Characterization and catalytic studies of PVD synthesized Mo/V/Nb/Te oxide catalysts

Multicomponent Mo/V/Nb/Te oxide catalysts were synthesized using physical vapor deposition (PVD) onto the substrates of Si wafers and of honeycomb cordierites. The PVD films were prepared at different metal deposition sequences and were subsequently calcined at different temperatures and environment (air or N₂). The PVD films on Si wafer were characterized using several surface techniques, including X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and secondary ion mass spectrometry (SIMS). In parallel, PVD films on cordierites were evaluated in a fixed-bed reactor for the selective oxidation of propane to acrylic acid. The combined synthesis, characterization, and reactor studies provided clear evidence that the surface compositions and the catalytic properties of the Mo/V/Nb/Te oxide catalysts depend strongly on the metal deposition sequence, calcination temperature, and calcination environment.     

Preparation, interconversion and characterization of nanometer-sized molybdenum carbide catalysts

Nanometer-sized molybdenum carbide particles have been shown to have hydrogenation and other catalytic properties similar to those of the more-expensive noble metals. However, current preparation techniques for molybdenum carbide require the high-temperature reduction of unsupported molybdenum oxide in the presence of CH₄ and H₂. Although this method is effective, it yields particles of relatively large size, 11 nm. Different, simpler, synthesis procedures starting with ammonium heptamolybdate impregnated on carbon yield carbides of different stiochiometries. Particle sizes are in the 2–10 nm range, which are generally smaller than those previously obtained, and can be altered by changing the method of preparation. The focus of this work is to characterize the new procedures, specifically the bulk transitions of the new starting material to the different molybdenum carbides, using time-resolved X-ray diffraction, temperature-programmed reduction, and temperature-programmed desorption.     

Iron Oxide Nanoparticles Employed as Seeds for the Induction of Microcrystalline Diamond Synthesis

Iron nanoparticles were employed to induce the synthesis of diamond on molybdenum, silicon, and quartz substrates. Diamond films were grown using conventional conditions for diamond synthesis by hot filament chemical vapor deposition, except that dispersed iron oxide nanoparticles replaced the seeding. X-ray diffraction, visible, and ultraviolet Raman Spectroscopy, energy-filtered transmission electron microscopy , electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to study the carbon bonding nature of the films and to analyze the carbon clustering around the seed nanoparticles leading to diamond synthesis. The results indicate that iron oxide nanoparticles lose the O atoms, becoming thus active C traps that induce the formation of a dense region of trigonally and tetrahedrally bonded carbon around them with the ensuing precipitation of diamond-type bonds that develop into microcrystalline diamond films under chemical vapor deposition conditions. This approach to diamond induction can be combined with dip pen nanolithography for the selective deposition of diamond and diamond patterning while avoiding surface damage associated to diamond-seeding methods.     

Gas-phase surface functionalization of multi-walled carbon nanotubes with vacuum UV photo-oxidation

For bulk processing of carbon nanotubes, an important first step in adhesion to the nanotubes is often liquid-phase functionalization through chemical oxidation with acids (e.g., nitric and sulfuric), peroxides and/or potassium permanganate. In comparison, gas-phase photo-oxidation can be an alternative to introduce oxygenated functional groups on the surfaces of carbon nanotubes without the generation of liquid waste. In the present study, vacuum UV photo-oxidation of multi-walled carbon nanotube (MWNT) paper was investigated downstream from an Ar microwave plasma. X-ray Photoelectron Spectroscopy (XPS) was used to detect the carbon- and oxygen-containing functional groups in the top 2–5 nm of the sample's surface. The current results are compared to previous investigations using MWNT powder and single-walled carbon nanotube (SWNT) paper showing decreased levels of oxidation in MWNT samples.     

