Control of Oxygen Partial Pressure by means of H[sub 2]–H[sub 2]O–O[sub 2] or CO–CO[sub 2]–O[sub 2] Gas Mixtures

Abstract

The electrical conductivity of SrTiO 3 as a function of p O 2 was investigated in the two gaseous systems H 2 -O 2 and CO-O 2 . The experimental results showed different conductivity values for these two different systems. The oxygen partial pressure was, therefore, checked carefully and it turned out that the control of p O 2 at reducing conditions is complicated by disequilibrium states when using CO-CO 2 -O 2 gas mixtures. A sample and an oxygen sensor that are positioned side by side may be exposed to p O 2 values that differ by up to 2 log units because of their different catalytic behavior with respect to the oxidation of CO. At intermediate p O 2 conditions, e.g., between 10 −5 and 10 −13 bar at T = 973 K, low concentrations of H 2 ͑CO͒ and O 2 may result in p O 2 gradients of up to 10 log units per centimeter in both H 2 -O 2 and CO-O 2 systems.

Simultaneous electrical impedance and optical absorption spectroscopy for rapid characterization of oxygen vacancies and small polarons in doped ceria

“…Remarkably, the out-of-equilibrium experiment lasts less than 10 minutes and enables access to the intermediate p O 2 conditions (<10 −5 bar) that are challenging to achieve by regular means. 34 Previously Stefanik 24 used an electrochemical oxygen pumping technique to control p O 2 in the 1 to 10 −15 bar range, however the experimental temperature was bounded by a 600 °C lower limit at which it took about 20 days to characterize the PCO10 conductivity (Stefanik's results shown for comparison in Fig. 5a as blue squares).…”

“…21,22,32,33 Furthermore, the pO 2 range that may be obtained in equilibrium experiments by mixtures of O 2 /Ar or reactive gases has an extended gap at intermediate pO 2 conditions, for instance, between 10 À5 to 10 À13 bar at 700 1C, as a very fine control over gas concentrations in the mixture is required in that range. 24,34 This leads to several major drawbacks that limit the experimentally accessible temperature, pO 2 and thus d ranges where PCO (and similar MIECs) can be characterized and utilized in applications.…”

Active Tuning of Optical Constants in the Visible–UV: Praseodymium‐Doped Ceria—a Model Mixed Ionic–Electronic Conductor

“…Accessing intermediate p O 2 values in the range of 10 –5 –10 –20 bar is challenging by inert or reactive gases mixing. [ 52 ] An alternative approach is via electro‐chemical pumping of oxygen directly into a thin film, as previously reported, [ 28,53 ] that creates an effective p O 2 in the thin film oxide in a finely controlled manner. That can enable access to all intermediate p O 2 values, and the corresponding optical constants, but at the price of a more complex device structure requiring a solid electrolyte layer and electrodes.…”

Nonstoichiometry in oxides and its control

“…This means that for intermediate partial pressures of oxygen, between 10 -5 and 10 -10 bar, mixtures CO-CO 2 or CO 2 -H 2 must be adopted. These mixtures are known for sluggish equilibrium in the gas phase, which consequently depends on catalysis effect of the sample [24].…”