In heterogeneous catalysis, supports play a crucial role in modulating the geometric and electronic structure of the active metal phase for optimizing the catalytic performance. A γ-Al2O3 nanosheet ...that contains 27% pentacoordinate Al(3+) sites can nicely disperse and stabilize raft-like Pt-Sn clusters as a result of strong interactions between metal and support. Consequently, there are strong electronic interactions between the Pt and Sn atoms, resulting in an increase in the electron density of the Pt sites. When used in the propane dehydrogenation reaction, this catalyst displayed an excellent specific activity for propylene formation with >99% selectivity, and superior anti-coking and anti-sintering properties. Its exceptional ability to maintain the high activity and stability at ultrahigh space velocities further showed that the sheet construction of the catalyst facilitated the kinetic transfer process.
•This study explored the influence of added nano-aluminas on CaO-activated GGBFS.•The effect of the nano-alumina on strength depended on the crystallinity of alumina.•The dissolving γ-alumina led to ...noticeable increase in the formation of C-S-H.•The nano-aluminas caused fast gypsum consumption, resulting in ettringite formation.•Adding γ-alumina with gypsum was most effective for early strength improvement.
This study investigated the role of crystallinity of added nanometer-sized Al2O3 (nano-alumina) (i.e., poorly crystalline nano-alumina (γ-alumina) vs. crystalline nano-alumina (α-alumina)) and the interaction of nano-aluminas with added gypsum in the CaO-activated ground granulated blast furnace slag (GGBFS) binder system. In the results, the use of γ-alumina evidently increased the dissolution degree of silicon (Si) from GGBFS regardless of adding gypsum, which was the main ingredient of calcium silicate hydrate (C-S-H), and thus led to a noticeable increase of C-S-H. However, the sole use of α-alumina without added gypsum barely affected the C-S-H formation, although the added α-alumina alongside gypsum also had a similar effect on the dissolution of Si. In addition, due to the amorphous nature of γ-alumina, the dissolution of γ-alumina was faster than that of α-alumina. Thus, the use of γ-alumina produced ettringite at the faster rate, as well achieving the greater and faster strength increase, although the added α-alumina was also slightly beneficial in accelerating the formation of ettringite. Thus, overall, the use of γ-alumina was significantly more advantageous in improving strength by increasing the quantities of reaction products (C-S-H and/or ettringite) than that of α-alumina.
Improving the thermal stability and modifying the porous properties of γ-alumina by incorporating various transition and rare earth metals in alumina have been widely studied. However, less attention ...has been paid to investigating the co-doping process for increasing the alumina thermal stability and surface area in temperatures ranging from 1000° to 1200°C. In this research, pure, Zr-doped, and Zr-La doped granules with an average diameter of 1.5–2 mm were synthesized using the sol-gel granulation method. XRD results indicated that the introduction of either Zr or Zr-La dopants improves the thermal stability of transition aluminas up to 1200 °C. DTA/TG results revealed that the incorporation of 1 wt% zirconium along with 1 wt% lanthanum retarded α-Al2O3 phase transformation temperature to around 1335 °C. Compared to the pure sample, the BET surface area of alumina containing 1 wt% zirconium, 1 wt% zirconium along with 1 wt% lanthanum, and 1 wt% zirconium along with 3 wt% lanthanum increased to 234.4, 232.4, and 215.6 m2/g at 750 °C, respectively. Moreover, the sample containing 1 wt% of Zr along with 1 wt% of La dopants maintained a higher specific surface area after calcination at 1000 °C (133.6 m2/g) and 1200 °C (64.7 m2/g). According to NMR and HRTEM investigations, the capability of the co-doped Zr-La alumina to preserve the mesoporous structure and thermal stability as well as suppressing α-alumina formation up to 1200 °C is the result of zirconium and lanthanum dopants occupying tetrahedral and octahedral vacancies in the structure of transition alumina. The reported Zr-Al co-doped alumina granules in this study with high thermal and porous structural stability can be good candidates as a support for industrial catalysts.
