Spinel CuFe2O4: a precursor for copper catalyst with high thermal stability and activity

Spinel CuFe₂O₄ has been studied as a precursor for copper catalyst. The spinel CuFe₂O₄ was effectively formed on the SiO₂ by calcination in air at 800 °C with the atomic ratio of Fe/Cu = 2. The spinel CuFe₂O₄ on the SiO₂ was reduced to fine dispersion of Cu and Fe₃O₄ particles by the H₂ reduction at 240 °C. After H₂ reduction at 600 °C, sintering of Cu particles over the CuFe₂O₄/SiO₂ (Fe/Cu = 2) was inhibited significantly, while fatal sintering of Cu particles over the Cu/SiO₂ (Fe/Cu = 0) occurred. The CuFe₂O₄/SiO₂ catalyst exhibited much higher activity and thermal stability for steam reforming of methanol (SRM), compared with the Cu/SiO₂ catalyst. The spinel CuFe₂O₄ on the SiO₂ can be regenerated after an intentional sintering treatment by calcination in air at 800 °C where the activity is also restored completely. Based on these findings, we propose that spinel CuFe₂O₄ is an effective precursor for a high performance copper catalyst in which the immiscible interaction between Cu and Fe (or Fe oxide) plays an important role in the stabilization of Cu particles.     

The promotion effects of Pd on Fe-Cu-Co based catalyst for higher alcohols synthesis

To increase the ability of Fe-Cu-Co based catalyst for hydrogen activation, the catalyst loaded with Pd was studied in the conversion of syngas to higher alcohols. X-ray diffraction (XRD), N₂ physisorption, H₂ temperature-programmed reduction (TPR) and H₂ temperature-programmed desorption (H₂-TPD) were applied to characterize the catalysts. The results of XRD showed that the Pd-loaded Fe-Cu-Co based samples were mainly composed of CuFe₂O₄ and CuO. After reduction, metallic Cu and Fe along with minor CuFe₂O₄ were identified, and the amount of CuFe₂O₄ decreased with the increase of the Pd content. H₂-TPR revealed that Pd facilitated the reduction of Fe-Cu-Co based catalyst. H₂-TPD confirmed that Pd enhanced the ability of Fe-Cu-Co based catalyst for H₂ activation. Therefore, the activity of the catalyst and the selectivity of alcohols were greatly improved. Over the Fe-Cu-Co based catalyst loaded with 0.5 wt.% Pd, the selectivity and the time-space yield of alcohols reached 58.7% and 1.53 g mL^− 1 h^− 1 at 350 °C, 6.0 MPa, GHSV = 10,000 h^− 1 and n(H₂)/n(CO) = 2.4.     

High-temperature close coupled catalysts Pd/Ce-Zr-M/Al2O3 (M = Y, Ca or Ba) used for the total oxidation of propane

A series of high-temperature close coupled catalysts Pd/Ce-Zr-M/Al₂O₃ (M = Y, Ca or Ba) were prepared by ultrasonic-assisted successive impregnation. The catalysts were subjected to a series of characterization measurements. The results of activity evaluation show that Y is the best promoter for propane total oxidation, especially at the calcination temperature of 1100 °C. It is interesting that although the BET specific surface areas and the dispersion of Pd species decrease, the Y-promoted catalyst calcined at 1100 °C shows higher catalytic activity than the corresponding one calcined at 900 °C and better sulfur-resisting performance. The results of TEM, TPHD and CO chemisorption indicate that Y can remarkably increase the dispersion of Pd species. However, the dispersion is hard to be connected with the activity increase as the calcination temperature is elevated from 900 to 1100 °C. The change of active phases and the interaction between Pd species and the supports may account for the activity enhancement. Combined with XRD, H₂-TPR and O₂-TPD results, it is deduced that the coexistence of metallic Pd and PdO species in the catalysts calcined at 1100 °C may be also favorable to C₃H₈ oxidation. In a word, Pd/Ce-Zr-Y/Al₂O₃ is indeed a promising high-temperature close coupled catalyst applicable to high temperature.     

Preparation and characterization of CuFe2O4 bulk catalysts

This work is devoted to the investigation of the influence of the preparation process on the physical-chemical properties of a copper spinel applied as catalyst for hydrocarbon (HC) oxidation. Samples of CuFe₂O₄ mixed oxide belonging to the inverted spinel type structure have been obtained by high temperature solid state reaction and by wet chemical synthesis methods, applying nitrate and citrate precursor procedures, within the thermal range from 150 to 700 °C.The bulk catalysts were characterized by XRD, TPR, FTIR, SEM-EDX, TEM and Mössbauer spectroscopy. Propane combustion was performed as a reaction test.In this work it is demonstrated that the calcination temperature and chemical synthesis affect the crystal properties and cation distribution in the spinel structure, microstructure, surface area and reducibility; which are among the most relevant physical chemical properties for the catalytic activity. The materials obtained by wet chemical procedure, through nitrate and citrate routes, are better suited for propane combustion. This feature is assigned to the microstructure, to the presence of nanometric size particles and also to the cation distributions in the spinel sublattices of these materials.     

