Sulfur Poisoning and Regeneration of Pd Catalyst under Simulated Emission Conditions of Natural Gas Engine 2007-01-4037

Palladium-based catalyst can be employed for natural gas exhaust clean up due to its high activity for light hydrocarbon oxidation. Unfortunately, trace amounts of sulfur in the natural gas feed severely deactivate the catalyst.

In this paper, SO₂ adsorption over a monolithic Pd/Al₂O₃ oxidation catalyst is monitored in a time-resolved manner in the presence of 100 ppm SO₂ under simulated aging conditions of a natural gas engine, which is correlated with the oxidation activity for CO and hydrocarbons such as CH₄, C₂H₆ and C₃H₈. The SO₂ adsorption is saturated in 0.5 h at 400°C and 100,000 h^-1. The molar ratio of adsorbed SO₂ and Pd is about 2/1, indicating SO₂ molecules adsorbed, or transferred to the Al₂O₃ support. The oxidation activity gets stabilized upon saturation of sulfur adsorption, and the hydrocarbon oxidation activity cannot recover even when 100 ppm SO₂ is completely removed from the stream. The light-off temperatures (T₅₀) of hydrocarbons shift 50-100°C higher after SO₂ poisoning.

When the gas stream was switched to the fuel-rich mode, 15% of the adsorbed SO₂ molecules were released from the poisoned catalyst at 400°C. No H₂S was detected in the outlet stream in the reducing atmosphere. Only traces of SO₂ molecules were detected when the regenerating temperature increased to 550°C. The poisoned Pd catalyst was reactivated to some degree, but suffered from a significant deactivation in 30 min even in the absence of SO₂, regardless of regenerating temperature. The results revealed the existence of reversible and irreversible sulfur in the reducing atmosphere. A mechanism of sulfur poisoning and regeneration is proposed.