Outgassing Temperature Research Articles

ChemInform Abstract: VARIATIONS OF HEAT OF IMMERSION OF FERRIC OXYHYDROXIDES IN WATER WITH STRUCTURAL CHANGES BY HEATING

AbstractDie Beziehung zwischen der Oberflächenaktivität von 04‐, β‐ und 7‐FeOOH und seinen Strukturänderungen beim Erhitzen wird durch Bestimmung der lmmersionswärmen in Wasser sowie der Adsorption von Wasserdampf untersucht.

Variations of Heat of Immersion of Ferric Oxyhydroxides in Water with Structural Changes by Heating

Abstract The relationship between the activity of ferric oxyhydroxides (α-, β-, and γ-FeOOH) and structural changes has been investigated by determining the heat of immersion in H2O and H2O vapor adsorption. For each ferric oxyhydroxide, the monolayer capacity of H2O adsorption and the heat of immersion decreased with the progress of transformation into α-Fe2O3. Above the transformation temperature the heat of immersion increased with the outgassing temperature. The heat of immersion of α-Fe2O3 obtained by outgassing each ferric oxyhydroxide at 400 and 500 °C increased with the accompanying increase in crystallite size. β-FeOOH showed higher heat of immersion and larger monolayer capacity per unit surface area than other ferric oxyhydroxides. It is presumed that the higher heat of immersion and larger monolayer capacity observed for β-FeOOH are ascribed to micropores in β-FeOOH crystals produced by the removal of molecular water originally contained.

Surface Structure of BeO Powders Obtained by Calcination of Sulfate

A study was made on the compositions and amounts of gases desorbed from BeO powders in the temperature range between 200° and 1100°C and the adsorption isotherms of water at 25°C on the powders which had been pretreated at 600°, 800° and 1000°C. The BeO powders were obtained by calcination of sulfate. These measurements showed that the BeO powders were changed in the surface structure or surface state as the calcination temperature was raised and the following conclusions were obtained from the present study.1) The BeO powders, calcined at lower temperature, adsorb or absorb SO and SO2 and are disordered in their surface structure. The structure are sensitive to the pretreatments.2) The amounts of gases desorbed from S900 BeO and S1200 BeO in the temperature range between 800° and 1000°C are much more than those between 600° and 800°C. The amounts of SO and SO2 desorbed from S900 BeO and those of CO and CO2 desorbed from S1200 BeO increase from 800°C, reach maximum at about 900°C and then decrease with increasing outgassing temperature.3) The surface structures are more ordered with the calcination temperature to about 1000°C but above this temperature they become again disordered, particularly at 1200°C.4) The monolayer capacities of physisorbed water are not affected by the calcination temperature.

Surface Structures of BeO Powders Derived from Hydroxide

Surface structures of nonsinterable BeO powders have been studied on the compositions and amounts of gas desorbed from the BeO powders and the adsorption isotherms of water at 25°C. The BeO powders were derived from hydroxide. As a result of our study, following conclusions were obtained.1) The beryllium hydroxide, precipitated homogeneously from sulfate solution, contained many sulfate ions. SO and SO2 gases were desorbed from the BeO powders which had been calcined at temperature less than 1000°C, but little desorption of those occurred above 1000°C.2) Surface structures of the BeO powders, calcined lower temperature, were sensitive to outgassing temperature3) Surface structures of the BeO powders were not sensitive to calcination temperature.4) The largest amount of the chemical adsorption of water was found in the BeO powder which was calcined at 1000°C.5) Outgassing temperature affected strongly the monolayer capacities of physical adsorption of water vapor on the BeO powders which were calcined at lower temperature, but calcination temperature affected weakly them.

Heats of Wetting of Various BeO Powders in Water

In order to study nature of BeO powders, calcined at various temperatures, we have measured the heats of wetting (ΔHi) at 25°C in water. They were produced from sulfate and hydroxide. The BeO powders produced from sulfate and calcined at 900°C was named S900BeO and that produced from hydroxide and calcined at 800°C, H800BeO and so on.1) ΔHi of S1100BeO and S1200BeO increases monotonously as outgassing temperature is raised, but ΔHi of S900BeO, S1000BeO, H800BeO and H1000BeO reaches maximum at outgassing temperature of 800°C ΔHi of H1200BeO and H1300BeO outgassed at 800°C is nearly equal to that outgassed at 1000°C.2) ΔHi of the BeO powders produced from hydroxide is larger than that from sulfate in the region of this study but outgassed above 800°C.3) ΔHi of the BeO powders made from sulfate differs one another with calcination temperature. Above outgassing temperature of 600°C, ΔHi of the BeO powders increases as calcination temperature is raised. ΔHi of H1000BeO is the largest among those of the BeO powders produced from hydroxide but the differences in ΔHi among them are small.

