The rising awareness of environmental protection on a global level is leading to more stringent automobile emissions regulations. In addition, there are calls to reduce the use of precious metals as catalysts due to concerns about resource depletion.Recently, the number of hybrid vehicles and vehicles featuring idling engine stop functionality is increasing as fuel-efficient vehicles rapidly becoming the norm for all models. In these vehicles the amount of NOx emissions increases when the engine restarts after an idling stop and it is difficult to reduce the use of precious metals in the catalyst. Consequently, it is necessary to develop a catalytic technology that can make effective use of the Rh component because this is essential to NOx conversion.In this study, an examination was conducted using the following two approaches for the purpose of reducing the amount of transient NOx.Approach (1) It was found that the use of a Praseodymium (Pr)-doped oxygen storage capacity (OSC) material as a support for precious metals resulted in accelerating metalation of the precious metals that cause an increase in NOx conversion under air-fuel ratio switching conditions.Approach (2) It was found that Praseodymium (Pr) and Yttrium(Y)-doped Zirconium oxide works as a precious metal support to accelerate the steam reforming reaction, which results in higher NOx conversion.In this study, a mechanism was investigated to improve NOx conversion under transient conditions from the perspective of structural changes to the Praseodymium-doped OSC material and its effects on the state of precious metals. In addition, another mechanism was also investigated to improve NOx conversion via the steam reforming reaction due to the effects of additional elements and structural changes to the precious metals.