Laboratory for Physical ChemistryThe home page of the Department of Physical and Organic Chemistry is located at http://www-k3.ijs.muzej.si/ijs-dept-k3.html
Laboratory for Physical ChemistryHead: dr. Ivan Kobal
Concerning the investigations of elementary processes in surface reactions and in gas phase the angle-resolved thermal desorption study of reaction sites on stepped Pt(s)2(111)(100) surface, quantum chemical description of oxygen adsorption on Pt(111) single crystal surface and interpretation of kinetic isotope effects in catalytic reactions were precceded. At low atomic oxygen and CO coverages of above mentioned stepped surface the CO2 is mostly formed on step sites and desorbs at 360 K. With increasing atomic oxygen coverage the oxidation is enhanced. On the other hand, an increase of the CO coverage switches the reaction to the terrace sites at CO = 0.08, the peak temperature is shifted to lower values by about 100 K. The oxygen adsorption energy on step sites is higher than that terraces, therefore, the activation energy of the CO oxidation on step sites may be higher. When CO is adsorbed on step sites, it pushes the oxygen atoms towards terrace sites, resulting the reaction site switching to the terrace sites.
The chemisorption of oxygen on Pt(111) was studied using ab initio molecular orbital and density functional methods. The cluster model with Pt atoms fixed at bulk lattice sites was employed for modelling a Pt surface where Pt atoms were described with 18-electron relativistic effective core potentials. The triplet state was found to be the lowest lying electronic state for all studied species, the degree of higher spin states were of minimal importance. The calculated atomic oxygen adsorption energy and O-surface distance agree satisfactorily with experimental values. Population analysis reveals that net charge transfer of ~0.9 electron ocurrs from Pt3 to oxygen, where net reduction of 5d population in all three Pt atoms corresponds with increased 2p population in O atom. Asymptotic value of molecular interaction energy is close to the experimental adsorption energy, whereas the optimized intermolecular oxygen distance coincides with equilibrium bond lenght of oxygen molecule.
In 1996 we worked on the field of the Density Functional Theory (DFT). We wanted to understand why the GGA approximation in comparison to LSD approximation yields longer bond lengths (but not H-H bond lengths), lowers atomization energies and increases energy barrier at transition state of the chemical reaction. First we defined the distributions of the key density variables in the expression for the exchange-correlation energy (rs-Seitz radius of an electron, s-reduced density gradient) and calculated them for a group of atoms, molecules and crystals. We found out that the values of these variables are limited on the values of rs between 0 and 10 and values of s between 0 and 3 for all the systems for different physical processes (atomization, infinitezimal changes in the bond lengths). We believe that the GGA approximation is still valid for these ranges of density variables. Then we substituted the distribution functions with carefully chosen averages of the density variables. Based on the averages we derived an inequality, which tells us how the reduced density gradient influences the final result. The acquired knowledge will be used in the construction of new density functionals.
In the field of electrochemistry three main research areas are of interest: degradation of orthopedic implants and related investigations of biomaterials, replacement of mercury from alkaline batteries and replacement and/or improvement of galvanic coatings by PVD produced coatings.
So far no systematic research related to the problem of degradation of orthopedic implants, i.e. artificial hips, has been performed in Slovenia. Our new three-year project, which proceeds in collaboration with the Orthopedic Clinic Ljubljana, therefore distinguishes as a new research field within medicine, as well as within material science. In vivo and in vitro investigations are performed. The changes in composition, morphology and microstructure of the metal implant surface, and the impact of the implant degradation on the biological environment, i.e. tissue, are followed (in vivo). In vitro research includes the investigations of the biocompatible materials in artificial physiological solutions using electrochemical and surface analytical methods.
The fact that the mercury from the used alkaline batteries implies a serious ecological impact has led to the restriction of the use mercury as a corrosion inhibitor in the MnO2/Zn batteries. Investigations are now devoted to the selection of the ecologically safe compounds, which would replace the mercury and overtake its role of corrosion inhibitor. In our laboratory the perfluoro organic compounds are tested for this purpose. Investigations proceed in collaboration with the Iskra-baterije factory.
Due to environmental problems related to the production of galvanic coatings, numerous projects dealing with a complete or partial replacement of galvanic chromium and nickel coatings with alternative, PVD produced coatings are currently among the top priorities in many research laboratories in Europe. Researchers from the Department of thin films and surfaces are already involved in this process and currently work on the development of clean technologies able to produce suitable Cr and Ni PVD coatings. In our laboratory the corrosion properties of these coatings are investigated.
