Al2O3 Surface Research Articles

Monolithic integrated all-GaN-based µLED display by selective area regrowth.

This work demonstrates an all-GaN-based µLED display with monolithic integrated HEMT and µLED pixels using the selective area regrowth method. The monochrome µLED-HEMT display has a resolution of 20 × 20 and a pixel pitch of 80 µm. With the optimized regrowth pattern, the µLED-HEMT achieves a maximum light output power of 36.2 W/cm2 and a peak EQE of 3.36%, mainly due to the improved crystal quality of regrown µLED. TMAH treatment and Al2O3 surface passivation are also performed to minimize the impact of nonradiative recombination caused by the dry etching damage. With a custom-designed driving circuit board, images of "HKUST" are successfully shown on the µLED-HEMT display.

Purification mechanism of microbial metabolism in kitchen-oil wastewater enhanced by cationic vacancies on γ-Al2O3

The use of catalyst materials to mediate the enhancement of microbial degradation in wastewater is a new economic and energy saving breakthrough in water treatment technology. In this study, γ-Al2O3, which is commonly used as catalyst/carrier, is used as biological filler to treat kitchen-oil wastewater with low biodegradability, and the COD removal rate is about 50 %. It is found that the complexation of cationic vacancies on Al2O3 surface with extracellular polymeric substance (EPS) secreted by microorganisms in wastewater lead to the polarization of electron distribution on biofilm. The efficient degrading bacteria are enriched on reaction interface and obtain electrons to maintain electron dynamic balance by enhancing the transmembrane metabolism of pollutants. The aluminum vacancies on Al2O3 surface accelerate the microbial degradation of pollutants. The cationic vacancies in the structure of catalyst accelerate the acquisition of exogenous electrons by microorganisms without the addition of external energy, which provides a new idea for catalytic fillers to enhance wastewater degradation.

Ion solvation as a predictor of lanthanide adsorption structures and energetics in alumina nanopores

Adsorption reactions at solid-water interfaces define elemental fate and transport and enable contaminant clean-up, water purification, and chemical separations. For nanoparticles and nanopores, nanoconfinement may lead to unexpected and hard-to-predict products and energetics of adsorption, compared to analogous unconfined surfaces. Here we use X-ray absorption fine structure spectroscopy and operando flow microcalorimetry to determine nanoconfinement effects on the energetics and local coordination environment of trivalent lanthanides adsorbed on Al2O3 surfaces. We show that the nanoconfinement effects on adsorption become more pronounced as the hydration free energy, ΔGhydr, of a lanthanide decreases. Neodymium (Nd3+) has the least exothermic ΔGhydr (−3336 kJ·mol−1) and forms mostly outer-sphere complexes on unconfined Al2O3 surfaces but shifts to inner-sphere complexes within the 4 nm Al2O3 pores. Lutetium (Lu3+) has the most exothermic ΔGhydr (−3589 kJ·mol−1) and forms inner-sphere adsorption complexes regardless of whether Al2O3 surfaces are nanoconfined. Importantly, the energetics of adsorption is exothermic in nanopores only, and becomes endothermic with increasing surface coverage. Changes to the energetics and products of adsorption in nanopores are ion-specific, even within chemically similar trivalent lanthanide series, and can be predicted by considering the hydration energies of adsorbing ions.

Effect of water molecular behavior on adhesion properties of asphalt-aggregate interface

