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

Abstract

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 NO x (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.

The Effects of Platinum Dispersion and Pt State on Catalytic Properties of Pt/Al2O3 in NH3 Oxidation

“…One strategy to produce such catalysts involves direct NP synthesis and stabilization in the pores of hierarchical (micro‐mesoporous) zeolites as demonstrated for CO oxidation Pt [40] and CH 4 oxidation Pd catalysts [41] . Another viable strategy involves synthesis of a protective zeolite shell on top of Pt nanoparticles rendering an active and stable core‐shell catalyst [42] . In both cases the zeolite can be functionalized with transition metals active in NH 3 ‐SCR and play a role of the SCR catalyst layer in close proximity to Pt sites reducing the complexity of the monolith washcoating procedure.…”

“…[41] Another viable strategy involves synthesis of a protective zeolite shell on top of Pt nanoparticles rendering an active and stable core-shell catalyst. [42] In both cases the zeolite can be functionalized with transition metals active in NH 3 -SCR and play a role of the SCR catalyst layer in close proximity to Pt sites reducing the complexity of the monolith washcoating procedure. Both strategies are proved to be working, however the catalyst synthesis procedure becomes more complex.…”

Progress on Selective Catalytic Ammonia Oxidation (NH3‐SCO) over Cu−Containing Zeolite‐Based Catalysts

“…[37][38][39] E.g., Dhillon et al [37] applied a hybrid layer design with a bottom mixed layer of Pt/Al 2 O 3 and Cu-SSZ-13, and a top Cu-SSZ-13 layer to obtain high activity and N 2 selectivity in NH 3 -SCO. Still, N 2 yield below 80 % was found in the range of 623-773 K. Recently, Ghosh et al [40] proposed a Pt/Al 2 O 3 @Cu-ZSM-5 core-shell catalyst. Evaluation of such catalyst shows excellent NH 3 oxidation activity (full NH 3 conversion at ca.…”

“…Gao et al [12,48,49] employed Cu-SSZ-13 catalysts with varying Cu loadings and n(Si)/n(Al) ratios to examine their catalytic properties using NH 3 and NO oxidation and standard NH 3 -SCR. At higher Cu loadings (ion-exchange, IE) level of 90 % versus Reprinted from [40] with the permission of American Chemical Society. Figure 2.…”

“… (a) Steady‐state NH 3 oxidation over core‐shell catalysts (CS) with varying shell thickness; NH 3 conversion (open symbol), N 2 selectivity (closed symbol); (b) SEM micrograph for core‐shell catalysts with shell thickness ∼0.5 microns (CS‐0.5) and ∼1.2 microns (CS‐1.2). Reprinted from with the permission of American Chemical Society.…”