Characterization of fused Fe-Cu based catalyst for higher alcohols synthesis and DRIFTS investigation of TPSR

Fused Fe-Cu based catalyst for higher alcohols synthesis (HAS) is characterized by XRD, TG-DTA, H₂-TPD and DRIFTS of CO adsorption. The results of XRD reveal that the fused Fe-Cu based catalyst consists of Cu₂O, CuFeO₂ and CuFe₂O₄ species. After reduction, the metallic Fe and Cu are the main species, but minor CuFeO₂ and CuFe₂O₄ species are also present. H₂-TPD shows that in comparison with the pure Fe- or Cu-based sample, the ability of Fe-Cu based catalyst for activation of H₂ is higher and the stronger metal-hydrogen bonds are formed. DRIFTS of CO adsorption indicates that CO is adsorbed on both metal and metal ion sites, where the dissociation of CO to C* and O* species and the formation of CO₂ are observed. In situ DRIFTS investigation of CO + H₂-TPSR over the Fe-Cu based catalyst shows that the dissociative activation of H₂ is more difficult than the activation of CO, and carbonaceous and hydrocarbon fragment species only appears after the dissociative activation of H₂. In addition, HAS over the Fe-Cu based catalyst is very complicated, where various intermediates including = CH₂, − CHO, − OOCH, − OH and − C(= O)-R exist.     

Selective synthesis of mixed alcohols from syngas over catalyst Fe2O3/Al2O3 in slurry reactor

The Fe₂O₃/Al₂O₃ catalyst was studied to selectively synthesize mixed alcohols from syngas in a continuously stirred slurry reactor with the oxygenated solvent Polyethylene Glycol-400 (PEG-400). The selectivity of mixed alcohols in the products reached as high as 95 wt.% and the C₂₊ alcohols (mainly ethanol) was more than 40 wt.% in the total alcohol products at the reaction conditions of 250 °C, 3.0 MPa, H₂/CO = 2 and space velocity = 360 ml/g_cat h. The hydrogen temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) measurements of the catalyst confirmed that the FeO phase was responsible for the high selectivity to mixed alcohols in the process. And the oxygenated solvent PEG-400 was also necessary for the selective synthesis of mixed alcohols in the reaction system.     

Iron oxide supported sulfated TiO2 nanotube catalysts for NO reduction with propane

Sulfated TiO₂ nanotubes and a series of iron oxide loaded sulfated TiO₂ nanotubes catalysts with different iron oxide loadings (1 wt%, 3 wt%, 5 wt% and 7 wt%) were prepared and calcined at 400 °C. The physico-chemical properties of the catalysts were studied by using XRD, N₂-physisorption, Raman spectroscopy, SEM-EDX, TEM, XPS, and pyridine adsorption using FTIR and H₂-TPR techniques. It was observed that iron oxide was highly dispersed on the sulfated TiO₂ nanotube support due to its strong interaction. The activity of these catalysts in the catalytic removal of NO with propane was also studied in the temperature range of 300–500 °C. Highest activity (90% NO conversion) was observed with 5 wt% iron oxide supported on sulfated TiO₂ catalyst at 450 °C. Selective catalytic reduction of NO activity of the catalysts was correlated with iron oxide loading, reducibility, and the Brönsted and Lewis acid sites of the catalysts. The catalyst also showed good stability under studied reaction conditions that no deactivation was observed during the 50 h of reaction.     

Deep desulfurization of fuels catalyzed by surfactant-type decatungstates using H2O2 as oxidant

