Intensified production of biodiesel using a spinning disk reactor

This work achieves continuous transesterification of soybean oil and methanol in a spinning disk reactor. The effects of the methanol-to-oil molar ratio, catalyst type, catalyst concentration, reaction temperature, flow rate, and rotational speed were investigated. Optimal yield of 96.9% was obtained with a residence time of 2–3 s at a molar ratio of 6, potassium hydroxide concentration of 1.5 wt%, temperature of 60 °C, flow rate of 773 mL/min, and rotational speed of 2400 rpm. The production rate of 1.86 mol/min was high compared to that of other reactors for continuous transesterification process, indicating that a spinning disk reactor is a promising alternative method for continuous biodiesel production.     

Surface X-ray scattering of high index plane of platinum containing kink atoms in solid-liquid interface: Pt(3 1 0) = 3(1 0 0)-(1 1 0)

Surface structure of Pt(3 1 0) = 3(1 0 0)-(1 1 0), which contains kink atoms in the step, has been determined with the use of in situ surface X-ray scattering (SXS) in the double layer region (0.50 V(RHE)) in 0.1 M HClO₄. Clean Pt(3 1 0) surface has pseudo (1 × 1) structure on which lateral displacements of 2-9% and 0.3-1% are found along a and b directions, respectively, whereas the surfaces of Pt(1 1 0) = 2(1 1 1)-(1 1 1) and Pt(3 1 1) = 2(1 0 0)-(1 1 1) are reconstructed to (1 × 2) according to previous reports. Interlayer spacing between the first and the second layers d₁₂ is contracted about 5% compared with the bulk spacing, whereas those between underlying layers are expanded down to fourth layer. Fully adsorbed CO has no effect on the surface structure of Pt(3 1 0). This result differs from that on Pt(1 1 1), where d₁₂ is expanded after CO adsorption.     

Redox behavior of hemin at p-GaAs(1 0 0) electrode

Electrochemical behavior of hemin on p-GaAs(1 0 0) electrodes was examined by cyclic voltammetry (CV) and impedance spectroscopy (EIS) in phosphate buffer solutions (PBS) at pH 7.45. CV investigations in 0.6 mM hemin in PBS revealed a pair of reversible peaks at −0.44 and −0.32 V vs. SCE resulting in stable adsorbed species. EIS spectra analysis pointed out that these adsorbed species bring significant changes in the semiconductor surface state population and the potential drop distribution between the semiconductor space charge region and the Helmholtz layer.     

Surface enhanced infrared spectroscopy—Au(1 1 1-20 nm)/sulphuric acid—new aspects and challenges

The structure and orientation of water molecules on Au(1 1 1-20 nm) film electrodes in contact with aqueous sulphuric acid solution was studied by surface enhanced infrared reflection-absorption spectroscopy (SEIRAS) employing an attenuated total reflection (ATR) configuration (ATR-SEIRAS) with a vertical spectroelectrochemical cell. The spectrum of interfacial water is strongly dependent on electrode potential, ionic strength and pH. Coadsorption of hydronium ions with weakly hydrogen-bonded water molecules was found at E<E_pzc. At E>E_pzc strong hydrogen-bonding among water molecules and to coadsorbed sulphate species exists. Based on pH-dependent electrochemical, spectroscopic and in situ STM investigations, a new model is suggested to represent the ordered () adlayer at maximum sulphate coverage θ=0.2: a Zundel or hydrated hydronium ion H₅O₂⁺, in which one proton is shared between two water molecules, bridges adjacent sulphate species via hydrogen-bonds along the main diagonal of the () unit cell. This alternating arrangement gives rise to a long-range ordered, 2D network of sulphate and water species interconnected by hydrogen-bonds, and capable to form hydrogen-bonds with second-layer water species. The suggested model is consistent with all experimental observations as well as predictions from quantum-chemical and molecular dynamic simulations.     

