Studium katalyzátorů Pt-CeO2 připravených magnetonovým naprašováním

Title: Investigation of magnetron sputtered Pt-CeO2 thin film catalyst for fuel cell applications. Author: Mgr. Mykhailo Vorokhta Department: Department of Surface and Plasma Science Supervisor: Prof. RNDr. Vladimír Matolín, DrSc. [email protected] Abstract: This doctoral thesis focuses on magnetron sputtered Pt-doped CeO2 thin films that were found to be very active catalysts in proton exchange membrane fuel cells (PEMFC). The films were prepared on different substrates (silicon wafers, carbon nanotubes and flat carbon substrates) and investigated mainly by means of Hard x-ray photoelectron spectroscopy (HAXPES). The HAXPES method showed that deposition of the Pt doped cerium oxide catalyst layers on carbon nanotubes and flat carbon substrates by magnetron sputtering leads to growth of a highly porous "Pt-Ce-O" solid solution film, where platinum is mostly in ionic states, Pt2+ , Pt4+ . The results obtained showed that the mechanism of film growth is strongly influenced by interaction of the Ce atoms with the substrate and their oxidation by oxygen containing RF plasma. The formation of Ptn+ states together with the porous character of the catalyst films are used to explain the high activity of Pt-CeO2 catalyst in PEMFCs. Keywords: magnetron sputtering, cerium oxide, Pt, XPS, SRPES.Název práce: Studium katalyzátorů Pt-CeO2 připravených magnetonovým naprašováním. Autor: Mgr. Mykhailo Vorokhta Katedra / Ústav: Katedra fyziky povrchů a plazmatu Vedoucí doktorské práce: Prof. RNDr. Vladimír Matolín, DrSc. [email protected] Abstrakt: Doktorská práce se zabývá studiem Pt dopovaných tenkých vrstev CeO2 připravených magnetronovým naprašováním, u kterých bylo zjištěno, že představují velice aktivní katalyzátor pro palivové články s proton-vodivou membránou (PEMFC). Tenké vrstvy Pt- CeO2 byly naprašovány na různé substráty (křemíkové a uhlíkové substráty, uhlíkové nanotrubky) a byly zkoumány převážně pomocí fotoelektronové spektroskopie buzené tvrdým rentgenovým zářením (HAXPES). Výsledky získané metodou HAXPES ukázaly, že příprava katalytických vrstev oxidu ceru dopovaných Pt na různých uhlíkových substrátech a nanotrubkách metodou magnetronového naprašování vede k růstu vysoce porézních vrstev Pt- Ce-O s platinou v iontovém stavu Pt2+ , Pt4+ . Získané výsledky také ukázaly, že mechanismus růstu vrstvy Pt-CeO2 je silně ovlivněn interakcí atomů Ce se substrátem a jejich oxidací v kyslíkovém plazmatu. Vznik Ptn+ stavů společně s porézním charakterem katalytických vrstev slouží k vysvětlení vysoké aktivity katalyzátorů na bázi Pt-CeO2 pro palivové články PEMFCs. Klíčová slova:...Katedra fyziky povrchů a plazmatuDepartment of Surface and Plasma ScienceFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Structural transformations and adsorption properties of PtNi nanoalloy thin film electrocatalysts prepared by magnetron co-sputtering

This is the final peer-reviewed manuscript accepted for publication in Electrochimica Acta Citation of the published version is: Electrochimica Acta 251, 427–441 (2017

Surface Composition of a Highly Active Pt3Y Alloy Catalyst for Application in Low Temperature Fuel Cells

Currently, platinum is the most widely used catalyst for low temperature proton exchange membrane fuel cells (PEMFC). However, the kinetics at the cathode are slow, and the price of platinum is high. To improve oxygen reduction reaction (ORR) kinetics at the cathode, platinum can be alloyed with rare earth elements, such as yttrium. We report that Pt3Y has the potential to be over 2 times more active for the ORR compared with Pt inside a real fuel cell. We present detailed photoemission analysis into the nature of the sputtered catalyst surface, using synchrotron radiation photoelectron spectroscopy (SRPES) to examine if surface adsorbates or impurities are present and can be removed. Pretreatment removes most of the yttrium oxide in the surface leaving behind a Pt overlayer which is only a few monolayers thick. Evidence of a substochiometric oxide peak in the Y 3d core level is presented, this oxide extends into the surface even after Ar+ sputter cleaning in-situ. This information will aid the development of new highly active nanocatalysts for employment in real fuel cell electrodes

