Subsequent Surface Reactions Research Articles

In chemico methodology for engineered nanomaterial categorization according to number, nature and oxidative potential of reactive surface sites.

Methanol probe chemisorption quantifies the number of reactive sites at the surface of engineered nanomaterials, enabling normalization per reactive site in reactivity and toxicity tests, rather than per mass or physical surface area. Subsequent temperature-programmed surface reaction (TPSR) of chemisorbed methanol identifies the reactive nature of surface sites (acidic, basic, redox or combination thereof) and their reactivity. Complementary to the methanol assay, a dithiothreitol (DTT) probe oxidation reaction is used to evaluate the oxidation capacity. These acellular approaches to quantify the number, nature, and reactivity of surface sites constitute a new approach methodology (NAM) for site-specific classification of nanomaterials. As a proof of concept, CuO, CeO2, ZnO, Fe3O4, CuFe2O4, Co3O4 and two TiO2 nanomaterials were probed. A harmonized reactive descriptor for ENMs was obtained: the DTT oxidation rate per reactive surface site, or oxidative turnover frequency (OxTOF). CuO and CuFe2O4 ENMs exhibit the largest reactive site surface density and possess the highest oxidizing ability in the series, as estimated by the DTT probe reaction, followed by CeO2 NM-211 and then titania nanomaterials (DT-51 and NM-101) and Fe3O4. DTT depletion for ZnO NM-110 was associated with dissolved zinc ions rather than the ZnO particles; however, the basic characteristics of the ZnO NM-110 particles were evidenced by methanol TPSR. These acellular assays allow ranking the eight nanomaterials into three categories with statistically different oxidative potentials: CuO, CuFe2O4 and Co3O4 are the most reactive; ceria exhibits a moderate reactivity; and iron oxide and the titanias possess a low oxidative potential.

Effect of the Microstructure of Composite CoMoS/Carbon Catalysts on Hydrotreatment Performances

Several CoMoS/carbons were successfully synthetized using hydrothermal-type approaches and a subsequent surface reaction for the promotion step with Co. The effect of the microstructure of carbons used as a matrix for CoMoS-based catalysts was studied in the case of hydrotreating reactions. It was found that 1D (nanofibers/tubes) or 2D (nanosheets) carbon nanostructures may influence the characteristics of the CoMoS crystallites (dispersion, promotion rate and orientation of the slabs) and as a consequence the catalytic properties. In particular, the HDS mechanism of the substituted 4-MethylDiBenzoThiophene (4-MDBT) was found to be microstructure-dependent, as well as the selectivity of 3-MethylThiophene (3-MT) HDS/2,3-DiMethylBut-2-eNe (2,3-DM2BN) hydrogenation.

In situ construction of inorganic component-rich polymers as interfacial stabilizers for high-rate lithium metal batteries

Lithium-metal batteries (LMBs) have attracted special interest owing to their high energy densities. Nevertheless, the unfavorable reactivity of lithium metal (Li metal) leads to the uncontrolled growth of dendrites and formation of unstable solid electrolyte interlayers, which shorten the cycle life of Li metal and limit its wide application. Herein, a conformal and thin protective layer, composed of Sn, an Sn–Li alloy, lithium fluoride (LiF), and polyethylene oxide (PEO) (denoted as Li-Sn-LiF@PEO), is constructed in situ on Li metal via a facile one-step method. This conformal artificial layer not only reduced the subsequent surface reactions of lithium with the electrolytes, but also effectively accommodated the large volume change of the Li-metal anode and suppressed the growth of Li dendrites. Symmetric cells with Li-Sn-LiPEO-modified Li-metal anodes exhibited small voltage hysteresis and outstanding plating/stripping cyclability over 400 h at high current densities and capacities of 20 mA cm−2 and 20 mAh cm−2. When coupled with NCA/NCM523/LiFePO4 cathodes, the corresponding LMBs exhibited remarkable cycling life and superior rate properties. These encouraging results demonstrate a novel and promising method for long-life and large-scale stable LMBs.