Oxygen electroreduction on Ag(1 1 1): The pH effect

The rotating ring disk method (RRDE) is applied to investigate the pH effect on oxygen reduction reaction (ORR) on Ag(1 1 1) single crystal surface in 0.1 M KOH and 0.1 M HClO₄. In 0.1 M KOH, the ORR proceeds through 4e⁻ reaction pathway with a very small (0.5-2.5%) peroxide formation in the entire potential range. In 0.1 M HClO₄ the onset potential for the ORR is shifted for ca. 400 mV toward the higher overpotentials compared to the 0.1 M KOH solution. At the low overpotentials, in 0.1 M HClO₄ the ORR proceeds entirely as a 2e⁻ process, i.e, 100% H₂O₂ formation. At higher overpotentials, the initial mixed a 2e⁻ and 4e⁻ reduction is followed by the potential region where the ORR proceeds entirely as a 4e⁻ process, with H₂O formation as a final product. The pH dependent shift in the onset of the ORR as well as the reaction pathway has been explained based on both: a thermodynamic analysis of pH independent rate determining step, and on the pH dependent change in availability of surface active sites and adsorption energies of molecular oxygen and reaction intermediates.     

Adsorption of 2-X-benzoic acids (X = fluoro, chloro, and bromo) on Au(1 0 0) electrode in acidic media studied by IRAS

Adsorption behaviors of 2-fluoro-, 2-chloro-, and 2-bromobenzoic acids on Au(1 0 0) electrode in 0.1 M HClO₄ have been investigated by using in situ reflection adsorption IR spectroscopy and differential capacity measurements. It is found for the 2-substitued benzoic acids that the flat lying adsorbed species is present at negative potentials, and the vertically adsorbed benzoate with both oxygen atoms oriented toward the metal surface is present at positive potentials. A new adsorbed state due to the vertical orientation was observed around 0.75 V only in 2-fluorobenzoic acid solution, as well as the case of benzoic acid. This indicates that the new band is due to the formation of an ordered adsorption layer of the vertical 2-fluorobenzoate, and the formation of the arrangement adsorption is dependent on the size of the adsorbed molecule.     

Numerical investigation of kinetics of free‐radical polymerization on spinning disk reactor

Spinning disk reactor (SDR) technology has been demonstrated to achieve significant enhancements in free radical polymerization rates. The effect on the rate may be attributed to the unique hydrodynamic environment experienced by the reacting polymer films. The rotating surface of a SDR promotes extension of the polymer chains, which may prevent the propagating chains from terminating through bimolecular reactions. In this article, a numerical simulation is performed to investigate how reduced rates of termination would affect time conversion behavior of free radical initiated styrene polymerization. Comparisons have been made between model predictions and SDR experimental results. The closeness between predicted and experimental results yields strong evidence that the hydrodynamic regime created on the surface of a SDR can lead to reduced rates of bimolecular termination. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 693–699, 2003     

Structure and electrochemical characterization of electrolytic Ni + Mo + Si composite coatings in an alkaline solution

Ni + Mo + Si coatings were obtained by nickel deposition from a bath containing suspension of molybdenum and silicon powders. These coatings were obtained in galvanostatic conditions, at the current density of j_dep = −0.100 A cm⁻². For determination of the influence of phase composition and surface morphology of obtained coatings on changes of corrosion resistance, these coatings were modified in argon atmosphere by thermal treatment at the temperature of 1100 °C during 1 h. A scanning electron microscope was used for surface morphology characterization of the coatings. Chemical composition of obtained coatings was determined by X-ray fluorescence spectroscopy method. Phase composition investigations were conducted by X-ray diffraction method. It was found that the obtained coatings are composed of three phase structures, i.e., nickel, molybdenum and silicon. Phase composition for the Ni + Mo + Si coatings after thermal treatment is markedly different. The main peaks corresponding to the Ni and Mo coexist with the new ones corresponding to new phases: Mo₅Si₃, NiSi, Mo₂Ni₃Si and Ni₆Mo₆C_1.06.Electrochemical corrosion resistance investigations were carried out in the 5 M KOH, using potentiodynamic and electrochemical impedance spectroscopy methods. On the basis of these investigations it was found that Ni + Mo + Si coatings after thermal treatment are more resistant in alkaline solution than Ni + Mo + Si as-deposited coatings. The reason of this is presence of silicides in the coatings.     

