(7fg) Methane Oxidation over, and Regeneration of, Sulfur-Treated Bimetallic Pd/Pt Catalysts

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 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 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. Catalysts with a small particle size or high Pd content tended to have greater activity for oxidizing sulfur species at low temperatures and as a result formed more aluminum sulfate species, which only decomposed at high temperatures. Large particle size or low Pd content catalysts tended to form more low-temperature decomposing and desorbing species, such as molecular SO₂ and aluminum surface sulfite, which tended to inhibit the CH₄ oxidation reaction over a broader temperature range. DRIFTS studies also proved that some sulfur species were oxidized during the SO₂-treatment or the low-temperature portion of the CH₄ oxidation reaction. To understand more specifically how the Pd:Pt mole ratio influences the type of sulfur species formed during SO₂ oxidation, DRIFTS, temperature-programmed oxidation, and steady-state oxidation studies were conducted on catalysts with different precious metal composition but similar precious metal crystallite particle sizes in order to decouple the mole ratio and particle size effects. During surface studies, it was found that catalysts with a high Pd content tended to be more active for oxidizing sulfur species at low temperatures and as a result produced more SO₃ and SO₄ species. On the other hand, reactor studies showed that catalysts with a low Pd content appeared to have greater activity for oxidizing sulfur species at a low temperature because more conversion of SO₂ to SO₃ species was detected at the reactor outlet. Since these results appear to contradict one another, studies are now focused on investigating how the oxidation products formed on the catalyst surface influence the catalyst’s apparent activity for the SO₂ oxidation reaction. Temperature-programmed oxidation, desorption, and reduction 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 SO₂regeneration 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.

Research Interests: catalysis pertaining to emissions reduction, catalyst decay resistance, and battery cell failure rate reduction

Teaching Interests: reaction engineering, battery safety, and air pollution