Mohamed Abdel-Khalik

The effect of impregnation conditions on the surface structure of silica-supported CuO catalysts

CuO/SiO2 catalysts with varying amounts of copper were prepard using meso- and microporous silica supports at pH > 10 and pH = 4.5. Structural and textural changes were followed using X-ray diffraction, TG and DTA techniques. Impregnation for periods > 10 days at high pH produces crystalline catalysts with two distinct peaks at d-spacings of 2.33 and 2.03 A resulting from a surface silicate which is structurally stable up to 800°C. At copper concentrations > 5% CuO also forms. Catalysts prepared at pH = 4.5 are amorphous to X-rays in spite of the presence of CuO which may either be < 50 A or from a surface solid solution. The copper ammine complex, if adsorbed on mesoporous silica, attains its maximum coordination number as [Cu(NH3)4(H2O)2]2+, whereas on microporous silica it loses the two water molecules as a result of pore restrictions. The surface complex releases its coordinated ammonia exothermally in the temperature range 200–400°C, whereas chemisorbed ammonia is evolved endothermally at ∼280°C. Ligand water is evolved at <200°C. An exotherm at ∼545°C is observed for all catalysts, resulting form the shrinkage of the solid/void matrix which disappears upon aging. Increase of copper content to 22.7% at high pH lowered the temperature of constant weight attainment from 1000°C for the pure silica to 750°C.

Effect of thermal treatment on catalytic and surface characteristics of a diluted supported CoAl2O3 catalyst

The sintering of a diluted sample of CoAl2O3 hydrocracking catalyst was investigated in the temperature range 300–450 °C. Both activity (A) and metallic surface area (Sco) measured through chemisorption of hydrogen showed essentially the same sintering trends: deactivation below 400 °C and activation above this temperature. In the temperature region below 400 °C, the activation energy of the sintering process, determined from both activity and chemisorption data, was found to be 14.70 kcal/mole. This is thought to be due mostly to surface diffusion. On the other hand, activation occurred at 450 °C as a result of the increased migration of metal atoms on the support surface. This indicates that a solid solution with lowered melting point might be formed, and the sintering step becomes rate-determining in this temperature region. Changes in the total surface area and in the pore-size distribution of the support during the heat treatment were also studied.

Surface and catalytic characteristics of thermally and chemically activated bentonite catalysts used in the polymerization of styrene

Abstract The most active bentonite samples in the polymerization of styrene were found to be one thermally treated for 2 hr at 550 °C and one chemically treated with 35% ( w w ) H2SO4. Surface studies of various bentonite samples showed that the characteristic pore system and Lewis acid sites resulting from the removal of the adsorbed water were responsible for the catalytic activity of the thermally treated bentonite. Overheating could destroy both the catalyst activity and the exposed surface. Upon activating bentonite with H2SO4, it was found that both the produced pore system and the creation of new Bronsted acid sites could be responsible for the catalytic activity. The different factors affecting the polymerization reaction over the most active samples and the dependence of the structure of the produced polymer on the pretreatment of the bentonite were also investigated.

Mechanical properties in relation to the microstructure of fibre reinforced portland clinker pastes

Abstract Portland clinker was mixed with various water/clinker ratios ranging between 0.7 to 0.2 covering the range for both “normal” and “low” porosity pastes. These pastes were reinforced with five kinds of fibres, all with weight percentages of 0.5 and 3.0. All the samples were tested for compressive and tensile strengths, total porosity, microstructure and degree of hydration. Fibre reinforcement led to an increase in the total porosity and a decrease in the degree of hydration as compared with the neat pastes. Also, compressive strength has to be sacrificed to a certain extent in order to obtain better flexural strength. SEM furnished direct evidence that fibre reinforcement could affect the pore structure, the habit and shape of the hydration products, as well as their spatial distribution. The indication gained is that the mechanical behavior of the mix is due mainly to changes in the physicochemical properties induced by the presence of fibres.

Autoclaved slag-clinker-sand pastes surface area, pore structure and compressive strength

Abstract Surface areas, total pore volumes and pore size distributions were measured from the adsorption of both nitrogen and water vapour on hydrothermally hardened slag-clinker-sand pastes. The compressive strength of the hardened specimens could be related to the pore structure and pore size distribution curves, and the interesting result obtained is that mesopores affect the compressive strength of the autoclaved pastes more than micropores, thus providing another factor to be considered in addition to the total porosity of the paste.