Exploiting micro-scale structural and chemical observations in real time for understanding chemical conversion: LEEM/PEEM studies over CeOx-Cu(111)
Proper consideration of length-scales is critical for elucidating active sites/phases in heterogeneous catalysis, revealing chemical function of surfaces and identifying fundamental steps of chemical reactions. Using the example of ceria thin films deposited on the Cu(111) surface, we demonstrate the benefits of multi length-scale experimental framework for understanding chemical conversion. Specifically, exploiting the tunable sampling and spatial resolution of photoemission electron microscopy, we reveal crystal defect mediated structures of inhomogeneous copper–ceria mixed phase that grow during preparation of ceria/Cu(111) model systems. The density of the microsized structures is such that they are relevant to the chemistry, but unlikely to be found during investigation at the nanoscale or with atomic level investigations. Our findings highlight the importance of accessing micro-scale when considering chemical pathways over heteroepitaxially grown model systems.
Redox-mediated conversion of atomically dispersed platinum to sub-nanometer particles
(článek, J. Mater. Chem. A)
The stability and the conversion of atomically dispersed Pt2+ species to sub-nanometer Pt particles has been investigated as a function of the Sn concentration in Pt‒CeO2 films by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and angle-resolved X-ray photoelectron spectroscopy in combination with density functional calculations. The deposition of Sn onto the Pt‒CeO2 films triggers the reduction of Ce4+ cations to Ce3+ yielding Sn2+ cations. Consecutively, the redox coupling between the Ce3+ and Pt2+ species triggers the reduction of Pt2+ species yielding sub-nanometer Pt particles. The onset of reduction of Pt2+ species is directly related to the concentration of Ce3+ centers which, in turn, is controlled by the concentration of Sn2+ cations in the Pt‒CeO2 film. On average, formation of 6 Ce3+ centers corresponding to the adsorption of 3 Sn atoms gives rise to the reduction of one Pt2+ species. The analysis of the depth distribution of Sn atoms in the Pt‒CeO2 films revealed preferential adsorption of Sn2+ at the surface followed by diffusion of Sn2+ ions into the bulk at higher Sn coverages. Density functional modeling suggested that adsorption of three Sn atoms in the vicinity of the Pt2+ species results in a rearrangement of the local coordination accompanied by substantial destabilization of the Pt2+ species relative to the adsorption of Pt0 atoms. The formation of sub-nanometer Pt particles is coupled with re-oxidation of two Ce3+ centers per one Pt2+ species reduced. Annealing of the Pt‒CeO2 films in the presence of metallic Sn also leads to reduction of the Pt2+ species due to thermally triggered oxidation of metallic Sn residues followed by diffusion of Sn2+ into the bulk. Annealing of the Pt‒CeO2 films to temperatures above 600 K results in a loss of Sn yielding sub-nanometer Pt particles supported on nearly stoichiometric and Sn-free CeO2 films.
Unraveling the surface state and composition of highly selective nanocrystalline Ni–Cu alloy catalysts for hydrodeoxygenation of HMF
(článek, Catal. Sci. Technol.)
The selective hydrodeoxygenation (HDO) of 5-hydroxymethylfurfural (HMF) to 2,5-dimethylfuran (DMF) is an important step in cellulosic biomass upgrading to biofuels, where bimetallic oxophilic catalysts have shown promising performance. Well controlled bimetallic NiCu and NiCu3nanocrystals supported on carbon are shown to give high yields and selectivities to DMF. To shed light on the active phase, near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) was used to characterize the surface composition of these highly selective base-metal catalysts under reducing conditions relevant to the HDO reaction. Reactions were performed in a continuous flow reactor under reasonable conditions of 33 bar and 180 °C. The Ni alloys were significantly more selective for DMF compared to monometallic Ni or Cu catalysts. With a well-controlled surface composition, the nanocrystal NiCu3/C catalyst exhibited a maximum DMF yield of 98.7%. NAP-XPS characterization showed that the Ni–Cu nanocrystals were completely reduced below 250 °C in H2; this, together with bulk thermodynamic calculations, implies that the catalysts were completely reduced under the reaction conditions. NAP-XPS also indicated that the NiCu3 nanocrystal structure consisted of a Cu-rich core and a 1:1 molar Ni:Cu shell.
