Simulated Stoichiometric Exhaust Conditions

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 

Extraordinarily effective promotion by electropositive modifiers (alkalies or alkaline earths) of Pt-group metals (Pd, Pt, and Rh) during reactions related to TWCs has been firstly reported by Yentekakis, Lambert, and coworkers at the end of 1990s [39-44]. Indeed, it has been shown that the CO and hydrocarbons oxidation activity, but more importantly the de-NO activity and N2-selectivity, of noble metals can be substantially enhanced by alkali or alkaline earths. For example Pt, the noble metal mostly studied in this turn, demonstrated an enhancement on NO reduction rate by up to two orders of magnitude accompanied by a notable increase of N2-selectivity upon optimal promotion by Li, Na, K, 

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

Pt/A PVACL Pt(10NaVA Pt(5NaVACL Pt(10Na)/ACL Pt(15NaVACL Commercial 

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Several parameters that affect catalytic methane oxidation on a natural gas vehicle were investigated with laboratory aged noble metal catalysts and a simulated vehicle exhaust. These include the air/fliel control strategy, the noble metal loading, the role of base metals and the exhaust hydrocarbon composition. The catalytic performance of several formulations was compared both slightly fuel-rich of the stoichiometric point and under extreme lean 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. 

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. 

Simulations have been performed with time-invariant and cyclic feeding at typical exhaust gas conditions. The simulations with oscillatory feeding allow a comparison of the time average conversions with the conversions during time-invariant feeding as a function of temperature, frequency, and amplitude. The stoichiometric equivalence ratio of the reactants is defined relative to the stoichiometry of the global reaction ... 

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