Local Catalytic Ignition during CO Oxidation on Low-Index Pt and Pd Surfaces: A Combined PEEM, MS, and DFT Study

Shedding light on light-off: Photoemission electron microscopy, DFT, and microkinetic modeling were used to examine the local kinetics in the CO oxidation on individual grains of a polycrystalline sample. It is demonstrated that catalytic ignition (“light-off”) occurs easier on Pd(hkl) domains than on corresponding Pt(hkl) domains. The isothermal determination of kinetic transitions, commonly used in surface science, is fully consistent with the isobaric reactivity monitoring applied in technical catalysis

Strain dependent light-off temperature in catalysis revealed by planar laser-induced fluorescence

Understanding how specific atom sites on metal surfaces lower the energy barrier for chemical reactions is vital in catalysis. Studies on simplified model systems have shown that atoms arranged as steps on the surface play an important role in catalytic reactions, but a direct comparison of how the light-off temperature is affected by the atom orientation on the step has not yet been possible due to methodological constraints. Here we report in situ spatially resolved measurements of the CO production over a cylindrical-shaped Pd catalyst and show that the light-off temperature at different parts of the crystal depends on the step orientation of the two types of steps (named A and B). Our finding is supported by density functional theory calculations, revealing that the steps, in contrast to what has been previously reported in the literature, are not directly involved in the reaction onset but have the role of releasing stress.The authors thank the Knut and Alice Wallenberg Foundation, the Swedish Research Council, the Swedish Foundation for Strategic Research, and the Crafoord Foundation. Support by the MAX IV staff is gratefully acknowledged. The calculations were performed at C3SE through a SNIC grant. J.E.O. acknowledges support from the Spanish Ministry of Economy (MAT2013-46593-C6-4-P) and the Basque Government (IT621-13).Peer Reviewe

Methane oxidation over Pd and Pt studied by DFT and kinetic modeling

Methane dissociation and subsequent formation of water and carbon-dioxide over Pd and Pt are investigated with density functional theory calculations and microkinetic modeling. Adsorption energies for reaction intermediates and activation barriers for CH4 dissociation and water formation are calculated for the (111), (100), (211) and (321) facets. The dissociative adsorption of methane is found to be the rate determining step on all considered facets. The results show that Pt has higher catalytic activity than Pd and that the (100) surface is the most active facet at moderate temperatures for both Pd and Pt. At low temperatures, the reaction is limited, in particular on Pd(100), by poisoning of OH-groups

Metal-oxide sites for facile methane dissociation

Experimental and theoretical studies have lately revealed that under-coordinated Pd-sites in the PdO(101) surface act as efficient centers for methane dissociation. Here, the density functional theory is used to explore the underlying reason for the low activation energy by systematically investigating a range of hypothetical metal-oxides in the PdO structure. Low activation energies are calculated for metal-oxides where the initial state is stabilized via reduced Pauli repulsion between the methane molecule and the site of dissociation. The low activation energy is found to be an atomic property and facile methane dissociation is predicted for single Pd atoms with appropriate ligands. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Oxidation at the Subnanometer Scale

Metals are commonly oxidized under ambient conditions. Although bulk oxidation has received considerable attention, far less is known about oxidation at the subnanometer scale. This is unfortunate, as metal particles used in heterogeneous catalysis typically range from subnanometer to some nanometers. Here, density functional theory calculations are used to explore oxidation of gas-phase transition metal clusters in the range from the dimer to the dodecamer. Comparisons with the corresponding bulk systems uncover that the decomposition temperature of stoichiometrically oxidized clusters may be lower than for the bulk. Despite pronounced variations in ground state geometries, oxidized clusters closely mimic energetic trends across the periodic table valid for bulk systems

The Active Phase of Palladium during Methane Oxidation

The active phase of Pd during methane oxidation is a long- standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts

Strain dependent light-off temperature in catalysis revealed by planar laser-induced fluorescence

Understanding how specific atom sites on metal surfaces lower the energy barrier for chemical reactions is vital in catalysis. Studies on simplified model systems have shown that atoms arranged as steps on the surface play an important role in catalytic reactions, but a direct comparison of how the light-off temperature is affected by the atom orientation on the step has not yet been possible due to methodological constraints. Here we report in situ spatially resolved measurements of the CO2 production over a cylindrical-shaped Pd catalyst and show that the light-off temperature at different parts of the crystal depends on the step orientation of the two types of steps (named A and B). Our finding is supported by density functional theory calculations, revealing that the steps, in contrast to what has been previously reported in the literature, are not directly involved in the reaction onset but have the role of releasing stress

Confluences, liens, rencontres : synergies d’expression française

Les différents volets qui composent ce livre déclinent les mots-clés affichés dans le titre. La première partie de l’ouvrage recueille les contributions qui portent sur les « confluences » entre le lexique et le discours, dans une perspective linguistique et/ou discursive ; la deuxième est un « lien » vers le passé et permet donc de jeter un coup d’œil diachronique sur la langue et la littérature françaises ; ensuite, la partie sur les « rencontres » entre langues et cultures analyse les dimensions traductive et didactique dans une perspective de réci- procité des langues-cultures ; enfin, le dernier volet reste ouvert sur d’autres « synergies » linguistiques et disciplinaires