Electrochemical investigations of the oxidation-reduction of furfural in aqueous medium: Application to electrosynthesis

The present study concerns the electrochemical properties of furfural in aqueous medium on noble (Au and Pt) and non-noble (Pb, Cu and Ni) metal electrodes. The anodic and cathodic reactions are investigated by cyclic voltammetry on Au, Pt and Ni electrodes and during prolonged electrolyses on Pt, Pb and Cu in order to find the optimum conditions for a paired electrosynthesis. Anodic reactions are controlled by diffusion in the range of the stability of the solvent (water). Beside these limits, the gas evolution competes with the conversion of furfural. The best conditions for preparative electrooxidation (Ni anode, 0.5 M NaOH, j=0.8 mA cm⁻¹) gave furoic acid in a 80% yield and furfuryl alcohol was obtained by electroreduction in a 55% yield on Cu cathodes at pH 10 and 30 mA cm⁻².     

Structure-texture correlation in ultra-high-performance concrete: A nuclear magnetic resonance study

The pore size distribution in three reactive powder concrete formulations has been studied by nuclear magnetic relaxation of protons. Confirming the discrete and fractal features of the distribution for this kind of concrete, each formulation is assessed a surface fractal dimension, which reveals the hierarchy of pores. The experimental results evidence a dependence between this dimension and both the filling ratio of cement grains and the reactivity of silica fume. ²⁹Si nuclear magnetic resonance (NMR) allows us to draw a relationship between the amount of calcium silicate hydrates (C-S-H) and this surface fractal dimension.     

Dynamic adsorption on activated carbons of SO2 traces in air: I. Adsorption capacities

Sulphur dioxide is an atmospheric pollutant which, among numerous others, has to be eliminated by habitacle filters. Breakthrough curves of low concentration SO₂ streams through beds of activated carbons have been obtained. Two carbons were studied, an activated PAN fiber (CF) and a granulated activated carbon (CN) under SO₂ concentrations lower than 100 ppm. Carbon CN used ‘as received’ is able to trap SO₂ in air at concentrations as low as 2.5 ppm. At this concentration, the adsorption of SO₂ is essentially irreversible. The fraction of reversibly adsorbed SO₂ rapidly increases when SO₂ content in air increases from 2.5 to 100 ppm. As expected, the amounts of SO₂ adsorbed per gram of carbon are much smaller than in the case of high SO₂ contents in air (>1000 ppm). The presence of water in carbon micropores enhances both reversible and irreversible adsorption of SO₂. The reversibly adsorbed part is physisorbed while the irreversibly adsorbed part results in oxidation of SO₂ at the carbon surface. This oxidation was evidenced by TPD from carbon samples after adsorption. The mechanism of SO₂ adsorption is discussed in relation to the mechanisms proposed in literature for high SO₂ contents (>1000 ppm).     

Electrochemical deposition of lead dioxide in the presence of polyvinylpyrrolidone: A morphological study

In this work, we investigate the effect of polyvinylpyrrolidone (PVP) on the morphology and the interfacial properties of lead dioxide (PbO₂). The electrodeposition of lead dioxide was achieved in the presence of PVP on Pt and Ti substrates under constant current density from solutions containing Pb(NO₃)₂ and NaF in HClO₄. Scanning electron microscopy (SEM) showed that the morphology and particle size of PbO₂ are strongly affected by the concentrations of Pb(NO₃)₂, PVP and HClO₄. It seemed that PVP can control both the morphology and particle size of lead dioxide and increases the overpotential for oxygen evolution during the electrodeposition of lead dioxide. The resulting lead dioxide was composed of nano-metric globular particles aligned in rice shaped structure with diameter in the range of 30-50 nm. It was suggested that the growth of PbO₂ crystals is affected by the aqueous network of self-assembled surfactant formed on the electrode surface during the electrodeposition process. The electrochemical impedance spectroscopy (EIS) was used to investigate the interfacial behavior of deposited lead dioxide in 0.5 M H₂SO₄ solution. The EIS results revealed a typically porous electrode behavior consisted of a straight line, at high frequency region, turning to a potential dependent semicircle, at low frequency region, the diameter of which being decreased with increasing potential increments. The X-ray diffraction (XRD) patterns show that samples were composed of β-PbO₂.     