The effect of heat treatments on the semiconductor properties of a commercially electrodeposited manganese dioxide

AbstractThe temperature dependence of d.c. conductivity and thermoelectric power has been studied for a series of thermally treated samples of an electrodeposited, microcrystalline, γ‐phase MnO2. Measurements were made on samples compacted at 3.86 × 105 KN/m2. Measurements of a.c. impedance up to 50 MHz were made on the powder samples to test for particle homogeneity.The results indicated that a hopping electron transport model was probably appropriate for these samples and that the majority carriers were electrons, possibly ionised from donor sites provided by oxygen deficiency. Conductivity, surprisingly, was found to increase in an oxygen environment and it is suggested that oxygen adsorption greatly enhances the electron mobility.The d.c. conductivity and thermoelectric power were found to be strongly dependent on sample outgassing temperature. The former reached a maximum at about 320 °C, the latter became less negative with increasing outgassing temperature. It is tentatively proposed that carrier density and “surface” mobility increased as terminal OH groups forming strong traps on manganese sites were removed by thermal treatment.Measurements of a.c. impedance on samples outgassed at 35 °C and 365 °C displayed dispersion around 20 MHz indicative of a high resistance surface skin possibly due to a hydroxylated and reduced surface respectively.

Studies of the Properties of the Basic Sites on the Silica-alumina Surface through the Reaction of the Basic Sites with Copper(II) Complexes Containing 2,2′-Bipyridine

Abstract The substitution reactions of complexes with basic sites on the surfaces of silica-alumina and alumina and the properties of the basic sites were studied through the adsorption of the complexes onto the surfaces. The hydroxyl groups with basic strengths of two kinds, H0>9.3 and 7.2<H0<9.3, exist on the silica-alumina surface pretreated at 300°C. They are present in a cluster state. The basic strength becomes higher as the outgassing temperature becomes lower. The complexes predominantly interact with the O2− ions on the surface pretreated at a relatively high temperature, resulting in new chemical species. The basic strength of the O2− ions is lower than that of the hydroxyl groups on the surface pretreated at a lower temperature. The hydroxyl groups on the alumina surface outgassed at 300°C are distributed, on the whole, homogeneously in comparison with those of the silica-alumina surface, and their basic strength is H0>9.3. The nature of the basic sites on the alumina surface tends to vary with the outgassing temperature, similarly to those on the silica-alumina surface. These analyses were performed on the basis of the ESR spectra for complexes in basic solutions.

The Surface Properties of Rutile Prepared by the Hydrolysis of Titanium Tetrachloride. I. The Heat of Immersion in Water

Abstract The surface properties of rutile prepared by hydrolyzing titanium tetrachloride have been studied by measuring the heat of immersion in water as a function of the outgassing temperature. The heat of immersion showed an extremely large value at outgassing temperatures between 300 and 400°C. However, when the rutile was calcined at 800°C, it showed a heat-of-immersion curve similar to those hitherto reported. In order to examine the phenomenon found on uncalcined rutile, the amounts of water physically and chemically adsorbed on the surface were estimated by measuring the water-vapor adsorption. We found that hydroxyl groups were formed in bulk as well as on the surface by means of the chemisorption, resulting in the formation of a hydrous structure because of the low crystallinity of the sample at temperatures below 400°C.

Adsorption on porous thorium oxide modified by water

A sample of thorium oxide containing pores in the micro and transitional size range has been subjected, in a vacuum microbalance system, to various outgassing temperatures and water vapor treatments. The adsorptions of nitrogen and argon were used to investigate the pore structure. Changes in the pore structure of the solid and the nature of the bound water were observed to alter significantly the energetics and extent of nitrogen and argon adsorption. These properties emphasize the difficulties involved in characterizing an active solid of this type. An outgassing temperature as high as 400°C was required to eject water blocking entry to the micropores. As expected, nitrogen is able to penetrate the partially hydrated fine pore structure better than argon. Comparison (by means of t-plots) of nitrogen adsorptions showed the importance of carefully selecting the temperature of outgassing and the standard reference isotherm. This becomes especially true for measurement of the micropore volume; recommendations are included.

