H.R. Oswald

Comparative experimental investigations of the water-splitting reaction with iron oxide Fe1−yO and iron manganese oxides (Fe1−xMnx)1−yO

Abstract The oxidation of the non-stoichiometric iron oxide Fe1−yO and mixed iron manganese oxides (Fe1−xMnx)1−yO (x ≤ 0.3) by water forms molecular hydrogen. The course of this reaction was studied in a tubular fixed bed reactor at various temperatures by means of on-line mass spectrometry of the gas phase and by structural investigations of the solid products with powder X-ray diffraction and scanning electron microscopy.

Thermal transformations of vaterite and calcite

Abstract The polymorphic transition and the thermal decomposition of vaterite are characterized. The effect of the morphology of the parent vaterite upon the polymorphic transition as well as upon the decomposition of pseudomorphous ex-vaterite, i.e. “spherulitic” calcite, is presented. It is shown for the first time that vaterite can be decomposed into calcium oxide and carbon dioxide before being transformed into calcite. The nature of the evaluated kinetic parameters and derived mechanistic interpretations are discussed.

Temporary phase segregation processes during the oxidation of (Fe0.7Mn0.3)0.99O in N2H2O atmosphere

A two-step thermochemical cycle with the ternary metal oxide system (Fe1 − xMnx)3O4(Fe1 − xMnx)1 − yO is applied to convert solar energy to chemical energy. Experimental investigations on the water splitting reaction of (Fe1 − xMnx)1 − yO revealed temporary formation of a manganese rich rock salt phase and an iron rich spinel phase due to phase segregation processes.

Mechanism and kinetics of thermal decompositions of solids governed by phase boundary processes

Abstract Based on structural, morphological, and thermoanalytical investigations the structural reaction mechanism including topotactic relations and the respective phase boundary process controlled macroscopic reaction mechanism of the thermal decomposition of single crystalline Ni(SCN) 2 (C 5 H 5 N) 2 to Ni(SCN) 2 is presented. From the results obtained and from the comparison with further decomposition processes of model compounds, a general approach for the evaluation of specific kinetic parameters, in particular of direction dependent rate constants of solid state decompositions governed by phase boundary processes is derived.

Preparation, crystal structure, thermal and catalytical behaviour of copper diammine divanadate

Abstract A new copper diammine divanadate Cu(NH 3 ) 2 (VO 3 ) 2 has been synthesized and structurally characterized. The unit cell is monoclinic, space group Pc, a = 5.616(2) A , b = 5.591(1) A , c = 11.331(2) A , β = 90.91(2)°, V = 355.7(3) A 3 , Z = 2. The thermal decomposition has been studied by thermogravimetry and simultaneous mass spectrometry (TG/MS) in different atmospheres, i.e. H 2 , N 2 , He and O 2 . As solid products elemental copper on V 2 O 3 , Cu 0.95 V 2 O 5 and CuV 2 O 6 have been obtained, and depending on the actual atmosphere different volatile products such as NH 3 , H 2 O and N 2 . Independent of the pretreatment, reoxidation of the product phases leads to the ternary oxide CuV 2 O 6 . The microstructures have been characterized by scanning electron microscopy (SEM) and high resolution electron microscopy (HREM). The product decomposed in hydrogen has been tested with respect to its catalytic activity for the reduction of NO with CO.

Morphological observations on the thermal decomposition of calcium carbonate

Morphological observations of sintering and decomposition processes of polycrystalline CaCO3 have been carried out as function of temperature, pressure and atmosphere. The investigations prove,that the properties of the solid products formed are strongly influenced by phenomena occurring before and during the decomposition and which, in turn, are governed by the actual experimental conditions.

The thermal behaviour of pure and nickel-doped hydromagnesite in different atmospheres

In nitrogen or hydrogen atmospheres the decomposition of hydromagnesite (Mg5(OH)2(CO3)4 · 4H2O) to magnesia occurred in two steps and it stopped at temperatures lower than 750 K. In a carbon dioxide atmosphere, the second decomposition step was divided into two parts and shifted towards higher temperatures. Between the two parts, a strongly exothermic process was observed at 790 K which was identified as the crystallization of magnesite (MgCO3). This phenomenon was also obeserved for nickel-doped hydromagnesite in a carbon dioxide atmosphere. Under hydrogen, the carbon dioxide evolved from nickel-doped hydromagnesite was catalytically reduced to methane in an exothermic process. Due to this reduction the over-all reaction became exothermic.