An Atom‐Pair Design Strategy for Optimizing the Synergistic Electron Effects of Catalytic Sites in NO Selective Reduction

AbstractEffective adsorption and speedy surface reactions are vital requirements for efficient active sites in catalysis, but it remains challenging to maximize these two functions simultaneously. We present a solution to this issue by designing a series of atom‐pair catalytic sites with tunable electronic interactions. As a case study, NO selective reduction occurring on V1−W1/TiO2 is chosen. Experimental and theoretical results reveal that the synergistic electron effect present between the paired atoms enriches high‐energy spin charge around the Fermi level, simultaneously rendering reactant (NH3 or O2) adsorption more effective and subsequent surface reactions speedier as compared with single V or W atom alone, and hence higher reaction rates. This strategy enables us to rationally design a high‐performance V1−Mo1/TiO2 catalyst with optimized vanadium(IV)‐molybdenum(V) electronic interactions, which has exceptional activity significantly higher than the commercial or reported catalysts.

An Atom-Pair Design Strategy for Optimizing the Synergistic Electron Effects of Catalytic Sites in NO Selective Reduction.

Effective adsorption and speedy surface reactions are vital requirements for efficient active sites in catalysis, but it remains challenging to maximize these two functions simultaneously. We present a solution to this issue by designing a series of atom-pair catalytic sites with tunable electronic interactions. As a case study, NO selective reduction occurring on V1 -W1 /TiO2 is chosen. Experimental and theoretical results reveal that the synergistic electron effect present between the paired atoms enriches high-energy spin charge around the Fermi level, simultaneously rendering reactant (NH3 or O2 ) adsorption more effective and subsequent surface reactions speedier as compared with single V or W atom alone, and hence higher reaction rates. This strategy enables us to rationally design a high-performance V1 -Mo1 /TiO2 catalyst with optimized vanadium(IV)-molybdenum(V) electronic interactions, which has exceptional activity significantly higher than the commercial or reported catalysts.

Silane-Based SAMs Deposited by Spin Coating as a Versatile Alternative Process to Solution Immersion.

Functionalization of silica surfaces with silane-based self-assembled monolayers (SAMs) is widely used in material sciences to tune surface properties and introduce terminal functional groups enabling subsequent chemical surface reactions and immobilization of (bio)molecules. Here, we report on the synthesis of four organotrimethoxysilanes with various molecular structures and we compare their grafting by spin coating with the one performed by the conventional solution immersion method. Strikingly, this study clearly demonstrates that the spin coating technique is a versatile, fast, and more convenient alternative process to prepare robust, smooth, and homogeneous SAMs with similar properties and quality as those deposited via immersion. SAMs were characterized by PM-IRRAS, AFM, and wettability measurements. SAMs can undergo several chemical surface modifications, and the reactivity of amine-terminated SAM was confirmed by PM-IRRAS and fluorescence measurements.

Electron penetration triggering interface activity of Pt-graphene for CO oxidation at room temperature

Achieving CO oxidation at room temperature is significant for gas purification but still challenging nowadays. Pt promoted by 3d transition metals (TMs) is a promising candidate for this reaction, but TMs are prone to be deeply oxidized in an oxygen-rich atmosphere, leading to low activity. Herein we report a unique structure design of graphene-isolated Pt from CoNi nanoparticles (PtǀCoNi) for efficiently catalytic CO oxidation in an oxygen-rich atmosphere. CoNi alloy is protected by ultrathin graphene shell from oxidation and therefore modulates the electronic property of Pt-graphene interface via electron penetration effect. This catalyst can achieve near 100% CO conversion at room temperature, while there are limited conversions over Pt/C and Pt/CoNiOx catalysts. Experiments and theoretical calculations indicate that CO will saturate Pt sites, but O2 can adsorb at the Pt-graphene interface without competing with CO, which facilitate the O2 activation and the subsequent surface reaction. This graphene-isolated system is distinct from the classical metal-metal oxide interface for catalysis, and it provides a new thought for the design of heterogeneous catalysts.

Influence of substrate morphology on alcohol sensing behaviour of p-type Co3O4

An eco-friendly approach was adopted to fabricate porous MgAl2O4 ceramic substrate for designing a gas sensor. The active alcohol sensing component Co3O4 nanoparticles (Co3O4 NPs) were coated on dense and porous MgAl2O4 substrates using a wet chemical based dip coating technique followed by annealing in air at 400 °C. The morphology of the Co3O4 NPs found to be different for the porous and dense substrate. The Co3O4 NPs coated MgAl2O4 substrate-based sensor exhibited better alcohol sensitivity at low-temperature as compared to dense substrate counterpart. The porous substrate offers more active sites for gas adsorption and subsequent surface reaction leading to good sensitivity.

