Nepheline group materials obtained by a thermal treatment of K2CO3-supported nanosized sodalite are identified as cost-effective materials which show promising activity and durability toward diesel soot combustion.
Aggregates (80 nm) of sodalite nanocrystals with crystallite sizes ranging from 20 to 40 nm have been synthesized from a sodium aluminosilicate solution at low temperature, without adding any organic additives, while paying attention to the key factors for the synthesis of nanosized zeolite crystals. The physical properties of nanosized sodalite crystals were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, 29Si solid-state magic-angle spinning (MAS) NMR, and N2 adsorption. As expected, the external surface area of nanosized sodalite crystals is significantly increased compared with that of microsized sodalite crystals. The size of synthesized sodalite crystals can be controlled from 20 nm to 10 microm. It is found that the preparation of a homogeneous aluminosilicate solution followed by the formation of an aluminosilicate hard gel by adjusting the initial composition, for example, SiO2/Al2O3 and Na2O/H2O ratios, is critical for synthesis.
The possible mechanism of soot combustion catalyzed by potassium carbonate loaded on aluminosilicate was elucidated to understand the surface reaction in solid–solid–gas triphasic catalysis. Potassium species on aluminosilicate showed high catalytic performance for the oxidation of carbon black by gaseous oxygen. Aluminosilicate helped in stabilizing the alkali cation on the surface. The carbonate ion played a critical role in enhancing the catalytic performance by acting as a supplier of active electrons to gaseous oxygen. To the best of our knowledge, this is the first report on the essential role of carbonate on the catalytic activity of an alkali cation for an oxidation reaction, although carbonate (carbon dioxide) is widely recognized to hinder the catalytic performance of the alkali compound for base‐catalyzed reactions.
We report the successful preparation of alkali-carbonatesupported aluminosilicate catalysts for diesel soot combustion. From the views of practical feasibility as well as economic and environmental concerns, utilizing alkali metals is preferable for diesel soot combustion. We overcome the drawback that the alkali compounds used as catalysts are easily soluble in water. Na 2 CO 3 was stabilized on sodium-and potassium-containing nepheline ((Na, K)-nepheline) with water tolerance. This result suggests that carbonates are stabilized on the supports with higher energy than hydration energy and van der Waals force by thermal treatment of K 2 CO 3 /Na-nepheline via solid ion exchange. Na 2 CO 3 on the surface of thermally treated K 2 CO 3 /Na-nepheline is likely to be a catalytically active species for carbon black combustion as (Na, K) mixed carbonate. Na 2 CO 3 stabilized on (Na, K)-nepheline is recovered by CO 2 , which is likely to be derived from catalytic action on combustion of carbon black.
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