Showing papers on "Mixed oxide published in 2014"

Low-Temperature Selective Catalytic Reduction of NOx with NH3 over Novel Mn−Zr Mixed Oxide Catalysts

Abstract: Novel Mn–Zr mixed oxide catalysts have been prepared by the citric acid method for the low-temperature selective catalytic reduction (SCR) of NOx with ammonia in the presence of excess oxygen. They have been characterized by a series of techniques, specifically N2 adsorption–desorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). It was found that an Mn(0.5)–ZrOx-450 (Mn/(Mn + Zr) mole ratio of 0.5) catalyst showed the highest activity, giving 100% NOx conversion at 100 °C with a space velocity of 30 000 h–1. XRD results suggested that an Mn–Zr solid solution was formed in the Mn(0.5)–ZrOx-450 catalyst, with highly dispersed MnOx. TPR data indicated a strong interaction between the zirconium oxide and manganese oxide, which improved the reduction ability of the MnOx. The TPD results indicated that an appropriate NH3 adsorption ability was beneficial for the low-temperature SCR. The catalyst showed a c...

One-Step Conversion of Biomass-Derived 5-Hydroxymethylfurfural to 1,2,6-Hexanetriol Over Ni-Co-Al Mixed Oxide Catalysts Under Mild Conditions

Abstract: The conversion of biomass-derived 5-hydroxymethylfurfural (HMF) was examined over Ni Co Al mixed oxide catalysts derived from corresponding hydrotalcite-like compounds (HTlcs). 1,2,6-Hexanetriol (1,2,6-HT) was obtained in 64.5% yield under mild reaction conditions. The catalysts were characterized by X-ray. powder diffraction (XRD), CO2 temperature-programthed desorption (CO2 TPD), N2 physical adsorption, and H2 temperatureprogrammed reduction (H2 TPR), and the reaction product distribution was correlated with the catalyst composition and reaction conditions. The reaction pathway was proposed based on the results. In all cases, the conversion of HMF proceeds according to a pathway that begins with the aldehyde group being hydrogenated to form 2,5-dihydroxymethylfuran (2,5-DHF). This product then undergoes a ring-opening reaction to form 1,2,6-HT. A synergetic effect between Ni and Co was observed, which significantly promoted catalytic activity and selectivity.

Cu-Based Catalyst Resulting from a Cu,Zn,Al Hydrotalcite-Like Compound: A Microstructural, Thermoanalytical, and In Situ XAS Study

Abstract: A Cu-based methanol synthesis catalyst was obtained from a phase pure Cu,Zn,Al hydrotalcite-like precursor, which was prepared by co-precipitation. This sample was intrinsically more active than a conventionally prepared Cu/ZnO/Al2O3 catalyst. Upon thermal decomposition in air, the [(Cu0.5Zn0.17Al0.33)(OH)2(CO3)0.17]⋅mH2O precursor is transferred into a carbonate-modified, amorphous mixed oxide. The calcined catalyst can be described as well-dispersed "CuO" within ZnAl2 O4 still containing stabilizing carbonate with a strong interaction of Cu(2+) ions with the Zn-Al matrix. The reduction of this material was carefully analyzed by complementary temperature-programmed reduction (TPR) and near-edge X-ray absorption fine structure (NEXAFS) measurements. The results fully describe the reduction mechanism with a kinetic model that can be used to predict the oxidation state of Cu at given reduction conditions. The reaction proceeds in two steps through a kinetically stabilized Cu(I) intermediate. With reduction, a nanostructured catalyst evolves with metallic Cu particles dispersed in a ZnAl2 O4 spinel-like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7 nm) of this catalyst are partially embedded leading to lower absolute activity in comparison with a catalyst comprised of less-embedded particles. Interestingly, the exposed Cu surface area exhibits a superior intrinsic activity, which is related to a positive effect of the interface contact of Cu and its surroundings.