Oxidation of dibenzothiphene (DBT) in model oil with H₂O₂ using surfactant-type decatungstates Q₄W₁₀O₃₂ (Q = (CH₃)₃NC₁₆H₃₃, (CH₃)₃NC₁₄H₂₉, (CH₃)₃NC₁₂H₂₅ and (CH₃)₃NC₁₀H₂₁) as catalysts was studied. The surfactant-type decatungstates were synthesized and characterized. The suitable reaction condition of deep desulfurization was suggested: n(DBT):n(catalyst):n(H₂O₂) = 1:0.01:3, 60 °C for 0.5 h, under which the DBT conversion can reach 99.6% with [(CH₃)₃NC₁₆H₃₃]₄W₁₀O₃₂ as catalyst. The length of carbon chains of quaternary ammonium cations played a vital role in the catalytic activity of surfactant-type decatungstates, that is, the longer the carbon chain of quaternary ammonium cation of a catalyst was, the better the activity of this catalyst showed. [(CH₃)₃NC₁₆H₃₃]₄W₁₀O₃₂ exhibited the best catalytic performance and can be recycled for six times without significant decrease in catalytic activity. Using benzothiphene (BT) and 4,6-dimethyldibenzothiphene (4,6-DMDBT) as substrates in model oil, surfactant-type decatungstates also showed high catalytic activity. During desulfurization process, BT conversion can reach 99.6% at 3.25 h, while 99.4% of 4,6-DMDBT conversion reached at 1.25 h, with the temperature of 60 °C under atmospheric pressure. The sulfone can be separated from the oil using N,N-dimethylformamide (DMF) as an extractant, and the sulfur content can be lowered from 1000 to 4 ppm. For real diesel, the sulfur removal can reach 93.5% after five times extraction.     

CO preferential oxidation over Au/MnOx-CeO2 catalysts prepared with ultrasonic assistance: Effect of calcination temperature

The Au/MnO_x-CeO₂ catalysts used for CO preferential oxidation were prepared by deposition-precipitation with ultrasonic assistance. The effect of calcination temperature (150-350 °C) on the structures and catalytic performance of the catalysts was systematically investigated. It is found that the catalyst Au/MnO_x-CeO₂ calcined at 250 °C exhibits the best catalytic performance, giving not only the highest CO conversion of 90.9% but also the highest selectivity of oxygen to CO₂ at 120 °C. The results of XRD, TEM and XPS indicate that this catalyst possesses the smallest particle size, the highest dispersion of Au species and the largest amount of surface adsorbed oxygen species, which are favorable to CO oxidation. The H₂-TPR results reveal that the selectivity of oxygen to CO₂ is mainly determined by the reducibility of Au species in the catalysts. The strong interaction between Au species and the support in Au/MnO_x-CeO₂-250 decreases its capability for H₂ dissociation and oxidation, leading to high selectivity of oxygen to CO₂.     

Fixed- and fluidised-bed synthesis of coiled carbon fibres on an in situ generated H2S-modified Ni/Al2O3 catalyst from a NiSO4/Al2O3 precursor

NiSO₄/Al₂O₃ and NiO/Al₂O₃ catalyst precursors were formed by calcination of NiSO₄·6H₂O/Al₂O₃ at 500 and 800 °C, respectively. The catalyst precursor was reduced under H₂ and N₂ and then reacted under C₂H₂, H₂ and N₂ at 650 °C. Coiled carbon fibres were formed in fixed- and fluidised-bed reactors using the NiSO₄/Al₂O₃ catalyst precursor. Thermodynamic modelling using an infinite equilibrium stage construction predicted complete reduction of NiSO₄ to Ni and simultaneous H₂S formation occurs in both fixed- and fluidised-bed systems. XRD measurements confirmed that Ni was the only catalytically active crystalline species present at concentrations >0.5 wt.% (XRD detection limit) post-reduction, however XRF and XPS measurements additionally detected the presence of small quantities (<0.9 wt.% S) of S species. S is adsorbed onto the Ni surfaces during reduction when H₂S is released and dissociates on the Ni surface. Non-coiled carbon fibres produced on the Ni/Al₂O₃ catalyst formed from the NiO/Al₂O₃ precursor demonstrated that modification of Ni/Al₂O₃ with S is required for coiled carbon fibre synthesis.     

Synthesis, characterization and performance of CuO/La2O3 composite catalyst for ammonia catalytic oxidation

This work considers the oxidation of ammonia (NH₃) by selective catalytic oxidation (SCO) over a CuO/La₂O₃ composite catalyst at temperatures between 150 and 400 °C. A CuO/La₂O₃ composite catalyst was prepared by co-precipitation of copper nitrate and lanthanum nitrate at various molar concentrations. This study also considers how the concentration of influent NH₃ (C₀ = 1000 ppm), the space velocity (GHSV = 92,000 l/h), the relative humidity (RH = 12%) and the concentration of oxygen (O₂ = 4%) affect the operational stability and the capacity for removing NH₃. The catalysts that were characterized using FTIR, XRD, UV-Vis, BET and PSA, have shown that the catalytic behavior is related to the copper (II) oxide, while lanthanum (III) oxide may serve only to provide active sites for the reaction during a catalyzed oxidation run. The experimental results show that the extent of conversion of ammonia by SCO in the presence of the CuO/La₂O₃ composite catalyst was a function of the molar ratio. The ammonia was removed by oxidation in the absence of CuO/La₂O₃ composite catalyst, and around 93.0% NH₃ reduction was achieved during catalytic oxidation over the CuO/La₂O₃ (8:2, molar/molar) catalyst at 400 °C with an oxygen content of 4.0%. Moreover, the effect of the reaction temperature on the removal of NH₃ in the gaseous phase was also monitored at a gas hourly space velocity of under 92,000 h^− 1.     

