Changjin Tang

Investigation of the physicochemical properties and catalytic activities of Ce0.67M0.33O2 (M = Zr4+, Ti4+, Sn4+) solid solutions for NO removal by CO

NO removal by CO model reaction was investigated over a series of ceria-containing solid solutions, prepared by an inverse co-precipitation method, to explore the relationship between the physicochemical properties and catalytic performances of these catalysts. The synthesized samples were studied in detail by means of XRD, Raman, TEM, UV-Vis spectroscopy, N2-physisorption, H2-TPR, OSC, XPS and in situ FT-IR technologies. These results indicate that the incorporation of Zr4+, Ti4+ and Sn4+ into the lattice of CeO2 leads to a smaller grain size and enhanced reduction behavior. Furthermore, the catalytic performance test shows that the activities and selectivities of these solid solutions are higher than pure CeO2 and that the Sn4+-doped sample shows the best results. The reason may be that: (1) the decrease in grain size results in an enlargement of the BET specific surface area and an increase of surface Ce3+. The former is conducive for sufficient contact between the catalyst and reactant molecules and the latter contributes to the adsorption of COx species; (2) the enhanced reduction behavior is beneficial in generating more surface oxygen vacancies during the reaction process, which can weaken the N–O bond to promote the dissociation of NOx effectively. Finally, in order to further understand the nature of the catalytic performances for these samples, a possible reaction mechanism is tentatively proposed.

Ceria-based catalysts for low-temperature selective catalytic reduction of NO with NH3

Selective catalytic reduction of NO with NH3 (NH3-SCR) is a powerful technique for the abatement of NOx from stationary sources, and the currently used VOx/TiO2-based catalysts are widely applicable for medium-temperature conditions but not suitable for NH3-SCR operated at low temperatures. Recently, low-temperature NH3-SCR has attracted considerable attention owing to the vast demand in industrial furnaces and its energy-conserving feature. During the past years, a great many studies have demonstrated that ceria-based catalysts are potential candidates as catalysts for low-temperature NH3-SCR. Herein we summarize the recent advances in the application of ceria-based catalysts for low-temperature NH3-SCR. The review begins with a brief introduction of the general guideline for low-temperature NH3-SCR and the interaction between the reactants and CeO2. The different roles of ceria as a pure support/active species, bulk doping component and surface modifier are discussed. As well, the mechanistic investigations (active sites, intermediates, reaction mechanism) and SO2/H2O tolerance are emphasized. Lastly, the perspectives on the opportunities and challenges of ceria-based catalysts for low-temperature NH3-SCR in future research are presented.

NO reduction by CO over CuO–CeO2 catalysts: effect of preparation methods

This work is mainly focused on investigating the influence of preparation methods on the physicochemical and catalytic properties of CuO–CeO2 catalysts for NO reduction by CO model reaction. Five different preparation methods have been used to synthesize CuO–CeO2 catalysts: mechanical mixing method (MMM), impregnation method (IM), grinding method (GM), hydrothermal treatment method (HTM) and co-precipitation method (CPM). All of these samples were characterized by a series of techniques such as N2-physisorption, XRD, LRS, H2-TPR, ICP-AES, XPS, in situ FT-IR and NO + CO model reaction. The obtained results show that the catalytic performances of these CuO–CeO2 catalysts can be ranked by CuCe-IM > CuCe-CPM > CuCe-GM > CuCe-HTM > CuCe-MMM, which is in agreement with the orders of the surface oxygen vacancy concentration, reducibility and Cu+ content, suggesting that the synergistic effect between Cu+ species and surface oxygen vacancies of these CuO–CeO2 catalysts plays an important role in this model reaction. In order to further understand the synergistic effect, a possible reaction model is tentatively proposed.