Ce-substituted LaNiO3 mixed oxides as catalyst precursors for glycerol steam reforming

Abstract: Mixed oxides of general formula La 1− x Ce x NiO 3 were prepared by self-combustion method and taken as precursors of Ni-supported catalysts for an experimental investigation of steam reforming of glycerol. The mixed oxide precursors and the corresponding derived fresh catalysts were characterized by XRD, TPR and TEM while the used catalysts were investigated by FE-SEM, TEM and TPO/TGA-MS. All catalysts were investigated on glycerol steam reforming with special focus on the resistance to deactivation. The precursors led to active catalysts, producing hydrogen-rich gas stream. Deactivation was seen to occur irrespective of catalyst composition. Substitution of 50% La by Ce provided more deactivation-resistant catalyst, allowing only minor amount of more reactive carbon deposits. It was suggested to be associated with the formation of CeO 2 –La 2 O 3 solid solutions, as evidenced by XRD, which would act as an oxygen buffer and therefore facilitate the removal of carbon deposits onto the catalyst surface.

Synthesis of glycerol carbonate from glycerol and urea over tin-tungsten mixed oxide catalysts

Abstract: Tin-tungsten mixed oxide catalysts with varying their mole ratio were prepared by co-precipitation method. The catalysts physico-chemical properties were derived from FT-infrared, Laser Raman, X-ray diffraction, UV–Vis DRS, BET surface area and temperature-programmed desorption of NH3. The catalysts activities were evaluated for the synthesis of glycerol carbonate from glycerol and urea. The activity results showed that Sn-W mixed oxide catalysts are highly active for selective formation of glycerol carbonate. Sn-W catalyst with 2:1 molar ratio exhibited about 52% of glycerol conversion with >95% selectivity towards glycerol carbonate. The active catalyst was subjected to calcination at different temperatures and evaluated for their activity in glycerol carbonate synthesis. The activity of the catalysts depends on mole ratio of Sn/W and treatment temperature which are influencing the surface-structural characteristics of the catalysts. Different reaction parameters such as glycerol to urea molar ratio, reaction temperature and catalyst loading were studied and optimum conditions were established. The catalysts showed consistent activity upon repeated use.

Aerobic Oxidation of Biomass-Derived 5-(Hydroxymethyl)furfural into 2,5-Diformylfuran Catalyzed by the Trimetallic Mixed Oxide (Co–Ce–Ru)

Abstract: The paper deals with oxidation of the biomass-derived model molecule 5-(hydroxymethyl)furfural (HMF) catalyzed by the trimetallic mixed oxide RuCo(OH)2CeO2. The catalyst RuCo(OH)2CeO2 was prepared through alkali hydrolysis of RuCl3, Co(NO3)2, and Ce(NO3)2 and characterized by X-ray diffraction and transmission electron microscopy techniques. RuCo(OH)2CeO2 showed high catalytic activity for aerobic oxidation of HMF under mild conditions (in the case of atmospheric oxygen pressure). Various reaction parameters such as the reaction temperature, catalyst amount, solvent, and oxidant were explored. Results demonstrated that the oxidant and solvent showed a remarkable effect on the aerobic oxidation of HMF to 2,5-diformylfuran (DFF). Under optimal conditions, DFF was obtained in a high yield of 82.6% with HMF conversion of 96.5% after 12 h at 120 °C. More importantly, the catalyst could be reused several times without significant loss of its catalytic activity.

Nanocrystalline Ce1−xRuxO2 – Microstructure, stability and activity in CO and soot oxidation

Abstract: Nanocrystalline (4–5 nm) Ce 1− x Ru x O 2 mixed oxides ( x = 0.03–0.16) were synthesized using water in oil microemulsion method. Morphology, microstructure and a phase composition of the samples subjected to heat treatment in oxidizing and reducing atmosphere were investigated by TEM, SEM-EDS-EBSD, XRD and XPS. Oxide with x = 0.11 was structurally stable in oxidizing atmosphere up to 550 °C but above this temperature it decomposed into Ru deficient, nanosized Ce 1− x Ru x O 2 and large (few μm) RuO 2 crystals. No phase separation was observed for Ce 0.97 Ru 0.03 O 2 even after heating at 800 °C. Doping with Ru decreases the size of ceria particles and strongly hinders their sintering at high temperatures. In hydrogen atmosphere a segregation of small (∼1 nm) Ru crystallites occurred at the surface of the Ce 0.89 Ru 0.11 O 2 mixed oxide. Only small increase of the mean crystallite size of Ru (to 2 nm) occurred after reduction at 1000 °C. The unique resistance of Ru to sintering is assigned to a special epitaxial orientation Ru (0 0 2)∥CeO 2 (1 1 1), which persisted up to the highest temperature of reduction, due to very strong surface bonding. Contrary to a situation in oxidizing atmosphere, doping with Ru had no significant effect on the sintering of ceria in hydrogen. Partial substitution of Ru for Ce strongly enhances the reducibility of ceria at low temperatures and its activity in catalytic combustion of CO and soot.

