IrSn Bimetallic Clusters Confined in MFI Zeolites for CO Selective Catalytic Reduction of NOx in the Presence of Excess O2

We developed a simple strategy for preparing IrSn bimetallic clusters encapsulated in pure silicon zeolites via a one-pot hydrothermal synthesis by using diethylamine as a stabilizing agent. A series of investigations verified that metal species have been confined successfully in the inner of MFI zeolites. IrSn bimetallic cluster catalysts were efficient for the CO selective catalytic reduction of NOx in the presence of excess O2. Furthermore, the 13CO temperature-programmed surface reaction results demonstrated that NO2 and N2O could form when most of the CO was transformed into CO2 and that Sn modification could passivate CO oxidation on the IrSn bimetallic clusters, leading to more reductants that could be used for NOx reduction at high temperatures. Furthermore, SO2 can also influence the NOx conversion by inhibiting the oxidation of CO. This study provides a new strategy for preparing efficient environmental catalysts with a high dispersion of metal species.

Inexpensive and easily replicable precipitation of CuO nanoparticles for low temperature carbon monoxide and toluene catalytic oxidation

Herein CuO nanoparticles (NPs) with nanostructures were prepared by precipitation method using hydrate copper sulfate (CuSO₄.5H₂O) and sodium hydroxide followed by heat treatment at 400 °C. The as-prepared CuO NPs with nanostructures were investigated using X-ray diffraction (XRD), Fourier Transformed Infra-red spectroscopy (FTIR), Raman spectroscopy, Scanning electron microscopy (SEM), X-ray photochemical spectroscopy (XPS), Energy dispersive spectroscopy (EDS), and Ultra-violet-visible (UV-vis) spectroscopy. In order to evaluate the reducibility, temperature programmed reduction (H₂-TPR) was applied. More importantly, CuO NPs was successfully tested as catalyst towards the total conversion of carbon monoxide (CO) and toluene (C₇H₈). Both XRD and Raman analysis as well as FTIR show that the sample exhibited a monoclinic spinel structure. SEM images indicate that CuO NPs are well-covered by grains size exhibiting homogeneous morphology composed of very fine interconnected particles with an apparent porosity. The sample was made up of Cu and O, according to the XPS and EDS measurements. The band gap energy obtained from optical property analysis is ∼2.65 eV. The catalytic performances of CuO NPs can be assigned to the combined effects of crystal structure, morphology, surface oxygen mobility, redox property and the higher specific surface area (∼87 m²/g). More precisely XPS and H₂-TPR data suggests that, the conversion of CO and C₇H₈ over CuO NPs follows a Mars-van Krevelen type mechanism. More importantly CuO NPs catalysts is reusable and exhibited good stability in the prolonged isothermal test. Thus, CuO NPs is confirmed as an efficient and inexpensive catalysts for CO and C₇H₈ conversion at low temperatures.

Fabrication of Stable Cu-Ce Catalyst with Active Interfacial Sites for NOx Elimination by Flame Spray Pyrolysis

The complete conversion of NOx to harmless N2 without N2O formation is crucial for the control of air pollution, especially at low temperatures. Cu-based catalysts are promising materials due to their low cost and high activity in NO dissociation, even comparable to noble metals; however, they suffer from low stability. Here, we established a Cu-Ce catalyst in one step with strong metal–support interaction by the flame spray pyrolysis (FSP) method. Almost 100% NO conversion was achieved at 100 °C, and they completely transferred into N2 at a low temperature (200 °C) for the FSP-CuCe catalyst, exhibiting excellent performance in NO reduction by CO reaction. Moreover, the catalytic performance can stay stable, while 23% NO conversion was lost in the same condition for the one made by the co-precipitation (CP) method. This can be attributed to the synergistic effect of abundant active interfacial sites and more flexible surface oxygen created during the FSP process. The flame technology developed here provides an efficient way to fabricate strong metal–support interactions, exhibiting notable potential in the design of stable Cu-based catalysts.

