Anwar Amin

Structure and phase changes in thermally treated mixed hydroxides of Mg and Al-effect of compaction

Abstract Mechanically mixed (I) and coprecipitated (II), hydroxides of magnesium and aluminium were investigated in both powder and compact forms in the temperature range 200–1000°C using DTG, DTA and X-ray techniques. Upon thermal treatment of the mechanically mixed hydroxide, the solid material gives rise to products which at first become poorly crystalline and then gradually pass into the crystalline state indicating a mixture of MgO, θ, δ and α—alumina together with a new spinel. This new spinel is stable in the temperature range 400–800°C—three of its distinct d -spacings are at 4.87, 3.86 and 3.74 A. Coprecipitated hydroxides (II) proved to constitute a new species—probably a hydrated spinel. The spinel MgAl 2 O 4 commences to form at a temperature as low as 200°C. Compaction with either 10 or 30 tons in. −2 decreases the crystallinity of products from (I) dehydrated ⩽500°C, whereas it favours crystallization for the products from II. Above ∼500°C, compaction has no appreciable effect on crystallization.

Effect of Autoclaving On Zinc-oxide in the Presence of Sulfate-ions

Abstract The effect of autoclaving a zinc oxide preparation containing SO 2− 4 under 5 and 10 atmospheres is studied by combining X-ray diffraction, differential thermal analysis, thermogravimetry and IR spectroscopy. Textural measurements are also carried out on the parent samples and those produced in the temperature range 200–1000°C. A new phase of a basic carbonatesulphate, including ammonia in its coordination shell, is observed in the original preparation and having its d distances at 11.060, 8.954 and 2.714 A. This is transformed to another phase at ∼180°C which is also the main phase characterizing the autoclaved samples, and belongs to a basic zinc oxide—sulphate possessing d distances at 7.055, 2.468 and 2.805 A. Autoclaving the oxide preparation under 10 atm gives hexagonal zinc oxide of high purity and crystallinity at 1000°C. An empirical formula is given for the oxide preparation which describes the different decomposition stages observed. At ∼390°C, a reversible reduction process comprising oxygen evolution is observed. Autoclaving increases the area of the parent oxide and at temperatures below 600°C is a function of the structural changes. The autoclaving pressure is insignificant ⩾600°C. Pore structure analysis showed all the samples to be predominantly mesoporous, coexisting with some micropores except that autoclaved under 5 atm and heated at 250°C which is predominantly microporous. Autoclaving under 5 atm causes narrowing of the pores for products below 600°C. Autoclaving has little effect on the average pore radius ⩾600°C. Evaluation of the average pore radius from the constructed t -curves for parallel-plate pore idealization is discussed.

Effect of compaction and acid treatment on the surface area and porosity of kaolinite, alumina and silica gel

X-ray analysis, surface area and porosity of kaolinite, silica gel and alumina as powder or compacted at 50 and 100 kN cm−2, before and after treatment with 4 N HCl, have been studied. All samples of kaolinite exhibit the diffraction pattern of aluminium silicate hydrate (Al2Si2O5 (OH)4); for alumina, the diffraction pattern of alpha basic aluminium oxide (α-Al2O(OH)2) and for silica gel all the X-ray diffraction patterns are of an amorphous phase. A slight decrease in the intensity of the diffraction lines for the compacted solids are due to enhancement of solid-solid interfaces. For kaolinite and alumina, generally compaction is associated with a loss in surface area due the adhesion of neighbouring particles and blocking of a significant fraction of micropores. Acid treatment is also accompanied by a slight change in surface area. For silica gel there is no appreciable change in the texture parameters between powder and compacted solids. Analysis of the nitrogen adsorption isotherms using the αs method of Sing indicated the presence of narrow pores and mesopores in the original kaolinite powder. For all other samples of kaolinite and alumina, both powder and compacted forms before and after acid treatment, mesopores only are present. For silica gel, all samples essentially contained micropores. A correlation between the amount of N2 adsorbed for compacted samples relative to the uncompacted samples at the same relative pressure (F-plot) was established to follow changes in the shape of the isotherm consequent to compaction and/or acid treatment. These changes show up clearly as deviations from the horizontal line, providing information on the surface area and any capillary condensation that occurs.

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.

Textural variations in CeO2 on thermal treatment in various atmospheres

Abstract The products of the thermal treatment of precipitated CeO 2 (pH 8.75) were obtained by heating in a reducing (hydrogen), an oxidizing (oxygen) or an inert (nitrogen) atmosphere in the temperature range 150–550 °C. Surface area measurements of the samples were carried out using nitrogen adsorption at 77 K. Structural characterizations were performed by using X-ray diffraction (wide angle), thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The lattice parameter measurements were achieved by means of the X-ray diffractometer technique. X-ray analysis showed that the samples are well-crystallized f.c.c. CeO 2 . DTA and TGA showed only physically adsorbed water. A small exothermic peak at about 290 °C and a broad exothermic peak at about 520 °C result from a small autoreduction process and a sintering process respectively. Both processes are accompanied by a decrease in the lattice parameter, the autoreduction process in a hydrogen atmosphere only, whereas the sintering process is independent of the prevailing atmosphere. Heating the samples at 150 °C in any atmosphere increases the surface area. At higher temperatures, the surface area is a function of the variations in the solid texture imposed by the prevailing atmosphere. Oxygen is strongly adsorbed onto the surface, probably in a dissociated state. Pore structure analysis showed that the original sample and the products obtained in the various atmospheres possess both mesopores and micropores. At 550 °C oxygen facilitated the production of a product with a large surface area and an average pore radius of 13.2 A, whereas a hydrogen atmosphere enhanced sintering and gave a product with a wider average pore size.

