4.3.4 Removal of NO
Laboratory experiments have shown that, under the conditions in the catalytic converter, the decomposition of NO to O2 and N2 over noble metal catalysts is too slow to be significant. When the A/F ratio is stoichiometric (or below stoichiometry), NO can be removed by reduction with CO and/or hydrocarbons. For simplicity we shall consider only reduction with CO, as with the oxidation reaction, the situation with hydrocarbons is considerably more complicated.
In principle, a variety of products can be formed, specifically:
In addition, H2 produced from the water-gas shift or steam reforming reactions can reduce NO to N2, N2O or NH3:
Which reactions should a catalyst ideally promote?
The aim of the catalyst is to selectively promote reactions 6 and 7 to produce N2, rather than N2O (a greenhouse gas) or NH3 (a potentially serious general pollutant).
The NOx activities of Rh, Pt and Pd are shown in Figure 15. It is evident that Rh has the highest activity, particularly under net reducing conditions (low A/F). So why is Rh superior? To answer this question, we need to consider the mechanism of the reaction.
The catalytic reduction of NO by CO and/or H2 over a variety of surfaces has been the subject of a great deal of research. Application of various surface-science techniques has provided some understanding of the elementary steps involved, but the exact mechanism is still controversial. One view is that the first step is the dissociative chemisorption of NO. (You should note that although we describe this as dissociative chemisorption strictly it does not meet the definition, as NO is in fact first adsorbed associatively and then dissociates on the surface.) The O atoms produced are then removed by the reducing agents CO or H2. The N atoms can combine to give N2, react with chemisorbed NO to give N2O (particularly important at low temperatures), or react with chemisorbed H atoms to form NH3. These and other processes that may be involved are listed below.
NO(ad) N(ad) + O(ad)
In the following steps we have assumed, for simplicity, that all products are desorbed as quickly as they are produced. You should recognise, however, that adsorbed species, no matter how transient, will be formed initially.
Surface reactions and desorption
Reactions with hydrogen
Figure 16 compares the rate of the NO–CO reaction over a Rh(111) single crystal with that over a Rh/Al2O3-supported catalyst.
After the reaction on Rh(111), the surface was found to have a high coverage of N atoms. What does this suggest about the mechanism?
It suggests that N atom combination (reaction 21) may be the rate-limiting step in the overall NO–CO reaction on Rh(111).
This seems to be the case, particularly at higher temperatures. At lower temperatures, the concentration of NO(ad) increases and reactions 22 and 23 would then be expected to contribute to N atom removal.
The elementary steps 21–23 all require surface mobility of N atoms (to encounter either NO(ad) or other adsorbed N atoms). Although this process may occur on surfaces that are extensive on the atomic scale, such as those of single crystals or large supported crystallites, it has been argued that such mobility will be insignificant on or between the small highly dispersed particles of Rh in the automotive catalyst. Therefore, we might expect the rate-limiting steps and the observed kinetics in the cases of Rh(111) and supported Rh to be different. The Arrhenius-type plots in Figure 16 confirm that this is so: over Rh/Al2O3, the reaction has a higher activation energy (the plot in Figure 16 has a larger gradient) and a lower rate (at a given temperature) than over Rh(111).
What then is the rate-limiting step with the supported catalyst? One suggestion is NO dissociation (reaction 19) but there is a more radical alternative, involving a different overall mechanism. Infrared spectra for NO adsorbed on Rh/Al2O3 (Figure 17) show bands at 1 743 cm−1 and 1 825 cm−1, which have been taken as evidence of a dinitrosyl species, O=N–N=O, formed by reaction 26:
This step provides a means, other than diffusion of N(ad), of accomplishing the most important task in the reduction of NO, namely the pairing of two nitrogen atoms on the surface. Once formed, the dinitrosyl species is thought to lose its two oxygen atoms by way of an N2O intermediate; for example:
To summarise Whichever mechanism is correct – NO pairing to form a dinitrosyl species, or NO dissociation followed by N-atom combination and N2 desorption – both require catalytic sites that can not only bind NO but also donate charge to the adsorbate. In the first case, this charge would be used to coordinate the two NO molecules. In the second case, it would be transferred into the partially vacant 2* antibonding orbital of NO (Figure 18), weakening the N–O bond and hence facilitating dissociation.
On examining the electronic structures of the noble metals, that of rhodium is found to be particularly suitable for facilitating charge transfer to adsorbed NO, with the uppermost occupied electron levels of the metal at higher energy than the partially vacant 2* antibonding orbital of NO (Figure 18). For Pt (and also Pd), however, the situation is reversed. Vacant metal levels lie at energies below that of NO 2*, so charge will drain from this orbital to the surface, strengthening the N–O bond.
This picture provides an appealingly simple explanation for the observation that rhodium is the most active of the noble metals for NOx removal, particularly under-net reducing conditions (Figure 15). In fact, not only do Pt and Pd show lower activities, they are also less selective, with Pt, for example, promoting ammonia formation (reaction 17). Rhodium is therefore the essential ingredient in the automotive catalyst for NOx control. Because it is so active, the amount required is small – about 0.1 mass % of the catalyst, or 0.1–0.2 g, highly dispersed over the surface – but even so, catalytic converters account for around 90% of world rhodium demand (see Box 1).
Box 1 Supply and demand of precious metals
Pt, Pd and Rh form part of the platinum group metals, along with Ru, Os and Ir; they are closely related in terms of chemical and physical properties, and occur together in nature. The metals are all quite rare, and are found in only a few comparatively concentrated deposits – principally in South Africa, the former USSR, Canada and the USA. In South Africa, deposits are mined specifically for the platinum group metals, whereas in Canada and the former USSR, they are produced as byproducts from nickel and copper mining.
Separation of the metals is a long and difficult process because they, and their compounds, are so chemically similar in nature. This difficulty in separation, together with the costs incurred in refining the metals to the high purity required, makes their production an expensive business. In addition, huge amounts of the ore have to be mined and processed; for example, it takes approximately 350 kg of South African ore to yield just 1 g of platinum.
Rhodium is essentially a by-product of platinum mining, and is by far the most expensive metal in the platinum group, with a price of approximately $18 250 per kg in July 1995 (compared with Pt ($15 300) and Pd ($5 500)). Because the ratio of Rh to Pt used in automotive catalysts is richer than the mine ratio, the shortfall in Rh came, until recently, from Pt mined for other uses, for example, jewellery. Automotive catalysts are a major application for platinum group metals, the percentage of total demand being 87% for Rh and 34% for Pt (1992). Up to date prices of precious metals are always available on the world wide web.