The catalytic oxidation of CO on the surface of NMs such as platinum, palladium and rhodium. In order to describe the process, the metal surface consists of active sites were denoted as “*” The catalytic reaction cycle begins with the adsorption of CO and O2 on the
H + C
R1 R2 R3 R4 R5
Olefin oxide
Aldehyde Acid CO CO2
metal oxides for the treatment of exhaust gases from internal combustion engine
surface of platinum, whereby the O2 molecule dissociates into two O atoms (X* indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site *):
O2 + 2* 2O* CO+ * CO*
The adsorbed O atom and the adsorbed CO molecule then react on the surface to form CO2, which, being very stable and relatively unreactive, interacts only weakly with the platinum surface and desorbs almost instantaneously:
CO* + O* CO2 + 2*
Note that in the latter step the adsorption sites on the catalyst are liberated, so that these become available for further reaction cycles. Figure 1.11 shows the reaction cycle along with a potential energy diagram. Once these radicals are available, the reaction with CO to CO2 follows instantaneously.
The activation energy of the gas phase reaction will be roughly equal to the energy required to split the strong O–O bond in O2, i.e. about 500 kJ mol–1. In the catalytic reaction, however, the O2 molecule dissociates easily – in fact without an activation energy – on the surface of the catalyst. The activation energy is associated with the reaction between adsorbed CO and O atoms, which is of the order of 50–100 kJ mol–1. Desorption of the product molecule CO2 costs only about 15–30 kJ mol–1 (depending on the metal and its surface structure). It can be seen that the most difficult step of the homogeneous gas phase reaction, namely the breaking of the O–O bond is easily performed by the catalyst.
Consequently, the ease with which the CO2 molecule forms determines the rate at which the overall reaction from CO and O2 to CO2 proceeds. This is a very general situation for catalyzed reactions, hence the expression: A catalyst breaks bonds, and lets other bonds form. The beneficial action of the catalyst is in the dissociation of a strong bond, the subsequent steps might actually proceed faster without the catalyst [98].
Figure 1.11 Reaction cycle and potential energy diagram for the catalytic oxidation of CO by O2 [98]
Figure 1.12 shows the reaction path ways based on the investigations. This is analogous in parts to those proposed by others and is one of the possible reaction pathways. Here M(a)–O and M(b)–O are considered as two types of active sites on metal oxides namely acidic and basic sites respectively. Where M(a) as surface active acidic site and O(b) as basic active site on metal oxides. M(a) is considered as an acidic site which is electron deficient. CO having lone pair of electrons directing the C-end of CO gets chemisorbed
metal oxides for the treatment of exhaust gases from internal combustion engine
with acidic site of metal oxide to form a bond as shown in Eq. (1).The adsorbed CO interacts with the lattice oxygen of the metal oxide. The partially bonded CO2 gets desorbed leaving the reduced acidic metal oxide on the surface as shown in Eq. (2).
Subsequently reduced site takes oxygen from the gas phase to fill the oxygen vacancy as seen in Eq. (3). The oxygen molecule takes electrons from the basic site forming O- species in Eq. (4). The adsorbed species may interact to give intermediate as shown in Eq.(5), subsequently giving CO2 and regeneration of the catalyst in Eqs. (6) and (7). If acidic and basic sites are present on the same metal oxide, then the reaction paths ways follow as below. The Eq. (8) indicates the presence of both acidic and basic sites on the same metal oxide. The carbon monoxide adsorbed on the acidic site and oxygen on basic site to form intermediate as shown in Eq. (9) and finally CO2 gas will desorbs as in Eq. (10), regenerating the active sites as seen in Eq. (11) [34].
Figure 1.12 Reaction pathways of CO oxidation over the metallic oxides [34]
1.4.3 Mechanism of the reduction of NOx
Two main chemical reaction pathways of HC-SCR (hydrocarbon-selective catalytic reduction) are complete oxidation of hydrocarbons and selective reduction of NOx by oxygenated species that are produced from such hydrocarbons (as seen Figure 1.13). On the basis of complete oxidation pathway, methoxy radical which is variously derived from i-propoxy radical, acetate and acetyl radical is the essential intermediate species for this process.
Methoxy radical can be oxidised by oxygen to generate water vapor directly. Along with water vapor formation, carbon dioxide is indirectly produced from methoxy radical
metal oxides for the treatment of exhaust gases from internal combustion engine
via formyl radical. In terms of selective reduction of NOx route, nitromethane is the first intermediate which contain both carbon and nitrogen species. Nitromethane is created by either the reaction between surface acetate and nitrogen dioxide or the reaction of surface acetyl radical and nitrate. Both surface acetate and acetyl radical are derivative products that generate from the same source, acetaldehyde. Acetaldehyde appears as the surface intermediate species which is produced from propane by oxidation processes via i- propanol and i-propoxy radical species. Once nitromethane is formed, it is further chemically converted to nitrogen through nitromethylene, formaldiminoxy, nitrile N-oxide, cyanide and isocyanate respectively [99].
Figure 1.13 Chemical reaction pathways of selective catalytic reduction of NOx by propane [99]
Model studies performed on Pt/BaO/Al2O3 suggested that the first step is the oxidation of NO to NO2, which is active species being adsorbed on the surface, even though kinetic studies could not distinguish whether surfaces nitrites are formed first and then oxidized to nitrates or whether both species are formed directly by a disproportionation mechanism (Figure 1.14). However, the final species that is strongly held on the surface and accounts for the majority of NOx stored appears to be a nitrate species, in particular at high temperature due to the low thermal stability of nitrite. Whatever is the true mechanism, it must be underlined that the kinetics and the extent of storage are heavily affected by the presence of water and CO2 in the exhausts: CO2 slows down the NOx adsorption kinetics as the reaction can more appropriately be seen as a transformation of surface carbonates into nitrates, e.g. CO2 strongly competes with NOx for the adsorption sites. This competition, on the other hand, increases the rate of NOx releases under the rich-spike. The effect of water is more controversial in that promotion of NOx adsorption was observed below 250
◦C by addition of small amounts of water (1%), whereas at higher temperature an
metal oxides for the treatment of exhaust gases from internal combustion engine
inhibition effect was observed. However, such promoting effect was not seen when both water CO2 were co-fed [67].
Figure 1.14 Principle of operation of an NSR catalyst: NOx are stored under oxidising conditions (1) and then reduced on a TWC when the A/F is temporarily switched to rich conditions (2) [67]