presence of the sulphate ions and suggested the gradual lowering through heat-treatment of the symmetry of the sulphate ions by complex formation. Surface acidity was favored when the sulphate ions were in a bidentate state of ligation. The latter ligands seemed in turn to be stabilized by water. The strongest acidity was displayed in systems where a high concentration of bidentated sulphate ions was present in conjunction with high surface areas and micropore volumes.

Characterization of hydrous zirconia gels: Adsorption and related studies

A series of zirconia gels has been prepared at different pH values by the addition of aqueous ammonia to aqueous solutions of zirconyl chloride. The gels were air dried at 110°C and their surface characteristics and structure studied with the aid of infrared spectroscopy, nitrogen adsorption and simultaneous DTA, TGA and DTG methods. Infrared studies suggest the formation of compounds of the type ZrO2 · xH2O · yNH3. Preparation pH values of 8.02 and 10.01 produced high area gels with considerable pore volumes, whereas pH values of 4.50 and 7.01 led to unpredictable areas with ill-defined textures reflecting the metastability of the systems. Differential thermal analysis curves revealed two distinctive endotherms allowing the separate identification of interstitial and cordinatively bound water. As the pH of precipitation increased, the amount of coordinated water decreased.

Characterization of microporous zirconia gels

Abstract Microporous zirconia gels were prepared from zirconyl chloride by the reaction in aqueous solutions with either ammonium carbonate or bircarbonate. Under conditions of low pH the ammonium bicarbonate precipitated gels tended to develop microporosity and an ill-defined mesoporosity. With the aid of FTIR spectroscopy and thermal analysis, it has been shown that carbonato ligands were present in most of the freshly prepared gels. All the gels were found to be X-ray amorphous when dried at room temperature and when heated for five hours up to 400°C. The exothermal transformation, which generally occurred in the temperature range 420°C − 450°C was associated with the ‘glow phenomenon’ and resulted in the develpment of crystallinity. The presence of residual water ligands and carbonato ligands served to protect the metastable material and delay the onset of the glow phenomenon.

Mechanism of cation uptake by zirconia gels

Abstract A study has been made of the uptake of the cations Cr 3+ , Co 2+ , Ni 2+ , Cu 2+ and Ca 2+ by zirconia gels prepared at pH values of 3.75, 7.45 and 9.00 using zirconyl chloride as starting material and ammonia solution as precipitating agent. The base exchange capacities of the gels as measured by the uptake of Na + and Cl − ions were found to depend on the preparative pH. The anionic capacity decreases and the cationic capacity increases when precipitation occurs in a basic medium. Before the isoelectric point (≈5.0), the uptake of Cu 2+ , Co 2+ , and Ni 2+ seems to involve complex formation by exchange of water molecules of the aquo ion with gel OH groups. The K d values appear to depend not only on the first hydrolysis constant but also may be affected by the textural characteristics of the gels brought about by differences in preparative pH values. The uptake of Cr 3+ is suggested to follow a different mechanism mainly involving electrostatic attraction between negatively charged anions on the surface and the aquo chromium ion as well as any other hydrolyzed species formed in solution. The uptake of Ca 2+ appears to involve electrostatic interaction. After the isoelectric point, the uptake involves a simple ion-exchange mechanism. The effect of equilibrium pH as well as the preparative pH on the distribution ratio K d has been discussed.