Gas-powder flow and powder accumulation in a packed bed: II: Numerical study

A mathematical model is proposed to describe gas-powder flow in a bed packed with particles. The model is the same as the two fluid model developed on the basis of the space-averaged theorem in terms of the governing equations but extended to consider the interactions between gas, powder and packed particles, as well as the static and dynamic holdups of powder. In particular, a method is proposed to determine the boundary between dynamic and stagnant zones with respect to powder phase, i.e., the profile of the powder accumulation zone. The validity of numerical modeling is confirmed by comparing the predicted and measured distributions of powder flow and accumulation under various flow conditions. On this basis, the role of gas-powder and powder-particle forces relative to the gravity force in controlling the powder flow and accumulation is analyzed.     

Kinetics of sodium borohydride direct oxidation and oxygen reduction in sodium hydroxide electrolyte: Part II. O2 reduction

In order to mimic the operation of the air-cathode in a direct borohydride alkaline fuel cell, we studied the oxygen reduction reaction (ORR) in sodium hydroxide solution containing traces of borohydride. The activity of several ORR electrocatalysts, namely carbon-supported platinum, gold, silver and manganese oxide, has been investigated using slow-scan linear voltammetry. Whereas platinum is one of the best electrocatalyst in pure sodium hydroxide, none of the classical electrocatalysts: gold, silver and platinum, exhibit sufficient selectivity towards the ORR. When BH₄⁻ is present in solution, the potential taken by electrodes using such materials is a mixed potential, following the competition between the ORR and the NaBH₄ hydrolysis and/or oxidation. Conversely, manganese oxide-based electrocatalysts exhibit very interesting behaviour towards the ORR in alkaline medium; while their intrinsic ORR activity in pure sodium hydroxide is quite as good as that for platinum, they still display a remarkable selectivity for this reaction when the electrolyte contains traces of sodium borohydride.As a result, carbon-supported manganese oxide-based nanoparticles seem very interesting materials to be used in direct borohydride fuel cell.     

Porosity of recycled concrete with substitution of recycled concrete aggregate: An experimental study

In this paper, we present the experimental analysis of samples of recycled concrete (RC) with replacement of natural aggregate (NA) by recycled aggregate originating from concrete (RCA). The results of the tests of mechanical properties of RC were used for comparison with tests of mercury intrusion porosimetry (MIP), in which the distribution of the theoretical pore radius, critical pore ratio, the surface area of the concrete, threshold ratio and average pore radius were studied at ages of 7, 28 and 90 days. The results showed some variation in the properties of the RC with respect to ordinary concrete. Porosity increases considerably when NA is replaced by RCA. Additionally, a reduction in the mechanical properties of the RC is seen compared with ordinary concrete when porosity increases.     

Synthetic carbons activated with phosphoric acid: II. Porous structure

Synthetic activated carbons were prepared by H₃PO₄ activation of a chloromethylated and sulfonated copolymer of styrene and divinylbenzene, using an impregnation weight ratio of 0.75 and carbonization temperatures in the 400-1000 °C range. Other impregnation ratios (0.93 and 1.11) were also used at a carbonization temperature of 800 °C. The porous texture of the resulting carbons was characterized by N₂ adsorption at −196 °C and CO₂ adsorption at 0 °C. All carbons exhibited a multimodal pore size distribution with maxima in the micropore and meso/macropore regions. Maxima in pore volume were attained at 900 °C for micropores and at 500 and 900 °C for mesopores. The mesopore volume was less sensitive than the micropore volume to changes in the impregnation ratio. It is concluded that the porous texture is not a prime factor in determining the outstanding cation exchange capacities of these carbons.     

