Mechanism of the Effect of Thermal Treatment on the Formation of Strong Acid Sites in the Surface Layers of Sulfated Metal Oxides

Strong acid catalysts were synthesized by the impregnation of hydrated ZrO₂ and TiO₂ with sulfuric acid followed by thermal treatment at different temperatures. The surface acidity and crystallochemical characteristics of the catalysts were studied by potentiometry and X-ray diffraction analysis, respectively. It was found that the surface acidity gradually increased as the temperature of thermal treatment was increased from 350 to 600°C for SO^2– ₄/ZrO₂ or to 200°C for SO^2– ₄/TiO₂; this increase correlated with the degrees of crystallinity of the samples. A hypothesis was proposed to explain the gradual accumulation of acid sites in the surface layer in the course of thermal treatment. It was assumed that, because of crystallographic changes that caused the weakening or even rupture of Zr–O–S and Ti–O–S bonds in modified surface layers, these layers exhibited an enhanced reactivity in contact with water vapor. Subsequently, this resulted in the formation of strongly acidic grafted M–O–SO₃–H⁺ groups.     

The influence of the active component and support nature, gas mixture composition on physicochemical and catalytic properties of catalysts for soot oxidation

Physicochemical and catalytic properties of compositions Fe(Ce)–Mn–O/support (gamma-, theta-, alpha-Al₂O₃, SiO₂ as the support) and Pt/CeO₂/theta-Al₂O₃ for oxidation of soot were characterized. It was established that the phase composition of the initial catalysts depended mainly on the nature of the active component and preparation conditions. Non-isothermal treatment of the soot–catalyst compositions at the temperature up to 1000 °C resulted in a change in the phase composition depending mainly on the final treatment temperature. The catalyst surface area was determined by the support nature. It was established that catalyst activities for oxidation of soot are determined by both catalyst nature and composition of gas mixture. The process of the soot oxidation is thought to involve oxygen from the catalyst surface. The higher proportion of weakly bound surface oxygen, the higher was the catalyst activity. An increase in the oxygen concentration from 5% O₂/N₂ to 15% O₂/N₂ is shown to lead to a decrease of the temperature of the soot oxidation. The influence of the oxygen concentration on the process of soot oxidation becomes weaker in the presence of water vapor. Results showed that the presence of NO in the gas mixture favors a decrease in the oxidation temperature of the soot, the higher being the nitrogen oxide concentration, the more pronounced effect. Introduction of SO₂ in amount of 50 ppm in the gas mixture has no noticeable effect on the process of the soot oxidation. Among the catalysts under study, Fe–Mn–K–O/gamma-Al₂O₃ is most effective to oxidation of the soot at otherwise identical conditions.     

Schiff base complex sol–gel method for LaCoO3 perovskite preparation with high-adsorbed oxygen

A novel Schiff base complex sol–gel method has been used to prepare LaCoO₃ producing high ratios of adsorbed (or surface) oxygen (α) to lattice oxygen (β). The as-prepared gels, characterized by Fourier transform infrared spectroscopy (FTIR), showed that both lanthanum and cobalt ions were complexed before calcinations. IR spectra revealed that CO₃²⁻ and NO₃⁻ presented on the sample surfaces during heat treatment. High-resolution transmission electron microscopic (HRTEM) images of all samples showed resolved lattice fringes with the inter-planar spacing 0.37–0.39 nm of the (0 1 2) plane in hexagonal perovskite. BET surface areas of LaCoO₃ nano-crystals were 11.7–18.6 m²/g. Ratios of adsorbed (or surface) oxygen (α) to lattice oxygen (β) quantified by X-ray photoemission spectroscopy showed that LaCoO₃ prepared by the Schiff base complex method produced higher ratios when bases had higher nitrogen content in molecules. Carbonate and nitrate which were resulting from the oxidation of functional groups in the Schiff base complex, can produce gaseous compounds and leave vacant sites for oxygen in the gas phase to adsorb.     

