Alumina Overcoat Research Articles

Sinter‐Resistant Nickel Catalyst for Lignin Hydrogenolysis Achieved by Liquid Phase Atomic Layer Deposition of Alumina

AbstractLignin hydrogenolysis is a key step in the sustainable production of renewable bio‐based chemicals and fuels. Heterogeneous metal catalysts have led to high yields but they rapidly deactivate, notably due to nanoparticle sintering and carbonaceous deposit formation. While these deposits can be removed by regeneration, sintering is irreversible and a significant barrier to commercialization. Here, simple liquid phase atomic layer deposition is used to deposit an alumina layer to protect nickel particles from sintering. In the gas phase, it is proved that alumina can prevent sintering during reduction up to 600 °C. This catalyst for hydrogenolysis of extracted lignin in batch and continuous operation is used. In batch, the overcoated catalyst maintains high monomer yields with little sintering over four cycles of reuse while the yield obtained with the catalyst without an overcoat reduces to half and severe sintering occurs. In a continuous flow reactor, deactivation rates are three times lower for the catalyst with the alumina overcoat. Microscopy images confirm that the alumina overcoat largely preserves nickel particle sizes after ten days of operation. The results demonstrate that catalyst overcoating with metal oxides substantially slows irreversible deactivation during lignin hydrogenolysis, which could facilitate the development of continuous lignin upgrading.

Elucidating the Roles of Amorphous Alumina Overcoat in Palladium-Catalyzed Selective Hydrogenation.

Amorphous alumina overcoats generated by atomic layer deposition (ALD) have been shown to improve the selectivity and durability of supported metal catalysts in many reactions. Several mechanisms have been proposed to explain the enhanced catalytic performance, but the accessibilities of reactants through the amorphous overcoats remain elusive, which is crucial for understanding reaction mechanisms. Here, we show that an AlOx ALD overcoat is able to improve the alkene product selectivity of a supported Pd catalyst in acetylene (C2H2) hydrogenation. We further demonstrate that the AlOx ALD overcoat blocks the access of C2H2 (kinetic diameter of 0.33 nm), O2 (0.35 nm), and CO (0.38 nm) but allows H2 (0.29 nm) to access Pd surfaces. A H–D exchange experiment suggests that H2 might dissociate heterolytically at the Pd–AlOx interface. These findings are in favor of a hydrogen spillover mechanism.

Is Coating Oxide on Porous Metal Thin-Film for Low-Temperature Solid Oxide Fuel Cell Cathode a Panacea for Performance Enhancement?

The Pt thin-film cathode thermo-mechanically reinforced by using atomic layer-deposited (ALD) alumina overcoat for low-temperature solid oxide fuel cells was investigated on electrochemical and electrical performances and its area specific resistance (ASR) with respect to cell dimension was evaluated. The nanoporous Pt thin-film cathode was fabricated by direct current magnetron sputtering with a deposition pressure of 50 mTorr. And alumina overcoat was deposited with thermal ALD 30 cycles onto the nanoporous Pt thin-film cathode. Compared to bare Pt thin-film cathode, ALD alumina-overcoated Pt thin-film cathode provided ~ 3-times lower activation ASR and ~ 2-times lower effective electrical conductivity after 4-h operation at 550 °C. Despite increases in the thermo-mechanical stability of Pt thin-film cathode by ALD alumina overcoating, it was evaluated that the total ASR of ALD alumina-overcoated Pt thin-film cathode was lower than that of the bare Pt thin-film cathode only if cell dimension is below 0.95 cm2 because of its lower current collecting performance in the in-plane direction.

Understanding Pore Formation in ALD Alumina Overcoats.