Synergistic Effects in Porous Mn–Co Mixed Oxide Nanorods Enhance Catalytic Deep Oxidation of Benzene

Abstract: A series of Mn–Co mixed oxide nanorods with homogeneous worm-like pores were facilely prepared by a sol–gel chelating method. With incorporating Mnn+ into Co3O4, the formation of solid solution with spinel structure inhibits the growth of nanoparticles which is benefit to keep smaller crystal size and higher surface area. XPS and H2-TPR results indicate that there are more high manganese valence (Mn3+ and Mn4+) and adsorbed oxygen species as well as low-temperature reducibility for the mixed oxide catalysts, as a result of the strong synergistic effect between Mn and Co species in solid solution, which will play a key role in catalytic activity. The as-prepared catalysts were used for catalytic deep oxidation of benzene which is a typical carcinogenic VOC. The catalytic activities over the mixed oxides with varied mole ratio are much higher than that on the single MnOx or Co3O4. The Mn5Co5 sample showed the best activity with T90% for benzene conversions into CO2 were low to 237 °C at a high space velocity of 120,000 mL g−1 h−1.

Molybdenum modified CeAlOx catalyst for the selective catalytic reduction of NO with NH3

Abstract: A series of MoO3 doped CeAlOx mixed oxide catalysts with different Mo/Al molar ratios were prepared by the simple coprecipitation method and used for selective catalytic reduction of NO with NH3. The Ce-Mo-AlOx catalyst with the Mo/Al molar ratio of 0.5 exhibited excellent activity and high H2O or/and SO2 poisoning resistance at 250 °C. XRD revealed that the molybdenum oxide existed in either highly dispersed or amorphous phases on the catalyst surface. BET analysis results showed that the total pore volume and the average pore diameter of the CeAlOx catalyst was improved by the addition of MoO3. As determined by the H2-TPR and NH3-TPD, the redox capacity and total acidity of the CeAlOx catalyst were also enhanced by the introduction of MoO3, which are critical for the NH3-SCR reaction. The SCR reaction mechanism was also studied by the in situ DRIFTS, the coordinated NH3 and ionic NH4+ species together with the monodentate and bidentate nitrate were active intermediates on the CeMo0.5AlOx catalyst surface during the NH3-SCR reaction.

Zinc/Lanthanum Mixed-Oxide Catalyst for the Synthesis of Glycerol Carbonate by Transesterification of Glycerol

Abstract: ZnO/La2O3 mixed oxides are prepared by a coprecipitation method at different molar ratios and used as catalysts for the synthesis of glycerol carbonate by transesterification of glycerol with dimethyl carbonate (DMC). X-ray diffraction, N2 adsorption, transmission electron microscopy, scanning electron microscopy, and thermal-programmed desorption methods were used for characterization of the catalysts. The surface area and porosity of the catalysts prepared by the precipitation method proved to be better than those prepared by combustion and modified-citrate methods. Zn4La1 and Zn2La1 (mole ratio Zn:La of 4:1 and 2:1, respectively) were found to be the best proportions in view of both higher activity and higher selectivity. The rates offered by these catalysts were substantially high compared to reported Mg/La and other catalysts. The effects of parameters such as the temperature, catalyst loading, and mole ratio of DMC to glycerol were examined, and a suitable kinetic model was proposed for the Zn4La1 c...

