Background:
Thermal decomposition of iron-bearing organometallic complex
acetyl ferrocene, (C5H4COCH3)Fe(C5H5), leads to hematite (α-Fe2O3) nanoparticles. Presence
of maliec anhydride, C4H2O3 as co-precursor during thermal decomposition modifies
the size of the particles as well as the quantity of the reaction product significantly.
Objective:
Kinetic analysis of the solid-state thermal reaction of acetyl ferrocene in the
presence of varying amount of co-precursor maliec anhydride under inert reaction atmosphere
has been studied in order to understand the reaction mechanism involved behind the
formation of hematite and the role of co-precursor in the reaction process. For this purpose,
reaction kinetic analysis of three mixtures of acetyl ferrocene and maliec anhydride has
been carried out.
Methods:
Thermogravimetry under non-isothermal protocol with multiple heating rates
has been employed. The data are analyzed using model-free iso-conversional kinetic techniques
to estimate the activation energy of reaction and reaction rate. The most-probable
reaction mechanism has been identified by master plot method. The kinetic triplets (activation
energy, reaction rate, most probable reaction mechanism function) have been employed
to estimate the thermodynamic triplets (ΔS, ΔH and ΔG).
Observations:
Acetyl Ferrocene (AFc) undergoes thermal decomposition in a four-step
process leaving certain residual mass whereas maliec anhydride (MA) undergoes complete
mass loss owing to melting followed by evaporation. In contrast, the (AFc1-x-MAx) mixtures
undergo thermal decomposition through a two-step process, and the decompositions are
completed at much lower temperatures than that in AFc. The estimated activation energy
and reaction rate values are found strongly dependent on the extent of conversion as well
as on the extent of mixing. Introduction of MA in the solid reaction atmosphere of AFc in
one hand reduces the activation energy required by AFc to undergo thermal decomposition
and the reaction rate, while on the other hand varies the nature of reaction mechanism involved.
Results:
The range of reaction rate values estimated for the mixtures indicate that the activated
complexes during Step-I of thermal decomposition may be treated as ‘loose’ complex
whereas ‘tight’ complex for the Step-II. From the estimated entropy values, thermal
process of (AFc1-x-MAx) mixture for Steps I and II may be interpreted as ‘‘slow’’ stage.
Conclusion:
Variation of Gibb’s free energy with the fraction of maliec anhydride in the
mixtures for Step-I and II indicate that the thermal processes of changing the corresponding
activated complexes are non-spontaneous at room temperature.
