Converter
3.3 Process Model of the TWC
3.3.2 Kinetics of the TWC
The reaction rate coefficient is also determined from the Arrhenius-Ansatz.
The rate is dependent on the occupancies of the reacting species, if Langmuir- Hinshelwood kinetics are applied:
rreaction,LH =Areac,LHe
−Ereac
ℜTs θiθj (3.14) As for the Eley-Rideal kinetics, the rate is dependent on the occupancy of the
Table 3.1: Reaction scheme of the TWC model.
O2(g) + 2∗ ↔ 2O∗ (3.15)
CO(g) +∗ ↔ CO∗ (3.16)
H2(g) + 2∗ ↔ 2H∗ (3.17)
NO(g) +∗ ↔ NO∗ (3.18)
C3H6(g) + 4∗ ↔ C3H5∗∗∗+ H∗ (3.19) CO∗+ O∗ → CO2(g) + 2∗ (3.20) 2H∗+ O∗ → H2O(g) + 3∗ (3.21) C3H5∗∗∗
+ 5O∗ → 3CO∗+ 2H2O(g) + H∗+ 4∗ (3.22)
NO∗+∗ ↔ N∗+ O∗ (3.23)
2N∗ → N2(g) + 2∗ (3.24) Ce2O3+1
2O2(g) → 2CeO2 (3.25)
Ce2O3+ H2O(g) + 2∗ → 2CeO2+ 2H∗ (3.26) C3H6(g) + 12CeO2 → 3CO(g) + 3H2O(g) + 6Ce2O3 (3.27) CO(g) + 2CeO2 → CO2(g) + Ce2O3 (3.28) H2(g) + 2CeO2 → H2O(g) + Ce2O3 (3.29)
The reaction scheme has been mainly derived from [54]. ∗denotes a vacant site on the noble metal. Superscript∗stands for adsorbed species (on the noble metal). The number of∗represents the number of vacant sites occupied by the adsorbed species.
adsorbed species and on the concentration of the gaseous species. Theoreti- cally, similar considerations should be made as for the adsorption rates. How- ever, for the sake of simplicity, also the Arrhenius-Ansatz is applied. Notice
that the unit of the pre-exponential factorAreac,ERhas to be adapted accord- ingly.
rreaction,ER=Areac,ERe
−Ereac,LH
ℜTs θicj (3.30) Table3.1shows the reaction scheme used in the model. Reactions on the noble metal surface (3.15–3.24) are modelled using Langmuir-Hinshelwood kinetics. Reactions on the ceria (3.25–3.27) follow the Eley-Rideal mechanism.
In [6], also the adsorption of CO, NO, and NO2on the ceria surface has been taken into account. Additionally, species adsorbed on the noble metal can be oxidised by oxygen adsorbed on the ceria surface. This has been omitted here in order to keep the model as simple as possible.
Oxygen Storage
Oxygen can adsorb on the noble metal (3.15) and on the ceria (3.25). The des- orption has been assumed to be very weak. Therefore, it has been neglected in the parametrisation of the models. The adsorption and dissociation are mod- elled in one step, where the adsorption is the rate determining step.
No exchange of oxygen between the noble metal and the ceria has been modelled. For simplicity, the two phases have been kept separated, which is certainly not the case in reality. Reducing species adsorbed on the noble metal react with oxygen from the ceria at the phase boundary [6]. Additionally, it has been shown that in reality, oxygen is exchanged between the noble metal and the ceria [95].
The ceria mainly contributes to the oxygen storage capacity. Oxygen storage in the bulk has not been taken into account, i. e., the total storage capacity has been collected in one surface capacity.
NO Reduction
The reduction of NO occurs via adsorption (3.18), dissociation (3.23) and N2
formation (3.24). This implies that vacant surface sites are necessary. During lean operation, the surface is mainly occupied by oxygen. Therefore, the NO dissociation is hindered. Because of the abundance of oxygen, the back reaction of (3.23) is additionally dominant. Hence, oxygen is preferred for the oxidation of reducing species. Under rich operation, the NO reduction is efficient, as long as the surface is rather free. With increasing deactivation, presumably by carbonaceous species, also the NO dissociation becomes more inhibited. This leads to a reduced NO conversion under rich operation, as has been reported in e. g. [29].
Other compounds of N and O, such as N2O or NO2 have been neglected.
Reaction schemes including NO2have been presented in [53] and [6].
CO Oxidation
The CO oxidation occurs on the noble metal via (3.16) and (3.20), and on the ceria via (3.28). The first reaction path is straightforward and widely applied in the literature, see e. g. [54], [6], or [31]. The CO2adsorption has been omitted here, no back reaction of (3.20) occurs. The oxidation with oxygen from ceria might also be modelled with CO adsorbed on the noble metal instead of the gaseous form, as has been proposed in [24]. However, in [95], it has been observed that only a limited amount of oxygen on the ceria is accessible for the oxidation of adsorbed CO on the noble metal. In [6], even the adsorption of CO on the ceria and on the oxidised noble metal surface (to form OCO) had been accounted for. From a phenomenological and thus much simplified point of view it is important that the presence of ceria significantly increases the CO oxidation. This has been taken into account here by the application of the Eley-Rideal mechanism, where the two reactants do not compete for the same surface sites.
