Temperature programmed isotopic exchange

Fig. 20. Temperature-programmed isotope exchange over two lanthana-based catalysts for OCM (after 148).

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). 

Temperature-programmed isotopic exchanges (TPIE) of H2 with hydroxyl groups of several oxides were carried out by Hall et a/., 1 1 and more recently, by Martin and Duprez.In both studies, the temperature was raised at 2°Cmin during TPIE. The activation energies were... 

Table 3. Temperature-Programmed Isotopic Exchange of H2 with O H groups of oxides.

Figure 81. Temperature dependence of isotope molecules fraction (left) and isotope variables (right) in the course of temperature-programmed isotope exchange experiment for 70%Lao,8Sro,2Feo 8Coo,20x+ 30%Ceo 9Gdo [Ox composite sintered at 900 C. Temperature ramp 5 /min from 100 to 750 C, p02 1.7 Torr.

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]... 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

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. 

Wilmer H, Genger T, Hinrichsen O (2003) The interaction of hydrogen with alumina-supported copper catalysts a temperature-programmed adsorption/temperature-programmed desorption/isotopic exchange reaction study. J Catal 215 (2) 188-198... 

Figure 18.3 Temperature-programmed desorption and isotopic exchange using 2 on Sr-doped LaSrCoOs perovskite. (Reprinted with permission from Ref. [16]. Copyright 2013, American Chemical Society.)...

Adsorption results shown in Figure 12.6 indicate that a parallel H2 adsorption process takes place simultaneously with H2 chemisorption on Ft when the catalyst has been evacuated at temperatures as low as 423 K. The mechanism of this phenomenon still remains as an open question. To enlighten the nature of this process and the species involved, two t3qjes of experiments have been carried out isotopic exchange under steady-state conditions and temperature programmed desorption tests after D2 adsorption, CO adsorption, and CO-D2 co-adsorption. 

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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