Asphalt-aggregate interfaces are prone to damage by moisture, leading to a decrease in interfacial adhesion. However, the precise mechanism governing water molecule behavior at these interfaces remains poorly understood. To address this, we employed molecular dynamics simulation to investigate the interaction of water molecules with aggregates and asphalts, as well as the adhesion properties of the interfaces. Simulations revealed that thin water films formed on SiO2, CaO, and MgO surfaces, while water molecules aggregated on Al2O3, Fe2O3, and CaCO3 surfaces due to hydrogen bonding and physical adsorption. Aged asphalt would adsorb more water due to more hydrogen bonds of water molecules with sulfoxide and carbonyl. Furthermore, the adhesion between asphalt and aggregates followed the order: alkaline aggregates > neutral aggregates > acid aggregates under dry conditions. However, in moist environments, the adhesion between asphalt and alkaline aggregates decreased sharply due to stable occupancy of water molecules in the interfacial gap, limiting the interaction between asphalt and aggregates. Additionally, the adhesion of interfaces decreased with increasing temperature due to higher mobility of water molecules, which pushed asphalt molecules further away from the aggregate layer. Conclusively, the molecular behavior of water is the key cause of adhesive damage at asphalt-aggregate interface under moisture condition. Type of aggregate, aging state of asphalt, water content, and temperature all affected the water molecular behavior, ultimately affecting interfacial adhesion.

Adsorption of Ferritin at Nanofaceted Al2O3 Surfaces.

The influence of nanoscale surface topography on protein adsorption is highly important for numerous applications in medicine and technology. Herein, ferritin adsorption at flat and nanofaceted, single-crystalline Al2O3 surfaces is investigated using atomic force microscopy and X-ray photoelectron spectroscopy. The nanofaceted surfaces are generated by the thermal annealing of Al2O3 wafers at temperatures above 1000 °C, which leads to the formation of faceted saw-tooth-like surface topographies with periodicities of about 160 nm and amplitudes of about 15 nm. Ferritin adsorption at these nanofaceted surfaces is notably suppressed compared to the flat surface at a concentration of 10 mg/mL, which is attributed to lower adsorption affinities of the newly formed facets. Consequently, adsorption is restricted mostly to the pattern grooves, where the proteins can maximize their contact area with the surface. However, this effect depends on the protein concentration, with an inverse trend being observed at 30 mg/mL. Furthermore, different ferritin adsorption behavior is observed at topographically similar nanofacet patterns fabricated at different annealing temperatures and attributed to different step and kink densities. These results demonstrate that while protein adsorption at solid surfaces can be notably affected by nanofacet patterns, fine-tuning protein adsorption in this way requires the precise control of facet properties.

Nucleation and Layer Closure Behavior of Iridium Films Grown Using Atomic Layer Deposition.

Understanding the initial growth process during atomic layer deposition (ALD) is essential for various applications employing ultrathin films. This study investigated the initial growth of ALD Ir films using tricarbonyl-(1,2,3-η)-1,2,3-tri(tert-butyl)-cyclopropenyl-iridium and O2. Isolated Ir nanoparticles were formed on the oxide surfaces during the initial growth stage, and their density and size were significantly influenced by the growth temperature and substrate surface, which strongly affected the precursor adsorption and surface diffusion of the adatoms. Higher-density and smaller nanoparticles were formed at high temperatures and on the Al2O3 surface, forming a continuous Ir film with a smaller thickness, resulting in a very smooth surface. These findings suggest that the initial growth behavior of the Ir films affects their surface roughness and continuity and that a comprehensive understanding of this behavior is necessary for the formation of continuous ultrathin metal films.

In-situ constructed ultrathin hydrotalcite derivative supported PtIn catalyst for isobutane direct dehydrogenation

The stable and ultrathin hydrotalcite nanosheets with thickness of ca. 1.0 nm have been built on the surface of Al2O3 by sharing Al element and using layer growth inhibitor formamide. Based on the topological transformation characteristics of hydrotalcite and the structural memory effect of calcined product, the supported PtIn bimetallic catalyst was achieved by stepwise incipient wetness impregnation method followed by calcination and reduction, and evaluated in isobutane direct dehydrogenation (IDH) to isobutene. The derived ultrathin bimetallic catalyst possesses excellent IDH activity under extreme conditions, in which the isobutane conversion and isobutene selectivity can remain above 63% and 92% respectively, and the turnover frequency is as high as 13.45 s−1. The excellent activity can be attributed to the formation of ultrathin hydrotalcite precursor, which can greatly enhance the specific surface area, interaction among metal components, surface acidity of weak- and medium-acid sites and surface In3+/In0 ratio, and further effectively inhibit carbon deposition and improve dispersion and stability of active Pt species in the corresponding derivative.

