In situ crack-healing behavior of Al2O3/SiC composite ceramics under static fatigue strength

A three-point bending (rectangular bar) specimen was made from sintered Al₂O₃/SiC composite ceramics and commercial Al₂O₃ material, upon which a semi-elliptical surface crack of 100 μm in diameter (aspect ratio ≒ 0.9) was introduced through an indentation method. The following materials were subjected to the following crack-healing treatment: Al₂O₃/SiC composite ceramics (under 1573 K temperature, 1 h crack-healing time) and monolithic Al₂O₃ (under 1373 K or 1723 K temperature, 1 h crack-healing time) designed to heal the crack samples.     

Nanocrystallites-forming hierarchical porous Ni/Al2O3–TiO2 catalyst for dehydrogenation of organic chemical hydrides

Dehydrogenation of organic chemical hydrides has been improved by reconstructing the catalyst in the form of hierarchical porous structure nanocatalyst, in which the economical Ni was adopted as catalytic component and nano Al₂O₃–TiO₂ hybrid composite as support. The Al₂O₃–TiO₂ composite was prepared by spontaneous self-assembly of nano Al₂O₃ and TiO₂ aggregates by hydrolysis of tetra-n-butyl-titanate under continuous agitation. The multi-scaled distribution of Al₂O₃–TiO₂ aggregates with hierarchy could be observed in dynamic light scattering spectrometer. The aggregates are comprised of nano-sized γ-Al₂O₃ and anatase TiO₂ crystallites with sizes of about 5 and 7 nm, respectively. The surface modulation by TiO₂ could be verified in FTIR Spectra. The migration of Ti species and crystallite growth were hindered by the Al₂O₃ skeleton and the hierarchical porous structure was sustained during the thermal related process. The multi-scaled distributed pores were confirmed by both TEM analysis and N₂ adsorption results. The results of dehydrogenation experiments showed that the hierarchical porous structure nano Ni/Al₂O₃–TiO₂ exhibited superior catalytic performance to Ni/Al₂O₃ with the optimum conversion of 99.9% at 400 °C, while the catalyst of Ni/Al₂O₃ exhibited only 16.5% under the same condition.     

Fuel cell grade hydrogen production via methanol steam reforming over CuO/ZnO/Al2O3 nanocatalyst with various oxide ratios synthesized via urea-nitrates combustion method

Hydrogen production via steam reforming of methanol has been studied over a series of CuO/ZnO/Al₂O₃ catalysts synthesized by the combustion method using urea as fuel. Furthermore, the effect of alumina loading on the properties of the catalyst has been investigated. XRD analysis illustrated the crystallinity of the Cu and Zn oxides decreases by enhancing alumina loading. BET showed the surface area improvement and FESEM images revealed lower size distribution by increasing the amount of alumina. EDX results gave approximately the same metal oxide compositions of primary gel for the surface of the nanocatalysts. Catalytic performance tests showed the well practicability of catalysts synthesized by the combustion method for steam reforming of methanol process. Alumina addition to the CuO/ZnO catalyst caused the higher methanol conversion and the lower CO generation. Among different compositions the sample with molar component of CuO/ZnO/Al₂O₃ = 4/4/2.5 showed the best performance which without CO generation at 240 °C its methanol conversion decreased from 90 to 60% after 90 h.     

Improvement in dye-sensitized solar cells employing TiO2 electrodes coated with Al2O3 by reactive direct current magnetron sputtering

A novel surface modification method was carried out by reactive dc magnetron sputtering to fabricate TiO₂ electrodes coated with Al₂O₃ for improving the performance of dye-sensitized solar cells (DSSCs). The Al₂O₃-coated TiO₂ electrodes had been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–vis spectrophotometer, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The study results revealed that the modification to TiO₂ increases dye absorption amount, reduces trap sites on TiO₂, and suppresses interfacial recombination. The impact of sputtering time on photoelectric performance of DSSCs was investigated. Sputtering Al₂O₃ for 4 min on 5-μm thick TiO₂ greatly improves all cell parameters, resulting in enhancing the conversion efficiency from 3.93% to 5.91%. Further increasing sputtering time decreases conversion efficiency.     

