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Journal ArticleDOI

Mechanistic investigation of the enhanced NH3-SCR on cobalt-decorated Ce-Ti mixed oxide: In situ FTIR analysis for structure-activity correlation

TL;DR: In this article, a series of transition metals (Co, Cu and Fe) were selected to decorate Ce-Ti mixed oxide to elevate the low-temperature activity of selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) reaction.
Abstract: A series of transition metals (Co, Cu and Fe) were selected to decorate Ce-Ti mixed oxide to elevate the low-temperature activity of selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) reaction, by adjusting the ratio of surface Ce 3+ species and oxygen vacancies. Among them, Co-Ce-Ti sample exhibited the excellent low-temperature activity and broadened temperature window, which could be attributed to the improvement of the physico-chemical properties and the acceleration of the reactions in the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. Owing to the different ionic sizes of Co 2+ and Ce 4+ , the lattice distortion of Ce-Ti mixed oxide was greatly aggravated and subsequently increased the ratio of Ce 3+ and the surface adsorbed oxygen, which benefited the generation of adsorbed NO x species and improved the reaction in the L-H mechanism. Meanwhile, the coordinatively unsaturated cationic sites over the Co-Ce-Ti sample induced more Lewis acid sites and enhanced the formation of the adsorbed NH 3 species bounded with Lewis acid sites, which were considered as the crucial intermediates in E-R mechanism, and therefore facilitating the reaction between the adsorbed NH 3 species and NO molecules. The enhancements in both the reactions from L-H and E-R mechanisms appeared to directly correlated with the improved deNO x performance on the Co-Ce-Ti sample, and the L-H mechanism could be the dominate one at low temperatures due to its rapid reaction rate.
Citations
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Journal ArticleDOI
TL;DR: This review summarizes the latest SCR reaction mechanisms and emerging poison-resistant mechanisms in the beginning and subsequently gives a comprehensive overview of newly developed SCR catalysts, including metal oxide catalysts ranging from VOx, MnOx, CeO2, and Fe2O3 to CuO based catalysts.
Abstract: Selective catalytic reduction with NH3 (NH3-SCR) is the most efficient technology to reduce the emission of nitrogen oxides (NOx) from coal-fired industries, diesel engines, etc. Although V2O5-WO3(MoO3)/TiO2 and CHA structured zeolite catalysts have been utilized in commercial applications, the increasing requirements for broad working temperature window, strong SO2/alkali/heavy metal-resistance, and high hydrothermal stability have stimulated the development of new-type NH3-SCR catalysts. This review summarizes the latest SCR reaction mechanisms and emerging poison-resistant mechanisms in the beginning and subsequently gives a comprehensive overview of newly developed SCR catalysts, including metal oxide catalysts ranging from VOx, MnOx, CeO2, and Fe2O3 to CuO based catalysts; acidic compound catalysts containing vanadate, phosphate and sulfate catalysts; ion exchanged zeolite catalysts such as Fe, Cu, Mn, etc. exchanged zeolite catalysts; monolith catalysts including extruded, washcoated, and metal-mesh/foam-based monolith catalysts. The challenges and opportunities for each type of catalysts are proposed while the effective strategies are summarized for enhancing the acidity/redox circle and poison-resistance through modification, creating novel nanostructures, exposing specific crystalline planes, constructing protective/sacrificial sites, etc. Some suggestions are given about future research directions that efforts should be made in. Hopefully, this review can bridge the gap between newly developed catalysts and practical requirements to realize their commercial applications in the near future.

800 citations

Journal ArticleDOI
TL;DR: In this paper, a samarium-doped CeO2-TiO2 mixed oxide (CeTi-O2) catalyst was synthesized for the NH3-SCR reaction.
Abstract: The Sm doped CeO2-TiO2 mixed oxide catalyst, which exhibited excellent activity and tolerance to H2O and SO2 in the NH3-SCR reaction, was synthesized. The reasons for the high activity and SO2 resistance of the catalyst were investigated by a series of characterization. The H2-TPR and O2-TPD results suggested that the reducibility and oxygen storage capacity (OSC) of CeTi catalyst were promoted by the addition of Sm species, which was beneficial for improving the activity of catalyst. The in situ DRIFTS results revealed that the adsorptive ability of NOx species and activation ability of NH3 were enhanced by Sm doping, which was also propitious to enhance the activity. XPS combined with DFT calculated results confirmed that the transfer of electron by Sm2++Ce4+⇌Sm3++Ce3+ circles occurred in the SmCeTi catalyst. The redox circles may be the reason of the good SO2 tolerance of the SmCeTi catalyst, for which suppressed the electron transferring from adsorbed SO2 to Ce4+. Through in situ DRIFTS and TG-DSC results, it can be concluded that the sulphation of catalyst was lowered by samarium doping into CeTi catalyst. Consequently, the SmCeTi catalyst exhibited significant SO2 tolerance ability.

