Izabela Czekaj

Evaporation of urea at atmospheric pressure.

Aqueous urea solution is widely used as reducing agent in the selective catalytic reduction of NO(x) (SCR). Because reports of urea vapor at atmospheric pressure are rare, gaseous urea is usually neglected in computational models used for designing SCR systems. In this study, urea evaporation was investigated under flow reactor conditions, and a Fourier transform infrared (FTIR) spectrum of gaseous urea was recorded at atmospheric pressure for the first time. The spectrum was compared to literature data under vacuum conditions and with theoretical spectra of monomolecular and dimeric urea in the gas phase calculated with the density functional theory (DFT) method. Comparison of the spectra indicates that urea vapor is in the monomolecular form at atmospheric pressure. The measured vapor pressure of urea agrees with the thermodynamic data obtained under vacuum reported in the literature. Our results indicate that considering gaseous urea will improve the computational modeling of urea SCR systems.

Modelling Catalyst Surfaces Using DFT Cluster Calculations

We review our recent theoretical DFT cluster studies of a variety of industrially relevant catalysts such as TiO2, γ-Al2O3, V2O5-WO3-TiO2 and Ni/Al2O3. Aspects of the metal oxide surface structure and the stability and structure of metal clusters on the support are discussed as well as the reactivity of surfaces, including their behaviour upon poisoning. It is exemplarily demonstrated how such theoretical considerations can be combined with DRIFT and XPS results from experimental studies.

The Cr X-ray absorption K-edge structure of poorly crystalline Fe(III)-Cr(III)-oxyhydroxides

Abstract Poorly crystalline solids play an important role in many low-temperature geochemical processes, such as trace element speciation and biomineralization. Yet, the structures of many such naturally occurring phases are poorly understood. X-ray absorption spectroscopy is a powerful tool that permits chemically and spatially resolved investigations of poorly crystalline materials. In this study, we compare structural and electronic information derived from different regions of chromium K-edge X-ray absorption spectra for a series of poorly ordered iron(III)-chromium(III)-oxyhydroxides. These phases regularly form after the reduction of Cr(VI) by Fe(II) and often dictate the long-term fate of Cr in the environment. The distinct parts of the X-ray absorption spectrum, namely the pre-edge region, the near edge (XANES) region, and the extended (EXAFS) region, provide complementary information about the local chemical environment of Cr. Analysis of the XANES and EXAFS spectra showed that the structure around Cr in the Cr-poor sample is primarily composed of edge-sharing octahedra, whereas the octahedra in the Cr-rich samples are connected by edge-sharing and cornersharing linkages. The analysis of non-local transitions in the pre-edge spectra indicated the absence of Cr clustering at low Cr substitution. This study demonstrates the advantage of complementary pre-edge, XANES, and EXAFS analysis to deduce information on the medium-range environment around Cr in poorly ordered solids.

X-ray Absorption and Emission Spectroscopy of CrIII (Hydr)Oxides: Analysis of the K-Pre-Edge Region

Pre-edge spectral features below the main X-ray absorption K-edge of transition metals show a pronounced chemical sensitivity and are promising sources of structural information. Nevertheless, the use of pre-edge analysis in applied research is limited because of the lack of definite theoretical peak-assignments. The aim of this study was to determine the factors affecting the chromium K-pre-edge features in trivalent chromium-bearing oxides and oxyhydroxides. The selected phases varied in the degree of octahedral polymerization and the degree of iron-for-chromium substitution in the crystal structure. We investigated the pre-edge fine structure by means of high-energy-resolution fluorescence detected X-ray absorption spectroscopy and by 1s2p resonant X-ray emission spectroscopy. Multiplet theory and full multiple-scattering calculations were used to analyze the experimental data. We show that the chromium K-pre-edge contains localized and nonlocalized transitions. Contributions arising from nonlocalized metal-metal transitions are sensitive to the nearest metal type and to the linkage mode between neighboring metal octahedra. Analyzing these transitions opens up new opportunities for investigating the local coordination environment of chromium in poorly ordered solids of environmental relevance.

CH4 combustion cycles at Pd/Al2O3--important role of support and oxygen access.

