Selective oxidation of ammonia to nitrogen on bi-functional Cu-SSZ-13 and Pt/Al2O3 monolith catalyst

The selective oxidation of NH3 to N-2 on bi-functional mixed and dual-layer Cu-SSZ-13 and Pt/Al2O3 washcoated monolith catalysts is investigated under several reaction conditions to probe the effects of the added mass transfer barrier, in terms of the SCR loading (layer thickness), on the activity and selectivity of the catalyst. The combination of Cu-SSZ-13, itself shown to have high activity in reducing NOx, to N-2 with NH3, and Pt/Al2O3 is effective in converting the NOx produced by the very active but poorly selective precious metal catalyst. The data reveal a trade-off between NH3 conversion and N-2 selectivity due to the mass transport barrier afforded by the added Cu-SSZ-13 top-layer: NH3 conversion decreases while N-2 selectivity increases as Cu-SSZ-13 loading is increased. In the presence of feed NOx the NOx conversion of the dual-layer catalyst increases with an increase in the Cu-SSZ-13 washcoat loading. In the absence of feed NOx the dual-layer catalyst has a high selectivity to N-2 due to the selective reaction in the Cu-SSZ-13 layer between NH3 from the bulk and counter-diffusing NOx formed in the underlying Pt/Al2O3. For the same catalyst loadings, the mixed washcoat catalyst gives a higher NH3 conversion at high space velocity than the dual-layer catalyst. This is attributed to NH3 conversion on Pt/Al2O3 sites not impeded by a SCR catalyst top-layer. In the presence of feed NOx the dual-layer catalyst shows a high NOx to N-2 conversion as a result of the high SCR activity of the Cu-SSZ-13 top-layer. The mixed catalyst is more selective to NOx because the Pt catalyst in proximity to the SCR catalyst leads to the oxidation of NH3 back to NOx. Finally, we demonstrate that a hybrid catalyst having dual-layer and mixed catalyst attributes exhibits good performance by exploiting the beneficial effects of both catalyst types. (C) 2016 Elsevier B.V. All rights reserved