249 citations

Journal ArticleDOI
TL;DR: In this article, the Co3O4 quantum dots (QDs)/TiO2 nanobelts (NBs) hybrid was constructed via a facile hydrothermal method for simultaneous H2 and O2 productions from pure water, with high evolution rates of 41.8 and 22.0
Abstract: Solar-light driven water splitting to hydrogen and oxygen without sacrificial agents has gained tremendous attention due to the clean and renewable energy supply of the future. Herein, we report construction of Co3O4 quantum dots (QDs)/TiO2 nanobelts (NBs) hybrids via a facile hydrothermal method for simultaneous H2 and O2 productions from pure water, with high evolution rates of 41.8 and 22.0 μmol h−1 g−1, respectively, which are significantly enhanced compared with TiO2 NBs and Co3O4 materials. The Co3O4 QDs not only improve light sensitivity but also change the work function of TiO2, promoting the transfer of electrons from TiO2 to Co3O4 QDs and H2 generation on the surface of Co3O4 QDs. Moreover, the size effect of Co3O4 QDs (∼3 nm) facilitates the electron trapping due to the shorter pathway, and the generation of heterojunctions favors to suppress the recombination of photo-excited carries.

198 citations

Journal ArticleDOI
TL;DR: In this paper, the authors reviewed the recent research progress on supported transition and mixed transition metal oxide catalysts for the LT-SCR reaction and discussed the influence of operating conditions and promoters on the performance.
Abstract: The importance of the low-temperature selective catalytic reduction (LT-SCR) of NOx by NH3 is increasing due to the recent severe pollution regulations being imposed around the world. Supported and mixed transition metal oxides have been widely investigated for LT-SCR technology. However, these catalytic materials have some drawbacks, especially in terms of catalyst poisoning by H2O or/and SO2. Hence, the development of catalysts for the LT-SCR process is still under active investigation throughout seeking better performance. Extensive research efforts have been made to develop new advanced materials for this technology. This article critically reviews the recent research progress on supported transition and mixed transition metal oxide catalysts for the LT-SCR reaction. The review covered the description of the influence of operating conditions and promoters on the LT-SCR performance. The reaction mechanism, reaction intermediates, and active sites are also discussed in detail using isotopic labelling and in situ FT-IR studies.

182 citations

Journal ArticleDOI
TL;DR: In this article, a novel CuInS2 quantum dots (CIS-QDs)/Bi2WO6 3D composites were successfully synthesized through facile deposition process, followed by low temperature calcination.
Abstract: Novel CuInS2 quantum dots (CIS-QDs)/Bi2WO6 3D composites were successfully synthesized through a facile deposition process, followed by low temperature calcination. The ternary p-type CIS-QDs were highly dispersed onto the surface of flower-like n-type Bi2WO6 nanosheets to form p-n heterojunction and simultaneously tune the behaviors of photogenerated charge carriers, resulting in higher photocatalytic efficiencies of toluene degradation and Cr(VI) reduction under visible light irradiation, which are 3.0 and 8.5 times higher than those of Bi2WO6, respectively. The photoelectrochemical investigations indicate that the introduction of CIS-QDs synergistically enhanced the harvesting efficiency of solar energy in the p-n heterojunction system with the internal electric field and reduced transfer barrier of photoinduced charge carriers by forming the unique Bi S bonds between the CIS-QDs and flower-like Bi2WO6.