Micro-contacted self-assembled tungsten oxide nanorods for hydrogen gas sensing
(článek, International Journal of Hydrogen Energy)
Electron beam lithography was used to fabricate platinum μ-contacts over tungsten oxide nanorods formed on a mica substrate. This made possible the measurement of sensorial response of these self-assembled tungsten oxide nanorods to hydrogen gas for the first time. The nanorods were prepared by thermal evaporation from an oxide source. Consequently, two types of conductometric sensors were assembled: a) percolating network of nanorods and b) set of individually contacted WO3 nanorods. The preparation procedures are described in detail and the comparison of response of both types of assemblies is given. The first sensorial measurements revealed a good response of the b) type of sensor and the minimum repeatedly detected concentration of H2 was 50 ppm.
Candle Soot as Efficient Support for Proton Exchange Membrane Fuel Cell Catalyst
(článek, Fuel Cells)
Candle soot deposited from the candle flame was used as a catalyst support for an anode catalyst in a proton exchange membrane fuel cell. The results showed that Pt/soot hybrids prepared by magnetron sputtering of 5 nm platinum films on candle soot exhibit very high mass activity in the fuel cell, which is more than one order of magnitude higher than that for commercial catalyst. The elementary preparation, high surface-to-volume ratio, good conductivity and hydrophobicity make candle soot a promising type of the support for PEMFCs catalyst.
Morphology and CO Oxidation Reactions on Anion Doped CeOXFY/Rh(111) and CeOX/Rh(111) Inverse Catalysts
(článek, J. Phys. Chem. C)
Doping cerium oxide with additives is a common procedure that improves stability of cerium oxide-based catalysts. We prepared fluorine-doped cerium oxide samples in the form of inverse catalysts on Rh(111) and compared their electronic, chemical, and morphological properties with fluorine-free CeOX samples. By means of X-ray photoelectron spectroscopy (XPS), we followed the formation of oxygen vacancies and the depletion of fluorine after exposure of CeOXFY to CO and O2 gases at elevated temperatures. According to Ce 3d XPS spectra, the ability to create oxygen vacancies is not altered by fluorine atoms. Our results from low energy electron diffraction (LEED) and atomic force microscopy (AFM) show that fluorine affects mainly the morphology of the layers. Unlike the CeO2 layers, fluorine-doped samples form 3D islands, which are partially rotated with respect to Rh [11̅0] direction due to stretching of the lattice constant caused by cerium oxide reduction. The possibility for creation stable Ce3+ sites without reducing the oxygen storage capacity makes anion doping a perspective tool for defect engineering in cerium oxide-based catalysts.
Magnetron sputtered Ir thin film on TiC-based support sublayer as low-loading anode catalyst for proton exchange membrane water electrolysis
(článek, Int. J. Hydrog. Energ.)
Proton exchange membrane (PEM) water electrolysis (PEMWE) is getting more attention in recent years as a promising alternative in context of energy storage from renewables. However high prices of platinum and iridium, currently considered to be the state-of-the-art electrocatalysts, prevent wider commercialization of this technology. In this paper, we present unconventional and cost-effective preparation method of anode catalyst, containing low amount of noble metal. Thin Ir film is magnetron sputtered on TiC-based support sublayer, hot-pressed on anode side of Nafion® N115 PEM. Following three parameters were systematically varied and their impact on PEMWE in-cell performance was evaluated: total TiC-based support material loading on the PEM, ionomer content within the support sublayer and Ir catalyst loading on top of the support sublayer. In addition, TiC-based sublayer underwent accelerated aging procedure, followed by photoelectron spectral analysis to prove its ability to withstand high anodic potentials. Remarkable PEMWE in-cell performances were obtained, considering amount of used Ir; 1.74 V (with ∼80 μg cm−2 of Ir), 1.72 V (with ∼160 μg cm−2 of Ir) and 1.71 V (with ∼240 μg cm−2 of Ir) at 1 A cm−2 and 80 °C.
