The balance of acidity and redox capability over modified CeO2 catalyst for the selective catalytic reduction of NO with NH3

Effects of impregnation sequence on the NH3-SCR activity and hydrothermal stability of a Ce-Nb/SnO2 catalyst

Hydrothermal stability is crucial for the practical application of deNOx catalyst on diesel vehicles, for the selective catalytic reduction of NOx with NH3 (NH3-SCR). SnO2-based materials possess superior hydrothermal stability, which is attractive for the development of NH3-SCR catalyst. In this work, a series of Ce-Nb/SnO2 catalysts, with Ce and Nb loading on SnO2 support, were prepared by impregnation method. It was found that, the NH3-SCR activities and hydrothermal stabilities of the Ce-Nb/SnO2 catalysts significantly varied with the impregnation sequences, and the Ce-Nb(f)/SnO2 catalyst that firstly impregnated Nb and then impregnated Ce exhibited the best performance. The characterization results revealed that Ce-Nb(f)/SnO2 possessed appropriate acidity and redox capability. Furthermore, the strong synergistic effect between Nb and Sn species stabilized the structure and maintained the dispersion of acid sites. This study may provide a new understanding for the effect of impregnation sequence on activity and hydrothermal stability and a new environmental-friendly NH3-SCR catalyst with potential applications for NOx removal from diesel and hydrogen-fueled engines.

Hydrothermal Aging Treatment Activates V2O5/TiO2 Catalysts for NOx Abatement

Thermal stability is crucial for the practical application of deNO_x catalysts. Vanadia-based catalysts are widely applied for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). Generally, hydrothermal aging at high temperatures induces the deactivation of deNO_x catalysts. However, in this work, a remarkable increase in low- and medium-temperature NH₃-SCR activity was observed for a V₂O₅/TiO₂ catalyst after hydrothermal aging treatment, especially at 750 °C for 16 h. After the vanadia-based catalyst was hydrothermally treated at 750 °C, the specific surface area decreased and the surface VO_x density and surface V ratio increased significantly. Therefore, the aged catalyst presented more abundant polymeric vanadyl species than the fresh one. Furthermore, the redox capability was improved markedly after hydrothermal treatment due to the strong interaction of vanadia and titania, contributing to the NH₃-SCR reaction. 750 °C is the optimal temperature to activate the V₂O₅/TiO₂ catalyst, improving the SCR performance significantly. This study provides an in-depth understanding of vanadia-based catalysts for practical applications.

Remarkable enhancement in the N2 selectivity of NH3-SCR over the CeNb3Fe0.3/TiO2 catalyst in the presence of chlorobenzene

The simultaneous removal of NO_x and dioxins is the frontier of environmental catalysis, which is still in the initial stage and poses several challenges. In this study, a series of CeNb₃Fe_x/TiO₂ (x = 0, 0.3, 0.6, and 1.0) catalysts were prepared by the sol-gel method and examined for the synergistic removal of NO_x and CB. The CeNb₃Fe_0.3/TiO₂ catalyst exhibits an optimum catalytic performance, with an NO_x conversion greater than 95% at 260-380 °C. It also exhibits an optimal CB oxidation activity, in which CB promoted both the NO_x conversion and N₂ selectivity below 250 °C. Moreover, the more favorable ratios of Ce⁴⁺ to Ce³⁺ and plentiful surface-adsorbed oxygen species are the reasons why CeNb₃Fe_0.3/TiO₂ catalyst has better catalytic activity than other catalysts at the lower temperature. Simultaneously, owing to the modulation of Fe to the redox properties of Ce and Nb, the large number of oxygen vacancies and acid sites was generated, and the CeNb₃Fe_0.3/TiO₂ catalyst is beneficial to NO_x reduction and CB oxidation. Furthermore, the results of in situ DRIFTS study reveal the NH₃-SCR reactions over CeNb₃Fe_0.3/TiO₂ catalysts are mainly conformed to by the L-H mechanism (< 350 °C) and E-R mechanism (> 350 °C), respectively, and the multi-pollutant conversion mechanism in the synergistic reaction was systematically studied.

Vanadium-bearing steel slag catalysts for the selective catalytic reduction of NOx by NH3

In this paper, denitration catalysts were prepared by different modification methods using vanadium-bearing steel slag as raw material.

Catalytic performance and mechanistic evaluation of sulfated CeO2 cubes for selective catalytic reduction of NOx with ammonia

Sulfated CeO₂ cubes were prepared by the impregnation of CeO₂ cubes by ammonium sulfates, and further evaluated in selective catalytic reduction of NO_x with ammonia (NH₃-SCR). Catalytic activity tests indicated that NO_x reduction conversions and N₂ selectivity of sulfated CeO₂ cubes could be significantly improved compared to pure CeO₂ cubes. The synthesized sulfated CeO₂ cubes were further characterized by atom-resolved high angle annular dark-field (HAADF) imaging, Fourier-transform infrared spectroscopy (FTIR) by pyridine adsorption, and temperature-programmed reduction by H₂ (H₂-TPR). The characterization results showed that sulfates were primarily dispersed through the corners, edges, and surfaces of CeO₂ cubes, and did not significantly affect the crystal structures of CeO₂ cubes. Sulfation treatment could create and strengthen Brønsted acid sites originated from the protons on surface sulfates, further facilitating ammonia adsorption and activation. The kinetic data indicated that the apparent reaction order of NO, O₂, and NH₃ was 0.95 to 1.01, -0.01 to 0.00, and -0.18 to -0.15, respectively. It could speculate that gaseous phase NO involving in NO catalytic oxidation was the rate-determining step over sulfated CeO₂ cubes for NH₃-SCR reaction. The presence of NH₃ slightly inhibited the SCR reaction rate due to the competitive adsorption blocking NO oxidation sites.

