WO3/CeO2/TiO2 Catalysts for Selective Catalytic Reduction of NOx by NH3: Effect of the Synthesis Method

WO3/CeO2/TiO2, CeO2/TiO2 and WO3/TiO2 catalysts were prepared by wet impregnation. CeO2/TiO2 and WO3/TiO2 showed activity towards the selective catalytic reduction (SCR) of NOx by NH3, which was significantly improved by subsequent impregnation of CeO2/TiO2 with WO3. Catalytic performance, NH3 oxidation and NH3 temperature programmed desorption of wet-impregnated WO3/CeO2/TiO2 were compared to those of a flame-made counterpart. The flame-made catalyst exhibits a peculiar arrangement of W-Ce-Ti-oxides that makes it very active for NH3-SCR. Catalysts prepared by wet impregnation with the aim to mimic the structure of the flame-made catalyst were not able to fully reproduce its activity. The differences in the catalytic performance between the investigated catalysts were related to their structural properties and the different interaction of the catalyst components

Flame-Made WO₃/CeO_<i>x</i>‑TiO₂ Catalysts for Selective Catalytic Reduction of NO_<i>x</i> by NH₃

Materials based on a combination of cerium–tungsten−titanium are potentially durable catalysts for selective catalytic NO_<i>x</i> reduction using NH₃ (NH₃–SCR). Flame-spray synthesis is used here to produce WO₃/CeO_<i>x</i>-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/CeO_<i>x</i>-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 % CeO_<i>x</i>-90 mol % TiO₂ exhibits the highest NO_<i>x</i> 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