Flame-Made WO3/CeOx-TiO2 Catalysts for Selective Catalytic Reduction of NOx by NH3
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Abstract
Materials based on a combination of cerium–tungsten−titanium are potentially durable catalysts for selective catalytic NOx reduction using NH3 (NH3–SCR). Flame-spray synthesis is used here to produce WO3/CeOx-TiO2 nanoparticles, which are characterized with respect to their phase composition, morphology, and acidic properties and are evaluated by NH3–SCR. HR-TEM and XRD revealed that flame-made WO3/CeOx-TiO2 consists of mainly rutile TiO2, brannerite CeTi2O6, cubic CeO2, and a minor fraction of anatase TiO2. These phases coexist with a large portion of amorphous mixed Ce–Ti phase. The lack of crystallinity and the presence of brannerite together with the evident high fraction of Ce3+ are taken as evidence that cerium is also present as a dopant in TiO2 and is well dispersed on the surface of the nanoparticles. Clusters of amorphous WO3 homogeneously cover all particles as observed by STEM. Such morphology and phase composition guarantee short-range Ce–O–Ti and Ce–O–W interactions and thus the high surface concentration of Ce3+. The presence of the WO3 layer and the close Ce–O–W interaction further increased the Ce3+ content compared to binary Ce–Ti materials, as shown by XPS and XANES. The acidity of the materials and the nature of the acid sites were determined by NH3 temperature-programmed desorption (NH3-TPD) and DRIFT spectroscopy, respectively. TiO2 possesses mainly strong Lewis acidity; addition of cerium, especially the presence of surface Ce3+ in close contact with titanium and tungsten, induces Brønsted acid sites that are considerably increased by the amorphous WO3 clusters. As a result of this peculiar element arrangement and phase composition, 10 wt % WO3/10 mol % CeOx-90 mol % TiO2 exhibits the highest NOx reduction efficiency, which matches that of a V2O5–WO3/TiO2 catalyst. Preliminary activity data indicate that the flame-made catalyst demonstrates much higher performance after thermal and hydrothermal aging at 700 °C than the V-based analogue despite the presence of the rutile phase. Ce3+ remains the dominating surface cerium species after both aging treatments, thus confirming its crucial role in NH3–SCR by Ce–W–Ti-based catalysts.
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