The effect of sulfur dioxide on the activity of hierarchical Pd-based catalysts in methane combustion
(článek, Appl. Catal. B)
SO2 poisoning of methane oxidation over alumina-supported, Pd@CexZr1-xO2 nanoparticle catalysts was systematically studied by means of advanced PhotoElectron Spectroscopy (PES) methods. The Pd@CexZr1-xO2 units were synthesized and deposited on two modified-alumina supports, i.e. high surface area modified alumina and a model alumina prepared by Atomic Layer Deposition (ALD) of alumina on Indium Tin Oxide (ITO)/quartz slides. The model support was designed to be suitable for PES analysis and was stable to high temperature treatments (850 °C). Characterization of the high-surface-area (HSA) catalysts by X-Ray Diffraction (XRD), N2 physisorption, CO chemisorption and Transmission Electron Microscopy (TEM) indicated formation of CeO2–ZrO2 (CZ) mixed-oxide crystallites that stabilize the Pd active phase against sintering. Correlation of methane-oxidation rates with PES results demonstrated two distinct mechanisms for deactivation by SO2. Below 450 °C, the presence of SO2 in the feed lead to partial reduction of the active PdO phase and to the formation of sulfates on the Pd. Above 500 °C, poisoning by SO2 was less severe due to spillover of the sulfates onto the oxide promoter. Pd@ZrO2 catalysts showed the best resistance to SO2 poisoning, outperforming analogous Pd@CZ mixed-oxide catalysts, because there was less sulfate formation and the sulfates that did form could be removed during regeneration.
Ambient Pressure XPS and IRRAS Investigation of Ethanol Steam Reforming on Ni-CeO2(111) Catalysts: An In Situ Study of C-C and O-H Bond Scission
(článek, Phys. Chem. Chem. Phys.)
Ambient-Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Infrared Reflection Absorption Spectroscopy (AP-IRRAS) have been used to elucidate the active sites and mechanistic steps associated with the ethanol steam reforming reaction (ESR) over Ni-CeO2(111) model catalysts. Our results reveal that surface layers of the ceria substrate are both highly reduced and hydroxylated under reaction conditions while the small supported Ni nanoparticles are present as Ni0/NixC. A multifunctional, synergistic role is highlighted in which Ni, CeOx and the interface provide an ensemble effect in the active chemistry that leads to H2. Ni0 is the active phase leading to both C-C and C-H bond cleavage in ethanol but it is also responsible for carbon accumulation. On the other hand, CeOx is important for the deprotonation of ethanol/water to ethoxy and OH intermediates. The active state of CeOx is a Ce3+(OH)x compound, that results from extensive reduction by ethanol and the efficient dissociation of water. Additionally, we discuss an important insight into the stability and selectivity of the catalyst by its effective water dissociation, where the accumulation of surface carbon can be mitigated by the increased presence of surface OH groups. The co-existence and cooperative interplay of Ni0 and Ce3+(OH)x through a metal-support interaction, facilitates oxygen transfer, activation of ethanol/water as well as the removal of coke.
Experimental and Theoretical Study on the Electronic Interaction between Rh Adatoms and CeOx Substrate in Dependence on a Degree of Cerium Oxide Reduction
(článek, J. Phys. Chem. C)
The electronic metal–substrate interaction plays an important role in the surface reactions on supported metal nanoparticles. We study the interaction between rhodium clusters and cerium oxide substrate having various stoichiometries, CeOx (2.00 > x > 1.67), by means of photoelectron spectroscopy. Our results show that rhodium deposited on substrates with stoichiometry of 2.00 > x > 1.93 induces reduction of cerium oxide. On the other hand, cerium oxide with a higher degree of reduction (1.93 > x > 1.67) becomes partially oxidized after the Rh deposition. Density functional theory simulations of Rh adatom adsorbed on the CeO2(111) surface and on an oxygen vacancy at the CeOx(111) surface show that there is a charge transfer between rhodium and cerium oxide substrate. While Rh adsorption on the CeO2(111) surface leads to electron depletion at the Rh adatom and cerium oxide reduction, Rh adsorption on the oxygen vacancy at CeOx(111) leads to electron accumulation about the Rh adatom and partial oxidation of cerium oxide. The results clearly demonstrate that the electronic metal–substrate interaction occurs on Rh/CeOx systems and strongly depends on the stoichiometry of cerium oxide. These findings could be beneficial for designing catalysts with specific properties.
