Simulated Exhaust Conditions

Although perovskite oxides have been proposed as potential TWCs for automotive emissions control, most of the studies up to now are focusing on model reactions, such as those described previously, and only few studies have been devoted on automotive emissions control under simulated or real exhaust conditions. Table 25.6 summarizes such efforts. 

Perovskite-type oxide Feed stream composition Reference 

05O3 Real exhaust conditions [87] 

The three-way catalytic activity of Pd-substituted (LaFeo.TyCoo.uPdo.oeOs) and Pd-supported (Pd/LaFeo,8Coo.203) perovskites was comparatively investigated by Zhou et al. [85] Table 25.7 summarizes their conversion performance, in terms of T50 and T90, under fuel rich (S = 0.9) and stoichiometric (S = 1.0) conditions. 

The Pd-supported perovskites appeared to be more active than Pd-substituted perovskites, especially in relation to NO reduction. The higher activity of Pd/ LaFeo,8Coo.203 catalysts was associated with the easier reduction of Pd species, compared to Pd in the lattice of Pd-substituted perovskite, which is difficult to be reduced due to the strong Pd-0 interactions [85]. Moreover, the TWC activity of two versions of Pd-doped perovskites was investigated under lean, stoichiometric and rich simulated exhaust conditions [89]. Pd-doped perovskites were found to exhibit similar conversion performance with a commercial Pd-rich TWC catalyst, demonstrating, however, enhanced N2-selectivity. 

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The promotional effect of Ce02-based mixed oxide solutions on NM/AI2O3 washcoat under simulated exhaust conditions is the subject of numerous reports [13,14,25,30,36-38]. For instance, both the three-way catalytic... 

Figure 25.4 Three-way catalytic performance He at 1 bar) and T=450 °C F, = 3200 cm of "fresh" (a) and "aged" (b) TWC samples at (STP)/min. A AI2O3, ACL Al203-Ce02-La203 simulated exhaust conditions (0.1% NO-fO.7% (Reprinted with permission from Ref. [36]. CO + 0.1067% C3H6 + 0.78% O2, balanced with Copyright 2011, Elsevier.)...

Application of Perovskites in Exhaust Emission Control S77 Table 25.6 Perovskite-type oxides investigated under simulated exhaust conditions. 

Boukos, N. (2011) Thermal aging behaviour of Pt-only TWC converters under simulated exhaust conditions efiect of rare earths (CeC>2, La203) and alkali (Na) modifiers. Top. Catal, 54,1124-1134. 

Matsuka, V., Konsolakis, M Lambert, R.M., and Yentekalds, l.V. (2008) In situ DRIFTS study of the effect of stmcture (Ce02-La203) and surfece (Na) modifiers on the catalytic and surfece behaviour of Pt/ Y Al203 catalyst under simulated exhaust conditions. Appl Catal B, 84, 715-722. 

Konsolakis, M., Macleod, N., Isaac, J., Yentekakis, I.V., and Lambert, R.M. (2000) Strong promotion by Na of Pt/Y-Al203 catalysts operated under simulated exhaust conditions. /. Catal, 193, 330-337. 

The most complete study on the oxidation of CO and hydrocarbons was reported by Kuo et al. (91). Their study was done on a copper chromite catalyst under conditions that simulate exhaust gases. They found that CO oxidation kinetics is very accurately represented as first order in CO... 

For all the above studies, the cycle time used for rich and lean phase, 15 and 30 min, respectively, was long and far from being realistic. In addition to such long cycle studies, to get an estimate of the catalyst performance under realistic conditions, we have also done second type of studies known as short cycle studies. In these studies, the total cycle time was kept relatively short ( s) to simulate the actual exhaust conditions, and... 

Figure 4. Laboratory reactor system used to measure the performance of catalysts under cycled conditions. Two simulated exhaust streams of differing compositions are alternately fed to the reactor ( ).

The concentration of nitromethane in automobile exhaust using nine hydrocarbon test fuels under simulated driving conditions ranged from < 0.8 to 5.0 ppm (Seizinger Dimitriades, 1972). 

The different catalyst models within the simulation environment ExACT are used for the simulation of combined aftertreatment systems, when exhaust conditions for a catalyst are influenced by its upstream component and changes in one catalyst affect all components further down the line. An application example is given in the following section and in Chatterjee et al. (2006). It investigates a combined system of DOC and SCR catalyst. Further examples for such combined systems to be investigated are DOC and NSRC or combinations of different catalyst technologies with DPF. 