Effect of the Measurement Temperature on the Dispersive Component of the Surface Free Energy of a Heat Treated SiO2 Xerogel

The surface free energy of a monolithic silica xerogel treated at 1000°C has been measured by inverse gas chromatography in the temperature range 25–150°C using n-alkanes. Values of the dispersive component, _S ^D, vary from 49.07 mJ·m^–2 at 25°C to 17.20 mJ·m^–2 at 150°C. The _S ^D value obtained at 25°C is lower than that found for amorphous and crystalline silicas but higher than that found for glass fibres meaning that the heat treatment at 1000°C changes drastically the structure of the silica xerogel showing a surface similar to a glass. However, the higher value of _S ^D in comparison to glass fibres can be attributed to the mesoporous structure present in the silica xerogel. In the temperature range of 60–90°C there exists an abrupt change of the _S ^D values as well as in the dispersive component of the surface enthalpy, h _S ^D. Such abrupt change can be attributed to an entropic contribution of the surface free energy.     

Adsorption of Cu(II) and Co(II) complexes on a silica gel surface chemically modified with 2-mercaptoimidazole

The isotherms of adsorption of MeX₂ (Me = Cu²⁺, Co²⁺; X = Cl^–, Br^–, ClO ₄ ^– ) by silica gel chemically modified with 2-mercaptoimidazole (SiMI) were studied in acetone and ethanol solutions, at 25 °C. Covalently attached 2-mercaptoimidazole molecule to silica gel surface adsorbs MeX₂ from solvent by forming a surface complex. The metal is bonded to the surface through the nitrogen atom of attached 2-mercaptoimidazole. At low loading, the electronic and ESR spectral parameters indicated that the Cu²⁺ complexes are in a distorted-tetragonal symmetry field. The d-d electronic transition spectra showed that for Cu(ClO₄)₂ complex, the peak of absorption did not change for any degree of metal loading and for Cl^– and Br^– complexes, the peak maxima shifted to higher energy with lower metal loading. The CoX₂(X = Cl^–, Br^–, ClO ₄ ^– ) analogues possess a distorted-tetrahedral field.     

Surface and Subsurface Oxygen on Platinum in a 0.5 M H2SO4 + 0.01 M CuSO4 Solution

The formation of a copper adatom layer on polycrystalline platinum in a copper sulfate solution is studied by cyclic voltammetry in different cycling ranges at 0.1 V s^–1. The copper adatom deposition kinetics is controlled by the following factors. The substrate's top layer structure during the oxygen exit onto the surface may be unstable at anodic limits E _a = 0.90–1.35 V. The concentration of copper oxides (active centers) may be higher at E _a = 0.8–0.95 V. The balance between different adsorption sites differs in different cycling conditions. Of importance is the number of complexes O_ss–Pt _n –SO₄ and O_ss–Pt _n –O_c, where O_ss is subsurface oxygen and O_c is chemisorbed oxygen.     

Cobalt–silicon mixed oxide nanocomposites by modified sol–gel method

Cobalt–silicon mixed oxide materials (Co/Si=0.111, 0.250 and 0.428) were synthesised starting from Co(NO₃)₂·6H₂O and Si(OC₂H₅)₄ using a modified sol–gel method. Structural, textural and surface chemical properties were investigated by thermogravimetric/differential thermal analyses (TG/DTA), XRD, UV–vis, FT-IR spectroscopy and N₂ adsorption at −196 °C. The nature of cobalt species and their interactions with the siloxane matrix were strongly depending on both the cobalt loading and the heat treatment. All dried gels were amorphous and contained Co²⁺ ions forming both tetrahedral and octahedral complexes with the siloxane matrix. After treatment at 400 °C, the sample with lowest Co content appeared amorphous and contained only Co²⁺ tetrahedral complexes, while at higher cobalt loading Co₃O₄ was present as the only crystalline phase, besides Co²⁺ ions strongly interacting with siloxane matrix. At 850 °C, in all samples crystalline Co₂SiO₄ was formed and was the only crystallising phase for the nanocomposite with the lowest cobalt content. All materials retained high surface areas also after treatments at 600 °C and exhibited surface Lewis acidity, due to cationic sites. The presence of cobalt affected the textural properties of the siloxane matrix decreasing microporosity and increasing mesoporosity.     

