Iva Matolínová

Creating single-atom Pt-ceria catalysts by surface step decoration

Single-atom catalysts maximize the utilization of supported precious metals by exposing every single metal atom to reactants. To avoid sintering and deactivation at realistic reaction conditions, single metal atoms are stabilized by specific adsorption sites on catalyst substrates. Here we show by combining photoelectron spectroscopy, scanning tunnelling microscopy and density functional theory calculations that Pt single atoms on ceria are stabilized by the most ubiquitous defects on solid surfaces—monoatomic step edges. Pt segregation at steps leads to stable dispersions of single Pt2+ ions in planar PtO4 moieties incorporating excess O atoms and contributing to oxygen storage capacity of ceria. We experimentally control the step density on our samples, to maximize the coverage of monodispersed Pt2+ and demonstrate that step engineering and step decoration represent effective strategies for understanding and design of new single-atom catalysts.

Platinum-Doped CeO2 Thin Film Catalysts Prepared by Magnetron Sputtering

The interaction of Pt with CeO(2) layers was investigated by using photoelectron spectroscopy. The 30 nm thick Pt doped CeO(2) layers were deposited simultaneously by rf-magnetron sputtering on a Si(001) substrate, multiwall carbon nanotubes (CNTs) supported by a carbon diffusion layer of a polymer membrane fuel cell and on CNTs grown on the silicon wafer by the CVD technique. The synchrotron radiation X-ray photoelectron spectra showed the formation of cerium oxide with completely ionized Pt(2+,4+) species, and with the Pt(2+)/Pt(4+) ratio strongly dependent on the substrate. The TEM and XRD study showed the Pt(2+)/Pt(4+) ratio is dependent on the film structure.

Interaction of Au with CeO2(111): A photoemission study

We have studied the adsorption of low dimensional gold on ceria, produced by evaporation onto the surface. The interaction of gold with CeO(2)(111) layers was investigated with x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and resonant photoelectron spectroscopy (RPES). Gold was deposited in steps onto a 1.5 nm thick CeO(2)(111) layer epitaxially grown on a Cu(111) substrate. The RPES showed a partial Ce(4+)-->Ce(3+) reduction, observed as a resonant enhancement of the 4f level of the Ce(3+) species. This can be explained by possible creation of a new Au(+) ionic state. The observed effects are stronger for Au deposition at room temperature than at 250 degrees C. The obtained results are in agreement with already published density functional theory calculations reporting weakening of bond between the oxygen and the Ce atoms in ceria caused by the presence of gold.

Epitaxial Cubic Ce2O3 Films via Ce-CeO2 Interfacial Reaction.

Thin films of reduced ceria supported on metals are often applied as substrates in model studies of the chemical reactivity of ceria based catalysts. Of special interest are the properties of oxygen vacancies in ceria. However, thin films of ceria prepared by established methods become increasingly disordered as the concentration of vacancies increases. Here, we propose an alternative method for preparing ordered reduced ceria films based on the physical vapor deposition and interfacial reaction of Ce with CeO2 films. The method yields bulk-truncated layers of cubic c-Ce2O3. Compared to CeO2 these layers contain 25% of perfectly ordered vacancies in the surface and subsurface allowing well-defined measurements of the properties of ceria in the limit of extreme reduction. Experimentally, c-Ce2O3(111) layers are easily identified by a characteristic 4 × 4 surface reconstruction with respect to CeO2(111). In addition, c-Ce2O3 layers represent an experimental realization of a normally unstable polymorph of Ce2O3. During interfacial reaction, c-Ce2O3 nucleates on the interface between CeO2 buffer and Ce overlayer and is further stabilized most likely by the tetragonal distortion of the ceria layers on Cu. The characteristic kinetics of the metal-oxide interfacial reactions may represent a vehicle for making other metastable oxide structures experimentally available.

A resonant photoemission applied to cerium oxide based nanocrystals

Cerium 4f level occupation determines the properties of cerium oxide based catalysts in a significant way. The Ce 4f level of nanosized cerium oxide particles was investigated with the use of resonant photoelectron spectroscopy in the Ce 4d-4f photoabsorption region. A strong interaction of ceria with different additives, e.g. Pd and Sn, led to a partial Ce4+-->Ce3+ transition that was observed as a significant resonance enhancement of 4f photoemission intensity. Increases of the CO oxidation catalytic activity were observed simultaneously. The ratio of resonance enhancement of Ce photoemission intensity DCe(3+)/DCe(4+) was used to monitor Ce(3+) and Ce(4+) state occupation. The relative parameter DCe(3+)/DCe(4+) was found to be particularly useful in the case of photoemission studies of nanopowder ceria catalysts.

Methanol adsorption and decomposition on Pt/CeO2(111)/Cu(111) thin film model catalyst.