Preparation and Performance of Cerium-Based Catalysts for Selective Catalytic Reduction of Nitrogen Oxides: A Critical Review

Selective catalytic reduction of nitrogen oxides with NH3 (NH3-SCR) is still the most commonly used control technology for nitrogen oxides emission. Specifically, the application of rare earth materials has become more and more extensive. CeO2 was widely developed in NH3-SCR reaction due to its good redox performance, proper surface acidity and abundant resource reserves. Therefore, a large number of papers in the literature have described the research of cerium-based catalysts. This review critically summarized the development of the different components of cerium-based catalysts, and characterized the preparation methods, the catalytic performance and reaction mechanisms of the cerium-based catalysts for NH3-SCR. The purpose of this review is to highlight: (1) the modification effect of the various metal elements for cerium-based catalysts; (2) various synthesis methods of the cerium-based catalysts; and (3) the physicochemical properties of the various catalysts and clarify their relations to catalytic performances, particularly in the presence of SO2 and H2O. Finally, we hope that this work can give timely technical guidance and valuable insights for the applications of NH3-SCR in the field of NOx control.

Poisoning Effects of Alkali and Alkaline Earth Metal Doping on Selective Catalytic Reduction of NO with NH3 over the Nb-Ce/Zr-PILC Catalysts

The poisoning effects of alkali metals (K and Na) and alkaline earth metals (Ca and Mg) on catalytic performance of the 2Nb4Ce/Zr-PILC catalyst for the selective catalytic reduction of NOx with NH3 (NH3-SCR) were investigated, and physicochemical properties of the catalysts were characterized by means of the X-ray diffraction XRD (XRD), Brunner−Emmet−Teller (BET), hydrogen temperature-programmed reduction (H2-TPR), X-ray Photoelectron Spectroscopy (XPS), ammonia temperature-programmed desorption (NH3-TPD), and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) techniques. Doping of M (M = K, Na, Ca, and Mg) deactivated the 2Nb4Ce/Zr-PILC catalyst according to the sequence of 0.8 K > 0.8 Na > 0.8 Ca > 0.8 Mg (M/Ce molar ratio = 0.8). The characterization results showed that the decreases in redox ability, NH3 adsorption, Ce3+/Ce4+ atomic ratio, and amount of the chemisorbed oxygen (Oβ) were the important factors influencing catalytic activities of the alkali metal-and alkaline earth metal-doped samples. Consequently, compared with the Mg- and Ca-doped samples, doping of K caused the 2Nb4Ce/Zr-PILC sample to possess the lowest redox ability, NH3 adsorption, and amount of the Oβ species, which resulted in an obvious deactivation effect.

The comparative study on the catalytic activity of Cu–M/Ce0.8Zr0.2O2 (M = W, Nb, Cr and Mo) catalysts with dual-function for the simultaneous removal of NO and CO under oxygen-rich conditions

The Cu–M/Ce_0.8Zr_0.2O₂ (M = W, Nb, Cr and Mo) catalysts can simultaneously catalyze NH₃-SCR and CO oxidation reactions, and the Cu–Nb/Ce_0.8Zr_0.2O₂ catalysts show the best catalytic performance.

Effects of calcination temperature on physicochemical property and activity of CuSO4/TiO2 ammonia-selective catalytic reduction catalysts

CuSO₄/TiO₂ catalysts with high catalytic activity and excellent resistant to SO₂ and H₂O, were thought to be promising catalysts used in Selective catalytic reduction of nitrogen oxides by NH₃. The performance of catalysts is largely affected by calcination temperature. Here, effects of calcination temperature on physicochemical property and catalytic activity of CuSO₄/TiO₂ catalysts were investigated in depth. Catalyst samples calcined at different temperatures were prepared first and then physicochemical properties of the catalyst were characterized by N₂ adsorption-desorption, X-ray diffraction, thermogravimetric analysis, Raman spectra, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption of NH₃, temperature-programmed reduction of H₂ and in situ diffuse reflectance infrared Fourier transform spectroscopy. Results revealed that high calcination temperature had three main effects on the catalyst. First, sintering and anatase transform into rutile with increase of calcination temperature, causing a decrement of specific surface area. Second, decomposition of CuSO₄ under higher calcination temperature, resulting in disappears of Brønsted acid sites (S-OH), which had an adverse effect on surface acidity. Third, CuO from the decomposition of CuSO₄ changed surface reducibility of the catalyst and favored the process of NH₃ oxidation to nitrogen oxides (NO_x). Thus, catalytic activity of the catalyst calcined under high temperatures (≥600°C) decreased largely.