Showing papers on "Mixed oxide published in 2017"

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

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.

MOF-Derived Hollow Cage Nix Co3-x O4 and Their Synergy with Graphene for Outstanding Supercapacitors.

Abstract: Highly optimized nickel cobalt mixed oxide has been derived from zeolite imidazole frameworks. While the pure cobalt oxide gives only 178.7 F g-1 as the specific capacitance at a current density of 1 A g-1 , the optimized Ni:Co 1:1 has given an extremely high and unprecedented specific capacitance of 1931 F g-1 at a current density of 1 A g-1 , with a capacitance retention of 69.5% after 5000 cycles in a three electrode test. This optimized Ni:Co 1:1 mixed oxide is further used to make a composite of nickel cobalt mixed oxide/graphene 3D hydrogel for enhancing the electrochemical performance by virtue of a continuous and porous graphene conductive network. The electrode made from GNi:Co 1:1 successfully achieves an even higher specific capacitance of 2870.8 F g-1 at 1 A g-1 and also shows a significant improvement in the cyclic stability with 81% capacitance retention after 5000 cycles. An asymmetric supercapacitor is also assembled using a pure graphene 3D hydrogel as the negative electrode and the GNi:Co 1:1 as the positive electrode. With a potential window of 1.5 V and binder free electrodes, the capacitor gives a high specific energy density of 50.2 Wh kg-1 at a high power density of 750 W kg-1 .

Remarkable activity and stability of Ni catalyst supported on CeO2-Al2O3 via CeAlO3 perovskite towards glycerol steam reforming for hydrogen production

Abstract: The effects of CeO 2 -Al 2 O 3 mixed oxide preparation methods on the physical and chemical properties of CeO 2 -Al 2 O 3 , and the catalytic performance toward glycerol steam reforming (GSR) over the CeO 2 -Al 2 O 3 -supported Ni catalysts were investigated. Three CeO 2 -Al 2 O 3 supports were prepared by a single-step sol-gel (SS) of ceria and alumina precursors, followed by the impregnations of ceria precursors on both a xerogel Al 2 O 3 (IS) and a commercial Al 2 O 3 (IC). The characterization techniques used were N 2 adsorption-desorption, H 2 -chemisorption, XRD, H 2 -TPR, NH 3 -TPD, XANES, XPS and TGA. The sol-gel derived supports generate not only a well-developed mesoporous structure, but also the incorporation of the Ce species into the Al 2 O 3 structure forming CeAlO 3 perovskite. This formation suppresses the interaction between Ni and Al 2 O 3 , thereby increasing the number of active and Bronsted acid sites to improve the bifunctional metal-acid properties of Ni/CeO 2 -Al 2 O 3 in the hydrogenolysis and dehydrogenation-dehydration of condensable intermediates that produce more H 2 . The formation of CeAlO 3 is remarkably promoted by the single-step sol-gel preparation method, and the catalyst supported on the SS-derived oxide exhibits excellent activity and strong resistances to coke formation, phase transformation, and nickel sintering.

A highly effective catalyst of Co-CeO2 for the oxidation of diesel soot: The excellent NO oxidation activity and NOx storage capacity

Abstract: In the catalytic combustion of diesel soot, the role of NO in the exhaust emission and the simultaneous reduction of NO by soot are two fascinating problems. Herein, the Co-CeO2 catalyst prepared by the citrate acid sol-gel method was developed to catalyze the NOx-assisted soot oxidation. The effect of the Co amount on the physicochemical and catalytic properties of the Co-CeO2 catalysts was investigated in detail. When the molar ratio of Co/Ce was 50/50, the Co50Ce50 mixed oxide catalyst exhibited the lowest soot oxidation temperature (Tm of 335 °C) in the feed gas of NO + O2, and the presence of water can improve obviously the catalytic combustion of soot with Tm of 310 °C. The presence of Co can improve the reduction of surface oxygen of ceria, and Ce can promote the reduction of Co3O4 to CoO, resulting in the enhancement of the catalytic activity of Co-CeO2 for NO oxidation and its capacity for NO2 storage as the surface nitrates. The soot oxidation on the Co-CeO2 catalyst can be promoted by “NO2-assistance”: the nitrate species are preferentially formed on the Co active sites and stored on the catalyst surface, and then NO2 produced by NO oxidation and the decomposition of nitrate stored on the surface at lower temperature, reacts with soot. That is, the stored NOx species initiates the beginning of soot oxidation, and the NO oxidation ability of the catalyst is responsible for the extensive soot oxidation by NO + O2, in which NO is reduced by carbon in soot combustion. The combination of high NO oxidation activity and high NO2 storage capacity makes the Co50Ce50 oxide catalyst as an efficient catalyst for the NOx-assisted soot oxidation.

