Nataliya Tsud

Support nanostructure boosts oxygen transfer to catalytically active platinum nanoparticles

Interactions of metal particles with oxide supports can radically enhance the performance of supported catalysts. At the microscopic level, the details of such metal-oxide interactions usually remain obscure. This study identifies two types of oxidative metal-oxide interaction on well-defined models of technologically important Pt-ceria catalysts: (1) electron transfer from the Pt nanoparticle to the support, and (2) oxygen transfer from ceria to Pt. The electron transfer is favourable on ceria supports, irrespective of their morphology. Remarkably, the oxygen transfer is shown to require the presence of nanostructured ceria in close contact with Pt and, thus, is inherently a nanoscale effect. Our findings enable us to detail the formation mechanism of the catalytically indispensable Pt-O species on ceria and to elucidate the extraordinary structure-activity dependence of ceria-based catalysts in general.

XPS, ISS and TPD study of Pd–Sn interactions on Pd–SnOx systems

Abstract The sensitivity and selectivity of SnO2 based gas sensors could be improved by doping of small amount of transition metals. In this work we used X-ray photoelectron spectroscopy, ion scattering spectroscopy, and thermal desorption techniques to investigate Pd evaporated on SnOx thin layer substrate, prepared by spray pyrolysis. The evolution of Pd/SnOx layer morphology with increasing amount of Pd deposits was studied using the XPS inelastic background shape analysis. The observations are compared to the results obtained from natural SnO2 crystal and metallic Sn substrates. A strong Pd–Sn bimetallic interaction was observed, resulting in the formation of PdSn alloy of noble metal-like electronic structure. This feature also corresponds to the presence of two CO desorption states with low energy peaks at approximately 390 K. The relation of our results with the operation mechanism of gas detection are discussed.

Adsorption of Histidine and Histidine-Containing Peptides on Au(111)

The adsorption of histidine (His) and three His-derived peptides on Au(111) has been studied by soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the nitrogen and oxygen K edges. The peptides were glycyl-histidine (Gly-His), glycyl-histidine-glycine (Gly-His-Gly), and glycyl-glycyl-histidine (Gly-Gly-His) and were adsorbed at saturated coverage on the Au(111) surface from aqueous solution. Coverages of 1 and 0.5 monolayers (ML) of His were adsorbed by evaporation in vacuum and compared with 1 ML deposited from solution. There were no significant chemical differences between the monolayers deposited in vacuum or from solution. The Au 4f core level shift indicates that a chemisorption rather than a physisorption bond is formed. In both deposited phases, His bonds to the gold surface in anionic form via the imino nitrogen atom of the imidazole ring and the oxygen atoms of the carboxylate group. N and O K-edge NEXAFS indicate that the ring and carboxylate triangle of adsorbed His are tilted at approximately 35 degrees and approximately 27 degrees, respectively, with respect to the Au(111) surface. The peptides bond to the gold surface in a mode similar to the single His molecule, via the imino and carboxylate groups, while the peptide group is at a steep angle to the surface. However, the peptides adsorb with a higher atomic density, consistent with the peptide groups being above the surface. There are also differences between Gly-His-Gly and Gly-Gly-His, implying that the sequence within the peptide has a significant influence on the bonding geometry.

CO adsorption on palladium model catalysts: XPS Pd–Al2O3 interaction study

The metal–substrate interaction (MSI) represents one of the most important effects determining the properties of supported catalysts. It is expected to be one of the driving forces of the size effect in catalysis. In this work we investigated the MSI by X-ray photoelectron spectroscopy (XPS) in the case of small Pd particles deposited on γ- and α-alumina substrates. We compared the binding energy and initial state variations as a function of Pd coverage. The initial state has been found to be shifted to higher positive values for a more strongly interacting substrate (γ-Al2O3), whilst for the inert-like sapphire substrate the MSI was less important. The initial state shift value was associated with the electron transfer in the substrate clusters direction. It was shown that the substrate–metal charge transfer could be responsible for partial CO dissociation on γ-Al2O3 supported Pd particles.

XPS and TDS study of CO interaction with Pd–AlOx systems

Abstract The metal–substrate and metal–metal interactions (MSI, MMI) represent important effects determining the properties of supported catalysts, gas sensors and gettering alloys. We investigated the MSI and MMI effects by the X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) in the case of Pd films deposited on Al 2 O 3 and Al substrates. The study shows that the particle-size dependent metal–substrate interaction plays an important role in CO–Pd chemisorption, namely, in the case of “aluminium rich” Pd–aluminium oxide interface. CO chemisorption exhibits a low-temperature desorption feature at 360 K characteristic for Pd–Al and very small Pd particles. The MSI is explained by the formation of a Pd–Al intermetallic interface exhibiting a strong bimetallic Pd–Al interaction.

Functionalization of Oxide Surfaces through Reaction with 1,3-Dialkylimidazolium Ionic Liquids.

