Size effects on χ- to α-Al2O3 phase transformation

Nano-scaled χ-Al₂O₃ powders with d₅₀ mean particle sizes from 17 to 314 nm were prepared to investigate the size effect on their phase transformation. Structural properties and crystallization behavior as a function of thermal treatments of various-sized χ-Al₂O₃ particles were examined by DTA, XRD and TEM characterizations. It was confirmed that the decrease of particle size allows for stable α-Al₂O₃ formation at relatively low temperature. Furthermore, the phase transformation route of χ-Al₂O₃ to α-Al₂O₃ was also modified due to the decrease of particle size. A critical size of χ-Al₂O₃ that determines the phase transformation behavior was found to be around 40 nm. For particles larger than 40 nm, a transition phase of κ-Al₂O₃ is formed before obtaining final α-Al₂O₃ phase. Nevertheless, for those smaller than the critical size, starting χ-Al₂O₃ particles have to grow to 40 nm and then directly transform to α-Al₂O₃ bypassing κ-Al₂O₃ at a temperature as low as 1050 °C.     

Performance of Pd catalysts supported on nanocrystalline α-Al2O3 and Ni-modified α-Al2O3 in selective hydrogenation of acetylene

Nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ have been prepared by sol–gel and solvothermal methods and employed as supports for Pd catalysts. Regardless of the preparation method used, NiAl₂O₄ spinel was formed on the Ni-modified α-Al₂O₃ after calcination at 1150 °C. However, an addition of NiO peaks was also observed by X-ray diffraction for the solvothermal-made Ni-modified α-Al₂O₃ powder. Catalytic performances of the Pd catalysts supported on these nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ in selective hydrogenation of acetylene were found to be superior to those of the commercial α-Al₂O₃ supported one. Ethylene selectivities were improved in the order: Pd/Ni-modified α-Al₂O₃–sol–gel > Pd/Ni-modified α-Al₂O₃-solvothermal ≈ Pd/α-Al₂O₃–sol–gel > Pd/α-Al₂O₃-solvothermal  Pd/α-Al₂O₃-commerical. As revealed by NH₃ temperature program desorption studies, incorporation of Ni atoms in α-Al₂O₃ resulted in a significant decrease of acid sites on the alumina supports. Moreover, XPS revealed a shift of Pd 3d binding energy for Pd catalyst supported on Ni-modified α-Al₂O₃–sol–gel where only NiAl₂O₄ was formed, suggesting that the electronic properties of Pd may be modified.     

Sintering kinetics of α-Al2O3 powder

The sintering kinetics of α-Al₂O₃ powder are reviewed in this paper. The initial sintering of α-Al₂O₃ micropowder and α-Al₂O₃ nanopowder is all controlled by grain boundary diffusion. The sintering kinetics dominate up to a relative density of 0.77, where the coarsening kinetics dominate during further densification. Herring's scaling law can be used to predict the approximate sintering temperature of α-Al₂O₃ powder and demonstrates that if the particle size can be reduced to <20 nm, sintering below 1000°C may be possible. ©     

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.     

Effect of the reducing agent on the hydrodechlorination of dioxins over 2 wt.% Pd/γ-Al2O3

2-Propanol and molecular H₂ (in methanol (MeOH) and MeOH–water) were examined as reducing agents for the liquid phase hydrodechlorination (HDC) of dioxins over 2 wt.% Pd/γ-Al₂O₃. Different amounts of NaOH were added to the reaction mixtures. The 2-propanol and H₂(g)/MeOH systems presented similar HDC activity. Notwithstanding, Pd sintering and graphitic carbon directly bonded to Pd on catalyst surface was observed on samples used with H₂(g)/MeOH. The addition of water to H₂(g)/MeOH decreased Pd sintering and favored dissolution of sodium compounds. However, dioxin degradation efficiency diminished. By contrast, 2-propanol acting both as reducing agent and solvent provided hydrogen to the HDC reaction, avoided metal sintering and Pd–C formation. Besides, almost complete dioxin degradation under mild reaction conditions was obtained. Kinetic experiments of dioxin HDC with 2-propanol showed a maximum net reaction rate and turnover frequency (TOF) for a given initial concentration of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). After that value, both reaction rate and TOF decreased. On the other hand, reaction rates and TOFs of dioxin-like polychlorinated biphenyls (DL-PCBs) linearly increased with concentration.     

