Adsorbed Hydroxyl Groups Research Articles

Formation and properties of paraffin wax submicron emulsions prepared by the emulsion inversion point method

Paraffin wax-in-water submicron emulsions stabilized by Span 80/Tween 80 were prepared by the emulsion inversion point (EIP) method. Stable emulsions with droplet diameters about 700 nm could be formed when the hydrophilic–lipophilic balance (HLB) values are between 9.5 and 10.3 and the surfactant-to-oil ratio is above 0.2. Increased emulsification temperature and cooling rate were found to improve the emulsion properties. Either mixing all components together or addition of oil to the aqueous surfactant dispersion could not produce stable emulsions. Optical microscopy and scanning electron microscopy (SEM) were used to study the morphology of the emulsion droplets. The droplets were solid at room temperature, showing bright spots under polarized light and an irregular spheres shape under SEM. Due to homogeneous nucleation in the finely dispersed emulsion droplets, the emulsion shows a large degree of undercooling (15 °C) in the differential scanning calorimetry (DSC) thermogram. The emulsions became unstable with heating, but remained stable under cooling. The electrophoretic properties of emulsion droplets showed a negative zeta potential, which was increased with the pH of the system due to the adsorption of hydroxyl groups.

Influence of carbonate intercalation in the surface-charging behavior of Zn–Cr layered double hydroxides

The influence of interlayer composition in the surface charge and reactivity of layered double hydroxides (LDHs) has been explored. With this purpose, a chloride-intercalated Zn–Cr-LDH has been synthesized by the constant pH coprecipitation method and afterwards exchanged with carbonate to obtain solids with different Cl −/CO 3 2− ratios. The solids structure has been characterized by elemental chemical analysis, powder X-ray diffraction and infrared spectroscopy, while its surface-charging behavior and reactivity have been studied by acid–base potentiometric titrations and electrophoretic mobility determinations. The chloride-intercalated sample shows an increasing hydroxyl adsorption with increasing pH and decreasing support electrolyte concentration and the particles present positive electrophoretic mobility in the measured pH range. As carbonate content increases in the samples, the total OH − uptake diminishes and the samples show an isoelectric point at pH around 10. When the gallery is totally occupied by carbonate anions, the OH uptake vs. pH curves registered at different electrolyte concentrations merge at around pH 10. A LDH–water interface model has been used to give an interpretation to the experimental data. The model indicates that as carbonate content increases, the sample behavior becomes similar to that of a metal (hydr)oxide and that surface (bi)carbonate anions undergo acid–base reactions.

Iron(III) and manganese(II) substituted hydroxyapatite nanoparticles: Characterization and cytotoxicity analysis

Calcium hydroxyapatite (HA) is the main inorganic component of natural bones and can bond to bone directly in vivo. Thus HA is widely used as coating material on bone implants due to its good osteoconductivity and osteoinductivity. Metal ions doped HA have been used as catalyst or absorbents since the ion exchange method has introduced new properties in HA which are inherent to the metal ions. For example, Mn2+ ions have the potential to increase cell adhesion while Fe3+ ions have magnetic properties. Here, Fe(III) substituted hydroxyapatite (Fe-HA) and Mn(II) substituted hydroxyapatite (Mn-HA) were produced by wet chemical method coupled with ion exchange mechanism. Compared with pure HA, the colour of both Fe-HA and Mn-HA nanoparticles changed from white to brown and pink respectively. The intensity of the colours increased with increasing substitution concentrations. XRD patterns showed that all samples were single phased HA while the FTIR spectra revealed all samples possessed the characteristic phosphate and hydroxyl adsorption bands of HA. However, undesired adsorption bands of carbonate substitution (B-type carbonated HA) and H2O were also detected, which was reasonable since the wet chemical method was used in the synthesis of these nanoparticles. FESEM images showed all samples were elongated spheroids with small size distribution and of around 70 nm, regardless of metal ion substitution concentrations. EDX spectra showed the presence of Fe and Mn and ICP-AES results revealed all metal ion substituted HA were non-stoichiometric (Ca/P atomic ratio deviates from 1.67). Fe-HA nanoparticles were paramagnetic and the magnetic susceptibility increased with the increase of Fe content. Based on the extraction assay for cytotoxicity test, both Fe-HA and Mn-HA displayed non-cytotoxicity to osteoblast.

