A. Eliyas

Kinetics of ethylene oxidation over a silver catalyst in the presence of dichloroethane

Abstract The kinetics of ethylene oxidation over a silver catalyst in the presence of dichloroethane has been studied by the circulation flow method with a gradientless reactor. Dichloroethane inhibits the total oxidation reaction to a greater extent than the partial oxidation reaction. In this way it promotes the selectivity for ethylene oxide. The promoter was added to the feed gas stream in trace quantities ∮.2 to 28 ppm. The experiments were carried out under steady-state conditions, at atmospheric pressure and in the interval 240 – 292°C. Additional factors that could influence the selectivity, temperature, feed composition and degree of conversion of ethylene, were also investigated. It was established that the influence of the conversion is negligible. A linear model of the influence of the dichloroethane concentration on the selectivity could be applied to most series of the experiments. The selectivity-promoting effect of dichloroethane is stronger at higher temperatures. The selectivity is most sensitive to promoter concentration when the feed gas stream has the composition 50% O2, 25% C2H4 and 25% Ar. A kinetic model consisting of two rate equations - for the partial and for the complete oxidation is proposed, including a dependence on the partial pressure of dichloroethane. The model describes satisfactorily the influence of all factors on the selectivity. The role of dichloroethane is to inhibit the dissociative adsorption of oxygen on the silver surface.

A kinetic model of steady state ethylene epoxidation over a supported silver catalyst

Abstract A kinetic study of selective ethylene oxidation to ethylene oxide over a silver catalyst has been carried out by the circulation flow method. The silver was promoted by a Ca additive and was supported on alumina. The experiments were conducted under steady state conditions in the temperature interval 210 – 292°C and at atmospheric pressure. As a result of this kinetic study two rate equations, for the partial and for the complete oxidation reactions, are proposed. A reaction scheme is put forward, according to which adsorbed molecular oxygen produces ethylene oxide, whereas the atomic oxygen is responsible for the complete oxidation reaction. The empirical kinetic model corresponds to a Rideal-Eley type of mechanism. Selectivity decreased with temperature increase and with decrease of the oxygen content in the feedstock.

Visible light photocatalytic activity of TiO2 deposited on activated carbon

Four photocatalyst samples, prepared from beech sawdust, were synthesized by an original method, combining pyrolysis and impregnation — two of them: TiO2 + activated carbon and other two — only activated carbon. The pyrolysis process has been carried out at two different temperatures — 680°C and 830°C. The prepared samples were characterized by a series of methods — XRD, BET, SEM and DTA/TG. The most important result was achieving visible light photocatalytic activity with an azo dye pollutant for both materials. The TiO2/AC-680°C sample demonstrated higher activity under visible light illumination than the TiO2/AC-830°C sample. The visible light activity was attributed to the active carbon component in the composite materials, which was evidenced by the photocatalytic tests with bare carbon (without any TiO2) manifesting visible light activity. The AC-680°C carbon was superior to the AC-830°C under visible illumination probably due to its higher specific surface area and porous texture. UV-light testing of the photocatalytic activity revealed that the TiO2/AC-680°C sample was higher than that of the TiO2/AC-830°C under polychromatic UV-A illumination (320–400 nm with a maximum at l = 365 nm). The TiO2/AC-680°C sample was also more efficient with the monochromatic UV-C illumination (l = 254 nm).Graphical abstract

CdS/ZnS nanocomposites: from mechanochemical synthesis to cytotoxicity issues.

CdS/ZnS nanocomposites have been prepared by a two-step solid-state mechanochemical synthesis. CdS has been prepared from cadmium acetate and sodium sulfide precursors in the first step. The obtained cubic CdS (hawleyite, JCPDS 00-010-0454) was then mixed in the second step with the cubic ZnS (sphalerite, JCPDS 00-005-0566) synthesized mechanochemically from the analogous precursors. The crystallite sizes of the new type CdS/ZnS nanocomposite, calculated based on the XRD data, were 3-4 nm for both phases. The synthesized nanoparticles have been further characterized by high-resolution transmission electron microscopy (HRTEM) and micro-photoluminescence (μPL) spectroscopy. The PL emission peaks in the PL spectra are attributed to the recombination of holes/electrons in the nanocomposites occurring in depth associated with Cd, Zn vacancies and S interstitials. Their photocatalytic activity was also measured. In the photocatalytic activity tests to decolorize Methyl Orange dye aqueous solution, the process is faster and its effectivity is higher when using CdS/ZnS nanocomposite, compared to single phase CdS. Very low cytotoxic activity (high viability) of the cancer cell lines (selected as models of living cells) has been evidenced for CdS/ZnS in comparison with CdS alone. This fact is in a close relationship with Cd(II) ions dissolution tested in a physiological solution. The concentration of cadmium dissolved from CdS/ZnS nanocomposites with variable Cd:Zn ratio was 2.5-5.0 μg.mL(-1), whereas the concentration for pure CdS was much higher - 53 μg.ml(-1). The presence of ZnS in the nanocrystalline composite strongly reduced the release of cadmium into the physiological solution, which simulated the environment in the human body. The obtained CdS/ZnS quantum dots can serve as labeling media and co-agents in future anti-cancer drugs, because of their potential in theranostic applications.

