M. Stoyanova

Degradation of phenolic waste waters over Ni-oxide.

The low-temperature oxidation of phenol in an aqueous phase in the presence of Ni-oxide system has been studied. The effects of pH, temperature and the catalyst amount on the efficiency of the oxidation process have been investigated. The results obtained show that phenol is completely oxidized to harmless products--carboxylic acids and CO2 at ambient temperature and pH 6.0-7.0. The simple scheme for this synthesis of the Ni-oxide system and its regeneration is a prerequisite for its application to the purification of waste waters containing toxic compounds.

Low-temperature iron-modified cobalt oxide system. Part 2. Catalytic oxidation of phenol in aqueous phase

Abstract Liquid-phase oxidation using a solid catalyst provides a potential method for removal of dissolved toxic organic pollutants from waste waters. Low-temperature heterogeneous oxidation of phenol on a cobalt oxide system modified with Fe has been studied. The conversion of phenol into nontoxic compounds occurs in air-equilibrated aqueous media. The major hydroxylated aromatic intermediates have been identified by UV–VIS spectroscopy and HPLC: catechol, hydroquinone and hydroxyhydroquinone. An oxidation mechanism has been proposed based on the results of the kinetic investigations and on the data obtained by IR and UV spectroscopic studies of the sample. The results obtained show that the Co–Fe-oxide system is a promising catalyst for practical application.

Low-temperature iron-modified cobalt oxide system: Part I. Preparation and characterisation

Abstract The cobalt oxide system, modified with Fe was prepared by a solution technique at a room temperature. The periodic method of precipitation with inverse order of feeding the precipitator to the system (Me 2+ →OH − +OCl − ) is used, which favors the formation of amorphous and highly disperse oxide system with non-stationary surface properties. The fresh sample as well as the samples obtained by its thermal treatment at different temperatures were characterized by Mossbauer and Infrared spectral analysis, X-ray diffraction and chemical analyses. A comparative consideration of the composition and structure of individual and modified with Fe cobalt oxide system has been carried out in order to clarify the influence of Fe as a modifying additive on the activity and selectivity of the sample. The results obtained show that the inclusion of Fe leads to the appearance of new active sites in the fresh samples — Fe(III) ions in octahedral coordination and to increasing the mobility of the reactive oxygen. The synthesized oxide system shows suitable properties for catalytic oxidation of toxic organic and inorganic compounds in an aqueous medium.

Low-temperature catalytic oxidation of water containing 4-chlorophenol over Ni-oxide catalyst

Abstract The catalytic oxidation of 4-chlorophenol (4-CP; chosen as an aromatic model pollutant) over Ni-oxide system in an aqueous medium has been studied. Experiments were carried out to investigate the effect of such parameters as pH, temperature, and catalyst dosage both on the reaction efficiency and on the reaction mechanism. The current concentrations of 4-CP as well as those of reaction intermediates—hydroquinone, benzoquinone, and 4-chlorocatechol—during the kinetic investigations were monitored by UV-Vis spectral analysis. Pseudo first-order kinetics with respect to the parent compound was observed. Experimental results demonstrated that 4-CP could be completely converted into the environmentally harmless CO 2 , H 2 O, and mineral acid under certain conditions. A probable kinetic model for the degradation pathway is proposed.

Low-temperature methanol oxidation on a higher nickel oxide in gaseous and aqueous phase. Part II

Abstract The IR-spectroscopy analysis brought about the conclusion that methoxyl groups are formed on the surface of Ni-oxide during adsorption of methanol at 25°C. Further on, they are oxidized partially to CO 2 by surface active oxygen at temperatures 25–35°C. O − forms (taking part in the oxidation process) are observed on the surface of the oxide. The methoxyl groups undergo a complete oxidation to form formiate complexes in the presence of oxygen in gaseous phase at temperatures higher than 35°C. The latter give the products of complete oxidation – CO 2 and H 2 O.

Mixed Ni-Mn-oxide systems as catalysts for complete oxidation. Part I. Preparation and characterization

A method for synthesis of new Ni-Mn-oxide systems is proposed. The synthesized systems have been characterised by a variety of methods including chemical analysis, infrared (IR) spectroscopy, X-ray diffraction (XRD) analysis, thermal analysis, and X-ray photoelectron spectroscopy (XPS). The results obtained reveal that the synthesized catalytic system can be successfully applied as an active phase in low temperature catalysts for complete oxidation carried out both in aqueous medium and in gas phase. The synthesis operating conditions yield hydrophilic and amorphous catalytic systems with high active oxygen (∼8%), taking part in oxidation processes. The Ni-Mn-oxide system is characterized by high thermal stability and its composition is kept constant up to 550 °C. Experimental data indicate that modifying a separate Ni-oxide system with Mn one can prepare new catalytic systems with high active oxygen content, larger specific area and higher thermal stability.

Catalytic oxidation of cyanides in an aqueous phase over individual and manganese-modified cobalt oxide systems.

The possibility for purification of wastewaters containing free cyanides by applying of a new method based on cyanides catalytic oxidation with air to CO(2) and N(2) at low temperature and atmospheric pressure was investigated. On this purpose, individual and modified with manganese Co-oxide systems as active phase of environmental catalysts were synthesized. The applied method of synthesis favours the preparation of oxide catalytic systems with high active oxygen content (total-O* and surface-O*(s)) possessing high mobility, and the metal ions being in a high oxidation state and in an octahedral coordination-factors determining high activity in reactions of complete oxidation. The catalysts employed were characterized by powder X-ray diffraction, Infrared spectroscopy, and chemical analysis. The effect of pH of the medium and catalyst loading on the effectiveness of the cyanide oxidation process, expressed by the degree of conversion (alpha, %), by the rate constant (k, min(-1)), and COD was studied. The results obtained reveal that using catalysts investigated a high cyanide removal efficiency could be achieved even in strong alkaline medium. The higher activity of the manganese promoted catalytic sample could be explained on the basis of higher total active oxygen content and its higher mobility both depending on the conditions, under which the synthesis of catalyst is being carried out.

Low-Temperature Catalytic Oxidation of Sulfide Ions in Aqueous Solutions on a Ni-Oxide System

Catalytic oxidation of sulfide ions in aqueous solutions by air oxygen has been investigated using a Ni-oxide system as a catalyst. The kinetics and the selectivity of the oxidation process were studied by varying the pH, catalyst amount and reaction temperature. A reduction/oxidation mechanism of the reaction has been supposed comprising interaction between the surface active oxygen of the catalyst and HS− and reoxidizing of the reduced catalyst by the dissolved oxygen. The results obtained show that the Ni-oxide system is a promising catalyst for practical application.