H₂–SCR NO_x catalytic activity and N₂-selectivity of Pd-exchanged perovskites

Abstract

As demonstrated in the literature, palladium exchanged perovskites offers enhanced catalytic performance in TWC applications [1]. Concerning the elimination of nitrogen oxides under lean conditions over perovskite catalysts, the use of hydrogen as a reducing agent has been less studied [2]. Considering the stringency of the legislation for emission in engines, the use of hydrogen as a reducing agent to eliminate NO_x under lean conditions and at relatively low temperatures may be a feasible solution in the forthcoming future. In this work we compare two palladium exchanged perovskite catalysts (La_0.95Ce_0.05Fe_0.95Pd_0.05O₃ and BaTi_0.95Pd_0.05O₃ both calcined at 900°C) in terms of their catalytic activity regarding the NO-reduction employing H₂ as reducing agent. The effect of H₂O + CO₂ or CO to the catalytic activity is also reported paying special attention to the N₂-selectivity of the catalysts. Results Both catalysts showed similar NO-conversion capability (70-75%) under a feedstock composition of 720 ppm NO, 5% O₂ and 1% H₂. Under these conditions, the catalytic performance of the BaTi_0.95Pd_0.05O₃ catalyst was at low temperatures compared to the La_0.95Ce_0.05Fe_0.95Pd_0.05O₃ catalyst (150°C vs. 225°C). In the presence of H₂O and CO₂ in the feedstock, the maximum NO-conversion achieved at 240°C with the La_0.95Ce_0.05Fe_0.95Pd_0.05O₃ was 55% whilst 70% with the BaTi_0.95Pd_0.05O₃ catalyst. Addition of 0.25% CO to the feedstock (without H₂O and CO₂) reduced the NO-conversion capability of both catalysts drastically (50% with BaTi_0.95Pd_0.05O₃ and 60% with La_0.95Ce_0.05Fe_0.95Pd_0.05O₃ catalyst). N₂-selectivity was higher with BaTi_0.95Pd_0.05O₃ (over 80%) compared to La_0.95Ce_0.05Fe_0.95Pd_0.05O₃ (65-75%) at temperatures below 200°C under lean conditions, indicating a more selective character at low temperatures for single-site catalyst. N₂-selectivity of both catalysts was very little influenced in the presence of H₂O and CO₂ in the feedstock, whereas it was reduced down to 50-60% with the addition of CO. [1] R. Karita, H. Kusabe, K. Sasaki, Y. Teraoka, Catal. Today 119 (2007) 83-87. [2] I. Twagiraschema, M. Engelman-Pirez, M. Frere, L. Burylo, L. Gengembre, C. Dujardin, P. Granger, Catal. Today 119 (2007) 100-105.