表面活性剂为碳源制备纳米碳化钼

以表面活性剂为碳源,通过程序升温反应法成功制备了α-Mo2C,并用粉末X-射线衍射（XRD）、扫描电子显微镜（SEM）、热失重分析（TG）和透射电子显微镜（TEM）等现代分析技术,对所制备的α-Mo2C进行了表征。结果表明：不同的表面活性剂高温分解所得碳原子或碳基作碳源,所生成的α-Mo2C的颗粒粒径明显不同。制得的α-Mo2C是六方晶,α-Mo2C的颗粒粒径达到纳米级,并且用聚乙烯吡咯烷酮作碳源生成的Mo2C颗粒粒径小于用双十八烷二甲基氯化氨作碳源生成的Mo2C颗粒粒径。     

Effects of Microwave Processing on Fiber-Matrix Adhesion. II. Enhanced Chemical Bonding of Epoxy to Carbon Fibers

The effect of microwave processing on the chemical interactions occurring between the carbon fiber surface and the epoxy matrix constituents was investigated using X-ray Photoelectron Spectroscopy (XPS). Monofunctional model compounds selected to duplicate the matrix constituents were exposed to the carbon fibers at temperatures similar to those encountered during composite processing. After solvent extraction, chemisorbed species were quantified by XPS. Differences were apparent in the C 1s and O 1s core electron regions of the microwave treated samples when referenced to the same elemental regions of thermally (convection) treated samples. Specifically, the atomic percentage of oxygen (in the form of carbon oxides) was increased to a greater degree when using the microwave treatment as opposed to the thermal treatments. The microwave treatment resulted in a substantial increase in the amount of chemical interaction between the fiber surface and the epoxy resin and amine components of the matrix. An epoxy resin/amine hardener adduct compound was also used to investigate the possible interaction of the adduct hydroxyl group with the carbon fiber surface. XPS results indicate a low to insignificant interaction of the hydroxyl with the carbon fiber surface under the conditions used in this study.     

Effects of Microwave Processing on Fiber-Matrix Adhesion. II. Enhanced Chemical Bonding of Epoxy to Carbon Fibers

The effect of microwave processing on the chemical interactions occurring between the carbon fiber surface and the epoxy matrix constituents was investigated using X-ray Photoelectron Spectroscopy (XPS). Monofunctional model compounds selected to duplicate the matrix constituents were exposed to the carbon fibers at temperatures similar to those encountered during composite processing. After solvent extraction, chemisorbed species were quantified by XPS. Differences were apparent in the C 1s and O 1s core electron regions of the microwave treated samples when referenced to the same elemental regions of thermally (convection) treated samples. Specifically, the atomic percentage of oxygen (in the form of carbon oxides) was increased to a greater degree when using the microwave treatment as opposed to the thermal treatments. The microwave treatment resulted in a substantial increase in the amount of chemical interaction between the fiber surface and the epoxy resin and amine components of the matrix. An epoxy resin/amine hardener adduct compound was also used to investigate the possible interaction of the adduct hydroxyl group with the carbon fiber surface. XPS results indicate a low to insignificant interaction of the hydroxyl with the carbon fiber surface under the conditions used in this study.     

The TPB Properties of Plasma-Treated Carbon Fiber-Reinforced Polystyrene Composites

Dielectric barrier discharges (DBD) in ambient air are used on carbon fiber to improve the fiber surface activity. Carbon fibers with length of 75 µm are placed into the plasma configuration. The interaction between modified carbon fibers and polystyrene was studied by the three-point bending (TPB) test. The chemical and physical changes induced by the treatments on carbon fiber surface are examined using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Contact angles of the plasma-treated carbon fiber and XPS results reveal that the carbon fibers modified with the DBD at an atmospheric pressure show a significant increase in oxygen and nitrogen concentration. These results demonstrate that the surfaces of the carbon fiber are more active, hydrophilic after plasma treatments using a DBD operating in ambient air.     

