Zeolite ZSM5 catalysts for abatement of nitrogen oxide

Abstract: Airborne pollutants from the combustion of fossil fuels area global problem. Emission of nitrogen oxides (NOx) is increasing with the worldwide increase in theuse of energy. Atmospheric and photochemical reactions linknitrogen oxides to hydrocarbons and tropospheric ozone. Theemission of NOxhas to be tackled urgently in order to limit theharmful effects of anthropogenic activity on theenvironment.The subject of this thesis is catalytic nitrogen oxideabatement through direct decomposition and reduction by methaneover ion-exchanged zeolite ZSM5. The work covers catalyticconversionand surface intermediates, including correlationswith the level of exchanged Cu2+cations and Ni2+or Pd2+co-cations. Special attention is given to thealuminium content of the support and changes in structuralparameters. It was found that NOxconversion over cation-exchanged ZSM5 is stronglyinfluenced by the ion-exchange procedure and by the abovematerial parameters.Characterization of Cu-ZSM5 reveals that approximately twomolecules of water per Cu2+ion desorb at temperatures between 150 and 350oC, in addition to the conventional dehydration atlower temperatures. The desorbed water comes from thedecomposition of Cu(OH)2. Decomposition of hydroxylated copper ions resultsin the formation of Cu2+-O-Cu2+dimers, which are suggested to be the activesites for catalytic decomposition of NO.Acid sites are important for the dispersion of copper ionson the catalyst surface. Acid sites are also important for theinteraction between copper species and the zeolite. Increasedacidity leads to a stronger interaction between the exchangedcation and the framework, i.e. the exchanged cations becomemore resistant to mobility. The stronger bond between theexchanged cations and lattice oxygen also preventsdealumination of the catalyst and decreases the thermalexpansion at higher temperatures.The temperature of dehydroxylation of acid sites on H-ZSM5overlaps with the light-off temperature for NO reduction overCu-ZSM5. Bridged nitrato groups ligated to Cu2+-O-Cu2+dimers act as site blockers below the light-offtemperature. At the light-off temperature zeolite latticevibrations destabilize surface nitrates and open the sites forcatalytic reactions via short-lived N2O3intermediates.The same lattice movementsdecompose OH-groups on the H-form of the zeolite and it wassuggested that zeolite ZSM5 should be noted mainly for itsflexibility rather than its narrow channels with strongelectrostatic fields or metal exchange sites with opencoordination.Keywords:NO, NO2, Decomposition, Reduction, Methane, ZSM5,Ion-exchange, Cu, Pd, Ni