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.