Hristo Kolev

Corrosion Performance of Carbon Steel in Simulated Pore Solution in the Presence of Micelles

This study presents the results on the investigation of the corrosion behavior of carbon steel in model alkaline medium in the presence of very low concentration of polymeric nanoaggregates [0.0024 wt % polyethylene oxide (PEO)113-b-PS70 micelles]. The steel electrodes were investigated in chloride free and chloride-containing cement extracts. The electrochemical measurements (electrochemical impedance spectroscopy and potentiodynamic polarization) indicate that the presence of micelles alters the composition of the surface layers (i.e., micelles were indeed absorbed to the steel surface) and influences the electrochemical behavior of the steel, i.e., the micelles lead to an initially increased corrosion resistance of the steel whereas no significant improvement was observed within longer immersion periods. Surface analysis, performed by environmental scanning electronic microscopy, energy-dispersive x-ray analysis, and x-ray photoelectron spectroscopy, supports and elucidates the corrosion performance. The product layers in the micelles-containing specimens are more homogenous and compact, presenting protective ?-Fe2O3 and/or Fe3O4, whereas the product layers in the micelles-free specimens exhibit mainly FeOOH, FeO, and FeCO3, which are prone to chloride attack. Therefore, the increased “barrier effects” along with the layers composition and altered surface morphology denote for the initially increased corrosion resistance of the steel in chloride-containing alkaline medium in the presence of micelles.

Reduced graphene oxide and MoP composite as highly efficient and durable electrocatalyst for hydrogen evolution in both acidic and alkaline media

Materials based on earth-abundant elements can be developed for hydrogen evolution reactions to meet the future demand for eco-friendly and renewable energy sources based on hydrogen. MoP nanoparticle assemblies in composite with reduced graphene oxide (rGO) have been introduced with superior electrocatalytic performances in hydrogen generation from both acidic and alkaline media. The composite drives HER with a current density ∼210 mA cm−2 at a potential of 400 mV vs. RHE and requires only ∼80–90 mV to start hydrogen evolution in both media. This composite remains stable and durable for hydrogen evolution in an acidic medium after 20 hours of chrono-potentiometric studies at a current density of 100 mA cm−2. Electrochemical hydrogen evolution studies show that an rGO based composite synthesized by an aniline complexation method exhibits higher catalytic efficiency than a composite synthesized by the citric acid complexation method. Detailed mechanistic investigations in both acidic and alkaline media have been carried out using impedance spectroscopy and the Tafel slope.

Transition metal modified activated carbons from biomass and coal treatment products as catalysts for methanol decomposition

Copper, iron and cobalt supported on activated carbon materials are compared as catalysts in methanol decomposition in view of their potential application as intelligent carriers of hydrogen. The activated carbon supports were obtained from renewable agriculture waste (peach shell) or coal treatment by-products (coal tar pitch). The parent activated carbons and their transition metal modifications were characterized by nitrogen physisorption, XRD, UV–Vis, FTIR, Mössbauer spectroscopy and TPR in hydrogen, and the surface functional groups were determined by the Böhm method. It was assumed that the formation of transition metal modified activated carbon catalysts is a complex process which proceeds during the preparation procedure with the activity of the support and also during the catalysis by the influence of the reaction medium. The decisive effect of carbon basal planes over the texture and surface functionality of the support on the formation of transition metals active phase was assumed. Among the studied materials, cobalt modifications exhibited excellent catalytic activity and selectivity in methanol decomposition to H2 and CO despite the nature of the activated carbons used.

Microstructure, surface chemistry and electrochemical response of Ag|AgCl sensors in alkaline media

Characterization of the Ag/AgCl electrode is a necessary step toward its application as a chloride sensor in a highly alkaline medium, such as concrete. The nucleation and growth of AgCl on Ag in 0.1xa0M HCl was verified through cyclic voltammetry. Ag anodization was performed at current densities, determined by potentiodynamic polarization in the same (0.1xa0M HCl) medium. The morphology and microstructure of the AgCl layers were evaluated via electron microscopy, while surface chemistry was studied through energy-dispersive spectroscopy and X-ray photoelectron spectroscopy. At current density above 2xa0mA/cm2, the thickness and heterogeneity of the AgCl layer increased. In this condition, small AgCl particles formed in the immediate vicinity of the Ag substrate, subsequently weakening the bond strength of the Ag/AgCl interface. Silver oxide-based or carbon-based impurities were present on the surface of the sensor in amounts proportional to the thickness and heterogeneity of the AgCl layer. It is concluded that a well-defined link exists between the properties of the AgCl layer, the applied current density and the recorded overpotential during Ag anodization. The results can be used as a recommendation for preparation of chloride sensors with stable performance in cementitious materials.