The laser chemistry group has been involved in the development of laser based photothermal techniques and their application in chemical analysis. The most significant achievements of the group in the past year are the development of a CO laser based differential thermal lens spectrometer and the development of thermal lens spectrometric (TLS) detection for HPLC analysis of trans-beta carotene.
The development of the first differential TLS spectrometer for measurements in the 5 - 7 m IR spectral region enabled up to 20 times reduction of the contributions of the solvent absorbance to the measured thermal lens effect. This resulted in three times lower limits of detection in measurements of pesticides and organic acids (such as tuberculostearic acid) extracted into organic solvents. In addition the differential TLS technique enabled measurements in aqueous samples where the conventional IR transmission techniques cannot be used.
Due to very high sensitivity of TLS, a smaller injection loop (10 l), which enables higher resolution of HPLC separation, was used for analysis of trans-beta-carotene in blood plasma. As a result, a 250 fold improvement in absolute limits of detection was achieved in addition to the reduction of the time required for completion of analysis from 50 to 15 min., compared to the at present used routine HPLC method with UV-Vis detection.
Research on TLS detection in combination with ion chromatography was also carried on. The emphasis in this field was on the applicability of specific reagents for TLS detection of ions and species such as Fe(II), Fe(III), Mn(II), Cu(II), and Co(II), which play an important role in chemical reactions in the aerosols.
In addition to ion chromatography and HPLC, methods based on biosensors were applied to improve the selectivity of TLS detection. A sensor operating on the principle of Acetylcholinesterase inhibition by pesticides and TLS detection of decay products of acetylholine iodide reaction with Acetylholinesterase was developed. Combination of the biosensor and TLS enables application of smaller sample volumes, shorter time of analysis and lower limits of detection for determination of organophosphate and carbamate pesticides in water.
Among other photothermal techniques, the photoacoustic spectroscopy (PAS) was successfully applied in investigations of secondary effects in thin layer chromatography (TLC). The PAS was used for measurements of thermal diffusivities of TLC plates which are essential to determine the thickness of the layer, which is probed at a given frequency of laser light modulation. Preliminary investigations have demonstrated that the PAS can be used to study the differences in in-depth concentration distribution of compounds separated on a TLC plate. In case of conventional reflectance measurements, which can probe only the surface of a TLC plate, such differences often result in erroneous densitometric validation of TLC chromatograms.
Besides the experimental work, the group has theoretically investigated the problem of simultaneous effects of thermal lens and beam deflection in highly absorbing samples. A new theoretical model was developed, which enables the calculation of the contributions from each of the two effects under different experimental conditions, and optimization of parameters such as pump beam radius and modulation frequency to improve the reproducibility and the accuracy as well as limits of detection of photothermal measurements.
The research activities performed on radon field can be divided in three different parts. In the first one the correlation between indoor radon concentrations in the kindergartens and schools (in the last few years indoor radon concentrations in all the 730 kindergartens and 890 schools in Slovenia were measured) and parameters which influencing them have been sought. These parameters are geology, climatic factors, age of the building, quality of the building construction, building materials and working regime (ventilation rate). So far Slovenia was devided into four geographic units (Alps, Fore-Alps, Dinarides, Pannonian basin) in order to look for the correlation between indoor radon concentration and different geology. It was found that a statistically significant difference exists between the units. The majority of high radon concentrations in buildings were detected in the Dinarides. Tha aim of this work is to predict high radon level risk areas in Slovenia.
Second part of radon research was focused on remedial actions in some high radon level kindergartens and schools. Namely, data form our previous studies indicate that about 10,000 children attend a kindergarten or school with radon levels which exceeded 400 Bq/m3, the proposed Slovene radon action level.
Third part of our radon research based on special measurements of radon and its decay products concentrations in two environments where one can expect high radon concentration. These are karst caves and spas. In year 1996 we started to measure radon, attached and unattached radon decay product concentrations, equillibrium factor and meteorological parametrs (temperature, humidity, pressure) in the Postojna Cave and some Slovene spas. Long-term measurements with a complementary measurement techniques will give necessary data for real working time dose estimations.
The study of carbon transfer in karst areas started in order to understand how karst world records environmental changes. The study included measurements of 13C and 14C contents in various components froming the karst system. Also, measurements of the environmental factors influencing to speleothem formation, as tempertures and humidity of air, temperature of water at the surface and in the cave, drip rates, etc... were measured during an entire year to see seasonal variations. The investigations will continue.