New Approach to the Investigation of Mechanisms and Apparent Activation Energies for the Reduction of Metal Oxides Using Constant Reaction Rate Temperature-Programmed Reduction

Abstract

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.

Low Effective Activation Energies for Oxygen Release from Metal Oxides: Evidence for Mass‐Transfer Limits at High Heating Rates

“…carbon, hydrogen), [22][23][24][25] there are some values of activation energies for direct metal oxide decomposition at low heating rates that have been published. carbon, hydrogen), [22][23][24][25] there are some values of activation energies for direct metal oxide decomposition at low heating rates that have been published.…”

“…[23,29,30] In particular, two primary factors could contribute to the observed lower effective activation energies: size effects, and high heating rates. [23,29,30] In particular, two primary factors could contribute to the observed lower effective activation energies: size effects, and high heating rates.…”

Kissinger kinetic analysis of data obtained under different heating schedules

“…This method has been often used for the kinetic analysis of experimental data obtained under Hi-Res conditions by a number of authors [26][27][28][29][30]. However, it must be pointed out that Salin and Seferis [12] did not demonstrated if α m is independent of the experimental parameters selected in the dynamic heating rate algorithm as demanded by the Kissinger method. Thus, the validity of this method must be reanalysed.…”

“…The most conventional SCTA method is the Constant Rate Thermal Analysis (CRTA) where the reaction rate is maintained constant at a value previously selected. The kinetic analysis of solid state reactions by means of CRTA has been extensively described in literature [6][7][8][9][10][11][12]. A different approach to SCTA is the High Resolution TGA (Hi-Res) also known as dynamic heating rate method.…”

Oxidation and reduction in copper/zinc oxides by mechanical milling

“…Reduction is accompanied by an extra temperature rise of a few degrees, indicating that the reduction proceeds very quickly and exothermically (the ∆H 0 for CuO reduction is of the order of −10 5 J/mol). In these samples the presence of a CuO species is assumed that is easily reducible where Cu 0 acts as a catalyst for further reduction of the remaining CuO [22]. The second reduction peak of Cu 2 O/ZnO/s.a and the reduction profile of Cu/ZnO/s.a.…”