Comparing The Fresh And Hydrothermally Aged SCR Catalysts
5 shows the XPS spectrum of the fresh and aged Cu-BEA-Nb and Cu-BEA-CeNb catalysts. The Nb3d signals were fitted with a doublet peaks in the BE range 206.78–207.87 eV for Nb3d5 and of 210.05–210.86 eV for Nb3d3 (Dzwigaj et al. 2010). These values could be assigned to the isolated mononuclear Nb2O5, Nb5+ in the framework of BEA zeolite. The Nb3d signal intensities of Cu-BEA-Nb and Cu-BEA-CeNb aged catalysts decreased compared to those of the fresh catalysts.
Fig. 6 represents the TEM images of the fresh SCR catalysts. By comparing the EDS electron images of the catalysts, the Cu, Ce, and Nb particles on catalyst surface were distinguished. As shown in Fig. 6(a), copper may disperse as Cu2+ or CuO species on the catalyst surface. In Figs. 6(c) and (d), Ce and Nb were observed, because of metal oxides of CeO2 and Nb2O3 as shown in the XRD results (Fig. 2) and XPS data of Figs. 4 and 5. Comparing Figs. 6(a)–(d), the fresh BEA zeolite, showing column shape with the size of 20-30 nm was …show more content…
8 shows the SEM images of both fresh and hydrothermally aged SCR catalysts at 600 and 700 °C. The SEM images clearly show the morphology changes with the additives of Ce and Nb. For the fresh catalysts, the surface shape is regularly arranged and then it becomes flat as the amount of additives increase. There is no big difference in the surface shape between the fresh and 600 °C aged catalysts. After aging at 700 °C, some agglomeration of the catalyst was observed. Comparing Figs. 8(a-3)–(d-3), 700 °C-aged Cu-BEA catalyst shows the largest size, indicating strong aggregation of particles. The CuO is characteristic of a charge transfer from the metal ion toward the copper, and the peak of CuO of the aged catalysts was weak as shown in Fig. 3 (Gaudinet al. 2016). A part of Cu also converted to CuO after hydrothermal aging (Kang et al. 2016). The other catalysts all show sintering phenomenon. The catalyst shape becomes irregular, and the particle size becomes much larger than that of fresh catalyst. These results show that the ion-exchanged Cu-BEA SCR catalysts with additives of Ce damaged significantly after hydrothermal aging at 700 °C, probably because of the sintering of particle, which is too large, thus damaged the zeolite structure (refer Fig. 2, XRD results and Fig. 4, XPS results). However, the Cu-BEA-Nb catalyst retained the zeolite structure as shown in the XPS results in Fig.