A CuO–CeO2–TiO2 catalyst for selective catalytic reduction of NOx with NH3 (NH3-SCR) at low temperatures was prepared by a sol–gel method and characterized by X-ray diffraction, Brunner–Emmett–Teller surface area, ultraviolet–visible spectroscopy, H2 temperature-programmed reduction, scanning electron microscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The CuO–CeO2–TiO2 ternary oxide catalyst shows excellent NH3-SCR activity in a low-temperature range of 150–250 °C. Lewis acid sites generated from Cu2+ are the main active sites for ammonia activation at low temperature, which is crucial for low temperature NH3-SCR activity. The introduction of ceria results in increased reducibility of CuO species and strong interactions between CuO particles with the matrix. The interactions between copper, cerium and titanium oxides lead to high dispersion of metal oxides with increased active oxygen and enhanced catalyst acidity. Homogeneously mixed metal oxides facilitate the “fast SCR” reaction among Cu2+–NO, nitrate (coordinated on cerium sites) and ammonia (on titanium sites) on the CuO–CeO2–TiO2 catalyst at low temperatures.Keywords: acid sites; CuO−CeO2−TiO2; DRIFTS; NH3-SCR; strong interactions;

Increasingly stringent regulations in many countries require effective reduction and control of NOx emissions. To meet these limits, various methods have been exploited, among which the selective catalytic reduction of NOx using ammonia as the reductant (NH3-SCR) is the most favored technology. High catalytic activity, N2 selectivity and resistance to deactivation by sulfur, alkaline metals and hydrothermal conditions are the optimal properties of a successful SCR catalyst. Rare earth oxides, particularly CeO2, have been increasingly used to improve the catalytic activity and resistance to deactivation of deNOx catalysts, both modifying traditional vanadium catalysts, and also developing novel catalysts, especially for low temperature applications. This review summarized the open literature concerning recent research and development progresses in the application of rare earths for NH3-SCR of NOx. Additionally, the roles of rare earths in enhancing the performance of NH3-SCR catalyst were reviewed.The low or high temperature activity of CeO2 catalysts can be adjusted by the addition of various compounds: 1) acidic component (such as WO3, TiO2, Nb2O5, sulfate and phosphate) improves the NOx conversions of catalyst at high temperatures; 2) transition metal (such as CuO, MnO2 and NiO) modification enhances the NOx conversions of catalyst at low temperatures but decreases the activity of catalyst at high temperature

•BaO modification promoted the activity for propene oxidation, while sulfation inhibited it.•Propene oxidation on Pt/Al2O3 undergoes the path from acrylates to carboxylates and formates and oxidized into CO2 and H2O.•The improving effect is related to the active oxygen at Pt–Ba interface and the active enolic species formed during the reaction.•The inhibition is related to the alteration of partial Pt0 to Ptδ+, strong adsorption of propene on the catalysts surface and poisoning effect caused by intermediate CO.BaO and SO42− modified Pt/Al2O3 catalysts were prepared by a two-step wetness incipient method. The textural properties were characterized by BET, XRD, ex situ FTIR. Propene oxidation activity under various redox ratio (S = [O2]/4.5[C3H6]) was evaluated. The oxidation process and intermediate species were studied by detailed in situ FTIR experiments including propene adsorption CO IR and propene TPO. It is observed that the addition of BaO promotes the catalytic oxidation of propene over Pt/Al2O3, while sulfation results in the deactivation of Pt/Al2O3. The good activity of Pt/BaO/Al2O3 is ascribed to the weakened propene adsorption, the formation of reactive enolic species and easy oxidation of intermediate CO by active oxygen at Pt–Ba interface. On the contrary, the strong adsorption of propene as well as the intermediate CO poisoning are proved to be the main reasons for the deactivation of Pt/SO42−/Al2O3. A general reaction scheme is proposed based on these results.Reaction pathway of propene oxidation on (a) Pt/Al2O3, (b) Pt/BaO/Al2O3 and (c) Pt/SO42−/Al2O3 catalysts.

WO3–CeO2–TiO2 catalysts for NO (nitrogen monoxide) reduction by ammonia were prepared by a sol–gel method. The catalysts were characterized by BET, XRD, Raman, NH3/NO adsorption and H2-TPR to investigate the relationships among the catalyst composition, structure, redox property, acidity and deNOx activity. WO3–CeO2–TiO2 catalysts show a high activity in a broad temperature range of 200–480 °C. The low-temperature activity of catalysts is sensitive to the catalyst composition especially under low-O2-content atmospheres. It may be related to the synergistic effect between CeOx and WOx in the catalysts. On one hand, the interaction between ceria and tungsten oxide promotes the activation of gaseous oxygen to compensate the lattice oxygen consumed in NH3-SCR (selective catalytic reduction) reaction at low temperatures. Meanwhile, the Brønsted acid sites mainly arise from tungsten oxides, Lewis acid sites mainly arise from ceria. Both of the Brønsted and Lewis acid sites facilitate the adsorption of NH3 on catalysts and improve the stability of the adsorbed ammonia species, which are beneficial to the NH3-SCR reaction.

