Selective oxidation of ammonia on mixed and dual-layer Fe-ZSM-5 + Pt/Al2O3 monolithic catalysts

Abstract

The development of improved Ammonia Slip Catalysts (ASCs) was pursued by changing the composition and architecture of the critical functions, Selective Catalytic Reduction (SCR) and oxidation, needed in the catalyst for high NH3 oxidation activity and selectivity to N2. Selected space velocities and reactant compositions were used to probe chemical and physical processes that limit the performance of ASCs. On a single component Pt/Al2O3 catalyst, at the lower space velocity (66kh−1) light-off was characterized by a sharp increase in NH3 conversion to >98%, while at high space velocity (265kh−1) the transition in conversion was more gradual and the limiting conversion was below 98%, indicative of transverse transport limitations. The light-off temperature of ammonia oxidation on Pt/Al2O3 catalyst decreased with Pt loading from 0.7 to 10g Pt/ft3 monolith and, in general, the selectivity to N2 decreased while that of N2O and NOx increased. A dual-layer ASC comprising a top layer of Fe-ZSM-5 and a bottom layer of Pt/Al2O3 resulted in higher selectivity and yield to N2, due to SCR reactions between the counter diffusing NOx formed in the Pt/Al2O3 and NH3 reactant in the Fe-zeolite layer. However, the diffusion resistance provided by the Fe-ZSM-5 layer inhibited the overall ammonia conversion at high space velocity. When Fe-zeolite and Pt/Al2O3 particles were mixed and washcoated as a single layer, this led to an increase in NH3 conversion at high space velocity due to a decrease in the diffusion barrier that was observed with the dual-layer structure. When SCR and oxidation catalyst particles were contiguous in the washcoat structure as in single-layer mixed catalysts, the N2 yield was lower due to Pt-catalyzed NH3 oxidation, compared to the dual-layer ASC, which was especially apparent at low space velocity. The dual-layer catalyst was superior to the mixed layer catalyst at high temperatures, exhibiting lower NOx and higher N2 yields whereas the mixed catalyst out-performed the dual-layer catalyst at low temperature by exhibiting lower N2O and higher N2 yield.