(484e) Sulfur Oxidation Studies with Bimetallic Pd/Pt Catalysts

Bimetallic Pd/Pt catalysts are good automotive-engine oxidation aftertreatment catalyst candidates due to their improved activity and potential sintering resistance relative to the monometallic formulations. Any of these catalyst systems have to endure extended times on stream and tolerate exposure to high temperatures, water, and trace sulfur. Of note, the literature states that the Pd/Pt bimetallic benefits only exist in the absence of sulfur. Previous studies with bimetallic Pd/Pt catalysts show that sulfur sorption and oxidation characteristics are influenced by both Pd:Pt mole ratio and precious metal particle size. Here, studies were conducted with mono- and bimetallic Pd/Pt/Al₂O₃ catalysts to investigate more specifically how SO₂ oxidation activity is influenced by the Pd:Pt mole ratio. Diffuse reflectance infrared Fourier transform spectroscopy (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. During surface studies, it was found that catalysts with a higher Pd content tended to have greater activity for oxidizing sulfur species at low temperatures and as a result formed more SO₃ and SO₄ species. However, reactor studies showed that catalysts with a lower 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. Due to this obvious disconnect, 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. This work provides evidence that the SO₂ oxidation characteristics, as well as the potential for sulfur decay, for mono- and bimetallic Pd/Pt catalysts are heavily influenced by the Pd:Pt mole ratio.