Electrochemical characterization of irreversibly adsorbed germanium on platinum stepped surfaces vicinal to Pt(1 0 0)

The electrochemical behavior of germanium irreversibly adsorbed at stepped surfaces vicinal to the Pt(1 0 0) pole is reported. The process taking part on the (1 0 0) terraces is evaluated from charge density measurements and calibration lines versus the terrace dimension are plotted. On the series Pt(2n − 1,1,1) having (1 1 1) monoatomic steps, the charge involved in the redox process undergone by the irreversibly adsorbed germanium is able to account for (n − 0.5) terrace atoms, thus suggesting some steric difficulties in the growth of the adlayer on the (1 0 0) terraces. Conversely, no steric problems are apparent in the series Pt(n,1,0) in which more open (1 0 0) steps are present on the (1 0 0) terraces. In this latter case the charge density under the germanium redox peaks is proportional to the number of terrace atoms. Some comparison is made with other stepped surfaces to understand the behavior and stability of germanium irreversibly adsorbed on the different platinum surface sites.     

Capacitance dispersion in EIS measurements of halides adsorption on Au(2 1 0)

Electrochemical interfaces that display dispersive characteristics do not present the purely capacitive behaviour predicted by the theory of ideally polarised interfaces. For interfaces involving solid electrodes, capacitance dispersion phenomena in the double layer (dl) region are usually attributed to the structural characteristics of the electrode surface as well as to the interfacial region. This paper presents a study of the dispersive characteristics, in the double layer potential region, of interfaces constituted by Au(2 1 0) electrodes and KF, KCl, KBr and KI aqueous solutions. The study was realised by using electrochemical impedance spectroscopy (EIS), with the utilisation of a constant phase element (CPE) to provide an electrical analogue model of the interface. The fitting results are compared with capacitance curves obtained by chronocoulometry, in order to analyse the relationship between the CPE and the interfacial capacitance. For all the systems analysed, evidence for the occurrence of dispersive phenomena in the potential regions associated with phase transition processes (e.g. adsorption and superficial species rearrangement) is observed. On the other hand, in the potential regions where such phenomena do not occur, the interface presents almost pure capacitive behaviour. These observations provide evidence of the strong contribution of the solution properties to the capacitance dispersion.     

Selective etching of n-InP(1 0 0) triggered at surface dislocations induced by nanoscratching

This work investigates selective dissolution of n-InP(1 0 0) triggered by surface defects. Nanometer size grooves were produced by sensitizing an InP surface with a mechanical contact patterning technique, i.e. nanoscratching at low loading forces at the threshold between elastic and plastic contacts. Subsequently, site-selective electrochemical dissolution was triggered in a 1 M HCl electrolyte. Electrochemical characterization shows that there is an onset voltage for selective dissolution at the scratch-induced defects and that it is possible to define a processing potential window where dissolution occurs only inside the nanoscratches. Scanning electron microscopy (SEM) micrographs reveal that for optimized forces, highly confined dissolution in well-defined channels of 125 nm width can be obtained. Transmission electron microscopy (TEM) cross sectional analysis of the scratches before and after electrochemical experiments shows that the dislocations generated are confined in a well-defined volume below the surface, and that exactly this dislocated volume is dissoluted. Indeed, the site-selective activation must be ascribed to these surface dislocations. Based on contact mechanics, the lateral resolution and the morphology of the dissolved groove can be explained by the dissolution time and the tip radius and the morphology by the applied load.     

In situ STM study of underpotential deposition of bismuth on Au(1 1 0) in perchloric acid solution

The underpotential deposition (UPD) of Bi on Au(1 1 0) was investigated in HClO₄ solution using in situ scanning tunneling microscopy. The UPD of Bi occurred in three steps. A structure, in which Bi atoms formed dimers, was found for the first UPD adlayer. A (1 × 1) image was obtained by STM at the second UPD peak. For the third UPD peak, Bi atoms formed an incommensurate adlayer, and stripes of Bi were observed on terraces. After the third UPD, a structural reconstruction caused by adsorbed Bi was observed.     