Rock Salt Ni/Co Oxides with Unusual Nanoscale-Stabilized Composition as Water Splitting Electrocatalysts

Abstract: The influence of nanoscale on the formation of metastable phases is an important aspect of nanostructuring that can lead to the discovery of unusual material compositions. Here, the synthesis, structural characterization, and electrochemical performance of Ni/Co mixed oxide nanocrystals in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is reported and the influence of nanoscaling on their composition and solubility range is investigated. Using a solvothermal synthesis in tert-butanol ultrasmall crystalline and highly dispersible Ni x Co1− x O nanoparticles with rock salt type structure are obtained. The mixed oxides feature non-equilibrium phases with unusual miscibility in the whole composition range, which is attributed to a stabilizing effect of the nanoscale combined with kinetic control of particle formation. Substitutional incorporation of Co and Ni atoms into the rock salt lattice has a remarkable effect on the formal potentials of NiO oxidation that shift continuously to lower values with increasing Co content. This can be related to a monotonic reduction of the work function of (001) and (111)-oriented surfaces with an increase in Co content, as obtained from density functional theory (DFT+U) calculations. Furthermore, the electrocatalytic performance of the Ni x Co1− x O nanoparticles in water splitting changes significantly. OER activity continuously increases with increasing Ni contents, while HER activity shows an opposite trend, increasing for higher Co contents. The high electrocatalytic activity and tunable performance of the nonequilibrium Ni x Co1− x O nanoparticles in HER and OER demonstrate great potential in the design of electrocatalysts for overall water splitting.

Highly dispersed CuyAlOx mixed oxides as superior low-temperature alkali metal and SO2 resistant NH3-SCR catalysts

Abstract: For NOx removal from the exhaust gases of municipal solid waste (MSW) incinerators by selective catalytic reduction (SCR) technology, a suitable SCR catalyst which is active at low temperatures and robust to the presence of alkali metals and SO2 is highly desired. In this contribution, we report the successful fabrication of a highly dispersed CuyAlOx (y = 2–4) mixed oxides for NH3-SCR catalyst using high surface area, flower-like highly dispersed AMO-Cu-Al-CO3 LDHs precursors. The influence of the Cu/Al ratio (2, 3, 4, and 5), calcination temperature (400, 500, 600, and 700 °C), and testing temperature (150, 200, 250, and 300 °C) on the activity of the CuyAlOx mixed oxide catalysts were systematically investigated. Among all samples, Cu4AlOx showed the highest NOx conversion of 91.1% at 200 °C. After being thermally treated at 700 °C, the NOx conversion of Cu4AlOx was still as high as 84.7%, which is much higher than that of the control catalyst 10 wt% CuO/γ-Al2O3 (57.5%). XRD and HR-TEM analyses suggested that the highly dispersed CuO nanoparticles are the active species for the SCR reaction. The catalytic De-NOx performance of Cu4AlOx in the presence of alkali metals (K and Na) and SO2 was also studied. In the presence of 50 ppm SO2, the NOx conversion of Cu4AlOx (78.4%) was much higher than that of CuO/γ-Al2O3 (48%). The selectivity of NOx conversion to N2 and resistance to H2O (and co-existence of H2O and SO2) for Cu4AlOx catalyst were also evaluated. In all, we have demonstrated that the newly obtained Cu4AlOx catalyst not only possesses higher thermal stability and higher low temperature (150–250 °C) catalytic activity, but also has much better alkali metal (K and Na), SO2, and H2O resistance than a conventional CuO/γ-Al2O3 catalyst.

