NOx removal efficiency and ammonia selectivity during the NOx storage-reduction process over Pt/BaO(Fe, Mn, Ce)/Al₂O₃ model catalysts. Part II: Influence of Ce and Mn–Ce addition

Abstract

It was previously demonstrated in the first part of this work that NOx storage-reduction process over Pt/BaO/Al₂O₃ model catalyst is limited by the reduction step, with ammonia emission since H₂ is not fully consumed. The stored NOx reacts preferentially with the introduced H₂ giving NH₃, than with NH₃ in order to produce N₂. Mn addition favors the NOx reduction with ammonia leading to better conversion and selectivity, but only at 400 °C. In Part II, a special attention was focused on the role of Ce and Mn–Ce addition in regard to the NOx conversion and the ammonia emission in the 200–400 °C temperature range. With ceria modified Pt/20Ba/Al catalyst, significant improvements are obtained from 300 °C. In addition to the enhancement of the NOx + NH₃ reaction, the ammonia selectivity is maintained at a lower level compared with Pt/Ba(Mn)/Al catalysts, even in the case of a large H₂ excess. It is attributed to the ammonia oxidation into N₂ via the available oxygen at the catalyst surface. A synergetic effect is observed between Mn and Ce when they are added simultaneously in Pt/Ba/Al catalyst.

Introduction

NOx storage reduction (NSR) catalysts are a possible way to reduce NOx for diesel and lean burn engines [1]. They work mainly in lean condition. During these periods, NOx are oxidized over precious metals and stored on basic compounds such as barium oxides, mainly as nitrates. Periodically, the catalyst is submitted to short periods for few seconds in rich conditions in order to reduce the trapped NOx into N₂ [2], [3]. In addition with deactivation by sulfur poisoning [4], [5] and thermal aging [6], [7], [8], another problem can be the NOx reduction selectivity. Indeed, in addition to N₂O, NH₃ emission can be observed [9], [10]. This work is mainly focused on this ammonia emission. In the first part of this study [11], NOx removal efficiency of Pt/Ba/Al model catalyst was studied using lean/reach cycling condition with H₂ as reducer and CO₂ and H₂O in the feed stream. It was established that the NOx reduction selectivity strongly depends on the hydrogen conversion which was introduced in the rich pulses: NH₃ is emitted since hydrogen is not fully converted, whatever the NOx conversion rate. Moreover, the ammonia selectivity increases with the amount of unconverted hydrogen. This study has showed that Pt/Ba/Al catalyst is able to reduce NOx into N₂ using NH₃ as reducer, but the ammonia formation rate via the NOx reduction by H₂ is higher than the ammonia reaction rate with NOx to form N₂. It was also showed that H₂O inhibits the ammonia formation because it limits the formation of CO via the reverse WGS reaction, CO being a precursor for the isocyanate formation, which leads to ammonia after hydrolysis. In absence of other introduced C-compounds, CO₂ also favors the ammonia formation via the isocyanate route, with intermediate formation of CO via the reverse WGS reaction.

Then, the influence of iron and manganese, both commonly proposed in NSR formulation, was studied. Fe is reported to improve the catalyst sulfur resistance because it leads to the inhibition of bulk barium sulfates formation [12], [13]. Fe is also reported to be active in NOx SCR by ammonia [14]. However, we showed that Fe addition leads to a strong catalyst deactivation after successive tests, probably due to interaction between iron and platinum. Mn can also participate in the NOx storage [15], [16] and is active for the NOx reduction by NH₃ [14], [17], [18]. In fact, we found that Mn addition induces different behaviors depending on the temperature test. At low temperature (200–300 °C), Mn is a poison for the reduction step. By contrast, at 400 °C, Mn favors the NOx reduction with ammonia, even if the introduced hydrogen is not fully converted, leading to a significant enhancement of the NOx conversion and N₂ selectivity. However, if a large hydrogen excess is introduced, the ammonia selectivity becomes very close with Pt/20Ba/Al and Pt/20BaMn/Al. Thus, the NOx conversion can be improved but the low temperature activity is still a problem. In this second part, influences of Ce and Ce–Mn addition were studied, especially toward ammonia emission. Cerium compounds are well known in automotive catalysis for their oxygen storage/release behavior [19]. However, some interesting cerium properties were put in evidence for NOx-trap systems. Ceria is claimed to improve the barium stability, with an inhibiting effect for the barium aluminate formation [20]. Barium–cerium interaction was evidenced by BaCeO₃ formation even if this specie is decomposed under NO₂–H₂O and destabilized under CO₂ [8]. Migration of Ba ions through CeZrOx compound was also observed by Liotta et al. [21] in Pt–CeZrOx/Ba–Al₂O₃ catalyst. This Ba–Ce interaction could allow a better control of the Ba dispersion as well as an improvement of the resistance to SO₂ poisoning [22]. These interesting properties toward sulfur poisoning regeneration are attributed to lower cerium sulfates stability compared with barium sulfates [23], [24]. In addition, ceria compounds are able to store NOx [25], [26], [21]. For example, Ba/CeO₂ material has exhibited a higher NOx storage efficiency than Ba/Al₂O₃ in the 200–400 °C temperature range [20]. Similar results were obtained by Lin et al. [27] who investigated the effect of La or Ce addition on the NOx storage properties of Pt/Ba–Al₂O₃. The low temperature efficiency of ceria based storage material was also demonstrated with MnOx–CeO₂ oxide [28]. Besides, cerium addition could improve the NOx removal efficiency. Indeed, Pt/MgO–CeO₂ catalyst was found to be active for low-temperature NO SCR by H₂ of NO [29]. Concerning the ammonia selectivity, cerium could lead to lower emission. Indeed, in a recent work about the NOx storage reduction (NSR) behavior of Pt/Ce_xZr_1−xO₂ catalysts, it was observed that the ammonia selectivity decreases with the increase of the cerium loading [30]. Thus, the aim of the present work is to examine the influence of Ce addition on the NSR efficiency of Pt/Ba/Al model catalyst, with a special attention for the ammonia emission. Furthermore, association of Mn and Ce addition is also studied.