Controlling Heteroepitaxy by Oxygen Chemical Potential: Exclusive Growth of (100) Oriented Ceria Nanostructures on Cu(111)
(článek, J. Phys. Chem. C)
A novel and simple method is presented for the preparation of a well-defined CeO2(100) model system on Cu(111) based on the adjustment of the Ce/O ratio during growth. The method yields micrometer-sized, several nanometers high, single-phase CeO2(100) islands with controllable size and surface termination that can be benchmarked against the known (111) nanostructured islands on Cu(111). Furthermore, we demonstrate the ability to adjust the Ce to O stoichiometry from CeO2(100) (100% Ce4+) to c-Ce2O3(100) (100% Ce3+), which can be readily recognized by characteristic surface reconstructions observed by low-energy electron diffraction. The discovery of the highly stable CeOx(100) phase on a hexagonally close packed metal surface represents an unexpected growth mechanism of ceria on Cu(111), and it provides novel opportunities to prepare more elaborate models, benchmark surface chemical reactivity, and thus gain valuable insights into the redox chemistry of ceria in catalytic processes.
Phosphorus poisoning during wet oxidation of methane over Pd@CeO2/Graphite model catalysts
(článek, Appl. Catal. B: Enviromental)
The influence of phosphorus and water on methane catalytic combustion was studied over Pd@CeO2 model catalysts supported on graphite, designed to be suitable for X-ray Photoelectron Spectroscopy / Synchrotron Radiation Photoelectron Spectroscopy (XPS/SRPES) analysis. In the absence of P, the catalyst was active for the methane oxidation reaction, although introduction of 15% H2O to the reaction mixture did cause reversible deactivation. In the presence of P, both thermal and chemical aging treatments resulted in partial loss of activity due to morphological transformation of the catalyst, as revealed by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analysis. At 600 °C the combined presence of PO43− and water vapor caused a rapid, irreversible deactivation of the catalyst. XPS/SRPES analysis, combined with operando X-ray Absorption Near Edge Structure (XANES) and AFM measurements, indicated that water induces severe aggregation of CeO2 nanoparticles, exposure of CePO4 on the outer layer of the aggregates and incorporation of the catalytic–active Pd nanoparticles into the bulk. This demonstrates a temperature–activated process for P-poisoning of oxidation catalysts in which water vapor plays a crucial role.
Influence of the Ce - F Interaction on Cerium Photoelectron Spectra in CeOxFy Layers
(článek, Chem. Phys. Lett.)
Fluorine atoms have been reported as an impurity in cerium oxide; however, their influence on the electronic structure and chemical properties is not well understood yet. We show the changes in Ce 3d photoelectron spectra features induced by fluorine. We introduce a fitting procedure which allows to evaluate the concentration of fluorine in the cerium oxide layer. A correlation between the fluorine concentration and the characteristic Ce-F interaction doublets added to Ce 3d spectra fits is presented in this paper. We studied also the interaction of the prepared layers with small doses of CO and O2 gases at elevated temperatures.
Mechanistic Insights of Ethanol Steam Reforming over Ni-CeOx(111): The Importance of Hydroxyl Groups for Suppressing Coke Formation
(článek, J. Phys. Chem. C)
We have studied the reaction of ethanol and water over Ni-CeO2-x(111) model surfaces to elucidate the mechanistic steps associated with the ethanol steam reforming (ESR) reaction. Our results provide insights about the importance of hydroxyl groups to the ESR reaction over Ni-based catalysts. Systematically, we have investigated ethanol reaction on Ni-CeO2-x(111) at varying Ce3+ concentrations (CeO1.8-2.0) with absence/presence of water using a combination of soft X-ray photoelectron spectroscopy (sXPS) and temperature programmed desorption (TPD). Consistent with previous reports, upon annealing, metallic Ni formed on reduced ceria while NiO was the main component on fully oxidized ceria. Ni0 is the active phase leading to both the C-C and C-H cleavage of ethanol but is also responsible for carbon accumulation or coking. We have identified a Ni3C phase that formed prior to the coke formation. At temperatures above 600K, the lattice oxygen from ceria and the hydroxyl groups from water act cooperatively in the removal of the coke through a strong metal-support interaction between nickel and ceria that facilitates oxygen transfer.
