Morphology effects of Co3O4 on the catalytic activity of Au/Co3O4 catalysts for complete oxidation of trace ethylene

Au/Co₃O₄ catalysts with different morphologies (nanorods, nanopolyhedra and nanocubes) were successfully synthesized and evaluated for ethylene complete oxidation. We found that support morphology has a significant effect on catalytic activity, which is related to the exposed planes of different morphological Co₃O₄. HRTEM revealed the Co₃O₄-nanorods predominantly exposes {110} planes, while the dominant exposed planes of Co₃O₄-nanopolyhedra and -nanocubes are {011} and {001} planes, respectively. Compared with {011} and {001} planes, {110} planes exhibit the maximum amount of oxygen vacancies, which play a major role in ethylene oxidation. Therefore, Au/Co₃O₄-nanorods exhibits extraordinary catalytic activity, yielding 93.7% ethylene conversion at 0 °C.     

Synthesis and analysis of multi-walled carbon nanotubes/oxides hybrid materials for polymer composite applications

Nanotubes-based nanocomposites to be used as polymer reinforcing/flame-retardant additives are synthesized by decomposition of isobutane at 600 °C. Catalytic chemical vapor deposition (CCVD) is carried out over 17 wt%Fe-catalysts supported on various oxides (Al₂O₃, MgO, CaO, SrO or BaO) reduced at 600 °C. Catalysts utilized and carbonaceous deposits obtained are systematically characterized by the use of several analysis techniques, in order to investigate the influence of catalyst specifics on reaction yield, selectivity and characteristics (crystallinity and purity) of the grown nanotubes. The results show that the support greatly influences the catalyst performance. The lack of metallic iron renders Fe/SrO- and Fe/BaO-catalysts inactive. Fe/Al₂O₃ catalysts exhibit the highest catalytic activity, but give rise to scarce selectivity and large metallic impurity contents. Contrarily, using Fe/MgO and Fe/CaO catalysts leads to lower yields, but allows reducing impurities and remarkably improving selectivity and nanotube crystallinity.     

Role of ZnO in Cu/ZnO/Al2O3 catalyst for hydrogenolysis of butyl butyrate

For hydrogenolysis of butyl butyrate (BB), a series of Cu/ZnO/Al₂O₃ catalysts with different metal compositions were prepared, and characterized by N₂O chemisorption for measuring Cu surface area and by chromatographic experiment for determining the heat of BB adsorption. As a result, the presence of ZnO in Cu-based catalysts was found to enhance the catalytic activity of Cu due to dual function of ZnO. The Cu surface area was linearly correlated with the butanol productivity, demonstrating that ZnO exerts the structural function in Cu/ZnO/Al₂O₃ catalysts. Additionally, the role of ZnO as a chemical contributor was revealed such that its presence leads to lower activation energy of the surface reaction, thus resulting in higher Cu catalytic activity obtained at a low temperature such as 200 °C. Consequently, optimizing the Cu/Zn ratio in Cu/ZnO/Al₂O₃ catalyst is required to tune its structural and chemical characteristics of Cu metals, and thus to obtain a higher activity on the hydrogenolysis reaction.     

Pressureless sintered Al2O3–SiC nanocomposites

Al₂O₃ + 5 vol% SiC composite ceramics were prepared via a conventional powder processing route followed by pressureless sintering. Commercially available Al₂O₃ and SiC powders were milled together in an aqueous suspension. The slurry was freeze granulated, and green bodies were obtained by cold isostatic pressing of the granules. Pressureless sintering was carried out in a nitrogen atmosphere at 1750 and 1780 °C. Near full density (>99%) was achieved at 1780 °C. Densification at the lower sintering temperature was promoted by smaller additions of MgO. Vickers hardness and indentation fracture toughness varied around 18 GPa and 2.3 MPa m^1/2 after sintering at 1780 °C. Transmission electron microscopy revealed that the SiC particles were located predominantly to the interior of the matrix grains and well distributed throughout the composite microstructures. The intragranular particles had sizes in the range 50–200 nm while the intergranular particles were larger, typically 200–500 nm in diameter.     

The effects of processing parameters on formation of nano-spinel (MgAl2O4) from LiCl moltensalt

Nano-MgAl₂O₄ particles were successfully synthesized at 850 °C using the molten-salt method, and the effects of processing parameters, such as temperature, holding time and amount of salt on the crystallization of MgAl₂O₄ were investigated. Nano-alumina, magnesia and lithium chloride were used as starting materials. LiCl molten salt provided a liquid medium for reaction of Al₂O₃ and MgO to form MgAl₂O₄. The results demonstrated that MgAl₂O₄ started to form at about 650 °C and that, after the temperature was increased to 1000 °C, the amounts of MgAl₂O₄ in the resultant powders increased with a concomitant decrease in Al₂O₃ and MgO contents. After washing with hot-distilled water, the samples heated for 3 h at 850 °C were single-phase MgAl₂O₄ with 30–50 nm particle size. Furthermore, the synthesized MgAl₂O₄ particles retained the size and morphology of the Al₂O₃ powders, which indicated that a template formation mechanism dominated the formation of MgAl₂O₄ by molten-salt method.     