AlOX thin films deposited by atomic layer deposition (ALD) have previously been used to increase both stability and selectivity of supported palladium catalysts and are known to develop nanoscale porosity upon heating. Understanding the factors that affect ALD thin-film porosityenables future design of layered catalytic structures with tunable nanoscale features on industrially-relevant high-surface-area materials. In this study, porous and nonporous aluminum oxide supports with and without palladium nanoparticles were overcoated with thin films of 2-7 nm AlOX by ALD deposited at temperatures of 100, 200, and 300 °C. Hydroxyl loss and changes in surface chemistry were observed upon heating the films, and changes in surface area and pore volume of the annealed films were correlated to AlOX deposition temperature and the presence of Pd. Crystallization of the overcoat to γ-Al2O3 is shown to occur separately from hydroxyl loss and pore formation. A mechanistic understanding of pore formation in AlOX ALD films is obtained by reference to studies of the structural transformations accompanying the formation of transition aluminas from hydroxide precursors. Additionally, a direct and tunable correlation is established between pore development and the overall hydroxyl content of AlOX ALD coatings.

Ni-alumina dry reforming catalysts: Atomic layer deposition and the issue of Ni aluminate

Abstract A catalyst consisting of 2 wt% Ni supported on a commercially available transition alumina is modified using TMA-H2O ALD cycles to deposit thin alumina overcoats on the catalyst, and it is then investigated for the catalytic dry reforming of methane (DRM) at 700 °C. Highly dispersed Ni rapidly sinters to form bulk Ni particles on the uncoated catalyst. In contrast, deposition of an alumina overcoat by ALD significantly lowers the rate of Ni sintering, and also lowers the propensity towards carbon deposition during DRM. Additionally, it is experimentally demonstrated that the Ni aluminate (NiAl2O4) spinel phase is unstable under the present DRM conditions and slowly undergoes reduction to metallic Ni and Al2O3. Slow reduction of the Ni2+ from NiAl2O4 is proposed as the origin of the large increase in DRM activity observed for the alumina-overcoated Ni catalysts.

Slowing the Kinetics of Alumina Sol-Gel Chemistry for Controlled Catalyst Overcoating and Improved Catalyst Stability and Selectivity.

Catalyst overcoating is an emerging approach to engineer surface functionalities on supported metal catalyst and improve catalyst selectivity and durability. Alumina deposition on high surface area material by sol-gel chemistry is traditionally difficult to control due to the fast hydrolysis kinetics of aluminum-alkoxide precursors. Here, sol-gel chemistry methods are adapted to slow down these kinetics and deposit nanometer-scale alumina overcoats. The alumina overcoats are comparable in conformality and thickness control to overcoats prepared by atomic layer deposition even on high surface area substrates. The strategy relies on regulating the hydrolysis/condensation kinetics of Al(s BuO)3 by either adding a chelating agent or using nonhydrolytic sol-gel chemistry. These two approaches produce overcoats with similar chemical properties but distinct physical textures. With chelation chemistry, a mild method compatible with supported base metal catalysts, a conformal yet porous overcoat leads to a highly sintering-resistant Cu catalyst for liquid-phase furfural hydrogenation. With the nonhydrolytic sol-gel route, a denser Al2 O3 overcoat can be deposited to create a high density of Lewis acid-metal interface sites over Pt on mesoporous silica. The resulting material has a substantially increased hydrodeoxygenation activity for the conversion of lignin-derived 4-propylguaiacol into propylcyclohexane with up to 87% selectivity.

Catalyst stabilization by stoichiometrically limited layer-by-layer overcoating in liquid media

The use of metal oxide overcoats over supported nanoparticle catalysts has recently led to impressive improvements in catalyst stability and selectivity. The deposition of alumina is especially important for renewable catalysis due to its robustness in liquid-phase conditions. However, there are limited reports of work on alumina deposition and stabilization that goes beyond atomic layer deposition (ALD). Here, we present a layer-by-layer deposition technique for the controlled formation of conformal alumina overcoats in the liquid phase. This technique is easy to perform in common wet chemistry conditions. Alternated exposure of the substrate to stoichiometric amounts of aluminum alkoxide and water in liquid-phase conditions leads to the formation of a porous overcoat that was easily tunable by varying synthesis parameters. The deposition of 60 Al2O3 layers onto Al2O3-supported copper nanoparticles suppressed irreversible deactivation during the liquid-phase hydrogenation of furfural – a key biomass-derived platform molecule. The porous overcoat leads to highly accessible metal sites, which significantly reduces the partial site blocking observed in equivalent overcoats formed by ALD. We suggest that the ease of scalability and the high degree of control over the overcoat’s properties during liquid-phase synthesis could facilitate the development of new catalyst overcoating applications.