In Situ and Theoretical Studies for the Dissociation of Water on an Active Ni/CeO2 Catalyst: Importance of Strong Metal–Support Interactions for the Cleavage of O–H Bonds
(článek, 2015 Angewandte Chemie)
Water dissociation is crucial in many catalytic reactions on oxide-supported transition-metal catalysts. Supported by experimental and density-functional theory results, the effect of the support on O-H bond cleavage activity is elucidated for nickel/ceria systems. Ambient-pressure O 1s photoemission spectra at low Ni loadings on CeO2 (111) reveal a substantially larger amount of OH groups as compared to the bare support. Computed activation energy barriers for water dissociation show an enhanced reactivity of Ni adatoms on CeO2 (111) compared with pyramidal Ni4 particles with one Ni atom not in contact with the support, and extended Ni(111) surfaces. At the origin of this support effect is the ability of ceria to stabilize oxidized Ni2+ species by accommodating electrons in localized f-states. The fast dissociation of water on Ni/CeO2 has a dramatic effect on the activity and stability of this system as a catalyst for the water-gas shift and ethanol steam reforming reactions.
Faceting Transition at the Oxide-Metal Interface: (13 13 1) Facets on Cu(110) Induced by Carpet-Like Ceria Overlayer
(článek, 2015 J. Phys. Chem. C)
Structural transitions affect electronic structure of materials and consequently their catalytic properties. We report the observation of faceting of a low index metal surface at an oxide–metal interface in a catalytically relevant system of ceria on Cu(110). We observe formation of (13 13 1) facets on the Cu(110) surface covered by ceria upon annealing above 500 °C. The faceting transition occurs in spite of a weak adsorbate–substrate interaction, which manifests itself in ceria adopting a carpet-like growth mode. We rationalize the surface faceting under such conditions by oxide overlayer-induced modification of the roughening temperature of Cu(110). We describe the carpet-like ceria film in terms of elasticity theory and show that the specific structure of the ceria supported on Cu(13 13 1) can lead to a periodic modulation of the electronic structure of the ceria–copper interface. The reported structural transition indicates that surface faceting of metal can occur at the oxide–metal interface at relatively low temperatures with possible consequences for the catalytic properties of the interface. The oxide overlayer induced faceting transition can be expected to occur for other oxide–metal combinations and, as such, has perspective applications in preparation of functional oxide–metal nanostructures.
Structural and electronic properties of manganese-doped Bi2Te3 epitaxial layers
(článek, 2015 New J. Phys. 17 013028 )
We show that in manganese-doped topological insulator bismuth telluride layers, Mn atoms are incorporated predominantly as interstitials in the van der Waals gaps between the quintuple layers and not substitutionally on Bi sites within the quintuple layers. The structural properties of epitaxial layers with Mn concentration of up to 13% are studied by high-resolution x-ray diffraction, evidencing a shrinking of both the in-plane and out-of plane lattice parameters with increasing Mn content. Ferromagnetism sets in for Mn contents around 3% and the Curie temperatures rises up to 15 K for a Mn concentration of 9%. The easy magnetization axis is along the c-axis perpendicular to the (0001) epilayer plane. Angle-resolved photoemission spectroscopy reveals that the Fermi level is situated in the conduction band and no evidence for a gap opening at the topological surface state with the Dirac cone dispersion is found within the experimental resolution at temperatures close to the Curie temperature. From the detailed analysis of the extended x-ray absorption fine-structure experiments (EXAFS) performed at the MnK-edge, we demonstrate that the Mn atoms occupy interstitial positions within the van der Waals gap and are surrounded octahedrally by Te atoms of the adjacent quintuple layers.
