Flame-Made WO3/CeOx-TiO2 Catalysts for Selective Catalytic Reduction of NOx by NH3

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• 作者：Katarzyna A. Michalow-Mauke ; Ye Lu ; Kazimierz Kowalski ; Thomas Graule ; Maarten Nachtegaal ; Oliver Kr枚cher ; Davide Ferri
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：October 2, 2015
• 年：2015
• 卷：5
• 期：10
• 页码：5657-5672
• 全文大小：870K
• ISSN：2155-5435

文摘

Materials based on a combination of cerium鈥搕ungsten鈭抰itanium are potentially durable catalysts for selective catalytic NO_x reduction using NH₃ (NH₃鈥揝CR). Flame-spray synthesis is used here to produce WO₃/CeO_x-TiO₂ nanoparticles, which are characterized with respect to their phase composition, morphology, and acidic properties and are evaluated by NH₃鈥揝CR. HR-TEM and XRD revealed that flame-made WO₃/CeO_x-TiO₂ consists of mainly rutile TiO₂, brannerite CeTi₂O₆, cubic CeO₂, and a minor fraction of anatase TiO₂. These phases coexist with a large portion of amorphous mixed Ce鈥揟i phase. The lack of crystallinity and the presence of brannerite together with the evident high fraction of Ce³⁺ are taken as evidence that cerium is also present as a dopant in TiO₂ and is well dispersed on the surface of the nanoparticles. Clusters of amorphous WO₃ homogeneously cover all particles as observed by STEM. Such morphology and phase composition guarantee short-range Ce鈥揙鈥揟i and Ce鈥揙鈥揥 interactions and thus the high surface concentration of Ce³⁺. The presence of the WO₃ layer and the close Ce鈥揙鈥揥 interaction further increased the Ce³⁺ content compared to binary Ce鈥揟i materials, as shown by XPS and XANES. The acidity of the materials and the nature of the acid sites were determined by NH₃ temperature-programmed desorption (NH₃-TPD) and DRIFT spectroscopy, respectively. TiO₂ possesses mainly strong Lewis acidity; addition of cerium, especially the presence of surface Ce³⁺ in close contact with titanium and tungsten, induces Br酶nsted acid sites that are considerably increased by the amorphous WO₃ clusters. As a result of this peculiar element arrangement and phase composition, 10 wt % WO₃/10 mol % CeO_x-90 mol % TiO₂ exhibits the highest NO_x reduction efficiency, which matches that of a V₂O₅鈥揥O₃/TiO₂ catalyst. Preliminary activity data indicate that the flame-made catalyst demonstrates much higher performance after thermal and hydrothermal aging at 700 掳C than the V-based analogue despite the presence of the rutile phase. Ce³⁺ remains the dominating surface cerium species after both aging treatments, thus confirming its crucial role in NH₃鈥揝CR by Ce鈥揥鈥揟i-based catalysts.