Acidic alumina overcoating on platinum nanoparticles: Close metal–acid proximity enhances bifunctionality for glycerol hydrogenolysis

Bifunctional catalysts that contain both metal and acidic functions have been widely used in renewable biomass conversions. The bifunctionality closely depends on the distance between the metal and acid sites. However, the metal–acid proximity effect has rarely investigated in biomass conversions. In this work, we precisely deposited a porous Al 2 O 3 overcoat onto a Pt/Al 2 O 3 catalyst using atomic layer deposition to improve the proximity between the Pt metal and the alumina acid sites by increasing the area of the metal–acid interface. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of pyridine chemisorption confirmed that the overall catalyst acidity did not change considerably after applying the alumina overcoat. In the aqueous-phase, hydrogenolysis of glycerol was used to demonstrate that the alumina overcoat significantly improved the activity approximately 2.8-fold, as well as the selectivity to 1,2-propanediol (1,2-PD) at high conversions. DRIFTS measurements of CO chemisorption indicated that the Pt-alumina interface had greater area for alumina coated Pt/Al 2 O 3 than for the uncoated analog. Moreover, we used the hydrogenation of acetol, the key reaction intermediate in glycerol hydrogenolysis, as a control experiment to confirm that the observed activity improvement in the hydrogenolysis of glycerol could be attributed to the enhancement of the dehydration reaction step, which requires acidic function. In brief, our work provides solid evidence that close metal–acid proximity enhances bifunctionality, thus improving the catalytic activity. Deposition of Al 2 O 3 on Pt/Al 2 O 3 catalyst using ALD can significantly enhance the activity and selectivity to 1,2-propanediol in the hydrogenolysis of glycerol through the metal–acid proximity effect.

Influence of the Evanescent Field Decay Length on the Sensitivity of Plasmonic Nanodisks and Nanoholes

We evaluate and compare the sensitivity of gold nanodisks on silica substrates and nanoholes made in silica-supported gold films, two of the most common sensor structures used in plasmonic biosensing. An alumina overcoat was applied by atomic layer deposition (ALD) to precisely control the interfacial refractive index and determine the evanescent plasmonic field decay length. The results are in good agreement with analytical models and biomolecular binding experiments for the two substrates. We found that nanodisks outperform nanoholes for thin dielectric coatings ( ∼20 nm). The optimum nanoplasmonic transducer element for a given biorecognition reaction can be chosen based on experimentally determined bulk sensitivities/noise levels and theoretically estimated evanescent field decay lengths.

Operando X‐ray Absorption Spectroscopy Studies of Sintering for Supported Copper Catalysts during Liquid‐phase Reaction

AbstractOperando X‐ray absorption spectroscopy is used to measure simultaneous changes in catalyst structure and changes in catalytic activity versus time during the liquid phase hydrogenation of furfural over supported copper catalysts. This approach allows the size of the copper nanoparticles to be monitored continuously versus time‐on‐stream, such that these changes in dispersion can be accounted for in the calculation of turnover frequency. It is shown that sintering of the copper nanoparticles is the predominant mode of catalyst deactivation for a Cu/γ‐Al2O3 catalyst throughout its time‐on‐stream, leading to irreversible loss of catalytic activity. In contrast, this mode of deactivation is eliminated by atomic layer deposition of an alumina overcoat; however, deposition of carbonaceous deposits in the small pores of the overcoat leads to deactivation that is reversible upon calcination of the catalyst.

Stabilization of Copper Catalysts for Liquid‐Phase Reactions by Atomic Layer Deposition

Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid-phase catalytic reactions (e.g., hydrogenation of biomass-derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid-phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X-ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ-Al2 O3 . Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under-coordinated copper atoms on the nanoparticle surface.

Stabilization of Copper Catalysts for Liquid‐Phase Reactions by Atomic Layer Deposition

AbstractAtomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid‐phase catalytic reactions (e.g., hydrogenation of biomass‐derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid‐phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X‐ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ‐Al2O3. Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under‐coordinated copper atoms on the nanoparticle surface.