Textural, structural, and morphological characterizations and catalytic activity of nanosized CeO2–MOx (M = Mg2+, Al3+, Si4+) mixed oxides for CO oxidation

The present work focuses on the combination of ceria with another oxide of different ionic valences from period 3 (Mg(2+), Al(3+), and Si(4+)) using coprecipitation method, followed by calcination at 450 and 750°C, respectively. The textural, structural, morphological and redox properties of nanosized ceria-magnesia, ceria-alumina and ceria-silica mixed oxides have been investigated by means of N(2) physisorption, XRD, Raman, HRTEM, DRS, FT-IR, and H(2)-TPR technologies. XRD results of these mixed oxides reveal that only nanocrystalline ceria (ca. 3-6nm for the 450°C calcined samples) could be observed. The grain size of ceria increases with the increasing calcination temperature from 450 to 750°C due to sintering effect. The highest specific surface area is obtained at CeO(2)-Al(2)O(3) mixed oxides when calcination temperature reaches 750°C. Raman spectra display the cubic fluorite structure of ceria and the existence of oxygen vacancies, and displacement of oxygen ions from their normal lattice positions in the ceria-based mixed oxides. DRS measurements confirm that the smaller the grain size of the ceria, the higher indirect band gap energy. H(2)-TPR results suggest that the reductions of surface and bulk oxygen of ceria were predominant at low and high calcination temperature, respectively. Finally, CO oxidation were performed over these ceria-based mixed oxides, and the combination of CeO(2)-Al(2)O(3) exhibited highest activity irrespective of calcination temperature, which may due to excellent textural/structural properties, good homogeneity, and redox abilities.

Synthesis, characterization, and catalytic performance of copper-containing SBA-15 in the phenol hydroxylation.

A series of copper-containing SBA-15 samples were successfully synthesized via evaporation-induced self-assembly route. The resulting materials were characterized by X-ray diffraction (XRD), (29)Si MAS NMR spectroscopy, transmission electron microscopy (TEM), N(2) sorption, inductively coupling plasma-atomic emission spectrometer (ICP-AES), thermogravimetry, and differential thermal analysis (TG-DTA), Fourier-transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectra (UV-vis) and X-ray photoelectron spectroscopy (XPS). The results indicated that: (1) all the samples exhibited typical hexagonal arrangement of mesoporous structure; (2) copper ions could be incorporated into the framework of SBA-15; (3) the addition of urea in the hydrothermal stage efficiently reduced the leaching of copper and improved the thermal stability of the mesoporous materials. Catalytic performances of the obtained materials were evaluated in the hydroxylation of phenol with H(2)O(2). The catalytic tests showed that the synthesized materials exhibited high activity for this reaction and copper ions in the framework were more active than copper species in the extra-framework position. The nitric acid treatment on the samples removed the bulk CuO species, which resulted in a dramatic increase in the catalytic activity.

Promotional effect of doping SnO2 into TiO2 over a CeO2/TiO2 catalyst for selective catalytic reduction of NO by NH3

TixSn1−xO2 was prepared by a co-precipitation method, and a series of CeO2/TixSn1−xO2 samples were prepared to investigate the effect of doping SnO2 into TiO2 for selective catalytic reduction of NO by NH3. The results of catalytic tests suggested that the catalyst with the optimal molar ratio (Ti : Sn = 1 : 1) exhibited the best catalytic performance. Moreover, the NO removal efficiency of CeO2/Ti0.5Sn0.5O2 was higher than that of CeO2/TiO2. The obtained samples were characterized by BET, XRD, H2-TPR, XPS, NH3-TPD and in situ DRIFT. The results revealed that the introduction of SnO2 resulted in the formation of rutile-type Ti0.5Sn0.5O2 solid solution with larger specific surface area and better thermal stability. The interactions between CeO2 and the Ti0.5Sn0.5O2 support could improve the redox performance of the catalyst, which was beneficial to the enhancement of catalytic activity at low temperature. Furthermore, doping SnO2 enhanced the surface acid sites and weakened the adsorption of nitrates, which played an important role in the catalytic reaction process. Finally, in situ DRIFT demonstrated that the competition adsorption happened between bridging nitrates and NH3 gas and the selective catalytic reduction of NO by NH3 proceeded mainly via the Eley–Rideal mechanism over Ce/TiO2 and Ce/Ti0.5Sn0.5O2.