Electrochemical oxidation of organic pollutants in water at metal oxide electrodes: A simple theoretical model including direct and indirect oxidation processes at the anodic surface

Abstract The electrochemical oxidation of organics in water at metal oxide electrodes was investigated with the aim to discuss the correlations between the instantaneous current efficiency ICE and operative conditions by considering both the hypothesis of a direct oxidation process and of an indirect process mediated by adsorbed hydroxyl radicals or chemisorbed “oxygen”, in order to explicit the main differences expected between these cases. Thus, a simple theoretical model was discussed, as an extension of previous studies of Comnnellis and co-workers which were focused on indirect oxidation paths [C. Comninellis, Electrochim. Acta 39 (1994) 1857; O. Simond, V. Schaller, Ch. Comninellis, Electrochim. Acta, 42 (1997) 2009], concerning both the cases of mass transfer control and oxidation reaction control and mixed kinetic regimes. A very good agreement, between theoretical predictions and experimental data pertaining to the electrochemical oxidation of oxalic and formic acid at IrO2–Ta2O5, was observed.

Generation of O− Radical Anions on MgO Surface: Long-Distance Charge Separation or Homolytic Dissociation of Chemisorbed Water?

O− radical anions were observed over a partially hydroxylated MgO surface after illumination by monochromatic light with λ = 280 and 303 nm. As all corner oxygen atoms are covered with adsorbed hydroxyl groups after activation at 450 °C, this process is initiated by selective excitation of 3-coordinated complexes [Mg2+−O2−]3C containing chemisorbed water. A new mechanism for generation of the O− radical anions is suggested. It is based on homolytic dissociation of chemisorbed water followed by migration of a hydrogen atom, probably, to a different nanoparticle rather than on long-distance separation of charges. The surface structure remaining after its detachment consists of an •OH radical stabilized near the corner oxygen atom. The imminent charge transfer generates a hole at the corner oxygen atom stabilized by a hydroxyl group [OH−···Mg2+−O−]3C. DFT simulation of this structure showed that it reproduces the main characteristics of the radical anions O−3C. The overall structure is electrically neutral. The charge of the hole is compensated by the nearby hydroxyl group. So, no long-distance charge separation is required.

Synthesis and Oxygen Reduction Electrocatalytic Property of Pt-on-Pd Bimetallic Heteronanostructures

Platinum-on-palladium bimetallic heterogeneous nanostructures were prepared using a sequential synthetic method, in which 3-nm Pt particles grew on the surfaces of 5-nm Pd nanoparticles. Electrochemical study of carbon-supported Pt-on-Pd heteronanostructures shows not only enhancement in electrocatalytic activity for oxygen reduction reaction (ORR) but also much improved stability in comparison to a commercial platinum catalyst (E-TEK, 20 wt % Pt, diameter: 2.5 nm). The greatly suppressed hydroxyl adsorption on active sites by introducing Pd was attributed to the enhanced activity, while the retention of active surface area, morphology, and composition because of the large overall bimetallic particle size and unique architectures could be the key factors for the much improved stability over 30,000 cycles. Our work shows heterogeneous platinum-on-metal bimetallic nanostructures offer new opportunities to the design of hierarchically ordered multifunctional fuel cell catalysts.