A first principle study of SO3 decomposition on silver nano-clusters: Implications toward hydrogen production

In this work, based on first principles density functional theory, we have investigated the interaction of SO₃ molecule on three different substrates; (i) clean Al₂O₃ surface (0001) (ii) an isolated Ag₆ cluster and (iii) Ag₆ clusters deposited on the Al₂O₃ surface. All calculations were carried out using the plane wave based pseudopotential method under the framework of density functional theory. For the clean Al₂O₃ surface, the SO₃ molecule was adsorbed in parallel orientation on the surface resulting in an elongation of the S–O bond from 1.44 to 1.52 Å with interaction energy of 1.67 eV. In contrast, the interaction of SO₃ with Ag₆ was found to be weak with 0.4 eV interaction energy and 1.47 Å as the largest S–O bond length. Remarkably, when SO₃ molecule interacted with Ag₆ cluster deposited on the Al₂O₃ support, the binding was found to be higher than both Al₂O₃ and Ag₆ clusters in their isolated state. In particular, upon adsorption of SO₃ on Ag₆/@Al₂O₃, the S–O bond length was found to increases from 1.44 to 1.64 Å and the interaction energy was estimated to be 2.00 eV. As the bond elongation bears the signature of bond weakening, a comparison of the above three results clearly suggests that the dissociation barrier of S–O bond on the Ag₆@Al₂O₃ support will be significantly lower than that on the isolated Ag₆ or Al₂O₃ surface. The nature of chemical interaction of SO₃ on these three systems has been discussed based on the electronic density of states analysis.     

Hydrogen production by steam reforming of dimethyl ether over ZnO–Al2O3 bi-functional catalyst

A series of ZnO–Al₂O₃ catalysts with various ZnO/(ZnO + Al₂O₃) molar ratios have been developed for hydrogen production by dimethyl ether (DME) steam reforming within microchannel reactor. The catalysts were characterized by N₂ adsorption-desorption, X-ray diffraction and temperature programmed desorption of NH₃. It was found that the catalytic activity was strongly dependent on the catalyst composition. The overall DME reforming rate was maximized over the catalyst with ZnO/(ZnO + Al₂O₃) molar ratio of 0.4, and the highest H₂ space time yield was 315 mol h⁻¹·kg_cat⁻¹ at 460 °C. A bi-functional mechanism involving catalytic active site coupling has been proposed to account for the phenomena observed. An optimized bi-functional DME reforming catalyst should accommodate the acid sites and methanol steam reforming sites with a proper balance to promote DME steam reforming, whereas all undesired reactions should be impeded without sacrificing activity. This work suggests that an appropriate catalyst composition is mandatory for preparing good-performance and inexpensive ZnO–Al₂O₃ catalysts for the sustainable conversion of DME into H₂-rich reformate.     

Improved performance of dye-sensitized solar cells with TiO2/alumina core-shell formation using atomic layer deposition

Alumina (Al₂O₃) shell formation on TiO₂ core nanoparticles by atomic layer deposition (ALD) is studied to suppress the recombination of charge carriers generated in a dye-sensitized solar cell (DSSC). It is relatively easy to control the shell thickness using the ALD method by controlling the number of cycles. An optimum thickness can be identified, which allows tunneling of the forward current while suppressing recombination. High-resolution TEM measurements show that a uniform Al₂O₃ shell is formed around the TiO₂ core particles and elemental mapping of the porous TiO₂ layer reveals that the Al₂O₃ distribution is uniform throughout the layer. The amount of dye absorption is increased with increase in the shell thickness but electrochemical impedance spectroscopic (EIS) measurement shows a drastic increase in the resistance. With an optimum Al₂O₃ thickness of 2 nm deposited by ALD, a 35% improvement in the cell efficiency (from 6.2 to 8.4%) is achieved.     

