Understanding the Behavior of Bi-functional Catalyst in Selective Catalytic Oxidation of Ammonia to Nitrogen
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Comprehensive steady-state experimental and modeling study of the selective oxidation of ammonia on washcoated multi-functional Pt/γ-Al2O3+metal-exchanged zeolite monolith is reported. The catalysts are compared in terms of their activity and selectivity for a range of temperature, catalyst architecture, catalyst composition, space velocity, and feed compositions (NH3 + O2; NH3 + NO+ O2) in the presence and absence of H2O and CO2. SCR functions used in this thesis are Cu-ZSM-5, Fe-ZSM-5 and Cu-SSZ-13. The Pt loading of the monolith used are in the range of 0.7 – 10 g/ft3 of monolith. A single component Pt/Al2O3 catalyst, at the lower space velocity (66k hr-1) light-off was characterized by decrease in the light-off temperature of ammonia oxidation with increase in Pt loading from 0.7 to 10 g Pt/ft3 monolith and, in general, the selectivity to N2 decreased while that of N2O and NOx increased. In absence of feed NO, a dual-layer ASC resulted in higher selectivity to N2, due to selective reaction in the Fe-zeolite layer between NH3 from the bulk and counter-diffusing NOx formed in the underlying Pt/Al2O3. Even with feed NO, the dual layer catalyst showed low NOx yield and high N2 yield due to SCR of NO by NH3. When Fe-ZSM-5 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, however, the N2 yield was lower due to Pt-catalyzed NH3 oxidation, compared to the dual-layer ASC. The mixed catalyst out-performed the dual-layer catalyst at low temperature by exhibiting lower N2O and higher N2 yield which was attributed to NO migration and reduction on the SCR. Most of the trends were predicted by a 1+1 D reactor model that includes the Pt/Al2O3 ammonia oxidation and Fe-ZSM-5 kinetics and the relevant transport processes. Kinetic parameters are estimated from single catalytic component experiments while the reacting species effective diffusivity in the alumina and zeolite layers are estimated from the simulation of multi-layer experiments in which inert top layers of γ-Al2O3 and Na-exchanged ZSM-5 were added to an underlying Pt/γ-Al2O3 layer. ASC comprising Cu-SSZ-13+Pt/Al2O3 showed conversion and selectivity trends similar to that of Fe-ZSM-5+Pt/Al2O3. In absence of feed NO, NH3 conversion decreased while N2 selectivity increased as Cu-SSZ-13 loading was increased. In presence of feed NO, both NH3 and NOx conversion of the dual-layer catalyst increased with an increase in the Cu-SSZ-13 washcoat loading (thickness). We also demonstrated that a hybrid catalyst having dual-layer and mixed catalyst attributes exhibits good performance by exploiting the beneficial effects of both catalyst types. Finally, experimental and modeling study of CO oxidation on a Pt/Al2O3 revealed a shift from kinetic to mixed washcoat diffusion with increasing temperature. Experiments conducted on the Pt/Al2O3 layer coated with γ-Al2O3 layer of different thicknesses were used to estimate the effective diffusivity of CO in the γ-Al2O3.