Fundamental deactivation mechanisms of Fe-BEA as NH3-SCR catalyst - Experimental studies and kinetic modeling
Sammanfattning: Due to globalization and an increasing transport sector the interest for more fuel efficient combustion engines operating under lean conditions has increased. The products formed during the burning process in internal combustion engines are major contributants to global air pollution, where carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOX) are the major toxic components regulated in many countries. One effective way to reduce nitrogen oxides in lean environments is selective catalytic reduction with ammonia (NH3-SCR). Metal-exchanged zeolites have in this connection proven to be very active and promising catalysts for NOX reduction. Several challenges arise when using these materials in exhaust aftertreatment systems for vehicles. One problem is thermal deactivation due to the high-temperature conditions in connection with the regeneration of the particulate filter which in addition to the SCR catalyst is an important component in the aftertreatment system. In this licentiate thesis, the thermal stability of iron-based zeolite beta, Fe-BEA, as NH3-SCR catalyst is evaluated with several different experimental techniques. Based on the experimental results a kinetic model is developed to describe the kinetics and the fundamental deactivation mechanisms for Fe-BEA after hydrothermal treatment with focus on the dynamics of the active iron sites. Cordierite supported Fe-BEA samples were hydrothermally treated at 600 and 7000C for 3-100 h to capture the effect of time and temperature on the ageing process. The samples were characterized with BET, XPS, XRD and NH3-TPD. The catalytic performance of the samples with respect to NO and NH3 oxidation, and NOX reduction (NH3-SCR) was studied by flow reactor experiments. The catalytic performance was correlated with structural changes of the zeolite and the iron phases. The results showed that the NOX reduction at low temperatures is more sensitive to changes in the oxidation state of iron caused by the hydrothermal ageing than at higher temperatures. Furthermore, a maximum in activity for NO oxidation and an increased oxidation state of iron indicate Fe2O3 particle growth. This was further investigated with DRIFT spectroscopy which showed that the change of the nature of the iron species in Fe-BEA proceeds in two steps; (i) milder ageing results in a decreased amount of isolated iron species due to migration, and (ii) more sever aging results in a continuous migration and formation of larger iron oxide particles. High surface coverage of ammonia inhibits the SCR reaction. However, the inhibition effect is not significantly affected by the hydrothermal treatment. Furthermore, the possibility to regenerate the catalyst by treatment with hydrogen was investigated. The H2-treatment showed a reversed trend compared to the hydrothermally treated samples. Increased NOX reduction at low temperatures was observed indicating that the H2-treatment results in the formation of isolated iron species in the zeolite. A kinetic ageing model was developed based on the experimental results for H-BEA and Fe-BEA. The ageing model describes the experiments well for both H-BEA and Fe-BEA, before and after hydrothermal treatment, by decreasing the density of active sites. Furthermore, the model showed that the spillover rate of ammonia, inhibiting the NOX reduction is independent of the site density and depends only on the fraction of free sites, indicating that a constant number of Brønsted sites buffer each active iron site is constant and unaffected by the hydrothermal treatment.
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