Study of silica supported PtxNi1-x catalysts by ion scattering spectroscopy

As supported Pt_xNi_1–x catalysts are used for hydrogenation reactions, it seemed necessary to assess the surface composition of the reduced samples. To approach the usual reduction conditions we applied in situ reduction in a reaction chamber (1 mbar H₂ up to 500 °C) placed in ultra high vacuum recipient (UHV: 10^–9 to 2.10^–10mbar). All ion scattering spectroscopy measurements were performed in UHV. Charging of the samples was avoided by electron bombardment (5 eV). The variation of the surface composition was determined after subsequent sputtering, thermal treatment at 500 °C and during oxygen adsorption. A comparison with previous results of surface compositions of binary alloys (polycrystalline foils [1, 2] and single crystals Pt_xNi_1–x [3]) is given.Dedicated to Professor Dr. rer. nat. Dr. h. c. Hubertus Nickel on the occasion of his 65th birthday     

The Relationship between the State of Active Species in a Ni/Al2O3Catalyst and the Mechanism of Growth of Filamentous Carbon

The formation of active particles and their changes in the course of 1,3-butadiene decomposition on a Ni/Al₂O₃catalyst at temperatures from 400 to 800°C were studied by high-resolution electron microscopy. It was found that carbon filaments of different types were formed at 400–800°C. The growth of thin filaments (20–30 nm in diameter) takes place at 400–600°C on a conical Ni particle located at the growing end of the filament, whereas di-symmetrical filaments 50–100 nm in diameter grow on biconical metal particles. As the carbonization temperature was increased to 700–800°C, graphite nanotubes 5–20 nm in diameter were formed. It was found that the mechanism of formation and the structure of filaments are related to the state of catalytically active species, which consist of a solid solution of carbon in the metal. It is suggested that the metastable surface nickel carbide Ni₃C_{1 – x} is an intermediate compound in the catalytic formation of graphite filaments from 1,3-butadiene. Upon termination of the reaction, the metastable Ni₃C_{1 – x} microphase is decomposed with the formation of hexagonal nickel microinclusions. The role of epitaxy in the nucleation and growth of a graphite phase on the metal is discussed. Models are presented for the growth of structurally different carbon filaments depending on the formation of active metal species at various temperatures. Considerable changes in the structure of carbon and the formation of nanotubes at 700–800°C are related to the appearance of a viscous-flow state of metal–carbon particles.     

Polypropylene Films in a Direct-Current Discharge

The effect of dc discharge treatment on polypropylene films—both commercial and stabilizer- and plasticizer-free as prepared under laboratory conditions (with the rac-Et(Ind)₂ZrCl₂–methylaluminoxane catalytic system)—was studied. The discharge treatment was shown to hydrophilize the polypropylene surface in both cases. Negative charged states were detected on the modified surface and their relation to the polypropylene hydrophilization was established. By the thermostimulated relaxation and depolarization techniques, it was shown that the observed charges are induced by injection of plasma electrons into the polymer. It was found that severe changes took place throughout the bulk of the polypropylene film synthesized over the metallocene catalyst, as indicated by enhancement of its crystallinity and a change in the molecular mass distribution.     

Potential-dependent chemisorption of carbon monoxide on platinum electrodes: new insight from quantum-chemical calculations combined with vibrational spectroscopy

Density functional theory (DFT) using the finite cluster approach is utilized to compute binding energies, bond geometries, and vibrational properties of carbon monoxide adsorbed on Pt(111) as a function of the external interfacial field, focusing attention on the metal–CO bond itself. Comparison with electrode potential-dependent frequencies for the metal–CO (ν_M–CO) as well as the much-studied intramolecular C---O (ν_CO) vibration, as measured by in-situ Raman and infrared spectroscopy, facilitate their interpretation in terms of metal-chemisorbate bonding for this archetypal electrochemical system. Decomposing the calculated metal–CO binding energy and vibrational frequencies into individual orbital and steric repulsion components enables the role of such quantum-chemical interactions to the field- (and hence potential-) dependent bonding to be assessed. No simple relationship between the field(F)-dependent binding energies and the ν_M–CO frequencies is evident. While the DFT ν_M–CO–F slopes are negative at positive and small–moderate negative fields, reflecting the prevailing influence of back-donation, a ν_M–CO–F maximum is obtained at larger negative fields for atop CO, and a plateau for hollow-site CO. This Stark-tuning behavior reflects largely offsetting field-dependent contributions from π and σ surface bonding, and can also be rationalized on the basis of changes in the electrostatic component of ν_M–CO from increasing M–CO charge polarization. A rough correlation between the field-dependent ν_M–CO frequencies and the corresponding bond distances, r_M–CO, is observed for hollow and atop CO in that r_M–CO shortens towards less positive fields, but becomes near-constant at moderate–large negative fields. A more quantitative correlation between the field-dependent C---O frequencies and bond lengths is also evident. In harmony with earlier findings (and unlike the ν_M–CO–F behavior), the ν_CO–F dependence is due chiefly to changes in the back-donation bonding component. The overall vibrational frequency-field behavior predicted by DFT is also in semi-quantitative concordance with experimental potential-dependent spectra.     