Mössbauer study of iron-based perovskite-type materials as potential catalysts for ethyl acetate oxidation

La-Sr-Fe perovskite-type oxides were prepared by the nitrate-citrate method. The basic object of this study is layered Ruddlesden-Popper phase LaSr3Fe3O10. The phase composition and structural properties of the obtained materials are investigated by Mossbauer spectroscopy, X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and temperature programmed reduction (TPR). The preliminary catalytic tests show a high potential of these materials for volatile organic compounds (VOCs) elimination as they possess high conversion ability and selectivity to total oxidation of ethyl acetate. Catalytic performance of LaSr3Fe3O10 is depended on the stability of structure and Fe4+-oxidation state.

Reaction kinetics and mechanism of complete methane oxidation on Pd/Mn 2 O 3 catalyst

A Pd/Mn2O3 catalyst highly active in the reaction of complete methane oxidation has been prepared by supporting palladium on nanosized Mn2O3. The nanosized Mn2O3 particles have been obtained by calcination of finely divided Mn3O4, synthesized by advanced wet chemical procedure. The very high activity manifested by the Pd/Mn2O3 catalyst (in the presence of 20000xa0pm water vapor and gaseous hourly space velocity of 25000xa0h−1 the catalyst bed temperature for reaching 50% conversion degree is 430xa0°C) is explained based on the nature of the active species: PdO clusters stabilized on Mn2O3. The role of Mn2O3 is to maintain the palladium in its oxidized state, acting as a high capacity storage of the oxygen species. The Langmuir–Hinshelwood or Eley–Rideal mechanisms are most probable over pure Mn2O3, while the addition of palladium to Mn2O3 changes reaction mechanism to Mars–van Krevelen whereupon water molecules are competing with the methane molecules for the oxidized adsorption sites, accompanied by a slow desorption of products (rate-limiting step).

Potentiometric Response of Ag/AgCl Chloride Sensors in Model Alkaline Medium

The stability and reproducibility of an Ag/AgCl sensors’ response in an alkaline medium are important for the application of these sensors in cementitious materials. The sensors’ response, or their open circuit potential (OCP), reflects a dynamic equilibrium at the sensor/environment interface. The OCP response in an alkaline medium is affected by the presence of hydroxide ions. The interference of hydroxide ions leads to inaccuracies or a delay in the sensors’ response to a certain chloride content. In this article, the potentiometric response (or OCP evolution) of the chloride sensors is measured in model solutions, resembling the concrete pore water. The scatter of the sensors’ OCP is discussed with respect to the interference of hydroxide ions at varying chloride concentration in the medium. The deviation of the sensor’s response from its ideal performance (determined by the Nernst law) is attributed to dechlorination of the AgCl layer and the formation of Ag2O on the sensor’s surface. Results from the surface XPS analysis of the AgCl layer before and after treatment in alkaline medium confirm these observations in view of chemical transformation of AgCl to Ag2O.

The Onset of Chloride-Induced Corrosion in Reinforced Cement-Based Materials as Verified by Embeddable Chloride Sensors

The need for an accurate determination of the chloride threshold value for corrosion initiation in reinforced concrete has long been recognized. Numerous investigations and reports on this subject are available. However, the obtained chloride threshold values have always been, and still are, debatable. The main concern is linked to the methods for corrosion detection and chloride content determination in view of the critical chloride content itself. In order to measure the chloride content, relevant to the corrosion initiation on steel, destructive methods are used. These traditional methods are inaccurate, expensive, time consuming and noncontinuous. Therefore, the application of a cost-effective Ag/AgCl ion selective electrode (chloride sensor) to measure the chloride content directly and continuously is desirable. The advantage would be an in situ measurement, in depth of the concrete bulk, as well as at the steel/concrete interface.