CoAl2O4 spinel catalyst for soot combustion with NOx/O2

Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NO_x + O₂ stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N₂ adsorption–desorption measurements. The CoAl₂O₄ spinel was able to oxidise soot as fast as the reference Pt/Al₂O₃ catalyst. Its catalytic activity can be attributed to a high NO_x chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO₂.     

Synthesis and electrochemical characterization of nano-CeO2-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

LiMn₂O₄ spinel cathode materials were coated with 0.5, 1.0, and 1.5 wt.% CeO₂ by a polymeric process, followed by calcination at 850 °C for 6 h in air. The surface-coated LiMn₂O₄ cathode materials were physically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron microscopy (XPS). XRD patterns of CeO₂-coated LiMn₂O₄ revealed that the coating did not affect the crystal structure or the Fd3m space group of the cathode materials compared to uncoated LiMn₂O₄. The surface morphology and particle agglomeration were investigated using SEM, TEM image showed a compact coating layer on the surface of the core materials that had average thickness of about 20 nm. The XPS data illustrated that the CeO₂ completely coated the surface of the LiMn₂O₄ core cathode materials. The galvanostatic charge and discharge of the uncoated and CeO₂-coated LiMn₂O₄ cathode materials were measured in the potential range of 3.0-4.5 V (0.5 C rate) at 30 °C and 60 °C. Among them, the 1.0 wt.% of CeO₂-coated spinel LiMn₂O₄ cathode satisfies the structural stability, high reversible capacity and excellent electrochemical performances of rechargeable lithium batteries.     

Synthesis of spinel LiMn2O4 with manganese carbonate prepared by micro-emulsion method

Micro-spherical particle of MnCO₃ has been successfully synthesized in CTAB-C₈H₁₈-C₄H₉OH-H₂O micro-emulsion system. Mn₂O₃ decomposed from the MnCO₃ is mixed with Li₂CO₃ and sintered at 800 °C for 12 h, and the pure spinel LiMn₂O₄ in sub-micrometer size is obtained. The LiMn₂O₄ has initial discharge specific capacity of 124 mAh g⁻¹ at discharge current of 120 mA g⁻¹ between 3 and 4.2 V, and retains 118 mAh g⁻¹ after 110 cycles. High-rate capability test shows that even at a current density of 16 C, capacity about 103 mAh g⁻¹ is delivered, whose power is 57 times of that at 0.2 C. The capacity loss rate at 55 °C is 0.27% per cycle.     

Fe-promoted CuO/CeO2 catalyst: Structural characterization and CO oxidation activity

In the present paper, effect of iron on activity of catalyst CuO/CeO₂ in selective CO oxidation in H₂-containing gases mixture was investigated. Catalysts were prepared by wet impregnation and calcination at 400 °C, characterized by ICP, BET, H₂-TPR, XRD and TEM. The addition of iron to Cu/CeO₂ catalyst improved the catalytic activity and selectivity for CO oxidation. Discussion of the results showed that the synergistic effect is correlated to better reducibility and dispersion of copper in the presence of the iron metal additive.     

Toward a CO2-free ethylene oxide process: Homogeneous ethylene oxide in gas-expanded liquids

Among industrial chemical processes, ethylene oxide manufacture emits the largest amount of CO₂ (∼2-3 million tons/yr), as byproduct from the burning of both the ethylene (feed) and ethylene oxide. Further, the conventional silver-based catalytic process presents safety challenges due to the formation of explosive ethylene oxide/O₂ mixtures in the gas phase. By judicious choice of the catalyst (methyltrioxorhenium), oxidant (H₂O₂) and reaction medium (methanol/water), a homogeneous liquid phase catalytic system has been demonstrated that eliminates CO₂ formation while producing ethylene oxide at >90% selectivity at near-ambient temperatures. Given its high volatility, the ethylene oxide is easily recovered from the reaction phase by distillation. The vicinity of the gaseous ethylene feed to its critical temperature (9 °C) is exploited to significantly increase its solubility in the liquid reaction phase by facile compression beyond the critical pressure of ethylene (∼50 bar). Since H₂O₂ is stable at typical reaction temperatures (40 °C or less), potentially explosive ethylene oxide/O₂ mixtures are avoided in the gas phase. In addition to the potential of arresting the carbon footprint of a large-scale industrial process, the demonstrated process concept shows how gas-expanded liquids can be generally exploited in homogeneous catalysis to enhance productivity.     

