Enhanced production of benzyl alcohol in the gas phase continuous hydrogenation of benzaldehyde over Au/Al2O3

Exclusive hydrogenation of benzaldehyde to benzyl alcohol in gas phase continuous operation (393–413 K, 1 atm) was achieved over Au/Al₂O₃, Au/TiO₂ and Au/ZrO₂. Synthesis of Au/Al₂O₃ by deposition–precipitation generated a narrower distribution (2–8 nm) of smaller (mean = 4.3 nm) Au particles relative to impregnation (1–21 nm, mean = 7.9 nm) with increased H₂ uptake under reaction conditions and higher benzaldehyde turnover. Switching reactant carrier from ethanol to water resulted in a significant enhancement of selective hydrogenation rate over Au/Al₂O₃ with 100% benzyl alcohol yield, attributed to increased available reactive hydrogen. This response extends to reaction over Au/TiO₂ and Au/ZrO₂.     

Dehydration of 2,3-butanediol into 3-buten-2-ol catalyzed by ZrO2

Vapor-phase catalytic dehydration of 2,3-butanediol was investigated over metal oxides such as CeO₂, La₂O₃, Yb₂O₃, ZrO₂, Al₂O₃, TiO₂, ZnO, Fe₂O₃, NiO, and Cr₂O₃. In the dehydration of 2,3-butanediol, 3-buten-2-ol was preferentially produced over monoclinic ZrO₂ along with major by-products such as butanone and 3-hydroxy-2-butanone. Over ZrO₂ calcined at 900 °C, 3-buten-2-ol was produced with a maximum selectivity of 59.0% at 300 °C without producing 1,3-butadiene.     

Evaluation of water treatment sludge as a catalyst for aqueous ozone decomposition

A new, novel, efficient, and stable green catalyst has been successfully used as a catalyst in aqueous ozone decomposition in acidic medium. The catalyst was characterized by using X-ray fluorescence (XRF), transmission electron microscope (TEM), scanning electron microscope (SEM), and X-ray diffraction (XRD) techniques. The sludge mainly consists of various metal and non-metal oxides. The effect of various experimental parameters such as catalyst loading, initial ozone concentrations, and various metal oxide catalysts on the decomposition of ozone was investigated. The decomposition of dissolved ozone was substantially enhanced by increasing the catalyst loading from 125 to 750 mg and by increasing the initial ozone concentration. The ozone decomposition efficiencies of Al₂O₃, SiO₂, TiO₂, Fe₂O₃, ZnO, and sludge have been studied and the efficiencies of these catalysts were found to be in the following order: ZnO ≈ sludge > TiO₂ > SiO₂ > Al₂O₃ ≈ Fe₂O₃. The catalytic stability was also investigated for up to four successive cycles and it was found that the catalyst was stable and ozone did not affect the catalyst morphology and its composition. However, the surface area of the catalyst increased after 1st cycle then it became stable. It was concluded that the sludge powder used in this study was a promising catalyst for aqueous ozone decomposition.     

Cost-effective large-scale synthesis of oxygen-defective ZnO photocatalyst with superior activities under UV and visible light

A cost-effective solution method was developed to produce ZnO photocatalyst in large quantity, through the conversion of ε-Zn(OH)₂ to ZnO in NaOH solutions. Experimental results indicated that the concentrated NaOH solution (4 mol L⁻¹) promoted the rapid formation of ZnO owing to the enhanced dissolution-precipitation reactions. The large-scale synthesis was also achieved with high-yield and solvent-recyclability. Structural analysis based on X-ray photoelectron spectroscopy, electron spin resonance and photoluminescence revealed that the as-prepared ZnO photocatalyst was rich in oxygen vacancies (V_O). The V_O-rich ZnO photocatalyst exhibited improved visible-light absorption, higher photocurrent responses and superior activities toward the degradation of rhodamine B under both UV (λ~254 nm) and visible-light illumination (λ>420 nm) compared to commercial ZnO and P25 TiO₂ powders, as well as good cycle stability. Based on the results of photoluminescence and active species detection, the V_O-enhanced photocatalytic activity was attributed to the generation of V_O-isolated level in the band structure. Under UV light, the V_O-level could promote charge separation by trapping the photoinduced electrons, while under visible-light, the V_O-level improved visible-light absorption and facilitated the charge generation. The presently developed synthesis may potentially benefit the large-scale production and low-cost application of ZnO photocatalyst for solar energy utilization.     

Sintering mechanisms of Yttria with different additives

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B₂O₃, YF₃, Al₂O₃, ZrO₂, and TiO₂, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B₂O₃ and YF₃ could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO₂, Al₂O₃ and ZrO₂ appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La₂O₃ added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.     

