Cerium oxide powders are widely used and are of fundamental importance in catalytic pollution control and energy production due to the unique chemical properties of CeO2. Processing steps involved in ...catalyst preparation, such as high-temperature calcination or mechanical milling processes, can alter the morphological and chemical properties of ceria, heavily affecting its final properties. Here, we focus on the tuning of CeO2 nanopowder properties by mild- and high-energy milling processes, as the mechanochemical synthesis is gaining increasing attention as a green synthesis method for catalyst production. The textural and redox properties were analyzed by an array of techniques to follow the aggregation and comminution mechanisms induced by mechanical stresses, which are more prominent under high-energy conditions but strongly depend on the starting properties of the ceria powders. Simultaneously, the evolution of surface defects and chemical properties was followed by Raman spectroscopy and H2 reduction tests, ultimately revealing a trade-off effect between structural and redox properties induced by the mechanochemical action. The mild-energy process appears to induce the largest enhancement in surface properties while maintaining bulk properties of the starting materials, hence confirming its effectiveness for its exploitation in catalysis.
Low-Energy Ion Scattering (LEIS or ISS) is used to selectively analyze the atomic composition of the outer atomic layer of surfaces. In addition, the spectrum gives (non-destructively) the in-depth ...distribution. Using a double toroidal energy analyzer with parallel energy detection and time-of-flight filtering a high sensitivity and mass resolution of LEIS is achieved. This is demonstrated for a highly dispersed catalyst of Pt/Au on γ-alumina. The improved depth resolution is illustrated for self-assembled monolayers of alkanethiols (12–20 carbon atoms) on gold. Even for these low Z carbon atoms a clear shift of 8
eV/carbon atom is observed (using 1.5
keV
4He
+ ion scattering). This opens many new possibilities for studies of ultra-thin diffusion barriers, high-k dielectrics and biosensors.
Commercial rutile TiO
particles capped with Al
O
and ZrO
layers, which are widely used in white pigments, can serve as a starting material for the fabrication of visible light-responsive ...photocatalysts toward gas-phase NO oxidation. The as-received TiO
with iron impurities exhibited reduced photocatalytic activity, and the activity was boosted by the deposition of additional iron comparable in quantity to the intrinsic iron impurity level. Analyses using X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectroscopy, and low-energy ion scattering spectroscopy revealed that the deposited iron and intrinsic impurity iron are dissimilar in terms of location, oxidation states, and interaction with TiO
. This suggests that tracking the structure and impurity levels of photocatalyst elements can be crucial for understanding structure-activity relationships of real catalysts.
Very simple and economical SiO 2 supported ionic liquid phase (SILP) materials are efficient catalysts for the addition of CO 2 to epoxides, producing cyclic carbonates in high yields (up to 99%) and ...selectivities (up to 99%). A range of ionic liquid (IL) concentrations (5–100 wt%), SiO 2 -supported 1- n -butyl-3-methylimidazolium halides (SBMIm·X: X = Cl, Br and I) ( 1–7 ) and 1-ethyl-3-(3-(trimethoxysilyl)propyl)-imidazolium halides (SEPIm·X) ( 8 , 9 ), were prepared and fully characterised. These hybrid materials are very active catalysts under mild reaction conditions (low temperature and atmospheric pressure or adsorbed CO 2 ). Under the optimal reaction conditions (S = 3.34 mmol, cat/S = 0.50, P CO2 = 5 bar, T = 80 °C), the best SILP system yields maximum conversion in just 30 min and can be reused at least five times without a noticeable decrease in activity and selectivity. The catalytic system is also active when using a CO 2 gas mixture from an industrial exhaust in both batch and continuous flow systems. A detailed structural and electronic analysis indicates that increasing the IL and water concentrations induces a solvation effect through the contact ion pair of the IL that drives the anions (Cl, Br and I) to the deeper regions of the confined space of SiO 2 . The catalytic performance is directly related to the presence of the nucleophilic Br anion on the outermost exposed layer of the hybrid material.
