A careful choice of the surface coverage of iron phthalocyanine (FePc) on Ag (110) around the single monolayer allows us to drive with high precision both the long-range supramolecular arrangement ...and the local adsorption geometry of FePc molecules on the given surface. We show that this opens up the possibility of sharply switching the catalytic activity of FePc in the oxygen reduction reaction and contextual surface oxidation in a reproducible way. A comprehensive and detailed picture built on diverse experimental evidence from scanning tunnelling microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, coupled with density functional theory calculations, sheds new light on the nature of the catalytically active molecule-surface coordination and on the boundary conditions for its occurrence. The results are of relevance for the improvement of the catalytic efficiency of metallo-macrocycles as viable substitutes for platinum in the cathodic compartment of low-temperature fuel cells.
Acetonitrile is used as an effective, non-corrosive selectivity enhancer for the direct synthesis of hydrogen peroxide (HP). Palladium catalysts supported by activated carbon or poly-divinylbenzene ...are poisoned by acetonitrile, which makes them very little productive. Conversely the catalysts supported by strongly acidic ion-exchange resin are much more selective and productive in the presence of acetonitrile, and for relatively long time. TEM shows that the size of their metal nanoparticles can be decreased by a reconstruction process, possibly sustained by leached palladium(II) species. HP decomposition experiments show that acetonitrile inhibits the OO bonds splitting.
•Conventional selectivity enhancers for the direct synthesis of hydrogen peroxide are very corrosive.•Like established enhancers, such as the halide ions, acetonitrile is a good ligand to Pd(II).•The effect of acetonitrile was studied in Pd on carbon and ion-exchange resins catalysts.•Acetonitrile inhibits O-O splitting and works as an enhancer only with Pd catalysts supported on ion-exchangers.••The metal nanoparticles are reconstructed through a process of release-and-capture of Pd.
Iron phthalocyanines (FePc) adsorbed onto a Ag(110) substrate self-assemble into different monolayer phases going from rectangular to different oblique phases, with increasing molecular density. We ...have investigated the oxygen uptake capability of the different phases and their associated magneto-structural changes. Our study combines scanning tunneling microscopy and spectroscopy (STM/STS), X-ray magnetic circular dichroism (XMCD), and density functional theory (DFT) calculations. STM measurements reveal that the oxygenation reaction of the FePc/Ag(110) generally involves a displacement and a rotation of the molecules, which affects the electronic state of the Fe centers. The oxygen intercalation between FePc and the substrate is greatly obstructed by the steric hindrance in the high-density phases, to the point that a fraction of oblique phase molecules cannot change their position after oxidizing. Depending on the oxidation state and adsoption geometry, the STS spectra show clear differences in the Fe local density of states, which are mirrored in the XAS and XMCD experiments. Particularly, XMCD spectra of the oxidized phases reflect the distribution of FePc species (nonoxygenated, oxygenated-rotated, and oxygenated-unrotated) in the different cases. Sum rule analysis yields the effective spin (m s eff) and orbital (m L) magnetic moments of Fe in the different FePc species. Upon oxygenation, the magnetic moment of FePc molecules increases about an order of magnitude, reaching m TOT ∼ 2.2 μB per Fe atom.
Iron(II) phthalocyanine (FePc) self-assembly on Ag(110) has been studied in ultrahigh-vacuum conditions at room temperature by means of scanning tunneling microscopy (STM) and density functional ...theory (DFT) calculations. For submonolayer to monolayer coverage, FePc molecules lie parallel to the Ag(110) surface, arranged in rows running along the 001 direction. Two similar yet distinct ordered phases are formed, the c(10 × 4) and p(10 × 4) superstructures. The latter is characterized by two equivalent equilibrium configurations of the constituent FePc units, that interconvert by means of a concerted transformation wherein molecules belonging to adjacent rows collectively rotate in opposite directions around the molecular axis perpendicular to the surface. The FePc adsorption site for both superstructures and the transition mechanism between the two configurations in the p(10 × 4) phase have been inferred from high resolution STM images and rationalized by means of DFT calculations. In the case of multilayer films a (1 ± 4, 4 ∓3) phase is observed, whose stacking geometry has been revealed by STM analysis. The p(10 × 4)/c(10 × 4) → (1 ± 4, 4 ∓3) coverage-dependent transformation is analyzed by DFT calculations, and shown to be driven by the overlayer−substrate interaction. The inclusion of the dispersion contribution to this interaction is found to be essential to correctly reproduce the observed phenomenon.
Multicomponent supramolecular self-assemblies of exceptional long-range order and low defectivity are obtained if C60 and 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (TPP-NH2) are assembled on ...Ag(110) by sequential evaporation in the submonolayer range of TPP-NH2 and fullerene on the substrate surface and subsequent annealing. A (±2 −3, 6 ± 3) array consisting of supramolecular stripes of a 1:1 C60/TPP-NH2 2D adduct develops at 410 K (the low temperature, LT, phase). If the LT phase is annealed at 470 K, then a 3:1 fullerene/TPP-NH2 (±3 −5, 5 ± 5) nanoporous array (the HT phase) forms, with each pore containing a single porphyrin molecule. Phase separation occurs by annealing the HT phase at 520 K. Structural models are proposed and discussed on the basis of the experimental scanning tunneling microscopy results.
Linearly polarized soft X-ray absorption spectroscopy experiments both on Ni L2,3 and O K edges have been carried out on NiO ultrathin films on Pd(100), from the c(4 × 2) defective monolayer up to ...fully relaxed films. In the defective monolayer a dichroic effect on the Ni L2,3 edge has been detected, and it has been associated to anisotropic structural effects (e.g., local crystal field symmetry) due to the spatial confinement. This produces a change in the energy sequence of the Ni 3d levels resulting in different transition probabilities for the various levels at different polarization. A support to this interpretation has been also obtained by theoretical computations. In the case of thicker films, a clear dichroic effect, reversed with respect to the one observed for a bulk like NiO(100) surface, has been observed for the L2 white line at all the explored thicknesses. This has been associated to a different spin structure of the ultrathin films, where the domains are preferentially aligned to the surface plane instead of being perpendicular to it. However, we have not observed the inversion of dichroism even for very thick fully relaxed films. The dependence of the dichroic effect on the preparation procedure of the films has been also explored. For the thicker films the dichroic effect is independent from the actual oxygen pressure used during the growth, while remarkable differences are found in the case of the thinner films.
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