Surface redox polymerized SPEEK–MO2–PANI (M = Si, Zr and Ti) composite polyelectrolyte membranes impervious to methanol

We reported sulfonated poly(ether ether ketone) (SPEEK, 61% degree of sulfonation)–metal oxides (MO₂:SiO₂, TiO₂ and ZrO₂)–polyaniline composite membranes. Metal oxides were incorporated into the swelled SPEEK membrane by sol–gel method and cured by thermal treatment. SPEEK–metal oxide membranes surfaces were modified with polyaniline (PANI) by a redox polymerization process. It was observed that water retention capacity of membrane was increased and methanol permeability was reduced due to synergetic effect of metal oxides and surface modification with polyaniline. These composite membranes showed extremely low methanol permeability (1.9–1.3 × 10⁻⁷ cm² s⁻¹), which was lower than till reported values either for SPEEK–metal oxide or SPEEK/PANI membranes. Relatively high selectivity parameter (SP) values at 343 K of these membranes, especially S–SiO₂–PANI and S–TiO₂–PANI, indicated their great advantages over Nafion117 (N117) membrane for targeting on moderate temperature applications due to the synergetic effect of MO₂ and PANI in SPEEK matrix. S–TiO₂–PANI and N117 showed comparable cell performance in direct methanol fuel cell (DMFC).     

A New Predictive Equation for the Surface Tensions of Aqueous Mixed Ionic Solutions Conforming to the Linear Isopiestic Relation

The linear isopiestic relation has been used, together with the fundamental Butler equations, to establish a new simple predictive equation for the surface tensions of the mixed ionic solutions. This newly proposed equation can provide the surface tensions of multicomponent solutions using only the data of the corresponding binary subsystems of equal water activity. No binary interaction parameters are required. The predictive capability of the equation has been tested by comparing with the experimental data of the surface tensions for the systems HCl–LiCl–H₂O, HCl–NaClO₄–H₂O, HCl–CaCl₂–H₂O, HCl–SrCl₂–H₂O, HCl–BaCl₂–H₂O, LiCl–NaCl–H₂O, LiCl–KCl–H₂O, NaCl–KCl–H₂O, KNO₃–NH₄NO₃–H₂O, and LiCl–NaCl–KCl–H₂O at 298.15 K; KNO₃–NH₄Cl–H₂O, KBr–Sr(NO₃)₂–H₂O, NaNO₃–Sr(NO₃)₂–H₂O, NaNO₃ –(NH₄)₂SO₄–H₂O, KNO₃–Sr(NO₃)₂– H₂O, NH₄Cl–Sr(NO₃)₂–H₂O, NH₄Cl– (NH₄)₂SO₄–H₂O, KBr–KCl–H₂O, KBr–KCl–NH₄Cl–H₂O, KBr–KNO₃– Sr(NO₃)₂–H₂O, KBr–NH₄Cl–Sr(NO₃)₂–H₂O, KNO₃–NH₄Cl–Sr(NO₃)₂–H₂O, and NH₄Cl–(NH₄)₂SO₄–NaNO₃–H₂O at 291.15 K; and KBr–NaBr–H₂O at temperatures from 283.15 to 308.15 K. The agreement is generally quite good.     

Anionic Effects on Raman OD Stretching Spectra for Alcoholic LiX Solutions (X = Cl−, Br−, I−, ClO4 −, NO3 −, and CH3COO−)

Raman OD stretching spectra of alcoholic LiX (X = Cl^–, Br^–, I^–, ClO₄ ^–, NO₃ ^–, and CH₃COO^–) solutions (alcohols = methanol and ethanol) were measured in the liquid state at room temperature and in the glassy state at liquid nitrogen temperature. The effects of the anions on the Raman OD stretching spectra in these alcoholic solutions are investigated and the structural changes of the solutions are discussed. It is shown that the structure-breaking effects of anions on the intrinsic alcoholic structure increase in the order: Cl^– < Br^– < I^– < ClO₄ ^–. From spectral changes, it seems that CH₃COOLi exerts little effect on the liquid structure of the alcohol.     

