Decoding the Reactivity Enhancement of Ln2Ce2O7 Compounds (Ln = Yb, Y, Tb, and Gd) for Soot Combustion: The Remarkable Contribution of Variable Valence A-Sites

The impact of variable valence A-sites on the redox property and reactivity of Ln2Ce2O7 compounds in soot particulate combustion has been investigated. It was observed that Yb2Ce2O7, Y2Ce2O7, and Gd2Ce2O7 formed a rare earth C-type phase, while Tb2Ce2O7 formed a solid solution phase. Both Tb2Ce2O7 and Yb2Ce2O7 possess dual valence state A-sites, resulting in significantly more surface vacancies. Additionally, the advantageous solid solution phase structure of Tb2Ce2O7 leads to even more surface vacancies than Yb2Ce2O7, which is crucial to generate active oxygen sites. Moreover, the introduction of NO into the reaction feed enhances combustion activity by producing active surface monodentate nitrates. A catalyst with higher numbers of surface vacancies exhibits improved NO oxidation ability and better NO2 utilization efficiency. Consequently, the Tb2Ce2O7 compound demonstrates not only the best soot combustion activity, but also an optimal NOx-assistance effect. Therefore, it is concluded that variable valence A-site is the intrinsic factor to improve the reactivity of Ln2Ce2O7 catalysts.

The controlled engineering of surface oxygen defects on Bi2Zr2O7 compounds for catalytic soot combustion by adjusting the preparation methods

The quantity of surface oxygen vacancies/defects is critical to promote the reactivity of metal oxide catalysts. Therefore, for the controlled engineering of Bi2Zr2O7 with rich surface defects for soot combustion, four different methods have been adopted. Bi2Zr2O7 compounds with a defective fluorite phase but with varied surface vacancy concentrations have been successfully synthesized by various methods. The best catalyst (Bi2Zr2O7-CP) was fabricated by a facile co-precipitation method. Both O2- and O22- were the active surface sites whose number positively correlated to the number of surface oxygen vacancies and determined the activity. Moreover, a sample with more surface vacancies usually had weaker Zr-O bonds, which could be the intrinsic factor to enhance the activity. In addition, a novel and simple method has been developed to accurately titrate the absolute amount of soot reactive oxygen sites and calculate the TOF values. In conclusion, by optimizing the preparation methods, Bi2Zr2O7 catalysts with rich surface defects can be tuned, which may help in designing more applicable soot oxidation catalysts.

Interfacial Electron Engineering of PdSn-NbN/C for Highly Efficient Cleavage of the C-C Bonds in Alkaline Ethanol Electrooxidation

The splitting of the C-C bonds of ethanol remains a key issue to be addressed, despite tremendous efforts made over the past several decades. This study highlights the enhancement mechanism of inexpensive NbN-modified Pd1 Sn3 -NbN/C towards the C-C bonds cleavage for alkaline ethanol oxidation reaction (EOR). The optimal Pd1 Sn3 -NbN/C delivers a catalytic activity up to 43.5 times higher than that of commercial Pd/C and high carbonate selectivity (20.5%) toward alkaline EOR. Most impressively, the Pd1 Sn3 -NbN/C presents good durability even after 25 200 s of chronoamperometric testing. The enhanced catalytic performance is mainly due to the interfacial interaction between PdSn and NbN, demonstrated by multiple structural characterization results. In addition, in situ ATR-SEIRAS (Attenuated total reflection-surface enhanced infrared absorption spectroscopy) results suggest that NbN facilitates the C-C bonds cleavage towards the alkaline EOR, followed by the enhanced OH adsorption to promote the subsequent oxidation of C1 intermediates after doping Sn. DFT (density functional theory) calculations indicate that the activation barriers of the C-H bond cleavage in CH3 CH2 OH, CH3 CHOH, CH3 CHO, CH3 CO, CH2 CO, and the C-C bond cleavage in CH3 CO, CH2 CO, CHCO are evidently reduced and the removal of adsorbed CH3 CO and CO becomes easier on the PdSn-NbN/C catalyst surface.

