Why Pt Survives but Pd Suffers From SO_<i>x</i> Poisoning?

Abstract

Pd is more prone to sulfation compared to Pt. Given the chemical similarity between Pt and Pd, the radical divide in their tendencies for sulfation remains a puzzle. We explain this intriguing difference using an extensive first-principles thermodynamics analysis and computed bulk and surface phase diagrams. In practically relevant temperatures and O₂ and SO₃ partial pressures, we find that Pt and Pd show significantly different tendencies for oxidation and sulfation. PdO formation is favored even at low oxygen chemical potential; however, PtO₂ formation is not favorable in catalytically relevant conditions. Similarly, PdSO₄, and adsorbed SO₃ and oxygen species on clean and oxidized surfaces are highly favored, whereas PtSO₄ formation does not occur at typical temperature and pressure conditions. Finally, several descriptors are identified that correlate to heightened sulfation tendencies, such as the critical O chemical potential for bulk oxide and surface oxide formation, chemical potentials O and SO₃ for bulk sulfate formation, and SO₃ binding strength on metal surface-oxide layers, which can be used to explore promising sulfur resistant catalysts