Temperature-Induced Ordering of Metal/Adsorbate Structures at Electrochemical Interfaces

The influence of temperature changes in water-based electrolytes on the atomic structure at the electrochemical interface has been studied using in situ surface X-ray scattering (SXS) in combination with cyclic voltammetry. Results are presented for the potential-dependent surface reconstruction of Au(100), the adsorption and ordering of bromide anions on the Au(100) surface, and the adsorption and oxidation of CO on Pt(111) in pure HClO(4) and in the presence of anions. These systems represent a range of structural phenomena, namely metal surface restructuring and ordering transitions in both nonreactive spectator species and reactive adsorbate layers. The key effect of temperature appears to be in controlling the kinetics of the surface reactions that involve oxygenated species, such as hydroxyl adsorption and oxide formation. The results indicate that temperature effects should be considered in the determination of structure-function relationships in many important electrochemical systems.

Hydroxyl radicals electrochemically generated in situ on a boron-doped diamond electrode

The hydroxyl radicals electrochemically generated in situ on a boron-doped diamond (BDD) electrode have been investigated for the first time in different electrolyte media, over the whole pH range between 1 and 11. A more extensive characterisation of BDD electrochemical properties is very important to understand the reactivity of organic compounds towards electrochemical oxidation on the BDD electrode, which is related to their interaction with adsorbed hydroxyl radicals due to water oxidation on the electrode surface. An oxidation peak corresponding to the transfer of one electron and one proton was observed in pH <9 electrolytes, associated with the water discharge process and electrochemical generation of hydroxyl radicals, which can interact and enhance the electro-oxidation of organic compounds. In pH >9 electrolytes the electrochemical generation of hydroxyl radicals was not observed; ammonia buffer electrolyte gave a pH-independent peak corresponding to the ammonia oxidation reaction. Additionally, for most pH values studied, a few small peaks associated with the electrochemical interaction between non-diamond carbon species on the doped diamond electrode surface and the electrolyte were also seen, which suggests that the doped diamond is relatively unreactive, but not completely inert, and the electrogenerated hydroxyl radicals play a role as mediator in the oxidation of organics.

Tracking hydroxyl adsorption on TiO 2 (1 1 0) through secondary emission changes

In surface science, rutile TiO 2 continues to be one of the most studied surfaces and in the catalysis field numerous groups study how adsorbates interact with this surface. All groups face the difficult problem of reproducibility due to surface preparation unknowns like defect concentration and the continuous aging of the crystals. Recent studies, using STM imaging, showed that hydroxyl adsorption takes place even in very good vacuum conditions. Upon adsorption, the surface electric field is reduced and the work function decreases. We found that this change may be readily detected in the onset energy of the secondary electrons. By following the onset region of secondary electron emission it is possible to track hydroxyl adsorption in quantities well below the detection level of XPS and LEIS. With this knowledge, we show that the time elapsed after surface preparation and water partial pressure should be accounted in the study of TiO 2 surfaces.

Autocatalysis by the intermediate surface hydroxide formed during hydrogen peroxide reduction on silver electrodes

Recent electrochemical studies of the cathodic reduction of hydrogen peroxide (H 2O 2) on silver electrodes in acidic electrolyte (HClO 4) revealed a novel autocatalytic reaction path. Adsorbed hydroxyl groups, OH ad, were proposed to act as the catalyst. To gather further clarity about this mechanism, in particular about the presence and nature of the postulated adsorbate OH ad, in the present work surface science experiments on Ag(1 1 1) electrodes are evaluated. It is concluded that the species OH ad is identical with the surface AgOH formed in alkaline solution as a relatively stable intermediate in the anodic oxidation of OH − to surface-Ag 2O.

