It is important to remember that the reactions discussed in sections 4.2–4.4, and often studied separately, occur simultaneously in the presence of all the other exhaust constituents. This may have an effect on the efficiency of the individual reactions. For example, kinetic experiments have demonstrated that NO has a strong inhibiting effect on the rate of the reaction of CO with O2: Figure 19 illustrates the point for a Rh/Al2O3 catalyst.
With reference to Figure 19, compare the temperatures at which each reaction reaches a rate of 1×10−6 mol CO (g cat)−1 s−1 (the dotted line)
The green line labelled CO–O2 in Figure 19 shows the CO conversion rate for the CO–O2 reaction in the absence of NO. This approaches a value of 1×10−6 mol CO (g cat)−1 s−1 at about 230 °C. When NO is present (the CO–NO–O2 mixture) the overall CO conversion rate decreases, and approaches a value of 1×10−6 mol CO (g cat)−1 s−1 only at about 290 °C. In fact, this is close to the temperature at which the CO–NO reaction, shown as the dashed line in Figure 19, becomes significant. Therefore, the inhibition of the CO–O2 reaction is believed to be due to blocking of the active sites by adsorbed NO.