Protective properties of Al2O3 + TiO2 coating produced by the electrostatic spray deposition method

Mechanical resistance of Al₂O₃ + TiO₂ nanocomposite ceramic coating deposited by electrostatic spray deposition method onto X10CrAlSi18 steel to thermal and slurry tests was investigated. The coating was produced from colloidal suspension of TiO₂ nanoparticles dispersed in 3 wt% solution of Al₂(NO₃)₃, as Al₂O₃ precursor, in ethanol. TiO₂ nanoparticles of two sizes, 15 nm and 32 nm, were used in the experiments. After deposition, coatings were annealed at various temperatures, 300, 1000 and 1200 °C, and next exposed to cyclic thermal and slurry tests. Regardless of annealing temperature and the size of TiO₂ nanoparticles, the outer layer of all coatings was porous. The first five thermal cycles caused a rapid increase of aluminum content of the surface layer to 30–37 wt%, but further increase in the number of thermal cycles did not affect the aluminum content. The oxidation rate of coating-substrate system was lower during the thermal tests than during annealing. The oxidation rate was also lower for smaller TiO₂ particles (15 nm) forming the coating than for the larger ones (32 nm). The protective properties of Al₂O₃ + TiO₂ coating against intense oxidation of substrate were lost at 1200 °C. Slurry tests showed that coatings annealed at 1000 °C had the best slurry resistance, but thermal tests had weakened this slurry resistance, mainly due to decreasing adhesion of the coating.     

ZnO-capped nanorod gas sensors

This paper reports on the synthesis of pristine α-Fe₂O₃ nanorods and Fe₂O₃–ZnO core–shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core–shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5–200 ppm) at different temperatures (150–250 °C). The XRD and SEM revealed the excellent crystallinity of the Fe₂O₃–ZnO core–shell nanorods, as well as their uniformity in terms of shape and size. The Fe₂O₃–ZnO core–shell nanorod sensor showed a stronger response to ethanol than the pristine Fe₂O₃ nanorod sensor. The response (i.e., the relative change in electrical resistance R_a/R_g) of the core–shell nanorod sensor was 22.75 for 100 ppm ethanol at 200 °C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe₂O₃–ZnO core–shell nanorods was 15.96 s, which was shorter than that of the pristine nanorod sensor (22.73 s). The core–shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe₂O₃–ZnO core–shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe₂O₃–ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe₂O₃–ZnO interface.     

Photoluminescence mechanism of annealed ZnO/Zn/Al2O3 sandwich structures deposited on glass substrates

ZnO/Zn/Al₂O₃ sandwich structures are grown on glass substrates by magnetron sputtering. The effect of Al₂O₃ layers on optical properties of ZnO/Zn/Al₂O₃ sandwich structures is investigated. Results indicated that as the deposition time of Al₂O₃ increases, violet peak centered at 402 nm gradually shifted to 412 nm and the intensity firstly decreases and then increases. We discuss the intensity change and shift of violet peak relating to V_Zn defects and the band alignment of ZnO/Zn/Al₂O₃ sandwich structures, respectively. We proposed that ZnO/Zn/Al₂O₃ sandwich structures can be approximately regarded as a quasiquantum-well-like structure. So the electron tunneling from Zn to Al₂O₃ layer is suppressed and the photogenerated carriers can be confined in the Zn Fermi level. In order to further understand the effect of posttreatment on optical properties of samples, samples are annealed in vacuum at 350 °C for 1 h. PL emissions are weakened with the increase of Al₂O₃ deposition time. Interestingly, at a same deposition condition, PL emissions are still improved after posttreatment. Combined Al₂O₃ layer modulation with annealing treatment, steady PL properties can be effectively improved.     

Thermodynamic calculation for the activity and mechanism of Mn/TiO2 catalyst doped transition metals for SCR at low temperature

The introduction of transition metals in Mn/TiO₂ catalysts played significant roles in oxidative abstraction of hydrogen from adsorbed ammonia during the selective catalytic reduction (SCR). Thermodynamic calculation studies showed that the SCR performance was in accordance with the ammonia oxidation with transition metals, and the reaction tendency for the ammonia oxidation was decreased in the following order: CuO > Co₃O₄ > NiO > Fe₂O₃ > Cr₂O₃ > ZnO > La₂O₃ > CeO₂ > ZrO₂. In addition, Mn/TiO₂ catalyst doped metal (Fe and Cu) oxides enhanced performance for NO_x conversion, being approximately 100% at 453 K.     

