In recent years, a growing awareness on the environmental impact caused by mankind has led to a growing development of pollution abatement systems. The Selective Catalytic Reduction process (SCR) has been widely implemented and is currently the leading technology for NOx abatement in lean-burn Diesel engines. Nevertheless, these systems are not anymore able to guarantee emissions in line with the more and more restrictive legislations. In particular, the engine and the combustion gases require between 3 and 5 minutes to achieve the temperatures at which the SCR catalysts become active. Thus, during this phase, also known as cold-start, the abatement systems are ineffective, causing huge emissions. Specifically, in these conditions: • The Diesel Oxidation Catalyst (DOC), upstream of the SCR system, is not operative, reducing so the NO2 availability for the very effective Fast SCR reaction. • The urea, used as forerunner of the ammonia which acts as reducing agent in the SCR reactions, decomposes at temperature close to 180 °C making the NOx reduction to diatomic nitrogen impossible. • The chemistry and reaction mechanism of the Standard SCR reaction at low temperature are still not fully elucidated. Other alternative NOx reduction technologies, like the NOx storage and reduction (NSR) catalysts, are not active as well at these temperatures. As a consequence, several studies are in progress in order to find a solution to this problem. One technology in particular has attracted the automotive companies’ interest, i.e. the Passive NOx Adsorber (PNA). This system is not able to reduce the NOx to N2, but nevertheless it is able to store at low temperature the nitrogen oxides and release them at higher temperatures at which the downstream SCR is active. For this reason, in 2001 the Ford Motor Company had proposed for the first time the combination of these two systems in order to manage the cold-start emissions. The last and even more restrictive legislations (2015 European Union with the RDE test approval which went into effect since 2017 for the emission reduction even at low temperature) led to the improvement of this technology. The most efficient formulation was discovered and patented in 2015 by the Johnson Matthey company, which is based on palladium zeolites (Pd-zeolites). The aim of the present thesis, in collaboration with this company, has been the characterization and the study of the effects of the real operative conditions on the performance of PNA systems based on the palladium chabazite (Pd-CHA), a specific zeolite with pores of small dimension. In particular, in the first chapter a detailed introduction about the environmental issues for NOx and the technology developed for their abatement up to the cold-start topic and the PNA explanation and possible application is described. In chapter 2 the sample characteristics and the used instrumentation for the experimental thesis work are described. Chapter 3 provides a preliminary characterization of the Pd-CHA systems based on experimental runs, under both dynamic and steady-state conditions, also in comparison with previous studies in the literature. In chapter 4 the performance of this catalytic formulation for the NOx storage (NO and NO2) is studied systematically in presence of different chemical species (oxygen, water and carbon monoxide) and at different adsorption temperatures, in order to address the real operative conditions.
Nei ultimi anni, un crescente interesse nell’impatto ambientale causato dell’uomo ha permesso un grande sviluppo dei sistemi di abbattimento dei inquinanti. Il processo Selective Catalytic Reduction (SCR) è stato ampiamente sviluppato ed è attualmente la principale tecnologia per l’abbattimento degli NOx nei motori Diesel a combustione magra. Ciò nonostante, questi sistemi non sono più sufficienti per garantire emissioni in regola con le sempre più restrittive legislazioni. In particolare, il motore e i gas di scarico richiedono tra i 3 e i 5 minuti per raggiungere le temperature alle quali i catalizzatori SCR diventano attivi. Per questo motivo, durante questa fase, nota come cold-start, i sistemi di abbattimento sono inefficaci, causando significative emissioni. Nello specifico, in queste condizioni: • Il sistema Diesel Oxidation Catalyst (DOC), che precede il sistema SCR, non è operativo riducendo la disponibilità di NO2 per la reazione più efficace di Fast SCR. • L’urea, impiegata come precursore dell’ammoniaca la quale svolge la funzione di agente riducente nelle reazioni SCR, decompone a temperature intorno ai 180°C rendendo impossibile la riduzione ad azoto biatomico dei NOx. • La chimica e il meccanismo della reazione Standard SCR a basse temperature non sono ancora del tutto chiari. Tecnologie alternative per la riduzione degli NOx, come il sistema NOx storage and reduction (NSR), non sono a loro volta attivi a queste temperature. Di conseguenza, diversi studi sono in corso al fine di trovare una soluzione a questo problema. Una tecnologia in particolare ha attratto l’interesse delle compagnie del settore automobilistico, cioè i Passive NOx Adsorbers (PNA). Questo sistema non è in grado di ridurre gli NOx a N2, ma tuttavia è in grado di adsorbire a basse temperature gli ossidi di azoto e rilasciarli a più alte temperature alle quali l’SCR è attivo. Per questo motivo nel 2001 la Ford Motor Company ha proposto per la prima volta la combinazione di questi due sistemi al fine di controllare le emissioni in cold-start. Le ultime e ancor