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Communication Dans Un Congrès Année : 2017

Why reaction rate does not always obey Arrhenius law during gas-solid reactions?

Résumé

Gas-solid reactions are extensively studied by thermal analysis. To describe the kinetics of these reactions, the reaction rate dα/dt (i.e. the derivative of the extent of conversion vs. time) is almost exclusively expressed by the product of a temperature function k(T) by a mathematical function f(α) depending on the kinetic model. In general, the temperature function k(T) is supposed to follow Arrhenius law with a pre-exponential term and the apparent activation energy Eapp according to : cf. abstract file. Non-Arrhenius behavior has been observed in many cases such as for example CaO hydroxylation and carbonation. Simon suggested that other k(T) functions than Arrhenius equation may be equally used. Moreover, the use of the Arrhenius equation in model-free methods when such equation is not relevant will necessarily induce Eapp variation with the extent of conversion. Galwey has recently stated that such variations are inconsistent with the Arrhenius activation model and he wonders how and why this variation. To precise the origin of such a complexity, it is necessary to come back to the mechanism of growth of the solid product phase which can be decomposed into a series of elementary steps: adsorption/desorption, external interface reaction, internal interface reaction, diffusion of species transferring from an interface to the other. Several reasons may be invoked to explain the non-Arrhenius behavior of k(T) function (in case of single reaction): when the reaction conditions are far from equilibrium, and if a rate-determining step i governs the kinetics, the rate equation involves concentration terms which can be expressed by means of the equilibrium constants Kj (i and j are elementary steps of the growth mechanism, i ≠ j). Due to adsorption and/or desorption steps, it comes out that the rate may depend of temperature through terms deriving from Langmuir isotherm equation, and thus that it will not follow Arrhenius equation, when the reaction conditions are near the equilibrium, the rate will never follow Arrhenius equation since the rate of the opposite reaction of the elementary rate-determining step cannot be neglected compared to that of the direct one. This is why we generally propose to write the rate equation using a function which accounts for possible complexity of the rate with thermodynamic variables. Examples of rate equations where the temperature term is complex will be presented to illustrate these theoretical considerations.
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Dates et versions

hal-01916410 , version 1 (08-11-2018)

Identifiants

  • HAL Id : hal-01916410 , version 1

Citer

Loïc Favergeon, Michèle Pijolat. Why reaction rate does not always obey Arrhenius law during gas-solid reactions?. International Confederation for Thermal Analysis and Calorimetry congress (ICTAC 2016), International Confederation for Thermal Analysis and Calorimetry, Aug 2016, Orlando, United States. pp.99 / KIN-3. ⟨hal-01916410⟩
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