Electrochemical characterization of kinked Pt(7 5 1) surface in acidic media

Kinked Pt(7 5 1) surface was prepared and its electrochemical behaviors under different pretreatment conditions in acidic media were investigated systematically by using cyclic voltammetry. The results demonstrated that the upper limit of potential scanning and cooling atmospheres after the Pt(7 5 1) having been flame-annealed significantly influence the voltammetric behavior of Pt(7 5 1) electrode. The electric charge of hydrogen adsorption-desorption slightly increases with increasing the upper limit of potential scanning. Different cooling atmospheres give rise impacts to the surface structure of Pt(7 5 1) electrode, but hardly change the amount of hydrogen adsorption-desorption sites on the electrode. In addition, the so-called third oxidation peak appears near −0.08 V in H₂SO₄ media and −0.05 V in HClO₄ solution because of the presence of (1 1 0) terrace sites on this surface, and a plausible mechanism for the formation of this current peak is discussed. The results are of importance in understanding the electroadsorption properties of the kinked Pt(7 5 1) surface, as well as in further exploration of this kinked electrode in electrocatalysis.     

Heteroepitaxial growth and characterization of 3C-SiC films on on-axis Si (1 1 0) substrates by LPCVD

Cubic SiC (3C-SiC) film has been deposited on Si (1 1 0) substrate by the low pressure chemical vapor deposition (LPCVD) with gas sources of SiH₄, C₃H₈ and carrier gas of H₂. The 3C-SiC crystalline film can be confirmed through the observations using reflection high-energy electron diffraction (RHEED) images. The X-ray diffraction (XRD) pattern and the rocking curve indicate that the (1 1 1) plane of SiC film is parallel to the surface of the Si (1 1 0) substrate and the film is of high crystallinity. The results of the field emission scanning electron microscope (FESEM) images show that the film has smooth surface morphology. Transmitted electron diffraction (TED) pattern and high resolution transmission electron microscope (HRTEM) image further confirm the high quality of the film.     

Electroreduction of nitrate ions on Pt(1 1 1) electrodes modified by copper adatoms

Kinetics and mechanism of nitrate ion reduction on Pt(1 1 1) and Cu-modified Pt(1 1 1) electrodes have been studied by means of cyclic voltammetry, potentiostatic current transient technique and in situ FTIRS in solutions of perchloric and sulphuric acids to elucidate the role of the background anion. Modification of platinum surface with copper adatoms or small amount of 3D-Cu crystallites was performed using potential cycling between 0.05 and 0.3 V in solutions with low concentration of copper ions, this allowed us to vary coverage θ_Cu smoothly. Following desorption of copper during the potential sweep from 0.3 to 1.0 V allowed us to estimate actual coverage of Pt surface with Cu adatoms. Another manner of the modification was also applied: copper was electrochemically deposited at several constant potentials in solutions containing 10⁻⁵ or 10⁻⁴ M Cu²⁺ and 5 mM NaNO₃ with registration of current transients of copper deposition and nitrate reduction.It has been found that nitrate reduction at the Pt(1 1 1) surface modified by copper adatoms in sulphuric acid solutions is hindered as compared to pure platinum due to induced sulphate adsorption at E < 0.3 V. Sulphate blocks the adsorption sites on the platinum surface and/or islands of epitaxial Cu(1 × 1) monolayer thus hindering the adsorption of nitrate anions and their reduction. The extent of inhibition weakly depends on the copper adatom coverage. Deposition of a small amount of bulk copper does not affect noticeably the rate of nitrate reduction.Nitrate reduction on copper-modified Pt(1 1 1) electrodes in perchloric acid solutions occurs much faster as compared to pure platinum. The steady-state currents are higher by 4 and 2 orders of magnitude at the potentials of 0.12 and 0.3 V, respectively. The catalytic effect of copper adatoms is largely caused by the facilitation of nitrate adsorption on the platinum surface near Cu_ad and/or on the islands of the Cu(1 × 1) monolayer (induced nitrate adsorption).Hydrogen adatoms block the adsorption sites on platinum for NO₃⁻ anion adsorption and inhibit reactions of nitrate reduction even at moderate surface coverage.The products of nitrate reduction in sulphuric and perchloric acids are essentially the same (NO and ammonia) irrespective of the presence or absence of Cu on the platinum surface.     