Bipolar high-power impulse magnetron sputtering synthesis of high-entropy carbides

In this study, we report high-entropy carbides synthesis with reactive bipolar high-power impulse magnetron sputtering (HiPIMS). Uncontrolled microstructure and stoichiometry development with reactive gas flow rate are major limitations of conventional direct current (DC) and radio frequency (RF) magnetron sputtering of multicomponent carbides. With HiPIMS these chemically disordered crystals structurally and compositionally transform from a carbon-deficient metallic (C/M < 1), to a stoichiometric ceramic zone (C/M ∼ 1), and to a nanocomposite embodiment (C/M > 1), as a function of the carbon content during HiPIMS deposition. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, and nanoindentation hardness measurements are combined to demonstrate the three regions of synthesis domain. HiPIMS provides access to metallic, ceramic, and composite carbides with great control over the microstructure and stoichiometry, which is elusive in case of conventional DC and RF magnetron sputtering. Notably, the stoichiometric ceramic zone maintains a constant carbon to metal ratio (C/M ∼ 1) over an extended amount of methane flow before transitioning to a nanocomposite microstructure (C/M > 1). The transition zone breadth depends on materials affinity for carbon that correlates with valence electron concentration (VEC). As such, synthesis conditions for new high-entropy carbides can be understood and predicted based on VEC.     

XPS studies of directly fluorinated HDPE: problems and solutions

X-ray photoelectron spectroscopy (XPS) applied to the study of fluorinated polymer surfaces presents several problems related both to peak assignment and to degradation. In this work, we analyse extensively the question of XPS peak assignments in this kind of surfaces. We conclude that in this kind of surfaces using binding energy differences between fluorine and carbon is better than using absolute binding energies. Also a useful relation between fluorine photoelectron energy vs. polymer composition expressed through the atomic ratio fluorine/carbon (F/C) was found. A protocol for data treatment is proposed and applied to a XPS study of the degradation induced by X-ray on high-density polyethylene surfaces modified by direct fluorination. Results obtained for the degradation, namely the atomic ratio F/C obtained by two different methods, combined with angle resolved X-ray photoelectron spectroscopy (ARXPS) were used to study the fluorine concentration profile in depth, producing self-consistent results.     

The effect of surface energy on the heat transfer enhancement of paraffin wax/carbon foam composites

The influence of carbon foam surface energy on heat transfer through paraffin wax/carbon foam composite was investigated. Carbon foam samples were surface treated and their corresponding surface energy values were measured. A theoretical model was formulated to analyze the mass of paraffin wax absorbed for both pristine and surface activated carbon foam samples based on the concept foam wettability. An experimental study was carried out for heating of the wax/carbon foam composite samples to study the phase change heat transfer due to the melting of wax within the foam matrices. The above studies showed that a greater mass of wax was absorbed within the activated carbon foam samples as compared to the pristine sample which can be due to their greater wettability. This resulted in an improvement in heat transfer rate for the activated samples. The total energy storage rate for the activated composite samples was compared with the pristine sample for the same heating duration and an enhancement of more than 18% was observed for the two activated samples. These studies revealed that the surface energy of carbon foams can play an important role in improving the overall thermal performance of wax/carbon foam composites.     

Development of an environmentally friendly protective coating for the depleted uranium-0.75 wt% titanium alloy: Part III: Surface analysis of the coating

Molybdenum oxide-based conversion coatings have been formed on the surface of the depleted uranium-0.75 wt% titanium alloy using either concentrated nitric acid or fluorides for surface activation prior to coating formation. The acid-activated surface forms a coating that offers corrosion protection after a period of aging, when uranium species have migrated to the surface. X-ray photoelectron spectroscopy (XPS) revealed that the protective coating is primarily a polymolybdate bound to a uranyl ion. Rutherford backscattering spectroscopy (RBS) on the acid-activated coatings also shows uranium dioxide migrating to the surface. The fluoride-activated surface does not form a protective coating and there are no uranium species on the surface as indicated by XPS. The coating on the fluoride-activated samples has been found to contain a mixture of molybdenum oxides of which the main component is molybdenum trioxide and a minor component of an Mo(V) oxide.     

Comparative study of fluorinated single- and few-wall carbon nanotubes by X-ray photoelectron and X-ray absorption spectroscopy

Pristine and ball-milled samples containing single-wall carbon nanotubes (SWCNTs) and few-wall carbon nanotubes (FWCNTs) have been fluorinated at room temperature using gaseous BrF₃ as a fluorinating agent. X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to estimate the chemical composition and to probe the electronic structure of the fluorinated CNTs. Analysis of the XPS C 1s spectra revealed that fluorinated carbon atoms in SWCNTs are bounded with one CF-group at least while most of the fluorinated carbon atoms in FWCNTs are surrounded by bare carbon atoms only. The ball-milling of the samples during 1 hour has insignificant effect on CNT length and more likely produces defects in CNT surface layers. These defects increase fluorination ability of CNTs and provide access for fluorine atoms to the subsurface layers of FWCNTs. NEXAFS investigation revealed that some of CNTs, which probably constitute interior of FWCNTs or CNT ropes, are not fluorinated during the conditions used and the fluorine atoms interact more strongly with CNT surfaces having a larger curvature.     

