Regeneration of a sulfur-poisoned methane combustion catalyst: Structural evidence of Pd4S formation

Abstract

Decomposition of PdSO4, a species responsible for the deactivation of Pd-based methane combustion catalysts, was studied using a PdSO4/Al2O3 model system. PdSO4 was observed to behave differently under different reaction conditions. The decomposition of PdSO4 under inert atmosphere probably involved only one reaction step and resulted in the formation of metallic palladium. Under H2-containing atmosphere, the decomposition of PdSO4 resulted eventually in the formation of Pd4S, which is probably one of the many possible sulfur-containing palladium species that can be formed during regeneration of a sulfur-poisoned Pd-rich methane combustion catalyst. The formation of Pd4S can provide a reasonable explanation to the threshold temperature of sulfur removal from the catalyst, as well as to the residual sulfur present in the catalyst after regeneration under reductive atmosphere. Overall, the results obtained in the study provide deeper insight into the regeneration process of Pd-based catalysts, possibly enabling development of a more efficient regeneration strategy.

Decomposition of Al2O3-Supported PdSO4 and Al2(SO4)3 in the Regeneration of Methane Combustion Catalyst: A Model Catalyst Study

“…Hydrogen consumption started around 250 °C, but no sulfur release was observed with a quadrupole mass spectrometer detector before the temperature range of 400-500 °C. The result is well in line with previous observations [26]. The decomposition of sulfate combinations began, however, at about a temperature of 100 °C lower than that for the individual…”

“…Al 2 O 3 -supported PdSO 4 decomposes at a lower temperature compared to individual supported Al 2 (SO 4 ) 3 . Hydrogen consumption started around 250 • C, but no sulfur release was observed with a quadrupole mass spectrometer detector before the temperature range of 400-500 • C. The result is well in line with previous observations [26]. The decomposition of sulfate combinations began, however, at about a temperature of 100 • C lower than that for the individual one, and the amount of Al 2 (SO 4 ) 3 in the catalyst affected this directly-the higher the Al 2 (SO 4 ) 3 content, the lower the decomposition temperature.…”

“…The analysis relies on the mass signals of O 2 and SO 2 recorded during the thermal decomposition. It has also been suggested that in the absence of oxygen, sulfate species could decompose via a one-step mechanism [26]. The decomposition of Al 2 O 3 -supported PdSO 4 was initiated at 600 • C in He gas, whereas the corresponding temperature under 10% O 2 /He was 800 • C. The oxygen concentration also affected the quantitative sulfur content-the lower oxygen concentration corresponded to better sulfur removal (Table 1) due to the longer time period at temperatures required for the decomposition.…”

Fundamentals of Sulfate Species in Methane Combustion Catalyst Operation and Regeneration—A Simulated Exhaust Gas Study

“…Thus, the regenerated active Pd/PdO may form on top of less active PdSO4. It was decided to perform regeneration at 500 • C due to the threshold temperature observed by several researchers [25,29]. Simulated exhaust gas contains reducing agents such as CH 4 , CO and NO, which can be expected to decrease the decomposition temperature of the sulfur species.…”

“…They successfully used reductive methane pulses to partially regenerate the catalyst at 550 • C; such temperature could also be achieved in a real engine, but complete regeneration was achieved at 600 • C, which requires additional thermal energy, and may cause a fuel penalty. A reason for partial regeneration of the sulfur-poisoned catalyst has been proposed in a recent study [29], where PdSO 4 has been observed to decompose under a reductive atmosphere to Pd 4 S. Hence, small quantities of sulfur will always remain in the regenerated catalyst. They concluded also that alternately reductive (rich) and oxidative (lean) pulses result in better sulfur removal in a regenerated catalyst compared to rich-only conditions.…”