The Re2Sn2O7 (Re = Nd, Sm, Gd) on the enhancement of fire safety and physical performance of Polyolefin/IFR cable materials

Polyolefin (PO) cables used in confined spaces need to have low smoke, low heat release, low toxic gas release and excellent physical properties. In this work, a series of rare earth stannates Re₂Sn₂O₇ (RES, Re = Nd, Sm, Gd) with high temperature catalytic performance were prepared by hydrothermal method for synergistic flame retardant PO/IFR. The flame retardancy, heat release, smoke density, toxic gas release and physical properties of PO composites were thoroughly studied in detail. The RES could enhance the vertical burning rating and the limiting oxygen index (LOI) of PO/IFR composites. Moreover, the residual char of the thermogravimetric analysis increased from 9.7% to 11.4 wt% after the RES added in PO/IFR system. Interestingly, the PO/IFR system containing Gd₂Sn₂O₇ exhibits the lowest peak heat release rate of 233.7 kW/m². Excellent flame resistance due to the formation of a complete and compact protective char layer. In addition, the toxic release of PO during combustion is also effectively reduced by introducing the RES. The tube furnace combustion test shows that the emission of carbon oxide (CO) and hydrogen cyanide (HCN) of PO/IFR/Gd₂Sn₂O₇ are the lowest. It can be attributed to the catalytic effect of rare earth elements and the blocking effect of the dense char layer. In addition, compared with the PO/IFR composites, the PO/IFR/RES system demonstrate higher mechanical properties and volume resistivity. Therefore, the addition of RES has a positive effect on improving the physical properties and fire safety properties of the PO/IFR cable composites, especially suitable for using in confined spaces.

MnOx Supported on Hierarchical SAPO-34 for the Low-Temperature Selective Catalytic Reduction of NO with NH3: Catalytic Activity and SO2 Resistance

The ethanol dispersion method was employed to synthesize a series of MnOx/SAPO-34 catalysts using SAPO-34 with the hierarchical pore structure as the zeolite carrier, which were prepared by facile acid treatment with citric acid. Physicochemical properties of catalysts were characterized by XRD, XPS, BET, TEM, NH3-TPD, SEM, FT-IR, Py-IR, H2-TRP and TG/DTG. NH3-SCR performances of the hierarchical MnOx/SAPO-34 catalysts were evaluated at low temperatures. Results show that citric acid etching solution at a concentration of 0.1 mol/L yielded a hierarchical MnOx/SAPO-34-0.1 catalyst with ca.15 wt.% Mn loading, exhibiting optimal catalytic activity and SO2 tolerance at low temperatures. Almost 100% NO conversion and over 90% N2 selectivity at 120 °C under a gas hourly space velocity (GHSV) of 40,000 h−1 could be obtained over this sample. Furthermore, the NO conversion was still higher than 65% when 100 ppm SO2 was introduced to the reaction gas for 4 h. These could be primarily attributed to the large specific surface area, high surface acidity concentration and abundant chemisorbed oxygen species provided by the hierarchical pore structure, which could also increase the mass transfer of the reaction gas. This finding suggests that the NH3-SCR activity and SO2 poisoning tolerance of hierarchical MnOx/SAPO-34 catalysts at low temperatures can be improved by controlling the morphology of the catalysts, which might supply a rational strategy for the design and synthesis of Mn-based SCR catalysts.

Pyrochlore oxides as visible light-responsive photocatalysts

This perspective describes the use of pyrochlore oxides in photocatalysis with focus on the strategies to enhance their activity.

One-pot synthesis of layered mesoporous ZSM-5 plus Cu ion-exchange: Enhanced NH3-SCR performance on Cu-ZSM-5 with hierarchical pore structures

Hierarchical ZSM-5 zeolite with meso- and micro-pore structures was successfully prepared through a facile one-pot hydrothermal synthesis method using bifunctional template. After copper ion-exchange, it was applied for the selective catalytic reduction of NO with NH₃ (NH₃-SCR). Compared with conventional Cu-ZSM-5 catalyst containing only micropores, the hierarchical catalyst with ca. 2 wt.% Cu loading displayed significantly improved catalytic performance. Particularly, the hierarchical zeolite catalyst also displayed excellent hydrothermal stability and sulfur resistance that exhibited great potential in practical application. Characterization techniques such as XRD, N₂ physisorption, temperature programmed desorption/reduction (TPD/TPR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) were comprehensively used to reveal the relationship between zeolite structure and catalytic properties. It was concluded that the hierarchically porous structure could not only improve the mass transfer of reactant/product but also provide larger specific surface area, higher surface acidity, larger NO adsorption capacity. And we found that bidentate nitrate species was more active in Cu-ZSM-5-meso than Cu-ZSM-5-C, which were all beneficial to the NH₃-SCR reaction. This work can provide a guideline to design other high performance hierarchical zeolites with different crystalline structures (such as CHA, LTA) for efficient catalytic NOx removal processes.