Preparation of binary washcoat deposited on cordierite substrate for catalytic applications

The deposition of binary M–ZrO₂ (M = Al₂O₃, SiO₂ or TiO₂) as secondary support on cordierite substrate by dip-coating was investigated and the as-prepared M–ZrO₂/cordierite was characterized by means of XRD and SEM techniques. After two successive dip-coating processes, the highest washcoat loading of 25.1% was achieved in Al₂O₃–ZrO₂/cordierite, followed by 18.7% in TiO₂–ZrO₂/cordierite and then 17.4% in SiO₂–ZrO₂/cordierite. The optimal parameters to prepare Al₂O₃–ZrO₂ washcoat on cordierite were investigated in detail. After introduction of noble metal active components (Rh and Pd) to the secondary support, the obtained Rh/M–ZrO₂/cordierite and Pd/M–ZrO₂/cordierite were further tested as monolithic catalysts for the decomposition of nitrous oxide and the deep oxidation of benzene, respectively.     

Characteristics of ZnO-based varistor ceramics doped with Al2O3

The influence of Al₂O₃ doping in the range 0.00–0.83 mol% on the microstructure and current–voltage characteristics of ZnO-based varistor ceramics sintered at 1200 °C for 2 h was studied. The threshold voltage V_T (V/mm) increased up to a dopant level of about 0.08 mol% Al₂O₃; the nonlinear coefficient α was significantly increased by additions of up to 0.04 mol% Al₂O₃, although larger additions of Al₂O₃ caused it to decrease; and the leakage current increased sharply with increasing amounts of Al₂O₃. Doping with Al₂O₃ up to about 0.12 mol% Al₂O₃ resulted in a significantly decreased ZnO grain size, which is mainly responsible for the significantly increased threshold voltage, V_T. No ZnAl₂O₄ spinel phase was detected in any of the samples, and EDXS and WDXS analyses showed that most of the added Al₂O₃ distributed between the Zn₇Sb₂O₁₂ spinel phase and the ZnO phase, while only trace amounts were detected in the Bi₂O₃-rich phase. The spinel phase incorporates an appropriate amount of Al₂O₃; however, with an increasing amount of added Al₂O₃, more of it remains outside the spinel phase in the Bi₂O₃-rich liquid, where it can incorporate into the growing ZnO grains at the sintering temperature. The amount of Al in the ZnO grains was determined. A mechanism for the grain growth inhibition resulting from the small amounts of Al₂O₃ in the Bi₂O₃-rich liquid phase is also proposed.     

Synthesis of zirconia–alumina and alumina–zirconia core–shell particles via a heterocoagulation mechanism

This study describes the synthesis of core–shell particles, consisting of a ZrO₂ or Al₂O₃ submicron nucleus coated by a nanolayer of Al₂O₃ or ZrO₂, respectively. The oxide layers around the cores are deposited via a heterocoagulation route, based on the attraction of oppositely charged core and shell particles.TEM micrographs clearly show a homogeneous Al₂O₃ shell (originating from boehmite or γ-Al₂O₃ particles) around the ZrO₂ cores and in the other case, a ZrO₂ layer (originating from hydrothermally prepared ZrO₂) around the submicron Al₂O₃ cores. From PCS measurements, it can also be deduced that the cores are enwrapped by a shell and it is calculated that the thickness of the Al₂O₃ shell is about 30–35 nm and the ZrO₂ layer is approximately 80 nm. The coated powders are additionally characterized by XRD.     

Catalytic decomposition of CH2Cl2 over supported Ru catalysts

Ru/TiO₂ and Ru/Al₂O₃ were prepared by wet impregnation of TiO₂ and Al₂O₃, and tested in the catalytic decomposition of dichloromethane (DCM). Ru/TiO₂ catalyst presents the higher activity than Ru/Al₂O₃ catalyst, with 50% and 90% conversion occurring at 235 and 267 °C, respectively. Moreover, the higher stability on Ru/TiO₂ catalyst is observed, which can be related to ready removal of Cl species produced during DCM decomposition. The chlorine uptake on Ru/TiO₂ catalyst is estimated at 240 °C to be 0.36 mmol Cl/g_cat, while on Ru/Al₂O₃, the value is 1.46 mmol Cl/g_cat.     

Synthesis of magnetically recoverable ferrite (MFe2O4, M Co,Ni and Fe)-supported TiO2 photocatalysts for decolorization of methylene blue

Effects of ferrite materials as supports (CoFe₂O₄, NiFe₂O₄, and Fe₃O₄) on nano-TiO₂ were elucidated by their use in the oxidation of methylene blue. These photocatalysts, which were synthesized by co-precipitation, were characterized by XRD, SEM, EDS and VSM. The crystalline phase of TiO₂ onto magnetic MFe₂O₄ was formed by anatase and rutile. TiO₂/CoFe₂O₄ exhibited the strongest magnetic property of the prepared catalysts, and the photocatalytic efficiencies followed the order TiO₂/CoFe₂O₄ > TiO₂/NiFe₂O₄ > TiO₂/Fe₃O₄. MB decolorization was enhanced with the amount of TiO₂ on the photocatalyst, and was moderately affected by the extent of structural distortion of ferrite supports.     