Preparation of biobased wrinkled surfaces via lignification-mimetic reactions and drying: a new approach for developing surface wrinkling

A simple and green approach to surface wrinkling via a lignification-mimetic reaction and drying is reviewed. A skin layer is synthesized on a chitosan (CS) film via immersion in a methanol solution containing a phenolic acid (PH) and a subsequent surface reaction with horseradish peroxidase (HRP), mimicking wood lignification. A surface relief with micron-scale wrinkles is formed upon drying as a result of inhomogeneous shrinkage. The wrinkle size of this system is predominately determined by the hardness of the skin layer, and the hardness in turn can be controlled by varying both the molecular structure of the PH and the immersion temperature. The crosslinking structure in the skin is composed of ionic bonding between the CS and an oligophenolic residue generated by the HRP-catalyzed surface reaction on the CS, and the quantity of the ionic bonds determines the skin hardness. Furthermore, the wrinkle direction can be highly controlled by the external stress during drying. Notably, this is a totally biobased system that involves green materials and processes. Recently, we have developed a simple and green approach to surface wrinkling via lignification mimetic reaction and drying. A skin layer is synthesized on a chitosan film via immersion in a methanol solution containing a phenolic acid and subsequent surface reaction by horseradish peroxidase, mimicking wood lignification. A surface relief with micron-scale wrinkles is formed upon drying and as a result of inhomogeneous shrinkage. This focus review overviews this innovative approach and its detailed mechanism.

Ab Initio Study of Growth Mechanism of 4H–SiC: Adsorption and Surface Reaction of C2H2, C2H4, CH4, and CH3

Silicon carbide is a semiconductor material with ideal properties for high-temperature and high-power applications. The epitaxial layer fabrication is usually performed using chemical vapor deposition (CVD) under a hydrogen rich atmosphere and high temperature. At such conditions the surface of the growing layer is expected to be passivated by the abundantly present hydrogen. In this work, we use quantum chemical density functional theory (B3LYP and M06-2X) and transition state theory to study surface reactions related to the deposition of carbon on the (0001) surface of 4H–SiC. We show that it is unlikely for an adsorption to occur on a passivated site unless the hydrogen termination is removed. We propose that unterminated sites can be effectively created during the CVD by an abstraction process. We provide details of the adsorption process of active carbon species, namely CH3, CH4, C2H2, and C2H4 gases, and their subsequent surface reactions such as desorption, abstraction of neighboring surface hydrogens and dimer formation. The reaction rates and sticking coefficients are provided for the temperature range of 298–2500 K. Finally, entire reaction paths from adsorptions to stable surface products are presented and discussed.

Atomic Layer Deposition of Zinc Oxide: Diethyl Zinc Reactions and Surface Saturation from First-Principles

Zinc oxide thin films grown via atomic layer deposition have been under intense research for the past few years. Here, we present a comprehensive density functional theory study on the atomic layer deposition of zinc oxide. The adsorption of diethyl zinc and subsequent surface reactions are studied on an ideal (100) ZnO surface as well as on a stepped surface to compare ideal and nonideal surface structures. Our results show that diethyl zinc adsorbs and reacts rapidly on the surface to form monoethyl zinc. Our calculations also show that the initial ligand-exchange reactions are preferred on the planar surface over the step surface. Further reaction from monoethyl zinc to adsorbed zinc atoms has a high reaction barrier. We present two surface structures for the saturated zinc oxide surface at the end of the diethyl zinc pulse corresponding to low and high temperature approximations that are in good agreement with the experiments.

ALMA OBSERVATIONS OF A GAP AND A RING IN THE PROTOPLANETARY DISK AROUND TW HYA

ABSTRACT We report the first detection of a gap and a ring in 336 GHz dust continuum emission from the protoplanetary disk around TW Hya, using the Atacama Large Millimeter/Submillimeter Array (ALMA). The gap and ring are located at around 25 and 41 au from the central star, respectively, and are associated with the CO snow line at ∼30 au. The gap has a radial width of less than 15 au and a mass deficit of more than 23%, taking into account that the observations are limited to an angular resolution of ∼15 au. In addition, the 13CO and C18O J = 3 − 2 ?> lines show a decrement in CO line emission throughout the disk, down to ∼10 au, indicating a freeze-out of gas-phase CO onto grain surfaces and possible subsequent surface reactions to form larger molecules. The observed gap could be caused by gravitational interaction between the disk gas and a planet with a mass less than super-Neptune (2 M Neptune ?> ), or could be the result of the destruction of large dust aggregates due to the sintering of CO ice.

