The redox kinetics of undoped ceria (CeO_(2−δ)) are investigated by the electrical conductivity relaxation method in the oxygen partial pressure range of −4.3 ≤ log(pO_2/atm) ≤ −2.0 at 1400 °C. It is demonstrated that extremely large gas flow rates, relative to the mass of the oxide, are required in order to overcome gas phase limitations and access the material kinetic properties. Using these high flow rate conditions, the surface reaction rate constant k_(chem) is found to obey the correlation log(k_(chem)/cm s^(−1)) = (0.84 ± 0.02) × log(pO_2/atm) − (0.99 ± 0.05) and increases with oxygen partial pressure. This increase contrasts the known behavior of the dominant defect species, oxygen vacancies and free electrons, which decrease in concentration with increasing oxygen partial pressure. For the sample geometries employed, diffusion was too fast to be detected. At low gas flow rates, the relaxation process becomes limited by the capacity of the sweep gas to supply/remove oxygen to/from the oxide. An analytical expression is derived for the relaxation in the gas-phase limited regime, and the result reveals an exponential decay profile, identical in form to that known for a surface reaction limited process. Thus, measurements under varied gas flow rates are required to differentiate between surface reaction limited and gas flow limited behavior.
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机译:通过电导率弛豫方法在1400°C的氧分压范围为-4.3≤log(pO_2 / atm)≤-2.0的条件下研究了未掺杂二氧化铈(CeO_(2-δ))的氧化还原动力学。已经证明,相对于氧化物的质量,需要极大的气体流速,以便克服气相的限制并获得材料的动力学性质。使用这些高流速条件,发现表面反应速率常数k_(chem)符合相关性log(k_(chem)/ cm s ^(-1))=(0.84±0.02)×log(pO_2 / atm) -(0.99±0.05),并随氧气分压而增加。这种增加与主要缺陷种类,氧空位和自由电子的已知行为形成了鲜明对比,这些缺陷的浓度随氧分压的增加而降低。对于所采用的样品几何形状,扩散太快而无法检测到。在低气体流速下,松弛过程受到吹扫气体向/从氧化物供应/去除氧气的能力的限制。导出了气相受限状态下弛豫的解析表达式,结果揭示了指数衰减曲线,其形式与表面反应受限过程中已知的形式相同。因此,需要在变化的气体流速下进行测量以区分表面反应受限行为和气体流动受限行为。
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