Effect of Pt pre-sintering on the durability of PtPd/Al2O3 catalysts for CH4 oxidation

Abstract

Unburned CH4 is a primary component of natural gas and its contribution to the greenhouse effect is 20 times that of CO2. Durable oxidation catalysts are critical for developing natural gas vehicles (NGV). Pd-based catalysts with Pt promotion have been widely used for CH4 oxidation due to their activity, durability, and tolerance in H2O and sulfur. However, significant drawbacks including poor durability and ineffective synthesis protocols related to the Pt:Pd ratio, exist. Here, we achieve significant improvements in the activity and durability of PtPd bimetallic catalysts for CH4 oxidation by simply pre-sintering the Pt particles before the addition of Pd. Energy dispersive X-ray spectroscopy coupled with scanning transmission electron microscopy, X-ray diffraction, and diffuse reflectance infrared Fourier-transform spectroscopy studies demonstrate that sequential impregnation of Pd after pre-sintering of Pt led to a segregated PdO layer on top of the pre-sintered metallic clusters, while the co-impregnated PtPd catalysts (without pre-sintering of Pt) exhibited homogeneous distribution of Pt and Pd. Re-oxidation experiments after regeneration by fuel-rich combustion, demonstrated the rapid Pd to PdO re-oxidation behavior for the pre-sintered catalysts compared to that for the co-impregnated catalysts, which we thought important for long-term durability due to the facile catalytic cycle needed for CH4 oxidation. All results suggest that the PdO surface layer on top of the Pt clusters is critical for long-term durability of CH4 oxidation; herein, important insight on the critical nature of the molecular level design of bimetallic clusters towards the development of more durable Pd-based catalysts for CH4 oxidation, is provided.