Synthesis and Stabilization of Supported Metal Catalysts by Atomic Layer Deposition

Supported metal nanoparticles are among the most important catalysts for many practical reactions, including petroleum refining, automobile exhaust treatment, and Fischer-Tropsch synthesis. The catalytic performance strongly depends on the size, composition, and structure of the metal nanoparticles, as well as the underlying support. Scientists have used conventional synthesis methods including impregnation, ion exchange, and deposition-precipitation to control and tune these factors, to establish structure-performance relationships, and to develop better catalysts. Meanwhile, chemists have improved the stability of metal nanoparticles against sintering by the application of protective layers, such as polymers and oxides that encapsulate the metal particle. This often leads to decreased catalytic activity due to a lack of precise control over the thickness of the protective layer. A promising method of catalyst synthesis is atomic layer deposition (ALD). ALD is a variation on chemical vapor deposition in which metals, oxides, and other materials are deposited on surfaces by a sequence of self-limiting reactions. The self-limiting character of these reactions makes it possible to achieve uniform deposits on high-surface-area porous solids. Therefore, design and synthesis of advanced catalysts on the nanoscale becomes possible through precise control over the structure and composition of the underlying support, the catalytic active sites, and the protective layer. In this Account, we describe our advances in the synthesis and stabilization of supported metal catalysts by ALD. After a short introduction to the technique of ALD, we show several strategies for metal catalyst synthesis by ALD that take advantage of its self-limiting feature. Monometallic and bimetallic catalysts with precise control over the metal particle size, composition, and structure were achieved by combining ALD sequences, surface treatments, and deposition temperature control. Next, we describe ALD oxide overcoats applied with atomically precise thickness control that stabilize metal catalysts while preserving their catalytic function. We also discuss strategies for generation and control over the porosity of the overcoats that allow the embedded metal particles to remain accessible by reactants, and the details for ALD alumina overcoats on metal catalysts. Moreover, using methanol decomposition and oxidative dehydrogenation of ethane as probe reactions, we demonstrate that selectively blocking low coordination metal sites by oxide overcoats can provide another strategy to enhance both the durability and selectivity of metal catalysts.

Subnanometer Palladium Particles Synthesized by Atomic Layer Deposition

Monodispersed palladium nanoparticle catalysts were synthesized by atomic layer deposition (ALD) using alternating exposures of Pd hexafluoroacetylacetonate (Pd(hfac)2) and formalin on an alumina support. The size of the ALD Pd particles could be tuned by adjusting the preparation conditions. Conventional ALD conditions produced Pd particles with an average size of 1.4 nm. Removal of surface hydroxyls from the alumina support by a chemical treatment using trimethyl aluminum (TMA) before performing Pd ALD led to nanoparticles larger than 2 nm. Ultrasmall (subnanometer) Pd particles were synthesized using low-temperature metal precursor exposures, followed by applying protective ALD alumina overcoats. The ALD Pd particles were characterized by transmission electron microscopy, extended X-ray absorption fine structure, and diffuse reflectance infrared Fourier transform spectroscopy techniques. The Pd loadings were measured by X-ray fluorescence. The catalytic performance of ALD Pd particles of different sizes was compared in the methanol decomposition reaction. The specific activity (normalized by Pd loading) of the ultrasmall Pd particles was higher than those of the larger particles. Considering the metal dispersion factor, the turnover frequency (TOF) of the ultrasmall Pd particles is comparable to that of the medium-sized (1.4 nm, on average) Pd particles synthesized under standard ALD conditions. The large Pd particles (>2 nm) are a factor of 2 less active than the smaller Pd particles.