Dynamic corrosion of Al2O3-ZrO2-SiO2 and Cr2O3-containing refractories by molten frits. Part II: Microstructural study

In this work results on dynamic corrosion studies of fused cast Al₂O₃-SiO₂-ZrO₂ and isostatically pressed and sintered Cr₂O₃-based refractories by two crystalline (transparent) frits are described. Experiments have been performed using the “Merry Go Round” test at ≅1500 °C.Microstructural and mineralogical analyses of selected areas from the corroded regions of the studied refractories were performed by reflected light optical microscopy and scanning electron microscopy with analysis by X-ray dispersive energy.Significant differences between the corrosion mechanisms acting in the two types of materials were found. In the fused cast Al₂O₃-SiO₂-ZrO₂ specimens corrosion took place by the dissolution of alumina and zirconia in the frit and in the glass formed by the reaction between the frit and the refractory. In the Cr₂O₃-based materials the corrosion process was controlled by the capillar penetration of the molten frit through the open pores. The reaction between the ZnO from the frits and Cr₂O₃ led to the formation of spinel (ZnCr₂O₄), a high-melting point bonding phase that retarded the frit penetration. Results are discussed using the relevant phase equilibrium diagrams.     

The functionalities of Pt/γ-Al2O3 catalysts in simultaneous HDS and HDA reactions

A Pt/γ-Al₂O₃ catalyst was tested in simultaneous hydrodesulfurization (HDS) of dibenzothiophene and hydrodearomatization (HDA) of naphthalene reactions. Samples of it were subjected to different pretreatments: reduction, reduction–sulfidation, sulfidation with pure H₂S and non-activation. The reduced catalyst presented the best performance, even comparable to that of Co(Ni)Mo catalysts. All catalyst samples were selective to the HDS reaction over HDA, and to the direct desulfurization pathway of dibenzothiophene HDS over the hydrogenation reaction pathway of HDS. The effect of H₂S partial pressure on the functionalities of the reduced Pt/γ-Al₂O₃ catalyst was studied. The results showed that an increase in H₂S partial pressure does not cause poisoning, but an inhibition effect, without changing the catalyst selectivity. Accordingly, the activity trends were ascribed to adsorption differences between the different reactive molecules over the same catalytic active site. TPR characterization along with a thermodynamics analysis showed that the active phase of reduced Pt/γ-Al₂O₃ is constituted by Pt⁰ particles. However, presulfidation of the catalyst leads to a mixture of PtS and Pt⁰ which has a negative effect on the catalytic performance without changing catalyst functionalities.     

Dynamic corrosion of Al2O3-ZrO2-SiO2 and Cr2O3-containing refractories by molten frits. Part I: Macroscopic analysis

Manufacturing of enamels and frits has undergone dramatic changes since the 1980s. This has required significant efforts in research and development. Typical compositions of frits for ceramic tiles are silica-based with fluxing agents; some of the components are highly corrosive. Improvements in the production of frits would imply the selection of the most adequate refractories as a function of the chemical composition of the considered frit and the fabrication procedure.The refractories currently used in frit furnaces are Al₂O₃-ZrO₂-SiO₂ (AZS) fused cast materials and Cr₂O₃-based materials. In this work, results on dynamic corrosion studies of AZS and Cr₂O₃-based materials by two ZnO-containing frits are described. Experiments have been performed using the “Merry Go Round” test at ≅1500 °C. Macroscopic results are analysed in terms of the remaining volume after the tests, as usually done in the glass industry. The significance and limits of such an approach are discussed.     

