Investigating the Ag–Support Interaction in Silver Catalysts for Soot Combustion: The Importance of Coordination of Al3+ Ions in the Support

This study shows how important the coordination of Al3+ ions in the silver support is for the overall activity in soot combustion. Five silver catalysts with a silver content of 14.7 wt.% were prepared using the following supports: α-Al2O3, which has only octahedrally coordinated Al3+, θ-Al2O3, which has both octahedrally and tetrahedrally coordinated Al3+, and zeolites, which contain only tetrahedrally coordinated Al3+: 10X, 13X, and 5A. The analysis of the diffraction patterns showed that silver on the surface of catalysts made with the first four supports was mainly in the metallic form, except for Ag/5A in which there was a lack of reflections from Ag0 in the XRD pattern. Nevertheless, the difference in the activity of the support and the catalyst as well as the EDX results indicate the presence of silver on the catalyst. The SEM-EDX analysis showed that the silver dispersion strongly depends on the support and that even the zeolites with large silver particles on the surface have silver evenly distributed across the surface. The activity of the catalysts decreased in the following series: Ag/Al 1200 > Ag/5A ≈ Ag/13X > Ag/10X ≈ Ag/Al 550. Time-of-Flight Secondary Ion Mass Spectrometry was used to delve into the reason why the catalyst with the low-surface area α-Al2O3 support yielded a better catalyst than that obtained using the high-surface area alumina support and showed that different ratios of secondary ions were emitted from the two surfaces.