Maria Ziolek

Catalytic ozonation and methods of enhancing molecular ozone reactions in water treatment

This paper presents a review of catalytic ozonation and methods of enhancing molecular ozone reactions in water treatment. It is also an attempt to propose general ideas about mechanisms governing catalytic ozone reactions. Catalytic ozonation is a new means of contaminants removal from drinking water and wastewater. Its application is mainly limited to laboratory use. However, due to successful results further investigation is to be carried out. The majority of models proposed represent more of a speculative approach to the problem than a hypothesis based on experimental data. It is therefore useful to provide a summary of the accomplishments concerning catalytic ozonation and methods of enhancing molecular ozone reactions that were published so far. A survey of the application of several homo- and heterogeneous catalysts, their activity and the parameters influencing the efficiency of catalytic systems is presented here as a short overview, the aim of which is to raise awareness of possible new approaches to water purification.

Niobium-containing catalysts: the state of the art

Abstract This review article is devoted to the materials containing niobium, which have been discovered or developed in the past few years and exhibit the potential application in heterogeneous catalysis. Niobium oxides and mixed oxides as well as sulfides, nitrides (oxynitrides), carbides (oxycarbides), and phosphates are considered. Among the catalytic processes in which Nb-containing materials were tested, liquid and gas phase oxidation is described in details, and the role of niobium in the prevention of the catalyst from SO2 poisoning is mentioned.

Synthesis and characterization of niobium-containing MCM-41

A mesoporous niobium-containing silicate of the MCM-41 type was prepared for the first time, according to our knowledge. Structure, mechanical stability, catalytic activity, and cation exchange properties were characterized and compared with those for aluminum-containing MCM-41 materials.

Oxidative properties of niobium-containing mesoporous silica catalysts

Abstract Oxidative properties of MCM-41 molecular sieves containing niobium in the framework or/and in the extra framework positions were studied by means of H2-TPR, FTIR+NO, ESR techniques and were tested in the oxidation of dibutyl sulphide with hydrogen peroxide. The application of various Nb-sources in the synthesis of NbMCM-41 materials allows the estimation of a role of oxalate ions in the generation of active centres. The activity of Nb-containing mesoporous molecular sieves in the oxidation of thioethers with H2O2 requires the presence of Nb–O− species in the framework. The growth of a concentration of niobium (from Si/Nb=32 to Si/Nb=16) results in the decrease of the number of active centres due to the release of oxygen during the activation procedure. Niobium species in the extra framework positions of MCM-41 catalysts are not active in the dibutyl sulphide oxidation with H2O2 unless they are leached to the solution. Oxidative centres on the catalyst surface cause the formation of nitrite/nitrate species after the adsorption of NO.

Acidity study of Nb-containing MCM-41 mesoporous materials. Comparison with that of Al-MCM-41

The acidity of Nb-MCM-41 molecular sieves as well as their hydrogen forms was studied using FTIR spectroscopy with pyridine and 2,6-dimethylpyridine as probe molecules and in test reactions (isopropanol and cumene decomposition). The number of Lewis acid sites (LAS) and Brønsted acid sites (BAS) was estimated from the intensity of IR bands at 1450 and 1549 cm-1 respectively. The acidity of niobium-containing materials was compared with that of Al related molecular sieves. The parent Al- and Nb-MCM-41 mesoporous materials studied in this work did not show Brønsted acidity which only appeared after ammonium exchange followed by calcination. The number and the strength of BAS was much higher on H,Al-containing material than that on H,Nb-containing molecular sieves. H,Nb-MCM-41 exhibited a higher concentration of LAS than H,Al related material.

Epoxidation of cyclohexene on Nb-containing meso- and macroporous materials

Abstract Various niobium-containing meso- and macroporous materials were prepared and characterised by means of XRD and H2-TPR techniques, scanning electron microscopy (SEM), and sorption measurements. They were tested in the liquid phase oxidation of cyclohexene to epoxide with hydrogen peroxide. The effect of various parameters such as a nature of solvent, material structure, atmosphere on the activity and selectivity of the reaction was studied. NbMCM-41 and NbSM-2 materials are highly active in the epoxidation.

Nb-containing mesoporous molecular sieves — a possible application in the catalytic processes

Abstract Nb-containing MCM-41 mesoporous molecular sieves were modified with ammonium and copper cations and characterised by ESR, FTIR and H 2 -TPR techniques. The prepared materials were tested in the following catalytic reactions: NO decomposition, reduction of NO with NH 3 , hydrosulphurisation of methanol, and oxidation of thioethers with hydrogen peroxide. The oxidising character of the NbMCM-41 mesoporous molecular sieves was revealed, which induces their application as very active and selective catalysts in the oxidation of thioethers with H 2 O 2 to sulphoxides. HNbMCM-41 samples, which exhibit Lewis acidity after dehydroxylation, are selective catalysts in the methanethiol synthesis in the reaction between methanol and hydrogen sulphide. Copper modified NbMCM-41 sieves indicate the higher reducibility of cupric cations than that observed when AlMCM-41 matrix for copper is used. The higher reducibility of cupric ions does not cause a higher activity in the catalytic NO decomposition.

Influence of sulfur dioxide adsorption on the surface properties of metal oxides

The effect of SO2 chemisorption at 623 K on various metal oxides (CeO2, MgO, Al2O3, ZrO2, TiO2 anatase and rutile) has been studied by IR spectroscopy and catalytic tests: cumene cracking and isopropanol decomposition, IR spectroscopic studies of lutidine and pyridine adsorbed on SO2 treated metal oxides surfaces show the increase or the creation of Bronsted acidity. Test reactions evidence that the strength of such sites depends on the metal oxide, being very weak on MgO and important on TiO2. Formation of hydrogensulfite species or modification of the redox character of surface properties explains the SO2 chemisorption effects.

Influence of hydrogen sulfide adsorption on the catalytic properties of metal oxides

Abstract The effect of sulfidation by H 2 S at 623 K of various metal oxides (MgO, TiO 2 rutile and anatase, ZrO 2 , CeO 2 , Al 2 O 3 ) on their catalytic properties towards isopropanol decomposition, cumene decomposition and methanol hydrosulfurization has been studied. The results are discussed according to the H 2 S adsorption modes. H 2 S coordination mainly occurs on TiO 2 samples and generates strong acid sites. The dissociative adsorption of H 2 S on alumina does not significantly influence its acidity. Replacement of O 2− by S 2− ions as in the case of MgO, ZrO 2 and CeO 2 mainly affects their basic or redox properties.

U.v./vis and i.r. spectroscopic study of hydrogen sulphide adsorption on faujasite-type zeolites

Abstract Adsorption of H 2 S on sodium X- and Y-type zeolites was investigated by combined i.r. and u.v./vis transmission spectroscopy. The first molecules adsorbed on NaX dissociate completely under formation of S 2− (indicated by a 250 nm band) and OH (3635 cm −1 ), the S 2− species being unreactive towards O 2 or SO 2 . At higher coverages dissociation to HS − and H + as well as physical adsorption occurs. Adsorption of H 2 S on NaY is much less pronounced, most probably proceeding through a mechanism different from adsorption on NaX.