The rising awareness of environmental protection on a global level is leading to more stringent automobile emission regulations. In addition, due to concerns about resource depletion, there are calls to reduce the use of precious metals as catalysts that convert HC, CO and NOx emissions from engines. In recent years, with fuel-efficient vehicles rapidly becoming the norm, the number of hybrid vehicles and vehicles featuring idle stop functionality is increasing. In such vehicles, restarts after stops increase NOx emissions, and it is difficult to reduce the use of precious metals. Consequently, the development a technology that can make effective use of the Rh component essential to NOx conversion is necessary. We examined the following two approaches for reducing transient NOx. 【Approach①】We found that Praseodymium-doped oxygen strage capacity (OSC) material as support of precious metals result in accelerating metalation of the precious metals that cause increase of NOx conversion under Air/Fuel switching conditions for three-way catalysts. 【Approach②】We found that Praseodymium and Yttrium-doped oxygen storage capacity (OSC) material works as a precious metal support to accelerate the steam reforming reaction, which results in higher NOx conversion. In this study, we have investigated a mechanism to improve NOx conversion under transient conditions from a perspective of structural changes of the Praseodymium-doped OSC material and its effects on state of precious metals. And a mechanism to improve NOx conversion by steam reforming reaction from effects of additional elements and structural change of precious metals.