Estimation of activated carbon adsorption efficiency for organic vapours: I. A strategy for selecting test compounds

Strategies for developing quantitative structure-affinity relationships (QSAfR) for the prediction of break-through performance of 31 chlorinated hydrocarbons on activated carbon have been studied. Two different approaches for the selection of a limited set of compounds for modelling were evaluated through the predictive power of the resulting QSAfR models. When the model was based on a training-set selected without a rational strategy, the developed QSAfR model showed poor predictive performance. Accordingly, such models have a limited capability to produce information concerning the important adsorbate related parameters influencing adsorption. By using a strategy where multivariate data analytical techniques are used in conjunction with statistical experimental design to select a balanced set of compounds for break-through performance evaluation, it was possible to develop QSAfR models with high predictive capability.     

Two new siloxanic proton conducting membranes: Part II. Proton conductivity mechanism and NMR study

The synthesis and structural characterization of two types of membranes with formulas {Si(CH₃)₃O[Si(CH₃)HO]_21.26-[Si(CH₃)((CH₂)₃SO₃H)O]_1.8-[Si(CH₃)((CH₂)₃Si(CH₃)₂O-)-O]₁₄-Si(CH₃)₃}_n (A) and {Si(CH₃)₃O[Si(CH₃)HO]_21.26-[Si(CH₃)((CH₂)₃SO₃H)O]_1.8-[Si(CH₃)((CH₂)₃(Si(CH₃)₂O-w₎)-Ov_][Si(CH₃)((CH₂)₃Si(CH₃)₂O-)-O_]14−vSi(CH₃)₃}_n (B), (w=20.31), were previously proposed.The ac electrical response of A and B was fully characterized in the 40 Hz-2 MHz frequency region by studying the impedance spectra in the medium and low frequency regions by equivalent circuits and complex dielectric spectra at high frequency in terms of dielectric relaxation modes. Results demonstrated that A and B conduct ionically by means of a proton exchange event which occurs via a vehicular mechanism between neighboring water clusters formed by water molecules aggregated around each sulfonic acid group of the siloxane side chains. The proton conductivities at 115 °C of ca. 1.9 × 10⁻³ and 1.8 × 10⁻⁴ S cm⁻¹ of fully hydrated membranes A and B, respectively, classify these silicone networks as good proton conductors.Membrane B was chosen for a closer investigation using NMR spectroscopy. Solid state ²⁹Si MAS NMR experiments gave further insight about the three-dimensional structure. Proton diffusion measurements provided some encouraging results about proton dynamics of this membrane signaling the great potential of siloxanic based proton conductors.     

Adsorption and electrooxidation of ethylene glycol and its C2 oxidation products on a carbon-supported Pt catalyst: A quantitative DEMS study

Aiming at a better understanding of ethylene glycol oxidation, the adsorption and oxidation of ethylene glycol and its incomplete C₂ oxidation products glycol aldehyde, glyoxal, glycolic acid, glyoxylic acid and oxalic acid on carbon supported Pt catalysts were investigated by on-line differential electrochemical mass spectrometry (DEMS) under continuous electrolyte flow. This includes adsorption transients at different, constant potentials, oxidative removal (‘stripping’) of the resulting adsorbates, and potentiodynamic bulk oxidation/reduction of the respective molecules. The data show a pronounced influence of the different functional groups on the adsorption and oxidation characteristics, with hydroxyl and carboxylic functions resulting in lower adsorption rates and pronounced potential effects, while aldehyde functions lead to high adsorption rates at all potentials. The potential effects in the adsorption rate are mainly ascribed to surface blocking by H_upd species. For aldehydes and acids, CO₂ formation occurs already at potentials below the onset of OH_ad formation, which is ascribed to the decomposition of the carboxylic group or of the diol groups of hydrated aldehydes. The contributions of different reaction pathways, including: (i) ‘direct’ oxidation to CO₂, (ii) indirect oxidation to CO₂ via formation and further oxidation of CO_ad, and (iii) incomplete oxidation to more highly oxidized C₂ species, with the possibility of their further reaction via re-adsorption and reaction along (i)–(iii), are discussed.     

