Investigation on Degradation Process of PdRuIr/CZ “Pseudo-Rh” Catalysts Used for Motorcycles

<div class="section abstract"><div class="htmlview paragraph">Platinum (Pt), palladium (Pd), and rhodium (Rh) are used as active substances in exhaust gas purification catalysts for automobiles. Among these, Rh is an essential element because it efficiently promotes a NOx reduction reaction. On the other hand, the price of Rh has been rising in recent years. From the perspective of the supply risk of rare resources, there is an urgent need to develop technologies to replace or reduce the amount of Rh used in catalysts. We focused on the pseudo-rhodium alloy developed by the ACCEL program of the Japan Science and Technology Agency (JST), and then investigated the application of the pseudo-rhodium alloy on the catalysts of our motorcycles and also the degradation process.</div><div class="htmlview paragraph">A nanosized PdRuIr alloy supported on a ceria-zirconia solid solution (PdRuIr/CZ) was prepared and assembled into a motorcycle for emissions measurement. The PdRuIr/CZ catalyst with an alloy loading of 4.0 g/L had initial properties comparable to the Rh supported on a CZ (Rh/CZ) catalyst with a Rh loading of 0.3 g/L, but after degradation treatment, emissions increased and were inferior to the Rh/CZ. X-ray diffraction and transmission electron microscopy of the catalyst powder showed that the alloy particles increased in size and underwent phase separation after degradation treatment. Furthermore, it was confirmed that iridium (Ir) was oxidized during the specimen preparation process. It is speculated that phase separation proceeds as iridium undergoes repeated oxidation and reduction during the catalyst production process and exposure to exhaust gases.</div><div class="htmlview paragraph">We have clarified the degradation process of PdRuIr/CZ catalysts and concluded that iridium oxidation is one of the major factors causing phase separation of the alloy.</div></div>