Pavel Čičmanec

Localization of Cu+ sites and framework Al positions in high-silica zeolites: Combined experimental and theoretical study

Three types of Cu+ sites are distinguished from the analysis of TPD spectra of CO in high-silica zeolites and the details about the structure and coordination of Cu+ at these sites are obtained from calculations.

Kissinger equation versus glass transition phenomenology

The applicability of the Kissinger equation for the evaluation of apparent activation energy corresponding to glass transition kinetics is examined. Theoretically simulated data based on the generally accepted Tool–Narayanaswamy–Moynihan model were used to represent relevant cases of structural relaxation behavior. The values of the apparent activation energy determined by the Kissinger equation were, despite the linearity of the dependencies, in major disagreement with the original values of ∆h* used for the simulation of the source data. Furthermore, a large dependence of the ∆hKis* evaluation (performed using the Kissinger equation) on the thermal history of the glass was found. The latter represents an unacceptable systematic error in the methodology, implying the incorrectness of the Kissinger equation usage for the evaluation of “glass transition activation energy”. This study addresses the currently widespread (incorrect) usage of the Kissinger equation for the above-mentioned purpose.

Characterization of Cu+ sites in FER: Combined computational and experimental TPD study

CO-TPD spectra of CO/Cu+/FER system were investigated and analyzed by a combination of experimental and theoretical techniques. Three Cu+ site types, characterized by adsorption energies -122, -76 and -59 kJ/mol were found using a combined quantum mechanical/interatomic potential function method. Three Cu+ site types with CO adsorption energies -121, -86, and -62 kJ/mol can be distinguished from the analysis of TPD spectra of CO/Cu+/FER. TPD spectra can be interpreted in terms of the computational results: CO desorbs first from the Cu+ sites in the perpendicular channel cavity. Sites in the main channel and on the intersection have the largest desorption energy. Information about the framework aluminum distribution in FER can be also obtained from the analysis of CO-TPD spectra.

Combined oxidative and non-oxidative dehydrogenation of n-butane over VOX species supported on HMS

The combination of oxidative and non-oxidative dehydrogenation of n-butane as an attractive possibility for production of C4 olefins was studied over VOX based catalyst. Long-term activity and selectivity to desired products could be achieved over the catalysts with well dispersed monomeric vanadium oxide species supported on mesoporous silica support.

Study of thermodynamic characteristics of CO adsorption on Li exchanged zeolites

The experimental thermodynamic characteristics of CO probe molecule adsorption on lithium exchanged zeolites of MFI, FAU, MCM-58, MCM-22, MCM-36 and MCM-49 structure was investigated by joint volumetry-calorimetry method. Consideration was given to the interpretation of the heat evolved when a probe molecule was adsorbed on the surface. In particular, the number and strength of adsorption sites were discussed as functions of zeolite structure and concentration of extra-framework cation. In addition, the obtained thermodynamic data and energy distribution functions were discussed together with results obtained from FTIR spectroscopy and theoretical DFT calculations.

Structural relaxation of amorphous Ge38S62 studied by length dilatometry and calorimetry

The structural relaxation of Ge38S62 glass has been studied by length dilatometry and calorimetry. The Tool-Narayanaswamy-Moynihan model was applied on obtained data of structural relaxation and parameters of this model were determined: Δh*= 483±2 kJ mol-1, ln(A/s)= -81±1, β= 0.7±0.1 and x=0.6±0.1. Both dilatometric and calorimetric relaxation data were compared on the basis of the fictive relaxation rate. It was found that the relaxation rates are very similar and well correspond to the prediction of phenomenological model.

Combined TPD and theoretical investigation of CO desorption from Cu-K-FER zeolite

Abstract The desorption of carbon monoxide from the Cu-K-FER zeolite was investigated by TPD. The obtained TPD curves exhibited significant differences compared to previously reported Cu-Na-FER TPD curves having the same copper content. The low temperature desorption peak was ascribed to a presence of heterogeneous dual cation Cu+…CO…K+ sites. The energetic parameters (i.e., energy and entropy of desorption) as well as distribution of all present Cu+ sites were obtained by the fitting of experimental data to the phenomenological model of TPD experiments. Results obtained from this model agree well with the results of quantum chemistry calculations.

Textural and morphology changes of mesoporous SBA-15 silica due to introduction of guest phase

Abstract The research focuses on study of guest phase effect on the surface area and pore volume of SBA-15 with the emphasis on elucidation of reasons for these changes. The changes of surface area and pore volume are evident from evaluated N2 adsorption isotherms of VOx-SBA-15 even for samples with relative low content of supported guest phase, which is “atomically” spread on the surface in the form of anchored monomeric vanadyl species. These species cannot block the pore with diameter of 10 nm, nevertheless the presence of such phase causes decrease in adsorbed nitrogen during physisorption. Comparison of guest phase amount with differences in adsorbed amount of nitrogen led to conclusion that each vanadyl complex prevents adsorption of about one or two N2 molecules in the layer and influences two adsorption layers. Significant pore blocking occurs in the VOx-SBA-15 materials only in the case of presence bulk oxide-like nanospecies. Re-structuralization of silica mimicking phase separation phenomena relying on spinodal decomposition of a system was observed by SEM/TEM analysis and adsorption isotherms inspection for materials with high vanadium content.