Finding an optimum surface chemistry for [Formula] systems as NOx storage materials
Author(s)
Advisor
Özensoy, EmrahDate
2010Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Titania promoted NOx storage materials in the form of BaO/TiO2/Al2O3 were
synthesized via two different sol-gel preparation techniques, with varying surface
compositions and morphologies [1, 2]. The influence of the TiO2 units on the NOx
storage component (8 - 20 wt. % BaO), the nature of the crystallographic phases,
thermal stabilities and the dispersion of the surface oxide/nitrate domains were
investigated.
The structural characterization of the synthesized NOx storage materials were
analyzed by means of BET surface area analysis, X-ray diffraction (XRD), ex-situ
Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X- ray
(EDX) and transmission electron microscopy (TEM). Comparative analysis of the
results showed that the TiO2/Al2O3 support material derived by the co-precipitation
of the corresponding hydroxides via the sol-gel technique, exhibited distinctively
more homogenous distribution of TiO2 domains.
The functionality/performance of these materials upon NOx and SOx
adsorptions were monitored by temperature programmed desorption (TPD) and insitu
Fourier transform infrared (FTIR) spectroscopy. An improved Ba surface
dispersion was observed for the BaO/TiO2/Al2O3 materials synthesized via the coprecipitation
of alkoxide precursors which was found to originate mostly from the increased fraction of accessible TiO2/TiOx sites on the surface. These TiO2/TiOx sites
functioned as strong anchoring sites for surface BaO domains and were tailored to
enhance surface dispersion of BaO. The relative stability of the NOx species
adsorbed on the BaO/TiO2/Al2O3 system was found to increase in the following
order: NO+
/N2O3 on alumina << nitrates on alumina < surface nitrates on BaO <
bridged/bidentate nitrates on large/isolated TiO2 clusters < bulk nitrates on BaO on
alumina surface and bridged/bidentate nitrates on TiO2 crystallites homogenously
distributed on the surface < bulk nitrates on the BaO sites located on the TiO2
domains.
The detailed study of the interaction of SOx with BaO/TiO2/Al2O3 ternary
oxide materials showed that titania (TiO2) was a promising candidate for improving
the sulfur tolerance on these type of surfaces. Adsorption of SOx on both pure Al2O3
and TiO2 showed that Al2O3 formed strongly bound SOx species, that were thermally
stable up to T > 1073 K. SOx adsorption directly altered stability of the nitrate
species on the Ti/Al (Protocol 1, Protocol 2) samples. SOx uptake properties of the
BaO/TiO2/Al2O3 materials were found to be strongly influenced by the morphology
of the TiO2/TiOx domains and the BaO loadings (8/20 wt% BaO). Consequently, the
presence of titania domains was seen to decrease the SOx desorption temperatures
and enhance the sulfur-tolerance of these materials by destabilizing the accumulated
sulfate species. SOx exposure on the synthesized materials led to a significant
decrease in the NOx adsorption capacities. The results obtained from FT-IR spectra
showed that the sulfur deposition on the NOx storage materials promoted by Ti
Keywords
NOx storage materialsNSR
LEANOX
BaO
Al2O3
TiO2
Anatase
Rutile
BaTiO3
Sulfur poisoning
DeNOx
NO2
SOx
FTIR
TPD