Fine-Tuning the Dispersion and the Mobility of BaO Domains on NO_x Storage Materials via TiO₂ Anchoring Sites

Abstract

In an attempt to control the surface dispersion and the mobility of BaO domains on NO x storage materials, TiO 2 /TiO x anchoring sites were introduced on/inside the conventional γ-Al 2 O 3 support matrix. BaO/TiO 2 / Al 2 O 3 ternary oxide materials were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies. The synthesized NO x storage materials were studied via XRD, Raman spectroscopy, BET surface area analysis, TPD, XPS, SEM, EDX-mapping, and in situ FTIR spectroscopy of adsorbed NO 2 . NO x uptake properties of the BaO/TiO 2 /Al 2 O 3 materials were found to be strongly influenced by the morphology and the surface structure of the TiO 2 /TiO x domains. An improved Ba surface dispersion was observed for the BaO/TiO 2 /Al 2 O 3 materials synthesized via the coprecipitation of alkoxide precursors, which was found to originate mostly from the increased fraction of accessible TiO 2 /TiO x sites on the surface. These TiO 2 /TiO x sites function as strong anchoring sites for surface BaO domains and can be tailored to enhance surface dispersion of BaO. TPD experiments suggested the presence of at least two different types of NO x species adsorbed on the TiO 2 /TiO x sites, with distinctively different thermal stabilities. The relative stability of the NO x species adsorbed on the BaO/TiO 2 /Al 2 O 3 system was found to increase in the following order: NO + /N 2 O 3 on alumina , nitrates on alumina < surface nitrates on BaO < bridged/bidentate nitrates on large/isolated TiO 2 clusters < bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on TiO 2 crystallites homogenously distributed on the surface < bulk nitrates on the BaO sites located on the TiO 2 domains.

TiO2–Al2O3 binary mixed oxide surfaces for photocatalytic NOx abatement

“…Titanium (IV) isopropoxide (TIP, 97%, Sigma-Aldrich) and aluminum-tri-sec-butoxide (ASB, 97%, Sigma-Aldrich) were used as the main ingredients in the preparation of the TiO 2 /Al 2 O 3 binary oxides via sol-gel method [16,18]. Three series of samples were prepared by varying the relative molar composition of the TiO 2 component in the TiO 2 /Al 2 O 3 binary oxide.…”

“…This alternative strategy was inspired by our recent studies on NSR technology which is used for the thermal catalytic after treatment of automotive NO x emissions [12,[16][17][18][19][20][21]. In couple of these former studies, we spectroscopically demonstrated that [12,[16][17][18][19][20][21] on the TiO 2 /Al 2 O 3 binary oxide surface, oxidized NO x species such as NO 2 (g) can readily undergo a thermal disproportionation reaction forming adsorbed nitrites and nitrates allowing effective solid state NO x storage. However NO(g) has a limited adsorption energy on many metal oxide surfaces compared to that of NO 2 , hindering the storage of NO in the solid (adsorbed) state.…”

Thermal evolution of structure and photocatalytic activity in polymer microsphere templated TiO2 microbowls

“…Calcination at 200 and 300 • C leads to the formation of an amorphous TiO 2 /TiO x structure, which starts to crystallize into a rather disordered anatase phase at 400 • C with a small average particle size, evident from the corresponding broad anatase XRD diffraction signals (ICDD Card No: 21-1272) in Fig. 4a and the characteristically intense anatase Raman scattering observed at 144 cm −1 [24][25][26]. At 500 • C, a well-ordered anatase phase with a larger average particle size is formed as can be seen from the sharp and intense anatase signals in both XRD (Fig.…”

“…4b. Formation of the rutile phase leads to the evolution of typical Raman scattering features at 236, 447, 612, 826 cm −1 [24][25][26]. Rutile phase becomes more crystalline and abundant at higher calcination temperatures.…”

“…This is caused by conversion of NO(g) into NO 2 (g) via photocatalytic oxidation. In addition, generated NO 2 (g) can also adsorb on the photocatalyst surface in the form of chemisorbed NO 2 , nitric/nitrous acid, nitrites and nitrates [24][25][26]33] and stored in the solid state, leading to a further decrease in the NO(g) signal. Furthermore, direct photocatalytic decomposition and photo-reduction of NO(g) forming N 2 (g) and/or N 2 O(g) cannot be ruled out [34].…”