Influence of Anions on Electrochemical Properties of Polypyrrole-Modified Electrodes

The influence of anions ClO₄ ^–, NO₃ ^–, Cl^–, SO₄ ^2–, and DDS^– (dodecyl sulfate) on the cyclic voltammetric response of polypyrrole-modified electrodes is studied. The change in the film composition is examined by electron probe microanalysis. It is established that essential changes in the shape of voltammograms take place during cycling if the anions are not sufficiently freely mobile in the polymer film and insertion of cations from the solution is necessary to guarantee electroneutrality of the system. Some differences between the mobility of Cl^– ions and ClO₄ ^– or NO₃ ^– ions are in good agreement with the results of semi-empirical quantum chemical calculations showing that the interaction of Cl^– and Br^– ions with pyrrole oligomers is stronger than that of NO₃ ^– or ClO₄ ^– ions. Nevertheless, it is established that the peak current determined from voltammograms increases linearly with the increase of the scan rate with very high correlation coefficient. It means that it is possible to describe the behavior of ClO₄ ^–, NO₃ ^– and Cl^– ions in the framework of the model of free ions. The redox behavior of the PPy films doped with anions of low mobility such as SO₄ ^2– and DDS^– depends essentially on the nature of cations in the test solution. It is found that the mobility of cations increases in the row Li⁺ < Na⁺ < K⁺ < Cs⁺. The mobility of DDS^– ions in the PPy in ethanolic solution is significantly higher and their electrochemical properties are quite similar to PPy|Cl or NO₃ ^– film in aqueous solution.     

Staphylococcus epidermidis adhesion to He, He/O(2) plasma treated PET films and aged materials: contributions of surface free energy and shear rate

Adhesion studies of bacteria (Staphylococcus epidermidis) to plasma modified PET films were conducted in order to determine the role of the surface free energy under static and dynamic conditions. In particular, we investigated the effect of the ageing time on the physicochemical surface properties of helium (He) and 20% of oxygen in helium (He/O₂) plasma treated polyethylene terephthalate (PET) as well as on the bacterial adhesion. Treatment conditions especially known to result in ageing sensitive hydrophilicity (hydrophobic recovery) were intentionally chosen in an effort to obtain the widest possible range of surface energy specimens and also to avoid strong changes in the morphological properties of the surface. Both plasma treatments are shown to significantly reduce bacterial adhesion in comparison to the untreated PET. However, the ageing effect and the subsequent decrease in the surface free energy of the substratum surfaces with time – especially in the case of He treated samples – seem to favor bacterial adhesion and aggregation. The dispersion-polar and the Lifshitz–van der Waals (LW) acid–base (AB) thermodynamic approaches were applied to calculate the Gibbs free energy changes of adhesion (ΔG_adh) of S. epidermidis interacting with the substrates. There was a strong correlation between the thermodynamic predictions and the measured values of bacterial adhesion, when adhesion was performed under static conditions. By decoupling the (ΔG_adh) values into their components, we observed that polar/acid–base interactions dominated the interactions of bacteria with the substrates in aqueous media. However, under flow conditions, the increase in the shear rate restricted the predictability of the thermodynamic models.     

Hybrid electron cyclotron resonance-radio-frequency plasma etching of III–V semiconductors in Cl2-based discharges. Part II: InP and related compounds

Electron Cyclotron Resonance (ECR) discharges of CCl₂F₂ or PCl₃ have been used to etch InP, InAs, InSb, InGaAs and AlInAs. The etch rates of these materials increase linearly with additional RF power level applied to the cathode and are in the range 50–180 Å · min^–1 for 50 W (DC bias 308 V), 10 mTorr, 38 CCl₂F₂/2 O₂ plasmas. The etch rates fall rapidly with increasing pressure or increasing O₂-to-CCl₂F₂ ratio. Polymeric surface residues up to 40 Å thick are found on all of these semiconductors when using Freon-based gas mixtures. Etching at practical rates is possible with only 100 V self-bias when using PCl₃ discharges, and the addition of microwave excitation under these conditions enhances the etch rates by factors of 2–9. At higher self-biases (300 V) etch rates of 3500–8000 Å · min^–1 are possible with PCl₃ although the surface morphologies are significantly rougher and the etching less anisotropic than with CCl₂F₂-based mixtures.     

