96 research outputs found
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
The effect of different InO(111) surface terminations on CO adsorption
InO-based catalysts have shown high activity and selectivity for
CO hydrogenation to methanol, however the origin of the high performance of
InO is still unclear. To elucidate the initial steps of CO
hydrogenation over InO, we have combined X-ray Photoelectron
Spectroscopy (XPS) and Density Functional Theory (DFT) calculations to study
the adsorption of CO on the InO(111) crystalline surface with
different terminations, namely the stoichiometric, the reduced, and the
hydroxylated surface, respectively. The combined approach confirms that the
reduction of the surface results in the formation of In ad-atoms and that water
dissociates on the surface at room temperature. A comparison of the
experimental spectra and the computed core-level-shifts (using methanol and
formic acid as benchmark molecules) suggests that CO adsorbs as a carbonate
on all surface terminations. We find that CO adsorption is hindered by
hydroxyl groups on the hydroxylated surface.Comment: 49 pages, 18 figure
Structure of the SnO2(110)-(4 x 1) Surface
Using surface x-ray diffraction (SXRD), quantitative low-energy electron diffraction (LEED), and
density-functional theory (DFT) calculations, we have determined the structure of the (4 × 1)
reconstruction formed by sputtering and annealing of the SnO2ð110Þ surface. We find that the
reconstruction consists of an ordered arrangement of Sn3O3 clusters bound atop the bulk-terminated
SnO2ð110Þ surface. The model was found by application of a DFT-based evolutionary algorithm with
surface compositions based on SXRD, and shows excellent agreement with LEED and with previously
published scanning tunneling microscopy measurements. The model proposed previously consisting of inplane
oxygen vacancies is thus shown to be incorrect, and our result suggests instead that Sn(II) species in
interstitial positions are the more relevant features of reduced SnO2ð110Þ surfaces
Stability, magnetic order, and electronic properties of ultrathin Fe3O4 nanosheets
We study the stability, magnetic order, charge segregation, and electronic properties of a novel three-layered Fe3O4 film by means of Hubbard-corrected density functional theory calculations. The stable film is predicted to consist of close-packed iron and oxygen layers, comprising a center layer with octahedrally coordinated Fe sandwiched between two layers with tetrahedrally coordinated Fe. The film exhibits an antiferromagnetic type I spin order. A charge analysis confirms that the stable structure has distinct charge segregation, with Fe2+ ions in the center layer and Fe3+ in the tetrahedral surface layers. Examination of the electronic band structures and densities of states shows that the bandgap is substantially reduced, from 2.4 eV for the bulk rocksalt to 0.3 eV for the film. The reduction in the bandgap is a consequence of the 2+ to 3+ change in oxidation state of Fe in the surface layers
Co3O4(100) films grown on Ag(100): Structure and chemical properties
Spinel type Co3O4(100) is successfully grown on Ag(100) at ultrahigh vacuum conditions and its structure, electronic and chemical properties are compared with those of Co3O4(111) grown on Ir(100). We find that the Co3O4(100) is unreconstructed. In contrast to the defect free Co3O4(111) surface the Co3O4(100) surface contains a high concentration of defects that we assign to subsurface cation vacancies analogous to those observed for Fe3O4(100). Our photoemission and absorption spectroscopy experiments reveal a very similar electronic structure of the Co3O4(111) and Co3O4(100) surfaces. The similar electronic structure of the two surfaces is reflected in the CO adsorption properties at low temperatures, as we observe adsorption of molecular CO as well as the formation of carbonate (CO3) species on both surfaces upon CO exposure at 85 K
Step dynamics and oxide formation during CO oxidation over a vicinal Pd surface.
In an attempt to bridge the material and pressure gaps - two major challenges for an atomic scale understanding of heterogeneous catalysis - we employed high-energy surface X-ray diffraction as a tool to study the Pd(553) surface in situ under changing reaction conditions during CO oxidation. The diffraction patterns recorded under CO rich reaction conditions are characteristic for the metallic state of the surface. In an environment with low excess of O2 over the reaction stoichiometry, the surface seems to accommodate oxygen atoms along the steps forming one or several subsequent adsorbate structures and rapidly transforms into a combination of (332), (111) and (331) facets likely providing the room for the formation of a surface oxide. For the case of large excess of O2, the diffraction data show the presence of a multilayer PdO with the [101] crystallographic direction parallel to the [111] and the [331] directions of the substrate. The reconstructions in O2 excess are to a large extent similar to those previously reported for pure O2 exposures by Westerström et al. [R. Westerström et al., Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 155410]
Redox behavior of iron at the surface of an single crystal studied by ambient-pressure photoelectron spectroscopy
We have studied the oxidation and reduction of iron in an Fe-doped MgO single crystal by , and using ambient-pressure XPS and NEXAFS. Surface charging of the crystal was rendered manageable by the elevated temperatures and the gas atmospheres. The oxidation state of iron was found to shift reversibly between the and states, with a strong asymmetry in the rates; while oxidation by or was nearly complete at , reduction by began at , and was still incomplete at . Grazing-incidence XRD characterization of the crystal indicated the presence of octahedral, nanoscale inclusions assigned to the magnesioferrite spinel . It is proposed that the redox behavior observed involves interconversion between the rock-salt and spinel phases, with the more open lattice containing enabling more rapid ion diffusion and thus more facile oxidation compared to reduction
Electrochemical Oxidation Of Size-Selected Pt Nanoparticles Studied Using In Situ High-Energy-Resolution X-Ray Absorption Spectroscopy
High-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) has been applied to study the chemical state of ∼1.2 nm size-selected Pt nanoparticles (NPs) in an electrochemical environment under potential control. Spectral features due to chemisorbed hydrogen, chemisorbed O/OH, and platinum oxides can be distinguished with increasing potential. Pt electro-oxidation follows two competitive pathways involving both oxide formation and Pt dissolution. © 2012 American Chemical Society
- …