On the Thermal Behavior of Giant Magnetoresistance Heads

The magnetic/mechanical spacing between the transducer and the disk significantly decreases due to thermal expansion of pole tips at stressed high temperature and high humidity tests. The protruded pole tips and alumina overcoat can cause head/disk contacts, resulting in thermal asperities and pole tip damage. The damage at the head–disk interface due to protruded pole tips and alumina overcoat may degrade the drive mechanical performance when flying height is below 10 nm. In this study the change in pole tin recession (PTR) with temperature and current in the writer coil, are measured using an optical profiler and an atomic force microscope for heads having a stack design with single and dual layers of writer coils. The pole tips protrude above the ABS surface by 3–4 nm when the temperature of the head is raised by 50°C. Heads with a single layer of writer coils exhibit significantly lower thermal PTR than those with dual layers of coils. The ABS profiles at elevated temperature generated using the finite element modeling of the differential thermal expansion of various layers in the head stack are in close agreement with the measured profiles. The thermal PTR and alumina overcoat protrusion can be reduced by optimizing the thermal expansion coefficient of the alumina basecoat and overcoat, the height of the head stack, and by replacing alumina by SiO2 and SiC.

Head design considerations for lower thermal pole tip recession and alumina overcoat protrusion

The magnetic/mechanical spacing between the transducer and the disk significantly decreases due to thermal expansion of pole tips at stressed high temperature and high humidity tests. The protruded pole tips and alumina overcoat result increased thermal asperities and can interfere with the higher disc asperities and be damaged due to high contact. The damage at the head-disk interface due to protruded pole tips and alumina overcoat may become a major roadblock in the drive mechanical performance when flying height is below 10 nm. In this study, the thermal PTR defined as change in PTR with temperature is measured using an optical profiler and an AFM for heads having stack design with single and dual layers of writer coil. The pole tips protrude above the ABS surface by 3–4 nm when the temperature of the head is raised by 50°C. Heads with single layer of writer coil exhibit significantly lower thermal PTR than those with dual layers of coils.

Stable encapsulation structures for Permalloy films

This paper presents a study of the effects of annealing on the micromagnetic properties of encapsulated Permalloy films. The stress relaxation during annealing causes observable changes in the micro-magnetic state of the Permalloy. The amount of stress relaxation is highly sensitive to the shape of the Permalloy film and the encapsulation material. After Permalloy films with negative magnetostrictive coefficient are annealed, the easy axis of bar-shape yokes remains in the transversal direction while those of ring-shape yokes and heart-shape yokes rotate so that the easy axis is normal to the film edge. The annealing properties of bar-shape Permalloy films in six different encapsulation structures are compared. Permalloy films encapsulated entirely in hardbaked photoresist are consistently more stable. The uneven external stress is absorbed by the relatively softer hardbaked photoresist buffer layer and is re-distributed evenly to the Permalloy. A better control of the anisotropy of the Permalloy yoke can be obtained by placing a hardbaked photoresist buffer layer between the alumina overcoat and the Permalloy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparison of magnetic fields of thin-film heads and their corresponding bit patterns using magnetic force microscopy

We have used dc-mode magnetic force microscopy to image the magnetic fringing fields of several thin-film heads and the bit patterns written with these heads. The images were taken with Si/sub 3/N/sub 4/ tips coated with 10-nm Fe and 5-nm Au. The heads and disks are typical industry standards. The heads had a variety of pole piece configurations. A large track separation was used so that the erase bands could be thoroughly studied. We were surprised to discover magnetic fields that correspond to layers in the alumina overcoat near the pole pieces. The magnetic force microscope images of the bit pattern show a definite twist at the track edge that points toward the trailing pole piece. We also observed disk magnetization patterns that remained after an ac erase procedure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

漸変溶射皮膜の耐熱衝撃性に及ぼす皮膜積層形態の影響

The purpose of this paper is to investigate the effect of coating configuration and heat treatment on the thermal shock resistant properties of the ceramic plasma sprayed coatings including the graded coatings.The results obtained are summarized as follows:1) The press sintered heat treatment induced a higher adhesion and cohesion strength between the substrate and Ni-based alloy by means of diffusion. However, similar results were not obesrved at the alloyalumina interface.2) The graded coating configuration, which consist of an alloy undercoat, an alumina overcoat and an intermediate graded layer, revealed superior thermal shock resistant properties. This was attributed to their better ability to relax the thermal stresses developed between the Ni-alloy and alumina layers.