In situ EC-STM studies of n-Si(1 1 1):H in 40% NH4F solution at pH 10

In situ electrochemical-scanning tunneling microcopy (EC-STM) was employed to investigate the etching dynamics of the moderately doped n-Si(1 1 1) electrode during cyclic voltammetric perturbation and at the seven different potentials including the open circuit potential (OCP) in 40% NH₄F solution at pH 10, which was prepared from 40% NH₄F and concentrated NH₄OH solution. The etching rate was significant at OCP and showed an exponential dependence on the potential applied to the silicon substrate electrode. Although some triangular pits were generated at the Si(1 1 1) surface, at the potentials more negative than OCP the site dependence in the removal of surface silicon atoms prevailed and led to the atomically flat Si(1 1 1):H surfaces with sharply defined steps of the step height 3.1 Å, where the interatomic distance of 3.8 Å was observed with a three-fold symmetry. At the potentials sufficiently more positive than OCP, macroporous hole was formed to limit further in situ EC-STM study. The results were compared with in situ EC-STM studies of the etching reaction of n-Si(1 1 1):H in the aqueous solution of dilute ammonium fluoride at pH 5, 40% NH₄F at pH 8, and 1 M NaOH reported in the literature.     

Oxidation of formaldehyde and ethanol on Au(1 1 1) and Au(1 1 1) modified by spontaneously deposited Ru in sulfuric acid solution

The oxidation of formaldehyde and ethanol on both pure Au(1 1 1) and Au(1 1 1) modified by approximately 0.3 monolayer (ML) of spontaneously deposited Ru was studied by cyclic voltammetry (CV) in 0.5 M H₂SO₄ solution containing either 0.25 M formaldehyde or 0.35 M ethanol. In situ scanning tunneling microscopy (STM) and CV were employed to characterize the Au(1 1 1) and Ru/Au(1 1 1) surfaces. The oxidation of HCHO on Ru/Au(1 1 1) commences at 0.1 V more negative potential than on pure Au(1 1 1). From 0.25 to 0.55 V vs. (Ag/AgCl), the reaction occurs with increasing current, showing a peak at a potential of 0.43 V. It is assumed that the increasing anodic activity of the Ru/Au(1 1 1) surface is associated with the oxidation of some reaction intermediates, facilitated by the presence of Ru in its metallic state. On the other hand, the oxidation of ethanol on Ru/Au(1 1 1) commences at 0.1 V more positive potential than on pure Au(1 1 1), and proceeds in the potential region from 0.2 to 0.5 V with significantly smaller currents, showing a peak at 0.43 V. This inhibiting effect is explained by the deactivation of the most active Au(1 1 1) step sites by high coverage with Ru islands. The appearance of a small peak at 0.43 V is most likely associated to the oxidation of some intermediates during ethanol oxidation at the Ru/Au step sites formed on the Au(1 1 1) terraces by the presence of a small coverage with Ru islands.     

Kinetic study of nitrate reduction on Pt(1 1 0) electrode in perchloric acid solution

The kinetics of electrocatalytic reduction of nitrate on Pt(1 1 0) in perchloric acid was studied with cyclic voltammetry at a very low sweep rate of 1 mV s⁻¹, where pseudo-steady state condition was assumed to be achieved at each electrode potential. Stationary current-potential curves in perchloric acid in the absence of nitrate showed two peaks at 0.13 V and 0.23 V (RHE) in the so-called adsorbed hydrogen region. The nitrate reduction proceeded in the potential region of the latter peak in the pH range studied. The reaction orders with respect to NO₃⁻ and H⁺ were observed to be close to 0 and 1, respectively. The former value means that the adsorbed NO₃⁻ at a saturated coverage is one of the reactants in the rate-determining step (rds). The latter value means that hydrogen species is also a reactant above or on the rds. The Tafel slope of nitrate reduction was −66 mV per decade, which is taken to be approximately −59 mV per decade, indicating that the rds is a pure chemical reaction following electron transfer. We discuss two possible reaction schemes including bimolecular and monomolecular reactions in the rds to explain the kinetics and suggest that the reactants in the rds are adsorbed hydrogen and adsorbed NO₃⁻ with the assistance of the results in our recent report for nitrate reduction on Pt(S)[n(1 1 1) × (1 1 1)] electrodes: the nitrate reduction mechanism can be classified within the framework of the Langmuir-Hinshelwood mechanism.