•Production of mesoporous alumina granules with a homogenous microstructure through a facile process.•Revealing the effect of Zr and Zr-La dopants on the phase stability and microstructural evolution of γ-alumina as a catalyst support.•A superior thermal, structural, and porous texture stability at elevated temperatures in Zr-La co-doped alumina.•Inhibition of α-alumina nucleation and phase transformation up to 1200 °C by zirconium and lanthanum co-doping in alumina.
In the present study, the deoxygenation of canola oil was performed in a continuous fix-bed reactor. Wormhole-like mesostructured γ-alumina with nano-sized crystalline domains and pores in the range ...of 8nm was one-pot synthesized using polymeric template assisted sol-gel method via evaporation-induced self-assembly (EISA). Nanoporous catalysts were prepared by employing the incipient wetness co-impregnation method followed by a calcination step, 15wt% MoO3 and 3wt% NiO or CoO, were impregnated on the nanoporous support. Both catalysts favored the hydrodeoxygenation reaction pathway, and the liquid hydrocarbons consisted mostly of C15–C18 n-alkanes. The effects of LHSV and temperature on the liquid product composition were investigated in the range of, LHSV: 1 to 3h−1, and temperature of 325 to 400°C while keeping other reaction conditions constant at a pressure of 450psi and H2/oil of 600mLmL−1. Slightly better catalytic activity was perceived for NiMo-S/γ-alumina at higher LHSV compared to CoMo-S γ-alumina catalyst. The liquid conversion on NiMo-S/γ-alumina is higher than that on CoMo-S/γ-alumina over the temperature range of 325 to 350°C. At 375°C, the conversion reached 100% over both catalysts. The production of green diesel range liquid products over NiMo-S/γ-alumina and CoMo-S/γ-alumina was found optimal at 325°C and 1h−1 LHSV.
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•Hydrodeoxygenation of canola oil to green diesel•High yield in C15–C18 normal alkane•Comparison of sulfided NiMo and CoMo supported on mesostructured γ-alumina•XPS analysis shows S more closely associated with Mo than Ni or Co.
The limitations of oxide/oxide ceramic matrix composites lie in the need to improve their mechanical properties. It is challenging to reconcile the conflict between matrix densification at high ...temperature and the potential thermal damage to fibers. The concept of using element doping to control the sintering behavior of matrix was initially applied to prepare alumina-based ceramic matrix composites (CMCs). To lower the sintering temperature of the Al2O3 matrix, MnO2 dopant was used as a sintering aid. As a result, the thermal damage to alumina fibers was decreased, and the mechanical properties of the composite were significantly enhanced. The effect of MnO2 content on the bending strength and micro-morphology of the composite was investigated. The composite exhibits optimal mechanical properties with a 0.01 wt% MnO2 doping, and its bending strength is approximately 405 MPa. The investigation aimed to determine the mechanism by which the fluctuation in micro-mechanical properties of the composites with different MnO2 contents. After the thermal aging at 1000℃ for 500 h, the bending strength of the N610/Al2O3-0.01 composite decreases to 297 MPa. The evolution and mechanism of thermal aging damage have been studied.
Identifying a crystal structure of alumina (Al2O3) scale is critical for evaluating the performance of heat-resistant alloys because α-Al2O3 is stable and protective against high-temperature ...oxidation and corrosion but θ- and g-Al2O3 provide poor oxidation resistance. Conventional methods to identify crystal structures of Al2O3 scales are time-consuming. Herein, the author proposes a method to rapidly identify crystal structures of Al2O3 scales on β-NiAl by obtaining their cathodoluminescence (CL) spectra. α-, θ-, and g-Al2O3 can be identified by detecting a sharp peak at 695.8 nm and 686.3 nm, and a broad peak at around 700 nm, respectively, in CL spectra. Concentrations of α-, θ-, and g-Al2O3 scales can be determined roughly from intensities of these peaks. This method can be applied to areas ranging from the millimeter to micrometer scale, and the acquisition time for the CL spectra was less than 10 s. The results indicate that obtaining CL spectra contributes to the identification of crystal structures of Al2O3 scales on heat-resistant alloys and a reduction in time to evaluate the performance of heat-resistant alloys.
