Seham A.A. Mansour

Ceria on silica and alumina catalysts: dispersion and surface acid-base properties as probed by X-ray diffractometry, UV-Vis diffuse reflectance and in situ IR absorption studies

Abstract X-ray diffractometry and UV-Vis diffuse reflectance spectroscopy revealed that fluorite-structured CeO 2 crystallites (mean size 22.3 nm) are dispersed on silica surfaces (CeSi) into microcrystallites (11.2-8.1 nm) and dispersed further on alumina surfaces (CeAl) into nanocrystallites ( x monolayers. Consequently, IR spectroscopy of adsorbed pyridine found Lewis acid sites to be far more strengthened on CeAl. Bronsted acid sites (proton-donors) were probed exclusively on CeAl. On the other hand, IR spectroscopy of adsorbed deuterated chloroform (CDCl 3 ) showed the originally moderate Lewis base sites (low-coordinated OH − and O 2− ) to be weakened on CeSi, but markedly strengthened on CeAl. Lewis base sites exposed on ceria surfaces assume a strong nucleophilic reactivity.

Characterization of the thermal genesis course of manganese oxides from inorganic precursors

Abstract NH4MnO4, Mn3O4 and Mn(NO3)2·6H2O were used as precursor compounds for the thermal genesis (at 150–600°C) of manganese oxides. Thermal events occurring during the genesis course were monitored by means of thermogravimetry and differential thermal analysis, in oxidizing and non-oxidizing atmospheres. Intermediate and final solid-phase products were characterized using X-ray diffractometry and infrared spectroscopy. Model manganese oxides were subjected to similar examinations for reference purposes. The results indicated that NH4MnO4 is almost completely decomposed near 120°C, giving rise to predominantly α-Mn2O3. The presence of K+ contaminant supports an oxidative conversion of α-Mn2O3 into KMn8O16+ at ⩾300°C. In contrast, the genesis of pure α-Mn2O3 from Mn(NO3)2·6H2O is not achieved unless the calcination temperature exceeds 500°C; β-MnO2 was the only detectable intermediate. Mn3O4, obtained at room temperature by the addition of aqueous Mn2+ to ammonia solution, was converted into α-Mn2O3 via the formation and subsequent decomposition of Mn5O8 at ⩾300°C.

ADSORPTION AND SURFACE-REACTIONS OF PYRIDINE ON PURE AND DOPED CERIA CATALYSTS AS STUDIED BY INFRARED-SPECTROSCOPY

Abstract Pyridine (Py) adsorption on ceria surfaces, prepared by thermal decomposition of diammonium hexanitratocerate at 400 °C, was studied by infrared spectroscopy and gravimetric techniques. At room temperature, Py was irreversibly adsorbed via coordination to Lewis sites of different acid strengths. High temperature calcination (at 800 °C) of ceria greatly reduced the Py adsorption capacity of the surface. Upon thermoevacuation at 100–400 °C, Py cracking occurred with formation of various surface species (among these caboxylates and nitrites) deriving from cleavage of the ring via attack of surface oxygen on adsorbed Py. Doping of ceria with Na + , Al 3+ and Cr 3+ ions (at 1 atom dopant per Ce atom) largely modified the Py adsorption capacity and surface reactions, thus revealing disparate impacts on acid/base properties of the surface.

Surface characterization of silica-supported cobalt oxide catalysts

Silica supported cobalt oxides were prepared by the impregnation method, using an aqueous solution of cobalt nitrate hexahydrate (Co(NO3)2 · 6H2O), then calcined at different temperatures (510, 620 and 870 K). Characterization of the samples was carried out by X-ray diffraction, N2-adsorption at −196°C, UV–Vis diffuse reflectance spectroscopy and KBr-IR spectroscopy of the calcination products. The surface acidity was studied by IR spectroscopy of adsorbed pyridine at different temperatures (300, 370, 470 and 570 K). Results indicated that Co3O4 is the stable phase on silica, however, dispersion of minor amount of cobalt oxide could not be ruled out. Results also indicated that the crystallinity of the formed Co3O4 increased by increasing the loading level and/or the calcination temperature. Furthermore, the surface area of the support was decreased by increasing the loading level and the calcination temperatures. It has been also found that the surface of the supported catalysts exposed strong different Lewis acid sites.

Thermal decomposition and the creation of reactive solid surfaces. V. The genesis course of the WO3 catalyst from its ammonium paratungstate precursor

Abstract The thermal decomposition of ammonium paratungstate (NH 4 ) 10 (H 2 W 12 O 42 ) · 7H 2 O to the onset of formation of WO 3 was studied using thermogravimetric and differential thermal analyses. Intermediate solid products were obtained by calcination at 155–500 ° C and subsequently analyzed using infrared spectroscopy and X-ray diffractometry. The gaseous products were identified by infrared spectroscopy. The results obtained together with molecular stoichiometry calculations, based on weight losses determined thermogravimetrically, could help in proposing a scheme for the course of decomposition.