Bio-based Wrinkled Surfaces Harnessed from Biological Design Principles of Wood and Peroxidase Activity.

A new and simple approach for surface wrinkling inspired by polymer assemblies in wood fibers is introduced. A hard skin is synthesized on a linear polysaccharide support that resembles the structural units of the cell wall. This skin, a wood mimetic layer, is produced through immersion in a solution containing phenolic precursor and subsequent surface reaction by horseradish peroxidase. A patterned surface with micron-scale wrinkles is formed upon drying and as a result of inhomogeneous shrinkage. We demonstrate that the design of the wrinkled surfaces can be controlled by the molecular structure of the phenolic precursor, temperature, and drying stress. It is noteworthy that this is a totally bio-based system involving green materials and processes.

Chemistry of Cu(acac)2 on Ni(110) and Cu(110) surfaces: Implications for atomic layer deposition processes

The thermal chemistry of copper(II)acetylacetonate, Cu(acac)2, on Ni(110) and Cu(110) single-crystal surfaces was probed under vacuum by using x-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Some data for acetylacetone (Hacac, CH3COCH2COCH3) adsorbed on Ni(110) are also reported as reference. Chemical transformations were identified in several steps covering a temperature range from 150 K to at least 630 K. The desorption of Hacac and a 3-oxobutanal (CH3COCH2CHO) byproduct was observed first at 150 and 180 K on Ni(110) and at 160 and 185 K on Cu(110), respectively. Partial loss of the acetylacetonate (acac) ligands and a likely change in adsorption geometry are seen next, with the possible production of HCu(acac), which desorbs at 200 and 235 K from the nickel and copper surfaces, respectively. Molecular Cu(acac)2 desorption is observed on both surfaces at approximately 300 K, probably from recombination of Cu(acac) and acac surface species. The remaining copper atoms on the surface lose their remaining acac ligands to the substrate and become reduced directly to metallic copper. At the same time, the organic ligands follow a series of subsequent surface reactions, probably involving several C–C bond-scissions, to produce other fragments, additional Hacac and HCu(acac) in the gas phase in the case of the copper surface, and acetone on nickel. A significant amount of acac must nevertheless survive on the surface to high temperatures, because Hacac peaks are seen in the TPD at about 515 and 590 K and the C 1s XPS split associated with acac is seen up to close to 500 K. In terms of atomic layer deposition processes, this suggests that cycles could be design to run at such temperatures as long as an effective hydrogenation agent is used as the second reactant to remove the surface acac as Hacac. Only a small fraction of carbon is left behind on Ni after heating to 800 K, whereas more carbon and additional oxygen remains on the surface in the case of Cu.

Effect of Li on the catalytic activity of MgO for the synthesis of flavanone

We have investigated the effects of Li on the structure, surface basicity and catalytic activity of MgO for the synthesis of flavanone. Introduction of low amounts of Li (i.e., ≤0.1 wt.%) was found to promote the rate of the Claisen–Schmidt condensation reaction, which is the first step in this process. However, at Li loadings above 0.1 wt.% a detrimental effect was observed, due to a concomitant decrease in surface area and increase in MgO crystallite size. A strong correlation was observed between surface-normalized basicity and catalytic activity. The increase in activity at higher levels of surface basicity can be attributed to the increased ability of Li–O − pairs to abstract a proton from the 2′-hydroxyacetophenone reactant, thus facilitating the adsorption and subsequent surface reactions of this molecule.

Study of Ammonia Formation During the Purge of a Lean NO x Trap

The aim of this work was to study the formation of ammonia during the regeneration step of a commercial lean NOxTrap catalyst in real conditions. Experiments were carried out on an engine test bench equipped with 2.2 L common rail diesel engine. The after-treatment system was composed of a commercial NSR monolith catalyst with a volume of 2.2 L. This material was composed of platinum, palladium and rhodium, as active components, with barium oxide and alumina as storage component and support, respectively. Catalytic measurements were carried out at 280 °C. Particular attention has been paid to formation of ammonia during the purge using hydrogen and CO as reducing agents. The time dependency of the extent of ammonia formation and the influence of hydrogen concentration were carefully examined. It was observed that the temperature rose in the first step of the regeneration process, after the switch in rich conditions, due to the occurrence of exothermic oxidation (H2/O2, CO/O2) accompanied with the releasing of available vacant noble metal sites which may provide a route for H2 and NO dissociation and/or subsequent surface reactions leading to the ultimate formation of ammonia. It was found that ammonia formation strongly depends on the purge duration and on the concentration of the reducing agent available during the purge.