Study of the correlation between flexible food packaging peeling resistance and surface composition for aluminum-metallized BOPP films aged at 60°C

ABSTRACT

This study investigated the correlation between surface composition and peeling resistance in food packaging films by studying the heat aging of fabricated films over varying periods of time. The films consisted of a layer of aluminum (Al) metallized, biaxially oriented polypropylene (BOPP) bonded with a polyurethane (PU) adhesive onto another polymeric layer of low-density polyethylene (LDPE). The Al metallized films were prepared by physical vapor deposition (PVD) and aged at 60°C for either 5 or 15 days. The resulting aluminum surfaces were analyzed using X-ray photoelectron spectroscopy (XPS) and found to contain aluminum oxide (Al₂O₃) and trihydroxide (Al(OH)₃). The XPS characterization also revealed a 29% increase in the Al(OH)₃ layer thickness of the aged sample relative to a non-aged sample. Atomic force microscopy (AFM) was applied on investigations of possible morphology changes. The aluminum and PU adhesive surface energies were also determined using contact angles measurements and the aluminum surface energy was found to increase by as much as 11.7% compared to the non-aged sample, while the PU adhesive surface energy was at least 65% higher than that of the metallic substrate. The peeling resistance of the laminated aluminum was determined by average peel strength measurements and it was found that the variation in peel strength was related to changes in the Al₂O₃ layer thickness. The delaminated samples were analyzed using scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) and showed the cohesive failure of the aluminum film.     

Aging effects of repeatedly glow-discharged polyethylene: influence on contact angle,infrared absorption,elemental surface composition,and surface topography

Aging effects of repeatedly oxygen glow-discharged polyethylene surfaces were determined by water contact angle measurements, infrared (IR) spectroscopy, X-ray photoelectron (XPS) spectroscopy, and surface topography determination. Glow-discharged surfaces were stored at room temperature and in liquid nitrogen for 8 days prior to the next glow-discharge (Gld) treatment. This cycle was repeated up to 13 times. Hydrophobic recovery of the polyethylene surface, as determined by contact angle measurements, became less when the number of glow-discharge treatments increased. Hydrophobic recovery was suppressed when the samples were stored in liquid nitrogen. After five glow-discharge treatments it was possible to detect the incorporation of hydroxy groups by IR spectroscopy, while XPS spectroscopy showed that repeated glow-discharge gives rise to a higher oxygen to carbon ratio and a broadening of the C_Is peak, suggesting the incorporation of C-O and C=O bonds in the surface layer of polyethylene. The surface roughness of repeatedly glow-discharge-treated polyethylene remained almost unaltered in the submicrometer range (0.30-0.35 μm), although scanning electron microscopy revealed fine-grained structures for samples after a large number of Gld treatments. It can be concluded from this study that repeated oxygen glow-discharge treatments of polyethylene create more stable, hydrophilic surfaces than can be obtained with only one treatment. However, even after repeated glow-discharge treatments, polymer surface dynamics cause a small hydrophobic recovery.     

Comparison of molybdenum carbide and tungsten carbide for the hydrodesulfurization of dibenzothiophene

The molybdenum and tungsten carbides (Mo₂C and W₂C) were synthesized, characterized and tested in hydrodesulfurization (HDS) of dibenzothiophene (DBT). The phase purity of these catalysts was established by X-ray diffraction (XRD), and the surface properties were determined by N₂ BET specific surface area (Sg) measurements, CO chemisorption and high-resolution transmission electron microscopy (HRTEM). The activities of catalysts were determined during the HDS of DBT at a temperature of 613 K and under a 6 MPa total pressure. Both molybdenum and tungsten carbides were active in HDS of DBT. The reactivity studies showed that molybdenum carbide was more active than tungsten carbide related to weight. However, W₂C was shown to possess stronger hydrogenating properties.     

