Krystyna Dyrek

An XRD and ESR study of V2O5/ZrO2 catalysts: influence of the phase transitions of ZrO2 on the migration of V4+ ions into zirconia

Abstract A series of undoped and potassium-doped catalysts for the oxidative dehydrogenation of propane, based on zirconia-supported V2O5 was investigated by XRD and ESR techniques. The chemical state of the surface and bulk vanadium species and of the ZrO2 matrix was investigated as a function of the calcination temperature and the presence of dopants. It was found that potassium improves the dispersion of surface vanadium species and affects the temperature of the phase transitions of zirconia. These two effects facilitate the incorporation of vanadium (IV) into the ZrO2 matrix. The formation of substitutional solid solution was revealed by Rietveld analysis.

Heteropolyanion-doped polypyrrole as catalyst for ethyl alcohol conversion

Abstract Heteropolyanion-doped polypyrrole can serve as a catalyst for ethyl alcohol conversion. The insertion of 12-molybdophosphoric anions into the polymer matrix significantly changes the selectivity of the catalyst with respect to unsupported crystalline H 3 PMo 12 O 40 . For example catalytic conversion of ethyl alcohol carried out on polypyrrole containing 75.5 wt.% of 12-molybdophosphoric anions and standardized by heating in helium, gives acetaldehyde as the main product (51%). On the acid-base centers ethylene (45%) and small amounts of diethyl ether (3.5%) are formed. Standardization of the catalyst by heating in air leads to heavy crosslinking of the polymer with simultaneous restoration of acid molecules. As a result, the selectivity and activity of this catalyst are similar to those of unsupported H 3 PMo 12 O 40 .

Spin dosimetry in catalysis research

Various aspects of the problem of reliability of the standards for spin dosimetry in solid samples containing transition metal ions are discussed. A method of preparing standards for d1 and d9 ions, based on vanadyl- and copper sulfate, is described. Reference samples with various spin concentration were prepared by uniform distribution of the paramagnetic substance in a diamagnetic, chemically unreactive matrix. The testing of the quality of standards was performed by statistical methods considering the following factors: reproducibility of the average chemical composition between preparations, macro- and microhomogeneity within preparations and precision of EPR measurements. The statistical analysis proved good quality of the standards produced by the elaborated method except for microinhomogeneity. Several examples are given to illustrate application of spin dosimetry in catalysis research. E.g., two different centers of reduced vanadium in vanadia-molybdena catalysts, V(IV) stabilized by oxygen vacancies and V(IV) stabilized by Mo(VI) ions, respectively, were identified. The kinetic model of redox processes occurring in V2O5−MoO3 catalysts upon interaction with oxygen and propylene was proposed. Quantitative determination of isolated Co(II) ions in CoO−MgO solid solutions revealed a strong correlation between the number of these ions and the amount of adsorbed O2− species.

ESR investigation of the redox processes occurring upon oxygen and propylene interaction with V2O5

Abstract ESR and chemical analysis were used to investigate the changes in reduction degree of vanadium pentoxide upon interaction with oxygen and propylene. It may be derived from our measurements that the redox processes accompanying interaction of oxygen at temperatures not exceeding 200 °C and propylene at temperatures up to 100 °C with partially reduced V 2 O 5 are restricted to the surface layers only. Assuming a model of two consecutive reactions of the first order the rate constants of the two-step oxidation processes, V 3+ → O 2 V 4+ → O 2 +V 5+ , and two-step reduction processes, → C 3 H 6 +V 4+ → C 3 H 6 V 3+ , were calculated by means of computer optimization. The kinetic data indicate that the activation energies of these reactions are 6.3 and 21 kJ/mol for the first and second steps of oxidation whereas for the first and second steps of reduction they are 33 and 59 kJ/mol, respectively. At temperatures exceeding 200 °C the rate of vanadium oxidation is determined by diffusion, the apparent activation energy of this process being 126 kJ/mol.

Electron paramagnetic resonance investigation of the paramagnetic centres in polycrystalline MoO3

The defect MoO3 was obtained by thermal decomposition of hexakisammonium heptamolybdate in vacuo at 630 K. Different signals present in the EPR spectrum of reduced MoO3 and those appearing in the course of oxidation, were attributed to Mo5+ ions in various symmetry, situated in different sites of the MoO3 lattice. The signal of Mo3+ ions with effective spin S= 1/2 was observed in the EPR spectrum of both reduced and reoxidized MoO3. A hole centre (Mo—O)5+ was found in reoxidized MoO3. Various methods of stabilization of reduced molybdenum in the MoO3 lattice were postulated.