Ethylene oxide oxidation over a supported silver catalyst: II. Kinetics of inhibited oxidation

Abstract The kinetics of the complete oxidation of ethylene oxide over a supported silver catalyst in the absence of dichloroethane was studied by the flow circulation method. The temperature range 100–300°C was investigated under steady-state conditions and at atmospheric pressure. The kinetic measurements showed that ethylene oxide oxidation starts at 190°C. Below this temperature, the ethene oxidation over a silver catalyst proceeds according to the parallel reaction scheme. At 300°C and an oxygen: ethylene oxide feed ratio of 1:2 catalytic decomposition of ethylene oxide to ethene and oxygen was observed. A sequential experimental design was applied for the determination of the parameters and the simultaneous discrimination of rival kinetic models. A kinetic model is proposed that describes the rate of uninhibited ethylene oxide oxidation in the temperature range 250–300°C. The model corresponds to a mechanism in which ethylene oxide and oxygen adsorption (dissociative and non-dissociative) are fast steps at equilibrium. The rate-limiting steps are ethylene oxide oxidation to formaldehyde and further to carbon dioxide and water with the participation of both atomic and molecular adsorbed oxygen. These steps are irreversible. The calculated kinetic constants offer the possibility of evaluating tentatively the rate constants of the irreversible steps and the equilibrium constants of the fast reversible steps.

Ozone Decomposition Reaction over α-Alumina-Supported Silver Catalyst: Comparative Study of Catalytic Surface Reactivity

An alumina-supported silver composite system has been investigated in the reaction of heterogeneous catalytic ozone decomposition. Perlite loaded with silver and silver modified zeolites have also been tested as catalysts for gas phase conversion of ozone to molecular oxygen. The catalysts have been activated by calcination and the changes occurring on the catalytic surface as a result of the reaction have been characterized in details using XPS. The SEM analysis has revealеd the role of surface morphology in the process of ozone decomposition on the surface of Ag/α-Al2O3 catalyst. A catalytic cycle is proposed that supports the hypothesis about the important role of the peroxide species as intermediates participating in the process and catalyzing the complete ozone conversion into molecular oxygen. It has been found out that the maximum conversion degree, achieved with the alumina-supported silver system, is up to 90%.

Microwave-assisted and conventional sol-gel preparation of photocatalytically active ZnO/TiO2/glass multilayers

AbstractFor the first time a combination of microwaves and/or the conventional treatment method was used to dry and heat multilayered sol-gel ZnO/TiO2/glass structures. Compact or porous TiO2 films were deposited as a bottom layer, covered with a ZnO film.The structures were characterized by X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDX) and Scanning Electron Microscopy (SEM). Only peaks of wurtzite ZnO crystalline phase were registered on the X-Ray diffractograms.The microwave irradiation leads to a formation of poorly crystallized multilayers with very small crystallites and enhanced surface roughness. This results in a better photocatalytic activity of these structures than the structures of the samples treated conventionally.It was established that the morphology of the bottom titania layer affects the reaction of photocatalytic degradation of Malachite Green dye (MG). The structures with the compact bottom TiO2 films showed higher activities than those on porous TiO2 films.This study offers an energy saving method of producing ZnO/TiO2/glass multilayered structures of various morphologies and pronounced photocatalytic properties. The method does not involve any calcination step, normally applied to achieve a good degree of crystallization. This makes the method suitable for protecting substrates of low thermal stability.

Modelling of the inhibiting effect of carbon dioxide on the selective oxidation of ethene over silver catalyst

Abstract The inhibiting effect of high feed carbon dioxide concentrations (12–33%) on ethene epoxidation and combustion reactions over Ag/Al2O3 catalyst was studied by the flow-circulation method. Kinetic measurements have been carried out under steady-state conditions, at atmospheric pressure and in the temperature range 483–565 K. It has been established that the feed carbon dioxide inhibits considerably both the epoxidation and the combustion of ethene, the effect on the latter being stronger. Therefore, the presence of carbon dioxide in the feed affects the selectivity favourably. The inhibiting effect is described quantitatively by a kinetic model consisting of two rate equations, one for the epoxidation and the other for the combustion reactions. The model is to be used when the carbon dioxide concentration in the reaction mixture is higher than 10% and the temperature is in the range 513–565 K. The model is empirical and it is compared with a mechanistic model derived on the basis of the assumed reaction mechanism. The empirical model fits the experimental results better.

Influence of feed water vapour on the selective oxidation of ethylene over silver catalyst

Abstract The selective oxidation of ethylene over supported silver catalyst was studied in the presence of 0.2–16% feed water vapour. In addition to silver, alumina and silica, the catalyst also contains Li, Na, K, Mg, Ca, Ba, Mn and Fe additives in trace amounts. Steady-state kinetic measurements were carried out using a glass flow-circulation system at atmospheric pressure and in the temperature range 210–264°C, varying the oxygen-to-ethylene concentration ratio in the feed. The results show that the partial oxidation of ethylene is inhibited by the presence of feed water but the complete oxidation is promoted by it. This result contradicts the results of other workers. The reason for this contradiction is probably the presence of the additives in the studied catalyst. The effect of water is stronger at low feed concentrations (0.2–1.4%), at which the process is carried out in the industrial reactor. This fact could be of great importance for selecting the optimum conditions for the industrial process. The presence of feed water has an unfavourable effect on the selectivity for ethylene oxide, unlike the effect of carbon dioxide in the feed established in previous work over the same catalyst. An explanation of the effect of water vapour on the oxidation of ethylene is proposed on the basis of the assumed reaction mechanism.

Thermal oscillations during the catalytic hydrogenation of nitrobenzene

Abstract Three commercial Kieselguhr-supported copper, nickel and coppernickel catalysts were investigated under severe conditions in the hydrogenation of nitrobenzene to aniline. Self-sustained oscillations of the catalyst bed temperature in the 438 – 593 K range were observed after a partial deactivation of the studied samples. The oscillation mode was changed upon external heating. The experimental results show that the oscillations are due to mass transfer retardation effects and the nature of the chemical processes. An explanation of the observed phenomenon is proposed, according to which a hot zone is formed in the front layer of the catalyst bed. The zone cycles up and down along the catalyst bed.