Effect of calcination temperature on the structure and catalytic performance of copper–ceria mixed oxide catalysts in phenol hydroxylation

Abstract: We report on highly active CuO@CeO2 catalysts prepared by the surfactant-template method and calcined at different temperatures. Then the obtained catalysts were characterized by means of various analytical techniques. Our findings show that the BET surface area and pore volume of the CuO@CeO2 catalyst measured by N2 adsorption–desorption are decreasing with the elevation of calcination temperature. From the results of XRD and XPS, we determined the oxidation state of copper in the copper–ceria mixed oxide catalysts. The CuO@CeO2 catalysts displayed good catalytic activity for the phenol hydroxylation using H2O2 as an oxidant. Moreover, we found that the catalytic activity is improved for high calcining temperature and the optimum conditions were obtained when the catalyst CuO@CeO2 is calcined at 800 °C, which lead to higher phenol conversion of 54.62% with 92.87% of selectivity for catechol and hydroquinone. More importantly, the catalyst seems to be easily recovered by simple centrifugation. The results of catalyst recycling illustrated that the catalytic activity remained high even after five cycles with slight Cu leaching and slight loss of activity. Finally, a possible mechanism in phenol hydroxylation by H2O2 over CuO@CeO2 catalyst was also proposed.

Rh-MnO Interface Sites Formed by Atomic Layer Deposition Promote Syngas Conversion to Higher Oxygenates

Abstract: Rhodium (Rh) catalysts are among the major candidates for syngas conversion to higher oxygenates (C2+oxy), with manganese (Mn) as a commonly used promoter for enhancing the activity and selectivity toward C2+oxy. In this study, we use atomic layer deposition (ALD) to controllably modify Rh catalysts with MnO, by depositing manganese oxide as a support layer or an overlayer, in order to identify the function of the Mn promoter. We also compare the ALD-modified catalysts with those prepared by coimpregnation. An ultrathin MnO support layer shows the most effective enhancement for C2+oxy production. Transmission electron microscopy, temperature-programmed reduction, and diffuse reflectance infrared Fourier transform spectroscopy characterization indicates that formation of Rh–MnO interface sites is responsible for the observed activity and selectivity improvements, while ruling out Rh nanoparticle size and alloy or mixed oxide formation as significant contributors. MnO overlayers on Rh appear to suffer from ...

Toluene catalytic combustion over copper modified Mn0.5Ce0.5 Ox solid solution sponge-like structures

Abstract: Catalytic combustion is a well-established way to remove the pollution of volatile organic compounds (VOCs), and the highly active catalyst is the key issue of this process. Herein, a series of sponge-like ternary mixed oxide were prepared by a facile surfactant modified coprecipitation approach. The structure properties were characterized via BET, XRD, Raman, TEM, EDS, H2-TPR, and XPS techniques. It was found that the Cu content significantly influences the catalytic activity in toluene combustion. The CMC-2 (Cu:Mn:Ce = 1:5:5) showed the best catalytic performance under the air atmosphere, and the complete conversion of toluene was achieved at 240 °C. The characterization reveals that a proper amount of Cu facilitates the incorporation of highly reducible Mn4+ into ceria lattice to form sponge-like MnCeOx solid solution structures. Such promotion effect of copper can improve the redox behavior, produce large amounts of Oα at the interface of CuOx/MnCeOx, both of which benefit the toluene combustion at low temperature. The findings of this work highlight the importance of Cu in preparation of the active MnOx−CeO2 catalyst for VOCs combustion.

Nanotubular Iridium-Cobalt Mixed Oxide Crystalline Architectures Inherited from Cobalt Oxide for Highly Efficient Oxygen Evolution Reaction Catalysis.

Abstract: Here, we report the unique transformation of one-dimensional tubular mixed oxide nanocomposites of iridium (Ir) and cobalt (Co) denoted as IrxCo1–xOy, where x is the relative Ir atomic content to the overall metal content. The formation of a variety of IrxCo1–xOy (0 ≤ x ≤ 1) crystalline tubular nanocomposites was readily achieved by electrospinning and subsequent calcination process. Structural characterization clearly confirmed that IrxCo1–xOy polycrystalline nanocomposites had a tubular morphology consisting of Ir/IrO2 and Co3O4, where Ir, Co, and O were homogeneously distributed throughout the entire nanostructures. The facile formation of IrxCo1–xOy nanotubes was mainly ascribed to the inclination of Co3O4 to form the nanotubes during the calcination process, which could play a critical role in providing a template of tubular structure and facilitating the formation of IrO2 by being incorporated with Ir precursors. Furthermore, the electroactivity of obtained IrxCo1–xOy nanotubes was characterized for...