Improved separation efficiency of photogenerated carriers for Fe2O3/SrTiO3 heterojunction semiconductor

To investigate the mechanisms of the improvement on separation efficiency of photogenerated carriers, a Fe₂O₃/SrTiO₃ heterojunction semiconductor with an improved separation efficiency was successfully prepared. The heterojunction semiconductor was characterized with X-ray diffraction (XRD), UV–vis absorption spectrum, scanning electron microscope (SEM) and surface photovoltage (SPV) spectroscopy. The energy band diagrams of Fe₂O₃ and SrTiO₃ were determined with X-ray photoelectron spectroscopy (XPS), based on which the conduction band offset (CBO) between Fe₂O₃ and SrTiO₃ was quantified to be 1.26 ± 0.03 eV. The recombination of photogenerated carriers was investigated with photoluminescence (PL) spectrum, which indicates that the formation of Fe₂O₃/SrTiO₃ decreases the recombination. Thus the improved separation efficiency is mainly due to the energy difference between the conduction band edges of Fe₂O₃ and SrTiO₃, and the decreased electron-hole recombination for Fe₂O₃/SrTiO₃.     

Hydrogen permeation on Al2O3-based nickel/cobalt composite membranes

Al₂O₃ was synthesized using the sol-gel process with aluminum isopropoxide as the precursor and primary distilled water as the solvent. Nickel and cobalt metal powders were used to increase the strength of the membranes. The Al₂O₃-based membranes were prepared using HPS following a mechanical alloying process. The phase transformation, thermal evolution, surface and cross-section morphology of Al₂O₃ and Al₂O₃-based membranes were characterized by XRD, TG-DTA and FE-SEM. The hydrogen permeation of Al₂O₃-based membranes was examined at 300–473 K under increasing pressure. Hydrogen permeation flux through an Al₂O₃-20wt%Co membrane was obtained to 2.36 mol m⁻² s⁻¹. Reaction enthalpy was calculated to 4.5 kJ/mol using a Van’t Hoff’s plot.     

Promotion of CO2 methanation activity and CH4 selectivity at low temperatures over Ru/CeO2/Al2O3 catalysts

The effect of CeO₂ loading amount of Ru/CeO₂/Al₂O₃ on CO₂ methanation activity and CH₄ selectivity was studied. The CO₂ reaction rate was increased by adding CeO₂ to Ru/Al₂O₃, and the order of CO₂ reaction rate at 250 °C is Ru/30%CeO₂/Al₂O₃ > Ru/60%CeO₂/Al₂O₃ > Ru/CeO₂ > Ru/Al₂O₃. With a decrease in CeO₂ loading of Ru/CeO₂/Al₂O₃ from 98% to 30%, partial reduction of CeO₂ surface was promoted and the specific surface area was enlarged. Furthermore, it was observed using FTIR technique that intermediates of CO₂ methanation, such as formate and carbonate species, reacted with H₂ faster over Ru/30%CeO₂/Al₂O₃ and Ru/CeO₂ than over Ru/Al₂O₃. These could result in the high CO₂ reaction rate over CeO₂-containing catalysts. As for the selectivity to CH₄, Ru/30%CeO₂/Al₂O₃ exhibited high CH₄ selectivity compared with Ru/CeO₂, due to prompt CO conversion into CH₄ over Ru/30%CeO₂/Al₂O₃.     

