Stefan Witkowski

Transformation of nitrogen structures in carbonization of model compounds determined by XPS

Abstract Nitrogen-containing chars were prepared by low-temperature carbonization (460 °C) of compounds containing pyrrolic, pyridinic, amino and cyano groups. The changes of precursor functionalities and transformation of nitrogen structures at 600 and 800 °C were determined by X-ray photoelectron spectroscopy. From all precursors, at low-temperature carbonization, three types of nitrogen structure were obtained: pyrrolic, pyridinic and quaternary, but from a precursor containing cyano groups this group was also observed. The concentration of pyrrolic and pyridinic structures decreases with increasing calcination temperature, whereas the amount of quaternary nitrogen increases. Quaternary nitrogen is the most stable form of the observed nitrogen structures, but an additional peak appeared at 403 eV for chars calcined at 800 °C.

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 and Raman investigations into anionic redox chemistry of nanoporous 12CaO·7Al2O3 interacting with O2, H2 and N2O

EPR and Raman spectroscopy jointed with temperature-programmed reduction (TPR) and oxidation (TPO) were used to elucidate of the anionic redox processes occurring during the interaction of dioxygen, nitrous oxide and dihydrogen with nanoporous 12CaO·7Al2O3. The results showed that hydrogen and oxygen enter the mayenite cages following a dissociative pathway involving hydride, hydroxyl and peroxide intermediates, respectively. Generation and annihilation of the cage O2− and O− radicals upon oxidative and reductive treatments, confined to the near to the surface region, were found to be reversible. The key intermediates of this process were identified and a detailed mechanism of the surface and cage reactions was proposed.

EPR Study of CO Adsorption onto CoZSM-5 Zeolite: Evidence for Spin Crossover upon Coordination and |z2, 2A1〉 Ground State

Adsorption of CO onto dehydrated cobalt-exchanged ZSM-5 zeolite was studied by CW-EPR techniques. It is shown that the reversible formation of a low spin carbonyl η1{Co(CO)n}7 adduct upon addition of carbon monoxide at pCO>50–60 Torr involves significant change in the spin state of Co2+ from S=3/2 to 1/2. The spin Hamiltonian parameters of the adduct gx=2.222, gy=2.184, gz=2.011, |Ax|=3.8 mT, |Ay|=3.2 mT, |Az|=7.9 mT were determined by a computer simulation and further discussed in terms of the possible ground state and molecular structure. It is shown that the η1{Co(CO)n}7 cage complex exhibits a C2v symmetry with |z2,2A1〉 ground state.

Tpr, EPR and UV-Vis studies of Ni(II) speciation in chrysoprase

TPR (Temperature Programmed Reduction) and EPR (Electron Paramagnetic Resonance) corroborated with UV-Vis methods were used to get a new insight into the status of nickel ions in chrysoprases from various localities. The investigation showed speciation of Ni into 2 : 1 highly dispersed phyllosilicates and extra-framework species grafted on the surface of chalcedony matrix. Both kinds of nickel exhibit a distorted octahedral coordination giving rise to a broad EPR spectrum with g = 2.17 and three characteristic d-d bands that can be parameterised with 10Dq = 8897cm - 1 and B = 953 cm - 1 .

Titanium dioxide doped with vanadium as effective catalyst for selective oxidation of diphenyl sulfide to diphenyl sulfonate

Diphenyl sulfide was oxidized to sulfoxide and sulfone over V-doped TiO2 using a 30% solution of H2O2. The TiO2 samples with different intended content of vanadium (0.02, 0.05, 0.1 and 0.18 mass%) were prepared by incipient wetness impregnation. Physicochemical properties of the V-doped TiO2 were characterized by chemical analysis (ICP-OES), X-ray diffraction (XRD/in situ HT-XRD), UV–Vis diffuse reflectance spectrometry (UV–Vis DRS), N2-sorption measurements, electron paramagnetic resonance and cyclic voltammetry. Both vanadium oxide loading and calcination temperature influenced the structure of the V-TiO2 samples. Vanadium species deposited on TiO2 decreased temperatures required for anatase to rutile phase transformation. The V-TiO2 samples were found to be efficient catalysts for oxidation of sulfides to sulfones. The sample with the lowest vanadium content (0.02VTiO2) presented among the studied catalysts the best catalytic properties with respect to high conversion of diphenyl sulfide to diphenyl sulfonate. An increase in vanadium loading resulted in decrease in catalytic activity of the samples. Also non-modified TiO2 presented significantly lower catalytic activity in comparison with 0.02VTiO2. This interesting effect was related to the formation of highly dispersed vanadium species catalytically active in Ph2S oxidation in the case of the samples with lower V-content. An increase in vanadium loading results in the formation of more aggregated V-species inactive, or less active, in the process of diphenyl sulfide oxidation.

Studies of Zn-Al-Ce mixed oxides as catalysts for diesel soot combustion

Studies of Zn-Al-Ce mixed oxides as catalysts for diesel soot combustion A series of Zn-Al-Ce mixed oxides was synthesized by a co-precipitation method. The obtained samples were characterized with respect to composition (XRF), structure (XRD, FT-IR) and texture (BET). Zn-Al-Ce mixed oxides were tested as catalysts of diesel soot combustion. The best catalytic activity was found for Zn2Ce oxide system, which operated in the temperature range of 350-500°C.