Zbigniew Sojka

Application of the genetic algorithm joint with the Powell method to nonlinear least-squares fitting of powder EPR spectra

The application of the stochastic genetic algorithm (GA) in conjunction with the deterministic Powell search to analysis of the multicomponent powder EPR spectra based on computer simulation is described. This approach allows for automated extraction of the magnetic parameters and relative abundances of the component signals, from the nonlinear least-squares fitting of experimental spectra, with minimum outside intervention. The efficiency and robustness of GA alone and its hybrid variant with the Powell method was demonstrated using complex simulated and real EPR data sets. The unique capacity of the genetic algorithm for locating global minima, subsequently refined by the Powell method, allowed for successful fitting of the spectra. The influence of the population size, mutation, and crossover rates on the performance of GA was also investigated.

Continuous wave electron paramagnetic resonance investigation of the hyperfine structure of 17O2− adsorbed on the MgO surface

The adsorption of molecular oxygen (enriched with 17O) onto high surface area MgO has been studied by electron paramagnetic resonance (EPR) spectroscopy. The oxide surface was pretreated in such a way so that surface trapped electron FS+(H) centers are produced. Subsequent dioxygen adsorption results in an electron transfer reaction from FS+(H) centers to O2, producing a surface stabilized superoxide (O2−) anion. The resulting EPR spectrum of the paramagnetic anion is complicated by the simultaneous presence of a high number of “normal” hyperfine lines along the principal axes and also by several off-axis extra features which have complicated previous interpretations of the Ayy and Azz components. By adopting a suitable adsorption procedure which suppresses the superoxide speciation, using a highly crystalline MgO material and controlling the isotopomer composition through appropriate 17O enrichments, the resolution of the EPR spectrum has been dramatically improved. Analysis of the 1H superhyperfine stru...

Molecular Aspects of Catalytic Reactivity. Application of EPR Spectroscopy to Stuies of the Mechanism of Heterogeneous Catalytic Reactions

Introduction The basic objective of mechanistic studies of real catalytic processes is to dissect the course of the reaction into individual steps; ascertain their sequence; and determine the stoichiometry, structure, and electronic states of active sites and intermediates. The electron paramagnetic resonance (EPR) technique is at present widely used to explore many of these principal aspects of heterogeneous catalysis and surface chemistry. The extreme sensitivity compared to the usual spectroscopic methods is perhaps its most acknowledged advantage and makes EPR best suited to investigate and characterize low-abundance active sites and intermediates appearing during catalytic reaction. Additional information can be drawn from the theoretical analysis of the experimental spin Hamiltonian parameters within the ligand field and from angular overlap or Newmans superposition models as well as by more sophisticated quantum chemical calculations. The purpose of this paper is to show how catalysis benefits fro...

Application of EPR spectroscopy for elucidation of vanadium speciation in VO x /ZrO2 catalysts subject to redox treatment

Application of EPR spectroscopy corroborated by spectra simulation in speciation studies of the tetravalent vanadium in supported VOx/ZrO2 catalyst has been discussed. Implementation of genetic algorithms into automated analysis of the EPR spectra has greatly improved the simulation efficiency. The performance of the new procedure has been benchmarked against common simplex method using the multi-component model and real EPR spectra of tetravalent vanadium in VOx/ZrO2 catalysts. The analysis has revealed speciation of vanadium into surface isolated and clustered vanadyl entities and isolated bulk VZrx ions due to formation of Zr1−xVxO2 solid solution in the near to surface region. The structural heterogeneity of vanadium can be controlled by the calcination temperature and the redox treatment.

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.

Supramolecular ordering of PTCDA molecules: the key role of dispersion forces in an unusual transition from physisorbed into chemisorbed state.

Adsorption and self-assembly of large π-conjugated 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on rutile TiO(2)(110) surface have been investigated using a combination of high-resolution scanning tunneling microscopy (STM), low-energy electron diffraction, and density functional theory calculations with inclusion of Grimme treatment of the dispersion forces (DFT-D). Evolution of the STM images as a function of PTCDA coverage is caused by transition of the adsorption mode from physisorbed single adspecies and meandering stripes into spontaneously ordered chemisorbed molecular assemblies. This change in the adsorption fashion is accompanied by significant bending of the intrinsically flat, yet elastic, PTCDA molecule, which allows for strong electronic coupling of the dye adspecies with the TiO(2) substrate. Extensive DFT-D modeling has revealed that adsorption is controlled by interfacial and intermolecular dispersion forces playing a dominant role in the adsorption of single PTCDA species, their self-organization into the meandering stripes, and at the monolayer coverage acting collectively to surmount the chemisorption energy barrier associated with the molecule bending. Analysis of the resulting density of states has revealed that alignment of the energy levels and strong electronic coupling at the PTCDA/TiO(2) interface are beneficial for dye sensitization purposes.

Photochemistry of the [Fe(CN)5N(O)SR]3- complex : A mechanistic study

Photochemical behaviour of the title complex (RS − = mercaptosuccinate) was defined as photodissociation and photooxidation–

Texture of zirconia obtained by forced hydrolysis of ZrOCl2 solutions

The structure and surface properties of ZrO2 strongly depend on its preparation. In the present work the impact of prolonged aging at basic conditions (pH = 9, T = 100°C, t = 48 h), on the phase composition and textural properties, obtained by calcination of the precipitate, was investigated using several techniques conjointly (DTA/TG, DSC, XRD, porosimetry). The thermal effects accompanying the ZrO2·xH2O gel formation, the coalescence of the particles and crystallization were evaluated and discussed in terms of the structural differences between the aged and non-aged samples.

Potassium at catalytic surfaces—stability, electronic promotion and excitation

Thermal desorption of potassium atoms, ions and Rydberg atoms from K-doped transition metal carbides and nitrides were invetigated. The results were rationalized in terms of Schottky cycle. The status of the promoter was evaluated in three aspects: stability at the surface, electronic promotion and non-equilibrium excitation. Their relevance to catalysis is briefly discussed.

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.