Understanding aluminum behaviour in aqueous alkaline solution using coupled techniques: Part II: Acoustic emission study

This work focuses on the behaviour of pure aluminum in alkaline media, by coupling both acoustic emission (AE) and direct hydrogen voltammetry to electrochemistry. We notably monitored, recorded and analyzed the acoustic emission activity generated by the aluminum electrode as a function of its polarization during a linear sweep voltammetry (from anodic to cathodic potentials) on pure aluminum in 4 M aqueous potassium hydroxide solution. Such in situ coupling of electrochemistry and acoustic emission shows a perfect correlation between the two signals. After careful analysis of the AE signal using a statistical treatment, and based on five relevant AE parameters (rise time, duration, amplitude, absolute energy, maximum frequency), we could separate various groups of AE signals occurring at the aluminum electrode. We further linked them to the different (and possibly concomitant) electrochemical phenomena, which are taking place upon polarization of the aluminum electrode in strong alkaline medium.First, we confirmed that hydrogen evolution initiates for potentials positive to aluminum open circuit potential in 4 M potassium hydroxide solution; such small but non-negligible hydrogen production occurs in parallel to aluminum oxidation. Second, aluminum oxides are present only around the open circuit potential; whereas they are eroded for high aluminum oxidation potentials, they are flaked off at high hydrogen evolution potentials. Such latter process is probably accelerated by the hydrogen evolution-induced alkalization of the electrolyte. Third, two modes of hydrogen evolution are recorded: one on the oxide, the other one on bare aluminum, the latter being the most efficient. This strong hydrogen evolution at very low electrode potential probably assists the removal of the brittle residual oxide/passive film present on aluminum (which we denote as hydrogen-assisted aluminum exfoliation corrosion), therefore causing the rapid erosion of the aluminum electrode. As a result, aluminum is never in immunity conditions in strong alkaline medium.     

Ettringite formation: A crucial step in cement superplasticizer compatibility

The rheology of cementitious system containing superplasticizer is the consequence of a physical process due to the electrostatic repulsion between particles, but also of a chemical process linked to the nature of the phases that are formed. Ettringite crystallization play as a key role in this matter and the nature of the sulfate phase added to control cement setting is as important as its dosage. Alkali sulfates, which provide only SO₄²⁻ ions, do not promote the formation of ettringite for which the presence of large amounts of Ca²⁺ is necessary. The adsorption of superplasticizer molecules on hydrated cement grains slows down the dissolution rates of the constituents and modifies the nature of the compounds formed. It could result in a modification of the ettringite morphology.     

Catalytic filamentous carbon: Structural and textural properties

Catalytic filamentous carbon (CFC) synthesized by the decomposition of methane over iron subgroup metal catalysts (Ni, Co, Fe or their alloys) is a new family of mesoporous carbon materials possessing the unique structural and textural properties. Microstructural properties of CFC (arrangement of the graphite planes in filaments) are shown to depend on the nature of catalyst for methane decomposition. These properties widely vary for different catalysts: the angle between graphite planes and the filament axis can be 0° (Fe-Co-Al₂O₃), 15° (Co-Al₂O₃), 45° (Ni-Al₂O₃), 90° (Ni-Cu-Al₂O₃). The textural properties of CFC depend both on the catalyst nature and the conditions of methane decomposition (T, °C). The micropore volume in CFC is very low, 0.001-0.022 cm³ g⁻¹ at the total pore volume of 0.26-0.59 cm³ g⁻¹. Nevertheless, the BET surface area may reach 318 m² g⁻¹. Results of the TEM (HRTEM), XRD, Raman spectroscopic, SEM and adsorption studies of the structural and textural properties of CFC are discussed.     

Cement composition and sulfate attack: Part I

Four cements were used to address the effect of tricalcium silicate content of cement on external sulfate attack in sodium sulfate solution. The selected cements had similar fineness and Bogue-calculated tricalcium aluminate content but variable tricalcium silicates. Durability was assessed using linear expansion and compressive strength. Phases associated with deterioration were examined using scanning electron microscopy and X-ray diffraction. Mineralogical phase content of the as-received cements was studied by X-ray diffraction using two methods: internal standard and Rietveld analysis.The results indicate that phase content of cements determined by X-ray mineralogical analysis correlates better with the mortar performance in sulfate environment than Bogue content. Additionally, it was found that in cements containing triclacium aluminate only in the cubic form, the observed deterioration is affected by tricalcium silicate content. Morphological similarities between hydration products of high tricalcium aluminate and high tricalcium silicate cements exposed to sodium sulfate environment were also observed.     

