(380b) Sulfur Dioxide Oxidation Studies with Precious Metal Catalysts: Sulfur Surface Species Stability Versus Adsorption Amount Impact on Activity Loss

Catalysts used in aromatics hydrogenation and automotive after-treatment systems are susceptible to sulfur poisoning. For these applications, precious metal-based catalysts are often used due to their high activity, though sulfur exposure can compromise activity. Researchers have studied bimetallic Pd/Pt catalysts due to their improved activity and potential sintering resistance relative to the monometallic formulations over time on stream. In the case of automotive after-treatment systems, catalysts have to endure extended times on stream and tolerate exposure to high temperatures, water, and trace sulfur species. Our previous studies with mono- and bimetallic Pd/Pt catalysts show that sulfur sorption and oxidation characteristics are influenced by both Pd:Pt mole ratio¹ and precious metal particle size². 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, in the absence of oxygen, 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, under oxidizing conditions, 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, further studies were focused on investigating how the oxidation products formed on the catalyst surface influence the catalyst’s apparent activity for the SO₂ oxidation reaction. During these studies, it became evident that the SO₂ oxidation characteristics, as well as the catalyst’s potential for sulfur decay, were heavily influenced by the catalyst precious metal composition and experimental gas environment. Both the catalyst precious metal composition and gas environment influenced the types of species formed and the amount of sulfur species adsorbed during the SO₂ oxidation reaction. This work provides evidence that the type of sulfur species formed on the catalyst surface, not necessarily the amount of sulfur adsorbed, has the greater impact on whether catalytic activity is lost during sulfur exposure.

1. Wilburn, Monique Shauntá, and William S. Epling. "SO2 adsorption and desorption characteristics of bimetallic Pd-Pt catalysts: Pd: Pt ratio dependency." Catalysis Today (2017).

2. Wilburn, Monique Shauntá, and William S. Epling. "SO2 adsorption and desorption characteristics of Pd and Pt catalysts: Precious metal crystallite size dependence." Applied Catalysis A: General 534 (2017): 85-93.