Role of Oxygen in Acetic Acid Decomposition on Pt(111)
We have investigated the role of co-adsorbed atomic oxygen during decomposition of acetic acid on Pt(111) by means of temperature programmed desorption (TPD) and synchrotron radiation photoelectron spectroscopy (SRPES). Reaction mechanisms have been established through identification of desorbing products and surface species formed during decomposition of acetic acid, both on Pt(111) and oxygen pre-exposed p(2×2)–O/Pt(111). Acetate and molecularly adsorbed acetic acid are formed on both samples during adsorption of acetic acid at 150 K. On p(2×2)–O/Pt(111), however, surface acetyl is identified as the principal species. The major decomposition channel for acetate and acetic acid involves formation of ketene and acetaldehyde at 222 K and this reaction is not affected by co-adsorbed oxygen. In the following reactions, partial decomposition of acetaldehyde yielded ethylene, ethylidene, ethylidyne, and small amounts of CO and methoxy on both samples. Large amounts of acetyl are formed on p(2×2)–O/Pt(111). Above 222 K, decomposition of acetate on Pt(111) yields acetic acid, hydrogen, methane, and CO. In contrast, the species desorbing from p(2×2)–O/Pt(111) are the products of acetyl decomposition. In particular, the reaction of acetyl with atomic oxygen and surface hydroxyl groups yields methanol and acetic anhydride at 300 and 450 K, and methane and CO2 at 390 K. Decomposition of acetic acid on both Pt(111) and p(2×2)–O/Pt(111) results in surface carbon from decomposition of ethylidyne and partial C–C bond cleavage in acetyl.
Hydrogen activation on Pt–Sn nanoalloys supported on mixed Sn–Ce oxide films
We have studied the interaction of H2 with Pt–Sn nanoalloys supported on Sn–Ce mixed oxide films of different composition by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The model catalysts are prepared in a three step procedure that involves (i) the preparation of well-ordered CeO2(111) films on Cu(111) followed by subsequent physical vapor deposition of (ii) metallic Sn and (iii) metallic Pt. The formation of mixed Sn–Ce oxide is accompanied by partial reduction of Ce4+ cations to Ce3+. Pt deposition leads to the formation of Pt–Sn nanoalloys accompanied by the partial re-oxidation of Ce3+ to Ce4+. Subsequent annealing promotes further Pt–Sn alloy formation at expense of the Sn content in the Sn–Ce mixed oxide. Adsorption of H2 on Pt–Sn/Sn–Ce–O at 150 K followed by stepwise annealing results in reversible reduction of Ce cations caused by spillover of dissociated hydrogen between 150 and 300 K. Above 500 K, annealing of Pt–Sn/Sn–Ce–O in a hydrogen atmosphere results in irreversible reduction of Ce cations. This reduction is caused by the reaction of hydrogen with oxygen provided by the mixed oxide substrate via the reverse spillover to Pt–Sn nanoalloy. The extent of the hydrogen and oxygen spillover strongly depends on the amount of Sn in the Sn–Ce mixed-oxide. We observe an enhancement of hydrogen spillover on Pt–Sn/Sn–Ce–O at low Sn concentration as compared to Sn-free Pt/CeO2. Although the extent of hydrogen spillover on Pt–Sn/Sn–Ce–O with high Sn concentration is comparable to Pt/CeO2, the reverse oxygen spillover is substantially suppressed on these samples.
Pt–CeOx thin film catalysts for PEMFC
Platinum is the mostly used element in catalysts for fuel cell technology, but its high price limits large-scale applications. Platinum doped cerium oxide represents an alternative solution due to very low loading, typically few micrograms per 1 cm2, at the proton exchange membrane fuel cell (PEMFC) anode. High efficiency is achieved by using magnetron sputtering deposition of cerium oxide and Pt of 30 nm thick nanoporous films on large surface carbon nanoparticle substrates. Thin film techniques permits to grow the catalyst film characterized by highly dispersed platinum, mostly in ionic Pt2+ state. Such dispersed Pt species show high activity and stability. These new materials may help to substantially reduce the demand for expensive noble-metals in catalytic applications. We measured Pt–CeOx thin film anode catalyst activity in a hydrogen PEMFC and compared it with performance of a standard reference cell. Photoelectron spectroscopy was used to investigate chemical composition of Pt–CeOx induced by the catalyst interaction with hydrogen. Nanostructured character of the catalyst was confirmed by electron microscopy.