Effect of foreign ion additives on ceria surface reactivity towards isopropanol adsorption and decomposition: An infrared investigation

Abstract Infrared spectroscopy was employed to investigate reactions of isopropanol vapour on pure and doped ceria catalysts. The results obtained reveal that isopropanol is irreversibly adsorbed (at 298 K) on strongly dehydroxylated pure ceria surfaces in the form of coordinated molecules and, probably, two different types of isopropoxide ions (terminal and bridge-bonded). The coordinated molecules are suggested to be of importance in the alcohol dehydration (at > / 573 K) to give propene, whereas the bridging isopropoxides to be the surface intermediate of the dehydrogenation (at > /423 K) to give acetone. At > 573 K, the acetone takes part in a surface-mediated bimolecular reaction to produce isobutene and methane, and acetate surface species. The dehydration and dehydrogenation activities of ceria are seen to reside at eus Ce4+-2− pair sites. The secondary reaction of acetone, however, is suggested to occur on Ce4+-OH− surface sites. Doping of ceria with Na+ ions suppresses alcohol dehydration as well as further reactions of acetone. Cr3+- and Al3+-doping improve the surface selectivity towards dehydrogenation and dehydration reactions respectively.

Decomposition of Cd(CH3COO)2 · 2H2O and creation of reactive solid surfaces - a spectrothermal investigation

The thermal decomposition course of cadmium acetate dihydrate (CdAc) was examined, on heating to 773 K at various rates (5–30 K min−1) by thermogravimetric and differential thermal analyses, and temperature shifts experienced by the events encountered as a function of the heating rate were employed in determining corresponding non-isothermal kinetic parameters (k, A and ΔE). Gas- and solid-phase products were identified by infrared spectroscopy and X-ray diffractometry. The results obtained showed CdAc to dehydrate at 360–423 K, and then decompose at 508–560 K giving CdO, and (CH3)CO and CO2 into the gas phase, encompassing two melting processes at 345 and 500 K. Other gas-phase products of CH3COOH, CO, CH4 and (CH3)2CHCH2 were also detected, however in minor proportions, and ascribed to reactions occurring at the gas-solid interface at the expense of some of the initial products. These results were found in good agreement with the results of previous investigations of metal acetates.

Thermal decomposition of calcium citrate tetrahydrate

Abstract The thermal decomposition of calcium citrate tetrahydrate in dynamic air or dry nitrogen has been studied thoroughly. Thermal events encountered throughout the decomposition range (room temperature to 1000°C) have been monitored by TG, DTA and DSC. The encountered events have been characterized by analysing the decomposition solid products using X-ray diffractometry, IR spectroscopy and scanning electron microscopy (SEM). Non-isothermal kinetic and thermodynamic parameters (A, k, ΔE, ΔH, Cp and ΔS) have been determined. The results show that Ca3(C6H5O7)2·4H2O dehydrates in two successive steps at 60–140°C and 140–190°C each involving release of two moles of water. The ultimate decomposition product has been found to be CaO which formed above 640°C in air or dry nitrogen flow. SEM examinations indicate the formation of fusion nuclei accompanying the second dehydration step. Decomposition pathways in air and nitrogen are proposed.

Spectroscopic and microscopic investigations of the thermal decomposition of nickel oxysalts: Part 2. Nickel nitrate hexahydrate

The thermal decomposition of nickel nitrate hexahydrate (NiNHH) under dynamic atmosphere of air has been thoroughly studied. Reactions occurring throughout the decomposition up to 600°C were monitored by means of thermogravimetry (TG) and differential thermal analysis (DTA). These reactions were characterized by analysis of the solid calcination products at different temperatures at which various intermediates were expected. Solid analysis was carried out using infrared spectroscopy and X-ray diffractometry. Non-isothermal kinetic parameters (ΔE, A and k) were determined. TG and DTA under nitrogen atmosphere has been carried out. The results indicate the formation of Ni(NO3)2 · nH2O (where n = 5.5, 4 and 2) and mixture of anhydrous and basic nickel nitrate as intermediate solid products. Basic nickel nitrate intermediate was formed in nitrogen atmosphere as well as in air, which was attributed to a hydrolysis process involving the water of dihydrate intermediate. The morphological changes throughout the decomposition course have been followed by scanning electron microscopy and the results are correlated with the obtained thermoanalytical results of the decomposition course of NiNHH.

Acid properties of silica and alumina surfaces as probed by thermogravimetry and differential scanning calorimetry of temperature-programmed desorption of pyridine

Abstract Thermogravimetry and differential scanning calorimetry of the temperature-programmed desorption of pyridine from silica and alumina surfaces were carried out. Quantitative information on the concentration and strength of the surface acid sites was derived from the results obtained. The principal objective was to find out the merits and drawbacks of the two techniques in studying the acid-base properties of solid surfaces. The investigation shows that the use of these techniques can be successful provided that a number of requirements is fulfilled. These requirements were characterized and are discussed.