S. Hanafi

Surface properties of silicas produced by thermal treatment of rice-husk ash

Abstract Silica samples were prepared by firing rice-husk ash at various temperatures ranging from 500 to 1400°C for a period of 3 h. Adsorption of nitrogen was used to study the surface properties of the silica obtained at each firing temperature. The parameters derived were related to the pore structure and the degree of crystallinity of the silica samples. X-Ray diffraction analysis was also used for the identification of the silica phases produced by thermal treatment of rice-husk ash.

Surface area and pore structure of thermally treated silica gel

Fine silica gel, prepared by grinding coarse granules to a particle size < 100 μm, is heated in air at 100, 500, 600, 700, 800, 900, 1000, 1100, 1300, and 1400°C for 3 h. Pore structure is studied from variations in specific surface area, total pore volume and the mean hydraulic radii of the pore system available for nitrogen adsorption. These parameters are related as far as possible to changes in pore size, the distinct phases produced at each temperature, and the particle size of the crystallites as calculated from X-ray diffraction analysis.

Ettringite formation in compressed expansive cement pastes

Type I expansive clinker pastes of w/c=0.25 were compressed at pressures of 50, 200 and 500 Kg/cm2. Several parameters of the hardened pastes, measured at various curing ages, were compared with those of the non-compressed (control) pastes. Relative values (α values) were developed to express the changes due to compression in total porosity, non-evaporable water, compressive strength, as well as in specific surface area. Surface area changes are due to changes in the size and subsequent growth of the ettringite crystallites. This work is affected by the available pore space. SEM gave direct evidence in favour of a through-solution mechanism at least beyond one day of hydration.

Effect of fluorination on the surface texture of zirconia

Abstract Surface area measurements were performed on pure and fluorinated zirconia gels and their decomposition products in the temperature range ambient to 1000 °C by nitrogen adsorption at -196 °C and benzene and isopropanol adsorption at 35 °C. Impregnation by NH 4 F does not change the amorphous character of the parent gel. A mixture of the tetragonal and monoclinic modifications is produced at 500 °C although the corresponding product of the pure gel is amorphous. At temperatures above 500 °C only the monoclinic zirconia exists. Fluorination increases the nitrogen areas markedly at temperatures of 500 °C and above but decreases the benzene and isopropanol areas at temperatures less than 1000 °C and less than 500 °C respectively. The surface is modified, and the sites that are active for adsorbing benzene molecules also appear to be responsible for adsorbing isopropanol. Pore structure analysis shows that the pure parent material is microporous whereas at higher temperatures mesoporosity predominates. Fluorination of the gel is found to create mesopores in the parent material and to increase mesoporosity in samples produced at higher temperatures as observed from the adsorption of the organic probe molecules. The nitrogen data show that in addition to the creation of mesopores at low temperatures stabilization of the surface texture by fluorination is established at 500 °C and above, giving rise to products with nearly the same specific surface areas, Brunauer-Emmett-Teller c constants and average pore radii.

Morphology and surface area changes in the thermal dissociation of iceland spar

Abstract The morphological changes accompanying the thermal decomposition of large crystals of Iceland spar were traced by SEM, and the development of both the area and the pore structure was determined by gas adsorption. The significance of precharacterizing the morphology of the pores developed in thermal treatment is emphasized in the present investigation; this information is of utmost interest for the correct interpretation of the adsorption isotherm. Structure analysis of mesopores by gas adsorption was carried out, based on capillary condensation, and the results were discussed for cases in which the cummulative area ( S cum ) is appreciably smaller than S BET , and V cum is appreciably smaller than V p , and also in other cases when both S cum and V cum are approximately equal to S BET and V p . The interpretation of these cases is based on the morphology of the pore as revealed by SEM.

Vapor adsorption on expanding and nonexpanding clay minerals

Adsorption of water and cyclohexane vapors at 35°C was measured on montmorillonite (an expanding-type clay mineral) and on illite (a nonexpanding clay mineral), both heated in vacuo at various temperatures for 5 hr. Specific surface areas were estimated from the BET method (SBET), and from the cumulative values of pore structure analysis (Scum). Water, but not cyclohexane, was noticed to penetrate between the layers of montmorillonite, and in such a case Scum > SBET. For the nonexpanding illite lattice SBET for water was found to be larger than Scum due to the presence of micropores in the nonexpanding mineral. Cyclohexane, as a bulkier molecule, was inaccessible to parts of the pore structure in both minerals. Heats of immersion in both water and cyclohexane were measured, and it is suggested that these values should be referred to the area actually accessible to the adsorbate molecule. For water/montmorillonite this value should be Scum and not SBET.