Some surface and catalytic studies on chromic-oxide gel

Three different samples of precipitated chromic oxide gel were prepared, and their surface and catalytic properties were studied. The techniques of DTA, TGA, X-ray diffraction, and electron microscopy, together with adsorption measurements, have been used to provide information regarding the water content, porosity, and surface area of the samples. In particular, the manner in which these properties change as the pretreatment temperature is varied has been studied. The isomerization of n-but-1-ene, to cis-but-2-ene and trans-but-2-ene at 25 °C, has been used as a test reaction to examine the manner in which the catalytic activity is dependent upon pretreatment temperature. The initial cis-but-2-ene/ trans-but-2-ene product ratio can give indications as to the type of reaction mechanism operative. The present work suggests that there are two different types of active sites on the chromic-oxide catalysts. At relatively low pretreatment temperatures, the reaction appears to proceed via a carbonium ion type of mechanism; and the sites responsible for such activity are believed to be associated with surface hydroxyl groups. At higher out-gassing temperatures the initial product ratio is greatly increased, and the reaction mechanism is best explained in terms of an allylic type of intermediate. It is suggested that the active sites responsible for this latter type of behavior are surface sites that are strained, such as would be produced by the removal of water from two adjacent hydroxyl groups.

Hydrogen—deuterium exchange reactions on ion-exchanged X-type zeolites

The hydrogen—deuterium exchange reaction has been studied over a series of X-type zeolites. With CoX zeolites the activity depended linearly on the extent of ion-exchange showing that the Co 2+ ions do not enter preferentially the positions which are likely to be inaccessible to the reacting molecules. Enhanced activity was found after hydrogen treatment of CoX at 200 °C or CaX and MnX at 300 °C and to a lesser extent at temperatures about 50 K lower. This increase in activity was thought to arise from the displacement of water molecules from the active sites by chemisorbed hydrogen. Other evidence for the poisoning effects of residual water was obtained from the variation in the activity of a CoX zeolite with outgassing temperature. There was no correlation between activity and charge on the metal ions—CeX was only slightly more active than NaX and there was a large range in the activity of the zeolites containing different divalent ions (Ni ⪢ Zn > Co > Mn > Ca). It is possible that NiX, the only catalyst giving high rates below 0 °C, brings about the exchange by a mechanism involving the homolytic dissociation of hydrogen. The results show that the usefulness of deuterium for the study of the exchange reactions with hydrocarbons may be limited to a few of the more active zeolites such as NiX and possibly ZnX or CoX.

Infra-red studies of rutile surfaces. Part 1

Samples of rutile prepared by hydrolysis of TiCl4 and by combustion of Ti(isoPrO)4 have been investigated by infra-red spectroscopic techniques to provide information on their surface hydroxylation and hydration. After outgassing at ambient temperatures the rutile surface carries both hydroxyl ions and adsorbed molecular water. The depopulation of the surface caused by raising the outgassing temperature follows a pattern that suggests there are two types of surface site for the adsorbed molecular species. Complementary studies of pyridine adsorption indicate that the molecular water is held on the surface as a coordinating ligand and also show the presence of two types of hydroxyl species on the ambiently outgassed oxide. Further studies of the adsorption of thionyl chloride, sulphur dioxide and hydrogen chloride vapours support the contention that the surface hydroxyls are ionic in character. In the case of HCl adsorption, evidence for two types of coordinately bonded water was found after exposure of the oxide to the dry acid gas. It was also found that the sample prepared from TiCl4, which carried chlorine in its surface layers, was more readily dehydroxylated than the “chlorine free” material prepared from the metal alkoxide. The chlorine containing sample also aggregated on storage whereas the pure material did not.

Adsorption of water vapour on α-Fe2O3

The chemisorption of water vapour on de-hydroxylated α-Fe2O3 has been investigated by heats of immersion into water at 25°C. The properties of physically adsorbed multilayers on the hydroxylated surface have been studied using dielectric relaxation techniques in conjunction with adsorption isotherms. The heat of immersion of the completely hydroxylated substrate is –367 ergs/cm2 and increases with increased outgassing temperature to –1073 ergs/cm2 for outgassing at 375°C. Differentiation of the integral heat curve gives the initial heat of chemisorption to be at least –50 kcal/mol of water vapour. The electrical capacitance at 70 c/s to 300 kc/s of physically adsorbed water vapour on the hydroxylated substrate is constant within the B.E.T. monolayer, but rises sharply with the onset of the second layer. This behaviour suggests that the first layer of physically adsorbed water is immobile, but that succeeding layers are mobile. Confirming evidence as to the immobile nature of the physisorbed monolayer is given on an entropy basis. The integral entropy of adsorption obtained from multi-temperature isotherms at constant spreading pressure is –33.2 cal/mol K for half a monolayer. This value agrees favourably with the theoretical value of –36.8 cal/mol K calculated from statistical thermodynamics for a localized monolayer, but not with the value –15.9 cal/mol K derived for mobile adsorption. Characteristic relaxation frequencies of adsorbed water vapour were obtained from Cole-Cole are plots of dielectric constant against dielectric loss. The characteristic frequencies increase smoothly with coverage from 10 c/s to a value of 10 kc/s at B.E.T. coverages of three or more, suggesting the development of a hydrogen-bonded ice-like structure for those coverages.