Structure and form of solids as ruling factors for their thermochemical reactivity

The role of structural parameters strongly influencing the course of heterogeneous solid-state reactions is established. Owing to the close relationship between the form and reactivity of solids, due emphasis must be given to detailed morphological studies. This allows the derivation of consistent correlations between the reaction mechanism on a microscopic scale and the observed macroscopic changes.Our present interests are focused on the generation of solids with “tailor-made” specific chemical and physical properties. Starting with suitable precursors and appropriately selected experimental conditions, in particular with reactive gases such as molecular hydrogen, the controlled preparation of, for example, catalytically active finely dispersed metals or alloys, and also binary and ternary metal oxide and sulfide phases is demonstrated.As indispensable tools, thermoanalytical techniques are applied in combination with mass spectrometry and gas chromatography.ZusammenfassungStrukturelle Parameter spielen eine, große Rolle beim Ablauf heterogener Festkörperreaktionen. Wegen der engen Beziehungen zwischen Form und Reaktivität fester Stoffe sind detaillierte morphologische Untersuchungen besonders wichtig. Damit können eindeutige Zusammenhänge zwischen dem mikroskopischen Reaktionsmechanismus und den beobachteten makroskopischen Änderungen abgeleitet werden.Wir interessieren uns zur Zeit besonders für die Herstellung von Feststoffen mit “massgeschneiderten” spezifischen chemischen und physikalischen Eigenschaften. Ausgehend von geeigneten Vorprodukten und bei günstigen experimentellen Bedingungen, insbesondere beim Arbeiten mit reaktiven Gasen, wie molekularem Wasserstoff, wird die kontrollierte Herstellung z.B. von katalytisch aktiven Metallen oder Legierungen sowie binären und ternären Metalloxid- und -sulfidphasen möglich.Dabei haben sich thermoanalytische Methoden in Kombination mit Massenspektrometrie und Gaschromatographie als unverzichtbar erwiesen.РезумеУстановлена роль стр уктурных факторов, затрагивающих ход ге терогенных твердоте льных реакций. Вследствии т есной взаимосвязи ме жду формой и реакционной способн остью твердых тел, особое вн имание должно быть уд елено детальным морфологи ческим исследования м. Это позволило вывест и хорошие корреляции между реакционным механиз мом на микроскопичес ком уровне и наблюдаемым и макроскопичесими и зменениями. Главное внимание авторов было сосредо точено на получении т вердых тел с ”специально пригото вленными” специфиче скими химическими и физиче скими свойствами. Нач иная с выбора подходящих ис ходных веществ и подбора соо тветствующих экспер иментальных условий, особенно, при работе с такими реакционноспособны ми газами, как молекул ярный водород, проведено ко нтролируемое получение каталитич ески активных тонко диспергированных ме таллов или сплавов, на ряду с получением получением двойных и тройных окс идов и сульфидов мета ллов. При этом, как правило, терм оаналитические методы комбинировал ись с масс-спектромет рией и газовой хроматограф йей.

Reversible topotactic reduction of perovskite-related calcium manganese oxides

Abstract The perovskite-related solids CaMnO 3 resp. Ca 2 MnO 4 react upon heating in reducing atmosphere to grossly oxygen-deficient phases: CaMnO 2.5 , Ca 2 MnO 3.5 , and CaMnO 2 , Ca 2 MnO 3 . Thermogravimetry, X-ray diffraction and electron microscopy reveal the reversible, topotactic nature of the processes important for heterogeneous catalysis.

Generation of mixed metal oxides by use of an ultrasonic aerosol thermal decomposition process

Abstract A novel technique involving the pyrolysis of an ultrasonically atomized solution has been used to prepare finely dispersed submicron sized spinel-type Cu 1.1 Mn 1.9 O 4 powders with spherical morphology. The decomposition temperature of the aerosol as well as the total concentration of the solution have been varied to study the influence on the structural, thermochemical and morphological features. Together with X-ray diffractometry (XRD), thermal analysis (TA), and X-ray photoelectron spectroscopy (XPS) results it became obvious that the Cu 1.1 Mn 1.9 O 4 -spinel exists in a metastable state. Turnover-rates of catalytic CO-oxidation with molecular oxygen near room temperature gave good yields, but depended on the activation procedure of the spinel.