In situ probing of Pt/TiO2_{2} activity in low-temperature ammonia oxidation

The improvement of the low-temperature activity of the supported platinum catalysts in selective ammonia oxidation to nitrogen is still a challenging task. The recent developments in in situ/operando characterization techniques allows to bring new insight into the properties of the systems in correlation with their catalytic activity. In this work, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and operando X-ray absorption spectroscopy (XAS) techniques were applied to study Pt/TiO$_{2}$ catalysts in ammonia oxidation (NH$_{3}$ + O$_{2}$ reaction). Several synthesis methods were used to obtain samples with different size of Pt particles, oxidation state of Pt, and morphology of the support. Metal platinum particles on titania prepared by pulsed laser ablation in liquids exhibited the highest activity at lower temperatures with the temperature of 50% conversion of NH$_{3}$ being 150 °C. The low-temperature activity of the catalysts synthesized by impregnation can be improved by the reductive pretreatment. NAP-XPS and operando XANES data do not show formation of PtO$_{x}$ surface layers or PtO/PtO$_{2}$ oxides during NH$_{3}$ + O$_{2}$ reaction. Despite the differences in the oxidation state of platinum in the as-prepared catalysts, their treatment in the reaction mixture results in the formation of metallic platinum particles, which can serve as centers for stabilization of the adsorbed oxygen species. Stabilization of the bulk platinum oxide structures in the Pt/TiO$_{2}$ catalysts seems to be less favorable due to the metal–support interaction

Investigation of magnetron sputtered Pt-CeO2 thin film catalyst for fuel cell applications

Title: Investigation of magnetron sputtered Pt-CeO2 thin film catalyst for fuel cell applications. Author: Mgr. Mykhailo Vorokhta Department: Department of Surface and Plasma Science Supervisor: Prof. RNDr. Vladimír Matolín, DrSc. [email protected] Abstract: This doctoral thesis focuses on magnetron sputtered Pt-doped CeO2 thin films that were found to be very active catalysts in proton exchange membrane fuel cells (PEMFC). The films were prepared on different substrates (silicon wafers, carbon nanotubes and flat carbon substrates) and investigated mainly by means of Hard x-ray photoelectron spectroscopy (HAXPES). The HAXPES method showed that deposition of the Pt doped cerium oxide catalyst layers on carbon nanotubes and flat carbon substrates by magnetron sputtering leads to growth of a highly porous "Pt-Ce-O" solid solution film, where platinum is mostly in ionic states, Pt2+ , Pt4+ . The results obtained showed that the mechanism of film growth is strongly influenced by interaction of the Ce atoms with the substrate and their oxidation by oxygen containing RF plasma. The formation of Ptn+ states together with the porous character of the catalyst films are used to explain the high activity of Pt-CeO2 catalyst in PEMFCs. Keywords: magnetron sputtering, cerium oxide, Pt, XPS, SRPES

Interaction at the F₁₆CuPc/TiO₂ Interface: A Photoemission and X‑ray Absorption Study

The interfacial interaction and charge transfer dynamics between a F₁₆CuPc molecular thin film and rutile TiO₂(110) (1×1) surface have been studied by photoelectron spectroscopy (PES), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and resonant photoemission spectroscopy (RPES). The evolution of PES spectra as a function of F₁₆CuPc film thickness shows strong coupling between the molecules and the TiO₂ surface. Adsorbed molecules experience substrate mediated charge transfer. Electrons being pulled away from nitrogen atoms toward to carbon ring results in an opposite direction binding energy shift for C 1s and N 1s. Moreover, the molecule gets deformed due to their strong interaction with the TiO₂ surface. Ultrafast charge transfer from F₁₆CuPc molecules to the TiO₂ substrate takes place on the time scale of 10 fs due to their strong electronic coupling. The results pave the way for the design and realization of F₁₆CuPc based electronic devices

Functionalization of nanostructured cerium oxide films with histidine

The surfaces of polycrystalline cerium oxide films were modified by histidine adsorption under vacuum and characterized by the synchrotron based techniques of core and valence level photoemission, resonant photoemission and near edge X-ray absorption spectroscopy, as well as atomic force microscopy. Histidine is strongly bound to the oxide surface in the anionic form through the deprotonated carboxylate group, and forms a disordered molecular adlayer. The imidazole ring and the amino side group do not form bonds with the substrate but are involved in the intermolecular hydrogen bonding which stabilizes the molecular adlayer. The surface reaction with histidine results in water desorption accompanied by oxide reduction, which is propagated into the bulk of the film. Previously studied, well-characterized surfaces are a guide to the chemistry of the present polycrystalline surface and histidine bonds via the carboxylate group in both cases. In contrast, bonding via the imidazole group occurs on the well-ordered surface but not in the present case. The morphology and structure of the cerium oxide are decisive factors which define the adsorption geometry of the histidine adlayer

