Cheol-Woo W. Yi

Understanding Practical Catalysts Using a Surface Science Approach: The Importance of Strong Interaction between BaO and Al2O3 in NOx Storage Materials

The interaction of NO2 with BaO/Al2O3/NiAl(110) model NOx storage materials was investigated at BaO coverages corresponding to practical systems. The results obtained from these model systems show excellent correlation with those of practical systems. We found a specific, strong interaction between BaO and Al2O3, and this interaction profoundly influences the NOx chemistry. BaO readily forms a Ba−aluminate-like surface phase, and upon the interaction with NO2, Ba ions are pulled out from this surface layer to form Ba(NO3)2 clusters.

Reactivity of a Thick BaO Film Supported on Pt(111): Adsorption and Reaction of NO2, H2O, and CO2

Reactions of NO2, H2O, and CO2 with a thick (>20 monolayer equivalent (MLE)) BaO film supported on Pt(111) were studied with temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). NO2 reacts with a thick BaO layer to form surface nitrite-nitrate ion pairs at 300 K, while only nitrates form at 600 K. In the thermal decomposition process of nitrite-nitrate ion pairs, first nitrites decompose and desorb as NO. Then nitrates decompose in two steps: at lower temperature with the release of NO2 and at higher temperature, nitrates dissociate to NO+O2. The thick BaO layer converts completely to Ba(OH)2 following the adsorption of H2O at 300 K. Dehydration/dehydroxylation of this hydroxide layer can be fully achieved by annealing to 550 K. CO2 also reacts with BaO to form BaCO3 that completely decomposes to regenerate BaO upon annealing to 825 K. However, the thick BaO film cannot be converted completely to Ba(NOx)2 or BaCO3 under the experimental conditions employed in this study.