Abstract
AbstractThis work investigates the potentialities of the thermal analysis (TA) coupled with the mass spectrometry (MS) technique by studying the thermal decomposition of calcium oxalate monohydrate (CaC2O4·H2O). The aim of this work is twofold: to demonstrate, at first, the efficacy of coupling the thermal gravimetric (TG)‐MS experimental approach to check the presence of intermediate reactions beyond the conventional three‐step decomposition: dehydration, decarbonylation, and decarbonation; second, to test the reliability of different modeling approaches in determining the activation energy (E) including, as innovative alternative, the use of the MS signals. The TA is carried out at selected constant heating rates by recording both the thermal gravimetric analysis, differential thermal analysis) profiles, and, simultaneously from the MS data, the signals of the total ion current and the ion currents selected to monitor the release of the H2O, CO, and CO2 species. These experimental results are effective in determining the E through different modeling approaches based on the maximum reaction rate method and isoconversional procedures including both TG and MS signals. Coupling the TA‐MS technique has allowed us to check the concurrent presence of the dismutation reaction of carbon monoxide, occurring during the decarbonylation event, and to determine the corresponding E value (E = 169.7 kJ/mol). These results present a surprising correlation between the global enthalpy of the two reactions involved in this second step and their activation energy values. On the whole, the results satisfy the conventional constraints usually adopted to test the reliability of the E results and are consistent with the majority of the datasets published in the literature on this subject.
Highlights
Studies on reactions involving solid materials represent a difficult task due to the great variety of factors having different impacts on solid-state reconstruction, diffusion of reaction products and reagents, materials properties, and physical state of the evolving solid-state surface
Keeping in mind the parameters of the proposed maximum reaction rate (MRR) models, the thermal decomposition of calcium oxalate monohydrate (COM) has been characterized by identifying the remarkable points introduced in Sections 4.1 and 4.3.1: the flex temperature Tf of the TGband curve, the peak temperature Tp of both the DTG and differential thermal analysis (DTA)-band curves, the peak temperature Tp,MS of the TIC and the selected IC curves derived from the MS analysis
In all the TGA coupled with mass spectrometry (TG-MS) analysis recorded at the selected β the trend of the IC m/z 44 ion current presents the formation of a smaller band in the range 365–600◦C in correspondence to the oxalate decarbonylation reaction, besides the CO2 development occurring during the carbonate decomposition
Summary
Studies on reactions involving solid materials represent a difficult task due to the great variety of factors having different impacts on solid-state reconstruction, diffusion of reaction products and reagents, materials properties, and physical state of the evolving solid-state surface. Isothermal TGA measurements cannot rigorously be carried out due to the unavoidable presence of finite nonisothermal heat-up time associated with this constraint. At slow heating rates, the weight loss during heat-up time can affect significantly the results of kinetics elaboration.[1,2] at an isothermal regime, the nonzero extent of conversion appears critical and the investigations have to be carried out carefully. In constant heating rate analysis, these problems can be avoided by starting at a temperature reasonably below that at which the thermal decomposition starts.[3] The authors are aware that in kinetics it is a good practice to propose a combination of nonisothermal and isothermal analysis. Considering the focus of this work, TA has been limited to a constant heating rate in view of proposing an extended elaboration on further planned investigations
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