Practical applications of ionic liquids (ILs) often involve IL/oxide interfaces, but little is known regarding their interfacial chemistry. The unusual physicochemical properties of ILs, including their exceptionally low vapor pressure, provide access to such interfaces using a surface science approach in ultrahigh vacuum (UHV). We have applied synchrotron radiation photoelectron spectroscopy (SR-PES) to the study of a thin film of the ionic liquid [C6C1Im][Tf2N] prepared in situ in UHV on ordered stoichiometric CeO2(111) and partially reduced CeO2-x. On the partially reduced surface, we mostly observe decomposition of the anion. On the stoichiometric CeO2(111) surface, however, a layer of surface-anchored organic products with high thermal stability is formed upon reaction of the cation. The suggested acid-base reaction pathway may provide well-defined functionalized IL/solid interfaces on basic oxides.

XPS and ESD study of carbon and oxygen chemistry on TiZrV NEG

Abstract In this work, properties of TiZrV getter films prepared on stainless-steel substrates by magnetron sputtering were investigated. Changes of sample surface during thermal activation and interaction of the activated getter with CO were studied by means of XPS and ESD of neutrals. The XPS measurements reflect the disappearance of the superficial oxide layer coveing air-exposed TiZrV surfaces and the transformation of graphitic carbon to carbides during thermal activation. The dissociative CO adsorption on activated TiZrV getter was observed. Neutral products of electron-stimulated desorption were registered using quadrupole mass spectrometer. The ESD yield of CO and H 2 was measured during thermal activation as well as during CO and H 2 exposure of activated getter. The difference between the ESD yields for CO and H 2 after the exposure indicates possibly different pumping mechanisms for these gases. Hydrogen seems to be incorporated into the bulk while carbon monoxide stays chemisorbed on the getter surface. Although detectable activation proceeds already at temperatures above 160°C, the activation process is reasonably fast (a few hours) at higher temperatures (above 200°C).

Bonding of histidine to cerium oxide.

Adsorption of histidine on cerium oxide model surfaces was investigated by synchrotron radiation photoemission, resonant photoemission, and near edge X-ray absorption fine structure spectroscopies. Histidine was evaporated in a vacuum onto ordered stoichiometric CeO2(111) and partially reduced CeO1.9 thin films grown on Cu(111). Histidine binds to CeO2 in anionic form via the carboxylate group and all three nitrogen atoms, with the imidazole ring parallel to the surface. The amino nitrogen atom of the imidazole ring (IM) is deprotonated, and both IM nitrogen atoms form strong bonds via π orbitals, while the α-amino nitrogen interacts with the oxide via its hydrogen atoms. In the case of CeO1.9, the deprotonation of the amino nitrogen of the imidazole ring is less pronounced and N K-edge spectra do not show a clear orientation of the ring with respect to the surface. A minor reduction of the cerium surface on adsorption of histidine was observed and explained by charge exchange as a result of hybridization of the π orbitals of the IM ring with the f and d orbitals of ceria. Knowledge of histidine adsorption on the cerium oxide surface can be used for design of mediator-less biosensors where the histidine-containing proteins can be strongly bound to the oxide surface via the imidazole side chain of this residue.

Interaction of tungsten with CeO2(111) layers as a function of temperature: a photoelectron spectroscopy study

The interaction of tungsten with CeO(2)(111) layers grown on Cu(111) was studied in the temperature range between 300 and 870 K by photoelectron spectroscopy of the core levels and resonant valence band spectroscopy. The interaction was found to be very strong even at 300 K, leading to the formation of cerium tungstate Ce(6)WO(12) in which the metal atoms were in Ce(3+) and W(6+) chemical states. The growth was limited by the diffusion of W atoms into the ceria layer, so subsequent tungsten deposition led to formation of W suboxides with consecutively lower chemical oxidation states, i.e. W(4+), W(2+) and metallic W(0) with an almost negligible contribution of W(5+). Step-wise annealing of the layer showed that due to stimulated diffusion of tungsten into ceria at higher temperature, Ce(6)WO(12) was formed more easily. Larger W overlayer thicknesses needed higher annealing temperature to promote diffusion. The thickest sample studied, 1.4 nm W/CeO(2), was transformed by annealing to 870 K to the Ce(6)WO(12)/W system with a tungsten monoxide (WO) interface, whereas the rest of the tungsten was converted to the W(6 + ) oxidation state.

XPS and Ellipsometric Study of DLC/Silicon Interface

Diamond-like carbon (DLC) films were prepared by plasma-enhanced chemical vapour deposition from the mixture of methane and argon on silicon substrates. Films were characterised by multi-sample modification of variable angle spectroscopic ellipsometry. Ellipsometry showed that there is a transition interlayer between the DLC film and the silicon substrate that cannot be attributed to a thin silicon dioxide layer but rather to amorphous silicon and/or a modified oxide layer. TRIM calculations revealed that argon or carbon ions could not produce a significant layer of amorphous silicon because the depth of target atom displacements is below the thickness of a native oxide layer. The chemical composition of a DLC film profile including a DLC/silicon interface was studied by X-ray photoelectron spectroscopy (XPS) coupled with argon sputtering of the 34 nm thick DLC film. The DLC/silicon interface composed of less than 6% of oxygen and a gradually decreasing and increasing carbon and silicon percentage, respectively.