(147d) Effect of Ceria on the Sulfation and Desulfation Behavior of Lean NOx Trap Catalysts
Lean NOx Traps (LNTs) represent a promising technology for the abatement of NOx from lean exhaust emissions. A major issue remaining for LNT catalysts is that of deactivation due to sulfur poisoning. It is well known that ceria is able to store sulfur (as sulfate); hence, for ceria-containing LNTs, the ceria may help to protect the main NOx storage component from sulfur poisoning. To better understand the role of ceria in LNT catalysis, we are studying the impact of ceria addition on the sulfation and desulfation behavior of model LNTs. This study consists of three parts: (1) A comparison of the NOx storage capacity (NSC) of both ceria-free and ceria-containing model powder LNTs before and after sulfation; (2) An investigation of the regeneration behavior of sulfated catalysts by means of H2-TPR; (3) Observation of the sulfur species formed during LNT sulfation by means of DRIFTS.
NOx storage measurements were performed at 300 °C using two powder model LNT catalysts, Pt/BaO/Al2O3 (hereafter denoted as PBA) and a physical mixture of Pt/BaO/Al2O3 and Pt/CeO2 (74:26 weight ratio, denoted as PBAC). After pretreatment at 750 °C in H2, PBA was found to possess slightly higher intrinsic NSC than PBAC under continuous lean conditions (60 min duration). However, sulfation at 300 °C caused a 32% decrease in NSC for PBA whereas only 8% was observed for PBAC. Desulfation under rich conditions for 1hr at 700 °C did not fully restore NOx storage ability for both catalysts; PBA exhibited a bigger drop in NSC than PBAC after one sulfation-desulfation cycle. Desulfation studies by means of TPR using H2 as reductant with added H2O and CO2 showed that sulfur was released from both catalysts mainly as H2S. Release of H2S occurred at ~650 °C from PBA, whereas PBAC showed two discrete H2S release events at ~ 520 °C and ~650 °C. From this it follows that the H2S release at ~520 °C corresponds to desulfation of the ceria phase, with Ba desulfation occurring at the higher temperature. Significantly, PBAC displayed much lower H2S evolution from the Ba phase than PBA, suggesting that the presence of ceria in PBAC lessened the degree of sulfur accumulation on the Ba phase. According to DRIFTS, PBA exhibited much stronger sulfate bands than PBA after exposure to flowing SO2/NO/O2 in the presence of H2O, as evidenced by a ratio of 2.43 observed for the sulfate/nitrate band intensities for PBA, versus a corresponding ratio of 0.5 for PBAC. Based on these observations, it can be concluded that the addition of ceria to barium-containing LNTs mitigates the sulfur deactivation of the barium NOx storage component.