The presence of sulfur in the exhaust gas mixture causes a reduction in the activity of the three-way catalyst, particularly for the water-gas shift and steam reforming reactions – processes that are important mechanisms for the removal of CO and hydrocarbons under fuel-rich conditions. Sulfur also decreases the efficiency of NOx removal. The deleterious effect of exposure to SO2 on the catalytic activity of a commercial monolithic catalyst (Pt–Rh/CeO2–Al2O3) is evident in Figure 27. Notice, however, that the conversion efficiency recovers quite rapidly once the sulfur has been removed from the gas stream. This is significant because it suggests that a change in the sulfur content of fuel (average 208 mg l−1 in the UK), could achieve a reduction in emissions from vehicles currently in use.
The general resistance of Pt and Rh to sulfur poisoning, and their ability to recover from it, were two of the factors in the original decision in the 1970s to use noble metals rather than the less active, but cheaper, base-metal oxidation catalysts. (In terms of the more stringent legislation now in force, let alone that which will apply in the future, it is unlikely that base-metal activity (or durability) would be sufficient to meet emission control requirements, even if all the sulfur in fuel were to be removed.) Pd is more susceptible to long-term damage, and it is this susceptibility to poisoning that limits its use in the UK at the moment. Pd-only catalysts are, however, under development for the US market – where there are stricter controls over the contaminant levels in fuel.
Some understanding of the mechanism of catalyst deactivation by sulfur has been obtained by examining the effects of SO2 on the surface area of metals The physical adsorption of N2 can be used to determine the ‘total’ surface area of a catalyst. The capacity of a supported metal catalyst to chemisorb CO can be used as a measure of the free metal surface area. It is typically expressed as the ratio CO/M, where CO is the amount of CO chemisorbed by a fixed mass of catalyst (proportional to the number of surface metal atoms), and M is the metal content of the catalyst (proportional to the total number of metal atoms). The higher the value of CO/M, the higher the surface area of the metal. Table 2 gives values of the CO chemisorption capacity of a Pt–Rh/CeO2–Al2O3 model catalyst after ageing in a fuel-rich mixture, in the presence and absence of SO2.
Table 2 Effect of fuel-rich ageing on the metal-surface area (expressed as CO/M) for a Pt–Rh/CeO2–Al2O3 catalyst, in the presence and absence of SO2. The value of CO/M for a fresh sample is 0.79.
|Ageing temperature/°C||CO/M after ageing|
|In the presence of SO2||In the absence of SO2|
What is the effect of SO2 on the chemisorption capacity of the catalyst? What do you suggest is happening to the metal surface in the presence of SO2?
At a given ageing temperature, the CO chemisorption capacity of the catalyst is dramatically decreased in the presence of SO2. This suggests that some sort of sulfur species has been formed on the metal surface, blocking sites previously available to chemisorb CO, and possibly also inhibiting CO adsorption at neighbouring sites.
Because the effects are not permanent, this sulfur species is presumably relatively weakly adsorbed. As a result it is desorbed, and the activity of the catalyst is regenerated, when the gas-surface equilibrium is shifted in favour of the gas-phase by removal of SO2 from the gaseous mixture (Figure 27).
The production of hydrogen sulfide
Recently, there has been a great deal of interest in the interactions of sulfur with the three-way catalyst, not so much because of its impact on activity, but rather because of a smelly side-effect. Ever since catalytic converters were introduced, the odour of hydrogen sulfide (H2S), described as ‘smelling like rotten eggs’, has been an issue. This is particularly noticeable when a car in front accelerates after idling (at traffic lights or a roundabout, say) or decelerates sharply after cruising. After combustion in the engine, sulfur in the fuel is released to the exhaust gases as SO2. Under fuel-rich (reducing) conditions, this is converted into H2S over Pt, the suggested mechanism involving the formation of the metal sulfide as a reaction intermediate. It has been found, however, that the amounts of H2S generated when engine conditions become fuel-rich, following prolonged running under fuel-lean (or oxidising) conditions, are larger than expected. The catalyst can apparently ‘store’ sulfur under lean conditions and release it under rich conditions.