Efficient production of hydrogen and multi-walled carbon nanotubes from ethanol over Fe/Al2O3 catalysts

Fe/Al₂O₃ catalysts with different Fe loadings (10-90 mol%) were prepared by hydrothermal method. Ethanol decomposition was studied over these Fe/Al₂O₃ catalysts at temperatures between 500 and 800 °C to produce hydrogen and multi-walled carbon nanotubes (MWCNTs) at the same time. The results showed that the catalytic activity and stability of Fe/Al₂O₃ depended strongly on the Fe loading and reaction temperature. The Fe(30 mol%)/Al₂O₃ and Fe(40 mol%)/Al₂O₃ were both the effective catalyst for ethanol decomposition into hydrogen and MWCNTs at 600 °C. Several reaction pathways were proposed to explain ethanol decomposition to produce hydrogen and carbon (including nanotube) at the same time.     

Nanocatalysis on Supported Oxides for CO Oxidation

Active gold and palladium nanoparticles supported on a variety of oxides (CeO₂, ZrO₂, Al₂O₃, SiO₂, MgO and ZnO) were synthesized using laser vaporization and microwave irradiation methods. The catalytic activities for CO oxidation on the nanoparticle catalysts were evaluated and compared among different oxide supports. The effect of shape on the catalytic activity is demonstrated by comparing the activities of the Au and Pd catalysts deposited on MgO nanocubes and ZnO nanobelts. The Au/CeO₂ nanoparticles deposited on MgO nanocubes exhibit high catalytic activity and stability. The enhanced catalytic activity is attributed to the presence of a significant concentration of the corner and edge sites in MgO nanocubes. The Au- and Pd-doped Mn₂O₃ nanoparticles show promising results for the low temperature CO oxidation. Several approaches for incorporating the Au and Pd nanocatalysts within mesoporous oxide supports are presented and discussed.     

Hydrogen production by oxidative steam reforming of methanol using ceria promoted copper–alumina catalysts

The performance of different Cu/CeO₂/Al₂O₃ catalysts of varying compositions is investigated for the oxidative steam reforming of methanol (OSRM) in order to produce the hydrogen selectively for polymer electrolyte membrane (PEM) fuel cell applications. All the catalysts were prepared by co-precipitation method and characterized for their surface area, pore volume and oxidation–reduction behavior. The effect of various operating parameters studied are as follows: reaction temperature (200–300 °C), contact-time (W/F = 3–15 kg_cat s mol^− 1) and oxygen to methanol (O/M) molar ratio (0–0.5). The steam to methanol (S/M) molar ratio = 1.5 and pressure = 1 atm were kept constant. Among all the catalysts studied, catalyst Cu–Ce–Al:30–20–50 exhibited 100% methanol conversion and 179 mmol s^− 1 kg_cat^− 1 hydrogen production rate at 280 °C with carbon monoxide formation as low as 0.19%. The high catalytic activity and hydrogen selectivity shown by ceria promoted Cu/Al₂O₃ catalysts is attributed to the improved specific surface area, dispersion and reducibility of copper which were confirmed by characterizing the catalysts through temperature programmed reduction (TPR), CO chemisorption, X-ray diffraction (XRD) and N₂ adsorption–desorption studies. Reaction parameters were optimized in order to produce hydrogen with carbon monoxide formation as low as possible. The time-on-stream stability test showed that the Cu/CeO₂/Al₂O₃ catalysts were quite stable.     

Densification and characterization of SiO2B2O3CaOMgO glass/Al2O3 composites for LTCC application

A glass/ceramic composite using lead-free low melting glass (SiO₂B₂O₃CaOMgO glass) with Al₂O₃ fillers was investigated. X-ray diffraction analysis revealed that the anorthite and cordierite phase appeared in the sintered composites. The dilatometric analysis showed that the onset of shrinkage took place at ∼624 °C for all the samples and the onset temperature was independent on the content of glass. The low melting glass significantly promoted densification of the composites and lowered the sintering temperature to ∼875 °C. The addition of 50 wt% glass sintered at 875 °C showed ε_r of 7.3, tan δ of 1.15×10⁻³, TEC of 5.41 ppm/°C, thermal conductivity of 3.56 W/m °C, and flexural strength of 184 MPa. The results showed that the SiO₂B₂O₃CaOMgO glass/Al₂O₃ composites were strong potential candidates for low temperature cofired ceramic substrate applications.     