Corrosion mechanisms of Al2O3/MgAl2O4 by V2O5, NiO, Fe2O3 and vanadium slag

The effects of V₂O₅, NiO, Fe₂O₃ and vanadium slag on the corrosion of Al₂O₃ and MgAl₂O₄ have been investigated. The specimens of Al₂O₃ and MgAl₂O₄ with the respective oxides above mentioned were heated at 10 °C/min from room temperature up to three different temperatures: 1400, 1450 and 1500 °C. The corrosion mechanisms of each system were followed by XRD and SEM analyses. The results obtained showed that Al₂O₃ was less affected by the studied oxides than MgAl₂O₄. Alumina was only attacked by NiO forming NiAl₂O₄ spinel, while the MgAl₂O₄ spinel was attacked by V₂O₅ forming MgV₂O₆. It was also observed that Fe₂O₃ and Mg, Ni, V and Fe present in the vanadium slag diffused into Al₂O₃. On the other hand, the Fe₂O₃ and Ca, S, Si, Na, Mg, V and Fe diffused into the MgAl₂O₄ structure. Finally, the results obtained were compared with those predicted by the FactSage software.     

Phase diagram of the Al2O3-HfO2-Y2O3 system

The phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system was first constructed in the temperature range 1200-2800 °C. The phase transformations in the system are completed in eutectic reactions. No ternary compounds or regions of appreciable solid solution were found in the components or binaries in this system. Four new ternary and three new quasibinary eutectics were found. The minimum melting temperature is 1755 °C and it corresponds to the ternary eutectic Al₂O₃ + HfO₂ + Y₃Al₅O₁₂. The solidus surface projection, the schematic of the alloy crystallization path and the vertical sections present the complete phase diagram of the Al₂O₃-HfO₂-Y₂O₃ system.     

CO2 adsorption and activation over γ-Al2O3-supported transition metal dimers: A density functional study

Catalytic conversion of CO₂ to liquid fuels has the benefit of reducing CO₂ emission. Adsorption and activation of CO₂ on the catalyst surface are key steps of the conversion. Herein, we used density functional theory (DFT) slab calculations to study CO₂ adsorption and activation over the γ-Al₂O₃-supported 3d transition metal dimers (M₂/γ-Al₂O₃, M = Sc–Cu). CO₂ was found to adsorb on M₂/γ-Al₂O₃ negatively charged and in a bent configuration, indicating partial activation of CO₂. Our results showed that both the metal dimer and the γ-Al₂O₃ support contribute to the activation of the adsorbed CO₂. The presence of a metal dimer enhances the interaction of CO₂ with the substrate. Consequently, the adsorption energy of CO₂ on M₂/γ-Al₂O₃ is significantly higher than that on the γ-Al₂O₃ surface without the metal dimer. The decreasing binding strength of CO₂ on M₂/γ-Al₂O₃ as M₂ changes from Sc₂ to Cu₂ was attributed to decreasing electron-donation by the supported metal dimers. Hydroxylation of the support surface reduces the amount of charge transferred to CO₂ for the same metal dimer and weakens the CO₂ chemisorption bonds. Highly dispersed metal particles maintained at a small size are expected to exhibit good activity toward CO₂ adsorption and activation.     

HDS of benzothiophene and dihydrobenzothiophene over sulfided Mo/γ-Al2O3

The hydrodesulfurization (HDS) of benzothiophene (BT) and dihydrobenzothiophene (DHBT) was studied over a sulfided Mo/γ-Al₂O₃ catalyst at 5 MPa and 280 and 300 °C. In the absence of H₂S, benzothiophene reacted by hydrogenation to dihydrobenzothiophene and by hydrogenolysis to ethylbenzene (EB), and dihydrobenzothiophene reacted by hydrogenolysis to ethylbenzene. H₂S inhibited both hydrogenation and hydrogenolysis, but the latter much more strongly. The reverse inhibition was observed for 2-methylpiperidine (MPi). In the presence of H₂S and/or 2-methylpiperidine, dihydrobenzothiophene reacted to ethylbenzene as well as by total hydrogenation to octahydrobenzothiophene, and on to ethylcyclohexenes and ethylcyclohexane. Dihydrobenzothiophene did not react back to benzothiophene at and below 300 °C, while the equivalent tetrahydrodibenzothiophene reacted fast to an equilibrium with tetrahydrodibenzothiophene, due to stabilization of the vinylic bond by the alkyl groups. The observed products and kinetic results were explained by a model in which the CS bonds were mainly broken by hydrogenolysis.     

