Optimizing the dual-layer Pt/Al2O3 + Cu/SSZ-13 washcoated monolith: Selective oxidation of NH3 to N2

The state-of-the-art Ammonia Slip Catalyst (ASC) has a dual-layer washcoat architecture with a bottom layer of Pt/Al2O3 and a top layer of Cu/SSZ-13. A trade-off between the NH3 conversion and N-2 selectivity presents a challenge in the ASC design. While a sufficiently thick and active zeolitic top layer increases the N-2 selectivity, it also imposes a diffusion barrier to the reacting species in reaching the bottom Pt layer, lowering NH3 conversion. Here we describe a systematic study to identify the ASC architecture and composition that optimizes the tradeoff. The in-house synthesized ASC samples span the single layer Pt/Al2O3, conventional dual-layer Pt/Al2O3 + Cu/SSZ-13, uniform single layer of mixed Pt/Al2O3 + Cu/SSZ-13, and a hybrid design comprising a bottom layer of mixed Pt/Al2O3 + Cu/SSZ-13 and a thin top layer of Cu/SSZ-13. The overall Pt and Cu loadings are fixed across the series of samples with the Cu distributed between the two layers. The best results are obtained with the combination of a base mixed layer that provides for effective coupling between Pt and Cu active sites and a top Cu/SSZ-13 layer of an intermediate thickness and nominally half of the total Cu loading. This design has sufficient oxidation activity to convert the NH3 and reduction activity to limit NOx slippage. A 1 + 1 dimensional model which follows from our recent work [3] is effective in predicting most of the data and assists in converging on the best composition and architecture. The hybrid design exhibits a linearly decreasing dependence of the NH3 conversion and logarithmically increasing dependence of the N-2 selectivity on the top layer Cu loading. The intersection of the two functions is shown to provide a good balance between the two opposing performance variables. The model is used to identify the combination of Pt loading and Cu loading distribution giving the maximum N-2 yield for a specified temperature and space velocity.

Automated summary

The state-of-the-art Ammonia Slip Catalyst (ASC) has a dual-layer washcoat architecture with a bottom layer of Pt/Al2O3 and a top layer of Cu/SSZ-13. A trade-off between the NH3 conversion and N-2 selectivity presents a challenge in the ASC design. The best results are obtained with the combination of a base mixed layer that provides for effective coupling between Pt and Cu active sites and a top Cu/SSZ-13 layer of an intermediate thickness and nominally half of the total Cu loading.