The present thesis work focuses on the study of N2O formation and N2O decomposition/reduction over several catalysts. The investigation of N2O formation has been carried out with an FT-IR operando system, which allowed to monitor both gaseous species and species adsorbed on the catalytic surface. The activities of three Lean-NOX Trap (LNT) model catalysts Pt-Ba/Al2O3, Rh-Ba/Al2O3 and PtRh-Ba/Al2O3 have been tested with propylene as reducing agent performing temperature programmed reduction experiments (TPR) and alternate adsorption and reduction phases under isothermal conditions (Lean/Rich cycles). During the simulations of the lean/rich phases, the Rh containing catalyst proved to be the less efficient in both NOX storage and reduction of the adsorbed species and it was poorly reactive towards N2O formation. Conversely, on the Pt-based catalyst the greatest amount of N2O was detected and the catalyst showed the best storage and reduction activities. The bimetallic catalyst reactivity lay in between the two monometallic samples. Interestingly, isocyanates adspecies linked to hydrocarbons presence have been observed on all the catalysts surface confirming their possible involvement in the mechanism of N2O formation, as widely reported in literature. Concerning the activity on the N2O decomposition/reduction, TPR experiments have been performed over both LNT commercial and model catalysts and SCR (Selective Catalytic Reduction) commercial catalyst with four different reductants (H2, NH3, CO, C3H6), evaluating the effect of CO2 and H2O, too. SCR catalyst showed the worst reactivity, resulting in the lowest N2O conversion with respect to the other catalysts under all the operating conditions. Concerning the LNT catalysts, they proved to be significantly active towards N2O decomposition/reduction causing the complete N2O consumption with all the reducing agents under clean and non-clean conditions (i.e. presence of CO2 and H2O). Comparison among the LNT catalysts (commercial and models) pointed out a link between the presence of Rh in the catalyst formulation and a greater activity towards N2O reduction.
Il presente lavoro di tesi si propone di studiare i meccanismi alla base della formazione e della decomposizione e/o riduzione di N2O su diversi sistemi catalitici. Lo studio del meccanismo di formazione di N2O è stato condotto impiegando un impianto FT-IR operando, che ha permesso l’analisi sia delle specie gassose sia di quelle adsorbite sulla superficie catalitica. L’attività di tre catalizzatori modello LNT (Lean-NOX Trap) Pt-Ba/Al2O3, Rh-Ba/Al2O3 e PtRh-Ba/Al2O3 è stata testata impiegando propilene come riducente attraverso prove in programmata di temperatura (TPR) e fasi di adsorbimento e riduzione alternate (cicli Lean/Rich) a temperatura costante. Durante la simulazione delle fasi di accumulo e riduzione, il catalizzatore a base di Rh si è dimostrato il meno efficiente in termini di capacità di accumulo e in termini di riduzione delle specie adsorbite e il meno reattivo nella produzione di N2O. Il catalizzatore contenente Pt viceversa ha promosso la formazione della maggior quantità osservata di N2O, mostrandosi efficiente in termini sia di accumulo sia di riduzione. Il catalizzatore bimetallico ha mostrato invece una reattività intermedia. Sulla superficie di tutti i campioni testati è stata rilevata la presenza di specie isocianato derivanti dall’utilizzo di un riducente idrocarburico e un loro coinvolgimento nel meccanismo di formazione di N2O è ritenuto plausibile. In relazione invece agli studi di decomposizione e/o riduzione di N2O, il tema è stato approfondito attraverso prove in programmata di temperatura (TPR) sia su catalizzatori LNT (i modelli sopra citati e un commerciale) sia su catalizzatore SCR (commerciale) impiegando quattro diversi agenti riducenti (H2, NH3, CO, C3H6) e valutando l’effetto della presenza di CO2 e H2O. Il catalizzatore SCR ha esibito scarsa reattività riportando le più basse conversioni di N2O rispetto agli altri catalizzatori in tutte le diverse condizioni operative adottate. Al contrario i catalizzatori LNT si sono dimostrati altamente attivi nella decomposizione e riduzione di N2O, causandone un completo consumo indipendentemente dal tipo di riducente impiegato e dall’ambiente di reazione. Il confronto tra il catalizzatore commerciale e i modelli ha permesso di imputare alla presenza di Rh un’efficiente attività di riduzione di N2O.
Study of N2O formation and decomposition-reduction over commercial and model DeNOx catalysts
DRAGOTTO, SUSANNA;CHIONCHIO, FRANCESCA
2016/2017
Abstract
The present thesis work focuses on the study of N2O formation and N2O decomposition/reduction over several catalysts. The investigation of N2O formation has been carried out with an FT-IR operando system, which allowed to monitor both gaseous species and species adsorbed on the catalytic surface. The activities of three Lean-NOX Trap (LNT) model catalysts Pt-Ba/Al2O3, Rh-Ba/Al2O3 and PtRh-Ba/Al2O3 have been tested with propylene as reducing agent performing temperature programmed reduction experiments (TPR) and alternate adsorption and reduction phases under isothermal conditions (Lean/Rich cycles). During the simulations of the lean/rich phases, the Rh containing catalyst proved to be the less efficient in both NOX storage and reduction of the adsorbed species and it was poorly reactive towards N2O formation. Conversely, on the Pt-based catalyst the greatest amount of N2O was detected and the catalyst showed the best storage and reduction activities. The bimetallic catalyst reactivity lay in between the two monometallic samples. Interestingly, isocyanates adspecies linked to hydrocarbons presence have been observed on all the catalysts surface confirming their possible involvement in the mechanism of N2O formation, as widely reported in literature. Concerning the activity on the N2O decomposition/reduction, TPR experiments have been performed over both LNT commercial and model catalysts and SCR (Selective Catalytic Reduction) commercial catalyst with four different reductants (H2, NH3, CO, C3H6), evaluating the effect of CO2 and H2O, too. SCR catalyst showed the worst reactivity, resulting in the lowest N2O conversion with respect to the other catalysts under all the operating conditions. Concerning the LNT catalysts, they proved to be significantly active towards N2O decomposition/reduction causing the complete N2O consumption with all the reducing agents under clean and non-clean conditions (i.e. presence of CO2 and H2O). Comparison among the LNT catalysts (commercial and models) pointed out a link between the presence of Rh in the catalyst formulation and a greater activity towards N2O reduction.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/137421