Study of Photoregeneration of Zinc Phthalocyanine Chemiresistor after Exposure to Nitrogen Dioxide

In this work, we present a complex study of photoregeneration of a zinc phthalocyanine (ZnPc) sensor by illumination from light-emitting diodes (LEDs). It includes an investigation of photoregeneration effectivity for various wavelengths (412–723 nm) of incident light carried out at sensor operating temperatures of 55 °C. It is demonstrated that the efficiency of photoregeneration is increasing with a decrease in the light wavelength. In the region of longer wavelengths (723–630 nm), the regeneration degree (RD) was low and ranged from 12% to 15%. In the region of shorter wavelengths (518–412 nm), the RD rose from 35% for 518 nm to 94% for 412 nm. The efficiency of photoregeneration is also shown to be higher in comparison with the temperature regeneration efficiency. In order to understand the chemism of photoregeneration processes, the electrical measurements are supplemented with Raman and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) studies. The spectroscopic results showed that nitrogen dioxide bonds to the Zn atom in ZnPc in the form of NO2− and NO−, i.e., partial decomposition of NO2 molecules occurs during the interaction with the surface. NAP-XPS spectra proved that light illumination of the ZnPc surface is essential for almost complete desorption of NOx species. At the same time, it is demonstrated that in case of long-time exposure or exposure of a ZnPc chemiresistor with a high concentration of NO2, the oxygen, released due to the NO2 decomposition, slowly but irreversibly oxidizes the layer. This oxidation process is most probably responsible for the sensor deactivation observed in sensor experiments with high NO2 concentrations. Based on these studies, the mechanism of nitrogen dioxide interaction with zinc phthalocyanine both under LED illumination and in dark conditions is proposed, and a special method for the sensor operation called “constant exposure dose” is established

Surface sites on Pt–CeO2 mixed oxide catalysts probed by CO adsorption: a synchrotron radiation photoelectron spectroscopy study

By means of synchrotron radiation photoemission spectroscopy, we have investigated Pt–CeO2 mixed oxide films prepared on CeO2(111)/Cu(111). Using CO molecules as a probe, we associate the corresponding surface species with specific surface sites. This allows us to identify the changes in the composition and morphology of Pt–CeO2 mixed oxide films caused by annealing in an ultrahigh vacuum. Specifically, two peaks in C 1s spectra at 289.4 and 291.2 eV, associated with tridentate and bidentate carbonate species, are formed on the nanostructured stoichiometric CeO2 film. The peak at 290.5–291.0 eV in the C 1s spectra indicates the onset of restructuring, i.e. coarsening, of the Pt–CeO2 film. This peak is associated with a carbonate species formed near an oxygen vacancy. The onset of cerium oxide reduction is indicated by the peak at 287.8–288.0 eV associated with carbonite species formed near Ce3+ cations. The development of surface species on the Pt–CeO2 mixed oxides suggests that restructuring of the films occurs above 300 K irrespective of Pt loadings. We do not find any adsorbed CO species associated with Pt4+ or Pt2+. The onset of Pt2+ reduction is indicated by the peak at 286.9 eV in the C 1s spectra due to CO adsorption on metallic Pt particles. The thermal stability of Pt2+ in Pt–CeO2 mixed oxide depends on Pt loading. We find excellent stability of Pt2+ for 12% Pt content in the CeO2 film, whereas at a Pt concentration of 25% in the CeO2 film, a large fraction of the Pt2+ is converted into metallic Pt particles above 300 K

Synergy between Metallic and Oxidized Pt Sites Unravelled during Room Temperature CO Oxidation on Pt/Ceria

International audiencePt-based materials are widely used as heterogeneous catalysts, in particular for pollutant removal applications. The nature of the active Pt phase responsible for CO oxidation has long been investigated with a view at designing more efficient formulations. The state of Pt has often been proposed to differ depending on experimental conditions, e.g. metallic Pt poisoned with CO being present at lower temperature before light-off, while an oxidized Pt surface prevails above light-off temperature. In stark contrast with all previous reports, we show here that both metallic and oxidized Pt are present in similar proportions under reaction conditions at the surface of ca. 1 nm nanoparticles showing high activity at 30 °C. The simultaneous presence of metallic and oxidized Pt enables a synergy between these phases. The main role of metallic Pt phase is to provide strong adsorption sites for CO, while that of oxidized Pt supposedly supplies reactive oxygen. Our results emphasize the complex dual oxidic-metallic nature of supported Pt catalysts and its evolving nature under reaction conditions, warranting a whole rethink of the mechanism of other reaction and metals, particularly redox and electrochemical reactions