Effect of Mg-and Fe-ions on formation mechanism of aluminium titanate

The presence of Mg²⁺- and Fe³⁺-ions has an effect on the formation of Al₂TiO₅. Crystalline phases produced under the influence of the heat treatment have been identified in a heated X-ray diffraction chamber in the temperature range of 20–1500 °C. In the presence of Mg²⁺- and Fe³⁺-ions transitional phases are formed in the temperature range of 1000–1350 °C during Al₂TiO₅ formation. The XRD technique was used to identify the crystalline phases formed. On addition of MgO, chemical composition of the transitional phase formed is Mg_0.3Al_1.4Ti_1.3O₅, whereas on addition of Fe₂O₃ we have calculated a Powder Diffraction File card data for the transitional phase. Determination of the lattice parameters of the Al₂TiO₅ ceramics produced enabled verification of incorporation of Mg²⁺- or Fe³⁺-ions into the crystal lattice of Al₂TiO₅, i.e. the formation of Mg²⁺- and Fe³⁺-containing solid solutions.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Dibenzothiophene hydrodesulfurization using in situ generated hydrogen over Pd promoted alumina-based catalysts

Catalytic hydrodesulfurization (HDS) of dibenzothiophene (DBT) was carried out in a temperature range of 320-?400 °C using in situ generated hydrogen via steam reforming of ethanol and the effect of some organic additives was studied for the first time. Four kinds of alumina-based catalysts, i.e. Co?-Mo/Al₂O₃, Ni-Mo/Al₂O₃ and their corresponding Pd promoted catalysts Pd-?Co-?Mo/Al₂O₃ and Pd-?Ni-?Mo/Al₂O₃, prepared through incipient impregnation method, were used for the desulfurization process. Catalytic activity was investigated in a batch autoclave reactor in the complete absence of external hydrogen gas. Experiments showed that organic additives like diethylene glycol (DEG), phenol, naphthalene, anthracene, o-xylene, tetralin, decalin and pyridine can affect the HDS activity of the catalysts in different ways, and only naphthalene is inhibitive for the catalytic activity towards HDS. The results showed that Ni-based catalysts are more active than Co-based ones while Pd shows a high promotion effect. DBT conversion of up to 97% was achieved with Pd-?Ni-?Mo/Al₂O₃ catalyst at 380 °C temperature and 13 h reaction time. Catalyst systems followed the HDS activity order of: Pd-?Ni-?Mo/Al₂O₃ > Ni-?Mo/Al₂O₃ > Pd-?Co-?Mo/Al₂O₃ > Co?-Mo/Al₂O₃ at all conditions. Qualitative analysis of the products stream was carried out using GC?-MS technique. The present HDS process using in situ generated hydrogen might be applied as an alternative approach for the catalytic HDS of DBT on industrial level due to its cost effectiveness, mild operating conditions and high activity of the catalysts.     

Non-oxidative methane coupling using Cu/ZnO/Al2O3 catalyst in DBD

Non-oxidative methane coupling into higher hydrocarbons was investigated in dielectric-barrier discharge (DBD) conditions using a stationary catalytic bed (Cu/ZnO/Al₂O₃). The experiments were carried out at the frequency of about 6 kHz, at 240 °C, at the pressure of ∼1220 hPa and with the overall gas flow rate 2 NL/h (0 °C, 1013 hPa). The effects of gas composition on the conversion, the effect of packing on the obtained products and activity of the catalyst under plasma conditions during long-term experiments were studied. Hydrocarbons from 2 to 5 atoms of carbon were identified in the outlet gas. It was found that in the presence of catalyst in plasma zone, overall methane conversion decreased, however the conversion towards ethane was higher, as compared to the process without packing.     

Synthesis and characterization of Al2O3 nanopowders by a simple chitosan-polymer complex solution route

Al₂O₃ nanopowders were synthesized by a simple chitosan-polymer complex solution route. The precursors were calcined at 800–1200 °C for 2 h in air. The prepared samples were characterized by XRD, FTIR and TEM. The results showed that for the precursors prepared with pH 3–9 γ-Al₂O₃ and δ-Al₂O₃ are the two main phases formed after calcination at 800–1000 °C. Interestingly, when the precursor prepared with pH 2 was used, α-Al₂O₃ was formed after calcination at 1000 °C, and pure α-Al₂O₃ was obtained after calcination at 1200 °C. The crystallite sizes of the prepared powders were found to be in the range of 4–49 nm, as evaluated by the XRD line broadening method. TEM investigation revealed that the Al₂O₃ nanopowders consisted of rod-like shaped particles and nanospheres with particle sizes in the range of 10–300 nm. The corresponding selected-area electron diffraction (SAED) analysis confirmed the formation of γ- and α-Al₂O₃ phases in the samples.     

