Hannes Kannisto

On the performance of Ag/Al2O3 as a HC-SCR catalyst – influence of silver loading, morphology and nature of the reductant

This study focuses on the performance of Ag/Al2O3 catalysts for hydrocarbon selective catalytic reduction (HC-SCR) of NOx under lean conditions, using complex hydrocarbons as reductants. The aim is to elucidate the correlation towards the silver loading and morphology, with respect to the nature of the reductant. Ag/Al2O3 samples with either 2 or 6 wt% silver loading were prepared, using a sol–gel method including freeze-drying. The catalytic performance of the samples was evaluated by flow reactor experiments, with paraffins, olefins and aromatics of different nature as reductants. The physiochemical properties of the samples were characterized by scanning electron microscopy/energy dispersive X-ray spectroscopy, scanning transmission electron microscopy/high angle annular dark field imaging, X-ray photoelectron spectroscopy and N2-physisorption. The 2 wt% Ag/Al2O3 sample was found to be the most active catalyst in terms of NOx reduction. However, the results from the activity studies revealed that the decisive factor for high activity at low temperatures is not only connected to the silver loading per se. There is also a strong correlation between the silver loading and morphology (i.e. the ratio between low- and high-coordinated silver atoms) and the nature of the hydrocarbon, on the activity for NOx reduction. Calculated reaction rates over the low-coordinated step and high-coordinated terrace sites showed that the morphology of silver has a significant role in the HC-SCR reaction. For applications which include complex hydrocarbons as reductants (e.g. diesel), these issues need to be considered when designing highly active catalysts.

Synthesis and functionalization of SSZ-13 as an NH3-SCR catalyst

Hydrothermal synthesis of Na-SSZ-13 using a low concentration of the structure-directing agent N,N,N-trimethyl-1-adamantanimonium hydroxide and functionalization of the SSZ-13 framework through the introduction of Cu and Fe via ion exchange have been carried out. The prepared samples were characterized by XRD, SEM, and UV-vis spectroscopy, N2 sorption, XRF and NH3-TPD. It was found that relatively large iron oxide particles and well-dispersed Cu2+ species are formed as a result of ion exchange with iron and copper, respectively. Furthermore, the catalytic activity for NH3 oxidation, NO oxidation and selective catalytic reduction of NOx with ammonia (NH3-SCR) was investigated both in the absence and in the presence of water. The functionalization of the SSZ-13 structure enhances the catalytic activity for NH3-SCR significantly. While the NH3-SCR activity is negligible for the Na-SSZ-13 sample, the Cu-SSZ-13 sample is highly active especially at low temperatures, and the Fe-SSZ-13 sample shows the highest activity at elevated temperatures.