Preparation and characterization of CeO2–Fe2O3 catalysts for the selective catalytic reduction of NO with CO

The catalytic performance of CeO2–Fe2O3 catalysts for the selective catalytic reduction of NO by CO was studied.

The enhancement of NH3-SCR performance for CeO2 catalyst by CO pretreatment

CO pretreatment was found to effectively improve the SCR performance of CeO₂, with over 90% at about 300 °C. The larger specific area and the decrease of CeO₂ crystallization indicated the modification of the surface structure after CO pretreatment. Abundant Ce³⁺ species and active oxygen, better reducibility, and the higher surface adsorption capacity were mainly responsible for its enhanced SCR performance. DRIFT analysis revealed the presumed coexistence of two reaction routes that the L-H mechanism was related to the reaction temperature, while the reaction rate of E-R route was almost directly proportional to the NO concentration at a certain temperature, based on the kinetic calculation. In addition, the CO-pretreated CeO₂ also exhibited a better poisoning tolerance for SO₂ and H₂O and excellent thermal stability and circularity. Graphical abstract The process of NH₃-SCR reaction over CeO₂-CO catalyst.

A review of the catalysts used in the reduction of NO by CO for gas purification

The reduction of NO by the CO produced by incomplete combustion in the flue gas can remove CO and NO simultaneously and economically. However, there are some problems and challenges in the industrial application which limit the application of this process. In this work, noble metal catalysts and transition metal catalysts used in the reduction of NO by CO in recent years are systematically reviewed, emphasizing the research progress on Ir-based catalysts and Cu-based catalysts with prospective applications. The effects of catalyst support, additives, pretreatment methods, and physicochemical properties of catalysts on catalytic activity are summarized. In addition, the effects of atmosphere conditions on the catalytic activity are discussed. Several kinds of reaction mechanisms are proposed for noble metal catalysts and transition metal catalysts. Ir-based catalysts have an excellent activity for NO reduction by CO in the presence of O₂. Cu-based bimetallic catalysts show better catalytic performance in the absence of O₂, in that the adsorption and dissociation of NO can occur on both oxygen vacancies and metal sites. Finally, the potential problems existing in the application of the reduction of NO by CO in industrial flue gas are analyzed and some promising solutions are put forward through this review.

Insights into the precursor effect on the surface structure of γ-Al2O3 and NO + CO catalytic performance of CO-pretreated CuO/MnOx/γ-Al2O3 catalysts

NO reduction by CO was investigated over CO-pretreated CuO/MnO_x/γ-Al₂O₃ catalysts with different metal precursors (nitrate and acetate). It was found that the catalyst prepared from acetate salts (Cu/Mn/Al-A) exhibited significantly higher activity than counterpart catalyst from nitrate precursors (Cu/Mn/Al-N). XRD, XPS and in situ DRIFT were carried out to approach the nature for the different catalytic performance. For both catalysts, copper mainly existed as CuO, but the status of manganese oxide was markedly different. Mn(IV) was predominant in Cu/Mn/Al-N and Mn(III) was enriched in Cu/Mn/Al-A. As a result, different dispersion behaviors of manganese oxide on γ-Al₂O₃ were displayed, which induced inconsistent Cu-Mn contact. The catalyst obtained from acetate precursor exhibited enriched Cu-Mn contact and thus more Cu⁺-□-Mn^3+/2+ entities would be produced after CO pretreatment, leading to promoted NO dissociation and favorable performance in NO reduction by CO. The present study sheds light on the effective tuning of Cu-O-Mn interfacial sites in CuO/MnO_x/γ-Al₂O₃ via modulating the dispersion behaviors of surface components.

