The mechanisms and kinetics of the reduction of powdered Fe2O3 and Fe3O4 samples have been investigated
under nonisothermal conditions to provide a detailed insight into the processes occurring. Both conventional
linear heating temperature-programmed reduction (TPR) and constant rate temperature-programmed reduction
(CR-TPR) techniques were utilized. Fe2O3 was found to reduce to Fe in a two-step process via Fe3O4. The
mechanism of the prereduction step of Fe2O3 to Fe3O4 was found to follow an nth order expression where
nucleation or diffusion was not the rate-controlling factor while the main reduction step to metal was described
by a model involving the random formation and growth of nuclei. A CR-TPR rate perturbation method,
“rate-jump”, was applied to the measurement of variations in apparent activation energy throughout the reduction
processes, under near-equilibrium conditions and the activation energy measurements are compared with
those obtained under conventional linear heating conditions.
A new system has been developed for the study of both bulk and surface metal oxides by temperature
programmed reduction (TPR) under both conventional linear heating and constant rate thermal analysis (CRTA)
conditions. It is shown that constant rate temperature-programmed reduction (CR-TPR) is capable of producing
higher resolution of overlapping events, provides more insight into reduction mechanisms, and allows easier
quantification of reduction processes than conventional TPR. The CR-TPR curves for both bulk and supported
copper oxides confirmed that reduction followed a nucleation or autocatalytic mechanism. Bulk nickel oxide
was found to reduce via a similar mechanism. Advantages of the CR-TPR “rate-jump” technique to determine
reaction energetics are illustrated by investigation of the apparent activation energy (E
a) of CuO reduction,
and the results are compared with those obtained under linear heating conditions. Both approaches yield
reasonable values of E
a under the appropriate experimental conditions employed. However, the CR-TPR
“rate-jump” technique allows variations in E
a to be measured as a function of the extent of reduction, revealing
changes in the reaction mechanism or kinetics. Our results suggest it is possible to estimate the apparent
activation energy of both the nucleation and growth stages involved in reduction. The validity of the “rate-jump” technique employed is confirmed using the thermal decomposition of CaCO3, a widely investigated
process. The TPR system uses a hygrometer cell to monitor the production of H2O as the sample is reduced.
The sample temperature is controlled by a computer in such a way that the production of H2O, i.e., the rate
of reduction, can be maintained at a constant preselected value for CR-TPR experiments. Important instrumental
features include a fast response furnace, direct temperature measurement, a sensitive specific detector, and
control and data analysis software developed specifically for this work.
Thermally induced reactions are of great importance in the manufacture and characterization of a very wide
range of increasingly complex materials covering areas as diverse as ceramics and heterogeneous catalysts.
Subsequently, there is a need for improved thermoanalytical methods that can provide enhanced resolution
and a greater understanding of the energetics and mechanisms involved. This paper describes a new solid
insertion probe mass spectrometer (SIP-MS) system that is designed to meet these needs by operating high
vacuum with small sample masses. The SIP-MS system supports both conventional linear heating and a
range of sample-controlled thermal analysis (SCTA) techniques including constant rate thermal analysis
(CRTA). Its ability, in conjunction with the latter technique, to obtain reliable apparent activation energy
measurements throughout a process under near-ideal experimental conditions is demonstrated. In addition,
the system can discriminate between different reaction mechanisms and provide information on the often
complex solid-state reactions found in calcination processes.
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