HC Oxidation
The HC are represented by propene, C3H6. This choice follows the model pre- sented in [54]. Often, propene or propane are chosen as a replacement for all HC. In [6], C2H2and C2H4have been used to represent slow and fast oxidised hydrocarbons. With the approach chosen here, the specific properties of dif- ferent hydrocarbons (alkanes, alkenes, aromates etc.) are collected in the one chosen species, C3H6. Hence, also properties such as coking, which are more attributed to alkanes, see [10], [25], or [13], are modelled with propene.
The oxidation of C3H6in the model takes place on the noble metal via (3.19) and (3.22) and on the ceria via (3.27). The scheme has been adopted from [54].
The adsorption on the noble metal includes the dissociation, where the breaking of one C-H bond is involved. The oxidation to adsorbed CO, H, and gaseous water has been modelled in one step. A very detailed scheme, where the break- ing of each C-H bond was taken into account, has been presented in [31]. It has been assumed that one adsorbed C3H5 molecule occupies three sites on the noble metal. This is again a rough assumption, based on phenomenolog- ical observations. It will be shown that in this model, the deactivation under rich operation mainly occurs because of site occupation by hydrocarbons. In reality, it is not entirely clear, what species actually occupies the sites. From exhaust gas measurements, see also Figure3.1, it is evident that some carbona-
ceous species are involved. It has been assumed that one surface site is used per C atom, which leaves space for interpretation about the form of the adsorbed species. Above all, this assumption has brought about the best simulation re- sults. In [62], it has been shown that “coking” with CHx indeed occurs on a Rh/alumina/ceria catalyst, when exposed to propane pulses. Additionally, it has been shown there that the conversion rate for the steam-reforming reaction rapidly decreases after the first pulse because of coking.
HC oxidation on the ceria is implemented using an Eley-Rideal mechanism.
No competition for vacant surface site between the reactants occurs. As on the noble metal, C3H6is oxidised to gaseous CO and water.
H2Oxidation and H2O Dissociation
The oxidation of the H2 occurs, like the CO and the HC, both on the noble metal via (3.17) and (3.21), and on the ceria via (3.29). The mechanism is in both cases simple and straightforward.
In [54], a reaction path on the ceria has been proposed, where adsorbed hy- drogen (on the noble metal) and oxygen (on the ceria) react to form OH on the ceria as an intermediate product. Two OH ions may react to water, leaving an oxidised and an empty site on the ceria:
H∗+ OCe ↔ OHCe (3.31)
2OHCe ↔ VCe+ OCe+ H2O(g) (3.32) The corresponding back reactions describe the adsorption and dissociation of water, providing the necessary steps for the water-gas shift and the steam- reforming reactions. Also other authors have described the occurrence of OH, see e. g. [34] or [98]. In [62], it has even been shown that hydroxyl is very mobile and spillover to the noble metal occurs.
In [34], it has been stated that the OH occupancy is responsible for the deac- tivation of the water-gas shift reaction under rich operation. That investigation was made with Rh/Al2O3catalysts, where only the water-gas shift reaction was focused on, i. e., hydrocarbons were not present. Considering the transient be- haviour of the aged TWC in Figure3.1, it can be seen that the water-gas shift reaction is deactivated rather fast after the lean–rich step (approximately after 5 s). However, the deactivation of the steam-reforming reaction is much slower (approximately20 safter the step). Hence, another mechanism must be respon- sible for this deactivation. Since no increased water or hydrogen concentrations have been observed after the rich–lean step, it has been concluded that the OH occupancy is only responsible for the “fast” deactivation of the water-gas shift reaction.
The goal of this investigation was to keep the reaction scheme as slim as pos- sible. Therefore, the water adsorption and dissociation was considered more on a phenomenological level. It is described by reaction (3.26). Hence, water is adsorbed on the ceria and dissociated to adsorbed hydrogen on the noble metal and oxygen on the ceria, if vacant sites are available both on the noble metal and on the ceria. Under lean operation, the occupancy on the surfaces by oxy- gen inhibits the adsorption of water. Additionally, hydrogen from the water dissociation is immediately oxidised because of the abundance of oxygen. Un- der rich operation, water adsorption is inhibited, if the noble metal surface is coked, i. e., covered with hydrocarbons. It will be shown that this very sim- ple approach suffices to describe the rather complex behaviour of the H2, CO, and HC concentrations during a lean–rich step. The same mechanism, which inhibits the water-gas shift reaction will be shown to also hinder the NO dissoci- ation. It is clear that this is a purely phenomenological approach, but hopefully it provides some inspiration for more detailed reaction schemes.
The Role of Sulfur
Basically, sulfur plays an important role in the deactivation of the TWC, espe- cially during rich operation, see e. g. [29]. It occurs in the exhaust mainly as SO2and is assumed to inhibit the water-gas shift and the steam-reforming reac- tions, especially on Pd and ceria [70]. Additionally, it has been found to hinder even NO reduction under rich operation [29]. Exactly these phenomena are fo- cused on here and can be reproduced by the TWC model (see Figures3.7–3.9).
For all experiments presented in this thesis, a fuel with a rather low sulfur con- tent (17 wtppm) was used. Therefore, sulfur was neglected. However, there is no contradiction of this thesis with the cited work, since it is still possible that sulfur promotes the mechanisms presented here. This means that the TWC is deactivated by stored carbonaceous species, but the storage mechanism itself can be enhanced by the presence of sulfur. In fact, it has been observed in [10]
that SO2promotes the coking of the TWC, when alkanes are present.