Enhanced Selective Oxidation of Ammonia in a Pt/Al2O3@Cu/ZSM-5 Core–Shell Catalyst

The ammonia slip catalyst (ASC) is an essential final step in the emission control system and involves the selective oxidation of NH3 to N2. The state-of-the-art ASC has a dual-layer architecture composed of a Pt/Al2O3 (PGM) bottom layer and a metal (Fe, Cu)-exchanged zeolite (M-Z) top layer. The PGM layer provides high NH3 oxidation activity; however, the desired N2 product is achieved over a narrow temperature range just above light-off, whereas the reaction byproducts N2O and NOx (i.e., NO and NO2) appear at intermediate and high temperatures, respectively. An advantage of the M-Z catalyst is the selective lean reduction of NO via conversion of NH3 to N2 over a broad temperature range. Although recent studies demonstrate the effectiveness of the dual-layer design, further advances are needed to reduce the PGM loading and ASC volume while enhancing low-temperature activity. In this study, the dual-layer concept is scaled down to the level of a single core–shell (CS) catalyst particle, Pt/Al2O3@Cu/ZSM-5, composed of a PGM core and a M-Z shell, with the intent to meet the aforementioned challenges. The CS catalyst was realized by rational design of key synthesis steps, the most critical being the initial growth of an intermediate silicalite-1 layer to prevent Al leaching during the secondary growth of the ZSM-5 shell. Characterization of the CS spherical catalyst reveals a mesoporous PGM core (ca. 40 μm diameter) that is active and a nearly dense zeolitic shell (ca. 1 μm thick). Evaluation of the CS catalyst in a fixed-bed reactor shows excellent NH3 oxidation activity and N2 selectivity. In addition, we obtained an unanticipated enhancement of the Pt/Al2O3 performance within the CS configuration that gives an exceptional light-off of the NH3 oxidation. Our findings reveal that the CS catalyst has an equivalent activity to that of a conventional Pt/Al2O3 catalyst containing 3 times higher Pt loading. Further, a dual-layer ASC composed of a bottom layer containing the seeded core Pt/Al2O3 and a Cu-SSZ-13 top layer achieves the same performance as a dual-layer ASC having 3 times higher Pt loading but with unmodified Pt/Al2O3. The enhanced activity of the Pt/Al2O3 catalyst is attributed to a modification of the reducibility of oxides of Pt crystallites owing to the overgrowth of silicalite-1 and ZSM-5 layers in the CS configuration. Finally, the separate impacts of H2O in the feed and of hydrothermal aging (HTA) on catalyst performance are reported. H2O in the feed is shown to have a negligible impact on conversion and product distribution. The silicalite-modified Pt/Al2O3 catalyst is more resilient to HTA treatment than conventional Pt/Al2O3.