title: In situ characterization during the synthesis and reaction of ceria-based mesoporous catalysts for NH3-selective catalytic reduction (SCR) applications
Creators: name: Shen, Jun
Publisher: name: UNSPECIFIED
Year: 2023

description: Nitrogen oxides (NOx) are major pollutants of the air environment, which are known as the major causes of haze, photochemical smog, acid rain, ozone depletion, and the greenhouse effect. Selective catalytic reduction using ammonia as a reductant (NH3-SCR) is proved to be an effective method to remove nitrogen oxides (DeNOx). Previous studies have reported that cerium-based oxide catalysts own the advantages of wide operating temperature windows and high catalytic activities in NH3-SCR reactions. Mesoporous materials (e.g. SBA-15) are considered as a suitable support material for metal oxides, providing a high surface area and stabilization effect. Thus, this work addresses the synthesis of SBA-15 supported ceria-based catalysts and their application for the NH3-SCR reaction. First, the simplest system was considered by loading only ceria onto the mesoporous SBA-15 (CeO2/SBA-15). In order to achieve a good dispersion of ceria on the inner surface, two kinds of samples were prepared by solid-state impregnation method following the “medium-synthesis” route (asSBA-CeO2) and the “post-synthesis” route (tfSBA-CeO2). Catalytic tests indicated that the DeNOx performance was enhanced by the assistance of template P123 in asSBA-CeO2. In situ characterizations, which were performed to elucidate the effect of P123 during the synthesis process, revealed that the good dispersion effect of asSBA-15 supported ceria was due to the presence of template P123, playing a physical role to confine the growth of ceria and a chemical role in catalytically reducing the ceria simultaneously. After the synthesis, the catalysts were applied for the NH3-SCR reaction and the mechanism was studied. In situ DRIFT spectra of CeO2/SBA-15 were recorded to monitor the surface species changes as a function of feeding gas, time, and temperature. The results showed that two active species, -NH2 and NO-, were involved. In addition, the analysis of features at about 2100-2200 cm-1, typically assigned to triple bonds, made it possible to deduce the formation of the side product N2O by over-oxidation of NH3. Moreover, the co-existence of the L-H and E-R routes for the NH3-SCR reaction on CeO2/SBA-15 was proved. Considering the poor reactivity of bare CeO2 for the NH3-SCR reaction, a second metal oxide was added to form a MOx-CeO2 couple, which was expected to improve the catalytic performance due to the synergistic effect of M-O-Ce species. Firstly, a wide selection of secondary metal elements was employed to prepare SBA-CeCuO, SBA-CeMnO, SBA-CeNiO, SBA-CeMgO, and SBA-CeLaO, while SBA-CeO2 served as a reference. The catalytic tests showed that the mixture with variable metals (Cu, Mn, Ni) resulted in enhanced DeNOx performance, while the mixture with permanent metals (Mg, La) exhibited a decreased performance. The observed behavior revealed that the redox ability rather than the acidity contributed to the NH3-SCR reactivity. Based on the catalytic performance and due to its wide application in various catalytic systems, the CuO-CeO2 catalyst was selected for detailed analysis. Template-free SBA-15 and as-made SBA-15 were employed as support precursors to form CuO-CeO2/SBA-15 and the synthesis process was monitored with in situ characterizations including Raman, DRIFT, and DR UV-Vis spectroscopy. The results specified the synthesis to follow two different routes, i.e. solid thermal decomposition for the template-free SBA-15 prepared sample and a hydrothermal route for the as-made SBA-15 prepared sample. The former route resulted in a mixed CuO-CeO2 phase with strong redox ability, leading to a low optimal temperature window but poor N2 selectivity, while the latter route led to separated CuO and CeO2 phases with moderate redox properties, resulting in a high and broad working temperature window. These findings provided evidence for the presence of a more comprehensive research pattern, connecting catalyst synthesis, structure, property, and catalytic performance. Another bimetallic oxide catalyst, MnOx-CeO2 supported on SBA-15, was prepared due to its favorable DeNOx performance. In situ characterizations were applied to study the whole process from the catalyst synthesis to its application for the NH3-SCR reaction. The results for the synthesis process revealed two different preparation routes when using a similar template strategy as for the CuO system. Based on in situ DRIFT spectra the mechanism of NH3-SCR on MnOx-CeO2/SBA-15 was shown to follow mainly the E-R route and the quick regeneration of -NH2 by strong redox properties contributed to the high reactivity. Atomic layer deposition (ALD) was employed as another promising method to load active metal oxides onto mesoporous materials. Firstly, VOx/TiO2/SBA-15 catalysts were prepared by ALD to explore the feasibility of using mesoporous powder as a deposition substrate. The results showed that the prepared sample owns a high surface area as expected and the surface-loaded VOx could be quantitatively controlled by the number of ALD cycles. Secondly, several atomic layers of SiO2 were deposited on prepared CeO2/SBA-15 to explore the activity of Ce-O-Si sites although SiO2 itself is known as an inert support. Due to the sandwich structure of SiO2/CeO2/SiO2 and the controllable amount of Ce-O-Si sites by ALD, Ce-O-Si was shown to be the active site for the NH3-SCR reaction and the NOx conversion was proportional to the SiO2 coverage until full coverage of the surface. In addition, the SiO2-covered sample showed a better SO2 resistance than bare CeO2/SBA-15. Besides, VOx/CeO2/SBA-15 was prepared by depositing VOx on CeO2/SBA-15 via the common ALD and a site-selective ALD (SSALD) method. This SSALD method is proposed here for the first time. As a crucial step it involves the pretreatment of the substrate with the target reaction gas (NH3-SCR) before the ALD procedure. The results showed that the SSALD-coated sample exhibited a better DeNOx performance than the common ALD-coated sample. IR analysis of the catalyst surface revealed that the substrate pretreated with the SCR experimental gas showed the presence of NHx and NxOy replacing hydroxyl groups. The NHx species was inert to the vanadium precursor and retained the NH3-active sites after coating, while the NxOy species was desorbed and free water was transformed into -OH groups, resulting in more active VOx species deposition. During the study of the synthesis process by in situ characterizations, the synthesis reaction was realized to be important for determining the final catalyst structure and further the reactivity behavior in the target NH3-SCR reaction. It is reasonable to hypothesize that there exists a correlation between the synthesis reaction and the target catalytic reaction. To test this hypothesis, CeO2/SBA-15 samples were prepared by the SSI method and calcined in different gas atmospheres, from reductive NH3 to oxidative O2. Among the prepared samples, the sample calcined in the target reaction gas showed the best catalytic performance. Raman and DR UV-Vis spectra of the sample exposed to the target reaction gas revealed the presence of a moderate amount of oxygen vacancies, which is proposed to contribute to the high NOx conversion due to the optimized adsorption of NH3. In all, the correlation between synthesis reaction and target catalytic reaction is an interesting topic to be explored for other reactions, and in situ spectroscopy applied to both synthesis and reaction is a powerful tool to elucidate the underlying chemistry.