4.5.6 Catalyst deterioration: summary
An ability to withstand mild deactivation is built into the design of the catalytic converter. However, severe deactivation could prevent the system from meeting emissions legislation.
The major causes of deactivation are thermal damage and poisoning.
High temperatures may cause sintering of the metals and/or the support; this can be prevented to some extent by the addition of ceria as a structural promoter. Damaging interactions between the noble metals, or with the support, can also occur at high temperatures. The interaction between Rh2O3 and γ-Al2O3 can be slowed down by first supporting the Rh2O3 on ZrO2.
Lead is a severe poison, particularly for Pd, and is believed to associate with the noble metal.
Phosphorus from engine oil can contaminate the catalyst and cause deactivation.
Sulfur present in fuel has two major undesirable effects. It can cause deactivation of the catalyst, and it also leads to generation of H2S.
Sulfur is oxidised to SO2, which is believed to block sites on the metal surface, forming a weakly adsorbed species that is desorbed when the sulfur is removed from the gas stream.
Sulfur can also be stored under fuel-lean conditions by Al2O3, and especially CeO2, in the support, and released as H2S under fuel-rich conditions. The sulfur is believed to be stored as Ce2(SO4)3. On going to fuel-rich conditions, this species decomposes, releasing SO2, which is converted into H2S over the noble metal. It has been found that adsorption of SO2, and hence the storage capabilities of a Pt–Rh/CeO2–Al2O3 catalyst, are reduced in the presence of phosphorus. The preferential formation of a Ce–P–O species (possibly CePO4) inhibiting formation of the Ce–S–O species (Ce2(SO4)3) has been proposed.
Refer back to the CO/M values listed in the final column of Table 2. How would you explain the variation in these CO/M values with increasing ageing temperature?
The decrease in the value of CO/M observed as the ageing temperature increases indicates that the free metal surface area of the catalyst is decreasing with increasing temperature. This is likely to be due to sintering of the noble-metal particles, and/or the support.