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- Chen, Lin, 1990
(författare)
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On the Character of Active Sites in Copper Exchanged Zeolites during NH3-SCR
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nitrogen oxides (NOx) are formed during combustion in diesel engines and one current approach to reduce NOx to N2 is selective catalytic reduction with NH3 as reducing agent (NH3-SCR). To acquire both high activity and selectivity, the active site is often atomically dispersed metal atom, so-called single-site catalysts. In the case of NH3-SCR, one example is Cu-exchanged chabazite (Cu-CHA). This is a kind of ion-exchanged zeolites that over the past decade has emerged as a promising candidate for NH3-SCR thanks to its good performance over a wide temperature window (473-773 K) and a high hydrothermal stability. In this thesis, the mechanisms for solid-state ion exchange of copper into CHA and activation of oxygen during NH3-SCR in Cu-CHA have been studied through density functional theory (DFT) in combination with ab-initio thermodynamics and molecular dynamics. Firstly, the energetic conditions for functionalization of CHA with copper ions from Cu2O(111) via the formation of Cu(NH3)2+ are explored. It is found that the diamine complexes form easily on Cu2O(111) and diffuse with low barriers over the surface and inside the CHA framework. The charge neutrality of the systems is maintained via counterdiffusion of H+ in the form of NH4+ from the zeolite to the Cu2O surface where water can be formed. The efficient solvation of Cu+ and H+ by ammonia renders the ion-exchange process exothermic. The dynamic character of Cu ion sites in CHA in the presence of ammonia is also highlighted. Secondly, the activation of oxygen, one crucial step during NH3-SCR reactions, is investigated. The character of ligand-solvated Cu ions under SCR conditions is explored and Cu(NH3)2+ is found to be the preferable species under low-temperature operation conditions (below 523 K). Direct dissociation of O2 is found to be facile over a pair of Cu(NH3)2+, whereas dissociation on a single Cu(NH3)2+ species is unlikely due to a high activation energy. Theffect of Al distribution on the stability of the pair formation in CHA is also investigated. The Al distribution is found to strongly affect the propensity to form the pairs in CHA. At temperatures above 623 K, copper sites are preferably framework-coordinated without any ligands. In this case, the NH3-SCR catalytic cycle is completed over framework-coordinated Cu sites. The character of active sites during NH3-SCR is consequently condition-dependent, where the low temperature reaction occurs on Cu-solvated species. The NH3-SCR process in Cu-CHA is consequently just at the border between homogeneous and heterogeneous catalysis.
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| 2. |
- Chen, Zhe, 1987-
(författare)
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Surface Integrity of Broached Inconel 718 and Influence of Thermal Exposure
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Inconel 718 is a nickel-based superalloy that is extensively used as a disc material in gas turbine engines. The service life of gas turbine discs is normally governed by the modes of material degradation and fatigue failure since they work mostly at high temperatures and are subjected to cyclic mechanical loadings. Fatigue failures often start with the initiation of cracks at the surface and the precise details of the failure process significantly depend on the surface conditions. In turbine disc production, one of the last manufacturing steps is to broach root fixings, commonly of fir-tree design, for blade mounting. It has always been a challenge when machining Inconel 718 due to its high strength retention at elevated temperatures, rapid work hardening, as well as low thermal conductivity. This usually leads to rapid tool wear, and consequently shorter tool life, and at the end to the deterioration of the surface integrity of the machined components.The aim of this licentiate thesis is to increase our knowledge about the surface integrity, especially microstructure and residual stresses, of broached Inconel 718 and its stability under thermal exposure. This knowledge can later be used for analyzing the initiation and propagation of fatigue cracks in broached Inconel 718, particularly in the case of high temperature fatigue, thereby giving a better understanding of the failure mechanism of gas turbine discs from a fatigue point of view.A broaching operation has been performed using similar cutting conditions as that used in turbo machinery industries for producing fir-tree root fixings. In addition, service damages were analyzed in a retired dis of Inconel 718. Surface defects, severe plastic deformation and generation of high tensile residual stresses have been found to be the main damages to the surface integrity caused by the broaching operation. The machining induced plastic deformation was found to accelerate the microstructural degradation beneath the broached surface when subjected to thermal exposure. The surface tensile residual stresses can be completely removed after short thermal exposure, the tensile layer in the sub-surface region, however, exhibited a high resistance to stress relief at high temperatures. The damage analysis on the retired disc indicated that presence of the highly deformed layer on the machined surface is negative for preventing the occurrence of fretting fatigue in turbine discs.
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