Twc Catalyst Definition
As explained above, Figure 5c,d shows the relative variation of the inverse quality factor (empty) as a function of the level of TWC oxygen storage. In Figure 5c, from a fully oxidized TWC, it is noted that the relative variation of Q0−1 increases with a gradual release of oxygen, due to increasing losses, as explained above. Unlike the resonant frequency, the relative change in Q0−1 decreases at higher temperatures. Since the absolute amplitude of the no-load reverse quality factor signal is not affected by the catalyst temperature (Figure 4c), this effect is due to the general increase in dielectric losses of the (fully) oxidized catalyst (see also Figure 4c), which is the state of the catalyst referenced in Figure 5. Even at 544 °C, the relative change observed for Q0−1 is at least a factor of 100 times higher than the Fres signal and even higher at lower catalyst temperatures. In this respect, the quality factor has a clear advantage over the resonance frequency. Machida, M., Murakami, K., Hinokuma, S., Uemura, K., Ikeue, K., Matsuda, M., Chai, M., Nakahara, Y., Sato, T.: AlPO4 as a support to minimize the Rh threshold load in automotive catalytic converters. Chem. Mater.
21, 1796-1798 (2009) The experimental results also show that the maximum relative amplitude of the fres increases continuously with higher temperatures and reaches its maximum of ~1% at the highest catalyst temperature of 544°C. In addition, the resonance frequency appears to be largely linear for weakly reduced catalysts and becomes increasingly nonlinear for highly reduced catalysts at high temperatures (>400 °C). A closer look at the resonance frequency reveals a slight S-shaped dependence on the level of oxygen storage, with the highest sensitivity at 50% of the total capacity (e.g. 345°C and 396°C). This S-shaped behavior can be explained by the distribution of the electric field of the TE111 mode, which has its maximum at the center of the symmetric geometry (see also Figure 1). Since oxygen storage/release takes place from front to rear of the catalyst, maximum RF sensitivity must be expected when the oxygen storage process takes place in the center of the catalyst. In addition to the resonance frequency, the reverse quality factor also provides information about the degree of oxygen storage of a TWC. It has been observed that the amplitude of the resonance frequency is very low at catalyst temperatures around the extinguished light of the catalyst. Here, the quality factor has obvious advantages over the resonant frequency.
It is much more sensitive to the activation of the oxygen storage component, since the losses in the resonator increase sharply as soon as the reduction of cerium oxide occurs. With the quality factor, the temperature required for the beginning of the oxygen storage capacity can also be easily determined. In addition, the increase in this characteristic temperature can be observed due to the aging of the catalytic converter and therefore the quality factor provides an additional opportunity to evaluate the aging state of the catalytic converter in the vehicle. Solid-solid phase transitions: are extreme forms of sintering that occur at very high temperatures and lead to the transformation of one crystalline phase into another. Phase transformations usually occur in the bulk washing sheath and significantly reduce the surface area of the catalyst. The powders of 10A2B and Fe-10A2B were produced by wet process. H3BO3 (Wako Pure Chemicals) was dissolved in water and boehmite (Disperal, Sasol) with a molar ratio of Al: B = 20: 4.8 was added with vigorous stirring. An excess of 20% H3BO3 was used because it evaporates during subsequent drying and calcination processes. The manure obtained was dried for 12 hours at 120 ° C, crushed and heated for 1 h at 300 ° C. Finally, the powder obtained was calcined in air at 1000 ° C for 5 h to complete the reactions in the solid state to a phase 10A2B. γ-Al2O3 was produced from boehmite as a reference by drying and calcining at 600 °C.
Lanthanum (3 mol%) was impregnated in 10A2B and γ-Al2O3 using an aqueous solution of La(NO3)3 (Wako Pure Chemicals), followed by drying and calcination at 600 °C for 3 h in air. It is known that the addition of La stabilizes γ-Al2O3 against phase transformation to α-Al2O3 [23–25]. Fe-10A2B was produced similarly to 10A2B using H3BO3, Fe(NO3)3 (Wako Pure Chemicals) and Boehmit. Solid solutions of CeO2-ZrO2 in the ratios 1:9 and 4:6 M were prepared as reference media by coprecipitation method. To aqueous solutions containing Ce(NO3)3 and ZrO(NO3)2 (Wako Pure Chemicals), a 12% NH3 solution was added with vigorous stirring until pH reached 10. The precipitates were washed with water, dried and calcined at 700 °C for 3 h to form single-phase solid solutions with tetragonal structure similar to fluorite (see Fig. S1 in Electronic Additive (ESM)). For the upper and lower layers of two-layer honeycomb catalysts, compounds prepared with a CeO2:ZrO2-M ratio of 1:9 or 4:6 were used. In order to meet the stringent emission regulations of modern automobiles, efficient exhaust after-treatment systems are required. In gasoline-powered vehicles, three-way catalytic converters (TWCs) predominate because they allow the conversion of unburned hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) when the engine is running stoichiometric [1,2]. Modern TWCs consist of active metals (Pt, Rh and) and promoters (Ceria zirconia) on an alumina support.
Nanocrystalline cerium oxide serves as an oxygen storage material to cushion lean engine variations [2,3,4]. The effect of carrier material on TWC performance was investigated with single-layer honeycomb catalysts (type A). Figure 3 shows the catalytic extinction curves of the stoichiometrically simulated gas mixture (A/F = 14.6) for honeycomb catalysts containing/10A2B or/γ-Al2O3. Both catalysts showed very similar light extinction curves for CO, C3H6 and NO. To compare their differences, Table 2 lists the light extinction temperatures (T50), which were defined as the temperature at which the conversion of each gas species reaches 50%, and the apparent conversion at 400 °C (η 400) used to evaluate performance after the extinction temperature. T50 values for/10A2B were 6-8°C higher than those of /γ-Al2O3 for each gas species. However, the former achieved η400 values 3 to 4 % higher than the latter, with CO and C3H6 being particularly close to the complete distance. It can be emphasized that the differences of η,400 values are too small to be correlated with catalytic activity. However, the difference in η 400 values between/10A2B and/Al2O3 is reproducible and corresponds to the activity trend of the two-layer honeycomb catalysts described in the next section. Indeed, the η400 value is considered a measure of activity after shutdown, which is a very important feature given the strict exhaust gas regulations. Catalytic light extinction curves of CO-C3H6-H2-NO-O2-H2O-N2 and NH3/N2O via single-layer honeycomb catalysts (type C, mode I) at A/F = 14.
Dotted lines/CeO2-ZrO2. 20°C min−1 temperature ramp NOx sensors are extremely expensive and are typically only used when a compression-ignition engine is equipped with a selective catalytic converter (SCR) or NOx absorber in a feedback system.