Metal–support interface engineering of Ni catalysts for improved H2 storage performance: Grafting alkyltriethoxysilane onto commercial alumina

Tuning metal–support interface in supported Ni catalysts is a promising approach for overcoming the agglomeration of Ni particles and the low reducibility of Ni species. This study describes a facile two-step method to produce active Ni catalysts for the hydrogenation of benzyltoluene. In the first step, alkyltriethoxysilanes (CnTES, n = 1, 3, and 8) were grafted onto the surface of commercial Al2O3. The maximum Si/Al ratio of the prepared CnTES-on-Al2O3 materials was determined to be approximately 2.9 mol.%, which can vary depending on the surface properties of Al2O3. The second step was H2 reduction of the Ni precursor loaded on the CnTES-on-Al2O3 materials to remove alkyl substituents from the grafted CnTES and form Ni particles on the surface of silica grafted onto alumina (SGA). The hydrogenation performance of the obtained Ni/SGA_CnTES catalysts was improved with increasing Si/Al ratio owing to higher Ni dispersion and more abundant metallic Ni. The activity of the Ni/SGA_2.9CnTES (Si/Al = 2.9 mol.%) catalysts exhibited a volcano-shaped relationship with the length of the alkyl substituent of grafted CnTES, reaching the maximum for C3TES. Notably, Ni/SGA_2.9C3TES showed excellent ability for the adsorption of H2 and substrate, as well as high stability, resulting from the reduced agglomeration of Ni particles and the modulated metal–support interface. Therefore, the demonstrated synthesis approach can render Ni catalysts servable to promote the commercialization of liquid organic hydrogen carrier system.

Rh Nanoparticles Dispersed on ZrO2–CeO2 Migrate to Al2O3 Supports to Mitigate Thermal Deactivation via Encapsulation

Full-scale monolithic three-way catalysts (TWCs) comprising Rh, oxygen-scavenging ZrO2–CeO2 (ZC), and γ-Al2O3 as a binder component were studied after real engine aging. The fatal irreversible deactivation that occurred under stoichiometric-lean-rich perturbation at 1000 °C for 40 h (SLR aging) was attributed to the complete encapsulation of Rh nanoparticles by ZC, leading to the physical blockage of gas adsorption. Preaging the catalyst under a rich condition at 1000 °C for 40 h (R aging) drastically mitigated this deactivation, i.e., the catalyst with R–SLR combined aging sustained its catalytic performance much better than the catalyst with SLR aging at the same temperature (1000 °C) and total time (80 h). X-ray mapping and high-temperature environmental electron microscopic analyses suggested that R aging promoted the migration of Rh nanoparticles across the ZC surface toward the boundary with the Al2O3 binder. Owing to the strong bonding with the Al2O3 surface, Rh nanoparticles were trapped at or near the boundary. Consequently, these Rh nanoparticles were unlikely to be fully covered by ZC even under the SLR aging condition because the encapsulation was induced through repetitive oxygen release/storage cycles at the Rh/ZC interface. Thus, we propose that Rh nanoparticles in contact with ZC and Al2O3 played crucial roles to hinder the encapsulation caused by SLR aging at 1000 °C. Rh nanoparticles supported on the dual-oxide support of ZC and Al2O3 were subjected to engine aging and chassis dynamometer tests. The deterioration extents of the TWC and oxygen storage capacity performances were successfully mitigated using this dual-oxide support formulation.