Enhancing the thermoelectric properties of super-lattice Al2O3/ZnO atomic film via interface confinement

Aluminum oxide (Al₂O₃)/zinc oxide (ZnO) thin films deposited via atomic layer deposition (ALD) are demonstrated to enhance their thermoelectric properties by manipulating them with a nano-thick Al₂O₃ interface. The overall superlattice structure is tuned by varying the ZnO ALD sequence and the Al₂O₃ ALD sequence while maintaining the same composition. An aluminum-doped zinc oxide (AZO) thin film is deposited at 250 °C, and the Al₂O₃ thickness in the superlattice is gradually increased from 0.13 nm to 1.23 nm. The total film composition is fixed at 2% AZO. We observe that an efficient superlattice structure is made with a specific Al₂O₃ thickness. The thermal conductivity is significantly decreased from 0.57 W/mK to 0.26 W/mK as the thickness of the Al₂O₃ layer is increased. Additionally, the absolute Seebeck coefficient is increased from 14 μV/K to 65 μV/K. This may be caused by the interface confinement effect and interface scattering between the ZnO layer and the Al₂O₃ layer. The figure of merit ZT value is 0.14 for the most efficient structure.     

Electrical properties and microstructural evolution of ZrO2–Al2O3–TiN nanocomposites prepared by spark plasma sintering

Electroconductive ZrO₂–Al₂O₃–25 vol% TiN ceramic nanocomposites were prepared by spark plasma sintering at 1200 °C for 3 min. The electrical resistivity of the composites decreased from 4.5 × 10^?4 Ω m to 3 × 10^?5 Ω m as the Al₂O₃ content in the ZrO₂–Al₂O₃ matrix increased from 0 to 100 vol%. SEM images graphically presented the microstructural evolution of the composites and a geometrical percolation model was applied to investigate the relationship between the electrical property and the microstructure. The results indicated that the addition of Al₂O₃ to ZrO₂–TiN improved the electrical conductivity of the material by tailoring the structure from “nano–nano” type for ZrO₂–TiN to “micro–nano” type for ZrO₂–Al₂O₃–TiN.     

Selective gas-phase conversion of maleic anhydride to propionic acid on Pt-based catalysts

Pt-based catalysts, supported on Al₂O₃, SiO₂ and SiO₂–Al₂O₃, were prepared by incipient wetness impregnation and tested in the gas phase hydrogenation of maleic anhydride at atmospheric pressure and 240 °C. In these conditions, the hydrogenolytic activity pattern was: Pt/SiO₂ > Pt/Al₂O₃ > Pt/SiO₂–Al₂O₃, which is just the opposite of the support acidity trend. These metal Pt-based catalysts showed high selectivity to propionic acid, which was always higher than 80%. The selectivity pattern to this product was: Pt/Al₂O₃ > Pt/SiO₂ > Pt/SiO₂–Al₂O₃. Both activity and selectivity patterns may be explained on the basis of metal-support interaction and support acidity.     

Microstructures and material properties of fibrous HAp/Al2O3–ZrO2 composites fabricated by multi-pass extrusion process

Fibrous HAp/Al₂O₃–ZrO₂ composites were fabricated using the multi-pass extrusion process. In the 3rd and 4th passed extrusion bodies, fibrous microstructures were obtained. The 3rd and 4th passed Al₂O₃–ZrO₂ cores used as reinforcement, were about 35 and 4.5 μm in diameter, respectively. In the bodies sintered at over 1400 °C, the HAp decomposed and was transformed to β-TCP and TTCP, in which large numbers of pores were observed. The values of bending strength, Vickers hardness and fracture toughness of the 3rd passed HAp/Al₂O₃–ZrO₂ composites were 178 MPa, 325 Hv, and 3.4 MPa m^1/2, respectively while the values of the 4th passed bodies were 190 MPa, 405 Hv and 3.8 MPa m^1/2.     

Microstructure and mechanical properties of Al2O3/Y3Al5O12/ZrO2 hypereutectic directionally solidified ceramic prepared by laser floating zone

Al₂O₃/Y₃Al₅O₁₂/ZrO₂ directionally solidified ceramic has been considered as a promising candidate for ultrahigh temperature structural materials due to its excellent performance even close to its melting point. In this work, laser floating zone (LFZ) solidification experiments were performed on Al₂O₃/Y₃Al₅O₁₂/ZrO₂ hypereutectic with the solidification rates between 2 μm/s and 30 μm/s. The full eutectic lamellar microstructure is obtained with hypereutectic composition. The solid/liquid interface morphology is investigated. The microstructure characteristic is discussed based on the solid/liquid interface. The variation of lamellar spacing with different compositions and solidification rates was reported and discussed by considering an irregular eutectic growth model. The maximum hardness and fracture toughness are 19.06 GPa and 3.8 MPa m^1/2, respectively. The toughening mechanism of ZrO₂ is discussed based on the scenario of the crack propagation pattern.