Adsorption Properties of As(III) and Cr(VI) in Water Environment by Calcined Gibbsite

The adsorption properties of As(III) and Cr(VI) by gibbsite (GB) calcined at 200 to 1150 degrees C was investigated on basis of their specific surface area, number of hydroxyl groups, surface pH and adsorption isotherms. The amount of As(III) and Cr(VI) adsorbed on the calcined GB at 300 or 400 degrees C was the highest. In the case of the calcination temperature was more than 700 degrees C, the amount adsorbed decreased with the increasing calcination temperature. In a single solution system, the amount of As(III) adsorbed on calcined GB was higher than that of Cr(VI). The amount of As(III) and Cr(VI) adsorbed on calcined GB was higher in a binary solution system than those in a single solution system. The pH in solution after As(III) and Cr(VI) adsorption was greater than that before adsorption. These results indicated that the adsorption mechanism of As(III) and Cr(VI) was as followed: the hydroxyl groups on calcined GB were exchanged to As(III) and Cr(VI) and they adsorbed n the calcined GB surface. In the case of coexistence of As(III) and Cr(VI), they could be removed better by calcined GB because the pH in binary solution was lower than that in single solution. Both As(III) and Cr(VI) in water environment could be removed by the calcined GB simultaneously.

Modeling Phosphate Adsorption by Agricultural and Natural Soils

The nonideal competitive adsorption model (NICA) was developed from the Langmuir adsorption theory and the Freundlich empirical equation. It is well suited to describing ion adsorption in complex systems like soils, which have multiple ions, highly heterogeneous surfaces, and a variety of adsorption sites on the particle surfaces. It has been successfully used to model the binding of protons and metal ions to humic substances, but its application for the adsorption of anions to heterogeneous surfaces has not been documented. The purpose of this study was to adapt the NICA model to describe hydroxyl and phosphate adsorption. Results show that by considering two types of surface sites, the NICA model can provide an excellent fit (R2 &gt; 0.99) of the hydroxyl adsorption data obtained from −11 to −4 of log[OH], which corresponds to soil pH from 3 to 10. By using the parameters generated from hydroxyl adsorption, including adsorption maxima (Qmax,OH), binding strength (KOH), and nonideality (m), the NICA model gave a remarkable goodness of fit (R2 &gt; 0.98) for the phosphate adsorption data obtained at different pH values. The model sensitivity test showed that the Type 1 surface (SOH2+) has up to 100 times greater contribution to phosphate adsorption than the Type 2 surface (SOH). Thus, the model may be simplified into a three‐parameter model by only considering the Type 1 surface for phosphate adsorption in acidic soils. The adapted NICA model can thus describe phosphate adsorption combined with hydroxyl adsorption and the parameters (nPO4,1/nOH,1 &lt; 1, where nPO4,1 accounts for nonideality of PO43− and nOH,1 accounts for nonideality of OH−, both on a Type 1 surface) reveal the multidentate binding of phosphate. It provides a promising tool for analyzing competitive anion adsorption processes in soils.

H 2-induced promotion of CO oxidation over unsupported gold

The kinetics and mechanism of the preferential oxidation of carbon monoxide in the presence of hydrogen (PrOx) over an unsupported gold powder (mean particle size ∼20 nm and free of silver) have been investigated using flow fixed bed catalytic testing and diffuse reflectance infrared Fourier transform spectroscopy coupled to mass spectrometry ( operando DRIFTS or DRIFTS-MS). It is shown that the presence of H 2 has a favourable effect on the oxidation of CO, either by strongly accelerating the reaction or by preventing the catalyst deactivation, depending on the conditions used. Variation of the hydrogen partial pressure has allowed us to determine partial reaction orders for both CO oxidation and H 2 oxidation under PrOx conditions. An infrared band at ∼2113 cm −1, corresponding to on-top CO adsorption on metallic gold, has been observed below 150 °C. In addition, adsorbed hydroxyl groups gradually develop simultaneously to gas-phase water in the course of the reaction at increasing temperatures. The promotional effect of hydrogen is ascribed to highly oxidative H x O y intermediates formed from the interaction between H 2 and O 2 on the gold surface.