(Fe, Cr)(OH)3 Coprecipitation in Solution and on Soil: Roles of Surface Functional Groups and Solution pH

The simultaneous precipitation of (Fe, Cr)(OH)3 nanoparticles in solution (homogeneous) and on soil surfaces (heterogeneous), which controls Cr transport in soil and aquatic systems, was quantified for the first time in the presence of model surfaces, i.e., bare and natural organic matter (NOM)-coated SiO2 and Al2O3. Various characterization techniques were combined to explore the surface-ion-precipitate interactions and the controlling mechanisms. (Fe, Cr)(OH)3 accumulation on negatively charged SiO2 was mainly governed by electrostatic interactions between hydrolyzed ion species or homogeneous (Fe, Cr)(OH)3 and surfaces. The elevated pH through protonation of Al2O3 surface hydroxyls resulted in higher Cr/Fe ratios in both homogeneous and heterogeneous coprecipitates. Due to ignorable NOM adsorption onto SiO2, the amounts of (Fe, Cr)(OH)3 precipitates on bare/NOM-SiO2 were similar; contrarily, attributed to favored NOM adsorption onto Al2O3 and consequently carboxyl association with metal ions or (Fe, Cr)(OH)3 nanoparticles, remarkably more heterogeneous precipitates harvested on NOM-Al2O3 than bare-Al2O3. With the same solution supersaturation, the total amounts of homogeneous and heterogeneous precipitates were similar irrespective of the substrate type. With lower pH, decreased electrostatic forces between substrates and precipitates shifted (Fe, Cr)(OH)3 distribution from heterogeneous to homogeneous phases. The quantitative knowledge of (Fe, Cr)(OH)3 distribution and the controlling mechanisms can assist in better Cr sequestration in natural and engineered settings.

Using metal precursors to passivate oxides for area selective deposition

Although it has long been known that metal-containing compounds can serve as catalysts for chemical vapor deposition (CVD) of films from other precursors, we show that metal-containing compounds can also inhibit CVD nucleation or growth. For two precursors A and B with growth onset temperatures TgA < TgB when used independently, it is possible that B can inhibit growth from A when the two precursors are coflowed onto a substrate at a temperature (T) where TgA < T < TgB. Here, we consider three precursors: AlH3⋅NMe3 (Tg = 130 °C, Me = CH3), Hf(BH4)4 (Tg = 170 °C), and AlMe3 (Tg = 300 °C). We find that (i) nucleation of Al from AlH3⋅NMe3 is inhibited by Hf(BH4)4 at 150 °C on two oxide surfaces (Si with native oxide and borosilicate glass), (ii) nucleation and growth of HfB2 is inhibited by AlMe3 at 250 °C on native oxide substrates and on HfB2 nuclei, and (iii) nucleation of Al from AlH3⋅NMe3 is inhibited by AlMe3 at 200 °C on native oxide substrates. Inhibition by Hf(BH4)4 is transient and persists only as long as its coflow is maintained; in contrast, AlMe3 inhibition of HfB2 growth is more permanent and continues after coflow is halted. As a result of nucleation inhibition, AlMe3 coflow enhances selectivity for HfB2 deposition on Au (growth) over Al2O3 (nongrowth) surfaces, and Hf(BH4)4 coflow makes it possible to deposit Al on Al nuclei and not on the surrounding oxide substrate. We propose the following criteria to identify candidate molecules for other precursor–inhibitor combinations: (i) the potential inhibitor should have a higher Tg than the desired film precursor, (ii) the potential inhibitor should be unreactive toward the desired film precursor, and (iii) at the desired growth temperature, the potential inhibitor should adsorb strongly enough to form a saturated monolayer on the intended nongrowth surface at accessible inhibitor pressures.

