Spiros Zafeiratos

When a Metastable Oxide Stabilizes at the Nanoscale: Wurtzite CoO Formation upon Dealloying of PtCo Nanoparticles

Ambient pressure photoelectron and absorption spectroscopies were applied under 0.2 mbar of O2 and H2 to establish an unequivocal correlation between the surface oxidation state of extended and nanosized PtCo alloys and the gas-phase environment. Fundamental differences in the electronic structure and reactivity of segregated cobalt oxides were associated with surface stabilization of metastable wurtzite-CoO. In addition, the promotion effect of Pt in the reduction of cobalt oxides was pronounced at the nanosized particles but not at the extended foil.

The role of carbon species in heterogeneous catalytic processes: an in situ soft x-ray photoelectron spectroscopy study

High pressure x-ray photoelectron spectroscopy (XPS) is used to characterize heterogeneous catalytic processes. The success of the new technique is based on the possibility of correlating the catalytic activity and the electronic structure of an active surface. The dynamic character of a catalyst surface can be demonstrated impressively by this technique. In this contribution the basics of high pressure XPS will be discussed. Three examples of heterogeneous catalytic reactions are presented in this contribution: the selective hydrogenation of 1-pentyne and the oxidation of ethylene over Pd based catalysts and the dehydrogenation of n-butane over V based catalysts. It is shown that the formation of subsurface carbons plays an important role in all the examples. The carbon incorporated changes the electronic structure of the surface and so controls the selectivity of the reaction. A change of the educts in the reaction atmosphere induces modifications of the electronic surface structure of the operation catalysts.

XPS characterization of the electrochemically generated O species on a Au electrode evaporated on Y2O3-stabilized ZrO2(100)

Abstract X-ray photoelectron spectroscopy (XPS) was used to study the electrochemically generated oxygen species on a gold surface which was evaporated on an yttria stabilized zirconia (YSZ) (100) single crystal solid electrolyte. It was found that electrochemical oxygen pumping to the gold electrode surface, causes the formation of two oxygen chemisorbed states. Their binding energies, ranging between 528.8–529.2 and 530.2–531.2 eV, are affected by the Au surface treatment (Ar + sputtering) and the existence of preadsorbed OH ad radicals.

XPS study of Pd particle growth on different alumina surfaces

Abstract X-ray Photoelectron spectroscopy (XPS) was used to investigate the Pd particle formation on Al2O3 substrates. Palladium was deposited step by step in situ using an electron beam evaporator on single crystalline alumina specimens which had been cleaned by heating in air. The (0001), (1–102) and (11–20) orientations of alumina were used for Pd deposition with no further treatment. In addition, the (0001) and (1–102) substrates were also Ar∗ ion sputtered before deposition, to change the surface stoichiometry and order. The Pd growth process was monitored by means of the modified Auger parameter and the Full Width at Half Maximum of the Pd3d 5 2 peak. The results obtained on the different substrates are compared and the influence of the substrate on the Pd particle growth is discussed.

A photoelectron spectroscopy study of Au thin films on ZrO2 (100)

Abstract The formation and electronic properties of Au deposits on yttria-stabilized ZrO 2 (100) were studied by X-ray and ultra-violet photoelectron spectroscopy (XPS, UPS) from submonolayer coverages up to a fully conducting thin film across the substrate at room temperature and at 680 K. The results show that at both temperatures Au exhibits a 3D particle growth, which is more pronounced at 680 K. The substrate temperature affects the particle density and average coordination number of the gold atoms, however, the interaction between gold and the substrate is very weak at both temperatures. Annealing up to 780 K of Au films deposited at 300 K, results only in particle coalescence and does not affect the chemical state of either Au or zirconia.

Study of CO adsorption on Rh/alumina model catalysts in dependence on substrate orientation

Abstract The interaction of a CO molecular beam with rhodium vapour-deposited onto (0001), (1–102) and (11–20) α-alumina was investigated. The dependence of the CO sticking coefficient on the sample temperature and the relative CO coverage for different Rh deposits were calculated from the intensity of the reflected CO molecular beam. The diffusion of CO over the uncovered substrate to the metal is discussed in relation to the Rh particle size and density.

The interaction of carbon monoxide with Rh/Al2O3 model catalysts: influence of the support structure

Thermal desorption spectroscopy (TDS) has been used to investigate the interaction of CO with rhodium particles evaporated by electron bombardment onto differently oriented single-crystal alumina supports. Rhodium particle populations of similar particle size and density were deposited onto the (0001), (1-102) and (11-20) α-alumina substrates. The CO TDS results depend on the crystallographic orientation of the support. The CO desorption peak shapes and activation energies, as well as the CO2 production which accompanies CO desorption, are compared. The data set obtained enables a discussion of the influence of the alumina support structure on the adsorptive and catalytic activity of the rhodium particles.

Ruthenium Active Catalytic States: Oxidation States and Methanol Oxidation Reactions

New opportunities for narrowing the pressure and material gaps in studies of catalytic systems have been provided by the advances in synchrotron-radiation-based X-ray photoelectron spectroscopy (XPS). The recent achievements in this respect are illustrated by studies revealing the dynamics of the ruthenium oxidation state and the related to that activity and selectivity of the Ru catalyst in methanol oxidation reaction. The microscopy anisotropic oxide growth or oxide reduction, resulting in coexistence of transient RuOx and RuO2 states in a wide temperature–pressure range, evidenced by the XPS provided the basis for identifying the catalytically active states by in-situ high-pressure XPS studies of methanol oxidation. It has been shown that the nonequilibrium transient ruthenium oxides evolve under the specific reaction conditions of methanol oxidation and dramatic changes in the selectivity is correlated to relatively small differences in the oxygen content of the RuOx states, controlled by reactants partial pressures and temperatures.