RHEED and XPS study of cerium interaction with SnO2 (110) surface
Interaction of cerium with thin epitaxial film of tin dioxide with (110) surface orientation was investigated by methods of X-ray Photoelectron Spectroscopy (XPS) and Reflection High Energy Electron Diffraction (RHEED). Strong interaction of cerium and tin leads to cerium diffusion into the tin dioxide bulk and formation of tin–cerium mixed oxide indicated by highly non-stoichiometric cerium oxide Ce 3d core level spectrum. This process is more pronounced with higher temperature during the deposition of cerium. With higher amount of deposited cerium, the tin dioxide substrate is gradually saturated with cerium atoms and the cerium atoms start to create 3D epitaxial clusters of cerium oxide with the (100) and (111) crystallographic planes parallel to the surface. The clusters show a higher oxidation state than the cerium atoms located in the tin oxide bulk. Oxygen required for the creation of these clusters comes from the tin dioxide substrate, which is partially reduced to metallic tin in this process. At the end of the experiment, we obtained a multilayer system consisting of layers of partially reduced tin dioxide, tin–cerium mixed oxide and top layer of partially reduced epitaxial grains of cerium oxide.
Ordered Phases of Reduced Ceria As Epitaxial Films on Cu(111)
Changes of stoichiometry in reducible oxides are inevitably accompanied by changes of the oxide structure. We study the relationship between the stoichiometry and the structure in thin epitaxial films of reduced ceria, CeOx, 1.5 ≤ x ≤ 2, prepared via an interface reaction between a thin ceria film on Cu(111) and a Ce metal deposit. We show that the transition between the limiting stoichiometries CeO2 and Ce2O3 is realized by equilibration of mobile oxygen vacancies near the surface of the film, while the fluorite lattice of cerium atoms remains unchanged during the process. We identify two surface reconstructions representing distinct oxygen vacancy ordering during the transition, a (√7 × √7)R19.1° reconstruction representing a bulk termination of the ι-Ce7O12 and a (3 × 3) reconstruction representing a bulk termination of CeO1.67. Due to the special property to yield ordered phases of reduced ceria the interface reaction between Ce and thin film ceria represents a unique tool for oxygen vacancy engineering. The perspective applications include advanced model catalyst studies with both the concentration and the coordination of oxygen vacancies precisely under control.
Comment on "Ordered Phases of Reduced Ceria As Epitaxial Films on Cu(111)"
(komentář - a přidání podstatných dat navíc - k vlastnímu článku, Journal of Physical Chemistry C, 2014, 118 (9), 5058−5059 )
Jelikož jde o komentář k vlastnímu článku, příspěvek nemá samostatný abstrakt.
Preparation of Magnetron Sputtered Thin Cerium Oxide Films with a Large Surface on Silicon Substrates Using Carbonaceous Interlayers
The study focuses on preparation of thin cerium oxide films with a porous structure prepared by rf magnetron sputtering on a silicon wafer substrate using amorphous carbon (a-C) and nitrogenated amorphous carbon films (CNx) as an interlayer. We show that the structure and morphology of the deposited layers depend on the oxygen concentration in working gas used for cerium oxide deposition. Considerable erosion of the carbonaceous interlayer accompanied by the formation of highly porous carbon/cerium oxide bilayer systems is reported. Etching of the carbon interlayer with oxygen species occurring simultaneously with cerium oxide film growth is considered to be the driving force for this effect resulting in the formation of nanostructured cerium oxide films with large surface. In this regard, results of oxygen plasma treatment of a-C and CNx films are presented. Gradual material erosion with increasing duration of plasma impact accompanied by modification of the surface roughness is reported for both types of films. The CNx films were found to be much less resistant to oxygen etching than the a-C film.