Adsorption and desorption of methanol on Pt particles on a CeO(2)(111)/Cu(111) thin film surface and on an ion-eroded Pt(111) single crystal were investigated by X-ray photoelectron spectroscopy and soft X-ray synchrotron radiation photoelectron spectroscopy (PES). Resonant PES was used to determine the occupancy of the Ce 4f states with high sensitivity. Multilayers of methanol were adsorbed at low temperature and subsequently desorbed by heating to 600 K. Methanol desorption is accompanied by the formation of chemisorbed methoxy -OCH(3). Cerium oxide surface is strongly reduced by methanol, which was detected via the transition Ce(4+) --> Ce(3+) and an increase of the Ce 4f electronic state occupancy. Partial C-O bond scission and formation of atomic carbon was observed on the Pt particles as well as on the rough Pt(111) surface. On Pt/CeO(2)(111), all traces of surface carbon and residual hydrocarbons disappear at 500 K.

Anode Material for Hydrogen Polymer Membrane Fuel Cell: Pt – CeO2 RF-Sputtered Thin Films

The interaction of Pt with CeO 2 layers was investigated using photoelectron spectroscopy. Pt-doped CeO 2 layers 30 nm thick were deposited by radio-frequency (rf) magnetron sputtering using a composite CeO 2 -Pt target on a carbon diffusion layer of micropolymer membrane fuel cell covered by double-wall carbon nanotubes. The laboratory X-ray photoelectron spectroscopy and synchrotron radiation soft X-ray photoemission spectra showed the formation of cerium oxide with completely ionized species Pt 2+,4+ embedded in the film. A small amount of Pt 0 is present on the film surface only. Hydrogen/air fuel cell activity measurements normalized to the amount of used Pt revealed specific power up to 30 W/mg(Pt). The activity of this material is explained by high activity of embedded Pt 2+,4+ cations toward H 2 dissociation and formation of protonic hydrogen. Very high specific power and low cost together with planar deposition techniques make the material promising for microfabrication of fuel cells to power mobile systems.

Study of Pd–In interaction during Pd deposition on pyrolytically prepared In2O3

Abstract In2O3 belongs to the group of metal oxides used successfully for gas detection, with sensitivity for CO and H2 comparable with that of SnO2 and excellent for O3. The sensing properties of these sensors could be greatly improved by adding small amount of transition metals such as Pd, Pt, Au, etc. In this work, we used X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) to study the growth of Pd particles on In2O3 substrates prepared by spray pyrolysis method. Surface stoichiometry and its changes during Ar+ ion bombardment used for surface cleaning were checked with XPS and ISS. The Pd growth was studied stepwise in situ using the micro-electron beam evaporation source (MEBES) with controlled evaporation rate. XPS and ISS measurements were carried out with special attention to metal–substrate interaction (MSI), which was observed for Pd–In and resulted in the formation of Pd–In alloy with noble metal-like electronic structure. Further, we studied reduction of these samples after oxidation in air. A lower reduction temperature than with bulk PdO was observed. The results are compared with the experiment of Pd deposition performed on a pure In foil.

Au+ and Au3+ ions in CeO2 rf-sputtered thin films

The interaction of gold with CeO2 layers was investigated using photoelectron spectroscopy. 65 nm thick Au doped CeO2 layers were deposited by rf-magnetron sputtering on a Si(0 0 1) substrate by using a composite CeO2–Au target. The laboratory XPS and synchrotron radiation soft x-ray and hard x-ray photoemission spectra showed the formation of stoichiometric Ce4+ (CeO2) and the appearance of new Au+,3+ states with ionized Au species in excess of 50% of the total Au amount. Depth resolved measurements, by varying the emission angle or photon energy, indicated the formation of an Au0 overlayer and deeper Au+,3+ species. The formation of Au+,3+ ions was found to be strongly dependent on the cerium oxide stoichiometry. The amount of ionized Au can be reversibly decreased and increased by surface reduction (removal of O by sputtering) and subsequent surface re-oxidation.

A study of tungsten oxide nanowires self-organized on mica support

Self-organized straight nanowires of WO3 have been epitaxially grown on muscovite mica in low super-saturation conditions. Morphology, structure and chemical composition of the prepared nanostructures have been investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). SEM permits us to observe nanowire networks and substrate electron channeling patterns (ECP) simultaneously and thus to determine crystallographic direction of nanowire growth. The experimental results show that straight WO3 nanowires are orientated preferentially parallel to two of three high symmetry crystallographic directions of mica [100] and [110] or [100] and [Formula: see text]. XRD and XPS measurements indicated self-assembly of very thin nanowires of hexagonal tungsten bronze in the first stage of growth followed by the formation of stoichiometric WO3. The growth mechanism has been studied as a function of different preparation conditions with special focus on the role of potassium ions present on the mica surface. Based on obtained results a growth model of tungsten oxide nanowires on mica is proposed.