Catalytic Oxidation of Soot on a Novel Active Ca-Co Dually-Doped Lanthanum Tin Pyrochlore Oxide

A novel active Ca-Co dually-doping pyrochlore oxide La_2−xCa_xSn_2−yCo_yO₇ catalyst was synthesized by the sol-gel method for catalytic oxidation of soot particulates. The microstructure, atomic valence, reduction, and adsorption performance were investigated by X-ray powder diffraction (XRD), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), H₂-TPR (temperature-programmed reduction), and in situ diffuse reflection infrared Fourier transformed (DRIFTS) techniques. Temperature programmed oxidation (TPO) tests were performed with the mixture of soot-catalyst under tight contact conditions to evaluate the catalytic activity for soot combustion. Synergetic effect between Ca and Co improved the structure and redox properties of the solids, increased the surface oxygen vacancies, and provided a suitable electropositivity for oxide, directly resulting in the decreased ignition temperature for catalyzed soot oxidation as low as 317 °C. The presence of NO in O₂ further promoted soot oxidation over the catalysts with the ignition temperature decreased to about 300 °C. The DRIFTS results reveal that decomposition of less stable surface nitrites may account for NO₂ formation in the ignition period of soot combustion, which thus participate in the auxiliary combustion process.

Highly dispersed Pd on macroporous SmMn2O5 mullite for low temperature oxidation of CO and C3H8

The catalytic behavior of a palladium catalyst supported on macroporous SmMn2O5 mullite (Pd/SMO-EG&M) for CO and C3H8 oxidation was measured under lean-burn conditions. Different analytical techniques including XRD, Raman, BET, CO chemisorption, SEM, FTEM, XPS, TPD, TPR and CO + O2 pulse were undertaken to evaluate its physical and chemical properties. It was concluded that the crystal structure, morphology and specific surface area (SSA) of SmMn2O5 remained unchanged after Pd addition. The Pd/SMO-EG&M exhibited a low complete transformation temperature for CO (105 °C) and C3H8 (350 °C) oxidation. Such remarkable oxidation activity was attributed to high Pd dispersion (38.4%), which improved the reducibility and mobility of oxygen species, as revealed by TPR and TPD measurements. The high activity of oxygen species for Pd/SMO-EG&M above 250 °C accelerated the oxidation capacity as well. In a word, our study indicates that the macroporous Pd-mullite catalyst has potential applications in exhaust purification for gasoline vehicle.

Ni/Ln2Zr2O7 (Ln = La, Pr, Sm and Y) catalysts for methane steam reforming: the effects of A site replacement

The Ln₂Zr₂O₇ phase varies from pyrochlore to defective fluorite with decreasing r_A/r_B, thus resulting in Ni/Ln₂Zr₂O₇ catalysts with improved performance.

H2 adsorption and dissociation on PdO(101) films supported on rutile TiO2 (110) facet: elucidating the support effect by DFT calculations

To explore metal oxide-support interactions and their effect, H2 adsorption and dissociation on PdO(101)/TiO2(110) films with different film thicknesses, in comparison with that on pure PdO(101) surface without TiO2(110) support, were studied by density functional theory calculation. A monolayer PdO(101) film supported on TiO2 facet shows different properties to a pure PdO(101) surface. On the monolayer PdO(101)/TiO2(110) film, TiO2 support leads to stronger molecular adsorption of H2 on coordinatively unsaturated Pd top sites than that on a pure PdO surface. H2 dissociation with the formation of OH was preferred thermodynamically but slightly unfavorable kinetically on the monolayer PdO film due to the TiO2 support effect. Graphical abstract On the monolayer PdO(101)/TiO2(110) film, the TiO2 support effect leads to stronger H2 molecular adsorption on coordinatively unsaturated Pd top sites than on pure PdO surface. H2 dissociation with the formation of OH is preferred thermodynamically but slightly unfavorable kinetically on the film due to the TiO2 support effect.

Mesoporous Y2Sn2O7 pyrochlore with exposed (111) facets: an active and stable catalyst for CO oxidation

Mesoporous Y₂Sn₂O₇ pyrochlore with exposed (111) facets was successfully synthesized via a simple hydrothermal method and showed superior catalytic performance for CO oxidation.

Thermally stable ultra-small Pd nanoparticles encapsulated by silica: elucidating the factors determining the inherent activity of noble metal catalysts

1.1 nm Pd nanoparticles embedded in silica nanospheres were prepared by an improved one-step reverse micelle method, which show superior activity and thermal stability. Pd active surface area is the determining factors for the activity.

Synthesis and characterization of Co–Al–Fe nonstoichiometric spinel-type catalysts for catalytic CO oxidation

CoAlFe nonstoichiometric spinel-type oxides derived from hydrotalcites precursors exhibit high activities for catalytic CO oxidation.