Klaus Bohmhammel

Thermodynamic investigation of the magnesium–hydrogen system

Abstract Thermodynamic properties of the magnesium hydride–magnesium system have been investigated using both calorimetric and equilibrium pressure measurements. Based on calorimetric measurements, on average a satisfactory agreement between experimental and calculated enthalpy and entropy values for the formation of MgH2 is achieved; systematic deviations are found only at higher temperatures (∼480°C). Experimental determination of PCIs (absorption and desorption mode) has been carried out in the temperature range 403–520°C, using Mg powders of different origin and particle size. According to measurements at 526°C, a minor dependence of H2 equilibrium pressure over MgH2 on the particle size of Mg powders used exists.

Thermodynamic optimization of the system magnesium–hydrogen

On the basis of our own calorimetric data and of known equilibria data the phase equilibria in the system Mg–H have been completely calculated by means of an optimization. Both for α-MgHx and MgH2 optimized free enthalpy functions are obtained.

Optimization of thermodynamic data of the Ni–Si system☆

Abstract The thermodynamic properties of the solid compound phases in the Ni–Si system were optimized by means of the non-linear Bayes algorithm. A consistent set of thermodynamic data consisting of the enthalpies and entropies of formations and the heat capacity functions is derived. The validity of the data is shown by computation of a well investigated chemical reaction.

Kinetic investigations on the basis of isothermal DSC measurements of hydrogenation and dehydrogenation of magnesium hydride

Abstract Results of kinetic investigations on the basis of isothermal DSC measurements by means of defined pressure changes are presented and interpreted for the Mg/MgH 2 system. From an evaluation of the experimentally obtained DSC-signals, the dependence of reaction rate on hydrogen pressure p 0 , temperature and sample morphology could be described. Under certain conditions, it is possible to visualise individual rate-determining steps and, thus, to arrive at conclusions on the reaction mechanism.

Calorimetric investigations into enzymatic urea hydrolysis

Abstract Enzymatic urea hydrolysis has been investigated calorimetrically as a function of urea concentration, the buffer system and the pH value. It is shown that the reaction follows a complex mechanism. Unambiguous conclusions on the reaction sequences are made possible by extensive thermodynamic calculations. The most important conclusion from the experimental results and thermodynamic calculations is the absolute necessity of knowledge of the experimental conditions, i.e. urea concentration, pH value and buffer system, for the meaningful discussion of calorimetric data. The molar reaction enthalpy of the enzymatic partial reaction is determined to be ΔRH1 = (−26.5 ± 1.5) kJ per mole urea. Thermokinetic evaluation by means of a simple consecutive reaction model allows the velocity constants to be calculated. For the enzymatic partial reaction, parameters Km and vmax are determined in phosphate buffer and HEPES buffer.

Mechanism of the silicide-catalysed hydrodehalogenation of silicon tetrachloride to trichlorosilane

A mechanism of transition metal silicide-catalysed hydrodehalogenation of silicon tetrachloride to trichlorosilane is proposed. The overall reaction includes electron transfer steps from the metal silicide catalyst to adsorbed silicon tetrachloride molecules. A silylene species (SiCl2), which is formed from SiCl4 and the catalyst surface, is proposed. A hydrogen molecule injects electrons into the solid, whereby hydrogen chloride is generated on the surface. Trichlorosilane results from the oxidative addition of HCl to chemisorbed SiCl2.

Formation of transition metal silicides by solid–gas reactions: thermodynamic and kinetic considerations

Abstract Alternative synthesis routes to nickel silicides are discussed. Results of the reactions of Ni with a SiCl4/H2 gas mixture (1), of NiSi with NiCl2 (2) as well as of Si with nickel halides (3) are presented. Kinetics of the reactions (1) and (2) were studied in detail by in situ electrical resistance measurements and isothermal X-ray powder diffraction, respectively. Kinetic evaluation of the electrical resistance measurement data points to island and layered silicide growth in process (1). XRD data analysis leads to the conclusion that process (2) proceeds via transient metastable nickel silicide phases, which are interrelated with the thermodynamically stable silicide phases Ni2Si, Ni3Si2 and NiSi.

Calorimetric investigations into enzyme catalysed glucose oxidation

Abstract The study presents the results of calorimetric investigations into enzyme catalysed glucose oxidation. It was shown that the convertible amount of glucose is limited by the availability of the oxidant or mediator: O2, K3[Fe(CN)6] and benzoquinone were used as mediators. The measurable glucose concentration range was much expanded by using benzoquinone. Independently of the mediator used, the enthalpy value for the oxidation of glucose in the presence of the enzyme glucose oxidase was found to be ΔRH = −125 ± 2 kJ mol−1.

Hydrodehalogenation of chlorosilanes in the presence of metal silicides: experimental studies of gas and solid phase composition related to thermodynamic calculations

Thermodynamic calculations show, in accordance with experimental investigations, that transition metals in an H2–SiCl4 atmosphere in the temperature range 600–1200 K will react to form metal silicides. With rising temperature, stoichiometry will change in the direction of silicides with a higher silicon content. The silicides formed in situ act both as a catalyst and a silicon source for the hydrogenation reaction of SiCl4 to form silanes. On the basis of the formation enthalpy of the silicides, systematization of silicide formation is possible.

Problems associated with using thermal measurement principles in enzymatic reactions

Abstract The primary intention when applying themoanalytical techniques to enzymatic reactions is the derivation of reliable thermochemical and thermokinetic information for selected reactions. The enzymatic oxidation of glucose was selected as a model reaction. Caloric measurements showed that the extrapolated thermal power at t = 0 is proportional to the concentration of the substrate in GOD-catalysed glucose oxidation, so that measurement can be restricted to the determination of the initial power. The range of measurable glucose oxidation was initially limited by the oxygen content of the solution (0–1.4 mmoll −1 ). This concentration range has been expanded considerably by adding mediators. The additional use of mutarotase resulted in an increased reaction rate.