Vapor-phase grafting of functional silanes on atomic layer deposited Al2O3

Fundamental studies are needed to advance our understanding of selective adsorption in aqueous environments and develop more effective sorbents and filters for water treatment. Vapor-phase grafting of functional silanes is an effective method to prepare well-defined surfaces to study selective adsorption. In this investigation, we perform vapor phase grafting of five different silane compounds on aluminum oxide (Al2O3) surfaces prepared by atomic layer deposition. These silane compounds have the general formula L3Si–C3H6–X where the ligand, L, controls the reactivity with the hydroxylated Al2O3 surface and the functional moiety, X, dictates the surface properties of the grafted layer. We study the grafting process using in situ Fourier transform infrared spectroscopy and ex situ x-ray photoelectron spectroscopy measurements, and we characterize the surfaces using scanning electron microscopy, atomic force microscopy, and water contact angle measurements. We found that the structure and density of grafted aminosilanes are influenced by their chemical reactivity and steric constraints around the silicon atom as well as by the nature of the anchoring functional groups. Methyl substituted aminosilanes yielded more hydrophobic surfaces with a higher surface density at higher grafting temperatures. Thiol and nitrile terminated silanes were also studied and compared to the aminosilane terminated surfaces. Uniform monolayer coatings were observed for ethoxy-based silanes, but chlorosilanes exhibited nonuniform coatings as verified by atomic force microscopy measurements.

Effect of Ar fast atom beam irradiation on alpha-Al2O3 for surface activated room temperature bonding

This study focuses on the surface-activated bonding of sapphire (alpha-Al2O3) wafers at RT. In the surface activation process, Ar fast atom beam (FAB) irradiation is used as a physical sputtering method. The bond strength estimated by the crack opening method is approximately 1.63 J m−2. The binding state of the activated alpha-Al2O3 surface is determined using angle-resolved X-ray photoelectron spectroscopy. The results reveal the existence of two binding energies of Al2p (73.9 and 74.0 eV) on the surface of the FAB-irradiated wafer, indicating that the surface activation changes the binding state of the utmost alpha-Al2O3 surface. This implies that the contact of the changed Al2O3 surface contributes to the formation of a strong bond interface.

Achieving homogeneous Li deposition of 3D dendrite-free lithium metal anode through atomic layer deposition surface modification

Herein, two coating amorphous Al2O3 and Li3PO4 with different properties were firstly demonstrated for Li/Nickel Foam (Li/NF) composite anode via atomic layer deposition (ALD) method to form a free-standing Li host material for dendrite-free Li anode. Impedance test shows that Li nucleates easily on Li3PO4 surface compared with NF and Al2O3 surface. Significantly, scanning electron microscope (SEM) observation of tracing the same site shows that a large number of lithium dendrites were deposited on NF and Al2O3 surface, while no dendrite growth was observed on Li3PO4 surface. The test of cells shows that Li@Li3PO4@NF anode presents better cycling stability and rate capability. Li3PO4 reduces the nucleation overpotential and the surface energy barrier, which can increase the overall surface lithiophilic, so that the entire surface is deposited uniformly. This work promotes the uniform deposition of lithium and sheds light on the development of ultrastable lithium metal anode.

Effects of Phosphorus Addition on the Hydrophobicity and Catalytic Performance in Methane Combustion of θ-Al2O3 Supported Pd Catalysts

A series of xPθ-Al2O3 supports modified with different amounts of phosphorus element were prepared and taken as supports of palladium catalysts for methane catalytic combustion. The impacts of phosphorus additives on the hydrophobicity of Pd/xPθ-Al2O3 and its performance of methane catalytic combustion in the absence of and presence of 8% water were systematically studied. It was found that the hydrophobicity of xPθ-Al2O3 changed with the increase of phosphorus content, which had a significant effect on the activity of methane catalytic combustion. The incorporation of phosphorus replaced the hydroxyl groups on the surface of Al2O3 in the form of phosphates, thus changing the density of hydroxyl groups of Al2O3 support. TGA, NH3-TPD, IR, and XPS were employed to illustrate the process of phosphate replacement. xPθ-Al2O3 with less than 1 wt.% phosphorus content had better hydrophobicity than the unmodified θ-Al2O3 and Pd/xPθ-Al2O3, therefore had better performance for methane catalytic combustion, which was attributed to the substitution of hydroxyl groups on the surface of θ-Al2O3 by PO43− and HPO42−. However, when the phosphorus content of Al2O3 was higher than 1 wt.%, the substitution of H2PO4− began to dominate, which would lead to poorer hydrophobicity and catalytic performance. This work will guide the design of methane catalytic combustion catalysts resistant to water inhibition problem.

