Changing morphology of BaO/Al2O3 during NO2 uptake and release.

The changes in the morphology of Ba-oxide-based NO(x)() storage/reduction catalysts were investigated using time-resolved X-ray diffraction, transmission electron microscopy, and energy dispersed spectroscopy. Large Ba(NO(3))(2) crystallites form on the alumina support when the catalyst is prepared by the incipient wetness method using an aqueous Ba(NO(3))(2) solution. Heating the sample to 873 K in a He flow results in the decomposition of the Ba(NO(3))(2) phase and the formation of both a monolayer BaO film strongly interacting with the alumina support and nanocrystalline BaO particles. Upon NO(2) exposure of these BaO phases at room temperature, small (nanosized) Ba(NO(3))(2) crystals and a monolayer of surface nitrate form. Heating this sample in NO(2) results in the coalescence of the nanocrystalline Ba(NO(3))(2) particles into large crystals. The average crystal size in the reformed Ba(NO(3))(2) layer is significantly smaller than that measured after the catalyst preparation. Evidence is also presented for the existence of a monolayer Ba(NO(3))(2) phase after thermal treatment in NO(2), in addition to these large crystals. These results clearly demonstrate the dynamic nature of the Ba-containing phases that are active in the NO(x)() storage/reduction process. The proposed morphology cycle may contribute to the understanding of the changes observed in the performances of these catalysts during actual operating conditions.