Endowing Carbon Black Pigment Particles with Primary Amino Groups

A readily synthetic route to endow the surface of hydrophobic carbon black (CB) particles with primary amino groups has been developed. The adsorption of the water-borne polyelectrolyte copolymer poly(vinyl amine-co-vinyl amide) (PVAm) on CB was studied as function of pH and polyelectrolyte concentration. The amino-functionalization process of CB with PVAm strongly depends on pH. As shown by electrokinetic measurement as function of pH, the content of accessible primary amino groups on the surface of CB increases with increasing the OH- concentration. The alternative functionalization of the CB surface with an anionic polyelectrolyte is demonstrated for the adsorption of poly(ethylene-alt-maleic anhydride) (PEMSA). About 50 mg of PVAm can be adsorbed on 1.0 g of CB, which is sufficient to completely alter the surface net charge. The aim of the introduced functional amino groups is their ability to act as anchoring groups for subsequent surface reactions improving the CB's process ability, compatibility, and adhesion properties.

On the preparation and application of novel PVDF–POSS systems

Novel systems based on poly(vinylidene fluoride) (PVDF) and polyhedral oligomeric silsesquioxane (POSS) have been prepared by grafting amino-containing POSS (POSS–NH2) onto the surface of modified PVDF (PVDFm). Namely, the approach consists of a preliminary modification of PVDF by a chemical treatment with an alkaline solution, in order to obtain unsaturations, and a subsequent surface reaction of PVDFm with POSS molecules characterized by an amino group as reactive side. The level of polymer unsaturation, measured by Raman spectroscopy, turned out to be finely tuned by varying the dehydrofluorination reaction time. The surface grafting of POSS has been studied by SEM-EDS analysis, Raman and XPS spectroscopy. Indeed, while the level of modification of PVDFm has been found to decrease as a consequence of the reaction with POSS, thus indicating a likely saturation of double bonds by silsequioxane molecules, the Si concentration turned out to increase with increasing the concentration of the polymer surface unsaturation. The feasibility of the application of POSS grafting onto the surface of membranes, based on PVDFm, has also been verified.

Nanocrystalline ZnO thin film growth by ultrasonic spray from a non-aqueous solution

In the present work, zinc oxide (ZnO) thin films are deposited by the ultrasonic spray method. In order to study the influence of the nature of the precursor solution, zinc acetate and zinc nitrate salts dissolved in methanol are used respectively to prepare two non-aqueous starting solutions. The influence of substrate temperature on the structural growth kinetics and optical properties of ZnO films is also investigated. For this purpose, the substrate temperature is varied in the range of 250—400°C. X-ray diffraction (XRD) analysis reveals that at low temperature the films that are grown contain nanocrystallites with a privileged orientation depending on the nature of the starting precursor; these privileged orientations correspond to the plane (100) for the films deposited with zinc acetate and (002) for films prepared with zinc nitrate precursor. However, with increasing deposition temperature, the privileged orientation disappears. The growth kinetics is closely related to the substrate temperature and to the nature of the starting solution. The scanning electron microscopy (SEM) observations suggest that the liquid droplets react once they reach the substrate surface when zinc acetate precursor is used. However, in the case of zinc nitrate solution, the droplets spread first on the substrate surface followed by the subsequent surface reaction. This difference is explained in terms of the dissociation enthalpy of both solutions.

Utilization of the Three-Dimensional Volcano Surface To Understand the Chemistry of Multiphase Systems in Heterogeneous Catalysis

CO hydrogenation is used as a model system to understand why multiphase catalysts are chemically important in heterogeneous catalysis. By including both adsorption and subsequent surface reactions, kinetic equations are derived with two fundamental properties, the chemisorption energies of C and O (DeltaHC and DeltaHO, respectively). By plotting the activity against DeltaHC and DeltaHO, a 3-D volcano surface is obtained. Because of the constraint between DeltaHC and DeltaHO on monophase systems, a maximum can be achieved. However, if multiphase systems are used, such a constraint can be released and the global maximum may be achieved.