Effect of flourination on the surface texture of silica gel

The adsorption of nitrogen at -195 °C and of cyclohexane and methanol at 35 °C was measured on pure and fluorinated amorphous silica which had been treated in the temperature range 150–600 °C. X-ray diffraction, differential thermal analysis and IR spectroscopy were used to characterize the structure. When the silica gel is impregnated with a concentrated NH4F solution (NH4)2SiF6 is formed which decomposes endothermally at 280 °C. Partial fluorination decreases the surface area markedly, whereas more extensive fluorination causes complications due to the presence of the decomposition products of (NH4)2SiF6 on the surface. The organic probe molecules do not measure the true surface area, and methanol adsorption is strongly affected by the presence of water at temperatures of 500 °C and below and by the pore size at temperatures above 500 °C. Pore structure analysis using the nitrogen adsorption data shows that the pure gel consists of mesopores of two different average sizes coexisting with some micropores. These pores are partially inaccessible to the methanol molecules and behave as narrow pores. Partial flourination causes narrowing of the pores probably as a result of the increase in the bond angles produced by changes in the oxygen hybridization. The significance of evaluating the average pore radius at different relative pressures for P/P0 > 0.90 is discussed.

Compaction versus Surface Parameters of Certain Solid Catalysts

The effect of compaction at 0.31–3.1 t/cm2 on Al(OH)3, Mg(OH)2 and their mixed hydroxide was studied by nitrogen adsorption. With Al(OH)3, compaction gave no significant changes up to 1.24 t/cm2, but above 1.55 t/cm2 a considerable decrease in SBET and an increase in pore radius occurred with small changes in pore volume. This could be ascribed to the presence of free water between the structural layers in the material which normally prevents their contact and a consequent destruction of the pore structure. With Mg(OH)2, compaction at low pressures decreased SBET and increased the pore dimensions as a result of adhesion between neighbouring particles, leading to a blocking of that fraction of the micropore structure originally accessible to nitrogen molecules. Increasing compaction led to a marked increase in the adsorption capacity as a consequence of plastic deformation associated with the breakage of fragile primary particles and the creation of new surfaces. Compaction of the mixed hydroxide led at first to an increase in both the SBET and Vp values (due to fragmentation of the particles), followed by a loss of SBET due to the presence of a mixture of particles in the system which increase the compression ability of the latter. Complete pore structure analysis showed that samples of Al(OH)3 powder when compacted at 0.31 and 1.55 t/cm2 were microporous. All other samples contained mainly mesopores.

Effect of halide impregnation on the structure and surface characteristics of rutile

Titanium dioxide (TiO2, rutile) subjected to fluoride (1 M and 2 M NH4F) and iodide (1 M KI) treatment has been investigated by combining several approaches: structural and textural characterization as well as surface acidity and basicity. Fluorination reduces the crystallinity at both concentrations studied, the higher concentration being more effective. Iodination has the same effect. Fluoride ions replace all basic OH groups whereas iodination does not lead to full replacement due to competition with the Ti4+ substrate. A distinct exotherm is observed at 228°C and 235°C, respectively, for the F− and I−treated samples, resulting from energetic changes leading to the activated dissociation of water and restoration of the basic OH groups. A surface compound is apparently formed on fluorination and iodination, and this decomposes endothermally at 270–275°C. In addition, impregnation with I− appears to increase the photosensitivity of the TiO2 leading to the formation of a carbonate with atmospheric CO2 which generates an endotherm at 285°C and whose existence has been confirmed by IR spectral analysis. Specific surface areas and pore structure analyses demonstrate differences not only upon increasing the F− concentration but also between the F− and I− treated samples, where the addition of the latter markedly reduced the surface parameters. Two groups of pore sizes could be recognized with most samples. Increasing the F− concentration increased the sintering temperature to 550°C, the value for the low F− sample being 400°C.

Adsorption of nitrogen, water and certain organic vapours on fully and partially hydroxylated silica gels with different porosity characters

Abstract The adsorption of several adsorbate molecules, namely nitrogen, water, carbon tetrachloride, benzene and methanol, on fully and partially hydroxylated silica samples was investigated. The samples tested had different porosity characters, i.e. they were microporous (Gasil 200), mesoporous (Gasil 35) and non-porous (TK800), in order to elucidate the effect of the porosity of the gel on specific and non-specific adsorbate-adsorbent interactions. Nitrogen adsorption, which is assumed to involve non-specific interactions, is influenced to a certain extent by the specific interaction between the nitrogen quadrupole and the surface hydroxyl. The smaller polar water molecules can penetrate into pores which are too narrow to accommodate larger adsorbate molecules. The marked reduction of the water uptake upon dehydroxylation of the silicas substantiates the important role of the specific interaction in the adsorption of water on silica surfaces. The carbon tetrachloride molecule appears to be a more sensitive probe than the nitrogen molecule for detecting microporosity, its only drawback being its larger size. Benzene adsorption depends mainly on the interaction between the mobile π bonding and surface hydroxyls. Finally, non-specific interactions appear to have a marked effect on the adsorption of polar methanol molecules on silica surfaces which involve essentially specific interactions.