Role of Pore Structure on the Durability of Pt Diesel Oxidation Catalysts

Arnab Ghosh, Hien P Pham, Matthew Melton, Sivakumar Challa, Deepak Kunwar, Andrew DeLaRiva, Abhaya K. Datye

University of New Mexico

Platinum (Pt) is an active component in diesel oxidation catalysts (DOCs) but it undergoes Ostwald ripening to form large particles at elevated temperatures due to significant vapor phase transport of PtO2. The role of the pore structure on trapping of Pt species and confining them inside the pores of mesoporous support have not been studied. Here we examined the effect of different pore structures on the emission of Pt to the vapor phase using accelerated aging protocols for Diesel Oxidation Catalysts. We used a thin film of catalyst to enable measurements of emission of Pt and we compared these observations with aging in a crucible and in a packed bed. As expected, mesoporous silica supports slowed the emission of PtO2 vapor from the catalyst due to the slower transport through the pore structure. However, to our surprise, when the catalysts were aged in a boat, the open pore Pt/Davisil silica performed better in CO oxidation than the mesoporous silica. XRD crystallite size measurements confirm that the mean crystallite size is larger in the mesoporous silica compared to the Davisil silica. We conclude that the particle size distribution after high temperature aging is determined by the nucleation of the Pt from PtO2 vapor. The mesoporous silica supports do not allow nucleation and growth of Pt within the pores, hence larger particles of Pt tend to form on the limited external surface of these mesoporous silica supports. This work provides clues for improving the performance of Pt catalysts subjected to high temperature aging.