Identification of the role of metal oxide component of combined metal oxide–SSZ-13 catalysts in improving low-temperature selective catalytic reduction of NOx by ammonia

Tahrizi  Andana, Pacific Northwest National Laboratory

Identification of the role of metal oxide component of combined metal oxide–SSZ-13 catalysts in improving low-temperature selective catalytic reduction of NOx by ammonia

Tahrizi Andana 1, Kenneth G. Rappé 1, Feng Gao 2, Yong Wang 1

1 Energy & Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99354, USA

2 Physical & Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99354, USA

 

Driven by the need to maximize the efficiency of low-temperature selective catalytic reduction (SCR) of NOx by ammonia, various alternatives to making use of faster reduction scheme independently of diesel oxidation catalyst (DOC) function are currently being pursued. Among the viable options to minimize the dependency is integrating DOC and SCR functions in the form of a combined catalyst containing oxidation-active and SCR-active components that allow for a self-sustaining fast SCR reaction thanks to the in-situ generation of NO2.

Ceria-based catalysts have been widely studied as the oxidation-active component for their optimum compromise between activity and affordability. While metal-exchanged SSZ-13 is the current state-of-the-art SCR component, H-SSZ-13 has initially been used to help elucidate the contribution of metal oxide to SCR reaction. Altering the chemistry of ceria through introduction of heterocations (e.g., zirconium or manganese) determines metal oxide component participation in the overall SCR reaction. For example, combining ceria-manganese oxide with H-SSZ-13 results in significantly improved light-off reduction performance under standard SCR feed conditions, surpassing that of H-SSZ-13 under fast SCR conditions. This is partially attributed to direct participation of ceria-manganese oxide in the SCR reaction. In contrast, combination of ceria-zirconia – an inactive material for SCR – and H-SSZ-13 results in low-temperature activity parallel to that of H-SSZ-13 under fast SCR conditions. Such a synergy originates from the sequential steps of NO oxidation over ceria-zirconia, to closely generate NO2 for fast SCR reaction over H-SSZ-13. These steps constitute the widely hypothesized “bifunctional mechanism”, identified for most combined metal oxide–zeolite systems [1].

Aside from the chemistry of the secondary component, proximity of active sites of the two components plays equally a crucial role in the success of the combined catalytic system. The dependency on the degree of intimacy between the oxide and zeolite components arises from the participation of unstable NO2 intermediate (e.g., nitrous acid/HONO) during the primary reduction step (i.e., formation of NH4NO2) [2]. Varying coupling techniques (e.g., dual-bed arrangement, physical mixture, precipitation and impregnation) helps determine whether this requirement is valid for all combinations. This research is focused on identifying and capitalizing on a synergistic interaction between metal oxide and zeolite SCR component towards superior low-temperature SCR performance.

 

References:

[1] A.Y. Stakheev, A.I. Mytareva, D.A. Bokarev, G.N. Baeva, D.S. Krivoruchenko, A.L. Kustov, M. Grill, J.R. Thøgersen, Catalysis Today, 258 (2015) 183-189

[2] M. Salazar, R. Becker, W. Grünert, Applied Catalysis B: Environmental, 165 (2015) 316-327