The Mechanism of Hydrocarbon Oxygenate Reforming: C--C Bond Scission, Carbon Formation, and Noble-Metal-Free Oxide Catalysts
(článek, ChemSusChem, 2014, 7 (1), pp 77–81 )
Towards a molecular understanding of the mechanism behind catalytic reforming of bioderived hydrocarbon oxygenates, we explore the C--C bond scission of C2 model compounds (acetic acid, ethanol, ethylene glycol) on ceria model catalysts of different complexity, with and without platinum. Synchrotron photoelectron spectroscopy reveals that the reaction pathway depends very specifically on both the reactant molecule and the catalyst surface. Whereas C--C bond scission on Pt sites and on oxygen vacancies involves intermittent surface carbon species, the reaction occurs without any carbon formation and deposition for ethylene glycol on CeO2(111).
Evidence for two growth modes during tungsten oxide vapor deposition on mica substrates
The morphology, the structure and the orientation of tungsten oxide nanorods grown on mica are investigated as a function of the deposition time. The previous results are recalled to point out the changes with the nanorod thickness. The investigations were conducted by Atomic Force Microscopy (AFM) and Reflection High Energy Electron Diffraction (RHEED). The results evidence two successive growth modes. In the first stage thin and long nanorods were formed. They grew layer by layer with a hexagonal tungsten bronze structure and two different (1−10) and (2–10) planes parallel to the mica surface. In the second stage, as the deposition time increased thin nanorods with the (1−10) orientation grew in thickness when the others preserve their morphology and structure. In the discussion the difference between the two growth modes is emphasized. In the first stage the nanorod growth proceeds mainly by the surface diffusion of KxWO3 species. In the second stage the growth is due to the by direct impinging of WO3 molecules on some thin nanorods having already the (1−10) orientation, leading to growth of thick nanorods with a monoclinic structure.
Sol–gel preparation of alumina stabilized rare earth areo- and xerogels and their use as oxidation catalysts
A new sol–gel synthesis route for rare earth (Ce and Pr) alumina hybrid aero- and xerogels is presented which is based on the so-called epoxide addition method. The resulting materials are characterized by TEM, XRD and nitrogen adsorption. The results reveal a different crystallization behavior for the praseodymia/alumina and the ceria/alumina gel. Whereas the first remains amorphous until 875° C, small ceria domains form already after preparation in the second case which grow with increasing calcination temperature. The use of the calcined gels as CO oxidation catalysts was studied in a quartz tube (lab) reactor and in a (slit) microreactor and compared to reference catalysts consisting of the pure rare earth oxides. The Ce/Al hybrid gels exhibit a good catalytic activity and a thermal stability against sintering which was superior to the investigated reference catalyst. In contrast, the Pr/Al hybrid gels show lower CO oxidation activity which, due to the formation of PrAlO3, decreased with increasing calcination temperature.
HAXPES study of CeOx thin film–silicon oxide interface
Investigation of cerium oxide thin film deposition on silicon and silicon oxide is important due to many possible applications of cerium oxide based micro-systems in electronics and catalysis. rf-Magnetron sputtering is technologically the most suitable method of preparation of such systems. Mechanism of film growth is strongly influenced by interaction of Ce atoms with the substrate and their oxidation by oxygen containing rf plasma. We show using hard X-ray photoelectron spectroscopy with high information depth that cerium is reducing silicon oxide by forming complex silicate phase at the interface with Ce in the 3+ state. For this reason composition of very thin films of cerium oxide is strongly influenced by thin film–substrate interaction. A coating of the silicon oxide substrate by an intermediate thin carbon film provides conductive substrate for electrocatalytic applications and decreases the silicon oxide substrates–cerium oxide interaction essentially.
Nové katalyzátory pro palivové články
(populární článek, Vesmír 2013/7-8, vol.92(143), pp 421-423)
The current heavy traffic causes serious air pollution problems, and in addition the sources of conventional fuels would be worked out soon. Hydrogen represents an alternative carrier of energy which can be produced and distributed worldwide. The efficient way how to transform hydrogen to electricity is a use of fuel cells where electricity is produced using chemical reactions. In case of very promising proton exchange membrane fuel cells the chemical reactions are catalysed by catalysts which are usually made from noble metals. Therefore development of novel cheaper catalysts is crucial for future massive utilisation of such source of clean energy.