Suzy A. Selim

Thermal decomposition of ammonium metavanadate

Abstract Ammonium metavanadate is subjected to thermal treatment in the range 180–550°C and structural and surface area changes are studied by X-ray diffraction and N 2 adsorption. DTA and TGA show the existence of a large endotherm (260°C) possibly composed of several stages, and a much smaller one (340°C) which is responsible for evolution of the molecule of water associated with the V 2 O 5 initially formed. Three exotherms also appear and explanations for their presence are given. The phases formed, as well as the specific surface areas, are determined for the products obtained in a vacuum and in the presence of water vapour, and changes in surface areas are related to the phase transformations and dehydration of the products formed during thermal treatment.

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.

Surface properties of thermally decomposed ammonium metavanadate under various atmospheres

Abstract Adsorption of nitrogen was measured on the decomposition products of ammonium metavanadate produced by heating in the presence of air, oxygen and water vapour (4.6 mm Hg). The products were identified by X-ray diffraction. No reduction of the vanadium pentoxide is observed by heating in air in the temperature range 350–450°C whereas in oxygen or water vapour (4.6 mm Hg) a crystalline product close to V 2 O 5 is produced, possessing two characteristic lines at d -distances of 3.520 and 3.325 A — this product appears to exist in the hydrated form in the latter atmosphere. Changes in surface area are related to both the phase transformations and the extent to which it is accomplished. Pore structure studies reveal the products to possess characteristic pore widths which do not permit capillary condensation to take place. A method of correcting the reference data is introduced for shifts of ∼10% between S BET and S t areas — a significant criterium for pore analysis.

Effect of thermal treatment on the structure and texture of titania

AbstractAnatase and rutile pigments, from two sources (B and T) were thoroughly purified from sulphate and chloride contaminants, thermally treated in the temperature range 150 to 550 ° C, and investigated using thermogravimetric (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), nitrogen and organic vapour adsorption techniques. TG analysis reveals two main dehydration steps, the first results from physically adsorbed water and the second from structural and ligand water. The number of ligand water molecules released through a unit surface area (nm2) is in the range 4.50 to 5.15, being evolved in the temperature range 250 to 300 ° C. Two dehydroxylation endotherms appear for the anatase samples in the temperature range 350 to 420 ° C which seem to arise from the presence of two types of hydroxyls. No transformation from the anatase to rutile sructure occurred in the temperature range investigated. Estimation of crystallite sizes showed a marked increase at temperatures >250 ° C for anatase (B), being greatest for the (101) plane, and >400 ° C for rutile (B), where the three planes (101), (110) and (111) increased distinctly. Maximum anisotropy was observed for the anatase heated at 550 ° C. Nitrogen adsorption data revealed a marked decrease in the specific area and total pore volume by thermal treatment ⩾400 ° C for anatase and ⩾250 ° C for rutile whereby it retains a nearly stable

NiO-Al2O3 catalysts prepared at high pH variation of structure and texture upon thermal treatment

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

value with an average range of 2.62 nm. The anatase (B) samples are composed of a mixture of both meso- and micropores whereas for rutile (B) microporosity appeared only for the sample heated at 150 ° C, becoming predominantly mesoporous at higher temperatures. Most heated samples exhibited two group sizes in the mesopore range resulting from their existence in the form of particles constituted from a collection of small particulates. The rutile (B) products are generally characterized by possessing a wider pore system than those from anatase. The anatase (T) samples are predominantly microporous at 150 ° C and become mesoporous at 550 ° C — the reverse is true for rutile (T). Cyclohexane and benzene adsorption measure only a fraction of the nitrogen area. Specific interaction (H-bonding) is believed to exist in some cases between the cyclohexane molecules and titania surface hydroxyls, as well as some enhanced adsorption which is believed to occur with benzene adsorbate.

Effect of fluorination on the surface texture of zirconia

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

A series of five catalysts of NiO-Al2O3 of varying content (IA-VA) were prepared at pH ∼ 10 and dried at 80°C using neutral mesoporous alumina composed of μ-Al2O3 and poorly crystalline boehmite. Structural changes produced upon heat treatment up to 700° C were followed using XRD, TG and DTA techniques. The surface texture of the samples was investigated by nitrogen adsorption at 77 K. The effect of soaking period on the resulting preparation and on its thermally treated products was also investigated (III Aa).The presence of nickel in the form of the amine complex had a marked effect on the crystalunity of the support changing the boehmite to the well crystalline form-an effect which was not observed in the absence of the nickel amine complex. The presence of NH3 alone in the medium resulted in the appearance of two broad bands covering d distance ranges of 0.545 to 0.521 nm and 0.432 to 0.386 nm that also appeared in the XRD patterns of the catalyst samples. The ammonia penetrates between the layer structure of the support thus changing its interlayer distance.Soaking for 7 days was found to represent non-equilibrium conditions compared to soaking for 15 days.NiAl2O3was characterized in the catalyst samples and NiO appeared in samples heated at 225° C and increased at 325° C resulting from the decomposition of the physically adsorbed surface species [Ni (NH3)6](NO3)2 and [Ni (NH3)6](OH)2, respectively. The latter became pronounced for higher nickel concentrations as observed from DTA and DTG. The former decomposes exothermally at around 260° C and the latter endothermally at 325° C after losing the NH3 ligand in the temperature range 245 to 265° C.At nickel contents greater than 4.6% and at temperatures greater than 225° C a surface compound appeared with characteristic d distances in the ranges 0.202 to 0.2034 nm and 0.2363 to 0.2348 nm depending on treatment temperature. It results from the attack of the nickel amine complex on two adjacent hydroxyl groups.An increase in the surface parameters of Al2O3 was observed upon soaking in NH4OH solution alone and from pore analysis is found to contain two groups of mesopore sizes. The presence of the least amount of nickel content (1.5%) produced a marked increase in both specific area and total pore volume accompanied by a decrease in average pore radius. These changes reflect the structural changes of the support observed upon impregnation in the nickel-amine complex. Variations of the surface parameters of the catalyst samples with further increase in nickel content depend on the nickel species formed. However, at a nickel content of 9.6% more pores are being exposed that result from the penetration of more nickel ions between the support particulates. At still higher nickel contents blocking of the narrower pores and narrowing of the wider pores occurs as their VI−t plots indicate. Despite this narrowing the catalyst samples are still predominantly mesoporous retaining, in most cases the existence of two groups of pore sizes.Variations in soaking period seems to affect the texture of the low temperature samples but for samples treated at temperatures above 500° C the surface parameters are comparable irrespective of the period of soaking.