Table of Contents

Abstract: n/a

Abstract: TS-1 and Fe-MFI systems have been chosen as cases studies for describing the behavior of single-site catalysts. We will report a concise review of the firmly established knowledge and of the open problems concerning the structure and the reactivity of Ti and Fe sites in TS-1 and Fe-MFI in partial oxidation catalysts. Some new experimental and theoretical results will also be described.

Abstract: Some aspects of the reaction mechanisms in multistep selective (amm)oxidation reactions over oxide surfaces are discussed evidencing some of the challenges for surface science and theory in describing these reactions, and for applied catalysis in order to have a more in deep identification of the key factors governing surface reactivity and which may be used to improve catalytic performances. In particular, the role of chemisorbed species in the modification of the surface reactivity and the presence of multiple pathways of reaction are evidenced by comparing the behavior of V-based catalysts in C3-C4 alkanes and alkene oxidation.

Abstract: The catalytic properties of LaCoO3 in aqueous oxidation are explored as a function of doping. Both Sr substitution for La and Fe/Ni substitution for Co are studied. The reaction of interest is the aqueous epoxidation of crotyl alcohol with hydrogen peroxide. The reaction products are measured using GC, and the decomposition of hydrogen peroxide is studied using the volume of oxygen evolved during reaction. Strong variations in the activity to epoxide formation are observed, with Fe-doped samples being rather inactive in the reaction. SR VUV photoemission is used to explore the surface reactivity of the ceramic catalysts in aqueous solution, using H2O as a probe molecule. These measurements are complemented by XANES measurements designed to probe the local defect structure and XPS measurements of surface composition. We relate the observed catalytic activity to the defect structure of the doped materials. In Ni-doped materials, oxygen vacancies appear to be the predominant defect, whereas in Fe-doped samples, electron holes are stabilized on Fe, leading to very different catalytic behaviour. Surface studies in combination with AA measurements reveal some dissolution of the catalyst into solution during the reaction. The surface reactivity to water is influenced by the TM d electron count, with water binding more strongly to Fe-doped materials than to those containing Ni. The influence of these factors on the rate of the unwanted hydrogen peroxide decomposition reaction and hence on activity in epoxidation is discussed.

Abstract: The density functional theory (DFT) based hybrid-method B3LYP has been used to study the interaction of the nitromethane molecule (CH₃NO₂) with the Au(111) surface. The perfect Au(111) surface has been represented by a rather large cluster model, Au₂₂, that was in turn used to extract information about the preferred adsorption geometry of the CH₃NO₂ species. In order to extract energetic information about the stability of adsorbed nitromethane, calculations were also performed for the gas-phase CH₃NO₂ molecule. The results obtained here are used to interpret experimental data. The computed geometry for adsorbed CH₃NO₂ agrees with the structure proposed from a previous experimental work.

Abstract: The H₂/O₂ reaction on a polycrystalline palladium foil has been studied. The experimental methods used were Laser-Induced Fluorescence (LIF) and microcalorimetry. The reaction was also simulated with the Chemkin software package. The water production maximum occurs at 40% H₂ and the OH desorption has its maximum at 10% H₂, at a total pressure of 100 mTorr (13 Pa) and a catalyst temperature of 1300 K. It is concluded, in agreement with the simulations, that the main water-forming reaction at these experimental conditions is OH+H→H₂O. From the water production maximum the quotient of the initial sticking coefficient for hydrogen and oxygen (S_H2(0)/S_O2(0)) is calculated to be 0.75(±0.1).

Abstract: Alumina supported CuCl₂, the basic catalyst for ethylene oxychlorination, has been investigated by UV-Vis spectroscopy, EPR, EXAFS and XANES in a wide range (0.25-9.0 wt%) of Cu concentration. We have evidenced that, at low Cu content, the formation of a surface aluminate species takes place. The formation of this surface copper aluminate stops at 0.95 wt% Cu / 100 m²; at higher Cu concentrations excess copper chloride precipitates directly from solution during the drying step forming an highly dispersed CuCl₂^.H₂O, phase, overlapping progressively the surface aluminate. Depletion tests and IR spectroscopy of adsorbed NO have demonstrated that the latter is the only active phase. A complete catalytic cycle has then been performed on CuCl₂/Al₂O₃ catalyst. EPR, XANES and EXAFS, have been used to demonstrate that the ethylene oxychlorination reaction: C₂H₄ + 2HCl + ½ O₂ --> C₂H₄Cl₂ + H₂O follows a three steps mechanism: (i) reduction of CuCl₂ to CuCl (2CuCl₂ + C₂H₄ --> C₂H₄Cl₂ + 2CuCl), (ii) oxidation of CuCl to give an oxychloride (2CuCl + ½ O₂ --> Cu₂OCl₂) and (iii) closure of the catalytic circle by rechlorination with HCl, restoring the original CuCl₂ (Cu₂OCl₂ + 2HCl --> 2CuCl₂ + H₂O). Finally, we have shown that time resolved, in situ, spectroscopy is a very promising technique to investigate the interplay between catalyst activity and oxidation state of copper.

Abstract: The growth of Pt deposits on Co(0001) was followed by STM and XPS. The chemical reactivity of the resulting surface was checked by CO adsorption. Pt grows as dendritic islands on the Co terraces whereas forming stripes at the Co step edges. Annealing the sample has no apparent effect on the STM pictures. However, XPS suggests that a limited dilution of Pt in Co takes place. The adsorption of CO on the surface is drastically affected by the presence of Pt even for minute traces. The adsorption energy on the Pt areas is decreased by 40 %. The maximum coverage on the Co areas is also decreased. This indicates that Pt impurities diluted in Co have a high passivating power as a consequence of the induced electronic changes.

Abstract: This paper provides a review of recently obtained spectroscopic results, ab-initio calculations and catalytic activity tests from the authors´ laboratory of factors affecting the performance of materials used as automobile catalysts. Issues addressed include the preparation of (Ce,Zr)O_x/Al₂O₃ (CZA) mixed oxide supports and the use of new active metals, such as Pd and Cu, for three-way catalysts (TWCs); the performance of non-noble transition metals, such as Co or Ag, for pollution control under lean conditions; and the use of Pt/BaO_x/Al₂O₃ (PtBa) for engine exhaust gas treatment under oscillating lean-stoichiometric conditions.

Abstract: The structure and local electron properties of Au atoms deposited on the TiN (001) surface has been theoretically analyzed using a periodic slab model and density functional based calculations. The surface is described by means of a 2x2 cell five layers thick, on which gold atoms are added. Deposition of single atoms on the surface, (θ = 0.25 ML), shows that the preferred site is on-top of Ti atoms, with a metal-surface distance of 2.49 Å. The computed adsorption energy for this site is -1.92 eV, only slightly lower than that lying between two Ti surface atoms (-1.90 eV). The on-top nitrogen sites are less favorable by about 0.4 eV. The calculations were carried out using the Perdew-Wang 91 exchange correlation functional and ultra soft pseudopotentials, with electronic states represented by a plane-wave expansion.

Abstract: Ab initio simulations of the Ag/a-Al₂O₃(0001) interface have been performed for periodic slab models. We have considered Al- and O-terminated corundum surfaces, low and high substrate coverages by silver, as well as the two preferred Ag adsorption sites. The two different terminations give rise to qualitatively different results: silver physisorption on the Al-terminated substrate and chemisorption on O-terminated one. The latter could be treated as a possible model for the defective Al-terminated substrate, where the outermost aluminium ions are removed (completely or partly). This makes O-terminated surface highly reactive towards a deposited metal, in order to restore initial corundum stoichiometry.