Diesel oxidation NOx adsorption catalysts
Vieitez Calo, Sara
Worldwide usage of diesel engines has been rising rapidly and, accordingly, more stringent emission standards are introduced to lower the concentrations of diesel exhaust pollutants. \ud NOx abatement is crucial, especially in diesel engines where controlling NOx is extremely difficult as oxygen is always in excess. The title and objective of this thesis arises from a novel approach, the diesel oxidation NOx adsorption catalyst (DONAC), where NOx is to be stored by a diesel oxidation catalyst (DOC).\ud Supported platinum catalysts are the most used technology for diesel oxidation catalysts. Hence, the investigation of the oxidation properties of supported platinum catalysts and particularly, how these properties are affected by the support material, is of great importance. This work addresses not only the effect of the support, platinum loading, preparation method, calcination conditions and platinum precursor, but also the effect of the titania phase composition (anatase or anatase-rutile). A series of supported platinum catalysts including Pt/TiO2, Pt/SiO2, Pt/Al2O3, Pt/TiO2-SiO2 and Pt/SiO2-Al2O3 were prepared by non-aqueous impregnation and chemical vapour impregnation. In order to determine the effect of the support on the structural and electronic properties of the supported platinum catalysts detailed characterisation including XRD, BET, CO chemisorption, NH3-TPD, H2-TPR, TGA, DRIFTS, Raman, XPS, SEM, TEM and EDX was performed. Two relevant oxidation reactions, total oxidation of propane and oxidation of nitric oxide to nitrogen dioxide, were studied.\ud It is found that the activity of supported platinum catalysts is greatly affected by the support material, in general, their performance is enhanced by addition of SiO2 to the TiO2 support. The activity of the platinum catalysts depends strongly on oxidation state, with platinum in metallic state being most active. However, it is not only the platinum oxidation state but a combination of several factors including metal dispersion, surface area, morphology and phase composition of the support that explains the variations in catalytic activity. Morphology and phase composition play an active role in the redox properties of the support and its interaction with supported metal particles. The high propane oxidation activity of Pt supported on anatase TiO2 is attributed to highly reactive oxygen species within the support.\ud DeNOx technologies based on the concept of NOx storage have proven to be effective, however further studies are needed to succeed in finding an optimum NOx storage system. A series of metal (Cu, Fe and Pt) -exchanged zeolites with different compositions (SiO2/Al2O3 = 5.1 - 50) and frameworks (Y, ZSM-5 and BETA) were prepared by two different exchange methods (WIE and CVI). The DOE method was applied to investigate the way in which preparation experimental variables affect the NOx storage capacity of CuII/ZSM-5 (30). Evaluation of NOx storage capacity, in the presence or absence of O2, of metal-exchanged zeolites was carried out. In order to correlate the physicochemical properties of the metal-exchanged zeolites with their NOx storage performance, characterisation including XRD, BET, NH3-TPD, H2-TPR, DRIFTS, UV-Vis DRS, XPS, TEM and EDX was performed.\ud It is found that the NOx adsorption/desorption capacity and the stability of those adsorption species formed, is greatly affected by the nature of the exchanged metal and the structure and acidity of the zeolite support. Due to the intrinsic adsorption capacity of the parent zeolite, H+-ZSM-5(23), CuII/ZSM-5(23) catalysts prepared by WIE and CVI exhibit high NOx storage capacities. The initial redox state, Cu2+/Cu+ and Fe2+/Fe3+ ratio, and bonding strength of the metal ions to the framework affect the reducibility of metal species and consequently the storage capacity.\ud The preparation method is found to be crucial in controlling metal loading, nature and distribution of active species and crystallites size. The statistical approach allowed for a significant enhancement in the storage capacity of CuII/ZSM-5(30) and, in particular, the amount of NOx desorbed at high temperature. Temperature is found to have the greatest effect on the NOx storage capacity. In addition, its influence is dependent on the copper precursor concentration.\ud Among the zeolite based catalysts studied, potential candidates for application under real working conditions, those that exhibit greater NOx desorption at high temperatures (T>200oC) were found.
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