Effect of surface hydroxyl groups on the reactions of i-propyl radicals adsorbed on silica at 77 K

The reaction of propylene adsorbed on silica gel with gaseous hydrogen atoms has been studied. Product selectivity [C3H8]/[C6H14], is a function of surface coverage, hydrogen atom concentration and out-gassing temperatures. Selectivity decreases as out-gassing temperatures are increased. This is interpreted in terms of increased surface diffusion due to decreased radical hydroxyl interactions. Exposure to saturated water vapour shows that surface dehydration is irreversible at high out-gassing temperatures. When propylene is diluted with ethane or propane to monolayer coverage product selectivity increases. The order of effectiveness in limiting surface diffusion is C3H8 > C2H6.

Nature of Acid Sites on the Surface of Silica-alumina. I. The Relation between the Acid Property of Sites and the Heat of Immersion

Abstract The acidity and the acid strength of the sites of such solid catalysts as silica gel, alumina, and silica-alumina were determined by the n-butylamine titration method using the Hammett indicators. The distribution of the acid sites on the surfaces of these catalysts was found to vary with the outgassing temperature, and also with the alumina content in the case of silica-alumina. The heats of immersion of these catalysts in water and in an aqueous solution of n-butylamine were measured. The heat value corresponding to the difference between these two cases, as well as the number of acid sites, showed a maximum at contents of 10–20%. These phenomena are discussed with reference to the results of the diffuse reflection measurements.

The heat of immersion of zinc oxide in water

The immersional heats of zinc oxide in water have been measured in connection with the variation of the outgassing temperature of the sample. The heats show a maximum on the sample treated at 400° to 500°C. This phenomenon can be explained in terms of chemisorption which occurs on the dehydrated site on the surface, followed by physical adsorption on the surface hydroxyl groups due to hydrogen bonding.

The heat of immersion of zinc oxide in water

The immersional heats of zinc oxide in water have been measured in connection with the variation of the outgassing temperature of the sample. The heats show a maximum on the sample treated at 400° to 500°C. This phenomenon can be explained in terms of chemisorption which occurs on the dehydrated site on the surface, followed by physical adsorption on the surface hydroxyl groups due to hydrogen bonding. The heats of adsorption and chemisorption of water molecules on the surface of zinc oxide were calculated separately from the data of the immersional heat and the water content.

Heats of immersion in the thorium oxide-water system. III. Variation with specific surface area and out-gassing temperature

Heats of immersion in the thorium oxide-water system. III. Variation with specific surface area and out-gassing temperature

Heats of Immersion Studies on Anion‐Treated Ferric Oxide

AbstractMicrocalorimetry was used to obtain heats of immersion data for αFe2O3 (hematite) treated with SO4‐2, H2PO4‐1 or MoO4‐2 at pH 4.0. The data were compared with values from a reference OH saturated iron oxide. Using water adsorption data, the net integral heats of adsorption as functions of outgassing temperature (25 to 470C) were calculated. The energies of adhesion were also determined. Anion penetration lowered the surface energy of hematite, as manifested by a lowering of the immersional heats with increasing outgassing temperature. At pH 4.0 the relative bonding strengths of the anions on hematite were concluded to be MoO4‐2 > H2PO4‐1 > SO4‐2 > OH‐1. The heat of immersion values after 25C outgassing indicated limited anion effect as well as a physical interaction of water with αFe2O3. The molybdate ion had the greatest effect on surface energy at all outgassing temperatures.The energy of rehydration (hydroxylation) of the αFe2O3 surface was also calculated from the data.

484. Heats of immersion. VII: The immersion of silica, alumina, and titania in hexane—variation with particle size and outgassing temperature: W. H. Wade and N. Hackerman, J. Phys. Chem., 66, Oct. 1962, 1823–1826

484. Heats of immersion. VII: The immersion of silica, alumina, and titania in hexane—variation with particle size and outgassing temperature: W. H. Wade and N. Hackerman, J. Phys. Chem., 66, Oct. 1962, 1823–1826