Stabilization of Catalytically Active Cu+ Surface Sites on Titanium–Copper Mixed‐Oxide Films

Abstract: The oxidation of CO is the archetypal heterogeneous catalytic reaction and plays a central role in the advancement of fundamental studies, the control of automobile emissions, and industrial oxidation reactions. Copper-based catalysts were the first catalysts that were reported to enable the oxidation of CO at room temperature, but a lack of stability at the elevated reaction temperatures that are used in automobile catalytic converters, in particular the loss of the most reactive Cu+ cations, leads to their deactivation. Using a combined experimental and theoretical approach, it is shown how the incorporation of titanium cations in a Cu2O film leads to the formation of a stable mixed-metal oxide with a Cu+ terminated surface that is highly active for CO oxidation.

Kinetic Study of Oxidative Dehydrogenation of Ethane over MoVTeNb Mixed-Oxide Catalyst

Abstract: A MoVTeNb multimetallic mixed oxide was studied for the oxidative dehydrogenation of ethane, a promising alternative for catalytic ethylene production. Lab-scale steady-state experimental reaction data were obtained according to a 3k experimental design to investigate the simultaneous effect of temperature (400–480 °C) and space–time [23–70 gcat h (mol of ethane)−1]. A fixed-bed reactor at atmospheric pressure was employed, feeding a mixture of ethane, oxygen, and nitrogen. Ethane conversion varied from 17 to 85%, whereas selectivity for ethylene and COx varied from 94 to 76% and from 4.0 to 24%, respectively. These types of analyses are useful for determining the optimum reaction conditions to enhance the catalytic performance of the mixed oxides presented herein.

A study of Ni–Al–O mixed oxides as catalysts for the oxidative conversion of ethane to ethylene

Abstract: A series of Ni–Al–O mixed oxides with different aluminum content ranging from 1% to 50% (metal atomic content) were synthesized via a facile co-evaporation route and tested in ethane oxidative dehydrogenation. In samples with Al loading up to 30%, the crystal structure of NiO is retained and the interaction between nickel oxide and alumina leads to smaller crystal sizes, higher surface areas and decreased reducibility, along with a gradual decrease in the amount of desorbed oxygen observed by O 2 -TPD measurements. At higher aluminum loadings the formation of an amorphous spinel-like precursor phase is evidenced. Correlation between the catalytic performance of Ni–Al–O mixed oxide and their oxygen desorption properties showed that as the excess oxygen per catalyst surface area decreases, the catalysts become less active in the activation of ethane, but more selective toward ethylene. The catalyst containing 30% aluminum represents the optimum composition among Ni–Al–O mixed oxides studied in this work. Further optimization of the catalyst preparation method with the use of an organic aluminum precursor led to the development of a catalyst presenting up to 85% selectivity toward ethylene. The enhanced performance of this material was correlated with the further reduction of catalyst over-stoichiometry and oxygen lability induced by the organic precursor, according to O 2 -TPD and isotopic oxygen exchange experiments respectively.

Effects of noble metals doped on mesoporous LaAlNi mixed oxide catalyst and identification of carbon deposit for reforming CH4 with CO2

Abstract: BACKGROUND The effect of the B cation on the surface properties and catalytic activity in the dry reforming reaction over La0.4M0.6Al0.2Ni0.8O3 (M=noble metal) perovskite-type oxides with surface area 3.26–4.14 m2 g–1 and rhombohedral structure was studied. RESULTS Among LaAlxNi1-xO3 series, LaAl0.2Ni0.8O3 had the highest catalytic activity, but suffered a slow deactivation with time-on-stream (TOS). It is observed that all samples presented similar activity at low reaction temperatures (500–600°C), while at higher temperatures (600–850°C) the prepared solid was more active and perovskite phase was transformed into Ni0 or La2O2CO3. It was found that among the noble metal samples, La0.4Rh0.6Al0.2Ni0.8O3 possessed the highest surface area and surface oxygen concentration and the best low-temperature reducibility. For the Rh catalyst the CH4 and CO2 conversions were 89.1 and 86.2%, which were the most resistant against coke deposition and showed very high stability without decrease in reforming and remained constant during the 3000 min TOS. The following order of activity was observed: La0.4Rh0.6Al0.2Ni0.8O3 > La0.4Ru0.6Al0.2Ni0.8O3 > LaAl0.2Ni0.8O3> La0.4Ir0.6Al0.2Ni0.8O3 ≥La0.4Pt0.6Al0.2Ni0.8O3 > La0.4Pd0.6Al0.2Ni0.8O3. CONCLUSIONS It is believed that the high surface area and surface oxygen concentration and good low-temperature reducibility were responsible for the good catalytic performance of the La0.4Rh0.6Al0.2Ni0.8O3 sample. © 2013 Society of Chemical Industry