Commercial rutile TiO2 particles capped with Al2O3 and ZrO2 layers, which are widely used in white pigments, can serve as a starting material for the fabrication of visible light-responsive ...photocatalysts toward gas-phase NO oxidation. The as-received TiO2 with iron impurities exhibited reduced photocatalytic activity, and the activity was boosted by the deposition of additional iron comparable in quantity to the intrinsic iron impurity level. Analyses using X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectroscopy, and low-energy ion scattering spectroscopy revealed that the deposited iron and intrinsic impurity iron are dissimilar in terms of location, oxidation states, and interaction with TiO2. This suggests that tracking the structure and impurity levels of photocatalyst elements can be crucial for understanding structure–activity relationships of real catalysts.
Undesired reactions at electrode/electrolyte interfaces impose challenges in the durability of Li-ion battery. Traditional strategies of interfacial stabilization involve coating with inactive oxide ...films on aggregated powders of active cathode oxides. Despite generating gains in electrode performance, the lack of control of film growth of existing methods limits the ability to design its chemical structure and enhance functionality. The complexity of these coated materials also complicates efforts to define the specific chemical and structural features that determine function. Core–shell heterostructures at the nanocrystal level offer opportunities for precise control of chemistry and homogeneity. This ability is demonstrated with the compositional and structural tailoring of passivating layers based on Al3+, grown conformally onto LiCoO2 nanoplates, using thermal treatments. They result in heterostructures from core–shell (LiCoO2 nanoplates@2 nm aluminum oxide) to LiCo1–x Al x O2 gradient structures composed by an Al-rich outer layer on a Co-rich core. While all samples presented improvements in electrochemical performance compared to the bare material, the LiCo1–x Al x O2 gradient heterostructure presented the greatest advantage compared to pure aluminum oxide shells. The presence of a high Al/Co ratio at the surface, combined with the structural epitaxy and presence of Li throughout the particle, was considered to be critical to the best electrode properties and electrode/electrolyte interface stabilization. This work advances our ability to build complex heterostructures that both offer engineering solutions and create novel fundamental insight into the origins of battery durability.
•Low energy ion scattering performed on a multi-component display glass surface.•The as-formed glass surface is significantly depleted in aluminum.•All melt surfaces are significantly depleted of ...modifiers compared to the fracture surface.•Aluminum and silicon signals are resolved via peak fitting.•The composition of the drawn fiber surface closely resembles the fracture surface.•Recovery of aluminum on the surface occurs after washing with alkaline detergents.
Flat panels displays (FPDs) are commonly manufactured on highly-engineered glass substrates known as display glasses. As FPD pixel sizes decrease and pixel densities increase, the surface composition and surface properties of these glasses have an increasingly important impact on device yield, influencing static electricity buildup and discharge, particulate adhesion, rate of contamination, and device lifetime. Here, we apply low energy ion scattering (LEIS) to the analysis of Eagle XG®, a widely used display glass. Surfaces were treated with production-line relevant chemistries including acids, bases, etchants, industrial detergents, and plasmas. The resulting surfaces were compared to as-formed melt surfaces, fracture surfaces, and fibers formed from remelted Eagle XG®. LEIS revealed the elemental composition of the outermost atomic layer of these materials, detecting all major Eagle XG® constituents except boron. The surface composition of the glass differed as a function of forming process used to fabricate it as well as surface treatment. The surface concentration of aluminum on the as-formed melt surface differs significantly from the bulk composition (1–5% vs. 30–31% Al2O3 surface coverage, respectively). HCl treatment depleted the surface of all species except silica. HF treatment depleted modifier species from the glass surface to a lesser extent. An alkaline industrial detergent produced an increase in alumina relative to the as-formed glass surface (8–12% vs. 1–5% Al2O3 surface coverage, respectively). Treatment with an atmospheric-pressure plasma had no detectable impact on the elemental surface composition of the glass. Aluminum and silicon generally give overlapping signals in LEIS, and these signals could only be resolved here through a combination of optimized experimental conditions and data fitting. Various approaches to this data analysis were explored, including a guided least-squares approach referred to herein as informed sample model approach (ISMA), wherein the pure spectral components required for the fit were mathematically derived from the sample spectra. Most commercial display glasses contain both Al and Si, but there is little discussion of the deconvolution of these LEIS signals in the technical literature.