EPR study of the mobility of paramagnetic species on the surface and in the bulk of solids

Abstract The temperature dependence of EPR spectra provides information on the mobility of paramagnetic species at the gas (liquid)/solid interface and in the bulk of solids. Changes in the environment of molecules on solid surfaces caused by their motion occurring upon thermal treatment at various temperatures are observed. Superoxide radical can migrate from Co(III) to Mg(II) surface sites of the CoO–MgO solid solutions. In aqueous solutions transition metal ions coordinate water molecules, forming aquacomplexes which are usually free to tumble within the liquid medium. Their mobility is, however, strongly modified in the vicinity of the solid surface or inside the narrow pores. In solids the migration of paramagnetic species from the surface into the bulk is controlled by the temperature of thermal treatment. In the case of V 2 O 5 –ZrO 2 catalyst this process is strongly influenced by the phase transitions occurring in the solid matrix and by the presence of alkali metals.

Standards for EPR measurements of spin concentration

Abstract Analytical aspects of the problem of standards for quantitative determination of the number of spins by EPR are discussed. A method of preparing the standards for V(IV) and other d 1 ions, based on vanadyl sulfate, is elaborated. The quality of the standards was tested by statistical methods.

Helical superstructure and charged polarons contributions to optical nonlinearity of 2-methyl-4-nitroaniline crystals studied by resonance Raman, electron paramagnetic resonance, circular dichroism spectroscopies, and quantum chemical calculations.

The Raman excitation profiles of solid 2-methyl-4-nitroaniline (MNA) reveal several band enhancements by intermolecular and intramolecular charge transfer states. Calculated excited- and ground-state molecular geometries and excited state distortions qualitatively determined from Raman spectra indicate multiple vibrations leading to MNA dissociation. Also, overtones and combination tones can generate charged polarons, as detected by electron paramagnetic resonance after the exposure to 980 and 1550 nm laser diodes. The MNA space group Ia (C(s)(4)) is nonchiral; however, the electronic circular dichroism (CD) spectra of solution, KBr pellet, and single crystal were recorded. The crystal chirality is elucidated by room-temperature dynamic disorder, possible helical superstructure along the [102] polar axis, and charged polarons presence. The CD spectra ab initio calculations for MNA neutral and negatively charged monomers, dimers, and trimers, lying along the helix, confirmed the chirality. The role of these findings toward efficient optical nonlinearity and electric conductivity failure is discussed.

Oxidation of propene over silica supported MoO3-CoO, MoO3-NiOand MoO3-MnO catalysts

Abstract The Raman spectra of MoO 3 -MeO/SiO 2 catalysts (Me = Co, Ni, Mn) revealed the presence of molybdate phases and the EPR spectra indicated changes in the oxidation state of molybdenum occurring on interaction with reaction mixtures containing various C 3 H 6 /O 2 ratios. Co and Ni enhanced the reducibility of the catalysts while Mn had practically no influence on the rate of catalyst reduction. It has been found that the concentration of Mo(V) influences the propene conversion and selectivity to acetaldehyde, as well as the total oxidation and coke deposition or ethene formation on the catalyst surface. On the other hand, selectivity to acrolein depends on the Mo(VI) concentration. The catalytic results were discussed in terms of the nucleophilic and electrophilic character of the oxygen species and the stability of the surface intermediates.

Electron spin resonance investigation of oxygen interaction with V2O5MoO3 catalysts

The changes in reduction degree of V2O5MoO3 catalysts occurring in the course of oxygen adsorption were investigated by electron spin resonance and chemical analysis. Two different types of paramagnetic centers, involving reduced vanadium ions stabilized by oxygen vacancies and substitutional Mo6+ ions, have been found in partially reduced catalysts. The kinetics of oxidation of such catalysts investigated by electron spin resonance revealed a delay in oxidation of the centers stabilized by Mo6+. The role of segregation processes occurring upon redox treatment of the V2O5MoO3 catalysts in determination of the stability of particular centers has been discussed.