Effects of Ba loading and calcination temperature on BaAl2O4 formation for BaO/Al2O3 NOx storage and reduction catalysts

The effect of thermal treatment on the structure and chemical properties of Ba-oxide-based NOx storage/reduction catalysts with different Ba loadings was investigated using BET, TEM, EDS, TPD and FT-IR techniques. On the basis of the present and previously reported results, we propose that moderate (<∼873 K) temperature calcinations result in a single monolayer (ML) ‘coating’ of BaO on the alumina surface. At high Ba loading in excess of that required for a full monolayer ‘coating’ (>8 wt.% BaO), small (∼5 nm) particles of ‘bulk’ BaO are present on top of the one ML BaO/Al2O3 surface. We did not observe any detectable morphological changes upon higher temperature thermal treatment of 2 and 8 wt.% BaO/Al2O3 samples, while dramatic changes occurred for the 20 wt.% sample. In this latter case, the transformations included BaAl2O4 formation at the expense of the bulk BaO phase. In particular, we conclude that the surface (ML) BaO phase is quite stable against thermal treatment, while the bulk phase provides the source of Ba for BaAl2O4 formation.