175 citations

References
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Journal ArticleDOI
TL;DR: This review attempts to summarize the recent progress in the rational design and fabrication ofheterojunction photocatalysts, such as the semiconductor-semiconductor heterojunction, the semiconductors-metal heterojunctions, the silicon-carbon heteroj junction and the multicomponent heteroj conjunction.
Abstract: Semiconductor-mediated photocatalysis has received tremendous attention as it holds great promise to address the worldwide energy and environmental issues. To overcome the serious drawbacks of fast charge recombination and the limited visible-light absorption of semiconductor photocatalysts, many strategies have been developed in the past few decades and the most widely used one is to develop photocatalytic heterojunctions. This review attempts to summarize the recent progress in the rational design and fabrication of heterojunction photocatalysts, such as the semiconductor–semiconductor heterojunction, the semiconductor–metal heterojunction, the semiconductor–carbon heterojunction and the multicomponent heterojunction. The photocatalytic properties of the four junction systems are also discussed in relation to the environmental and energy applications, such as degradation of pollutants, hydrogen generation and photocatalytic disinfection. This tutorial review ends with a summary and some perspectives on the challenges and new directions in this exciting and still emerging area of research.

3,013 citations

Journal ArticleDOI
TL;DR: In this paper, the open literature concerning chemical and mechanistic aspects of the selective catalytic reduction of NO by ammonia (SCR process) on metal oxide catalysts is reviewed, and the results of spectroscopic studies of the adsorbed surface species, adsorption-desorption measurements, flow reactor and kinetic experiments are analyzed.
Abstract: The open literature concerning chemical and mechanistic aspects of the selective catalytic reduction of NO by ammonia (SCR process) on metal oxide catalysts is reviewed. Catalytic systems based on supported V2O5 (including the industrial TiO2-supported V2O5–WO3 and/or V2O5–MoO3 catalysts) and catalysts containing Fe2O3, CuO, MnOx and CrOx are considered. The results of spectroscopic studies of the adsorbed surface species, adsorption–desorption measurements, flow reactor and kinetic experiments are analyzed. The proposed reaction mechanisms are described and critically discussed. Points of convergence and of disagreement are underlined.

1,946 citations

Journal ArticleDOI
TL;DR: In this paper, a study of the core-electron X-ray photoelectron (X-p) spectra of the f0 compounds La2O3, LaMO3(M = Fe and Co), CeO2, and BaCeO3 is described.
Abstract: A study of the core-electron X-ray photoelectron (X-p.e.) spectra of the f0 compounds La2O3, LaMO3(M = Fe and Co), CeO2, and BaCeO3 is described. Results on the chelate species [La(tmhd)3] and [Ce(tmhd)4](tmhd = 2,2,6,6-tetramethylheptane-3,5-dionato) are included for comparison. Special precautions have been taken to ensure an optimal degree of surface purity of the samples. Satellite structure has been observed for the 4p, in addition to the 3d and 4d, signals in both the lanthanum(III) and cerium(IV) compounds. These satellites arc discussed in terms of coexcitations of the charge-transfer type, principally O 2p→ metal 4f transitions. In the cerium(IV) oxides the satellites are apparently due to energy-gain (representing ‘ shake-down ’) rather than energy-loss (shake-up) processes.

841 citations

Journal ArticleDOI
Songbo Wang1, Lun Pan1, Jiajia Song1, Wenbo Mi1, Ji-Jun Zou1, Li Wang1, Xiangwen Zhang1 
TL;DR: Using anatase TiO2, the most important and widely studied semiconductor, it is demonstrated that metal vacancies (VTi) can be introduced in undoped oxides easily, and the presence of VTi results in many novel physiochemical properties.
Abstract: Defects are critically important for metal oxides in chemical and physical applications. Compared with the often studied oxygen vacancies, engineering metal vacancies in n-type undoped metal oxides is still a great challenge, and the effect of metal vacancies on the physiochemical properties is seldom reported. Here, using anatase TiO2, the most important and widely studied semiconductor, we demonstrate that metal vacancies (VTi) can be introduced in undoped oxides easily, and the presence of VTi results in many novel physiochemical properties. Anatase Ti0.905O2 was synthesized using solvothermal treatment of tetrabutyl titanate in an ethanol–glycerol mixture and then thermal calcination. Experimental measurements and DFT calculations on cell lattice parameters show the unstoichiometry is caused by the presence of VTi rather than oxygen interstitials. The presence of VTi changes the charge density and valence band edge of TiO2, and an unreported strong EPR signal at g = 1.998 presents under room temperatu...

517 citations

Journal ArticleDOI
TL;DR: In this article, a series of Mn-Ni/TiO 2 catalysts were prepared by adopting incipient wetness technique and investigated for low-temperature SCR of NO with NH 3 in the presence of excess oxygen.

458 citations