Ahmed K. H. Nohman

Thermal and chemical events in the decomposition course of manganese compounds

Abstract Thermogravimetry and differential thermal analysis, infrared spectroscopy (solid phase and gas phase), and X-ray diffraction were used to characterize the pathways of the decomposition of the hydrated acetate, oxalate and nitrate of manganese in air. The non-isothermal activation energy ( ΔE , kJ mol −1 ) was determined for the thermal processes monitored throughout the decomposition course. The results showed that Mn ( CH 3 COO ) 2 ·4 H 2 O dehydrates in three steps up to 130 °C, and then decomposes to a mixture of manganese oxides Mn 3 O 4 (major) and Mn 2 O 3 (minor) through the intermediates Mn(OH)CH 3 COO and/or MnOCH 3 COO and MnCO 3 . MnC 2 O 4 ·2 H 2 O dehydrates in one step at 150 °C, and then decomposes via MnCO 3 into Mn 3 O 4 (hausmannite structure) at 350 °C. Mn ( NO 3 ) 2 ·4 H 2 O undergoes stepwise dehydration up to 175 °C, and decomposes above 200 °C via an unstable oxynitrate intermediate yielding MnO 2 ; this decomposes at about 550 °C to the α-Mn 2 O 3 phase.

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.

Surface to bulk characterization of phosphate modified aluminas

Abstract γ-Al2O3, dried alumina gel, as well as their phosphated forms using (NH4)2HPO4 were prepared by wet impregnation and calcined at 870 K. Resulted samples were subjected to investigate the consequent bulk [X-ray powder diffractometry (XRD), diffuse reflectance spectroscopy (DRS) and infrared spectroscopy (IR)] and surface (texture, N2-adsorption and surface acid properties, pyridine adsorption). Results indicated no detectable bulk phase changes due to phosphation. However, the phosphated gel sample reveals the highest SBET. Surface stabilization of phosphate species by γ-Al2O3 or gel is indicated, leading to modifications on surface hydroxyl and hence surface acidity. The phosphated gel sample exhibits the strongest acidity (both Bronsted and Lewis).

Spectro-thermal investigation of the decomposition intermediates developed throughout reduction of ammonium paratungstate

Abstract The thermal decomposition course of ammonium paratungstate (APT) in hydrogen was studied using thermogravimetric and differential thermal analyses. X-ray diffractometry, infrared spectroscopy and diffuse reflectance spectroscopy were used to characterize and identify the intermediate solid products. It was found that ammonium paratungstate decomposes to tungsten metal W 0 through five main steps encompassing different tungsten intermediate compounds. Ammonium tungsten bronze (NH 4 ) 0.33 WO 3 , which precedes the formation of WO 3 , is relatively the most stable intermediate (250–550°C) encountered through reduction of APT to tungsten metal W 0 . The influence of hydrogen spillover on the reduction behavior commences to be effective just after formation of the bronze intermediate.

Characterization of ammonium tungsten bronze [(NH4)0.33WO3] in the thermal decomposition course of ammonium paratungstate

Abstract Ammonium tungsten bronze, (NH 4 ) 0.33 WO 3 , was characterized in the calcination product of ammonium paratungstate at 400°C for 2 h in a static atmosphere of air. X-ray diffractometry, thermogravimetry, and infrared and UV–vis diffuse reflectance spectroscopies found the yellowish orange product to also contain hexagonal tungsten trioxide, WO 3 . On heating up to 500°C, the bronze together with the hexagonal WO 3 were transformed completely into the yellowish green monoclinic tungsten trioxide.

Temperature-programmed characterization studies of thermochemical events occurring in the course of decomposition of MnII oxysalts

The thermal decomposition course of the MnII oxysalts Mn(NO3)2·4H2O, MnC2O4·2H2O and MnCO3 has been examined in different gaseous atmospheres by thermogravimetry and differential thermal analysis. The solid-phase decomposition products at some selected temperatures were characterized by various bulk and surface analytical techniques. The results were used to assign the thermochemical events encountered in the decomposition course. A complex reaction interface chemistry is implied. It is concluded that the thermochemical behaviour of MnII is largely dependent on the nature of the anionic atmosphere and the volatile component being released.