In order to investigate the reaction pathway of propane total oxidation on Pt/CeO2–ZrO2 (Pt/CZ) catalyst and the promoting effect of Pt, CeO2–ZrO2 (CZ) mixed oxides were prepared by sol–gel method and Pt was impregnated. The catalysts were characterized by means of XRD, BET, TPR, in situ FTIR spectroscopy of propane adsorption/transformation/desorption and propane catalytic oxidation activity experiments. The propane oxidation activity is greatly enhanced by the addition of Pt while there is no obvious structure change based on XRD and BET. It is shown by the adsorption experiments that at 250 °C, propane can react with oxygen-containing species; there is obvious destruction of CeO bonds, from which oxygen consumption can be inferred, on both CZ and Pt/CZ catalysts, as well as the generation of some bicarbonate ions on CZ and carbonate on Pt/CZ. Moreover, there are more δ(CH) and δ(CH2) species on CZ than Pt/CZ. When oxygen was introduced into the reactor, the CeO bonds re-formed immediately on Pt/CZ but not on CZ. The propane desorption results show that the intermediate products and desorption process are almost the same on both catalysts. Furthermore, some bidentate carbonate has been detected on both catalysts during the total oxidation process, and it is thought to be the intermediate product of the reaction. Based on the IR results, a possible propane oxidation pathway and the role of Pt as well as the possible electrochemical mechanism have been proposed.Graphical abstractHighlights► The addition of Pt increases propane oxidation activity at the low temperature. ► The oxidation of Ce3+ and regeneration of Ce4+ is important to the oxidation reaction. ► Bidentate carbonate species are thought to be the intermediate of the oxidation reaction. ► The desorption of products is mainly related to CeO2–ZrO2 support.

A serial of surfactant-templated mesoporous TiO2 films with the thickness over several micrometers have been successfully synthesized by one-step dip-coating and subsequent evaporation induced self-assembly method. Three different pre-condensed TiO2 sols in the presence of surfactant (Pluronic F127) micelles with high viscosities were employed as the precursors for dip-coating. By treating the films in liquid paraffin as “shape protector” at certain high temperature for sufficient time, thick mesoporous films can be kept crack-free after calcinations. By employing the size-controlled titanium-oxo clusters in the sols as building blocks for self-assembly, the final obtained films represent tunable mesostructures. The mesoscopic characteristics of the films, such as Brunauer–Emmett–Teller surface areas, pore size distributions and pore wall crystallizations, have been comparatively studied. The results demonstrate that such tunable mesoscopic characteristics are greatly dependent on the structural and shape parameters of the initial formed inorganic clusters.

Binary Ce–Zr (CZ), trinary Ce–Zr–Pr (CZP), Ce–Zr–Nd (CZN) mixed oxides were prepared by coprecipitation. The structural and textural properties were characterized by the X-ray diffraction (XRD) analysis, Brunauer–Emmett–Teller (BET) method, Raman and X-ray absorption near-edge spectra (XANES) techniques, while the oxygen storage capacity (OSC) was evaluated under both dynamic and static conditions at 500 °C. The doping of Pr or Nd cations causes the lattice deformation of the tetragonal Zr-rich mixed oxides to form a pseudocubic structure and prevents the phase demixing after calcination in flowing steam/air at 1050 °C for 5 h. After the hydrothermal ageing treatment, the doped samples show higher BET surface areas and better oxygen mobility. Pr exists mainly in the form of trivalent cations in the aged CZP and functions primarily as the doping element with large ionic radius instead of redox couple Pr3+/Pr4+, which may introduce more Ce3+ species and hereby more lattice defects. Among the aged samples, CZP shows the best oxygen storage capacity and the fastest oxygen release rate.The doping of Pr or Nd causes the lattice deformation of tetragonal ceria–zirconia to form a pseudocubic structure. After hydrothermal ageing, the Pr doped sample shows a better oxygen storage capacity and a faster oxygen release rate, where Pr functions primarily as large Pr3+ doping cation instead of redox couple Pr3+/Pr4+ to introduce more Ce3+ species and lattice defects.

A MnOx–NbOx–CeO2 catalyst for low temperature selective catalytic reduction (SCR) of NOx with NH3 was prepared by a sol–gel method, and characterized by NH3–NO/NO2 SCR catalytic activity, NO/NH3 oxidation activity, NOx/NH3 TPD, XRD, BET, H2-TPR and in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The results indicate that the MnOx–NbOx–CeO2 catalyst shows excellent low temperature NH3-SCR activity in the temperature range of 150–300°C. Water vapor inhibits the low temperature activity of the catalyst in standard SCR due to the inhibition of NOx adsorption. As the NO2 content increases in the feed, water vapor does not affect the activity in NO2 SCR. Meanwhile, water vapor significantly enhances the N2 selectivity of the fresh and the aged catalysts due to its inhibition of the decomposition of NH4NO3 into N2O.Download full-size image