4.3.3 Removal of hydrocarbons
Figure 14 shows a comparative study for hydrocarbon oxidation over single-metal catalysts: it can be seen that Rh, Pd and Pt all give high conversions for A/F ratios at and above stoichiometry. Again (as in the case of CO), in the current (1996) UK three-way catalytic converter, Pt is the main component responsible for oxidation of the hydrocarbons. On noble metal surfaces, alkane adsorption is dissociative, whereas unsaturated and aromatic hydrocarbons adsorb either dissociatively or associatively as -complexes. The subsequent oxidation process is thought to be considerably more complicated than the oxidation of CO, and we shall not consider it in any detail.
When the engine exhaust gas composition is reducing (fuel-rich), hydrocarbons compete effectively with CO for oxygen, and they can also react with water vapour to produce CO and H2 – a reaction known as steam reforming:
This is catalysed by Rh and/or Pt with ceria and, as in the case of the water-gas shift reaction, the combination Pt–Rh/CeO2–Al2O3 is particularly active. As we noted earlier, the H2 produced may react preferentially with any O2 present, thus reducing the amount of oxygen available to react with hydrocarbons and CO. In addition, the CO produced adds to the burden of carbon monoxide to be removed.