Sulfur Poisoning and Regeneration of Pd Catalyst under Simulated Emission Conditions of Natural Gas Engine

Abstract

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, SO 2 adsorption over a monolithic Pd/Al 2O3 oxidation catalyst is monitored in a time-resolved manner in the presence of 100 ppm SO 2 under simulated aging conditions of a natural gas engine, which is correlated with the oxidation activity for CO and hydrocarbons such as CH 4, C 2H6 and C 3H8. The SO 2 adsorption is saturated in 0.5 h at 400°C and 100,000 h -1 . The molar ratio of adsorbed SO 2 and Pd is about 2/1, indicating SO 2 molecules adsorbed, or transferred to the Al 2O3 support. The oxidation activity gets stabilized upon saturation of sulfur adsorption, and the hydrocarbon oxidation activity cannot recover even when 100 ppm SO 2 is completely removed from the stream. The light-off temperatures (T 50 ) of hydrocarbons shift 50-100°C higher after SO 2 poisoning. When the gas stream was switched to the fuel-rich mode, 15% of the adsorbed SO 2 molecules were released from the poisoned catalyst at 400°C. No H 2S was detected in the outlet stream in the reducing atmosphere. Only traces of SO 2 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 SO 2, 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.