Spinelisation and properties of Al2O3–MgAl2O4–C refractory: Effect of MgO and Al2O3 reactants

The effect of particle size of MgO and Al₂O₃ on the spinel formation associated with permanent linear change on reheating (PLCR) and microstructure of Al₂O₃–MgAl₂O₄–C refractory is investigated as a function of heating cycle at 1600 °C with 2 h holding at each cycle. It was found that rate of spinel formation and associated volume expansion is very much dependent on the reactivity and particle size of the reactant. When the reactants are very fine and reactive there is considerable amount of spinel formation, whereas coarser reactants with lower reactivity show negligible formation of spinel phase and associated expansion. Magnesia and alumina with moderate reactivity develops optimum PLCR of the refractory. It continuously increases with the number of heating cycles. The SEM photomicrographs show that in Al₂O₃–MgAl₂O₄–C refractory the spinel phase is formed in between the calcined bauxite grain and the EDX analysis indicates that the spinel phase formed is stoichiometric in nature.     

Characterization and electrocatalytic properties of PtRu/C catalysts prepared by impregnation-reduction method using Nd2O3 as dispersing reagent

A simple impregnation-reduction method introducing Nd₂O₃ as dispersing reagent has been used to synthesize PtRu/C catalysts with uniform Pt-Ru spherical nanoparticles. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis have been used to characterize the composition, particle size and crystallinity of the catalysts. Well-dispersed catalysts with average particle size about 2 nm are achieved. The electrochemically active surface area of the different PtRu/C catalysts is determined by the CO_ad-stripping voltammetry experiment. The electrocatalytic activities of these catalysts towards methanol electrooxidation are investigated by cyclic voltammetry measurements and ac impedance spectroscopy. The in-house prepared PtRu/C catalyst (PtRu/C-03) in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 30 °C display a higher catalytic activity and lower charge-transfer resistance (R_t) than that of the standard PtRu/C catalyst (PtRu/C-C). It is mainly due to enhanced electrochemically active specific surface, higher alloying extent of Ru and the abundant Pt⁰ and Ru oxides on the surface of the PtRu/C catalyst.     

Selective oxidation of methanol to dimethoxymethane over acid-modified V2O5/TiO2 catalysts

Selective oxidation of methanol to dimethoxymethane (DMM) was conducted in a fixed-bed reactor over an acid-modified V₂O₅/TiO₂ catalyst. The influence of the acid modification on its structure, redox and acidic properties, and catalytic performance for methanol oxidation were investigated. The results indicated that the content of vanadia in the catalyst exhibits a vital influence on the dispersion of vanadium species, while the acid modification can enhance its surface acidity. Proper amounts of the acid (W() = 15%) and V₂O₅ (W(V₂O₅) = 15%) components loaded in the acid-modified V₂O₅/TiO₂ catalyst are able to build a bi-functional circumstance that is favorable for the formation of DMM with high activity and selectivity. As a result, for the selective oxidation of methanol, the H₂SO₄-modified V₂O₅/TiO₂ catalyst gives a much higher DMM yield at 150 °C than the unmodified one.     

Comparisons of graphite and spinel Li1.33Ti1.67O4 as anode materials for rechargeable lithium-ion batteries

The aim of this work was to compare the electrochemical behaviors and safety performance of graphite and the lithium titanate spinel Li_1.33Ti_1.67O₄ with half-cells versus Li metal. Their electrochemical properties in 1 M LiPF₆/EC + DEC (1:1 w/w) or 1 M LiPF₆/PC + DEC (1:1 w/w) at room and elevated temperatures (30 and 60 °C) have been studied using galvanostatic cycling. At 30 °C graphite has higher reversible capacity than Li_1.33Ti_1.67O₄ when using the LiPF₆/EC + DEC as electrolyte. At 60 °C graphite declines in cell capacity yet Li_1.33Ti_1.67O₄ remains almost unchanged. In a propylene carbonate (PC) containing electrolyte, graphite electrode exfoliates and loses its mechanical integrity while Li_1.33Ti_1.67O₄ electrode is very stable. An accelerating rate calorimeter (ARC) and microcalorimeter have been used to compare the thermal stability of lithiated lithium titanate spinel and graphite. Results show that Li_1.33Ti_1.67O₄ may be used as an alternative anode material offering good battery performance and higher safety.