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Enhanced Selective Oxidation of Ammonia in a Pt/Al2O3@Cu/ZSM-5 Core–Shell Catalyst
ACS Catalysis ( IF 11.3 ) Pub Date : 2020-03-03 , DOI: 10.1021/acscatal.9b04288 Rajat Subhra Ghosh 1 , Thuy T. Le 1 , Tanguy Terlier 2 , Jeffrey D. Rimer 1 , Michael P. Harold 1 , Di Wang 3
ACS Catalysis ( IF 11.3 ) Pub Date : 2020-03-03 , DOI: 10.1021/acscatal.9b04288 Rajat Subhra Ghosh 1 , Thuy T. Le 1 , Tanguy Terlier 2 , Jeffrey D. Rimer 1 , Michael P. Harold 1 , Di Wang 3
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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.
中文翻译：
Pt / Al 2 O 3 @ Cu / ZSM-5核-壳催化剂中氨的选择性氧化增强
氨逃逸催化剂（ASC）是排放控制系统中必不可少的最终步骤，涉及将NH 3选择性氧化为N 2。最新的ASC具有双层结构，该双层结构由Pt / Al 2 O 3（PGM）底层和金属（Fe，Cu）交换的沸石（MZ）顶层组成。PGM层提供高的NH 3氧化活性；然而，所需的N 2产物是在刚刚高于起燃温度的狭窄温度范围内获得的，而反应副产物N 2 O和NO x（即NO和NO 2）分别出现在中温和高温下。MZ催化剂的优点是在较宽的温度范围内通过将NH 3转化为N 2来选择性地贫化还原NO 。尽管最近的研究证明了双层设计的有效性，但仍需要进一步的发展来减少PGM的负载和ASC的体积，同时增强低温活性。在这项研究中，双层概念被缩小到单个核-壳（CS）催化剂颗粒Pt / Al 2 O 3的水平@ Cu / ZSM-5，由PGM内核和MZ外壳组成，旨在应对上述挑战。CS催化剂是通过合理设计关键合成步骤实现的，最关键的是中间Zillite-1层的初始生长，以防止ZSM-5壳二次生长期间铝的浸出。CS球形催化剂的表征揭示了具有活性的中孔PGM核（直径约40μm）和接近致密的沸石壳（约1μm厚）。在固定床反应器中对CS催化剂的评价显示出优异的NH 3氧化活性和N 2选择性。此外，我们获得了Pt / Al 2 O 3的意外增强CS配置内的出色性能，可出色熄灭NH 3氧化。我们的发现表明，CS催化剂具有与传统Pt / Al 2 O 3催化剂相同的活性，而传统的Pt / Al 2 O 3催化剂的Pt含量高3倍。此外，由包含籽晶芯Pt / Al 2 O 3的底层和Cu-SSZ-13顶层组成的双层ASC的性能与Pt负载高3倍但未经改性的双层ASC相同Pt / Al 2 O 3。Pt / Al 2 O 3的活性增强该催化剂归因于由于CS构型中silicalite-1和ZSM-5层的过度生长而导致的Pt微晶氧化物还原性的改变。最后，报告了进料中H 2 O和水热老化（HTA）对催化剂性能的单独影响。进料中的H 2 O对转化率和产物分布的影响可忽略不计。硅沸石改性的Pt / Al 2 O 3催化剂比常规的Pt / Al 2 O 3对HTA处理更具弹性。
更新日期：2020-03-04
中文翻译：
Pt / Al 2 O 3 @ Cu / ZSM-5核-壳催化剂中氨的选择性氧化增强
氨逃逸催化剂（ASC）是排放控制系统中必不可少的最终步骤，涉及将NH 3选择性氧化为N 2。最新的ASC具有双层结构，该双层结构由Pt / Al 2 O 3（PGM）底层和金属（Fe，Cu）交换的沸石（MZ）顶层组成。PGM层提供高的NH 3氧化活性；然而，所需的N 2产物是在刚刚高于起燃温度的狭窄温度范围内获得的，而反应副产物N 2 O和NO x（即NO和NO 2）分别出现在中温和高温下。MZ催化剂的优点是在较宽的温度范围内通过将NH 3转化为N 2来选择性地贫化还原NO 。尽管最近的研究证明了双层设计的有效性，但仍需要进一步的发展来减少PGM的负载和ASC的体积，同时增强低温活性。在这项研究中，双层概念被缩小到单个核-壳（CS）催化剂颗粒Pt / Al 2 O 3的水平@ Cu / ZSM-5，由PGM内核和MZ外壳组成，旨在应对上述挑战。CS催化剂是通过合理设计关键合成步骤实现的，最关键的是中间Zillite-1层的初始生长，以防止ZSM-5壳二次生长期间铝的浸出。CS球形催化剂的表征揭示了具有活性的中孔PGM核（直径约40μm）和接近致密的沸石壳（约1μm厚）。在固定床反应器中对CS催化剂的评价显示出优异的NH 3氧化活性和N 2选择性。此外，我们获得了Pt / Al 2 O 3的意外增强CS配置内的出色性能，可出色熄灭NH 3氧化。我们的发现表明，CS催化剂具有与传统Pt / Al 2 O 3催化剂相同的活性，而传统的Pt / Al 2 O 3催化剂的Pt含量高3倍。此外，由包含籽晶芯Pt / Al 2 O 3的底层和Cu-SSZ-13顶层组成的双层ASC的性能与Pt负载高3倍但未经改性的双层ASC相同Pt / Al 2 O 3。Pt / Al 2 O 3的活性增强该催化剂归因于由于CS构型中silicalite-1和ZSM-5层的过度生长而导致的Pt微晶氧化物还原性的改变。最后，报告了进料中H 2 O和水热老化（HTA）对催化剂性能的单独影响。进料中的H 2 O对转化率和产物分布的影响可忽略不计。硅沸石改性的Pt / Al 2 O 3催化剂比常规的Pt / Al 2 O 3对HTA处理更具弹性。
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