Temperature-programmed oxygen isotopic exchange

In the case of temperature-programmed oxygen exchange (TPIE), data analysis was carried out using specially derived equations. Thus, by transformation of isotope equations (30) and (31), the time-independent isotope-mechanistic equation was obtained [51-60]... 

Studies of the homomolecular and heterolytic exchange processes are generally in the form of the measurement of rates under isothermal conditions. However, studies have also been made of temperature programmed isotopic exchange, in which the oxide is subjected to a temperature ramp under the reaction atmosphere, and the partial pressures of various isotopic oxygen species is determined as a function of temperature (e.g., Refs. 20-21). The photoactivation of oxygen exchange has also been reported in a number of studies which have been performed under UV irradiation (e.g.. Refs. 18, 22, 23). 

The following sections provide a kinetic analysis of the transient responses based on an atomic state for the chemisorbed oxygen, 0(s). We show that this approach allows us to account for the qualitative features of the results described above, the temperature dependence of the rate of isotope scrambling under steady-state conditions, and results ftom temperature programmed desorption (TPD) experiments performed at very low pressure. The steady-state exchange and TPD experiments are described in Sec. 3.1.3.. The kinetics of isotope exchange of O2 (gas) with oxide materials have been reviewed by Ceilings and Bouwmeester. Readers are referred to this work and references therein for a more comprehensive discussions of the mechanisms and kinetics involved in more complex systems. 

Isotope oxygen exchange was carried out in a static installation with MS control both in the temperature-programmed and isothermal modes as described elsewhere [60-63]. The surface/bulk oxygen mobility for samples pretreated in O2 at 500 °C was characterized by the temperature-programmed desorption (TPD) in He flow. Reactivity of perovskites was estimated by temperature-programmed reduetion (TPR) by H2 or CH4 (1% in Ar, heating rate of 5 K/min from 25 up to 880 °C) earried out in kinetie installations equipped with GC and PEM-2M analyzer (IR absorbance and electrochemieal sensors) [11, 60-63]. 

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