Rhodium-alumina catalysts with only 0.002 wt % Rh had a good activity for converting NO to nitrogen using simulated exhaust but inadequate oxidation activity. The latter situation can be improved by the addition of Pt or Pd, which have a well known ability for CO and hydrocarbon oxidation. However, more ammonia was formed under reducing conditions using R-Rh or Pd-Rh and also the delicate balance of reactions (CO + NO, CO + H2, etc.), which determines NO removal on the lean-side of the stoicheiometric point (i.e., excess of oxygen), was upset. Deposition of Pt... 

To summarize the laboratory data comparing Pd and Pt/Rh commercial catalysts, we found that an increase in the sulfur content in simulated exhaust results in a loss of both lightoff and warmed-up activity. The lightoff activity of the Pd is generally better than the Pt-Rh catalyst regardless of the sulfur content in the feedstream. Under warmed-up conditions, the loss of activity for HC, CO and NOx in the presence of sulfur was greater under slightly rich conditions than under lean conditions for both Pd and Pt-Rh catalysts. 

The conversions of HC, CO, and NOx on the Pd and Pd/Sr catalysts plotted as a function of X in simulated exhaust gases at 300°C are shown in Figs.l and 2, respectively. The catalytic activity on the Pd/Sr catalyst was superior to that on the Pd catalyst, in particular, under reducing conditions defined as A.<1. The conversion of HC, as representation of hydrocarbon oxidation activity, on the catalysts with alkaline earth metals and that with alkali metals plotted as a function of X in simulated exhaust gases are shown in Figs.3 and 4,... 

Cerium-, copper-cerium coexchanged ZSM-5, copper-MCM-22, copper- and cerium-EMT type zeolite, copper-FAU type zeolite and copper-Beta exhibit an activity of the same order as that of copper-ZSM-5 in NOx reduction under simulated Diesel exhaust conditions. Propene was used as the reducing agent. The catalysts were used in a powder form and their activities tested in a fixed-bed flow reactor at a space velocity of 50 000 H . Copper-SAPO-34 and cerium- and gallium-EMT type zeolite have a moderate activity under the same conditions. The presence of water vapor inhibits the activity of EMT-zeolites. When an ageing procedure is carried out on copper-EMT type zeolite, dealumination occurs. The increase of the Si/Al ratio of the zeolite does not limit the dealumination process. The exchange of the zeolite with lanthanum prevents the zeolite from dealumination but leads to a loss of the catalytic activity. 

The activity of the fresh and aged catalysts for CO, HC and NO ahatment was measured in a synthetic exhaust gas which simulated reaction conditions around the stoichiometry (R=l) with a pulsation frequency of IHz and an amphtude of 0.05. The TWC performance was measured with a catalyst inlet temperature of 450°C. The light-off performance was determined in experiments where the exhaust gas temperature at the catalyst inlet was increased continuously. 

Supported catalysts were evaluated for initial oxidation activity on a plug flow reactor at various space velocities using a standard reactant mix consisting of 1% carbon monoxide, 1.25% oxygen, 1000 ppm nitric oxide, and 250 ppm propylene (all on a dry basis), 10% water, and nitrogen the balance. This poison-free mix was used to simulate automotive exhaust conditions for lean operation. Temperature was varied from 100° to 600 °C, and conversions of carbon monoxide and propylene were determined as a function of temperature. Space velocity was varied by using 1-4 monolithic pieces, each with a volume of 6 cm3. At a total flow rate of 7500 cm3/min, space velocities of 18,750, 25,000, 37,500, and 75,000/hr could be attained. 

The behaviour of TWC s under continuous operation has been extensively studied. Due to the step-like response of the oxygen sensors the gas composition oscillates with a frequency of about 1 Hz around the stoichiometric set point. Therefore, most studies focus on the behaviour of catalytic converters under oscillating exhaust gas composition. In particular, the contribution of ceria to the dynamic behaviour of automotive catalysts under transient air/fuel conditions [3, 4, 5, 6, 7, 8] has been investigated. Binary gas mixtures have been applied to clarify the mechanisms of the periodic operation effects over different noble metal catalysts [9,10,11,12]. Muraki et al. used simulated exhaust gas to examine the performance of noble metals on a-Al Oj [9]. 

Measurements at steady-state conditions were used to determine the axial concentration profiles, which were established when a stoichiometric CO/Oj mixture (2 vol% CO, 1 vol% Oj) or a simulated exhaust gas with A, = 1 were fed into the converter. Stepwise reduction of the honeycomb length resulted in 12 axial data points, which were used to estimate the model parameters. 

Rb, Cs, or Ba, either under model three-way reactions [39-42] or under simulated car exhaust conditions [43,44]. 

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