Effect of thermal treatment on the structure and texture of differently impregnated NiO/SiO2 catalysts

Abstract NiO/SiO2 catalysts were prepared with Ni contents ranging from 2–15% using a microporous silica support at pH ~11.5. The role of the method of preparation on the resulting catalyst is also investigated. Structural and textural changes were followed using X-ray diffraction, TG and DTA techniques—the surface area measurements were carried out on the parent catalysts and those produced in the temperature range 250–1000°C. Impregnation of the silica gel in the nickel ammine complex solution (catalyst series 1N–4N) with subsequent drying at 80°C overnight produced crystalline catalysts with two distinct peaks at d-spacings of 2.035 and 2.349 A resulting from a surface silicate. This is easily destroyed by thermal treatment at 250°C for Ni contents ⩽ 10% but is stable to this temperature for the higher Ni content. Drying the catalyst at room temperature (3Nb) gives rise to an amorphous product. A non-crystalline catalyst is also obtained when concentrated ammonia solution is added to the adsorbed nickel salt (3Nc). At high Ni content, the hydroxo ligand becomes significant and results in a surface compound in which one silanol group is attacked. This gives rise to a crystalline product at 500°C with characteristic d-spacings at 2.201 and 2.049 A which, subsequently, produces a poorly crystalline NiO product at 1000°C. The presence of this hydroxo ligand is manifested by a small endotherm at 260°C. At Ni contents below 15% but greater than 2% a small exotherm is observed at ~ 500°C resulting from a reduction process. Entrained SO42− ions present as an impurity are evolved at temperatures & > 750°C and can be estimated by TG analysis. The specific surface area decreases with Ni contents ⩽ 5% but increases for higher Ni contents. Catalyst samples containing 15% Ni possess the highest specific area at all temperatures. Pore structure analysis showed that microporosity increased with increase in Ni content for the catalyst series 1N–4N. Samples from preparations 3Nb and 3Nc showed more mesoporosity than that of 3N. Thermal treatment causes widening of the pores for catalysts 1N–3N becoming predominantly mesoporous, co-existing with some micropores. Catalyst samples with 15% Ni remained predominantly microporous-mesoporosity increasing only at 1000°C.

Structural and surface aspects of thermally treated phosphated and sulphated silica gel

Abstract Silica gel submitted to phosphate and sulphate treatments and heated in the temperature range 150–600°C has been studied by adsorption of N 2 at − 195°C and C 6 H 12 and CH 3 OH at 35°C. Structural characterizations were performed by XRD and TG. Silica gel, impregnated with ammonium phosphate solution and heated to 300°C forms (NH 4 ) 8 H 2 (P 3 O 10 ) 2 ·3 H 2 O, with characteristic bands at d -distances of 5.305 A, 3.08 A and 3.76 A. At 150°C, bands at 4.33 A and 3.91 A may point to a condensed phosphate adsorbed on silica. Dimerization of (NH 4 ) 2 SO 4 is favoured upon adsorption on silica and (NH 4 ) 3 H(SO 4 ) 2 is the main product at 300°C. At 600°C, both impregnated silica species are amorphous to X-rays. Thermal treatment of phosphated silica is accompanied by a continuous decrease in N 2 -areas whereas the reverse results from sulphate treatment. Phosphation appears to favour dehydroxylation and enhances sintering. The organic probe molecules do not measure the true area. Methanol adsorption is largely affected by the decomposition products of the impregnating salts on the surface at temperatures Pore structure analysis from N 2 -adsorption data shows that phosphate treatment blocks some of the narrower pores leading to higher average pore radii compared with the sulphated samples. Shrinkage of phosphated silica commences at 500°C.

Surface properties of montmorillonite, an expanding-type clay mineral

Abstract The adsorption of nitrogen at -196 °C, and of water and cyclohexane vapours at 35 °C, was measured on montmorillonite samples prepared by heating the clay mineral at various temperatures between 30 and 1000 °C for 5 h in vacuo. Specific surface areas were estimated from both the BET method (SBET) and the cumulative values obtained from pore structure analysis, known as Scum. The results obtained indicate in general that water and nitrogen are able to penetrate between the layers due to the expansion properties of the mineral, and in such cases Scum >SBET. Cyclohexane, with a much bulkier molecule having a non-polar character, is unable to produce such an effect. Heats of immersion were also measured in water and in cyclohexane, and it is suggested that, for adsorbates which actually penetrate between the layers (e.g. H2O), the heat values should be referred to Scum and not to SBET.

Effect of structure on the mechanism of dehydration of clay minerals

Abstract The kinetics of the isothermal dehydration of two clay minerals, attapulgite and powdered vermiculite were studied both in vacuo and in the presence of constant water vapour pressure. The mechanism of dehydration was found to be interfacial where diffusion plays a dominant role and it is called “activated diffusion”. A comparison is made with some layered structure clay minerals.