Catalytic surface development of novel nickel plate catalyst with combined thermally annealed platinum and alumina coatings for steam methane reforming

Catalytically active surface of small nickel (min 99 wt%) plates for steam methane reforming was enhanced by successive temperature programmed oxidation−reduction (TPO−TPR) pretreatment and combined physical vapor deposition of Pt and Al₂O₃. The effect of annealing time, temperature, order and number of coatings on the catalytic activity was investigated by means of a pulse technique at the reaction temperature of 760 °C. The most active and stable surface phases resulted after the successively deposited layers of Pt, Al₂O₃, and Pt had been annealed for 12 h onto 2-cycle TPO−TPR pretreated nickel plate at the temperature of 700 °C in a circulating atmosphere of N₂. The durability performance of the so-prepared surface phases on a specifically structured plate catalyst element (diameter 43 mm and length 42 mm) was tested in a tubular reactor for some 70 h in temperature range 500−650 °C. Deactivation was mainly caused by carbon surface deposition.     

Stabilized zirconia-based sensor utilizing SnO2-based sensing electrode with an integrated Cr2O3 catalyst layer for sensitive and selective detection of hydrogen

A highly selective hydrogen (H₂) sensor has been successfully developed by using an yttria-stabilized zirconia (YSZ)-based mixed-potential-type sensor utilizing SnO₂ (+30 wt.% YSZ) sensing electrode (SE) with an intermediate Al₂O₃ barrier layer which was coated with a catalyst layer of Cr₂O₃. The sensor utilizing SnO₂ (+30 wt.% YSZ)-SE was found to be capable of detecting H₂ and propene (C₃H₆) sensitively at 550 °C. In order to enhance the selectivity towards H₂, a selective C₃H₆ oxidation catalyst was employed to minimize unwanted responses caused by interfering gases. Among the examined metal oxides, Cr₂O₃ facilitated the selective oxidation of C₃H₆. However, the addition or lamination of Cr₂O₃ to SnO₂ (+30 wt.% YSZ)-SE was found to diminish the sensing responses to all examined gases. Therefore, an intermediate layer of Al₂O₃ was sandwiched between the SE layer and the catalyst layer to prevent the penetration of Cr₂O₃ particles into the SE layer. The sensor using SnO₂ (+30 wt.% YSZ)-SE coated with a catalyst layer of Cr₂O₃ as well as an intermediate layer of Al₂O₃ exhibited a sensitive response toward H₂, with only minor responses toward other examined gases at 550 °C under humid conditions (21 vol.% O₂ and 1.35 vol.% H₂O in N₂ balance). A linear relationship was observed between sensitivity and H₂ concentration in the range of 20–800 ppm on a logarithmic scale. The results of sensing performance evaluation and polarization curve measurements indicate that the sensing mechanism is based on the mixed-potential model.     

Enhanced performance of Ca-doped Pt/γ-Al2O3 catalyst for cyclohexane dehydrogenation

The promotion effects of Ca on catalytic performance of Ca-doped Pt/Al₂O₃ catalyst were investigated by varying Ca content and impregnation orders. The Ca-doped Pt/Al₂O₃ catalyst with atomic rate Ca/Pt = 5 exhibited the highest catalytic activity and stability. Detailed analyses show that low Ca content (Ca/Pt ＜ 5) could not efficiently promote hydrogen spillover from Pt-Al interface to the support and reduce the strong acid sites on the support. However, high Ca content (Ca/Pt > 5) decreases the initial activity due to the coverage of Ca on the support and increases the amount of Pt strongly interacting with Ca, which inhibits Pt reduction. Furthermore, the Ca promotion effect is more pronounced when Ca is added prior to Pt due to surface modification of Ca on Al₂O₃ support. This modified catalyst possesses more dispersed Pt, lower acidity and larger amount of spilled-over hydrogen.     