Mn-Modified CuO, CuFe2O4, and γ-Fe2O3 Three-Phase Strong Synergistic Coexistence Catalyst System for NO Reduction by CO with a Wider Active Window

A series of samples with the precursor's molar ratio of {KMn₈O₁₆}/{CuFe₂O₄} = 0, 0.008, 0.010, 0.016, and 0.020 were successfully synthesized for selective catalytic reduction of NO by CO. The physicochemical properties of all samples were studied in detail by combining the means of X-ray photoelectron spectroscopy, H₂-temperature-programmed reduction, scanning electron microscopy mapping, X-ray diffraction (XRD), N₂ physisorption (Brunauer-Emmett-Teller), NO + CO model reaction, and in situ Fourier transform infrared spectroscopy techniques. The results show that three phases of γ-Fe₂O₃, CuFe₂O₄, and CuO, which have strong synergistic interaction, coexist in this catalyst system, and different phases play a leading role in different temperature ranges. Mn species are highly dispersed in the three-phase coexisting system in the form of Mn²⁺, Mn³⁺, and Mn⁴⁺. Because of the strong interaction between Mn²⁺ and Fe species, a small amount of Cu²⁺ precipitates from CuFe₂O₄ and grows along the CuO(110) plane, which has better catalytic performance. Mn³⁺ can inhibit the conversion of γ-Fe₂O₃ to α-Fe₂O₃ at high temperature and then increases the high-temperature activity. The synergistic effect between Mn⁴⁺ and the surfaces of three phases generates active oxygen species Cu²⁺-O-Mn⁴⁺ and Mn⁴⁺-O-Fe³⁺, which can be more easily reduced to some synergistic oxygen vacancies during the reaction. Furthermore, the formed synergistic oxygen vacancies can promote the dissociation of NO and are also propitious to the transfer of oxygen species. All of these factors make the appropriate manganese-modified three-phase coexisting system have better catalytic activity than the manganese-free catalyst, making NO conversion rate reach 100% at around 250 °C and maintain to 1000 °C. Combining comprehensive analysis of various characterization results and in situ infrared as well as XRD results in the equilibrium state, a new possible NO + CO model reaction mechanism was temporarily proposed to further understand the catalytic processes.

NO x reduction by CO over ASC catalysts in a simulated rotary reactor: effect of CO2, H2O and SO2

The influence of CO₂, H₂O and SO₂ on the NO reduction by CO over Fe/Co activated semi-coke catalyst was investigated in a simulated rotary reactor. The results showed that, in the simulated rotary reactor, the influence of CO₂ and H₂O on the NO adsorption was significant at low temperatures, and the inhibition became weak when increasing the temperature. However, the NO adsorption efficiency could not be improved by increasing temperature after catalyst sulfur poisoning. The heavily inhibited NO adsorption process, which was due to the competitive adsorption and formation of the sulfate, resulted in a low NO reduction efficiency in the presence of CO₂, H₂O or SO₂. The in situ DRIFT study showed that the dominant effect of CO₂, H₂O and SO₂ on the NO adsorption was the inhibition of the free nitrate ions formation. In addition, the introduction of CO₂, H₂O and SO₂ could not change the route of NO reduction, but just reduced the degree of the NO + CO reduction.

The influence of CO₂, H₂O and SO₂ on the NO reduction by CO over Fe/Co activated semi-coke catalyst was investigated in a simulated rotary reactor.

Effects of the Fe/Ce ratio on the activity of CuO/CeO2–Fe2O3 catalysts for NO reduction by CO

Copper catalysts on Fe-loaded ceria were studied for NO reduction by CO.

Investigation of SO2tolerance of Ce-modified activated semi-coke based catalysts for the NO + CO reaction

The SO₂tolerance mechanism of Ce-modified activated semi-coke based catalysts for the NO + CO reaction.

In situ DRIFTS study of the NO + CO reaction on Fe–Co binary metal oxides over activated semi-coke supports

Activated semi-coke loaded with Fe and Co species by a hydrothermal method exhibited excellent CO-deNO_x performance.

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

The promotional effect of SnO₂ in a Ce/TiO₂ catalyst for NH₃-SCR reaction.

Searching for cheaper catalysts with high activity and stability in Ce–M–O systems (M = Fe, Co, Ni)

A cheaper catalyst with enhanced catalytic activity and stability was investigated among Ce–M–O (M = Fe, Co, Ni) systems.