The noble metals do not retain any sulfur under these conditions. However, adsorption studies with both model catalysts and the commercial (fully formulated) catalyst have shown that sulfur storage under lean conditions can occur by interaction of SO2 with both CeO2 and Al2O3 in the support. Under typical conditions, CeO2 provides the preferred adsorption sites, and the sulfur storage by the catalyst has been found to increase with increasing Ce content. XPS studies (some results of which are collected in Table 3) have been used to study the nature of the species involved.
Table 3 XPS data recorded for various cerium and sulfur compounds.
|Ce 3d||S 2p|
|CeO2||550 °C, air||881.7||–|
|CeO2||20 °C, SO2||881.6||168.0|
|CeO2||550 °C, SO2||882.8||168.8|
Comparing the S 2p binding energies with the typical values for SVI and SIV compounds included in Table 3, how would you assign the sulfur species present when CeO2 is exposed to SO2 at 20 °C and 550 °C?
The typical values suggest that at 550 °C, the sulfur exists as SVI. The value at 20 °C is slightly harder to assign, but it may be attributed to SIV associated with a highly charged cation such as CeIV or CeIII.
Comparing the Ce 3d binding energies with those for CeO2 in air and for Ce(NO3)3.6H2O, what do you conclude about the Ce species present when CeO2 is exposed to SO2 at 20 °C and 550 °C?
The Ce 3d binding energy of CeO2 in SO2 at 20 °C is similar to that of CeIV oxide (CeO2 in air). The value at 550 °C agrees reasonably well with that for CeIII in Ce(NO3)3.6H2O, suggesting that reduction of some of the surface sites to CeIII may have occurred.
Hence, interpretation of the XPS data suggests that, at room temperature, SO2 is adsorbed on CeO2, possibly to form a sulfite species (that is, a species containing SIV). At 550 °C the CeIV oxide, CeO2, appears to participate in a redox reaction with SO2; the sulfur is oxidised to SVI and the cerium is reduced to CeIII, possibly resulting in the formation of cerium(III) sulfate, Ce2(SO4)3.
The sulfate/sulfite species formed under fuel-lean conditions with the cerium and aluminium in the support decompose in fuel-rich conditions to release the SO2/SO3 species, which are then converted into H2S over the noble metal component of the catalyst. Catalysts that do not contain noble metals do not produce H2S.
Although it has been known for some time that automobile exhaust catalysts can produce H2S in fuel-rich exhaust streams, there has been an increase in the levels emitted in recent years. This is believed to be a consequence of the considerable improvements made in catalyst activity over the years. However, during one study into this effect, it was noted that H2S emissions from ‘engine-aged’ catalysts, that is, those that had been ‘on the road’ for 50,000 miles, were much lower than those from the fresh catalyst. In addition, the H2S ‘spikes’ when going from lean to rich conditions were found to be much smaller on the engine-aged catalyst.
Examination of an aged catalyst revealed traces of the usual poisons, including S and Pb from the fuel, and phosphorus (and zinc) from the engine oil. This suggested that at least one of these components can reduce the storage of sulfur by the Pt–Rh/CeO2–Al2O3 system. Phosphorus appears to be a likely candidate, because it has been found that phosphorus-doped Pt–Rh/CeO2–Al2O3 catalysts exhibit a lower capacity for adsorption of SO2 than an undoped reference catalyst. This suggests that the phosphorus is somehow ‘interfering’ with the component of the catalyst that adsorbs the SO2 – the ceria. Indeed, it has been proposed that a Ce–P–O species (possibly CePO4) is formed, which is more stable than, and hence inhibits the formation of, a Ce–S–O species (for example, Ce2(SO4)3). This would have the effect of reducing the storage capabilities of the ceria, and hence of reducing the size of the H2S spike on going from lean to rich conditions.
To summarise In view of the discussion above, it would seem that several different strategies could be used to reduce H2S emissions: (i) decreasing the Ce surface area (however, this will have detrimental effects on the catalyst activity); (ii) improving the A/F control; (iii) reducing the sulfur content of the fuel; (iv) including an H2S scavenger in the catalyst (in the USA nickel is used, but in Europe this is prohibited because of concern that it could lead to the formation and release of carcinogenic nickel carbonyl).