New approach to the investigation of mechanisms and apparent activation energies for the reduction of metal oxides using constant reaction rate temperature-programmed reduction

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.