Photocatalytic removal of organic dyes by titanium doped alumina nanocomposites: Using multivariate factorial and kinetics models

Hazardous containing different groups of dyes from various industries are extremely dangerous and causing unfavorably susceptible dermatitis, skin diseases, transformation, and malignant growth. To overcome the hazards caused by water pollution, eco-friendly composites (TiO2-Al2O3) of titanium oxide (TiO2) and alumina (Al2O3) were synthesized with different proportional ratios (0.25/1, 0.16/1, 0.125/1, and 0.1/1) by kneading method and characterized through X-ray diffraction, scanning electron microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Ultraviolet-Visible (UV–Vis) spectrophotometry and high resolution transmittance electron microscopy (HR-TEM). Experimental finding showed that the sample TiO2 / Al2O3 with a molar-ratio of 0.1/1 had the higher photodegradation efficacy for malachite green and from the ratio 0.1/1, the higher photodegradation efficiency was achieved for eosin Y. The comparative studies of TiO2-Al2O3 samples were done for basic malachite green and acidic eosin Y and it was estimated that due to adsorbent, Al2O3 surface basicity it showed the highest adsorption for acidic eosin Y and hence had a higher photo degradation removal up to 90% for the eosin Y than for the basic malachite green with 80% photo degradation removal. Reaction mechanism for photodegradation were finest to define by Langmuir isotherms which trailed by pseudo-second order kinetics. The t/qt vs t plots for pseudo-second order kinetic produced high regression coefficients (R2 =0.963), which stated the adsorbate sorption's followed the pseudo second order kinetics. Thermodynamic parameters of dye degradation were calculated by the Van't Hoff equation that indicated the adsorption method was exothermic. The ΔH֯ parameters were found to be -54.44 kJ mol−1. The negative ΔH֯ recommended the exothermic biosorption process. Residuals plots for responses, interaction plots for responses and inter-contour plots were utilized to study the optimum points. Furthermore, accredited to experimental results and a reaction mechanism is planned for the photodegradation of organic dyes with TiO2-Al2O3 photocatalyst. Thus the broad band gap of inert semiconductor TiO2 can be narrower by heterogeneous coupling with Al2O3 through kneading method and showed the excellent photodegradation of organic dyes in the visible region without producing any secondary pollutant.

Effect of plasma sprayed flyash based composite coatings on corrosion resistance

With the aim of exploring the possibility of achieving a low-cost thermal spray coating to prevent wear, erosion and corrosion. In the current study, flyash-Al2O3 and flyash-SiC composite coatings were effectively created using the air plasma spray process on substrates of Al6061 alloy. NiCr material is used as the bond coat to improve the bond strength between the coat and the substrate. Taguchi's DoE method is applied to for spray process parameters optimization. In addition, the developed coating is subjected to microstructure analysis and long-term immersion corrosion testing (1 year) in an aqueous environment to assess corrosion properties.The results revealed that the over a certain test period, the developed flyash-SiC coating has greater corrosion resistance than the uncoated and flyash-Al2O3 coated Al6061. It is noticed that the corrosion resistance of the flyash-Al2O3 coating shifts to a negative value with respect to the uncoated substrate. The uncoated sample is extensively pitted and locally corroded, as shown by the SEM image of the corroded surface. Flyash-corroded Al2O3's surface exhibits extensive degradation in the form of peeling, breaking, and cracking of the splats. With flyash-SiC composite coating a very minor corrosion splat deterioration is seen.

Understanding the Effect of Single Atom Cationic Defect Sites in an Al2O3 (012) Surface on Altering Selenate and Sulfate Adsorption: An Ab Initio Study.

