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B, 79 Mavrikakis, M. Bianchi, C. Loschen, C. Box , Nicosia, Cyprus. An attempt is made to describe the essential theoretical and experimental features of these techniques through several examples. In this chapter, the regeneration of the activity of aged com- mercial and model three-way catalysts through thermo-chemical and acid-washing treatments, and how these are related to the recov- ery of OSC and oxygen mobility, are also reviewed. The latter pro- vides fundamental and important details on the mechanisms of TWC deactivation in an effort to extend the life cycle of commercial three-way catalysts by providing an improved design for their chemi- cal composition and structural properties.
Though many attempts to under- stand the basic properties of these materials have been made, the reasons for their good heat resistance and high OSC are still not very well understood. One of the main reasons for this is that stud- ies were performed in a fragmentary and not always quantitative way.
In particular, studies estimating the transient rate of oxygen uptake and release under real driving conditions were rarely per- formed, in spite of the importance of gas emission control. In particular, comprehensive studies that quantitatively describe the transient kinetics of oxygen storage and release via the participation of bulk and surface oxy- gen diffusion processes as a function of fundamental properties of.
This fundamental knowl- edge would permit the design of the next generation of advanced OSC materials. Pulse and step-change transient OSC measurements, as well as transient 18O-isotopic techniques, can provide the means to effec- tively characterize commercial and model TWC materials for their intrinsic oxygen storage and release kinetics under dynamic condi- tions.
Values for the most reactive labile oxygen and the total reac- tive oxygen referred to as OSC and OSCC, respectively, of a given TWC material are usually estimated after applying alternating pulses and step changes to the feed-gas composition. Equation 3. Table 3. Its value depends on the gas-flow system Fig. According to Eq. A carrier gas e. Figure 3. The V5 valve is equipped with a gas sampling loop of known volume. The gas loop is used to store the quantity mol of the oxidizing or reducing gas to be injected into the He carrier gas stream, and then to the reactor through the use of a six-way chromatographic valve V3.
The exit stream from the microreactor is connected to an online mass spectrometer for gas analysis. In the experimental set-up shown in Fig. During this step, oxygen is stored on both the Pd metal and the ceria—zirconia support. Step 3: Following step 2, a He flow is used to flush the gas lines and the reactor volume from gas-phase oxygen.
Step 4: During this step, one CO pulse e. During this step, several dynamic phenomena occur, namely:. Step 5: Following step 4, the gas lines and reactor volume are flushed with a He flow to eliminate gas-phase oxygen. Re-oxidation of the catalyst sample takes place as in step 2. However, this methodology may not. Alternatively, oxygen storage capacities can be measured dur- ing step 6 using successive O2 pulses. During the latter pulse-oxygen treatment, the consumption of oxygen is estimated.
A correction for PdO forma- tion is possible via independent metal dispersion measurements. In addition, any small amounts of CO or carbon accumulated during the CO pulsing can be measured during the oxygen pulsing, and this quantity is subtracted from the total amount of oxygen consumed. The Ar response represents the dynamics of a non-adsorbing and non- reacting gas when pulsed using the same amount as CO through the reactor with the catalyst present. The area difference between the Ar and the CO response curves shaded area in Fig.
In addition, integration of the CO2 transient response gives the amount of CO2. C3H8, have rarely been used. O2 pulse : 0. These are described by the following equations,29,32,34,37— During CO pulsing, at. On partially reduced noble-metal nanoparticles, the Langmuir—Hinshelwood mechanism may also take place through the interaction between adsorbed O—S1 and CO—S1 species, Eq.
If this is not the case, then several other reaction paths must be considered. The latter species are strongly bound on the support, and desorb at ele- vated temperatures. It is appropriate to point out that the extent of each of the reactions 3. This transient kinetics is influenced by the chemical state of the catalyst surface, the concen- tration of CO in the gas phase, the reaction temperature, and the initial amount of stored oxygen in the solid catalyst sample. Reactions 3. Bernal et al. The frequency the number of step changes per second of switch- ing is within the 0.
The second step in the experimental procedure in Table 3. The first peak arises during the CO step and the second one during the O2 step. This is illustrated in Fig. Efstathiou and S. Adapted with permission from Lambrou et al. Adapted with permission from Boaro et al. The O2 peak is delayed compared to the CO2 2 peak Fig. Boaro et al. It is also likely that in other metal-oxide OSC materi- als or supported noble-metal catalysts, the CO2 2 peak represents the reaction of oxygen with carbon, the latter originating from CO disproportionation, Eq.
For supported noble metals, it is also possible that during the He purge, following the CO step, some strongly adsorbed CO on the noble metal remains, which then readily reacts with oxygen dur- ing the O2 step to form CO2. The concentrations of CO2 and SO2 used were similar to those encountered in the exhaust-gas composition of a gasoline-driven car. It was found that the amount of CO2 produced Fig. It was suggested that the presence of CO2 or SO2 in the oxidizing gas mixture, under which oxygen is. When the CO oxidation reaction was carried out under transient conditions OSC measurements , oxygen diffusion plays an impor- tant role in the enhancement of the redox properties of ceria— zirconia compared to ceria.
It was found that the performance of solid ceria—zirconia for CO oxidation was significantly better than that of ceria, with optimum performance in the middle composi- tional range, especially with low-surface area samples, for which participation of lattice oxygen is more likely to occur. In addition, the actual change in composition of the exhaust gas for a TWC between lean and rich occurs on a millisecond scale.
In this respect, only a limited number of studies have presented OSC data on a millisecond scale. Volume 86 , Issue 11 November, Pages Related Information. Close Figure Viewer. Browse All Figures Return to Figure. Previous Figure Next Figure. Email or Customer ID. Forgot password? Old Password. New Password. Password Changed Successfully Your password has been changed.