Identifying a crystal structure of alumina (Al2O3) scale is critical for evaluating the performance of heat-resistant alloys because α-Al2O3 is stable and protective against high-temperature ...oxidation and corrosion but θ- and g-Al2O3 provide poor oxidation resistance. Conventional methods to identify crystal structures of Al2O3 scales are time-consuming. Herein, the author proposes a method to rapidly identify crystal structures of Al2O3 scales on β-NiAl by obtaining their cathodoluminescence (CL) spectra. α-, θ-, and g-Al2O3 can be identified by detecting a sharp peak at 695.8 nm and 686.3 nm, and a broad peak at around 700 nm, respectively, in CL spectra. Concentrations of α-, θ-, and g-Al2O3 scales can be determined roughly from intensities of these peaks. This method can be applied to areas ranging from the millimeter to micrometer scale, and the acquisition time for the CL spectra was less than 10 s. The results indicate that obtaining CL spectra contributes to the identification of crystal structures of Al2O3 scales on heat-resistant alloys and a reduction in time to evaluate the performance of heat-resistant alloys.
Biomaterials—the materials used for the manufacturing of medical devices— are part of everyday life. Each one of us has likely had the experience of visting a dentist’s office, where a number of ...biomaterials are used temporarily or permanently in the mouth. Devices that are more complex are used for to support, heal, or replace living tissues or organs in the body that are suffering or compromised by different conditions. The materials used in their construction are metals and metallic alloys, polymers—ranging from elastomers to adhesives—and ceramics.Within these three cases, there are materials that are inert in the living environment, that perform an active function, or that are dissolved and resorbed by the metabolic pathways. Biomaterials are the outcome of a dynamic field of research that is driven by a growing demand and by the competition among the manufacturers of medical devices, with innovations improving the performance of existing devices and that contribute to the development of new ones. The collection of papers forming this volume have one particular class of of biomaterial in common, ceramic (bioceramic) composites, which as so far been used in applications such as orthopaedic joint replacement as well as in dental implants and restorations and that is being intensively investigated for bone regeneration applications. Today’s bioceramic composites (alumina–zirconia) are the golden standard in joint replacements. Several manufracturers have proposed different zirconia–alumina composites for use in hip, knee, and shoulder joint replacements, with several other innovative devices also being under study. In addition, bioceramic composites with innovative compositions are under development and will be on the market in years to come. Something that is especially interesting is the application of bioceramic composites in the regeneration of bone tissues. Research has devoted special attention to the doping of well-known materials (i.e., calcium phosphates and silicates) with bioactive ions, aiming to enhance the osteogenic ability and bioresorbability of man-made grafts. Moreover, high expectations rely on hybrid biopolymer/ceramic materials that mimic the complex composition and multiscale structure of bone tissue.
•Nano γ–alumina (20 nm) was first reported for Al2O3 PLOT column.•Highly performance and good selectivity different with either bulk or commercial alumina columns.•Easily coated with dynamic method ...under relative lower coating pressure.•Film thickness could be tuned by coated again, which was not possible on bulk alumina.•The γ crystallite structure and size remained constant of nano γ–alumina whereas specific surface area, pore volume and average half pore width changed obviously.