Comparative study on the catalytic CO oxidation properties of CuO/CeO2 catalysts prepared by solid state and wet impregnation

A series of CuO/CeO2 catalysts were prepared by solid state impregnation (SSI) and wet impregnation (WI) methods and characterized by X-ray diffraction, H2 temperature-programmed reduction (H2-TPR), laser Raman spectroscopy (LRS), in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and X-ray photoelectron spectroscopy (XPS). XPS and H2-TPR results showed that SSI increased the dispersion of the copper species on the catalyst surface, which benefited the reduction of CuO species. LRS results indicated that a higher concentration of oxygen vacancies was obtained by the SSI method unlike the WI method. CO oxidation results showed that at a given CuO loading, the activity of the catalysts prepared by SSI was higher than that of their counterparts prepared by WI. Based on the combined characterization results, it was suggested that the enhanced activity was closely related to the higher concentrations of oxygen vacancies and Cu+-CO species on the catalysts. Last, a possible synergetic mechanism was proposed for CO oxidation over the CuO/CeO2 catalysts.

Effect of CO-pretreatment on the CuO–V2O5/γ-Al2O3 catalyst for NO reduction by CO

The influence of CO-pretreatment on the properties of CuO–V2O5/γ-Al2O3 catalysts was investigated in the reduction of NO by CO. Catalytic performance results showed that the pretreated CuO–V2O5/γ-Al2O3 exhibited extremely high activity and selectivity. For example, NO conversion can be remarkably enhanced from 13.8% to 100.0% for the 03Cu01V catalyst. For the catalyst characterization, the XRD results suggested that copper oxide and vanadium oxide were highly dispersed on the surface of γ-Al2O3 and the TPR results gave evidence for the existence of Cu2+–O–V5+ species. The XPS and EPR results demonstrated that Cu2+ and V5+ were reduced to lower valence states (Cu2+ → Cu+, V5+ → V4+) by the CO-pretreatment, which was proved by in situ FT-IR to be beneficial to the adsorption of CO and dissociation of NO. In addition, the interaction between the dispersed copper oxide and vanadium oxide species upon the γ-Al2O3 support before and after CO-pretreatment was tentatively discussed, using the concept of SSOV (surface synergetic oxygen vacancy) which was proposed elsewhere. According to this concept, the dispersed Cu2+–O–V5+ species could be reduced to Cu+–ϒ–V4+ (ϒ represents an oxygen vacancy) by CO-pretreatment and it was considered to be the primary active species for the reaction. Based on the discussion of the experiment results, a possible mechanism was proposed.

Engineering the NiO/CeO2 interface to enhance the catalytic performance for CO oxidation

In this work, NiO/CeO2 catalysts were synthesized with tunable CeO2 crystal facets ({110}, {111} and {100} facets) to study the crystal-plane effects on the catalytic properties. Kinetic studies of CO oxidation showed that NiO/CeO2 {110} had the lowest activation energy. Furthermore, the obtained samples were characterized by means of TEM, XRD, Raman, N2-physisorption, UV-Vis DRS, XPS, H2-TPR and in situ DRIFTS technologies. The results demonstrated that the geometric and electronic structures of the nickel species were dependent on the NiO/CeO2 interfaces, which had an influence on the synergetic interaction of absorbed CO and active oxygen species, and then the generation of the formate intermediate played an important role in the catalytic performance. The possible interface structures of nickel species on the CeO2 {110}, {111} and {100} surface were proposed through the incorporation model, suggesting that the advantageous NiO/CeO2 {110} interface facilitated CO adsorption/activation and active oxygen species formation, leading to the best catalytic performance.

Promotional effect of CO pretreatment on CuO/CeO2 catalyst for catalytic reduction of NO by CO

The CuO/CeO2 catalysts were investigated by means of X-ray diffraction (XRD), laser Raman spectroscopy (LRS), X-ray photoelectronic spectroscopy (XPS), temperature-programmed reduction (TPR), in situ Fourier transform infrared spectroscopy (FTIR) and NO+CO reaction. The results revealed that the low temperature (<150 °C) catalytic performances were enhanced for CO pretreated samples. During CO pretreatment, the surface Cu+/Cu0 and oxygen vacancies on ceria surface were present. The low valence copper species activated the adsorbed CO and surface oxygen vacancies facilitated the NO dissociation. These effects in turn led to higher activities of CuO/CeO2 for NO reduction. The current study provided helpful understandings of active sites and reaction mechanism in NO+CO reaction.