Nickel on a macro-mesoporous Al2O3@ZrO2 core/shell nanocomposite as a novel catalyst for CO methanation

A novel nickel catalyst supported on Al₂O₃@ZrO₂ core/shell nanocomposites was prepared by the impregnation method. The core/shell nanocomposites were synthesized by depositing zirconium species on boehmite nanofibres. This contribution aims to study the effects of the pore structure of supports and the zirconia dispersed on the surface of the alumina nanofibres on the CO methanation. The catalysts and supports were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H₂ temperature-programmed reduction (H₂-TPR), nitrogen adsorption–desorption, and thermogravimetry and differential thermal analysis (TG-DTA). The catalytic performance of the catalysts for CO methanation was investigated at a temperature range from 300 °C to 500 °C. The results of the characterization indicate that the metastable tetragonal zirconia could be stably and evenly dispersed on the surface of alumina nanofibres. The interlaced nanorods of the Al₂O₃@ZrO₂ core/shell nanocomposites resulted in a macropore structure and the spaces between the zirconia nanoparticles dispersed on the alumina nanofibres formed most of the mesopores. Zirconia on the surface of the support promoted the dispersion and influenced the reduction states of the nickel species on the support, so it prevented the nickel species from sintering as well as from forming a spinel phase with alumina at high temperatures, and thus reduced the carbon deposition during the reaction. With the increase of the zirconia content in the catalyst, the catalytic performance for the CO methanation was enhanced. The Ni/Al₂O₃@ZrO₂-15 exhibited the highest CO conversion and methane selectivity at 400 °C, but they decreased dramatically above or below 400 °C due to the temperature sensitivity of the catalyst. Ni/Al₂O₃@ZrO₂-30 exhibited a high and constant rate of methane formation between 350 °C and 450 °C. The excellent catalytic performance of this catalyst is attributed to its reasonable pore structure and good dispersion of zirconia on the support. This catalyst has great potential to be further studied for the future industrial use.     

Electrochemical performance of LiCoO2 cathodes by surface modification using lanthanum aluminum garnet

LiCoO₂ particles were coated with various wt.% of lanthanum aluminum garnets (3LaAlO₃:Al₂O₃) by an in situ sol–gel process, followed by calcination at 1123 K for 12 h in air. X-ray diffraction (XRD) patterns confirmed the formation of a 3LaAlO₃:Al₂O₃ compound and the in situ sol–gel process synthesized 3LaAlO₃:Al₂O₃-coated LiCoO₂ was a single-phase hexagonal α-NaFeO₂-type structure of the core material without any modification. Scanning electron microscope (SEM) images revealed a modification of the surface of the cathode particles. Transmission electron microscope (TEM) images exposed that the surface of the core material was coated with a uniform compact layer of 3LaAlO₃:Al₂O₃, which had an average thickness of 40 nm. Galvanostatic cycling studies demonstrated that the 1.0 wt.% 3LaAlO₃:Al₂O₃-coated LiCoO₂ cathode showed excellent cycle stability of 182 cycles, which was much higher than the 38 cycles sustained by the pristine LiCoO₂ cathode material when it was charged at 4.4 V.     

Evaluation of Ni/Y2O3/Al2O3 catalysts for hydrogen production by autothermal reforming of methane

Ni/xY₂O₃–Al₂O₃ (x = 5, 10, 15, 20 wt%) catalysts were prepared by sequential impregnation synthesis. The catalytic performance for the autothermal reforming of methane was evaluated and compared with Ni/γ-Al₂O₃ catalyst. The physicochemical properties of catalysts were characterized by X-ray diffraction (XRD), Transmission electron microscope (TEM), X-Ray Photoelectron Spectrometer (XPS), Thermo Gravimetric Analyzer (TGA) and H₂-temperature programmed reduction techniques (TPR). The decrease of nickel particle size and the change of reducibility were found with Y modification. The CH₄ conversion increased with elevating levels of Y₂O₃ from 5% to 10%, then decreased with Y content from 10% to 20%. Ni/xY₂O₃–Al₂O₃ catalysts maintained high activity after 24 h on stream, while Ni/Al₂O₃ had a significant deactivation. The characterization of spent catalysts indicated that the addition of Y retarded Ni sintering and decreased the amount of coke.     