Composition, structure and surface acid-base behaviour of manganese oxide dispersed on silica

Abstract Silica-supported manganese oxides were derived by calcination at 870 and 1270 K of manganese intrate-impregnated and manganese oxide-coated silicas (at 10 wt.%Mn). Characterization of the products was carried out by X-ray powder diffraction, volumetric analysis of nitrogen adsorption at 77 K, and infrared spectroscopy of pyridine adsorption at 300–670 K. The results indicate that the impregnated silicas evolve highly accessible surfaces dispersing non-crystalline MnO x and Mn 2 SiO 4 -like species, and consequently expose strong Lewis and Bronsted acid sites. Thus, the potential activity of these materials in acid-catalyzed reactions is implied.

Thermoanalytic resolution of hydrogen-influenced reductive events in the decomposition course of ammonium paratungstate

Abstract Hydrogen-influenced reductive events in the decomposition course of ammonium paratungstate (APT) were resolved by thermal analysis (TG and DTA) as a function of the heating atmosphere (air, N2 and H2) and rate (2-20°C min−1). A kinetic characterization of the events was carried out non-isothermally. The results indicated that the reductive events begin after the complete decomposition of APT into WO3 near 400°C, and lead eventually to the formation of W0 at 700–800°C. Thus, it was concluded that a pre-calcination of APT-containing materials might develop upon reduction to monolayer-type W0 catalysts.

Consequences of foreign salt additives on the structure and texture of some metal oxides

Abstract The influence of foreign salt additives, namely (NH2)2CO and NH4NO3, on the structure and texture of ZrO2, Nb2O5, La2O3 and ThO2, produced by thermal decomposition of their nitrates, has been investigated. Thermal behaviour of oxide precursors (metal nitrates) as well as additive salts (pure and impregnated into the oxides) was explored by thermogravimetry. Accordingly, the calcination products of the additive-free and additive-containing oxides were obtained at the appropriate temperatures. The final oxide materials, with and without additives, were characterized by X-ray diffractometry. Slight structural modifications were detectable, viz. a retardation to crystallinity and particle size decrease. Texture assessment (surface area and porosity) was carried out via analysis of nitrogen adsorption isotherms (at −196°C) adopting BET- and t-methods. The additives, particularly of urea, were found to improve notably the surface area and porosity. This was correlated with the structural modifications conceded by the material bulk.

Thermochemical adsorptive and catalytic events occurring during isopropanol decomposition over MnOx-modified aluminas. In situ infrared spectroscopic studies

Abstract Adsorptive and catalytic events occurring at gas/solid interfaces established during isopropanol decomposition over pure and MnO x -modified aluminas were monitored and characterized by in situ infrared spectroscopic measurements. On pure alumina the alcohol adsorbs dissociatively and non-dissociatively at room temperature, giving rise respectively to isopropoxides coordinated to Lewis acid sites and isopropanol molecules hydrogen-bonded to hydroxyl (and oxide) sites. Those adsorbed species withstand outgassing at room temperature, but are completely eliminated near 200°C. Meanwhile, the alcohol catalytic dehydration is commenced at 150°C and gas-phase propene is thus released. Quantitative conversion into the alkene is effected near 225°C. The modification with MnO x stabilizes the isopropoxide species to thermo evacuation at well above 200°C, but activates their conversion into surface carboxylates at > 300°C. Moreover, the isopropanol dehydration selectivity of alumina is critically suppressed and acetone becomes the dominant gas-phase product. Thus, a strong dehydrogenation selectivity is generated. These and other MnO x -influenced alterations to the adsorptive and catalytic behaviours of alumina are discussed on the basis of available characterization results for the catalysts and mechanistic aspects established in the field.