Diesel oxidation catalysts (DOCs) are typically composed of a Pd and Pt combination, as these bimetallic systems show improved CO and hydrocarbon oxidation activity as well as sintering resistance compared to their monomeletallic counterparts. However, sulfur poisoning is still a common degradation mode. Furthermore, SO2 oxidation to SO3 over a DOC can lead to different types and extents of degradation for downstream selective catalytic reduction (SCR) and lean NOx trap (LNT) catalysts. Previous work has shown that Pt:Pd ratio and particle size both influence intermediate sulfur surface species formation and stabilities when exposed to SO2 [1-3], and likely oxidation also. Here, a series of Pd-Pt/γ-Al2O3 catalysts were prepared using incipient wetness impregnation with different metal ratios. To obtain different particle sizes, the catalysts were calcined at different temperatures and different times. Steady-state SO2 oxidation studies were performed with catalysts having different precious metal ratios and similar particles sizes or the same metal ratio but different particle sizes. To obtain the kinetic parameters, different concentrations of SO2, O2 and SO3 were used. Global reaction orders with respect to SO2, O2 and SO3 were estimated, and found to be 1st, 0 and –½, respectively. SO2 adsorption and subsequent temperature programmed desorption studies, accompanied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), were used to identify surface species formed during SO2 adsorption and oxidation. Those observed included aluminum sulfites and sulfates, as well as PdSO4. The distribution of these species changed as a function of particle size and precious metal ratio. These sulfur species can be correlated to extents of observed loss in catalyst activity, with an example being CH4 oxidation which will be discussed.

SO2 oxidation over monometallic and bimetallic Pt and Pd catalysts

2019 Spring National Meeting

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