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- Clemens, Anna, 1986, et al.
(författare)
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Reaction-driven Ion Exchange of Copper into Zeolite SSZ-13
- 2015
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 5:10, s. 6209-6218
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Tidskriftsartikel (refereegranskat)abstract
- We have used several techniques to characterize Cu-SSZ-13 before and after exposure to reaction conditions relevant for NO reduction by NH3-SCR to address the increase in NH3-SCR activity observed as a function of time on stream. Specifically, we focus on characterizing copper species in samples prepared with solid state ion exchange (SSIE) having varying Cu loadings from 0.7 to 5.2 wt %. X-ray diffraction shows that CuO species that likely remain from the SSIE synthesis on the outside of the zeolite crystallites are significantly reduced by exposure to reaction conditions. High-resolution X-ray diffraction (HR-XRD) further reveals a corresponding increase of Cu2O species. X-ray photoelectron, Auger electron, and ultraviolet -visible spectroscopy show an increase in both Cu(II) and Cu(I) species inside the zeolite pores, with a preserved long-range order of the zeolite structure as revealed by HR-XRD.
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- Feng, Yanyue, 1993, et al.
(författare)
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Synthesis and Characterization of Catalytically Active Au Core─Pd Shell Nanoparticles Supported on Alumina
- 2022
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 1520-5827 .- 0743-7463. ; 38:42, s. 12859-12870
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Tidskriftsartikel (refereegranskat)abstract
- A two-step seeded-growth method was refined to synthesize Au@Pd core@shell nanoparticles with thin Pd shells, which were then deposited onto alumina to obtain a supported Au@Pd/Al2O3 catalyst active for prototypical CO oxidation. By the strict control of temperature and Pd/Au molar ratio and the use of l-ascorbic acid for making both Au cores and Pd shells, a 1.5 nm Pd layer is formed around the Au core, as evidenced by transmission electron microscopy and energy-dispersive spectroscopy. The core@shell structure and the Pd shell remain intact upon deposition onto alumina and after being used for CO oxidation, as revealed by additional X-ray diffraction and X-ray photoemission spectroscopy before and after the reaction. The Pd shell surface was characterized with in situ infrared (IR) spectroscopy using CO as a chemical probe during CO adsorption-desorption. The IR bands for CO ad-species on the Pd shell suggest that the shell exposes mostly low-index surfaces, likely Pd(111) as the majority facet. Generally, the IR bands are blue-shifted as compared to conventional Pd/alumina catalysts, which may be due to the different support materials for Pd, Au versus Al2O3, and/or less strain of the Pd shell. Frequencies obtained from density functional calculations suggest the latter to be significant. Further, the catalytic CO oxidation ignition-extinction processes were followed by in situ IR, which shows the common CO poisoning and kinetic behavior associated with competitive adsorption of CO and O2 that is typically observed for noble metal catalysts.
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- Härelind, Hanna, 1973, et al.
(författare)
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Influence of the Carbon–Carbon Bond Order and Silver Loading on the Formation of Surface Species and Gas Phase Oxidation Products in Absence and Presence of NOx over Silver-Alumina Catalysts
- 2012
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 2:8, s. 1615-1623
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Tidskriftsartikel (refereegranskat)abstract
- The influence of carbon–carbon bond order, here systematically represented by prototypical C2H6, C2H4, and C2H2, on the formation of oxidation products and surface species in the absence and presence of NO has been studied for silver-alumina catalysts with different silver loadings (2 and 6 wt %). The catalysts were prepared with a sol–gel method including freeze-drying, which results in small silver species uniformly distributed throughout the alumina matrix. The performance of the catalysts was investigated by temperature programmed extinction-ignition experiments using a continuous gas flow reactor. The evolution of surface species during reactant step-response experiments was studied in situ by diffuse reflection Fourier transform infrared spectroscopy. The results show that activation for oxidation generally proceeds more easily with increasing bond order of the hydrocarbon. For example, C2H2 shows the highest conversion at low temperatures. Furthermore, the use of hydrocarbons with high bond order, that is, C2H2, as reductant for lean NOx reduction results not only in the highest peak activity but also in considerable high activity in a wide temperature range mainly thanks to high activity at low temperature. With increasing silver loading, the oxidation reactions are favored such that both the hydrocarbon and the NO activation occur at lower temperatures. Several types of adsorbates, for example, carbonate, acetate, formate, enolic and isocyanate/cyanide surface species, are present on the catalyst during reaction. Generally the nature of the surface species and likely also the surface processes are more influenced by the carbon–carbon bond order than the silver loading. This needs to be considered when designing catalysts for emission control systems. Especially for applications using homogeneous fuels with short hydrocarbons, this may provide opportunities to tailor the catalyst functionality for the needs at hand.
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