Tortorelli, Miriam (2014) Innovative technology for NOx direct decomposition. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Resource language: English
Title: Innovative technology for NOx direct decomposition
Date: 2014
Number of Pages: 127
Institution: Università degli Studi di Napoli Federico II
Department: Ingegneria Chimica, dei Materiali e della Produzione Industriale
Scuola di dottorato: Ingegneria industriale
Dottorato: Ingegneria chimica
Ciclo di dottorato: 26
Coordinatore del Corso di dottorato:
Date: 2014
Number of Pages: 127
Keywords: NOx direct decomposition, Cu-ZSM5, IR
Settori scientifico-disciplinari del MIUR: Area 09 - Ingegneria industriale e dell'informazione > ING-IND/27 - Chimica industriale e tecnologica
Aree tematiche (7° programma Quadro): NANOSCIENZE, NANOTECNOLOGIE, MATERIALE E PRODUZIONE > Integrazione di tecnologie per applicazioni industriali
Date Deposited: 10 Apr 2014 20:08
Last Modified: 26 Jan 2015 11:52

Collection description

Direct decomposition of NO to N2 and O2 would in principle constitute the most attractive solution to remove NOx since it does not require the use of a reducing agent. The main limitation to the application of this reaction is the slow kinetics. Indeed, the Cu-ZSM5, the only catalyst able to activate the NO decomposition providing reaction rates three orders of magnitude higher than the other catalysts at fairly low temperatures, shows lower performance compared to those reported for the traditional SCR process. In addition, the deactivation by water represents another unsolved issue common to the use of Cu-ZSM5 in all De-NOx processes. This PhD thesis focuses on these two aspects proposing an alternative way to carry out the process which overcomes the kinetic limits associated to a continuous flow process and analyzing the effect of water mostly on the adsorption of NO. The catalyst is prepared by incorporating metal cations into zeolite according to ion exchange procedure. Lanthanum has been exchanged in addition to copper to provide better hydrothermal stability to the zeolite. The PhD work can be divided into three main sections: 1) a very fundamental study of the copper sites and their interaction with NO carried out at Bulgarian Academy of Science using FTIR spectroscopy at low temperature; 2) a quantitative and qualitative investigation of the adsorption capacity of the zeolite under condition close to that of flue gases (higher temperature and presence of water) and 3) the development of a novel experimental rig for a non-steady-state process to overcome the kinetic limitation. A detailed investigation of the copper sites and of the nature and evolution of the NO adsorbed species with time has been performed by in-situ FTIR using a IR cell cooled at 100K studying both 14NO adsorption and 14NO+15NO co-adsorption. In addition to the well-known nitrosyl species on Cu2+ and Cu+, two kinds of Cu3+–NO mononitrosyls have been detected produced as a result of disproportionation of Cu2+ ions from associated sites: Cu2+–O–Cu2+ → Cu+–O–Cu3+. In the presence of NO, the Cu3+–NO complexes are reduced to Cu+–NO and Cu2+–NO. At low temperature, the Cu3+–NO mononitrosyls are able to accept a second NO ligand forming thus Cu3+(NO)2 species. The new findings support the hypothesis that Cu+ ions are active sites in NO decomposition and the dinitrosyl species, reaction intermediates. A preliminary study of the adsorption of NO and co-adsorption with water has been carried out in a continuous flow reactor at temperatures close to those typical of flue gases in order to determine the adsorption capacity of the catalyst under real conditions and the effect of water in the absence or presence of O2 in the feed. TPD experiments have been performed after each adsorption test monitoring all the desorbed NOx species. The effect of water on NO adsorption on LaCu-ZSM5 has been also qualitatively studied by in-situ FTIR identifying the copper sites for NO adsorption displaced by water. Water has been found to displace NO from copper sites, strongly reducing its adsorption, except for nitrate-like species which are partially preserved also in the presence of water. O2 addition limits the negative effect of water because it increases the nitrates formation. Lanthanum co-exchange results in improving the NO adsorption capacity of the zeolite both in the absence and in the presence of water. The relative thermal stability of NO and H2O adsorbed species have been also determined. At about 300°C water is removed from zeolite, while NO is still adsorbed on copper as bridged nitrates. The novel process proposed in this thesis is based on both good adsorption and catalytic properties of the Cu-exchanged ZSM5. The decomposition reaction is carried out under unsteady state (batch) conditions after the zeolite has been saturated with NO. Indeed, the limitations to high NO conversion, related to a NO partial pressure determined by combustion gas emissions, can be theoretically overcome carrying out the reaction in a closed system desorbing large amount of NO in a suitably small volume. To this end a proper experimental rig for testing the feasibility of a cyclic process has been designed and developed. The test procedure involves i) adsorption of NO (in a typical concentration of flue gases from stationary or mobile sources) on LaCu-ZSM5 at 50°C up to saturation of the zeolite, ii) catalysts heating under batch conditions up to the decomposition temperature (480°C), iii) catalyst cooling down to NO adsorption temperature again. It has been verified that in the step i) NO can be effectively adsorbed on LaCu-ZSM5 up to the zeolite saturation and the adsorption proceeds through a complete removal of NOx from the gas phase. In the step ii) NO is converted into N2 and O2which are the only products exiting the reactor regenerating at the same time the zeolite which is ready for a new cycle after cooling down the system back to the adsorption temperature. The adsorption/decomposition cycles can be repeated without any catalyst deactivation even in the presence of O2. It must be pointed out that a very high NO conversion, much higher than those obtained under flowing (steady state) reaction conditions, is reached in all cycles and that no NO emissions have been detected in all steps of the process.


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