Adsorption is a promising under-the-sink selenate remediation technique for distributed water systems. Recently it was shown that adsorption induced water network re-arraignment control adsorption energetics on the (012) surface. Here, we aim to elucidate the relative importance of the water network effects and surface cation identity on controlling selenate and sulfate adsorption energy using density functional theory calculations. Density functional theory (DFT) calculations predicted the adsorption energies of selenate and sulfate on nine transition metal cations (Sc-Cu) and two alkali metal cations (Ga and In) in the (012) surface under simulated acidic and neutral pH conditions. We find that the water network effects had larger impact on the adsorption energy than the cationic identity. However, cation identity secondarily controlled adsorption. Most cations decreased the adsorption energy weakening the overall performance, the larger Sc and In cations enabled inner-sphere adsorption in acidic conditions because they relaxed outward from the surface providing more space for adsorption. Additionally, only Ti induced Se selectivity over S by reducing the adsorbing selenate to selenite but not reducing the sulfate. Overall, this study indicates that tuning water network structure will likely have a larger impact than tuning cation-selenate interactions for increasing adsorbate effectiveness.

One-Pot Fabrication of Nanocomposites Composed of Carbon Nanotubes and Alumina Powder Using a Rotatable Chemical Vapor Deposition System.

The fabrication of multi-dimensional nanocomposites has been extensively attempted to achieve synergistic performance through the uniform mixing of functional constituents. Herein, we report a one-pot fabrication of nanocomposites composed of carbon nanotubes (CNTs) and Al2O3 powder. Our strategy involves a synthesis of CNTs on the entire Al2O3 surface using a rotatable chemical vapor deposition system (RCVD). Ehylene and ferritin-induced nanoparticles were used as the carbon source and wet catalyst, respectively. The RCVD was composed of a quartz reaction tube, 5.08 cm in diameter and 150 cm in length, with a rotation speed controller. Ferritin dissolved in deionized water was uniformly dispersed on the Al2O3 surface and calcinated to obtain iron nanoparticles. The synthesis temperature, time, and rotation speed of the chamber were the main parameters used to investigate the growth behavior of CNTs. We found that the CNTs can be grown at least around 600 °C, and the number of tubes increases with increasing growth time. A faster rotation of the chamber allows for the uniform growth of CNT by the tip-growth mechanism. Our results are preliminary at present but show that the RCVD process is sufficient for the fabrication of powder-based nanocomposites.

Hydrogen Production from Biogas: Development of an Efficient Nickel Catalyst by the Exsolution Approach

Hydrogen production from biogas over alumina-supported Ce1−xNixO2−x catalysts was studied in a temperature range of 600–850 °C with an initial gas composition of CH4/CO2/H2O of 1/0.8/0.4. To achieve a high and stable hydrogen yield, highly dispersed Ni catalysts were prepared through the exsolution approach. A solid solution of Ce1−xNixO2−x was firstly formed on the surface of Al2O3 and then activated in H2/Ar at 800 °C. The genesis and properties of the Ce1−xNixO2−x/Al2O3 catalysts were established using X-ray fluorescence analysis, thermal analysis, N2 adsorption, ex situ and in situ X-ray diffraction, Raman spectroscopy, electron microscopy, EDX analysis, and temperature-programmed hydrogen reduction. The performance of Ce1−xNixO2−x/Al2O3 catalysts in biogas conversion was tuned by regulation of the dispersion and reducibility of the active component through variation of content (5–20 wt.%) and composition (x = 0.2, 0.5, 0.8) of Ce1−xNixO2−x as well as the mode of its loading (co-impregnation (CI), citrate sol–gel method (SG)). For the 20 wt.% Ce1−xNixO2−x/Al2O3 catalyst, the rate of the coke formation decreased by a factor of 10 as x increased from 0.2 to 0.8. The optimal catalyst composition (20 wt.% Ce0.2Ni0.8O1.8/80 wt.% Al2O3) and preparation mode (citrate sol–gel method) were determined. At 850 °C, the 20 wt.% Ce0.2Ni0.8O1.8/Al2O3-SG catalyst provides 100% hydrogen yield at full CH4 conversion and 85% CO2 utilization.