Effects of platinum high-temperature redispersion on Pt/Al2O3 diesel oxidation catalyst for nitric oxide oxidation and its reaction pathway

Abstract

High-temperature redispersion of platinum nanoparticle is inevitable for Pt/Al2O3 diesel oxidation catalyst (DOC), yet its effects on NO catalytic oxidation and its reaction mechanism are still indeterminate. Results of this work indicate that high-temperature redispersion not only decrease the nano-size of platinum particle, but also increase the crystallinity. The smaller size of platinum nanoparticle is beneficial for the reduction of high valence Ptδ+ species to form Pt0 active phase and hence improving NO oxidation performance. The higher crystallinity of platinum particle is beneficial to maintaining Pt0 state but adverse to the reduction of Ptδ+ species during long-time use. In-situ Fourier transform infrared spectroscopy results prove that NO oxidation on Pt/Al2O3 catalyst is mainly through the intermediates of nitrite and bridging nitrate. It will convert to chelated nitrates if the decomposition of nitrite and bridging nitrate intermediates are not able to occur promptly. Chelated nitrates are stable and hard to decompose which therefore will restrain NO oxidation reaction. The smaller Pt nano-size is conducive to the decomposition of nitrite/nitrate intermediates and therefore boost NO oxidation reaction, simultaneously the higher Pt crystallinity will lead to the generation and accumulation of chelated nitrates consequently cover the active sites and restrain NO oxidation reaction.