Oxidation and Photo-Oxidation of Water on TiO2Surface

The oxidation and photo-oxidation of water on the rutile TiO2(110) surface is investigated using density functional theory (DFT) calculations. We investigate the relative stability of different surface terminations of TiO2 interacting with H2O and analyze the overpotential needed for the electrolysis and photoelectrolysis of water. We found that the most difficult step in the splitting of water process is the reaction of a H2O molecule with a vacancy in the surface to form an adsorbed hydroxyl group (OH*). Comparison to experiment shows that the computed overpotential for O2 evolution (0.78 V) is available under the experimental conditions required for both oxygen and hydrogen evolution.

Adsorption of Water on Cu2O and Al2O3 Thin Films

The initial stages of water condensation, approximately 6 molecular layers, on two oxide surfaces, Cu2O and Al2O3, have been investigated by using ambient pressure X-ray photoelectron spectroscopy at relative humidity values (RH) from 0 to >90%. Water adsorbs first dissociatively on oxygen vacancies producing adsorbed hydroxyl groups in a stoichiometric reaction: Olattic + vacancies + H2O = 2OH. The reaction is completed at ∼1% RH and is followed by adsorption of molecular water. The thickness of the water film grows with increasing RH. The first monolayer is completed at ∼15% RH on both oxides and is followed by a second layer at 35−40% RH. At 90% RH, about 6 layers of H2O film have been formed on Al2O3.

Essential Explanation of the Strong Mineralization Performance of Boron-Doped Diamond Electrodes

Electrochemical oxidation of p-nitrophenol was examined using differentanodic materials, including T/boron-doped diamond (BDD), Ti/SnO2-Sb/PbO2, and Ti/SnO2-Sb anodes. The results demonstrated that Ti/BDD anodes had a much stronger mineralization performance than the other two anodes. Furthermore, it was found that hydroxyl radicals could mainly exist as free hydroxyl radicals at BDD anodes, which could react with organic compounds effectively. This implied that the dominant mechanism for a much higher mineralization capacity of BDD anodes would be attributed to the existence of free hydroxyl radicals in the BDD anode cell rather than adsorbed hydroxyl radicals on the BDD anode. To further corroborate this hypothesis, electrochemical oxidation of p-substituted phenols (p-nitrophenol, p-hydroxybenzaldehyde, phenol, p-cresol, and p-methoxyphenol) was examined at the Ti/BDD, Ti/SnO2-Sb/ PbO2, and Ti/SnO2-Sb anodes, respectively. The study revealed that for Ti/BDD electrodes, the degradation rate of p-substituted phenols (k) increased with the increase of Hammett's constant (sigma), which confirmed the dominance of free hydroxyl radicals at BDD anodes and its effective reaction with organics therein. For Ti/SnO2-Sb/PbO2 electrodes, the degradation rate of p-substituted phenols (k) increased with the increase of initial surface concentration gamma (representing the adsorption capacity of phenols to electrode surface), which indicated that organic compounds mainly reacted with adsorbed hydroxyl radicals at PbO2 anodes. For Ti/SnO2-Sb electrodes, however, k increased with the increase of the integrated parameter S (representing the effects of both sigma and gamma), which implied that organic compounds reacted with both adsorbed hydroxyl radicals and free hydroxyl radicals at SnO2 anodes.

Microstructure and superhydrophilicity of anodic TiO 2 films on pure titanium

The superhydrophilicity of TiO 2 prepared by anodization of pure titanium was investigated and the underlying mechanism was explored. The water contact angle of the as-anodized oxide decreases with the conversion voltage and ultraviolet (UV) light illumination, whereas the annealed oxide exhibits further low contact angles regardless of the conversion voltage and UV illumination. The O 1s X-ray photoelectron spectroscopy spectra of the oxides have a shoulder peak due to adsorption of hydroxyl groups and the intensity of the peak increases with annealing and UV illumination; these observations suggest that the interaction between the surface of the oxide and the hydroxyl groups is enhanced.