Densification ability of combustion-derived Al2O3 powders

Nanocrystalline Al₂O₃ powders containing different amounts of MgO (0.1–5.0 mol%) or added boehmite (AlOOH) have been synthesized by combustion synthesis from aluminium nitrate and magnesium nitrate, using urea or sucrose as fuels. The as synthesized alumina powders were deagglomerated, compacted by dry pressing and sintered at 1625 °C for 2 h. For comparison purposes, a commercial high purity α-Al₂O₃ powder (ACC) was also processed following the same route. The sintered materials were characterized for bulk density (BD), apparent porosity (AP), and water absorption (WA) capacity, microstructure using SEM, and XRD phase composition. In comparison to boehmite, the MgO had a considerable effect on the densification behaviour of combustion-synthesized powder.     

Effect of annealing process on the growth and surface properties of Au–ZnO nanowire films grown by chemical routes

Au–ZnO nanowire films have been synthesized by chemical routes, electrochemical deposition (ECD) and chemical bath deposition (CBD) techniques, on zinc foil followed by annealing in air at 400 °C. X-ray diffraction patterns reveal formation of the ZnO wurtzite structure along with binary phases Au₃Zn and AuZn₃. Scanning electron microscopy shows the presence of ZnO nanowires having several micrometers in length and less than 120 nm in diameter synthesized by ECD and in the range of 70–400 nm using the CBD technique. During the annealing process, different surface morphologies originating from different catalytic effects of Au atoms/layers were observed. In addition, the effect of synthesis routes on crystalline quality and optical properties were studied by Raman and photoluminescence spectrometers indicating varying concentration of defects on the films. The Raman results indicate that Au–ZnO nanowire film prepared by chemical bath deposition route had better crystalline quality.     

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.     

Anticorrosion efficiency of ZnxMgyAl2O4 core–shell spinels in organic coatings

Anticorrosion pigments were synthesized by reaction of metal aluminum lamellar particles whose surface is oxidized to Al₂O₃ during the first stage and by subsequent reaction with ZnO and/or MgO at 800–1150 °C producing a thin spinel layer that is chemically bonded to the metal core of the pigment particles. Core–shell spinels were synthesized: MgAl₂O₄/Al; Mg_0.8Zn_0.2Al₂O₄/Al; Mg_0.6Zn_0.4Al₂O₄/Al; Mg_0.4Zn_0.6Al₂O₄/Al; Mg_0.2Zn_0.8Al₂O₄/Al; and ZnAl₂O₄/Al. The prepared pigments were characterized by means of X-ray diffraction analysis and scanning electron microscopy. The synthesized anticorrosion pigments were used to prepare epoxy coatings that were tested upon application for their physical–mechanical properties, anticorrosion properties and resistance against a chemical environment. All of the synthesized pigments exhibit good anticorrosion efficiency in epoxy coatings. Compared to lamellar kaolin and metal core of aluminum without coverage, the protective function of the synthesized pigments in coatings is demonstrably better.     

Ultraviolet irradiated ozone treatment of a metal catalyst for the large-scale synthesis of single-walled carbon nanotubes with small, uniform diameters

Large-scale synthesis of single-walled carbon nanotubes with small diameters and narrow distribution was performed using catalytic decomposition of C₂H₂ at 800 °C by introducing an ultraviolet irradiated ozone (UV-ozone) treatment on an as-prepared Fe-Mo/MgO catalyst (APC). The UV-ozone treatment effectively suppressed metal migration and the agglomeration of the Fe-Mo catalyst on the MgO support material at high temperature (800 °C). During UV-ozone treatment, active oxygen species were adsorbed onto the APC and generated hydroxyl groups. The hydroxyl groups prevented the formation of large catalytic metal nanoparticles at 800 °C by acting as a surfactant. We also investigated whether the Mo species prevented the metal sintering of iron species into the MgO lattice.     

Oxygen isotope equilibration and exchange as probes for differing dioxygen interactions with pure and rhodia-promoted CeO2 and Al2O3

Comparisons are made between the catalytic activities of CeO₂, Al₂O₃ and Rh₂O₃ when pure, or in the case of CeO₂ and Al₂O₃ when promoted by rhodia dispersed thereon, in respect of: (a) activity at 290 K for homomolecular oxygen isotope equilibration of an equimolar (¹⁸O₂ + ¹⁶O₂) probe gas, the so-called R₀ process, (b) activity under T-ramp for heterophase oxygen isotope exchange between surface lattice and O₂ highly enriched in ¹⁸O, which is shown to occur predominantly by single-stay exchange of both oxygen atoms of dioxygen (the so-called R₂ process). This revised version was published online in June 2006 with corrections to the Cover Date.