This research is focused on the analysis of adsorbed CH4 intermediates at oxidized Pd9 nanoparticles supported on γ-alumina. From first-principle density functional theory calculations, several configurations, charge transfer and electronic density of states have been analyzed in order to determine feasible paths for the oxidation process. Methane oxidation cycles have been considered as a further step at differently oxidized Pd nanoparticles. For low oxidized Pd nanoparticles, activation of methane is observed, whereby hydrogen from methane is adsorbed at one oxygen atom. This reaction is exothermic with adsorption energy equal to -0.38 eV. In a subsequent step, desorption of two water molecules is observed. Additionally, a very interesting structural effect is evident, mainly Pd-carbide formation, which is also an exothermic reaction with an energy of -0.65 eV. Furthermore, oxidation of such carbidized Pd nanoparticles leads to CO2 formation, which is an endothermic reaction. Important result is that the support is involved in CO2 formation. A different mechanism of methane oxidation has been found for highly oxidized Pd nanoparticles. When the Pd nanoparticle is more strongly exposed to oxidative conditions, adsorption of methane is also possible, but it will proceed with carbonic acid production at the interface between Pd nanoparticles and support. However, this step is endothermic.

Solving the puzzle of Li4Ti5O12 surface reactivity in aprotic electrolytes in Li-ion batteries by nanoscale XPEEM spectromicroscopy

The safe operation and long life-span of Li-ion batteries rely on a stable electrode–electrolyte interface. However, determining the thermodynamic stability window of such an interface is challenging due to the different (electro)chemical reactivities of the electrode components. Here we demonstrate a holistic experimental and theoretical approach to elucidate the nature and origin of the multiple reactions at such complex interfaces, which remain a major obstacle for the development of next generation Li-ion batteries. We applied X-ray photoemission electron microscopy (XPEEM) on Li4Ti5O12 electrodes to solve, with nanoscale resolution, its controversial surface reactivity in carbonate-based electrolytes. Local X-ray absorption spectroscopy (XAS) is performed upon cycling on individual carbon and Li4Ti5O12 particles, while maintaining their working environment, as in the commercial-like electrode composition. Despite the theoretical prediction of a stable electrochemical interface, we find that electrolyte reduction occurs solely on Li4Ti5O12 particles during lithiation at 1.55 V vs. Li+/Li. With the support of density functional theory (DFT) calculations, we show that this behavior is caused by the solvents adsorbed on the Li4Ti5O12 outer planes driven by the Li-ion insertion. The DFT results indicate that Li-ion insertion leads to a shift of the LUMO of the adsorbed solvents to energies below the Fermi level position of lithiated Li7Ti5O12 and thus to chemical instability. Simultaneously, at the same potential, we detect a competing reaction that leads to the partial dissolution of the electrolyte by-product layer. Such a finding has to be considered for other insertion materials and needs to be addressed in surface engineering to mitigate side reactions and design safe and long-lasting batteries.

SnO2 Model Electrode Cycled in Li-Ion Battery Reveals the Formation of Li2SnO3 and Li8SnO6 Phases through Conversion Reactions

SnO2 is an attractive negative electrode for Li-ion battery owing to its high specific charge compared to commercial graphite. However, the various intermediate conversion and alloy reactions taking place during lithiation/delithiation, as well as the electrolyte stability, have not been fully elucidated, and many ambiguities remain. An amorphous SnO2 thin film was investigated for use as a model electrode by a combination of postmortem X-ray photoelectron spectroscopy supported by density functional theory calculations and scanning electron microscopy to shed light on these different processes. The early stages of lithiation reveal the presence of multiple overlapping reactions leading to the formation of Li2SnO3 and Sn0 phases between 2 and 0.8 V vs Li+/Li. Between 0.45 V and 5 mV vs Li+/Li Li8SnO6, Li2O and Li xSn phases are formed. Electrolyte reduction occurs simultaneously in two steps, at 1.4 and 1 V vs Li+/Li, corresponding to the decomposition of the LiPF6 salt and ethylene carbonate/dimethyl carbonate solvents, respectively. Most of the reactions during delithiation are reversible up to 1.5 V vs Li+/Li, with the reappearance of Sn0 accompanied by the decomposition of Li2O. Above 1.5 V vs Li+/Li, Sn0 is partially reoxidized to SnO x. This process tends to limit the conversion reactions in favor of the alloy reaction, as also confirmed by the long-term cycling samples.