Studies of the surface wettability and hydrothermal stability of methyl-modified silica films by FT-IR and Raman spectra

Fourier transform infrared (FT-IR) and Raman spectroscopy were employed to study the hydrothermal stability and the influence of surface functional groups on the surface wettability of methyl-modified silica films. The surface free energy parameters of the silica films were determined using the Lifshitz-van der Waals/acid–base approach. The thermal decomposition mechanisms of the CH₃ groups in the methyl-modified silica material are proposed. The results show that with the increase of methyltriethoxysilane (MTES)/tetraethylorthosilicate (TEOS) ratio, the surface free energy and surface wettability of the silica films decrease greatly. This is mainly because of the contribution of the acid–base term; the intensity of Si–CH₃ groups increases at the expense of the intensity of O–H groups in the samples. The surfaces of the methyl-modified silica films exhibited predominantly monopolar electron-donicity. The contact angle on the silica film surface reaches its maximum value when calcination is performed at 350 °C. Thermogravimetric analysis implies that some low molecular weight species, such as H₂, CH₄, and C, are eliminated upon thermal decomposition of the –CH₃ groups. The Si–CH₃ and –CH₃ vibrational bands diminish in intensity as the calcination temperature is increased, disappearing completely when the calcination temperature is increased to 600 °C. When the calcination temperature is increased to 750 °C, the free carbon and CSi₄ species will be formed.     

Formation and decomposition of thin rhodium oxide films

Interaction of O₂ with Rh (poly) and Rh (100) has been studied by Auger Electron Spectroscopy and thermal desorption method at O₂ pressures of 10^–5–10³ Pa and at 400–1600 K. At P(O₂)<10^–5 Pa chemisorption of O₂ occurs, at P(O₂)=10^–5–10^–1 Pa surface oxides are formed, at P(O₂)>1.0 Pa a bulk Rh₂O₃ oxide starts to grow. The growth of rhodium oxide film proceeds via the Cabrera-Mott mechanism. Its decomposition occurs via a mechanism including electron transfer across the oxide film, O₂ desorption from the surface layer and rearrangement of the oxide layer.     

UV-Visible and IR Spectroelectrochemical Studies of FeVO4 Sol-Gel Films for Electrochromic Applications

The sol-gel synthesis route, in combination with dip-coating deposition, was used for the preparation of FeVO₄ films. TEM measurements of Fe/V (1 : 1)-oxide films heated at 400°C reveal that the films consist of a triclinic FeVO₄-I and an orthorhombic FeVO₄-II phases with a grain size of up to 50 nm. The electrochromic properties of the films were tested in 1 M LiClO₄/propylene carbonate (PC) using various electrochemical techniques and in-situ UV-visible spectroelectrochemical measurements. The best compromise between the charge capacity per film thickness (Qd^–1 = –0.14 mC cm^–2 nm^–1), electrochemical stability (>1000 cycles) and optical modulation (T_vis = 0.15) was achieved in the potential range of 4.80 to 1.80 V vs. Li, which suggests that FeVO₄ films can be used as counter-electrodes in electrochromic devices.Extensive IR-spectroscopy studies of FeVO₄ films in charged/discharged states revealed the following spectra changes: (i) small charging (–0.01 mC cm^–2 nm^–1) leads to a variation in the intensity of all the vibrational bands without shifting their frequencies, (ii) higher chargings bring about the intensity and frequency changes of bridging V—O···Fe and V···O···Fe stretchings showing that vanadium, and probably also iron, are involved in the insertion/extraction processes, (iii) below 500 cm^–1 broad absorption appears due to the Li⁺—O modes, which also remained in the IR spectra of discharged (bleached) states revealing the irreversible lithiation, and (iv) charging to –0.30 and –0.50 mC cm^–2 nm^–1 leads to the amorphisation of the film structure.