Influence of M II/M III ratio in surface-charging behavior of Zn–Al layered double hydroxides

Zn–Al layered double hydroxides with different M II/M III ratios were synthesized by the coprecipitation method and the obtained solids were characterized by powder X-ray diffraction, infrared spectroscopy, acid–base potentiometric titrations, electrophoretic mobility and modeling of the electrical double layer. As general behavior, increasing hydroxyl adsorption with increasing pH and decreasing supporting electrolyte concentration takes place between pH 7 and 12. The electrophoretic mobility of the samples are in all cases positive and it decreases when the pH becomes higher than 9. A net positive charge of the particles, with a point of zero charge (PZC) around 11.8, is deduced from electrophoretic data. A modified MUSIC model, where OH −/Cl − exchange at structural sites and proton desorption from surface hydroxyl groups are taken into account, is used to estimate the deprotonation constants, the number of surface groups, the number of sites with structural charges an the affinity constants for Cl − and OH − of the samples with different M II/M III ratios. The modeling of potentiometric titrations and electrophoresis data indicates that anions between the layers and negatively charged groups in the particle surface balance most of the structural charge at the particles surface. It shows an important variation of the number of structural sites with the M II/M III ratio and an important role of surface protonation–deprotonation sites to the electrical behavior and exchange properties of LDHs.

Oxidation of toluene on Bi-doped PbO2 studied by electrochemical impedance spectroscopy and UV spectrophotometry

Oxidation of organics in the potential region of O2 evolution is supposed to proceed through the oxidation of water to hydroxyl radicals, which then may either be further oxidized to give molecular oxygen or interact with organic molecules in an oxygen transfer reaction. Therefore, the electrode material must ensure (1) the preferential adsorption of the organic compound, (2) the production of adsorbed hydroxyl radicals able to react with this compound in a selective oxidation reaction (with as little as possible oxygen evolution), and (3) a long-term stability. In the present paper, the oxidative decomposition of toluene in sulfuric acid solution on PbO2 coatings deposited on Ti substrate from acidic nitrate + fluoride baths containing Pb2+ and Bi3+ is investigated by voltammetry, electrochemical impedance spectroscopy, and UV spectrophotometry. The chemical composition and structure of the catalytic coatings is characterized with X-ray photoelectron spectroscopy and X-ray diffraction. The catalytic activity is estimated both from current density vs potential and polarization resistance vs potential plots using measurements on the same electrodes in sulfuric acid without toluene to eliminate the oxygen evolution reaction that proceeds in parallel to the oxidation of toluene. A skeletal reaction mechanism of the process is proposed to account for the steady-state and transient response of the catalytic electrodes during oxidation of toluene.

Roles of adsorbed OH and adsorbed H in the oxidation of hydrogen and the reduction of UO2+2 ions at Pt electrodes under non-conventional conditions

The roles of adsorbed hydroxyl radicals, OH, at a high temperature and adsorbed hydrogen atoms, H, in an acidic solution were investigated in the electrochemical reactions on Pt electrode by using potentiodynamic polarisation experiment, cyclic voltammetry and constant-potential electrolysis combined with UV/VIS analysis. From the analysis of the polarisation curves obtained from Pt electrode in a 0.185 M H3BO3 solution at 473 K, it was found that the reducing capability of dissolved hydrogen is significantly enhanced due to the increases of the mass transfer and the electron transfer rates. Especially, it is suggested that the stable Pt-OHad plays a significant role in the passivation reaction in the potential range from 0.60 to 0.75 VSHE. From the analyses of the experimental results for the electrochemical reduction of UO 2 2+ ions on Pt surface in a 1.0 M HClO4 solution, it is recognised that the reduction reaction of UO 2 2+ to U4+ ions is strongly dependent on the hydrogen atoms adsorbed on Pt electrode (indirect reduction of UO 2 2+ ) as well as on the electrons transferred from Pt electrode (direct reduction of UO 2 2+ ). In addition, the reduction mechanism of UO 2 2+ ions involved in Pt-Had is also proposed.