Revisiting the catalytic activity of a doped SrFeO3 for water pollutants removal: Effect of light and temperature

Abstract: A Sr0.85Ce0.15FeO3-δ perovskite-type mixed oxide was prepared by solution combustion synthesis from citric acid and carefully characterized for the structure by X-ray diffraction coupled with Rietveld analysis, for the microstructure/morphology by HRTEM and SEM, for texture by N2 uptake at low temperature, for surface charge by ζ potential measurements and for reactive species by Electron Paramagnetic Resonance (EPR). Photocatalytic activity was tested in the presence of simulated solar light towards Orange II and Rhodamine B as model pollutants. Despite this compound is known as photocatalyst, a good reactivity was evidenced in the 55–80 °C temperature range without light irradiation, indicating that it can work as well in the dark after thermal-activation. Therefore, experimental data obtained in this work suggest to carefully reconsidering the role of this mixed oxide as photocatalyst. EPR spin-trapping tests allowed to deepen the aspects related to reactive species generated by Sr0.85Ce0.15FeO3-δ, revealing that the temperature increase promotes hydroxyl radicals and singlet oxygen production. Furthermore, an interesting selectivity towards the degradation of different dyes was found.

Ambient-temperature NO oxidation over amorphous CrOx-ZrO2 mixed oxide catalysts: Significant promoting effect of ZrO2

Abstract: A series of novel CrOx-ZrO2 mixed oxide catalysts are prepared via a sol-gel method. Within a range of Cr/Zr atomic ratios, the mixed oxides maintain high surface area, homogeneous amorphous phases. As compared to CrOx-only catalysts formed using the same method, the addition of zirconia greatly enhances the catalytic performance for ambient-temperature, low-concentration NO oxidation. X-ray Photoelectron Spectroscopy (XPS) and Electron Paramagnetic Resonance (EPR) analyses indicate an electronic effect of ZrO2 addition to the oxidation state of Cr. That is, ZrO2 addition induces an increase in surface concentrations of Cr6+. Rapid deactivation of a pre-reduced catalyst, coupled with the fact that a deactivated catalyst contains lower concentrations of surface Cr6+, provide rather strong evidence for a Mars-van Krevelen NO oxidation mechanism. Such a mechanism is also consistent with in situ DRIFTS observations.

Isothermal and cyclic oxidation behavior of free standing MCrAlY coatings manufactured by high-velocity atmospheric plasma spraying

Abstract: In the present paper the high temperature oxidation behavior of a free standing NiCoCrAlY coating produced by high-velocity atmospheric plasma spraying (HV-APS) is investigated and compared with that produced by conventional low pressure plasma spraying (LPPS). Isothermal thermogravimetric experiments at 1000 and 1100 °C in synthetic air revealed a lower oxidation rate of the HV-APS than the LPPS coating. Both coatings formed oxide scales based on alpha alumina, however, in the LPPS coating incorporation of coarse mixed Y/Al-oxide pegs into the scale occurred, increasing the oxidation rate by providing short circuit paths for oxygen diffusion probably due to higher diffusivities in the mixed oxide and/or along the interfaces between mixed oxide and alumina. In the HV-APS coatings most of the yttrium was tied-up in sub-μm Y-containing oxide particles and only minor amounts of mixed Y/Al oxide precipitates were found in the alumina surface scale. Cyclic air oxidation tests at 1100 °C revealed a lower oxidation rate and better scale adherence for the HV-APS coating. The results thus show that HV-APS is a promising method for the processing of MCrAlY coatings. The specific yttrium distribution in form of fine oxide precipitates in the HV-APS material prevents the formation of deleterious Y-rich oxide pegs and promotes formation of a slowly growing, protective alumina scale with excellent adherence.

Influence of Mg–Al Mixed Oxide Compositions on Their Properties and Performance in Aldol Condensation

Abstract: The influence of chemical composition of Mg–Al mixed oxides on their properties and catalytic performance in aldol condensation of furfural and acetone was studied. Pure alumina, Mg–Al layered double hydroxides with a wide range of Mg/Al molar ratios from 0.5 to 15, and pure magnesia were prepared by the same synthesis method, and corresponding oxides were produced by calcination of the as-prepared samples at T = 450 °C. The physicochemical properties of the samples were investigated by XRD, N2 physisorption, SEM, CO2-TPD, and NH3-TPD, and their catalytic performance was evaluated in aldol condensation of furfural and acetone. SEM images revealed that the Al content in the samples influenced the size and the morphology of both large agglomerates and plate-like crystals. The CO2-TPD measurements proved that the concentration of basic sites and the distribution of their strengths was determined by the Mg/Al molar ratio in the Mg–Al mixed oxides. However, over the whole range of Mg/Al compositions studied he...