In situ crack-healing behavior of Al2O3/SiC composite ceramics undercyclic-fatigue strength

Alumina/silicon carbide (Al₂O₃/SiC) composite ceramics were sintered and subjected to three-point bending. A semicircular surface crack 50–320 µm in diameter was made on each sample. Crack-healing behavior was systematically studied, as a function of cyclic-fatigue strength, crack-healing temperature, healing time, and crack size, and the bending strength of the crack-healed sample from room temperature to 1500 °C were investigated. Four main conclusions were drawn from the present study.

(1)

Crack-healed Al₂O₃/SiC sample exhibited very high cyclic-fatigue limit of 700 MPa.     

Preparation of activated carbon supported Fe–Al2O3 catalyst and its application for hydrogen production by catalytic methane decomposition

Activated carbon (AC) supported Fe–Al₂O₃ catalysts were prepared by impregnation method and used for catalytic methane decomposition to hydrogen. The XRD and H₂-TPR results showed that ferric nitrate on AC support was directly reduced to Fe metal by the reducibility of carbon at 870 °C. The loading amount and Fe/Al₂O₃ weight ratio affect the textural properties and catalytic methane decomposition. The surface area and pore volume of the catalyst decrease with the loading of Fe and Al₂O₃. Mesopores with size of about 4.5 nm can be formed at the loading of 20–60% and promote the catalytic activity and stability. The mesopores formation is thought that Fe accelerates burning off of carbon wall and enlarging pore sizes during the pretreatment. When the Fe/Al₂O₃ ratio is 16/24 to 24/16 at the loading of 40%, the resultant catalysts show narrow mesopore distributions and relative high methane conversion. Al₂O₃ as the promoter can improve catalytic activity and shorten transitional period of AC supported Fe catalyst.     

Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite

The photocatalytic hydrogen production from aqueous methanol solution was investigated with ZnO/TiO₂, SnO/TiO₂, CuO/TiO₂, Al₂O₃/TiO₂ and CuO/Al₂O₃/TiO₂ nanocomposites. A mechanical mixing method, followed by the solid-state reaction at elevated temperature, was used for the preparation of nanocomposite photocatalyst. Among these nanocomposite photocatalysts, the maximal photocatalytic hydrogen production was observed with CuO/Al₂O₃/TiO₂ nanocomposites. A variety of components of CuO/Al₂O₃/TiO₂ photocatalysts were tested for the enhancement of H₂ formation. The optimal component was 0.2 wt% CuO/0.3 wt% Al₂O₃/TiO₂. The activity exhibited approximately tenfold enhancement at the optimum loading, compared with that with pure P-25 TiO₂. Nano-sized TiO₂ photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

Influence of the method of synthesis on hydrogen adsorption properties of mesoporous binary B2O3/Al2O3 gel systems

Three series of binary oxide systems B₂O₃/Al₂O₃ were prepared and the effect of alumina on dispersion of boron (B₂O₃) component was investigated. The aim of the study was to achieve a maximum dispersion of B₂O₃ in the Al₂O₃ a gel matrix that would lead to increased sorption capacity on boron oxide. Many attempts were made to establish the preparation conditions that would lead to a maximum dispersion of B₂O₃ in the Al₂O₃ gel matrix needed to increase the hydrogen sorption capacity on boron oxide. The systems were characterized by X-ray diffraction, SEM, TEM and low temperature nitrogen adsorption. Hydrogen adsorption was tested in the volumetric system.Results of the study showed that the amount of hydrogen adsorbed on B₂O₃ depended not only on the surface area of the system but also on the separation of B₂O₃ domains in Al₂O₃ gel network. Irrespective of the method of synthesis of the binary oxide system, the dispersion of B₂O₃ phase reflected in the amount of hydrogen adsorbed was the highest for the systems of the lowest B/Al molar ratios studied, i.e. for B/Al = 0.25.