Laser machining of Al2O3-ZrO2 (3%Y2O3) eutectic composite

In this work we present the study of the interaction between NIR pulsed laser and Al₂O₃-ZrO₂ (3%Y₂O₃) eutectic composite. The effect produced by modifying the reference position as well as the working conditions and laser beam features has been studied when the samples are processed by means of pulse bursts.The samples were obtained by the laser floating zone technique using a CO₂ laser system. The laser machining was carried out with a Q-switched Nd:YAG laser at its fundamental wavelength of 1064 nm with pulse-widths in the nanosecond range.Geometric dimensions, i.e. ablated depth, machined width and removed volume as well as ablation yield of the resulting holes have been studied. We have described and discussed the morphology, composition and microstructure of the processed samples.     

Effect of CO2 in the feed stream on the deactivation of Co/γ-Al2O3 Fischer–Tropsch catalyst

The influence of CO₂ on the deactivation of Co/γ-Al₂O₃ Fischer–Tropsch (FT) catalyst in CO hydrogenation has been investigated. The presence of CO₂ in the feed stream reveals a negative effect on catalyst stability and in the formation of heavy hydrocarbons. The CO₂ acts as a mild oxidizing agent on cobalt metal during Fischer–Tropsch synthesis. During FT synthesis on Co/γ-Al₂O₃ of 70 h, the CO conversion and C₅₊ selectivity in the presence of CO₂ decreased more significantly than in the absence of CO₂. CO₂ is found to be responsible for the partial oxidation of surface cobalt metal at FT synthesis environment with the co-existence of generated water.     

Matrix–filler interactions in polysilazane-derived ceramics with Al2O3 and ZrO2 fillers

Interactions between a poly(vinyl)silazane and Al₂O₃ or Y₂O₃-stabilised ZrO₂ fillers were studied during the fabrication of polysilazane-derived bulk ceramics in order to investigate the influence of oxide fillers on resulting properties. Specimens were produced by coating of the filler powders with the polysilazane, warm-pressing of the resulting composite powders, and pyrolytic conversion in flowing N₂ at various temperatures between 1000 °C and 1400 °C. Significant differences in densification were observed, depending on the filler used. Reactions between the polysilazane-derived matrix and Al₂O₃ or ZrO₂ at temperatures ≥1300 °C resulted in the formation of Si₅AlON₇ or ZrSiO₄, respectively. Reactivity in the polysilazane-derived component was a result of SiO₂ contamination caused primarily by adsorbed species on the filler particle surface. Knowledge of polysilazane/filler interface processes is found to be decisive for the prediction of properties such as shrinkage and porosity, which heavily influence performance of a material.     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

Ru/γ-Fe2O3-Al2O3催化剂在磁稳定床中 苯选择性加氢性能

设计建造了磁稳定床加氢实验装置,以磁性氧化铝为载体,通过浸渍法制备了蛋壳型钌基磁性Ru/γ-Fe₂O₃-γ-Al₂O₃微球催化剂,详细考察了磁性催化剂的制备参数、磁稳定床的操作参数对苯选择性加氢的影响。结果表明磁稳定床的链式操作状态提高了环己烯的选择性,证实了所研制的蛋壳型钌基磁性微球催化剂适用于磁稳定床中苯的选择性加氢工艺,具有较好的应用前景。     

Synergetic effect of combined use of Cu–ZnO–Al2O3 and Pt–Al2O3 for the steam reforming of methanol

Commercial Cu–ZnO–Al₂O₃ catalysts are used widely for steam reforming of methanol. However, the reforming reactions should be modified to avoid fuel cell catalyst poisoning originated from carbon monoxide. The modification was implemented by mixing the Cu–ZnO–Al₂O₃ catalyst with Pt–Al₂O₃ catalyst. The Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture created a synergetic effect because the methanol decomposition and the water–gas shift reactions occurred simultaneously over nearby Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalysts in the mixture. A methanol conversion of 96.4% was obtained and carbon monoxide was not detected from the reforming reaction when the Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture was used.