Carbon black-supported molybdenum sulfide catalysts

Four carbon black samples differing in surface area, pH and surface properties (oxygen functionality) were pore volume impregnated with aqueous molybdate solutions as to achieve a Mo loading of 0.5 Mo atoms per nm² support surface area. Dispersion measurements obtained by means of X-ray photoelectron spectroscopy, dynamic oxygen chemisorption and transmission electron microscopy, indicated the presence of highly dispersed molybdate in all precursor samples, which upon sulfidation was converted into molybdenum sulfide with a particle size varying between 3.5 and 13.5 nm dependant on the type of carbon black support. To explain these dispersion differences the interaction between molybdate ions and the carbon surface was studied by means of FTIR and XPS. No major changes were observed in the oxygen functionality of the carbon black upon loading with molybdate. Some minor changes were, however, observed by means of FTIR which could point to a chemical reaction between an aryl ether functional group and the molybdate ions.     

Synthesis and characterization of microporous carbons templated by ammonium-form zeolite Y

Emerging applications such as gas storage require porous carbon materials with tailored structural and surface properties. Template synthesis approach to porous carbons offers opportunities for tailoring these properties. In this study, ammonium-form zeolite Y (NH₄Y) was used as a template and furfuryl alcohol (FA) was employed as a carbon precursor to prepare microporous carbons by simple impregnation method. The effects of synthesis conditions such as carbonization temperatures and heating rates on the pore structure of the microporous carbons were investigated. The thermal behaviors of FA-NH₄Y mixtures and zeolite/carbon composites were studied by thermogravimetric analysis (TGA). The physical, structural, and surface properties of the microporous carbons were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), elemental analysis, and physical adsorption of nitrogen. Microporous carbons with high surface areas, pore volumes and nitrogen-containing surface functional groups can be readily synthesized.     

Degradation of Interfacial Chemistry of Epoxy/Silane/Aluminium Interfaces as a Result of Aqueous Attack

The degradation of a thin layer of adhesive on a grit-blasted aluminium substrate, as a result of aqueous attack, was investigated and compared with the behavior of the adhesive on a grit-blasted aluminium substrate treated with γ-glycidoxypropyl trimethoxy silane (GPS). The degradation study was achieved by examining aluminium coupons treated with adhesive that had been immersed in water at 25°C and an elevated temperature (50°C) for various treatment times ranging between 10 min and 1 day. All samples were characterized using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS and ToF-SIMS data indicated that the adhesive layer on both types of substrate was readily displaced by water. This is shown to be a two-stage process with bond rupture being identified by ToF-SIMS analysis and the displacement of the organic phase occurring at a later stage, as indicated by the XPS analysis, which showed a reduction in surface carbon concentration. When the substrates were directly in contact with water, a hydration process occurred and hydrated oxide species were formed on the surfaces. The results indicated that the hydration process was a postfailure event.     

Degradation of Interfacial Chemistry of Epoxy/Silane/Aluminium Interfaces as a Result of Aqueous Attack

The degradation of a thin layer of adhesive on a grit-blasted aluminium substrate, as a result of aqueous attack, was investigated and compared with the behavior of the adhesive on a grit-blasted aluminium substrate treated with γ-glycidoxypropyl trimethoxy silane (GPS). The degradation study was achieved by examining aluminium coupons treated with adhesive that had been immersed in water at 25°C and an elevated temperature (50°C) for various treatment times ranging between 10 min and 1 day. All samples were characterized using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS and ToF-SIMS data indicated that the adhesive layer on both types of substrate was readily displaced by water. This is shown to be a two-stage process with bond rupture being identified by ToF-SIMS analysis and the displacement of the organic phase occurring at a later stage, as indicated by the XPS analysis, which showed a reduction in surface carbon concentration. When the substrates were directly in contact with water, a hydration process occurred and hydrated oxide species were formed on the surfaces. The results indicated that the hydration process was a postfailure event.