BaO/Al2O3/NiAl(110) Model NOx Storage Materials. The effect of BaO film thickness on the amorphous-to-crystalline Ba(NO3)2 phase transition
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
The reaction of NO2 with BaO (0.15 – 2 ML and > 30 ML)/Al2O3(12 ML)/NiAl(110) model NOx storage materials was studied. A thick (~12 ML), ordered Al2O3 film was prepared as the support oxide on a NiAl(110) substrate in order to minimize the effect of the intermixing between the two oxide phases (BaO and Al2O3) on the NOx chemistry of BaO. The growth of a thick alumina film, prepared by atomic oxygen deposition onto NiAl(110), follows a layer-by-layer growth mode and the resulting film is much more stable when exposed to NO2 than the ultra-thin alumina films studied before. The interaction of NO2 with the model NOx storage systems at low coverages of BaO show fundamentally different behaviors from a thick BaO film, as nitrite species form at low exposures of NO2, followed by nitrate formation at high NO2 exposures. In contrast, on the thick BaO layer nitrite-nitrate ion pairs form at 300 K under UHV conditions (PNO2 ~ 1 x 10-9 Torr). However, at elevated NO2 pressures (≥ 1 x 10-5 Torr) the thick BaO film is gradually converted into amorphous Ba(NO3)2 at 300 K. Raising the temperature of the samples with ΘBaO > 1 ML after NO2 exposure (in the absence of gas phase NO2) leads to the phase transformation of the amorphous Ba(NO3)2 layer into crystalline Ba(NO3)2 particles in the temperature range of 500 – 600 K. No phase transformation is observed in samples with ΘBaO < 1 ML.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 949085
- Report Number(s):
- PNNL-SA-62867; 19198; KC0302010; TRN: US200907%%312
- Journal Information:
- Journal of Physical Chemistry. C, Vol. 113, Issue 2; ISSN 1932-7447
- Publisher:
- American Chemical Society
- Country of Publication:
- United States
- Language:
- English
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