An alumina porous layer open-tubular (Al2O3 PLOT) column coated with γ–alumina nanoparticles (20 nm) for highly volatile hydrocarbons (C1 to C5) separation was described. Relative to the coating of bulk alumina, this column was easily coated with dynamic method under 0.4 or 0.6 MPa for 0.53 mm or 0.32 mm capillary, respectively. And the thickness of coating layer could be tuned by repeating the coating process after column was dried and activated at 300 °C for 3 h. The effect of deactivation agents on the physicochemical properties of nano γ–alumina was characterized by X–ray diffraction (XRD) measurement and nitrogen adsorption–desorption isotherms. The influences of deactivation agents, film thickness, conditioning and column dimensions on the inertness, polarity, selectivity and elution order of C1 to C5 separation were investigated in detail. The crystallite structure and size of nano alumina were not affected by the deactivation agents and remained constant during the column making processes, whereas specific surface area, pore volume and average half pore width altered significantly. The specific surface area decreased to 125.4 m2 g–1 or 174.0 m2 g–1 and the average half pore size distributions decreased to 1.6–8.4 nm or 2.4–14.3 nm when it was deactivated with potassium chloride or sodium sulfate solution, respectively. The deactivation agents and its concentrations impacted significantly on the retention performance of column. The column deactivated with sodium sulfate solution exhibited stronger polarity and lower selectivity than which deactivated with potassium chloride solution although both columns showed good inertness. The length, internal diameter and film thickness of the column had less influence on the selectivity and resolution for C1 to C5 hydrocarbons separation, whereas the conditioning temperature and time had an obvious influence. The column had distinguished polarity and selectivity which was different from either bulk or commercial alumina columns. Typically, the hydrocarbons were baseline separated with resolutions ranging from 1.65 to 15.33 within 9 min under programmed temperature below 100 °C, and the tailing factors ranging from 1.02 to 1.07.
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In this thesis, we build and solve various mathematical models to describe the interplay between alumina particles and a cryolite bath that takes place in a Hall-Héroult cell, which is the ...foundational process for aluminium production. We aim to understand the heat and mass transfer mechanisms taking place in order to gather insights into the aluminium production process and use these insights to suggest process improvements. First, we examine the problem of adding a cold alumina particle to a bath of hot cryolite. We develop a spherically symmetric model to describe heat and mass transfer between the particle and the surrounding material, allowing for freezing and melting of the cryolite close to the particle and its subsequent dissolution. We solve this model numerically using the method of lines using the early-time asymptotic solution to initiate the numerical scheme. We explore the behaviour as we vary the key dimensionless parameters. We also consider the model in the limits of small superheat and of small Stefan number. Using these asymptotic solutions, we find approximate expressions for the freezing, melting, and dissolution timescales, with freezing being the fastest effect and dissolution the slowest. Also, using the early-time solution, we determine constraints on the parameters for which shell growth is possible. Our results predict that increasing the superheat or decreasing the particle size should decrease the time it takes for a particle to dissolve, in accordance with practical experience. Furthermore, we note that the temperature changes are localised to a 5--10 particle radii. Second, we formulate a one-dimensional model for the phase-changing infiltration into a porous alumina raft placed on top of the cryolite. We simplify our model in the limit of small superheat. We solve the equations using the same approach as in Chapter 2 with the method of lines, using the early-time asymptotic solution to initiate the numerical scheme. We find that the early-time problem can exhibit a stable and an unstable solution and in certain parameter regions, these solutions disappear altogether. Furthermore, depending on the heat loss through the top of the raft, the model can undergo a discontinuous change in behaviour which corresponds to the clogging of the pore space. Additionally, near clogging, we find power-law behaviour of the frozen cryolite fraction and the location of the infiltration and solidification fronts. Third, we formulate a model for the disintegration of a spherical clump of alumina particles. We solve the problem numerically with the method of lines and see that, for certain parameters, the solution behaves like a travelling dissolution front, while in other regimes there is bulk dissolution. We investigate the travelling dissolution front by using the method of matched asymptotic expansions to reduce the full problem. The asymptotic expressions match well with the numerical solutions up to the order that would be expected. In Chapter 5 we formulate a model for the phase-changing infiltration into a spherical clump of alumina particles, which physically corresponds to a larger batch that is entirely enclosed by the cryolite. We find that many of the results in Chapter 3 carry over, such as the multiple solutions for early time, the late time clogging, or the fact that the phase-changing front is slower than the isothermal one. The presence of clogging in this model is especially interesting as it is caused by the slowing down of the front, as opposed to the cooling at the top, which was the case in Chapter 3. Finally, we summarize the findings and offer possible extensions to our models in Chapter 6 and present our mathematical findings in an approachable way in Chapter 7. The alumina feeding is a process with many interacting parts producing complex behaviour. In this thesis we shed light on a few problems, but the research is not over.
