Characterization of NOx Storage Materials by Temperature-Programmed Desorption and Diffuse Reflectance Infrared Spectroscopy
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Date
2004-04-14
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Abstract
The goal of this research was to elucidate the chemical nature of NO2 adsorption sites on basic metal oxides (MgO, g-Al2O3, MgO/Al2O3, BaO/Al2O3, and a hydrotalcite-derived Mg-Al oxide) and in Na- and Ba-exchanged faujasite (NaY and BaY). The mixed oxides were characterized by powder x-ray diffraction (XRD) and surface area analysis using the single-point BET method. Temperature-programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to characterize the surface species produced by NO2 and CO2 adsorption. Our results evidence that NO2 chemisorbs on basic metal oxides in two forms: (1) NO2 bound as nitrite species that decompose at moderate temperatures (180-300°C) evolving NO and leaving oxygen on the surface and (2) surface nitrate species that decompose at high temperatures (400-500°C) with concomitant evolution of NO and O2. Basicity, as measured by CO2 TPD, is not the principal characteristic of surface sites for NO2 adsorption; however, competitive adsorption of CO2 and NO2 reveals that there is 20-50% overlap of the two site populations. Site competition between CO2 and NO2 is most significant for the more strongly basic sites on BaO/Al2O3.
Exposure of NaY and BaY to a simulated diesel exhaust gas (containing 20% CO2, 12% O2, 1000 ppm NO2 in He) at 30°C demonstrated that these materials have closely similar NOx adsorption capacities; however, NO2 binds much more strongly to BaY than NaY, as evidenced by TPD. For NaY, ~80% of the adsorbed NO2 desorbed as NO and NO2 at less than 100°C and the remainder desorbed as NO and O2 at 200°C. For BaY, ~60% of the adsorbed NO2 desorbed as NO and NO2 at 160°C, while the remainder of the NOx desorbed as NO + O2 at 370°C. Simulated NOx storage-reduction experiments revealed that Pt-impregnated BaY could function effectively at diesel exhaust temperatures albeit in the absence of sulfur-containing gases.
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Keywords
NOx Storage Reduction, Basic Oxides, DRIFTS, TPD, CO2 Adsorption, NO2 Adsorption, Zeolites
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Degree
MS
Discipline
Materials Science and Engineering