Trace elements in clinker: I. A graphical representation

The trace element content of clinkers (and possibly of cements) can be used for the qualitative identification (i.e. manufacturing works). Several hundred clinker sorts have been analysed (by replicated quarterly samples, collected from all Hungarian cement factories, as well as from factories in eight foreign countries) to determine their Mg, Sr, Ba, Mn, Ti, Zr, Zn and V content. The first six elements come from the main raw materials and are of dactylogrammatic value, while the last two elements mainly come from fuel (used tires and heavy fuel oil, respectively) and cannot be used for identification. In this paper, a graphical method is presented to facilitate the visualisation of the trace element content.     

Understanding aluminum behaviour in aqueous alkaline solution using coupled techniques: Part I. Rotating ring-disk study

Rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) measurements have been undertaken to study the behaviour of pure aluminum electrodes in alkaline media. The measurements did consist of linear sweep voltammetry from anodic to cathodic potentials on 4N, 5N or 5N5-aluminum samples in 4 M aqueous potassium hydroxide solution. In the potential range studied (−0.7 V versus NHE to −2.5 V versus NHE) the aluminum undergoes oxidation/dissolution into aluminates anions at high electrode potential while it yields strong hydrogen evolution at low potentials. Thanks to the RRDE technique, we show that hydrogen starts to evolve from the aluminum electrode even above the open circuit potential. Also, the oxidation state of superficial aluminum varies according to the electrode potential: whereas non-conducting aluminum oxides are present above the open-circuit potential hindering hydrogen evolution reaction (HER), they tend to disappear below the ocp, due to the strong hydrogen evolution, following the probable porous oxide layer blow up induced by the hydrogen bubbles formation. In consequence at very low potential, HER occurs on bare aluminum, HER kinetics being much faster than on oxide-covered aluminum.     

An identification study of vermiculites and micas: Adsorption of metal ions in aqueous solution

This study deals with the identification of vermiculite in the presence of mica. Three natural silicates were first subjected to expansion by rapid heat treated at 400 °C. Then, the starting silicates and their derived heat-treated products were analyzed by X-ray diffraction, FT-IR spectroscopy, N₂ adsorption at − 196 °C, and helium density measurement. The adsorption of cadmium, chromium, and lead was also carried out. Two of the silicates are composed of mainly biotite and the other of biotite and vermiculite. In general, the results of X-ray diffraction, FT-IR spectroscopy, and helium density measurements are well in agreement. The heat treatment at 400 °C has little effect on the chemical composition, structure, and texture of the silicates. The development of microporosity is small in the silicates. Surprisingly, the adsorption capacity toward chromium and lead is larger for the biotite silicate than for the vermiculite containing silicate.     

Pyrrole nanoscaled electropolymerization: Effect of the proton

Polypyrrole (Ppy) nanowires are electrochemically synthesized using the nanochannels of a proton-modified natural zeolite clinoptilolite (HNZ). The synthetic inorganic structures Y zeolite and clay montmorillonite (M) in acid form (HY and HM) are also employed for pyrrole polymerization. The generation of Ppy with electrochemical activity is favored in a strongly acidic nanoscaled environment, in comparison with the polymer synthesized in the hosts without previous proton modification. It is proposed that the reduction/oxidation responses are a consequence of the polymer protonation/deprotonation where the H⁺ ions possibly act as charge transfer promoters. The well defined redox signals of Ppy included in HNZ, compared to the HY and HM, suggest that the polymeric units are held more tightly to the channels walls due to its complex framework. The TEM image of HNZ-Ppy reveals the presence of the polymer nanowires inside the natural zeolite framework. It is also found that the one electron oxidation process do not depend on first order monomer concentration but lower (0.38) for Ppy growth in HNZ, which is related to the spatial restriction of the host.