Methane oxidation over, and regeneration of, sulfur-treated bimetallic Pd/Pt catalysts

Monique Shauntá  Wilburn, University of Virginia; University of Houston

Methane oxidation over, and regeneration of, sulfur-treated bimetallic Pd/Pt catalysts

Monique Shauntá Wilburn and William S. Epling

Department of Chemical and Biomolecular Engineering

University of Houston; Chemical Engineering Department

University of Virginia

Automotive-engine oxidation aftertreatment catalysts have to endure lengthy durations on stream and tolerate exposure to high temperatures, water, and trace sulfur. In view of these challenging operating conditions, bimetallic Pd/Pt catalysts have been a focal point of many aftertreatment catalyst research studies due to their improved activity and potential resistance to sintering when compared to the monometallic formulations. However, the literature states that these bimetallic benefits only exist when sulfur is not included in the feed gas stream. Since engine exhaust typically contains trace sulfur species, the potential for sulfur-induced activity loss cannot be ignored.  Here, CH4 oxidation (combustion) experiments were conducted on mono- and bimetallic Pd/Pt/Al2O3 catalysts to assess 1) catalytic activity after sulfur exposure and 2) effectiveness of regeneration methods. Preliminary studies with SO2-treated catalysts[1] showed that the CH4 oxidation reaction completely ceased until all low-temperature desorbing and decomposing sulfur species could be liberated from the catalyst.

 

SO2 reactor and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) adsorption and temperature-programmed desorption (TPD) studies were conducted to help identify the species inhibiting the CH4 oxidation reaction at low temperatures. The results showed that SO2 sorption characteristics and oxidation activity depend on both precious metal crystallite particle size and Pd:Pt mole ratio. In general, the amount of SO2 adsorbed and later desorbed during TPD decreased with increasing particle size or Pt content in the bimetallic Pd/Pt catalysts. Temperature-programmed oxidation, desorption, and reduction processes as models for possible catalyst regeneration were evaluated in terms of sulfur release and CH4 oxidation performance recovery. In general, for the bimetallic samples the effectiveness of SO2 regeneration methods decreased with increasing Pt content. Also, for bimetallic catalysts with higher Pt content, the associated sintering effects from the temperature-programmed regeneration methods were more detrimental to catalytic activity than the sulfur exposure.

 

[1] : M.S. Wilburn, W.S. Epling / Applied Catalysis B: Environmental 206 2017 p. 589-598