Sulfation of a PdO(101) methane oxidation catalyst: mechanism revealed by first principles calculations

Abstract

PdO efficiently catalyzes the oxidation of methane but suffers tremendously from sulfur poisoning that lowers its catalytic activity. In this work, first principles calculations were performed to reveal the mechanism of PdO(101) sulfation and how the active sites for methane activation are altered upon the formation of SO_y (y = 2 to 4) species on the surface. The results suggest that under typical experimental conditions with a high O₂/SO₂ gas ratio, the formation of SO₄-decorated PdO(101) is favored and contributes significantly to the poisoning of PdO(101) as it blocks the coordinatively unsaturated Pd atoms that were identified to play a crucial role in the activation of methane. At a low temperature regime, SO₂ oxidation forming SO₃ and SO₄ species is highly exothermic via the Eley–Rideal and Langmuir–Hinshelwood mechanisms but is limited by the high activation barrier for O₂ dissociation. On the other hand, the Mars–van Krevelen mechanism has low exothermicity but provides facile elementary steps. From these results, insights into the design of PdO-based sulfur poisoning-resistant methane oxidation catalysts were drawn.