The self-terminating chemistry of atomic layer deposition (ALD) ideally enables the growth of homogeneously distributed materials on the atomic scale. This study investigates the ALD of zinc oxide ...(ZnO) on mesoporous zirconium oxide (ZrO2) using zinc acetylacetonate Zn(acac)2 and synthetic air in a fixed-bed powder ALD reactor. A broad variety of methods, including thermogravimetry analysis, scanning electron microscopy with energy-dispersive X-ray spectroscopy, low-energy ion scattering, X-ray absorption near-edge structure, X-ray photoelectron spectroscopy, in-situ diffuse reflectance infrared Fourier transform spectroscopy–mass spectrometry, and density functional theory calculations, were used to analyze the reactant and the resulting samples. The factors affecting the zinc loading (wt %) on ZrO2 were investigated by varying the ALD reaction temperature (160–240 °C), the calcination temperature of zirconium oxide (400–1000 °C), and the ALD cycle number (up to three). The studied process showed self-terminating behavior with the areal number density of zinc of approximately two atoms per square nanometer per cycle. Zinc was distributed throughout ZrO2. After the Zn(acac)2 reaction, acac ligands were removed using synthetic air at 500 °C. In the following cycles, already-deposited ZnO acted as nuclei for further ZnO growth. This study demonstrates the potential of Zn(acac)2 as an ALD reactant and provides an initial understanding of ZnO growth via ALD on high surface area porous particles as an example for catalytic applications.
Ionic liquid (IL) hybrid organosilicas based on 1-n-butyl-3-(3-trimethoxysilylpropyl)imidazolium cations associated with hydrophilic and hydrophobic anions decorated with well-dispersed and ...similar-sized (1.8–2.1 nm) Pd nanoparticles (Pd-NPs) are among the most active and selective catalysts for the partial hydrogenation of conjugated dienes to monoenes. The location of the sputter-imprinted Pd-NPs on different supports, as determined by RBS and HS-LEIS analysis, is modulated by the strength of the contact ion pair formed between the imidazolium cation and the anion, rather than the IL hybrid organosilica pore size and surface area. In contrast, the pore diameter and surface area of the hybrid supports display a direct correlation with the anion hydrophobicity. XPS analysis showed that the Pd(0) surface component decreases with increasing ionic bond strength between the imidazolium cation and the anions (contact ion pair). The finding is corroborated by changes in the coordination number associated with the Pd–Pd scattering in EXAFS measurements. Hence, the interaction of the IL with the metal surface is found to occur via IL contact pairs (or aggregates). The observed selectivities of ≥99% to monoenes at full diene conversion indicate that the selectivity is intrinsic to the electron-deficient Pd metallic surfaces in this “restricted” ionic environment. This suggests that IL hybrid organosilica/Pd-NPs under multiphase conditions (“dynamic asymmetric mixture”) operate akin to catalytically active membranes: i.e., far from the thermodynamic equilibrium. Detailed kinetic investigations show that the reaction rate is zero order with respect to hydrogen and is dependent on the fraction of catalyst surfaces covered by either the substrate and/or the product. The reaction proceeds via rapid inclusion and sorption of the diene to the IL/Pd metal surface saturated with H species. This is followed by reversible hydride migration to generate a π-allyl intermediate. The reductive elimination of this intermediate, the formal rate-determining step (RDS), generates the alkene that is rapidly expelled from the IL phase to the organic phase.
Low energy ion scattering (LEIS) probes the elemental composition of the outermost atomic layer of a material and provides static depth profiles of the outer
ca.
10 nm of surfaces. Its extreme ...surface sensitivity and quantitative nature make it a powerful tool for studying the relationships between surface chemistry and surface related phenomena such as wetting, adhesion, contamination, and thin film growth. The high depth resolution obtained in LEIS in its static and sputter depth profile modes are useful for studying the layer structures of thin films. LEIS instrumentation has improved significantly in recent years, showing dramatic increases in its sensitivity and further expanding its potential applications. In this article, we provide a practical introduction to the technique, including a discussion of the basic theory of LEIS, LEIS spectra, LEIS instrumentation, and LEIS applications, including catalysts, solid oxide fuel cells (SOFCs), and thin films in integrated circuits.
Noble gas ion scattering off of a ZrO
2
film on Si (left), and mock LEIS spectrum for this material (right).
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