KINETICS AND EQUILIBRIA OF REDOX SYSTEMS AT TEMPERATURES AS LOW AS 300°C

Abstract

ZrO2 oxygen sensors, gas mixtures, and conventional solid buffers have been used for decades to either control or measure oxygen fugacity (fO2) at high temperatures. In dry systems below ca. 700°C these techniques were used cautiously, if at all, due to doubt that there was any equilibration at lower temperatures. We have re-investigated these three types of redox systems in a study where each system (two different Y2O3-ZrO2 cells, four different gas mixtures, and four different dry solid buffers) was simultaneously cross-checked with the other to temperatures below 300°C and compared to JANAF data, extrapolated down to low temperatures.Steady and reproducible readings were observed down to T =< 300°C, from which we infer fast kinetics for all three systems. Specifically, we find equilibration of various CO2-H2 gas mixtures over the entire temperature range and to much lower temperature than previously predicted. We assign the reactivity (decomposition) of CO2 at low T to the catalytic action of Pt, whereby chemisorption of H2 on the platinum surface enhances the reactivity with CO2. This catalytic reactivity is diminished over time due to a long-term irreversible reaction of Pt with H2. Subsequent embrittling and aging after prolonged exposure to H2 explains erroneously high emf readings. Oxygen sensing of ZrO2 cells is linear in 1T-log f O2 space and Nernstian at high temperatures. However, for cells with a specific and complex trace element chemistry, one may observe a non-Nernstian behavior in the low T range, i.e., below 470° or lower, probably caused by partially blocked O2- migration, dependent on the H2 content in the gas mixture. Linearity and reproducibility of this deviation still allows, however, a useable calibration. Solid buffers of the metal-metal oxide type are known to alloy with noble metals and we therefore used AgPd electrodes, for consistency in all studies, including (IW), (IM), (FMQ), and (NNO). Whereas (IW) and (IM) can be used in the temperature range of consideration, (FMQ) and (NNO) react sluggishly. Complex defect structure of (FMQ) and age alteration of Ni surfaces by chemisorption of oxygen and/or Ag-Ni alloying of (NNO) may be the reason. Fast kinetics and successful redox sensing of CO2-H2 gas mixtures, of ZrO2 cells and of at least some solid buffers are therefore promising for future research on low-T redox equilibria.