The theoretical curves in the coordinates a vs. time for isothermal, and avs. temperature for non-isothermal experiments are calculated as functions of three kinetic parameters: activation energyE, pre-exponentical factorA and theg(α) function describing the mechanism of thermal decomposition of solids. The results show that conclusions not taking into consideration all three parameters can lead to information of little value concerning the mechanism of the decomposition and kinetic calculations. A correlation between non-isothermal and isothermal experiments, important for determination of the thermal stabilities of the compounds, is impossible without a knowledge of theg(α) function.
Supported catalysts contain often only small amounts of active component(s) which renders their characterization difficult, particularly because they usually contain a substantial amount of water. Thermal analysis (TA) coupled with mass spectrometry (MS) offers an interesting potential for characterizing such material, various steps of catalyst preparation as well as crucial properties of fresh and used catalysts can be investigated. Some examples illustrating the versatility of TA-MS in catalysis research, such as solid-state reactions occurring upon exposure of the precursors or catalysts to reducing, oxidizing or inert atmosphere, are presented in this study. The combined use of TA and MS allows in many cases a much more detailed interpretation of the observed phenomena than could be achieved by one of these methods alone.
Authors:M. Maciejewski, W. Emmerich, and A. Baiker
Pulse thermal analysis (PTA) is based on the injection of a specific amount of gaseous reactant into a carrier gas stream.
PTA provides the following advantages compared to conventional TA: (i) quantitative calibration of the mass spectrometric
signals allows increasing the sensitivity of TA measurements; (ii) monitoring of gas-solid processes with defined extent of
reaction i.e. the reaction can be stopped at any point between pulses, enabling elucidation of the relationship between the
composition of the solid and the reaction progress; (iii) simultaneous monitoring of changes in mass, thermal effects, composition
and amount of gaseous reactants and products under pulse conditions.
Authors:D. Brandova, M. Maciejewski, and W. Keller
Thermal analysis combined with mass spectrometry was applied to radiocarbon dating procedures (age determination of carbon-containing
samples). Experiments carried out under an oxygen atmosphere were used to determine carbon content and combustion range of
soil and wood samples. Composition of the shell sample and its decomposition were investigated. The quantification of CO2 formed by the oxidation of carbon was done by the application of pulse thermal analysis. Experiments carried out under an
inert atmosphere determined the combustion range of coal with CuO as an oxygen source. To eliminate a possible source of contamination
in the radiocarbon dating procedures the adsorption of CO2 by CuO was investigated.
Simultaneous thermal analysis
(TA) and evolved gas analysis by mass spectrometry (MS) and/or Fourier transform
infrared spectroscopy (FTIR) is a powerful hyphenated technique combining
direct measurement of mass loss and sensitive spectroscopic analysis. In the
present study the influence of several experimental parameters which may affect
the quantification of FTIR signals have been studied using a combined TA-FTIR-MS
system. Parameters studied include: sample mass (1-400 mg), carrier
gas flow rate (25-200 mL min-1), resolution
of the FTIR spectrometer (1-32 cm-1),
and location of injection of the calibrating gas.
MS analysis, which was not significantly affected
by the experimental conditions, was used as a reference for assessing the
accuracy of quantification by FTIR. The quantification of the spectroscopic
signals was verified by the decomposition (NaHCO3)
or dehydration (CuSO45H2O)
of compounds with well-known stoichiometry.
The systematic study of the parametric sensitivity
revealed that spectral resolution and carrier gas flow rate, which affect
the acquisition time in the IR-cell, are key parameters that must be adjusted
carefully for reliable quantification. The dependence of the reliability of
quantification on these parameters is illustrated and conditions leading to
proper quantification are discussed. As an example, for a standard spectral
resolution of 4 cm-1 and a FTIR gas cell
volume of 8.7 mL, the carrier gas flow must be lower than 100 mL min-1
for warranting accurate results (relative deviation <2%). The concentration
range of analyzed species is limited but can be extended by proper selection
of the wavenumber regions for molecules giving strong IR signals.
Authors:I. Gorzkowska, M. Maciejewski, and R. Rudnicki
A new method (based on DTA and TG) for the determination of the eutectic composition is proposed for systems in which one of the components is unstable in the vicinity of the eutectic melting temperature. The eutectic composition established by means of this method for the CaCO3-CaF2 system is in very good agreement with the results obtained in a classical way.