The NixCe0.75Zr0.25−xO2 solid solution and the RWGS

Abstract: Catalysts based on Ni, Ce and Zr, were prepared using different procedures. The performance of these solids was evaluated in the RWGS reaction. Raman spectroscopy, N2 physisorption, dehydrogenation of cyclohexane reaction, temperature programmed reduction, temperature programmed desorption of CO2 and temperature programmed surface reaction were employed to characterize these catalysts. It was shown that Ni is able to be incorporated into the Ce0.75Zr0.25O2 lattice by co-precipitation or by impregnation generating active RWGS catalysts. It was inferred that the higher the reducibility of the oxides based on Ce, Zr and Ni, the higher the activity in the RWGS reaction. The mixed oxides composed of Ce, Zr and Ni are also very promising components of catalytic systems employed in reactions where the RWGS is one of the steps, as for instance, processes which generate hydrocarbons from CO2. Catalysts based on Ni supported on NiCeZr are very active, stable and selective to CO at high temperatures. Two different sites can be considered for this reaction: one on the mixed oxide and the other on the interface between Ni and the oxide, being the latter the most active.

ZnO-modified zirconia as gold catalyst support for the low-temperature methanol steam reforming reaction

Abstract: Nanoscale zirconia (ZrO 2 ) is widely investigated both as catalyst support and catalyst. Bare ZrO 2 binds gold weakly due to lack of surface defects. In this study, nanoscale ZrO 2 was modified with zinc oxide (ZnO) at different amounts, and examined as support of gold catalysts for the methanol steam reforming (SRM) reaction. ZrO 2 with high specific surface area serves as dispersant of ZnO to create highly dispersed ZnO clusters, which provide more anchoring sites for strong gold-metal oxide bonding. Addition of ZnO also passivates the surface acidity of ZrO 2 , thus blocking the direct methanol decomposition to CO and H 2 . To prepare the highly dispersed ZnO clusters on ZrO 2 , a carbon hard-template method was used, followed by calcination at 550 °C to produce a highly porous mixed oxide solid. To investigate the modification of ZrO 2 by ZnO, the surface acidity was probed with isopropanol in temperature-programmed desorption (TPD) mode. Adsorption of the gold precursor on the ZnO-modified ZrO 2 was carried out by the anion adsorption method. Better dispersion, and better activity and stability of these gold catalysts were found than on either pure ZrO 2 or ZnO. Accordingly, the light-off temperature of SRM is lower and a wider temperature window for CO-free H 2 production up to moderately high temperatures (∼375 °C) is found.

Improved overall water splitting with barium tantalate mixed oxide composites

Abstract: The combination of effective charge carrier separation and improved electron transfer in highly crystalline barium tantalate composites modified with Rh–Cr2O3 core–shell co-catalyst systems induces enhanced activity for overall water splitting (OWS) with stoichiometric amounts of H2 and O2 (2 : 1). A sol–gel route employing complexing reagents was investigated to prepare selectively defined mixed oxide materials with improved surface areas and smaller particle sizes compared to the conventional solid state reaction (SSR). The catalytic activities of the materials are investigated in photocatalytic test reactions for hydrogen production and overall water splitting. The formation of Rh–Cr2O3 core–shell co-catalyst systems for water splitting is evidenced by transmission electron microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). Moreover, we developed new and highly active barium tantalate composites for hydrogen generation from aqueous methanol solutions under UV-light, which show the highest hydrogen evolution rate for a three-component composite consisting of Ba5Ta4O15/Ba3Ta5O15/BaTa2O6. Hydrogen rates of more than 6 mmol h−1 can be achieved without any co-catalyst. Using Rh–Cr2O3 core–shell co-catalysts on these three-component composites simultaneous generation of H2 and O2 from pure water splitting reaches rates up to 70% higher than for the pure Ba5Ta4O15.