Raouf Sh. Mikhail

The contact angle in mercury intrusion porosimetry

Abstract The general theory of the contact angle of mercury on complex, internal surfaces of solids, i.e. within pores that cannot be approximated as circular cylinders, is examined. A model for such pore surfaces, in which the pores are represented as cylinders with surfaces that are very rough and that contain void spaces corresponding to the entrances of branch pores, is developed. This model enables us to conclude that in the Washburn equation, for the pressure required to force mercury into a pore of radius r, the contact angle that should be used is 180°, particularly for macropores and mesopores in most practical solids.

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.

The ancient Egyptian balance

Abstract The oldest known balances are equal-armed instruments that have been found in Egypt and are represented on Egyptian drawings and reliefs. In its simplest form the beam was made of wood or stone and provided with a central boring for the balance suspension and two borings at the ends of the beam for the pan suspensions. The existence of a balance standard in the Fifth Dynasty is testified. A plummet line was hung parallel to the balance suspension so that the horizontal position of the beam could be checked by comparing the right angles formed between beam and plummet line. From the time of the Middle Kingdom the pans were suspended by four cords. In the New Kingdom tubular beams were introduced whose ends were shaped in lotus flower or papyrus form. The strings of the pans came out together from inside the beam and diverged to the sides, the lower beam ends acting as knife edges. In most cases a pointer is either missing or so short that it is hardly helpful. This type of elaborate standard balance which was also distributed in Syria, Greece and Persia, is often depicted in the Books of Death of the New Kingdom in the scene showing the “Death Tribunal” with “Weighing of the Heart”. Unclear drawings led to misinterpretations as to the design and mode of operation of these balances. By style analysis and comparison with finds exhibited in the Cairo Egyptian Museum, it was possible, however, to clarify details.

The effect of the variation in the pH and the carrier porosity on the surface properties of CuO-SiO2 catalysts

Abstract The surface areas of four series of CuO-SiO2 catalysts prepared at pH values larger than 10 and at pH 4.5 using mesoporous (59) and microporous (03) silica gels as supports were measured by nitrogen adsorption at 77 K. The surface area of the mesoporous silica gel was more strongly affected at pH 4.5, where the formation of micropores was favoured, than at pH values greater than 10, where the original mesoporosity was retained. The reverse behaviour was observed for gel (03). At high pH values the incorporation of about 1% Cu in either gel (59) or gel (03) affected the surface area. Increasing the copper content raised the surface area of gel (59) whereas that of gel (03) was decreased. A maximum in the surface area was observed at 400 °C for catalysts supported on silica (59) containing more than 1% Cu. The reverse behaviour was observed for such catalysts on silica (03). The location and the attachment of the copper, whether on the external surface or in the pore system, were significant parameters controlling the specific area. At low pH values the introduction of 1% Cu caused a decrease in the surface area of gel (59) and an increase in that of gel (03). A further increase in the copper content produced the same effect as that observed at high pH. Sintering was enhanced when microporous silica gel was the carrier. Pore structure analysis showed that catalysts supported on either carrier and containing about 1% Cu in the form of the ammine complex were predominantly microporous, whereas at higher copper concentrations both mesopores and micropores existed. Thermal treatment created mesoporosity in catalysts supported on carrier (03). The catalysts prepared at low pH values were predominantly microporous at temperatures up to 800 °C for carrier (03) and at temperatures below 800 °C for carrier (59).

Studies on alumina-silica cracking catalysts I. Surface area and pore structure

Abstract The effect of heat treatment under vacuum on the surface properties of pure and doped alumina-silica cracking catalysts has been determined from nitrogen adsorption isotherms. Doping with V 2 O 5 by impregnation leads to lower specific surface areas than those found in the corresponding unimpregnated samples. Analysis of the adsorption isotherm by means of the t -method reveals that, when the sample is originally microporous, doping causes blocking of a number of pores without appreciably affecting their size. For samples that are mesoporous with a narrow pore volume distribution, doping causes blocking of certain pores with a slight shift of the peak location to larger radius values. when the sample is mostly mesoporous with a wide pore volume distribution, pore blocking due to doping plays a minor role and the main effect is a change in the size of the pores as well as in the nature of the surface.

The effect of compaction of the surface characteristics of thermally treated mixed hydroxides of magnesium and aluminium

Abstract Adsorption measurements for cyclohexane and methanol at 35 °C were performed on thermally treated mechanically mixed (I) and coprecipitated (II) hydroxides of magnesium and aluminium heated in the temperature range 200–1000 °C. At all temperatures except 1000 °C precompaction increases the specific surface area of the mixed hydroxide I. This is more apparent from methanol adsorption data which are more sensitive to variations in the sizes of the micropores. The effect of compaction on the coprecipitated mixed hydroxide II shows no regular trend for surface area changes over the above temperature range owing to variations in the pore structure and to interfering factors which invariably influence the surface structure. At 1000 °C precompaction accelerates crystallization and sintering in both I and II as a result of the increased number of solid-solid contacts. A complete pore structure analysis showed both mixed hydroxides to be a mixture of mesopores and micropores. Compaction increases the micropore fraction as measured by cyclohexane at temperatures of 600 °C or less and this is reduced to zero at 1000 °C for both preparations. For methanol the precompaction treatment causes some widening of the micropores at 400 °C for I and at all temperatures except 500 °C for II. With a polar molecule such as methanol chemical or enhanced adsorption in micropores may take place. For practical purposes a means of differentiating between the two effects from pore structure analysis is highly desirable and this is offered in the present work.

Studies on alumina-silica cracking catalysts II. Catalytic activity

Abstract In the present investigation, alumina-silica gels, as well as V2O5 doped samples of various pore structures, have been examined for cumene cracking activity in a fixed-bed catalytic reactor connected to a flame ionization detector. On estimating the catalytically active sites by adsorption of pyridine on the catalysts, using a vacuum microbalance technique, a correlation between acid sites and OH content was noted. An attempt was made to relate the activity towards cumene cracking with the chemistry as well as the pore structure of the substrate. Doping with V2O5 leads to poisoning of some active (acid) sites on the surface. The rate of cumene cracking is approximately proportional to the surface area, and the rate per unit area is roughly proportional to the surface density of acid sites. The activity of a site in the cumene reaction depends not only on whether it is a Bronsted or Lewis acid site but also on the energetic state of the site. The effect of the pore structure thus appears to be indirect in the sense that it is not the accessibility of the reacting molecules to the site, but rather the energetics of the site; this is different in micropores and wider pores.