più restrittive legislazioni (2015 Unione Europea con l’approvazione del RDE test con entrata in vigore dal 2017 per la riduzione delle emissioni anche a basse temperature) hanno portato al miglioramento di questa tecnologia. La formulazione più efficiente è stata scoperta e brevettata nel 2015 dalla compagnia Johnson Matthey, la quale consiste in palladio supportato su zeoliti (Pd-zeolites). Lo scopo della presente tesi, proprio in collaborazione con questa ditta, è stato la caratterizzazione e lo studio degli effetti generati dalle condizioni operative reali di lavoro sulle performance del sistema PNA basato sulla palladio chabazite (Pd-CHA), un particolare tipo di zeolite a pori di piccole dimensioni. In particolare, nel primo capitolo viene fatta un’introduzione dettagliata sui problemi ambientali legati agli NOx e sulle tecnologie sviluppate per il loro abbattimento arrivando alla tematica del cold-start e alla descrizione dei PNA e la loro possibile applicazione. Nel secondo capitolo sono illustrate le caratteristiche del campione e della strumentazione utilizzata per il lavoro sperimentale della tesi. Nel terzo capitolo viene illustrata una preliminare caratterizzazione del sistema Pd-CHA basata su prove sperimentali, condotte in condizioni dinamiche e a step stazionari, comparata anche con precedenti studi già presenti in letteratura. Nel quarto capitolo vengono studiate sistematicamente le performance di questa formulazione catalitica per lo storage di NOx (NO e NO2) in presenza di diversi reagenti (ossigeno, acqua e monossido di carbonio) e a differenti temperature di adsorbimento tali da simulare le reali condizioni operative.
Study of Pd-zeolite systems as passive NOx adsorbers (PNA) for NOx removal in Diesel exhausts
FESTA, MATTIA;MICALE, GUIDO
2017/2018
Abstract
In recent years, a growing awareness on the environmental impact caused by mankind has led to a growing development of pollution abatement systems. The Selective Catalytic Reduction process (SCR) has been widely implemented and is currently the leading technology for NOx abatement in lean-burn Diesel engines. Nevertheless, these systems are not anymore able to guarantee emissions in line with the more and more restrictive legislations. In particular, the engine and the combustion gases require between 3 and 5 minutes to achieve the temperatures at which the SCR catalysts become active. Thus, during this phase, also known as cold-start, the abatement systems are ineffective, causing huge emissions. Specifically, in these conditions: • The Diesel Oxidation Catalyst (DOC), upstream of the SCR system, is not operative, reducing so the NO2 availability for the very effective Fast SCR reaction. • The urea, used as forerunner of the ammonia which acts as reducing agent in the SCR reactions, decomposes at temperature close to 180 °C making the NOx reduction to diatomic nitrogen impossible. • The chemistry and reaction mechanism of the Standard SCR reaction at low temperature are still not fully elucidated. Other alternative NOx reduction technologies, like the NOx storage and reduction (NSR) catalysts, are not active as well at these temperatures. As a consequence, several studies are in progress in order to find a solution to this problem. One technology in particular has attracted the automotive companies’ interest, i.e. the Passive NOx Adsorber (PNA). This system is not able to reduce the NOx to N2, but nevertheless it is able to store at low temperature the nitrogen oxides and release them at higher temperatures at which the downstream SCR is active. For this reason, in 2001 the Ford Motor Company had proposed for the first time the combination of these two systems in order to manage the cold-start emissions. The last and even more restrictive legislations (2015 European Union with the RDE test approval which went into effect since 2017 for the emission reduction even at low temperature) led to the improvement of this technology. The most efficient formulation was discovered and patented in 2015 by the Johnson Matthey company, which is based on palladium zeolites (Pd-zeolites). The aim of the present thesis, in collaboration with this company, has been the characterization and the study of the effects of the real operative conditions on the performance of PNA systems based on the palladium chabazite (Pd-CHA), a specific zeolite with pores of small dimension. In particular, in the first chapter a detailed introduction about the environmental issues for NOx and the technology developed for their abatement up to the cold-start topic and the PNA explanation and possible application is described. In chapter 2 the sample characteristics and the used instrumentation for the experimental thesis work are described. Chapter 3 provides a preliminary characterization of the Pd-CHA systems based on experimental runs, under both dynamic and steady-state conditions, also in comparison with previous studies in the literature. In chapter 4 the performance of this catalytic formulation for the NOx storage (NO and NO2) is studied systematically in presence of different chemical species (oxygen, water and carbon monoxide) and at different adsorption temperatures, in order to address the real operative conditions.| File | Dimensione | Formato | |
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Descrizione: Testo della tesi magistrale in Chemical Engineering di Festa Mattia e Micale Guido
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https://hdl.handle.net/10589/142728